Free Molecular Heating

Use the Free Molecular Heating simulation object to account for the heat generated by the interaction between a spacecraft and the low density gas molecules in low altitude orbits.

In low altitude orbits, the gas density is rarefied enough to be outside of the continuum mechanics regime.

The incident heat flux, q”, that is generated by the free molecules impacting the spacecraft surfaces, is computed as follows:

q” = 0.5ρV3				where:ρ is the atmospheric densityV is the spacecraft’s velocity

The amount of heat absorbed by a surface on the spacecraft, qα, is computed as follows:

q α= α q”				where:q” is the incident flux.α is the accommodation coefficient, 0 ≤ α ≤ 1.

The heat flux from the free molecular heating on the reflector and un-shadowed elements is equal.

In the Free Molecular Heating dialog box, you define:

• The part of the model to which free molecular heating applies.

• The accommodation coefficient.

• The heat flux computation method. You can specify one of the following:A constant or time-varying heat flux.A constant or time-varying velocity for the spacecraft and a constant or time-varying atmospheric density.A constant, time-varying, or altitude-varying atmospheric density.

• The heat flux direction.

• The Monte Carlo settings for computing free molecular fluxes.

When you specify a constant or time-varying density, the velocity is computed from the position of the spacecraft in the orbit. When you specify an altitude-varying density, both the density and velocity are determined from the orbit definition. The velocity is the spacecraft’s orbital velocity, and the density is computed from the spacecraft’s altitude.

When you use the Free Molecular Heating simulation object and request free molecular fluxes in the Results Options page of the Solution dialog box, you obtain additional results sets listed in the following table.

Orbital, Radiative and Free Molecular Source Fluxes options	Free molecular heating results set
Solar and Planet View Factors	No results sets
Absorbed Fluxes	Absorbed Radiative Flux, FMH – Elemental
Incident Fluxes	Incident Radiative Flux, FMH – Elemental
Transmitted Fluxes	No results sets
Reflected Fluxes	Reflected Radiative Flux, FMH – Elemental

Note:

Because the free molecular heating fluxes cannot be transmitted through a surface, Transmitted Radiative Flux, FMH – Elemental results are always 0.0 and are not output.

Where do I find it?

Application	Pre/Post
Command Finder	Free Molecular Heating
Simulation Navigator	Right-click the Simulation Object Container node→New Simulation Object→Free Molecular Heating

How do I

Create a Free Molecular Heating simulation object

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Inputs to expressions

Monte Carlo Settings

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Auto-generated expressions

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Simcenter 3D Thermal/Flow, Electronic Systems Cooling, and Space Systems Thermal boundary conditions

Thermal/Flow, Electronic Systems Cooling, and Space Systems Thermal

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Free Molecular Heating, Simcenter 3D 2021.1 Series

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