Analysis of periodic synthetic turbulence generation and development for direct numerical simulations applications
The introduction of directional biases and mismatches in the second-order statistics associated with the enforcement of periodicity is analyzed and quantified. Two strategies for mitigation of these disparities are proposed. The suggested development from synthetic to realistic turbulence approaches are subsequently validated and compared with the original methodologies using direct numerical simulations. The proposed strategies are capable of neutralizing the disparities in the second-order statistics at the injection region. The resulting flow statistics and spectral analysis evaluate.
Example of instantaneous fluctuations for different longitudinal positions for simulation. The colors of the scatter plots are related to the transversal position of the data. Hence, similar color implies that points lay together.
Characterisation and Design of Direct Numerical Simulations of Turbulent Statistically Planar Flames
This work aims to provide support for the design of reliable DNSs for statistically planar flames. Improved simulation design strategies are developed. Therefore, design criteria for the simulative domain are discussed. The gained mathematical relations for all of the relevant physical quantities were channelled into a deterministic calculation strategy for mesh features.“
Three-dimensional depictions and two-dimensional slices (three each) of flame fronts from three DNS-simulated premixed combustion cases (A-C). The slices show the inner patterns. The colour range for filtering the flame fronts is from 1000 K (blue) to 3000 K (red).
Simulation of Combustion in a Methane-Oxygen Rocket Engine
Why we are investigating Methane? What experiments we are using for validation? How we build our simulations? and What you probably also want to know?.
Longitudinal section of a subscale combustion chamber
Analysis of turbulence generation and dissipation in shear layers of methane–oxygen diffusion flames using DNS
Turbulent methane–oxygen diffusion flame is studied using a direct numerical simulation setup. The operating regime and turbulence characteristics are chosen to resemble those of a modern methane rocket combustor. Local flame characteristics and dimensionless numbers are defined and evaluated, and their relationship with the turbulent kinetic energy transport budget is studied. Positive net turbulence generation is observed in the reaction shear layer. It is found that the underlying mechanisms for these results are similar to those encountered in premixed flames, with pressure terms acting as the primary turbulent kinetic energy sources.
Observed and predicted values for the turbulence production by mean pressure.
An eddy dissipation concept performance study for space propulsion applications
Turbulent methane–oxygen diffusion flame is studied using a direct numerical simulation setup. The operating regime and turbulence characteristics are chosen to resemble those of a modern methane rocket combustor. Local flame characteristics and dimensionless numbers are defined and evaluated, and their relationship with the turbulent kinetic energy transport budget is studied. Positive net turbulence generation is observed in the reaction shear layer. It is found that the underlying mechanisms for these results are similar to those encountered in premixed flames, with pressure terms acting as the primary turbulent kinetic energy sources.
Observed and predicted values for the turbulence production by mean pressure.
Turbulent combustion statistics in a diffusion flame for space propulsion applications
The statistical properties of the relevant physical fields are examined to study the interactions between turbulence and combustion. This analysis is complemented by an investigation and quantification of the error sources in direct numerical simulations of turbulent diffusion flame. A method to estimate the statistical error is derived based on the classical inference theory. In addition, critical resolution criteria are discussed using a mesh sensitivity analysis.
Example of instantaneous field results: Water mass fraction.
Mixture fraction statistics in methane-oxygen turbulent combustion for space propulsion Conditions
Two high-pressure turbulent non-premixed methane-oxygen flames are simulated using the unsteady reactive solver EBI-DNS. The results are used to investigate the statistics of the mixture fraction in the frame of turbulent combustion for space propulsion applications. The capability of the beta distribution to capture the probabilistic behavior of the mixing process is evaluated as well as the validity of extended closure models for the mixture fraction variance.
Schematic of the entire simulation procedure.
Subgrid Turbulent Flux Models for Large Eddy Simulations of Diffusion Flames in Space Propulsion
Subgrid scale models for unresolved turbulent fluxes are investigated, with a focus on combustion for space propulsion applications. An extension to the gradient model is proposed, introducing a dependency on the local burning regimen. The dynamic behaviors of the model’s coefficients are investigated, and scaling laws are studied. The discussed models are validated using a DNS database of a high-pressure, turbulent, fuel-rich methane–oxygen diffusion flame. The operating point and turbulence characteristics are selected to resemble those of modern combustors for space propulsion applications to support the future usage of the devised model in this context.
Schematic of the simulation with subgrid modeling of unresolved turbulent fluxes.
Impacts of hydrocarbon impurities on heat transfer deterioration
Hydrocarbon impurities in Liquified Natural Gas can lead to unwanted occurrences like heat transfer deterioration and pseudo-boiling phenomena. It can pose a challenge for cooling channels in rocket engines. This study examines the impacts of hydrocarbon impurities on heat transfer deterioration in cooling channels.
Schematic of the test section and assessment of derived Nusselt number.
I. Nasser, C. Manfletti, and O. Haidn. "Influences of Hydrocarbon Impurities on Heat Transfer Deterioration for Supercritical LNG Flowing in Cooling Channel." Space Propulsion Conference. 2024.
Interactions between flame topology and turbulent transport in high-pressure premixed combustion
Direct numerical simulations of a turbulent premixed stoichiometric methane-oxygen flame were conducted. The chosen combustion pressure was 20 bar, to resemble conditions encountered in modern rocket combustors. The chemical reactions followed a finite rate detailed mechanism integrated into the EBI-DNS solver within the OpenFOAM framework. Flame geometry was thoroughly investigated to assess its interaction with the transport of turbulent properties.
