Non-premixed Combustion Process Research Articles

Experimental Investigation of Non-Premixed Combustion Process in a Swirl Burner with LPG and Hydrogen Mixture

In the modern world, issues related to the use of alternative fuels are becoming increasingly pressing. These fuels offer the potential to achieve significantly improved environmental and technological performance. Currently, among such fuels, biodiesel, ammonia, LPG, and hydrogen are considered the most promising options. LPG and hydrogen exhibit a high Lower Heating Value (LHV) and have a relatively low environmental impact. This article investigates the combustion of hydrogen-LPG mixtures in a diffusion burner. The main parameters under study include the proportion of hydrogen in the fuel, equivalence ratio, and vane angle. The analyzed parameters encompass NOx and CO concentrations. The studies have demonstrated that the addition of hydrogen can reduce greenhouse gas emissions, as the combustion product is clean water. The primary focus of this research is the examination of combustion processes involving flow swirl systems and alternative fuels and their mixtures. The studies indicate that flame stabilization is significantly influenced by several factors. The first factor is the amount of hydrogen added to the fuel mixture. The second factor is the degree of mixing between the fuel and oxidizer, along with hydrogen. Lastly, the equivalence ratio plays a crucial role. As the studies have shown, the maximum stabilization for a speed of 5 m/s is achieved at an angle of 60° and a hydrogen fraction of 40%, resulting in φLBO = 0.9. This represents an 8.0% improvement in stabilization compared to the baseline mode, primarily due to the substantial proportion of hydrogen. An analysis of flame photographs reveals that as the twist angle increases, a recirculation zone becomes more apparent. Increasing the blade angle and incorporating hydrogen leads to a reduction in CO concentrations in the exhaust gases. The analysis indicates that increasing the hydrogen proportion to 50%, compared to the absence of hydrogen, results in a 30% decrease in CO concentration. In our case, for the option φ = 0.3 and blade angles of 60°, the reduction in CO concentration was 28.5%. From the authors’ perspective, the most optimal vane angle is 45°, along with a hydrogen fraction of 30–40%. With these parameters, it was possible to achieve concentrations of NOx = 17–25 ppm, φLBO = 0.66, and CO = 130–122 ppm.

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.

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.

Hybrid regularized Lattice-Boltzmann modelling of premixed and non-premixed combustion processes

A Lattice-Boltzmann model for low-Mach reactive flows is presented, built upon our recently published model (Comb & Flame, 196, 2018). The approach is hybrid and couples a Lattice-Boltzmann solver for the resolution of mass and momentum conservation and a finite difference solver for the energy and species conservation. Having lifted the constant thermodynamic and transport properties assumptions, the model presented now fully accounts for the classical reactive flow thermodynamic closure: each component is assigned NASA coefficients for calculating its thermodynamic properties. A temperature-dependent viscosity is considered, from which are deduced thermo-diffusive properties via specification of Prandtl and component-specific Schmidt numbers. Another major improvement from our previous contribution is the derivation of an advanced collision kernel compatible of multi-component reactive flows stable in high shear flows. Validation is carried out first on premixed configurations, through simulation of the planar freely propagating flame, the growth of the associated Darrieus-Landau instability and three regimes of flame-vortex interaction. A double shear layer test case including a flow-stabilized diffusion flame is then presented and results are compared with DNS simulations, showing excellent agreement.

Numerical and Experimental Evaluation of a Dual-Fuel Dry-Low-NOx Micromix Combustor for Industrial Gas Turbine Applications

Abstract The dry-low-NOx (DLN) micromix combustion technology has been developed originally as a low emission alternative for industrial gas turbine combustors fueled with hydrogen. Currently, the ongoing research process targets flexible fuel operation with hydrogen and syngas fuel. The nonpremixed combustion process features jet-in-crossflow-mixing of fuel and oxidizer and combustion through multiple miniaturized flames. The miniaturization of the flames leads to a significant reduction of NOx emissions due to the very short residence time of reactants in the flame. The paper presents the results of a numerical and experimental combustor test campaign. It is conducted as part of an integration study for a dual-fuel (H2 and H2/CO 90/10 vol %) micromix (MMX) combustion chamber prototype for application under full scale, pressurized gas turbine conditions in the auxiliary power unit Honeywell Garrett GTCP 36-300. In the presented experimental studies, the integration-optimized dual-fuel MMX combustor geometry is tested at atmospheric pressure over a range of gas turbine operating conditions with hydrogen and syngas fuel. The experimental investigations are supported by numerical combustion and flow simulations. For validation, the results of experimental exhaust gas analyses are applied. Despite the significantly differing fuel characteristics between pure hydrogen and hydrogen-rich syngas, the evaluated dual-fuel MMX prototype shows a significant low NOx performance and high combustion efficiency. The combustor features an increased energy density that benefits manufacturing complexity and costs.

