Experimental investigations on overall cooling effect of ribbed channel with air bleeds

An experimental investigation on overall heat transfer performance of a rectangular channel, in which one wall has periodically placed oblique ribs to enhance heat exchange and cylindrical film holes to bleed cooling air, has been carried out in a hot wind tunnel at different mainstream temperatures, hot mainstream Reynolds numbers, coolant Reynolds numbers and blowing ratios. To describe the cooling effect of combined external coolant film with the internal heat convection enhanced by the ribs, the overall cooling effectiveness at the surface exposed in the mainstream with high temperature was calculated by the surface temperatures measured with an infrared thermal imaging system. The total mass flow rate of cooling air through the coolant channel was regulated by a digital mass flow rate controller, and the blowing ratio passing through the total film holes was calculated based on the measurements of another digital-type mass flow meter. The detailed distributions of overall cooling effectiveness show distinctive peaks in heat transfer levels near the film holes, remarkable inner convective heat transfer effect over entire channel surface, and visible conductive heat transfer effect through the channel wall; but only when the coolant Reynolds number is large enough, the oblique rib effect can be detected from the overall cooling effectiveness; and the oblique bleeding hole effect shows the more obvious trend with increasing blowing ratios. Based on the experimental data, the overall cooling effectiveness is correlated as the functions of Re_m (Reynolds number of hot mainstream) and Re_c (Reynolds number of internal coolant flow at entrance) for the parametric conditions examined.     

具有新型双层壁结构的透平叶片流热耦合研究

多通道壁面射流冷却结构是一种新型的燃气透平动叶内部冷却结构,具有消耗冷气少、压力损失小等优点。本文构建了简化的壁面射流冷却叶片与GE-E3冷却结构叶片模型,采用流热耦合方法对比研究了其流动与换热特性。结果表明,壁面射流冷却通道内的狭小空间抑制了横流的产生,冷气在冷却通道中形成了流向涡;前缘冷气流道中的大量冷气流经吸力侧冷却区,并从出口压力更小、面积更大的尾缘排出,使得前缘气膜孔出流的冷气流量和动量较小,冷气在叶片外表面的气膜覆盖特性更好;离心力的影响导致前缘冷气流道中叶根处的压力较低,叶根附近的气膜孔出现燃气主流入侵现象。相比于GE-E3叶片,壁面射流冷却叶片的前缘温度和温度梯度都较小,因此多通道壁面射流冷却在前缘具有更优异的冷却特性。     

Effects of Non-Darcian and Inlet Conditions on the Forced Convection Along a Vertical Plate With Film Evaporation

The present study analyzes theoretically the non-Darcian effects and inlet conditions of forced convection flow with liquid film evaporation in a porous medium. The physical scheme includes a liquid–air streams combined system; the liquid film falls down along the plate and is exposed to a cocurrent forced moist air stream. The axial momentum, energy, and concentration equations for the air and water flows are developed based on the steady two-dimensional (2-D) laminar boundary layer model. The non-Darcian convective, boundary, and inertia effects are considered to describe the momentum characteristics of a porous medium. The paper clearly describes the temperature and mass concentration variations at the liquid–air interface and provides the heat and mass transfer distributions along the heated plate. Then, the paper further evaluates the non-Darcian effects and inlet conditions on the heat transfer and evaporating rate of liquid film evaporation. The numerical results show that latent heat transfer plays the dominant heat transfer role. Carrying out a parametric analysis indicates that higher air Reynolds number, higher wetted wall temperature, and lower moist air relative humidity will produce a better evaporating rate and heat transfer rate. In addition, a non-Darcy model should be adopted in the present study. The maximum error for predictions of heat and mass transfer performance will be 21% when the Darcy model is used.     

