Experimental investigation on influence of boundary conditions on cooling effect and mechanism of crushed-rock layers

A series of experiments were carried out for the crushed-rock layers with open and closed boundaries, respectively. According to the measured temperature and calculated velocity distributions in the crushed-rock layers with the upper boundaries covered and exposed, it is found that, under the upper boundary covered, the heat transfer from the lower area to the upper area of the crushed-rock layer is by means of the natural convection and the thermal conduction among the crushed rocks when the temperature on the top surface of the crushed-rock layer is lower than that on the bottom surface. The heat transfer from the upper area to the lower area of the crushed-rock layer is mainly carried out by thermal conduction when the temperature on the top surface is higher than that on the bottom, and at that time the air in the crushed-rock layer almost kept static, which could obstruct the heat transfer. Therefore, the crushed-rock mass with closed boundaries can efficiently remove heat from its bottom surface during cold season and intakes a little heat during warm season. The net effect shows that it can be as a passive cooling method. Its characteristic is of thermal semiconductor. However, under the upper boundary exposed, when the wind of an average temperature 0.5 °C crosses over the top surface of the crushed-rock layer, the thermal transfer is mainly by the forced convection. Its characteristic is not of thermal semiconductor and its cooling effect is not obvious.