Burner Diameter Research Articles

Experimental and numerical study on the occurrence of off-stagnation peak in heat flux for laminar methane/air flame impinging on a flat surface

A combined experimental and numerical study has been conducted to investigate the occurrence of off-stagnation peak for laminar methane/air flame impinging on a flat surface. Experiments were conducted for three tube burners of internal diameter 8mm, 9.7mm and 12mm. Radial heat flux distributions were compared (experimentally) for different burner diameters under identical operating conditions (with firing rates of 0.25kW, 0.40kW and 0.50kW, ϕ=1 and H=40mm). An off-stagnation point peak in heat flux was observed for some of the configurations in the present study which is in accordance with the previous findings. This off-stagnation point peak is a function of stand-off distance between the exit plane of the burner and the plate and also the distance between flame-tip and the plate. A satisfactory explanation is presented to explain the existence of this off-stagnation peak with the help of results of numerical simulation carried out with commercial CFD code FLUENT. It is concluded that this off-stagnation peak in heat flux is primarily due to the peak in the axial velocity profile close to the impingement surface.

Flashback propensity of syngas fuels

This paper presents experimental measurements of the critical velocity gradient and flashback behavior of H 2–CO and H 2–CH 4 mixtures. Effects of H 2 concentration, external excitation, and swirl on the flashback behavior for flames of these fuel mixtures are discussed. For H 2 concentration burner and scaling studies the critical velocity gradient ( g F), defined as the ratio of the square of the laminar burning velocity to the thermal diffusivity of the mixture ( g F = S L 2 α ) , was used to quantify the flashback propensity of the flames. The critical velocity gradient of both H 2–CH 4 and H 2–CO flames changed nonlinearly with the increase in H 2 contents in the mixture. The critical velocity gradient ( g F) of 5–95% and 15–85% H 2–CO mixtures somewhat agreed with the scaling relation ( g F = c S L 2 α ) and yielded an average c value of 0.04. Similarly, values of a 25%H 2-75%CH 4 for different burner diameters were also fitted using the scaling relation yielding an average c value of 0.044. The g F values of 25–75% H 2–CO mixture showed non-linear variation with the S L 2 / α ratio (especially for S L 2 / α > 19 , 000 s - 1 ), and at a lower S L 2 / α ratios burner diameter had small effect on critical velocity gradient measurements. The opposite trend was observed for a 25–75% H 2–CH 4 mixture showing non-linear variation at a lower S L 2 / α ratios (for S L 2 / α > 5600 s - 1 ) and having less effect at higher S L 2 / α ratios. It was also determined that the effect of external excitation on the flashback propensity of H 2–CO flames with more than 5% H 2 was not significant. Flashback through two mechanisms and their dependence on combustor parameters were also identified for swirl stabilized H 2–CO flames.

Nozzle effect on heat transfer and CO emission of impinging premixed flames

Experiments were carried out to investigate the heat transfer and CO emission characteristics of a premixed LPG/air circular flame jet impinging upwards normal to a flat plate. The effects of nozzle diameter and nozzle arrangement on the heat transfer and CO emission under different fuel/air mixture flow rates ( Q mix), equivalence ratios ( Ф) and normalized nozzle-to-plate distances ( H/ d) were examined. For the effect of nozzle diameter, burners of nozzle diameters of d = 7.9, 9 and 10 mm were used, and for the effect of nozzle arrangement, a twin-nozzle burner and a triple-nozzle burner, each with a cross-sectional area equal to that of the 9 mm diameter burner, were investigated under different normalized jet-to-jet spacing, S/ d, of 3, 5 and 7. The heat transfer rate and CO emission index (EICO) are enhanced significantly with the decrease in the nozzle diameter for the single-nozzle flames. For the twin- and triple-nozzle flames, when the other operational conditions including Q mix, Ф and H/ d are invariant, the moderate S/ d of 5 gives the highest heat transfer rate, whereas the EICO increases with increasing S/ d. Comparison of the flames from all the burners shows that the highest heat transfer rate and EICO are obtained on the single-nozzle burner with the smallest nozzle diameter while the lowest heat transfer rate and EICO are obtained on the triple-nozzle burner with the smallest S/ d.

