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The aim of this paper is to propose an efficient numerical scheme to deal with heat transfer problems in pipe flow with a large length/diameter ratio. The generalized finite difference method (GFDM) is combined with a space stepping algorithm (SSA) for the solution. The SSA divides the solution domain into a number of overlapping sections in the length direction of the pipe. Due to the large Peclet number, the heat transport process is dominated by advection, which allows an approximate boundary condition to be applied at the downstream cross section. The problem is then solved section by section. Using the uniform distributed points, the solution matrix for each section does not change, and only the right hand side needs to be refreshed for the changing boundary conditions. This leads to a highly efficient scheme for problems with a large pipe length. To show the accuracy of the numerical scheme, a problem with available analytical solution is studied. Then the method is applied to problems of single pipe with different pipe wall temperature and flow Reynolds number, and concentric annular pipe. Numerical results confirm the stability and efficiency of the GFDM-SSA. The method can be applied to many real world transient heat transfer problems in which the pipe is heated or cooled along its length with arbitrary wall temperature for heat exchange and other purposes. [ABSTRACT FROM AUTHOR]

This communication concerns some doubtful results included in the above paper. [ABSTRACT FROM AUTHOR]

This paper is the second of a two-part series that presents experimental data and analysis on the liquid nitrogen quenching heat transfer process of a stainless-steel tube with an inner surface low-thermal conductivity thin-film coating. This paper focuses on the effects of different flow directions and two tube wall thicknesses. Additionally, this paper also provides an analysis on the chilldown thermal efficiency of the quenching process. Three flow directions with four different inner surface coating modifications, and three tube wall thicknesses were examined. The experimental data covers the Reynolds numbers ranging from 3500 to 140,000. The chilldown efficiency, along with chilldown time and LN 2 (liquid nitrogen) mass consumption were analyzed to assess the overall performance of the LN 2 line chilldown process. For thin tube wall cases, the chilldown efficiencies cover a range between 3% and 41%, and the maximum chilldown efficiency value is found for the tube with 3 L coating at Re = 5278 in the vertical up flow direction. For thick wall tube cases, the efficiencies cover a range between 4% and 52%, and the maximum chilldown efficiency value was found for the tube with 4 L coating at Re = 5308 in the vertical up flow direction. [ABSTRACT FROM AUTHOR]

In this paper, the gas turbine blades middle double swirl cooling structure under diverse number of nozzles N are constructed to explore the heat transfer and flow behavior. The k-ω turbulence model are applied and the inlet coolant Reynolds number is 12500. The N vary by 4 to 11 and two cases are investigated. For case 1, the cross-sectional area of each nozzle is 6 mm2 for all structures. For case 2, the cross-sectional area of total nozzles stays the constant at 42mm2 when the N is varied. Results show that, in case 1, when the N increases, the area of the high heat exchange zone decreases but the distribution is more uniform. Moreover, the thermal performance factor ξ gets larger, but the average Nusselt number Nu a decreases significantly. In case 2, when the N increases, the ratio of inclination to nozzle length increases for the jet near the outlet, Nu a is almost constant but the heat transfer uniformity is improved. The best ξ is obtained when the N is 8. For case 1, as the N is increased, the Nu a decreases significantly. Therefore, heat transfer and ξ should be considered by case 2 to select the optimum N. • Middle double swirl cooling models with different nozzle numbers are established. • Effects of nozzle number on flow and heat transfer characteristics are explored by two cases. • When the nozzle number is changed, the cross-sectional area of total nozzles in case 1 is changed, but it unchanged in case 2. • The results show that case 2 is more appropriate and the optimum nozzle numbers should be selected by the way of case 2. [ABSTRACT FROM AUTHOR]

• In this paper is provided an calculation expression for heat transfer during condensation inside tubes that provides an adequate fit with experimental data available from a total of 22 fluids, including refrigerants, water, and a variety of substances organic. This paper presents a mathematical deduction of a new improved model for heat transfer during condensation inside tubes. This new model has been developed with the aid of the Gaussian Equation for an infinite straight line, considering this relation with the differential equations that govern the heat transfer process. The proposed model was verified by comparison with available experimental data of 22 different fluids, including various refrigerants, water, and organic substances, which condense inside vertical, inclined and horizontal tubes. The proposed model is valid for an reduced pressure values ranging from 0.0008 to 0.91, values of Reynolds number for single-phase between 68 and 84,827 and for Reynolds number for two-phase between 900 and 59,4373, a range of internal diameters ranging from 2 to 50 mm, vapor quality from 0.01 to 0.99, P r values for single-phase from 1 to 18 and mass flux rates in the ranges of 3 to 850 kg/(m2s). The mean deviation found for the analyzed data for horizontal tubes was 11.8%, while for the vertical and inclined tubes data the mean deviation was 13.0%. In all cases, the agreement of the proposed model is good enough to be considered satisfactory for practical design. [ABSTRACT FROM AUTHOR]

This paper is the first of a two-part series that presents experimental data and analysis on the liquid nitrogen quenching heat transfer process of a stainless-steel tube with an inner surface low-thermal conductivity thin-film coating. The experimental data covers a wide range of flow mass fluxes (43 to 1077 kg/m2s). The experiments were conducted using test tubes with four different coating thicknesses to investigate the effects of various Teflon coating thicknesses. An analysis of the chilldown time and LN 2 mass consumption data are presented to evaluate the effects of the flow mass flux, axial distance from the tube inlet, and coating layer thickness. The chilldown time decreases with increasing flow Re. However, the chilldown mass consumption increases with increasing flow Re. In general, the larger the flow Reynolds number, the lower the percentage reduction in chilldown time is observed. For the percentage reduction in the chilldown time by the coating, the minimum is 27.8% for the tube with 7-layer coating at flow of Re = 100,000 and the maximum is 61.3% for the tube with 14-layer coating at flow of Re = 10,000. The percentage reduction in mass consumption due to coating falls in the range between 35.3% and 54.1%. [ABSTRACT FROM AUTHOR]

This paper explores the impact of hybridisation and geometric variations in micro heat exchangers featuring cylindrical fins. The primary aim of the study is to maximise the thermal conductance. Solid, half-hollow and hollow circular fins are modeled and mounted on a heat sink with a rectangular flow channel. A high-density heat flux of magnitude 500 W / cm 2 is dissipated on the bottom surface by a microelectronic device. Single-phase water of Reynolds number range of between 400 and 500 is supplied through the inlet of the heat sink to remove the heat at the bottom and in the inner walls of the hollows, while cool air stream of Reynolds number range 10 ≤ Re a ≤ 14 is allowed to convectively flow over the vertical micro cylindrical fins to take away extra heat deposited on the fins. The computation domain is descretised and computational fluid dynamic code applied to solve mathematical equations. The numerical results show that the thermal conductance in the half-hollow fins is 16.4% greater than in the solid fins, and 0.14% higher than the hollow fins in parallel flow, while in counter flow the half-hollow fins increase above the solid fins by 5% and by 0.5% higher than hollow fins. The integrated micro heat sink with half-hollow is superior in both parallel and counter flow. Experimental data is used to validate the numerical prediction. [Display omitted] • A groundbreaking method for cooling micro heat exchangers. • Dual cooling technique in integrated heat sink. • The application of cylindrical fins in an integrated heat sink. • A novel approach to enhance heat transfer efficiency in heat sinks. • A cutting-edge innovation in heat sinks with a hybrid configuration. [ABSTRACT FROM AUTHOR]

Rising temperature of gas turbine inlet brings great challenge for blade temperature tolerance, thus high effective cooling structure for turbine blade is vital. In this paper, aerothermodynamic features including heat transfer and pressure drop of a Coriolis-applied rotating smooth U channel are studied in detail. Both experiment and numerical methods are utilized in the study. A rotating cooling structure experiment rig system has been established and validated. The experiments were conducted in the rotating cooling structure experiment rig. Besides, RANS was chosen in the simulation. In the experiment and numerical studies, Reynolds number is constant at 16000 and Rotation number ranges from 0 to 0.024. The results indicate that Coriolis force plays a dominant and positive role on heat transfer ability on trailing wall, and that bend outlet impingement effect is the primary factor compared to Coriolis force for the Nusselt number of smooth leading surface. Besides, Coriolis force at bend region probably suppresses the formation of K–H vortices, thus boosts the flow stability, reduce pressure loss of channel and weakens second heat transfer peak of impingement at the bend outlet. What's more, the total performance of the Coriolis-applied smooth U channel at Ro of 0.006 and 0.024 are 2.18 % and 8.51 % higher than the channel at Ro of 0, which means that rotation is good for the Coriolis-applied smooth U channel and that the channel has superior performance and is encouraging in future rotating blade cooling application. • A Coriolis-applied smooth U channel is proposed and investigated. • Detail heat transfer experiments on the channel were implemented. • Simulations were conducted to unveil the heat transfer and pressure drop mechanisms. • Rotation enhances overall heat transfer of the Coriolis-applied smooth U channel. • Rotation brings pressure loss of the Coriolis-applied smooth U channel decreased. [ABSTRACT FROM AUTHOR]

