A study on the characteristics of frost distribution and growth on finned tube heat exchangers

Abstract

An air source heat pump (ASHP) unit is an energy-saving and environment-friendly device to utilize low-grade energy from ambient air for space/water heating. For the last 15 to 20 years, more and more countries have introduced national policies for promoting the wider uses of ASHP technology for space/water heating to alleviate the worsening global environmental issues and energy crisis. Therefore, future extensive use of ASHPs can be expected. However, for a space/water heating ASHP unit operated at an ambient temperature between -15 °C and 6 °C, and at a relative humidity of greater than 45%, frosting occurs on the surface of its outdoor coil. Frosting deteriorates its operational performance and energy efficiency, and reduces its output heating capacity. To alleviate the negative impacts of frosting on ASHP units, studies on frosting mechanism and characteristics, methods for suppressing frosting, defrosting methods and control strategies, and the operating characteristics for an ASHP unit during frosting and defrosting operations have been carried out. Although a large number of studies on frosting characteristics on FTHXs, the most commonly used structure for the outdoor coil in an ASHP unit, have been conducted and significant progresses made, the experimental investigations into the detailed characteristics of frost distribution and growth on FTHXs are still considered insufficient, which is the main subject area addressed in the research project reported in this Thesis. This Thesis begins with, firstly, describing an experimental setup including an environmental chamber, refrigerant supply system using R410A as refrigerant, wind box, and visualization system to facilitate the experimental work required in this research project. Then using the setup, the detailed frost distribution and growth, and the air side performances for two experimental FTHXs with different fin pitches under a fixed typical frosting condition were experimentally investigated. The study results demonstrated that the frost accumulated on an FTHX could be divided into two parts, one accumulated on the edge of windward fins and the other on the surfaces of fins and tubes. Furthermore, the ratios of the frost mass accumulated on the edge of windward fins to that on the entire FTHX surface were 13.7% (60 mins frosting period) and 12.5% (120 mins frosting period) for the two FTHXs, respectively. The frost thickness and average frost densities in these two parts were also evaluated and compared. Secondly, the effects of frosting conditions such as air temperature (Ta), relative humidity (RHa) and initial air face velocity (va,ini) on frost distribution and growth on the FTHXs were experimentally investigated. The results showed that the protruded thickness of frost on the edge of windward fins and the ratio of the frost mass on the edge of windward fins to that on the entire FTHX were both increased with the increases in Ta and RHa, but decreased with an increase in va,ini. In addition, the effects of RHa on the mass ratio were more significant than those of Ta and va,ini. Furthermore, the average frost density on the edge of windward fins was increased with an increase in RHa; and that on the surfaces of fins and tubes was increased with an increase in va,ini, but decreased with an increase in RHa. Thirdly, an experimental study on the differences in frost pattern, frost distribution and growth, and air side performances for an FTHX at two different fan control modes of constant air flow rate and constant outdoor air fan speed was carried out. The study results showed that compared to the use of the constant outdoor air fan speed control mode, the use of the constant air flow rate control mode can make the frost at the exit from leeward fins more compact and uniform, and lead to an accelerated frost growth on the surfaces of fins and tubes but an impeded frost growth on the edge of windward fins. At the end of a 60-min frosting period, the protruded thickness of frost on the edge of windward fins, and frost thickness at the entrance to the windward fins at the constant air flow rate control mode were 7.9% and 11.1%, respectively, smaller than those at the constant outdoor air fan speed control mode. In contrast, the frost mass on the FTHX and the average frost density on the surfaces of fins and tubes at the constant air flow rate control mode were 6.7% and 16.7%, respectively, larger than those at the constant outdoor air fan speed control mode. Furthermore, the average air side pressure drop, air velocity and total heat transfer rate of the FTHX at the constant air flow rate control mode were 134.5%, 82.4% and 13.4%, respectively, larger than those at the constant outdoor air fan speed control mode during the 60-min frosting period. Finally, a mathematical model for predicting frost distribution and growth on an FTHX with flat fins was established and experimentally validated. The model was made of two sub-models, one frosting model for the frost growth on the edge of windward fins and the other for the frost growth on the surfaces of fins and tubes. In addition, the established frosting model can also be used to predict the air side performances of an FTHX with flat fins under frosting conditions. The predicted air side performances such as the total heat transfer rate and pressure drop agreed well with the experimental data.