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 $t_{film}=0,5(t_{f}+t_{s})$<br /><br />In external forced convection, the film temperature is often used as a reference temperature. It is defined as the average of the surface temperature ($t_{s}$) and the free stream temperature ($t_{f}$), which is given by the formula $t_{film}=0,5(t_{f}+t_{s})$. This temperature represents the temperature of the fluid in the boundary layer and is used to calculate the heat transfer coefficient in forced convection.