A fully compressible four-equation model for multicomponent two-phase flow coupled with a realfluid phase equilibrium-solver is suggested. It is composed of two mass, one momentum, and one energy balance equations under the mechanical and thermal equilibrium assumptions. The multicomponent characteristics in both liquid and gas phases are considered. The thermodynamic properties are computed using a composite equation of state (EoS), in which each phase follows its own Peng-Robinson (PR) EoS in its range of convexity, and the two-phase mixtures are connected with a set of algebraic equilibrium constraints. The drawback of complex speed of sound region for the two-phase mixture is avoided using this composite EoS. The phase change is computed using a phase equilibrium-solver, in which the phase stability is examined by the Tangent Plane Distance (TPD) approach; an isoenergetic-isochoric (UVn) flash including an isothermal-isobaric (TPn) flash is applied to determine the phase change. This four-equation model has been implemented into an inhouse IFP-C3D software. Extensive comparisons between the four-equation model predictions, experimental measurements in flash boiling cases, as well as available numerical results were carried out, and good agreements have been obtained. The results demonstrated that this four-equation model can simulate the phase change and capture most real-fluid behaviors for multicomponent two-phase flows. Finally, this validated model was applied to investigate the behaviors of n-dodecane/nitrogen mixtures in one-dimensional shock and double-expansion tubes. The complex wave patterns were unraveled, and the effects of dissolved nitrogen and the volume translation in PR EoS on the wave evolutions were revealed. A three-dimensional transcritical fuel injection is finally simulated to highlight the performance of the proposed four-equation model for multidimensional flows.

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Further investigation into the collapse mechanism of multi-jet flash boiling sprays is crucial to improve the mixture formation in gasoline direct injection (GDI) engines. Herein, n-hexane flash boiling sprays from a five-hole GDI injector have been numerically studied using the homogeneous relaxation model (HRM). The collapse process has been validated by the experimental data in terms of spray penetration length and spray width. For the given case, as the individual jets were discharged from the nozzle, they became under-expanded, where a typical shock structure (primary cell) can be well observed for each individual jets. Then, strong interactions were found between the adjacent jets leading to the formation of secondary cells. It was found the jet-to-jet structure did not form a closed ring, indicating that the closedring jet-to-jet structure might be not the necessity for the flash boiling spray collapse. The pressure distribution by two cross-sections revealed the low-pressure region development process and the shock structure vibration characteristic. The initial generation reason for the crown structure in the spray tip was considered the flow separation of the different near-field collapse levels in this process.

Flash boiling has been widely investigated experimentally and numerically in the world for its non-negligible influences on air-fuel mixing, combustion and emission in gasoline engines. Nevertheless, the internal and external flash boiling, and the transition regime therein are still far fully understood. The present work is aimed to shed some light on this issue through numerical simulation approach. The simulation tool is based on homogeneous relaxation (HRM) model implemented in CONVERGE CFD solver. Through a series of case-sensitive simulations for hexane fuels with varied ambient pressure, the flashing scheme has transferred from the external flash state to internal flash state. By redefining the pressure ratio () based on the ratio of the saturation pressure at real-time temperature and real-time pressure, the phase change regions or the superheated aeras in the spray are clearly presented. It is found that the superheated or potential phase change regions have shifted gradually from the nozzle inside in the upper stream to the downstream spray during the transition of non-flashing state, external flash boiling to fierce internal flash boiling regime. Moreover, the timing for the transition of internal flash boiling and external flash boiling may correspond to the disappearance of Mach disk. The current research is expected to provide more insights to the understanding of the internal and external flash boiling.

The performance of siphon toilets in trains with varied pipe profiles were investigated. The profiles of the siphon pipe are varied based on the diameter of the pipe along the pipe length. The flushing process was simulated by using CFD method. The starting time, the duration, the siphon mass flow rate, the negative pressure in the siphon pipe were recorded and selected as siphon performance indexes. The results showed that for the same type of siphon pipe, the siphon performance becomes worse with the increase of the maximum pipe diameter as the minimum pipe diameter is 45mm. The simulation confirms that the performance of the siphon toilet with the pipe diameter varying from large to small then to large is the best; the performance of the siphon with a pipe diameter varying from small to large then to small is the worst. The qualitative results may provide some suggestions for relevant industrial designs.