Thermal protection with liquid film in turbulent mixed convection channel flows

In this numerical study, a channel flow of turbulent mixed convection of heat and mass transfer with film evaporation has been conducted. The turbulent hot air flows downward of the vertical channel and is cooled by the laminar liquid film on both sides of the channel with thermally insulated walls. The effect of gas–liquid phase coupling, variable thermophysical properties and film vaporization are considered in the analysis. In the air stream, the k–ε turbulent model has been utilized to formulate the turbulent flow. Parameters used in this study are the mass flow rate of the liquid film B, Reynolds number Re, and the free stream temperature of the hot air T_o. Results show that the heat flux was dramatically increases due to the evaporation of liquid water film. The heat transfer increases as the mass flow rate of the liquid film decreases, while the Reynolds number and inlet temperature increase, and the influences of the Re and T_o are more significant than that of the liquid flow rate. It is also found that liquid film helps lowering the heat and mass transfer rate from the hot gas in the turbulent channel, especially at the downstream.     

Mixed convection heat transfer enhancement through film evaporation in inclined square ducts

The investigation of mixed convection heat transfer enhancement through film evaporation in inclined square ducts has been numerically examined in detail. The main parameters discussed in this work include the inclined angle, the wetted wall temperature and the relative humidity of the moist air mixture. The numerical results of the local friction factor, Nusselt number and Sherwood number are presented for moist air mixture system. Attention was particular paid to the effects of latent heat transport on the heat transfer enhancement. Results show that the latent heat transport with film evaporation augments tremendously the heat transfer rate. The heat transfer rate can be enhanced to be 10 times of that without mass transfer, especially for a system with a lower temperature. Besides, better heat and mass transfer rates related with film evaporation are found for case with a higher wetted wall temperature. The increase in the relative humidity of moist air in the ambient causes the decrease in heat transfer enhancement.     

几种带冷却结构的双层管换热及流动对比

对相同质量流量下的光管、双层光管、带冷却结构（肋、扰流柱、凹坑、螺旋通道）的双层管等不同结构的管流动进行了流固耦合三维数值模拟,获取了固体壁温的分布特征;对各结构下,外层壁冷热侧温差、冷气温升、流动特性及综合换热效率进行了研究分析。研究结果表明：相同质量流量下,带螺旋通道双层管的外层壁冷热侧温差最小、综合换热效率最高;凹坑结构双层管与双层光管的流动及换热特性相似,流阻较小但换热效果也较差;扰流柱和肋结构双层管的流动换热特性相近,其温度分布均匀性、换热量介于双层光管和螺旋通道双层管之间,其流阻大且综合换热效率低。     

Falling film evaporation of ternary mixture under three different modes of heating

This numerical study deals with heat and mass transfer by evaporation under mixed convection in three different configurations of a ternary liquid film in a vertical channel. The ternary liquid mixture water-methanol-benzene falls along the right plate of channel while the other plate is kept thermally insulated. In the first configuration, a heat flux density is applied to the wall carrying the trickle film, while in the other configurations this same amount of heat is used to preheat the liquid film or the air at the inlet of the channel. The implicit finite difference scheme is used to solve the system of equations in both liquid and vapor phases. According to this study, it was observed that the evaporation efficiency is high when the mass fraction of volatile components is high or in the preheating state of ternary film.     

Thermodynamic study of wet cooling tower performance

An analytical model was developed to describe thermodynamically the water evaporation process inside a counter‐flow wet cooling tower, where the air stream is in direct contact with the falling water, based on the implementation of the energy and mass balance between air and water stream describing thus, the rate of change of air temperature, humidity ratio, water temperature and evaporated water mass along tower height. The reliability of model predictions was ensured by comparisons made with pertinent experimental data, which were obtained from the literature. The paper elaborated the effect of atmospheric conditions, water mass flow rate and water inlet temperature on the variation of the thermodynamic properties of moist air inside the cooling tower and on its thermal performance characteristics. The analysis of the theoretical results revealed that the thermal performance of the cooling tower is sensitive to the degree of saturation of inlet air. Hence, the cooling capacity of the cooling tower increases with decreasing inlet air wet bulb temperature whereas the overall water temperature fall is curtailed with increasing water to air mass ratio. The change of inlet water temperature does not affect seriously the thermal behaviour of the cooling tower. Copyright © 2005 John Wiley & Sons, Ltd.     

