Research PaperNumerical model of dehumidifying process of wet air flow in open-cell metal foam
Introduction
In the practical application of refrigeration and air conditioning systems, the evaporators always work under dehumidifying conditions [1], [2]. During the dehumidifying process, the condensates form small water droplets and grow up to greater ones, and the greater droplets will move down and drain out of the heat exchangers under the coupling effect of gravity, surface tension and air flow [3], [4], [5], [6]. The dehumidifying process would have a great influence on the performance of the evaporators and the air conditioning systems [7], [8], [9]. For improving the air conditioning system performance and the air quality, the mechanism of dehumidifying process should be revealed.
The existing dehumidification technique mainly includes chemical and mechanical dehumidification [10]. For the chemical dehumidification, it includes liquid and solid desiccant dehumidification [2], [11], [12], [13], [14], [15], [16], and the water vapor is moved from the air by transferring it towards a desiccant material through the adsorption or absorption. The liquid desiccant dehumidification has been proved to be an effective method to extract the moisture of air in air conditioning systems [10], [11], [12], [13], [14], and the mass transfer characteristics in the liquid desiccant was experimentally investigated [15]; the liquid-to-air membrane energy exchanger was designed to eliminate the desiccant solution aerosol carry-over problem [13], [16], and its high effectiveness has been certified [12], [13]; the solid desiccant wheels were widely used in desiccant/evaporative cooling systems for wider ranges of climates [17], [18], [19]. For the mechanical dehumidification, it utilises compression refrigeration systems to cool and dehumidify the wet air [10], and the dehumidifying process may be enhanced by adopting the evaporators with a large specific surface area, e.g. the fin-tube heat exchangers [1] and the metal foam heat exchangers [3], [9], [20].
Open-cell metal foam (as shown in Fig. 1) is one kind of porous media with high porosity (up to 98%), high thermal conductivity, high specific surface area (up to 10,000 m2 m−3) and a tortuous flow path to promote mixing, and is widely used in heat exchangers [21], [22], [23], [24]. Using metal foam to replace the conventional fins in the finned tube heat exchangers would enhance airside heat transfer, resulting in the improvement of heat exchanger performance [3], [20], [25], [26], [27]. Compared to the currently used finned tube heat exchanger, the heat transfer characteristics of dry air and wet air flow in the metal foam heat exchangers are enhanced significantly [3], [9], [26], [27], [28], [29], therefore, the open-cell metal foam heat exchangers have received much attention for application in dehumidifying apparatus in recent years [3]. For the metal foam heat exchangers, there is mass transfer from the water vapor of the wet air onto the metal fibers during the dehumidifying process, and the driving force of the mass transfer is the water molecule concentration difference between the wet air and the surface of metal fibers [3], [9]; moreover, there is also the heat transfer between the wet air and metal fiber due to the temperature difference [3], [9]. Therefore, in order to optimize the open-cell metal foam heat exchangers, it is necessary to investigate the heat and mass transfer characteristics during the dehumidifying process of wet air flow in metal foam.
For the airside heat and mass transfer characteristics for metal foam, the existing experimental research mainly focuses on the dry air [25], [30], [31], [32], [33], [34], [35]. Dai et al. [25] compared the performance between the metal foam heat exchangers and finned tube heat exchangers, indicating that the same performance with lighter and smaller size can be achieved by using the metal foam heat exchanger [25]. Hutter et al. [30] investigated the characterization of heat transfer in commercial metal foam filled tubular reactors; the heat transfer was found to increase with ligament diameter ascribed to the enhanced turbulent kinetic energy, and the volumetric heat transfer performances were 3 orders of magnitude higher compared to conventional batch reactors [30]. Dukhan and Chen [31] presented the heat transfer measurements inside rectangular blocks of aluminum foam subjected to the constant heat flux at one side, and proposed a two-dimensional analytical model for the heat transfer in metal foam, which had a good prediction accuracy for most of the data points [31]. Kamath et al. [32] reported the effects of thickness and thermal conductivity of high porosity foams on the heat transfer and pressure drop characteristics in a vertical channel; the foam thickness contributes to a significant increase in heat transfer and an insensitive effect in pressure drop. Mancin et al. [33], [34] investigated the heat transfer coefficient and pressure drop of air in different copper foam samples, and proposed a suitable copper foam for the design of innovative thermal management solutions for electronic cooling application; the permeability was found to increase with the decreasing PPI, while the form factor decreased with the increasing mean pore diameter [34]. Hsieh et al. [35] performed the experimental investigation on the heat transfer characteristics of aluminum-foam heat sinks, and the effects of porosity, pore density and air velocity were analyzed; the deduced temperature difference between the solid and gas phases clearly indicated the existence of non-local thermal equilibrium condition [35].
