A thermocouple is attached to a thin rectangular sheet of copper of thickness 0.5 mm and extent 50 × 150 mm. The copper sheet is initially at a temperature of 75◦ C and is suddenly exposed to still...

A thermocouple is attached to a thin rectangular sheet of copper of thickness 0.5 mm and extent 50 × 150 mm. The copper sheet is initially at a temperature of 75◦ C and is suddenly exposed to still air at 15◦ C. How long should one wait for the thermocouple to read 50◦ C? It may be assumed that the entire plate cools uniformly with time and the heat loss is by natural convection. Assume that the 150 mm edge is vertical. The heat transfer coefficient may be based on an average plate temperature of 62.5◦ C. Use suitable natural convection correlation from a book on Heat Transfer.