Replacing the L.H.S. in terms of the substantial derivative form we get,

Subtracting Eq. (23.1) from (23.2), we get, the heat energy equation is;

Let’s simplify the L.H.S.,

From the definition of enthalpy we have,

Therefore the heat energy equation is,  

Further simplification of this equation leads to,

Cancellation of similar terms from both the sides we have,