Impact of delta winglet vortex generators on the performance of a novel fin-tube surfaces with two rows of tubes in different diameters

To achieve heat transfer enhancement and lower pressure loss penalty, even pressure loss reduction, two novel fin-tube surface with two rows of tubes in different diameters are presented in this paper. Numerical simulation results show that the fin-tube surface with first row tube in smaller size and second row tube in larger size can lead to an increase of heat transfer and decrease of pressure drop in comparison with the traditional fin-tube surface with two rows of tubes in the same size. Based on this understanding, delta winglet pairs are punched out only from the larger fin area around the first transverse row of tubes in smaller size in the novel fin-tube surfaces. Delta winglet pairs used as longitudinal vortex generator are arranged either in “common flow up” or “common flow down” configurations. Numerical simulation results show that delta winglet pairs can bring about a further heat transfer enhancement and pressure drop decrease through the careful arrangement of the location, size and attack angle of delta winglet pairs either in “common flow up” or “common flow down” configurations. The traditional knowledge of heat transfer enhancement with necessary pressure drop increase is challenged by the present conclusion. The present work will be helpful to develop more compact, higher heat transfer efficiency, lower fan power and quieter heat exchanger of refrigeration and air condition system.     

WINGLET-TYPE VORTEX GENERATORS WITH COMMON-FLOW-UP CONFIGURATION FOR FIN-TUBE HEAT EXCHANGERS

This investigation stems from the area of augmentation of heat transfer by generating streamwise longitudinal vortices. The vortex generators are arranged in a common-flow-up configuration. Existing air-cooled condensers in geothermal power plants use fin-tube heat exchangers with circular tubes. The heat exchangers are huge, and often the cost of the condensers is more than one-third of the plant cost. The size of the condensers can be reduced through enhancement of heat transfer from fin surfaces. The enhancement strategy involves introduction of strong swirling motion in the flow field. The swirl can be generated by the longitudinal vortices. In this study, the longitudinal vortices are created by delta winglet-type vortex generators, which are mounted behind the tubes. An element of a heat exchanger has been considered for detailed study of the flow structure and heat transfer analysis. Biswas and colleagues have obtained significant enhancement of heat transfer by deploying the winglet pair behind each tube. In this study, a novel technique (Torii and colleagues [2]) has been utilized for the enhancement. The winglets are placed with a heretofore unused orientation for the purpose of augmentation of heat transfer. This orientation is called the common-flow-up configuration. The proposed method causes significant separation delay, reduces form drag, and removes the zone of poor heat transfer from the near wake of the tubes. The analyses of flow and heat transfer in the proposed configuration have been accomplished through a numerical solution of complete Navier-Stokes and energy equations.     

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.     

Augmentation of Heat Transfer by Creation of Streamwise Longitudinal Vortices Using Vortex Generators

This paper summarizes the current state of the art related to improvement of the heat exchanger surfaces using streamwise longitudinal vortices. Primarily, the improvements related to fin-tube cross-flow heat exchangers and the plate-fin heat exchangers have been addressed. Protrusions in certain forms, such as delta wings or winglet pairs, act as vortex generators, which can enhance the rate of heat transfer from the heat-exchanger surfaces that may be flat or louvered. The strategically placed vortex generators create longitudinal vortices, which disrupt the growth of the thermal boundary layer, promote mixing between fluid layers, and hence lead to augmentation in heat transfer. The flow fields are dominated by swirling motion associated with modest pressure penalty. Heat transfer is augmented substantially for all the proposed configurations of the longitudinal vortex generators, such as delta wings, rectangular winglet pairs, and delta winglet pairs, with varying degree of pressure penalty. Both computational and experimental investigations on flow and heat transfer in the heat exchanger passages with built-in vortex generators are revisited and summarized.     

