Influence Of Convective Heat Transfer Coefficient On External Wall Net Heat Gain Of Building

With the development of China’s economy,the acceleration of urbanization and the continuous improvement of people’s living standards,the demand for building energy is also increasing,especially heating and air conditioning.It is estimated that the ratio of building energy consumption to total social energy consumption will increase to 30%in the future.So,the use of energy in buildings is widely concerned by the society.It is of great significance to decrease the amount of solar radiation entering the room through the wall in summer for building energy conservation.Previous studies have mainly focused on improving the thermal performance of the envelope structure to reduce the solar energy absorption of the envelope structure,such as the development and improvement of new wall structure materials,exterior cladding materials,etc.In practice,the amount of heat that is actually transferred to the interior of the wall by the solar energy irradiated to the outer surface of the building envelope is affected not only by the performance of the wall itself,but also significantly affected by the surface convection heat transfer coefficient.And the convective heat transfer coefficient may vary within a wide range due to the different layout of buildings and the background wind field.Therefore,based on the typical summer climate conditions in Shanghai,this paper studies the effect of convection heat transfer coefficient on the absorption of solar radiation on the outer wall of the building in order to provide a theoretical basis for the possible role of building layout in building energy conservation.This paper takes the room in the middle of the middle layer of a common residential building with external insulation as the research object.Computational Fluid Dynamics(CFD)numerical simulation method is used to simulate the dynamic simulation of heat transfer process of enclosure structure.Under different weather conditions(no cloud,little clouds,partly clouds,cloudy),the influence of external surface heat transfer coefficient on solar energy actually passed into the south wall is analyzed.The research results show that the convection heat transfer coefficient of the external surface of the building has a great influence on the absorption of solar radiation heat by the envelope structure under the influence of solar radiation.Since the wall absorbs solar radiation only occurs in the daytime and the convective heat transfer is carried out throughout the day,the effect of the convective heat transfer coefficient on the heat gain of the wall presents a different pattern in day and night.When the convective heat transfer coefficient of the wall surface and the air increases increases,the net heat gain of the wall due to solar radiation in summer can be greatly reduced.The results show that under the same solar radiation intensity,the net heat gain of the south wall with a convection heat transfer coefficient of 19 W/(m~2·K)on the outer surface is about 40%less than that of 8.5 W/(m~2·K)(the condition of static wind or unsmooth airflow in the building group).It means that in the summer,even on cloudy days when the solar radiation intensity is small,if the building air flow is not smooth,there will be less convection heat transfer between the envelope structure and the surrounding air.It is still possible for the solar radiation actually absorbed by the wall to be greater than the heat gain under the conditions of a sunny day.And the heat accumulates in the wall and will cause the load on the envelope to increase.The formation of a well-ventilated building layout within the building group during the summer can significantly reduce the actual absorption of solar radiation by the wall,thereby realizing building energy efficiency.