Damage localization effects of the regeneratively-cooled thrust chamber wall in LOX/methane rocket engines

To investigate the damage localization effects of the thrust chamber wall caused by combustions in LOX/methane rocket engines, a fluid-structural coupling computational methodology with a multi-channel model is developed to obtain 3-demensioanl thermal and structural responses. Heat and mechanical loads are calculated by a validated finite volume fluid-thermal coupling numerical method considering non-premixed combustion processes of propellants. The methodology is subsequently performed on an LOX/methane thrust chamber under cyclic operation. Results show that the heat loads of the thrust chamber wall are apparently non-uniform in the circumferential direction. There are noticeable disparities between different cooling channels in terms of temperature and strain distributions at the end of the hot run phase, which in turn leads to different temperature ranges, strain ranges, and residual strains during one cycle. With the work cycle proceeding, the circumferential localization effect of the residual strain would be significantly enhanced. A post-processing damage analysis reveals that the low-cycle fatigue damage accumulated in each cycle is almost unchanged, while the quasi static damage accumulated in a considered cycle declines until stabilized after several cycles. The maximum discrepancy of the predicted lives between different cooling channels is about 30%.

Development of fire shutters based on numerical optimizations

This article deals with a prototype concept, real experiment and numerical simulation of a layered industrial fire shutter, based on some new insulating composite materials. The real fire shutter has been developed and optimized in laboratory and subsequently tested in the certified test room. A simulation of whole concept has been carried out as the non-premixed combustion process in the commercial final volume sw Pyrosim. Model of the combustion based on a stoichiometric defined mixture of gas and the tested layered samples showed good conformity with experimental results – i.e. thermal distribution inside and heat release rate that has gone through the sample.

Effect of improved accelerating method on efficient chemistry calculations in diesel engine

With detailed chemical kinetics being employed in combustion simulations, its major computational challenge is the time-intensive nature of chemical kinetics integration due to the large number of chemical species and wide range of chemical timescales involved. In this work, an extended tabulated dynamic chemistry approach with dynamic pruning method is carried out to simulate complex spray combustion for non-premixed combustion process. The thought of extended tabulated dynamic chemistry approach with dynamic pruning is achieved by selecting the optimum acceleration method as well as its error tolerances at different combustion stages depending on combustion characteristics involving the low-temperature combustion. The present method is applied to realistically complex combustion systems involving spray flame of n-heptane fuel and non-premixed combustion engine. Computation efficiency of the proposed method is compared with the results using different accelerating methods, including dynamical adaptive chemistry, in situ adaptive tabulation, and coupled method of tabulated dynamical adaptive chemistry. The results show that transient computational cost will decrease for low-temperature combustion by reducing ambient oxygen concentration clearly in spray flame. Meanwhile, very low computational efficiency is presented once the autoignition occurs, especially at the initial oxygen concentration of 21%. Based on the feature, extended tabulated dynamic chemistry approach with dynamic pruning with different dynamic adaptive chemistry error tolerances is proposed to improve computational efficiency. Extended tabulated dynamic chemistry approach with dynamic pruning with larger error tolerance [Formula: see text] improves around two times for decreased amplitude of transient computational cost at high-temperature combustion stage, and at the same time, the computational accuracy is also improved by comparing the important intermediate species obtained by direct integration. For applications in diesel engine, the results show that extended tabulated dynamic chemistry approach with dynamic pruning can accurately capture the first-stage ignition feature that determines the high-temperature combustion stage. In addition, extended tabulated dynamic chemistry approach with dynamic pruning with the smaller in situ adaptive tabulation error tolerance of 0.001 only used at the high-temperature combustion stage significantly improves the performance on diesel engine simulation with a larger chemistry mechanism. The present method further significantly improves computational efficiency with an overall speedup factor of 10 with high-accuracy compared with result using direct integration.