For the airside heat and mass transfer characteristics under dehumidifying conditions, the existing research mainly focuses on the fined tube exchangers, and there is only one published paper on wet air in metal foam [9]. For the finned tube heat exchangers, the heat transfer characteristics for four types of fins were investigated, covering the plain fin [1], [4], [36], [37], the wavy fin [38], [39], [40], [41], the slit fin [42], [43] and the louver fin [44], [45]; the finite-element method was adopted for simulating the heat and mass transfer characteristics of moist air in the heat exchanger by assuming no droplet on the fin surface [46]. For the metal foam heat exchanger, the heat and mass transfer characteristics of wet air flow in metal foam under dehumidifying conditions are much different from those for finned tube heat exchangers due to the complex structure of metal fiber [9]; the heat transfer capacity of copper foam heat exchangers under dehumidifying conditions is enhanced by 56–196% compared to that of the finned tube heat exchanger [9].
For the numerical model on the heat and mass transfer characteristics in open-cell metal foam, the existing research is mainly focused on the single phase convection [22], [24], [47], [48], [49], [50], [51], [52], [53]. The exhaustive reviews for the numerical investigations on the convection heat transfer in metal foam have been presented by a number of researches [3], [22], [23], [24]. For developing the numerical model, it is the promising and challenging task to determine the geometric characteristics [22], [24]. Different models have been considered, including cubic unit cell with slender cylinders, dodecahedron or a tetrakaidecahedron, interconnected hexagonal cells [24], [48]. Using such basic representations of the unit cell, the models for the pressure drop [49], [50], [51] and heat transfer [52], [53] in the metal foam were developed. Numerical studies of porous media have traditionally relied on the volume averaging approach [54], [55], [56], [57], [58]. For the open-cell metal foam, the representative elementary volume (REV) approach has shown encouraging results [47], [48], [49], [59]. A 3D numerical simulation methodology for the flow and heat transfer at the pore scale level of high porosity metal foam was presented [47], [60]. The existing models are mainly for the convection heat transfer in metal foam. However, until now, there is no available numerical model for predicting the heat and mass transfer characteristics of wet air in metal foam under dehumidifying conditions.
The purpose of the present study is to develop a numerical model for the dehumidifying process of wet air flow through metal foam, to validate the model based on the experimental data, and to analyse the influences of dehumidifying conditions and metal fiber surface wettability on the dehumidification characteristics in metal foam.
Section snippets
Modeling object and technical road map
The schematic diagram for the dehumidifying process in the open-cell metal foam is shown in Fig. 2. The actual open-cell metal foam is composed of three-dimensional interlaced pores and fibbers, as shown in Fig. 2(a).
The dehumidifying process of wet air in the open-cell metal foam involves the condensates formation, growth and movement in sequence. During the condensates formation and growth processes, the mass transfer occurs from the water vapor of the moist air onto the fiber surface, which
Simulation domain and solution methodology
The geometrical model and simulation domain is shown in Fig. 7. In the figure, the metal foam cell is tetrakaidecahedron, as did by Kopanidis et al. [60]; the maximum size (dps), side length (a) and the fiber diameter (dfd) of each tetrakaidecahedron cell can be determined based on the structure parameters of metal foam, i.e. the porosity (ε) and pore density (PPI):
The simulation is performed using commercially available Fluent software. The VOF-CSF
Experimental rig and uncertainty
The model was validated based on the experimental data of heat transfer rate and pressure drop of wet air flow in metal foam under dehumidifying conditions. The schematic diagram of the experimental rig for validating the model is shown in Fig. 8.
The experimental rig shown in Fig. 8 can be subdivided into two main parts, including wet air side system and cooling water system [9]. In the wet air side system, an air compressor (Greeloy GA-82Y) with a dryer is used to provide the low-humidity air
Conclusions
- (1)
The contact angle function was developed based on the force analysis of condensate on metal fiber.
- (2)
The model of mass transfer during condensate formation process was developed based on the heterogeneous nucleation rate and critical nucleation radius of condensate; the model of mass transfer during condensate growth was developed based on the species conservation of water vapor on phase interface.
- (3)
The numerical model of dehumidifying process of wet air flow in metal foam was proposed by
Acknowledgements
This study is supported by National Natural Science Foundation of China (No. 51576122), Natural Science Foundation of Shanghai (No. 15ZR1422000), the Foundation for Innovative Research Groups of the National Natural Science Foundation of China (No. 51521004) and Shanghai Academic Leaders Program (No. 16XD1401500).
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