This paper presents a comprehensive investigation of heat transfer enhancement and pressure drop in a Coiled Flow Reverser (CFR) with a twisted tape insert. Both laminar (10 < Re < 2000) and turbulent (12,000 < Re < 18,000) regimes have been studied using both experimental and numerical methods. The effects of twist ratio of twisted tape, inlet Reynolds number, and perforating the twisted tapes have been investigated. The study found that the best performance in the laminar regime is achieved with a twisted tape with a twist ratio of 15.73 and no holes on it at a Reynolds number of 2000. In the turbulent regime, the same twisted tape performs the best at a Reynolds number of 17,000. The highest Thermal Performance Ratio (TPR) in laminar regime is 1.55 and in turbulent regime is 0.99. In comparison to the straight pipe, all types of CFRs show better performance in heat transfer enhancement. Also, as the Reynolds number increases, the friction coefficient decreases in all configurations. The study reveals that the twisted tape insert enhances heat transfer performance while increasing pressure drop. This study provides valuable insights into the design and optimization of heat exchangers with CFR. The findings can be used to improve the efficiency and performance of various industrial processes, such as refrigeration systems, air conditioning, and power plants. • Fluid flow and heat transfer in a coiled flow reverser (CFR) were studied. • Effects of tube insert on the heat transfer enhancement of CFR were investigated. • Effects of perforation of tube insert on heat transfer enhancement of CFR were examined. • The effects of tube insert at laminar flow are more than turbulent flow. • Perforation has no significant effects on the thermal performance ratio. [ABSTRACT FROM AUTHOR]

With the development of science and technology, the heat dissipation problem of high-density electronic products has gradually emerged in engineering practice. In this paper, the enhanced heat dissipation process of fluid induced oscillation through flexible membrane is proposed, which belongs to the multi-field synergistic bidirectional coupling effect of thermal-fluid-solid coupling. Through the comparative numerical analysis of the flow induced oscillation of the flexible membrane and the fixed rigid fin in the flow channel, the mechanism of the fluid induced oscillation of the flexible polyethylene membrane is revealed. Based on the two-way fluid-solid coupling method, a numerical model for the heat transfer process of polyethylene membrane with thickness of 0.02 mm in the fluid channel was established. The temperature field, Nu (Nusselt number) and PEC (Performance comparison index) number of the rigid fin and polyethylene membrane installed in the channel under different Reynolds numbers were calculated respectively. The ratio of the length of the membrane to the height of the channel was adjusted for multiple groups of calculations to verify the positive effect of the polyethylene membrane installed in the channel on the heat exchange effect. The results show that in the fully developed area of heat transfer, the channel with polyethylene membrane is better than that with rigid fins in terms of turbulence and vortex shedding; at Re = 4400, the comprehensive enhanced heat transfer factor of heat exchange is the largest, up to 1.27; as the ratio of the height of the new flexible membrane to the height of the channel is 1, the heat transfer enhancement effect is best. In the next step, the correlation theory between oscillation characterization parameters and heat transfer enhancement effect under pulsating fluid excitation will be investigated. [ABSTRACT FROM AUTHOR]

Abstract Winglet pairs are promising longitudinal vortex generators which can be used to produce streamwise vortices that do not decay until further downstream and consequently increase heat transfer rate with comparatively lower pressure penalty. This paper deals with the effect of delta winglet vortex generator (DWVG) pairs on thermal and flow behaviors in a circular tube for Reynolds numbers (Re) range of 5000–25000. The DWVG pairs involved are the pitch ratio (PR = 9.6), four attack angles (α = 10°, 20°, 30° and 40°), three winglet height (h = 5 mm, 7.5 mm and 10 mm) and three spacing between leading edges (s = 10 mm, 15 mm and 20 mm). The experimental results indicate that the Nusselt number (Nu) increases with Re while friction factor (f) decreases with Re. Nusselt number and friction factors both are increasing with attack angle and winglet height, while the middle spacing yields the highest Nu and f. Maximum Nusselt number increment (Nu / Nu 0) with the DWVG pairs was observed as being 73% larger than that of smooth tube, while the maximum friction factor increment (f / f 0) was 2.5 times larger. Thermal enhancement factor (TEF) decreases with Re. The largest TEF obtained, 1.44, is with the combination of α30°s15 h 7.5 at Re = 5000. Compared with other types of VGs in published experimental research papers, the current DWVG pairs show better thermal performance than many of them. Vortices downstream of the DWVG are visualized with smoke flow for better understanding of the flow behavior. [ABSTRACT FROM AUTHOR]

Highlights • Method of images is successfully adopted to predict the LVG vortex dynamics. • A mixed LVG configuration is suggested to enhance the heat transfer performance. • LVG aspect ratio and channel height are very critical to the LVG effectiveness. Abstract This paper reports a numerical investigation of fluid flow and heat transfer in a rectangular channel with delta-shaped winglet longitudinal vortex generators (LVGs) under different configurations. In addition to the conventional common-flow-down configuration and the common-flow-up configuration, a unique mixed configuration is suggested. The "method of images" is successfully adopted to analyze the dynamics of the longitudinal vortices due to wall interference. The Nusselt number, friction factor and overall performance coefficient for the three configurations are compared at various Reynolds numbers (all less than 2200), LVG row numbers, channel heights, and LVG aspect ratios. It is found that the channel height and LVG aspect ratio are the two most critical factors influencing the effectiveness of the different LVG configurations, which may explain why inconsistent conclusions have been presented by previous studies comparing the performance of the common-flow-down and common-flow-up configurations. When the channel height is relatively small, the unique mixed configuration is found to be the most effective at enhancing fluid mixing (and therefore improving heat transfer) at streamwise cross sections. When the LVG aspect ratio becomes large, the common-flow-down configuration outperforms the common-flow-up configuration. This paper sheds some insight on how to design the optimal LVG configuration for enhancing heat transfer performance. [ABSTRACT FROM AUTHOR]

Highlights • Composite fin with high thermal conductivity coatings deliver large heat transfer enhancement. • At high Re typical of Engine Section Stator vanes, the coating thickness should be around 100 μm. • Composite vanes would be more effective at low Re, as predicted also for composite fins. • Using the internal web structure of vanes can improve heat transfer in long vanes. Abstract This paper main goal is aimed at a paradigm shift in the enhancement of heat transfer rate between finned surfaces and surrounding fluid by presenting a novel approach in composite fins. This approach consists in using high thermally conductive coatings on top of the finned substrate in order to increase the local temperature along the fin washed surface. In the present paper a high thermal conductivity coating has been applied to an aeroengine vane of different shape and dimensions subject to icing conditions at high Reynolds numbers and where the main aim was to keep the vane warm as to avoid icing in aerospace applications. Numerical simulations have been carried out to ascertain the range of thickness of the coatings to be used to maximise the wanted effect. Both short Engine Section Stator (ESS) and longer fan Outlet Guide Vane (OGV) have been modelled, having different final goals. In the case of longer OGV, an additional novel design modification has been suggested to enhance further the heat transfer along the vane length by the use of the internal webs. The experimental validation also carried out at much higher Reynolds numbers than that reported in Buffone et al. (2005), demonstrate that the novel concept of heat transfer enhancement in composite fins is a simple and yet powerful strategy in a wide range of Reynolds numbers. A fin analysis has been performed of both the present ESS vane the fins tested in Buffone et al. (2005) at much lower Reynolds numbers and shows that the improvement obtained with the coated fins tested in Buffone et al. (2005) is much larger than the coated ESS vanes investigated in the present study. This said, the present study demonstrates that the use of high conductive coatings in composite fins can keep the ESS vanes ice free, something that was not possible with uncoated vanes. It is important to note that the actual optimal thickness of the thermally conductive coatings is a function of Biot number, fin shape, dimensions and thermal conductivities of fin substrate and coating; depending on the application, a proper design of the fin substrate and coating should be carried out. [ABSTRACT FROM AUTHOR]

