Quantitative Study Of Convective Heat Transfer At The Interior Surfaces Of Building Envelopes During Night Natural Ventilation

Night ventilation is considered to be as a passive cooling technology that can effectively reduce the energy consumption of air conditioning.Understanding the cooling mechanism of ventilation process is a precondition for using the night ventilation technique properly.The convective heat transfer between the ventilation flow and the inner surfaces of building envelopes is crucial to the night cooling performance.In this study,the convection heat transfer at the inner surfaces of building envelopes during night ventilation through door and window is studied experimentally and numerically.The experiments are conducted in a full-scale test chamber.The convective heat flux and the convective heat transfer coefficient(CHTC)at the floor are derived from the measured temperature data.The effects of air change rate(ACH),the initial temperature difference between the ground and the inlet air and the vertical location of the vent on the convective heat transfer at the ground are then analyzed.The results indicate that the surface averaged CHTC at the ground of a room ventilated through door and window is not more than 7 W/(m2-K),and the local CHTC is less than 12 W/(m2·K)if the ACH is varied from 4.99 to 13.6 h-1 and the initial temperature difference between the ground and inlet air is varied from 6.86 to 12.5 ℃.The mathematical model of the convective heat transfer at the inner surfaces of envelopes of a night ventilated room is established according to the physical prototype of the test chamber.The model is then solved numerically and the numerical results are shown to agree well with the experimental results.So the numerical simulation is reasonable and reliable.Typical cases of night ventilation through door and window of a building with a gallery are investigated numerically.The results show that the surface averaged CHTC at the inner surfaces of building envelops is less than 3.1 W/(m2·K)and the local CHTC is less than 6 W/(m2·K)if the speed of inlet air is varied from 0.1 to 0.5 m/s and the initial temperature difference between the inner surfaces and inlet air from 3 to 7 ℃.Convection heat transfer correlations are obtained by using the multiple regression analysis of numerical simulations.These correlations can be used to predict the heat transfer at the inner surfaces of building envelopes during night ventilation through door and window.Clear temperature stratification along the height of the room develops during ventilation,and the temperatures of building envelopes keep decreasing,among which the floor temperature drops fastest.For a room ventilated from the south direction to the north direction,the surface averaged CHTC at the ground decreases slightly during the ventilation,while the surface averaged CHTC at the other five surfaces nearly keep constant.The convective heat transfer at the ground and the north wall is the strongest,the next is the east or west wall,then the south wall,and the ceiling is the weakest.The proportion of the convective heat flux at the ground to the total convective heat flux at the inner surfaces of building envelops decreases slowly during ventilation,and the proportions of the convective heat fluxes at the other surfaces vary slightly.Temperature efficiency does not change significantly with time.The faster the inlet air flows(greater ACH),the greater the CHTC at the inner surface of the envelope and the mean CHTC of the room,but the lower the temperature efficiency.The initial temperature difference is not the key factor affecting the heat transfer intensity at the inner surfaces of building envelops,and it also has no obvious influence on the mean CHTC of the room and the temperature efficiency.The vertical location of the inlet has a significant effect on the night cooling performance.The lower the inlet,the greater the surface averaged CHTC at the ground.The maximum temperature efficiency exists if the inlet locates at a certain height.