Turbulent Combustion Processes Research Articles

A robust autoregressive long-term spatiotemporal forecasting framework for surrogate-based turbulent combustion modeling via deep learning

This paper systematically develops a high-fidelity turbulent combustion surrogate model using deep learning. We construct a surrogate model to simulate the turbulent combustion process in real time, based on a state-of-the-art spatiotemporal forecasting neural network. To address the issue of shifted distribution in autoregressive long-term prediction, two training techniques are proposed: unrolled training and injecting noise training. These techniques significantly improve the stability and robustness of the model. Two datasets of turbulent combustion in a combustor with cavity and a vitiated co-flow burner (Cabra burner) have been generated for model validation. The effects of model architecture, unrolled time, noise amplitude, and training dataset size on the long-term predictive performance are explored. The well-trained model can be applicable to new cases by extrapolation and give spatially and temporally consistent results in long-term predictions for turbulent reacting flows that are highly unsteady.

Large eddy simulation of effects of oxidizer inlet temperatures on the transition routes before and after thermoacoustic instability in a subcritical hydrogen peroxide/kerosene liquid rocket engine

This paper presents the first numerical evidence of the intermittency routes that exist before and after the occurrence of thermoacoustic instability in a subcritical single-element liquid rocket engine burning liquid kerosene and decomposed hydrogen peroxide with increasing oxidizer inlet temperatures (T). Three-dimensional compressible large eddy simulation algorithms, combined with Euler–Lagrangian frameworks, are employed to model the spray turbulent combustion process in a high-pressure rocket combustor where a one-equation eddy viscosity sub-grid turbulence model and a PaSR sub-grid combustion model are used based on OpenFOAM. After verifying the numerical framework and achieving grid independence, we focus on (i) dynamical transition routes before and after the thermoacoustic regime, (ii) frequency-locking phenomena between acoustic perturbations, vortex dynamics, and combustion heat release, and (iii) the underlying physical mechanisms associated with different dynamical states. The results show that as we increase the oxidizer inlet temperature (700 K ≤ T ≤ 900 K), the system dynamics undergo a transition from a state of combustion noise to a period-1 limit cycle via intermittency. Furthermore, by further increasing the oxidizer inlet temperature (950 K ≤ T ≤ 1450 K), a second bifurcation occurs, causing a transition from a limit cycle state back to a combustion noise state also via intermittency. These bifurcation phenomena are attributed to frequency-locking interactions among pressure fluctuations, combustion heat release, and vortex dynamics. Evidence from combustion flow subsystems including mixture fraction, burning modes, and flame-induced vorticity sources provides additional insights into the complex instability mechanism.

Numerical investigation of combustion instability in a liquid rocket engine: Interaction effect between hydrodynamics and acoustic mode

In this paper, we present the numerical evidence of multiple bifurcations in thermoacoustic instability as we decrease the equivalence ratio (ϕ) from fuel-rich (ϕ = 1.5) to fuel-lean (ϕ =0.59) within a single-element coaxial liquid rocket engine burning hydrogen peroxide and methane. An axisymmetric compressible Large Eddy Simulation solver in an open-source platform OpenFOAM combined with a global methane chemical kinetical mechanism was used to model the turbulent mixing and combustion process. After the validation of grid independence and simulation results, the synchronization phenomenon between hydrodynamic instability, acoustics and combustion is the main focus of this paper. The results here first revealed that in a direction of decreasing equivalence ratios, the first transition to limit cycle state from combustion noise state was observed via intermittency. A second bifurcation was also observed, leading to quasi-period thermoacoustic instability from the limit cycle state. Intrinsic vorticity modes and natural acoustic modes could be regarded as oscillators. The competition and cooperation between the two modes led to a phenomenon beyond limit cycle states. The vortex dynamics of thermoacoustic systems are dominated by the interaction of intrinsic vorticity modes and natural acoustic modes and exhibit an adjustment of vortex shedding frequencies, resulting in different stability regions. Herein, the flame index was performed in more detail to distinguish between the premixed flame and diffusion flame. Results indicated that the burning mode was an important driver in promoting instability whilst it also served as a medium for competition and cooperation between the natural acoustic modes and intrinsic vorticity modes.

