Combustor Wall Research Articles

Wall Temperature Measurements in a Full-Scale Gas Turbine Combustor Test Rig With Fiber Coupled Phosphor Thermometry

Abstract Wall temperature measurements with fiber coupled online phosphor thermometry were, for the first time, successfully performed in a full-scale H-class Siemens gas turbine combustor. Online wall temperatures were obtained during high-pressure combustion tests up to 8 bar at the Siemens Clean Energy Center (CEC) test facility. Since optical access to the combustion chamber with fibers being able to provide high laser energies is extremely challenging, we developed a custom-built measurement system consisting of a water-cooled fiber optic probe and a mobile measurement container. A suitable combination of chemical binder and thermographic phosphor was identified for temperatures up to 1800 K on combustor walls coated with a thermal barrier coating (TBC). To our knowledge, these are the first measurements reported with fiber coupled online phosphor thermometry in a full-scale high-pressure gas turbine combustor. Details of the setup and the measurement procedures will be presented. The measured signals were influenced by strong background emissions probably from CO*2 chemiluminescence. Strategies for correcting background emissions and data evaluation procedures are discussed. The presented measurement technique enables the detailed study of combustor wall temperatures and using this information an optimization of the gas turbine cooling design.

Numerical Investigation of a High Momentum Jet Flame at Elevated Pressure: A Quantitative Validation with Detailed Experimental Data

The development of efficient low emission combustion systems requires methods for an accurate and reliable prediction of combustion processes. Computational Fluid Dynamics (CFD) in combination with combustion modelling is an important tool to achieve this goal. For an accurate computation adequate boundary conditions are crucial. Especially data for the temperature distribution on the walls of the combustion chamber are usually not available. The present work focuses on numerical simulations of a high momentum jet flame in a single nozzle FLOX® type model combustion chamber at elevated pressure. Alongside the balance equations for the fluid the energy equation for the solid combustor walls is solved. To assess the accuracy of this approach, the temperature distribution on the inner combustion chamber wall resulting from this Conjugate Heat Transfer (CHT) simulation is compared to measured wall temperatures. The simulation results within the combustion chamber are compared to detailed experimental data. This includes a comparison of the flow velocities, temperatures as well as species concentrations. To further assess the benefit of including the solid domain in a CFD simulation the results of the CHT simulation are compared to results of a CFD computation where constant temperatures are assumed for all walls of the combustion chamber.

NO emission performance assessment on a perforated plate-implemented premixed ammonia-oxygen micro-combustion system

• The flow field is found to vary greatly downstream of the perforated plate. • The conjugate heat transfer affects NO formation by relocating the flame. • The preferential diffusion is intensified due to the two-dimensionality flow. • The coupling effect of these factors above determines the NO generation rate. The present work examines the NO x emission characteristics of a premixed micro-combustion system with a perforated plate implemented. For this, a three-dimensional (3D) computational model involving a detailed chemical-kinetic mechanism for ammonia-oxygen combustion in the micro-combustor is developed. The model is first validated with the experimental measurements available in the literature before conducting comprehensive analyses. It is found that implementing a perforated plate in the micro-combustion system creates a flow recirculation zone downstream characterized by a low flame temperature and combustion speed. Meanwhile, the conjugate heat transfer between the combustion products and the inner combustor walls is shown to play a key role in the NO generation by relocating the flame in the axial direction and thus changing the chemical reaction rate. Furthermore, the preferential diffusion caused by the variation in the mass diffusivity of different species and the two-dimensionality flow is identified to vary significantly in comparison with the case in the absence of the perforated plate, especially in the vicinity of the recirculation zone. This diffusion effect results in the considerable drop in the N/O atomic ratio, primarily due to the reduction and increase of O 2 and H 2 O, together with less available N 2 , and consequently affecting the NO generation rate. This work confirms that the flow field, the conjugate heat transfer as well as the preferential diffusion effect could be regarded as the potential mechanisms leading to the NO x emission variation in the recirculation zones.