Testing fireproof materials in a combustion chamber

This article deals with a prototype concept, real experiment and numerical simulation of a combustion chamber, designed for testing fire resistance some new insulating composite materials. This concept of a device used for testing various materials, providing possibility of monitoring temperatures during controlled gas combustion. As a fuel for the combustion process propane butane mixture has been used and also several kinds of burners with various conditions of inlet air (forced, free) and fuel flows were tested. The tested samples were layered sandwich materials based on various materials or foams, used as fillers in fire shutters. The temperature distribution was measured by using thermocouples. A simulation of whole concept of experimental chamber has been carried out as the non-premixed combustion process in the commercial final volume sw Pyrosim. The result was to design chamber with a construction suitable, according to the international standards, achieve the required values (temperature in time). Model of the combustion based on a stoichiometric defined mixture of gas and the tested layered samples showed good conformity with experimental results – i.e. thermal distribution inside and heat release rate that has gone through the sample.

An investigation of in situ adaptive tabulation for premixed and nonpremixed combustion engine simulations with primary reference fuel mechanism

The object of this work is to enhance the ISAT performance for transient combustion simulations for both premixed and non-premixed combustion process. In this work, dynamic pruning (DP) strategy for not-frequently-accessed tabulated points has been employed. For DP approach, the action considered when the table is full is to delete the most recently used leaf (at the tail of most recently used leaf) and replace it by the new entry in ISAT. The performances of ISAT with DP are presented firstly both in a homogenous charged compression ignition (HCCI) engine and a diesel engine. In present study, we investigate the sensitivity of ignition and composition inhomogeneity to heat loss/chemistry model for HCCI combustion. Composition inhomogeneity is governed by heat loss, turbulence diffusion and chemical reaction. Meanwhile, the effect of composition and temperature inhomogeneity on chemistry acceleration with ISAT-DP is studied in detail. It is shown that, ISAT-DP method can effectively reduce computational cost and achieve a large speedup factor of around 2 compared with original ISAT method. Memory size in ISAT has a tight influence on computational efficiency. And an overall speed up factor of around 4 for diesel engine is also achieved.

Comparative study of thermal radiation properties models in turbulent non-premixed sooting combustion

ABSTRACTThis work presents a numerical study of radiation heat transfer and its interaction with gray and spectral radiation of combustion products and soot that are formed in a turbulent non-premixed combustion process. To this purpose, weighted sum of gray gases (WSGGM), Rayleigh, narrow-band model, and semi-empirical radiation properties models are considered. Two situations are studied −1-D radiation problem and non-premixed gaseous combustion process. The results demonstrate that the models exhibit discrepancies that are smaller than the available line-by-line or experimental data. A study of the factors that influence such a disagreement is presented, underscoring the role of the chemical kinetics of hydrocarbon species and soot oxidation.

Fitting narrow-band models to temperature-dependent, spectral absorption coefficients of fuel vapors

Accurate modeling of infrared radiation transport through fuel rich cores of fires and other non-premixed combustion processes requires computationally efficient processing of temperature-dependent, spectral absorption coefficients for major fuel vapor species. Spectrally resolved transmissivity band measurements in the mid-infrared and near-infrared have been taken in recent years for numerous small fuel molecules including but not limited to ethane, ethylene, and propylene for a range of temperatures relevant for combustion environments. This paper compares the spectral transmissivity measurements using FTIR for both ethane and ethylene with the HITRAN 2012 edition. Narrow band absorption coefficients and overlap parameters are derived by fitting the spectral transmissivity measurements for ethane, ethylene, and propylene with narrow band models for temperatures up to 1000K. The resulting fits provide a basis for calculating spectrally resolved infrared radiation transport in fuel rich cores of flames and other combustion processes where these species can be prevalent.

