Eddy Dissipation Concept Model Research Articles

CO formation characteristics in a centrally staged swirl combustor

Based on a validated numerical simulation methodology, this study focused on a centrally staged swirl combustor to investigate the combustion flow field inside the chamber under typical operating conditions. The CO formation characteristics of each region in the combustor were analyzed. In terms of elementary reactions, this study analyzed crucial reaction pathways contributing to CO formation in distinct regions utilizing the eddy dissipation concept model. Results reveal the occurrence of CO formation in the flame front and its rapid oxidation to form CO2 in the post-flame zone. CO2 partially dissociates into CO at high temperatures. Furthermore, air film quenching substantially impacts CO formation and emission, and air film greatly affects CO oxidation more than its formation. Local cooling and dilution effects from the air film contribute to the freezing of CO oxidation reactions. Therefore, a substantial quantity of CO remains unreacted, leading to increased CO emissions at the combustion chamber outlet.

Adaptive detached eddy simulation of turbulent combustion with the subgrid dissipation concept

Detached eddy simulation has become a widely used method in eddy simulations due to its balance between cost and accuracy. The recently developed subgrid dissipation concept (SDC) combustion model [Liu et al., “On the subgrid dissipation concept for large eddy simulation of turbulent combustion,” Combust. Flame 258, 113099 (2023)] is found to be more reasonable and accurate than the conventional eddy dissipation concept model in large eddy simulation (LES). In this paper, the SDC model is adapted to the ℓ2-ω adaptive detached eddy simulation framework, named DES-SDC. The required key quantities, including the fine structure mass fraction and dissipation rate, are appropriately blended across Reynolds-averaged Navier–Stokes and LES regions. The DES-SDC approach is validated using premixed bluff body stabilized flame, non-premixed swirl flame, and premixed swirl flame with complex geometry. It is much more tolerant to coarse mesh resolution than pure LES, yet it preserves the capability of resolving the key unsteady feature critical for the combustion process, as it is designed to be. The DES-SDC approach is relatively insensitive to the grid resolution. The present research provides a promising approach for accurately simulating practical unsteady turbulent combustion problems at an affordable computational cost.

Retrofit and application of pulverized coal burners with LNG and oxygen ignition in utility boiler under ultra-low load operation

An oxygen-rich and low NOx burner integrated with liquefied natural gas (LNG) was proposed to address unstable combustion and high NOx emissions from a 330 MW subcritical boiler under ultra-low load operation in China. To assess the effectiveness of the retrofit, Chemkin and Fluent softwares were utilized to construct a new NOx model and calculate NOx generation, based on the combustion of pulverized coal gas and LNG. Further, an eddy dissipation concept (EDC) model, which can reflect detailed chemical reactions, was applied to calculate gas-phase reactions in the furnace. The results showed that when performing the deep peak shaving after the retrofit, the combustion in the furnace was stable under 50% or more load, and NOx emission level at the furnace outlet was lower than 350 mg/m3 (6% O2 content, dry basis). Under 25% load, the oxygen-rich burner integrated with LNG was applied, and the pulverized coal flow entered the furnace in a state of high-intensity combustion, which effectively promoted the stability of combustion in the furnace. The reductive combustion state with reductive free radicals generated by LNG decomposition inhibited NOx formation. Consequently, NOx emissions from the furnace outlet decreased from 380 mg/m3 to 316 mg/m3.

Modified Reacting Solver: A Simplified Approach for Capturing the Molecular and Flow Diffusivities for the Nonpremixed MILD Flames.