Effects of the Burner Diameter on the Flame Structure and Extinction Limit of Counterflow Non-Premixed Flames

Experiments and numerical simulations were conducted to investigate the effects of the burner diameter on the flame structure and extinction limit of counterflow non-premixed methane flames in normal gravity and microgravity. Experiments were performed for counterflow flames with a large inner diameter ( d) of 50 mm in normal gravity to compare the extinction limits with those obtained by previous studies where a small burner ( d < 25 mm) was used. Two-dimensional (2D) simulations were performed to clarify the flame structure and extinction limits of counterflow non-premixed flame with a three-step global reaction mechanism. One-dimensional (1D) simulations were also performed with the same three-step global reaction mechanism to provide reference data for the 2D simulation and experiment. For microgravity, the effect of the burner diameter on the flame location at the centerline was negligible at both high ( ag = 50 s−1) and low ( ag = 10 s−1) strain rates. However, a small burner flame ( d = 15 mm) in microgravity showed large differences in the maximum flame temperature and the flame size in radial direction compared to a large burner flame ( d = 50 mm) at low strain rate. In addition, for normal gravity, a small burner flame ( d = 23.4 mm) showed differences in the flame thickness, flame location, local strain rate, and maximum heat release rate compared to a large burner flame ( d = 50 mm) at low strain rate. Counterflow non-premixed flames with low and high strain rates that were established in a large burner were approximated by 1D simulation for normal gravity and microgravity. However, a counterflow non-premixed flame with a low strain rate in a small burner could not be approximated by 1D simulation for normal gravity due to buoyancy effects. The 2D simulations of the extinction limits correlated well with experiments for small and large burner flames. For microgravity, the extinction limit of a small burner flame ( d = 15 mm) was much lower than that of a large burner flame when ag ≤ 20 s−1. For normal gravity, the extinction limit of a small burner flame ( d = 23.4 mm) was also much lower than that of the large burner flame when ag ≤ 35 s−1. The effects of the burner diameter on the flame structure and extinction limit of counterflow non-premixed methane flames were more important in normal gravity than in microgravity.

マイクロ拡散火炎の消炎限界

This paper presents a theory to predict the extinction limit of jet diffusion microflames. The constant-density approximation is introduced to separate flow field from combustion reaction; flow field then depends only on the jet Reynolds number, Re. An exact solution of the Navier-Stokes equation is adopted; the predicted flame shape based on the exact solution well agrees with experimental observation. An Arrhenius-type one-step global reaction is considered, adding the Damköhler number, Da, as an additional parameter to the Reynolds number. The extinction limit is obtained using an activation-energy asymptotics technique. The obtained extinction criterion, Da Re2 ≈ constant, reproduces experimental results that the jet exit velocity under the extinction condition is proportional to d-2, where d is the burner diameter.

Realization and modeling of continuous spin detonation of a hydrogen-oxygen mixture in flow-type combustors. 1. Combustors of cylindrical annular geometry

A comprehensive numerical and experimental study of continuous spin detonation of a hydrogen-oxygen mixture in flow-type cylindrical annular combustors 4 and 10 cm in diameter is performed. Hydrogen is injected through injectors, and oxygen is supplied as a continuous flow through an annular slot. The flow structure is studied with variations of the flow rates of the components of the mixture and the width of the slot for oxygen supply. The region of existence of continuous spin detonation is determined as a function of the fuel-to-air equivalence ratio and specific flow rates of the components with variations of the relative width of the slot and combustor diameter. A two-dimensional unsteady gas-dynamic problem of rotation dynamics of a transverse detonation wave with geometric parameters of the combustor corresponding to those used in experiments is solved numerically. A comparison with experiments is performed, and reasonable agreement is reached for the detonation velocity and mean pressure in the combustor. It is shown that the geometric size of the transverse detonation waves is underestimated because the gas-dynamic model does not involve the mixing process, and the number of waves is almost doubled.

