Predicting Human Thermal Comfort In A Transient Nonuniform .
Predicting Human Thermal Comfort in a Transient Nonuniform Thermal EnvironmentJohn P. Rugh1 , Robert B. Farrington1, Desikan Bharathan1, Andreas Vlahinos2, Richard Burke3,Charlie Huizenga4, and Hui Zhang41National Renewable Energy Laboratory, 1617 Cole Blvd., Golden, CO 84010, U.S.A.2Advanced Engineering Solutions, 4547 N. Lariat Drive, Castle Rock, CO 80104, U.S.A.3Measurement Technology Northwest, 4211 24th Ave. West, Seattle, WA 98199, U.S.A.4University of California, Berkeley, Center for Environmental Design Research, Berkeley, CA 94720, U.S.A.email: john rugh@nrel.govAbstractThe National Renewable Energy Laboratory (NREL) hasdeveloped a suite of thermal comfort tools to assist in thedevelopment of smaller and more efficient climate controlsystems in automobiles. These tools, which include a126-segment sweating manikin, a finite elementphysiological model of the human body, and apsychological model based on human subject testing, aredesigned to predict human thermal comfort in transientnonuniform thermal environments such as automobiles.The manikin measures the heat loss from the human bodyin the vehicle environment and sends the heat flux fromeach segment to the physiological model. Thephysiological model predicts the body’s response to theenvironment, determines 126 segment skin temperatures,sweat rates, and breathing rate, and transmits the data tothe manikin. The psychological model uses temperaturedata from the physiological model to predict the local andglobal thermal comfort as a function of local skin andcore temperatures and their rates of change. Results ofinitial integration testing show the thermal response of amanikin segment to transient environmental conditions.Key WordsThermal comfort, thermal manikin, human physiology,numerical modeling, automotive1.0 IntroductionAbout 26 billion liters of fuel (equivalent to about 9.5%of our imported crude oil) are used annually to coolvehicle passenger compartments in the United States[Rugh and Hovland 2003]. Europe would use about 6.9billion liters if all its vehicles were equipped with airconditioners. Japan uses about 1.7 billion liters to aircondition its vehicles. These numbers can be reducedsignificantly with advanced climate control systems thatreduce the solar load, efficiently and intelligently deliverconditioned air, and use more efficient equipment.However, there is a continuing need for sophisticatedtools to evaluate the effectiveness of advanced climatecontrol systems.The objective of this project is to develop computationaland physical models of human thermal physiology andthermal comfort to evaluate vehicle climate controlsystems. Specifically, NREL is developing a numericalmodel of human thermal physiology and psychology, anda thermal manikin that can be placed in vehicles. All willrespond to the transient and extremely nonuniformthermal environments inside vehicle cabins. Industry canthen use these tools to develop climate control systemsthat achieve optimal occupant thermal comfort withminimum power consumption [McGuffin et al. 2002].2.0 Human Thermal Physiological Numerical Model2.1 DescriptionThe NREL Human Thermal Physiological Model, athree-dimensional transient finite element model, containsa detailed simulation of human internal thermalphysiological systems and thermoregulatory responses.The model consists of two kinds of interactive systems: ahuman tissue system and a thermoregulatory system. onses, such as vasomotor control, sweating, andshivering. The human tissue system represents the humanbody, including the physiological and thermal propertiesof the tissues. The model was developed using thecommercially available finite element software ANSYS.This software can compute heat flow by conduction,convection, and mass transport of the fluid, which makesit practical for simulating human heat transfer.Human thermal response to an environment consists ofconvection within the circulatory and respiratory systems,and conduction within the tissues. The arms and legsconsist of bone, muscle, fat, and skin. There are
additional lung, abdominal, and brain tissues in the torsoand head segments. The model calculates the conductionheat transfer based on the temperature gradients betweenthe tissue nodes.body part is connected to its adjacent part with veins andarteries. In the limbs, the tissues are not connectedbetween parts. In the torso, which is modeled as anintegral part, all tissues are connected.Circulation heat transfer is modeled using a right-anglednetwork of pipe elements within each body segment. Thediameter of the pipes decreases from the center of eachsegment outward toward the skin and extremities. Theflow in the pipes is modeled as Poiseuille flow and aconvection coefficient is solved at each node in the pipenetwork. The diameters of the pipes in the skin layer canconstrict or dilate depending on temperature distribution.The equations that control vasoconstriction/dilation arebased on medical experiments [Smith 1991].The overall model consists of approximately 25,000nodes and 25,000 elements. Because the model is verydetailed, we can see a