Abstract This paper presents analyses of complex flows and heat transfer induced by twin oblique turbulent slot-jets of different directions (divergent, convergent or parallel) impinging a heated wall. A comparison of the heat transfer characteristics between perpendicular and three cases of twin oblique jets (parallel, convergent and divergent). The twin slot jets are located on a confining adiabatic wall at a distance of 8 slot jet width. Convective heat is investigated numerically examining the effect of Reynolds number (Re) and jet inclination angle (α). This problem is relevant to a wide range of practical applications including nuclear engineering devices, manufacturing, material processing, electronic cooling, drying paper or textile, tempering of glass, etc. All computations are performed using two dimensional large eddy simulations (LES) approach with Smagorinsky sub-grid scale (SGS) models. For all directions and inclinations of the jets, the location of the stagnation points is changed and hence, the location and magnitude of the maximum Nusselt number on the heated wall vary. When Reynolds number increases, Nusselt number is improved for all types of inclination. The averaged Nusselt number shows that the perpendicular impingement gives better heat transfer than that of the oblique jets. The poor heat transfer is obtained for the parallel oblique jets. For the same angle, divergent jets give smallest heat transfer than the convergent jets. Graphical abstract Twin-jets flow configuration. (a) Perpendicular (b) Divergent (c) Parallel (d) Convergent. Image 1 Highlights • Four types of flow patterns of impinging twin jet are studied by Large Dissipation Simulation. • Nusselt number is enhanced for all types of inclination for increasing Reynolds number. • Averaged Nusselt number of the perpendicular impingement gives better heat transfer than that of the oblique jets. • The poor heat transfer is obtained for the parallel oblique jets. • For the same angle, divergent jets give smallest heat transfer than the convergent jets. [ABSTRACT FROM AUTHOR]

The interaction of longitudinal vortices decreases the intensity of longitudinal vortices and inevitably affects the heat transfer performance of heat exchangers. In this paper, the effects of transverse distance of vortex generators (VGs) on the interaction of longitudinal vortices and the heat transfer performance are quantitatively studied. The increments of longitudinal vortex intensity, Nusselt number and friction factor resulted from the application of VGs are discussed in detail. The results show that the transverse distance of VGs obviously affects the interaction of longitudinal vortices, the heat transfer enhancement and the pressure loss characteristics of the heat exchanger. The interaction of co-rotating longitudinal vortices generated by VGs around the same tube is less affected by the transverse distance of VGs. While the interaction of counter-rotating longitudinal vortices generated by VGs around different tubes is closely related to the transverse distance of VGs. The interaction between counter-rotating longitudinal vortices plays a dominant role in the interaction process of longitudinal vortices. Optimal transverse distance of VGs exists for best heat transfer performance of the heat exchanger. Meanwhile, the transverse distance which leads to the worst heat transfer performance is also reported. For the largest Reynolds number studied in this paper, the maximum differences in the increments of intensity of longitudinal vortices, Nusselt number and friction factor for different transverse distance of VGs are 34.0%, 33.9% and 18.5%, respectively. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • The effect of variable baffle configurations on HX performance was investigated. • The influence of water mass flow and heat flux on the HTC was presented. • Experimental correlation for investigated configurations was developed. • Recognized the secondary fluid motion influence on heat transfer was investigated. • Compared the efficiency of the proposed shell-coil design with baffles with typical heat exchanger designs was presented. Abstract The paper presents the possibility of using passive intensification of heat transfer in the form of baffles to increase the energy efficiency of the shell and coil heat exchanger. The experiment was carried out by using a modular coil heat exchanger in the form of an electric heater. Water was used as a working fluid with constant thermal-flow parameters at the inlet of the module. It should be noted that experiments were made for a large range of power i.e. from 200 W to 1200 W and with mass flow rates from 0.01 kg−1 to 0.03 kg−1. For this flow range, Reynolds numbers were obtained in the field 150 < Re < 450. Consequently, all experiments were carried out for laminar flow conditions. This work shows, that due to the presence of mixed convection, natural convection has a significant effect on small values of Reynolds numbers and large values of Richard' number. Also, the baffles location has a significant influence on HX performance. The proposed solution of the shell coil exchanger with a baffle is characterized by better heat transfer efficiency on the shell side, but rather for lower values of Reynolds (Re < 150) numbers and high values of heat fluxes. For all configurations is noticeable the influence of the supplied heat flux value on the rate of heat transfer. The paper presents new experimental Nusselt numbers correlation on the shell side of the heat exchanger with core-baffles. This correlation depends on Grashoff and Dean numbers as well as dimensionless shell diameter (C/D 0), the correlation also includes Prandtl number. Experimental data were compared with selected correlations from literature as well as with own correlation. It was shown that only own semi-empirical correlation has satisfactory compliance with experimental results within 35% error band and with less than 8% of absolute deviation. The considered construction has markedly better heat transfer efficiency on the shell side for small Reynolds numbers (Re <150) comparing to the tube-in-tube exchanger with wire coil turbulator(over 20%). [ABSTRACT FROM AUTHOR]

In this paper, we experimentally investigated the effect of turbulent intensity on heat transfer in a square rotating channel. The grid generated turbulence is measured by hot-wire, and the heat transfer coefficient is measured by TLCs. In the experiment, the Reynolds number, based on the channel hydraulic diameter ( D = 80 mm ) and the bulk mean velocity ( V m = 1.82 m / s ), is 10,000, and the rotation number ranges from 0 to 0.52. The mean density ratio ( d . r . = ( T w - T b ) / T w ) is about 0.1 in the current work using transparent heater glass (Indium Tin Oxide) to provide uniform heat flux. Two different turbulent intensity of inlet air (0.6% and 5.5%) are taken into consideration to investigate the heat transfer distribution on the leading and trailing side. The results show that turbulent intensity has an effect on heat transfer on both leading and trailing side, especially at rotating conditions. At static conditions, the effect of turbulent intensity on heat transfer is not obvious. However, with the increase of rotation number, in case B with a medium turbulent intensity (Tu) of 5.5%, the Nu/Nu 0 is about 10% higher than that in the case A with low turbulent intensity of 0.6% with the rotation number of 0.52 at X/D = 2 on trailing side. The enhancement of case B decreases along X/D directions. On the leading side, the turbulent intensity has same effect on heat transfer with that on trailing side, but not as prominent as that on the trailing side. In current work, the turbulent intensities at different X/D directions are also presented to explain the phenomenon of heat transfer in the channel. More detail of results will be presented in this paper. [ABSTRACT FROM AUTHOR]

• The heat transfer process of NH 4 Cl fouling was studied experimentally. • The local heat transfer characteristics around tube wall was analyzed. • The effects of flow rates and temperature on heat transfer process were investigated. • The fouling resistance and average fouling thickness on the HET wall were obtained. The crystallization fouling of ammonium chloride (NH 4 Cl) often occurs on heat exchange tube bundles in petroleum refining equipment, which seriously reduces the heat transfer and operational efficiency of the equipment. In this paper, the heat transfer characteristics of heat exchange tube (HET) with NH 4 Cl crystallization fouling was studied experimentally. The effects of velocity, temperature and mass flow rate of hot and cold fluids on the overall and local heat transfer characteristics were analyzed. The results indicate that the local heat transfer coefficient around the tube wall decreases first and then increases with the increase of the circumferential angle (θ) under clean condition. At the position of 0°-45°, the local heat transfer coefficient is the largest. At 90°-135°, the local heat transfer coefficient is the smallest. Under fouling condition, the local heat transfer coefficient increases with the increase of circumferential angle. In the region of 0°-45°, the local heat transfer coefficient is the smallest. The region with the largest local heat transfer coefficient is 135°-180° The heat transfer coefficient (h), fouling resistance (R f) and average fouling thickness increase with the increase of Reynolds number (Re), temperature of hot fluid (T h) and mass flow rate of cold fluid (q m,c), but decrease with the increase of temperature of cold fluid (T c). [Display omitted] [ABSTRACT FROM AUTHOR]

• The pressure drop through polyurethane foams was studied. • A correlation for friction factor based on fiber diameter and permeability in polyurethane foams was obtained. • A correlation for Nusselt number based on fiber diameter in polyurethane foams was obtained. • Comparison of heat transfer and friction factor in polyurethane foams with metal foams was made. Foam-like materials have a large specific surface area and complex structure that promotes flow turbulence, which makes them useful for heat transfer. In the literature, heat transfer in metal foams (MF) with high thermal conductivity is mainly considered. In the present paper, an experimental study of the hydraulic and heat transfer characteristics of a rectangular channel filled with polyurethane foam (PUF) inserts with a thermal conductivity of 0.2 W⋅m−1⋅K−1 in the air flow was carried out. The open-cell PUF samples had 20 and 80 pores per inch (PPI), 0.97 and 0.98 porosity, 269 µm and 60 µm fiber diameter, respectively. The Reynolds number, based on the fiber diameter ranged from 0.04 to 50. From the results of hydraulic tests, friction coefficients were determined based on the fiber diameter and permeability. The Forchheimer coefficient was 0.198 and 0.318 for 20 and 80 PPI samples, respectively. The permeability was 1.889 10−7 ⋅m2 and 7.535 10−9 ⋅m2 for 20 and 80 PPI samples, respectively. The Nusselt number for both tested PUF samples was correlated with the Reynolds number based on the fiber diameter in a power law with the Reynolds number exponent and constant equal to 0.61 and 0.037 respectively. The intensity of heat transfer in the PUF samples was 7–12 times lower than that in MF. However, a significant heat transfer enhancement is still possible compared to the empty channel up to 7 times. The heat transfer performance of PUF was higher compared to the empty channel, but the thermal performance factor was lower than one. Nevertheless, in practical situations where the mass or cost of the heat exchanger is preferable, PUF can be considered as a heat transfer intensifier. [Display omitted] [ABSTRACT FROM AUTHOR]

