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.     

Vapor absorption by LiBr aqueous solution in vertical smooth tubes

Heat and mass transfer in a falling film vertical in-tube absorber was studied experimentally with LiBr aqueous solution. The presented results include the effect of solution flow rate, solution subcooling and cooling water temperature on the absorption in a smooth copper tube 16.05 mm I.D. and 400 mm long. The experimental data in the previous report for a 1200-mm-long tube was also re-examined and compared. It was demonstrated by the observation of the flow in the tube that the break down of the liquid film into rivulets leads to deterioration of heat and mass transfer at lower film Reynolds number or in longer tubes. An attempt to evaluate physically acceptable heat and mass transfer coefficients that are defined with estimated temperature and concentration at the vapor–liquid interface was also presented.     

Analytical investigation of two different absorption modes: falling film and bubble types

The objectives of this paper are to analyze a combined heat and mass transfer for an ammonia–water absorption process, and to carry out the parametric analysis to evaluate the effects of important variables such as heat and mass transfer areas on the absorption rate for two different absorption modes — falling film and bubble modes. A plate heat exchanger with an offset strip fin (OSF) in the coolant side was used to design the falling film and the bubble absorber. It was found that the local absorption rate of the bubble mode was always higher than that of the falling film model leading to about 48.7% smaller size of the heat exchanger than the falling film mode. For the falling film absorption mode, mass transfer resistance was dominant in the liquid flow while both heat and mass transfer resistances were considerable in the vapor flow. For the bubble absorption mode, mass transfer resistance was dominant in the liquid flow while heat transfer resistance was dominant in the vapor region. Heat transfer coefficients had a more significant effect on the heat exchanger size (absorption rate) in the falling film mode than in the bubble mode, while mass transfer coefficients had a more significant effect in the bubble mode than in the falling film mode.     

Effect of vapor flow on the falling-film heat and mass transfer of the ammonia/water absorber

Falling-film heat and mass transfer in an absorber can be influenced by the motion of the surrounding refrigerant vapor. In this study, the effect of the vapor flow direction on the absorption heat and mass transfer has been investigated for a falling-film helical coil absorber which is frequently used in the ammonia/water absorption refrigerators. The heat and mass transfer performance was measured for both parallel and countercurrent flow. The experiments were carried out for three different solution concentrations (3, 14, and 30%). The vapor in equilibrium with the solution is supplied to the test section. It is found that the falling-film heat and mass transfer is deteriorated in the countercurrent flow if the specific volume of the vapor solution is large. For the countercurrent flow, the high velocity of the vapor due to large specific volume seems to cause the unfavorable distribution of falling-film and reduce the heat and mass transfer performance of the ammonia absorber. The effect of vapor flow direction decreased with increasing concentration of ammonia solution since the specific volume of the ammonia vapor which is in equilibrium with the solution becomes smaller and the vapor velocity becomes lower.     

Gravity-driven flow of liquid films and droplets in horizontal tube banks

Liquid films falling over banks of internally cooled horizontal tubes are often used to absorb mass from a surrounding vapor. This arrangement is particularly suitable for absorption processes where the vapor has a high heat of absorption and where high transfer rates and low pressure drops are required, as is the case of absorption heat pumps and other chemical processes. When the liquid film presents a significant resistance to heat and mass transfer, understanding the motion of the film is critical. However, mathematical models of these types of systems in the literature have generally made use of many simplifying assumptions about the behavior of the falling liquid. The formation, detachment, and impact of droplets and the associated waves and film disturbances can all affect the mixing of the liquid and can enhance transfer rates accordingly. The objective of this paper is to identify and visually document these deviations from idealized film behavior and discuss their implications on the heat and mass transfer processes, which are important to consider in the development of mechanistic models of the absorption process.     

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.     

