A reference temperature for external forced convection is: The temperature of the solid surface ts The ambient temperature The film temperature defined as the average of the surface and free stream temperatures t_(film)=0,5(t_(f)+t_(s)) The temperature of the fluid tf

The correct answer is: The film temperature defined as the average of the surface and free stream temperatures In external forced convection, the film temperature is often used as a reference temperature. It is defined as the average of the surface temperature ( ) and the free stream temperature ( ), which is given by the formula . This temperature represents the temperature of the fluid in the boundary layer and is used to calculate the heat transfer coefficient in forced convection.