叉排管束与M-W引流丝网交替吸收器性能研究

叉排管束与M-W引流丝网交替吸收器是一种将热质耦合传递与绝热传质相结合而开发的新型吸收器。该文基于VOF方法建立溴化锂吸收器中溶液降膜吸收水蒸汽过程的CFD模型,展示在叉排管束与M-W引流丝网交替吸收器中和对比方案的光管束吸收器上溶液的流动状态以及温度和浓度分布云图;给出不同溶液进口喷淋密度和溶液浓度对2种吸收器传热传质性能的影响的模拟和实验结果,表明在计算范围内,叉排管束与M-W引流丝网交替吸收器的传热系数和传质系数的平均值可比光管束吸收器分别提高33.4%和55.4%。     

水平管降膜蒸发器传热传质实验研究

水平管降膜蒸发器因传质传热系数高而被广泛应用于淡化水处理中。搭建了水平管降膜蒸发传热实验台,通过对实验结果的归纳,得到了水平管降膜蒸发器的蒸发量随喷淋密度、蒸发温度、热通量的变化规律及热量利用率随蒸发温度的变化规律。结果表明,热通量范围不同时,蒸发量随喷淋密度的变化规律不同;蒸发量随热通量的增大而增大,随蒸发温度的增大而增大;热量利用率随蒸发温度的增大而增大。     

风冷垂直管降膜吸收器及其传热传质问题的研究

吸收器是吸收式制冷机的关键部分，传统的吸收器都是采用水冷却，该文提出了风冷吸收器的设计方案，建立了风冷吸收器降膜吸收过程中传热、传质耦合问题的物理数学模型。并在此基础上，对风冷垂直单管内溴化锂水溶液吸收过程的传热、传质问题进行了数值研究，得出了一些基本结论。这些结果对于垂直管降膜吸收过程的研究以及风冷吸收器的设计具有一定的指导意义。     

水平管外降膜蒸发传热实验研究

以水为工质,对直径为19 mm的铝黄铜管外降膜蒸发传热过程进行实验研究。实验通过测量管表面和饱和蒸气温度,计算得到平均和局部传热系数。由实验数据分析喷淋密度、蒸发温度、热流密度、管间距等参数对管外平均传热系数的影响,并与直径25.4 mm铝黄铜管降膜蒸发传热系数进行比较,讨论局部传热系数随周向角度的变化。结果表明,在实验范围内,管外平均传热系数随温度的升高而增大,随喷淋密度的增大先增大,后略微下降。小管径管的降膜蒸发传热系数大于大管径管的传热系数。     

海水淡化用波纹管降膜蒸发强化传热实验研究

通过利用水平管降膜蒸发换热试验台分别对Φ19×0.75 mm的波纹管和光滑管进行实验研究。实验在变喷淋密度(0.007~0.130 kg/m·s)、变热通量(52~143 k W/m~2)、变传热温差(1.5~10.0℃)、变蒸发压力(0.020~0.065MPa)条件下进行。通过实验数据得到波纹管和光滑管传热系数与各影响因素(喷淋密度、热通量、传热温差、蒸发温度)之间的变化规律。实验结果表明:在一定范围内,降膜蒸发器的传热系数K随喷淋密度γ、热通量Q的增大、蒸发温度T的升高而增大,随传热温差Δt的增大而降低。当喷淋密度大于0.178 kg/(m·s)时,总传热系数趋于稳定,当热通量大于130 k W/m~2时,总传热系数的增速明显变缓。此外,不凝气含量对传热系数K的影响显著,在同等实验条件下波纹管的传热系数比光滑管提高近30%。     

乙醇胺水溶液降膜吸收CO_2的数值研究

针对MEA(乙醇胺溶液)降膜吸收CO_2的过程中存在的传热传质问题,本研究建立了不同浓度MEA溶液降膜吸收CO_2的二维数学模型,得到了液膜内部温度场与浓度场分布,以及液膜内热通量、质量通量和CO_2吸收速率沿液膜下降方向的变化规律。结果表明:界面处温度在入口处迅速上升,随后呈指数规律下降;相界面处的热通量、质量通量、液膜流膜下降方向上CO_2吸收速率在入口处直线下降,随后小幅变化,说明吸收作用主要发生在入口段附近。在液膜中间位置处,热通量和质量通量的变化趋势在入口段附近差异很大,这可能是由反应放出的热量一部分被液体吸收所致。     

竖管内溴化锂溶液降膜蒸发数值研究

研究以太阳集热板制取的高温空气为热源,直接驱动竖管内溴化锂溶液降膜蒸发的传热性能。提出适用于竖管内溴化锂溶液降膜蒸发传热传质耦合的数学模型,根据合理的数值解法,运用MATLAB软件编程,计算出降膜区域内温度场和浓度场,入口Re数越大,换热效果相对降低,而传质效果增强,并得到量纲为1的降膜换热准则式。通过与经验公式对比发现,溴化锂溶液中传质对传热有抑制作用,且其影响不可忽略。     

开式吸收式热泵降膜吸收器热质传输研究

针对开式循环吸收式热泵系统中的吸收器,建立了竖直管降膜式吸收器模型,分析了吸收器内部的热质传输规律,并将拟合的传质系数与实验结果进行了比较.计算结果表明,溶液浓度升高最利于水蒸汽吸收,溶液温度升高有利于热的回收利用,增大降膜管径及增加降膜管长度均利于吸收过程的进行.计算结果可为此类吸收器的设计提供参考.     