Simulation of the transient response of heat driven refrigerators with continuous temperature control Simulation de la réponse transitoire des réfrigérateurs utilisant une source de chaleur avec maîtrise de la température en continu

A theoretical and experimental study is presented and a mathematical model is introduced for a heat driven refrigeration system operating with continuous temperature control. The model consists of a refrigerated space, an absorption refrigerator, operating irreversibly, a temperature sensor and a reference signal, and a power law control action. The steady-state behavior of the absorption refrigerator model is validated by direct comparison between theoretical results and experimental data. The model is then used to identify an optimal thermal conductance allocation, for a fixed total thermal conductance inventory, such that the refrigeration rate is maximized and the ‘pull-down' time is minimized. A simulation of the system operating in a transient mode is carried out to show that closed-loop operation results in a large reduction of fuel consumption, with respect to the ‘on–off' operation. Appropriate dimensionless groups are identified and the generalized results reported in charts using dimensionless variables.     

液氮薄膜在切应力作用下的降膜流动和传热模型

以钎焊板式换热器当中液氮薄膜为研究对象,通过建立在切应力作用下层流饱和蒸发液氮薄膜的传热特性的物理模型,推导出了无量纲液膜厚度和表面传热系数与气液界面切应力、界面对流换热强度、初始雷诺数和流动长度之间的非线性关系式.     

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.     

The role of surfactant adsorption rate in heat and mass transfer enhancement in absorption heat pumps

The importance of heat and mass transfer additives in absorption chillers and heat pumps has been recognized for over three decades. However, a universally accepted model for the mechanisms responsible for enhanced absorption rates has yet to be proposed. The Marangoni effect—an instability arising from gradients in surface tension at the liquid-vapor interface—is generally accepted as the cause of the convective flows that enhance transfer rates. Certain surfactant additives can significantly improve absorption rates and thus reduce the overall transfer area required by a given machine. Any means available that can increase the efficiency and acceptability of absorption machines is to be welcomed, as this technology provides an alternative to vapor compression systems which is both environmentally friendly and more versatile with regards to energy sources. This study investigates the rate at which a surfactant additive adsorbs at a liquid-vapor interface. The residence time of the falling liquid solution in an absorber is quite short. An effective additive must not only reduce the surface tension of the solution; it must do so quickly enough to cause the Marangoni instability within the short absorption process time. The entrance region of an absorber features a freshly exposed interface at which no surfactant has adsorbed. A numerical model is used to analyze surfactant relaxation rates in a static film of additive-laced solution. Kinetic parameters for the combination of the working pair LiBr-H₂O and the additive 2-ethyl-1-hexanol are derived from data in the literature for static and dynamic surface tension measurements. Bulk, interfacial and boundary parameters influencing relaxation rates are discussed for surfactant adsorption occurring in the absence of absorption, as well as for concurrent adsorption and stable vapor absorption. Initial solution conditions and absorption driving force are shown to impact the potential for instability in the effect they have on the rate of interfacial additive adsorption.     

Simulation study of a hybrid absorber–heat exchanger using hollow fiber membrane module for the ammonia–water absorption cycle

An innovative hybrid hollow fiber membrane absorber and heat exchanger (HFMAE) made of both porous and nonporous fibers is proposed and studied via mathematical simulation. The porous fibers allow both heat and mass transfers between absorption solution phase and vapor phase, while the nonporous fibers allow heat transfer between absorption solution phase and cooling fluid phase only. The application of HFMAE on an ammonia–water absorption heat pump system as a solution-cooled absorber is analyzed and compared to a plate heat exchanger falling film type absorber (PHEFFA). The substantially higher amount of absorption obtained by the HFMAE is made possible by the vast mass transfer interfacial area per unit device volume provided. The most dominant factor affecting the absorption performance of the HFMAE is the absorption solution phase mass transfer coefficient. The application of HFMAE as the solution-cooled absorber and the water-cooled absorber in a typical ammonia–water absorption chiller allows the increase of COP by 14.8% and the reduction of the overall system exergy loss by 26.7%.     

