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%.     

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.     

Heat and mass transfer enhancement of binary nanofluids for H2O/LiBr falling film absorption process

The objectives of this study are to measure the vapor absorption rate and heat transfer rate for falling film flow of binary nanofluids, and to compare the enhancement of heat transfer and mass transfer under the same conditions of nanofluids. The key parameters are the base fluid concentration of LiBr, the concentration of nanoparticles in weight %, and nanoparticle constituents. The binary nanofluids are H₂O/LiBr solution with nanoparticles of Fe and Carbon nanotubes (CNT) with the concentrations of 0.0, 0.01 and 0.1 wt %. The vapor absorption rate increases with increasing the solution mass flow rate and the concentration of Fe and CNT nanoparticles. It is found that the mass transfer enhancement is much more significant than the heat transfer enhancement in the binary nanofluids with Fe and CNT. It is also found that the mass transfer enhancement from the CNT nanoparticles becomes higher than that from the Fe nanoparticles. Therefore, the CNT is a better candidate than Fe nanoparticles for absorption performance enhancement in H₂O/LiBr absorption system.     

垂直管内TFE/NMP降膜吸收过程中热质传递试验研究

作为一种新型的吸收式制冷工质时-TFE/NMP（2，2，2-trifluo-roethanol/N-methylpyrolidone，中文名：三氟乙醇/氮甲基吡咯烷酮），因其良好的工作特性而被国际制冷界所重视，但有关吸收式制冷/热泵系统运行中的一个重要环节-TFE/NMP降膜吸收过程中的传热、传质现象却有人进行过研究。在国家自然科学基金的资助下，我们建立了单根管吸试验台以研究TFE/NMP降膜吸收过程中热、质传递规律。在不同TFE/NMP溶液流量和不同冷却水流量条件下，测得两组试验数据，对试验数据进行处理并对其数据结果加以分析后，得出垂直管内TFE/NMP降膜吸收过程中热量和质量传递规律的一些特性。     

Numerical Modelling of Flow Boiling Heat Transfer in Horizontal Metal‐Foam Tubes

The flow boiling heat transfer performance in horizontal metal‐foam tubes is numerically investigated based on the flow pattern map retrieved from experimental investigations. The flow pattern and velocity profile are generally governed by vapour quality and mass flow rate of the fluid. The porous media non‐equilibrium heat transfer model is employed for modelling both vapour and liquid phase zones. The modelling predictions have been compared with experimental results. The effects of metal‐foam morphological parameters, heat flux and mass flux on heat transfer have been examined. The numerical predictions show that the overall heat transfer coefficient of the metal‐foam filled tube increases with the relative density (1‐porosity), pore density (ppi), mass and heat 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.     

Effect of axial conduction on the ice crystal growth in laminar falling filmsEffet de la conduction axiale sur la vitesse de formation de cristaux de glace à l'intérieur de films tombants laminaires

A finite volume numerical code has been developed to numerically approximate the rate of ice crystal growth in a laminar falling film flowing down a cooled vertical plate. The governing energy equation contains the phase energy as the source term. Enhancement of heat transfer as a result of suspended ice crystals is accounted for in the use of effective values of thermal conductivity, viscosity, thermal diffusivity, and specific heat as function of volumetric concentration of ice crystals in the falling film. Nusselt number, overall heat transfer coefficients between the fluid and cooled plate, and ice crystal growth rate were calculated for different film thicknesses with and without axial diffusion. Nusselt number and ice crystal growth rates were found to be dependent on film thickness. Axial diffusion effects were found to be negligible for larger film thickness (large flowrate).     

Gas absorption with interfacial resistance in the case of temperature‐dependent physical,transport and reaction properties

Abstract

This work analyzes the problem of interfacial resistance to heat and mass transfer for a gas absorption system with first order reaction when the solubility, the diffusivity, and the reaction rate constant are dependent upon temperature. A film theory model is applied. Two different types of temperature‐dependent solubility relationships, the linear and the exponential approximations, are employed. The temperature‐dependent diffusivity and reaction rate constant are expressed in exponential form. Three cases of different solute concentrations are compared to study the influence of solubility on the system performance. One of these cases with the exponential temperature‐dependent solubility is further examined for the effects of diffusivity coefficient and of surface resistance on the enhancement factor, the temperature rise, and the solubility. The results indicate that the form of the temperature‐dependent solubility relationship plays an important role in the estimation of the surface temperature rise and hence the enhancement factor. The linear approximation predicts slightly higher values in the intermediate region of √M_o , while the exponential approximation results in dramatically higher values at very large √M_o. The surface resistance significantly affects the absorption rate, the interfacial temperature rise, and the solubility at small values of √M_o. The effect of the variation of diffusivity on the system performance is of secondary importance, especially when the values of √M_o are not large.     

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.     

Theoretical analysis of the thermal performance of a plate heat exchanger at various chevron angles using lithium bromide solution with nanofluid

Nanofluids technology has been rapidly developing over the last two decades. In this paper, the performance of a lithium bromide (LiBr) solution with and without nanoparticles in plate heat exchanger (PHE) for various chevron angles and mass flow rates was investigated. As a result, the heat transfer rate and the overall heat transfer coefficient in 60°/60° PHE is over 100% higher than that of 30°/30° PHE, and the effectiveness of the PHE in 60°/60° PHE is about 70% higher than that of 30°/30° PHE. By using nanoparticle in the working fluid, the heat transfer performance can increase significantly. The heat transfer rate of 3 vol.% nanofluids increased about 3–8% compare to that of LiBr solution for all chevron PHEs. Besides, the 60°/60° PHE using 3 vol.% nanofluids produced the largest heat transfer rate and heat exchange effectiveness under given operating conditions.     

