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     

Impact of leading edge delta-wing vortex generators on the thermal performance of a flat tube, louvered-fin compact heat exchanger

The effectiveness of delta-wing type vortex generators is experimentally evaluated by full-scale wind-tunnel testing of a compact heat exchanger typical to those used in automotive systems. The mechanisms important to vortex enhancement methods are discussed, and a basis for selecting a delta-wing design as a vortex generator is established. The heat transfer and pressure drop performance are assessed at full scale under both dry- and wet-surface conditions for a louvered-fin baseline and for a vortex-enhanced louvered-fin heat exchanger. An average heat transfer increase over the baseline case of 21% for dry conditions and 23.4% for wet conditions was achieved with a pressure drop penalty smaller than 7%. Vortex generation is proven to provide an improved thermal-hydraulic performance in compact heat exchangers for automotive systems.     

Experimental study on heat transfer enhancement on natural convection in a vertical plate by using longitudinal vortex generators arranged in rows

Longitudinal vortices are capable of producing beneficial effects in heat transfer enhancement. Experiments in natural convection heat transfer enhancement were done on a vertical flat heating plate using delta‐winglet longitudinal vortex generators (LVGs) arranged in rows. In an experimental range of Rayleigh number, the height and width of the winglet of the longitudinal vortex generator (LVG), the array form of the longitudinal vortex generators on the heat transfer performance were experimentally investigated, and the best height of the winglet of the longitudinal vortex generator was obtained. The results showed the change of the array form of the longitudinal vortex generators could affect the heat transfer effect. Finally by arranging some longitudinal vortex generator arrays with the appropriate interval, the whole heat transfer effect of the interval could reach a prime value. © 2006 Wiley Periodicals, Inc. Heat Trans Asian Res, 35(5): 351–358, 2006; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20119     

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.     

Evaporation performance of a new plate‐type heat exchanger with winding tubes on plate surfaces (estimation of performance of the heat exchanger)

A method for evaluating and predicting the performance of a newly developed plate‐type heat exchanger as an evaporator for water‐refrigerant systems such as chillers has been developed. The main component of the developed heat exchanger consists of plates packed together in a casing with winding tubes connected to both sides of the plates. Refrigerant flows inside the tubes, and water flows in the space between the plates. A herringbone‐like pattern is formed in this space by the cross sections of the winding tubes. The newly developed method estimates evaporation performance of the developed heat exchanger using new empirical correlations. There are correlations for heat transfer and pressure drop in winding‐tube banks on the water side, and correlation for the pressure drop on the refrigerant side. © 2004 Wiley Periodicals, Inc. Heat Trans Asian Res, 33(4): 245–257, 2004; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20009     

Optimization of the angle of attack of delta-winglet vortex generators in a plate-fin-and-tube heat exchanger

Delta-winglet vortex generators (VGs) are known to enhance the heat transfer between the energy-carrying fluid and the heat transfer surfaces in plate-fin-and-tube banks. In this study optimal angles of attack of the delta-winglets are investigated based on the Pareto optimal strategy. The optimization process combines a CFD analysis, genetic algorithms and the response surface methodology. The angle of attack of a pair a delta-winglet-type VGs mounted behind each tube is varied between β = ?90° and +90°. Three circular tube rows with inline and staggered tube arrangements are investigated for Reynolds numbers from 200 to 1200 (based on the inlet height and inlet velocity). The flow structure and heat transfer behavior is analyzed in detail for certain cases and the staggered and the inline tube arrangements are compared. Finally, for each of these arrangements the optimal sets of angles of attack for different Reynolds numbers are presented.     

Numerical investigation of the vortex tube performance in novel precooling methods in the hydrogen fueling station

In the present study, the potential of integrating a Ranque-Hilsch vortex tube (RHVT) in the precooling process for refueling high-pressure hydrogen vehicles in hydrogen refueling stations is investigated. In this regard, two novel precooling processes integrating a vortex tube are proposed to significantly reduce the capital expenditure and operating costs in hydrogen fueling stations. Then a numerical study of the RHVT performance is carried out for a high-pressure hydrogen flow to validate the feasibility of the proposed processes. Obtained results from the numerical simulation show that the energy separation effect also exists in the RHVT with hydrogen flow at the pressure level of tens of megapascals. Moreover, it is found that the energy separation performance of the RHVT improves as the pressure ratio increases. In other words, the temperature drop of the cold exit of RHVT decreases as the pressure ratio decreases in the refueling process, which just matches the slowing-down temperature rise during the cylinder charge. Based on the obtained results, it is concluded that the integration of a RHVT into the precooling process has potential in the hydrogen fueling station.     

