Thermodynamics
The classical Carnot heat engine
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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 ${\displaystyle 1/{\sqrt {k))}$ and not at Mach number equal to one as in the case of many other model such as Fanno flow. This fact applies to real gases as well as ideal gases.

For an important practical case of a gas flow through a long tube, the model has an applicability in situations which occurs in a relatively long distance and where heat transfer is relatively rapid so that the temperature can be treated, for engineering purposes, as a constant. This model has also applicability as upper boundary to Fanno flow.