Heat transfer from the Human Body

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Presentation transcript:

Heat transfer from the Human Body N.Chandrasekaran Group – B IIT Kharagpur

Objectives Mechanisms of heat loss Heat Transfer – Skin Heat Transfer – Skin(Clothing) Heat Transfer – Lungs Total Rate of Heat loss Defense mechanisms-Cold Defense mechanisms-Hot

Mechanisms of Heat loss

Why? It is a common experience to feel “chilly” in winter and “warm” in summer in our homes even when the thermostat setting is kept same.

Problem Qrad,winter = εσAs(Ts4 – T4surr,winter) Consider a person standing in a room maintained at 220C at all times. The inner surfaces of the walls, floors and ceiling of the house are at 10oc in winter and 25oC in summer. (Aperson = 1.4m2, Tperson = 300C, εskin = 0.95) Qrad,winter = εσAs(Ts4 – T4surr,winter) = 0.95 x 5.67x10-8 x 1.4 x [(30+273)4-(10+273)4)] = 152 W Qrad,summer= εσAs(Ts4 – T4surr,summer) = 0.95 x 5.67x10-8 x 1.4 x [(30+273)4-(25+273)4)] = 40.9 W (Heat loss due to Radiation)winter ≈ 4 X (Heat loss due to Radiation)summer

Heat Transfer – Skin Sensible heat loss Latent heat loss Convection Radiation Latent heat loss Evaporation QSkin = Qconvection + Qradiation + Qlatent

Heat Transfer - Skin(Clothing) Clothing complicates the Heat transfer from the body and hence experimental data is used. Effect: Serves as insulation and reduces both sensible and latent form of heat loss. Sensible heat Clothing > Environment

Heat Transfer - Lungs Sensible heat loss Latent heat loss Convection Evaporation QLungs = Qconvection + Qlatent

Total Rate of Heat loss QBody,total = QSkin + QLungs = (QSensible + Qlatent)skin + (QSensible + QLatent)Lungs = (QConvection + QRadiation + QLatent) Skin + (QConvection + QLatent)Lungs Total Heat transfer rate: (Under Steady Conditions) Light Office Work : 100 W Heavy Physical Work : 1000W

Defense mechanisms-Cold Reduction of skin temperature Increasing metabolic heat generation rate – shivering Fall in deep body temperature

Defense mechanisms-Hot Increase of skin temperature Release of water from sweat glands – evaporative cooling Rise in deep body temperature

References Heat Transfer A Practical Approach 2nd ed by Yunus A. Cengel http://hyperphysics.phy-astr.gsu.edu/Hbase/thermo/coobod.html http://www.drphysics.com/convection/convection.html

Questions