Effect of transverse fuel injection system on combustion efficiency in scramjet combustor

Supersonic mixing layer behind the hydrogen fueled wedge shaped strut has been demonstrated by means of combined fuel injection strategy. Numerical investigation on the Effect of wall transverse fuel injection system in addition with parallel fuel injection approach have been found suitable for stable flame and combustion performance. The experimental supersonic combustor observation was performed by Waidmann et al. in German Aerospace Centre (DLR), which has been utilised in validation section to examine the accuracy and applicability of present computational work. Non-reacting and reacting both types of flow field characteristics are analysed to complete the validation. Good agreement was found in the contour plots towards oblique shock incident location moreover expansion fan behaviour at the end tip of the strut was observed. Pressure graph near the combustor walls and mid plane of the combustor were well matched with numerical observation however velocity graph was best suitable for near wake region instead of far-field region. Thus Ansys 14.0 fluent software has been chosen to compare the experimental data from the literature with numerical analysis. Two-dimensional computational combustor geometry has been taken with involving steady state, SST k-ω two equation based turbulence model analysis. Finite rate-eddy dissipation chemistry based combustion model is used to analyse the combustion process. Parallel fuel injection location is engaged at the same point and the change of wall transverse fuel injection port position have been analysed. Variations can be visualised in the numerical analysis outcome of combustion and mixing performance. The combustion efficiency plot is divided in to two section i.e. rapid change in chemical reaction in all four cases and second is gradual consumption of fuel. The outcome shows that the abrupt change in 68 mm transverse fuel injector model with 96% combustion efficiency.

Bubblig Fluidized Bed Combustor (Dept.M)

A Bubbling Fluidized Bed Combustor (BFBC) is designed and set-up to investigate the combustion processes of sub-bituminous Egyptian coal particles and wood. The effect of changing the coal particles size, with keeping the fluidized air velocity constant, on: gas temperatures, heat flux to the combustor wall, radiant-convective overall heat transfer coefficient and combustion level of coal particles is investigated. The highest combustion level is found at coal particles size of 17 mm. The heat transfer coefficient in the case of coal combustion is extremely greater by 500% than that of wood combustion.

Assessment of numerical radiation models on the heat transfer of an aero-engine combustion chamber

Thermal radiation is the most dominant type of heat transfer inside the combustion chamber, which can directly affect the temperature distributions at the combustor walls. This paper provides a comprehensive analysis of the effects of two radiation models on the flame and liner-walls temperatures. A combustion chamber used in the Rolls-Royce-RB-183 turbofan engine was examined in this study by integrating a solid combustor model with the numerical fluid domain. The results indicated that the implementation of radiation models shrinks the flame peak-temperature and altered temperature distribution across the liner. The Discrete Ordinates Method (DOM) estimated a 10% higher temperature at the front part of the liner compared to the non-radiation model and 15% less than the P-1 radiation method. After the dilution zone, the DOM and P-1 models estimated respectively 15% and 25% reduction in the liner temperature compared to the non-radiation combustor. The radiation models have also down predicted the flame temperature by 200 K and more than 200 K for DOM and P-1 case respectively. The results also showed that the emissivity value had minimal effects on the combustor temperature distribution. The DOM considered being more accurate to estimate the combustor wall and flow temperatures compared to the P-1 radiation method.

Study on Combustion Characteristics of Premixed Methane-Oxygen in a Cylindrical Porous Media Combustor

A three-dimensional numerical and experimental study on methane-oxygen combustion has been conducted in a small scale porous-media cylindrical combustor which aims at extending the optimized controlling parameters for thermophotovoltaic power application. Different equivalence ratios, inlet velocities, porous-media porosities and wall thermal conductivities are simulated in the same configuration and the results show that porous-media improves the thermal characteristics of a combustor compared to the non-porous-media counterpart. There is about 350 K difference in peak wall temperature for the porous-media combustor over the non-porous-media combustor. The flame temperature first increases as porosity increases, and then slightly decreases as the porosity increases. At higher wall thermal conductivities, the effect of the change in porosity does not affect the mean temperature distribution on the combustor wall. Based on the findings, the porous-media combustor with equivalence ratio of 0.8, porosity of 0.5, inlet velocity of 0.35 m/s and wall thermal conductivity of 20 W/(m2219;K) are identified as key combustion conditions that can produce uniform and stable wall temperature for an efficient combustion chamber for the thermophotovoltaic system. With the inherent constraint associated with small-scale combustion this study will provide a premise for developing and enhancing the performance of the combustor under the given constraints.

