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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]

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]

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]

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]

• 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]

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 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]

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]

In this paper, the thermal-fluid characteristics of a pin-fin heat sink with delta winglet vortex generators in a cross flow are investigated experimentally and numerically. The effects of the Reynolds number of the flow, the angle of attack of the vortex generators, the shortest distance between the vortex generators, the distance between each vortex generator and the heat sink, the height of the vortex generators, and the configuration of the vortex generators on the performance of the heat sink are explored. The results show that the thermal resistance decreases as the Reynolds number increases, but that the magnitude of the decrease tends to diminish with increasing Reynolds numbers. By considering the thermal resistance and the pressure difference simultaneously, it was determined that an angle of attack of 30° is a better arrangement for the vortex generators. The thermal resistance produced with the shortest distance between the vortex generators equal to the length of the heat sink is lower than that produced with the shortest distance between the vortex generators greater than the length of the heat sink. The heat transfer of the heat sink is higher when the vortex generators are installed against the middle of the heat sink on both sides of it. Although increasing the height of the vortex generators can enhance the heat transfer, the pressure difference also increases. The thermal resistance of the heat sink with the vortex generators arranged in the common-flow-up configuration is lower than that of the heat sink with the vortex generators arranged in the common-flow-down configuration. [ABSTRACT FROM AUTHOR]

In this paper, some novel structures aimed to optimize the airside performance of H-type finned tube heat exchangers, such as longitudinal vortex generators, dimples/protrusions and grooves, are examined numerically. The effects of geometric parameters on the heat transfer, flow resistance and comprehensive thermal–hydraulic performances are investigated by means of Taguchi method. The optimum configuration of compound designs is determined on the basis of the optimizations of these structures. According to the results, for longitudinal vortex generators, dimples/protrusions and grooves, the heat transfer characteristics are significantly affected by the winglet type, vertical distance and groove diameter while the attack angle, dimple depth and groove type have prominent impacts on the flow resistance characteristics. The winglet type, vertical distance and groove number make the most contributions to the comprehensive thermal–hydraulic performance, respectively. In compound designs, the longitudinal vortex generator plays the most important role in the performance of heat exchangers. The comprehensive thermal–hydraulic characteristic of the optimal compound design exerts the improvements of 0.9–23.8%, 24.5–57.1% and 8.3–37.5% than those of LVGs, dimples/protrusions and grooves, respectively, when Reynolds number ranges from 4650 to 28,300. The enhancement of 4.5–16.6% can be achieved compared with the case built based on references. • The optimal parameters of longitudinal vortex generators, dimples/protrusions and grooves on H-type fins are determined by Taguchi method. • The innovative compound structures are designed based on the optimizations of each novel structure. • Longitudinal vortex generators have the primary impact on comprehensive thermal–hydraulic performances among three structures. • The optimal compound design can achieve significant improvements compared to single structure. [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]

In this paper, experimental investigation on heat transfer and friction characteristics of dual-pipe heat exchanger for single-phase forced convective flow with alternation of clockwise and counterclockwise twisted tape (ACCT tape) and typical twisted tape (TT tape) inserted in the inner tube has been carried out. And effects of different lengths of twisted tapes on heat transfer enhancement were studied through a circular tube using water as testing fluid in turbulent conditions with a range of Reynolds number between 3000 and 9000. In the dual-pipe heat exchanger, hot water was cooled in the inner tube and cold water was as cooling fluid between the inner tube and the outer tube. The results showed that the ACCT tapes performed better on heat transfer enhancement than the typical twisted tapes and the best effect appeared with the full-length ( l = 2400 mm) ACCT tape insert in comparison with the twisted tapes of other lengths. The maximum values of PEC (performance evaluation criteria) with the full-length ACCT tape insert reached 1.42 in experimental flowing conditions. Empirical correlations for Nusselt number and friction factors were established and found to fit the experimental data with a deviation within 8%. [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]