The present study investigates the applicability of the inlet boundary species Lewis number (combined effect of molecular and flow diffusion) for the nonpremixed moderate and intense low oxygen dilution (MILD) flames. A modified reactive solver named modifiedReactingFoam is developed by including the enthalpy flux in the energy equation and using modified model constants in OpenFOAM. The present solver is tested on the delft-jet-in-hot-coflow burner operating under a moderate and intense low oxygen dilution combustion environment. Along with the flame with Reynolds number 4100, eight other jet-in-hot-coflow flames are simulated to test the capability of the present proposed solver. The main aim of the current work is to investigate the efficacy of the proposed solver in predicting the velocity field, temperatures, and flame lift-off height for the considered flames with a significant reduction in computational time. The predictions with the modified eddy dissipation concept model are improved. However, a significant deviation is still observed in the downstream direction of the burner. The numerical simulations are performed with methane Lewis numbers of 0.9-1.14 by keeping the respective constant Lewis numbers for the inlet boundary species. The modifiedReactingFoam predictions at a methane Lewis number of 1.12 are in very close agreement with the experimental results. The maximum deviation in lift-off heights is within ±3% of the experimental results. The present modified solver outperformed the other combustion models in the literature and reduced the computational time up to 10 times with a combination of DLBFoam compared to the inbuilt solver.

Numerical investigation of premixed combustion of producer gas with hydrogen injection in a cyclonical chamber

ABSTRACT This paper employs computational fluid dynamics (CFD) simulations with an eddy dissipation concept model (EDC) and a skeletal reaction mechanism (SK17) to analyze the impact of hydrogen addition on the premixed combustion mode of producer gas (PG) fuel in a cyclonic flow combustion chamber. The results, validated against measured wall temperature data, demonstrate a minor discrepancy between simulation and experiment. The studied result showed that injection of hydrogen into PG fuel combustion elevates the temperature of the combustion process, which can be indicated by the average temperature across the combustion chamber, outlet temperature of exhaust gas, and reactive radical species. However, increasing H2 injection over 20% results in strongly increasing CO formations. Compared to a fixed thermal input case, hydrogen injection with a variable input power resulted in a higher combustion temperature and increased emissions of both CO and NO. The maximum CO emission reached approximately 1246 ppm, while NO peaked at around 75 ppm, both observed at a 30% hydrogen injection rate. The key finding of this study revealed that direct injection of hydrogen to the combustion under the condition of a given equivalence ratio is unfavorable due to insufficient oxygen for the reaction, leading to an increase in CO emissions. The optimal condition would be a hydrogen injection below 20%, which produces low CO and NO emissions.

Mechanism evaluations of conventional and suspended HVOF thermal spraying based on EDC and EDM combustion models

Suspended high-velocity oxygen fuel (SHVOF) thermal spraying technology can spray solid particles with nano or sub-micron scale, and effectively prepare high quality coatings by wrapping solid particles into suspension with solvent. The HVOF and SHVOF processes are numerically simulated in this study. The gas phase and discrete phase were simulated by the bidirectional coupling Euler-Lagrange method. The state equation of ideal gas and realizable k-ε turbulence model were used to simulate gas phase flow. The effect of eddy dissipation model (EDM) and eddy dissipation concept (EDC) model on turbulent combustion reaction was discussed. Due to that EDM model simplifies the Arrhenius rate of different reactions, there are errors in the prediction of spraying combustion process. To solve the problem, the chemical kinetic mechanism of multi-component combustion reaction for the propylene, ethanol and oxygen was introduced in EDC model for the first time in this study. This method has not yet been used in the modeling of thermal spraying. The suspended droplets are a mixture of ethanol and WC-12Co powder. Results show that the atomization and evaporation of suspension significantly disturb the flow field and reduce the internal pressure and temperature of spray gun. Compared with the EDM model, the EDC model can well predict the peak temperature in the combustion chamber, the prediction of particle temperature and velocity is higher, and the temperature and velocity decay are slower during flight. Compared with the median particle size d50 in actual, two combustion models can effectively predict the median particle size after drying, which is the premise of accurately predicting the flight characteristics of particles.

