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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]

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]

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]

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]

• 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]

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]

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 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 • 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]

• 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]

Abstract The advancements in the materials science and the ability of manufacturing new materials, make it possible for engineers to use the porous medium as a solid body that provide maximum surface contact with the fluid. In this paper, to increase the contact surface area and decrease the flow resistance, the solid tapered fins replace with porous one and its effect on convective heat transfer is investigated for different Reynolds number. To do this, first the heat transfer of the rigid tapered fins is studied and validated, then, the solid body is changed to the porous one and the influence of the Reynolds number was investigated on the heat transfer and pressure drop. The results show that in laminar flow the Nusselt number of the flow with porous medium is 33% higher and the pressure drop is 9.35% lower than the rigid one with the same conditions. Therefore the application of the porous medium at low velocities is much recommended but with increasing the velocity, the porous media is not preferable. Also, for the problem of flow around a bundle of porous tapered fins, an equation for Nusselt number is presented based on the Reynolds number. [ABSTRACT FROM AUTHOR]

In this paper, a numerical investigation is performed to study the heat transfer characteristics and the mixed convective flow past a square cylinder oscillating at different inclinations. The computations are carried out for a fluid with a Prandlt number of 0.7 at low Reynolds number ranging from 40 to 220. The tilt angle is varied from 0 (inline oscillation) to π/2 (transverse oscillation). The governing equations were solved using a finite-volume technique. The validity of the simulations is established by comparison with the available data from literature. The flow and heat transfer features are analyzed for different angles of oscillations. A critical oscillation angle at which the flow structure changes from inline oscillation mode to transverse oscillation mode has been determined. The effects of blockage ratio (β = h/H, the ratio between the side length of the cylinder and the height of the channel), Reynolds number Re , oscillating amplitude A m and oscillating frequency F c on the heat transfer characteristics are examined. The results show that the heat transfer is enhanced remarkably, when these parameters are increased. Furthermore, the rate of heat transfer from the cylinder increases when the angle of oscillation tends to π/2. For engineering applications, appropriate correlations have been proposed between the average Nusselt number and the different parameters. Finally, the interest of using the inclined oscillation is discussed. This approach has the advantage of widening the range of the oscillation amplitude. The results show that the heat transfer rate has been at least doubled. [ABSTRACT FROM AUTHOR]

In this paper, the MRT (Multiple Relaxation Time)-D2Q9 model and the MRT-D2Q5 are employed to simulate the flow and heat transfer of laminar confined impinging jet flow. For impinging jet flows, the paper considers the effect of Reynolds numbers and jet-impingement surface distance ratio ( H / W ) on the flow and temperature fields. As for the investigation of heat transfer behaviors on stagnation Nusselt number, it is evident that the effect of H / W is not significant. The Nu number increases dramatically with increasing Reynolds number. And what’s more, this study presents flow and heat transfer predictions of slot jets impinging on cylinder and obtains distribution of Nusselt numbers around the cylinder for different Reynolds number. [ABSTRACT FROM AUTHOR]

This paper constructs a three-dimensional model to numerically investigate the heat transfer and fluid flow characteristics of combined microchannel with cone-shaped micro pin fins (MCPF) for inlet Re number ranging from 147 to 637. Firstly, the cases of micro pin fins located in the upstream (case1), middle (case2), downstream (case3) of microchannel and distributed throughout the microchannel (case4) are studied with fixed number of fins. Thereafter, the dimensionless parameters of relative fin diameter, relative space between the fins and relative height of the fin are studied to optimize the design. The variations of apparent friction factor f app,ave and average Nusselt number Nu ave with inlet Re number have been conducted, and the thermal enhancement factor ( η ) has been adopted as evaluation criteria to determine the best comprehensive thermal-hydraulic performance of MCPF. The results show that the microchannel inserted cone-shaped micro pin fins improves the heat transfer accompanied with a higher pressure drop as compared with the smooth rectangular (Rec) microchannel. Microchannel with uniform fins distribution (case4) has the largest η and the bottom wall temperature is the lowest and well-distributed among all cases. When Re < 500, the overall thermal performance of case3 is better than case1 but when Re > 500, it displays a contrary tendency. The geometric configuration optimization of MCPF suggests that there is an optimum pin diameter, fin space and fin height that satisfy the demands of heat transfer and pressure drop. [ABSTRACT FROM AUTHOR]

This paper numerically compares the flow structures and thermal performance in a swirl chamber with three different inlet chambers. The cross section of inlet chambers are constant, convergent, and divergent along the streamwise direction, respectively. Parametric studies involve Reynolds number and the temperature ratio between cooling air and the wall of swirl chamber. Results indicate that shear layer vortices, corner vortices, and large-scale vortices appear in different sections of swirl chamber. Besides, Görtler vortices appear in convergent inlet chamber. The alternation of the inlet chamber changes more than the value of the Nusselt number. It also changes the flow structures and the distribution of Nusselt number. In comparisons, the swirl chamber with convergent inlet chamber possesses the highest thermal performance coefficient. With the increase of Reynolds number, the intensity of heat transfer is significantly enhanced. Different from former research, the rise of temperature ratio leads to relatively high Nusselt number due to the addition of multiple jet nozzles and the inlet chamber. In addition, empirical correlation among global average Nusselt number, Reynolds number, and temperature ratio is obtained to assist relevant research and design. [ABSTRACT FROM AUTHOR]