• Reduced gravity cryogenic line chilldown data presented. • Investigation is carried out on effect of low thermally conductive coatings and pulse flow. • Comparisons are made between 1-g and reduced gravity for similar conditions. • No appreciable degradation in chilldown efficiency at moderate-to-high Reynolds numbers in reduced gravity relative to terrestrial gravity. Future cryogenic propulsion systems will require efficient methods to chill down transfer lines and propellant tanks prior to transfer of propellant while in space. When chilling down the transfer line, there is an inherent tradeoff between propellant mass consumed and chilldown time to steady state, depending on mission constraints and desires. This paper examines an efficiency parameter based on a simplified energy balance that can be used to compare the performance of different chilldown methods and to determine the optimal chilldown method. The parameter, along with chilldown time and mass, is then applied to recent liquid nitrogen line chilldown experiments conducted in both terrestrial and microgravity to compare efficiencies between numerous continuous and pulse chilldown tests on bare as well as coated tubes across a range of thermodynamic conditions. Despite a well-known reduction in heat transfer in microgravity due to the absence of buoyancy-assisted cooling, results indicate no appreciable degradation in chilldown efficiency at moderate-to-high Reynolds numbers in microgravity relative to terrestrial gravity. [ABSTRACT FROM AUTHOR]

In this paper, the fluid flow behavior and cooling characteristics of a sweeping impingement jet were investigated. Also, the difference between free and confined impingement schemes was investigated. In addition, the conventional approach for the purpose of significantly reducing computational time by using a confined domain with a slip upper wall instead of an unconfined domain was evaluated. A 2D unsteady Reynolds averaged Navier-Stokes (URANS) simulation accompanied with the k-ω SST turbulence model is used in this study. The study has been conducted for a target wall with a constant heat flux of 3000 W/m2, jet-to-wall distance of 4, and a jet Reynolds number of 2500. The results show that the overall average cooling performance of the sweeping jet is better in the confined impingement scheme compared to that of the steady jet, while the steady jet is slightly better in the unconfined sweeping impingement scheme. Using a confined scheme with a slip upper wall does not reveal the complete thermal and flow behaviors, and the wall heat transfer distribution is very different from the unconfined domain. [ABSTRACT FROM AUTHOR]

This paper studied the buoyance-aided mixed convective heat transfer of air-cooling flow in a 5 m long and 1.2mX0.3 m cross-section vertical channel. One of the channel walls was uniformly heated and the rest three walls were unheated and insulated. Heat was transferred from the heated wall by convection and radiation, and heat was also transferred from unheated walls to the air flow by convection originated from their net radiation gain from the heated wall. The air flow rate, the inlet and outlet air flow temperatures, and local surface heat fluxes and temperatures on the heated wall were measured. The Reynolds number of air flow was from 2900 to 221,000 and Richardson number was changed from 0.006 to 90.6. The results showed that for walls with larger emissivity the total convection heat transfer was not deteriorated in mixed convection zone. Therefor a modified heat transfer correlation was developed for the aiding mixed convection in the asymmetrically heated channel considering both mixed convection and radiation. It predicted the experiment results with a deviation less than 18%. [ABSTRACT FROM AUTHOR]

This paper analyses the effect of spirally corrugation in a simple tube on the heat transferred and friction factor using numerical simulations. The simulations have been validated with experimental data available in the literature. The study compares the behaviour of both smooth and spirally corrugated tubes considering turbulent flow at four Reynolds numbers (15- 40 × 10 3 ) and two Prandtl numbers (2.9 and 4.3). The main novelty of this paper is to perform a 3-D simulation because some previous studies using similar geometry were restricted to a 2-D analysis. For the smooth and corrugated tubes, stainless steel tubes with an inner diameter of 18 mm, a length of 6 m and a wall thickness of 1 mm were used. The corrugated tube has a corrugation depth of 0.43 mm and a helical pitch of 15.86 mm. The meshing process was performed using ANSYS Workbench (v.17.0) with an unstructured grid with a refined mesh near the wall to ensure that the laminar viscous sub-layer was captured. Hence, a k-epsilon ( k − ε ) turbulence model with a near-wall treatment was used in the proposed simulations. Two grids were used to perform a grid sensitivity analysis. The results for the corrugated tube indicate that the numerical model predicts an average Nusselt number within a maximum relative error of 17% compared with the experimental data, and the differences in the Fanning factor are lower than 9%. [ABSTRACT FROM AUTHOR]

Active and passive cooling are the two possible methods for removing heat. An active cooling system is the one that involves the use of energy as opposed to passive cooling that uses no energy. Passive cooling methods are cost effective and more reliable than active cooling due to the absence of moving parts. Microchannel heat sink is one of high-tech devices that have widely considered passive cooling methods especially for electronics cooling. In this paper, the use of passive cooling methods in microchannel heat sink is comprehensively discussed. This paper also present the effects of some important parameters such as the type of channel types, surface roughness, fluid additives, and Reynolds number on the rate of heat transfer in microchannel heat sink. Finally, the conclusions and important summaries were presented according to the data collected. [ABSTRACT FROM AUTHOR]

This paper presents the performance investigation of a shell and helical coil type of Solution Heat Exchanger (SHX) in an ammonia–water vapour absorption system. In an absorption system, SHX is one of the major heat recovery components. The main objective of any heat exchanger design is to achieve minimum heat transfer area required for a given heat duty, as it governs the overall fixed cost content of such a system. The required surface area is decided by the overall heat transfer coefficient. Hence, the heat transfer coefficient (HTC) correlation plays a major role in optimizing the heat exchanger. In this paper, shell and helical coil type of SHX is investigated with more emphasis on the dimensionless correlation of shell side co-efficient, which decides the overall HTC and the size of heat exchanger. From the experimental study, shell side heat transfer coefficient of 510–650 W/m 2 K is obtained with the heat exchanger effectiveness of 0.84–0.9 for the tested conditions. A proposed Nusselt number correlation is compared with the experimental results. [ABSTRACT FROM AUTHOR]

• The flow and heat transfer characteristics of single and multiple-synthetic jet impingement cooling is presented. • The equivalent Reynolds and Nusselt numbers were determined for the purpose of describing multiple orifice configuration. • The merging jet process is presented and the merging point is determined. In this paper, the comparison between synthetic jet actuator with single axisymmetric orifice and multiple axisymmetric orifices is presented. The aim of this paper is to compare the flow and heat transfer characteristics of single and multiple synthetic jet impingements cooling. The multiple orifice configurations had a central orifice and a satellite orifices placed in the pitch circle. The synthetic jet actuator had central orifice with 0, 3, 4, 5 and 6 orifices placed in the diameters of the relative pitch circle p / d = 2.5, 3 and 4. A total of 13 various geometric configurations were tested experimentally. In the multiple orifice configurations, the total cross-section area was approximately equal to the cross-section area of the single orifice configuration. The input power delivered to the synthetic jet actuator was maintained constant and equal to 3.00 W. The synthetic jet actuator energetic efficiency, Reynolds number, equivalent Reynolds number, radial and axial distributions of pressure coefficient, and heat transfer characteristic were presented. The synthetic jet issuing from multiple orifice plate merged at some axial distance, creating a single axisymmetric jet. The merging jet axial distances were determined for all investigated configurations. In the present paper, the integral quantities averaged over all orifices cross section area, or over impingement plate much larger than total orifices area, were analyzed for the purpose of objective comparison. From all various investigated parameters obtained from five different and independent experimental methods, the following conclusion may be made: if the total orifice cross section area in the case of single and multiple orifice synthetic jet is similar and real power deliver to the SJA is maintained at the same level the resulting equivalent Reynolds number and equivalent dimensionless stroke length achieved similar values and the impinging jet equivalent Nusselt number also achieved comparable values and distributions. [ABSTRACT FROM AUTHOR]

• Unsteady wake dynamics past a blunt headed cylinder at incidence is studied. • Determined critical Reynolds number (Re) based on primary stability analysis. • Identification of various flow regimes during vortex shedding. • Evaluation of forced convective heat transfer for various Re and angles of incidence. • Entropy generation optimization at different Re and angles of incidence. In the present paper, a numerical investigation has been carried out to study the forced convective flow and heat transfer characteristics past a blunt-headed cylinder in crossflow. Employing air as an operating fluid, calculations are carried out for a range of Reynolds number (Re) from 40 to 160. The angle of incidence is varied in the range of 0∘ ≤ α ≤ 180 ∘. The thermofluid features of flow and heat transport are analysed in detail for different angles of incidence. To analyse the aerodynamic characteristics, several parameters such as drag and lift coefficients, moment coefficient, Strouhal number, recirculation length, and local time-averaged vorticity flux have been calculated. Furthermore, a stability analysis has been undertaken by using the Stuart Landau equation to enumerate the critical Reynolds number at each angle of incidence. Heat transfer characteristics are studied by computing local and time-averaged values of Nusselt numbers. When compared to a rectangular cylinder, a blunt-headed cylinder exhibits an enhanced heat transfer rate. In the end, an entropy generation analysis has been carried out to study the effects of Re and angle of incidence on the efficiency of thermofluid transport characteristics. [ABSTRACT FROM AUTHOR]