基于全板翅换热器溴化锂吸收式制冷机结构与性能研究

为了降低吸收式制冷机金属耗量，较小占用空间，提高制冷机传热传质性能，进而提高整机性能，在实验室建立了基于全扳翅式换热器的溴化锂吸收式制冷机，并在设计上采用了专利技术。介绍了这种新型制冷机的设计思想和结构以及传热性能，并与管理壳式吸收式制冷机进行了比较。     

Heat transfer and pressure drop characteristics of nanofluids in a plate heat exchanger

In this paper, the heat transfer characteristics and pressure drop of the ZnO and Al2O3 nanofluids in a plate heat exchanger were studied. The experimental conditions were 100-500 Reynolds number and the respective volumetric flow rates. The working temperature of the heat exchanger was within 20-40 degrees C. The measured thermophysical properties, such as thermal conductivity and kinematic viscosity, were applied to the calculation of the convective heat transfer coefficient of the plate heat exchanger employing the ZnO and Al2O3 nanofluids made through a two-step method. According to the Reynolds number, the overall heat transfer coefficient for 6 vol% Al2O3 increased to 30% because at the given viscosity and density of the nanofluids, they did not have the same flow rates. At a given volumetric flow rate, however, the performance did not improve. After the nanofluids were placed in the plate heat exchanger, the experimental results pertaining to nanofluid efficiency seemed inauspicious.     

溴化锂溶液中添加活性炭颗粒对制冷机出力的影响

为改善吸收式制冷机的性能,基于活性炭的高吸附性,研究在溴化锂水溶液添加适量活性炭颗粒形成的悬浮液的传热传质性能,分析活性炭/溴化锂悬浮液替代溴化锂水溶液作为吸收剂的可行性。研究表明,悬浮液的粘度随活性炭浓度线性增长,以悬浮液作为吸收剂的制冷机的出力随透湿系数、悬浮液吸附平衡常数及换热表面积呈近似线性增长关系,并且,以活性炭/溴化锂悬浮液作吸收剂的制冷机出力较以纯溴化锂溶液作吸收剂的制冷机出力大。     

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.     

Modeling of heat transfer and unsaturated flow in woven fiber reinforcements during direct injection-pultrusion process of thermoplastic composites

This paper provides a methodology for the modeling of heat transfer and polymer flow during direct thermoplastic injection pultrusion process. Pultrusion was initially developed with thermosets which have low viscosity. But the impregnation becomes a critical point with thermoplastics which exhibit higher viscosity. There are very few reported works on direct thermoplastic impregnation with injection within the die. In addition, the rare studies have not adequately addressed the issue of unsaturated flow in woven fiber reinforcements. The solution proposed here, models the polymer flow through dual-scale porous media. A heat transfer model is coupled to a flow model enriched with a sink term. Specific changes of variables are made so as to model the steady state solution of unsaturation along a continuous process. The sink term, added to the continuity equation, represents the absorption rate of polymer by the bundles. Data were measured on a pultrusion line and micrographs confirmed the modeling strategy with an unsaturated flow approach. The flow modeling coupled to heat transfer of the thermoplastic pultrusion process aims at determining the saturation evolution through the die so as to manufacture pultruded profiles with the lowest residual porosity.     

Performance of a metal hydride cooling system

The performance of a single-stage metal hydride cooling system working with the ZrMnFe/MmNi_4.5Al_0.5 pair has been evaluated based on heat transfer and reaction kinetics considerations. Results show that the initial and operating costs of the system have to be minimized by optimizing the bed thickness, effective thermal conductivity and overall heat transfer coefficients. Of the three operating temperatures (heat source, heat sink and refrigeration temperatures), refrigeration temperature has the greatest influence on system performance, as desorption during the refrigeration process normally controls the cycle time and specific alloy output. However, heat source and heat rejection temperatures assume importance at high refrigeration temperatures. The average COP of the above system lies between 0.45 and 0.50.     

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.     

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.     

Finite-element method for the effects of chemical reaction, variable viscosity, thermophoresis and heat generation/absorption on a boundary-layer hydromagnetic flow with heat and mass transfer over a heat surface

Summary The effects of variable viscosity, thermophoresis and heat generation or absorption on hydromagnetic flow with heat and mass transfer over a heat surface are presented here, taking into account the homogeneous chemical reaction of first order. The fluid viscosity is assumed to vary as an inverse linear function of temperature. The velocity profiles are compared with previously published works and are found to be in good agreement. The governing fundamental equations are approximated by a system of nonlinear ordinary differential equations and are solved numerically by using the finite element method. The steady-state velocity, temperature and concentration profiles are shown graphically. It is observed that due to the presence of first-order chemical reaction the concentration decreases with increasing values of the chemical reaction parameter. The results also showed that the particle deposition rates were strongly influenced by thermophoresis and buoyancy force, particularly for opposing flow and hot surfaces. Numerical results for the skin-friction coefficient, wall heat transfer and particle deposition rate are obtained and reported graphically for various parametric conditions to show interesting aspects of the solution.