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Recently, there has been an increasing emphasis on the efficient utilization of energy resources. Precise control of gas injection can aid in achieving this objective. However, measuring the temperature and concentration of gases simultaneously presents a challenge. Therefore, it is essential to monitor and track irregular behavior, such as combustion processes, in real-time without interfering with the combustion flow. This study proposes a non-contact optical measurement method that does not disrupt the combustion flow. Direct absorption spectroscopy (DAS) is used to extract the absorption line during optical measurements. Since the choice of absorption line extraction method can affect temperature measurement results, the performance of linear and non-linear interpolation methods is compared. Temperature measurements of stable flames were compared using a thermocouple and the computed tomography-tunable diode laser absorption spectroscopy (CT-TDLAS) method. The relative temperature error at the center of the measurement area between the two measurement methods was found to be approximately 2%. To overcome the real-time temperature measurement limitation of thermocouples, the combustion process of premixed flames is measured using computed tomography (CT) technology. Additionally, to evaluate the real-time measurement superiority of CT-TDLAS, the combustion state is assessed by comparing the intensity of a single-camera image with simultaneous measurement using CT-TDLAS. The results demonstrate that the image intensity of the flame captured by a single camera and the temperature field measured by CT-TDLAS exhibit the same trends during the extinguishing process. Consequently, the superiority of CT-TDLAS measurement for temperature measurement is demonstrated, and the real-time measurement superiority is confirmed.

A theoretical study was conducted to examine the flame transfer function (FTF) of a diverging premixed flame subject to uniform and/or convective velocity disturbances considering developing turbulent flame speed. The FTF under uniform velocity disturbances with developing flame speed is less oscillatory compared with constant flame speed, and the magnitude of the aforementioned FTF is decreased more rapidly with an increase in turbulent time scale or flame length. The peak overshoot of the FTF under convective velocity disturbances is observed in a certain frequency range, which is known to be a typical nature of the FTF of a diverging flame under convective velocity disturbances. Such peak overshoot is quite sensitive to how the turbulent flame speed varies along the flame front in case of developing flame speed while the amplitude of the peak overshoot rarely changes with the mean flame length in case of constant flame speed.

In industrial furnace applications, the interaction between the flame plume and the surface of the heat exchanger plays a crucial role in heat transfer efficiency, particularly given the current emphasis on energy conservation. Traditional research methods require adjusting independent variables to observe changes in dependent variables, leading to a large amount of experimental conditions. Hence, the equivalent simplification of problem complexity and the establishing of predictive models are crucial for applying premixed flames in engineering fields. In the present study, the three-dimensional computational fluid dynamics (CFD) method was employed alongside the assumption of isothermal flow to conduct a numerical investigation into the methane premixed impinging flat flame. The Reynolds number ( Re ) , the distance from the normalized nozzle to plate ( H / d ) , and the influence of the inlet temperature ( T in ) on the high-temperature isothermal impinging jet and local heat transfer characteristics were explored. The results indicated that the spread rate of the jet increased with rising Re , while the inlet temperature exhibited minimal effect. Furthermore, it was observed that the local Nusselt number increased with increasing Re and decreasing H / d . When appropriate adiabatic wall temperatures were taken into account, the local Nusselt number distribution was found to be independent of the inlet temperature. Additionally, a close relationship was identified between the spread rate of the jet and local Nusselt number. The surrogate model, specifically the Kriging model (KM), was utilized to analyze the interactive effects of variable on global heat transfer performance. The findings demonstrated that the average Nusselt number increased with increasing Re and decreasing H / d , with a slight increase also observed with rising T in . Sensitivity analysis revealed that the influence level of variables on the average Nusselt number followed the order of Re > H / d > T in . Moreover, KM with promising predictive capabilities was achieved through the infilled criterion aimed at minimizing maximum root mean square error (RMSE), thereby significantly reducing both maximum and average relative errors.

In this paper, computational fluid dynamics (CFD) numerical simulation is employed to analyze and discuss the effect of obstacle gradient on the flame propagation characteristics of premixed hydrogen/air in a closed chamber. With a constant overall volume of obstacles, the obstacle blocking rate gradient is set at +0.125, 0, and −0.125, respectively. The study focuses on the evolution of the flame structure, propagation speed, the dynamic process of overpressure, and the coupled flame–flow field. The results demonstrate that the flame front consistently maintains a jet flame as the obstacle gradient increases, with the wrinkles on the flame front becoming increasingly pronounced. When the blocking rate gradients are +0.125, 0, and −0.125, the corresponding maximum flame propagation speeds are measured at 412 m/s, 344 m/s, and 372 m/s, respectively, indicating that the obstacle gradient indeed increases the flame propagation speed. Moreover, the distribution of pressure is closely related to changes in the flame structure, with the overpressure decreasing in the obstacle channel as the obstacle gradient increases. Furthermore, the velocity vector and vortex distribution in the flow field are revealed and compared. It is found that the obstacle tail vortex is the main factor inducing flame evolution and flow field changes in a closed chamber. The effect of the blocking rate gradient on flow velocity is also quantified, with instances of deceleration occurring when the blocking rate gradient is −0.125.

