Study On Heat & Mass Transfer And Cooling Effect Of Underground Duct System

Underground duct system makes use of tunnels to cool the air and sends it to buildings above the ground through air supply system in order to reduce indoor temperature. This thesis is based upon theoretical analyses of heat and mass transfer between soil and air in the tunnel undergoing ventilation and cooling process. Experiments and tests were conducted concerning soil thermal conductivity as well as heat and mass transfer process. Furthermore, the effect of tunnel ventilation and cooling was simulated by means of CFD techniques.First,the formula of integrated heat transfer coefficient between air and tunnel wall is deduced theoretically,in accordance with the heat and mass transfer characteristics of soil and air in the tunnel. Then the heat conductivity coefficients of soil with different moisture contents were determined experimentally by use of the hot probe method,the results of which were applied to FLUENT software as simulated conditions. Three models,which simulating different soil moisture contents,different velocity of inlet air and different ventilation runtime respectively,are set to analyze the ultimate air temperature and the variation of soil temperature around the tunnels. An analytical comparison between the results of simulation models of different conditions and the data of field experiment provides theoretical principles for the practical application of air in underground duct system.Experimental results show that soil moisture content has a great influence on the heat transfer coefficient, which grows with the increase of soil moisture content. In addition, the phenomenon of heat and mass transfer between soil around the tunnel and air in the tunnel only occurs within a certain range of thickness near the soil. Simulation results suggest that with the increase of soil moisture content,the air temperature drop increases gradually. When the soil moisture content stands in the range of 10% to 20%,the temperature falls obviously,but when it stands above 20%,the temperature drop is increasing slightly. Besides,the air cooling effect turns out to be better when the mainstream wind speed is low in the tunnel. Finally,the temperature of tunnel wall is the highest and gradually reduces with the increase of depth into the soil. The thermal boundary layer thickness is mainly affected by the running time.