Flow, thermal, and emission characteristics of turbulent nonpremixed CH 4 flames were investigated for three burner heads of different cone heights. The fuel velocity was kept constant at 15 m/s, while the coflow air speed was varied between 0 and 7.4 m/s. Detailed radial profiles of the velocity and temperature were obtained in the bluff body wake at three vertical locations of 0.5D, 1D, and 1.5D. Emissions of CO 2 , CO, NO x, and O 2 were also measured at the tail end of every flame. Flames were digitally photographed to support the point measurements with the visual observations. Fifteen different stability points were examined, which were the results of three bluff body variants and five coflow velocities. The results show that a blue-colored ring flame is formed, especially at high coflow velocities. The results also illustrate that depending on the mixing at the bluff-body wake, the flames exhibit two modes of combustion regimes, namely fuel jet- and coflow-dominated flames. In the jet-dominated regime, the flames become longer when compared with the flames of the coflow-dominated regime. In the latter regime, emissions were largely reduced due to the dilution by the excess air, which also surpasses their production.

Liquefied petroleum gas (LPG) is widely used as a cooking fuel as it has higher energy content and produces lower emissions compared to other traditional fuels. Due to massive demand for LPG, aside from its limited reserve, performance improvement of the LPG cook-stoves is essential. In the present work, the thermal efficiency of a traditional cook stove has been studied both experimentally and numerically for LPG fuel. Based on the knowledge from the computational model concerning flow field and species transport parameters, the conventional cook-stove design has been modified for improving the efficiency. In the modified design of the stove, attachment of an annular metal plate insert and introduction of an extended spill-tray to close the gap around the burner are considered. The modifications result in favourable guidance of the flow of secondary air and hot product gases of combustion to ensure better heat transfer rate to the loading vessel. The thermal efficiency of the modified cook-stove is around 73.6%, which is about 4.7 percentage point improvement from that of an identical stove without the insert and extended spill-try.

Swirling impinging flame jets possess the potential of uniformly heating the target surface. Numerical simulations have been carried out for multiple reacting swirling flows arranged in square 3 × 3 array. The turbulence chemistry interactions in the flow field are modeled using RNG based k-ε/ eddy dissipation model (EDM) framework. Co-and-counterrotating configurations have been compared for interactions and spent gas release mechanism at fixed inter-jet spacing and separation distance. Multiple swirling impinging flames undergo strong interactions resulting in distortions of recirculation zones (RCZs) for all the surrounding but central flame. Co-swirling flames result in development of higher turbulence in the interaction regions as compared with counter-swirl case. Tilted heat flux contours have been noticed for co-swirling impinging flames, whereas heat flux impressions are straight for counter-swirl case. Effect of inter-jet spacing (C/D h = 5, 7, and 9) and separation distance (H/D h = 3, 5, 7, and 9) studied for co-swirl case revealed that peak heat fluxes decreased with increasing inter-jet spacing and separation distance. With increase in inter-jet spacing, the width of central flame increased due to increased suppression effect induced by the recirculating gases. Increase in separation distance resulted in decrease of width of central flame and thermal dilution taking place led to decrease in heat flux magnitudes.

To guide ultra-compact combustor (UCC) engineering, simulations were conducted about turbulent premixed combustion in a high acceleration field which is called high-g combustion, along with a detailed investigation on the evolution of turbulent premixed flame in a rotating tube of stoichiometric propane-air. The rotation of the tube was mimicked by modified momentum source term in the unsteady 2D simulations to decouple the centrifugal force and the Coriolis force, the latter of which was usually neglected in previous reports. A good agreement was found between the simulation result and experimental data, along with a discovery of the phenomenon that flame speed was accelerated by the imposed acceleration field. Further study indicated that the flame acceleration phenomenon can be attributed to the flame corrugation induced by the Rayleigh–Taylor instability (RTI). The Coriolis force was found to be non-negligible in high-g combustion since the Coriolis acceleration could be at the same magnitude as the centrifugal acceleration, and the observed flame speed was nearly 20% lower without the Coriolis force. The current study revealed that the high-g combustion in an open chamber due to the absence of pressure wave/flame front interaction could not be fully compatible with predictions derived from closed chamber experiments and that the Coriolis force could not be ignored in the high-g combustion process.