Numerical studies and validation of combustor and annular isolator interactions of hydrocarbon based axisymmetric dual combustion ramjet

Combustor Intake interactions study is very important for design of Ramjet/Scramjet propulsion systems. High Combustion-induced back pressures make the shock train propagate upstream further and interfere the flow of inlet and the scramjet would unstart. The study has more relevance especially for the propulsion systems like Dual Combustion Ramjet (DCR) due to its wider range operation of Mach number. In this paper, numerical simulation of flow field in an axisymmetric scramjet/ramjet combustor with annular isolator is carried out. Configuration chosen for simulation is Dual Combustion Ramjet, which has a dump subsonic combustor, main supersonic Combustor and an annular isolator. Objective of the work is to evolve a CFD procedure for combustor-isolator interaction and validation of the model with experimental data. Geometry of model DCR is taken from literature for simulations and studies are carried out for Isolator inlet Mach No. 1.79 and 2.23. Favre Averaged equations are solved using Commercial code–ANSYS FLUENT. Premixed composition of air and Kerosene is introduced into dump combustor. Reactive flow is modelled using 7 species and 4 step chemistry. Numerical simulations have been carried out with various turbulence models namely k–ε (Standard), (RNG) k–ε, kω-Standard, kω-SST and combustor wall pressures were compared with experimental results. Error in estimation of starting location of Shock train, Maximum Pressure, Average pressure in combustor is compared. (RNG) k–ε predicted shock train location with an error difference of 5% compared to k–ε (Standard). Further, effect on shock train position in isolator with change in gas generator equivalence ratio is also studied for free stream Mach numbers 4 and 5. Shock train moves upstream in Isolator with increase in gas generator equivalence ratio. Various parameters such as Wall static Pressure, Shock train location, length and static pressure along shock train centre, Velocity profile and Combustion efficiency are compared for reactive flow turbulence models.

Influence of wall heat loss on the emission characteristics of premixed ammonia-air swirling flames interacting with the combustor wall

The influence of wall heat loss on the emission characteristics of ammonia-air swirling flames has been investigated employing Planar Laser-Induced Fluorescence imaging of OH radicals and Fourier Transform Infrared spectrometry of the exhaust gases in combustors with insulated and uninsulated walls over a range of equivalence ratios, ϕ, and pressures up to 0.5 MPa. Strong influence of wall heat loss on the flames led to quenching of the flame front near the combustor wall at 0.1 MPa, resulting in large unburned NH3 emissions, and inhibited the stabilization of flames in the outer recirculating zone (ORZ). A decrease in heat loss effects with an increase in pressure promoted extension of the fuel-rich stabilization limit owing to increased recirculation of H2 from NH3 decomposition in the ORZ. The influence of wall heat loss resulted in emission trends that contradict already reported trends in literature. NO emissions were found to be substantially low while unburned NH3 and N2O emissions were high at fuel-lean conditions during single-stage combustion, with values such as 55 ppmv of NO, 580 ppmv of N2O and 4457 ppmv of NH3 at ϕ = 0.8. In addition, the response of the flame to wall heat loss as pressure increased was more important than the effects of pressure on fuel-NO emission, thereby leading to an increase in NO emission with pressure. It was found that a reduction in wall heat loss or a sufficiently long fluid residence time in the primary combustion zone is necessary for efficient control of NH3 and N2O emissions in two-stage rich-lean ammonia combustors, the latter being more effective for N2O in addition to NO control. This study demonstrates that the influence of wall heat loss should not be ignored in emissions measurements in NH3-air combustion, and also advances the understanding of previous studies on ammonia micro gas turbines.

Dual/scram-mode combustion limits of ethylene and surrogate endothermically-cracked hydrocarbon fuels at Mach 8 equivalent high-enthalpy conditions

This paper examines the scram/dual-mode combustion limits of hydrocabon fuels within a Mach 8, scramjet combustor. Flight-equivalent flows were delivered to the axisymmetric, cavity combustor via a reflected shock tunnel. Two scramjet fuels were examined: ethylene and a surrogate mixture representing endothermically cracked n-dodecane. Combustion modes were examined via static pressure sensors and through both chemiluminescence imaging, and planar laser induced fluorescence (PLIF) of the OH combustion radical in the combustor exhaust plume. Ethylene-fuelled experiments developed scram-mode combustion under reduced fuelling conditions, experiencing shock wave dominated flowfields. OH PLIF diagnostics indicated such combustion modes developed a ring-like structure of combustion products, primarily axisymmetrically adjacent to the combustor wall. Increased fuelling anchored combustion downstream of the fuel injector, while further increases instigated dual-mode combustion. In this mode, subsonic combustion regions combine with the supersonic coreflow to permit the transfer of information upstream with substantially increased pressure encountered. Optical diagnostics indicate broadly asymmetric, unsteady combustion features. The surrogate mixture representing endothermically cracked n-dodecane experienced rapid onset from no-combustion (optically confirmed) to fully developed dual-mode combustion at critical fuelling rates. OH PLIF signals and chemiluminescence of this fuel were weaker than comparable ethylene cases, indicating potential differences in combustion pathways.

