Question

11 We often see frost on a windshield of car at a clear cold night. A thin layer of water formed from moist ar dew point) on a windshield of a car in a parking lot is exposed to a clear sky at -17°C. The ambient air temperature and convection coefficient are 5°C and 10 W/m2.K. Assume that the emissivity of the thin layer of water is 0.9. Explain how frost can be formed even the ambient air temperature of 5°C is above the freezing temperature.

Answer 1

Date... solo Given, Temprature of this loyer, Ts = -14°C = 256K Temproture of air , Ta=5°C = 278K h=10 W/m2K Convection coefficient, Emmisiturity of thin {=0.9 layer, on the thin layer of water keyper Now, Heat fall m2 by Connection Q h (Ta- Too) 10 (278-256) 2 220 Walte / m2 Now, Heat Radiated by the the thin layer of water due to Radiation R {o T4 0.9 X5.67X108 8 [256] QA 219.172 watts/m2 Since, R

since, heat lost by radiation is more than the heat fall on the thin layer of water by convection due to this frost will formed.