Numerical investigation of fluid flow and heat transfer over louvered fins in compact heat exchanger

Numerical investigation of fluid flow and heat transfer characteristics over louvered fins and flat tube in compact heat exchangers is presented in this study. Three-dimensional simulations of single and double row tubes with louvered fins have been conducted. Simulations are performed for different geometries with varying louver pitch, louver angle, fin pitch and tube pitch and for different Reynolds number. Conjugate heat transfer and conduction through the fins are considered. The air-side performance of heat exchanger is evaluated by calculating Stanton number and friction factor. The results are compared with experiment and a good agreement is observed. The local Nusselt number variation along the top surface of the louver is calculated and effects of geometrical parameters on the average heat transfer coefficient is computed. Design curves are obtained which can used to predict the heat transfer and the pressure drop for a given louver geometry.     

Flow efficiency in multi-louvered fins

The paper studies the effect of Reynolds number, fin pitch, louver thickness, and louver angle on flow efficiency in multi-louvered fins. Results show that flow efficiency is strongly dependent on geometrical parameters, especially at low Reynolds numbers. Flow efficiency increases with Reynolds number and louver angle, while decreasing with fin pitch and thickness ratio. A characteristic flow efficiency length scale ratio is identified based on geometrical and first-order hydrodynamic effects, which together with numerical results is used to develop a general correlation for flow efficiency. Comparisons show that the correlation represents more than 95% of numerical predictions within a 10% error band, and 80% of predictions within a 5% error band over a wide range of geometrical and hydrodynamic conditions.     

Effects of winglets to augment tube wall heat transfer in louvered fin heat exchangers

The louvered fin heat exchanger, a type of compact heat exchanger, has been used heavily in the automotive and air conditioning industries for the last several decades. The majority of past research, aimed towards improving louvered fin exchanger efficiency, has focused on optimizing various parameters of the louvered fin. The experimental study presented in this paper concentrates instead on augmenting the heat transfer along the tube wall of the compact heat exchanger through the use of winglets placed on the louvers. The experiments were completed on a 20 times scaled model of an idealized louvered fin exchanger with a fin pitch to louver pitch ratio of 0.76 and a louver angle of 27°. The Reynolds numbers tested, based on louver pitch, were between 230 and 1016. A number of geometrical winglet parameters, including angle of attack, aspect ratio, direction, and shape, were all evaluated based on heat transfer augmentation, friction factor augmentation, and efficiency index (combination of both augmentations). In an attempt to optimize these winglet parameters, tube wall heat transfer augmentations as high as 39% were achieved with associated friction factor augmentations as high as 23%.     

Heat transfer and pressure drop correlations for compact heat exchangers with multi-region louver fins

An experimental study on the air-side heat transfer and pressure drop characteristics for brazed aluminum heat exchangers with multi-region louver fins and flat tubes was conducted. A series of tests were conducted for heat exchangers with different numbers of louver regions at the air-side Reynolds numbers of 400–1600 based on the louver pitch. The air-side thermal performance data were analyzed by using the effectiveness-NTU method. The characteristics of the heat transfer and pressure drop for heat exchangers with different geometry parameters were presented in terms of the Colburn j factor and Fanning friction f factor as function of the Reynolds number.     

Fin-tube junction effects on flow and heat transfer in flat tube multilouvered heat exchangers

Three-dimensional simulations of four louver-tube junction geometries are performed to investigate the effect on louver and tube friction and heat transfer characteristics. Three Reynolds numbers, 300, 600 and 1100, based on bulk velocity and louver pitch are calculated. Strong three-dimensionality exists in the flow structure in the region where the angled louver transitions to a flat landing adjoining the tube surface, whereas the flow on the angled louver far from the tube surface is nominally two-dimensional. Due to the small spatial extent of the transition region, its overall impact on louver heat transfer is limited, but the strong unsteady flow acceleration on the top louver surface augments the heat transfer coefficient on the tube surface by over 100%. In spite of the augmentation, the presence of the tube lowers the overall Nusselt number of the heat exchanger between 25% and 30%. Comparisons with correlations derived from experiments on full heat exchanger cores show that computational modeling of a small subsystem can be used reliably to extract performance data for the full heat exchanger.     