In this paper, the effect of vortex induced vibration (VIV) on convective heat transfer from an elastically mounted rigid circular cylinder in cross-flow is investigated numerically. The motion of cylinder is modeled by using a mass-spring-damping system. The effect of reduced velocity and damping ratio on the vortex formation, vortex shedding process, cylinder displacement amplitude, Nusselt number and the position of maximum local Nusselt number is studied. In this study the Reynolds Number and the mass ratio are 150 and 2, respectively. Furthermore, different values of reduced velocity and damping ratio are investigated ( U r = 3, 4, 5, 6, 7, 8 and   ζ = 0. 0.01, 0.05, 0.1). The numerical results demonstrate that the reduced velocity and damping ratio can affect the heat transfer considerably. The beating phenomenon is occurs at U r = 4 and ζ = 0.05 which leads to changes in the displacement amplitude and Nusselt number widely with respect to the time. In the beating phenomenon the total Nusselt number decreases in comparison with a stationary cylinder. The maximum heat transfer enhancement is obtained at U r = 4 , ζ = 0 . [ABSTRACT FROM AUTHOR]

In this paper, a two-dimensional numerical simulation is carried out to understand the effect of confinement (blockage ratio β ) on fluid flow and forced convective heat transfer characteristics past a blunt headed cylinder. Utilizing air as an operating fluid, flow simulations are carried out for wide ranges of blockage ratios ( 1 10 ≤ β ≤ 1 3 ) and Reynolds numbers ( 60 ≤ Re ≤ 200 ) . The flow characteristics and heat transport are analysed critically for different β . The functional dependence of C D (Drag Coefficient) and C L (Lift Coefficient) on blockage ratio is examined. It has been found that C D reduces with increasing Re, while the Strouhal number and the average Nusselt number show an increasing trend when the blockage ratio is increased. The average Nusselt number also increases with increasing Re . [ABSTRACT FROM AUTHOR]

The present paper deals with entropy generation study of CuO/water nanofluid flow over a spherical heat source that is installed inside a cylindrical channel with sudden expansion and contraction. To estimate the entropy generation rate; first, the governing equations are solved numerically to find the velocity and temperature fields in the domain where the flow is steady and laminar. Next, using the simulation data, the entropy generation number is calculated theoretically. The effects of nanofluid concentration, nanoparticle size, blockage ratio, and Reynolds number on the entropy generation have been investigated. The outcomes of this research elucidate that entropy generation number rises with an increase in the nanoparticle volume fraction and surface temperature of the heat source. On the other hand, it was found that the entropy generation is diminished by implementing finer nanoparticles and lowering the blockage ratio. In this study, a new correlation has also been presented to calculate the Nusselt number that is a function of Reynolds number, Prandtl number, and blockage ratio. [ABSTRACT FROM AUTHOR]

Inserts are used to enhance the heat transfer rates between the two fluids in heat exchanger tubes. A variety of tube inserts such as twisted tape, wire coil, swirl flow generator have been investigated for their effect on heat transfer rates and fluid friction. This paper reviews the works pertaining to the application of different class of tube inserts in order to comprehend the prevailing mechanism of fluid flow and heat transfer. An attempt has been made to elucidate the fluid flow behaviour sustained by the particular class of insert that controls the heat transfer rates across the thermal boundary layer attached to the tube wall. [ABSTRACT FROM AUTHOR]

In this paper, heat transfer in a carbon nanotube based fluid between two parallel rotating disks is investigated. The governing equations are solved by Least Square Method (LSM) using a semi-analytical code. Water is considered as the base fluid and two kind of carbon nanotube (CNTs), single-walled carbon nanotube (SWCNT) and Multi-walled carbon nanotube (MWCNT) are added as the second phase or additives to the base fluid. The problem is solved for different nanoparticles volume fraction and Reynolds numbers to study the effect of some parameters such as scaled stretching parameters (A 1 , A 2 ), Rotation parameter (Ω), Radiation parameter (Rd) and thermal Biot numbers (γ 1 ,γ 2 ) on the skin friction factors and Nusselt numbers. Results show that in most cases, SWCNTs leads to more heat transfer and temperature profiles compared to MWCNTs. [ABSTRACT FROM AUTHOR]

Jet impingement cooling is usually used in zones with extremely high thermal loads. The major factor that limits impingement heat transfer uniformity is the crossflow. The impingement cooling with return hole is applied to reduce the negative effect of crossflow on the target surface heat transfer in present paper. The influences of jet-to-plate spacing (H / D), impingement target surface curvature (α), temperature ratio (TR) and Reynolds number on flow field and heat transfer performance are investigated in detail. The results indicate that the correlation between the overall area average Nusselt number of target surface and H / D , the overall area average Nusselt number decreases by approximately 5.76% with H / D increases by one unit. Increasing the impingement target surface curvature is beneficial to expand the spanwise heat transfer. The overall area average Nusselt number increases by 295.5% for Reynolds number varying from 10,000 to 50,000 at TR = 0.910, and the overall area average Nusselt number increases by 8.68% for the TR varying from 0.668 to 0.910 at Re = 30,000. The results indicate that impingement cooling with return hole can achieve high heat transfer coefficient and uniform heat transfer distribution. [ABSTRACT FROM AUTHOR]

