Effect of nozzle inlet geometry in high temperature hydrocarbon liquid jets

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 Cd with respect to Tinj in the non-cavitating region below the boiling point shows that Cd increases with increasing chamfer depth C, but the Cd 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 Cd diminishes as the cavitation becomes stronger with increasing Tinj. The plot of Cd 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 Cd with respect to Re for various chamfer depths at each ΔP condition merge when Tinj is very high, implying that the trend of Cd with respect to Re at high Tinj conditions becomes independent of chamfer depth. The variation of Cd 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 Tinj 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 Tinj increases beyond the boiling point, and are determined only by the pressure difference between Pinj and Psat at Tinj.