Study On Human Heat Transfer Characteristics And Thermal Comfort In Asymmetric Radiation Cooling Environments

Radiant heating/cooling systems mainly absorb the heat from the indoor environment in the form of radiant heat exchange,with no condensation and operating equipment in the room,and have the advantages of quietness,comfort,and energy saving.However,when the radiant heating/cooling system is in operation in summer,there is a large temperature difference between the radiation terminal surface and other surfaces(e.g.external walls,windows,internal walls,and furniture),which creates an asymmetric radiation field of varying intensity indoors.In this asymmetric radiation environment,there is a difference in heat exchange between different parts of the human body and the cold radiation surface and the surface of the enclosure structure,which also affects the human thermal comfort to some extent.The application of the current uniform thermal comfort evaluation model has certain deviations in evaluating asymmetric radiation cooling environments.This paper analyzes the heat transfer characteristics of the human body,carries out experiments on human thermal response in asymmetric radiation environments with cooling ceiling radiation,explores the physiological and psychological laws of human body in asymmetric radiation cooling environment,and constructs a corresponding human thermal comfort evaluation model for the thermal comfort evaluation of various asymmetric environments,providing support for promoting the application of radiant cooling systems.Firstly,combining the heat transfer mechanism between the human body and the surrounding environment,considering the influence of asymmetric radiation on the human body local radiant heat exchange,the local view factor model between each segment of the human body and the surrounding radiant surface with different orientations was constructed.Therefore,the radiation heat exchange model between each part of the human body and the surrounding radiant surface was established.Based on 65 MN node model,and considering the influence of clothing thermal resistance,a clothing node was added.A human body thermoregulation model applicable to the asymmetric radiation environment of radiant cooling was established.and the model was experimentally validated.Secondly,experiments were designed to investigate the thermal response of human subjects exposed to the asymmetric environment of cooling radiation from the ceiling and thermal radiation from the external envelope for a long period of time,the thermal response of human body in the asymmetric radiation field is revealed.The results showed that the average skin temperature of the subjects gradually stabilized after 3 hours of exposure,and most of the local skin temperatures stabilized after 120 minutes,except for the hands,calf,and feet.The overall thermal sensation of the subjects stabilized after 150 minutes of exposure,and the overall thermal comfort and overall thermal acceptability gradually improved,while the heart rate gradually decreased.As the temperature of the inner surface of the external envelope structure increased,the local skin temperature of the subjects gradually increased,and the overall thermal sensation and overall thermal comfort decreased.The influence of the ceiling radiation temperature on the local and overall thermal sensation of the human body was less than that of the external envelope structural temperature.Thirdly,the thermal sensation model applicable to radiation cooling environment was established.Through analyzing the experimental results between human skin temperature and local thermal sensation,a regression equation was determined by analyzing the variation law of skin temperature and local thermal sensation,and the weight coefficients of each local regression equation were determined by the neural network stochastic gradient descent method.Taking into account the comprehensive influence of local thermal sensation,the stepwise linear regression analysis method was applied to analyze the local thermal sensation of the human body,and the weight coefficients of each local thermal sensation on the overall thermal sensation were determined,in descending order of the thigh,chest,back,upper arm,and head.Then,combining the weight coefficients of the human body’s cold and hot spot distribution density and the average skin temperature calculation formula,a prediction model of the overall thermal sensation in asymmetric radiation cooling environment was established and experimentally validated.The results showed that the model can accurately predict the overall thermal sensation of the human body while reducing the number of test points for local skin temperature.Finally,a human thermal comfort model in asymmetric radiation cooling environment was established based on the human thermos-physiological thermoregulation model and thermal sensation models,and the model was used to evaluate the human comfort in asymmetric radiation environments.For summer radiation cooling environment,the indoor personnel’s comfort is affected by environmental parameters,external envelope structures,and radiation terminals.The human body’s thermal response in radiation cooling environment was quantitatively analyzed.The results showed that in the asymmetric cooling environment with an inner surface temperature of the external envelope structure of 30°C and a ceiling radiation temperature of 18℃,when the air temperature was in the range of 24℃～28℃ and the relative humidity was distributed between 40%～70%,the human body had good thermal comfort.In the asymmetric cooling environment with an air temperature of 26℃,relative humidity of 60%,an inner surface temperature of 30℃ for external envelope structures,and a ceiling radiation temperature of 18℃,the use of Low-E glass for the windows could create a uniform and comfortable thermal environment indoors,while ordinary single-layer glass and double-layer hollow glass caused obvious thermal discomfort in the human body’s back and feet.In the asymmetric radiation cooling environment with the aforementioned air temperature,humidity,inner surface temperature of external envelope structures,and ceiling radiation temperature,an emissivity of the cold radiation panel is less than 0.1,which has an impact on the thermal physiology and psychological well-being of the human body.To solve the problem of insufficient laying area and low cooling capacity of the ceiling in the aforementioned asymmetric radiation cooling environment,a combination of radiation plates on the ceiling and upper side of the side walls can be used.