Modelling of a direct evaporative air cooler using artificial neural network

This paper proposes the use of artificial neural networks (ANNs) to predict various performance parameters of a direct evaporative air cooler. For this aim, an experimental evaporative cooler was operated at steady‐state conditions, while varying the dry bulb temperature and relative humidity of the entering air along with the flow rates of air and water streams. Using some of the experimental data for training, a three‐layer feed‐forward ANN model based on back propagation algorithm was developed. This model was used for predicting various performance parameters of the cooler, namely the dry bulb temperature and relative humidity of the leaving air, mass flow rate of the water evaporated into the air stream, sensible cooling rate, and effectiveness of the cooler. Then, the performance of the ANN predictions was tested by applying a set of new experimental data. The predictions usually agreed well with the experimental values with correlation coefficients in the range of 0.969–0.993, mean relative errors in the range of 0.66–4.04%, and very low root mean square errors. This study reveals that, as an alternative to classical modelling techniques, the ANN approach can be used successfully for predicting the performance of direct evaporative air coolers. Copyright © 2007 John Wiley & Sons, Ltd.     

Novel design and modelling of an evaporative cooling system for buildings

Passive evaporative cooling has great potential as an alternative to conventional air‐conditioning in arid hot climates because of its low cost and zero pollution. This paper describes a novel evaporative cooling system with an automatic wind‐tracking device to improve its operating efficiency. The design and operating principles are discussed. A mathematical model is simplified by the assumption of convective heat and mass transfer of staggered streamlets of water. A computer program has been developed to calculate the deflection and length of spray water streamlets, as well as evaporative water mass, minimum cooled water temperature and required cooling time. A typical example illustrates that approximately 20 kg water are evaporated and around 26 min are required for 980 kg of water to be cooled from 28°C to the wet bulb temperature of 19.2°C of ambient air in a typical arid hot climate (relative humidity = 0.30, dry bulb temperature = 32°C and wind velocity = 4 m s^?1). The application of adsorbents, would allow the evaporative cooling system to be applied in hot, humid climates, in addition to hot climates with low humidity. Copyright © 2006 John Wiley & Sons, Ltd.     

Experimental Study on Effectiveness of Celdek Packed Liquid Desiccant Cooling System

Dehumidifier and regenerator are the most important components in a liquid desiccant cooling system. Present paper is focused on study the effect of inlet process parameters on the effectiveness of dehumidifier and regenerator of liquid desiccant cooling system. Experimental study is performed with varying inlet process parameters; mass flow rate of air, desiccant solution flow rate, inlet air temperature, inlet solution temperature, inlet specific humidity and concentration of desiccant solution. Celdek structured pads as packing material and calcium chloride as liquid desiccant is investigated first time using counter flow of the desiccant solution and air. It is concluded from the results that the effectiveness of dehumidifier increases with solution flow rate, inlet specific humidity while decreases with increasing mass flow rate of air, inlet temperature of air and desiccant, temperature and concentration of desiccant solution. The effectiveness of regenerator increases with increasing solution flow rate and inlet desiccant concentration and it decreases with increasing inlet air temperature, air flow rate and inlet solution temperature. Present paper adds to effect of inlet specific humidity, inlet temperature of the air and solution on the effectiveness of desiccant cooling system on the past research.     