C-곡선상의 화염 소화 특성에 있어서 대향류 버너직경 효과

C-곡선상의 화염 소화 특성에 있어서 대향류 버너직경 효과

Characterization of wall temperature and radiation power through cylindrical dump micro-combustors

The micro-combustor (emitter) is a key component of the micro-thermophotovoltaic (TPV) system. An experimental study on the wall temperature and radiation power through the wall of a series of cylindrical dump micro-combustors was carried out. The effects of combustor diameter ( d), combustor length ( L), step height ( s), flow velocity ( u 0) and fuel–air equivalence ratio ( Φ) on the wall temperature distribution were investigated. ‘Emitter efficiency’ was defined and quantified based on the measured wall temperature. It was demonstrated that for this particular configuration, that is, a cylindrical micro-combustor with a backward-facing step, the two dimensionless ratios – L/ d and s/ d, are sufficient to determine the emitter efficiency, provided the flow velocity ( U e ) and Φ are known. Based on this result, the effects of the dimensionless step height ( s/ d) on the emitter efficiency were examined. It was shown that such a sudden flow expansion in the dump combustors does not favor the radiation through the outer wall. Finally, the position of the highest wall temperature and its variation with the flow Reynolds number were discussed. It was noted that the Reynolds number and the relative flow expansion ( s/ d) alone are inadequate to determine the relative position of the highest wall temperature.

Experimental and Numerical Investigation of Swirl Induced Self-Excited Instabilities at the Vicinity of an Airblast Nozzle

We report on the experimental and numerical investigation of swirl induced self-excited instabilities in the form of precessing helical structures at the vicinity of an Airblast Atomiser. Within the scope of this work, the increase of the knowledge of the fundamental factors governing the precessing vortex core phenomenon (PVC) (Gupta et al. 1984) by applying dual air-flow Airblast nozzles is aimed. This study concentrates on the experimental investigation of the impact of important parameters of a combustor system on the performance characteristics of this instability. In particular, in terms of this work the properties of the PVC are determined by applying two different Airblast Atomisers, one of them producing an attached swirl flame, and another producing a lifted swirl flame. Measurements are also performed for a confined and a non confined flame, aiming to determine the impact of the confining duct on the performance of the PVC. In order to gain some further knowledge, regarding the impact of this aerodynamic instability on combustion of gaseous fuel, the features of PVC are experimentally identified under reacting conditions, by employing a laser light sheet (LLS) measurement technique. By applying the LLS measurement technique, further investigation on the nature of the PVC is also attempted. The power spectral density (PSD) function of the flow field was determined on the basis of raw data provided by 3D Laser Doppler Anemometry (3D-LDA). In order to validate the measurement technique as well as the nature of the instability, the planar Mie-Scattering of the flow was evaluated by employing a high speed camera performing at 12 kHz. The precessing character of the flow was also confirmed by means of a numerical simulation using the 3D Reynolds Stress Model (3D-RSM). The results of the numerical investigation provided some useful information concerning the onset of the instability within the primary swirler as well as its size and amplitude. According to this analysis, a high frequency instability was confirmed within a region of about one burner diameter downstream of the burner exit. Finally an evaluation of the (LDA) method in terms of providing accurate (PSD) was performed for the case of a swirl flow field.

Study on premixed combustion in cylindrical micro combustors: Transient flame behavior and wall heat flux

The micro combustor is a key component of the micro thermophotovoltaic (TPV) system. Improving the wall temperature of the micro combustor is an effective way to elevate the system efficiency. An experimental study on the wall temperature and radiation heat flux of a series of cylindrical micro combustors (with a backward-facing step) was carried out. For the micro combustors with d = 2 mm, the regime of successful ignition (under the cold wall condition) was identified for different combustor lengths. Acoustic emission was detected for some cases and the emitted sound was recorded and analyzed. Under the steady-state condition, the effects of the combustor diameter ( d), combustor length ( L), flow velocity ( u 0) and fuel–air equivalence ratio ( Ф) on the wall temperature distribution were investigated by measuring the detailed wall temperature profiles. In the case that the micro combustor is working as an emitter, the optimum efficiency was found at Ф ≈ 0.8, independent of the combustor dimensions ( d and L) and the flow velocity. Under the experimental conditions employed in the present study, the positions of the peak wall temperature were found to be about 8–11 mm and 4–6 mm from the step for the d = 3 mm and d = 2 mm micro combustors, respectively, which are 8–11 and 8–12 times of their respective step heights. This result suggests that the backward-facing step employed in the combustor design is effective in stabilizing the flame position.

