Experiment Study On The Heat And Mass Transfer Characteristics Of The Green Roof

In this paper, the thermal insulation properties and dynamic evaporation cooling characteristics of light planting roof module are tested by experiments and analyzed through the data collection and theoretical study. Based on the testing and analysis, research has been taken to optimize the planting structure which aiming to develop passive evaporation and heat insulation planting roof module who suit for use in South China, to provide the basic data for engineering application, and create a practical way to materialize technology of passive evaporation cooling.The research, use comparative experiments devices, analyzed the thermal characteristics of the light planting earth humidifying module,dry module,sedum lineare planted module and water storage module. The testing and analyzed results show the feature of insulation properties,scatter thermal performances,evaporation cooling characteristics and the distribution of the heat transfer. The optimized structure was obtained through researches on two factors of the planting soil thinkness and the storage plate height, as well.On the side of thermal insulation characteristics:the bottom surface mean temperature of planting module is3.5℃～5.9℃lower than dry soil module’s,2.1℃～2.4℃lower than wet soil module’s,0.9℃～1.7℃lower than water retaining module’s under the laboratory test conditionings. And the bottom surface mean heat flux of planting module is1.9W/m2～4.1W/m2lower than dry soil module, lower than wet soil module, lower than water retaining module.On the side of radiating features:the radiating feature of planting module is similar with wet soil module and water retaining module, while dry soil module is better than all the module mentioned above. The bottom surface mean temperature of planting module is0.7℃～1.4℃higher than dry soil module’s,0.4℃～0.9℃higher than water retaining module’s,0.1℃～0.6℃lower than wet soil module’s; while the bottom surface mean heat flux of planting module is1.0W/m2～1.3W/m2higher than the dry soil module’s,1.1W/m2higher than the wet soil module’s,0.9W/m2～1.1W/m2lower than the water retaining module’s.On the side of dynamic evaporative cooling characteristics:planting module>water retaining module> wet soil module> dry soil module. The regression equation of evaporation heat flux and the initial moisture content of planting module is put forward respectively as followings:qxNaporation=-0.2648φ2+16.347φ-107.53, the correlation coefficient R2is0.875, while the regression equation of evaporation heat flux and the initial moisture content of wet soil module is qxnaporation=3.6697φ+26.713, the correlation coefficient R2is0.9293.On the side of heat flux distribution features:Under the laboratory conditions the ratio for the transfer heat flux with the net radiation flux are showed as follows:the wet soil module is1.3%, the dry soil module is2.0%, the planting module is1.0%and the water retaining module is1.5%; on the convective heat transfer heat flux ratio, the wet soil module is13.7%, the dry soil module is25.3%, the planting module is0.3%and the water retaining module is0.7%; on the evaporative cooling heat flux ratio, the wet soil module is27%, the dry soil module is14.1%, the planting module is48.2%and the water retaining module is42.5%; on the heat storage flux ration, the wet soil module is30.4%, the dry module is17.4%, the planting module is22.3%and the water retaining module is30.7%.On the side of the optimization of the height of planting soil:the best height of planting soil is75mm. The bottom surface mean temperature of75mm high planting soil module is6.7℃lower than the25mm’s,2.5℃lower than the50mm’s,0.4℃lower than the125mm’s.On the side of the optimization of the height of reservoir drainage board:the best height of reservoir drainage board is25mm. The bottom surface mean temperature of water retaining module with25mm high reservoir drainage board is1.0℃～1.2℃lower than the100mm’s,1.0℃～2.0℃lower than the75mm’s,0.1℃～0.2℃lower than the125mm’s.On the side of moisture content and thermal characteristics:compared with the wet soil module, the efficiency of cooling of the planting module reaches to the top when the moisture content of matrix is about23.8%during the heating stage, The regression equation of cooling efficiency difference and volumetric moisture content is△t=-0.0079φ2+0.3483φ-1.2157, the correlation coefficient R2is0.8208. The efficiency of radiating of the planting module is higher than the wet soil module only under the condition that the moisture content of matrix is about23.8%during the radiating stage.