This paper presents a numerical investigation of the flow-induced pitching, heaving or surging of an airfoil in a rectangular channel for heat transfer enhancement. The flow dynamics, airfoil fluttering response, and heat transfer performance are analyzed and discussed at four different Reynolds numbers (Re = 700, 1000, 1300 and 1500), three rotational axis locations (a x / C =0.25, 0.33 and 0.42), two airfoil perturbation modes (passive vs. active), and three combinations of multiple airfoil freedoms (pitch only, pitch and heave, and pitch and surge) with various stiffness coefficients. It is found that after Reynolds number reaches 1000, the pitch only airfoil starts the similar self-fluttering pattern, and the fluttering effect becomes the dominant influencer for increasing the Nusselt number compared to the Reynolds number effect. Placing the rotational axis at a x / C =0.42 achieves the highest thermal enhancement factor η of 1.4, while when a x / C =0.25, the airfoil oscillation diminish effect is vast because of its aerodynamic center. The passive pitching airfoil can achieve more than 6% of the thermal enhancement factor than the active ones. Adding another heave or surge degree of freedom to the pitching airfoil, the heat transfer enhancement of the channel can also be improved. The present work can facilitate future optimizations of airfoil-based flow-structure interaction system for heat exchanger channels. [ABSTRACT FROM AUTHOR]

• Thermal and hydrodynamic characteristics of staggered drop-shaped tubes were studied. • The effect of the number of tube rows N R on the performance was investigated. • The effect of increasing Reynolds number Re on the performance was analyzed. • The investigation covered 1.78 × 103 ≤ Re ≤ 18.72 × 103 and 1 ≤ N R ≤ 20. • New correlations for N u ¯ , f , and ε considering N R were developed. In this paper, for the first time, air-side correlations were developed and introduced to describe the average Nusselt number N u ¯ , friction factor f , and thermal − hydraulic performance ε of compact heat exchanger employing drop-shaped tubes considering the number of tube rows (N R). Drop-shaped tubes were arranged in a staggered formation and placed in 1 to 20 consecutive rows in the flow direction, each consisting of 7 tubes in the transverse direction. The Reynolds number (Re) varied from 1.78 × 103 to 18.72 × 103. The calculations were carried out using Ansys Fluent software package. Tube row and Reynolds number effects on the thermal and hydrodynamic characteristics of the studied heat exchangers were analyzed. The simulation results were in a good agreement with the available literature. The results showed that N u ¯ and ε increase, while f decreases as Re increases. Moreover, an increase in N R leads to an increase of f and a decrease of ε. The maximum values of N u ¯ were obtained for N R = 2, which were higher by 3.95–6.69% and 19.91–24.62% than those for N R = 1, and N R = 20, respectively. Heat transfer was stabilized only from the 19-row onwards. In addition, a 1-row heat exchanger had better thermal − hydraulic performance compared to that with a larger N R (ε for N R = 1 was about 22.32–27.24 times higher than those for N R = 20). The maximum deviation between the numerical results and the obtained correlations developed from them was ± 8.87%, which provides a useful reference for future studies and designs of heat exchangers with different row numbers of drop-shaped tubes. [ABSTRACT FROM AUTHOR]

This paper compares the thermohydraulic performance of varying curvature wavy microchannels, namely elliptical-wavy and sinusoidal-wavy microchannels. The amplitude of microchannels is fixed at 0.6 mm, and wavelengths are varied from 3 mm to 8 mm. Simulations are performed for the laminar flow of water (160 ≤ Re ≤ 900) considering three-dimensional conjugate heat transfer using ANSYS Fluent version 2020R2. The computational model has been validated by performing experiments on a serpentine-wavy microchannel, a special case of the elliptical-wavy microchannel, using micro-particle image velocimetry (μ-PIV) test facility. Both Nusselt number and friction factor increase with the increase in waviness. Elliptical-wavy microchannels have superior thermohydraulic performance than sinusoidal-wavy microchannels except for the channel of highest waviness at Re = 160. A microchannel with lower entropy generation has a higher thermohydraulic performance. Further, we observe centrifugal instabilities in both sinusoidal-wavy and elliptical-wavy microchannels at higher Reynolds numbers. Centrifugal instabilities in the elliptical-wavy differ from those in the sinusoidal-wavy microchannel by the asymmetric formation of additional vortices about the horizontal midplane in the channel cross-section. [ABSTRACT FROM AUTHOR]

This paper explores the effect of curved shape fin heights on open microchannel heat sink performance. Different geometries of rectangular, plano-convex, and plano-concave fins have been investigated by employing three-dimensional simulation. A variety of parameters including fin height, curve range coefficient, Reynolds number, and heat flux have been examined. At the same heat flux, Reynolds number, and fin height, the Nusselt number of the rectangular fin is lower than the two shapes of plano-convex and plano-concave. In Comparison of the two curved shapes, due to their geometries and fluid flow behavior, in the lower fin heights, the Nusselt number of the plano-concave is greater than the plano-convex. In the higher fin heights, the Nusselt number of the plano-convex is greater than the plano-concave. Even though at higher fins, where the Nusselt number of the plano-convex fins is greater than the rectangular and plano-concave shape, the plano-convex pressure drop is less than these two shapes. In a heat flux of 450 k W / m 2 , Reynolds number of 600, and a fins equivalent height of 0.6 mm, that the volume and mass of the fins are the same, there is an 18% difference between the maximum and minimum Nusselt number of plano-convex and plano-concave, by different curve coefficients. The obtained results are beneficial in designing novel shape fins in enhanced open microchannel heat sinks. [ABSTRACT FROM AUTHOR]

