Chapter 1, Problem 127P

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 an outdoor air temperature of $0^{\text{o}}\text{C,}$ for example, the wind-chill temperature is $-5^{\text{o}}\text{C}$ with 20 km/h winds and $-9^{\text{o}}\text{C}$ with 60 km/h winds. That is, a person exposed to $0^{\text{o}}\text{C}$ windy air at 20 km/h will feel as cold as a person exposed to $-5^{\text{o}}\text{C}$ 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 ${34}^{\text{o}}\text{C}\text{.}$ and For a convection heat transfer coefficient of 15 W/m^2. K, determine the rate of heat loss from this man by convection in still air at ${20}^{\text{o}}\text{C}\text{.}$ and What would your answer be if the convection heat transfer coefficient is increased to 30 W/m^2. K as a result of winds? What is the wind-chill temperature in this case?