Two-Dimensional Numerical Analysis of Membrane-Based Heat and Mass Cross-Flow Exchanger

ABSTRACT

A two-dimensional numerical simulation model for a membrane-based heat and mass exchanger was developed. The system model equations were used to determine the coupled heat and moisture transfer from the humid air to the high concentrated liquid desiccant solution (LiCl, lithium chloride) by means of a parallel stack hydrophobic permeable membrane. The two streams of air and liquid desiccant solution were arranged in cross-flow directions. The fourth-order Runge–Kutta method was employed to solve these system model equations in a steady-state condition. This model enables one to predict the latent effectiveness of a membrane-based parallel cross-flow exchanger for dehumidification purpose in response to air to liquid mass flow ratio and the mass transfer unit number.     

A numerical solution for cooling and dehumidification of air by a falling desiccant film in parallel flow

The heat and mass transfer process between falling liquid desiccant film and air in parallel flow heat exchanger is investigated numerically. The governing equations with appropriate boundary and interfacial conditions describing the physical problem are derived. The control volume approach is used to predict the outlet conditions for both the air and the desiccant solution. The effect of inlet conditions, mass flow rates and channel geometry on the air cooling and dehumidification processes is also predicted. The average Nusselt and Sherwood numbers for air flow are correlated in terms of Prandtl number, Schmidt number and channel geometry. Typical numerical experiments showed good agreement of the present results with the available data in literature. Moreover, a parametric study is conducted to illustrate the general effects of various variables on heat and mass transfer processes in cooling and dehumidification of air.     

Conjugate heat and mass transfer in a hollow fiber membrane module for liquid desiccant air dehumidification: A free surface model approach

Conjugate heat and mass transfer in a hollow fiber membrane module used for liquid desiccant air dehumidification is investigated. The module is like a shell-and-tube heat exchanger where the liquid desiccant stream flows in the tube side, while the air stream flows in the shell side in a counter flow arrangement. Due to the numerous fibers in the shell, a direct modeling of the whole module is difficult. This research takes a new approach. A representative cell comprising of a single fiber, the liquid desiccant flowing inside the fiber and the air stream flowing outside the fiber, is considered. The air stream outside the fiber has an outer free surface (Happel’s free surface model). Further, the equations governing the fluid flow and heat and mass transfer in the two streams are combined together with the heat and mass diffusion equations in membranes. The conjugate problem is then solved to obtain the velocity, temperature and concentration distributions in the two fluids and in the membrane. The local and mean Nusselt and Sherwood numbers in the cell are then obtained and experimentally validated.     

Comparative study between parallel and counter flow configurations between air and falling film desiccant in the presence of nanoparticle suspensions

A comparative numerical study is employed to investigate the heat and mass transfer between air and falling film desiccant in parallel and counter flow configurations. Nanoparticles suspensions are added to the falling film desiccant to study heat and mass transfer enhancements. The numerical results show that the parallel flow channel provides better dehumidification and cooling processes of the air than counter flow configuration for a wide range of pertinent parameters. Low air Reynolds number enhances the dehumidification and cooling rates of the air and high air Reynolds number improves the regeneration rate of the liquid desiccant. An increase in the channel height results in enhancing the dehumidification and cooling processes of air and regeneration rate of liquid desiccant. The dehumidification and cooling rates of air are improved with an increase in the volume fraction of nanoparticles and dispersion factor. Copyright © 2003 John Wiley & Sons, Ltd.     

An experimental study of a cross-flow type plate heat exchanger for dehumidification/cooling

