Ground-based experimental study on liftoff and turbulent transition characteristics of non-premixed jet flames

A ground-based study was performed to identify suitable conditions for space-based non-premixed jet flames. This research investigated the effects of nozzle diameter and nitrogen dilution on the liftoff and transition behaviors of propane and ethylene flames. Results showed that ignition method and height significantly influence ethylene flame behaviors but have minimal influence on propane fuel. For ethylene, the transition Reynolds number under single ignition (Re = 2000) was significantly higher than under multiple ignitions (Re = 1312), and a higher ignition position promoted earlier liftoff and transition. For propane, pure propane flames exhibited both liftoff and turbulent transition using a 0.4 mm nozzle. However, with the addition of 10 %-50 % nitrogen diluent, only flame liftoff was observed. For 0.5 mm and 0.6 mm nozzles, pure propane and cases with 10 % and 20 % diluent showed both liftoff and turbulent transition, while other conditions only showed flame liftoff. For ethylene fuel with 0.33 mm nozzle, neither flame liftoff nor transition occurred under any condition. For 0.4 mm nozzle, pure ethylene flames showed both liftoff and transition, while only liftoff was observed in diluted cases. For 0.5 mm nozzle, turbulent transition occurred in pure ethylene and with 10 % diluent, while only liftoff was observed in other cases. Froude number analysis revealed that propane laminar flames were buoyancy-controlled, while ethylene flames are in the buoyancy-controlled regime at low flow rates and in both buoyancy- and momentum-controlled regimes at high flow rates. Turbulent flames of both fuels remain buoyancy-controlled. Suitable fuels for in-orbit experiments were identified as 80 % propane/20 % nitrogen (using a 0.6 mm nozzle) and pure ethylene (0.4 mm nozzle). Under normal gravity, they exhibited maximum flame heights of 103 mm and 107 mm, peak combustion loads of 258 W and 268 W, and optimal ignition heights of 30 mm and 25 mm, respectively, with propane requiring single ignition and ethylene multiple ignitions. This study identified the experimental condition points for 80 % propane/20 % nitrogen and pure ethylene aboard the Tiangong Space Station and predicted the corresponding flame heights.