Experimental Investigation of Heat Transfer and Resistance Characteristics of a Finned Oval-Tube Heat Exchanger With Different Air Inlet Angles

The air inlet flow direction is not orthogonal to the heat exchanger surface in many cases. To study the performance of the heat transfer and pressure drop of a heat exchanger with different air inlet angles, this paper shows the experimental system about a finned oval-tube heat exchanger inclined toward the air incoming flow direction. The heat transfer and pressure drop characteristics of four air inlet angles (90°, 60°, 45°, and 30°) are studied separately for the Reynolds number ranging from 1300 to 13000 in this study. The experimental correlations of Nusselt number and resistance coefficient of the air side are acquired. The results show that the overall heat transfer coefficients become smaller and smaller with the decrease of the air inlet angles, while the pressure drops have significant changes. The heat transfer performances of the heat exchanger under the three inclined air inlet angles are worse than that at 90°. Among the three inclined angles, the performance at 45° is the best under identical mass flow rate criterion and at low Reynolds number under identical pressure drop criterion; that at 60° is the best at large Reynolds under identical pressure drop criterion. Finally, some conclusions are attained about the effects of the air inlet angles on the heat transfer and pressure drop performance of the finned oval-tube heat exchanger.     

Characterization of sodium flow over hexagonal fuel subassemblies

Steady flow of liquid sodium over a bundle of heat generating hexagonal subassemblies has been investigated. The cross flow pressure drop and heat transfer are characterized using the general purpose CFD code STAR-CD. Analysis has been carried out for both laminar and turbulent regimes of interest to liquid metal fast reactors. Turbulence has been modeled using low Reynolds number (Re) k-ε model. The estimated pressure drop and heat transfer coefficients are compared against that of a straight parallel plate channel. It is seen that in the low Reynolds number range, the pressure drop for the hexagonal path is nearly equal to that of the parallel plate channel for the same length. However, in the high Reynolds number range, the pressure drop of the hexagonal path is much higher than that in the parallel plate channel, the ratio being 2 at Re = 2000 while it is 3.6 at Re = 20,000. Two competing factors, viz., (i) jet impingement/flow development effect and (ii) flow separation effect are found to influence the average Nusselt number (Nu). In the laminar regime, the latter effect dominates leading to a decrease of the Nusselt number with an increase in the Reynolds number. However, in the turbulent regime, the former effect dominates leading to an increase in the Nusselt number with Reynolds number. The Nusselt number in the hexagonal path is about twice that of the parallel plate channel due to under development of velocity/temperature profiles and the recirculation associated with the hexagonal path due to the changes in flow direction. Detailed correlations for both the pressure drop and the average Nusselt number have been proposed.     

EXPERIMENTAL STUDIES ON FLOW IN CONVERGENT AND DIVERGENT DUCTS OF RECTANGULAR CROSS SECTION

This paper describes the experimental examination of the pressure drop and heat transfer of the flow in convergent and divergent ducts of rectangular cross section. The aspect ratio based on the dimensions of the large end of the duct was 0⋅1. It has been found that at a given convergent or divergent angle pressure drop decreases while heat transfer increases with increasing Reynolds number. Along a given duct of small convergent angle, pressure drop increases while heat transfer decreases along the duct. However, heat transfer may increase near the downstream end of ducts of high convergent angle. At a given Reynolds number, both pressure drop and heat transfer increase with increasing convergent angle. As for flow in divergent ducts, the effects of Reynolds number on pressure drop and heat transfer are somewhat similar to those of flow in a convergent duct. © 1997 by John Wiley & Sons, Ltd.     

Laminar Flow and Heat Transfer Phenomena Across a Confined Semicircular Bluff Body at Low Reynolds Numbers