The main aim of this paper is to investigate the effect of nozzle geometry on the flow and heat transfer from a pulsed jet into a concave surface. Experiments have been carried out for pulsed circular jet and numerical simulations were done for circular, elliptical, rectangular and square geometries. Numerical solution was performed for the frequency ranges of 25 H z to 100 H z , the jet Reynolds number of 7000 , the dimensionless nozzle distance to concave surfaces of 2 and 5. Numerical results show a consistent agreement with experimental results and previous works. Accordingly, the geometry of nozzle directly affects the air entrainment ratio. In addition, with the increase of aspect ratio of the nozzle, average Nusselt number (Nu ave) decreases. By pulsating the inlet jet with pulse frequency of 100 H z , the average Nu number for circular/square and elliptical/rectangular jets increases 22 % and 15 % , respectively. Generally, the pulsation causes a reduction and an increase of the Nu ave at low and high frequencies, respectively, compared to that of the steady jet. At low frequencies, the Nu ave of circular jet decreases significantly in comparison to that of the square and elliptical jets. However, at high frequencies, circular jet shows higher Nu ave compared to other ones. As the distance between the jet and the concave surface increases, the effect of the nozzle shape on Nu ave decreases. [ABSTRACT FROM AUTHOR]

This paper analyzes the fluid and heat flow within entrapped porous triangular cavities involving various thermal boundary conditions (case 1: hot horizontal walls and cold inclined walls; case 2: cold horizontal walls and hot inclined walls). Finite element based numerical study has been performed for various values of Prandtl number ( Pr m = 0.026 and 7.2 ), Darcy number ( Da m = 10 - 4 - 10 - 2 ), Reynolds number ( Re = 1 - 1000 ) at a high value of Grashof number ( Gr = 10 5 ). The upper cavity for the case 1 and the lower cavity for the case 2 show almost similar trends and vice versa based on the exact opposite thermal boundary conditions. Heat transfer is conduction dominant at Da m = 10 - 4 involving both Pr m whereas, convection heat transfer dominates at Da m = 10 - 2 especially for Pr m = 7.2 . Although the motion of the horizontal walls significantly influences the fluid flow field within the cavities, due to the decoupling between the fluid and thermal fields at the low Pe m ( Pe m = 0 . 026 , 0 . 26 and 2.6 ), conduction dominant heat transfer occurs. At the high Da m and Pr m = 7.2 , a pair of symmetric streamline or heatline cells are seen for Re ≤ 1 whereas, a bigger primary streamline or heatline cell is accompanied by a secondary streamline or heatline cell for 1 ⩽ Re ≤ 100 . At 500 ⩽ Re ⩽ 1000 , the single larger streamlines or heatline cells are seen especially for Pr m = 7.2 involving Da m = 10 - 2 . At Pr m = 0.026 , the average heat transfer rates ( Nu t ‾ , Nu b ‾ , Nu l ‾ and Nu r ‾ ) are almost constant with Re and Da m . At Pr m = 7.2 , the larger magnitudes of Nu t ‾ and Nu l ‾ are observed for Re ≤ 10 compared to those for Re = 100 involving Da m ⩾ 10 - 3 whereas, the magnitudes of Nu r ‾ are almost similar for all Re involving all Da m in the upper cavity (case 1) within Re ⩽ 100 . In the lower cavity (case 1), the magnitudes of Nu b ‾ and Nu r ‾ are larger for Re = 100 whereas, the magnitudes of Nu l ‾ are larger for Re ≤ 1 for almost entire range of Da m within Re ≤ 100 involving Pr m = 7.2 . The average Nusselt numbers are significantly larger at the high Re ( Re = 500 and 1000) involving all Da m (except for the horizontal walls involving Da m ⩽ 8 × 10 - 4 ) in the cases 1 and 2 at Pr m = 7.2 . [ABSTRACT FROM AUTHOR]

This paper investigates a Taylor-Couette-Poiseuille flow in an annular channel of a slotted rotating inner cylinder, corresponding to a salient pole hydrogenerator. The purpose of this study is to improve the understanding of flow and thermal phenomena in electrical machines using a simplified scale model. The validation of the numerical model for a specific configuration is first shown by comparing the results with the experimental data. A parametric study is also performed to investigate all main flow regimes and to derive correlations in terms of the Nusselt number distribution on the rotor pole face and sides. The results show that the Nusselt number is proportional to the tangential Reynolds number to the power 1/7 in the pole and inductive faces trailing side. This relationship is similar to the one encountered in classical Taylor-Couette-Poiseuille flows between two concentric and smooth cylinders. [ABSTRACT FROM AUTHOR]

In this paper, the performance of ethylene glycol and water based TiO 2 nanofluids as an automobile radiator coolant is determined experimentally. Forced convective heat transfer coefficient of TiO 2 nanofluids has been measured and compared with the data of base fluid in an automobile radiator. Nanofluids were prepared taking 40% ethylene glycol and 60% water with volume concentrations of 0.1%, 0.3% and 0.5% of TiO 2 nano powder. All the experiments were conducted in the range of Reynolds numbers from 4000 to 15,000. In all the experiments the nanofluids made to flow through the radiator tubes with elliptical cross section and airflows with constant speed in the crosswise direction in between the tubes of a tube bundle. Results demonstrate that increasing the fluid circulation rate can improve the heat transfer performance while the fluid inlet temperature to the radiator has little or no effect. Nanofluids investigated in the present work with low concentrations enhanced the heat transfer rate up to 37% in comparison with base fluid. The results of the investigation are presented graphically in terms of non-dimensional heat transfer coefficient as a function of volume concentration, temperature and flow rate. [ABSTRACT FROM AUTHOR]