Numerical investigation of combustion instability in a single-element liquid rocket engine: Intermittency routes before and after thermoacoustic instability

This paper provides the first numerical proof of intermittency routes existing before and after thermoacoustic instability, as we increase the oxidizer inlet temperature under fuel-lean (ϕ=0.91) and fuel-rich (ϕ=1.25) conditions in coaxial liquid rocket engine utilizing methane and hydrogen peroxide as propellant. Axisymmetric compressible Large Eddy Simulation algorithms combined with global chemical reaction mechanisms are used to simulate the non-premixed turbulent combustion process in high-pressure flows, where the one-equation eddy sub-grid turbulence model and the PaSR sub-grid combustion model are employed based on OpenFOAM. After verification of the grid independence as well as the global chemical mechanism against experimental measurements and existing simulations, the multi-bifurcation process of the system dynamics and the corresponding lock-in phenomenon are the main focus of this paper. The results suggest that with an increase in oxidizer inlet temperatures, the system dynamics at ϕ=0.91 conditions experience an initial shift from states of combustion noise to states of thermoacoustic instability through intermittent oscillation. A further rise in oxidizer inlet temperature leads to a second bifurcation from thermoacoustic instability states back to combustion noise states via intermittency, which is the first observation in the liquid rocket engine. Such an intermittency route before and after thermoacoustic instability is preserved at ϕ=1.25 conditions. Furthermore, a decrease in the equivalence ratio accelerates the initial bifurcation point for the onset of thermoacoustic instability. Flame index analysis is also employed to distinguish between diffusion and premixed flames, providing further insight into the underlying physical mechanism of the frequency-locking phenomenon between the pressure and combustion flow subsystems both in fuel-lean and fuel-rich conditions. Other details of the combustion flow fields also include the migration of the flame anchor point towards the injector with an increase in oxidizer inlet temperature.

Large eddy simulation of combustion instability in a subcritical hydrogen peroxide/kerosene liquid rocket engine: Intermittency route to period-2 thermoacoustic instability

This paper presents the first numerical evidence of an intermittency route to period-2 thermoacoustic instability in a subcritical single-element liquid rocket engine burning hydrogen peroxide/kerosene as we decrease the equivalence ratio (ϕ) from fuel-rich to fuel-lean. To achieve this, three-dimensional compressible large eddy simulation algorithms combined with the Euler–Lagrangian framework are used. A one-equation eddy sub-grid turbulence model with a partially stirred reactor sub-grid combustion model is employed to simulate the spray turbulent combustion process in a high-pressure liquid-fueled combustor based on open-source platform OpenFOAM. This paper focuses on examining the transition process of the dynamical states in the thermoacoustic system and the synchronization between multiple subsystems. The results indicate that, as the equivalence ratio reduces continuously (1.5 ≤ ϕ ≤ 0.5), the system dynamics shift from period-1 oscillations (ϕ = 1.5) to period-2 oscillations (ϕ = 0.5) via intermittency (1.3 ≤ ϕ ≤ 0.9). Under the equivalence ratio of 0.7 (ϕ = 0.7), a transient mode switching between period-1 and period-2 was also observed. The synchronization processes between the pressure and combustion subsystems in terms of phase-locking and frequency-locking are responsible for the emergence of complex dynamical states. The cycle snapshots analysis also provides more details on the synchronization processes between the pressure and the multiple subsystems, such as vortex dynamics, mixture fraction, and combustion heat release. In summary, this paper sheds light on the complex non-linear thermoacoustic oscillations and the underlying physical mechanisms related to the two-phase flow of spray combustion in liquid rocket engines using three-dimensional large eddy simulations, paving the way for developing passive or active control methods.