As an effective technology to reduce flow resistance, drilling holes on the surface of vortex generators had been studied extensively. In this paper, thermal performance of punched rectangular winglet vortex generators (PRWVGs) in turbulent regime (Reynolds number was in the range of 9,090 ∼ 21,210) was studied experimentally and numerically. Air was applied as the working fluid. The PRWVGs were inserted into the tube with three attack angles (α = 30 ° , 45 ° , 60 °). The punched holes were set in three different positions (g = 3 m m , 5 m m , 7 m m) in the vertical direction and at three different hole heights (d = 1 m m , 3 m m , 6 m m). The performance of PRWVGs was quantified by the heat transfer coefficient ratio (h / h 0), the friction factor ratio (f / f 0), and a combined thermal enhancement factor (T E F) of the two. The results revealed that h / h 0 as well as f / f 0 values of PRWVGs compared with those of planar VGs (without holes) were reduced. The flow and temperature fields of different cases were obtained by numerical simulations, and the experimental results were verified by analyzing the fluid flow behaviors. The results showed that the jet flow (defined as a jet emanating from the punched hole of a VG), would enhance the heat transfer efficiency in the recirculation area. And the T E F value of VGs was improved by punching holes on the surface of VGs. However, the shape of the jet flow depended on the position of the hole. When the position of the hole was lower in the vertical direction, the jet flow was effective in increasing its T E F value. Additionally, the size of the hole had a large effect on the T E F value. Besides, entropy generation analysis showed that PRWVGs were thermodynamically advantageous. When d = 1 m m , w = 5 m m , the Bejan number reached the minimum. Under this condition, the proposed PRWVGs performed best in practice. When the height of the hole was d = 1 m m , T E F was the maximum, which was 1.25 at α = 45 ° and R e = 9,090. [ABSTRACT FROM AUTHOR]

· Behavior of local heat transfer coefficient in plane diverging channels is described. • Local Stanton number should be based on flow parameters at diverging channel inlet. • Empirical equations for local Stanton number are based on Re and Kays parameter. • Kinematic structure and TKE in plane diverging channels are estimated experimentally. • Correlation between local heat transfer and local Reynolds stresses is revealed. Heat transfer in diverging channels is essentially different from the one in channels of constant cross section. The mechanism of heat transfer enhancement in diverging channels compared to the channels of constant cross section is based on increased turbulence intensity. The present paper suggests new approach to the analysis and generalization of heat transfer data. Heat transfer on the wall of a diverging channel should be considered similarly to heat transfer from a plate instead of heat transfer in a channel. The data on heat transfer coefficient on the wall of a plane diverging channel at different expansion angles can be generalized by the Stanton number as a function of the Reynolds number and Kays acceleration parameter, St∼ f (Re, K). This function has characteristic sections along the wall that are typical of laminar, transitional and turbulent boundary layers. Stanton and Reynolds numbers based on the distance from the inlet and the velocity at the diverging channel inlet can be employed for data generalization in all the considered ranges of geometries and flow regimes. Kinematic structure of flow is examined to reveal hydrodynamic mechanisms behind heat transfer formation. Optical measurements provide the profiles of velocities and turbulence parameters in characteristic sections of the channel. Mechanism of turbulent structure formation in diverging channels is examined. Correlation between hydrodynamics and heat transfer is analyzed. Correlation between local coefficients of heat transfer and near-wall Reynolds stress is revealed. [ABSTRACT FROM AUTHOR]

Enhancement of heat removal and reduction of fin size become a major concern in designing heat exchanger equipment. The present paper reports an experimental analysis to investigate the turbulent heat transfer performance of rectangular fin arrays, both solid and circular perforations along the length of the fins. The size and number of circular perforation have been varied. Tests have been conducted in a horizontal wind tunnel equipped with forced draft fan. The data are obtained by varying flow velocities while maintaining constant heat input and taken over a period of time. The Reynolds number is varied between 6 × 10 4 through 25 × 10 4 . The heat loss due to radiation and convection is taken into account during calculation. Thermal performances and effectiveness of perforation and equivalent surface solid fin arrays have been evaluated and compared. Results show remarkable heat transfer enhancement, lower thermal resistances, pressure drop and higher efficiencies, effectiveness for perforated fins with increasing number of perforations in addition to the considerable reduction in weight in comparison with solid fin arrays. [ABSTRACT FROM AUTHOR]

Impingement cooling is an important turbine blades cooling technique, but it is often not as efficient as required due to the deflection of downstream jets caused by crossflow. In this paper, a novel corrugated target surface is proposed to convert downstream oblique impingement into orthogonal impingement for higher heat transfer. To prove this design, the flow and heat transfer characteristics of impingement cooling with conventional flat target surface (Baseline case), full-corrugated (FC) target surface, and semi-corrugated (SC) target surface are compared over the jet Reynolds number ranging from 15,000 to 45,000. Results show that the Nusselt number of full-corrugated target surface is almost the same as that of flat target surface at the region corresponding to the first five jets, but its peak value and uniformity are significantly improved at the region corresponding to the last four jets, especially under high jet Reynolds numbers. In addition, with the increase of corrugation depth (H), the total heat transfer capacity and area-averaged Nusselt number increase, but this comes at the cost of increased friction loss. Comprehensively evaluating heat transfer and friction loss, only the FC with H = 0.8 D case always has better thermal performance than Baseline case under all computed conditions, and the maximum improvement can reach 5.8%. Last but not least, the SC with H = 0.8 D case is proposed based on the FC with H = 0.8 D case to further reduce the friction loss, and results show that its thermal performance is improved significantly due to the decreased friction loss and almost unchanged heat transfer. [ABSTRACT FROM AUTHOR]

