Numerical investigation and analysis of heat transfer enhancement in channel by longitudinal vortex based on field synergy principle

3-D numerical simulations were presented for laminar flow and heat transfer characteristics in a rectangular channel with vortex generators. The effects of Reynolds number (from 800 to 3 000), the attack angle of vortex generator (from 15° to 90°) and the shape of vortex generator were examined. The numerical results were analyzed based on the field synergy principle. It is found that the inherent mechanism of the heat transfer enhancement by longitudinal vortex can be explained by the field synergy principle, that is, the second flow generated by vortex generators results in the reduction of the intersection angle between the velocity and fluid temperature gradient. The longitudinal vortex improves the field synergy of the large downstream region of longitudinal vortex generator (LVG) and the region near (LVG); however, transverse vortex only improves the synergy of the region near vortex generator. Thus, longitudinal vortex can enhance the integral heat transfer of the flow field, while transverse vortex can only enhance the local heat transfer. The synergy angle decreases with the increase of Reynolds number for the channel with LVG to differ from the result obtained from the plain channel, and the triangle winglet performs better than the rectanglar one under the same surface area condition. __________ Translated from Journal of Xi’an Jiaotong University, 2006, 40(9): 996–1000 [译自: 西安交通大学学报]     

Numerical analysis on heat transfer enhancement by longitudinal vortex based on field synergy principle

Three-dimensional numerical simulation results are presented for a fin-and-tube heat transfer surface with vortex generators. The effects of the Reynolds number (from 800 to 2 000) and the attack angle (30° and 45°) of a delta winglet vortex generator are examined. The numerical results are analyzed on the basis of the field synergy principle to explain the inherent mechanism of heat transfer enhancement by longitudinal vortex. The secondary flow generated by the vortex generators causes the reduction of the intersection angle between the velocity and fluid temperature gradients. In addition, the computational evaluations indicate that the heat transfer enhancement of delta winglet pairs for an aligned tube bank fin-and-tube surface is more significant than that for a staggered tube bank fin-and-tube surface. The heat transfer enhancement of the delta winglet pairs with an attack angle of 45° is larger than that with an angle of 30°. The delta winglet pair with an attack angle of 45° leads to an increase in pressure drop, while the delta winglet pair with the 30° angle results in a slight decrease. The heat transfer enhancement under identical pumping power condition for the attack angle of 30° is larger than that for the attack angle of 45° either for staggered or for aligned tube bank arrangement. Translated from Journal of Xi’an Jiao Tong University, 2006, 40(7): 757–761 [译自: 西安交通大学学报]     

Numerical study of fluid flow and heat transfer in a flat-plate channel with longitudinal vortex generators by applying field synergy principle analysis

Three dimensional numerical simulations are performed on laminar heat transfer and fluid flow characteristics of a flat-plate channel with longitudinal vortex generators (LVGs). The effects of two different shaped LVGs, rectangular winglet pair (RWP) and delta winglet pair (DWP) with two different configurations, common-flow-down (CFD) and common-flow-up (CFU), are studied. The numerical results indicate that the application of LVGs effectively enhances heat transfer of the channel. According to the performance evaluation parameter, (Nu/Nu₀)/(f/f₀), the channel with DWP has better overall performance than RWP; the CFD and CFU configurations of DWP have almost the same overall performance; the CFD configuration has a better overall performance than the CFU configuration for RWP. The basic mechanism of heat transfer enhancement by LVGs can be well described by the field synergy principle.     

纵向涡强化片式散热器油侧热性能的数值研究

采用三维数值模拟的方法研究了在片式散热器一组通道内设矩形涡流发生器(rectangualr vortex generator,RVGs)时对油侧的传热性能影响。研究了相同入口条件(层流)和考虑重力作用的条件下,纵向涡发生器的宽度、高度、攻角和纵向间距等几何因素对竖直通道传热和压降的影响。结果表明:纵向涡发生器产生的涡旋导致边界层分离,传热效果得到强化;改变涡发生器的宽度和高度对传热的影响趋势相似;当几何条件相同下攻角为30°时整体强化传热效果最佳;随着纵向间距的增加,传热的强化效果先降低再提高,最后又降低;最佳纵向间距为160 mm。     

Numerical study on laminar convection heat transfer in a rectangular channel with longitudinal vortex generator. Part A: Verification of field synergy principle

