Rv Ravikrishna

Laser-induced fluorescence measurements and modeling of nitric oxide in counterflow partially premixed flames

Abstract Quantitative measurements of NO concentration ([NO]) have been obtained along the centerline of atmospheric pressure, methane–air, counterflow partially premixed flames using the laser-induced fluorescence (LIF) technique. The effect of partial premixing was studied by investigating flames with fuel-side equivalence ratios (φ B ) of 1.45, 1.6, 1.8, and 2.0 at a constant global strain rate near 20 s −1 . Linear LIF measurements of [NO] are corrected for variations in the electronic quenching rate coefficient by using major species profiles generated by an opposed-flow flame code and quenching cross-sections for NO available from the literature. Corrected linear LIF measurements of [NO] and temperatures measured using thin filament pyrometry are compared with numerical predictions from the opposed-flow flame code by utilizing the GRI (version 2.11) mechanism for the NO kinetics. The effect of radiative heat loss on code predictions is accounted for by using an optically thin radiation model. Reasonably good agreement exists between LIF [NO] measurements and predictions in all flames. In particular, all predictions fall within 10% of measurements at peak [NO] locations. Spatial separation was observed between regions where prompt-NO and thermal-NO dominate in the φ B = 1.45 flame. A previously modified rate coefficient for the prompt-NO initiation reaction improved agreement between predictions and measurements in the region dominated by prompt NO.

Laser-induced fluorescence measurements and modeling of nitric oxide in methane–air and ethane–air counterflow diffusion flames

Abstract Quantitative laser-induced fluorescence (LIF) measurements of nitric oxide concentrations [NO] have been obtained along the centerline in atmospheric pressure methane–air and ethane–air counterflow diffusion flames. These flames are highly diluted to avoid both soot formation and the influence of radiative heat losses on NO formation, thereby ensuring NO production mostly via the prompt mechanism. Linear LIF measurements of [NO] are corrected for variations in the electronic quenching rate coefficient by using major species profiles generated by an opposed-flow flame code and quenching cross-sections for NO available from the literature. Temperature measurements are also made in the methane–air counterflow diffusion flames by using thin SiC filament pyrometry. The excellent agreement between temperature measurements and model predictions verifies the efficacy of a new calibration method developed for thin filament pyrometry. Predictions using the GRI mechanism consistently underpredict peak [NO] in all flames. This result indicates a need for refinement of both the prompt-NO and CH kinetics, especially the rate coefficient for the prompt-NO initiation reaction. A modified rate coefficient proposed for the prompt-NO initiation reaction significantly improves agreement between modeling and measurements in both the methane–air and ethane–air counterflow diffusion flames. The remaining discrepancy in some flames can be attributed to a lack of refinement in the CH chemistry. Overall, the modified rate coefficient proposed here seems to be a good choice over a wide range of strain rates for both methane and ethane fuels.

Quantitative Laser-Induced Fluorescence Measurements and Modeling of Nitric Oxide in High-Pressure (6-15 atm) Counterflow Diffusion Flames

Laser-induced fluorescence (LIF) measurements of NO concentration ([NO]) have been obtained along the centerline of methane–air counterflow diffusion flames at 6 to 15 atm. This study is an extension of our previous work involving measurements of [NO] in similar flames at two to five atm, wherein we had used a counterflow premixed flame for calibration. For the flames studied here, a method based on computed overlap fractions is developed to calibrate [NO] measurements at higher pressures. The linear LIF measurements of [NO], which are corrected for variations in the electronic quenching rate coefficient, are compared with numerical predictions from an opposed-flow flame code utilizing two Gas Research Institute (GRI) chemical kinetic mechanisms (versions 2.11 and 3.0). The effect of radiative heat loss on code predictions is accounted for by using an optically thin radiation model. The revised GRI mechanism (version 3.0) offers a significant improvement in prompt-NO predictions for these flames compared to the older version (2.11), especially at pressures below eight atm. However, a consistent discrepancy remains in the comparisons, particularly at peak NO locations for pressures lower than six atm. The measurements display a continuing trend of decreasing NO concentration with increasing pressure at 6–15 atm as expected for flames dominated by prompt NO. The discrepancy between measurements and predictions decreases with rising pressure so that the revised GRI mechanism predicts [NO] with reasonable accuracy at pressures above six atm.

