The apparatus described herein uses a disc wafer-type rotor featuring channels disposed around its circumference and around the interior circumference of the rotor housing specifically to induce cavitation. The channels are shaped to control the size, oscillation, composition, duration, and implosion of the cavitation bubbles. The rotor is attached to a shaft which is driven by external power means. Fluid pumped into the device is subjected to the relative motion between the rotor and the device housing, and exits the device at increased temperature. The device is thermodynamically highly efficient, despite the structural and mechanical simplicity of the apparatus. Such devices accordingly provide efficient, simple, inexpensive, and reliable sources of distilled potable water for residential, commercial, and industrial use, as well as the separation and evaporation of other liquids.