溴化锂水溶液绝热吸收过程实验研究

该文提出预冷却绝热吸收的空冷溴化锂吸收式制冷循环的吸收器设计方案，设计加工了一个溴化锂水溶液绝热降膜吸收的循环实验装置。实验研究李在湍流情况下，溴化锂水溶液在竖真光管外绝热降膜吸收的传质性能。研究了溶液浓度、温度、吸收压力对水蒸气吸收速率和吸收系数的影响以及添加表面活性剂后对吸收性能的影响。     

低温地热双循环式发电系统横管喷淋降膜蒸发器传热性能实验研究

以R245fa为工质,搭建有机朗肯循环（ORC）发电系统横管喷淋降膜蒸发器传热测试平台,研究有机工质喷淋密度,地热水初温及流率等因素对管外换热系数的影响。实验结果表明：随着有机工质喷淋密度、地热水初温、地热水流率的增大,传热系数均先增大后减小。最后,根据实验结果,对现有横管喷淋降膜蒸发器的管外传热系数经验公式的参数进行修正。     

A coupled level set and volume-of-fluid simulation for heat transfer of the double droplet impact on a spherical liquid film

A coupled level set and volume-of-fluid method is applied to investigate the double droplet impact on a spherical liquid film. The method focuses on the analysis of surface curvature, droplet diameter, impact velocity, double droplets vertical spacing, the thickness of the liquid film of two liquid droplets after the impact on a spherical liquid film, and the influence of flow and heat transfer characteristics. The results indicate that the average wall heat flux density of the double liquid droplet impact on a spherical liquid film is greater than that of a flat liquid film. Average wall heat transfer coefficient increases with the increase in the liquid film’s spherical curvature. When the liquid film thickness is smaller, the average wall heat flux density of the liquid film is significantly reduced by the secondary droplets generated from the liquid film. When the liquid film thickness is larger, the influence of liquid film thickness on the average wall heat flux density gradually decreases. The average wall heat flux density increases with the increase in impact velocity and the droplet diameter; it also decreases with the increase in double droplets vertical spacing.     

Effects of film evaporation on laminar mixed convection heat and mass transfer in a vertical channel

A numerical study of finite liquid film evaporation on laminar mixed convection heat and mass transfer in a vertical parallel plate channel is presented. The influences of the inlet liquid mass flow rate and the imposed wall heat flux on the film vaporization and the associated heat and mass transfer characteristics were examined for air-water and air-ethanol systems. Predicted results obtained by including transport in the liquid film are contrasted with those where liquid film transport is neglected, showing that the assumption of an extremely thin film made by Tsay and Yan (Wärme- und Stoffübertragung 26, 23–31 (1990)) is only valid for a system with a small liquid mass flow rate. Additionally, it is found that the heat transfer between the interface and gas stream is dominated by the transport of latent heat associated with film evaporation. The magnitude of the evaporative latent heat flux may be five times greater than that of sensible heat flux.     

The effect of gravity on R410A condensing flow in horizontal circular tubes

Heat transfer characteristics of R410A condensation in horizontal tubes with the inner diameter of 3.78?mm under normal and reduced gravity are investigated numerically. The results indicate that the heat transfer coefficients increase with increasing gravitational accelerations at a lower mass flux, whereas their differences under varying gravity are insignificant at a higher mass flux. The liquid film thickness decreases with increasing gravity at the top part of the tube, whereas the average liquid film thickness is nearly the same under different gravity accelerations at the same vapor quality and mass flux. The local heat transfer coefficients increase with increasing gravity at the top of the tube and decrease with increasing gravity at the bottom. The proportion of the thin liquid film region is important for the overall heat transfer coefficients for the condensing flow. A vortex with its core lying at the bottom of the tube is observed under normal gravity because of the combined effect of gravity and the mass sink at the liquid–vapor interface, whereas the stream traces point to the liquid–vapor interfaces under zero gravity. The mass transfer rate under zero gravity is much lower than that of normal gravity.     

Investigations on heat and mass transfer characteristics of falling film horizontal tubular absorber

An experimental set-up is built incorporating only two principle components, viz, absorber and generator of vapor absorption refrigeration system (VARS) to investigate heat and mass transfer characteristics of absorber. The refrigerant, R134a (1,1,1,2-tetrafluroethane) is absorbed by R134a-DMAC (N,N-dimethylacetamide) solution flowing over the horizontal tubes arranged as tube bank. The effect of solution flow rate, coolant flow rate and temperature, heater load and concentration of R134a is studied. The performance parameters like solution exit temperature from tubes, state point temperatures, heat flux, mass flux, and overall heat and mass transfer coefficients are presented for different operating condition of absorber. For lower flow rate of the solution and higher flow rate of the coolant, the bulk solution temperature is found to decrease. The heat and mass transfer coefficients increase with mass flow rate of the solution. An increase in inlet temperature of coolant results into an increase in overall heat transfer coefficient and decrease in overall mass transfer coefficient.     

