Absorption process of a falling film on a tubular absorber: An experimental and numerical study

This paper describes an experimental and theoretical analysis of the combined heat and mass transfer processes that take place in the absorber of vapor absorption cooling systems. The effect of the main operating variables such as solution inlet temperature, solution flow rate, coolant inlet temperature, coolant flow rate and absorber vapor pressure on the performance of absorber is experimentally investigated. A mathematical model that takes account of the geometrical details of the solution and coolant flow is developed following the traditional heat exchanger analysis to obtain additional information on the performance of the absorber. The governing equations are solved analytically using Laplace Transformation technique. The variations of the concentration of solution and the temperature of the coolant and solution after each horizontal tube are analyzed. There is satisfactory agreement between the predictions of the analytical model and the experimental results. The serpentine arrangement of the coolant flow in the absorber tubes results in a temperature and concentration variation of the solution mainly along the height of the absorber.     

Validation of a model for the absorption process of H2O(vap) by a LiBr(aq) in a horizontal tube bundle, using a multi-factorial analysis

A multi-factorial experimental test of H₂O vapour absorption by a falling film of LiBr(aq) over an horizontal smooth tube absorber is presented. The response parameters of the study are the average convective heat transfer and mass transfer coefficients of the falling film. The response is expressed as a function of the factors used in the study. Also a mathematical model for the prediction of the performance of the absorber is exposed which takes into account the wetting effects. Finally the predictions from the model and the experimental multi-factorial runs are compared and the conclusions discussed.     

Analytic modelling of a falling film absorber and experimental determination of transfer coefficients

Accurate interpretation of the experimental data on falling film flows is a critical part of the investigations in the field of absorption energy system research. However, there is no theoretically proven way to determine experimental heat and mass transfer coefficients for non-isothermal absorption falling film flows. In this article, firstly, it is shown how the governing equations of a falling film absorber can be reduced to two ordinary differential equations and analytic expressions can be obtained for the temperature and concentration profiles along the absorber. Secondly, a new method is proposed to determine heat and mass transfer coefficients from experimental data and its application is demonstrated by reprocessing the experimental data from two experimental studies reported in the literature. The results show that some of the experimental data were misinterpreted by conventional methods and the errors were negligible only when heat and mass fluxes were small, which agrees with the fact that the obtained analytic solutions approach the conventional logarithmic heat and mass transfer equations in such conditions.     

Influence of absorber effectiveness on performance of vapour absorption heat transformers

The effects of mass transfer effectiveness of absorber on the performance of single stage vapour absorption heat transformers were studied by making a thermodynamic analysis. The refrigerant absorbent pairs considered were R21-DMF and R21-DMETEG. The variations in performance parameters such as coefficient of performance, exergetic efficiency, concentration difference and circulation ratio with mass transfer effectiveness of the absorber at various operating temperatures were computed. Increasing the effectiveness of absorber resulted in increases in coefficient of performance, exergetic efficiency and concentration differential across the absorber. The improvements in coefficient of performance and exergetic efficiency with mass transfer effectivensss of the absorber were more pronounced for the R21-DMF pair than for R21-DMETEG. Correlations are presented for quick estimation of performance under different operating conditions.     

Operating design conditions of a solar-powered vapor absorption cooling system with an absorber plate having different profiles: An analytical study

An effort has been devoted to analyze the collector performance parameters of a solar-assisted LiBr/H₂O vapor absorption cooling system with a flat-plate collector consisting of an absorber plate of different profiles. The effect of the collector fluid inlet temperature on the performance of solar collector, vapor absorption cycle, vapor absorption system and refrigerating efficiency has been studied for a wide range of design variables. A comparative study has also been made among the performance parameters of an absorber plate of different shapes with the variation of collector fluid inlet temperature. From the result, it can be highlighted that, at a particular collector fluid inlet temperature, the performances of a vapor absorption system attain a maximum value. Finally, an optimum collector fluid inlet temperature is determined by satisfying the minimization of volume of an absorber plate without affecting the cooling rate in the evaporator.     

