Chapter 1 Basics of Heat Transfer1-127A standing man is subjected to high winds and thus high convection coefficients. The rate of heat loss from this man by convection in still air at 20°C, in windy air, and the wind-chill factor are to be determined. Assumptions1 A standing man can be modeled as a 30-cm diameter, 170-cm long vertical cylinder with both the top and bottom surfaces insulated. 2The exposed surface temperature of the person and the convection heat transfer coefficient is constant and uniform. 3 Heat loss by radiation is negligible. AnalysisThe heat transfer surface area of the person is Windy weather As= πDL= (0.3 m)(1.70 m) = 1.60 m² The rate of heat loss from this man by convection in still air is Qstill air= hAsΔT= (15 W/m²·°C)(1.60 m²)(34 - 20)°C = 336 WIn windy air it would be Qwindy air= hAsT= (50 W/m²·°C)(1.60 m²)(34 - 20)°C = 1120 WTo lose heat at this rate in still air, the air temperature must be 1120 W = (hAsT)still air= (15 W/m²·°C)(1.60 m²)(34 - Teffective)°C which gives Teffective= -12.7°C
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