R.P. Sampat
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6 records found
1
Low emissions and fuel flexibility are two important criteria required for gas turbine combustors to facilitate the energy transition to low-carbon fuels for propulsion and power applications. A jet-stabilized combustor, having both these characteristics, was operated with CH 4–H 2 fuel mixtures with H 2 varying from 0 to 100 % and with varying equivalence ratios (ϕ). Comprehensive measurements were carried out of the velocity field using Particle Image Velocimetry (PIV), temperature and gas composition by traversing probes in the chamber, and flame topology using chemiluminescence imaging. The flow field in this combustor consists of a jet that undergoes recirculation, generating Central and Peripheral Recirculation Zones (CRZ and PRZ). The recirculation ratio in the PRZ is found to be twice that of the CRZ. Increasing H 2 % for the same ϕ leads to higher NO x. Ultra-low ϕ flames could be stabilized only at H 2≥50 %, which in turn leads to low NO x due to low adiabatic flame temperatures. The combination of temperature, gas composition (CO/NO), and chemiluminescence images is used to identify the extent and location of the reaction zone. Distributed reaction zones, stabilizing at around 30 % of the length of the chamber, are achieved at lean conditions, whereas an increase in H 2 % makes the reaction zone more compact and shifts upstream towards the burner head. Flame kernels are extracted from the instantaneous chemiluminescence images, and probability distribution functions for their aspect ratio and axial location are constructed. It is seen that reducing ϕ leads to low aspect ratio kernels that tend to occur further downstream, whereas increasing H 2 % leads to higher aspect ratio kernels, stabilizing upstream. These flame kernel statistics are also used to identify ignition modes (autoignition/flame propagation) for varying fuel H 2 % and inlet ϕ based on a hypothesis of flame stabilization mechanisms.
The jet-in-coflow is a two-stream configuration having engineering applications in combustors and gas turbine engine exhausts. In practical systems, the coflow generates a boundary layer of the outer wall of the jet pipe and may also have a certain level of turbulence. In the current work, the evolution of this flow configuration is studied using an air-air turbulent jet in a low turbulence coflow (turbulence intensity < 6%), and the 2D velocity field is measured by planar particle image velocimetry. Cases of varying coflow ratio (ratio of coflow velocity to jet velocity) of 0 (turbulent free jet), 0.09, 0.15, and 0.33 are generated by keeping a constant velocity jet (Re = 14000) and varying the coflow velocity. The trends of jet centerline properties such as velocity decay, jet spread, and jet momentum of jet-in-coflow cases, scaled to represent an equivalent free jet, show deviations from that of the turbulent free jet. The radial profile of mean velocity shows a region of velocity deficit, compared to a turbulent free jet, on the coflow side in the jet-in-coflow cases. In contrast, the turbulence intensity and Reynolds shear stress profiles show an enhanced peak near the interface for the jet-in-coflow cases. Further, conditional statistics were extracted by detecting the interface between the jet and the surroundings, wherein the same trends are observed. The low turbulence levels of the coflow have little effect on the jet/coflow interface, as seen by the conditional enstrophy diffusion and tortuosity compared to a turbulent free jet. The differences at the jet/coflow interface of a jet-in-coflow with respect to a turbulent free jet are attributed to the boundary layer initially developed by the turbulent coflow over the pipe generating the jet, and these are seen throughout the near-to-intermediate field (0≤x/D≤40).
Autoignition characteristics of vitiated methane-air mixtures were studied by simulations in 0D reactor setup. Unlike most studies in literature, vitiation, in the context of flameless combustion, was generated as the hot combustion product of fresh reactants that also contained radicals that existed in equilibrium. Particularly, the effect of varying levels of vitiation and heat loss was studied on properties such as ignition delay time, reaction time scale, and the NOx and CO emissions. This revealed the most suitable conditions to achieve low emissions and distributed reaction zones for premixed reactants that are vitiated by exhaust gases. Further, a regime of multi-ignition was discovered where prior to the main ignition event, there is a pre-ignition event attributed to the initial pool of radicals in a vitiated mixture. The conditions of occurrence were mapped out, as well as the mechanism behind it was explained.
The mixing at the interface of the jet and the recirculation zone in a jet-stabilized combustor has an important role in determining the composition of the hot-diluted mixtures. Thus, the fluid mechanics of the turbulent-turbulent interface were studied in a canonical configuration of a turbulent-jet-in-turbulent-coflow. This is also a common configuration used to produce Flameless/MILD combustion under laboratory conditions. Although there is vast research on free turbulent jets, combustors operating in the Flameless Combustion regime would reach flow conditions where the ratio of coflow to jet velocity would increase. This work elucidates the evolution of such a flow field through Particle Image Velocimetry (PIV) measurements done along the axis of the jet in the range 0<x/D<42. Further, the interface is detected using an algorithm developed based on other image processing algorithms using vorticity as a criterion. This enables the assembly of conditional statistics with respect to the interface. The results show that the cases with higher coflow have a lower jet centerline velocity decay rate and reduced jet spreading. The mean axial velocity shows a region of deficit compared to the free jet near the interface region. Further, the case with higher coflow shows higher turbulence intensity and Reynolds Shear Stress close to the interface. The detailed results are presented as both unconditional and conditional statistics and the mechanism behind this effect is deduced.