Heat and mass transfer coefficients of falling-film absorption process

This paper deals with the method to calculate heat and mass transfer coefficients of falling film absorption process over vertical tube or plate type surface employed in absorption refrigeration system. The conventional log mean temperature/concentration difference method is criticized for lack of physical rationality, and for incorrect results from the calculation. A new method based on a simplified model is proposed and demonstrated by numerical simulations and comparison analysis.     

椭圆横管外降膜流动和换热特性分析

本研究基于VOF算法编写用户UDF(自定义函数),采用FLUENT软件建立了椭圆横管外降膜流动和换热的计算模型。根据CFD(计算流体力学)模型计算和分析了在不同长短轴比下管外降膜速度分布、压力分布、液膜内温度分布和管外换热分布的变化规律。研究结果表明:长短轴比的变化影响了管外液膜速度分布、压力分布和膜内温度分布;相比圆管,椭圆管的管外膜内液体流速更快。壁面压力沿周向逐渐减少并在X=0.9附近快速回升;随长短轴比e的增加,周向压力最小值位置逐渐向后推移。局部Nu数分布与压力分布在趋势上存在一致性。当e=1.65附近时,椭圆的换热性能最优;最后,通过对管形的研究分析,提出横管的传热分区模型。     

The first and second law analysis of a lithium bromide/water coil absorber

The aim of this paper is to study the irreversibilities in a coil absorber using lithium bromide solution and to determine the variation of the second law efficiency with some variables such as cooling water flow rate, solution flow rate, cooling water temperature and solution concentration. The influence of absorber performance parameters is examined on the basis of the first and second laws of thermodynamics for parallel and counter-current types. In this regard, the heat and mass transfer, the second law efficiency, the magnitude and place of exergy losses in two types of absorbers are estimated and discussed comprehensively. The results showed that increasing the cooling water flow rate and decreasing the cooling water inlet temperature increase the heat and mass transfer, and decrease the second law efficiency. The effect of the solution concentration on the efficiency in general is small. Whereas the irreversibility for the counter-current mode is greater than that of the parallel-current mode, the heat-mass transfer 3–19% and the second law efficiency 1–12% are higher.     

Free convection heat and mass transfer to steady flow in a semi-infinite vertical porous medium

Analytical solutions are derived for flow in a semi-infinite vertical porous medium with heat and mass transfer. When the temperature and mass concentration are uniform a constant pressure is possible and sustains a fully developed flow. Thereafter there is a small perturbation of the wall temperature and concentration, and the subsequent two-dimensional problem is tackled for large Prandtl number and free convection parameters and small Reynolds number. The heat transfer rate at the wall is discussed quantitatively.     

The effect of absorber packing height on the performance of a hybrid liquid desiccant system

A hybrid system consisting of vapour compression unit, a liquid desiccant system, and a flat solar hot water collector were designed, fabricated and tested. This combination allowed for a separate control of humidity and temperature without energy penalty. Various packing heights of the absorber component were tested to determine the optimal performance of the combined unit. A 1000 mm packing height with cross-sectional area of 600×600 mm, proved to be the best height that gives promising improvements in the coefficient of performance of the vapour compression unit.     

Numerical simulation of horizontal tube bundle falling film flow pattern transformation

It is important to study the falling-film pattern of a horizontal tube bundle in order to set up a heat and mass transfer model accurately. The falling-film pattern of a horizontal tube bundle is simulated in this paper. The technique is based on computational flow dynamics (CFD) for the two-phase flow of gas and water. The experimental results were used to validate the mathematical model. It indicates that the simulation results accord with experimental data well. The simulated results show that the flow pattern varies with different flow rates. Under the different flow rates, it observes the droplet, droplet-columnar, columnar, columnar-sheet and sheet flow patterns. The critical value is 0.0125 kg/s between droplet and columnar, and the critical value is 0.02 kg/s between columnar and sheet.     