Liquefied petroleum gas (LPG) is widely used in domestic cookstoves as it is a clean and high energy content fuel in comparison with other traditional cooking fuels. With the increasing demand of LPG, study and improvement of cookstove performance have become an important subject. In the present work, a numerical study of the flow and thermal fields for a domestic cookstove burner has been investigated and the performance of the stove is analyzed at different parametric conditions, like the equivalence ratio of the primary fuel–air mixture, fuel flow rate, thermal load height, and loading vessel size. The maximum thermal efficiency has been found for an equivalence ratio of 1.4 for the LPG–air mixture and at load height of 20 mm. The heat flux distribution at the bottom of the vessel is found to be bimodal with the higher peak occurring closer to the center of the vessel. The thermal efficiency of the stove increases with the rise in the fuel flow rate, and it decreases with reducing cooking vessel diameter. As the vessel diameter increases, the fraction of the total heat supplied through the vessel bottom increases. The radiative component of the heat flux is found to be much smaller compared to the convective component.

Non-premixed combustion was implemented in a micro-lobed combustion system, and its influence on combustion was studied using both experiments and simulations. The results show that a micro-lobed burner produces streamwise vortices with intensities that increase with the equivalence ratio of methane to oxygen (Φ). Due to the streamwise vortices and the increment of the contact area between methane and oxygen, the gasses mix well in the micro-lobed burner, giving it a larger OH mass fraction and higher temperatures than the micro-splitter burner. Moreover, the equivalence ratio greatly influences the combustion enhancement from the micro-lobed burner, especially near the burner exit. The maximum temperature difference between the two micro-burners at the Z/D = 0.01 cross section is 171 K, when Φ is 0.6. However, when the mixing enhancement caused by the streamwise vortices disappears, Φ has little influence on the combustion temperature of the micro-lobed burner, especially when Φ ≥ 1. In this case, the maximum temperature variation between the micro-lobed burner and micro-splitter burner remains nearly constant.

The effects of pulsation intensities on the flame characteristics of a 10 deg-backward-inclined jet flame in the crossflow were investigated in a wind tunnel. The jet and the crossflow Reynolds numbers were 1527 and 2165, respectively. The jet-to-crossflow momentum flux ratio was 0.10. A loudspeaker was used to acoustically excite the jet flame. The excitation Strouhal number was 0.73, while the jet pulsation intensities varied from 0 to 1.26. The flame behaviors were studied through photography techniques. The flame temperatures were measured using a fine-wire R-type thermocouple. The combustion-induced emissions were probed by a commercial multi-gas analyzer. The jet flames were categorized into five characteristic modes with increasing pulsation intensities. Mode I was characterized by a yellowish down-washed recirculation flame, a blue neck flame, and a yellow tail flame. Modes II and III featured a split yellow tail flame, a yellowish recirculation flame, and a blue neck flame. Mode IV was characterized by a blue down-washed recirculation flame and neck flame, as well as a split yellow tail flame. Mode V was identified by a single yellow tail flame and the absence of the down-washed recirculation flame. When the jet flames were excited beyond mode I, the combustion-induced pollutants of carbon monoxide and nitric oxide were significantly reduced. However, the excited jet flame in mode V displayed low temperatures in the near-tube region.

We have developed the novel oxygen-enriched burner using the self-induced oscillating phenomenon of jet flow which has excellent radiation heat transfer characteristics. We revealed that the flame property suitable for radiation heat transfer is obtained by extending the oscillation cycle. By optimizing the connecting tube length and diameter, we got the stable oscillating flame in 1 s-oscillation period. At this time, the relationship between the burner structure including the connecting tube and the oscillation period can be easily evaluated by the dimensionless tube length and Strouhal number. In addition, combining multi-staged combustion technology, the burner can improve heat transfer efficiency by 9.1% compared with the conventional oxygen-enriched burner. We confirmed that the burner has a low NOx emission level below 55 ppm.