Quenching modes of local flame–wall interaction for turbulent premixed methane combustion in a constant volume vessel

The quenching mode of local flame–wall interaction (FWI) is investigated for its response to different levels of turbulence intensity as well as its effect on quenching distance, wall heat flux, and near-wall reaction. For that, direct numerical simulations of turbulent premixed methane combustion in a constant volume vessel are carried with initial Karlovitz numbers (Ka) of 1.0, 10.0, and 30.0. Local flame–wall quenching positions are identified based on the local fuel consumption speed during the turbulent combustion process, and the local FWI events have been classified into four quenching modes according to the flame–wall geometric relationships of quenching positions, namely head-on quenching (HOQ), oblique-wall quenching, side-wall quenching (SWQ), and back-on quenching (BOQ). The results show that in the case with higher initial Ka, the flame surface shows a more complicated wrinkled structure due to the flame–turbulence interaction. Meanwhile, the local quenching distance defined based on the identified quenching position is strongly influenced by the near-wall flow, and the range of the local quenching mode extends further to BOQ. However, for all three cases, HOQ and near-HOQ modes account for the majority of local FWI. Wall heat flux and heat release rate (HRR) of near-wall reaction yield high values for the FWI under HOQ or BOQ and are low for SWQ. In addition, there is a discrepancy in the near-wall transportation of some species under different quenching modes, which further leads to the difference in FWI-induced near-wall reaction regarding its total and elementary HRR.

Combustion Instabilities and its Control Techniques for Liquid Propellant Rocket Engine

It is a great challenge for us to understand and to predict the concept of liquid propellant rocket engines combustion instability due to high complex turbulent combustion process. Traditional methods have been adopted to eliminate the combustion instabilities which will increase system damping. Such methods usually suffer deficiencies with lack of study on fundamental mechanism. Difficulties have been faced during the process of various phenomena such include spray formation process, turbulent multiphase flow conditions, hydrodynamics of injection and transport characteristics of individual droplets. The ultimate aim of every engine design is to prevent occurrence of this combustion instability and to maintain reliable operation during entire span of the combustion process. This article is focused on occurrence of various type of combustion instabilities in liquid propellant rocket engine and its preventing methods in detail.

DESIGN AND MODELING OF AN 80 BAR OXY-COMBUSTOR FOR DIRECT FIRED SUPERCRITICAL CO2 APPLICATIONS

As supercritical CO<sub>2</sub> power cycles for fossil energy power generation continue to generate interest, there are significant issues and unanswered questions regarding injector design, flame stabilization, wall heat transfer, CO emissions, combustion dynamics, and other combustion phenomenon. For natural gas, direct-fired cycles with carbon capture it is believed that computational fluid dynamics (CFD) modeling will play an essential role in the combustor design process. To accurately model turbulent reacting flows at these unique conditions, experimental data are needed to validate CFD codes and sub-models and are currently lacking at conditions relevant for these cycles. This paper presents the conceptual design and CFD simulations of an experimental 80 bar oxy-combustion facility and test article currently under construction at National Energy Technology Laboratory (NETL). The facility is targeted towards the testing of a single-injector, direct-fired sCO<sub>2</sub> combustor at the 100 kW thermal output level. While these conditions do not reflect the actual Allam cycle operating conditions (300 bar) they are viewed as a stepping stone in the model validation process at supercritical conditions. Reynolds Averaged Navier-Stokes as well as Large-Eddy Simulation are used to model the turbulent combustion process and aid in the design of the combustor and injector. Process parameters including oxidizer preheat temperature and combustor flowrate are investigated.

