《candidatefinal1.doc》由会员分享，可在线阅读，更多相关《candidatefinal1.doc（28页珍藏版）》请在三一文库上搜索。
1、Candidacy ExamA dynamic model of the human/cooling system/clothing/environment systemZhengxiang Pu2002, MayThe aim of this dissertation is to develop a mathematical model to improve the human body thermal comfort prediction. The system researched includes human body, personal cooling system, clothin
2、g and environment, so the mathematical model includes human thermoregulatory model, model of heat and mass transfer from skin through clothing to environment and thermal comfort prediction model.Air temperature, radiant temperature, humidity and air movement are the four basic environmental paramete
3、rs that affect human response to thermal environments. Combined with the personal parameters of metabolic heat generated by human activity and clothing worn by a person, they provide the six fundamental factors which define human thermal environments. If personal cooling system is available, the flu
4、id flow speed, cooling tube distribution density and fluid inlet temperature have significant effects on the human thermal comfort. In specific situations, other factors will be influential, such as posture and degree of acclimatization to heat. The six fundamental factors therefore provide a minimu
5、m requirement for a useful conceptual basis upon which a realistic consideration of human thermal environments can be based.The interaction of all six fundamental factors and other additional applicable factors will be considered in the model to improve the human body thermal comfort prediction. 1.
6、Literature ReviewsSimple thermal models have been extensively used in the assessment of human in thermal environments. For example, effective temperature index or the wet bulb globe temperature indexes are rational indices of heat stress. They provide a method of integrating relevant factors to give
7、 a single number, which is related to how humans respond to the combined effects of the factors. This method is relatively simple and the index can be obtained from the use of charts or graphs. However the development of digital computers has provided a facility, which allows more detailed models to
8、 be used in practical application.Thermal models are often considered to be those which provide a rational representation of the human body involving both heat transfer between the body and the environment, anthropometry and thermal properties of the body and a dynamic representation of the human th
9、ermoregulatory system.1.1 Thermoregulatory models review Around 1970, when substantial data on thermoregulatory control functions became available in the literature, and with the increasing availability of computers, the development of physiological simulation models started. These models combined p
10、hysics of internal and external heat flow with internal body temperature regulation. The most well known physiological models of human thermoregulation were initiated by research for space travel. Stolwijk (196) published a 25 compartment thermoregulatory model, incorporating control functions for b
11、lood flow, sweating and metabolism for use by NASA. He was followed by many other authors who published relatively simple (Gagge et al., 1971, 1986) to more complex (Wissler, 1964, 1982; Werner, 1989; Werner and Webb, 1993) physiological models. These models all allowed the introduction of anthropom
12、etric data, and some acclimation, but these parameters were hardly validated for work in the heat.1.1.1 Fanger P.M.V. P.P.D. modelThe first step was done by fanger (1970) with the Predicted Mean Vote (PMV) and the Predicted Percentage of Dissatisfied (PPD). PMV is grounded on thermal balance S of th
13、e body, between heat produced by metabolism (M-W, where M is total metabolism, W is the external work) and heat exchanged with environment through skin and respiration (E+R+C+Res, where E is sweat evaporation, R and C are the radiative and convective heat transfer, Res is respiration heat transfer w
14、hich includes latent and sensible heats). S is related to PMV, a vote scale, which is equal to zero for neutrality (S=0), is positive or negative according to S sign, and varies from 3 (too cold) to +3 (too hot). PPD is deduced from PMV through a chart established after a great number of experiments
15、.Simulation using a PMV based control is easy to do because the great simplicity of PMV type outputs, but not very helpful because all ambience parameters are reduced to one index and the effects of each of them cannot be analyzed separately. Despite of the great improvements of PMV control versus u
16、sual technique, there are still many simplifications for such a method to be sufficient. PMV is used to determine how far from equilibrium the body is, but formulate used to compute it is only valid for equilibrium. A more important drawback of PMV is that it only applies to steady state, in homogen
