It is well known to heat liquid by internal friction either by rotating a body in a liquid reservoir as disclosed, for example, in my U.S. Pat. No. 4,424,797 or by forcing liquid through restricted orifices as disclosed in the patent to Horne et al. U.S. Pat. No. 4,344,567. Though rotating a body through liquid in a reservoir is effective to heat the liquid a problem of cavitation can arise where the rotor loses intimate contact with the liquid, and during such periods the heating process becomes highly inefficient.