Modelling a Turbulent Non-Premixed Combustion in a Full-Scale Rotary Cement Kiln Using reactingFoam

No alternatives are currently available to operate industrial furnaces, except for hydrocarbon fuels. Plant managers, therefore, face at least two challenges. First, environmental legislation demands emission reduction. Second, changes in the origin of the fuel might cause unforeseen changes in the heat release. This paper develops the hypothesis for the detailed control of the combustion process using computational fluid dynamic models. A full-scale mock-up of a rotary cement kiln is selected as a case study. The kiln is fired by the non-premixed combustion of Dutch natural gas. The gas is injected at Mach 0.6 via a multi-nozzle burner located at the outlet of an axially mounted fuel pipe. The preheated combustion air is fed in (co-flow) through a rectangular inlet situated above the attachment of the fuel pipe. The multi-jet nozzle burner enhances the entrainment of the air in the fuel jet. A diffusion flame is formed by thin reaction zones where the fuel and oxidizer meet. The heat formed is transported through the freeboard, mainly via radiation in a participating medium. This turbulent combustion process is modeled using unsteady Favre-averaged compressible Navier–Stokes equations. The standard k-ϵ equations and standard wall functions close the turbulent flow description. The eddy dissipation concept model is used to describe the combustion process. Here, only the presence of methane in the composition of the fuel is accounted for. Furthermore, the single-step reaction mechanism is chosen. The heat released radiates throughout the freeboard space. This process is described using a P1-radiation model with a constant thermal absorption coefficient. The flow, combustion, and radiative heat transfer are solved numerically using the OpenFoam simulation software. The equations for flow, combustion, and radiant heat transfer are discretized on a mesh locally refined near the burner outlet and solved numerically using the OpenFoam simulation software. The main results are as follows. The meticulously crafted mesh combined with the outlet condition that avoids pressure reflections cause the solver to converge in a stable manner. Predictions for velocity, pressure, temperature, and species distribution are now closer to manufacturing conditions. Computed temperate and species values are key to deducing the flame length and shape. The radiative heat flux to the wall peaks at the tip of the flame. This should allow us to measure the flame length indirectly from exterior wall temperature values. The amount of thermal nitric oxide formed in the flame is quantified. The main implication of this study is that the numerical model developed in this paper reveals valuable information on the combustion process in the kiln that otherwise would not be available. This information can be used to increase fuel efficiency, reduce spurious peak temperatures, and reduce pollutant emissions. The impact of the unsteady nature of the flow on the chemical species concentration and temperature distribution is illustrated in an accompanying video.

Are the available data from laboratory spray burners suitable for CFD modelling validations? A review

A systematic compilation and evaluation of turbulent spray flame combustion data acquired from lab-scale spray-burners and CFD models validated with these are presented. Previous review studies have been limited to presentations of the progress with validation targets and modelling methods. This work expands these by presenting the details of lab-scale spray-burners and the characteristics of their datasets, the computational methodologies developed to capture the turbulent spray combustion processes acquired from the burners and their validation with the dataset. Additionally, by critically evaluating the spray and inflow boundary conditions, the reacting and non-reacting features and the main flame characteristics and data, direct correlations between the effectiveness of the dataset from the lab-scale spray burners and the outcomes from the computational methodologies using them for validation are clearly elucidated. The evaluation indicates that the turbulence modelling and validation of the swirling non-reacting flows before the application of the combustion models are important. A better representation of the multi-mode nature and the flame-turbulence-chemistry interactions in the combustion models are required. Also, a clearer experimental presentation of the flame structure and modes would help with the development of the modelling techniques. To plug these research needs, this paper reinforces the importance of developing improved numerical techniques that can be validated by selecting an appropriate lab-scale spray-burner with a robust database. Together these can be used to improve the development of and reduce emissions from combustors burning future green fuels.

Numerical analysis of self-excited tangential combustion instability for an MMH/NTO rocket combustor

