Numerical Simulation Of Block Thermal Environment Based On Fractal Porous Medium Method

With increasing urbanization and human activities,mitigating the heat island effect has become one of the most severe challenges.This paper proposes a preliminary prediction of the block thermal environment using a porous media approach to optimize the block thermal environment.The effects of block building form,porous media parameters and anthropogenic heat on the block thermal environment are discussed.Optimization strategies are also proposed to"enhance ventilation"and"control heat gain".A standard CFD model and a porous media model are first developed for the block to simulate and calculate the microthermal environment of the block,and the accuracy of the simulation results is verified by the measured data.Compared with the standard CFD method,the porous media method is more efficient and less expensive than the standard CFD method in obtaining reliable results for the same block.The number of meshes can be reduced by 27.8%,the total number of iteration steps by 52.2%and the total computation time by about 66.7%.Secondly,the fractal porous media block models of point and slab buildings are reconstructed to investigate the effects of building morphology,porosity and fractal dimension on the thermal environment.The wind speed and temperature distributions are consistent within the point and slab blocks,but the thermal environment of the point block is relatively better when considered together.The blocks with different porosity obtained by varying the number of buildings and building height follow the pattern that the average wind speed increases and the average temperature decrease as the porosity increases.The average wind speed in the block is positively correlated with the fractal dimension change,and the average temperature is negatively correlated with the fractal dimension change.The block wind speed dispersion symbolizes the uniformity of the block wind speed distribution.The block wind dispersion can be reduced by increasing the block porosity and decreasing the block fractal dimension.The intensity of human metabolism,air conditioning,natural gas,and traffic anthropogenic heat emissions in the study area was estimated by statistical research and other methods.Taking the thermal environment distribution of an ideal point block under different kinds of anthropogenic heat distribution as an example,found that domestic heat emissions have the highest degree of impact on the block thermal environment compared to the remaining anthropogenic heat emissions.At 2 m height,the temperature rise caused by each 10 W/m~2 increase of domestic heat emission is about 0.3°C.Traffic heat emission mainly affects the street and the block environment,and the effect is gradually weakened with the increase in height.Finally,the porous medium model is used to analyze the thermal environment of the Xinjiekou block and compare the thermal environment in the cases of total anthropogenic heat emission,no anthropogenic heat emission,no domestic heat emission,no traffic heat emission and no metabolic heat emission concentration.The optimization strategies for the thermal environment of the block are proposed in terms of"enhancing ventilation"and"controlling heat gain"concerning the porous medium parameters and anthropogenic heat emission intensity.The research results of this paper can provide critical support for the effective prediction of the micro-thermal environment at the early stage of urban design,which is essential for energy saving and emission reduction,creating urban ecological areas,and achieving"carbon peaking and carbon neutral"as early as possible.