CONVECTION HEAT TRANSFER OF AN OPEN RECTANGULAR CAVITY PACKED WITH SPHERICAL PARTICLES IN AN AIR FLOW (EFFECT OF CAVITY LENGTH)

摘要

Forced-natural-mixed convection heat transfer characteristics of one-stage spherical particle layer are investigated experimentally. The one-stage spherical particle layer is provided in a rectangular cavity installed at the lower part of a rectangular channel. The test fluid (air) flows through the rectangular channel, and is heated by the bottom surface of the cavity via the particle layer. Three types of spherical particles, which have almost the same diameter of about 10 mm and different thermal conductivities are tested. The cavity length is varied in three steps of 20 mm, 43 mm and 90 mm, and the cavity depth is changed in four steps of 0 mm (flat plane), 2.5 mm, 5 mm, and 10 mm. The air velocity is ranged from 0.15 m/s to 2.5 m/s, and the temperature difference between the incoming air and the heating surface is varied between 10 K and 100 K. The particles placed on the heat transfer surface suppress the air flow near the heat transfer surface and reduce the heat transfer, while the particles also function as an extended heat transfer surface and a turbulence promoter. Since the particles have these two opposite effects, the heat transfer coefficient changes variously with changing of the experimental conditions. The heat transfer coefficient can be expressed as a non-dimensional heat transfer correlation, where the effects of the cavity length, the cavity depth, the air velocity and the temperature difference between the air and the bottom surface of the cavity are taken into consideration.