[Paper Review] Modeling and Simulation of Diesel Injection at Transcritical Conditions

Peter Ma, Matthias Ihme+1 more|arXiv (Cornell University)|May 20, 2017

Combustion and flame dynamics19 references3 citations

TL;DR

This study employs a compressible real-fluid solver with a diffused interface method to simulate diesel injection under transcritical conditions, using the Peng-Robinson equation of state and an entropy-stable finite-volume scheme. It achieves excellent agreement with experimental data for vapor penetration length and mixture fraction statistics, demonstrating robust prediction of turbulent mixing despite challenges from large density gradients and spurious pressure oscillations.

ABSTRACT

The need for improved engine efficiencies has motivated the development of high-pressure combustion systems, in which operating conditions achieve and exceed critical conditions. Associated with these conditions are large thermodynamic gradients and strong variations in transport properties as the fluid undergoes mixing and phase transition. Accurately simulating these real-fluid environments remains a main challenge. Different modeling approaches have been employed, which can be categorized as diffused and sharp interface methods. The objective of this study is to examine the diffused interface method for simulating diesel-fuel injection at conditions related to the supercritical regime. To this end, a recently developed compressible real-fluid solver for transcritical conditions is employed. Simulations of an ECN-relevant diesel-fuel injector are performed and predictions for instantaneous and statistical flow-field results are compared against available measurements. It is expected that results from this analysis will be useful in identifying limitations of current modeling techniques and in improving physical and numerical models for high-pressure injection systems.

Motivation & Objective

• To investigate the performance of a diffused interface method in simulating diesel fuel injection under transcritical conditions.
• To address challenges in modeling transcritical flows, including large thermodynamic gradients, strong property variations, and spurious pressure oscillations.
• To validate the numerical model against experimental measurements from the ECN Spray A benchmark case.
• To assess the accuracy of liquid penetration length predictions under varying threshold criteria.
• To evaluate the model's ability to capture turbulent mixing and mixture fraction statistics downstream of the injector.

Proposed method

• A compressible, multi-species, conservative finite-volume solver is used to solve the governing equations for mass, momentum, energy, and species transport.
• The Peng-Robinson equation of state is applied to model real-fluid behavior across the supercritical and transcritical regimes.
• An entropy-stable numerical scheme with a double-flux model is employed to suppress spurious pressure oscillations arising from nonlinearities in the equation of state.
• Large-eddy simulation (LES) is performed to resolve large-scale turbulent structures while modeling subgrid-scale stresses.
• The diffused interface method avoids explicit interface tracking, making it suitable for flows without distinct liquid-gas boundaries under transcritical conditions.
• Numerical dissipation is locally applied to stabilize regions with large density gradients, particularly near the injector nozzle.

Experimental results

Research questions

• RQ1 How accurately can a diffused interface method predict the flow structure and mixing behavior in diesel injection at transcritical conditions?
• RQ2 What is the sensitivity of liquid penetration length predictions to the choice of fuel mass fraction threshold?
• RQ3 How well does the numerical model reproduce experimental mixture fraction statistics in the turbulent mixing region?
• RQ4 To what extent do spurious pressure oscillations affect simulation stability, and how are they mitigated?
• RQ5 What are the limitations of the current model in capturing near-nozzle flow dynamics compared to experimental observations?

Key findings

• The simulation shows excellent agreement with experimental data for vapor penetration length, with predictions falling within experimental uncertainty bounds.
• The mean and root-mean-square (rms) values of mixture fraction along the centerline match well with Rayleigh scattering measurements, indicating accurate prediction of turbulent mixing.
• The simulation predicts a narrower jet spreading angle near the nozzle compared to experimental images, suggesting potential discrepancies in near-nozzle flow dynamics.
• Liquid penetration length is highly sensitive to the chosen threshold for fuel mass fraction, varying from 5 mm to 15 mm across thresholds from 0.95 to 0.4.
• The model successfully suppresses spurious pressure oscillations through the use of a double-flux model and entropy-stable scheme, enabling stable simulations under strong nonlinearities.
• The results confirm the capability of the current numerical framework to accurately model mixing and dispersion after the dense liquid jet fully disintegrates, despite challenges in near-nozzle resolution.