• The novel geometry of channel cooling was presented. • The channel cooling with shaped slit ribs are further explored. • The effect of the convergent and divergent angle on cooling performance are described. Described in this paper is a numerical investigation on the concept for improving thermal performance of internal cooling by placing the convergent and divergent slit ribs. Five different geometrical models are investigated, including the ribs of rectangular slits and trapezoidal slits with different convergent and divergent angles. The effects of slit shape and its convergent or divergent angles on thermal performance of internal cooling are evaluated with the Reynolds numbers ranging from 10,000 to 25,000. Results obtained show that the turbulence intensity in cases with the smallest angle trapezoidal slits are in the highest level, which produces the highest level of heat transfer enhancement and highest level of pressure loss. The thermal performance index, which comprehensively evaluates the thermal performance of internal cooling, shows that the thermal performance in cases with the smallest angle trapezoidal slits are in the highest level due to the increased heat transfer enhancement and limited increase of pressure loss. [ABSTRACT FROM AUTHOR]

• Internal heat transfer of film-cooled leading edge is investigated. • Two jet impinging designs: the normal jet and the tangential jet are studied. • Mainstream flow influence decreases with increasing jet Reynolds number. • CFD simulations show a good agreement with experimental data. This paper investigates internal heat transfer of film-cooled leading edge with mainstream flow. The semi-cylindrical leading edge model receives coolant through impinging jets located at the neighborhood rectangular channel. The leading edge has three rows of cylindrical film cooling holes: row 1 located along the stagnation line (0°) and rows 2 and 3 at ±40° measured from the stagnation line. All film cooling holes are at an inclined angle of 25° relatives to the surface. There are two impinging jet designs in this study: the normal jet and the tangential jet. The normal jet has one row of normal jet impinging holes. After jets impinging on the inner wall of the semi-cylinder stagnation line, coolant spreads out through film cooling holes. The tangential jet has two rows of tangential jet impinging holes. Swirl flow is generated when jets enter the semi-cylinder from two sides of the semi-cylinder. Mainstream Reynolds number is about 100,000 based on the outside diameter of the leading edge cylinder, and the mainstream turbulence intensity is about 7%. Leading edge detailed internal heat transfer distributions are measured by using transient liquid crystal method. Three different coolant jet Reynolds numbers are tested (Re j = 5000, 10,000 and 15,000), corresponding to averaged blowing ratios about 0.77, 1.54, and 2.31. The experimental results provide useful information for the jet impingement cooling design, especially the leading edge region is under the conditions of mainstream flow and external film extraction. CFD simulations are performed to present the velocity field and compare the heat transfer results with experimental data. [ABSTRACT FROM AUTHOR]

• A method is proposed to assess the efficiency of heat transfer enhancement techniques. • Reynolds number and ln(Nu e / Nu r)/ln(f e / f r) were taken as the abscissa and ordinate. • The data in higher levels in the coordinate will be more effective in energy saving. In this paper, a revised performance evaluation method was proposed to estimate the thermal hydraulic performance of various heat transfer enhancement techniques. The energy saving effectiveness for the enhanced structures can be identified via a plot. In the plot, Reynolds number and the efficiency index ln(Nu e / Nu r)/ln(f e / f r) are taken as the X and Y coordinates, respectively. The efficiency can be ranked into four different levels. For experimental data or simulation results in Level 1, enhanced heat transfer is accompanied with more power consumption. The data in Level 2 is that the heat transfer can be intensified at the same pumping power. In Level 3, it is characterized by the augmented heat transfer at the same pressure drop. The efficiency in Level 4 is the highest. The ratio of heat transfer augmentation is higher than the increment ratio of friction factor at the same Reynolds number. Four levels correspond to four horizontal regions with Level 1 at the bottom and Level 4 at the top. The data at higher levels in the ordinate will be more competitive for energy saving. The examples on how to use the method for evaluating the performance of enhanced surfaces and typical heat exchangers were also presented. [ABSTRACT FROM AUTHOR]

In this paper, thermohydraulic characteristics and enhanced heat transfer mechanism in water–cooling oil heat exchanger with various shapes and attack angles of vortex generators (VGs) and spacing of double VGs are numerically investigated at different Reynolds number (Re) and oil–water mediums. A CFD-aided method, conjugated heat transfer model, is performed to elaborate the correlation between heat transfer and flow characteristics. The quantitative and qualitative results manifest that the longitudinal and secondary vortices, induced by VGs, generate synergistically the tapered and elliptical enhanced heat transfer regions on the heat exchange fins, but the inhibiting effect still exists between both, particularly in high attack angle or oil medium. Elliptical winglet VG with 30° in oil medium with Re = 349 and 15° in water medium with Re = 2726 gains the maximum thermal performance factor, reaching 1.69 and 1.28, respectively, compared without VGs. Furthermore, the appropriate configuration of two core heat transfer regions is a cost-optimal method to obtain in-depth synergistic heat transfer. The spacing of 4 mm shows the best synergistic heat transfer, and the maximum heat flux of its was elevated to 2.2% and 8.6% more than that of single VG in oil and water medium, respectively. [ABSTRACT FROM AUTHOR]

The droplet impacting on a substrate is widely encountered in daily life and industrial applications. In this paper, the impact dynamics and heat transfer are numerically simulated for both single droplet and binary droplets on a hot superhydrophobic substrate. The dimensionless numerical model is built up through the transient 2D axisymmetric model with a volume of fluid (VOF) model. The effect of the Weber number, Reynolds number, size ratio, contact angle on the spreading dynamics and heat transfer is investigated in details. It is found that the spreading factor and contact time increase with the increasing Weber number. Besides, the maximum spreading factor and contact time of binary droplets are larger than those of a single droplet under the same Reynolds and Weber numbers. The transient heat transfer rate of a single droplet is larger than that of binary droplets impingement due to larger spreading surface area-to-volume. The total input heat of a single droplet is generally larger than that of binary droplets except at large Weber numbers, and the equal-sized binary droplets have the larger total input heat than un-equal-sized binary droplets. For binary droplets impact on a hot superhydrophobic surface, the hot substrate can promote the spreading and retard the receding due to thermo-capillary effect, and thus will enhance heat transfer between the droplet and the hot substrate. These findings may be helpful in gaining insights into the dynamics and heat transfer of binary droplets impact on the hot substrate. [Display omitted] • Spreading factor and contact time of binary droplets are higher than single droplet. • Heat transfer of single droplet is higher than binary droplets. • Heat transfer is the best for equal-sized droplet. • Thermal-capillary effect promotes the spreading and retards the receding. [ABSTRACT FROM AUTHOR]

Augmentation of heat transfer without increasing the pressure drop has been a challenge in turbine blade internal cooling. Ribs are commonly used to achieve this goal and it is imperative to keep the pressure loss as low as possible. In this paper, the thermohydraulic performances of a number of perforated ribs are examined and compared. Three geometrical features of the ribs include hole inclination angle (0°, 30°, and 45°), relative hole inlet height (0.2, 0.4, 0.6, and 0.8), and the variation of the cross-sectional area of the circular holes (convergent, straight, and divergent). The ribs are mounted in a rectangular channel (A R = 2:1) and flows with a range of Reynolds numbers (10,000 to 25,000) are examined. A validated SST Gamma-Theta model is employed to evaluate the turbulent heat transfer and pressure loss characteristics. It is found that the permeable rib labeled as Case #212, improves the performance between 4.35 and 6.39% for flows with Reynolds numbers between 10000 and 25000. Among the investigated configuration parameters, the hole inclination has the highest effect on the thermal performance enhancement. • Study on heat transfer and pressure drop characteristics of perforated ribs. • Role of each geometric parameter of the perforated rib is determined. • Flow structure is investigated to obtain fundamental reasons for performances. • Best performance by perforated ribs for Reynolds number from 10,000 to 25,000. [ABSTRACT FROM AUTHOR]

