Flame shape and size for a high-pressure turbulent non-premixed swirl combustion were experimentally investigated over a wide range of varying parameters including fuel mass flow rate, combustor pressure, primary-air mass flow rate, and nozzle exit velocity. A CFD simulation was conducted to predict the flame profile. Meanwhile, a theoretical calculation was also performed to estimate flame length. It was observed that flame length increased linearly with increasing fuel mass flow rate but decreased with the increment of combustor pressure in the power function. The flame diminished at a larger primary-air mass flow rate but remained unaffected by the increasing nozzle exit velocity. Considering the global effect of all parameters at a particular pressure, the flame length generally decreased as the primary-air to fuel ratio increased. This was attributed to the reduced air entrainment required to dilute the fuel to stoichiometric proportions. The CFD simulation offered a good prediction of the variation trends of flame length, although some deviations from experimental values were observed. The theoretical calculation estimated the trends of flame length variation particularly well. Nevertheless the difference between the theoretical and experimental results was found to be due to the swirl influence. Hence, a swirl factor was proposed to be added to the original equation for swirl flames.

Flame pulsation has a significant effect on combustion, and understanding its oscillatory behavior is important to the combustion community. An experiment was performed to analyze the pulsation characteristics of a swirl non-premixed flame under various parameters. The results showed that as fuel mass flow rate increased, the puffing frequency increased due to the decreased flame radiation fraction, and the puffing amplitude became smaller resulting in a more stable flame. With an increase in combustor pressure, the flickering frequency declined because of the increasing soot radiation, while the flickering amplitude uniformly increased, leading to more deteriorative flame stability. With an increment in mass flow rate of primary air, the puffing frequency decreased due to the enhanced mixing between fuel and primary air. Also, the puffing amplitude had an oscillating relationship with the mass flow rate of primary air. When the exit velocity of the injector was increased, the flickering frequency diminished nearly linearly because of the improving swirl intensity, and the flickering amplitude was approximately unaffected by injector exit velocity. Moreover, the measured puffing frequencies summarized over all cases varied within the range of 3-22 Hz, the predicted values from theoretical models based on non-swirl flame also fell within this range. The puffing frequency of swirl combustion was more sensitive to the variation in operating conditions than that of non-swirl combustion. Additionally, the obtained correlations indicated that the Strouhal number St was proportional to Fr −1.4 (the Froude number) and Re −2.9 (the Reynolds number), respectively.

The behavior of the pollutants NO and CO at elevated combustor pressure are of special importance due to the continuing trend toward developing engines operating at higher pressure ratios to yield higher thermal efficiency. An experiment was performed to examine the NO and CO emissions for a swirl convergent-divergent nozzle at elevated pressure. The NO and CO correlations were obtained. Meanwhile, the flame length, exhaust gas oxygen concentration, exit temperature and global flame residence time were also determined to analyze the NO and CO emission characteristics. The results showed that, with the increase in combustor pressure P, flame length decreased proportionally to P −0.49 ; exit O 2 volume fraction increased and exit temperature was reduced. The global flame residence time decreased proportionally to P −0.43. As pressure increased, the NO and Emission Index of NO (EINO) levels decreased proportionally to P −0.53 and P −0.6 respectively, which is mainly attributed to the influence of global flame residence time; the NO and EINO increased almost proportionally with the increase in global flame residence time. The EINO scaling EINO (ρu e /d) was proportional to Fr 0.42 , which indicated that compared with pure fuel, the fuel diluted with primary air can cause a decrease in the exponent of the Fr power function. At higher pressure, the CO and Emission Index of CO (EICO) decreased proportionally to P −0.35 and P −0.4 , respectively, due to the increased unburned methane and high pressure which accelerated chemical reaction kinetics to promote the conversion of CO to CO 2 .

An experiment was conducted to explore the flickering parameters under varying mass flow rate of fuelṁ F with spatial analysis and global analysis. The parameters include mean value, flickering weighted average frequency F, flickering coefficient of variation c v (firstly introduced), skewness s, and kurtosis k. From the spatial analysis, it was found that the brightest part of flame is located in its core, and the brightness gradually decreased from the inside out. The distributions of high levels of F, c v , s, and k are almost consistent, all lying in the flickering edge, which is a thin layer where the parameters sharply declined. From the global analysis, with the increment oḟ m F , the global F decreased slightly; the global c v declined uniformly, which means the oscillation amplitude diminished and thus the flame became more stable; the global s linearly reduced; and the global k also showed a decreasing trend. The decreasing global s indicates that the number distribution gradually became symmetric, and the decreasing global k indicates that the number distribution progressively became flat. Consequently, the number distribution progressively tends to normal distribution at largerṁ F . Keywords: flame flickering; partially premixed flame; varying mass flow rate of fuel; spatial analysis and global analysis; flickering weighted average frequency; flickering coefficient of variation; skewness and kurtosis

It is of practical significance to understand the flame puffing behavior under varying mass flow rate of primary air ṁpri. An experiment was conducted to study the impact of ṁpri on flame puffing in a swirl partially premixed combustor, the puffing behavior of six significant flame properties was examined. The results showed that almost every spectrum had two fundamental frequencies, which is different from the single-peak spectrum of non-swirl flame. The flame heat-release rate, flame area, and flame equivalent width had identical dominant frequency and sub-dominant frequency, both decreased with the increasing of ṁpri. It was attributed to the decreased overall flame temperature caused by the improved mixing of fuel and primary air. All measured frequencies were in the range of 3–14 Hz, but the predicted frequencies from the theoretical models based on non-swirl flame were larger than the measured. This indicates the puffing frequency of swirl flame was much more sensitive to the variation in ṁpri than the frequency of non-swirl flame. Moreover, the amplitude of flame length was the smallest in all properties, with the most weakened oscillating intensity. While the amplitude of the flame area and flame equivalent width were the largest, with the strongest oscillation level. Consequently, the flame puffing is mainly attributed to the oscillation in width direction.

The C 1ε = 1.6 standard k – ε equation combined with the steady flamelet model was applied to a methane/dimethyl ether swirl combustion field, and the effects of the dimethyl ether (DME) blending ratio and operating pressure on the flame behavior, including species variation, reaction zone behavior, and flame entrainment, were investigated. The results demonstrated that selected models could better reproduce the trends of the experimental measurements. The downstream reaction zone achieved better calculation accuracy than the outer shear layer of the first recirculation zone. The addition of DME accelerated the accumulation process of H 2 , O, H, and OH radicals. The intermediate radical CH 2 O was rapidly developed by the influence of the H extraction rate under a constant fuel volume flow rate. The reaction zone dimensions were approximately linearly and positively correlated with the DME blending ratio, whereas flame entrainment expressed a lower DME concentration dependence in the high-DME mass-dominated system. The operating pressure significantly impacted the distribution of reactive radicals in the turbulent flame; meanwhile, the flame and reaction zone length showed nonlinear inverse behavior with pressure variation, while the thickness of the reaction zone was always linearly and negatively correlated with pressure. Moreover, the peak flame entrainment rate also experienced a nonlinear decline with pressure elevation; however, the peak positions were not sensitive to pressure fluctuation. Concurrently, the response surface functions for the reaction zone dimensions were established covering the range of 0–1 for the DME blending ratio and 1–5 atm operating pressure, which could provide assistance for combustion condition optimization and combustion chamber design.