表面活性剂对溴化锂吸收式制冷机吸收过程强化机理的研究概况

吸收式制冷技术具有环保、节电和利用余热等不可替代的优点，在我国应用广范。溴化锂机组的吸收器是系统中换热面积最大、成本最高的换热部件，采用添加剂强化吸收器传热传质是一种不可缺少的手段。但是添加剂的强化机理却一直没有研究清楚，各国对添加剂的强化机理的研究很重视，已经有了不少研究成果，本文对国外添加剂对溴化锂制冷机吸收器的强化机理的研究进行简要介绍和分析。     

The effect of heat transfer additive and surface roughness of micro-scale hatched tubes on absorption performance

The objectives of this paper are to investigate the effect of heat transfer additive and surface roughness of micro-scale hatched tubes on the absorption performance and to provide a guideline for the absorber design. Two different micro-scale hatched tubes and a bare tube are tested to quantify the effect of the surface roughness on the absorption performance. The roughness of the micro-scale hatched tubes ranges 0.39–6.97 μm. The working fluid is H₂O/LiBr solution with inlet concentration of 55, 58 and 61 wt.% of LiBr. Normal Octanol is used as the heat transfer additive with the concentration of 400 ppm. The absorber heat exchanger consists of 24 horizontal tubes in a column, liquid distributor at the liquid inlet and the liquid reservoir at the bottom of the absorber. The effect of heat transfer additive on the heat transfer rate is found to be more significant in the bare tube than that in the micro-scale hatched tubes. It is found that the absorption performance for the micro-hatched tube with heat transfer additive becomes up to 4.5 times higher than that for the bare tube without heat transfer additive. It is concluded that the heat transfer enhancement by the heat transfer additive is more significant than that by the micro-scale surface treatment.     

Experimental studies on the characteristics of an absorber using LiBr/H2O solution as working fluidEtudes expérimentales sur les caractéristiques d'un absorbeur utilisant comme fluide actif le LiBr/H2O

The absorber is an important component in absorption machines and its characteristics have significant effect on the overall efficiency of absorption machines. This article reports on the results of experimental studies on the characteristics for a falling film absorber which is made up of 24 row horizontal smooth tubes. It shows that while the mass transfer coefficient is increased with the increase of spray density, the heat transfer coefficient is increased only in small spray density range. There is an optimum spray density between 0.005 and 0.055 kg s⁻¹ m⁻¹ spray density at which the heat transfer coefficient is maximum. The heat transfer coefficient (Nusselt number), which is traditionally expressed using Reynolds number and Prandtl number, was modified taking the effect of inlet solution concentration into account. The results can be used to optimize the future design of absorption machines having a falling film absorber and using LiBr/H₂O as working fluid.

Abstract

L'absorbeur est un composant important des systèmes à absorption et ses caractéristiques exercent des effets significatifs sur l'efficacité des machines à absorption. Cet article présent des résultats obtenus dans des études expérimentales sur les caracteristiques d'un absorbeur à film tombant composé d'une rangée de 24 tubes lisses horizontaux. Les auteurs montrent que le coefficient de transfert de masse augmente avec la densité de pulvérisation, le coefficient de transfert de chaleur augmente uniquement dans la gamme des densités de pulvérisation faibles. Il existe une densité de pulvérisation optimale (0,005–0,055 kg s⁻¹ m⁻¹) pour laquelle le coefficient de transfert de chaleur est maximal. Le coefficient de transfert de chaleur (nombre de Nusselt), qui est généralement exprimé en utilisant le nombre de Reynolds et le nombre de Prandtl, a été modifié en tenant compte l'effet de la concentration de la solution à l'entrée. A l'avenir, les résultats peuvent être utilisés pour optimiser la conception des systèmes à absorption à absorbeur à film tombant utilisant le LiBr/H₂O comme fluide actif.     

Experimental correlation of combined heat and mass transfer for NH3–H2O falling film absorptionCorrélation expérimentale entre le transfert de chaleur et de masse pour de l'absorption au NH3–H2O à film tombant

In this article, experimental analysis was performed for ammonia–water falling film absorption process in a plate heat exchanger with enhanced surfaces such as offset strip fin. This article examined the effects of liquid and vapor flow characteristics, inlet subcooling of the liquid flow and inlet concentration difference on heat and mass transfer performance. The inlet liquid concentration was selected as 5%, 10% and 15% of ammonia by mass while the inlet vapor concentration was varied from 64.7% to 79.7%. It was found that before absorption started, there was a rectification process at the top of the test section by the inlet subcooling effect. Water desorption phenomenon was found near the bottom of the test section. It was found that the lower inlet liquid temperature and the higher inlet vapor temperature, the higher Nusselt and Sherwood numbers are obtained. Nusselt and Sherwood number correlations were developed as functions of falling film Reynolds Re₁, vapor Reynolds number Re_v, inlet subcooling and inlet concentration difference with ±15% and ±20% error bands, respectively.     

