Rayleigh was probably1.40, the first to suggest a model for frictionless flow with a constant heat transfer. Rayleigh's work was during the time when it was debatable as to whether there are two forms of energies (mechanical, thermal), even though Watt and others found and proved that they are the same. Therefore, Rayleigh looked at flow without mechanical energy transfer (friction) but only thermal energy transfer. In Rayleigh flow, the material reaches choking point due to heat transfer, hence term ``thermally choked'' is used; no additional flow can occur.