Analysis Of Convective Heat Transfer At Inner Surfaces Of Building Enclosure During Buoyancy-driven Ventilation At Night

Night natural ventilation is a passive cooling technology which can effectively reduce the energy consumption of air-conditioning system.It can be driven by wind,buoyancy or wind and buoyancy.In many areas of our country,the frequency of static wind is high,even higher than the frequency of dominant wind direction.Therefore,the investigation of night buoyancy-driven ventilation is of distinct theoretical and practical significance.The energy saving effect of buoyancy-driven ventilation at night mainly depends on the convective heat transfer between the ventilation flow and the surface of the enclosure.The convective heat transfer between air flow and ground in a full-scale test chamber was measured,and the effects of indoor initial temperature,ground initial temperature and vents size on the heat transfer process were analyzed.The experimental results show that the indoor temperature decreases gradually,the temperature changes more and more slowly,and the vertical temperature distribution tends to be uniform during the buoyancy-driven single-sided ventilation.If the initial indoor temperature is higher(close to the initial ground temperature),the heat dissipation from the ground is less than that of heat exhaust by airflow,so the indoor temperature,airflow rate and temperature efficiency all decrease gradually.If the initial indoor temperature is obviously lower than the initial ground temperature,the heat dissipation from the ground at the early stage of ventilation is greater than that of heat exhaust by airflow,the indoor temperature first increases rapidly and then decreases gradually,both airflow rate and temperature efficiency first increase and then decrease.Compared with the case of small upper and big lower vents,the local convective heat transfer coefficient at the ground for the case of big upper and small lower vents is relatively small,the average convective heat transfer coefficient is larger,and the temperature efficiency is higher.If the outdoor temperature is about 12?,the indoor initial temperature is 13?35?,and initial the ground temperature is 33?41?,the average convective heat transfer coefficient at the ground is between 2?8 W/(m2·K),and the local convective heat transfer coefficient is less than 18 W/(m2·K).Combined with the physical prototype of the test chamber,a numerical model of convective heat transfer at the inner surface during the buoyancy-driven ventilation at night is proposed.The predictions of the model are in good agreement with the measured data,and the numerical method is reasonable.The convective heat transfer between the ventilation flow and the inner surfaces of building enclosure during the buoyancy-driven ventilation is then simulated.The effects of the initial temperature difference between the interior and the exterior,initial surface temperature,window characteristic and room depth on the convective heat transfer coefficient at inner surfaces of building enclosure and the average convective heat transfer coefficient of the room are analyzed.The simulation results show that the convective heat transfer coefficient at the ground is the largest,the north wall,the east wall,the west wall and the south wall decrease in turn,and the ceiling is the smallest;in the ventilation stability stage,the convective heat transfer coefficient of each surface remain nearly unchanged;the ground heat transfer accounts for the largest proportion of the total heat transfer in the room,the east wall,the west wall,the north wall and the south wall decrease in turn,the ceiling is the smallest,and the heat transfer ratio of each surface is almost constant during the ventilation.Though the size of the south window is different from that of the north window,the position of the neutral surface at the two windows is similar,and it almost does not change with time.The initial temperature difference between the interior and the exterior and the initial temperature of enclosure structure have little effect on the average convective heat transfer coefficient of the room,unlike the window characteristic and room depth.For the same area,the window height is greater,the average convection heat coefficient of the room is larger;the room depth is smaller,the average convective heat transfer coefficient of the room is larger.The convective heat transfer correlations of each surface and the room are induced by the simulation results.The former is only suitable for the buoyancy-driven ventilation through doors and windows of corridor buildings,while the latter is suitable for the buoyancy-driven ventilation or wind-driven ventilation of this kind of buildings.