Two-dimensional thermal analysis of a polygonal fin with two tubes on a square pitch

The boundary element method (BEM) has been used to investigate the two-dimensional temperature distribution and the flow of heat from a polygonal fin with two tubes on a square pitch. This numerical method is shown to be convergent, stable and consistent. The resultant heat flows from the fin and the tubes are presented in the form of fin performance ratios. The values of the two-dimensional fin performance ratios are almost identical to those obtained for a single radial rectangular fin of equivalent surface area. The one-dimensional fin performance indicators, fin performance ratio or fin efficiency can be used to predict the heat flows. However, the two-dimensional temperature distributions have revealed the existence of conductive paths between the two tubes depending upon the fin dimensions, the values of the heat transfer and material thermal conductivity, and the magnitude of the temperature differences between the two tubes and the surrounding air.     

Impact of different internal convection control strategies in a non-evacuated CPC collector performance

Over the last decade the technological advances observed in solar collector materials, namely better spectrally selective absorber coatings and ultra clear glass covers, contribute to performance improvements and translate into higher operational temperature ranges with higher efficiency values.While the use of Evacuated Tube Collectors (ETCs) is becoming widespread in the thermal conversion of solar energy, non-evacuated solar collectors still hold advantages at manufacturing, reliability and/or cost levels, making them interesting and competitive for a large range of applications, in particularly, in temperature ranges up to 80 °C. However, these advantages have not prevented the major drawback of these collectors when compared to ETCs: thermal losses due to internal convection which prevent their general use in the range of operating temperatures up to 150 °C.Insulation, double glazing or selective coatings can be used in non-evacuated collectors to reduce heat losses. To prevent internal convection losses in these solar collectors, different control strategies have been studied, such as the adoption of different inert gases within the collector cavity, physical barriers reducing air flow velocities over the absorber or cover surfaces or the use of concentration.In the present article, an assessment of adopting such internal convection control strategies in a CPC collector is presented. Each of the presented strategies is assessed in terms of the resulting collector optical and thermal characterization parameters and yearly collector yield. For this purpose, an integrated tool allowing the design, optical and thermal characterization of CPC collectors was developed. The results obtained provide valuable guidelines for anyone wishing to implement any of these strategies in a new collector design.     

Numerical study of energy separation in a vortex tube with different RANS models

The aim of the present paper is to investigate numerically the energy separation mechanism and flow phenomena within a vortex tube. A 3D computational domain has been generated considering the quarter of the geometry and assuming periodicity in the Azimuthal direction which was found to exhibit correctly the general behaviour expected from a vortex tube. Air is selected as the working fluid. The flow predictions reported here are based upon four turbulence models, namely, the k–?, k–ω and SST k–ω two-equations models and the second moment closure model (RSM). The models results are compared to experimental data obtained from the literature. Four cases have been considered by changing the inlet pressure from 200 up to 380 kPa. It has been observed that all the above mentioned models are capable of predicting fairly well the general flow features but only the advanced RSM model is capable of matching correctly the measured cold and hot outlet temperatures. All the other models over predict the mean temperature difference by values up to twice the measured data.     

两种不同结构的光伏光热一体化系统的性能分析

文章对光伏光热一体化热泵(PV/T-HP)系统和光伏光热一体化热管(HP-PV/T)系统进行了对比实验,并分析了这两种PV/T系统的热水温度、光热转化效率、光电转化效率、光伏电池板温度、火用效率以及一次能源节约效率等参数。研究结果表明:PV/T-HP系统的光热转化效率、光电转化效率相对较高;通过火用效率分析可知,以节约用电或发电为主要目的时,宜采用HP-PV/T系统;通过一次能源节约效率分析可知,PV/T-HP系统节约化石能源的能力要优于HP-PV/T系统,并且以供热为主要目的且需热量较大时,适宜采用PV/T-HP系统。     

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.     

Effect of a delta-winglet vortex pair on the performance of a tube–fin heat exchanger

The experimental analysis of the effects of delta-winglet vortex generators on the performance of a fin and tube radiator is presented. The winglets were arranged in flow-up configuration, and placed directly upstream of the tube. This is a hitherto untested configuration, but is thought to have certain advantages. In addition to vortex generation the flow is guided onto the tube surface increasing the localised velocity gradients and Nusselt numbers in this region. The study includes dye visualisation and full scale heat transfer performance measurements. The results are compared to a standard louvre fin surface. It was found that the winglet surface had 87% of the heat transfer capacity but only 53% of the pressure drop of the louvre fin surface.     

