降膜和泡式等不同吸收模式的分析研究

本文的目的是针对降膜和泡式两种吸收模式,分析氨水吸收过程的复合传热传质,并企图对传热传质面积等重要参数对吸收量的影响进行参数分析。用在冷却剂侧有偏置肋片（OSF）的板式热交换器来设计降膜和泡式吸收器。已经发现,泡式吸收器的局部吸收量总是大于降膜式,使此种吸收器的尺寸比降膜式约小48．7％。对降膜式吸收器、液体侧的传质阻力是决定性的,而气体侧的热热和传质阻力的也很大。对泡式吸收器,液体侧传质阻力是决定性的,而比泡式吸收器大,而对泡式吸收器,传质系数的影响比降膜式更大。     

R124-DMAC垂直管内鼓泡吸收传质性能试验研究

通过搭建垂直管内R124-DMAC鼓泡吸收可视化试验平台,对鼓泡吸收过程中的气液体积流率、溶液入口温度与溶液入口浓度对鼓泡吸收传质性能的影响进行研究。试验结果表明:提高溶液体积流率和降低气体体积流率,可以提高鼓泡式吸收器的传质性能,但提高溶液体积流率会增加吸收式制冷系统的溶液泵功率,降低系统性能系数;降低气体体积流率会降低吸收式制冷系统的制冷量。而溶液入口温度和浓度的降低,均可以较大幅度提高R124-DMAC鼓泡式吸收器的传质性能。     

吸收压力对LiBr斜板绝热吸收器的影响

吸收器作为溴化锂吸收式制冷机中重要的组成部分,其中进行的传热传质过程对机组的吸收效果和传热性能有着重要的影响．本文对蒸汽——溴化锂水溶液在传热传质分离的绝热吸收器中的吸收情况进行了实验研究,对进出口温差、浓度差、吸收率随吸收压力的变化关系进行了分析,并和数值结进行了对比。实验结果表明,在实验的条件范围内,斜板绝热吸收的传质系数比水平管降膜吸收的传质系数有很大的提高,传质得到了强化。     

LiBr喷雾吸收器的传热传质分离研究

吸收器是溴化锂吸收式制冷系统中最大的部件，换热面积占机组的40％左右。文章介绍了稳定性能好适合船用的喷雾吸收器的原理和特点，提出通过预冷却和绝热吸收，实现传热传质的分离，使两者可以分别得到强化。在Newman的基础上建立数学模型，对传热传质分离的吸收器进行分析和计算，得出结论。     

溴化锂吸收式制冷机吸收器的操作线方程、理论分析和实验研究

本文从定义吸收器的传热系数和传质系数出发,分析传热和传质的相互作用,得到了实用的、易于计算的操作线方程。引用斯蒂芬流的概念建立了一种更合理的吸收扩散模型。对描述过程的基本方程组进行简化和求解,得到了吸收器的传热量和传质量计算式,并计算了吸收器的传热系数和传质系数。对盘管吸收器的试验结果表明,所作的理论分析是正确的。其结果可供研究和设计吸收器参考。     

溴化锂风冷垂直降膜吸收过程数值模拟

通过对溴化锂溶液在降膜吸收过程中传热、传质特性的分析,建立了垂直降膜吸收过程的数学模型。在这个模型中,考虑了对流及变膜厚等因素对传热、传质性能的影响。并对风冷垂直管降膜吸收过程进行了数值模拟。得出的结论对垂直降膜吸收器的设计和优化具有指导意义。     

添加剂和纳米粒子强化溴化锂水溶液／氨水吸收特性研究进展

吸收式制冷以其节能、环保等诸多优点得到了越来越广泛的应用。本文总结与分析添加剂和纳米粒子强化溴化锂水溶液及氨水吸收特性的机制和相关实验研究的发展现状。针对吸收式制冷系统中吸收器传质系数和换热系数小而导致的制冷效率低的问题,很多学者进行了添加剂和纳米粒子对吸收过程影响的实验研究,并据此采取措施增大传质传热效率。实验主要包括以下几个方面：表面张力实验、静态池吸收实验、降膜吸收实验和氨水鼓泡吸收实验。实验结果均表明添加剂和纳米粒子可以提高吸收器中溴化锂水溶液及氨水的传热传质性能。该研究对于提高吸收式制冷系统的制冷效率有很大帮助,同时为该技术在实际系统中的应用奠定基础。     

热管式溶液吸收器传热传质过程的数值模拟

本文首次利用水重力热管，以溴经锂水溶液为工质，对在这外壁面上溶液降膜吸收水蒸气并移出吸收热的传热传质过程进行了数值模拟。结果表明，在一定条件下，所需热管加热段长度随膜雷诺数的增加而增加，随输出热温度的提高而减小，并且降膜平均传热和传质系数随膜雷诺数的增加而降低；利用热管作为吸收器的传热传质元件，其传热温差很小，但大较大浓差和较大雷诺数下，所需热管加热段长度太长。     

