Research progress on clothing ventilation mechanism and strategy
Outdoor workers often work in high temperature environments in summer.Due to their wearing heavy clothing,the heat loss of human body will be seriously hindered,which increases the risk of hyperthermia.Clothing ventilation strategy is an important technical mean to regulate the heat loss of human body.To increase the heat loss of human body by optimizing the compatibility of clothing fabric,adding clothing openings,and embedding micro-ventilation equipment is of great significance for the protection of health and safety of human.First of all,the evaluation index of clothing ventilation efficiency(i.e.clothing ventilation index),the measurement methods(the CR method and the LH method),the calculation formula of overall and local ventilation indexes,and the advantages and disadvantages of the measurement methods were introduced.The CR method needs to measure the microenvironment volume under clothing,while the LH method does not,and is suitable for measurement under dynamic condition.However,on strong wind conditions,as for the highly breathable clothing,the tracer gas is diffused through the clothing to the outside environment before it is evenly distributed in the microenvironment under clothing,resulting in a larger measurement result of the LH method.Secondly,the development of the fabric ventilation model,local ventilation model,dynamic ventilation model,passive ventilation model and active ventilation model was reviewed.The fabric ventilation model included two mass transfer modes:diffusion and convection.Such a model is represented by the three-node ventilation model,with the inner point referring to the inner layer of yarn,the outer point referring to the outer layer of yarn,and the empty point of air referring to the flowing air.The mass transfer between the inner point and the outer point is diffused,and the mass transfer between the outer point and the empty point of air is convected.The local ventilation model is a double-layer cylindrical model built with the torso or arm as the prototype.The inner cylinder is the torso or arm skin,and the outer cylinder is the clothing.The direction of air layer under clothing can be divided into horizontal one and vertical one.As for the dynamic ventilation model,the air flow inside and outside clothing during the movement needs to be considered.Most studies simplified the shape change of the air layer under clothing during movement into a sinusoidal fluctuation of its thickness,while Ghaddar et al.decomposed it into an air mass balance stage and an air convection stage.Passive and active ventilation clothing models mainly describe two kinds of clothing:passive ventilation and active ventilation ones.In the passive ventilation clothing model,each section of the human body was simplified into a cylinder by Ismail et al.and a three-dimensional cylinder ventilation model was established.The influence of human body surface shape on air flow was considered by other studies,which used a 3D scanner to obtain the clothed and naked human body surface,and then introduced fluid dynamics software for modeling.The modeling principle of the active ventilation model is similar to that of the passive ventilation model.In addition,it is also necessary to consider the change of the space shape under clothing caused by active ventilation and the exchange of heat and humidity after sweating.Thirdly,the influence mechanism of existing clothing ventilation strategies on human heat loss was discussed from three aspects:fabric permeability,garment opening and embedded ventilation equipment.The breathable fabrics are conducive to water vapor diffusion and convective heat transfer on the skin surface,and they can effectively increase the heat loss of the human body if they are configured on the part of the human body that is prone to sweating or forced convection.The locations where the clothing opening is large,the air layer under clothing is thick,and the skin sweat is heavy and forced convection is easy to occur are conducive to the heat loss of human body.According to different ventilation methods,active ventilation clothing is divided into micro fan ventilation clothing and air cooling ventilation clothing.The former consists of a micro fan and a battery,and the fan generates forced convection,which increases the heat loss of human body.The garment has the advantages of small size,light weight,low cost and wide application.The latter is composed of an air supply device,a cooling device and a hose to exchange heat by injecting cold air into the clothing microenvironment.The garment is heavy and expensive,and is mainly used in the aerospace field.In these two kinds of active ventilation suits,the ventilation speed,ventilation temperature,the thickness of the air layer under clothing,the opening position and the opening size are the important parameters affecting the heat loss of human body.Finally,two research prospects were proposed.First,it is necessary to establish a three-dimensional full-size dynamic clothing ventilation model and analyze the ventilation mechanism of clothing microenvironment based on the dynamic ventilation model and the three-dimensional static clothing ventilation model by considering more comprehensive boundary conditions and coupling with the human thermophysiological model.Second,it is necessary to develop an active ventilation suit coupled with a variety of heat exchange modes and discuss the influence mechanism of various cooling parameters coupled with a variety of heat exchange modes.
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