The characteristics of the planar premixed flame near the flammability limit were investigated by employing the Zel’dovich-Liñán two-step reaction model, focusing on the fast recombination regime. The structure of the reaction zone was numerically analyzed to examine the differences in flame characteristics between first-order and second-order recombination reactions, under the limit of a large chain branching Zel’dovich number, β . As the ratio of reaction rates between branching and recombination decreases, the concentration of chain carriers also decreases, leading to a reduction in flame speed, as expected. In the flammability limit condition where the flame speed is zero, there are notable differences in combustion characteristics between first and second recombination. As the flammable limit is approached, the ratio of reaction rates approaches the mathematical constant e in first-order recombination due to substantial fuel leakage from the reaction zone, whereas in second-order recombination, the ratio of reaction rates approaches zero as there is no fuel leakage. An asymptotic analysis was employed to derive the relevant expressions for the ratio of reaction rates at the flammable limit.

The behaviours of the three invariants of the velocity gradient tensor and the resultant local flow topologies in turbulent premixed flames have been analysed using three-dimensional direct numerical simulation data for different values of the characteristic Lewis number ranging from 0.34 to 1.2. The results have been analysed to reveal the statistical behaviours of the invariants and the flow topologies conditional upon the reaction progress variable. The behaviours of the invariants have been explained in terms of the relative strengths of the thermal and mass diffusions, embodied by the influence of the Lewis number on turbulent premixed combustion. Similarly, the behaviours of the flow topologies have been explained in terms not only of the Lewis number but also of the likelihood of the occurrence of individual flow topologies in the different flame regions. Furthermore, the sensitivity of the joint probability density function of the second and third invariants and the joint probability density functions of the mean and Gaussian curvatures to the variation in Lewis number have similarly been examined. Finally, the dependences of the scalar–turbulence interaction term on augmented heat release and of the vortex-stretching term on flame-induced turbulence have been explained in terms of the Lewis number, flow topology and reaction progress variable.

In this work, the behavior of a lean premixed stretched flame stabilized in a flat channel near a heated wall was studied. Dependences of the flame front position on the stretch rate parameter at temperatures of the heated wall of 1000 and 1200 K and the combustible mixture composition (ϕ = 0.7 and 0.6) were obtained experimentally. The reduced thermal diffusive model was used in numerical simulation for an explanation of obtained experimental results. Theoretical estimates are in qualitative agreement with the experiment. The performed qualitative analysis may be useful in estimation of the combustion product temperature and the residence time of the nanoparticles forming in combustion products before their impact with the hot wall.

Counterflow flames provide an ideal platform for understanding the flame structure and as a model to study the effect of physical and chemical perturbations on the flame structure. This article reviews the advances made in the understanding of combustion dynamics and chemistry through experimental and numerical studies in counterflow non-premixed and partially premixed flames. Key contributions on fundamental aspects such as extinction, ignition and effect of perturbations on the stability of diffusion flames are first summarized and analysed. The review then focuses on the progress made in the understanding of the effect of inert particles and flame suppressants on the flame characteristics. A review of detailed studies on edge flames facilitates further understanding of local quenching and re-ignition phenomena in highly turbulent flames. The influence of radiation model and unsteady flow-conditions on the flame kinetics and dynamics along with work on NOx kinetics has been discussed. The review also outlines that specific experiments need to be carried out over a wide range of conditions for further understanding and validation of numerical models.

Swirl combustion serves as a helpful flame stabilization method, which also affects the combustion and emission characteristics. This article experimentally investigated the effects of CO 2 microjets on combustion instability and NO x emissions in lean premixed flames with different swirl numbers. The microjets’ control feasibility was examined from three variables of CO 2 jet flow rate, thermal power, and swirl angles. Results indicate that microjets can mitigate the combustion instability and NO x emissions in lean premixed burners with different swirl numbers and thermal power. Still, the damping effect of microjets in low swirl intensity is better than that in high swirl intensity. The damping ratio of pressure amplitude can reach the maximum of 98%, and NO x emissions can realize the maximum reduction of 10.1×10 −6 at the swirl angle of 30°. Besides, the flame macrostructure switches from an inverted cone shape to a petal shape, and the flame length reduction at low swirl intensity is higher than that of high swirl intensity. This research clarified the control differences of mitigation of combustion instability and NO x emissions by microjets in lean premixed flames with different swirl numbers, contributing to the optimization of microjets control and the construction of high-performance burners.

We investigate the onset of thermoacoustic instabilities in a turbulent combustor terminated with an area contraction. Flow speed is varied in a swirl-stabilized, partially premixed combustor and the system is observed to undergo a dynamical transition from combustion noise to instability via intermittency. We find that the frequency of thermoacoustic oscillations does not lock-on to any of the acoustic modes. Instead, we observe that the dominant mode in the dynamics of combustion noise, intermittency and thermoacoustic instability is a function of the flow speed. We also find that the observed mode is insensitive to the changes in acoustic field of the combustor, but it varies as a function of upstream flow time scale. This new kind of thermoacoustic instability was independently discovered in the recent theoretical analysis of premixed flames. They are known as intrinsic thermoacoustic modes. In this paper, we report the experimental observation and the route to flame intrinsic thermoacoustic instabilities in partially premixed flame combustors. A simplified low-order network model analysis is performed to examine the driving mechanism. Frequencies predicted by the network model analysis match well with the experimentally observed dominant frequencies. Intrinsic flame-acoustic coupling between the unsteady heat release rate and equivalence ratio fluctuations occurring at the location of fuel injection is found to play a key role. Further, we observe intrinsic thermoacoustic modes to occur only when the acoustic reflection co-efficients at the exit are low. This result indicates that thermoacoustic systems with increased acoustic losses at the boundaries have to consider the possibility of flame intrinsic thermoacoustic oscillations.