Comparison of saturated and linear laser-induced fluorescence measurements of nitric oxide in counterflow diffusion flames

Abstract Quantitative measurements of NO concentrations ([NO]) have been obtained along the centerline of atmospheric ethane–air counterflow diffusion flames by using saturated and linear laser-induced fluorescence (LIF). In particular, four flames with strain rates varying from 5 to 48 s −1 were investigated while maintaining a constant fuel dilution in all cases. The utility of a broad-band laser-saturated fluorescence (LSF) technique is assessed by comparison to similar measurements of NO using linear LIF. The linear LIF measurements are corrected for variations in the local electronic quenching rate coefficient by using major species profiles generated by a diffusive flame code and available correlations for the quenching cross-sections of NO. The corrected LIF profiles compare favorably with the LSF profiles. A four-level model is used to investigate the effects of rotational energy transfer (RET) on the LSF measurements. The excellent comparison between the quenching-corrected linear LIF and the LSF measurements at locally fuel-lean to greater than stoichiometric mixture fractions verifies the validity of the LSF technique for these conditions. The slight but consistent discrepancy between the LSF and linear LIF measurements at local equivalence ratios above 1.6 may be attributed to a change in the collisional branching ratio from lean to rich stoichiometries and/or the need for further work on the electronic quenching cross-sections required for quantitative NO measurements under fuel-rich conditions.

Secondary breakup of a drop at moderate Weber numbers

We present volume of fluid based numerical simulations of secondary breakup of a drop with high density ratio (approx. 1000) and also perform experiments by injecting monodisperse water droplets in a continuous jet of air and capture the breakup regimes, namely, bag formation, bag-stamen, multibag and shear breakup, observed in the moderate Weber number range (20–120). We observe an interesting transition regime between bag and shear breakup for We=80, in both simulations as well as experiments, where the formation of multiple lobes, is observed, instead of a single bag, which are connected to each other via thicker rim-like threads that hold them. We show that the transition from bag to shear breakup occurs owing to the rim dynamics which shows retraction under capillary forces at We=80, whereas the rim is sheared away with flow at We=120 thus resulting in a backward facing bag. The drop characteristics and timescales obtained in simulations are in good agreement with experiments. The drop size distribution after the breakup shows bimodal nature for the single-bag breakup mode and a unimodal nature following lognormal distribution for higher Weber numbers.

Experimental and Numerical Studies in a Compact Trapped Vortex Combustor: Stability Assessment and Augmentation

Fundamental studies on a compact trapped vortex combustor indicate that cavity injection strategies play a major role on flame stability. Detailed experiments indicate that blow-out occurs for a certain range of cavity air flow velocities. An unsteady RANS-based reacting flow simulation tool has been utilized to study the basic dynamics of cavity vortex for various flow conditions. The phenomenon of flame blow-out at certain intermediate cavity air velocities is explained on the basis of transition from a cavity-stabilized mode to an opposed flow stagnation mode. A novel strategy is proposed for achieving flame stability at all conditions. This involves using a flow guide vane in the path of the main flow to direct a portion of the main flow into the cavity. This seems to result in a desirable dual vortex structure, i.e., a small clockwise vortex behind the vane and large counterclockwise vortex in the cavity. Experimental results show stable flame at all flow conditions with the flow guide vane, and pressure drop is estimated to be within acceptable limits. Cold flow simulations show self-similar velocity profiles for a range of main inlet velocities, and high reverse velocity ratios (−0.3) are observed. Such a high-velocity ratio in the reverse flow shear layer profile leads to enhanced production of turbulence imperative to compact combustors. Reacting flow simulations show even higher reverse velocity ratios (above −0.7) due to flow acceleration. The flame is observed to be stable, even though minor shear layer oscillations are present in the form of vortex shedding. Self-similarity is also observed in reacting flow temperature profiles at combustor exit over the entire range of the mainstream velocity. This indicates that the present configuration holds a promise of delivering robust performance invariant of the flow operating conditions.

