Computational optimization of a hydrogen direct-injection compression-ignition engine for jet mixing dominated nonpremixed combustion

Hydrogen (H2) nonpremixed combustion has been showcased as a potentially viable and preferable strategy for direct-injection compression-ignition (DICI) engines for its ability to deliver high heat release rates and low heat transfer losses, in addition to potentially zero CO2 emissions. However, this concept requires a different optimization strategy compared to conventional diesel engines, prioritizing a combustion mode dominated by free turbulent jet mixing. In the present work, this optimization strategy is realized and studied computationally using the CONVERGE CFD solver. It involves adopting wide piston bowl designs with shapes adapted to the H2 jets, altered injector umbrella angle, and an increased number of nozzle orifices with either smaller orifice diameter or reduced injection pressure to maintain constant injector flow rate capacity. This work shows that these modifications are effective at maximizing free-jet mixing, thus enabling more favorable heat release profiles, reducing wall heat transfer by 35%, and improving indicated efficiency by 2.2 percentage points. However, they also caused elevated incomplete combustion losses at low excess air ratios, which may be eliminated by implementing a moderate swirl, small post-injections, and further optimized jet momentum and piston design. Noise emissions with the optimized DICI H2 combustion are shown to be comparable to those from conventional diesel engines. Finally, it is demonstrated that modern engine concepts, such as the double compression-expansion engine, may achieve around 56% brake thermal efficiency with the DICI H2 combustion, which is 1.1 percentage point higher than with diesel fuel. Thus, this work contributes to the knowledge base required for future improvements in H2 engine efficiency.

A General Study of Counterflow Diffusion Flames for Supercritical CO2 Combustion

A counterflow diffusion flame for supercritical CO2 combustion is investigated at various CO2 dilution levels and pressures by accounting for real gas effects into both thermal and transport properties. The UCF 1.1 24-species mechanism is used to account the chemistry. The nature of important nonpremixed combustion characteristics such as Prandtl number, thermal diffusivity, Lewis number, stoichiometric scalar dissipation rate, flame thickness, and Damköhler number are investigated with respect to CO2 dilution and pressure. The results show that the aforementioned parameters are influenced by both dilution and pressure; the dilution effect is more dominant. Further, the result shows that Prandtl number increases with CO2 dilution and at 90% CO2 dilution, the difference between the Prandtl number of the inlet jets and the flame is minimal. Also, the common assumption of unity Lewis number in the theory and modeling of nonpremixed combustion does not hold reasonable for sCO2 applications due to large difference of Lewis number across the flame and the Lewis number on the flame drop significantly with an increase in the CO2 dilution. An interesting relation between Lewis number and CO2 dilution is observed. The Lewis number of species drops by 15% when increasing the CO2 dilution by 30%. Increasing the CO2 dilution increases both the flow and chemical timescales; however, chemical timescale increases faster than the flow timescales. The magnitudes of the Damköhler number signify the need to consider finite rate chemistry for sCO2 applications. Further, the Damköhler numbers at 90% sCO2 dilution are very small; hence, laminar flamelet assumptions in turbulent combustion simulations are not physically correct for this application. Also, it is observed that the Damköhler number drops nonlinearly with increasing CO2 dilution in the oxidizer stream. This is a very important observation for the operation of sCO2 combustors. Further, the flame thickness is found to increase with CO2 dilution and reduce with pressure.

Large Eddy Simulation of Multiple-Stage Ignition Process of n-Heptane Spray Flame

Large eddy simulation of n-heptane spray flames is conducted to investigate the multiple-stage ignition process under extreme (low-temperature, low oxygen, and high-temperature, high-density) conditions. At low oxygen concentrations, the first-stage ignition initiates in the fuel-rich region and then moves to stoichiometric equivalence ratio regions by decreasing the initial temperature. It is also clear that at high temperatures, high oxygen concentrations, or high densities, the reactivity of the mixture is enhanced, where high values of progress variable are observed. Analysis of key intermediate species, including acetylene (C2H2), formaldehyde (CH2O), and hydroxyl (OH) in the mixture fraction and temperature space provides valuable insights into the complex combustion process of the n-heptane spray flames under different initial conditions. The results also suggest that C2H2 appears over a wider range in the mixture fraction space at higher temperature or oxygen concentration condition, implying that it mainly forms at the fuel-rich regions. The initial oxygen concentration of the ambient gas has great influence on the formation and oxidization of C2H2, and the maximum temperature depends on the initial oxygen concentration. OH is mainly formed at the stoichiometric equivalence ratio region, which moves to high-temperature regions very quickly especially at higher oxygen concentrations. Finally, analysis of the premixed and nonpremixed combustion regimes in n-heptane spray flames is also conducted, and both premixed and nonpremixed combustion coexist in spray flames.

Large-eddy simulation of transcritical liquid oxygen/methane jet flames

A numerical method to perform large-eddy simulations (LES) of nonpremixed liquid oxygen/methane (LOx/CH4) combustion at supercritical pressures is presented and the computational results are compared with available experimental data. The injection conditions of the considered test case resemble those in typical liquid-propellant rocket engines (LRE). Thermodynamic nonidealities are modeled using the Peng–Robinson (PR) equation of state (EoS) in conjunction with a novel volume-translation method to correct deficiencies in the transcritical regime. The resulting formulation is more accurate than the standard cubic EoS's without deteriorating their good computational efficiency. The real-gas thermodynamics model is coupled with the steady laminar flamelet model (SLFM) for turbulent nonpremixed combustion to incorporate chemical reactions at reasonable computational cost in the LES. A reduced reaction mechanism, which is validated with respect to the full mechanism, is used to generate a flamelet library. A comparison of the LES result with available OH* measurements shows that important flow features are well predicted.