Numerical Simulation and Optimization of Enhanced Heat Transfer in Helical Oval Tubes: Effect of Helical Oval Tube Modification,Pitch Ratio,and Depth Ratio

ABSTRACT

Flow and heat transfer behaviors in the helical oval tube, alternate-twisted-direction helical oval tube and regularly spaced helical oval tubes were numerically investigated. The helical oval tubes with eight oval tube depth ratios (0.03, 0.04, 0.05, 0.06, 0.07, 0.10, 0.15, and 0.20) and nine oval tube pitch ratios (0.6, 0.8, 1.0, 1.5, 2.0, 2.5, 3.0, 3.5, and 4.0) were examined in turbulent regime, Reynolds number ranged from 5000 to 20,000. The computational results showed that fully developed periodic flow and heat transfer in helical oval tubes commenced at around entrance length to characteristic diameter of 8–9. The decreasing depth ratio and increasing pitch ratio helped to reduce the pressure loss of the tube heat exchanger. The maximum thermal performance of 1.30 was obtained by the use of the helical oval tube with depth ratio of 0.05 and pitch ratio of 0.6 at the lowest Reynolds number of 5000. At similar conditions, typical helical oval tubes offered better heat transfer rate and thermal performance than helical oval tubes with alternate axes and regularly spaced helical oval tubes.     

百叶窗结构参数对管带式散热器流动传热特性影响的研究

本文利用CFD计算软件star ccm＋,建立了管带式散热器空气侧的三维热固耦合模型,分析了不同空气流速和百叶窗结构参数对散热器流动和传热特性的影响,发现阻力损失主要集中在空气进入百叶窗的入口区域,百叶窗的前端传热效果较好,同时计算区域后半部分存在滞止区域;随着百叶窗倾角的增大,传热j因子先增后减,摩擦f因子一直增加,24°时传热j因子达到最大值;随着百叶窗厚度的增大,传热效果变差,摩擦f因子先增后减;百叶窗间距增加到一定程度,传热j因子和摩擦f因子增加缓慢,甚至减小。     

Visualization study of steam condensation in triangular microchannels

A visualization experiment is conducted to investigate the condensation of steam in a series of triangular silicon microchannels. The results indicate that droplet, annular, injection and slug-bubbly flow are the dominant flow patterns in these triangular silicon microchannels. With increased mass flow rate, or an increase in the hydraulic diameter under the same Reynolds number, the location at which the injection occurred is observed to move towards the channel outlet. The frequency of the injection increases, i.e. the flow of condensation instability is higher with increased inlet vapor Reynolds number, condensate Weber number and the prolongation of the injection location, or with a decrease in the hydraulic diameter of the channel. In addition, the wall temperature of the channel decreases along the condensation stream. The total pressure drop, the average condensation heat transfer coefficient and the average Nusselt number are observed to be larger with increased inlet vapor Reynolds number. Moreover, it is found that the condensation heat transfer is enhanced by a reduction in the channel scale.     

Experimental study on the unsteady laminar heat transfer downstream of a backwards facing step

The objective of this work is to study experimentally the unsteady heat transfer downstream of a backward-facing step in the 2-D laminar regime when the inlet flow is pulsated. To this aim, an experimental set-up has been prepared with water as the working fluid. The Reynolds number based on the hydraulic diameter of the inlet channel and average inlet velocity is 300. Inlet flow temperature is 30 °C and a region downstream of the step is heated up to 74 °C. Pulsation is achieved using a piston pump and heat transfer is studied up to a maximum pulsation Strouhal number of 1.2. The results obtained confirm previous numerical simulation work in the sense that pulsation could be used to partially recover the heat transfer efficiency that is lost in steady flow conditions downstream of a backward-facing step. It has also been confirmed that the behaviour of the averaged Nusselt number versus pulsation Strouhal number is of the resonant type. That is: the Nusselt number increases from the steady situation up to a certain value of the Strouhal number (0.41 in our case) and, then, it degrades as the frequency of the pulsation is further increased.     

Three‐dimensional turbulent forced convection around a hot cubic block exposed to a cross‐flow and an impinging jet

A numerical study of a three‐dimensional, turbulent, forced convection flow around a hot cubic block exposed to cross‐flow and an impinging jet is carried out. The standard k‐ε turbulence model is used to study the effects of Reynolds number ratio on the flow and heat transfer. For each value of the Reynolds number of the jet, the Reynolds number ratio is equal to 1, 1.5, and 2. The influence of the channel height and the jet axis location are also examined. The governing equations are solved by using Ansys Fluent software 14.5. Results show that the heat transfer increases with the increase in the Reynolds number ratio. At the top of the cube, better cooling occurs with an increase in the speed of the impinging jet. A reduction in the height of the channel and the displacement of the axis of the jet toward the channel inlet improve the heat transfer. Our simulations are compared with experimental data found in the literature, using different turbulence models.     

