Mixing field characteristics of multi-lateral jets injected into a round pipe flow

Jet in cross-flow is a well-studied and characterised fluid-mixing phenomenon. In several combustion applications, the use of laterally placed side-jets can be used to produce jets into a confined cross-flow (JICCF). These flows can be expected to have similar mixing as the traditional jet in cross-flow cases and therefore provide a potentially cost-effective means of optimising a combusting jet flow. However, there are limits to the data currently available on the fundamentals of JICCF. Hence, the current study investigates the flow structures formed in a round pipe flow modified by four equi-spaced side jets. Non-reacting, isothermal experiments are conducted in water on a central nozzle with four smaller jets located one central diameter upstream of the nozzle exit plane. The induced flow structures are visualised using Planar Laser Induced Fluorescence (PLIF). The operating conditions are varied to explore the role of jet injection to primary flow ratio, whilst the bulk flow rate is maintained at a constant level. The analysed data identify the formation of various flow regimes as the relative momentum-flux ratio induced via side jet injection is increased. The behaviour of the side jets within the main jet is substantially different from similar side-jet injection into an unconfined flow. The results show that several flow regimes can be discerned, namely: a non-impinging flow; impinging flow with no backflow; and impinging flow with backflow. It is found that the mixing trends and resulting regimes have consequences for the emerging near-field mixedness.