Experimental study of heat transfer and air flow in a refrigerated display cabinet

摘要

The heat transfer and air flow in a vertical open refrigerated display cabinet loaded with packages of test product was studied in order to analyse unsteady state phenomena. The product and air temperatures in the curtain were measured using tiny thermocouples, air velocity using a hot wire anemometer and PIV (Particle Image Velocimetry). A comparison between the air velocities measured by these two methods shows a good agreement at the internal side of the air curtain. However, these two methods give quite different result at the external side of the air curtain which can be explained by the complex air flow in this region. The temporal variation of the air temperature inside the air curtain is due partly to the "on/off" compressor cycle and partly to the introduction of ambient air via vortices. The latter phenomena contribute to the rapid velocity fluctuations which are greatest in the mixing layer of the air curtain. The product position in the cabinet is a determining factor of its temperature. The convective and radiative heat flux between the products located at the front of the display cabinet and the air was also measured. Near the air supply and return ducts, the convective heat transfer coefficient was higher than between these two positions because of the high instability of air flow near the inlet and return ducts. The radiative exchange is more significant at the top of the display cabinet than at the bottom.