Scramjet Combustor: Rayleigh Flow & Thermal Choking

Inlet Conditions

Inlet Mach number M₁ 2.60

Inlet stagnation temp T₀₁ 1000 K

Combustor

Fuel heat addition 60% of choking limit

State

SUPERSONIC — COMBUSTOR STARTED

Live Readouts

Exit Mach M₂

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Exit static T

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p₀₂ / p₀₁

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Choke margin

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How it works

Fuel injected into a constant-area scramjet combustor releases heat into an already-supersonic airstream (M₁ ≥ 2). For frictionless 1-D flow with heat addition in a fixed-area duct — Rayleigh flow — the stagnation temperature ratio to the sonic reference state is:

T0/T0* = (γ+1)M²[2+(γ-1)M²] / (1+γM²)²

On the supersonic branch (M > 1) this ratio rises monotonically as M falls toward 1. So adding heat always decelerates a supersonic flow toward Mach 1 — it can never accelerate it further. If enough heat is added that T0 would need to exceed T0* (the value at M=1), the duct cannot physically pass that much energy at constant area: the flow thermally chokes, M is pinned at 1, and the disturbance is forced upstream as a shock train that can spill out of the inlet — the classic scramjet failure mode called inlet unstart.

M₁ / T₀₁ — the flow state delivered by the inlet's oblique-shock compression, before fuel is added.
Fuel heat addition — heat release as a percentage of the maximum the duct can absorb before choking (100% = exactly M₂ = 1 at the exit; above 100% the choke point moves inside the duct).
Colour shows local static temperature; particle speed shows the local flow speed M·√(γRT) — both computed pointwise from the Rayleigh relation, not just interpolated for looks.
Readouts also track stagnation-pressure recovery p₀₂/p₀₁, which always drops when heat is added — the thermodynamic price of Rayleigh-flow combustion, independent of friction.

This constraint is why scramjets (unlike ramjets and turbojets, which decelerate flow to subsonic before burning) must keep combustor heat release carefully below the thermal-choking limit at every operating point, or the whole engine unstarts.

Cool supersonic inflow Heat release / combustion Peak temperature

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