Experimental research on a new solar pump-free lithium bromide absorption refrigeration system with a second generator

This paper is concerned with experimental research on a new solar pump-free lithium bromide absorption refrigeration system with a second generator. By using the second generator together with a lunate thermosiphon elevation tube, the required minimum driving temperature of the heat source is only 68 °C compared to above 100 °C in traditional absorption refrigeration systems. Based on the horizontal-tube falling-film method, the performance of the absorber can be enhanced by the second generator due to an increase in the differential concentration of the solution between the inlet and the outlet of the absorber and an increase in the temperature difference between the inlet and the outlet of the cooling water in the absorber. The yield of condensate with the second generator open is increased by 68% compared to that with the second generator closed. The performance of the evaporator is significantly improved due to the increase in temperature drop of the chilled water and the decrease in the outlet temperature of the chilled water. This leads to an improvement of the performance of the overall refrigeration system. The maximum coefficient of performance (COP) approaches 0.787.     

Gas-liquid mass transfer in a cross-flow hollow fiber module: Analytical model and experimental validation

The cross-flow operation of hollow fiber membrane contactors offers many advantages and is preferred over the parallel-flow contactors for gas-liquid mass transfer operations. However, the analysis of such a cross-flow membrane gas-liquid contactor is complicated due to the change in concentrations of both phases in the direction of flow as well as in the direction perpendicular to flow. In addition, changes in the volumetric flow rate of compressible fluid can also occur over the volume of membrane contactor. These hollow fiber membrane contactors resemble to the more conventional shell and tube cross-flow heat exchanges where a similar variation in the local driving force within the module occurs. Hence heat transfer analogy can be applied to predict the performance of these contactors.Analytical expressions are derived in this work to describe the mass transfer in these hollow fiber cross-flow contactors analogously to heat transfer in cross-flow shell and tube heat exchangers. CO₂ absorption experiments were carried out in a commercial as well as in the lab-made single-pass cross-flow hollow fiber membrane contactors to check the applicability of this heat transfer analogy under different conditions. Experimental results show that the derived analytical expressions can be applied to the cross-flow membrane gas-liquid contactor under the asymptotic conditions of negligible or small volumetric flow changes. However, in the case of significant changes in the flow rate of compressible fluid, the application of heat transfer analogy results into slight under predictions of the module performance. A more rigorous model is then required for an accurate prediction of the performance.     

Development of CIGS2 solar cells with lower absorber thickness

The availability and cost of materials, especially of indium can be a limiting factor as chalcopyrite based thin-film solar cells advance in their commercialization. The required amounts of metals can be lowered by using thinner films. When the thickness of the film decreases, there is possibility of remaining only in the small grain region because the coalescence of grains does not have an opportunity to enhance the grain size to the maximum. Solar cell performance in smaller grain chalcopyrite absorber deteriorates due to larger fraction of grain boundaries. Efforts are being made to reduce the thickness while maintaining the comparable performance. This work presents a study of preparation, morphology and other material properties of CIGS2 absorber layers with decreasing thicknesses up to 1.2 μm and its correlation with the device performance. Encouraging results were obtained demonstrating that reasonable solar cell efficiencies (>10%) can be achieved even for thinner CIGS2 thin-film solar cells.     