A priori direct numerical simulation assessment of MILD combustion modelling in the context of large eddy simulation

A priori Direct Numerical Simulation (DNS) assessment of the closures for the filtered reaction rate and the Favre-filtered scalar dissipation rate (SDR) for large eddy simulations (LES) of homogenous and stratified mixture MILD combustion has been performed. It has been found that the probability density function of the reaction progress variable can be adequately predicted using the beta function for both homogenous and stratified mixture MILD combustion. It has also been found that a scalar dissipation rate closure characteristic of passive scalar mixing may not be suitable for homogenous and stratified mixture MILD combustion. The predictions of different filtered reaction rate closures have been assessed with respect to DNS data in this study. The Eddy Dissipation Concept (EDC), Eddy-Break Up (EBU) model, and SDR-based reaction rate closures for Reynolds Averaged Navier-Stokes simulations have been extended for MILD combustion in the context of LES. The SDR-based reaction rate closure captures the qualitative behaviour but overpredicts the filtered reaction rate for both homogenous and stratified mixture MILD combustion. The EBU-based reaction rate and flame surface density (FSD) closures did not capture the qualitative behaviour of the filtered reaction rate for both homogenous and stratified mixture MILD combustion. By contrast, the EDC model with bridging functions, which ensures asymptotic behaviour, shows the potential to provide reasonable predictions of the filtered reaction rate for a wide range of filter widths in the cases investigated here.

Multi-Element Gaseous Methane–Oxygen Rocket Combustor Optimization for Modern Space-Flight Technology

In the realm of reusable rocket technology, the methane–oxygen propellant combination has gained prevalence in recent years. However, understanding of design parameters affecting the combustion performance of a methane–oxygen combustor is currently limited. The present study proposes a novel analytical and computational approach to design and optimize a combustor. The design of a seven-shear coaxial injector-based combustion chamber is optimized using an in-house developed code line with the aim of minimizing chamber length while preserving chamber performance. Numerical simulations are then carried out to ensure complete combustion within the chamber. Methane–oxygen reactions are modeled using laminar finite-rate and eddy dissipation concept models. An optimized design for the combustion chamber shows maximum characteristic velocity and optimal chamber length near the oxygen-to-fuel mass ratio of 2. The computational study shows that the eddy dissipation concept model can accurately capture the effect of turbulent mixing on gaseous methane–oxygen reactions. The eddy dissipation concept model yielded notable differences in flame characteristics compared to the laminar finite-rate model. The study indicates a need to optimize injector configuration. An optimized injector configuration is proposed by varying its geometric parameters, which exhibits substantial improvement in combustion performance compared to the initial configuration, utilizing practical recess, divergence, and an increased velocity ratio.

Cavity effects on spontaneous ignition of pressurized hydrogen jets

The release of pressurized hydrogen jets can lead to spontaneous ignition, which poses a major challenge to the safe utilization of hydrogen energy. Previous studies have primarily focused on the spontaneous ignition of pressurized hydrogen released into tubes. However, in real industrial scenarios, pressurized hydrogen is usually released into the atmosphere or a complex equipment environment. Therefore, the effect of obstacles on spontaneous ignition needs to be considered. In this study, the spontaneous ignition of sudden releases of pressurized hydrogen entering various cavity shapes was modeled using the standard k-ω turbulence model, the eddy dissipation concept (EDC) model and a detailed 21-step chemistry model. The effects of the cavities on the jet flow fields and the spontaneous ignition were analyzed for various cavity shapes and release pressures. For the pressures considered in this study, the cylindrical cavity did not lead to ignition. The highest hydrogen jet temperature occurred in the contact region due to the shock wave reflection, after which spontaneous ignition could not occur due to the flow divergence. The pressurized hydrogen entering hemispherical and conical cavities enhanced the hydrogen-air mixing which led to combustible regions and the formation of multidimensional shock waves, which significantly increased the spontaneous ignition probability. The cavities created a high-temperature region inside the cavity that experienced ignition but the flames did not spread out of the cavity. The conical cavity produced lower flame temperatures than the hemispherical cavity but prolonged the flame lifetime. Higher initial hydrogen release pressures resulted in more violent multidimensional shock wave interactions and higher contact surface temperatures, which lead to earlier ignition times but with little impact on the initial location of the spontaneous ignition. The results of this study provide a scientific foundation for hydrogen safety codes, safe equipment designs and the development of industry codes and standards.

