KAUST VSRP Research Project | King Abdullah University of Science and Technology (KAUST), Thuwal, Saudi Arabia | Oct – Dec 2025
J. Subburaj, T. A. Kashif, M. Vogl, S. Maddi, Z. Alyousef & A. Farooq, "A CFD Study of the Combustion Process in a Miniature Shock Tube at Different Oxyhydrogen Fill Pressures," AIAA SCITECH 2026 Forum, Orlando, FL, 12–16 January 2026.
DOI: 10.2514/6.2026-2606
As part of a Visiting Student Research Program (VSRP) at KAUST under the supervision of Prof. Aamir Farooq, I contributed experimental work to a study on combustion processes in a miniature shock tube. The research group developed a three-dimensional CFD model of a 6 mm-diameter, 165 mm-long shock tube charged with hydrogen–oxygen mixtures at varying equivalence ratios and nitrogen dilution levels. My contributions focused on two key experimental activities: deflagration-regime shock tube experiments with precisely controlled gas mixtures and gas chromatography (GC) analysis of electrolysis-generated gas samples. These experimental datasets provided the boundary conditions and validation data used in the published AIAA paper.
Initial validation of the CFD model revealed a major discrepancy: simulated pressure peaks reached roughly 120 bar, whereas experimental measurements showed only about 30 bar — a fourfold difference. After thorough analysis, the deviation was attributed to uncertainties in the molar composition of the explosive gas mixture used in earlier experiments. To resolve this, new experiments were designed using a precisely prepared mixture with known compositions, allowing a direct, apples-to-apples comparison with the simulation boundary conditions.
A fuel-rich oxyhydrogen mixture with an equivalence ratio of Φ = 1.5 was prepared in a large mixing vessel. The vessel and manifold piping were evacuated before each gas was introduced via partial pressures. The mixture was left to homogenise for over three hours.
| Species | Partial Pressure [Torr] | Cumulative Pressure [Torr] | Mole Fraction [%] |
|---|---|---|---|
| H₂ | 1 208 | 1 208 | 38.65 |
| O₂ | 402.67 | 1 610.67 | 12.88 |
| N₂ | 1 514.79 | 3 125.46 | 48.47 |
Experiments were conducted at three initial fill pressures — 1, 2 and 3 bar — with five repetitions per level (15 runs total). For each run:
No detonation events were observed at any fill pressure. The combustion remained in the deflagration regime throughout all 15 experiments. The absence of deflagration-to-detonation transition (DDT) may be attributed to:
This finding provided essential calibration data for the CFD model and confirmed the deflagration-regime behaviour predicted by the simulations at these conditions.
To quantify the actual composition of gas produced by an in-situ water electrolyser used to fill the shock tube, a systematic gas chromatography (GC) campaign was conducted. Knowing the exact molar fractions of H₂, O₂ and residual N₂ is critical for defining accurate CFD boundary conditions.
Raw mass-percentage data from the GC were converted to mole percentages using molar masses (H₂ = 2 g/mol, O₂ = 32 g/mol, N₂ = 28 g/mol). Data visualisation was performed with MATLAB.
| Species | Mass % Range |
|---|---|
| H₂ | 0.02 – 0.18 % |
| O₂ | 0.003 – 1.02 % |
| N₂ | 0.42 – 2.24 % |
The residual nitrogen in all samples confirmed that the flushing procedure does not fully eliminate atmospheric contamination — an important factor for accurate simulation boundary conditions. Considerable variability in H₂ and O₂ concentrations across pressures and replicates was observed, likely due to fluctuations in the electrolysis process, differences in sample collection timing, or incomplete gas mixing. Converting from mass to mole percentages significantly increases the apparent hydrogen fraction owing to its low molar mass (2 g/mol vs. 28–32 g/mol for N₂ and O₂).