The present study is concerned with the simulation of incompressible Newtonian fluid flow and heat transfer over a long semicircular bluff body in a channel at low Reynolds numbers. In particular, wall effects on the forced convection from a (heated) semicircular cylinder confined in a horizontal channel are investigated for Reynolds number = 1–40 and blockage ratio = 16.67–50% for air as the working fluid. Flow and thermal fields are found steady for the preceding range of settings. The onset of flow separation increases as the wall confinement increases. The size of the recirculation zone downstream of a semicircular cylinder is seen to increase almost linearly with Reynolds number for a fixed blockage ratio, but it decreases with increasing blockage ratio for a fixed Reynolds number. As expected, total drag coefficient and its components decrease with increasing value of Reynolds number. However, with increasing blockage ratio, the values of these drag coefficients increase. On the basis of equal projected area, the total drag coefficient for the present flow system is found to be greater than the corresponding drag in the case of the unconfined semicircular cylinder. Similarly, the overall drag in the case of a confined semicircular cylinder is found to be greater than that of a confined circular cylinder for the appropriate range of dimensionless control parameters. The maximum augmentation in heat transfer for blockage ratios of 25% and 50% is found to be approximately 16% and 51% with respect to the corresponding value at the blockage ratio of 16.67% at Reynolds number = 1. Finally, the correlations of wake length, drag coefficient, and average Nusselt number are obtained.     

Dynamic and thermal study of air flow control by chicanes with inclined upper parts in solar air collectors

A dynamic and thermal simulation for two-dimensional model is developed on air flow and heat transfer control by chicanes in solar air collectors. New chicane form is adopted with two parts: the first is orthogonal to the air flow direction and the second is titled (α=60°). It is apparent that the turbulence created by introducing chicanes, resulting in greater increase in heat transfer inside the dynamic air vein with a rise of 23%. The effect of the variation of the Reynolds number in the range of 100<Re<4500, on the convective heat transfer coefficient, the pressure drop and Nusselt number are analysed and have shown good agreements with the literature results. Therefore, the mass flow rates effect on the velocity magnitude, temperature and the turbulent intensity is analysed. The Reynolds number variation showed a substantial effect on the mechanism of vortex development and separation phenomenon.     

Comparative analysis of heat transfer and pressure drop in helically segmented finned tube heat exchangers

Four different semi-empirical models of heat transfer and pressure drop for helically segmented finned tubes in staggered layout were analyzed. The performance of a Helically Segmented Finned Tubes Heat Exchanger on an industrial scale was obtained and the predictions were compared with experimental data. The method used for thermal analysis is the Logarithmic Mean Temperature Difference (LMTD). Comparisons between predictions and experimental data show a precision greater than 95% in heat transfer for a combination between the Kawaguchi and Gnielinski models at a flue gas Reynolds number, based on the outside bare tube, of about 10,000. In the case of pressure drop, there is a precision of approximately 90% for the Weierman model at a Reynolds number, based on the outside bare tube, of about 10,000. And so, the results show that the best flow regime in which heat transfer and pressure drop are optimum, is for a Reynolds number (based on the outside bare tube) of about 10,000.     

Experimental Study of Passive Heat Transfer Enhancement in a Drag-Reducing Flow

Drag and heat transfer reduction were examined in a fully developed two-dimensional drag-reducing flow with different concentrations of surfactant additives. A row of low-profile mixers was used to enhance heat transfer in the flow. The effect of the low-profile mixers on heat transfer enhancement was examined in terms of the local Nusselt number for the surfactant concentration of 90 ppm. A comparison of the cases with and without low-profile mixers show that the average Nusselt number is enhanced by about 170% for Reynolds numbers of 7000, 12000, and 16200 with only a small pressure drop penalty. This increase in heat transfer performance might be related to the longitudinal vortices generated by the low-profile mixers, resulting in the enhancement of the bulk exchange of fluid between the main flow and the near-wall region. It was found that the enhancement of the low-profile mixers to the heat transfer rate starts to deteriorate in the streamwise direction at a certain location downstream of the mixers. The location of the onset of the deterioration shifts downstream with an increase in the Reynolds number.     

Multiobjective optimization study of jet impingement heat transfer through a porous passage configuration

The present study reports an optimized configuration of multijets impinging through porous passages, providing a viable solution for applications requiring localized heat transfer. The cascaded collision lattice Boltzmann numerical method is initially validated with the in-house experimental results of single jet impinging through a porous passage configuration. A multiobjective optimization study using Kriging-GA algorithm is conducted on a single jet impinging through a porous passage at a Reynolds number of 400, considering Darcy number, porosity, and porous passage height as variables and Nusselt number, nondimensional pressure drop as the conflicting objectives. The optimal parameters from the generated pareto plot are chosen attributing equal weightage to Nusselt number and nondimensional pressure drop. Finally, an optimal pitch for multijets impinging through optimized porous passages is determined to maximize heat transfer performance.     