Characterizing pulverized coal combustion for high-ash content Indian coal

ABSTRACT The present study performs three-dimensional (3D) numerical simulations of coal combustion of four samples with varying ash content in a drop tube furnace (DTF) to mimic the particle heating rates observed in industrial furnaces. The numerical framework adopted is validated with prior experimental and simulation results using a 3D cylindrical geometry and a very good match is obtained for the axial temperature distribution and particle burnout rate. The combustion performance of coal samples with varying ash contents of 2.3, 16.6, 24.1, and 37.2 wt%, is explored through temperature profiles, burnout rates, particle tracking, and mass fractions of various products of combustion. A user-defined function is used to specify the 14 species, 10-step solid particle combustion with a modified eddy dissipation concept model for volatile combustion. The range of ash content considered is for Indian coal which has a high degree of heterogeneity, making the evaluation of its combustion performance, a challenge. The present work aims to provide benchmark set of results showing the evolution of the evaporated, charred, and volatile mass for the three ash contents, which has not been attempted before. It is found that the high-ash content Indian coal samples have a tendency to produce less . A reduction in the is obtained for the coal sample with ash compared to that with ash. The results suggest that the combustion performance is enhanced when the ash content falls within an optimum range of .

A comparative study of combustion models for simulating partially premixed swirling natural gas flames

Combustion models are important for simulating chemical reaction properties and the turbulence-chemistry interaction (TCI) in combustion. However, little attention has been paid to the temperature/species distributions/velocity flow fields predicted by different combustion models and their requirements for mesh resolution. In the present work, a partially premixed swirling natural gas flame was separately simulated by two commonly-used combustion models in Reynolds-Averaged-Numerical-Simulation (RANS) simulations: the flamelet generated manifold (FGM) model and the eddy dissipation concept (EDC) model. It is found that the EDC model requires a higher quality of mesh than the FGM model from the temperature and species distributions perspective. It is possible for the EDC model to overpredict temperature, however, such a problem can be solved by improving the mesh quality. The combustion models do not differ obviously for the mesh refinement for flow fields. The further analysis find that the EDC model brings forward the reaction location (11 mm upstream than the FGM model). The weakened turbulence mixing and early chemical reactions make the EDC model derive discrepancies with measured values, resulting from the variations between its modelling principles and actual applications in the software ANSYS Fluent. The low turbulence viscosity makes the EDC model flame thicker. Overall, the FGM model showed better suitability and accuracy for such a partially-premixed swirling natural gas flame. This work connects the two common combustion models and partially premixed swirling flame characteristics in the RANS simulations, and highlights the effects on the temperature/species concentration/turbulence flow fields, in concerns of mesh resolution and reaction properties rather than sole applications, which can provide foundation for subsequent researchers.

On the subgrid dissipation concept for large eddy simulation of turbulent combustion

A new subgrid turbulent combustion model for LES is derived based on the eddy dissipation concept (EDC). To be consistent with the LES framework, the relationship between the subgrid quantities and the fine structure ones is built from the unresolved part of the energy cascade, hence the subgrid dissipation concept (SDC). The fine structure quantities are directly computed using the subgrid scale ones. Resolving local laminar flame structures is allowed if the grid resolution permits. The model is examined using premixed piloted jet flames and non-premixed swirl flame. Satisfactory predictions from the SDC model are obtained in the test cases, showing improvements with respect to the standard EDC model. The new SDC model is relatively insensitive to the grid resolution.