节流型微通道换热特性研究

为研究节流型微通道换热特性,设计并加工制作了突缩突扩结构的微通道实验件。采用控制变量法控制改变加热电压、质量流量、入口温度,通过实验数据对比分析研究了影响节流型微通道对流换热的规律。研究结果表明:随着质量流量的增加,微通道蒸发器的对流传热系数不断减小;随着雷诺数的增大努谢尔数不断增大,对流换热效果比较明显。     

Numerical Analysis of Flow and Thermal Performance of Liquid-Cooling Microchannel Heat Sinks with Bifurcation

Single-phase liquid-cooling microchannels have received great attention to remove the gradually increased heat loads of heat sinks. Proper changes of the flow path and/or heat transfer surface can result in much better thermal performance of microchannel heat sinks. In this study, a kind of rectangular straight microchannel heat sink with bifurcation flow arrangement has been designed, and the corresponding laminar flow and heat transfer have been investigated numerically. Four different configurations are considered. The effects of the bifurcation ratio (the initial channel number over the bifurcating channel number) and length ratio (the channel length before bifurcation over the bifurcation channel length) on laminar heat transfer, pressure drop, and thermal resistance are considered and compared with those of the traditional straight microchannel heat sink without bifurcation flow. The overall thermal resistances subjected to inlet Reynolds number and pumping power are compared for the five microchannel heat sinks. Results show that the thermal performance of the microchannel heat sink with bifurcation flow is better than that of the corresponding straight microchannel heat sink. The heat sinks with larger bifurcation ratio and length ratio provide much better thermal performance. It is suggested to employ bifurcation flow path in the liquid-cooling microchannel heat sinks to improve the overall thermal performance by proper design of the bifurcation position and number of channels.     

百叶窗翅片的传热和阻力性能试验研究

风洞试验台上对8种不同结构参数的百叶窗翅片进行传热和流动阻力的性能试验。分析比较了翅片长度、翅片间距、翅片高度对其传热和阻力性能的影响，其中翅片长度和翅片间距对无量纲传热j因子和摩擦阻力f因子影响较大，翅片高度影响较小。同时采用3√j/f因子综合评价了8种翅片的强化传热效果。结果表明，翅片长度对强化传热影响最为显著。     

Analysis of three-dimensional heat transfer in micro-channel heat sinks

In this study, the three-dimensional fluid flow and heat transfer in a rectangular micro-channel heat sink are analyzed numerically using water as the cooling fluid. The heat sink consists of a 1-cm² silicon wafer. The micro-channels have a width of 57 μm and a depth of 180 μm, and are separated by a 43 μm wall. A numerical code based on the finite difference method and the SIMPLE algorithm is developed to solve the governing equations. The code is carefully validated by comparing the predictions with analytical solutions and available experimental data. For the micro-channel heat sink investigated, it is found that the temperature rise along the flow direction in the solid and fluid regions can be approximated as linear. The highest temperature is encountered at the heated base surface of the heat sink immediately above the channel outlet. The heat flux and Nusselt number have much higher values near the channel inlet and vary around the channel periphery, approaching zero in the corners. Flow Reynolds number affects the length of the flow developing region. For a relatively high Reynolds number of 1400, fully developed flow may not be achieved inside the heat sink. Increasing the thermal conductivity of the solid substrate reduces the temperature at the heated base surface of the heat sink, especially near the channel outlet. Although the classical fin analysis method provides a simplified means to modeling heat transfer in micro-channel heat sinks, some key assumptions introduced in the fin method deviate significantly from the real situation, which may compromise the accuracy of this method.     

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.     

