Research On Heat Transfer Process Of Air Layer Under Clothing In Outdoor Environment

In the context of the normal management of the prevention and control of the new crown epidemic,medical workers will inevitably wear individual protective clothing,which is often closed and poorly permeable to moisture,and for medical staff working in outdoor environments,or other industries such as construction workers,traffic police and other outdoor workers,due to the characteristics of their clothing,work may cause thermal discomfort or heat stress in high temperature environments.Stress reactions.The study of heat transfer within the air layer under clothing is of great significance to the study of thermal comfort of workers in outdoor environments.This paper focuses on the heat transfer in the tiny air layer between human skin and clothing in an outdoor environment,exploring the heat transfer in the air layer of tightly fitting garments and loosely fitting garments.The air layer is often horizontal for tight fitting areas,such as arms and shoulders,and vertical for loose fitting areas,such as the back of the body.In order to improve the thermal environment of the under garment space of protective clothing,a radiation-cooled fabric with passive cooling capability is selected as the human garment in this paper.Finally,the results predicted by the proposed heat transfer model for the air layer of the garment are compared with the experimentally obtained temperature values,and a heat balance analysis is performed to verify the accuracy of the model.This paper specifically analyses the heat fluxes such as radiant and conductive heat on the outer surface of the garment under different air layer thicknesses,ambient temperature,ambient wind speed and solar radiation illumination in the horizontal direction air layer.For the vertical direction air layer heat transfer problem is mainly simulated with the help of CFD method to simulate the surface temperature of the garment and the temperature distribution inside the air layer under the garment under different environmental working conditions,and the experimental method is used to verify the relevant conclusions,which provides a theoretical basis for the design of garments with improved thermal comfort.The main conclusions of this paper are as follows.For the horizontal under-clothing air layer of tight-mouthed garments:（1）Under three different ambient temperatures of 16.5℃,20.3℃ and 21.3℃ in the transitional season,the total heat gain calculated by the single-layer garment heat transfer model proposed in this paper in the horizontal direction is 3%～9% higher than the total heat dissipation,with the solar radiation heat gain component accounting for the largest proportion,24.18%～29.09%;the radiation heat loss component of this radiation cooling fabric to the atmospheric window accounts for the largest proportion,21.68%～28.9% 21.68%～28.9%.This heat transfer model is reasonable for analysing the effect of the outdoor environment on the surface temperature of the garment,and the error is no more than 10%.（2）Under this experimental condition: I As the ambient temperature increases,the thermal conductivity（convection）,radiative heat flux,and total heat flux on the surface of the radiative cooling cloth all decrease,because the increase in ambient temperature causes one of the two layers to decrease.The temperature difference between the two heat transfer surfaces decreases,and the thermal driving force decreases.II Radiant heat from the outer surface of the garment increases with increasing solar irradiance,but the thermal conductivity term decreases with increasing solar irradiance.Clothing and skin decrease,and the driving force for heat conduction decreases.Total heat gain is more stable.III When the ambient wind speed is less than1.1m/s,the outer surface temperature of the clothing is less affected by the ambient wind speed,and the convective heat transfer coefficient does not change much;when the ambient wind speed is greater than 1.1m/s,the outer surface temperature of the clothing increases with the increase of the ambient wind speed.lower,and a bigger drop.At this time,the convection coefficient of the outer surface of the fabric increases,and the overall heat dissipation of the outer surface of the fabric will increase.（3）In the direction of the horizontal air layer at three different distances of 5mm,10 mm and 15 mm,the temperature decreases linearly with increasing air layer distance;this also confirms the one-dimensional nature of the temperature distribution along the air layer distance and is consistent with the temperature characteristics of heat transfer with thermal conductivity as the main mode of heat transfer.The temperature of the air layer within the garment is controlled by heat conduction.In addition,the temperatures predicted by the heat transfer model for single-layer garments presented in this paper correlate well with the experimental temperatures,with the average temperature difference between the predicted and experimental values being less than 1℃。（4）The surface temperature and the temperature within the horizontal air layer of the tight-lipped protective clothing with radiation-cooled fabric are lower than those of ordinary fabric garments under similar working conditions.The temperature gradient within the air layer is also greater than that of ordinary fabric garments.For the vertical air layer under the garment of a loose fitting garment:（1）Compared with the tight mouth garment,the air layer under the loose garment is strengthened due to convection,and the heat transfer in the air layer is no longer based on heat conduction,but on convection.（2）The temperature of the air layer under the clothes on the back of the human body tends to be high and low.The bottom is the air inlet,which accelerates heat dissipation,and the rising air absorbs the heat flow from the skin,resulting in a high temperature at the top.The airflow velocity in the vertical air layer is high in the middle and decreases in the sides.The lower the ambient temperature,the greater the temperature difference between the skin surface and the clothing surface,and the lower the skin surface temperature;the total heat flux on the skin surface decreases with the increase of the height of the air layer,and the higher the height of the air layer,the stronger the heat dissipation capacity of the human body.（3）The average value of the garment surface temperature decreases with the increase in the width of the air layer.When the thickness of the air layer is less than 12 mm,the temperature drop is small;when the thickness of the air layer under the clothes is greater than 12 mm,the temperature drop is large.