Research On Thermal Comfort Evaluation And Improvement Of Microenvironment In Safety Helmet

Safety helmet is a commonly used head protective appliance in dangerous working environment.However,in the hot summer,when wearing safety helmets,the staff generally feel stuffy in the head,which will reduce the staff's willingness to wear them,increase the thermal burden of the staff at work,affect the reaction speed and attention,and increase the risk of safety accidents.Therefore,it is particularly important to optimize the thermal environment in the safety helmets and the thermal comfort of the head after wearing helmets in summer.In order to deeply study the heat transfer inside the safety helmet,firstly,the human head wearing the safety helmet is simplified into a thermodynamic opening system.Based on the human body heat balance equation and heat transfer theory,the heat balance equation and heat transfer model of the system are proposed.In order to optimize the internal thermal environment of the safety helmet,reduce the heat gain of the safety helmet and increase the heat dissipation,various heat transfer processes are analyzed through the heat transfer model,and it is proposed to apply radiative cooling materials and forced ventilation on the safety helmet to optimize the internal thermal environment of the safety helmet and the thermal comfort of the head.In order to verify the practical application effect of the above two measures,this paper uses ordinary white safety helmets,radiative cooling safety helmets,white ventilation safety helmets and radiative cooling ventilation safety helmets to carry out comparative experiments.10 male college students were invited to participate in the experiment under the outdoor sunshine environment with Wet-Black-Globe-Temperature(WBGT)of 26?,27.5?,29?and 30?.During the experiment,the subjects wore safety helmets and walked up the treadmill at the speed of 4km/h to simulate the exercise with medium labor intensity.Each experiment lasted for 30min.During the experiment,the environmental conditions,the internal and external surface temperature of the safety helmet,the internal microenvironment temperature and the human forehead temperature were monitored,and the thermal sensation,thermal comfort and damp sensation of the subject's head were asked every 5 minutes.After the experiment,the data and results are analyzed.Finally,the numerical simulation of ventilation safety helmet is carried out to study the effects of ventilation mode,ventilation wind speed,vent position and hair on its heat dissipation performance.The main conclusions of this paper are as follows:(1)Through heat transfer analysis,it is concluded that after wearing safety helmet,the main heat dissipation mode is sweat evaporation,followed by convective heat dissipation and radiation heat dissipation,which is relatively small;The main ways of heat gain are solar radiation and head heat generation.(2)In the hot outdoor environment in summer,the application of forced ventilation on safety helmets can significantly optimize the internal thermal environment of safety helmets,reduce the temperature of human forehead,and significantly optimize the thermal feeling,thermal comfort and damp sensation of the head.The application of radiative cooling materials can also optimize the thermal environment in safety helmets,but the effect is not as significant as forced ventilation.At the same time,it has no obvious effect on reducing the temperature of human forehead and optimizing the thermal feeling,thermal comfort and damp sensation of head.In addition,both improvement measures can reduce the temperature of the outer surface of the safety helmet.(3)In the hot outdoor environment in summer,according to the pros and cons of the internal thermal environment of the four kinds of safety helmets,they are radiative cooling and ventilation safety helmets,white ventilation safety helmets,radiative cooling safety helmets and ordinary white safety helmets.(4)There is a strong linear correlation between the environmental UTCI index and the head thermal sensation,thermal comfort and damp sensation after wearing the safety helmet.R~2is greater than 0.5.The environmental UTCI value can be used to predict the head thermal sensation,thermal comfort and damp sensation after wearing the safety helmet.In addition,the head thermal sensation has more than medium correlation with the ambient temperature and solar radiation intensity,and has strong correlation with the microenvironment temperature in the safety helmet.(5)In the aspect of numerical simulation,the heat dissipation performance of ventilation safety helmet is compared by comparing the head skin temperature.In terms of ventilation mode,the ventilation safety helmet without baffle behind the air inlet has better cooling effect on the head skin than the ventilation safety helmet with baffle.In terms of ventilation wind speed,with the increase of ventilation wind speed,the head skin temperature decreases gradually,but the decline range decreases with the increase of wind speed.From the position of the vent,the vent set on the side has the best cooling effect on the head skin;Modifying the height of the air inlet has little effect on reducing the skin temperature of the head.Compared with the case without hair,when there is hair and the porosity is 0.9,the head skin temperature is lower and the temperature distribution is more uniform.