The use of rock pile thermal energy storage systems has been considered for solar energy and industrial process heat applications. a particular system involves a cylindrical container, 2 m long by 1 m in diameter, in which nearly spherical rocks of 0.03-m diameter are packed. the bed has a void space of 0.42, and the density and specific heat of the rock are rho=2300 kg/m^3 and cp=879 j/kg*k, respectively. consider conditions for which atmospheric air is supplied to the rock pile at a steady flow rate of 1 kg/s and a temperature of 90°c. the air flows in the axial direction through the container. if the rock is at a temperature of 25°c, what is the total rate of heat transfer from the air to the rock pile?