The horizontally oriented jet flame induced by rectangular source impinging upon the opposite wall is actually common in the chemical industry, but the related studies are limited. In this paper, the computational fluid dynamics codes are carried out to investigate the temperature profile in thermal impinging flow of the horizontally oriented methane jet flame with rectangular source, which the rectangular orifice is 400 mm 2 with three different aspect ratios (L/W = 1, 2, 4); besides, the jet velocities vary from 27.5 m/s to 125 m/s. As the horizontally oriented methane jet flame impinges on the vertical plate in front of the fuel orifice directly, the vertical temperature along the opposite plate is focused on. Results show that the temperature near the impingement point is the same for different jet velocities, but the temperature along the vertical direction is larger with increasing fuel jet velocity. Moreover, the orifice aspect ratio has a significant effect on the temperature, which increases with the aspect ratio at a given position for the momentum-controlled flame. The effective heat release rate on the basis of unburned fuel and ellipse flame shape hypothesis is put forward to correlate the temperature profile. Finally, a new correlation to illustrate the vertical temperature rising along the opposite plate is proposed in light of the orifice aspect ratio and fuel jet velocity, and the predictions obtained by the proposed model agree well with the numerical results, which is applicable for the horizontally oriented flame with rectangular source impinging upon the opposite wall.

The effects of acoustic excitation at resonance on the flame appearances, flame lengths, flame temperatures, and combustion product concentrations of combusting swirling dual-disk double-concentric jets were studied. The Reynolds number of the annular swirling air jet was varied, while it was fixed at 2500 for the central propane jet. The central fuel jet was acoustically forced by a loudspeaker, which was installed using downstream longitudinal irradiation. The central jet pulsation intensities were measured by a calibrated, one-component hot-wire anemometer. The instantaneous full-length and close-up flame images were captured to identify the characteristic flame modes. Long-exposure flame images were taken to measure the flame lengths. The axial and radial temperature distributions of flames were measured using a homemade, fine-wire R-type thermocouple. The concentrations of combustion products were measured by a gas analyzer. Four characteristic flame modes, blue-base wrinkled flame, yellow-base anchored flame, blue-base anchored flame, and lifted flame, were observed in the domain of central jet pulsation intensity and annular swirling jet Reynolds number. The lifted flame, which was formed at large central jet pulsation intensities, presented characteristics of a premixed flame due to significant mixing induced by violent, turbulent flow motions. It was short and stable, with high combustion efficiency and low toxic emissions, when compared with the unexcited flame and other excited characteristic flame modes, which presented characteristics of diffusion flame.

The aim of this paper was to examine the response of twin premixed flames formed in a counterflow configuration to the presence of an unsteady straining flow. We began by describing the problem mathematically using the thermodiffusive model with constant density and then adopted a finite elements approach to solve the problem numerically. The study has shown that the role of flow on flame propagation is determined by three main parameters, namely, flow amplitude A, strain rate ε, and fuel Lewis number Le F . For Le F ≥ 1, the flow is seen to promote flame extinction, while Le F < 1 the flow clearly enhances the flame reactivity. Qualitatively, it has been shown that for Le F = 1, there exists a critical value of A (that varies with ε) below which the reactivity decreases monotonically with A. For small Le F < 1, on the other hand, the reactivity was seen to increase with A. For Le F > 1, however, a nonmonotonic dependence, especially for small ε, is predicted.

The dry-low-NO x (DLN) micromix combustion technology has been developed originally as a low emission alternative for industrial gas turbine combustors fueled with hydrogen. Currently, the ongoing research process targets flexible fuel operation with hydrogen and syngas fuel. The nonpremixed combustion process features jet-in-crossflow-mixing of fuel and oxidizer and combustion through multiple miniaturized flames. The miniaturization of the flames leads to a significant reduction of NO x emissions due to the very short residence time of reactants in the flame. The paper presents the results of a numerical and experimental combustor test campaign. It is conducted as part of an integration study for a dual-fuel (H 2 and H 2 /CO 90/10 vol %) micromix (MMX) combustion chamber prototype for application under full scale, pressurized gas turbine conditions in the auxiliary power unit Honeywell Garrett GTCP 36-300. In the presented experimental studies, the integration-optimized dual-fuel MMX combustor geometry is tested at atmospheric pressure over a range of gas turbine operating conditions with hydrogen and syngas fuel. The experimental investigations are supported by numerical combustion and flow simulations. For validation, the results of experimental exhaust gas analyses are applied. Despite the significantly differing fuel characteristics between pure hydrogen and hydrogen-rich syngas, the evaluated dual-fuel MMX prototype shows a significant low NO x performance and high combustion efficiency. The combustor features an increased energy density that benefits manufacturing complexity and costs.

