Heat transfer augmentation in a wedge-ribbed channel using winglet vortex generators

Experimental investigations have been carried out to study the effect of combined wedge ribs and winglet type vortex generators (WVGs) on heat transfer and friction loss behaviors for turbulent airflow through a constant heat flux channel. To create a reverse flow in the channel, two types of wedge (right-triangle) ribs are introduced: wedge ribs pointing downstream and pointing upstream. The arrangements of both rib types placed inside the opposite channel walls are in-line and staggered arrays. To generate longitudinal vortex flows through the tested section, two pairs of the WVGs with the attack angle of 60° are mounted on the test channel entrance. The test channel has an aspect ratio, AR = 10 and height, H = 30 mm with a rib height, e/H = 0.2 and rib pitch, P/H = 1.33. The flow rate in terms of Reynolds numbers is based on the inlet hydraulic diameter of the channel ranging from 5000 to 22,000. The presence of the combined ribs and the WVGs shows the significant increase in heat transfer rate and friction loss over the smooth channel. The Nusselt number and friction factor values obtained from combined the ribs and the WVGs are found to be much higher than those from the ribs/WVGs alone. In conjunction with the WVGs, the in-line wedge pointing downstream provides the highest increase in both the heat transfer rate and the friction factor while the staggered wedge pointing upstream yields the best thermal performance.     

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.     

Experimental study on heat transfer in square duct with combined twisted-tape and winglet vortex generators

The article presents an experimental investigation on thermal performance enhancement in a constant heat-fluxed square duct fitted with combined twisted-tape and winglet vortex generators. The experiments are carried out for the airflow rate through the tested square duct fitted with both the vortex generators for Reynolds number from 4000 to 30,000. The effect of the combined twisted tape and rectangular winglet inserts on heat transfer and pressure drop presented in terms of respective Nusselt number and friction factor is experimentally investigated. The characteristics of the combined twisted-tape and winglet include two twist ratios (Y = 4 and 5), three winglet- to duct-height ratios, (R_B = 0.1, 0.15 and 0.2), four winglet-pitch to tape-width ratios, (R_P = 2, 2.5, 4 and 5) and a single attack angle of winglet, α = 30°. The experimental results reveal that the Nusselt number and friction factor for the combined twisted-tape and V-winglet increase with increasing R_B but decreasing R_P. The inserted duct at R_B = 0.2, R_P = 2 and Y = 4 provides the highest heat transfer rate and friction factor but the one at R_B = 0.1, R_P = 2 and Y = 4 yields the highest thermal performance. The application of combined vortex-flow devices gives thermal performance around 17% higher than the twisted tape alone.     

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     

Experimental study on heat transfer enhancement in the vertical nature convection by using delta‐winglet longitudinal vortex generators

This paper focuses on the study of heat transfer enhancement in natural vertical convection by using delta‐winglet longitudinal vortex generators. In the experimental range of Rayleigh numbers, the effect of attack angle, height, and width of the winglet of longitudinal vortex generator (LVG) on heat transfer performance was experimentally investigated. The results showed that there was an optimal attack angle and that the height and width can affect the heat transfer. In terms of array performance, it was shown that initial arrays could enhance the performance of later arrays. Moreover, the effects of LVG and low rectangular fins were compared. The results showed that the effect of LVGs was greater than that of low rectangular fins. © 2006 Wiley Periodicals, Inc. Heat Trans Asian Res, 35(6): 402–409, 2006; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20126     

Flow visualization of annular and delta winlet vortex generators in fin-and-tube heat exchanger application

This study presents flow visualization and frictional results of enlarged fin-and-tube heat exchangers with and without the presence of vortex generators. Two types of vortex generators and a plain fin geometry were examined in this study. For plain fin geometry at Re=500, the horseshoe vortex generated by the tube row is not so pronounced, and the horseshoe vortex separates into two streams as it flows across the second row and consequently loses its vortical strength. This phenomenon may supports the “maximum phenomenon” in low Reynolds number region reported by previous studies. With the presence of annular vortex generator, the presence of a pair of longitudinal vortices formed behind the tube is seen. The strength of the counter-rotating vortices increases with the annular height and the strength of the longitudinal vortices is so strong that may swirl with the horseshoe vortices and other flow stream. For the same winlet height, the delta winlet shows more intensely vortical motion and flow unsteadiness than those of annular winlet. This eventually leads to a better mixing phenomenon. However, it is interesting to know that the corresponding pressure drops of the delta winlet are lower than those of annular winlet. Compared to the plain fin geometry, the penalty of additional pressure drops of the proposed vortex generators is relatively insensitive to change of Reynolds number.     