Heat transfer characteristics of Fe 2 O 3 /water and Fe 2 O 3 /EG nanofluids were measured in a shell and tube heat exchanger under laminar to turbulent flow condition. In the shell and tube heat exchanger, water and ethylene glycol-based Fe 2 O 3 nanofluids with 0.02%, 0.04%, 0.06% and 0.08% volume fractions were used as working fluids for different flow rates of nanofluids. The effects of Reynold's number, volume concentration of suspended nanoparticles and different base fluids on the heat transfer characteristics were investigated. Based on the results, adding nanoparticles to the base fluid causes a significant enhancement of the heat transfer characteristics and thermal conductivity. This enhancement was investigated with regard to various factors; concentration of nanoparticles, types of base fluids, sonication time and temperature of fluids. In this paper, the effect of Fe 2 O 3 nanoparticles on the thermal conductivity of base fluids like ethylene glycol and water was studied. The thermal conductivity measurement was made for different concentrations and temperatures. As the concentration of the nanoparticles increased, there was a significant enhancement in thermal conductivity and overall heat transfer due to more interaction between particles. It was also observed that there was an improvement in the thermal conductivity of the base fluid as the temperature increased. The measurements also showed that the pressure drop of nanofluid was higher than that of the base fluid in a turbulent flow regime. However, there was no significant increase in pressure drop at laminar flow. [ABSTRACT FROM AUTHOR]

Jet impingement is exceedingly operational for numerous field applications including electronics cooling and process industry. Its implementation comfort makes it common for many applications. The paper deals with investigation of heat transfer characteristics of a cooling flat surface. It is cooled by air jet impinged at an inclination of 15–75° to target plate. For understanding variation in cooling performance, comprehensive experiments are performed with different configurations of inclined jets on hot surface. For simplicity in using air, it is recommended for verity of applications as cooling fluid. Air Inclined jet with Reynolds Number in the range of 2000 ⩽ Re ⩽ 20,000 is examined for the circular Inclined jet. The target to inclined jet perpendicular height ( H ) is varied from 0.5 ⩽ H / D ⩽ 6.8 for understanding effect of H on cooling performance of different locations on target plate. The investigation leads to equations for average Nusselt Number for inclined non-confined air jet for cooling applications. [ABSTRACT FROM AUTHOR]

• IR Thermography is applied to measure heat transfer from impinging swirling jets. • The swirl number and impingement distance influence the shape of the heat transfer distribution. • The Reynolds number affects essentially the magnitude of the heat transfer rates. • Four different regimes are identified as a function of the swirl degree and impingement distance. • Empirical correlation laws for the area-averaged Nusselt number in these regimes are derived. This paper reports on a parametric study of the impingement heat transfer from swirling jets issuing from a circular nozzle. The swirl velocity component is imparted to the flow via a tangential entry into a recirculation chamber located upstream of the nozzle; the degree of swirl is varied by changing the angle of the channels leading to the chamber with respect to the radial direction. The effects of the dimensionless impingement distance (H / D = 1 , 2, 3, 4, 6, 8, 10, 12, 14 with D being the nozzle diameter), the swirl number (S = 0 , 0.078 , 0.22 , 0.37 , 0.54) and the Reynolds number (Re = 20,800 , 31,100 , 41,500) on the spatial distribution of the heat transfer are investigated by means of infrared thermography and the heated thin foil sensor. The present results show that the swirl number and the impingement distance significantly influence the structure of the heat transfer distribution, while the Reynolds number affects essentially the magnitude. Four distinct ranges with different behaviours can be identified by distinguishing between weakly or non swirling jets (0 ≤ S ≤ 0.22) and moderately swirling jets (0.37 ≤ S ≤ 0.54) and between short and long impingement distances (1 ≤ H / D ≤ 4 and 6 ≤ H / D ≤ 14 respectively). A comparative assessment of the performance of the swirling jets is carried out by focusing on the area-averaged Nusselt number distributions. It is found that in the range of short impingement distances and over relatively small target areas an enhancement of the heat transfer rates can be obtained at a small extent by adding a weak swirl with no detrimental effect on the uniformity of the distribution and at a larger extent by a moderate swirl with a deterioration of the uniformity. For long impingement distances no enhancement is observed, although swirl yields a significant reduction of the non-uniformity. Finally, correlation laws of the area-averaged Nusselt number as a function of the control parameters are derived in the four regimes identified. [ABSTRACT FROM AUTHOR]