Investigations on the self-excited oscillations in a kerosene spray flame

A laboratory scale gas turbine type burner at atmospheric pressure and with air preheat was operated with aviation kerosene Jet-A1 injected from a pressure atomiser. Self-excited oscillations were observed and analysed to understand better the relationship between the spray and thermo-acoustic oscillations. The fluctuations of CH ∗ chemiluminescence measured simultaneously with the pressure were used to determine the flame transfer function. The Mie scattering technique was used to record spray fluctuations in reacting conditions with a high speed camera. Integrating the Mie intensity over the imaged region gave a temporal signal acquired simultaneously with pressure fluctuations and the transfer function between the light scattered from the spray and the velocity fluctuations in the plenum was evaluated. Phase Doppler anemometry was used for axial velocity and drop size measurements at different positions downstream the injection plane and for various operating conditions. Pressure spectra showed peaks at a frequency that changed with air mass flow rate. The peak for low air mass flow rate operation was at 220 Hz and was associated with a resonance of the supply plenum. At the same global equivalence ratio but at high air mass flow rates, the pressure spectrum peak was at 323 Hz, a combustion chamber resonant frequency. At low air flow rates, the spray fluctuation motion was pronounced and followed the frequency of the pressure oscillation. At high air flow rates, more effective evaporation resulted in a complete disappearance of droplets at an axial distance of about 1/3 burner diameters from the injection plane, leading to a different flame transfer function and frequency of the self-excited oscillation. The results highlight the sensitivity of the self-excited oscillation to the degree of mixing achieved before the main recirculation zone.

Ignition of turbulent swirling n-heptane spray flames using single and multiple sparks

This paper examines ignition processes of an n-heptane spray in a flow typical of a liquid-fuelled burner. The spray is created by a hollow-cone pressure atomiser placed in the centre of a bluff body, around which swirling air induces a strong recirculation zone. Ignition was achieved by single small sparks of short duration (2 mm; 0.5 ms), located at various places inside the flow so as to identify the most ignitable regions, or larger sparks of longer duration (5 mm; 8 ms) repeated at 100 Hz, located close to the combustion chamber enclosure so as to mimic the placement and characteristics of a gas turbine combustor surface igniter. The air and droplet velocities, the droplet diameter, and the total (i.e. liquid plus vapour) equivalence ratio were measured in inert flow by phase Doppler anemometry and sampling respectively. Fast camera imaging suggested that successful ignition events were associated with flamelets that propagated back towards the spray nozzle. Measurements of ignition probability with the single spark showed that localised ignition inside the spray is more likely to result in successful flame establishment when the spark is located in a region of negative velocity, relatively small droplet Sauter mean diameter, and mean equivalence ratio within the flammability limits. Ignition with the single spark was not possible at the location where the multiple spark experiments were performed. For those, the multiple spark sequence lasted approximately 1 to 5 s. It was found that a long spark sequence increases the ignition efficiency, which reached a maximum of 100% at the axial distance where the recirculation zone had maximum width. Ignition was not feasible with the spark downstream of about two burner diameters. Visualisation showed that small flame kernels emanate very often from the spark, which can be stretched as far as 20 mm from the electrodes by the turbulent velocity fluctuations. These kernels survive very little time. Successful overall ignition occurs at a random time from the spark initiation and, as in the case of the single spark, success is associated with kernels that move without getting extinguished towards the bluff body. The results demonstrate that the energy deposited by multiple sparks and spark stretching in a turbulent flow can have a spatially far-reaching effect to initiate combustion.

A novel meso-scale combustion system for operation with liquid fuels

A novel meso-scale combustion concept utilizing a flow-blurring injector to produce fine fuel droplets, a counter-flow heat exchanger to preheat reactants using product gas enthalpy, and porous inert medium to homogenize reactants and stabilize the flame is presented. The overall system is 30 mm long and 17 mm in diameter, with combustor diameter of 10 mm. For atmospheric pressure operation on kerosene fuel, the combustion system achieved heat release rate of up to 460 W, pertaining to energy density of 90 MW/m 3 based on the total volume and 230 MW/m 3 based on the combustor volume. The combustor performance was determined from surface temperatures measured by an infra-red camera, product gas temperature measured by an R-type thermocouple probe, and carbon monoxide (CO) and nitric oxide (NO x ) concentrations measured by a miniature sampling probe attached to gas analyzers. A comprehensive Computational Fluid Dynamics (CFD) model incorporating conjugate heat transfer, radiation heat exchange, flow and heat transfer in PIM, and heat release by combustion was developed and validated in this study. The CFD model accurately predicted the thermal performance of the combustion system. A 1D, burner-stabilized flame model incorporating detailed chemical kinetics was used to predict the flame structure. Model predictions are compared with measured CO and NO x concentrations, and subsequently, analyzed to identify the key reactions affecting the NO x production.