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 on air-side performance of an integrated fin and micro-channel heat exchanger

A new type of aluminum heat exchanger with integrated fin and micro-channel has been proposed. The air-side heat transfer and flow characteristics of the integrated fin and micro-channel heat exchanger are systematically analyzed by a 3D numerical simulation. The effect of flow depth, fin height, fin pitch and fin thickness at different Reynolds number is evaluated by calculating Colburn factor j and Fanning friction factor f. A parametric study method is used to analyze the fin designed parameters affecting the performance of the heat exchanger. The results show that the contribution ratio of the fin geometries in descending order is flow depth, fin pitch, fin height and fin thickness. The air-side performance of the integrated fin and micro-channel heat exchanger is compared with that of the multi-louver fin micro-channel heat exchanger and the wavy fin micro-channel heat exchanger.     

Heat transfer enhancement and pressure drop for fin-and-tube compact heat exchangers with wavy rectangular winglet-type vortex generators

In the current work, heat transfer enhancement and pressure loss penalty for fin-and-tube compact heat exchangers with the wavy-up and wavy-down rectangular winglets as special forms of winglet are numerically investigated in a relatively low Reynolds number flow. The rectangular winglets were used with a particular wavy form for the purpose of enhancement of air side heat transfer performance of fin-and-tube compact heat exchangers. The effect of Reynolds numbers from 400 to 800 and angle of attack of 30° of wavy rectangular winglets are also examined. The effects of using the wavy rectangular winglet, conventional rectangular winglet configuration and without winglet as baseline configuration, on the heat transfer characteristics and flow structure are studied and analyzed in detail for the inline tube arrangements. The results showed that the wavy rectangular winglet can significantly improve the heat transfer performance of the fin-and-tube compact heat exchangers with a moderate pressure loss penalty. In addition, the numerical results have shown that the wavy winglet cases have significant effect on the heat transfer performance and also, this augmentation is more important for the case of the wavy-up rectangular winglet configuration.     

Turbulent heat transfer enhancement in a triangular duct using delta‐winglet vortex generators

The augmentation of convective heat transfer of a turbulent flow using delta‐winglet vortex generators (VG) in a triangular duct was experimentally investigated. Two side walls of the heated test section are electrically heated with a constant heat flux while the lower wall is indirectly heated. Single, double, and triple pairs of VG are utilized. Each pair of VG was punched on one wall of the test duct. The effects of the number of VG pairs, the VG angle of attack, the VG location from the leading edge of the test duct, the VG geometry, and Reynolds number are examined in this paper. The results indicate that the Nusselt number and friction factor are relatively proportional to the size, number, and the inclination angle of the VG. The Nusselt number increases and the friction factor decreases as the Reynolds number increases. The present results were compared with the available literature and they show good agreement. Correlation equations of Nusselt number and friction factor for turbulent flow are developed, for the cases studied, as a function of Reynolds number and VG angle of attack. © 2011 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley Online Library ( wileyonlinelibrary.com/journal/htj ). DOI 10.1002/htj.20382     

Application of Combined Enhanced Techniques for Design of Highly Efficient Air Heat Transfer Surface

In order to reduce the size and cost of heat exchangers, an air-side wavy fin-and-tube heat transfer surface with three-row tubes needs to be replaced by two-row tubes with some appropriate enhancing techniques. The major purpose of the present paper is to search for such new structure by numerical simulation. First, longitudinal vortex generators of Delta-winglet type are tried. The influence of number and of arrangement of the winglets on the performance of the heat transfer surface is studied in detail. The numerical results show that the fin with two winglets aligned spanwise in the front and rear of each tube (Fin W6) has higher heat transfer capability than other enhanced structures with vortex generators, but it still unable to meet the heat transfer requirement. Then a combination design of the longitudinal vortex generator with slotted protruding parallel strips is proposed and different variations of their arrangement are tried. Finally we come to such a combination (C3), which is based on Fin W6 with additional eight protruding strips situated at five positions (grouped by 1, 2, 2, 2, and 1) along the flow direction. Fin C3 can satisfy the requirements for heat transfer rate of the original wavy fin of three-row tubes with a mild increase in pressure drop, and its volume and material reduce to about 67% of the original one.     