Experimental and Numerical Study of Jet Impingement Cooling for Improved Gas Turbine Blade Internal Cooling With In-Line and Staggered Nozzle Arrays

Abstract Internal cooling of gas turbine blades is performed with the combination of impingement cooling and serpentine channels. Besides gas turbine blades, the other turbine components such as turbine guide vanes, rotor disks, and combustor wall can be cooled using jet impingement cooling. This study is focused on jet impingement cooling, in order to optimize the coolant flow, and provide the maximum amount of cooling using the minimum amount of coolant. The study compares between different nozzle configurations (in-line and staggered), two different Reynold's numbers (1500 and 2000), and different stand-off distances (Z/D) both experimentally and numerically. The Z/D considered are 3, 5, and 8. In jet impingement cooling, the jet of fluid strikes perpendicular to the target surface to be cooled with high velocity to dissipate the heat. The target surface is heated up by a direct current (DC) power source. The experimental results are obtained by means of thermal image processing of the captured infra-red (IR) thermal images of the target surface. Computational fluid dynamics (CFD) analysis were employed to predict the complex heat transfer and flow phenomena, primarily the line-averaged and area-averaged Nusselt number and the cross-flow effects. In the current investigation, the flow is confined along with the nozzle plate and two parallel surfaces forming a bi-directional channel (bi-directional exit). The results show a comparison between heat transfer enhancement with in-line and staggered nozzle arrays. It is observed that the peaks of the line-averaged Nusselt number (Nu) become less as the stand-off distance (Z/D) increases. It is also observed that the fluctuations in the stagnation heat transfer are caused by the impingement of the primary vortices originating from the jet nozzle exit.

Development of an ultra-high capacity hydrocarbon fuel based micro thermoelectric power generator

The objective of the present study is to develop a fossil-fuel based alternative energy storage system for electrochemical batteries. A novel perforated-plate based microcombustor, used as a heat source for thermoelectric power generation, is developed in the present work. It has been shown to produce a high heat-flux with improved temperature uniformity. A superior thermal performance is achieved due to simultaneous flame-flame interaction and flame-impingement on the combustor walls. The performance of the microcombustor, in terms of heat-flux and surface temperature distribution, is compared with a theoretical model developed using the inverse heat conduction technique. A high heat-flux of 56 kW/m2 with a significantly lower coefficient of variance is achieved. The performance of the integrated system is experimentally investigated through detailed thermal and flame stability characteristics. An electric power output of 21.2, 22.4 and 23.5 W, with an overall conversion efficiency of 3.01, 2.82 and 2.68 %, is achieved for mixtures with equivalence ratios of 0.8, 0.9 and 1.0 respectively at a mixture velocity of 1.0 m/s. The novelty of the present study lies in the development of a high power system and its performance characterisation at various operation conditions, making it a suitable alternative for various standalone, rural, and portable applications.

Experimental investigations on the temperature equilibrium process inside a detonation tube operating in a valveless scheme

As the previous studies have clarified that the temperature of detonation combustor wall will increase with the increase of operating frequency, but the mechanisms are still not clarified. An experimental work has been conducted to investigate the temperature equilibrium process of the internal wall in a pulse detonation tube, which was operated in a valveless scheme without the purge process. Ethylene and oxygen-enriched air with an oxygen volume fraction of 50% were used as fuel and oxidizer, respectively. The injections of fuel and oxidizer were both controlled by pressure oscillations in the detonation tube. The heat transfer process inside the detonation tube was studied by employing a fast response thermocouple installed near the internal wall. The results indicate that the heat transfer process in an individual detonation cycle in multi-cycle operation contains a heating up process and a cooling down process. Heat transferred from the burned gases to the tube wall in the heating process, while it reversed into the fresh mixture in the cooling down process. The equilibrium temperature was determined by the competition of the two processes. The cooling duration decreased when the operating frequency increased. The heating up duration was influenced by the expansion waves and the wall temperature, and it decreased with the increase of the wall-temperature. In addition, the temperature differences, among the burned gases, the fresh mixture, and the tube wall, were the main factors that influenced the heat load and the equilibrium process of the wall-temperature.