Extinction of laminar jet diffusion microflames

This paper presents a theory to predict the extinction limit of laminar jet diffusion microflames, defined as flames established on submillimeter-diameter burners. The classical Burke–Schumann (BS) theory is first extended to include the effect of one-step, finite-rate chemistry. Then, a theory of diffusion-flame extinction is applied using activation-energy asymptotics to predict the extinction limit. The present theory correctly reproduces experimental observations, i.e., u L ∼ d −2, where u L is the fuel jet velocity at the extinction limit (lower limit) and d the burner diameter. According to the BS theory, the gradient of mixture fraction at the flame-sheet location is infinite at the burner rim, and it decreases with increasing axial distance to the minimal value at the flame tip. Therefore, local extinction initiates at the burner rim, and extinction occurs when the mixture-fraction gradient at the flame tip is greater than a critical value. This view of microflame extinction is supported by the results of experiments and numerical simulations. It is found that the present theory can be applied for various types of fuels, those are, methane, propane, and butane.

Experimental Validation of Froude-Modeling-Based Physical Scaling of Water Mist Cooling of Enclosure Fires

Fire suppression experiments were conducted in two geometrically similar enclosures of 3-to-1 ratio under fire products collectors to quantify water mist cooling of propane fires, to evaluate the scaling applicability of Froude modeling for water mist cooling of enclosure fires. The parameters considered in the evaluation were: enclosure size, door opening size, water mist spray condition, fire size, fire location and fireshielding condition. The two enclosures measured 1.22x1.22x1.22 m high and 3.66x3.66x3.66 m high. Two door opening sizes were tested for each enclosure: 0.30x0.61 m high and 0.61x0.61 m high for the Scale-1 enclosure and 0.91x1.83 m high and 1.83x1.83 m high for the Scale-3 enclosure. Propane fires were established on 0.3-m and 0.91-m diameter burners respectively for the two scaled enclosures. Four corresponding pairs of propane supply rates were selected according to the scaling requirement, producing 10 to 50 kW at Scale-1 and 150 to 800 kW at Scale-3. Two water mist nozzles operating respectively at 13.8 and 43.7 bar were used to produce the water mist sprays in the two enclosures. In each enclosure, nine nozzles were arranged in a 3x3 pattern at the ceiling level with equal nozzle-to-nozzle and nozzle-to-wall spacing. The results show that Froude modeling can be used to scale water mist cooling of enclosure fires under the present experimental conditions.

An experimental and numerical study of stagnation point heat transfer for methane/air laminar flame impinging on a flat surface

A combined experimental and numerical study has been conducted to determine the stagnation point heat transfer for laminar methane/air flame impinging on a flat surface. Effects of Reynolds number, equivalence ratio and burner diameter on stagnation point heat flux were examined experimentally at different separation heights. Maximum stagnation point heat flux was obtained when the flat surface was closest to the tip of the inner premixed reaction zone. Heat flux decreased along the axial direction when the separation distance was further increased from the tip of inner reaction zone. There was a secondary rise in heat flux at the stagnation point at larger separation distances. Correlations were developed for stagnation point Nusselt number. Numerical simulations were carried out using a commercial CFD code (FLUENT) for laminar methane/air flame impinging on a flat surface for various separation distances. Results were compared with those found experimentally. The reason for conducting the simulations was to (a) gain more insight into how the presence of the plate affects the flame and the flow and temperature fields and (b) to explain the reason for high heat flux when the tip of the inner reaction zone was very close to the stagnation point.

