The role of backpressure on discharge coefficient of sharp edged injection orifices

Sharp edged injection orifices used in thrust chambers and thermal regulation system of liquid rocket engines have to maintain a steady flow to ensure optimum performance. These orifices are subjected to backpressure and elevated temperature during actual operation. The sharp edged orifices do not always run full due to the orifice geometry and/or flow conditions. In some cases, the flow may change its characteristics from attached to detached flow or vice versa. Length of the orifice in relation to its diameter and the backpressure have been observed to play an important role in the abrupt changes in the flow characteristics. In order to comprehensively characterize this phenomenon and to clearly identify the governing parameters, discharge characteristics were determined experimentally for sharp edged injection orifice of 0.6 and 1.4 mm diameters with length-to-diameter ratios in range between 1.4 and 11.5. Cavitation numbers were varied in the range of 0.06 to 20.9 while Reynolds numbers were varied in the range of 13 000 to 62 500. The backpressure for the injection orifice was varied upto 3.6 MPa. The effect of backpressure, Reynolds number, cavitation number, length-to-diameter ratio, and injection pressure drop-to-backpressure ratio on discharge characteristics was brought out. The experimental investigation gives the detailed flow characteristics of sharp edged orifices in the presence of backpressure.