太阳能溴化锂吸收式制冷系统溶液降膜吸收流动分析

溴化锂降膜式吸收器能在较小液流量和较小温差下获得较高的热流密度和传热传质系数,尤其是当液膜沿着水平管外作降膜流动时,传热传质效果更佳。为此建立溴化锂降膜吸收器溶液吸收过程流动的物理模型,通过对模型假设简化,对其进行数值求解,从而进行流动分析。与实验结果分析相结合,使得对吸收式制冷系统的分析更加全面。     

溴化锂填料吸收器中绝热吸收过程的实验研究

针对传热、传质分离的填料吸收器，设计、加工了一个溴化锂绝热降膜吸收的循环实验装置；实验研究了溴化锂水溶液在填料层上的绝热吸收特性；分析了溶液温度、浓度、降膜雷诺数对吸收效率和传质系数的影响。     

板式膜反转降膜吸收器设计与性能研究

板式膜反转降膜吸收器是一种将板式降膜吸收和膜反转技术相结合而开发的新型吸收器,合理设计与掌握其吸收性能对今后这种吸收器的工程应用十分重要.为此,通过建立、求解板式膜反转降膜吸收过程的数学模型,确立了设计条件下最佳吸收器结构;对于所设计的板式膜反转吸收器进行了不同吸收压力、溶液流量、进口浓度、进口温度及冷却条件下传热传质性能的计算,并与竖板降膜吸收器进行了比较.     

Heat and mass transfer in vertical tubular bubble absorbers for ammonia-water absorption refrigeration systems

A model is developed for calculation of simultaneous heat and mass transfer processes in vertical bubble absorbers as used for ammonia-water absorption refrigeration systems. Some preliminary experiments have been performed in an absorber without heat removal. The results from these experiments are compared with the literature and give a first indication about the methods for prediction of the absorption process. Experiments have also been performed with simultaneous heat removal. The internal diameters of the absorbers tested were 10.0, 15.3, and 20.5 mm. The mass transfer coefficients resulting from these experiments are correlated by a modified Sherwood relation. An interative procedure is presented which allows design of vertical tubular bubble absorbers for ammonia-water absorption refrigeration systems.     

Effect of pressure and concentration on performance of a vertical falling-film type of absorber and generator using lithium bromide aqueous solutions

Experiments on an absorber and generator in an absorption refrigerating machine were made using a vertical falling-film type of stainless steel column. Three lithium bromid e aqueous solutions (40, 55 and 60wt% LiBr) were used as working fluid. The experimental apparatus was operated at 1.3 kPa (the pressure for a practical absorber) and 5.3 kPa (the intermediate pressure between absorber and generator). The measured absorption (evaporation) rate decreased with reducing pressure an d increasing concentration of LiBr in the falling liquid. The rate agreed with the values obtained from the analysis of heat and mass transfer in a falling film. Therefore, a falling-film type of absorber and generator can be designed and operated by a consistent method.     

Visual experimental research on the effect of nozzle orifice structure on R124–DMAC absorption process in a vertical bubble tube

A visual experimental platform for R124–DMAC bubble absorption in a vertical tube absorber was designed and built for this research. The bubble behaviors, flow pattern characteristics and distributions are observed and the bubble absorption heights (BAHs) were measured when the two kinds of different structure nozzles (single-orifice or multi-orifice nozzle) were applied in the absorber. The results showed that the BAH will heighten with increases of vapor flow rate and nozzle flow area. Based on visual experimental observations, the BAH or bubble absorption performance was significantly affected by the velocity of vapor from the nozzle rather than by the nozzle structure. The proportion of slug flow in BAH or the BAH can be decreased by using a multi-orifice nozzle in the absorber under the same flow area condition. However, the flow resistance of the vapor through the nozzle will increase, which has a negative action on the performance of absorption refrigeration systems. So, using multi-orifice nozzle does not improve the absorption performance of the bubble absorber under the same nozzle flow resistance condition.     

A simple model for falling film absorption on vertical tubes in the presence of non-absorbables

Most water–lithium bromide (LiBr) absorption chillers have a purge system to remove non-absorbable gases that cause a reduction in cooling capacity. Generally, the non-absorbables are originated in corrosion/passivation processes inside the machine, but leaks can also be a source of concern. However, since leaks must be corrected immediately to avoid machine deterioration, this study is mostly aimed at the non-absorbables evolved during operation. This paper analyses the effect of inlet non-absorbable air concentration, outlet purge velocity, absorber pressure and cooling water temperature on the falling film absorption process inside a vertical tube absorber, based on a simple transport coefficient model. This model consists of three ordinary differential equations solved with as method for initial-value problem, and a set of auxiliary equations. The study shows that the effect of non-absorbables can be significant, and furthermore provides a quantitative framework to aid in purge design. The nominal working conditions in this study were a solution Reynolds number of 100, an absorber pressure of 1.3 kPa, a cooling water temperature of 35 °C and an inlet solution concentration of 62% LiBr by weight. The results indicate that a minimum vapour velocity is required to sweep the non-absorbables along the absorber towards the purge, thus preventing reduced absorption fluxes. At a cooling water temperature of 35 °C, an inlet air concentration of 20% (by mole) resulted in a 61% reduction in mass absorption flux.     