The thermal and dehumidification behaviour of a standard cross-flow type plate heat exchanger, intended for use as a dehumidifier/cooler, has been investigated both experimentally and numerically. Three sets of experiments have been carried out where air is blown into the primary and secondary sides of the exchanger, while water and liquid desiccant were being sprayed in a counter flow arrangement. The first set represents the indirect evaporative cooling of the primary stream by the secondary air stream. The second set is with liquid desiccant only and no indirect evaporative cooling. In the third set of experiments the primary air stream is indirectly evaporatively cooled by the secondary air stream and dehumidified by the liquid desiccant sprayed into the primary side of the exchanger. The above experiments indicate that the heat exchanger performs well when used with liquid desiccant. Furthermore, for an exchanger angle of 45°, there is an optimum value of air mass flow rate at which the effectiveness and dehumidification efficiency of the plate heat exchanger are maxima. To investigate the effect of the ambient air conditions on the PHE performance, further experiments were carried out using a heater element and a humidifier. The results show that under laboratory conditions the exchanger effectiveness and dehumidification efficiency increase with increasing primary air inlet temperature and humidity ratio. The experimental results were used to validate a computer model developed for the cross-flow type plate heat exchanger/dehumidifier. Comparison indicates that the numerical results are in good agreement with the experiments.     

Research progress in liquid desiccant air-conditioning devices and systems

The developments on liquid desiccant air-conditioning systems were illustrated and summarized in this paper. In order to obtain a better dehumidification (or humidification) performance, liquid desiccant should be cooled (or heated) rather than air. Two fundamental modules were proposed, including basic spray module with extra heat exchanger and total heat recovery device, which could be combined to set up various kinds of liquid desiccant air processors. The operating principle of heat pump-driven outdoor air processor as well as heat-driven outdoor air processor was analyzed. The COP_air of the heat pump (or power)-driven outdoor air processor could be as high as 5.0 both in summer and in winter operating conditions. The COP_air of the hot water-driven processor (65°C–80°C) was 1.19 and 0.93, respectively, using evaporative indoor exhaust air or cooling water to cool the dehumidification process. The liquid desiccant air processor-based temperature and humidity-independent control air-conditioning system could save 20%–30% operating energy compared with the conventional air-conditioning system.     

Control performance of a dedicated outdoor air system adopting liquid desiccant dehumidification

Liquid desiccant is energy efficient for dehumidification in air-conditioning systems. In this study, a novel dedicated outdoor air system (DOAS) adopting lithium chloride solution as liquid desiccant is proposed to process supply air. The DOAS mainly consists of a membrane-based total heat exchanger, a liquid dehumidifier, a regenerator and a dry cooling coil. It can realize independent temperature and humidity controls for supply air. Control strategies for the supply air dehumidification and cooling process as well as the desiccant solution regeneration process in the DOAS are developed and verified. The control performances of the proposed dedicated outdoor air system are investigated at different operation conditions by simulation tests. The results show that the DOAS is more suitable for hot and humid climates. The effects of the total heat exchanger on the performance of the DOAS are also evaluated. It can improve the system energy performance by 19.9–34.8%.     

Analytical solution of combined heat and mass transfer performance in a cross-flow packed bed liquid desiccant air dehumidifier

Heat and mass transfer between air and liquid desiccant in a cross-flow packed bed dehumidifier is investigated. Analytical solutions of air and desiccant parameters as well as enthalpy and moisture efficiencies are given in the present study, based on the analogy between the combined heat and mass transfer process in the cross-flow dehumidifier and the heat transfer process in the cross-flow heat exchanger. The results given by the analytical solution are compared with numerical solutions and experimental findings. Good agreement is shown between the analytical solutions and the numerical or experimental results. The analytical solutions can be used in the optimization of the cross-flow dehumidifier.     

Research progress in liquid desiccant air-conditioning devices and systems

The developments on liquid desiccant air-conditioning systems were illustrated and summarized in this paper. In order to obtain a better dehumidification (or humidification) performance, liquid desiccant should be cooled (or heated) rather than air. Two fundamental modules were proposed, including basic spray module with extra heat exchanger and total heat recovery device, which could be combined to set up various kinds of liquid desiccant air processors. The operating principle of heat pump-driven outdoor air processor as well as heat-driven outdoor air processor was analyzed. The COP_air of the heat pump (or power)-driven outdoor air processor could be as high as 5.0 both in summer and in winter operating conditions. The COP_air of the hot water-driven processor (65°C–80°C) was 1.19 and 0.93, respectively, using evaporative indoor exhaust air or cooling water to cool the dehumidification process. The liquid desiccant air processor-based temperature and humidity-independent control air-conditioning system could save 20%–30% operating energy compared with the conventional air-conditioning system.     