Model for absorption of water vapor into aqueous LiBr flowing over a horizontal smooth tube

A model for absorption of water vapor into aqueous LiBr flowing over a horizontal smooth tube is developed. The flow is divided into three regimes: (1) falling film in contact with the tube, (2) drop formation at the bottom of the tube, and (3) drop fall between the tubes. Governing equations are formulated for each flow regime, and the variations of solution temperature, LiBr mass fraction, mass absorption rate and heat transfer rate are discussed including the effect of inlet subcooling. It is shown that the temperature variation across the film exhibits a nonlinear profile near the top of the tube and this effect leads to the necessity of a two-dimensional formulation in the falling film regime for accurate prediction. As has been observed previously, the mass fraction boundary layer at the vapor/liquid interface is found to be very thin and this explains the low absorption flux. The model predicts that significant absorption takes place in the drop formation regime with a considerable variation of temperature and mass fraction.     

From measurements of hydrodynamics to computation of species transport in falling films

Falling films on horizontal tube banks are widely used in absorption heat pumps. Liquid film heat and mass transfer models in the literature are based on simplistic assumptions about such flows. Droplets forming under the tubes fall on subsequent tubes, and the ensuing waviness and mixing on the liquid film affect absorption. High-speed flow visualization, image analysis, and computations are used to understand the effect of hydrodynamics on local absorption phenomena. Image analysis mathematically describes the interface, and the surface area and volume of the droplets. These flows are also numerically analyzed using the volume-of-fluid method for the liquid–vapor interface. The 3-D model corroborates with high fidelity the droplet formation, detachment and impact observed in experiments. Fully resolved 3-D computations of the coupled fluid flow and heat and mass transfer reveal the local gradients and transfer rates, and their spatial and temporal variations in the falling-film and inter-tube regions.     

Development of a slug flow absorber working with ammonia-water mixture: part I—flow characterization and experimental investigation

This study deals with an experimental investigation for a counter-current slug flow absorber, working with ammonia–water mixture, for significantly low solution flow rate conditions that are required for operating as the GAX (generator absorber heat exchanger) cycle. It is confirmed that the slug flow absorber operates well at the low solution flow rate conditions. From visualization results of the flow pattern, frost flow just after the gas inlet, followed by slug flow with well-shaped Taylor bubble, is observed, while dry patch on the tube wall are not observed. The liquid film at the slug flow region has smooth gas–liquid interface structure without apparent wavy motion. The local heat transfer rate is measured by varying main parameters, namely, ammonia gas flow rate, solution flow rate, ammonia concentration of inlet solution and coolant inlet conditions. The heat transfer rate while absorption is taking place is higher than that after absorption has ended. The absorption length is greatly influenced by varying main parameters, due to flow conditions and thermal conditions.     

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.     

Experimental verification of H2O/LiBr absorber bundle performance with smooth horizontal tubes

A model for absorption of water vapor into LiBr flowing over horizontal smooth tubes is developed to predict the absorption performance of a tube bundle. The performance of a horizontal smooth tube absorber is calculated and compared with experimental data. The calculation results of absorber performance are found to vary considerably depending on mass diffusivity. The literature value of mass diffusivity, of the order of 10⁻⁹ m² s⁻¹, causes the model to deviate from the experimental results especially for solution side heat transfer coefficient. A parametric study varying mass diffusivity shows that a value of 1.0 × 10⁻¹⁰ m² s⁻¹ gives a better agreement with the experimental results. The effects of absorber capacity, heat transfer coefficient of droplets ejected from the system (slinging) and number of tubes in the bundle on the absorption performance are also discussed.