Laminar flow and heat transfer of non-newtonian falling liquid film on a horizontal tube with variable surface heat flux

An analysis is performed to study the laminar flow and heat transfer of non-Newtonian falling liquid film on a horizontal tube for the case of variable surface heat flux. The inertia and convection terms are taken into account. The governing boundary layer equations are solved numerically using an implicit finite difference method. Of particular interest are the effects of the mass flow rate Γ, the concentration C of carboxymethylcellulose (CMC) solutions, the exponent m for the power-law surface heat flux, and the tube diameter D on the film thickness profiles, as well as on the local and average Nusselt numbers. It was found that an increase in the mass flow rate Γ and exponent value m increases the local and average heat transfer rates. Finally, the present simulation is found to be in good agreement with previous experimental and numerical results for Newtonian films.     

Effects of inlet conditions on film evaporation along an inclined plate

The evaporation of falling water liquid film in air flow is used in different solar energy applications as drying, distillation and desalination, and desiccant systems. The good understanding of the hydrodynamics and heat exchange in falling liquid film and gas flow, with interfacial heat and mass transfer, can be applied in improving solar systems performance. The solar system performance is dependent on the operating conditions, system conception and related to several physical parameters, where the effects of some of these parameters are not completely clarified. In the present numerical study, we examine the effects of inlet conditions on the evaporation processes along the gas–liquid interface. The liquid film streams over an inclined plate subjected to different thermal conditions. Liquid and gas flows are approached by two coupled laminar boundary-layers. The numerical solution is obtained by utilizing an implicit finite-difference box method. In this analysis an air–water system is considered and the coupled effects of inclination, inlet liquid mass flow rate and gas velocity are examined. The results show that, for imposed heat flux or uniform wall temperature, the effect of inclination is highly dependent on the liquid mass flow rate and gas velocity. An increase in the liquid mass flow rate causes an enhancement of the effect of inclination on the heat and mass transfer. The inclination affects the heat and mass transfer, especially at lower gas velocities. In the range of inclination angles of 0–10°, an increase in the inclination improves the evaporation by increasing the vapor mass flow rate. The maximum effect of inclination is nearly achieved at an inclination angle of 10°.     

垂直窄缝通道内环状流沸腾传热特性

本研究基于液膜和蒸汽的质量、动量和能量方程,建立了均匀热流垂直窄缝通道内环状流沸腾传热模型,通过相关文献估算环状流起始点处液膜厚度,利用有限差分法对环状流模型方程组进行数值求解,得到沿流道环状流区域的液膜厚度,并进一步预测了局部沸腾传热系数,结果表明:环状流区域的局部沸腾传热系数随质量流量和干度的增加而增加,与Kenning关联式对比,模型预测沸腾传热系数较关联式计算值偏低。将不同工况下的226组两相环状流实验数据与模型预测结果进行对比,平均绝对误差为18.2%。     

Evaporative heat transfer characteristics of a water spray on micro-structured silicon surfaces

Experiments were performed to evaluate the evaporative heat transfer characteristics of spray cooling of water on plain and micro-structured silicon surfaces at very low spray mass fluxes. The textured surface is made of an array of square micro-studs. It was found that the Bond number of the microstructures is the primary factor responsible for the heat transfer enhancement of evaporative spray cooling on micro-structured silicon surface in the present study. A qualitative study of evaporation of a single water droplet on plain and textured silicon surface shows that the capillary force within the microstructures is effective in spreading the deposited liquid film, thus increasing the evaporation rates. Four distinct heat transfer regimes, which are the flooded, thin film, partial dryout, and dryout regimes, were identified for evaporative spray cooling on micro-structured silicon surfaces. The microstructures provided better cooling performance in the thin film and partial dryout regime and higher liquid film breakup heat flux, because more water was retained on the heat transfer surface due to the capillary force. Heat transfer coefficient and temperature stability deteriorated greatly once the liquid film breakup occurred. The liquid film breakup heat flux increases with the Bond number. Effects of surface material, system orientation and spray mass flux were also addressed in this study.     

微细圆管内CO2两相沸腾换热实验研究

本文对CO_2在水平微细管内流动沸腾特性进行实验研究。实验结果表明:热流密度增加对强化核态沸腾换热和高干度区域流型转变具有显著影响,随着热流密度的增加换热系数增加,对摩擦压降影响很小;质量流率对于换热系数的影响较小,但随着质量流率的增加摩擦压降大幅增加,质量流率的大小直接决定了换热过程所经历流态;饱和温度升高换热系数相应升高,摩擦压降减小,且对流态转变特性有重要影响。在同样工况下摩擦压降最大值先于换热系数最大值出现,理论分析采用的流态形式与实际CO_2管内流动流动沸腾换热流态基本一致。     

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.