被动蒸发冷却式双层皮玻璃幕墙传热过程分析

本文于夏秋两季,在广州这样夏热冬暖、隔热为主的低纬度地区进行了被动蒸发冷却式双层皮玻璃幕墙相关系列实验.研究不同朝向被动蒸发冷却式双层皮玻璃幕墙试验房热通道冷却与对比房热通道无冷却的防热比较.实验旨在通过试验房和对比房在相同喷水流量下不同朝向的防热效果对比,测试空腔内冷却对双层皮玻璃幕墙防热性能的影响,检验低纬度夏热冬暖地区双层皮玻璃幕墙试验房和对比房在不同朝向对环境的热适应性.然后通过研究其传热过程,对双层皮玻璃幕墙热工性能进行分析,计算被动蒸发式双层皮幕墙节约的能量和隔热效率.分析表明,该新型双层皮玻璃幕墙在夏秋的隔热效果比普通的双层皮幕墙效果更佳.     

Evaporative Cooling and Heat Transfer Augmentation Related to Reduced Condenser Temperatures

In recent articles evaporative cooling has proved to be the utmost air-side heat transfer augmentation. This kind of augmentation, when applied to steam condensers in power plants or to refrigerants in air conditioner systems, results in a lowering of the condensing temperature, even below ambient levels. This yields a remarkable increase of thermodynamic efficiencies of both processes and, therefore, a reduction of energy consumption. The combined heat and mass transfer processes in an evaporatively cooled device are very complex, due to the vast numbers of parameters which must be considered. Previous studies have assumed it impossible to perform any analysis unless many simplifying assumptions are made. The present article indicates that analysis can be performed where none of the assumptions originally used by Merkel are made. It is demonstrated by experiments that plate-fin tube and bare tube condenser performances can be predicted reliably. The insensitivity of the analysis to geometric design eliminates the need for extensive model tests and for production of correction and design factors for the design of evaporative condensers.     

Heat Transfer Enhancement of Spray Cooling on Flat Aluminum Tube Heat Exchanger

This study provides an experimental analysis on the heat transfer performance of a flat aluminum tube microchannel heat exchanger with spray cooling. The effects of water spraying rate, airflow rate, and relative humidity were investigated. The test results show that the heat transfer performance increased with increasing the water spraying rate but without the penalty of increased flow resistance at low spray conditions. This effect is further enhanced by increasing the water spraying rate. However, when the spraying rate is high, part of the nonevaporated drops attached to the fin surface and formed a liquid film, which caused the flow passage to become narrower. Further increase in the spraying rate resulted in part of the flow passages being blocked by the nonevaporated water drops and caused a region of poor heat transfer. The friction coefficient jumped drastically at this condition. This phenomenon deceased gradually with increasing airflow rate. High inlet air humidity resulted in the water accumulation phenomenon appearing at lower water spraying rates. The evaporative cooling effect decreased and flow friction increased. The test results just described show that the water spray is able to significantly improve the air-side heat transfer performance. The optimum spray rate for each airflow rate must be carefully determined.     

Boiling Heat Transfer Enhancement Using Micro-Machined Porous Channels for Electronics Cooling

Boiling heat transfer enhancement for a passive electronics cooling design is presented in this paper. A novel pool boiling enhancement technique is developed and characterized. A combination of surface modification by metallic coating and micro-machined porous channels attached to the modified surface is tested and reported. An experimental rig is set up using a standard BGA package with 12 mm × 12 mm thermal die as a test surface. The limiting heat flux for a horizontally oriented silicon chip with fluorocarbon liquid FC-72 is typically around 15 W/cm². Boiling heat transfer with the designed enhancement techniques is investigated, and the factors influencing the enhancement are analyzed. The metallic coated surface at 10°C wall superheat has a heat flux six times larger than an untreated chip surface. Micro-machined porous channels with different pore sizes and pitches are tested in combination with the metallic coated surface. The boiling heat flux is seven times larger at lower wall superheat compared to the plain chip surface. Maximum critical heat flux (CHF) of 38 W/cm² is obtained with 0.3 mm pore diameter and 1 mm pore pitch. A ratio of pore diameter and pore pitch is found to correlate well with the heat transfer enhancement obtained by experiments. Structures with smaller pore diameter to pitch ratio and larger pore opening are found to have higher heat transfer enhancement in the tested combination.     