Longitudinal heat conduction study of a uniform power density apparatus for the experimental determination of the heat transfer coefficient

A combined experimental and numerical analysis was performed to accurately evaluate the effect of longitudinal heat conduction along an aluminium test section on the local heat transfer coefficient (HTC) experimental determination. The test section contains a rectangular channel with fins typical of compact heat exchangers. The hydraulic diameter of the channel is around 2 mm. The fluid stream is heated by flat electrical resistances clamped to the outer walls of the test section. The test section contains four zones with different fin pads each, in order to be able to generate data for several geometries at the same time at exactly the same mass flow rate.It was found that longitudinal conduction can lead to a quite uneven distribution of the heat flux to the fluid at the beginning and end of each zone of the test section. Three phenomena were found to be the cause for the observed uneven heat distribution, i.e. the longitudinal gradient of temperature along the wall induced by the fluid temperature rise, the presence of heated and non-heated zones, and the stepwise non-uniformity of the HTC along the test section induced where the fin pad changes.The study has shown that the elementary data reduction methodology, assuming that the surface power density generated by the electrical resistance and the heat flux density reaching the fluid are equal and uniform along the channel, is too far from the reality at the beginning and end of each zone of the test section, especially at low mass flow rates.     

Diurnal response of an open roof solar regenerator system for absorption air-conditioning

This paper presents a time-dependent heat and mass transfer analysis of an open roof surface as a solar collector-regenerator system for absorption air-conditioning. The system consists of water evaporation from a lithium chloride solution (LiClH₂O) flowing on the roof of a building. The analysis takes into account the variation of the solution temperature-concentration and hence the water evaporation from the absorbent solution along the flow length of the regenerator and the periodic variation of the solar intensity and the ambient air temperature. The effects of operational parameters, viz. solution flow rate, regenerator length, humidity ratio and the inlet solution conditions, on the time dependence of the water evaporation have been investigated. It is ascertained that about 2.5–4.0 kg of water can be evaporated per unit solar regenerator area per day under typical operating conditions, and for every kg of water evaporated in the regenerator, 1 kg of water can be evaporated in the evaporator of the absorption cooling system. The overall average daily COP of the cooling system is found to be in the range 0.36–0.57 for a typical hot and dry climate, and hence, the system is more attractive for solar air-conditioning.     

Phase change in the cathode side of a proton exchange membrane fuel cell

A three-dimensional steady state two-phase non-isothermal model which highly couples the water and thermal management has been developed to numerically investigate the spatial distribution of the interfacial mass transfer phase-change rate in the cathode side of a proton exchange membrane fuel cell (PEMFC). A non-equilibrium evaporation-condensation phase change rate was incorporated in the model which allowed supersaturation and undersaturation take place. The most significant effects of phase-change rate on liquid saturation and temperature distributions are highlighted. A parametric study was also carried out to investigate the effects of operating conditions; namely as the channel inlet humidity, cell operating temperature, and inlet mass flow rate on the phase-change rate. It was also found that liquid phase assumption for produced water in the cathode catalyst layer (CL) changed the local distribution of phase-change rate. The maximum evaporation rate zone (above the channel near the CL) coincided with the maximum temperature zone and resulted in lowering the liquid saturation level. Furthermore, reduction of the channel inlet humidity and an increase of the operation temperature and inlet mass flow rate increased the evaporation rate and allowed for dehydration process of the gas diffusion layer (GDL) to take place faster.     

Laminar mixed convection heat and mass transfer in vertical rectangular ducts with film evaporation and condensation

The investigation of mixed convection heat and mass transfer in vertical ducts with film evaporation and condensation has been numerically examined in detail. This work is primarily focused on the effect of film evaporation and condensation along the wetted wall with constant temperature and concentration on the heat and mass transfer in rectangular vertical ducts. The numerical results, including the distributions of dimensionless axial velocity, temperature and concentration distributions, Nusselt number as well as Sherwood number are presented for moist air mixture system with different wall temperatures and aspect ratios of the rectangular ducts. The results show that the latent heat transport with film evaporation and condensation augments tremendously the heat transfer rate. Better heat transfer enhancement related with film evaporation is found for a system with a higher wall temperature.     

