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.     

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.     

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.     

Modeling of Air to Air Enthalpy Heat Exchanger

The thermal performance of a Z-shaped enthalpy heat exchanger utilizing 45-gsm Kraft paper as the heat and moisture transfer surface for heating, ventilation, and air conditioning (HVAC) energy recovery is experimentally investigated through temperature and moisture content measurements. A mathematical model is developed and validated against the experimental results using the effectiveness-NTU method. In this model the paper moisture transfer resistance is determined by paper moisture permeability measurements. Results showed that the paper moisture transfer resistance is not constant and varies with moisture gradient across the paper. Furthermore, the model is used to predict the heat exchanger performance for different heat exchanger flow configurations. The results showed that higher effectiveness values are achieved when the heat exchanger flow path width is reduced. Temperature and moisture distribution in the heat exchanger is also studied using a computational fluid dynamics package (FLUENT). To model the moisture transfer through the porous materials a nondimensional sensible–latent effectiveness ratio was developed to obtain the moisture boundary conditions on the heat exchanger surface.     

Three dimensional numerical modeling of simultaneous heat and moisture transfer in a moist object subjected to convective drying

The drying behavior of a moist object subjected to convective drying is analyzed numerically by solving heat and moisture transfer equations. A 3-D numerical model is developed for the prediction of transient temperature and moisture distribution in a rectangular shaped moist object during the convective drying process. The heat transfer coefficients at the surfaces of the moist object are calculated with an in-house computational fluid dynamics (CFD) code. The mass transfer coefficients are then obtained from the analogy between the thermal and concentration boundary layer. Both these transfer coefficients are used for the convective boundary conditions while solving the simultaneous heat and mass transfer governing equations for the moist object. The finite volume method (FVM) with fully implicit scheme is used for discretization of the transient heat and moisture transfer governing equations. The coupling between the CFD and simultaneous heat and moisture transfer model is assumed to be one way. The effect of velocity and temperature of the drying air on the moist object are analyzed. The optimized drying time is predicted for different air inlet velocity, temperature and moisture content. The drying rate can be increased by increasing the air flow velocity. Approximately, 40% of drying time is saved while increasing the air temperature from 313 to 353 K. The importance of the inclusion of variable surface transfer coefficients with the heat and mass transfer model is justified.     

Combined heat and mass transfer in natural convection on a vertical flat plate

In analyzing the combined heat and mass transfer in natural convection, most of the surface conditions are either maintained at an uniform wall temperature and uniform wall concentration or subjected to an uniform heat flux and uniform mass fluc. Other conditions are seldom investigated. This study is to investigate the effects of the coupled thermal and mass diffusion on the natural convection of a vertical plate for a moist air system. The surface conditions of the plate are uniform heat flux and uniform relative humidity. A finite difference numerical method is used to solve the governing equations simultaneously. The results that the relative humidity of the surface is both larger and smaller than that of the ambient are examined in detail.     

Theoretical and experimental investigations of a liquid desiccant filmed cellulose fibre heat and mass exchanger

This paper presented theoretical and experimental investigations of a liquid desiccant filmed cellulose fibre heat and mass exchanger, a new type of exchanger with the potential to be an alternative to a conventional exchanger. Owing to the complexity of the desiccant assisted heat and mass transfer and difficulty in determining its associated parameters, work started from the simulation of a clear fibre exchanger by developing a dedicated numerical model, and its validation by using the data from the manufacturer of the exchanger. Further to this, laboratory testing was carried out with the same exchanger, but filmed with a liquid desiccant fluid, i.e. LiCl. Comparison between the data of the clear and desiccant filmed exchangers suggested the use of correction factors for heat and mass transfer resistances with desiccant operation. A revised model for the desiccant filmed exchanger was then established taking into account the correction factors. By using the updated model, influence of geometrical sizes and operating conditions of the liquid desiccant filmed exchanger on the exchanger efficiency were studied and the optimal values of these were obtained. The results indicated that the exchanger efficiencies (heat, mass and enthalpy) are largely dependent upon the exchanger channel length, air flow rate and less related to the exchanger channel height, intake air temperature and intake‐to‐outgoing air moisture content difference. It was also suggested that the air speed across the channels should be in the range 0.5–1.5 m s^?1. The height of air channel (passage) should be set at 6.5 mm or below and its length should be 1.0 m or more. A simulation was carried out under UK typical summer operation conditions, i.e. the intake air streams at 30°C db and 70% rh and outgoing air streams at 24°C db and 50% rh, and the results indicated that the exchanger with the above recommended geometrical sizes can achieve an energy efficiency of 87%, which is 30% higher than for non‐desiccant filmed operation. Copyright © 2009 John Wiley & Sons, Ltd.     