发动机冷却水腔内沸腾传热的模拟研究

从单相流观点出发研究了两种计算过冷流动沸腾传热的思路:分区描述法和叠加计算法.提出了两个基于分区描述法的沸腾模型A和沸腾模型B;修正了基于叠加计算法的Chen沸腾模型和BDL沸腾模型中对流传热项的计算方法.利用这些沸腾模型进行了缸盖鼻梁区冷却水腔沸腾传热的数值模拟,并与试验结果进行了对比分析.结果表明:采用分区描述法和叠加计算法进行发动机冷却水腔内过冷流动沸腾传热计算均是可行且有效的方法;采用沸腾模型A和修正的BDL模型的预测精度比另两个沸腾模型要高;提高流速和过冷度均能强化沸腾传热的能力,提高压力后则在更高的壁面温度下才出现沸腾传热.     

一种多孔介质太阳能吸热器传热研究

为了研究塔式太阳能多孔介质吸热器的传热传质特性,建立吸热器稳态传热模型,选择适合多孔介质太阳能吸热器的体积对流换热系数模型,采用数值方法求解,并分别分析孔隙密度、孔隙率和入口空气速度对温度场的影响。文中技术可以为同类型太阳能吸热器的设计和改造提供参考。     

考虑冷却水管对钢坯传热影响的步进梁式连续加热炉数学模型

在充分考虑了冷却水管对钢坯传热影响的情况下建立了步进梁式连续加热炉数学模型,并利用"黑匣子实验"对所建立的数学模型进行了实测验证。在验证数学模型正确可信的基础上分析了冷却水管对钢坯传热过程和钢坯温度场的影响。所做工作在一定程度上揭示了"水管黑印"产生的规律和特点,对进一步提高钢坯加热质量有重要的参考价值。     

Experimental Assessment of Heat Transfer Correlations for a Small-Scale Evaporative Condenser

This work presents an experimental analysis of a small-scale evaporative condenser, performed on a calorimetric test facility, where heat transfer coefficients are measured and compared to some well-known correlations from the literature. The external flow of air and spray water ranged from 2.2 kg/min to 8.7 kg/min and 4.8 kg/min to 15.0 kg/min, respectively, keeping an average air to water ratio of approximately 1:2, followed by condensation temperatures ranging from 26 to 36°C. The flow pattern map is first determined, followed by the identification of the transition regions based on the void fraction concept. The overall heat transfer coefficient for the condensation zone calculated after the experimental data acquired in the present research was compared to six literature correlations, and the one developed by Tovaras, Bykov, and Gogolin in 1984 provided the better agreement. Local and mean values of the refrigerant heat transfer coefficients did not vary significantly for both single-phase superheated vapor and subcooling liquid. Results are still particular to the evaporative condenser assessed in the present work, and full-scale analysis must be performed in order to build more general correlations.     

Heat Transfer Enhancement in Shell-and-Tube Heat Exchangers Using Porous Media

A numerical simulation has been carried out to investigate the heat transfer enhancement in a shell-and-tube heat exchanger using a porous medium inside its shell and tubes, separately. A three-dimensional geometry with k-? turbulent model is used to predict the heat transfer and pressure drop characteristics of the flow. The effects of porosity and dimensions of these media on the heat exchanger's thermal performance and pressure drop are analyzed. Inside the shell, the entire tube bundle is wrapped by the porous medium, whereas inside the tubes the porous media are located in two different ways: (1) at the center of the tubes, and (2) attached to the inner wall of the tubes. The results showed that this method can improve the heat transfer at the expense of higher pressure drop. Evaluating the method showed that using porous media inside the shell, with particular dimension and porosity can increase the heat transfer rate better than pressure drop. Using this method inside the tubes leads to two diverse results: In the first configuration, pressure loss prevails over the heat transfer augmentation and it causes energy loss, whereas in the second configuration a great performance enhancement is observed.     

