Teoretiska och experimentella studier av frostbildning i luftspalt. (Theoretical and experimental studies of frost between two plates).

摘要

Frost has a complex structure and frost formation is influenced by several parameters, according to a literature study. A basic understanding of the frost formation process would be valuable in many situations, e.g. when optimizing equipment. A mathematical model for frost growth in a humid air stream between two plates, where one is cold, has been developed, based on mass balance, heat balance, and diffusion of water vapor in the frost. At the calculations, the space between the two plates is divided into a number of equally long elements. For each element, the balance equations and the equation for the diffusion of water vapor in the frost are formed. A new description is introduced by using a variable which denotes the part of the mass flux that is increasing the frost thickness. A computer program was developed, that with the use of experimental values from the literature, calculates the frost thickness, the frost density, the temperature of the frost surface, the mass flux to the frost layer, and the mass transfer coefficient. The results show good agreement between the values obtained from the model and the values obtained in experiments. An experimental facility has been built to investigate the frost growth between two cold plates, under conditions similar to those in an air to air heat exchanger. The frost thickness, the frost mass, the change in relative humidity with corresponding temperatures, the change in the temperature of the humid air and temperatures between the plates, can be measured. From these values, e.g. the heat flux, the mass flux, the heat transfer coefficient, the mass transfer coefficient, and the effective thermal conductivity of the frost can be calculated. The results of the experiments show how the frost formation depends on different parameters like time, the temperature of the cold surfaces, the inlet humidity, and the inlet air velocity.