Experimental And Numerical Investigations Of Frost Formation With Low Temperature And High Relative Humidity

Frost formation is a very common phenomenon in cold and wet air. Frost on heat exchanger increasesthermal resistance and blocks air flow passage,which reduces the system energy efficiency. Therefore, investigations of frost formation, especially influences of surface characteristics and simulation of frost layer growth are essential for exploring anti-frosting methods and designing heat exchangers operating with frosting conditions.Experimental and numerical investigations of frosting process and numerical investigation of frost layer growth on fin surfaces have been carried out. The main works of this paper are summarized as follows.Frosting process on hydrophilic surface, bare aluminum surface and hydrophobic surface with low temperature and high relative humidity is experimentally investigated. The experimental conditions are-20~-5oC cooling surface temperature,-5~2oC humid air dry-bulb temperature, 85% relative humidity and 0.31~0.92 m/s humid air velocity. The experimental results show that the condensate water droplets freezes earlier and the frost crystal grows faster on the bare aluminum surface with lower cooling surface temperature; the frost layer grows faster with lower cooling surface temperature and higher air velocity; the condensate water droplets freezing process is delayed and the height of frost layer grows slower on hydrophobic surface; frosting process on different surfaces are tend to be the same, when the humid air temperature and cooling surface temperature are low enough; the hydrophilic surface has less residual meltwater, thus hydrophilic surface can be used for the facilities in frosting/defrosting circulation in order to restrain frost formation.Frost layer growth process on horizontal cooling surface is numerically investigated using computational fluid dynamics software. The numerical results show that frost height and density grow with time; the frost on the front end of the cooling surface is denser because of higher water vapor concentration; the main heat transfer type is heat conduction in the frost layer while heat convection is the main heat transfer type in air flow region; The calculation results are compared with the experimental results, with more than 90% of the frost height data in the relative error from-15% to +40% and all of the frost mass data in the error from-30% to +20%.Frost layer growth on fin-tube heat exchanger surfaces is numerically investigated using the same frost growth predicting model used in predicting frost layer growth on horizontal cooling surface. The numerical results show that the frost distribution on fin surface and the air pressure drop are in good agreement with the experimental result, which means the frost growth predicting model can be applied to predicting frost layer growth on heat exchanger surfaces and predicting airflow resistance.