The heat transfer performances of pipes with twisted tape inserts having periodically spherical dimple-protrusion patterned walls are numerically investigated by Ansys Fluent under a constant heat flux of 5 W/cm2. The inserts having three different densities (N = 0, 30, and 45) and three different kinds of arrangement (composed of only dimples, only protrusions, and a combination of dimples and protrusions) with a constant twist ratio of 3 were tested for Reynolds numbers (Re) within the range from 3000 to 27000. Comparing the turbulence models proposed the validated realizable k-ϵ model coupled with enhanced wall function to employ on the numerical simulations. While the highest Nusselt number (Nu) and friction factor (f) values for the presented study were found for the dense protruded twisted tape (D-PT) arrangement at Re values lower than 18000 and for dense dimpled-protruded twisted tape (D-DPT) arrangement for the Re greater than 18000, dimpled-protruded twisted tape (DPT) and dimpled twisted tape (DT) sequences reached the lowest f values of 0.365 and 0.322 at Re = 3000, respectively. D-DPT and D-PT sequences reached the highest thermal performance factor (TPF) values of 1.508 and 1.478 at Re = 3000, respectively. All patterned twisted tapes performed better Nu than conventional twisted tapes in the analyzing range and performed better TPF than typically twisted tapes in the Re region of less than 6000. [Display omitted] • The effect of DT, PT, DPT, D-DT, D-PT, and D-DPT on heat transfer enhancement performance was numerically investigated. • All patterned twisted tapes performed better TFT than typically twisted tapes in the Re region of less than 6000. • D-DPT and D-PT sequences reached the highest TPF values of 1.508 and 1.478 at Re = 3000, respectively. [ABSTRACT FROM AUTHOR]

• Introduction of air swirling can either enhance or reduce the heat transfer rate depending on operating condition. • Heat transfer enhancement is achieved with the appropriate level of air swirling when the complete combustion occurs in the inner reaction zone. • Impinging inverse flame jet with an appropriate level of air swirling possesses the advantages of both premixed and diffusion flame jets. • Impinging inverse flame jet is a desirable option for impingement heating applications. This paper presents an experimental study to explore the feasibility of utilizing a swirling inverse diffusion flame (IDF) jet for impingement heating. The investigation concentrates on the impinging flame structure and impingement heat transfer. The effects of air swirling, the air jet Reynolds number, Re a , the global equivalence ratio, Φ, and the non-dimensional burner-to-plate distance, H/d a , on the impinging flame shape and impingement heat transfer are investigated experimentally. The Experiments show that introduction of air swirling can either enhance or reduce the heat transfer rate depending on operating condition. Under-swirling produces no evident heat transfer enhancement due to insufficient air/fuel mixing, while over-swirling reduces heat transfer rate because of the dilution of the entrained cold ambient air. Heat transfer enhancement is achieved with the appropriate level of air swirling when the complete combustion occurs in the inner reaction zone around a stoichiometric condition. The investigation shows that the impinging inverse flame jet with an appropriate level of air swirling possesses the advantages of both premixed and diffusion flame jets with blue appearance, ultra-low NO x emission, high flame temperature, good self-stabilization capability, and enhanced heat transfer rate. Therefore, it is a desirable option for impingement heating applications. [ABSTRACT FROM AUTHOR]

This paper numerically qualifies the effect of rib orientation on heat transfer and flow performances of mist/steam in square channels with inclined ribs. Four types of rib orientations are investigated. The study is conducted in contrast to steam with Reynolds number ranged from 10,000 to 60,000. Secondary flow patterns and droplet trajectories are analyzed to reveal the underlying reasons bringing about heat transfer enhancement due to mist/steam. The results indicate that rib orientation greatly determines secondary flow pattern in channels, and the distribution of secondary flow triggered by N type ribs is opposite to that by P type ribs. Such secondary flow forces droplets at different locations to assemble near different side walls. The places where water drops gather in a large quantity provide splendid heat transfer enhancement. And this effect gets more obvious at higher Reynolds number. For mist/steam, both heat transfer coefficient and overall heat transfer performance in the first passage are remarkably stronger than that in the second one. In general, NN channels and PP channels filled with mist/steam have the best heat transfer performance among the investigated cases. [ABSTRACT FROM AUTHOR]