Influence of combined non-uniform wire coil and twisted tape inserts on thermal performance characteristics

In this paper, heat transfer, friction factor and thermal performance behaviors in a tube equipped with the combined devices between the twisted tape (TT) and constant/periodically varying wire coil pitch ratio are experimentally investigated. The periodically varying three coil pitch ratios were arranged into two different forms: (1) D-coil (decreasing coil pitch ratio arrangement) and (2) DI-coil (decreasing/increasing coil pitch ratio arrangement) while the twisted tapes were prepared with two different twist ratios. Each device alone is also tested and the results are subjected for comparison with those from the combined devices. The experiments were conducted in a turbulent flow regime with Reynolds numbers ranging from 4600 to 20,000 using air as the test fluid. Compared to each enhancement device, the heat transfer rate is further augmented by the compound devices. Over the range investigated, the highest thermal performance factor of around 1.25 is found by using DI-coil in common with the TT at lower Reynolds number. In addition, the empirical correlations of the heat transfer (Nu) and pressure drop (f) are also presented.     

带错排V肋和反V肋矩形通道强化传热机理及规律研究

采用热色液晶瞬态测量技术研究了带45°V肋和45°反V肋的矩形通道端壁的传热特性,分析V肋诱导产生二次流强化传热机理及其传热系数分布规律。通道进口雷诺数变化范围是10 460~32 100,肋高与当量直径的比为0.13,肋间距与肋高的比为10。实验结果表明:带V肋和反V肋矩形通道传热系数随着雷诺数的增大而增大;正V肋诱导产生沿V肋从中间向两侧发展的二次流,反V肋片诱导气流沿肋方向产生从两边流向中间的二次流;斜置V肋诱导产生的二次流增强了通道的传热能力;带V肋通道的传热强于带反V肋通道。     

水平方形管进口区域单面冷却凝结换热

对方形管进口区蒸汽单侧冷却凝结进行可视化观测及参数测量，发现随蒸汽雷诺数(Re为1669～5553)的提高，凝结液成膜方式、发展演化和稳定性均与低雷诺数下由液滴、液桥合并形成的稳定液膜有较大差异。液膜不同流动形态，如周期性断裂、局部失稳、小溪流，对换热的影响十分显，进口区域存在的高换热特性正是由于液膜流动方式的不稳定性所致。高蒸汽雷诺数(Re为5553)时，蒸汽流动的脉动性、界面切应力及Marangoni效应是导致液膜断裂的主要原因。     

Pulsating flow and convective heat transfer in a cavity with inlet and outlet sections

This paper deals with the study of 2-D, laminar, pulsating flow inside a heated rectangular cavity with different aspect ratios. The cooling liquid (water with temperature dependent viscosity and thermal conductivity) comes and leaves the cavity via inlet and outlet ports. The flow topology is characterised by the large recirculation regions that exist at inner corners of the cavity. These low velocity regions cause the heat transfer to be small when compared, for instance, to that of a straight channel. We study the effect that a prescribed pulsation at the inlet port has on the cavity heat transfer. This pulsating boundary condition, of the unsteady Poiseuille type, is described by its frequency and the amplitude of the pressure gradient. The time averaged Reynolds number of the flow, based on the hydraulic diameter of the inlet channel, is 100 and we consider that the dimensionless pulsation frequency (Strouhal number) varies in the range from 0.0 to 0.4. We show that the prescribed pulsation enhances heat transfer in the cavity and that the mechanism that causes this enhancement appears to be the periodic change in the recirculation flow pattern generated by the pulsation. Regarding the quantitative extent of heat transfer recovery, we find that appropriate selection of the pulsation parameters allows for the cavity to behave like a straight channel that is the configuration with the highest Nusselt number.     

Numerical investigations of developing flow and heat transfer in raccoon type microchannels under inlet pulsation

The present study investigates numerically the simultaneously developing unsteady laminar fluid flow and heat transfer inside a two dimensional wavy microchannel caused by a sinusoidal varying velocity component at an inlet. The flow was both thermally and hydro dynamically developing while the channel walls were kept at a uniform temperature. The simulation was performed in the laminar regime for Prandtl number 7(water) and Reynolds number ranging from 0.1 to 100. A Wavy microchannel having non-dimensional hydraulic diameter 1 with varying pulsating amplitude and frequency represented by the Strouhal number was designed for the given Reynolds number range. Based on the comparison with steady flow in a wavy channel it was found that imposed sinusoidal velocity at the inlet can provide improved heat transfer performance at different amplitudes (0.2, 0.5, 0.8) and frequencies (1, 5, 10) while keeping the pressure drop within acceptable limits.