Flame front geometry classifications as a function of the principal curvatures 𝜅1 and 𝜅2.
The role of turbulence in the characteristic velocity and length of rocket combustors
High-fidelity direct numerical simulations are conducted to investigate combustion performance in high pressure methane-oxygen diffusion flames with different inlet turbulence. The results are post-processed to investigate the influence of turbulence on characteristic velocity and length. It is found that despite the eminent fuel-rich non-premixed configuration of the flame, a substantial amount of heat release takes place in premixed lean conditions.
Analysis of turbulent mixing in a methane–oxygen recessed injector for space propulsion“
Turbulent mixing in a methane–oxygen recessed injector is studied using direct numerical simulations. The operating point is chosen to be fuel-rich and at high pressure to recreate a representative environment for space propulsion applications. The results are used to investigate the transport of the turbulent mixture fraction statistics and the validity of conventional transport models. It is observed that molecular diffusion is only relevant near the boundary layer of the injection recess cavity and at the recirculation zone.
Flame front resolution: The probability density function of the flame front resolution within the recess segment and the combustion chamber for different temperature ranges is an example of instantaneous flame front resolution.
Combustion regimes in turbulent non-premixed flames for space propulsion
The main purpose of this research was to study the flame development in the frame of modern space propulsion systems. A novel definition of the stoichiometric oxidizer-to-fuel ratio was proposed to capture the effects of significant carbon monoxide formation. The proposed definition is better suited to reaction engines, since the amount of carbon dioxide in equilibrium is small compared to carbon dioxide, altering the main assumption in which the classical definition of stoichiometric conditions is grounded.
Details of normalized heat release right after the reactive shear layers merge.
Investigation of Sidewall and Reynolds Number Effects in a Ribbed Square Duct
The paper provides novel insights into the interplay between Reynolds numbers, roughened surfaces, and boundary conditions in turbulent flows, laying a foundation for a deeper understanding of the flow induct with high roughness.
Visualization of the vertical structures near a riblet. Iso surfaces of the scalar Q with increasing value from grey to red.
Effects of injection recess in methane turbulent combustion for space propulsion
Direct numerical simulations (DNS) are conducted to analyze the effect of recess in the mixing and combustion performance of gaseous methane oxygen injection systems. The recess length is varied from 0 to 2 injector diameters to analyze the sensitivity of various physical processes to this geometrical feature. It is found that the injection recess enhances the development of Kelvin–Helmholtz instabilities (KHI), due to the more consistent jet flow disposition at the injection plane, and the higher velocities.
Schematic of a recessed shear coaxial injection system lR = recess length, di = inner (oxidizer) injection diameter, and do = outer (fuel) injection diameter.
Numerical investigation of rocket engine cooling channel heat transfer for different LNG under trans-critical conditions
This study investigates the impacts of hydrocarbon impurities on LNG thermophysical and transport properties. It estimates the required relative roughness to improve the cooling capabilities with appropriate pressure drops for different LNG mixtures flowing in the rocket engine cooling channel. This study provides a systematic understanding of the relationships between cooling channel surface roughness, Nusselt Number, and LNG composition, offering designers the ability to optimize heat transfer in rocket engine cooling systems.
Dimensionless pressure drop and Nusselt number with a variant sand grain wall roughness for different LNG mixtures.
CFD-analysis of the effect of a cooling film on flow and heat transfer characteristics in a GCH4/GOX rocket combustion chamber
This work numerically reconstructs combustion behaviour in a film cooled rocket combustion chamber. The turbulence model and the eddy dissipation concept reaction model were used for the simulative approaches. Investigations of possible variations of the numerical setup and analyses of inner flow processes were conducted. Such variations in particular simulation settings as film species and mass flow were performed. The results were compared to each other and to experimental data. It was concluded that properties of the core flow are sensible to changes in thermal conditions.
Temperature distribution at Simytry combustion chamber.
A comprehensive investigation of heat transfer in a HARR of a rocket engine using LNG coolant
Small quantities of hydrocarbon impurities in liquefied natural gas (LNG) play a significant role in regenerative engine cooling in propulsion systems. Conjugate heat transfer in a high aspect ratio cooling channel is numerically studied at a supercritical pressure of 10 MPa, for pure methane and two different LNG with a 97.5% and 88.5% methane mole fraction.
contours at different cross sections of HARCC. (Left: pure LNG, right: rich LNG).
A Comprehensive Analysis of LNG as Coolant in Cooling Channel Under Supercritical Pressure Conditions
This study is focused on understanding the effect of impurities in LNG flow inside the cooling channels of rocket engines. LNG cooling systems' flow evolution and cooling capability are investigated.
Vapor-liquid equilibrium of methane butane mixture.“
Heat transfer and combustion simulation of a 7-element GOX/GCH 4 rocket combustor
Within the framework of the SFB-TRR 40 Summer Program 2017 the simulation of the flow inside an experimental rocket thrust chamber was undertaken in the Chair of Turbomachinery and Flight Propulsion (LTF) of the TUM. The rocket combustor is operated with gaseous oxygen (GOX) and gaseous methane (GCH4) and the tests were carried out at the experimental test bench of the LTF. The combustor’s injector consists of seven individual coaxial injector elements, while the chamber and nozzle segments are water cooled. The results presented in this work were obtained with 3D RANS simulations using an adiabatic Flamelet formulation for the chemistry modeling.
Temperature field in the thrust chamber using the standard k- epsilon model (the black line corresponds to the stoichiometric mixture fraction):