This paper proposed a swirl chamber with circumferential ribs to avoid the impacting of cross-flow to jets, thus enhancing the penetration ability of jet and swirl cooling heat transfer intensity. To reveal the mutual effects between jets and cross-flow, the heat transfer features and flow structures under different Reynolds numbers of smooth swirl chamber and novel rib-roughened swirl chambers were studied and compared using 3D steady numerical analysis. The influence of rib height ratio on flow and heat transfer features was investigated further. The investigation revealed that for smooth swirl chamber, the cross-flow seriously weakened the heat transfer intensity. In contrast, the novel rib-roughened swirl chamber had an excellent suppression effect on cross-flow, thus greatly enhancing the penetration ability of jet and strengthening the swirl cooling heat transfer intensity. Under the Reynolds number of 10,500, for smooth swirl chamber, the peak number of circumferentially-averaged Nusselt number decreased from 80 to 58 from the first pitch to the last pitch. While for the case that rib height ratio is equal to 0.5, it increased from 80 to 120. In addition, the suppression effects of circumferential ribs on cross-flow increased when the rib height ratio increased, likewise the heat transfer enhancement effect. When the rib height ratio is less than 0.5, the Nusselt number ratio was larger than the friction factor ratio, while when the rib height ratio is larger than 0.5, the Nusselt number ratio was less than the friction factor ratio. The best thermal performance can be achieved when the rib height ratio was 0.5. Besides, the heat transfer enhancement effect of circumferential ribs was more significant under a higher Reynolds number in the studying range. [ABSTRACT FROM AUTHOR]

All studies regarding the double passages channel use a rigid baffle to separate the two passages. In the current paper, we use a flexible separator and inspected its role in enhancing the performance of the heat transfer. The cold fluid is forced upward in the channel which is heated isothermally by different temperatures on its walls. A flexible sheet is fixed from its two ends and equally separate the channel to two passages. The problem is investigated numerically using the finite element method coupled with the arbitrary Lagrangian-Eulerian approach. The problem is casted in dimensionless form and several pertained parameters have been inspected, like Reynolds number in each passage, elasticity of the separating sheet and the temperature ratio of the channel walls. Results are promising where it is found that the enhancement of heat transfer is possible passively in either passage with the aid of using a flexible separator rather than rigid separator. A 6% enhancement in the Nusselt number of the left passage, which comprises the higher temperature, is obtained by making the separator flexible. The flexible sheet is found to increase the pressure drop to 200% in some cases which leading to decline the thermal performance criterion to lower than unity. However, the thermal performance criterion TEC was feasible in the right passage which subjects to lower temperature. • A monolithic finite element approach using high-order schemes in space and time is utilized. • Heat transfer enhancement is possible passively in a passage with the aid flexible separator rather than rigid separator. • A 6% enhancement in the Nusselt number of the left passage is obtained by making the separator flexible. • The flexible sheet increases the pressure drop to 200% in some cases leading to decline the thermal performance criterion. • Thermal performance criterion TEC was feasible in the right passage which subjects to lower temperature. [ABSTRACT FROM AUTHOR]

• Flow and heat transfer characteristics of jet array impingement with effusion holes are investigated. • A total of 300 simulations are tested for five hole arrangements. • The radial acceleration, turbulence, and jet potential core strongly affect impingement heat transfer. • Predictions of previous Nusselt number correlations for jet array impingement are evaluated. • A new Nusselt number correlation applicable to five hole arrangements is proposed. Impingement/effusion cooling systems have been studied extensively over the past few decades to constantly improve cooling performance as the turbine inlet temperature of modern gas turbine engines has increased. However, a Nusselt number correlation is less developed for an impingement/effusion cooling system with effusion holes in its impingement plate than for jet array impingement without effusion holes. Moreover, Nusselt number correlations, available for a wide range of geometric and flow parameters with various hole arrangements, are sparse for gas-turbine casing cooling systems. Therefore, this study investigated the effects of these parameters on the flow and heat transfer characteristics of an impingement/effusion cooling system that uses a jet array of effusion holes to cool a gas-turbine casing. The circular holes in the parallel jet and impingement plates were arranged in five different hole arrangements. Air forced through holes in the jet plate struck the impingement plate then discharged through the effusion holes of the impingement plate. Three-dimensional simulations were performed to examine the effects of hole pitch, distance between the jet and impingement plates, ratio of total jet area to heat transfer area, and Reynolds number based on the jet hole diameter on impingement heat transfer. A total of 300 simulations were run for the five hole arrangements to obtain the local and average Nusselt numbers on the impingement plate. Our results show that the radial acceleration and turbulence near the impingement plate, and the length of the jet potential core strongly affect the impingement heat transfer. This study also evaluated previous Nusselt number correlations for jet array impingement. This paper presents a new correlation to predict the average Nusselt number of jet array impingement applicable to the five hole arrangements. [ABSTRACT FROM AUTHOR]

A novel type of pipe insert was developed to improve convection heat transfer in the present work. Heat transfer performance and pressure drop was numerically studied. Pipe inserts with different dimensionless spacer length (S/D = 3.33, S/D = 5, S/D = 6.67, S/D = 8.33, S/D = 10, S/D = 11.67) were investigated at the Reynolds number range of 100–1750. Liquid water was used as the working fluid. The results showed that the maximal Nusselt number was enhanced by 3.4–10.3 times as that of the smooth tube. The friction factor resulted in an increase of 5.6–13.5 times. Performance Evaluation Criterion (PEC) values were approximately 1.91–4.33. The Nusselt number increased with the decrease in dimensionless spacer length. A small spacer length resulted in a high heat transfer coefficient. However, it also brought a high flow resistance and eventually deteriorated the heat transfer performance. Therefore, a suitable dimensionless spacer length S/D = 6.67 was recommended in this paper. The effect of pipe shape (R/D = 3.33, R/D = 5, R/D = 6.67) on the thermal characteristic was also investigated in this study. It indicated that the pipe shape had a little impact on the heat transfer performance in the laminar regime. Compared with other inserts, pipe inserts can transfer more heat for the same pumping power for their structure, because their special structure make it possible for the fluid to flow from the central region to the wall region, which reconstructs the velocity profile and temperature profile in the tube and results in a high heat transfer performance. [ABSTRACT FROM AUTHOR]

