If we consider a volume of fluid V whose boundaries move with the fluid such that its mass M is conserved, then the integral of Eq. (VI-2) for this volume leads to
where A and 
have the same meaning as in all the preceding
surface integrals. The quantity 
represents
the outward flux of heat at a given point on the surface in this
expression. If we consider a closed system, perfectly insulated
from all radiation or any other heat transfer such that
is zero at all points on the surface, then we
see that
for a thermally insulated system.