Rarified flows in heated passages are shown to have instabilities under certain conditions similar to those occurring with laminar continuum flows. As in the case of continuum flows these instabilities occur where the rate of change of pressure drop with mass flow rate at constant heat input is negative. As gases become more highly rarified, the slope of the pressure drop with flow rate no longer becomes negative and flows are stable. Stability regions are defined by the ratio of outlet to inlet temperature, Knudsen number at the inlet to the passage, and the product of Reynolds number at the inlet and diameter-to-length ratio.

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