Comparative study on homogeneous NO-reburning in flameless and swirl flame combustion

NO-reburning technology can effectively decrease NOx emissions during combustion. Despite extensive studies have been conducted on NO reburning under traditional combustion, homogeneous NO-reburning characteristics have not been systematically examined in flameless combustion (FLC). This study performs a systematic comparative investigation of homogeneous NO-reburning for the first time via experiments and simulations in a 20-kW furnace. The influences of combustion mode, air-preheating temperature (Ta), and equivalence ratio (Φ) are studied in detail. The experimental results show that FLC can improve NO-reduction efficiency by more than 35% compared to swirl flame combustion (SFC). Moreover, the results also show that reducing Ta from 723 K to 573 K can enhance NO-reburning during FLC, while the NO-reburning is insensitive to Φ in the range of 0.65–0.89. Furthermore, a detailed numerical simulation that couples the eddy-dissipation concept model and a well-verified skeletal mechanism for NO-reburning is validated and performed, to further reveal the reduction pathways of NO. By combining flameless combustion with NO-reburning, this study confirms the preponderance of flameless combustion under low-temperature air conditions (Ta ≤ 573 K) for considerable NO reduction by reburning. The results offer novel fundamental insights into the homogeneous NO-reburning characteristics.

Numerical investigation on the combustion characteristics of powder fuel under different regulation parameters

A comprehensive numerical study is undertaken to investigate the operating feature in a powder fueled scramjet combustor. The effect of regulation parameters on the flow field and combustion mechanism is examined systematically. The numerical simulation method is developed for multi-species gas-solid two-phase turbulent combustion in the Euler-Lagrange frame. The particle surface reaction model and the Eddy-dissipation Concept model are utilized for powder fuel combustion. The combustor flow field, flame structure, and species distribution in the powder fueled scramjet combustor are revealed in detail. The numerical simulation results show that the combustion organization scheme of tandem double-cavity can provide an excellent flame stabilization effect. The powder fuel achieves continuous combustion and sufficient release heat in the supersonic airflow. The combustion flow field features and flame distribution are strongly influenced by the transport solid-gas ratio and the equivalent ratio. The increase of both regulation parameters improves the combustion efficiency of powder fuel in the region from the injection hole to the downstream cavity. Nevertheless, the large equivalent ratio significantly leads to higher total pressure losses in the combustor.

Numerical study on pollutant emissions characteristics and chemical and physical exergy analysis in Mild combustion

This analytical study appraises the emission characteristics of different pollutants such as CO and NOX as well as conducting chemical and physical exergy analyses on conventional and Mild combustion regimes. The RANS turbulence approach and the k-ε RNG model were utilized in this study to simulate turbulence, whereas the eddy dissipation concept (EDC) model and the GRI–Mech 2.11 chemical mechanism were adopted to treat the turbulence-chemistry interaction. This study evaluated local and global pollutant emissions and exergy efficiency along the reacting jet and explored the effects of temperature and chemical species on the physical and chemical exergies in MILD combustion in comparison to conventional combustion. It is found that the overall exergy efficiency of MILD combustion rises by 1, 3, and 4% for oxygen mass fractions of 3, 6, and 9% in hot oxidizer coflow, with the NOx emissions are 81, 72, and 55% lower than conventional combustion, respectively. Chemical exergy is greater than physical exergy near the fuel nozzle due to high CH4 and H2 concentrations. Chemical exergy reduces along the burner due to chemical reactions, while physical exergy increases due to the increased temperature. However, owing to the trade-off between these two exergies along the flame jet, the total exergy approaches a constant value at large distances from the burner. The maximum total exergy occurs at a distance of 0.12 m from the fuel nozzle, with chemical exergy accounting for 83% of the total exergy. In the outlet (0.5 m from the fuel nozzle), chemical exergy accounts only for 30% of the total exergy. It can also be concluded that the exergy is stabilized at a distance of 0.3 m from the nozzle (60% of the domain length). Despite a higher local CO emission in conventional combustion than in MILD combustion, the CO emission substantially decreases at a smaller distance from the nozzle in conventional combustion due to the high O2 concentration.

