Dynamic flow behavior during fuel-offloaded process in control valve for unit pump fuel system

Highlights: • We build an optical test rig based on a diesel unit pump fuel system. • We complete a two-phase unsteady model using dynamic mesh. • We introduce a parameter relative brightness RB to quantify the cavitation. • As the valve opens, cavitation occurs and induces choking flow. • During choking, the mass flow is stable but the discharge coefficient drops. - Abstract: As an important part of high-pressure fuel systems, the control valve adjusts the pressure by regulating the fuel delivery quantity and the withdrawal of fuel. This study presents both experimental and numerical investigations into the flow characteristics within the control valve of a diesel unit pump fuel system (UPS). To observe the fuel flow within the control valve, an optical test rig based on a baseline UPS is built. Further, numerical simulations based on Computational Fluid Dynamics (CFD) method are conducted based on the homogenous two-phase unsteady flow model using dynamic mesh. The results show that during the valve opening period cavitation occurs, directly affecting the fuel-offloading process of the high-pressure fuel line and delaying the time for injector needle seating to cut off fuel injection. There are two cavitation regions: the conical region and the downstream of the conical region. Due to great pressure differential between the inlet and outlet of the control valve, the cavitation in the conical region induces choking flow. Once choking flow occurs, the mass flow rate maintains stable even as the pressure differential continues rising, but the discharge coefficient (C_d) declines. This choking flow increases the time delay of stop fuel injection.