板式间接蒸发冷却换热器的层流特性研究

间接蒸发冷却换热器与传统空调相比具有环保与节能的优点。由于蒸发冷却过程的传热传质机理复杂,本文针对板式间接蒸发冷却换热器,建立了三维稳态传热传质数学模型,并确定了合适的边界条件。通过数值模拟得到了压力场、温度场和浓度场的分布,并讨论了通道间距、速度、温度以及相对湿度等因素对换热效果的影响,为间接蒸发冷却换热器的设计提供了理论指导。     

Improved Prediction of Shell Side Heat Transfer in Horizontal Evaporative Shell and Tube Heat Exchangers

This paper presents an improved prediction method for the heat transfer and pressure drop in the shell side of a horizontal shell and tube evaporator. The results from an experimental test program are used in which a wide range of evaporating two-phase shell side flow data was collected from a TEMA E-shell evaporator. The data are compared with shell side heat transfer coefficient and pressure drop models for homogeneous and stratified flow. The comparison suggests a deterioration in the heat transfer data at low mass fluxes consistent with a transition from homogeneous to stratified flow. The pressure drop data suggest a stratified flow across the full test range. A new model is presented that suggests the transition in the heat transfer data may be due to the extent of tube wetting in the upper tube bundle. The new model, which also takes into account the orientation of the shell side baffles, provides a vast improvement on the predictions of a homogenous type model. The new model would enable designers of shell side evaporators/reboilers to avoid operating conditions where poor heat transfer could be expected, and it would also enable changes in process conditions to be assessed for their implications on likely heat transfer performance.     

Optimization of Water Consumption in Hybrid Evaporative Cooling Air Conditioning Systems for Data Center Cooling Applications

In the light of its potential to reduce energy consumption when coupled to the condenser of existing vapor compression systems, evaporative cooling technology still faces questions relating to its water consumption pattern and the effect of water quality on its performance. This paper presents an experimental investigation of the water consumption characteristics in a split air-conditioning system employing evaporative precooling of air flowing over the condenser coils. The effect of the cooling pad thickness on the coefficient of performance of the system is investigated by varying the pad thickness from 5 to 15 cm in step size of 5 cm. Condition of the exit air from the pad shows that evaporative cooling can be employed as a standalone method for cooling of data centers, with adequate humidity control systems in place. Results obtained on water consumption shows that the volume of water consumed does not increase proportionally with time, rather it increases by a factor of 1.8, 2.5, and 3.2, for 2, 3, and 4 hours of operation respectively, relative to 1hour of operation. Two parameters; salinity and turbidity were used to study the effect of water quality on the evaporative cooling process. Additionally, a method of optimizing the pump operation is introduced in this paper. Running the pump intermittently could cut the energy consumed by the water pump in such a hybrid system by up to 20%.     

Channel Wall Cooling by Evaporative Falling Water–Ethanol and Water–Methanol Films

Abstract

The thermal protection by evaporative cooling is among the most efficient cooling methods, which provides higher wall temperature reduction owing to the phase conversion of the heat-transfer fluid. Thus, the purpose of this study is to investigate the capacity of pure water, water–ethanol, and water–methanol liquid mixtures to thermally protect a channel wall. To achieve the goals, an implicit finite difference in house code is developed and exploited to solve the coupled governing equations in both liquid and gas phases. The influence of water mass fraction in binary mixture is studied. The results revealed the convenient liquid and water mass fraction to have low wall temperature. It was found that both latent and sensible heat are important in this process. In addition, the presence of alcohols in the mixture with high concentration ensures a better cooling and thermal protection of the wall. A maximum wall temperature drop of 12?K was achieved by pure methanol liquid film versus an elevation of 6?K by pure water film. Moreover, it is highlighted that a pure ethanol film can be avoided by using only 25% of methanol with water, which permitted keeping approximately the same temperature drop.     

多孔表面管沸腾传热试验研究

针对烧制成多孔表面管,进行了传热性能研究,试验表明：多孔管可以显著地强化多孔侧沸腾传热,民同规格光滑管传热性能试验对比,其沸腾给热系数比光滑管提高５－６倍。     

多孔介质燃烧_换热器内燃烧和传热的数值模拟

通过建立二维数值模型研究了多孔介质燃烧-换热器内的燃烧和传热。研究系统配置对燃烧-换热器热效率和压力降的影响。结果表明,换热管的纵向距离对燃烧器内温度分布、传热速率和压力损失有显著的影响。减小换热管纵向距离,热效率和压力损失增大,而换热管的水平距离对热效率和压力损失的影响很小。另外,增大小球直径导致热效率增大和压力损失的急剧减小。数值模型的有效性通过实验进行验证。     

