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Modeling for predicting frost behavior of a fin-tube heat exchanger with thermal contact resistance.

Article Excerpt
INTRODUCTION

Frost formation on the cold surface of a heat exchanger operated at below freezing temperature causes a decrease in the air flow rate and increase in the frost layer temperature resulting in reduced thermal performance. The complex geometry of fined tube heat exchanger coil results in uneven working fluid and air temperature distribution inside the coil and around the fins and tubes. This will lead to uneven frost formation and growth on the coil surface.

Although, much about fin-tube heat exchangers has been studied for a long time, the thermal contact resistance has not been fully investigated and often has been overlooked because the heat transfer through the interfaces is a complex phenomenon. Most of the models previously reported, did not consider dynamic variation of parameters inside the heat exchanger, such as: air temperature, humidity and fin-tube temperature.

A study on the effect of thermal contact resistance in a fin- tube heat exchanger was first attempted by Dart (1959). In his study, thermal contact resistances were tested with several samples with two passages, which were one for cold and the other for hot water. To minimize the influence of the natural convection, the tube was placed in an adiabatic chamber. The results were compared to that in soldered fins. Jones et al. (1975) studied frost formation on flat plates and found that increase of relative humidity of ambient air increases the frost formation rate, but increase of air velocity decreases the frost formation rate. Schneider (1978) investigated frost formation on cylindrical surfaces experimentally and found that frost formation is independent from Reynolds number (air velocity).

Abuebid (1984) investigated the thermal contact resistance with plate-fin tube heat exchangers placed in a vacuum. He performed an error analysis but the error band was narrower. Niederer (1986) performed experiments to investigate the frosting and defrosting effects on the heat transfer in heat exchangers. He found that frost accumulation on the coil surface reduced the air flow rate and the heat exchanger capacity. Eckels and Rabas (1987) predicted the thermal contact conductance, varying the number of fin, the fin thickness, and the diameter of tube in the wet and the dry fin-tube heat exchangers. In addition, they improved the empirical method, based on Dart's method, including error analysis.

Sami et al. (1989) developed a model to calculate the frost thickness and frost density for flat surfaces. Model results showed that increase of relative humidity or decrease of surface temperature accelerates frost formation. Kondepudi and O'Neal (1990) comprehensively discussed the effects of frost on fin efficiency, overall heat transfer coefficient, pressure drop, and surface roughness of extended surface heat exchangers. They suggested that more models are highly needed to determine effects of frost on fin performance. Rite and Crawford (1991) investigated the effects of various parameters on frost formation and performance of a domestic refrigerator-freezer fin tube evaporator. They concluded that the frosting rate increases for higher air humidity, temperature, flow rate and lower refrigerant temperature and the air side pressure drop increases as frost forms on the evaporator coil for a constant air flow rate.

Sherif et al. (1993) used a differential continuous model that includes the air side heat transfer coefficients taken from literature and the Lewis analogy to calculate frost thickness and surface temperature for flat plates operating under forced convection. Stubblefield et al. (1996) investigated the heat loss by thermal contact resistance using the insulated pipe and presented a simple method to predict the effect of contact resistance. Salgon et al. (1997) theoretically predicted the thermal contact conductance which was presented as a function of contact pressure and compared with experimental data. Lee et al. (2003) studied on flat plates experimentally and suggested correlations for frost thermal conductivity. According to their correlations frost thermal conductivity varies with its density.

Yan et al. (2003) investigated the performance of flat plate fin tube heat exchangers under frosting conditions experimentally. They concluded that the frost formation is greater for a lower air flow rate, and the rate of pressure drop increases rapidly as the relative humidity increases. Jeong et al. (2004) evaluated the thermal contact conductance using the experimental-numerical method for various fin-tube heat exchangers with 9.52 mm tube. Their results imply that the thermal contact resistance increases as the expansion ratio and the number of fin increases. Seker et al. (2004) developed a mathematical model to calculate the unsteady thermal characteristics of fin tube heat exchangers under forced convection conditions. The model was partially based on utilizing empirical correlations for predicting frost thermal conductivity and the amount of water vapor increasing the frost density. Their results were in good agreement with reported literature. Jeong et al. (2006) also investigated the thermal contact conductance using the experimental-numerical method in the fin-tube heat exchangers with 7 mm tubes. They revealed that the factors such as fin type, manufacturing type of the tube and etc. have a large effect on the thermal contact resistance in fin-tube heat exchanger with 7 mm tube. They concluded that the thermal contact conductance in the case of wide slit fin is larger than that of normal slit fin and that in the case of plate fin is largest of all fin types. Byun et al. (2007) numerically examined the effects of the heat exchanger type, refrigerant, inner tube configuration, and fin geometry on the evaporator performance of a fin-tube heat exchanger. They found that for the given inlet air temperature of 27[degrees]C and relative humidity of 50%, the heat transfer rate of the cross counter flow type heat exchanger is about 3% higher than that of the cross-parallel flow type. Also they compared three kinds of refrigerant, R-22, R-134a, and R -410A, and their effect on performance of the heat exchanger.

The purpose of this paper is to investigate the...