Performance of cylindrical and planar meso­scale combustor with double narrow slit flame holder for micropower generator

This research compared the performance of a cylindrical mesoscale combustor against two planar mesoscale combustors, which include the shape of the flame front, temperature of the combustor axis and the combustor wall, and the resulting flammability limit. The combustor used has a circular, square and rectangular cross-section. All three combustors have the same cross-section area and combustion chamber volume. The flame holder used is a double narrow slit. The fuel used is liquefied petroleum gas with a pure oxygen oxidizer. The experiment results showed that the cylindrical combustor produces a more even flame shape that fills the combustion chamber and there is no clear separation between the sides of the flame on each side of the narrow slit. A high ratio of the entrance to average velocity results in a large adverse pressure gradient which generates vortex and recirculation behind the flame holder which gives the mixture a longer chance in the combustion chamber (prolonged residence time). The flame front shape affects the temperature of the combustor axis. The flame front shape that fills the entire combustion chamber has a higher flame temperature than the separate flame front shape. The circular combustor has the highest average axis temperature, but it has the lowest combustor wall temperature. This fact shows that the circular combustor has the smallest heat loss from the flame to the combustor wall. Furthermore, a circular mesoscale combustor has the most extensive stability map. For the same volume, the circular combustor has a lower surface area to volume ratio, thus the heat loss is also low. The dead zone area also becomes narrower, only at a low reactant rate. Rectangular combustors have the largest surface area to volume ratio, thus the losses are also the biggest. Despite the narrowest flammability limits, rectangular combustors have the highest average wall temperatures

Post-test analysis of the LAPCAT-II subscale scramjet

A subscale flight experiment configuration propelled by a Mach 8 supersonic combustion ramjet (scramjet) was designed within the framework of the European Commission co-funded Long Term Advanced Propulsion Concepts and Technologies II project. The focus of this design exercise was to verify by ground testing the ability of the proposed scramjet engine to produce adequate thrust for hypersonic level flight. Experiments performed in DLR’s High Enthalpy Shock Tunnel Göttingen confirmed precedent CFD predictions of the total thrust and demonstrated the operability of the vehicle. Yet, significant discrepancies between the CFD analyses, which were performed to design the vehicle, and subsequent detailed measurements of the pressure distribution in the combustor were observed and could not be resolved so far. This paper focuses on a further analysis of these residual discrepancies. It was found that the CFD predictions of the combustor pressure distribution are sensitive to the configuration of the intake boundary layer. Particularly, different assumptions for the location of the laminar to turbulent boundary layer transition strongly influence cross-flow structures which develop on the intake and which are able to trigger different combustion modes in the combustor. While the effect on total vehicle performance remains limited, a significant impact on the structure and magnitude of the surface pressure distribution was observed. i.e., the large combustor peak pressures, which occur in the experiment, can be explained by the occurrence of a strong shock train in the vicinity of the combustor wall.

Parametric analysis on the combustion and thermal performance of a swirl micro-combustor for micro thermophotovoltaic system

A swirl micro-combustor which has strong capability to stabilize the flame has been developed and is promising to be used in micro thermophotovoltaic power generators in this work. Heat recirculation plays a requisite role in flame stability and thermal performance of micro combustors. Thus, the effects of three parameters (center inlet radius w1, swirler width w2, and step height w3) that determine the size and location of the recirculation zones in a premixed hydrogen/air flame are investigated numerically. The results show that inner recirculation zone and corner recirculation zone generated by a swirler are critical for stabilizing the flame. The parameter w1 has an insignificant influence on the thermal performance. Enlarging the inner recirculation zone by decreasing w2 provokes enhanced heat transfer, and increases the temperature level and temperature non-uniformity of combustor walls. The size of corner recirculation zone is increased by increasing w3, which raises the outer wall temperature uniformity. Based on the parametric analysis, the newly designed micro combustor possesses a high wall temperature level. Additionally, the energy output increment is between 9.1% and 14.2% at hydrogen mass flow rate of 0.0006 g/s when stoichiometric hydrogen/air ratio ranges from 0.6 to 1.4.