fairly complete picture oftemperature distribution. An example for the hand isshown in Figure 1. For this simulation, blood flows intothe supply arteries at 1380 cc/hr at 37 C. The hand’smuscle and skin tissues generate heat at 750 W/m3 and1005 W/m3, respectively. The hand is exposed to a lowertemperature environment that is applied as a heat loss of100 W/m2 on its exposed surfaces. The resultingtemperature distribution on the external surfaces is shownin Figure 1. The tip of the pinky attains the lowesttemperature; the palm remains warm.The human thermoregulatory system is modeled usingvasoconstriction/dilation, sweating, shivering, andmetabolic changes. The vasoconstriction/dilationresponse varies with skin and core temperatures, and witheach body segment, because of the diameter of the pipes.The sweating response is a function of skin and coretemperatures, and the number of sweat glands in eachsegment. The degree of shivering depends on skin andcore temperatures, and the amount of muscle in eachsegment. The cardiac output or flow through the pipenetwork is a function of the metabolic rate and skin andcore temperatures [Smith 1991].2.2 StatusThe physiological model was generated in sections usingANSYS. The sections consist of hand, lower arm, upperarm, foot, lower leg, and thigh, one each for the left andright sides. The body is developed as a torso together withneck and head. The limbs consist of bone, muscle, fat,and skin, each surrounding the previous layer. Specialtissues for abdomen, lung, and brain are introduced in thetorso and head volumes. Each part is generatedindividually and is populated with arteries and veins. Theprimary blood vessels join via capillaries placed adjacentto the skin layer. The blood vessel diameters are sized toallow blood to flow to each body part at an overallnominal pressure difference of 70 mmHg between theblood supply and return. The tissues are modeled usingANSYS Solid70 elements and the blood flow pipes useFluid116 elements. Tissue properties are taken fromtables provided by Gordon et al. [1976]. The overallmasses and mass distribution for each part in the modelcompare favorably to those of a human. Deviations arenominally less than 5%.An additional pipe network to simulate airflow throughthe trachea and lungs is also included in the torso. EachFigure 2 shows the temperature distribution over the bodyas well as a cross sectional view of the torso. Theabdomen is warmer because of internal heat generationand relative isolation from the environment. The lungarea remains cool because of breath flow. The brain massreaches a moderate temperature between the two.2.3 Technical ChallengesGiven a set of heat flux boundary conditions on the skin,the model requires about 2 min to arrive at the steadystate temperature distribution. We expect to cut this timein half by streamlining the model and eliminating thelarge amount of input/output that occurs during a normalANSYS run.Future work will include modifying the model to run as atransient over a given amount of time and integrating withthe data stream from the manikin. We have simulated thisoperation with one segment of the manikin and acorresponding model in ANSYS. We plan to extend thatoperation to the entire manikin.The model can also be operated independently of themanikin to verify its behavior against published data.However, the model is currently configured to interactwith the environment using only a heat flux boundarycondition. To run as a stand-alone model, the interactionshould occur via radiation, convection, and conduction.Convection should also include sensible and latent heattransfer. To accommodate all these modes of heattransfer, the model requires additional surface shellelements. Additional work is needed to implement theseelements in the model. Our capabilities are therefore
limited in comparing model predicted data withexperimental results at this time.3.0 Human Thermal Physical Model – The ADvancedAutomotive Manikin (ADAM)3.1 DescriptionThe manikin acts as a heat transfer sensor that mimics areal three-dimensional body. It senses the difficult-tomodel local sweat evaporation, convection, and radiationprocesses that are highly dependent on localmicroclimate. The manikin can also be clothed toaccurately depict the sweat transport of a clothed humanand analyze other clothing effects. The manikin isprimarily designed as an integrated tool for use with theHuman Thermal Physiological Model and HumanThermal Comfort Model, but it can also operate as astand-alone device to test clothing or environmentsfollowing traditional control schemes.The manikin is designed with the following generalcapabilities and characteristics: Detailed spatial and rapid temporal control ofsurface heat output and sweating rate.Surface temperature response time approximatinghuman skin.Human-like geometry and weight with prostheticjoints to simulate the human range of motion.Breathing with inflow of ambient air and outflowof warm humid air at realistic human respirationrates.Complete self-containment, including batterypower, wireless data transfer, and internal sweatreservoir for at least 2 hr of use with no