This study presents numerical computation results on laminar convection heat transfer in a rectangular channel with a pair of rectangular winglets longitudinal vortex generator punched out from the lower wall of the channel. The effect of the punched holes and the thickness of the rectangular winglet pair to the fluid flow and heat transfer are numerically studied. It is found that the case with punched holes has more heat transfer enhancement in the region near to the vortex generator and lower average flow frictional coefficient compared with the case without punched holes. The thickness of rectangular winglet can cause less heat transfer enhancement in the region near to the vortex generator and almost has no significant effect on the total pressure drop of the channel. The effects of Reynolds number (from 800 to 3000), the attack angle of vortex generator (15°, 30°, 45°, 60° and 90°) were examined. The numerical results were analyzed from the viewpoint of field synergy principle. It was found that the essence of heat transfer enhancement by longitudinal vortex can be explained very well by the field synergy principle, i.e., when the second flow generated by vortex generators results in the reduction of the intersection angle between the velocity and fluid temperature gradient, the heat transfer in the present channels will be enhanced. Longitudinal vortices (LVs) improve the synergy between velocity and temperature field not only in the region near LVG but also in the large downstream region of longitudinal vortex generator. So LVs enable to enhance the global heat transfer of channel. Transverse vortices (TVs) only improve the synergy in the region near VG. So TVs can only enhance the local heat transfer of channel.     

Numerical investigation of turbulent flow and heat transfer in a channel with novel longitudinal vortex generators

A novel combined longitudinal vortex generator (LVG), comprising a rectangular wing mounted with an accessory rectangular wing, is developed and the turbulent flow and heat transfer characteristics are numerically analyzed. The influences of six main parameters of the combined rectangular winglet pair (CRWP) on heat transfer enhancement and fluid flow resistance characteristics in a rectangular channel are examined. The parameters include the location of accessory wing on the main wing and geometric sizes of the accessory wing. The Reynolds number range is from 2000 to 16,000. The numerical results show that in the range of the present study, the increase of the six parameters can result in the increase of heat transfer and pressure drop. Specially, the pressure drop decreases at large value of the distance of accessory wing from the channel bottom. In comparison with RWP, the CRWP generates vortices with larger area and lower core. Furthermore, the accessory wings generate vortices that swirl downward the channel bottom and disturb the boundary layer growth more effectively. Hence the heat transfer is enhanced. The numerical result for the Nusselt number of the channel agrees well with the experimental result, indicating the reliability of the present numerical predictions.     

Investigation on laminar convection heat transfer in fin-and-tube heat exchanger in aligned arrangement with longitudinal vortex generator from the viewpoint of field synergy principle

3-D numerical simulation results are presented for laminar flow heat transfer of the fin-and-tube surface with vortex generators. The effects of Reynolds number (from 800 to 2000), the attack angle (30° and 45°) of delta winglet vortex generator are examined. The numerical results are analyzed from the viewpoint of field synergy principle. It is found that the inherent mechanism of heat transfer enhancement by longitudinal vortex can be explained by the field synergy principle, the second flow generated by the vortex generators results in the reduction of the intersection angle between the velocity and fluid temperature gradient. In addition, the heat transfer enhancement of delta winglet with the attack angle of 45° is larger than that of 30°, while the delta winglet with the attack angle of 45° results in an increase of the pressure drop, however, the delta winglet with the attack angle of 30° results in a slight decrease.     

Discussion on the convective heat transfer and field synergy principle

The convective “heat” transfer is actually mainly carried out by the motion of hotter or colder particles from one system into another system. Therefore, the best convective “heat” (strictly speaking, internal energy) transfer is the case where velocity vectors are always perpendicular to the isothermal surfaces (or isotherm in 2D cases). This conclusion has been named “field synergy principle”. In this paper, some field synergy exact solutions are presented to further develop the principle. The concrete physical meanings of the derived analytical solutions are analyzed. The method of separating variables with addition and other extraordinary approaches are adopted in the derivation.     