Numerical investigations of flow and heat transfer enhancement in a corrugated channel using nanofluid

In this paper, heat transfer and pressure drop characteristics of copper–water nanofluid flow through isothermally heated corrugated channel are numerically studied. A numerical simulation is carried out by solving the governing continuity, momentum and energy equations for laminar flow in curvilinear coordinates using the Finite Difference (FD) approach. The investigation covers Reynolds number and nanoparticle volume fraction in the ranges of 100–1000 and 0–0.05 respectively. The effects of using the nanofluid on the heat transfer and pressure drop inside the channel are investigated. It is found that the heat transfer enhancement increases with increase in the volume fraction of the nanoparticle and Reynolds number, while there is slight increase in pressure drop. Comparisons of the present results with those available in literature are presented and discussed.     

Numerical investigation for improved heat transfer characteristics in micro‐fin tubes

Generally, internal micro‐fin tubes are used for increasing the life and performance of electronic devices. The micro‐fins enhance the heat transfer rate by increasing the surface area with an increase of the pressure drop. In this study, heat transfer and pressure drop are analyzed by varying Reynolds number with the increase in the number of fins in tubes. Heat transfer and pressure drop, together with turbulence kinetic energy of micro‐fin tubes (helical and straight) and a smooth tube, have been evaluated for different Reynolds numbers (60 000, 40 000, 20 000, and 2000) at a constant temperature of 350 K, which clearly establishes laminar to turbulent flow. It is observed that the helical micro‐fin tube has a better result compared with the straight micro‐fin tube and smooth tube at Reynolds numbers 60 000, 40 000, and 20 000 at velocity 2, 1, and 0.5 m/s, respectively. This study is an attempt to establish a comparison of different micro‐fin geometries with varying Reynolds numbers, concluding that a high Reynolds number is suitable for the same.     

Measurement of performance of plate-fin heat sinks with cross flow cooling

This work assesses the performance of plate-fin heat sinks in a cross flow. The effects of the Reynolds number of the cooling air, the fin height and the fin width on the thermal resistance and the pressure drop of heat sinks are considered. Experimental results indicate that increasing the Reynolds number can reduce the thermal resistance of the heat sink. However, the reduction of the thermal resistance tends to become smaller as the Reynolds number increases. Additionally, enhancement of heat transfer by the heat sink is limited when the Reynolds number reaches a particular value. Therefore, a preferred Reynolds number can be chosen to reduce the pumping power. For a given fin width, the thermal performance of the heat sink with the highest fins exceeds that of the others, because the former has the largest heat transfer area. For a given fin height, the optimal fin width in terms of thermal performance increases with Reynolds number. As the fins become wider, the flow passages in the heat sink become constricted. As the fins become narrower, the heat transfer area of the heat sink declines. Both conditions reduce the heat transfer of the heat sink. Furthermore, different fin widths are required at different Reynolds numbers to minimize the thermal resistance.     

Heat transfer and pressure drop characteristics of flow in convergent and divergent ducts

The heat transfer and pressure drop characteristics of the flow in convergent and divergent ducts of rectangular crosssection are obtained through the simulation of the flow by a three-dimensional parabolic model. The results show that in both convergent and divergent flows heat transfer decreases and pressure drop increases sharply near the entrance region of the ducts. Generally, the Nusselt number increases with increasing convergent/divergent angle, aspect ratio, or Reynolds number, and the pressure drop increases with increasing convergent/divergent angle or decreasing aspect ratio or Reynolds number in both flows. However, an increasing convergent/divergent angle may also result in a lower pressure drop owing to the recovery of static pressure from dynamic pressure. Furthermore, the pressure drop in a divergent flow is generally lower than that in a convergent flow except in the entrance region. For divergent flows with high divergent angle or high Reynolds number, flow separation may occur.     