粘土多孔砖瞬态传热数值分析

利用自然对流和热传导的物理耦合模型,控制方程采用有限容积法,求解算法使用SIMPLE算法,研究了粘土多孔砖在周期性条件下的瞬态传热规律.通过计算得到粘土多孔砖壁温特性,同时也研究了通过粘土多孔砖的热流变化规律,并与相同尺寸实心砖的传热过程比较.发现实心砖和粘土多孔砖的内壁面温度的延迟相差不大.而粘土多孔砖与实心砖平均热流相差很大.实心砖的平均热流约比粘土多孔砖平均热流大64%,说明粘土多孔砖节能效果显著.     

旋转对涡轮工作叶片内冷通道换热影响的研究

为了获得涡轮工作叶片内冷通道的换热特性,采用非结构化网格及standard k-ε紊流模型,求解三维N-S方程,对带60°肋和气膜孔出流的旋转矩形通道内的三维流场进行了数值模拟,近壁面采用增强壁面函数处理.气动参数为:通道入口雷诺数60000,罗斯贝数0.22,气膜孔总出流比0.22.重点分析了静止通道和不同旋转方向的旋转通道的换热系数分布.计算结果表明,静止通道由于肋的存在使换热显著增强,不同的旋转方向对换热强化的效果不同,顺时针旋转时的增强效果要高于逆时针旋转时的增强效果.     

The Effect of Conjugate Heat Transfer on Film Cooling Effectiveness

The film cooling effectiveness of a two-dimensional gas turbine endwall is compared for the cases of conjugate heat transfer and an adiabatic wall condition using five common turbulence models. The turbulence models employed in this study are: the RNG k–ε model, the realizable k–ε model, the standard k–ω model, the SST k–ω model, and the RSM model. The computed flow field and surface temperature profiles along with the film effectiveness for one and two cooling slots at different injection angles are presented. The results show the strong effect of conjugate heat transfer on the film effectiveness compared to the adiabatic case and also compared to the effectiveness values obtained from analytically solvable models.     

Forced Convection Heat Transfer in Porous Structure: Effect of Morphology on Pressure Drop and Heat Transfer Coefficient

A light-weight structure with sufficient mechanical strength and heat transfer performance is increasingly required for some thermal management issues. The porous structure with the skeleton supporting the ambient stress and the pores holding the flowing fluid is considered very promising, attracting significant scientific and industrial interest over the past few decades. However, due to complicated morphology of the porous matrices and thereby various performance of the pressure drop and heat transfer coefficients(HTC), the comprehensive comparison and evaluation between different structures are largely unclear. In this work, recent researches on the efforts of forced convection heat transfer in light-weight porous structure are reviewed; special interest is placed in the open-cell foam, lattice-frame, structured packed bed, and wire-woven structures. Their experimental apparatus, morphological of the porous structures, effect of morphology on pressure drop and HTC, and further applications are discussed. The new method which measure morphology accurately should be paid more attention to develop more accuracy correlation. Also, the most research focused on low Reynolds number and existing structure, while very few researchers investigated the property of forced convection heat transfer in high velocity region and developed new porous structure.     

多孔介质气体润滑轴承气热耦合研究

考虑气体温度梯度对工质物性及轴承热稳定性的影响,以及润滑气体和多孔介质的传热耦合,在满足Darcy定律条件下三维求解雷诺方程,计算了轴承间隙和多孔介质中气体的压力及温度分布。分析了3个给定供给压力(0.5、0.65和0.8 MPa)、3种轴承偏心率(0.2、0.5、0.8)对轴承的承载能力、偏位角及润滑流量的影响,计算结果表明：在不同的轴偏心率下均存在有一段轴承最佳渗透系数范围,表征该范围承载能力最强;偏心率越大,轴承承载能力越高,最佳渗透系数区间也越明显;轴承压缩数越大承载能力也越高;间隙中温度分布不均匀导致轴承承载能力降低,通过调整压力和偏心率增加介质的润滑流量,及时导出摩擦产生热量降低间隙中气膜温度,可使轴承平衡稳定运行。