A numerical and experimental study of Polycyclic Aromatic Hydrocarbons in a laminar diffusion flame

During the process of biomass gasification tars are formed which exit the gasifier in vapor phase. Tar condensation creates problems like fouling and plugging of after-treatment, conversion and end-use equipment. Gasification tars consist mainly of Polycyclic Aromatic Hydrocarbons (PAHs). Former research has shown the possibilities and difficulties of tar conversion by partial combustion. Basic studies to investigate the oxidation of tars in non-premixed combustion processes are expected to give more insight in this problem. In this paper the ability of the flamelet-generated manifold (FGM) approach to numerically model multi-dimensional, laminar, non-premixed flames with the inclusion of PAH chemistry is investigated. Modeling detailed PAH chemistry requires the employment of large reaction mechanisms which lead to expensive numerical calculations. The application of a reduction technique like FGM leads to a considerable decrease (up to two orders) in the required computation time. A 1D numerical validation shows that the improvements achieved by implementing a varying Lewis number for the progress variable Y are significant for PAH species with a large Lewis number, such as C10H8. Considerable improvements are found near the flame front and on the fuel side of the flame. A comparison has been made of FGM results with qualitative Planar Laser Induced Fluorescence (PLIF) measurements. A laminar CH4/N2–air co-flow flame has been doped with two dopants, benzene and toluene, at three different concentrations. A set of filters was used in order to qualitatively distinguish the small (1–2 rings) and large (3 or more rings) aromatic species. The results show that the model is able to capture the major flame characteristics typical for PAH formation in multi-dimensional laminar non-premixed flames.

Numerical modeling of fires on gas pipelines

When natural gas is released through a hole on a high-pressure pipeline, it disperses in the atmosphere as a jet. A jet fire will occur when the leaked gas meets an ignition source. To estimate the dangerous area, the shape and size of the fire must be known. The evolution of the jet fire in air is predicted by using a finite-volume procedure to solve the flow equations. The model is three-dimensional, elliptic and calculated by using a compressibility corrected version of the k - ξ turbulence model, and also includes a probability density function/laminar flamelet model of turbulent non-premixed combustion process. Radiation heat transfer is described using an adaptive version of the discrete transfer method. The model is compared with the experiments about a horizontal jet fire in a wind tunnel in the literature with success. The influence of wind and jet velocity on the fire shape has been investigated. And a correlation based on numerical results for predicting the stoichiometric flame length is proposed.

Characteristics of non-premixed oxygen-enhanced combustion: I. The presence of appreciable oxygen at the location of maximum temperature

The presence of appreciable molecular oxygen at the location of maximum temperature has been observed in non-premixed oxygen-enhanced combustion (OEC) processes, specifically in flames having a high stoichiometric mixture fraction ( Z st ) produced with diluted fuel and oxygen-enrichment. For conventional fuel–air flames, key features of the flame are consistent with the flame sheet approximation (FSA). In particular, the depletion of O 2 at the location of maximum temperature predicted by the FSA correlates well with the near-zero O 2 concentration measured at this location for conventional fuel–air flames. In contradistinction, computational analysis with detailed kinetics demonstrates that for OEC flames at high Z st : (1) there is an appreciable concentration of O 2 at the location of maximum temperature and (2) the maximum temperature is not coincident with the location of global stoichiometry, O 2 depletion, or maximum heat release. We investigate these phenomena computationally in three non-premixed ethylene flames at low, moderate, and high Z st , but with equivalent adiabatic flame temperatures. Results demonstrate that the location of O 2 depletion occurs in the vicinity of global stoichiometry for flames of any Z st and that the presence of appreciable O 2 at the location of maximum temperature for high Z st flames is caused by a shift in the location of maximum temperature relative to the location of O 2 depletion. This shifting is attributed to: (1) finite-rate multi-step chemistry resulting in exothermic heat release that is displaced from the location of O 2 depletion and (2) the relative location of the heat release region with respect to the fuel and oxidizer boundaries in mixture fraction space. A method of superposition involving a variation of the flame sheet approximation with two heat sources is shown to be sufficient in explaining this phenomenon.

Prediction of extinction and reignition in nonpremixed turbulent flames using a flamelet/progress variable model: 1. A priori study and presumed PDF closure