Single cavity trapped vortex combustor dynamics - Part-2: Simulations

The first part described a versatile TVC test rig capable of a continuously variable length-to-depth ratio (L/D) of the cavity and optical access through quartz plates provided on three sides for visualization. Flame stabilization in the single cavity TVC was successfully achieved with methane as fuel and the range of flow conditions for stable operation were identified. From these, a few cases were selected for detailed experimentation, the results of which were presented in part-1. The results indicated that reducing L/D ratio and increasing cavity-air velocity favour stable combustion. In the present paper, numerical simulations are performed to ascertain reasons for some of the trends. The predicted temperatures at the exit showed reasonably good agreement with measured values. The experiments are also performed for different flow conditions to ascertain stability limits of the combustor. Insight from these set of experiments along with simulations has highlighted the importance of air and fuel injection strategies in the cavity. It was observed in the experiments that for certain cases involving moderate cavity-air velocity, the flame tend to blowout whereas at higher and lower cavity-air velocities, the flame was observed to be stable. This observation could be explained based on understanding obtained from simulations. From a mixing and combustion efficiency standpoint, it is desirable to have a cavity vortex that is anti-clockwise. However, natural tendency for flow over a cavity is to form a vortex that is clockwise. The tendency to blowout at higher inlet flow velocities is thought to be because of these two opposing effects. This basic understating of cavity flow dynamics can be used for further design improvements in future to improve flame stability at higher inlet flow velocities.

Mixing Enhancement in a Compact Trapped Vortex Combustor

Previous studies on a single-cavity, compact trapped vortex combustor concept showed good flame stability for a wide range of flow conditions. However, achieving good mixing between cavity products and mainstream flow was still a major challenge. In the present study, a passive mixing enhancement strategy of using inclined struts along with a flow guide vane is presented and experimentally tested at atmospheric pressure conditions. Results show excellent mixing and consequently low values of the combustor exit pattern factor in the range of 0.1 and small flame lengths (5–7 times the main-duct depth). The pressure drop is small in the range of 0.35%, and NOx levels of the order of 1–2 ppm are achieved. The flame stability is excellent, and combustion efficiency is reasonable in the range of 96%. The effectiveness of the proposed strategy is explained on the basis of in-situ OH chemiluminescence images and prior numerical simulations of the resulting complex flow field. The flow guide vane is observed to lead to a counterclockwise cavity vortex, which is conducive to the rise of cavity combustion products along the inclined struts and subsequent mixing with the mainstream flow.

Single cavity trapped vortex combustor dynamics - Part-1: Experiments

In the present work, a water-cooled, modular, atmospheric pressure Trapped Vortex Combustor (TVC) test rig is designed and fabricated for reacting and non-reacting flow experiments. The unique features of this rig consist of a continuously variable length-to-depth ratio (L/D) of the cavity and optical access through quartz plates provided on three sides for visualization. Flame stabilization in the single cavity TVC was successfully achieved with methane as fuel and the range of flow conditions for stable operation were identified. From these, a few cases were selected for detailed experimentation. Reacting flow experiments for the selected cases indicated that reducing L/D ratio and increasing cavity-air velocity favour stable combustion. The pressure drop across the single cavity TVC is observed to be lower as compared to conventional combustors. Temperatures are measured at the exit using thermocouples and corrected for radiative losses. Species concentrations are measured at the exit using an exhaust gas analyzer. The combustion efficiency is observed to be around 97-99 % and the pattern factor is observed to be in the range of 0.08 to 0.13. High-speed imaging made possible by the optical access indicates that the overall combustion is fairly steady, and there is no vortex shedding downstream.

Laser-Induced Fluorescence Measurements and Modeling of Nitric Oxide in High-Pressure Counterflow Diffusion Flames

Abstract Quantitative laser-induced fluorescence (LIF) measurements of NO concentration ([NO]) have been obtained along the centerline of prompt-NO dominated, methane-air counterflow diffusion flames at two to five atm, Global strain rates of 20, 30 and 40 s−1 were investigated at each pressure, with the addition of a 15 s−1 case at three and four atm. Linear LIF measurements of [NO] are corrected for variations in the electronic quenching rate coefficient by using major species profiles generated by an opposed-flow flame code and quenching cross-sections for NO available from the literature. Corrected linear LIF measurements of [NO] are compared with numerical predictions from the opposed-flow flame code by utilizing the GRI (version 2.11) mechanism for the NO kinetics. The effect of radiative heat loss on code predictions is accounted for by using an optically thin radiation model. A modest decrease in predicted temperature owing to radiative heat loss causes a significant decrease in predicted [NO], indicating the temperature sensitivity of the prompt-NO kinetics. Comparisons between [NO] measurements and predictions show that the GRI mechanism underpredicts prompt-NO by a factor of two to three at all pressures. The underprediction peaks at 2 to 3 atm, and decreases with pressure from 3 to 5 atm. Although the GRI mechanism does not display this trend, predictions with a modified rate coefficient for the prompt-NO initiation reaction give qualitative agreement with the experimentally observed variation. However, modifying the prompt-NO initiation reaction is not sufficient to account for the differences between measurements and predictions, thus indicating a need for refinement of the CH chemistry.