In order to enhance the heat transfer performance of the battery thermal management system, to obtain a more uniform temperature distribution and to achieve the lightweight design requirements, a new liquid cooling plate design approach with primary and secondary combined fins synergistic phase change materials and nanofluid is proposed in this paper. Based on the traditional square fins, 10 new improved fin design options are proposed in this paper. Firstly, the thermal and flow characteristics of the 10 new improved fins are compared and analyzed, and the optimization is carried out. Next, the optimal model is obtained by considering the primary fin size and introducing the secondary fins of the partitioned combination for optimization. Compared to the traditional square fins, the average temperature is reduced by 0.288 °C and the pressure drop is reduced by 1.287 Pa (17.42%). Then, based on the optimal model, the thermal performance of the combined filling method and thickness of PCM is discussed and analyzed for different Reynolds number conditions. The results indicated that the smaller the Reynolds number, the more significant the heat dissipation effect of the filled PCM. The optimal model with PCM filling reduces the mass by 41.4 g (68.86%) compared to the model without PCM, with significant lightweighting effect. Finally, to further improve the thermal performance of the system, different types of nanoparticles with different volume fractions are introduced into the system. Compared to pure water liquid cooling, the Nusselt number can be increased by 9% to 78%, and the heat dissipation effect is significantly improved. • Based on the traditional directional fins, 10 new and improved fins are proposed. • Thermal performance of the system was improved after introduction the secondary fins. • A new liquid cooling plate with PCM synergistic combination fins was proposed. • The mass of the optimal model after filling the PCM was reduced by 41.4 g (68.86%). • Nusselt number of system can be improved by 9% to 78% after introduction nanofluid. [ABSTRACT FROM AUTHOR]

Highlights • The BRs could induce the recirculations in both vertical and spanwise directions. • Heat transfer enhancement induced by the BRs is more sufficient. • The integrated manufacturing of BRs was accomplished in a MCHS by MEMS technology. Abstract The heat transfer and flow characteristics of the microchannel heat sink (MCHS) with bidirectional ribs (BRs) are experimentally and numerically studied in the present paper. The BR, composed of vertical rib (VR) and spanwise rib (SR), can interrupt the thermal boundary layer and induce recirculation in both vertical and spanwise directions. Its cooling effectiveness is compared with that of the widely-used VR and SR for the Reynolds number ranged from 100 to 1000. The results show that the Nussalt number of the microchannel with BRs (BR-MC) is up to 1.4–2 and 1.2–1.42 times those of microchannels with VRs (VR-MC) and SRs (SR-MC), respectively. This implies that the BR can strengthen the heat transfer more sufficiently. Meanwhile, the utilizing of BR gives rise to the larger pressure drop penalty due to its broader obstruction areas. In addition, the higher relative rib height of VR (e VR) and relative rib width of SR (e SR) are revealed to enhance the heat transfer but induce pressure drop in the BR-MC. The thermal enhancement factor can keep larger than 1 when e VR < 0.316 and 0.026 < e SR < 0.357. [ABSTRACT FROM AUTHOR]

Highlights • The heat transfer and flow characteristics of two-turn ribbed serpentine channel were analyzed. • Heat transfer distribution is studied with the transient liquid crystal measurement technique. • The inlet Reynolds numbers are varied from 5,000 to 20,000. • Pressure coefficient distribution of two-turn ribbed serpentine channel was researched. Abstract This paper experimentally and numerically investigates the heat transfer and flow characteristics of a two-turn ribbed serpentine channel with lateral outflow. The heat transfer coefficient was measured by a transient liquid crystal technique. Experiments were carried out at Reynolds numbers between 5,000 and 20,000 and rotation numbers of 0 and 0.03. The results indicate that with increasing inlet Reynolds number, the high-Nusselt number regions of the middle and lateral outflow channels gradually move to upstream ribs. The inlet channel shows the highest increase rate of the Nusselt number; whereas, the rate is lowest at downstream turning area. The rotation increases the trailing surface Nusselt number of the inlet and lateral outflow channels, and the increase rate is more prominent for higher values of the Reynolds number. However, the rotation has negative effects on the averaged area Nusselt number of the middle channel in cases of Re ≤ 17,000. The pressure coefficients decrease the in inlet and middle channels along the flow direction; whereas, they slightly increase in the lateral outflow channel. There is a long low-velocity vortex formed in the lateral outflow channel, and the vortex size is considerably reduced by the rotation. Accordingly, the rotation has the most positive effects on the pressure coefficient of the lateral outflow channel. [ABSTRACT FROM AUTHOR]

Highlights • A model with optimized anode-side prediction was developed for PEM-based dehumidification. • Optimal anode-side exchange current density and heat/mass transfer coefficient were derived by the multi-parameter fitting. • The model showed a close trend to experiments, especially in the current prediction. • At higher airflow rates or larger electrical fields, the prediction deviation could be reduced by more than 50%. • The electrical field has the largest influence on anode-side electrochemical process. Abstract Electrolytic dehumidification based on a polymer electrolyte membrane (PEM) is promising due to the fast humidity control, safe operation and ultra-compactness. However, predictions with existing methods deviated widely, mainly due to the inaccurate calculations of the anode-side over-potential and heat/mass transfer. In this paper, an improved theoretical model was developed, by optimizing the anode-side prediction using newly developed equations for the exchange current density and heat/mass transfer coefficient. The equations were derived by the multi-parameter fitting, with a database obtained by making the simulation results as close as possible to the experimental data under various operating conditions. Compared to the results of previous models, the developed model showed a data trend that was much closer to the experimental data. The overall errors for the moisture removal rate and operating current under most conditions were less than 15% with acceptable average errors of 10.4% and 7.1%, respectively. Particularly, this model showed much higher accuracy at higher air flow rates or larger electrical fields, especially for the current prediction. When the air Reynolds number was above 2 or the applied voltage was above 3 V, the deviation of the prediction could reduce by more than 50% with the new model. And the common problem of overestimation for current prediction (up to 4–5 times) in existing models could be solved. In addition, the parameter analysis of the optimal exchange current density and heat/mass transfer coefficient was conducted. It could be found that the applied voltage during dehumidification had the largest influence. The influences of the anode air inlet temperature, flow rates and inlet relative humidity were also significant. But the effects of cathode air temperature and flow rates were slight. This research significantly improves the prediction accuracy and provides a good guidance for the performance optimization of electrolytic dehumidification systems. [ABSTRACT FROM AUTHOR]

Highlights • The thermal-hydraulic performance of twisted oval tube bundle is investigated in cross flow. • The influences of geometry parameters and Re are studied. • Twisted oval tube is a streamlined tube that is independent of the angle of attack. • Correlations are presented for Nu and Eu. Abstract A numerical research is conducted to investigate air side heat transfer and pressure drop performance of twisted oval tube bundles with in line layout in cross flow. The effects of five geometric parameters (the ratio of outer major axis to outer minor axis (A/B), twist pitch length (S), transverse tube pitch (S t), longitudinal tube pitch (S l) and number of longitudinal tube rows (Z)) on the air side heat transfer and pressure drop performance are examined in the range of Reynolds number (Re) from 500 to 23,000. The velocity field and temperature field are repeated periodically every S /2 along the length of the twisted oval tube, which reveals that the twisted oval tube is a streamlined tube that is independent of the angle of attack. Twisted oval tube bundles perform excellent heat transfer performance compared with other tube bundles (circular tube bundle with the same tube layout as the twisted oval tube bundle, elliptic tube bundle with staggered manner, spirally corrugated tube bundle with in line layout) in cross flow. And the staggered tube layout is beneficial to enhance heat transfer performance compared with in line tube layout. Correlations for Nusselt number (Nu) and Euler number (Eu) from this paper are presented. [ABSTRACT FROM AUTHOR]

Highlights • The relationship between the turbulent spot characteristics and the conditions of water injection, used to initiate the spot. • Thermal visualization of a puff and a turbulent spot on an isothermal flat surface. • The predictive formulas for the Nusselt number of turbulent spot. Abstract This paper investigated the change in the thermal and structural characteristics of turbulent spot footprints, affected by Reynolds number, jet velocity and duration using thermochromic liquid crystals. The turbulent spot was induced on an isothermal plate by a small pulsating water jet through a 1 mm diameter hole in an upward direction across the mainstream flow, having velocities of 0.128, 0.160, and 0.192 m/s. The jet was set to Reynolds number values of 3300, 5500, 7700, 9900, 13,100, and 16,400 and dimensionless injecting durations of 0.5, 0.7, 0.9, 4.7, 9.5, 23.6 and 47.3. The results showed that the Re of the jet strongly affected on the characteristics of the turbulent spot. The Nu increased approximately 14% when the injecting duration changed from 4.7 to 47.3 while the ε s / τ inj decreased by 12 times. The spot area, the heat transfer area, the velocity of the spot leading edge, the half spreading angle, and the non-dimensional spot propagation parameters increased approximately 75, 60, 3, 70, and 85%, respectively, but the velocity of the spot trailing edge decreased about 20% at τ = 260. Meanwhile, the variation of jet velocity caused a different shape of disturbance. Furthermore, the predictive formulas for the Nusselt number of the turbulent spot, induced from the injecting duration between 4.7 and 47.3, were also provided and the mechanism of the flow and heat transfer discussed. [ABSTRACT FROM AUTHOR]