17、ous conditions. A similar result is given when all the body is at neutrality and when simultaneously feet are too hot and head too cold.A better accuracy in the determination of physiological parameters is given by the use of models, including heat exchanges and physiological thermoregulation proces
18、ses. The thermal network of the body is made up from a set of nodes, the number of which varying from 2 to about 250. Internal heat exchanges are due to thermal conduction between contiguous layers and to forced convection through blood flow. External heat exchanges include sensible (conduction, con
19、vection, radiation) and latent heat (sweat evaporation, respiration). Physiological reactions, in charge of the maintain of a constant internal temperature, are shivering and vasoconstriction, against cold environment, sweat and vasodilatation against hot climate.1.1.2 Stowijk and Hardy 25-node mode
20、lStowijk is one of the forerunners in the research of human thermoregulatory model and his model has provided the basis and inspiration for much of the work on thermal modeling. Stowijk and Hardy 25-node model mathematically represents the geometric and thermal characteristics of the body itself as
21、the passive system, and the temperature information and integration and related physiological controls as the controlling system.The passive system of the model consists of appropriate sized cylinders representing trunk, arms, hands, legs, and feet; the head is represented as a sphere. Each of these
22、 has four concentric layers or compartments representing core, muscle, fat and skin layers. An additional central blood compartment, representing the large arteries and veins, exchanges heat with all other compartments via convective heat transfer occurring with the blood flow to each compartment. T
23、he model assumes that the body is symmetric to reduce the number of calculations. The six segments, four compartments each segment and the central blood compartment make a total 25 nodes. For each of 25 compartments, heat balance equations was developed to account for heat flow into and out of the c
24、ompartment, via conduction and convection, and the metabolic heat production within the compartment. The model is based on a standard, 1.72m, 74.4 kg man with a volume of m3. The controlling system in body temperature regulation consists of three parts. The first part contains the sensing mechanisms
25、 which recognize the thermal state of the controlled system. The second part receives information regarding the thermal state, integrates it and sends out appropriate effector commands to the various effector systems. The third part of the controlling system receives the effector commands and modifi
26、es them according to the conditions at the periphery before translating such commands into effector action. In its original form, Stowijk and Hardy 25-node model represents only the nude body.1.1.3 Nishi and Gagge 2-node modelA simplified version of Stowijk and Hardy model was also developed by Nish
27、i and Gagge (1977) for us in practical application, the model includes equations for the thermal resistance provided by clothing over the body, the passive system is two concentric cylinders of appropriate dimension and thermal properties. These represent an inner core an outer shell. The controllin
28、g system is represented by a similar system to that used by Stowijk and Hardy but with only 2 nodes to control.1.1.4 Wissler modelWissler (1985) describes a model, which computes 225 temperatures in 15 elements plus , and lactate concentrations. The model is an order of magnitude larger than the Sto
29、wijk and Hardy model and it is well validated for hot and cold environments and both atmosphere and hyperbaric environments. The detail provided allows its use in specialist areas such as for cold water immersion and diving, where it has been used in application.1.1.5 Werner model Werner (1990) prov
30、ides probably the most sophisticated thermal model available. Its three-dimensional and involves 63 types of tissues with the temperature grid in the body (1cm for the trunk, 0.5cm for other parts) represented by 400,000 points. “ an unresolved problem is the control strategy of the system, that is
31、the question whether the inhomogeneous pattern of effector distribution is maintained in the cold and warmth or whether active modification and control is distributed”. The model requires a very powerful computer to perform the many calculations involved. It provides detailed results and is supporte
32、d by a program of research which develops knowledge about both passive and controlling properties. 1.2 Clothing models reviewClothing provides a thermal resistance between the human body and its environment; so one functional role of clothing is to maintain the body in an acceptable thermal state, i
33、n a variety of environments. In some cases, Personal Protective Equipment (PPE) is worn to reduce persons exposures to hazards when the administrative controls are not feasible or effective in reducing these exposures to acceptable level. The thermal behavior of clothing in an (active) person is com
