Hi all I was reading some material about two pahse flow with heat transfer. In the moumentum balance equations, along with the habituel terms, a term is introduced and named as the momentum due to virtual mass production.

Can some one explain what is it exactly?

Virtual mass is defined as the sum of "real" mass and "added" mass. The notion of "added" mass has been introduced to represent a fluid force exerted by the fluid particles around an "accelerating" body. The body feels a force as if its mass is increased by the amounnt of the "added" mass.

The "added" mass force is of "inviscid" origin. And therefore it doesn't vanish even in the case of potential or inviscid flows. It occurs only when the body is accelerating (e.g., oscillation). The added mass force can be significant when the density of the fluid is comparable to the density of the body. So, you hardly see the added mass mentioned in the textbooks dealing with aerodynamics or gas dynamics. But you see lots of treatises on it in textbooks on hydrodynamics.