Abstract Sidewall proximity, characterized by the gap distance (G) between border pin-fin column and sidewall in a pin-finned microchannel with in-line arrangement, plays a significant role on pressure drop and heat transfer characteristics. To better understand the thermal performance and explore the underlying mechanisms, a comprehensive comparison is numerically developed among three representative microchannels with gap-to-diameter ratios (G / D) of 0.6, 1.0 and 1.4, respectively. The Reynolds number investigated in this paper varies from 13 to 202. It is found that the gap distance severely influences flow distribution, streamline structure, velocity field and temperature distributions in a pin-finned microchannel. At a fixed Reynolds number, pressure drop of microchannel is continuously decreased while heat transfer is first enhanced and then reduced with the increase of gap distance. Among the three models, the microchannel with G / D = 1.0 possesses a comparatively superior heat transfer performance. In addition, extremely low local Nusselt numbers on both the base surface and the pin-fin surface near sidewall seriously deteriorate the overall heat transfer performance of the microchannel with G / D = 0.6. Furthermore, unremarkable heat transfer performance is also observed from the microchannel with G / D = 1.4 for its obvious decline of local Nusselt number on inner regions in spite of a rise on border region. Taking heat transfer and pressure drop into account simultaneously, the results show that a very small gap distance (i.e., G / D = 0.6) should be avoided for design of a pin-finned microchannel. Microchannels with middle gap distances (i.e., G / D = 0.9, 1.0, 1.1) have a relatively better overall thermal performance, which separately provide a superiority of 10.6–13.6% (G / D = 0.9), 10.0–13.5% (G / D = 1.0), 8.2–14.4% (G / D = 1.1) compared to the microchannel with G / D = 0.6. Finally, new correlations of friction factor and Nusselt number are developed by considering the effects of sidewall proximity. Highlights • Sidewall proximity influences on a pin-finned microchannel are observed. • Details of flow characteristics and heat transfer are discussed. • Suitable sidewall proximity is of importance to improve thermal performance. • New correlations of friction factor and Nusselt number are developed. [ABSTRACT FROM AUTHOR]

Highlights • Impingement/effusion cooling is numerically investigated. • The effects of film cooling hole diameter and location are investigated. • The relationship between effusion air distribution and flow development are studied. • The enhanced heat transfer mechanism in impingement/effusion cooling is studied. Abstract This paper numerically investigates the effects of the film cooling hole diameter and the film cooling hole location on the impingement/effusion cooling performance inside a concave target channel. Three film cooling hole rows are established on the target surface under two arrangements. In the first arrangement, the inclined angle between the film cooling hole axis and jet hole axis is 0°, −60°, 60° respectively. In the second arrangement, the inclined angle is 0°, −30°, 30° respectively. In the first effusion hole arrangement, numerical simulations are conducted under three Reynolds numbers. In the second effusion hole arrangement, numerical simulations are only conducted under the middle Reynolds number. Four film cooling hole diameters of 0.4D, 0.6D, 0.8D, 1.0D are studied based on a fixed jet hole diameter of D = 10 mm. The heat transfer performance, pumping power and overall performance are evaluated and compared. Effusion air distribution and static pressure distribution are analysed. Flow development inside the target channel is compared and discussed. Nusselt number distribution is evaluated and compared. Results show that the effusion air mass flow rate and the flow development inside the target channel are obviously affected by the film cooling hole diameter and film cooling hole location. The heat transfer performance is also significantly affected by the film cooling hole diameter and film cooling hole location. [ABSTRACT FROM AUTHOR]

Abstract Sticking probability (P) and deposit bond strength (ξ) are two of the most important factors that determine the fouling process, but no calculation correlations for both P and ξ could be found in current models. Thus, the fouling process could not be described by specific formulas. This paper analyzed each parameter in the Kern-Seaton fouling model, and a correlation of dry matter concentration (C b ′) was developed, depending on the water quality test. Test data suggested that the sticking probability (P) in such combined fouling ranged from 0.96445×10−3 to 6.20781×10−3. The deposit bond strength (ξ) of fouling ranged from 0.99953×107 N·s/m2 to 3.51186×107 N·s/m2. Furthermore, the calculation correlations of P and ξ in types of f (Re , N s , α , e / D i) and f (f , j) for two-dimensional enhanced tubes were developed, respectively, based on long-term fouling data. Results indicated that the sticking probability (P) was consistently decreasing in both the j -factor and frictional factor (f), but it was consistently increasing in the temperature of heat transfer surface (T). Both the sticking probability (P) and deposit bond strength (ξ) were more affected by frictional drag performance of the tubes than the heat transfer performance. The sticking probability (P) was negatively related to the specific geometric parameters (N s , α , e / D i) of the tubes, but positively related to the Reynolds number, Re , which was completely opposite to the deposit bond strength (ξ). The parameters of e / D i and α affected both the sticking probability (P) and the deposit bond strength (ξ) more than the starting number of N s. [ABSTRACT FROM AUTHOR]

Abstract The heat transfer performance and friction loss of two-pass ribbed channels are highly influenced by the combined effect of rib orientation and bend shape. This paper, for the first time, numerically investigates this effect for four rib orientations, two rib angles and eight different bend shapes at Reynolds number of 30,000. The results show that the rib orientation determines the rotation direction and the transverse position of secondary flow in straight passages, while the bend shape determines the delivery of upstream secondary flow and the generation of local secondary flow. The key to heat transfer enhancement in the second passage is to promote local secondary flow using the energy from upstream secondary flow with the same rotation direction. The upstream secondary flow with the same rotation direction should be guided to a proper position, in order to enhance the dominant secondary flow in the second passage and reduce energy loss due to the undesirable secondary flow interaction. The impact of different bend shapes on the secondary flow has been summarized. The round inner corner for the bend is preferred for the significant friction loss reduction and the excellent secondary flow energy delivery capacity. Meanwhile, the secondary flow generated by the square outer corner can significantly enhance the cooling performance at the entrance of the second passage for some certain rib orientations. An optimized combination of 60° rib orientation and bend shape can either raise thermal performance factor by 10% in the second passage or reduce the friction loss by 22.3% in the whole channel, compared with the existing experimental design. By contrast, it is found cooling performance and pressure loss are not sensitive to bend shape for the rib angel of 45°. The optimized bend shape is inspired by an intuitive visualization of secondary flow evolution, which provides insight on the geometry optimization for heat and mass transfer applications. [ABSTRACT FROM AUTHOR]

Abstract To overcome the deficiencies of parallel-flow shell and tube heat exchanger with round rod baffles (RRB-STHX), STHX with a new type of anti-vibration baffle (hexagon clamping baffle) and equilateral triangle cross sectioned wire coils (HCBetsw-STHX) is proposed in this paper. Response surface methodology (RSM) is adopted to investigate its thermal-hydraulic performance. Baffle distance, coil pitch, coil diameter and Reynolds number are considered as four design parameters. Analysis is conducted based on CFD to acquire the objective functions (Nu H , Nu H / Nu R , f H , f H / f R and PEC) for different combination of design parameters. The RSM with Central Composite design is used to identify the relationships between the objective functions and the design parameters. Variances of the linear term, quadratic term and interactive term for design parameters in the response variables are analyzed. The mechanism of heat transfer enhancement of HCBetsw-STHX is illustrated from the view of field synergy theory. The results indicate that the heat transfer enhancement of HCBetsw-STHX is quite better than that of RRB-STHX. In the given design space, the Nu H / Nu R is in the range of 1.335–1.720; the f H / f R is in the range of 5.462–12.936. The most significant factor on Nu H is Reynolds number, while the most significant effect factor on Nu H / Nu R , f H , f H / f R and PEC is coil pitch. The sensitivity of three objective functions (Nu H , f H and PEC) to design parameters is explored. In addition, the design parameters optimization for the maximum value of Nu H and PEC and that for the minimum value of f H are also carried out and corresponding flow structures are shown and illustrated. Highlights • A new parallel-flow heat exchanger with hexagon clamping baffles and wire coils is proposed. • Parameter sensitivity of factors on thermal-hydraulic performance is explored. • Design parameters optimization is explored to maximize the Nu and PEC and to minimize the f based on the generated response surface. [ABSTRACT FROM AUTHOR]

