Frost Growth Characteristics Of The Finned-Tube Heat Exchanger

The air source heat pump(ASHP) system has the advantages of high efficiency, safety and environmental protection, etc. But frost formation has always been one of the main problems that restrict the development and application of the ASHP units. The frost layer not only increases the thermal resistance between air and the refrigerant, but also reduces the airflow rate through the evaporator, which reduces the coefficient of performance(COP) of the ASHP system. Therefore, the study of the frost growth characteristics on the finned tube heat exchanger under frosting conditions has a guiding significance for the efficient operation of the ASHP units.In this paper the dynamical performance of the finned tube heat exchanger under different frosting conditions was investigated numerically and experimentally. A quasi-steady state mathematical model of the frost growth was established, and the frost layer in the infinitely small volume was assumed to be uniform. The frost growth characteristics on the finned tube heat exchanger were simulated under different air temperature, relative humidity, air velocity and wall temperature by using the energy conservation equation and mass transfer equation. In order to verify the accuracy of the model, a test set-up was established in the psychrometric rooms to investigate the dynamic performance under frosting conditions of the finned tube heat exchangers. The deionized water–ethylene glycol mixture(50% by volume) was used as refrigerant and a precise low temperature thermostat bath was used as a chiller unit. The wall temperature, air pressure drop and inlet/outlet air temperature and humidity were measured. Based on the experimental results, the frost thickness on the fins, frost mass, air pressure drop and the heating capacity were analyzed. The main conclusions are as follows:(1) The numerical results agree well with the experimental data and the maximum deviation is within 30% for frost thickness, frost mass, air pressure drop and heating capacity.(2) For different refrigerant temperature, the frost thickness, frost mass, air pressure drop, heating capacity all increase with the decrease of the refrigerant temperature. But the stale operation time decreases and the decay rate of the heating capacity increases with the decrease of the refrigerant temperature. The stable operation time is about 20 min, 35 min, 70 min, respectively,when the refrigerant temperature is at-25℃、-20℃、-15℃.(3) For different face velocities, the frost mass, heating capacity, frost density, stable operation time all increase with the increase of the face velocity, but the face velocity has no appreciable effect on frost thickness. The results showed that after frosting 75 minutes, the maximum heating capacity was reduced by 14.5%, 32.5% and 48%, respectively, when the face velocity is at 2.0m/s、1.7m/s、1.4m/s.(4) For different air temperature(≤0℃) and relative humidity, the experimental results showed that the frost thickness, air pressure drop, heating capacity all decrease with the decrease of the air temperature and relative humidity, but frost mass increase with the decrease of the air temperature and relative humidity, which means that the lower the air temperature and the relative humidity, the bigger the density of the frost layer.(5) For the three flat, wavy and louver finned-tube heat exchangers, the experimental results showed that the average heating capacity of the wavy finned-tube heat exchanger in the whole frosting cycle is the maximum, followed by the louver and flat finned-tube heat exchanger.(6) For different fin spacing(1.6mm-2.8mm) heat exchangers, the growth rate of frost thickness increases with the increase of the wavy fin spacing, which nearly have no effect on the frost accumulation within the same frosting time. In addition, through comprehensive comparison of the heat transfer effect among different fin spacing heat exchangers, the experimental results indicated that the average heating capacity of the wavy finned-tube heat exchanger with 1.9mm fin spacing is the maximum.