Isothermal flow is a model of compressible fluid flow whereby the flow remains at the same temperature while flowing in a conduit.[1] In the model, heat transferred through the walls of the conduit is offset by frictional heating back into the flow. Although the flow temperature remains constant, a change in stagnation temperature occurs because of a change in velocity. The interesting part of this flow is that the flow is choked at 1 / k {\displaystyle 1/{\sqrt {k}}} and not at Mach number equal to one as in the case of many other model such as Fanno flow.