This study presented the numerical simulation of the tangential combustion instability in a hypergolic liquid bipropellant rocket thrust chamber, which applied fuel liquid film cooling method and unlike impinging injectors. The liquid spray was modeled using Lagrangian approach, while the gas was regarded as Euler phase. Stress-blended eddy simulation and finite rate/eddy–dissipation model were adopted to simulate the turbulent combustion process. Consistent with the experiment results, this work successfully simulated the transformation of tangential combustion instability from standing mode to spinning mode. The mean pressure, amplitude and frequency of limit cycle oscillation were in good agreement with the experiment. There was a detailed analysis about the flow field, Rayleigh index, and driving mechanism of the combustion instability. It was found that the oscillation began with hot spots of heat release rate due to the interaction between the spray of impinging injectors and cooling fuel jet. More than that, cooling fuel jet also contributed to drive the oscillation. In the standing mode, injectors in the inner and outer rings drive the oscillation together, while the spinning mode is mainly driven by injectors in the outer ring. The pressure wave is subsonic and its Mach number is close to 1. It was shown that the pressure wave contained a complex structure divided into three parts. This led to the in-phase of the pressure along the axial direction and the double-peak characteristic of the downstream pressure signal. Besides, a positive feedback closed-loop system associated with periodic oxidizer/fuel ratio was believed to sustain the combustion instability. The oscillation can be maintained when pressure, heat release and oxidizer/fuel ratio are coupled together. The analysis results indicate that rotating detonation is an implication to tangential combustion instability.

Reduced modeling of the NOx formation based on the reaction-diffusion manifolds method for counterflow diffusion flames

The accurate prediction of the NOx formation has gained great attention in view of clean combustion. In this direction, a reliable reduction technique for the chemical kinetics is important to capture the NOx formation accurately with reduced computational costs for the practical turbulent combustion processes. This work focuses on the hierarchical construction of Reaction-Diffusion Manifolds (REDIM) to include the NOx chemistry, which is well-known to be governed by very slow chemical reactions. Based on the hierarchical structure of the REDIM method, a two-dimensional (2D) and a three-dimensional (3D) REDIM model are generated, without any principle extension of the REDIM method. Sample calculations of NOx formation in methane/air non-premixed counterflow flames verify the REDIM method for both steady and transient processes.

Applying the Random Forest Method to Improve Burner Efficiency

Fuel power plants are one of the main sources of pollutant emissions, so special attention should be paid to improving the efficiency of the fuel combustion process. The mathematical modeling of processes in the combustion chamber makes it possible to reliably predict and find the best dynamic characteristics of the operation of a power plant, in order to quantify the emission of harmful substances, as well as the environmental and technical and economic efficiency of various regime control actions and measures, and the use of new types of composite fuels. The main purpose of this article is to illustrate how machine learning methods can play an important role in modeling and predicting the performance and control of the combustion process. The paper proposes a mathematical model of an unsteady turbulent combustion process, presents a model of a combustion chamber with a combined burner, and performs a numerical study using the STAR-CCM+ multidisciplinary platform. The influence of various input indicators on the efficiency of burner devices, which is evaluated by several parameters at the output, is investigated. In this case, three possible states of the burners are assumed: optimal, satisfactory and unsatisfactory.

Numerical Simulation of Mixing and Combustion Characteristics based on Air Turbine Rocket Engine Rocket Afterburner

The gas combustion efficiency of the afterburner of the Air Turbo Rocket (ATR) engine directly affects the performance of the engine. In order to study the reaction mechanism of mixing combustion of gas and air in the afterburner, the paper numerically simulates the turbulent mixing and combustion process of rich fuel gas and air, and analyzes the influence of air intake mode and intake parameters on engine performance. The results show that: The both sides air intake is better than the one sides air intake; The air incidence angle is 90°, the engine performance is ideal; The air intake position is selected between the length to diameter ratio of the afterburner from 1.0 to 1.5, and the combustion efficiency of the afterburner is higher.

The effect of fuel injection pressure and combustion chamber configuration on fuel spray, combustion and emissions of a direct injection diesel engine: a CFD approach

In the present study, the combined impacts of injection pressure and piston bowl configuration on fuel–air mixing, combustion and emissions of a diesel engine have been analysed numerically. An investigation has been conducted at four different fuel injection pressures and three different combustion chamber configurations for the same bowl volume to have a constant compression ratio. The in-cylinder transient flow and subsequent turbulent combustion process have been modelled using computational fluid dynamics code AVL FIRE. The predicted results indicate that the brake-specific fuel consumption (BSFC) is decreased by 4.19%, 8.96% and 11.92% for HCC, 5.77%, 9.92% and 13.68% for CCC, and 5.13%, 9.19% and 12.92% for TRCC when engine operates with an injection pressure of 215, 230 and 250 bar, respectively compared to 200 bar at full load operation. Soot emissions for CCC and TRCC engines are decreased by 23.4% and 33.43%, respectively, compared to baseline HCC engines.