externalconnections.Rugged, durable, low-maintenance construction.The geometry of the manikin was designed to match the50th percentile American male. The manikin isapproximately 1.75 m tall. A NURBS digital model of thehuman body was reshaped in CAD to comply with the50th percentile target and allow the manikin to be digitallymanufactured.The manikin’s fundamental components are the 126individual surface segments, each with a typical surfacearea of 120 cm2. Each segment is a stand-alone devicewith integrated heating, temperature sensing, sweatdistribution and dispensing, and a local controller tomanage the closed loop operation of the zone. Thesweating surface is all-metal construction optimized forthermal uniformity and response speed. Variable porositywithin the surface provides lateral sweat distribution andflow regulation across the zone. Distributed resistancewire provides uniform heating, and is backed up by aninsulative layer that improves structural rigidity. Thesingle zone controller, including flow control valving, ismounted directly behind the zone [Burke et al. 2003].The manikin’s skeleton is composed of laminated carbonfiber, which supports its structure, houses all internalcomponents, and provides mounting locations for surfacesegments. The joints connect the skeleton parts to givethe manikin a human-like form. The adjustable frictionjoints are pre-tensioned so it can be posed in specifichuman positions. The wiring harness and sweat tubes passthrough the joints.In a vehicle, the manikin can operate with no externalcabling; rather, it uses an internal battery power source(four internal NiMH battery modules in the torso andthighs) and a wireless communication system. Forwireless communication, data are transferred via 900MHz spread spectrum transceivers. For applications thatdo not require wireless operation, the system can beplugged into an external power supply andcommunication port for continuous operation and batterycharging.3.2 StatusAll segments are complete, although sensor, heater, andfluid problems have delayed the full operation of 11segments. The manikin has been assembled (Figure 3),and full testing has been initiated. The breathing systemhas not yet been incorporated. More details are availablein Burke et al. [2003].3.3 Technical ChallengesIn very dry conditions, evaporation rates may be so highthat the segment surface is not completely wetted by thesweat. If the locations where the temperatures aremeasured are not wetted, an incorrect temperature readingmay result. The segment emissivity is lower than skinemissivity, which increases the heat loss to theenvironment. The response time of the manikin to atransient environment and to a change in the set pointtemperatures will be carefully assessed. When themanikin is seated, certain segments will recess into itsinterior. The Human Thermal Physiological Model willneed to recognize this condition and adjust appropriately.More details are available in Burke et al. [2003].4.0 Human Thermal Comfort Empirical Model4.1 DescriptionWe performed 109 human subject tests under a range ofsteady-state and transient thermal conditions to explore
the relationship between local thermal conditions andperception of local and overall thermal comfort. The testsinclude collection of core and local skin temperatures aswell as subjective thermal perception data obtained via asimple form. These data are used to develop a predictivemodel of thermal comfort perception [Zhang et al. 2003].The human subject testing was conducted in theControlled Environmental Chamber at the University ofCalifornia-Berkeley. The subject first stayed in atemperature-controlled water bath for 15 min to decreasethe time needed for the body to reach a stable, repeatableinitial condition. After drying, the subject was fitted witha harness of skin surface temperature sensors under a thinleotard. In each test, an air sleeve was connected to anindividual segment of the leotard and supplied controlledtemperature air to provide local heating and cooling. Thesubject voted his or her overall and local thermalsensation and comfort approximately every minute (seeFigure 4). After 10–20 minutes, the local heating orcooling was removed. The procedure was repeated forother segments.The skin temperature measurement harness had 28 finegauge thermocouples to measure the skin temperatures atstandardized locations on the body. The thermocoupleswere soldered onto an 8 mm copper disk and taped to theskin to allow very fast response during temperaturetransients.A wireless sensor was used to measure core temperature.The subject swallowed the sensor before the test. As thesensor passed through the digestive tract, it provided ahigh-resolution measurement of the core temperature.We used the 9-point thermal sensation scale, extendingthe ASHRAE scale with “very cold” and “very hot.” Weused an independent comfort scale rather than thecombined Bedford scale since the highly asymmetric andtransient conditions produced during the test meant that acold or hot sensation could be quite comfortable.4.2 StatusThe subject testing and analysis are complete. Details areavailable in Zhang et al. [2003].4.3 Technical