空气制冷循环特性及一种新型涡流制冷装置研究

通过与蒸汽压缩制冷循环的比较,论述了空气制冷循环的特性,特别是空气制冷循环对解决环保问题的优良特性以及适用温度范围宽广的特点。并且提出一种新型涡流管制冷装置系统。     

The development of active vortex generators from shape memory alloys for the convective cooling of heated surfaces

A study of the convective heat transfer enhancement of heated surfaces through the use of active delta wing vortex generators is reported in this paper. The surface-mounted vortex generators (VGs) change their shape to intrude further into the flow at high temperatures to enhance heat transfer, while maintaining a low profile at low temperatures to minimise flow pressure losses. The VGs are made from shape memory alloys and manufactured in a selective laser melting process. Experiments have been carried out in a rectangular duct supplied with laminar-transition air flow. In the test section, a single, and a pair of active delta wing VGs were placed near the leading edge of a heated plate and tested separately for their heat transfer enhancement effects using infrared thermography. The pressure difference across the test section was also measured to determine the pressure drop penalty associated with the obstruction caused by the vortex generators in their active positions. Promising shape memory response was obtained from the active VG samples when their surface temperatures were varied from 20 °C to 65 °C. The vortex generators responded by increasing their angles of attack from 10° to 38° and as the designs were two-way trained, they regained their initial position and shape at a lower temperature. At their activated positions, maximum heat transfer improvements of up to 90% and 80% were achieved by the single and double wings respectively along the downstream direction. The flow pressure losses across the test section, when the wings were activated, increased between 7% and 63% of the losses at their de-activated positions, for the single and double VG respectively.     

A brief survey and economical analysis of air cooling for electronic equipments

The techniques used in the cooling of electronic equipments vary widely depending on the particular application. Cooling with liquids and impinging air jets become important where classical cooling techniques may be insufficient, while a single or array jet is employed where highly localised cooling is desired. This paper provides a comparative survey of advanced methods of cooling for electronic systems and an economical analysis of cooling electronic equipments using slot and circular jets.     

Forced Convective Air Cooling from Electronic Component Arrays in a Parallel

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Enhancement of radiation cooling by means of reflectors

It is shown that radiation cooling to a clear sky can be substantially enhanced by coupling a reflector to the radiator. The class of reflectors considered in this study is the CPC truncated on the side of the exit aperture. It is estimated that, under real circumstances, the cooling power per unit area of entrance aperture can be nearly doubled, with the help of a truncated CPC trough reflector, and the equilibrium limit temperature can be brought some 24°C further down. A truncated CPC dish reflector can still improve on the equilibrium temperature but at the cost of some loss of cooling power. The device can still operate satisfactorily in the day time.     

风冷式冷渣器的实验研究

测试了一种用于煤粉炉炉渣冷却的风冷式冷渣器 ,结果表明 ,此设备仅用空气冷却炉渣 ,起到了节水的作用 ,同时利用炉渣的余热预热了进入炉膛的部分空气 ,有效地利用了炉渣的余热。此设备在较低负荷下冷却效果较好 ,高负荷下不能满足排渣温度要求 ,需要作进一步改进。     

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。     

Enhancement of tidal generators by superconducting energy storage and Jaya-based sliding-mode controller

This article presents a novel control technique of Jaya-based super-sliding controller that is applied on superconducting magnetic energy storage system (SCMES). The SCMES will be integrated to operate with tidal turbine generators (TTGs). The penetration of TTGs has recently grown due to distributed generation contributions to the energy sector. To alleviate the potential system instability due to TTG dynamic performance, SCMES is connected to TTG to support the dynamic operational conditions. SCMES is implemented to act quickly during both cases of power generation and absorption, so it can realize any required mismatch between the supply and load power at dynamic transient periods. To enhance such capability, this article proposes the application of Jaya-based super-sliding control technique to control and tune the system dynamic gains. The proposed technique implements incorporated co-operative algorithms using enhanced Jaya competitive algorithm and super-sliding method to facilitate the exchange of energy between the SCMES, the TTG, and the power network. It allows smooth dynamic performance, power quality enhancement, and robust dynamic reference tracking. Jaya-based super-sliding controller is developed and proposed for accurate and reliable control and tunning of system parameters. The proposed controller ensures the superb performance of the synergy of SCMESs and TTGs. Digital simulations have validated the effectiveness of the proposed system with the optimal dynamic parameters and minimization of the global control error.     