Effect of wavy plate geometry configurations on the temperature and flow distributions

As fluid flowing through the wavy plate, breaking and destabilizing in the thermal boundary layer are induced. In the present study, the numerical investigation on the heat transfer and flow distributions in the channel with various geometry configuration wavy plates under constant heat flux conditions is considered. A finite volume method with the structured uniform grid system is used to solve the turbulent model. Effects of geometry configuration of wavy plates, wavy plate arrangements, and air flow rates on the temperature and flow developments are considered. The sharp edge of wavy plate has a significant effect on the flow structure and heat transfer enhancement. The results of this study are expected to lead to guidelines that will allow the selected wavy plate geometry configuration for designing heat exchanger which increase thermal performance.     

Experimental Study on Air Cooling Via a Multiport Mesochannel Cross-Flow Heat Exchanger

The air-side heat transfer and flow characteristics of cross-flow multiport slab mesochannel heat exchanger are investigated experimentally in this article. The multiport slab mesochannel heat exchanger consists of 15 finned aluminum slabs; each slab contains 68 flow channels of 1 mm circular diameter. The cold deionized water at a constant mass flow rate was forced to flow through the mesochannels, whereas the hot air at different velocities was allowed to pass through the finned passages of the heat exchanger core in cross-flow orientation. The heat transfer and fluid flow key parameters were examined in the region of the air-side Reynolds number in the range of 972–2758, with a constant water-side Reynolds number of 135. The effect of air-side Reynolds number on air-side Nusselt number was examined and a general correlation of Nusselt number with Reynolds number was obtained. The Nusselt number value was found to be higher in comparison with other research works for the corresponding Reynolds number range. The multiport mesochannel flat slab geometry has offered uniform temperature distribution into the core. This uniform temperature distribution leads to higher heat transfer over stand-alone inline flow tube bank.     

A General Matrix Approach to Model Steady-State Performance of Cross-Flow Heat Exchangers

A steady-state performance model of multirow multipass cross-flow tubular heat exchangers is developed. The proposed matrix approach uses the concepts of local effectiveness, energy balance, and number of transfer units (NTU) applied to every pass/row in the cross-flow heat exchanger to predict thermal performance. The method can predict the total effectiveness of assemblies of heat exchangers. Several circuiting configurations, such as overall counter-cross-flow, overall parallel cross-flow, and fluids in parallel in one of the streams, were considered. Predictions of the steady heat transfer performance of selected multirow multipass cross-flow heat exchangers are obtained by applying the general matrix approach. The heat exchanger geometries selected for the comparative study represent common cross-flow heat exchanger configurations used in industry. For these heat exchangers the overall heat exchanger effectiveness values were computed for various capacity rate ratios and NTU values. The validity of the matrix approach was then verified by comparing the resulting predictions with those obtained using the P-NTU approach and the Domingos method for the selected complex cross-flow heat exchanger configurations.     

