Thermal Effect Simulation in High-Pressure Injection System Transient Flows 2004-01-0532
Temperature variations due to compressibility effects of the liquid fuel were evaluated, for the first time in high-pressure injection system simulation, by employing the energy conservation equation, in addition to the mass-continuity and momentum-balance equations, as well as the constitutive state equation of the liquid. To that end, the physical properties (bulk elasticity modulus, thermal expansivity, kinematic viscosity) of the fluid were used as analytic functions of pressure and temperature obtained by interpolating carefully determined experimental data. Consistent with negligible thermal effects of heat transfer and viscous power losses involved in the flow process, the equation of energy was reduced to a state relation among the fluid thermodynamic properties, leading to a barotropic flow model. A comparison between isentropic and isothermal evolutions in the pure liquid regions was carried out for evaluating the influence of the temperature variation simulation on the macroscopic results given by local pressure time-histories. Furthermore, for cavitation analysis, different thermodynamic transformations of the vapor-liquid mixture were considered and compared.
A recently developed conservative numerical model of general application, including a comprehensive thermodynamic approach to the simulation of cavitation, based on a barotropic flow model, was applied and assessed through the comparison of prediction and measurement results on a diesel injection system performance.
A conventional pump-line-nozzle system was considered for this purpose, being relevant to the model evaluation due to its pressure wave dynamics and also because it was subject to severely cavitating flow conditions at part loads. Predicted time-histories of injector needle lift and pressure at two pipe locations, for two engine loads at the same pump speed, were compared to experimental results, substantiating the validity and robustness of the conservative model taking thermal effects into account in high-pressure injection system transient flow simulation with great degree of accuracy, even in the presence of cavitation induced discontinuities, with minor oscillation problems. The thermal effects due to the temperature variations in the liquid fuel and in the cavitating mixture were analyzed and discussed.