Review of effect of meso scale combustor wall material, Catalytic material, & Porous media on flame Stabilization

In recent trends demand of energy for portable devices like notebook, microsatellite, laptops, UAV are increasing at high rate, a high energy density hydrocarbon fuel creates opportunity to alternate chemical Li-ion batteries with micro combustion based power generation system. Mainly, this review focus on experimental, mathematical modelling techniques, and numerical simulation performed on heat recirculating combustor to find the effect of combustor wall material, catalytic material and porous media. It also summarizes the electricity generation possibilities with micro combustor turbine, thermo electric material & thermo-photovolatic combustion system. The effect of combustor wall material, catalytic material, and porous media on flame stabilization are enlisted with its summary.

Soot formation and combustion characteristics in confined mesoscale combustors under conventional and oxy-combustion conditions (O2/N2 and O2/CO2)

This work investigated soot formation and combustion characteristics in confined mesoscale combustors of two diameters 4 and 6 mm between conventional and oxy-combustion conditions using high resolution transmissions electron microscopy, thermogravimetric analysis and exhaust gases analysis. Results demonstrated that by increasing O2 concentration, the combustion performance can be improved significantly. In 4 mm combustor, both ethylene conversion and mean wall temperature for N2 diluted flame were larger for higher O2 concentration because the reduction of inert diluted gas decreased the heat loss and more O2 content contributed to reactions of more fuel. For N2 diluted flames in 6 mm combustor, soot nanostructure under 21% O2 exhibited amorphous structures but typical fullerene-like structures with more organized fringes could be observed under 30% and 40% O2. The soot carbonization degree increased notably with higher O2 concentration. For the same O2 concentration and combustor, ethylene conversions and temperatures for CO2 diluted flames were always less than those from N2 diluted flames due the larger specific capacity of CO2 than N2. Higher CO concentrations were also detected in exhaust gas from the flame with CO2 dilution than with N2 because CO2 directly participated in the reaction CO2+H⇄CO+OH.

Analysis of the maximum flight Mach number of hydrocarbon-fueled scramjet engines under the flight cruising constraint and the combustor cooling requirement

Scramjet is the optimal propulsion system for hypersonic vehicles, and it is important to study its maximum flight Mach number. Upper limit and its influence factors of the flight Mach number of dual-mode scramjet used in hypersonic vehicles are investigated in this paper. Scramjet performance analysis model with the constant-area heating process, cooling requirement analysis model of the combustor and cruise model of the scramjet-powered hypersonic vehicle are established. Specific impulse of dual-mode scramjet engine with hydrocarbon fuel is similar to that of liquid oxygen kerosene rocket engine when the freestream Mach number is above 10. The ratio of hypersonic vehicle reference area to scramjet inlet capture area is introduced to establish the size relationship between the scramjet engine and the hypersonic vehicle. Considering the balance of thrust and drag, smaller ratio of hypersonic vehicle reference area to scramjet inlet capture area leads to higher flight Mach number that hypersonic vehicle can cruise. When the angle of attack is 10 degrees, the ratio of hypersonic vehicle reference area to scramjet capture area should be less than 5.5 to ensure that the vehicle can work at Mach 8. The maximum length to diameter ratio of the combustor is used to indicate active cooling requirements, and it is less than 5 under the cooling requirement when flight Mach number is above 9. Reducing the heat flux at the combustor wall and increasing the fuel heat sink is helpful to increase the allowable combustion chamber length, so as to increase the maximum flight Mach number of the scramjet engine on the premise of meeting thermal protection requirement.

Numerical Investigation of the Combustion in an Improved Microcombustion Chamber with Rib

This study proposes an improved microcombustor with a rectangular rib to improve the temperature level of the combustor wall. Moreover, the OH mass fraction, temperature distribution, and outer wall temperature of the original and improved combustors of premixed hydrogen/air flames are numerically investigated under various inlet velocities and equivalence ratios. Results show that the improved microcombustor enhances heat transfer between the mixture and wall because its recirculation zone is larger than that of the original, thereby resulting in high wall temperature. Conversely, thermal resistance in the horizontal direction increases with upstream and downstream step lengths. Consequently, the outer wall temperature decreases with step length in the improved combustor. A high equivalence ratio (e.g., 0.6) may result in the destruction of the combustor because the wall temperature has exceeded the acceptable temperature of wall material quartz. Therefore, the improved microcombustor is recommended for micro-thermo-photovoltaic systems.