Effects of Non-Darcian and Inlet Conditions on the Forced Convection Along a Vertical Plate With Film Evaporation

The present study analyzes theoretically the non-Darcian effects and inlet conditions of forced convection flow with liquid film evaporation in a porous medium. The physical scheme includes a liquid–air streams combined system; the liquid film falls down along the plate and is exposed to a cocurrent forced moist air stream. The axial momentum, energy, and concentration equations for the air and water flows are developed based on the steady two-dimensional (2-D) laminar boundary layer model. The non-Darcian convective, boundary, and inertia effects are considered to describe the momentum characteristics of a porous medium. The paper clearly describes the temperature and mass concentration variations at the liquid–air interface and provides the heat and mass transfer distributions along the heated plate. Then, the paper further evaluates the non-Darcian effects and inlet conditions on the heat transfer and evaporating rate of liquid film evaporation. The numerical results show that latent heat transfer plays the dominant heat transfer role. Carrying out a parametric analysis indicates that higher air Reynolds number, higher wetted wall temperature, and lower moist air relative humidity will produce a better evaporating rate and heat transfer rate. In addition, a non-Darcy model should be adopted in the present study. The maximum error for predictions of heat and mass transfer performance will be 21% when the Darcy model is used.     

Heat Transfer Characteristics of Laminar Flow in Internally Finned Tubes under Various Boundary Conditions

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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.     

Thermally developing flow and heat transfer in rectangular microchannels of different aspect ratios

Laminar convective heat transfer in the entrance region of microchannels of rectangular cross-section is investigated under circumferentially uniform wall temperature and axially uniform wall heat flux thermal boundary conditions. Three-dimensional numerical simulations were performed for laminar thermally developing flow in microchannels of different aspect ratios. Based on the temperature and heat flux distributions obtained, both the local and average Nusselt numbers are presented graphically as a function of the dimensionless axial distance and channel aspect ratio. Generalized correlations, useful for the design and optimization of microchannel heat sinks and other microfluidic devices, are proposed for predicting Nusselt numbers. The proposed correlations are compared with other conventional correlations and with available experimental data, and show very good agreement.     

Entropy generation in a binary gas mixture in the presence of thermal and solutal mixed convection

Steady-state, laminar, fully-developed mixed convection of a binary non-reacting gas mixture flowing upwards in a vertical parallel-plate channel has been investigated from the point of view of the second law of thermodynamics. Analytical expressions are derived for the entropy generation rate for two combinations of boundary conditions: uniform wall temperature with uniform wall concentration (UWT-UWC) and uniform wall heat flux with uniform wall concentration (UHF-UWC). These expressions include three sources of irreversibility: heat conduction, fluid friction and species diffusion. In the UWT-UWC case, the entropy generation rate depends on the thermal and solutal boundary conditions and the mean velocity while in the UHF-UWC case it also depends on the streamwise coordinate. For humid air, the contribution of fluid friction is negligible for both cases while heat conduction and species diffusion effects appear to be of comparable orders of magnitude.     

Passive heat and moisture removal from a natural vented enclosure with a massive wall

Simultaneous transport of heat and moisture by conjugate natural convection in a partial enclosure with a solid wall is investigated numerically. Moist air motions are driven by the external temperature and concentration differences imposed across enclosures with different ambient moisture conditions. The Prandtl number and Schmidt number used are 0.7 and 0.6, respectively. The fluid, heat and moisture transports through the cavity and solid wall are, respectively, analyzed using the streamlines, heatlines and masslines, and the heat and mass transfer potentials are also explained by the variations of overall Nusselt and Sherwood numbers. The numerical simulations presented here span a wide range of the main parameters (heat and mass diffusion coefficient ratios, solid wall thickness and thermal Rayleigh numbers) in the domain of aiding and opposing buoyancy-driven flows. It is shown that the heat transfer potential, mass transfer potential, and volume flow rate can be promoted or inhibited, depending strongly on the wall materials and size, thermal and moisture Rayleigh numbers.     

