Effect of nozzle inlet geometry in high temperature hydrocarbon liquid jets

Highlights: • Orifices of different chamfer depths were used to study high-temperature hydrocarbon liquid injection. • Laminar to turbulent transition was observed for orifices with higher chamfer depth. • Effect of chamfer depth on discharge coefficient diminished with increasing fuel injection temperature. • Discharge coefficients with respect to cavitation number for various nozzle inlets converged gradually. • Hydraulic characteristics became independent of chamfer depth in strong cavitation conditions. An experimental study was conducted to investigate the effect of orifice inlet geometry on discharge and cavitation characteristics of high-temperature hydrocarbon liquid jets. The fuel was heated to 553 K (280 °C) using an induction heater, at an upstream pressure of up to 1.0 MPa, and injected to atmospheric pressure conditions through variously chamfered plain orifices of diameter 0.7 mm and length 4.3 mm. Hydraulic characterization in terms of fuel temperature was carried out by introducing the discharge coefficient, and the macroscopic internal flow characteristics were correlated with Reynolds number and cavitation numbers. The variation of C_d with respect to T_inj in the non-cavitating region below the boiling point shows that C_d increases with increasing chamfer depth C, but the C_d trend nearly converges to a maximum value when the relative chamfer depth reaches 20% of the orifice diameter. In the cavitating region, on the other hand, the effect of chamfer depth on the mass flow rate or C_d diminishes as the cavitation becomes stronger with increasing T_inj. The plot of C_d with respect to Re shows laminar to turbulent transition in high chamfer depth cases at ΔP = 0.3 MPa, and this reveals that the internal flow in the current orifice configuration at relatively low velocity conditions remains laminar even at high Re for higher chamfer depths. Furthermore, the curves of C_d with respect to Re for various chamfer depths at each ΔP condition merge when T_inj is very high, implying that the trend of C_d with respect to Re at high T_inj conditions becomes independent of chamfer depth. The variation of C_d with respect to cavitation number for various chamfer depths converges gradually under cavitating conditions, and this suggests that the degree of cavitation (as quantified by the cavitation number) inside fuel injectors of different chamfer depths becomes closer as T_inj reaches very high values. It can, therefore, be concluded that because of mass flow choking the hydraulic characteristics represented by the discharge coefficient of high temperature hydrocarbon liquid jets become independent of chamfer depth in strong cavitation conditions as T_inj increases beyond the boiling point, and are determined only by the pressure difference between P_inj and P_sat at T_inj.