Investigation of the partially premixed turbulent combustion through the PRECCINSTA burner by large eddy simulation

The turbulent combustion processes within full annular PRECCINSTA combustor were investigated numerically in the present study. Dynamic thickened flame model was implemented to simulate the partially premixed turbulent methane-air combustion processes. The governing equations considering wall heat loss were solved by using the large eddy simulation method. In order to validate the implemented model, the temperature and main species distribution profiles within the burner were compared to the experimental data and good agreement was observed. The vortex motion characteristics of the cold flow field, the characteristics of the flame surface area, heat release rate and velocity vector changes with time during the circumferential cross-fire process were studied. Results showed that considering the wall heat loss significantly improved the prediction accuracy of the temperature and intermediate species distributions causing flame structure change from “M-shaped” to “V-shaped”. Numerical analysis proved that the circumferential velocity in different direction induced by the swirling flow near the inner and outer walls of the annular combustor are coupled with the combustion heat expansion, causing difference gain effects to the flame propagation on two sides of the combustor.

Investigation of NOx emission under different burner structures with the optimized combustion model

As restrictions on NOx (nitrogen oxides) emission become increasingly stringent, many efforts have been put into the development of NOx control strategies. The burner structures of the heating furnace can affect NOx formation by the means of changing the flame temperature. To simulate the turbulent combustion process, the Steady Diffusion Flamelet (SDF) model is used and coupled with detailed mechanisms. It is found that some key flamelet parameters of the SDF model will greatly affect the accuracy of the simulation results. In this study, an efficient optimization procedure is proposed to optimize the combustion model parameters with the surrogate assisted evolutionary algorithm K-RVEA. The surrogate model is used to reduce the computational time of the optimization procedure. Using the optimized model parameters, the temperature field and the concentration fields obtained by the simulations are in good agreement with the measurements. Based on this, the investigation of the impact of the length of pilot wall and jet wall on NOx emission is carried out. The results show that the outlet NOx concentration reaches a local minimal value when the length of pilot wall and jet wall are equal. The length of monotonic increasing interval, where the outlet NOx concentration increases when increasing the length of the pilot wall, is 30 mm. If the length of pilot wall is relatively 30 mm longer than that of the jet wall, the outlet NOx concentration will be further reduced. The proposed optimization procedure and obtained results will benefit the improvement of the burner structure of the heating furnace.

Impacts of the horizontal swirl and axial tumble on the turbulent kinetic energy and combustion process of a natural gas engine

ABSTRACT Natural gas (NG) engines are restricted by slow combustion rates. An appropriate flow condition is considered to be the most promising way to improve the combustion rate of NG. In this study, decoupling analysis of the impacts of swirl and tumble on turbulent kinetic energy (TKE) and combustion process of a spark ignition (SI) NG engine was conducted by simulation. A 3D model was established and boundary conditions of the most economical operating point (1200 r/min, 100% load) were obtained through tests, and the rationality of the case was verified. Then, the flow field of the original engine was decoupled at the intake valve closing time to obtain the decoupled swirl field and the decoupled tumble field. Finally, the swirl field and the tumble field were amplified and applied to the SI NG engine. The results showed that high swirl or tumble can obtain high TKE, but the contribution mechanism of them to TKE is significantly different. According to the analysis of the spatial motion structure and energy conversion, the TKE in tumble field is much higher than that in swirl field with the same intensity. In addition, the impacts of swirl and tumble on the combustion process of the SI NG engine are both divided into two phases, and high swirl or tumble can obtain higher thermal efficiency. The combustion rate of NG first increased with the increase in initial flow field strength. When the swirl ratio at intake valve closing time (SR0) and the tumble ratio at intake valve closing time (TR0) exceeded 2.0 and 1.25, respectively, the combustion rate is kept in a fluctuating equilibrium state. At this time, the improvement of swirl and tumble on NG combustion process was brought into full play, the maximum indicated thermal efficiency reached 43.18% and 44.39%, respectively. Finally, the conclusion was drawn by comparing swirl and tumble in different aspects where high tumble shows greater potential in achieving super-high thermal efficiency.