• Self-excited rotational squares enhance heat and mass transfer in a microchannel. • Transport details are revealed under all 5 distinct rotation modes with 5 < Re < 300. • Both local and overall enhancement efficiencies are quantitatively determined. • Phase diagrams of enhancement index are drawn relative to Re and axial position. • Underlying mechanisms are analyzed based on cylinder dynamics and flow structures. The flow disturbance technique is an effective method to enhance the heat and mass transfer efficiency in microchannels. This paper numerically investigates the influence of two side-by-side freely rotatable square cylinders on microchannel flow, heat transfer, and mass transfer under a Reynolds number (Re) range of 5‒300. Results indicate that the cylinders show five different vortex-induced rotation modes, namely the rotation mode (Re < ∼10), oscillation mode (∼10 < Re < ∼70), stationary mode (∼70 < Re < ∼90), random mode (∼90 < Re < ∼250), and reversal rotation mode (Re > ∼250). The vortices generated by rotations of the cylinders disturb the flow field significantly, thus promoting heat and mass transfer in the microchannel. As Re increases, the average Nusselt number (Nu) of the channel first rises slowly then sharply, specifically, from 3.50 when Re = 5 to 8.35 when Re = 75, and then to 42.45 when Re = 300. Under a small Re , the rotating cylinders only affect the local heat transfer near them, while only when Re > 100 can the cylinders significantly promote the heat transfer in a relatively long distance in the microchannel. This behavior should be attributed to the insufficient vortex disturbance under small Reynolds numbers. Mass transfer is scrutinized by releasing fluorescein at the upper half of the channel entrance, and mixing efficiency (η) is introduced to characterize the mixing situation. We show that when Re = 100, the substances in the channel are almost completely mixed (η = 0.98) compared with η = 0.84 for stationary cylinders; beside, as Re increases, the position where the mixture reaches a steady-state gradually moves forward in the channel, and when R > 100, the distribution of η along the channel seldom changes with Re. Considering fluid pressure loss, heat transfer enhancement index φ and mass transfer enhancement index ϕ are introduced to compare heat and mass transfer capability between microchannels set with rotatable cylinders and stationary cylinders, and phase diagrams relative to Re and x * are drawn. Analysis reveals that the freely rotatable cylinders better improve the heat and mass transfer with a maximum φ of 1.6 at Re ≈ 25, x * ≈ 12 or Re ≈ 100, and maximum ϕ of 1400 at Re ≈ 50, x * ≈ 10‒25. [ABSTRACT FROM AUTHOR]

A comparative experimental study on the heat transfer characteristics of steam and air flow in rectangular channels roughened with parallel ribs was conducted with the use of an infrared camera. The effects of Reynolds numbers ( Re ), rib spacing ratio ( P / e ) and rib angles ( α ) on steam and air convective heat transfer were obtained and compared. The blockage ratio ( e / D h ) of the tested channel is 0.078 and the aspect ratio ( W / H ) is fixed at 3.0. In the paper, the Reynolds number for both coolants ranges from 3000 to 15,000, the rib spacing ratios were 8, 10 and 12, and rib angles were 90°, 75°, 60°, and 45°, respectively. For a square channel with 90° ribs with wider rib spacing ratio ( P / e = 10 and 12) respectively give a area-averaged heat transfer about 8.4% and 11.4% more than P / e = 8 model. The heat transfer enhancement of both steam and air increased with decreasing the rib angle from 90° to 45°. Although the heat transfer distributions of steam and air flow in the rib roughness channels were similar to each other in all the investigated channels, the steam flow can obtain a high convective heat transfer enhancement capability. The area-averaged Nusselt numbers of steam flow at a Reynolds number of 12,000 were approximately 13.9%, 14.2%, 19.9%, and 23.9% higher than those of air flow for rib angles of 90°, 75°, 60°, and 45°, respectively. [ABSTRACT FROM AUTHOR]

The present work studies the enhancement of heat transfer on a cylinder impinged by a rectangular jet in presence of a metallic grid on the slot exit. The experiments are carried out to cool a smooth cylinder, electrically heated, with a rectangular jet of air, at three Reynolds numbers in turbulent flow, i.e. Re = 5100, 10,600 and 15,300, where the Reynolds number is defined with the cylinder diameter. Three metallic grids, identified by the French classification as NF06-10-20, are employed with the solidity ratio ranging from 24.3% to 67.7%, and the same diameter of the mesh filament (0.6 mm). Fluid dynamic measurements, performed with a hot film anemometer, are done to justify the thermal results. Heat transfer measurements are presented as local and mean Nusselt numbers. The comparison with the case without grid, indicated as NF00, is done at the same mass flow rate of the jet and the same power consumption of the wind tunnel to move the jet. The main goal of the paper is to evaluate how the solidity ratio of the metallic grids influences the heat transfer. [ABSTRACT FROM AUTHOR]