34、plex and dynamic, not fully understand and difficult to quantify. Thats not say that much is not known, there has been much theoretical and empirical research. Factors affecting the thermal behavior of clothing will include the dry thermal insulation, transfer of moisture and vapor through clothing,
35、 heat exchange with clothing (conduction, convection, radiation, evaporation and condensation), compression (e.g. caused by high wind), pumping effect (e.g. caused by body movement), air penetration (e.g. through fabrics, vents and openings), subject posture and so on. An approach to assessing the t
36、hermal properties of clothing is to identify simple thermal models of clothing behavior and attempt to estimate values of quantities required for the thermal models. 1.2.1 A Simple Clothing ModelBodyClothingAir LayerEnvironmentHeat FlowFig. 1.1 A Simple Model with a Layer of Clothing InsulationThe d
37、ry thermal insulation value of clothing materials and clothing ensembles is of fundamental importance and has been extensively investigated. A simple thermal model is of a heated body with a layer of insulation. For the body to maintain equilibrium, heat flows from body to the skin and then through
38、clothing to the environment. The heat flow direction depends on the temperature of body and environment conditions.The simple model presented provides a representation of clothing in the stationary, comfortable or cold human body in many conditions, It provides only an approximation however to many
39、circumstance. An important limitation of the model shown here is that it does not consider wet clothing. Moisture can transfer heat between the body and the environment. This is particularly important when the skin sweats. 1.2.2 2-parameters modelBodyClothingAir LayerEnvironmentHeat FlowMoisture Flo
40、wFig. 1.2 2-Parameters Model with a Layer of Clothing InsulationA simple two-parameter model would consider both dry heat transfer and moisture transfer, which combine to provide the total effect.For most practical applications the simple dry insulation “model” is used to quantify clothing insulatio
41、n (e.g. fanger, 1970). For more specialist evaluations of clothing, the two- parameters model is needed especially for the hot environment where sweating and hence vapor permeation properties will be of great importance. For a more detailed representation of clothing, models are required which invol
42、ve other important factors of clothing thermal behavior. 1.2.3 More complex modelFactors which are not considered in two- parameters model may have a significant effect on the thermal properties of clothing. Kerslake (1972) noticed that the insulation is provided by the fabrics themselves and the la
43、yers of air trapped between the skin and clothing and the clothing layers. The insulation of clothing is mainly decided by the air trapped in and between them. Other important factors, including wind penetration, pumping, clothing ventilation, wicking, are very difficult to modeled and depends great
44、ly on the clothing design, thermal conditions of the body and a persons activity. More complex models of clothing can have a number of characteristics in addition to the simple two-parameters model. No fully comprehensive model exists and the possibility for this is restricted. The thermal propertie
45、s of fabrics change with the change of temperature, humidity. For a complex transient model, these factors are also required to be considered.1.3 Personal cooling system Occupational Safety & Health Administration (OSHA) requires the use of personal protective equipment (PPE) to reduce employees exp
46、osures to hazards when the administrative controls are not feasible or effective in reducing these exposures to acceptable levels. One risk of wearing PPE is that the prevention of heat loss from the body may lead to the suffering from heat stress. Three factors contribute to the level of heat stres
47、s: environment condition, work rate and properties of PPE. With the increase of PPE level to protect human body, it also increases the heat stress level. Personal cooling is one widely used way to alleviate heat stress, especially for those situations where cooling by the ambient air is not applicab
48、le. Personal cooling system technology was first utilized by NASA for the Apollo astronauts to prevent heat buildup under their space suits. The resulting space suit undergarment keeps an astronaut cool and comfortable during extravehicular activities from a spacecraft. Now several companies produce personal cooling systems for use in a wide range of applications. Its important to know the desired cooling capacity and the preferable cooling methods when one chooses cooling systems for persons wearing PPE. From the 60s of the last century, various research groups began to develop and ma