Analysis of effective wetting area of a horizontal generator for an absorption heat transformer

The use of thermal processes for water purification is a good alternative to reduce primary energy consumption. The efficiency in heat exchange process by falling film plays a very important role in the performance of a heat pump. The objective of this paper is to evaluate the efficiency of heat transfer on a tube bank, of a heat transformer designed for water purification with LiBr–H₂O as absorbent mixture, by digital image processing. To analyze the wetting area and falling film behavior of the mixture on a bank of sixteen horizontal tubes, of 1.22 cm outer diameter and 30 cm in length, experimental tests were performed. The results show the distribution of absorbent mixture, along the tubes of the bank, with 0 and 5° inclination respect to the horizontal, and different mass flow rates per unit length (0.006–0.034 kg/m s). The film thickness of LiBr–H₂O was determined by the digital image processing, obtaining an average value of 0.033 cm. The most regular fall of mixture, through the tubes of the bank, was obtained with the flow of 0.025 kg/m s. This mass flow was the one which gave the best results of efficiency of heat transfer, with values between 80 and 98%.     

Mixed free convection and mass transfer flow along a vertical needle

The combined free convection and mass transfer flow in a plume over a vertical needle is studied. This mixed type of flow is produced from a point heat-mass source at the tip of the needle. A numerical solution of the similarity equations of the problem under consideration is obtained. The velocity, temperature and concentration profiles are shown for different values of the dimensionless parameters entering into the problem under consideration. The flow field is greatly influenced by the dimensionless parameters α (heat-source strength) and β (mass-source strength).     

Simultaneous measurement of local film thickness and temperature distribution in wavy liquid films using a luminescence technique

Heat transfer in falling liquid film systems is enhanced by waviness. Comprehension of the underlying kinetic phenomena requires experimental data of the temperature field with high spatiotemporal resolution. Therefore a non-invasive measuring method based on luminescence indicators is developed. It is used to determine the temperature distribution and the local film thickness simultaneously. First results are presented for the temperature distribution measurement in a laminar-wavy water film with a liquid side Reynolds number of 126 flowing down a heated plane with an inclination angle of 2°. The measured temperature distributions are used to calculate the local heat transfer coefficient and the convective heat flux perpendicular to the wall for different points in the development of a solitary wave.     

Characteristics of falling film flow on completely wetted horizontal tubes and the associated gas absorption

Falling water films on completely wetted horizontal tubes of 16 mm diameter in a vertical row and the associated oxygen gas absorption were experimentally investigated in the Reynolds number range of 10<Re<150. It was found that the falling films form continuous sheets between the tubes when the tube spacing L_s is 2 mm, and that the film surfaces are smooth for Re<30 whereas fine random waves appear on the surfaces for Re>30. When L_s=5 mm or larger, dripping from each tube occurs and causes the larger amplitude waves which rapidly spread on the film on the lower tube.The Sherwood number, Sh, greatly increases with an increase in L_s from 2 to 5 mm and then levels off at L_s=10 mm or larger. When L_s=2 mm, Sh is in proportion to Re^1.15 for Re<30 and to Re^0.9 for Re>30. The Sh varies as Re^0.86 for the whole Re range at L_s=5-15 mm. Comparison of the present results with empirical equations for vertical tube absorber indicates that volume of horizontal tube absorber can be smaller than that of vertical tube absorber by a factor of 1/1.18-1/2.2.     

Mass transfer and free convection flow through A porous medium

A theoretical analysis of the steady free convective and mass transfer flow is presented, when a viscous and incompressible fluid flows through a porous medium occupying a semi-infinite region of the space bounded by an infinite vertical porous plate. The fluid is subjected to a normal suction velocity, and the heat flux at the plate is constant. The free-stream velocity is assumed constant.     

Effects of mass transfer and free convection on the unsteady mhd flow past a vertical porous plate with variable suction

In this paper we investigate the effects of mass transfer on the unsteady free convective flow of an electrically conducting and viscous incompressible fluid past an infinite vertical plate subjected to variable suction, in the presence of transverse magnetic field, when the free-stream velocity oscillates with time in magnitude but not in direction. In this analysis, the effects of the induced magnetic field is neglected. Approximate solutions to the transient flow, the amplitude and phase of skin-friction and the rate of heat transfer have been derived. During the course of the discussion, the effects of Gr, Gc, Sc, Ec, M and ω have been presented.