Effect Of Solar Radiation On Heat Transfer Calculation And Comfort Of Radiant System

As a new type of air conditioner terminal form,the radiant panels can be combined with the ventilation system to create a comfortable indoor environment.Since the radiant system has more comfortable and energy-saving characteristics than the traditional convection air conditioning system,it is widely used in buildings.In office buildings,large shopping malls,high-speed rail stations and other public buildings,the transparent envelope structure occupies a relatively large area,and the amount of incident solar radiation is correspondingly enormous.When a radiant air conditioning system is used in these buildings,solar radiation will not only affect the heat transfer calculation of the radiant panels,but also have a certain impact on the indoor thermal environment,so it is necessary to conduct in-depth research on these effects.In various manuals and standards,the calculation of heat transfer of the radiant panels only considers the convective heat transfer and long-wave radiation heat transfer between the cold surface and the environment,and does not consider the solar short-wave radiation directly absorbed by the radiant panel.A linear equation method is adopted to calculate the solar radiation energy directly absorbed by the cold radiant panels.The effects of room size and surface reflectance of visible light on the absorption of solar radiation energy by cold radiant panels were also studied.Based on this,the calculation method of the heat transfer amount of the radiant panel given by ASHRAE has been revised,which increases the directly absorbed solar radiation.According to the method revised and the method recommend by ASHRAE,the hourly heat transfer of the radiant panels on the summer design day is obtained.The results show that the more solar radiation entering the room,the greater the difference in the calculation of heat transfer between the two methods and the maximum difference reaches40%.Since the calculation of the cooling load of the radiant system is only revised on the basis of the traditional convection cooling load calculation method,the calculation result is deterministic.And the cooling capacity per unit area of the surface of the radiant panel calculated by the ASHRAE method is lower than the revised method,the calculated radiant panel laying area will be larger than the revised method of this paper,but the average PPD(predicted percentage dissatisfied)of the center section of the room is larger than those designed in this paper,which means it is less comfortable.Therefore,when designing a radiant system,it is necessary to consider the solar radiation directly absorbed by the cold radiant panels to correctly calculate the area of the laying radiant panel.In order to analyze the effect of solar radiation on thermal comfort of the room of the radiant system,this paper applied a method that combine the CFD simulation with the experiment,and the experiment verifies the accuracy of the model established in this paper.The simulation results show that solar radiation not only has important effects on the global thermal comfort of indoor environment of the radiant system but also alters local thermal comfort in the room.Under winter conditions,when there is no solar radiation,the PMV value shows a decreasing trend from the roof position to the window position.The closer to the glass curtain wall,and the asymmetry of radiation along the direction of the window continues to decrease.The minimum value is close to ?10 ℃.And in the presence of solar radiation,the PMV value on the indoor center cross section shows an increasing trend from the indoor center position to the window position.The closer to the window,the radiant asymmetry along the window direction gradually increases.The maximum value is close to6℃.In summer,regardless of the presence of solar radiation,the indoor PMV distribution trends are similar.The closer to the window,the greater the PMV value.As the glasses temperatures are higher,the radiation asymmetries along the direction of the window increase respectively.But the trend is more obvious when there is solar radiation,and the maximum radiant asymmetry exceeds 10 °C.The vertical temperature difference of the air is related to the temperature of the supply air and the location of the heat source.The temperature of surface irradiated by sun is relatively high,so the vertical temperature difference of the air nearby is small.In summary,since the cold radiant panels can directly absorb part of the incident solar radiation,the calculation result of its heat transfer is increased compared to the result only considering the convective heat transfer and long wave radiation heat transfer of the cold surface.When the temperature of the radiant panel is constant,the required area of the radiant panel is smaller than that of the ASHRAE method,but the comfort is better at the same time.The areas exposed to solar radiation have higher temperatures.These areas will not only increase the average indoor radiation temperature and reduce the radiant heat dissipation of the human body,but also cause local discomfort caused by radiant asymmetry and heated floor.The study found that reducing the surface temperature of the cold radiant panel to reduce the indoor mean radiant temperature and air temperature in the presence of solar radiation is an effective way to achieve thermal comfort for the body.