Radical-Farming Scramjet Engine

• The radical-farming scramjet engine is a hypersonic propulsion system that uses early hydrogen injection to create reactive hot pockets, reducing ignition delays at high Mach numbers.
• It employs a two-ramp cowl inlet with sonic hydrogen injectors and advanced IDDES turbulence models to capture shock interactions and distributed combustion.
• Enhanced combustion efficiency (>80% at φ=0.8–0.9) and stable autoignition in shock-induced regions demonstrate its potential for robust hypersonic propulsion.

The radical-farming type scramjet engine is a hypersonic airbreathing propulsion system employing staged-ignition, wherein limited quantities of hydrogen fuel are injected upstream in the inlet or isolator region to cultivate regions of high radical concentration (“hot pockets”). These hot pockets, enriched in highly reactive species such as H, O, OH, and HO₂, are convected downstream to seed rapid autoignition and promote stable, distributed combustion across multiple shock-induced hot spots. This configuration aims to minimize ignition delays while maintaining combustion stability at extreme flight Mach numbers (up to Mach 10), reducing sensitivity to short residence times typical of supersonic combustors (Plewacki et al., 22 Nov 2025).

1. Engine Architecture and Radical-Farming Concept

The radical-farming scramjet analyzed in Mach 10 conditions in the University of Queensland’s T4 Wind Tunnel has a two-ramp cowl inlet (9° and 3° ramps, total length 182 mm, height 26 mm, width 75 mm), leading to an internal cross-section optimized for hypersonic flow. Downstream, eight sonic transverse hydrogen injectors are located 94 mm from the inlet throat; each injector is 1.2 mm in diameter, inclined 45° to the local wall, spaced at 11.6 mm pitch, with fuel supplied at 300 K. The combustor and isolator maintain a constant cross-section of 26 mm × 75 mm and extend 380 mm from the injector plane to the nozzle. The exhaust section is a 9° single-ramp expansion nozzle (length 198 mm). Optical windows provide diagnostic access up to 120 mm downstream, enabling experimental validation with schlieren and OH-chemiluminescence imaging (Plewacki et al., 22 Nov 2025).

The radical-farming concept leverages early hydrogen injection to generate high-temperature, reactive pockets due to mixing and shock interactions. These pockets are instrumental in reducing local ignition delay, facilitating autoignition, and enabling stable supersonic combustion across distributed regions influenced by shock-induced mixing.

2. Governing Equations, Turbulence, and Reaction Modeling

Simulation of the radical-farming scramjet employs the compressible, multi-species Favre-filtered Navier–Stokes equations for mass, momentum, energy, and species conservation. The model includes both resolved (“bar” terms) and subgrid-scale (“prime” terms) contributions:

• Mass:

$$\frac{\partial \bar{\rho}}{\partial t} + \frac{\partial (\bar{\rho}\tilde u_i)}{\partial x_i} = 0$$

• Momentum, energy, and species equations include viscous, subgrid, and reaction source terms.

For turbulence closure, the Improved Delayed Detached Eddy Simulation (IDDES) model is used. This hybrid approach blends the Spalart–Allmaras RANS model with Cătriş density corrections in wall-bounded regions, transitioning to LES with the Vreman SGS model in detached and shear zones. Switching is governed by a hybrid length scale,

$$l_{\text{IDDES}} = l_{\text{RANS}} - f_d\max\left[0,\,l_{\text{RANS}}-C_{\text{DES}}\Delta\right],$$

where $f_d$ delays premature DES switching near walls and $\Delta$ is the grid spacing.

Combustion is modeled using a Finite Rate Chemistry (FRC) mechanism for H₂/O₂ (Burke et al. 2011), encompassing 12 species (H₂, O₂, H, O, OH, H₂O, HO₂, H₂O₂, Ar, He, N₂, M) and 27 elementary reactions. Rate constants follow Arrhenius kinetics, with molar production for each species given by:

$$\dot\omega_i = M_i\sum_{j=1}^{N_R}(\nu''_{ij}-\nu'_{ij}) \left[k_{j,f}\prod_{s}C_s^{\,\nu'_{sj}} - k_{j,r}\prod_{s}C_s^{\,\nu''_{sj}}\right],$$

where $\nu'_{ij}, \nu''_{ij}$ are stoichiometric coefficients and $C_s$ are molar concentrations.

3. Boundary Conditions and Test Parameters

Freestream conditions replicate Mach 10 flight at the UQ T4 facility: total enthalpy $H_0=4.3$ MJ/kg, $M_\infty=9.7$ (nominal), and measured $p_\infty=4.1$ kPa, $T_\infty=370$ K, $u_\infty=2\,830$ m/s. Oxygen mass flux is $\dot m_{O_2}=167.3$ g/s.

Hydrogen injection equivalence ratio ($\phi$) varies from 0.5 to 0.9. At $\phi=0.8$, $T_{H_2}=404$ K, $\rho_{H_2}=0.863$ kg/m³, $u_{H_2}=777$ m/s, and $\dot m_{H_2}=16.87$ g/s. Inlet boundaries impose Dirichlet conditions for air and hydrogen jets; the outlet uses extrapolation. Surfaces are no-slip, isothermal ($T=300$ K), with symmetry imposed on the centerplane (Plewacki et al., 22 Nov 2025).