Heat transfer characteristics of laminar methane/air flame impinging normal to a cylindrical surface

An experimental study has been conducted to determine the heat transfer characteristics of methane/air laminar flames impinging normal to a cylindrical surface. Effects of variations in the values of Reynolds number ( Re = 600–1300), equivalence ratio ( ϕ = 0.8–1.3), dimensionless separation distance ( H/ d = 1–5), and burner diameter to cylinder diameter ratio ( d/ D = 0.0538–0.1076) have been investigated. Three important configurations, viz., flame inner reaction zone far away, just touching and intercepted by the impingement surface, were examined in detail. High stagnation point heat fluxes were obtained when tip of the flame inner reaction zone just touched the target surface. Stagnation point heat fluxes were either zero or negative when the inner reaction zone was intercepted by the impingement surface. An off-stagnation peak in heat flux was obtained at moderate separation distances above the flame tip. Both stagnation point and peak heat fluxes increased with Re when the inner reaction zone length was less than the separation distance. Heat fluxes in the wall-jet region were high at high Re. Maximum heat fluxes were obtained for initially fuel-rich mixture conditions due to entrainment of the surrounding air. Smaller burner diameters produced high heat flux at the stagnation region for fixed Reynolds number and opposite trends were seen in the wall-jet region. A secondary rise in stagnation point heat flux was obtained at larger separation distances. This secondary rise in heat flux was quite significant for larger burner diameters and at low flow rates. Correlations were developed for stagnation point heat flux. Results were also compared with flat plate under identical operating conditions.

UNSTEADY MOTION OF POOL FIRE ON SMALL-SCALE BURNER

The flame motion of a pool fire on a small-scale burner has been experimentally investigated from the viewpoint of nonlinear dynamics, focusing on the relationship between the flame motion behavior and increasing burner diameter d. Hexane ( C 6 H 14) was used as a liquid fuel in the present study. For small burner diameters of up to d ≈ 10 mm , a stable conical flame was observed. The flame tip of the stable flame began to oscillate at low frequency (approximately 10 Hz) due to the buoyancy-driven hydrodynamic instability when d exceeded 13 mm. With further increase in d, the flame tip oscillation began to exhibit an interesting oscillation mode. These flame motions can be shown quantitatively by drawing an attractor, and evaluated by estimating the correlation dimension Dc. For d ≤ 10 mm , the attractor was a fixed point and Dc was approximately zero. When d reached 13 mm, the attractor became a limit cycle and Dc was estimated to be approximately unity, indicating a periodic motion. With larger burner diameters of up to d ≈ 50 mm , the trajectories of the attractor seemed to be rolled up and Dc approached approximately 2, indicating a quasi-periodic motion. These results indicate that the flame motion of the small-scale pool fire switches from stable to quasi-periodic, throughout periodic with increasing the burner diameter. The present results also show that a nonlinear analysis based on deterministic chaos theory, such as that using the attractor and the correlation dimension, would be a valid method by which to discuss the flame instability issue of the pool fire.

RESPONSES OF A LIFTED NON-PREMIXED FLAME TO ACOUSTIC FORCING. PART 2

The interaction between non-premixed lifted flames and the free jet structures is of paramount importance in the stabilization of combustion processes. So, by organizing or disorganizing the jet structures, acoustics must also act on flames by its ability either to amplify the flames' intrinsic drawbacks, or to change and improve the combustion regime. An earlier systematic study, carried out by the authors, concerns the responses of a non-premixed flame, lifted in its hysteresis zone, to sine-shaped forcing in a wide range of frequencies and amplitudes, under fixed conditions of burner diameter and flow rate. The present work is aimed at verifying the robustness of the physical mechanisms involved over a wider range of conditions. These conditions are chosen in a range of moderate Reynolds numbers (3000–6000) so that the flame stabilizes in the hysteresis zone. The influence of imposed oscillations on the flame is summarized in charts as a function of the forcing frequency and the fluctuation amplitude. Results show that the phenomena presented previously are still observed, but shifts of the response domain limits are noticed. Some objective global criteria are proposed to explain these shifts. A particular behavior (the “split-flame” described by Hertzberg et al.) is only observed for the case of small diameters.

Experimental studies of flame stability limits of CNG–air premixed flame

Abstract The stability aspects of laminar premixed CNG–air flames are investigated experimentally. Bunsen burners with two different port diameters, such as 12 and 15 mm, are employed to characterize the blowoff and flashback limits of the CNG–air premixed flames. For the two burners, the peak flashback limits occurs at a fuel–air ratio slightly richer than stoichiometric. However, the flashback limits are enhanced with increase in diameter of the burner, but the blowoff limit increases with increase in fuel concentration. Similar trends in increasing blowoff limits are obtained with further increase in burner diameter. The stability plots for burners with two different port diameters have been established, which can be used for designing and developing CNG/air combustion systems.