Question

1.23 Under conditions for which the same room tempera- ture is maintained by a heating or cooling system, it is not uncommon for a person to feel chilled in the win- ter but comfortable in the summer. Provide a plausible explanation for this situation (with supporting calcu- lations) by considering a room whose air temperature is maintained at 20°C throughout the year, while the walls of the room are nominally at 27°C and 14°C in the summer and winter, respectively. The exposed sur- face of a person in the room may be assumed to be at a temperature of 32°C throughout the year and to have an emissivity of 0.90. The coefficient associated with heat transfer by natural convection between the person and the room air is approximately 2 W/m².K. What is the ratio of the thermal resistance due to convection to the thermal resistance due to radiation in the summer? What is the ratio of thermal resistances in the winter?

Answer 1

1.237 goln - S = 305* winter condition Summer condition Room temperature Room temperature Ta - 90 - $93 0. Te = 20°c RW = nos -293 Person temperature - Tho = 32% To I 32 305* Room wall temperature Fra 14°C Tewas = 27°C =287 - 300K emmissivity Ce) 2018 heat transfer coefficient (n) = 2 W/m2K for convection proces, q=hA(OT) 2 AT e ... Thermal resistante - for convection creating a Rothie ) = ha ( for Radiation proceil 2- REA (1727) q=0.€ A (TIP+732) (Tit Te) (TI-T) 1. (E a Crita fe y CTATE)) Thermal resistant for Baton Radiation (Rthie) - с£я Стта Студі

Now Ratio of Resistance - for Summer condition he (Ratio) qumoney to 905273002) (305+36) 5.67X16XD.9 x (S05+3003(305+300) [(Ratio = 2.825 Ans mimer For winter condition :- - TE (305*+ 2873) ( 305+284) (Ratio) winter 567x16*x0.9% (505'+8872) (305+287) 2.649 = winter