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Flame brush thickness is a significant parameter that demonstrates the evolution of turbulent premixed flame. In this paper, the flame height and flame brush thickness of lean turbulent premixed Bunsen CH4/air and C3H8/air flames were investigated under low turbulence intensities. The turbulence flow field is generated and controlled by changing the outlet velocity and perforated plates, and it is measured using a hot wire anemometry system. The turbulent flame fronts are detected by OH-PLIF technique. Results show that under fuel-lean conditions, the characteristic turbulent Bunsen flame height and centerline flame brush thickness decrease with increasing equivalence ratio, while they increase with the augment of outlet velocity. Turbulence intensity has marginal effect on these two parameters. The variations of flame brush thickness indicate that there are three regions during the whole development of turbulent Bunsen flames under low turbulence intensity. The first is the turbulence dominating region, where the variations of horizontal flame brush thickness follow the turbulence diffusion theory. The second is the non-local effect manifesting region, where the equivalence ratio, which controls the flame instability wavelength, influences the horizontal flame brush thickness. The third is the flame tip region where the flamelets intersect and make the radial wrinkles offset while the axial wrinkles superimpose. This unique tip region of turbulent Bunsen flame can be demonstrated by the linear relationship between its flame height and centerline flame brush thickness. Turbulent premixed Bunsen flames suffer different non-local effects during the flame development to downstream. To minimize the non-local effect on the statistics of turbulent flame front structure, a new method based on the development time period is proposed. Compared with statistical analysis based on the whole development time, this method can better demonstrate the flame-turbulence interaction.

There is little knowledge about the inhibition efficiency of dimethyl methylphosphonate (DMMP) against turbulent flames, in which vortices of different sizes corrugate and stretch the flame front. This study is to investigate the performance and the mechanism of DMMP in inhibiting turbulent methane/air premixed Bunsen flames. Several important parameters are chosen to characterize the combustion of the flames, including flame brush height, flame brush thickness and turbulent burning velocity. It is found that the flame brush expands both horizontally and vertically with the increase of turbulent intensity. For all the flames, the centerline mean flame brush thickness increases linearly with flame height. Turbulent burning velocity decreases monotonically with the increasing of DMMP addition. However, the normalized turbulent burning velocity increases with the normalized turbulent intensity. Compared with the performance in laminar premixed flames, the inhibition efficiency depends on the turbulent intensity. For low-intensity turbulent flames, the inhibition efficiency is higher than that in laminar premixed flames, but the relative inhibition efficiency decreases with DMMP addition. This can be attributed to the relative size of the vortices and the flame front thickness: when the Kolmogorov length scale is less than the preheat zone thickness but greater than the reaction zone thickness, vortices reduce the burning velocity; when the Kolmogorov length scale is less than the reaction zone thickness, vortices facilitate the combustion. The trend is opposite in high-intensity turbulent flames, which is mainly attributed to the generating of the flame pockets in the flame tips.

This paper presents an experimental study of free flame patterns, impinging flame structures and flame impingement heat transfer characteristics of a partially premixed flame. The partially premixed flame is produced on a double-concentric-pipe burner, with both flow strength and mixedness of its inner and outer jets controlled by digital flowmeters. The aim of the study is, through examination of the effects of inner and outer jet Reynolds numbers (Rei, Reo), equivalence ratios (Φi, Φo) and nozzle-to-plate distance (H), to understand the heat transfer behaviors of such partially premixed flame and further identify the best impinging flame to achieve higher energy usage efficiency. A total of twelve different free flame patterns are observed by varying Rei-Reo-Φi-Φo, and W-shaped flame is the most representative flame under the conditions tested. The partially premixed dual flame is higher in stability than any single component of it, i.e. Bunsen flame or annulus flame. Further, leaner flammability limit is achieved by the dual flame due to mutual interaction which effectively supports leaner inner flame. Both inner and outer flames influence the radial heat flux distribution of W flame, while the contribution of inner flame becomes significant only when the difference in two flame heights is large. The outer flame globally dominates the total heat transfer rate, leading to general decay with increasing H. Comparison of W flame with single Bunsen and annulus flame shows that W flame has the favorable structure for higher energy efficiency due to double-pipe configuration and mutual flame interaction.

Effects of N2 dilution on laminar burning velocity (LBV), adiabatic flame temperature (AFT), intermediate radicals (CH3, H) and NOx emissions (NO, N2O) of methane were investigated using Bunsen burner under the condition of fuel-rich combustion (equivalence ratio 1.0 to 1.4). The Aramco mechanism was selected to analyze the effect of N2 blending on the adiabatic flame temperature and the intermediate radicals of CH4 premixed flame under the equivalence ratios (Φ = 0.8, 1.0, 1.2). The effect of different N2 doping ratios on the NOx emissions (NO, N2O) of methane was analyzed by using GRI 3.0 mechanism. The experimental and numerical results showed that as the equivalence ratio increased (from 1.0 to 1.4), the laminar burning velocity first increased and then decreased, reaching a maximum when the equivalence ratio was about 1.05. The mole fraction changes of intermediate radicals and NOx emissions also showed a trend of increasing first and then decreasing with the increasing of equivalence ratios. Under the same equivalence ratio, the mole fraction of intermediate radicals and NOx decreased as the N2 doping ratio increased. The sensitivity analysis showed that the major elementary reactions that inhibits the generation of intermediate radicals and NOx emissions were different under different equivalence ratios and N2 doping ratios. In addition, the sensitivity coefficients of the elementary reactions that mainly promotes or inhibits the generation of free radicals and NOx decreased with the increasing of the N2 addition.

