Condensation of a refrigerant CFC11 in horizontal microfin tubes (Proposal of a correlation equation for frictional pressure gradient)

In this chapter, a method for modeling transport phenomena in heat transfer devices is discussed. The modeling technique is based on the averaging method in which the heat transfer devices are treated as a fluid-saturated porous medium. A novel method for analytically determining the unknown coefficients resulting from the averaging is presented and this represents a significant improvement over experimental or numerical determination of these coefficients. The averaging method in turn yields analytical solutions for the velocity and temperature distributions which are useful in the thermal analysis of heat transfer devices. The modeling technique is elucidated for thermal design and optimization of microchannel heat sinks and internally finned tubes. By way of these case studies, the method is shown to be a promising tool for thermal analysis and optimization of heat transfer devices.

A porous medium approach for the thermal analysis of heat transfer devices

Major factors affecting heat transfer between the refrigerant and air in a plate finned-tube heat exchanger, otherwise known as an evaporator or condenser coil, are discussed. These factors are inside heat transfer coefficient, thermal contact resistance between the fin and the tube, and outside heat transfer coefficient. Correlations are presented to calculate the rate of heat transfer for each of these factors. Additional design considerations are presented and discussed for these three heat transfer mechanisms. In-tube augmentation, oil-refrigerant mixture effects, and frosting are also discussed, since they affect heat transfer in the coil. Tube circuiting procedures and standards available for rating coils are also presented.

Design Considerations for Air-Conditioning Evaporator and Condenser Coils

This study performs the comparative analysis of R32 and R410A condensation inside a 4.2 mm ID microfin tube: the effects of saturation temperature, vapour quality and mass velocities are evaluated. The heat transfer coefficients show a great sensitivity to vapour quality, a non-negligible sensitivity to saturation temperature/pressure, and a non-monotonic trend versus mass velocity. The microfin tube exhibits a maximum heat transfer enhancement with respect to the smooth tube around 3.5−3.6 at a mass velocity of 200 kg m−2s−1 and a vapour quality of 0.8. The optimum mass velocity for R32 and R410A condensation inside the microfin tube is in the range 150−300 kg m−2s−1. R32 exhibits condensation heat transfer coefficients 20 − 30% higher and frictional pressure drops 10 − 20% higher than those of R410A and it seems a valuable low GWP substitute for R410A.

Comparative analysis of microfin vs smooth tubes in R32 and R410A condensation

:A simulation program was developed to evaluate the heat transfer performance of a multi-pass fin-tube evaporator, considering the pressure drop in the distributor and capillary tubes. The effect of capillary tube length for each pass was analyzed with various inlet air flow types and distributions. The appropriate capillary tube length distribution and correlation were deter-mined for various inlet air flow types and distributions. The correlated results agreed well with the simulation, with an average error of less than 7%. By applying an optimal capillary tube length distribution, the heat transfer rate was increased by 4～5% compared to cases with uniform tube- length distributions, for each of the inlet air flow types and distributions considered in this study.Keywords:Fin-and-tube evaporator(휜-튜브 증발기), Simulation(시뮬레이션), Capillary tube(모세관), Distributor(분배기), Air flow distribution(공기유속 분포) †Corresponding author Tel.: +82-2-2220-0426; fax: +82-2-2295-9021 E-mail address: ksleehy@hanyang.ar.kr

Numerical Simulation of a Heat Pump Evaporator Considering the Pressure Drop in the Distributor and Capillary Tubes

The expression for the performance of a gas-cooled heat exchanger per unit of power expended to the flowing gas provides a useful figure of merit for extended-surface heat exchangers. As developed here, the expression accounts for the effects of both increased heat transfer coefficient and increased effective temperature difference with higher power expended. Previous expressions accounted for only the increase in heat transfer coefficient. The convenience of comparing different heat exchanger configurations, as well as different tube spacing in the same configuration, is shown.

B. M. Johnson

Papers

A Metho of Comparing Performance of Extended-Surface Heat Exchangers