We study in this paper the combined effect of heat loss and reversibility on the propagation of planar flames formed within the counterflow configuration. The problem has been formulated first using the thermodiffusive model with constant density and then solved numerically using finite elements. The impact of four main parameters, namely the reversibility r , the heat loss κ , the strain rate ε , and the activation energy β , on the propagation of planar flames has been discussed in details. The study has shown that planar flames under reversible conditions behave qualitatively similar to those observed for irreversible reactions, which agree with the asymptotic findings. In the presence of heat loss, the problem exhibits multiplicity of solutions whose number and stability were found to vary according to the strain rate ε . In addition, the study has predicted the existence of a certain value of the reversibility parameter r beyond which the impact of reversibility becomes negligible. Finally, we have examined the stability of the solutions and determined the domain of stability of solutions and their multiplicity for this problem.

The flame behavior and the thermal structure of gaseous fuel jets issued from rectangular nozzles of high and low aspect ratios with co-flowing air were experimentally studied. Two rectangular nozzles with aspect ratios AR = 36 and 3.27 and with side channels for co-flowing air were examined. Flame behaviors were studied by photography techniques. Flame temperatures were measured using a fine-wire thermocouple. The AR = 36 burner exhibited three characteristic flame modes: attached flame, transitional flame, and lifted flame. The AR = 3.27 burner presented three characteristic flame modes: diffusion flame, transitional flame, and triple-layered flame. High AR jets promoted entrainment and mixing in the region around the flame base, whereas low AR jets enhanced mixing in the regions along the flame edges. At low co-flows, at Re c < 1200, the low AR burner flames were shorter, but at Re c > 1200, the high AR burner flames became shorter and wider. At Re c > 950, the high AR burner recorded higher flame temperatures, compared to the low AR burner by over 100 °C. At high fuel jet Reynolds numbers and moderate co-flow, high AR burner flames presented better combustion performances when compared to low AR jet flames. The good combustion performance of the high AR jet flames was due to enhanced entrainment and mixing, which were induced by flame lifting. However, at low Re c and high co-flow, the low AR jet flames exhibited desirable flame characteristics due to improved entrainment and turbulence at the jet interfaces.

This paper aims to identify performance improvements in cooker-top gas burners for changes in its original geometry, with aspect ratios (ARs) ranging from 0.25 to 0.56 and from 0.28 to 0.64. It operates on liquefied petroleum gas (LPG) and five thermal power (TP) levels. Considering the large number of cooker-top burners currently being used, even slight improvements in thermal performance resulting from a better design and recommended operating condition will lead to a significant reduction of energy consumption and costs. Appropriate instrumentation was used to carry out the measurements and methodology applied was based on regulations from INMETRO (CONPET program for energy conversion efficiency in cook top and kilns), ABNT (Brazilian Technical Standards Normative) and ANP—National Agency of Petroleum, Natural Gas (NG) and Biofuels. The results allow subsidizing recommendations to minimum energy performance standards (MEPS) for residential use, providing also higher energy conversion efficiency and/or lower fuel consumption. Main conclusions are: (i) Smaller aspect ratios result in the same heating capacity and higher efficiency; (ii) higher aspect ratios (original burners) are fuel consuming and inefficient; (iii) operating conditions set on intermediate are lower fuel consumption without significant differences in temperature increases; (iv) Reynolds number lower than 500 provides higher efficiencies.

