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

2. 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

3. Mechanisms of Heat loss

4. 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.

5. 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

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

7. 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

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

9. 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

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

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

12. References
Heat Transfer A Practical Approach 2nd ed by Yunus A. Cengel

13. Questions