Numerical investigation of a fueled pre-chamber spark-ignition natural gas engine

Pre-chamber spark-ignition (PCSI) is a leading advanced ignition concept for internal combustion engines with the potential to enable diesel-like efficiency in medium-duty/heavy-duty (MD/HD) natural gas (NG) engines. By leveraging distributed ignition sources from multiple turbulent jets, the PCSI technology can deliver extremely short combustion duration in ultra-lean mixtures and significantly improve the engine thermal efficiency. However, in the automotive industry there is a lack of adequate science base and predictive simulation tools required for commercial development of PCSI engines. In this study, Reynolds-Average Navier-Stokes simulations are carried out to describe the combustion process in lean-burn NG engines, focusing on the combustion modeling approach. Two combustion models, multi-zone well-stirred reactor (MZ-WSR) and G-equation, are used to simulate the combustion process in an MD NG engine equipped with a fueled-PCSI system for four operating conditions close to the lean operating limit. A skeletal chemical mechanism and a laminar flame speed tabulation are used to compute the combustion accurately. Simulation results are compared with experimental data regarding measured cylinder pressure, heat release rate, and combustion duration. By dividing the PCSI combustion process into four distinct phases, the difference between the two models’ results for each phase is analyzed in detail. The MZ-WSR model overestimates the combustion duration for early flame kernel growth in the pre-chamber due to the lack of a specific formulation to take turbulence-chemistry interaction into account. Despite the prolonged combustion duration and low pressure built-up inside the pre-chamber, the model matches the combustion rate in the main-chamber. In contrast, the G-equation model delivers good agreements for the pre-chamber combustion and turbulent jet-driven combustion processes. However, the model starts to underestimate the combustion rate in the main-chamber, especially under ultra-lean mixture conditions. Finally, improvements are needed for both models to simulate the later combustion stage that occurred in the near-wall regions.

Numerical Simulation of Turbulent Non-premixed Combustion Processes for Methane and Dimethyl Ether Binary Fuels.

The C1ε = 1.6 standard k – ε equation combined with the steady flamelet model was applied to a methane/dimethyl ether swirl combustion field, and the effects of the dimethyl ether (DME) blending ratio and operating pressure on the flame behavior, including species variation, reaction zone behavior, and flame entrainment, were investigated. The results demonstrated that selected models could better reproduce the trends of the experimental measurements. The downstream reaction zone achieved better calculation accuracy than the outer shear layer of the first recirculation zone. The addition of DME accelerated the accumulation process of H2, O, H, and OH radicals. The intermediate radical CH2O was rapidly developed by the influence of the H extraction rate under a constant fuel volume flow rate. The reaction zone dimensions were approximately linearly and positively correlated with the DME blending ratio, whereas flame entrainment expressed a lower DME concentration dependence in the high-DME mass-dominated system. The operating pressure significantly impacted the distribution of reactive radicals in the turbulent flame; meanwhile, the flame and reaction zone length showed nonlinear inverse behavior with pressure variation, while the thickness of the reaction zone was always linearly and negatively correlated with pressure. Moreover, the peak flame entrainment rate also experienced a nonlinear decline with pressure elevation; however, the peak positions were not sensitive to pressure fluctuation. Concurrently, the response surface functions for the reaction zone dimensions were established covering the range of 0–1 for the DME blending ratio and 1–5 atm operating pressure, which could provide assistance for combustion condition optimization and combustion chamber design.