Numerical study on the effect of complex structural barrier walls on high-pressure hydrogen horizontal jet flames

Hydrogen has the potential to produce energy with no emissions, and therefore offers a strategic opportunity for rapid development of renewable energy storage. However, jet flames caused by the leakage of high-pressure hydrogen may cause casualties and damage equipment, so the safety issues associated with hydrogen incidents are a key issue restricting its development. Barrier walls can be an important mitigative action to reduce the hazard of hydrogen jet flames. Previous studies have evaluated the protective effect of different vertical or inclined arrangements of a single barrier wall in the vertical direction. However, in the vertical direction, a single barrier wall is not sufficient to reduce the hazards associated with high-pressure hydrogen jet flames. In this paper, a two-dimensional CFD model of high-pressure hydrogen jet flames are established. A simplified form of the hydrogen real gas equation of state is used to describe the complex behavior of high-pressure hydrogen. This CFD study employs the standard k-ε turbulence model, the eddy dissipation concept (EDC) model, and the discrete ordinate (DO) radiation model. The results display good agreement with the experimentally obtained flame shape and thermal radiation values, which confirms the applicability of the model. Based on this model, the characteristics of high-pressure jet flames are systematically studied under different forms of barrier walls and different pressures of hydrogen storage. Compared with the vertical barrier wall, barrier walls with complex structures can block the flames in front of the wall more effectively. This is proven through a remarkable decline in the vertical flame length and a reduction in the high temperature area. Comparing the five barrier wall patterns for their combined flame shape and flame temperature, it is more preferred to choose 60° Plate. The results of this study will help to better analyze the consequences of hydrogen jet flames and provide guidance for better design of barrier walls for mitigation.

Four-step global kinetics mechanism for diluted combustion fueled with kerosene

The aim of the present study was to develop a global kinetics mechanism capable of predicting characteristics of the combustion of kerosene diluted with combustion gases. A four-step model of the Aachen mechanism was developed via modification of a mechanism for lower molecular weight hydrocarbons. A simplified fuel was used to simulate kerosene, the thermodynamic properties of which were estimated based on the surrogate fuel in the Aachen mechanism. Combustion of this model fuel in a perfectly stirred reactor (PSR) operating under high-temperature air combustion (HiTAC) conditions was simulated using inflow mixtures preheated and diluted with nitrogen. In this new mechanism, the activation energy for the fuel consumption reaction and the pre-exponential factor for the hydrogen oxidation reaction were adjusted. The resulting model accurately predicted variations in temperature and major species over time. The four-step mechanism was also validated under conventional flame and moderate or intense low oxygen dilution (MILD) combustion conditions. The PSR simulations with the four-step mechanism accurately predicted variations in temperature and major species under MILD combustion conditions. In addition, this four-step mechanism was confirmed to be superior to other global mechanisms with regard to predicting the mole fractions of various species. Experimental trials were carried out with diluted kerosene flames formed above parallel jet burners composed of a fuel spray nozzle and an oxidizer nozzle, and it was found that NOx emission decreased with increasing distance between nozzles even for spray combustion. These diluted flames were numerically simulated using the eddy dissipation concept model combined with the four-step mechanism. The temperature distributions obtained from the simulations showed good agreement with the experimental data in the downstream region.

Characterization of pintle engine performance for GO2/kerosene propellants

Liquid rocket engine with a wide range of thrust variations has drawn much attention in recent years. A GO2/kerosene deep-throttling variable thrust engine using a liquid–gas pintle injector is presented and tested with aim of characterizing engine performance, regulating capability, and heat flux distribution. Three-dimensional numerical simulations are conducted to investigate the flame appearance and spray characteristics of the engine. The SST k-omega model is used for modeling turbulence, the Wave model is used for modeling the secondary breakup, and a multi-step quasi-global combustion mechanism and eddy dissipation concept model are used for modeling combustion. The GO2/kerosene pintle engine achieves stable combustion at 0.18 ∼ 4.35 MPa with a thrust of 21.30 ∼ 864.70 N. The thrust ratio of the pintle engine is 40.6:1 indicating a deep-throttling variable thrust capability. The thrust regulation of the pintle engine using the closed-loop control has high accuracy and response speed, as well as high disturbance suppression capability. The effects of combustor pressure, spray characteristics, and reactants concentration distribution on flame appearance and heat flux distribution of the engine are studied in detail. The engine equipped with a liquid–gas pintle injector could ensure a good atomization effect and high combustion efficiency by actively adjusting the propellant injection pressure drop coefficient. This work will provide a reference for the design of the pintle injector and thermal protection system of deep-throttling variable thrust rocket engines.