Previously conducted studies of the flamelet/progress variable model for the prediction of nonpremixed turbulent combustion processes identified two areas for model improvements: the modeling of the presumed probability density function (PDF) for the reaction progress parameter and the consideration of unsteady effects [Ihme et al., Proc. Combust. Inst. 30 (2005) 793]. These effects are of particular importance during local flame extinction and subsequent reignition. Here, the models for the presumed PDFs for conserved and reactive scalars are re-examined and a statistically most likely distribution (SMLD) is employed and tested in a priori studies using direct numerical simulation (DNS) data and experimental results from the Sandia flame series. In the first part of the paper, the SMLD model is employed for a reactive scalar distribution. Modeling aspects of the a priori PDF, accounting for the bias in composition space, are discussed. The convergence of the SMLD with increasing number of enforced moments is demonstrated. It is concluded that information about more than two moments is beneficial to accurately represent the reactive scalar distribution in turbulent flames with strong extinction and reignition. In addition to the reactive scalar analysis, the potential of the SMLD for the representation of conserved scalar distributions is also analyzed. In the a priori study using DNS data it is found that the conventionally employed beta distribution provides a better representation for the scalar distribution. This is attributed to the fact that the beta-PDF implicitly enforces higher moment information that is in excellent agreement with the DNS data. However, the SMLD outperforms the beta distribution in free shear flow applications, which are typically characterized by strongly skewed scalar distributions, in the case where higher moment information can be enforced.

Matching conditional moments in PDF modelling of nonpremixed combustion

The rate of generation of fluctuations with respect to the scalar values conditioned on the mixture fraction, which significantly affects turbulent nonpremixed combustion processes, is examined. Simulation of the rate in a major mixing model is investigated and the derived equations can assist in selecting the model parameters so that the level of conditional fluctuations is better reproduced by the models. A more general formulation of the multiple mapping conditioning (MMC) model that distinguishes the reference and conditioning variables is suggested. This formulation can be viewed as a methodology of enforcing certain desired conditional properties onto conventional mixing models. Examples of constructing consistent MMC models with dissipation and velocity conditioning as well as of combining MMC with large eddy simulations (LES) are also provided.

Sensitivity Analysis of a FGR Industrial Furnace for NOx Emission Using Frequency Domain Method

Preliminary study has shown that the flue gas recirculation (FGR) is one of the effective ways to reduce the nitric oxides (NOx) emission in industrial furnaces. The sensitivity of the NOx emission from a FGR industrial furnace to the change in three major furnace input variables—inlet combustion air mass flow rate, inlet combustion air temperature, and pressure head of the FGR fan—is investigated numerically in this study. The investigation is carried out in frequency domain by superimposing sinusoidal signals of different frequencies on to the furnace control inputs around the design operating condition, and observing the frequency responses. The results obtained in this study can be used in the design of active combustion control systems to reduce NOx emission. The numerical simulation of the turbulent non-premixed combustion process in the furnace is conducted using a moment closure method with the assumed β probability density function for the mixture fraction. The combustion model is derived based on the assumption of instantaneous full chemical equilibrium. The discrete transfer radiation model is chosen as the radiation heat transfer model, and the weighted-sum-of-gray-gases model is used to calculate the absorption coefficient.

Reduction of NOx in a Regenerative Industrial Furnace With the Addition of Methanol in the Fuel

The analysis of the combustion process and NOx emission in a gas-fired regenerative industrial furnace has been carried out numerically. The effect of the additive, methanol CH3OH, to the fuel on the NOx emission is studied. A moment closure method with the assumed β Probability Density Function (PDF) for the mixture fraction is used to model the turbulent non-premixed combustion process in the furnace. The combustion model is based on the assumption of instantaneous full chemical equilibrium. The P-1 model is chosen as the radiation model, and the Weighted-Sum-of-Gray-Gases Model is used to calculate the absorption coefficient. The numerical results showed that the use of CH3OH is effective in the reduction of NOx in a regenerative industrial furnace. The mechanism of NOx reduction by the use of CH3OH is also discussed.

Predictions of free jet fires from high pressure, sonic releases

A numerical model for predicting jet fires resulting from high pressure, sonic releases of natural gas is described. The model is based on solutions of the density-weighted forms of the fluid flow equations. It is capable of accurately resolving the near-field shock structure that occurs in these flows through the use of a compressibility corrected version of the k-ϵ turbulence model, and also includes sub-models for the flame lift-off height and a prescribed probability density function/laminar flamelet model of the turbulent non-premixed combustion process. Radiation heat transfer is described using an adaptive version of the discrete transfer method, with solutions of the radiation heat transfer equation obtained using a statistical narrow band approach. The complete model is demonstrated to yield plausible predictions of the structure of both the near-field non-reacting and subsonic combusting zones within wind blown fires, and to provide realistic predictions of flame lift-off heights, mean temperatures, trajectories and the radiation fluxes received about a number of field-scale jet fires.