A critical review of models of coupled heat and mass transfer in falling-film absorption

In absorption space-conditioning systems, the performance of the absorber is critical to the overall system performance, size, and first-cost. The objective of this paper is to provide a comprehensive review of the significant efforts that researchers have made to mathematically model the coupled heat and mass transfer phenomena that occur during falling-film absorption. A detailed review of the governing equations, boundary conditions, assumptions, solution methods, results, and validation of these investigations is presented. This review excludes experimental work in this area, the effect of additives, and the effect of non-absorbable gases. It is shown that most work found in the literature has focused on the particularly simplified case of absorption in laminar vertical films of water-lithium bromide. Fewer researchers have considered the important situations of wavy films, turbulent films, and films on horizontal tubes. Investigations of the ammonia-water fluid pair have been generally more empirical in nature and/or restricted to vertical laminar films. This review is used to highlight key areas which need attention such as film and vapor hydrodynamics, especially the non-periodicity, instability, and recirculatory motion of waves in the vertical wall case and droplets and waves in the horizontal tube case. Also the potential interaction of the heat and mass transfer process on the film hydrodynamics, surface wetting, heat transfer in the vapor phase, and common simplifications to the governing equations should all be considered carefully. Finally, emphasis must be placed on experimental validation of the local conditions and transfer processes within the absorber, not just overall transport values.     

空冷/水冷混合冷却竖管内LiBr溶液降膜吸收过程数值解

针对风冷和水冷联合冷却的竖管降膜吸收器,考虑汽液界面的阻力、变膜厚、横向对流和冷却水的冷却作用的影响,建立了降膜吸收过程中热质耦合数学模型和同心管环空内冷却水换热数学模型.计算了沿竖管内表面的液膜厚度、温度、浓度以及冷却水在混合冷却条件下的温度分布等参数.分析了冷却水进口温度、LiBr溶液Re数和PE数等参数对传热系数和吸收速率的影响.数学模型的计算结果与实验数据吻合较好.得出的结论对联合冷却吸收器的设计和优化具有指导意义.     

Combined momentum, heat and mass transfer in vertical slug flow absorbers

Numerical solutions have been obtained for the system of equations of momentum, heat and mass transfer describing the absorption of a refrigerant vapour from a Taylor bubble into the refrigerant-absorbent solution film around the bubble. The numerical results are compared with Nusselt's solution of the energy equation and with the penetration theory solution of the mass diffusion variation. Experimental data have been collected in vertical tubular absorbers in the slug flow region with the systems ammonia-lithium nitrate and ammonia-sodium thiocyanate. Four different absorber tubes have been tested with internal diameters of 10, 15, 20, and 25 mm. These data are compared with the numerical and theoretical results. The effect of the bubble nose on mass transfer is studied. Typical temperature profiles during the absorption process in absorption cooling/heating systems are shown.     

A study on numerical simulations and experiments for mass transfer in bubble mode absorber of ammonia and water

An absorber is a major component in the absorption refrigeration systems, and its performance greatly affects the overall system performance. In this study, both the numerical and experimental analyses in the absorption process of a bubble mode absorber were performed. Gas was injected into the bottom of the absorber at a constant solution flow rate. The region of gas absorption was estimated by both numerical and experimental analyses. A higher gas flow rate increases the region of gas absorption. As the temperature and concentration of the input solution decrease, the region of gas absorption decreases. In addition, the absorption performance of the countercurrent flow was superior to that of cocurrent. Mathematical modeling equations were derived from the material balance for the gas and liquid phases based on neglecting the heat and mass transfer of water from liquid to gas phase. A comparison of the model simulation and experimental results shows similar values. This means that this numerical model can be applied for design of a bubble mode absorber.     

Nanofibrous membrane-based absorption refrigeration system

This paper presents a study on the efficacy of highly porous nanofibrous membranes for application in membrane-based absorbers and desorbers. Permeability studies showed that membranes with a pore size greater than about one micron have a sufficient permeability for application in the absorber heat exchanger. Membranes with smaller pores were found to be adequate for the desorber heat exchanger. The membranes were implemented in experimental membrane-based absorber and desorber modules and successfully tested. Parametric studies were conducted on both absorber and desorber processes. Studies on the absorption process were focused on the effects of water vapor pressure, cooling water temperature, and the solution velocity on the absorption rate. Desorption studies were conducted on the effects of wall temperature, vapor and solution pressures, and the solution velocity on the desorption rate. Significantly higher absorption and desorption rates than in the falling film absorbers and desorbers were achieved.