• v 2 f turbulence model predict hydrodynamics and heat transfer of free surface liquid jet better compared to k − ϵ and k − ω models. • Lower jet to plate spacing (H / d) results in higher momentum before and after impact on impingement surface compared to high H / d. • For lower H / d , higher shear stress was observed throughout the impingement surface compared to higher H / d. • Nusselt number strongly dependent on jet to plate spacing for H / d ≤ 1. Numerical study is carried out to investigate hydrodynamic structures and heat transfer characteristics of vertical free surface liquid jet impingement on a horizontal flat plate. Different turbulent models (k − ϵ , k − ω , v 2 f) are compared to predict velocity field of the free surface jet, and the most suitable turbulence model is identified for prediction of flow physics of free surface jet impingement. A parametric study is performed to investigate the effect of Reynolds number (20000 − 60000), nozzle to plate spacing, H / d (0.5 − 2), and nozzle exit diameter (10.9, 15, and 23 mm) on the hydrodynamics, turbulence and heat transfer. The paper focuses on the underlying physics for jet impingement in the case of the lower jet to plate spacing, H / d (0.5 − 2). It was found that v 2 f turbulence model predicts flow field reasonably well compared to other turbulent models (k − ϵ and k − ω). Liquid jet radius before the impact and liquid film thickness after the impact have been examined. Velocity field, pressure and shear stress distribution on the flat plate are presented for free surface jet impingement. Further, turbulence intensity variation is examined and its effect on heat transfer is discussed. Jet to plate spacing, H / d = 0.5 resulted in around 20% higher Nusselt number compared to H / d ≥ 1 cases; and it is identified that a higher momentum before impact, compared to H / d ≥ 1, is the driving factor. Transition to turbulence is observed for higher Reynolds number and is explained by analyzing the variation in turbulence intensity along the flow streams of the free surface jet. [ABSTRACT FROM AUTHOR]

Highlights • Six heat transfer regimes identified. • Existence of a turbulent flow but laminar-like heat transfer regime for low Prandtl numbers. • Prediction of Nusselt number in turbulent flow and turbulent heat transfer regime. Abstract This paper investigates heat transfer regimes in fully developed plane channel flows without considering the buoyancy effect. Analyzing the governing thermal energy equation, aided by direct numerical simulation (DNS) data, six heat transfer regimes are identified including (i) laminar flow and laminar heat transfer (LamF-LamH), (ii) transitional flow and laminar-like heat transfer (TraF-LamH), (iii) transitional flow and transitional heat transfer (TraF-TraH), (iv) turbulent flow but laminar-like heat transfer (TurF-LamH), (v) turbulent flow and transitional heat transfer (TurF-TraH), and (vi) turbulent flow and turbulent heat transfer (TurF-TurH). One key result is the clarification of a TurF-LamH regime which exists only for low Prandtl number fluid (Pr ≪ 1). A critical non-dimensional number is determined as Re τ Pr 1 / 2 ≲ 50 where Re τ is the Reynolds number defined by the channel half-height δ and the frictional velocity u τ . In the TurF-LamH regime, the simplified thermal energy equation yields a prediction of Nusselt number as Nu ≈ 6.0. The clarification of the TurF-LamH regime provides valuable insight into the understanding of the Nusselt number data for liquid metals, which have very low Prandtl number. Another major finding is that, in the TurF-TurH regime, the Kader-Yaglom-style equation is shown to be better than the traditional power law correlations at predicting Nusselt number, for either low or high Prandtl numbers. No separate correlations are needed for the prediction of Nusselt number at low Prandtl numbers and high Prandtl numbers. Another advantage of the Kader-Yaglom-style equation is that the equation can be theoretically connected to the log-law for the mean velocity and the mean temperature. More importantly, in the TurF-TurH regime the prediction of Kader-Yaglom-style equation can be reliably extended to ultra high Reynolds numbers, for either low or high Prandtl numbers. [ABSTRACT FROM AUTHOR]

Abstract In the process of drilling, the tubing-casing annulus are easily to be eccentric. Heat transport phenomenon occurring in such tubes and annulus can be complex, and this paper investigates forced convection in a vertical eccentric annulus with different radius ratios and eccentricity (normalized by the radius difference). Physical experiments and numerical simulation have been conducted with constant inner tube inlet temperature and flow rate of hot water. Results show that with the increase of radius ratios, the convection heat transport coefficient increases. For a radius ratio of 1.875, the heat transport coefficient increases as the eccentricity increases when the Re > 2000 and it decreases when the Re < 2000. For a radius ratio of 2.25 and Re < 1500, the influence of eccentricity is trivial, but for Re > 1500, the increase of eccentricity strengthens the heat convection. For a radius ratio of 3.06, the heat transport coefficient increases as the eccentricity increases. An empirical correlation for Nusselt number is derived as a function of Reynolds number, Prandtl number and eccentricity. Highlights • For D o / D i = 1.875, the effect of eccentricity is identified into two zones. • For D o / D i = 3.06, the increase of eccentricity enhances the convective heat transport. • The increase of radius ratio strengthens the heat transport of annular channels. • With low eccentricity and low Reynolds number, the increase of radius ratio can hardly improve the heat exchange efficiency. • Three kind of empirical correlations are regressed through parametric analysis. [ABSTRACT FROM AUTHOR]

Highlights • Temperature distributions in electrokinetically driven microchannel flows with axially varying wall heat flux are obtained. • Both the Joule heating and axial conduction effects are considered. • Numerical results agree well with the analytical theory over practical ranges of problem data. • Results are of importance for efficient thermal management of microelectronics. Abstract There has been a growing interest in the development of microchannel heat sinks deploying electrically modulated fluid flow in recent years. The efficient design of such devices requires heat transfer models that can account for complex distributions of heat generation in microelectronics. In this paper, expressions are obtained for temperature distribution and Nusselt number of thermally developing mixed electroosmotic and pressure-driven flow through circular/slit microchannels of axially non-uniform wall heat flux. The heating section is considered to be of a finite length in order to simulate a physically more realistic situation. Both the Joule heating and axial conduction effects are considered in the model. By comparing the results for linear, sinusoidal, and exponential distributions of the wall heat flux with the predictions of full numerical simulations, it is shown that the analytical solutions presented are accurate up to a Péclet number of 10. This threshold is demonstrated to be larger than the maximum Péclet number encountered in practical applications involving electroosmotic pumping mechanisms. After justification of the model, a parametric analysis is executed, revealing that the average Nusselt number is a decreasing function of the EDL thickness and pressure-driven velocity, irrespective of the wall heat flux distribution. Moreover, whereas a higher Joule heating rate is accompanied by a smaller value of the average Nusselt number for pure electroosmotic and pressure-assisted flows, the opposite is true in the presence of a significant back pressure. [ABSTRACT FROM AUTHOR]

Highlights • Heat transfer and pressure loss in swirl cooling tubes with dimples were numerically obtained. • Total heat transfer performance of the swirl tubes with dimples can be increased. • Pressure loss of the swirl tubes with dimples can be decreased. • Dimple depths have appreciable influences on the heat transfer and pressure loss. • Detailed interactions of the swirling flow with dimples at the wall were provided. Abstract This paper presents a numerical study of heat transfer and pressure loss characteristics in a swirl cooling tube with five tangential inlet jets and with dimples on the tube inner surface. The swirl tube has a length of 20 times the tube diameter, and the Reynolds number based on the tube diameter ranges from 10,000 to 40,000. Polyhedral meshes as well as the SST k - ω turbulence model were chosen for the numerical calculations. A swirl cooling system with a smooth tube was investigated as baseline, and comparatively with dimples on the tube inner surface. The results showed different swirling flow and heat transfer patterns in the swirl tubes. Dimples on the tube inner surface can reduce the wetted-area averaged heat transfer coefficient on the tube inner surface. However, the total heat transferred in the dimpled swirl cooling tubes can be increased by up to 7.2% due to the increased heat transfer area and the interactions between the swirling flow and the dimpled wall. On the other hand, the pressure loss in the dimpled tube can be appreciably reduced by up to 17.6% compared with that of the smooth swirl tube. Detailed flow interactions between the jets, the tube wall and the dimples are illustrated to explain the heat transfer change and pressure loss reduction mechanisms. [ABSTRACT FROM AUTHOR]

Highlights • Falling film flows with natural and forced disturbances are simulated. • Velocity vector in a wave is obtained by LBM. • Dimensionalization to actual absorption by a velocity ratio. • Compare wave flow with laminar one on falling film absorption. Abstract The investigations on heat and mass transfer in the form of falling film flow were mainly focused on the simulation of heat and mass coupling adopting the continuity hypothesis of Navier-Stokes equations. Most of the previous works were done with laminar flow assumptions, or just deal with a very short smooth laminar section beyond the falling film entrance. In this paper, a study investigating the effects of wavy flow on stream absorption by falling liquid film is presented in the perspective of sorption refrigeration process. A multi-phase model lattice Boltzmann method is adopted to simulate the wavy falling liquid film flow, and the absorption process takes place in this flow field region. Absorption simulation was carried out using the laminar flow assumption with the semi-parabolic velocity distribution and the fluctuation results in the LBM simulation. When the overall simulation section is 1 m in length, with initial velocity 0.1 m/s for lithium bromide solution falling film flow, results show that the wave flow has apparent enhancement on heat and mass transfer. Local dimensionless numbers for mass (Sh) and heat (Nu) transfer with waves increase 4 times and 2 times, respectively, compared with the laminar flow. [ABSTRACT FROM AUTHOR]