Looks about right, you're probably OK with incompressible ideal gas, if the flow is fast enough to be fully compressible the fire may be the least of your worries! Did you use ALL of the decimal places in the operating density?
https://ansyshelp.ansys.com/account/Secured?returnurl=/Views/Secured/corp/v231/en/flu_ug/flu_ug_bcs_sec_operating.html Focus on the text just above eqn 7-68.
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