+Transient characteristics of a liquid-to-air membrane energy exchanger (LAMEE) experimental data with correlations

In this paper, the transient and steady-state performance of a liquid to air membrane energy exchanger (LAMEE) is investigated experimentally. The transient sensible, latent and total effectiveness are each presented for particular air and desiccant mass flow rate for both summer and winter supply air test conditions. After a step change in inlet conditions the effectiveness increases with time. Also, the transient effectiveness is assessed for various air and desiccant mass flow rates under summer test conditions and the LAMEE’s time constant is investigated as an important variable that depends primarily on the thermal capacity of the exchanger and liquid content. The calculated effectiveness shows considerable dependency on the air and desiccant mass flow rate. Finally, steady-state results reveal that as the desiccant mass flow rate increases or air mass flow rate decreases, the effectiveness increases.     

A new method for determining coupled heat and mass transfer coefficients between air and liquid desiccant

This paper presented the characteristic of liquid desiccant dehumidification based on NTU–Le model. The results showed that the Lewis number Le had little effect on air outlet humidity ratio during desiccant solution dehumidification process. A new method called h_D–Le separative evaluation method was developed for determining coupled heat and mass transfer coefficients between air and liquid desiccant, through which the heat and mass transfer coefficients between air and liquid desiccant were calculated to obtain from experimental inlet and outlet parameters of air and desiccant solution. The effects of the air volume flow rate, temperature, humidity ratio and the solution concentration, temperature on the Lewis number, heat and mass transfer coefficient were analyzed according to experimental data and the h_D–Le separative evaluation method. Based on the computation results, it was concluded that the Lewis number greatly depended on the operation parameters and conditions of the air and desiccant. In addition, the correlations of the heat and mass transfer coefficients were developed. The additional 74 groups of experiments validated the developed correlations by comparison of air/solution parameters change with the calculation data.     

Modelling and performance analysis of a cross-flow type plate heat exchanger for dehumidification/cooling

This paper describes the performance analysis of a cross-flow type plate heat exchanger for use as a liquid desiccant absorber (dehumidifier) and indirect evaporative cooler. The proposed absorber can be described as a direct contact, cross-flow, heat and mass exchanger, with the flow passages separated from each other by thin plastic plates. One air stream (primary air) is sprayed by liquid desiccant solution, while the other stream (secondary air) is evaporatively cooled by a water spray. Each thin plate, besides separating the water/air passage from the solution/air passage, also provides the contact area for heat and mass transfer between the fluids flowing in each passage. A parametric study for the primary air stream at 33°C, 0.0171 kg/kg humidity ratio and secondary air stream at 27°C and 0.010 kg/kg humidity ratio using calcium chloride solution was performed in this study. The results showed a strong dependence on the heat and mass transfer area, solution concentration and ratio of secondary to primary air mass flow rates. However, negligible differences were found between the performance of a counter flow and a parallel flow arrangement. The results demonstrate that the proposed absorber will not offset both the latent and sensible load of the primary air and, therefore, an auxiliary cooler or more dehumidification/indirect evaporative cooling stages will generally be required to meet the sensible and latent load in a typical comfort application.     

Coupled heat and mass transfer characteristic in packed bed dehumidifier/regenerator using liquid desiccant

The dehumidifier and regenerator are two key components in liquid desiccant air conditioning systems. The heat transfer driving force and the mass transfer driving force influence each other, the air and desiccant outlet temperatures or humidity ratio may exceed the air and desiccant inlet parameters in the dehumidifier/regenerator. The uncoupled heat and mass transfer driving forces, enthalpy difference and relative humidity difference between the air and desiccant are derived based on the available heat and mass transfer model and validated by the experimental and numerical results. The air outlet parameter reachable region is composed of the air inlet isenthalpic line, the desiccant inlet equivalent relative humidity line and the linkage of the air and desiccant inlet statuses. Except the mass flow rate ratio and the heat and mass transfer coefficients, the air and desiccant inlet statuses and flow pattern have great effects on the dehumidifier/regenerator performance. The counter flow configuration expresses the best mass transfer performance in the dehumidifier and the hot desiccant driven regenerator, while the parallel flow configuration performs best in the hot air driven regenerator.     

