Experiment And Simulation Of System Combining Displacement Ventilation With Cooling Ceiling With High Long Wave Transimitance Film

Currently, displacement ventilation and cooling ceiling system (DV/CC) has two main disadvantages: first, condensation will happen when the radiant panel temperature is lower than the dew point temperature of air around it;second, the downdraft from the cooling ceiling destroys the airstream shape of DV, which reduces indoor air quality. This paper puts forward a method to avoid condensation and downdraft: the radiant panel is covered and airproofed with the film whose long wave transmittance (LWT) is high, and there is a layer of air or vacuum between the undersurface of panel and the film. CC with the film is called CCF for short. The method is based on the theoretical analysis: because the layer is air or vacuum and has great heat resistance, the film temperature will be much greater than the panel temperature and near to the indoor air temperature. Generally, such high temperature can avoid condensation and reduce the downdraft. Although the film temperature is high, the radiant heat transfer between CC and indoor heaters or walls is almost not affected because the film LWT is high. Therefore, most of cooling capacity of CC can remain.This paper puts forward a simple method to test transmittance, emissivity and reflectivity of the film, and the film experiment used is tested by the method. The experiment in an office shows that the method to avoid condensation and downdraft is feasible. The radiant panel is tested in a simulation chamber, including three conditions: first, it is with no film;second, with the film;third, with the film and with the lattice between the undersurface of the panel and film. The simulation models of the conditions of the panel are established, and simulation result shows: first, even if the chilled water in the normal temperature range directly enter the panel, condensation will not happen when the panel is with the film;second, when condensation do not happen, the greatest cooling capacity of the panel with the film can be twice as that with no film, which can greatly decrease the area covered by the panel when the same cooling capacity is needed in a room and greatly decrease costof the panel.DV/CC is tested in a simulation chamber, including three conditions: first, CC is with no film;second, with the film;third, with the film and with the lattice between the undersurface of the panel and film. The testing result shows: in the same conditions, compared with CC, CCF decreases the cooling capacity of CC slightly, increases that of DV, increases the indoor air temperature, and the indoor upside air temperature increases more than the downside. The phenomenon of directional radiant cooling is found when CC is with the film and lattice.The simulation models of DV/CC and DV/CCF are established, and include: dividing zonal, the model of CC or CCF, the model of airflow, the model of radiant heat transfer, the model of wall heat balance and CFD model. By simulating, three conditions are compared: DV/CC, DV/CCF with the layer of air between the undersurface of the panel and the film, and DV/CCF with the layer of vacuum between the undersurface of the panel and the film. The result shows: when the total cooling capacity of air conditioning system is the same, compared with DV/CC, DV/CCF has more apparent character of displacement ventilation;downdraft is decreased greatly;contamination ratio decreases by 70 percent averagely;the indoor air quality is improved greatly.The ratio of radiant and convection heat transfer of air conditioning terminal device has great effect on indoor environment, which requires further study.