Condensation heat transfer and pressure drop of refrigerant R-410A flow in a vertical plate heat exchanger

Heat transfer and associated frictional pressure drop in the condensing flow of the ozone friendly refrigerant R-410A in a vertical plate heat exchanger (PHE) are investigated experimentally in the present study. In the experiment two vertical counter flow channels are formed in the exchanger by three plates of commercial geometry with a corrugated sinusoidal shape of a chevron angle of 60°. Downflow of the condensing refrigerant R-410A in one channel releases heat to the upflow of cold water in the other channel. The effects of the refrigerant mass flux, imposed heat flux, system pressure (saturated temperature) and mean vapor quality of R-410A on the measured data are explored in detail. The results indicate that the R-410A condensation heat transfer coefficient and associated frictional pressure drop in the PHE increase almost linearly with the mean vapor quality, but the system pressure only exhibits rather slight effects. Furthermore, increases in the refrigerant mass flux and imposed heat flux result in better condensation heat transfer accompanying with a larger frictional pressure drop. Besides, the imposed heat flux exhibits stronger effects on the heat transfer coefficient and pressure drop than the refrigerant mass flux especially at low refrigerant vapor quality. The friction factor is found to be strongly influenced by the refrigerant mass flux and vapor quality, but is almost independent of the imposed heat flux and saturated pressure. Finally, an empirical correlation for the R-410A condensation heat transfer coefficient in the PHE is proposed. In addition, results for the friction factor are correlated against the Boiling number and equivalent Reynolds number of the two-phase condensing flow.     

Direct numerical simulation of fluid flow and heat transfer in periodic wavy channels with rectangular cross-sections

Fully-developed flow and heat transfer in periodic wavy channels with rectangular cross sections are studied using direct numerical simulation, for increasing Reynolds numbers spanning from the steady laminar to transitional flow regimes. The results show that steady flow is characterized by the formation of symmetric secondary flow or Dean vortices when liquid flows past the bends. It is found that the patterns of Dean vortices may evolve along the flow direction, thus leading to chaotic advection, which can greatly enhance the convective fluid mixing and heat transfer. With increasing Reynolds numbers, the flow undergoes transition from a steady state to a periodic one with a single frequency, and subsequently to a quasiperiodic flow with two incommensurate fundamental frequencies. Within these unsteady regimes, the flow is characterized by very complex Dean vortices patterns which evolve temporally and spatially along the flow direction, and the flow symmetry may even be lost. Further increase in Reynolds number leads to chaotic flow, where the Fourier spectrum of the velocity evolution becomes broadband. The bifurcation scenario in wavy channels may thus share some common features with the well-known Ruelle–Takens–Newhouse scenario. Heat transfer simulation in all flow regimes is carried out with constant wall temperature condition and liquid water as the coolant. It is found that due to the efficient mixing in wavy channels, the heat transfer performance is always significantly more superior to that of straight channels with the same cross sections; at the same time the pressure drop penalty of wavy channels can be much smaller than the heat transfer enhancement. The present study shows that these wavy channels may have advantages over straight channels and thus serve as promising candidates for incorporation into efficient heat transfer devices.     

Numerical Analyses of Single‐Phase Pressure Drop and Forced Convective Heat Transfer Coefficient of Water–Ethanol Mixture: An Application in Cooling of HEV Battery Module

The present numerical analyses are related to the cooling of a hybrid electric vehicle (HEV) battery module by water–ethanol mixture. The fluid is passed through a cold plate consisting of two rectangular channels of 0.01 m depth, 0.015 m width, and 0.15 m length. The battery module is represented by a heater placed below the cold plate. The single‐phase pressure drop and single‐phase heat transfer coefficient for water, water–ethanol mixture of mass fraction of 25%, 50%, and 75%, and ethanol are determined numerically for different heat fluxes of 10, 15, 20, and 25 kW/m² and different Reynolds numbers 500, 1000, 1500, 2000, and 2500. To solve the Navier–Stokes equation, the pressure correction method was used and to solve the energy equation, the Lax–Wendroff explicit method is used. Numerical results obtained for water are compared with the literature correlations. The friction factor for water deviated by an average of 8.02% from the Lewis and Robertson equation. The Nusselt number for water deviated by 7.35% from the Churchill and Ozoe equation at lower Reynolds number 500 and at higher Reynolds number 2500, Nusselt number deviated by 13.68% from the Stephan equation. The results showed that the heat transfer coefficient increased with an increase in Reynolds number and heat flux. The effect of the increase in Reynolds number is more significant than the increase in heat flux. At higher ethanol mass fraction and higher Reynolds number the heat transfer coefficient increased with heat flux when compared to water. There is no significant decrease in heat transfer coefficient with an increase in ethanol mass fraction. The pressure drop increased and the heat transfer coefficient decreased with an increase in ethanol mass fraction.     