Heat and moisture transfer in fibrous thermal insulation with tight boundaries and a dynamical boundary temperature

Prefabricated, lightweight building elements are widely used in the building construction sector. Such elements consist of fibrous thermal insulation encapsulated between two metal sheets. Under various circumstances, moisture can appear in the insulation matrix. Since the temperature of the boundary metal sheets changes dynamically with meteorological conditions, heat and mass transfer between boundaries appear in this case. This paper presents a transient model of the heat and mass transfer, including the sorption and condensation processes. A numerical model considers the dynamical changing of the boundary temperatures. A parametric study considering different amplitudes of temperature change, different moisture masses and different thicknesses of the insulation matrix was made. It was found that a relatively small mass of water in the insulation matrix can result in a significantly increased average heat flux during a periodic cycle. The numerical code was verified with experiments, which showed good agreement with the numerics.     

果蔬预冷冰浆式湿冷热湿交换器性能实验

针对果蔬预冷设备应用场合,提出并设计了一套以冰浆作为载冷介质的湿冷热湿交换器,并搭建单体性能测试台架,以出风温度和相对湿度为指标,通过改变填料类型(金属、纸质填料)、载冷介质种类(冰浆、冷水)和喷淋流量进行了性能实验研究。结果表明:实验工况下,金属填料的换热性能较纸质填料好;以冰浆作为载冷介质相比以冷水的情况,可以获得更低的出风温度,但出风相对湿度也有所降低;随着进风干球温度的降低,出风温度明显降低,而出风相对湿度变化并不明显;在一定范围内,提高载冷介质的喷淋流量,有利于湿冷热湿交换器出风温度的降低和出风相对湿度的升高;低浓度的冰浆可以在湿冷热湿交换器中稳定运行,且降温效果较冷水湿冷热湿交换器更加明显,虽然相对湿度略有下降但仍然可保持在90%左右,适用于果蔬预冷和保鲜。     

Heat pipe heat exchanger for heat recovery in air conditioning

The heat pipe heat exchangers are used in heat recovery applications to cool the incoming fresh air in air conditioning applications. Two streams of fresh and return air have been connected with heat pipe heat exchanger to investigate the thermal performance and effectiveness of heat recovery system. Ratios of mass flow rate between return and fresh air of 1, 1.5 and 2.3 have been adapted to validate the heat transfer and the temperature change of fresh air. Fresh air inlet temperature of 32–40 °C has been controlled, while the inlet return air temperature is kept constant at about 26 °C. The results showed that the temperature changes of fresh and return air are increased with the increase of inlet temperature of fresh air. The effectiveness and heat transfer for both evaporator and condenser sections are also increased to about 48%, when the inlet fresh air temperature is increased to 40 °C. The effect of mass flow rate ratio on effectiveness is positive for evaporator side and negative for condenser side. The enthalpy ratio between the heat recovery and conventional air mixing is increased to about 85% with increasing fresh air inlet temperature. The optimum effectiveness of heat pipe heat exchanger is estimated and compared with the present experimental data. The results showed that the effectiveness is close to the optimum effectiveness at fresh air inlet temperature near the fluid operating temperature of heat pipes.     

Coupled heat and mass transfer in an application-scale cross-flow hollow fiber membrane module for air humidification

This study is a step forward from previous researches involving hollow fiber membrane contactors for air humidification. A real application-scale cross-flow hollow fiber membrane module is investigated. The air stream flows transversely across the fiber bundle while being humidified. The novelty is that the shell-and-tube module is converted to a parallel-plates heat mass exchanger in the model set up. The equations governing the heat and moisture transfer from the water to the air, through the membranes, are described. The equations are then normalized with newly defined dimensionless parameters, which summarize the operating conditions and have clear physical meanings. Following this step, the two-variable two-dimensional partial differential equations are numerically solved. Tests are conducted to validate the model. Effects of varying operating conditions on system performance are discussed. It is found that the system is dominated by mass transfer in membranes with a total Lewis number larger than 10. The packing density has a direct influence on performances. In contrast, the geometry of fiber packing arrangement has a negligible effect. This is tremendously different from the traditional metal tube bundles for sensible-only heat transfer.