Enhancement in Evaporative Effectiveness of an Evaporative Tubular Heat Dissipator Using Experimental Design Approach

An experimental investigation of evaporative effectiveness and mass transfer coefficient on a bundle of tubes of an evaporative tubular heat dissipator is presented. Based on the experiments, correlations of evaporative effectiveness and mass transfer coefficient are derived using multiple regression analysis. A statistical model is developed to correlate the operating variables using design of experiment approach by selecting central composite design of a response surface methodology. Results shown in this article indicate that as the cooling film flow rate increases, evaporative effectiveness and mass transfer coefficient increases provided that the air flow rate is constant which is flowing from underneath the tubes of the evaporative tubular heat dissipator. Derived correlations are helpful in improvement of the design of heat transfer devices and many other engineering applications. Consideration of relative humidity of upstreaming air as one of the operating variables leads to the contribution to heat and mass transfer study of evaporative tubular heat dissipators in the present investigation.     

Heating and cooling of a building by double hollow concrete slab

This paper presents the thermal performance, in the heating and cooling of a building, of a double hollow concrete slab, one of whose faces is exposed to solar radiation and ambient air while the other is in contact with room air at constant temperature. A blackwened network of pipes is laid on the top surface and glazed sutiably. the flow rate of water / air through pipes is kept constant. It is seen that there is a time difference of 10-12 h between the maximum/ minimum of the thermal flux extering the room and the solair temperature for any flow rate. the heat flux inside the room is reduced appreciably for higher infiltration when there is no water flow to heat the building. the effect of a water film on the performance of the wall/roof has also been discussed and found to be more effective for the reduction of the heat flux coming into the building.     

逆流开式发生器内部的热、质传递规律

针对以太阳能加热的空气为携热介质，以LiBr溶液为工作介质的填料塔型开式发生器，建立热质传递数学模型。对2种系统结构形式进行比较，并分别研究太阳能集热板温度、液气比、环境相对湿度以及填料层高度对溶液再生的影响，以揭示此类发生器内热、质传递的规律，为产品开发、设计提供理论帮助。     

Experimental study of a direct evaporative cooling approach for Li-ion battery thermal management

A desirable operating temperature range and small temperature gradient is beneficial to the safety and longevity of lithium-ion (Li-ion) batteries, and battery thermal management systems (BTMSs) play a critical role in achieving the temperature control. Having the advantages of direct access and low viscosity, air is widely used as a cooling medium in BTMSs. In this paper, an air-based BTMS is modified by integrating a direct evaporative cooling (DEC) system, which helps reduce the inlet air temperature for enhanced heat dissipation. Experiments are carried out on 18650-type batteries and a 9-cell battery pack to study how relative humidity and air flow rate affect the DEC system. The maximum temperatures, temperature differences, and capacity fading of batteries are compared between three cooling conditions, which include the proposed DEC, air cooling, and natural convection cooling. In addition, a DEC tunnel that can produce reciprocating air flow is assembled to further reduce the maximum temperature and temperature difference inside the battery pack. It is demonstrated that the proposed DEC system can expand the usage of Li-ion batteries in more adverse and intensive operating conditions.     

非能动安全壳竖直平板降膜流动特性数值模拟

核电安全日益受到关注,非能动系统作为第三代核电系统具有很高的安全性。采用FLUENT流体体积分数(volume of fraction,VOF)模型和k-ε湍流模型对非能动安全壳冷却系统(passive containment cooling system,PCCS)三维平板降膜流动进行数值模拟。结果表明:1)在降膜过程中有波动现象,最终波动趋于平缓;2)水与空气逆流流动过程中发生轻微的液滴夹带;3)降膜流动受重力、表面张力与壁面黏滞力共同作用,液膜厚度沿横向分布均匀,沿高度方向平均液膜厚度越来越小,并且受进口水流速度与入口宽度影响,水流量一定时增加进口水流速度与入口宽度,平均液膜厚度增大,空气入口流速对水膜厚度影响相对较小。