Numerical study of heat transfer enhancement by unilateral longitudinal vortex generators inside parabolic trough solar receivers

This study presents numerical computation results on turbulent flow and coupled heat transfer enhancement in a novel parabolic trough solar absorber tube, the unilateral milt-longitudinal vortexes enhanced parabolic trough solar receiver (UMLVE-PTR), where longitudinal vortex generators (LVGs) are only located on the side of the absorber tube with concentrated solar radiation (CSR). The novel absorber tube and the corresponding parabolic trough receiver with smooth absorber tube (SAT-PTR) are numerical studied by combining the finite volume method (FVM) and the Monte Carlo ray-trace (MCRT) method for comparison and verification from the viewpoint of field synergy principle (FSP). Then the effects of Reynolds number, heat transfer fluid (HTF) inlet temperature, incident solar radiation and LVG geometric parameters were further examined. It was found that the mechanism of heat transfer enhancement of this novel absorber tube can be explained very well by the field synergy principle, and that the proposed novel UMLVE-PTR has good comprehensive heat transfer performance than that of the SAT-PTR within a wide range of major influence factors of diverse working conditions and geometric parameters.     

A unified analysis on enhancing single phase convective heat transfer with field synergy principle

Numerical simulations were conducted to reveal the inherent relation between the filed synergy principle and the three existing mechanisms for enhancing single phase convective heat transfer. It is found that the three mechanisms, i.e., the decreasing of thermal boundary layer, the increasing of flow interruption and the increasing of velocity gradient near a solid wall, all lead to the reduction of intersection angle between velocity and temperature gradient. It is also revealed that at low flow speed, the fin attached a tube not only increases heat transfer surface but also greatly improves the synergy between the velocity and the temperature gradient.     

Numerical investigation into thermal mixing efficiency in Y-shaped channel using Lattice Boltzmann method and field synergy principle

This study employs the lattice Boltzmann method to simulate the thermal mixing efficiency of two-dimensional, incompressible, steady-state low Reynolds number flows in a Y-shaped channel. The effects of introducing a staggered arrangement of wave-like and circular obstacles into the mixing section of the Y-shaped channel are systematically examined. The simulation results demonstrate that both types of obstacle yield an effective improvement in the thermal mixing efficiency compared to that achieved in a Y-channel with a straight mixing section. Adopting the field synergy principle, it is demonstrated that the enhanced mixing efficiency is the result of an increased intersection angle between the velocity vector and the temperature gradient within the channel.     

纵向涡强化竖直平板自然对流换热的实验研究

对纵向涡强化竖直平板自然对流换热进行了实验研究。结果表明，在一定的Rayleigh数范围内，直角三角翼纵向涡发生器的攻角、翼高、翼宽等几何参数是影响强化换热的主要因素。存在最佳攻角；宽高比一定时，翼高和翼宽的变化会影响换热的效果。发现在直角三角翼阵列中前排直角三角翼产生的纵向涡可以强化后排直角三角翼纵向涡的换热。将直角三角翼与矩形低肋换热表面的性能作了对比性实验，在其他条件相同的情况下，直角三角翼强化换热的效果优于矩形低肋。     

倾斜封闭方腔内自然对流换热的场协同分析

采用场协同原理对倾斜封闭方腔内自然对流换热现象进行了研究。通过数值模拟获得了不同瑞利数(Ra=1~10~6)时倾斜封闭方腔内自然对流换热的协同关系,以及在一定Ra数下不同倾斜角φ对封闭方腔内自然对流换热的影响。结果表明:随着瑞利数Ra的增大,协同数Fc变小,其协同性越好。随着倾斜角φ的变化,当Ra≤10~3时,lg Fc的变化曲线近似呈现开口向上的抛物线型;当10~4≤Ra≤10~5时,lg Fc的变化曲线是单调的;当瑞利数Ra≥10~6时,lg Fc的变化曲线有2个极值和1个拐点。     

Numerical investigation of heat transfer enhancement on a porous vortex-generator applied to a block-heated channel

The numerical simulation is used to obtain the unsteady laminar flow and convective heat transfer in the block-heated channel with the porous vortex-generator. The general Darcy–Brinkman–Forchheimer model is adopted for the porous vortex-generator. The parameters studies including porosity, Darcy number, width-to-height ratio of porous vortex-generator and Reynolds number have been explored on heat transfer enhancement and vortex-induced vibration in detail. The results indicate that heat transfer enhancement and vortex-induced vibration increase with increasing Reynolds number and width-to-height ratio. However, the porosity has slight influence on heat transfer enhancement and vortex-induced vibration. When Darcy number is 10^?3 or 10^?4, installing a porous vortex-generator with B/h = 1.0 improves overall heat transfer the best along heated blocks, and has a strong reduction of vortex-induced vibration.     

