A housing (1-4) forms a heat-generation chamber (6) in which a driven rotor (11) works. A heat-absorption chamber (15) adjacent to this chamber allows fluid to flow through both. An externally-driven shaft (10) rotating in the housing turns the rotor, which has faces (11c-e) forming a gap (20) in conjunction with internal faces in the housing filled with the viscous fluid (21). Heat is generated by the shearing action in the gap. Part at least of the housing forming the heat-generation chamber is of material having a linear coefficient of expansion greater than that of the rotor.