In this paper, we provide a numerical study of the stability analysis of a planar premixed flame. The interaction of preferential diffusion and heat loss for a planar premixed flame is investigated using a thermodiffusive (constant density) model. The flame is studied as a function of three nondimensional parameters, namely, Damköhler number (ratio of diffusion time to chemical time), Lewis number (ratio of thermal to species diffusivity), and heat loss. A maximum of four solutions are identified in some cases, two of which are stable. The behavior of the eigenvalues of the linearized system of stabilty is also discussed. For low Lewis number, the heat loss plays a major role in stabilizing the flame for some moderately high values of Damköhler number. The results show the effect of increasing or decreasing Lewis number on adiabatic and nonadiabatic flames temperature and reaction rate as well as the range of heat loss at which flames can survive.

The effect of the electric field on laminar nonpremixed counterflow propane flames was analyzed computationally. The computations were conducted using ANSYS fluent platform associated with a detailed kinetic mechanism. The mechanism was supplemented with a set of three reactions accounting for the consumption/production of three chemi-ions. It was established that the position of the flame could be only controlled through altering the intensity of the applied electric field. The effect of the applied electric field was included within the reactive flow equations via introducing two distinct terms: a body force term that accounts for the electric field effects on the momentum of the reactive mixture, and an extra diffusion term that accounts for the mobility charged species, namely ambipolar diffusion. This study clearly shows that electric force provides a potential for controlling the location of propane flames without affecting their structure.

We investigate numerically the effect of heat loss and strain rate on the premixed flame edges encountered in a two-dimensional counterflow configuration for Lewis number higher than one. Under nonadiabatic conditions, multiple flame edges and multiple propagation speeds (positive and negative) are discussed. Different regions of multiple propagation speeds have been revealed ranging from two to four, depending on the value of the heat loss parameter and Damkohler number, which is inversely proportional to the strain rate. A combustion wave is modeled by connecting a strongly burning flame on one side of the burner to a weakly burning flame on the other side. These combustion waves are changing with increasing Dam number into flame edges with the fact that the strongly burning flame is the dominant.

Combined with the use of renewable energy sources for its production, hydrogen represents a possible alternative gas turbine fuel for future low-emission power generation. Due to the difference in the physical properties of hydrogen compared to other fuels such as natural gas, well-established gas turbine combustion systems cannot be directly applied to dry low NO x (DLN) hydrogen combustion. The DLN micromix combustion of hydrogen has been under development for many years, since it has the promise to significantly reduce NO x emissions. This combustion principle for air-breathing engines is based on crossflow mixing of air and gaseous hydrogen. Air and hydrogen react in multiple miniaturized diffusion-type flames with an inherent safety against flashback and with low NO x emissions due to a very short residence time of the reactants in the flame region. The paper presents an advanced DLN micromix hydrogen application. The experimental and numerical study shows a combustor configuration with a significantly reduced number of enlarged fuel injectors with high-thermal power output at constant energy density. Larger fuel injectors reduce manufacturing costs, are more robust and less sensitive to fuel contamination and blockage in industrial environments. The experimental and numerical results confirm the successful application of high-energy injectors, while the DLN micromix characteristics of the design point, under part-load conditions, and under off-design operation are maintained. Atmospheric test rig data on NO x emissions, optical flame-structure, and combustor material temperatures are compared to numerical simulations and show good agreement. The impact of the applied scaling and design laws on the miniaturized micromix flamelets is particularly investigated numerically for the resulting flow field, the flame-structure, and NO x formation.

At present, unless a boiler is especially designed to burn biomass, the levels of co-firing are generally limited to around 5% by mass. Higher levels of substitution sometimes lead to burner instability and other issues. In order to co-fire higher concentrations of biomass, a technique is required which can monitor flame stability at the burner level and optimize the combustion to ensure that local NO x is maintained below set limits. This paper presents an investigation of a system that monitored the combustion flame using photodiodes with responses in the ultraviolet (UV), infrared (IR), and visible (VIS) bands. The collected data were then processed using the Wigner–Ville joint time–frequency method and subsequently classified using a self-organizing map (SOM). It was found that it was possible to relate the classification of the sensor data to operational parameters, such as the burner airflow rate and NO x emissions. The developed system was successfully tested at pilot scale (500 kW t ), where the ability of the system to optimize the combustion for a variety of unseen coal/biomass blends was demonstrated.