It is well known that wind makes the cold air feel much colder as a result of the wind chill effect that is due to the increase in the convection heat transfer coefficient with increasing air velocity. The wind chill effect is usually expressed in terms of the wind chill temperature (WCT), which is the apparent temperature felt by exposed skin. For outdoor air temperature of 0°C, for example, the wind chill temperature is 25°C at 20 km/h winds and -9°C at 60 km/h winds. That is, a person exposed to 0°C windy air at 20 km/h will feel as cold as a person exposed to -5°C calm air (air motion under 5 km/h). For heat transfer purposes, a standing man can be modeled as a 30-cm-diameter, 170-cm-long vertical cylinder with both the top and bottom surfaces insulated and with the side surface at an average temperature of 34°C. For a convection heat transfer coefficient of 15 W/m2·K, determine the rate of heat loss from this man by convection in still air at 20°C. What would your answer be if the convection heat transfer coefficient is increased to 30 W/m2·K as a result of winds? What is the wind chill temperature in this case?
It is well known that wind makes the cold air feel
much colder as a result of the wind chill effect that is due to the
increase in the
air velocity. The wind chill effect is usually expressed in
terms of the wind chill temperature (WCT), which is the apparent
temperature felt by exposed skin. For outdoor air temperature
of 0°C, for example, the wind chill temperature is 25°C at
20 km/h winds and -9°C at 60 km/h winds. That is, a person
exposed to 0°C windy air at 20 km/h will feel as cold as a person
exposed to -5°C calm air (air motion under 5 km/h).
For heat transfer purposes, a standing man can be modeled
as a 30-cm-diameter, 170-cm-long vertical cylinder
with both the top and bottom surfaces insulated and with the
side surface at an average temperature of 34°C. For a convection
heat transfer coefficient of 15 W/m2·K, determine
the rate of heat loss from this man by convection in still air
at 20°C. What would your answer be if the convection heat
transfer coefficient is increased to 30 W/m2·K as a result of
winds? What is the wind chill temperature in this case?
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