The laminar flame speed in mixtures of partially dissociated NH3 in air under ambient temperature and pressure was studied via experimentation and kinetic modelling. The experiments were carried out using a vertical tube flame propagation method for low degrees of NH3 dissociation and a Bunsen burner method equipped with Schlieren imaging technique for high degrees of NH3 dissociation. Chemkin modelling incorporating three different reaction mechanisms, namely, the Okafor, Otomo, and Mathieu and Petersen mechanisms, respectively, was undertaken to predict the laminar flame speed. The degree of NH3 dissociation varied from 0 to 50% (0–37.5 %v/v H2 in the fuel mixture with a fixed H2/N2 ratio of 3) with equivalence ratios of 0.9–1.2. The experiments showed that a higher degree of NH3 dissociation led to a larger initial flame kernel size upon ignition and brighter flames. Regardless of the equivalence ratio, the laminar flame speed increased with increasing the degree of NH3 dissociation, from 0 to 50%, the maximum laminar flame speed increased from 7.8 to 22.7 cm·s−1 at the equivalence ratio of 1.10. The Okafor mechanism offered the best predictions of the experimental data while the Otomo and Mathieu and Petersen mechanisms over-predicted the laminar flame speed. The production of key radicals including OH, H, O, and NH2 was enhanced in the presence of H2 and, thus the conversion of NH2 to NH, HNO, and NNH, leading to a significant increase in the laminar flame speed.

Turbulence induced by actuators has been widely utilized as a effective means to enhance jet impingement heat transfer (JIHT). Flame impingement heat transfer (FIHT) has parital similarity to JIHT, and thus it is of interest to investigate the role of an actuator-induced turbulence to flame combustion as well as its FIHT. On a Bunsen burner that is incorporable with a ring-type turbulence actuator, a detailed experimental comparison of the flame appearance, reaction zone structure, thermal field and heat transfer of both laminar and turbulent flames under identical bulk flow conditions and equivalence ratios were conducted. Results show that the turbulent flame has a narrower stable-operation range than the laminar couterpart, and thus turbulence reduces the flame stability. A major difference between two flames exists at the reaction zone, which is identified to be a ‘brush’ structure in the turbulent case, instead of being a perfect cone in the laminar case. The flame brush changes the heat release behavior such that the temperature field of the turbulent flame shows lowered peak temperature while enlarged high-temperature zone. The changed heat release or temperature field reults in distinct local heat fluxes in the impinging flames at the stagnation region and near wall jet region. Consequently, the overall heat transfer rate of the turbulent flame becomes higher than the laminar flame. Accordingly, this study proposes that the actuator turbulence can be utilized as a useful technique to promote FIHT.

Ducted fuel injection (DFI) which mimics Bunsen burner, is a cutting-edge technology for reducing the soot formation of heavy-duty diesel engines. In this work, the Mie-scattering optical method was used to visualise impacts of ambient gas pressure on DFI and free spray characteristics with a single-hole injector. Ambient gas pressure varies from 3 to 12 MPa and diesel injection pressure from 40 to 120 MPa. Spray morphology, tip penetration, cone angle and area were discussed. The results indicated that DFI spray has a distinct velocity advantage over free spray, especially under high ambient pressure. As ambient pressure increases, DFI spray has better cone angle and area advantages than those of free spray. Increasing injection pressure helps to promote these advantages. Additionally, when the ambient pressure is high enough, a mushroom-shaped spray appeared at the duct outlet, diffusing actively around and forming a large-scale vortex. All of these characteristics of DFI spray can be interpreted as better serving the fuel-gas mixing as ambient pressure increases.

An experimental study was conducted on the measurement of the laminar burning velocity of alternative aviation fuels, in order to assess the feasibility of using the newly developed fuels in aviation applications. Two kinds of high performance hydrocarbon fuels (HPF-1 and 2), having higher density but moderate viscosity as compared with existing conventional aviation fuels, were created, and a Bunsen burner was manufactured to measure their laminar burning velocities at an elevated temperature of 550 K and atmospheric pressure. The measured laminar burning velocity of HPF-1 and 2 at the preheating temperature shows that the peak burning velocity of HPF-1 is approximately 125 cm/s, which is very close to that of the reference fuel (RF) and higher than Jet A-1, while the maximum laminar burning velocity of HPF-2 is shown to be approximately 130 cm/s, which is slightly higher (5–6 cm/s) than those of HPF-1 and RF. Compared with RF and Jet A-1, furthermore, the laminar burning velocity curves of HPFs shift to the fuel-rich side, with a corresponding decrease in the burning velocity on the lean side and an increase on the rich side. Therefore, the equivalence ratio where the laminar burning velocity reaches maximum (Φmax) increases slightly for the HPFs; Φmax for Jet A-1 and RF are around 1.07 and 1.08, respectively, while those for HPF-1 and 2 are 1.12 and 1.14, respectively. These fundamental data can be used to develop and validate combustion models to enable quantitative performance predictions in engine design for the newly developed fuels.