Enhancement of cooling capacity in a hybrid closed circuit cooling tower

Performance characteristics of the Hybrid Closed Circuit Cooling Tower (HCCCT) have been tested experimentally. Bare-type copper tubes and fin tubes were used in staggered arrangement at the heat exchanger of the HCCCT and the cooling capacity and pressure drop have been studied from various perspectives. The relevant temperatures and velocities were selected based on the typical East Asian meteorological constraints for the year round operation of the HCCCT. Performance characteristics were compared and found that for the fin tubes, cooling capacities were about 22% and 260% higher than those of the bare tubes in wet and dry modes, respectively. The pressure drop for the fin tubes was nearly two times higher than that of the bare tubes in both modes. The experimental results show considerable enhancement of cooling capacity.     

半圆柱面涡流发生器在矩形管道内抑垢实验

半圆柱面涡流发生器具有较好的强化传热效果,在换热器设备上具有较大的应用前景。为了获得半圆柱面涡流发生器的抑垢特性,采用离线称重法,研究了在流速为0.2m/s、体积质量为3 500mg/L、温度为50℃下不同排列间距、不同高度下的半圆柱面涡流发生器对CaSO4污垢结垢量的变化。结果表明:半圆柱面涡流发生器具有抑垢性,总结垢量要少于光片;相同高度下间距20mm的试片结垢量最少;相同间距下高为5mm的试片结垢量最小。     

Numerical visualization of air intake induced by free surface vortex

Free surface vortex control is vital in a pump sump system because the air absorbed by free surface vortex induces noise,vibration,and cavitation corrosion on the pumping system.In this study,the change of free surface vortex and air absorption in a pump intake has been investigated by the Volume of Fraction (VOF) method with steady multiphase flow model in order to represent the behavior of the free surface vortex exactly.The homogeneous free surface model is used to apply interactions of air and water.The results show that air intake by the free surface vortex motion can be visualized using the iso-sufface of air volume fraction.The vortices make an air column from the free surface to the pump intake.Also,it was found that the free surface vortex can be controlled by installing curtain walls.     

Performance enhancement of photovoltaic modules by nanofluid cooling: A comprehensive review

Only 15-20% of solar radiation incident on the photovoltaic (PV) cells is utilized which further reduces due to the rise in the temperature of the PV module and it also degrades the lifespan of the PV module. Therefore, numerous attempts were made to reduce this rising temperature of the PV module and different cooling techniques were employed. Nanofluid cooling is one of the potential cooling techniques for lowering the temperature of the PV module and augmenting the heat transfer by increasing the thermal conductivity of the nanofluid relative to the base fluid (BF). The experimental and numerical studies related to the cooling of PV cells with nanofluids have been reviewed. It was found that the heat transfer from the back of the PV module is enhanced with the augmentation in the concentration of nanoparticle in BF; however, some studies also demonstrate that the enhancement in the heat transfer also depends upon other factors such as the geometry at the rear of the PV module, nanoparticle material, nanoparticle size, BF, ambient conditions, etc. This review article also demonstrates the various issues with nanofluids such as instability, technological difficulties, high system costs, and the impossibility of finding a viable operational design which creates a barrier in the commercialization of the nanofluid cooling technique for PV modules.     

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。     

Optimization of microscale vortex generators in a microchannel using advanced response surface method

Mixing based on mass diffusion and advective flow at low Reynolds number is important on design of microscale vortex generators. We studied on the optimization of micromixer for the improvement of mass transport using an advanced response surface method to be closely approximate the real map of mixing performance. We considered four rib geometries simultaneously; rib angle, rib height, rib width, and rib spacing. The optimized microchannel was occurred at a micromixer configuration where θ, d/h, a/d, and b/d were 35.6°, 0.7, 0.127, and 1.10, respectively. The channel length to obtain the mixing uniformity over 95% was 1344 μm.