ChallengesThe test subject sample size was somewhat limited, anddid not include a wide variety of ages, weights, and bodycompositions. Since the potential number of testpermutations is very large, a subset of tests was carefullyselected. Correlating global thermal comfort proved to bea complicated process. Details are available in Zhang etal. [2003].5.0 Thermal Comfort Model Integration5.1 DescriptionEach tool discussed in this paper addresses an element ofthe total human comfort perception. The integratedthermal comfort prediction system consists of the thermalmanikin, Human Thermal Physiological Model, and theHuman Thermal Comfort Model. The manikin representsthe physical hardware components in this comforttoolbox. It provides a true body positioned in a vehicle tomeasure the transient thermal response with extremelyhigh spatial density. The finite element model providesthe manikin with a control algorithm that closely mimicshuman response. The real-time psychological comfortmodel uses this response to output the end goal of thesystem—human perceptions of local and global thermalcomfort versus time.The manikin is essentially a surface sensor that measuresthe rate of heat loss at each surface segment. The skinheat transfer rates are sent to the physiological model,which computes the skin and internal temperaturedistribution and surface sweat rates. This information isthen sent back to the manikin, which generates prescribedthe skin temperatures and surface sweat rates, andbreathing rate. This loop continues to provide a transientmeasurement tool. Within each period of the loop, thetemperature distribution and rate of temperature change issent to the psychological thermal comfort model. Thetemperature distribution and rates of change are thenconverted to perceptions of local and global sensation andcomfort, which are displayed graphically versus time on alaptop computer.Initial interface testing was conducted demonstratingphysiological model control of the manikin. Figure 5shows the temperature and heat flux response of a nonsweating manikin segment to increased thermalconvection. The increased heat loss is measured by themanikin and the set point temperature calculated by themodel drops accordingly. The heat loss from the bodyreturns to the steady state range and the skin temperaturesare cooler.The impact of a radiant load on a non-sweating manikinsegment is shown in Figure 6. When the radiant load isapplied, the manikin measures a heat gain into the bodyand passes this information to the physiological model.The model responds by increasing the skin temperatureset point and sweat rate. Since the sweat system was notfunctioning for this test, the manikin skin temperature
overshoots the set point temperature, which demonstratesthe importance of sweating on thermal manikins.Using an IR camera, the surface temperatures of a humanand the manikin with physiological model control arecompared in Figure 7. The human and the manikin bothhave a warmer chest and head and cooler extremities. Thedark regions on the manikin represent low temperatureareas and are due to malfunctioning segments.5.2 StatusAll components of the thermal comfort model and initialinterface testing are complete.5.3 Technical ChallengesSince the manikin and models have different timeconstants, the control software will need to be optimizedto minimize run time and maximize time step.Measuring low heat fluxes at low temperature differencesis difficult, and may cause control stability issues.Validating the entire process will be a challenge.6.0 ConclusionsWe have developed the three major systems necessary forpredicting thermal comfort in a transient, nonuniformthermal environment: a physiological model of the humanthermal regulatory system, a physical model (manikin) ofthe human body including heating and sweating, and anempirical model to predict local and global thermalsensation and comfort. Initial integration of the threecomponents has been completed and is being validated.Each component (as well as the interaction of the models)has presented its own challenges. We plan to use theintegrated models in vehicle applications to develop andevaluate fuel-saving climate control systems.7.0 ReferencesBurke, R.; Rugh, J.; and Farrington R. (2003) ADAM –the Advanced Automotive Manikin, 5th InternationalMeeting on Thermal Manikins and Modeling, Strasbourg,France.Rugh, J. and Hovland, V. (2003) National and Worl
2.0 Human Thermal Physiological Numerical Model 2.1 Description The NREL Human Thermal Physiological Model, a three-dimensional transient finite element model, contains a detailed simulation of human internal thermal physiological systems and thermoregulatory responses. The model consists of two kinds of interactive systems: a
Thermal Comfort Seven Factors Influencing Thermal Comfort 1 Activity level 2 Clothing 3 Expectation 4 Air temperature 5 Radiant temperature 6 Humidity 7 Air speed e974 c Dr. M. Zahurul Haq (BUET) Moist Air & Thermal Comfort ME 415 (2011) 13 / 19 Thermal Comfort ASHRAE Comfort Zone e264 c Dr. M. Zahurul Haq (BUET) Moist Air & Thermal Comfort ME .
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