Experimental–CFD Optimisation of Hydraulic Jet Reach for the GS Mark III Peatland Firefighting System

The use of Computational Fluid Dynamics (CFD) with surrogate-based optimisation is increasingly becoming common in the design of fluid delivery systems although there are very few cases where it has been applied to fixed water-based fire suppression systems. This paper seeks to propose an integrated four-stage engineering methodology incorporating field experiment, internal flow CFD using the Shear Stress Transport (SST) k-ω turbulence model in ANSYS Fluent, coherent-stream trajectory analysis and response surface methodology based on third-order polynomial regression. The proposed method is used to design the GS Mark III which is a fixed sprinkler nozzle system used to extinguish subsurface smouldering peatland fires. The validation of the coupled CFD and trajectory analysis model for 3, 5, and 7 bar using the field measurement shows the error of the model ranging from 2.55 to 3.58%. The coherent-stream trajectory is modelled by direct integration of the equations of motion, with aerodynamic deceleration represented by a single lumped coefficient calibrated against the field data, since neither a bluff-body drag coefficient nor a skin-friction closure reproduces the measured reach. The design of a parametric model involving 20 nozzle geometries in terms of diameter (5–15 mm) and discharge angles (0–67.5°) yields a surrogate with R2 = 0.988, a root mean square error (RMSE) of 1.19 m and a mean absolute error (MAE) of 0.92 m. The surrogate locates the optimum at a diameter of 15 mm and a discharge angle of 38.26°, although the fitted response is flat between approximately 31° and 46°, so any angle within that band performs equivalently within the resolution of the model. The CFD simulation of the optimum (exit velocity = 32.121 m/s) gives a coherent-stream jet distance of 46.91 m, within 1.01% of the surrogate prediction. This represents a 25.9% improvement over the baseline configuration within the same modelling framework (37.25 m against 46.91 m), obtained at 2.28 times the baseline discharge, which the water supply must be able to sustain.