水平管降膜蒸发器中不同形式的布液器对内部流场流动影响的数值模拟

应用Fluent两相流VOF模型,数值模拟水平管降膜蒸发器内部流场流动的过程。提出3种不同结构的布液器,分别对蒸发器内部流动特性进行了比较分析,得到气液两相图、速度云图、XY图等。模拟结果表明:中间开孔布液器使两部分蒸发管阵产生的蒸汽通过中间蒸汽排气口疏导出去,换热面大,使液体分配较均匀,更为合理,有利于蒸发器内部整体流场的优化设计。研究结果可为降膜蒸发器的优化设计提供理论依据。     

On the pressure drop and the velocity distribution in the cylindrical vortex chamber with two inlet pipes for the control of vortex flow

Vortex flow is applied to a cyclone dust collector, a vortex combustion chamber, and a vortex diode for vortex control. In order to apply the vortex flow to the industries, it is necessary to keep the stable flow condition and to estimate the response time of the transient flow process and also the intensity of the vortex flow. For control vortex flow, two types of vortex chamber with two inlet pipes were designed. One of them is to promote the vortex flow named as Co-Rotating Flow System and another one is to hinder the vortex flow named as Counter-Rotating Flow System. The pressure drops and the velocity distributions were measured for these vortex chambers. The estimation of the tangential velocity by the application of the angular momentum flux is compared with the measured velocity by a cylindrical Pitot-tube. The characteristics of the total pressure drop could be explained by introducing the circulation.     

Effect of number of tube rows on the air-side performance of crimped spiral fin-and-tube heat exchanger with a multipass parallel and counter cross-flow configuration

The air-side performance of crimped spiral fin and tube heat exchangers at high Reynolds number (3000–13,000) is investigated in this study. The test heat exchangers have a new type of multipass parallel and counter cross-flow water flow arrangement which is a combination of parallel cross-flow and counter cross-flow. The test samples are made from copper and aluminium with different number of tube rows (N_row = 2, 3, 4 and 5). The effects of number of tube rows and fin material on the heat transfer and friction characteristics are studied. The results show that no significant effect for either number of tube rows or fin materials on the heat transfer performance is found at high Reynolds number. In addition, the correlation of the air-side performances of this type of the heat exchangers at high Reynolds number is developed for industrial applications.     

Experimental study of condensation heat transfer of R-134a flow in corrugated tubes with different inclinations

This experimental study is performed to investigate condensation heat transfer coefficient of R-134a flow inside corrugated tube with different inclinations. Different inclinations of test condenser ranging from − 90^° to + 90^° and various flow mass velocities in the range of 87 to 253 [kg/m²s] are considered in this study. Data analysis showed that change in the tube inclination had a significant effect on condensation heat transfer behavior. At low mass velocities, and low vapor qualities, the highest condensation heat transfer coefficient was obtained for α = + 30^° which was 1.41 times greater than the least one obtained for α = − 90^°. The results also showed that at all mass velocities, the highest average heat transfer coefficients were achieved for α = + 30^°. Based on the experimental results, a new empirical correlation is proposed to predict the condensation heat transfer coefficient of R134a flow in corrugated tubes with different inclinations.     

Numerical study of the relationship between heat transfer enhancement and absolute vorticity flux along main flow direction in a channel formed by a flat tube bank fin with vortex generators

The secondary flow is frequently used to enhance the convective heat transfer. In this paper, the cross-averaged absolute vorticity flux in the main flow direction is used to specify the intensity of the secondary flow produced by vortex generators that are mounted on a three-row flat tube bank fin surfaces. The relationship between the intensity of the secondary flow and the strength of convective heat transfer is studied using a numerical method. The results reveal that cross-averaged absolute vorticity flux in the main flow direction can reflect the intensity of the secondary flow; a significant relationship between this cross-averaged absolute vorticity flux and span-averaged Nusselt number exists for the case studied. This cross-averaged absolute vorticity flux can account only for the secondary flow effects on convective heat transfer but cannot quantify the effects of developing boundary layer on convective heat transfer.     

A novel solar bi-ejector refrigeration system and the performance of the added injector with different structures and operation parameters

A novel solar bi-ejector refrigeration system was investigated, whose difference compared to the traditional system is that the circulation pump is replaced by a thermal injector. The new system works more stably and needs less maintenance work than the old one, and the whole system can more fully utilize the solar energy. The mathematical models for calculating the performance of the injector and the whole solar refrigeration system were established. The pressure rise performance of injector under different structure and operation parameters and the performance of solar bi-ejector refrigeration system were studied with R123. The results show that the discharged pressure of injector is affected by structure dimensions of injector and operation conditions. With increasing generation temperature, the entrainment ratio of ejector becomes better while that of injector becomes worse and the overall thermal efficiency of the solar bi-ejector refrigeration system first increases and then decreases with an optimum value of 0.132 at generation temperature of 105 °C, condensation temperature of 35 °C and evaporation temperature of 10 °C.