Investigation of a low emission peripheral vortex reverse flow (PVRF) combustor fuelled by LPG and ethylene

Peripheral Vortex Reverse Flow (PVRF) combustor is characterized by a strong peripheral recirculation zone resulting in stable combustion and low pollutant emissions. In this study, a PVRF combustor is investigated in the premixed and non-premixed modes with LPG and ethylene fuels at a maximum heat load of 6.25 kW and thermal intensity of 25 MW/m3-atm, relevant to gas turbine combustors. The fuel is injected co-axially, with respect to the air jet, using two different fuel injection diameters of 1 mm, and 2 mm. For both the fuels, the fuel injection velocity is higher as compared to the air injection velocity for fuel injection diameter of 1 mm, while it is lower for fuel injection diameter of 2 mm. CH* chemiluminescence images are acquired, and CO and NOx emissions are measured. Reynolds Averaged Navier Stokes (RANS) simulations are carried out at an equivalence ratio of 0.8 using the Eddy Dissipation Concept (EDC) model with a H2/CO/C1–C4 mechanism comprising of 50 species and 373 reactions. Delayed ignition and downstream shifting of the reaction zone facilitating higher product gas entrainment prior to reactions is observed in the non-premixed mode as compared to the premixed mode. NOx emissions were found to be lower in the non-premixed mode as compared to the premixed mode for both the fuels. This is possibly due to suppression of NO formation in the reaction zone via. prompt route which is also supported by lower CH* chemiluminescence intensity in the non-premixed mode. With LPG, the case with larger fuel injection diameter showed further delayed mixing as compared to the case with smaller fuel injection diameter, and resulted in even lower NOx emissions. For all the cases, CO emissions were of similar levels in the non-premixed and premixed modes suggesting that lower NOx emissions in the non-premixed mode is not at the expense of increase in CO emissions. Overall, the investigation shows that lower NOx emissions can be achieved in the non-premixed mode as compared to the premixed mode if more product gas entrainment is facilitated prior to the reactions by delayed mixing between fuel and oxidizer.

Influence of jet velocity and heat recuperation on the flame stabilization in a non-premixed mesoscale combustor: An exergetic approach

An experimental and numerical model to determine the exergy balance based on flow availability and availability transfer in the process of liquefied petroleum gas (LPG)/air combustion in mesoscale gas turbine combustor is developed to elucidate the second law efficiency and total thermodynamic irreversibility. In terms of developing an energy and exergy-efficient combustor design, the present work highlights the influence of vortex shedding and recirculation in the volumetric entropy production and the exergy efficiency. It is performed in a heat recuperative high-intensity LPG-fueled mesoscale combustor for mini-gas turbine applications. The combustor is operated at different thermal inputs ranging from 0.2 to 1.0 kW under range of equivalence ratios of ϕ = 0.4–1.23. The Favre-averaged governing equations are solved by using finite volume-based approach. The standard k–ε turbulence model with modified empirical constant, Cɛ1=1.6, is considered to model the turbulence quantities. The volumetric reaction-based eddy-dissipation concept model and a reduced skeletal model (50 species and 373 reactions) are used for turbulence–chemistry interaction. The design methodology, total volumetric entropy generation, destructive exergy due to thermodynamic irreversibility, exergy efficiency, flow recirculation, and mixing characteristics (reacting and non-reacting) are reported. The entropy generation rate due to thermal conduction is approximately 50% of the total entropy generation, while its contribution percentage due to chemical reaction is the smallest. The exergy efficiency reaches its peak with ηII = 79.41% at 1.0 kW under fuel-rich condition, while its minimum value of 41.49% is obtained at 0.2 kW under fuel-lean (ϕ = 0.8) condition.