Numerical investigation of pressure drop reduction without surrendering heat transfer enhancement in partially porous channel

The present study is to investigate the numerical simulation of steady laminar forced convection in a partially porous channel, with four dissimilar porous-blocks, attached to the strip heat sources at the bottom wall. The analysis is based on the Navier–Stokes equation in the fluid field, the Darcy–Brinkman–Forchheimer flow model in the porous field, and the energy equations for two thermal fields. The effects of variations of different parameters such as porous blocks Darcy numbers, arrangements of dissimilar blocks, Forchheimer coefficient, Reynolds number, thermal conductivity and Prandtl number are investigated and the velocity and temperature fields are presented and discussed. In the dissimilar partially porous channel, it is found that when the blocks sorted from the lowest to the highest Da in the flow direction, the total heat transfer enhancement is almost the same as in the similar porous channel (Nu/Nu_sim = 92%), while the total pressure drop is considerably lower (P/P_sim = 28%). In addition, reverse arrangement of porous blocks is suggested to prepare more uniform temperature gradient in all heat sources.     

Numerical investigations of curved square channel from the viewpoint of field synergy principle

A curved square channel in laminar flow is numerically investigated based on the classical Navier–Stokes equations from the viewpoint of the field synergy principle. The field synergy principle may accurately describe the curved channel has higher convective heat transfer rate in the case that the heat transfer surface is specified on the outer wall, rather than on the inner wall. The field synergy principle could also be responsible for that the curved channel can enhance the convective heat transfer significantly at the cost of the slight increase of the flow resistance. The field synergy number represents the degree of the synergy between the temperature gradient and the velocity vector, the higher field synergy number leads to the higher convective heat transfer rate under the same Reynolds number and Prandtl number. The field synergy number plays the same positive role in the convective heat transfer whether the fluid is heated or cooled.     

通道内设置涡发生器时的强化传热研究现状与展望

随着强化传热技术的研究发展，各种形式的涡发生器的强化传热效果日益受到国内外的重视。文章比较全面地介绍了近年来国内外关于通道内布置各类涡发生器时的强化传热研究状况，并提出了有待进一步开展的研究内容。     

Physical quantity synergy in laminar flow field and its application in heat transfer enhancement

On the basis of field synergy principle for heat transfer enhancement, physical quantity synergy in laminar flow field of convective heat transfer is analyzed according to physical nature of convective heat transfer between fluid and solid wall. Synergy regulation among physical quantities is revealed by mathematical expressions reflecting mechanism of heat transfer enhancement. Characteristic of heat transfer enhancement, which is directly associated with synergy angles α, β and γ, is also analyzed. Numerical simulation is made to verify the principle of physical quantity synergy developed in the paper.     

Numerical investigation of heat transfer using a novel punched vortex generator

ABSTRACT

Novel types of double delta winglets (DDWTs) and double delta winglet with holes (DDWTHs) have been developed. The influence of parameters, including the angle of DDWTs and the distance between DDWTs, on thermal enhancement and flow resistance characteristics in a rectangular channel is examined. The results reveal that the larger angle and increasing distance could make an active role in turbulent heat transfer enhancement, and slight influence on laminar flow. According to the field profile, the cross-flow and encircling flow are generated by DDWTs, which increases the mixing efficiency and reduces the thermal boundary layer thickness as well. Vortex generators, such as delta wings, rectangular wings, DDWTs, and DDWTHs, are compared. The delta wings and rectangular wings perform better heat transfer enhancement and higher resistance, while the DDWTs and DDWTHs have better overall heat transfer performance, especially for large Reynolds numbers.     

多种纵向涡发生器配置方案流动换热的数值模拟

为了深入挖掘三角翼纵向涡发生器在两个相对壁面布置的强化换热潜力,采用数值模拟方法,在雷诺数3 000～18 000的范围内研究了5种纵向涡发生器配置的流动换热情况,配置方式包括单面布置的共同上、下流配置,双面布置的共同上、下流配置,以及混合配置。结果表明：纵向涡可以很好地提高场协同效果,换热强度不完全取决于通道中的二次流强度,还取决于通道中的场协同性;在所有配置中,混合配置具有最高的二次流强度、最佳的场协同效果以及换热性能,可以将光滑通道的Nu提高28.3%～35.3%;另外4种配置可分别将光滑通道的Nu提高21.4%～32.0%,20.0%～29.2%,26.3%～34.3%和23.7%～28.0%;建议选用Re<6 000范围内的混合配置,此时其具有1.03～1.10的综合换热因子以及1.32～1.35的Nu/Nu₀。