The left side of the above equation represent the net heat flow in the x-direction. If we put the value 
of in the right side of the above equation,
In a similar way we can get the net heat flow in the y and z-directions,
As we know some heat is entering, some heat is leaving and some heat in generating in the volume element and as we have not considered any steady state assumption till now, thus because of all these phenomena some of the heat will be absorbed by the element. Thus the rate of change of heat energy 
within the volume element can be written as,
where, cp is the specific heat capacity at constant pressure (J/(kg·K)), ρ is the density (kg/m3) of the material, and t is the time (s).
We know all the energy term related to the above problem, and with the help of energy conservation,
On putting all the values in the above equation,
or,