Scaling transient heat transfer in impinging flames under compression-ignition engine-like conditions

Understanding transient heat transfer similarity in impinging flames is crucial for scaled experiments. This study investigates transient heat transfer similarity for impinging flames under conditions simulating small- and large-bore compression-ignition (CI) engines. Experiments were conducted in a constant volume vessel using injectors with a geometry similarity ratio of 0.72. Flame luminosity and temperature were captured using two-color pyrometry, while fast-response thermocouples measured local and spatial-averaged transient heat transfer. Three similarity rules (injection pressure, engine speed, and lift-off length) were evaluated, and a novel cumulative injection velocity parameter was introduced. Results indicate that similarity rules effectively predict flame development parameters, such as tip penetration, flame height, and radius under scaled engine-like conditions. Among the evaluated similarity rules, the engine speed similarity rule (S Rule) provided the best predictive accuracy for transient heat transfer. The new characteristic velocity parameter significantly improved accuracy in predicting transient heat transfer. The findings demonstrate the effectiveness of the new cumulative injection velocity parameter in transient heat transfer calculations, providing valuable guidance for scaling methodologies in CI engine design.