Mixed convection heat transfer of one-stage spherical particle layer packed in an open shallow rectangular cavity Original Research Article

Forced natural mixed convection heat transfer characteristics of a one-stage spherical particle layer, which is a simple model for porous media, are investigated experimentally. The one-stage spherical particle layer is provided in a rectangular cavity (length × WIDTH = 90 × 400 mm; depth is variable) installed at the bottom of a rectangular channel (whose cross-sectional dimensions are height × WIDTH = 100 × 400 mm). Air, as the test fluid, 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 (about 10 mm) and different thermal conductivities [0.21–22 W/(m K)], are tested. The cavity depth is changed in four steps of 0 (flat plane), 2.5, 5, and 10 mm. The air velocity ranges from 0.15 to 2.5 m/s, and the temperature difference between the incoming air and the heating surface varies between 10 and 100 K. The particle layer suppresses the airflow near the bottom surface of the cavity and decreases the heat transfer coefficient in the case of the particles having low thermal conductivity. It also functions as an extended surface and increases the heat transfer coefficient in the case of particles having large thermal conductivity. With a given particle, the heat transfer coefficient in the low air velocity region is dominated by both the air velocity and the temperature difference between the heating surface and air. However, it is almost unaffected by the cavity depth. In contrast, in the high air velocity region, the heat transfer coefficient is dominated by both the air velocity and cavity depth and increases with a decrease in cavity depth. An increase in the thermal conductivity of the particles brings an increase in the heat transfer coefficient. The heat transfer results can be expressed using nondimensional parameters for porous media; that is, the effective Nusselt number is correlated with the Reynolds number, the effective Prandtl number, and the ratio of the cavity depth to the particle diameter.