20-102 A solar collector consists of a horizontal copper tube of outer diameter 5cm enclosed in a concentric thin glass tube of 9cm diameter. Water is heated as it flows through the tube, and the annular space between the copper and glass tube is filled with air at 1atm pressure. During a clear day, the temperatures of the tube surface and the glass cover are measured to be 60C and 32C, respectively. Determine the rate of heat loss from the collector by natural convection per meter length of the tube. Answer: 17.4 W FIGURE P20-102