An experimental study of convective heat transfer in silicon microchannels with different surface conditions

An experimental investigation has been performed on the laminar convective heat transfer and pressure drop of water in 13 different trapezoidal silicon microchannels. It is found that the values of Nusselt number and apparent friction constant depend greatly on different geometric parameters. The laminar Nusselt number and apparent friction constant increase with the increase of surface roughness and surface hydrophilic property. These increases become more obvious at larger Reynolds numbers. The experimental results also show that the Nusselt number increases almost linearly with the Reynolds number at low Reynolds numbers (Re<100), but increases slowly at a Reynolds number greater than 100. Based on 168 experimental data points, dimensionless correlations for the Nusselt number and the apparent friction constant are obtained for the flow of water in trapezoidal microchannels having different geometric parameters, surface roughnesses and surface hydrophilic properties. Finally, an evaluation of heat flux per pumping power and per temperature difference is given for the microchannels used in this experiment.     

流体流过带折流板通道时的流动及传热数值研究

对常物性流体在通道内的周期性充分发展层流流动和换热特性进行了二维数值计算分析。所研究的通道是由两平行平板布置于中心线位置的一系列折流板构成。平行平板保持温度恒定，折流板则分成完全导热和绝热两种情况，对不同几何参数，Re数和Pr数下的流动和换热性能进行了数值研究。文章还给出了系统流函数图和局部换热系数分布情况。     

Heat Transfer Enhancement in a Pipe Using a Flow Inverter

Flow inversion, transferring a liquid from the wall region toward the center of the pipe or from the axis toward the heat transfer surface, improves heat transfer in the laminar flow regime. While a fully developed velocity profile is quickly established, a thin thermal boundary layer is preserved for a considerable distance in the pipe behind the flow inverter for highly viscous liquids. Thus the pressure drop is increased only locally (by the inverter itself), while heat transfer enhancement is also seen in a long straight section of the pipe. Two original flow inverter designs were tested in a long pipe (3 m in length) heated by condensing steam, using starch molasses as a working medium. Experiments carried out in the range of Reynolds number 4–60 and Graetz number 150–700 resulted in an increase of 20–35% in the heat transfer coefficient, accompanied by a 30–40% increase in the pressure drop. The experimental results confirm the numerical model prediction (within an extended Reynolds number range starting from 0.1). Thus an almost 40% increase in heat transfer can be expected at the optimal Graetz number in the range 50–100, using only one flow inverter located in the middle of a sufficiently long pipe.     

Boiling characteristics of ternary mixtures inside enhanced surface tubing

Two phase flow characteristics such as coefficient of heat transfer and pressure drop observed during boiling of ternary azeotropic refrigerant mixtures R-404A (R-125/RR--134a/R-143a:44/4/52), R-407B (R-32/R-125/R-134a: 10/70/20), R-407C (R-32/R-125/R-134a:23/25/52) and R-408A (R-22/R-125/R-143a:46/7/47) are presented and analyzed in this paper.Experiments showed that for Reynolds numbers higher than 4.5 E04, R-408A and R-404A appear to have greater heat transfer rates than the other blends under investigation. Furthermore, it is quite evident from this data that at higher Reynolds number R-404A and R-408A have the highest pressure drop while R-407 experiences the lowest pressure drop among the refrigerants under investigation.     

Experimental Study on the Air-Side Performance of a Multipass Parallel and Counter Cross-Flow L-Footed Spiral Fin-and-Tube Heat Exchanger

This study conducted experiments on the air-side performance of novel L-footed spiral fin-and-tube heat exchangers that were faced with airflow at high Reynolds numbers (3500–13,000). The examined heat exchangers have a multipass parallel-and-counter cross-flow type of water flow arrangement. This flow arrangement is a combination of the parallel cross-flow and the counter cross-flow. This type of water flow arrangement may be the best fit for the reverse-flow system, because it can provide constant heat-exchange effectiveness for every flow reversal direction at the same airflow rate. Ambient air was used as a working fluid on the air side and hot water for the tube side. This way the effect of the number of tube rows on the heat transfer and friction characteristics of L-footed spiral fin-and-tube heat exchangers was clearly observed. The effect of the fin's outside diameter on the pressure drop was also studied. The results show that the number of tube rows has no significant effect on the air-side heat transfer or on friction characteristics at high Reynolds numbers. However, the fin's outside diameter shows a significant effect on the pressure drop. The pressure drop increases as the fin's outside diameter increases for the same number of tube rows.