In this paper the flow of an incompressible Newtonian fluid past a stationary unconfined spheroid under the mixed convective heat transfer regime is studied numerically. The spheroid is assumed to be maintained at a constant wall temperature. The effect of Reynolds number (Re), Prandtl number (Pr), aspect ratio ( E ) and Richardson number (Ri) on the average drag coefficient and Nusselt number is investigated. The range of parameters considered in this study is: 10 ≤ Re ≤ 100; 0 < Ri < 2 and 0.5 ≤ E ≤ 1.5 and for two Prandtl numbers of 1 and 5. The drag coefficient increases monotonically with the increase of surface temperature for all Reynolds number. Prolate spheroids have higher drag coefficient and Nusselt numbers compared to oblate spheroids. The variation of Nusselt number with Ri at a fixed Re is non monotonic at large Re and this effect is stronger for prolate spheroids when compared to oblates. For aspect ratios greater than unity and for Re > 60, Nusselt numbers for small Ri (∼0.5) are lower compared to Nusselt numbers for Ri = 0. This difference in Nusselt numbers becomes appreciable for larger Prandtl numbers. A correlation for average Nusselt number was developed using least squares regression analysis. [ABSTRACT FROM AUTHOR]

This article presents an experimental investigation of the heat transfer characteristics in a typical rotor–stator system with cooling air inlet at low radius under different dimensionless flow rate (CW ) ranges from 1.32×104 to 4.87×104 at five different rotational speeds, i.e., 500, 1000, 1500, 2000 and 2500rpm with a large range of rotational Reynolds numbers 4.18×105 ⩽ Reω ⩽2.484×106. A transient thermochromic liquid crystal (TLC) technique is employed to obtain the detailed distribution of the Nusselt number (Nu) on the surface of the rotor with effective rotating radius of 350mm as well as with a maximum gap of 67mm between the rotor and stator. The relationship between the Nusselt number (Nu) and the turbulent flow parameter (λT ) which has been identified as the parameter governing heat transfer in the rotating disk system is experimentally explored. A numerical model is developed for the investigation of the flow structure inside the rotating cavity and provides a basis for further study in explaining the heat transfer behavior over the rotating disk. It is found that the heat transfer characteristics are strongly affected by the flow structure. Numerical results also clearly show the existence of three flow regimes inside the cavity, namely, viscous regime, co-dominated regime, and inertial regime. The heat transfer characteristics in a rotor–stator system are well explained by the flow structure obtained in the paper. [ABSTRACT FROM AUTHOR]

It is significant to develop an expression for the rarefaction correction to the continuum solution for heat transfer. An extended Reynolds analogy for slip and transition flow heat transfer in microchannels and nanochannels is proposed. A relationship between the rate of the dimensionless mean wall shear stress reduction and the rate of the dimensionless mean wall heat flux reduction may exist. The extended Reynolds analogy can be utilized to obtain an initial engineering approximation of the Nusselt number. As slip flow heat transfer is inherently complex, the proposed simple model may provide an insight into the mechanism of slip flow heat transfer. This paper cultivates the facility to perform the kind of engineering analysis which, although not exact, still provides useful information concerning real engineering needs. [ABSTRACT FROM AUTHOR]

Taylor–Couette–Poiseuille flow commonly occurs in rotating machinery in the annulus between the outer casing and rotating shaft as part of lubrication or thermal management systems. To improve efficiency of these machines, industry is beginning to work with low viscosity, high density fluids operating at high pressures, such as supercritical carbon dioxide turbines. Within existing literature, vast high quality heat transfer data exists for air under Taylor–Couette–Poiseuille flow regimes. However, for other fluids, these data are limited in their experimental coverage, especially at high Taylor numbers (i.e. > 1 × 10 10). This paper presents first of its kind experimental data for supercritical carbon dioxide. The data are analysed to provide insight into the heat transfer processes and to provide a Taylor–Couette–Poiseuille flow heat transfer correlation. Operating conditions of the experimental campaign cover pressures of 7 and 10 MPa, a temperature range of 315 to 333 K, and rotational speeds of 10 to 25 kRPM. This equates to Taylor number and axial Reynolds number ranges of 9.52 × 10 10 to 8.73 × 10 11 and 3.41 × 10 4 to 5.4 × 10 4 , respectively. The range of conditions tested covers the operational envelope of those found in thermal management systems for supercritical carbon dioxide turbine designs. It is found that heat transfer of supercritical carbon dioxide is significantly higher than that of ideal gases (e.g. air) for the same Taylor number range. Further findings show that change in fluid properties near the critical point (i.e. heat capacity and density) increase the Nusselt number from 1.5 × 10 3 at 10 MPa to 3.1 × 10 3 at 7 MPa at an effective Reynolds number of 2.5 × 10 6. A dimensional analysis shows the importance of capturing the change in fluid properties when near the critical point and to account for tangential energy transport. To capture this we introduce a non-dimensional parameter, Ψ , being the ratio of tangential energy flux to inertial energy flux. Including Ψ in heat transfer correlations improves their validity by capturing additional mechanisms that influence heat transfer. The suitability of correlations including Ψ are demonstrated using air and supercritical carbon dioxide data sets. Finally, analysing heat transfer trends across the range of Ψ tested provides insight on how heat transfer mechanisms vary with operating condition. This work provides a vital step forward for the unification of Taylor–Couette–Poiseuille flow heat transfer correlations across multiple experimental data sets. [ABSTRACT FROM AUTHOR]