3D numerical simulation of heat and mass transfer of fin-and-tube heat exchanger under dehumidifying conditions
Introduction
In HVAC systems many fin-and-tube heat exchangers work under dehumidifying conditions, and the working medium of the fin side consists of both air and a certain amount of vapor. When the temperature of the air-steam mixture is lower than its dew point temperature, steam starts to condense due to latent and sensible heat transfer. The fields of temperature, velocity, and humid air concentration become conjugated, thus resulting in a complex heat and mass transfer process. Therefore, in comparison to dry conditions, the performance of fin-and-tube heat exchanger under dehumidifying conditions is much different.
The available reports on the performance of fin-and-tube heat exchangers under dehumidifying conditions are mostly experimental. A large amount of experimental data and their corresponding correlations for heat and mass transfer coefficients are obtained through experiments. Ma et al. [1] carried out their experiment on 14 fin-and-tube heat exchangers with hydrophilic coatings, and the effects of pipe row numbers, fin pitch, and inlet relative humidity were investigated for both wavy and interrupted fins, and subsequently, the correlations in terms of the Colburn j factor and the Fanning f factor were proposed. Liu et al. [2] presented the heat transfer and friction characteristics of 9 plain fin-and-tube heat exchangers (tube diameter of 15.88 mm) and found that the effects of fin pitch on the sensible j factor were gradually diminished with the increase of tube row number. Wang et al. [3] tested 18 fin-and-tube heat exchangers under dehumidifying conditions and investigated the influences of fin pitch, pipe row number, pipe diameter, and the dimension of corrugation on heat exchanger performance. Consequently, many correlations for both the dimensionless heat transfer and friction factor, including geometrical parameters, were obtained [4], [5], [6], [7], [8], [9], [10], [11]. Kuvannarat et al. [12] studied the effects of fin thickness on heat exchanger performance under dehumidifying conditions and found that at a smaller fin pitch and a smaller row number of pipes, the heat transfer and flow characteristics of the heat exchanger for thicker fins were respectively 50% and 20% higher than that of thinner fins; however, these effects vanished with the increasing rows of pipes and fin pitches. Wang et al. [7] experimentally investigated the performance of a flat fin-and-tube heat exchanger for inlet relative humidities of 50% and 90%, and little change in friction factor was noticed. Moreover, the heat transfer factor was smaller than that of dry conditions at a lower Reynolds number and almost the same as that of dry conditions at higher Reynolds number (>2000).
However, in previous research, most of the experimental ranges were relatively small, and the correlations were induced for certain kinds of fin geometries; hence, the suitability of these correlations was restricted. In contrast, numerical simulation is more broadly applicable for fin-and-tube heat exchangers under dehumidifying conditions.
Comini et al. [13] considered the coupled heat transfer for fluid and solid regions and proposed a model to compute latent heat and condensing mass transfer at the interface of rectangular and wavy fins under dehumidifying conditions. During the computing process, the authors assumed that condensing liquid disappeared instantaneously; hence, neglected the effect of liquid on heat and mass transfer. Comini et al. [14] also employed the proposed model to study the performance of fin-and-tube heat exchanger with two rows of pipes, and it was found that for inline arrangements of tubes, the condensing mass flow rate for humid air was lower in the second row of pipe than that in the first row; however, it was almost the same for the staggered one. Furthermore, by ignoring the condensates from humid air, Comini et al. [13], [14], [15], [16], [17], [18] employed the aforesaid numerical method to investigate the performances of plate fins or fin-and-tube heat exchangers under dehumidifying conditions. An and Choi [19] numerically studied the characteristics of a wavy fin-and-tube heat exchanger under both dry and dehumidifying conditions, and the fin efficiencies were found to be constant at completely dry or humid regions; however, in the regions of unsaturated steam, the efficiencies started to decrease with increasing relative humidities. Benelmir et al. [20] simulated the moist condition performance for fin-and-tube heat exchangers using FLUENT software, and it was observed that the local condensation rate decreased along the circumferential direction from the upwind surface to the wake region and also decreased with increasing pipe row numbers. Croce et al. [21] assumed a perfect drainage condition at the vertical fin surface and compared their numerical results with the experimental data, and the effects of thermal resistance and gravity were found to be negligible. Zhuang et al. [22], [23] proposed a numerical model to predict the condensing droplet behaviors on the fin surfaces of fin-tube heat exchangers under dehumidifying conditions. The effects of operating conditions and fin geometries on heat and mass transfer characteristics were analyzed and validated with the experimental results. Dehbi et al. [24] integrated the condensation model with noncondensable gases in the FLUENT code by neglecting the thermal resistance of liquid films and treating condensation as sink terms in the governing equations. Consequently, the model was employed to study condensation with a large amount of noncondensable gases in an intermediate-size vessel.
It can be noticed from the brief reviews that most research focuses on the comprehensive performance (instead of a specific field information) of heat exchangers under dehumidifying conditions. In the current study, numerical simulation is conducted for a staggered five-row fin-and-tube heat exchanger under dehumidifying conditions. The effects of inlet relative humidities on temperature, as well as the velocity and concentration distributions of fins, are investigated, and the rate of condensation and heat transfer are obtained. The detailed distributions of temperature, velocity, and steam concentration are investigated. The results can be used to design hydrophilic or hydrophobic materials for fin surfaces, as well as to enhance the performance of heat exchangers under humid conditions.
Section snippets
Numerical method
The condensation of humid air is a process of both heat and mass transfer. When humid air encounters any environment with a temperature lower than its dew point temperature, condensation occurs. The condensates in heat exchangers ususlly make the flow and heat transfer process under humidifying conditions very complex; thus, the fin structures cannot be too complex. One of the most commonly used ways to increase fin efficiencies under humidifying conditions is surface treatment to change their
Heat transfer rate
The heat transfer during condensation consists of both latent heat and sensible heat (convection heat transfer rate). The total heat transfer rate () was calculated by Eq. (22).where and are the specific enthalpies of moist air at the inlet and outlet, respectively, and and are the mass flow rates of moist air at inlet and outlet, respectively. The specific enthalpy was calculated by Eq. (23).where t is the temperature of gas
Temperature distribution of a wet fin
Kundu et al. [30], [31], [32], [33], [34] have done much work on condensation on wet fins using analytical methods. In this section the temperature distribution of a one-dimensional plain wet fin is obtained using the numerical method in this paper. The results are compared with the analytical solutions in [30]. It’s seen in Fig. 2 that the numerical results agree well with the analytical ones.
Moist air condensation in 2D plate channel
The problem of convection-condensation of humid air in a 2D plate channel [20] was considered in this
Physical model
The physical model used in this paper is presented in Fig. 7(a). In order to avoid recirculation, the computational domain was extended upstream and downstream by two times and ten times of the streamwise fin length, respectively. Because of the periodic repetition of air channels in the z-direction, the neighboring two center planes of the fins were chosen as the upper and lower boundaries of the computational domain. In the spanwise direction, the computational region was bounded by two
Conclusions
Numerical simulation is conducted under dehumidifying conditions with the assumption of the instant disappearance of condensed water in the system. The effects of inlet velocity and relative humidity on heat and mass transfer are investigated. The distributions of temperature, species concentrations, and condensation mass rate at the centerline cross-section of the channel are discussed, and the performance curves for condensation mass flow rate and heat transfer rate are obtained. The main
Acknowledgements
This study was supported by the Key Project of International Joint Research of NSFC (51320105004) and the 111 Project (B16038).
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