Numerical study on laminar convection heat transfer in a channel with longitudinal vortex generator. Part B: Parametric study of major influence factors

This paper presents the influences of main parameters of longitudinal vortex generator (LVG) on the heat transfer enhancement and flow resistance in a rectangular channel. The parameters include the location of LVG in the channel, geometric sizes and shape of LVG. Numerical results show that the overall Nusselt number of channel will decrease with the LVGs’ location away from the inlet of the channel, and decrease too with the space between the LVG pair decreased. The location of LVG has no significant influence on the total pressure drop of channel. With the area of LVG increased, the average Nusselt number and the flow loss penalty of channel, especially when β = 45° will increase. With the area of LVG fixed, increasing the length of rectangular winglet pair vortex generator will bring about more heat transfer enhancement and less flow loss increase than that increasing the height of rectangular winglet pair vortex generator. With the same area of LVG, delta winglet pair is more effective than rectangular winglet pair on heat transfer enhancement of channel, and delta winglet pair-b is more effective than delta winglet pair-a. Delta winglet pair-a results in a higher pressure drop, the next is rectangular winglet pair and the last is delta winglet-b. The increase of heat transfer enhancement is always accompanied with the decrease of field synergy angle between the velocity and temperature gradient when the parameters of LVG are changed. This confirms again that the field synergy is the fundamental mechanism of heat transfer by longitudinal vortex. The laminar heat transfer of the channel with punched delta winglet pair is experimentally and numerically studied in the present paper. The numerical result for the average heat transfer coefficient of the channel agrees well with the experimental result, indicating the reliability of the present numerical predictions.     

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.     

Air-Cooling Enhancement with Delta Winglet Vortex Generators in Entrance Region of In-Line Array Electronic Modules

The objective of this study is to experimentally investigate the heat transfer enhancement by delta winglet vortex generators for air cooling in the entrance region of an in-line array of electronic modules. The study has been carried out when the winglet pairs are placed in front of all modules with attack angles of 10, 15 and 20 degrees. Each module has dimensions of 1.8 cm × 5.8 cm × 0.6 cm and generates heat at 5 W. The adiabatic heat transfer coefficients and the thermal wake functions for the modules with and without the generators are considered at different values of Reynolds number. It could be seen that the vortex generators could enhance the adiabatic heat transfer coefficients and reduce the thermal wake function and the module temperature significantly. Moreover, the correlations to predict the heat transfer data when the vortex generators are integrated have been developed. The temperatures of the modules in each row could be predicted from these correlations, and the results agree very well with the experimental data.     

Numerical study on a slit fin-and-tube heat exchanger with longitudinal vortex generators

A 3-D numerical simulation is performed on laminar heat transfer and flow characteristics of a slit fin-and-tube heat exchanger with longitudinal vortex generators. Heat transfer enhancement of the novel slit fin mechanism is investigated by examining the effect of the strips and the longitudinal vortices. The structure of the slit fin is optimized and analyzed with field synergy principle. The result coincides with the guideline ‘front coarse and rear dense’. The heat transfer and fluid flow characteristics of the slit fin-and-tube heat exchanger with longitudinal vortex generators are compared with that of the heat exchanger with X-shape arrangement slit fin and heat exchanger with rectangular winglet longitudinal vortex generators. It is found that the Colburn j-factor and friction factor f of the novel heat exchanger with the novel slit fin is in between them under the same Reynolds number, and the factor j/(f^1/3) of the novel heat exchanger increased by 15.8% and 4.2%, respectively.     

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.     

不同排列方式下三角翼波纹翅片管换热器的换热性能比较

应用三维数值模拟的方法对加装三角翼涡发生器的波纹翅片管换热器的流动换热特性进行了研究.3排换热圆管按顺排和叉排2种方式排列.结果表明:三角翼产生的纵向涡包括1个主涡和1个角涡.顺排布置时,纵向涡不但改善了尾迹区的换热,同时还大大强化了三角翼下游管排壁面的换热;叉排布置时,纵向涡在遇到后一个波谷时很快被抑制,换热的强化主要作用于尾迹区.ReD=3000时,与无三角翼的波纹翅片相比,三角翼波纹翅片的j、f,因子在顺排和叉排布置中分别增加了15.4%、10.5%和13.1%、7.0%.在不同排列方式下,三角翼产生的纵向涡均提高了波纹翅片管换热器的换热性能.     