Thermodynamic analysis of a counter flow adiabatic dehumidifier with different liquid desiccant materials

Liquid desiccant systems have been proposed as energy saving alternatives to the conventional vapor compression systems for handling the latent load. This paper presents the results from a study of the performance of a counter flow liquid desiccant dehumidifier. A heat and mass transfer theoretical model of an adiabatic packed column has been developed, based on the Runge-Kutta fixed step method, to predict the performance of the device under various operating conditions. Good agreement was found between experimental tests and the theoretical model, with the maximum deviation being ±2.9% in air outlet temperature, ±15.9% in air outlet humidity ratio and ±2.8% in solution outlet temperature. Following the model validation, the rate and the efficiency of the dehumidification process were assessed under the effects of variables, such as air temperature and humidity, desiccant temperature and humidity and air and desiccant flow rates. The three most commonly used liquid desiccant solutions, namely LiCl, LiBr and CaCl₂ were evaluated against each other. The results show that high absorber efficiency and system efficiency could be achieved under humid conditions, low air mass flow rates and LiCl as the desiccant solution.     

Fluid flow and heat mass transfer in membrane parallel-plates channels used for liquid desiccant air dehumidification

Fluid flow and convective heat mass transfer in membrane-formed parallel-plates channels are investigated. The membrane-formed channels are used for liquid desiccant air dehumidification. The liquid desiccant and the air stream are separated by the semi-permeable membrane to prevent liquid droplets from crossing over. The two streams, in a cross-flow arrangement, exchange heat and moisture through the membrane, which only selectively permits the transport of water vapor and heat. The two flows are assumed hydrodynamically fully developed while developing thermally and in concentration. Different from traditional method of assuming a uniform temperature (concentration) or a uniform heat flux (mass flux) boundary condition, the real boundary conditions on membrane surfaces are numerically obtained by simultaneous solution of momentum, energy and concentration equations for the two fluids. Equations are then coupled on membrane surfaces. The naturally formed boundary conditions are then used to calculate the local and mean Nusselt and Sherwood numbers along the channels. Experimental work is performed to validate the results. The different features of the channels in comparison to traditional metal-formed parallel-plates channels are disclosed.     

Conjugate heat and mass transfer in a cross-flow hollow fiber membrane contactor for liquid desiccant air dehumidification

The fluid flow and conjugate heat and mass transfer in a cross-flow hollow fiber membrane contactor are investigated. The shell-and-tube like contactor is used for liquid desiccant air dehumidification, where numerous fibers are packed into the shell and air flows across the fiber bank. To overcome the difficulties in the direct modeling of the whole contactor, a representative cell, which comprises of a single fiber, a liquid solution inside the fiber, and an air stream across the fiber, is selected as the calculation domain. The air stream in the cell is surrounded by an assumed outer free surface. The equations governing the fluid flow and heat and mass transfer in the two cross-flow streams are solved together with the heat and mass diffusion equations in the membrane. The friction factor and the Nusselt and Sherwood numbers on the air and stream sides are then calculated and experimentally validated.     

Investigation of heat and mass transfer in a gauze‐type structured packing liquid desiccant dehumidifier

In this experimental investigation, a packed bed column suitable for 5‐ton hybrid cooling system has been designed to study the absorption of water vapour from moist air by contact with aqueous solutions of calcium chloride. The packing material used in the study was two elements of the BXPEP structured packing and the height of the each element was 17 cm. This packed bed dehumidifier handles desiccant flow rates from 10 to 32 l/min. This paper presents results from a detailed experimental investigation of the heat and mass transfer between a liquid desiccant (calcium chloride) and air in a gauze‐type structured packing dehumidifier. The effects of different independent variables such as air inlet absolute humidity, desiccant inlet temperature, flow rate and its concentration on the performance of the dehumidifier have been investigated. Copyright © 2002 John Wiley & Sons, Ltd.     