4. Mixing Dynamics, Combustion Modes, and Diagnostic Indices

Transverse Injection and Shock–Fuel Interaction

Each hydrogen jet generates bow and barrel shocks that interact with inlet oblique shocks, producing hot-spot regions on the ramp. Coherent hydrogen lobes persist to $x/L\approx0.2$ but fragment rapidly ($L_{\text{mix}}\approx20$ mm), promoting turbulent mixing essential for radical production.

Mixedness, Flame Index, and CEMA

The Takeno mixedness parameter and flame index provide a quantitative map of combustion regimes:

• Mixedness parameter,

$$Z_{FO}= \begin{cases} \frac{Y_O}{j}, & \frac{Y_O}{j}\le Y_F \ -Y_F, & \frac{Y_O}{j}\ge Y_F, \end{cases} \quad j=\frac{m_Ov_O}{m_Fv_F},$$

with $Y_F$, $Y_O$ as hydrogen and oxygen mass fractions.

• Takeno flame index,

$G_{FO}=\nabla Y_F\cdot\nabla Y_O,$

with $G_{FO}>0$ (premixed), $G_{FO}<0$ (non-premixed), and near-zero (transitional).

Simulations reveal autoignition in partially premixed, positive $G_{FO}$ regions (“hot pockets”) with adjacent diffusion-controlled edges ($G_{FO}<0$).

Chemical Explosive Mode Analysis (CEMA) isolates regions of high chemical sensitivity, with the leading Jacobian eigenvalue $\lambda_\text{max}>0$ indicating locally explosive (autoignitive) chemical modes. These are spatially correlated with observed hot pockets just downstream of shock reattachment in the isolator.

5. Combustion Characteristics and Experimental Validation

Flow Structures and Validation

Computed centerplane schlieren contours align with experimental schlieren, accurately reproducing oblique and bow shock patterns. Simulated OH chemiluminescence iso-surfaces (at 1% mass fraction) match experimental recordings, confirming the ignition location behind primary shock reflections.

Pressure, Temperature, and Fuel Distribution

Wall-pressure coefficient,

$$C_p=\frac{p_w-p_\infty}{0.5\,\rho_\infty u_\infty^2},$$

matches UQ measurements within 2–5% for $\phi=0.5$ and 0.8. Increasing equivalence ratio ($\phi$) intensifies pressure and unsteadiness. Streamwise mean temperature increases from 370 K up to approximately 2,500 K ($x/L\sim0.4$–0.8), reaching peak $T\sim2,200$ K ($\phi=0.5$) to $2,800$ K ($\phi=0.9$), before cooling in the nozzle.

Cross-sectional hydrogen mass fraction maps illustrate the transition from lobe-like structures at $x/L=0.2$ to highly fragmented filaments at $x/L=0.4$–0.6, indicative of turbulent mixing. Water vapor contours verify near-complete reaction by $x/L\approx0.8$–1.0.

Flame Stabilization Mechanism

Autoignition is localized in hot pockets generated by expansion–reflection interactions in the isolator, verified by CEMA ($\Re[\lambda_\text{max}]>0$). The local Damköhler number,

$$Da=\frac{\lambda_{\max}}{\chi^{-1}}, \quad \chi=2\,\alpha\,|\nabla\zeta|^2,$$

with $\alpha$ as thermal diffusivity and $\zeta$ as mixture fraction, confirms $Da\gg1$ in these pockets, supporting an autoignition-controlled (as opposed to flame-propagation) stabilization mechanism.

6. Performance Metrics and Impact of Radical-Farming

Combustion efficiency, measured as fuel-to-air heat release, exceeds 80% for $\phi\ge0.8$, compared to $\sim65\%$ at $\phi=0.5$, attributed to enhanced upstream air–fuel mixing in the isolator. Wall-pressure and heat-flux remain within material limits, indicating that radical generation from early injection does not result in detrimental heat localization. Preliminary thrust analysis demonstrates positive net thrust for total dynamic pressures below 37 kPa and above 55 kPa, corroborating with prior full-scale IDDES studies.

A summary of key performance data is given below:

Metric	Value at $\phi=0.5$	Value at $\phi=0.8-0.9$
Combustion efficiency	~65%	>80%
Peak mean temperature	~2,200 K	~2,800 K
Wall-pressure agreement	within 2–5%	within 2–5%
Thrust (momentum flux)	Positive (select $p$)	Positive (select $p$)

7. Outlook and Implications for Hypersonic Propulsion

The radical-farming concept demonstrably reduces ignition delay and expands operable equivalence-ratio windows by leveraging radical-rich hot spots formed upstream via staged hydrogen injection. Simulations project stable combustion even at extremely short residence times typical in Mach 10 operation ($O(\mu\text{s})$). Diagnostics such as CEMA and the Takeno flame index enable quantitative regime identification, supporting detailed design optimization.

Future research directions include improved large-eddy simulation for finer turbulence–chemistry interaction fidelity and extension to advanced engine geometries and flight trajectories. The radical-farming approach is shown to enhance combustion efficiency and operational robustness, establishing a rigorous framework for future developments in hypersonic scramjet technology (Plewacki et al., 22 Nov 2025).