Heat transfer augmentation in a plate‐fin heat exchanger using a rectangular winglet

This study presents numerical computation results on laminar convection heat transfer in a plate‐fin heat exchanger, with triangular fins between the plates of a plate‐fin heat exchanger. The rectangular winglet type vortex generator is mounted on these triangular fins. The performance of the vortex generator is evaluated for varying angles of attack of the winglet i.e., 20, 26, and 37° and Reynolds number 100, 150, and 200. The computations are also performed by varying the geometrical size and location of the winglet. The complete Navier–Stokes equation and the energy equation are solved by the (Marker and Cell) MAC algorithm using the staggered grid arrangement. The constant wall temperature thermal boundary conditions are considered. Air is taken as the working fluid. The heat transfer enhancement is seen by introducing the vortex generator. Numerical results show that the average Nusselt number increases with an increase in the angle of attack and Reynolds number. For the same area of the LVG, the increase in length of the LVG brings more heat transfer enhancement than increasing the height. The increase in heat transfer comes with a moderate pressure drop penalty. © 2010 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley Online Library ( wileyonlinelibrary.com ). DOI 10.1002/htj.20318     

Numerical study of heat-transfer enhancement by punched winglet-type vortex generator arrays in fin-and-tube heat exchangers

The potential of punched winglet type vortex generator (VG) arrays used to enhance air-side heat-transfer performance of finned tube heat exchanger is numerically investigated. The arrays are composed of two delta-winglet pairs with two layout modes of continuous and discontinuous winglets. The heat transfer performance of two array arrangements are compared to a conventional large winglet configuration for the Reynolds number ranging from 600 to 2600 based on the tube collar diameter, with the corresponding frontal air velocity ranging from 0.54 to 2.3 m/s. The effects of different geometry parameters that include attack angle of delta winglets (β = 10 deg, β = 20 deg, β = 30 deg) and the layout locations are examined. The numerical results show that for the punched VG cases, the effectiveness of the main vortex to the heat transfer enhancement is not fully dominant while the “corner vortex” also shows significant effect on the heat transfer performance. Both heat transfer coefficient and pressure drop increase with the increase of attack angle β for the side arrangements; the arrays with discontinuous winglets show the best heat transfer enhancement, and a significant augmentation of up to 33.8–70.6% in heat transfer coefficient is achieved accompanied by a pressure drop penalty of 43.4–97.2% for the 30 deg case compared to the plain fin. For the front arrangements of VGs higher heat transfer enhancement and pressure drop penalty can be obtained compared to that of the side arrangement cases; the case with front continuous winglet arrays has the maximum value of j/f, a corresponding heat transfer improvement of 36.7–81.2% and a pressure drop penalty of 60.7–135.6%.     

矩形管内纵向涡强化传热研究

将纵向涡强化换热技术应用于矩形管槽,研究以水为换热介质在过渡流状态下的换热效果。实验结果表明有纵向涡发生器的换热效果明显优于无纵向涡发生器的情况。利用PHEON ICS计算软件对实验进行数值模拟,模拟值与实验值符合较好。在此基础上,改变纵向涡的翼高和形状来模拟,发现两者均为换热影响的因素,相比之下,高宽比为0.4纵向涡发生器的换热效果比高宽比为0.5和0.6的要好。而采用相同高宽的矩形翼时,N u高于三角翼,但其换热性能指标却低于直角三角翼。     

矩形通道内冲孔矩形小翼涡流发生器的流动换热特性

雷诺数Re=214～10 703时,通过数值模拟方法对布置有冲孔和无孔的两种矩形小翼涡流发生器的矩形通道进行了传热和流阻特性的研究。计算结果表明:在低雷诺数下,冲孔矩形小翼涡流发生器的传热因子j值与无孔矩形小翼涡流发生器相差不大,而在高雷诺数下,冲孔涡流发生器的传热因子j值略低于无孔涡流发生器,大约低1.03%～3.05%。在相同的雷诺数下,无孔矩形小翼涡流发生器的阻力因子f大于冲孔涡流发生器,而且随着雷诺数的增大二者的差距也越来越大。通过对比综合性能指标可知,两种通道的综合性能指标均随着雷诺数的增加而减小,而且冲孔矩形小翼涡流发生器的综合性能要优于无孔矩形小翼涡流发生器。