Theoretical and experimental investigation on the application of solar water heater coupled with air humidifier for regeneration of liquid desiccant

Theoretical and experimental investigation on the application of flat plate solar water heater coupled with air humidifier for regeneration of liquid desiccant has been presented in this work. The heated water from the storage tank of the solar heating system is circulated in a finned tube air heater. Hot air from the air heater is blown through a packing of a honeycomb type for the purpose of regeneration of calcium chloride (CaCl₂) solution. An experimental system has been designed and installed for this purpose. The system comprises a solar water heater with a storage tank connected to an air/water heat exchanger. Hot air from the heat exchanger is blown to the air humidifier, which functions in this study as a regenerator. Calcium chloride solution is applied as the working desiccant in this study. Solution concentration is determined at the end of regeneration process and the mass of evaporated water is evaluated. It is observed that the heating temperature varies, at day time, in a range of about 5 °C. This limited variation in hot water temperature demonstrates the importance of the storage tank to attain a nearly steady state operation of the system. Experimental results show that solution with 30% concentration can be regenerated up to 50% using solar energy. In the theoretical part of this study, a multiple-layer artificial neural network (ANN) model has been applied to study the performance of a solar liquid-desiccant dehumidification/regeneration system when calcium chloride solution is applied as the working desiccant. The experimental results of the present study are used to construct and test the ANN model. Then the model has been utilized to describe and analyze the effect of the inlet conditions of air on the regeneration process. Good agreement between the outputs from the ANN model and the corresponding results from the experimental data has been found. The proposed model can work well as a predictive tool to complement the experiments.     

转轮复合式空调系统的数值计算及能耗分析

主要研究了以太阳能作为再生热源的转轮除湿和蒸气压缩制冷相结合及转轮除湿、蒸气压缩和蒸发冷却相结合的2种复合式空调系统,同时对电能作为再生热源的上述空调系统进行研究,建立了系统的物理模型,并对系统性能参数进行数学描述。通过与相同条件下常规蒸气压缩空调系统的比较分析,得出复合式空调系统制冷剂质量流量分别减少50.20％和66.67％；压缩系统性能系数COP分别提高了26.49％和32.16％；压缩机能耗分别节省了62.64％和76.92％。电能作为再生热源时,总负荷能耗分别节省了32.68％和42.00％；当采用太阳能作为再生热源时,总负荷能耗节省更多的能量,分别为61.86％和71.16％(认为1kW电能等价于3kW热能)。研究还发现,室内相对湿度相同,随室内设计温度的提高,复合式系统压缩机能耗明显减少,节能率呈上升趋势；相反总负荷能耗的节能率呈下降趋势。干热气候条件下,系统节能较为明显：71.75％和85.96％(电能再生)。热湿气候条件下,系统节能不明显,甚至消耗更多能量,而采用太阳能时,复合式系统均具有明显节能效果。     

Improvement of Dehumidification Performance of Desiccant Rotors Regenerated at a Low Temperature

This paper presents a highly effective desiccant rotor that can be regenerated at a temperature between 20 and 30°C, corresponding to return air exhausted from conditioned spaces. The desiccant rotor consists of a honeycomb structure, which is coated with organic polymer desiccant materials. For a specific operating condition, the desiccant rotor functions as a rotary total heat exchanger. Desiccant rotors with thickness of 0.2 m and more lead to both higher dehumidification and temperature efficiencies compared to conventional total rotary heat exchangers in different states of the inlet process and regeneration airflows. Both the dehumidification and temperature efficiencies achieve 100% at a thickness of 0.4 m, and at rotational speeds between 100 and 300 rph. Dehumidification, together with cooling, is very effective. For the desiccant rotor with a thickness of 0.4 m, the humidity change of the process air corresponds closely to isothermal dehumidification. In terms of the dehumidification and cooling functions, the performance of the desiccant rotor with thickness of 0.2 m and more is very advantageous compared to conventional desiccant rotors and rotary total heat exchangers.