Experiments were done on a jet-stabilized combustor capable of producing the Flameless Combustion regime. It was operated using methane-hydrogen fuel admixtures at varying equivalence ratios. The combustor performance was analyzed based on the stabilization of the flame zone and the emissions. This work presents a unique, comprehensive measurement of temperature, gas composition, velocity field, and chemiluminescence signal in a jet-stabilized combustor. The recirculation regions are visualized through PIV measurements and the recirculation ratio is quantified. The instantaneous flame images are used to identify flame kernels and construct probability density functions of the aspect ratio, rotation angle, and location along the combustor axis. An increase in hydrogen content in the fuel mixture shifts the stabilization mechanism from autoignition to flame propagation. There is also an increase in NO emissions. A similar effect is seen with the increase in equivalence ratio from lean to stoichiometric condition. Distributed reaction regimes with ultra-low NO and moderate flame temperatures are achieved at very low equivalence ratios. Such mixtures are stabilized better with the addition of hydrogen to the fuel mixture.
This thesis provides fundamental information on the chemistry and flow physics of the phenomenon in a jet-stabilized combustor followed by measurements from the operation of one. The data and conclusions are a suitable reference for future engineers designing jet-stabilized combustors for low NOx emissions and high combustion efficiency. ...
Autoignition characteristics of vitiated methane-air mixtures were studied by simulations in 0D reactor setup. Unlike most studies in literature, vitiation, in the context of flameless combustion, was generated as the hot combustion product of fresh reactants that also contained radicals that existed in equilibrium. Particularly, the effect of varying levels of vitiation and heat loss was studied on properties such as ignition delay time, reaction time scale, and the NOx and CO emissions. This revealed the most suitable conditions to achieve low emissions and distributed reaction zones for premixed reactants that are vitiated by exhaust gases. Further, a regime of multi-ignition was discovered where prior to the main ignition event, there is a pre-ignition event attributed to the initial pool of radicals in a vitiated mixture. The conditions of occurrence were mapped out, as well as the mechanism behind it was explained.
The mixing at the interface of the jet and the recirculation zone in a jet-stabilized combustor has an important role in determining the composition of the hot-diluted mixtures. Thus, the fluid mechanics of the turbulent-turbulent interface were studied in a canonical configuration of a turbulent-jet-in-turbulent-coflow. This is also a common configuration used to produce Flameless/MILD combustion under laboratory conditions. Although there is vast research on free turbulent jets, combustors operating in the Flameless Combustion regime would reach flow conditions where the ratio of coflow to jet velocity would increase. This work elucidates the evolution of such a flow field through Particle Image Velocimetry (PIV) measurements done along the axis of the jet in the range 0<x/D<42. Further, the interface is detected using an algorithm developed based on other image processing algorithms using vorticity as a criterion. This enables the assembly of conditional statistics with respect to the interface. The results show that the cases with higher coflow have a lower jet centerline velocity decay rate and reduced jet spreading. The mean axial velocity shows a region of deficit compared to the free jet near the interface region. Further, the case with higher coflow shows higher turbulence intensity and Reynolds Shear Stress close to the interface. The detailed results are presented as both unconditional and conditional statistics and the mechanism behind this effect is deduced.
Experiments were done on a jet-stabilized combustor capable of producing the Flameless Combustion regime. It was operated using methane-hydrogen fuel admixtures at varying equivalence ratios. The combustor performance was analyzed based on the stabilization of the flame zone and the emissions. This work presents a unique, comprehensive measurement of temperature, gas composition, velocity field, and chemiluminescence signal in a jet-stabilized combustor. The recirculation regions are visualized through PIV measurements and the recirculation ratio is quantified. The instantaneous flame images are used to identify flame kernels and construct probability density functions of the aspect ratio, rotation angle, and location along the combustor axis. An increase in hydrogen content in the fuel mixture shifts the stabilization mechanism from autoignition to flame propagation. There is also an increase in NO emissions. A similar effect is seen with the increase in equivalence ratio from lean to stoichiometric condition. Distributed reaction regimes with ultra-low NO and moderate flame temperatures are achieved at very low equivalence ratios. Such mixtures are stabilized better with the addition of hydrogen to the fuel mixture.
This thesis provides fundamental information on the chemistry and flow physics of the phenomenon in a jet-stabilized combustor followed by measurements from the operation of one. The data and conclusions are a suitable reference for future engineers designing jet-stabilized combustors for low NOx emissions and high combustion efficiency.
The jet-in-hot-coflow is a canonical combustion setup, which has been used in several studies to study Flameless/MILD combustion and auto-ignition of fuels. However, the NOx and CO emission measurements from these combustion setups were not possible due to the entrainment of laboratory air and a lack of a well-defined physical system limit. These limitations have been overcome by a new enclosed jet-in-hot-coflow setup. The combustor was operated by injecting a mixture of CH4-Air in the central jet, and the coflow comprised of hot products from CH4-Air combustion in burners upstream. The coflow composition was further controlled by adding diluents such as N2 and CO2. Measurements were done using stereoscopic particle image velocimetry, suction probe gas analysis, thermocouples, and chemiluminescence imaging. Increasing central jet velocity and equivalence ratio led to lower NOx and a reaction zone that enlarged and shifted downstream. The reduction in NOx emission was attributed to the returning mechanism. Adding CO2 and N2 as diluents in the coflow resulted in a longer combustion zone and reduced temperatures in the combustion chamber, leading to decreased NOx production and increased reburning. These experiments provide relevant flowfield and emissions data for modelers and help characterize combustion regimes such as Flameless/MILD.
Flameless Combustion is an interesting low NOx combustion technology for gas turbine engines. In order to design systems for stringent performance standards, it is important to understand emission formation in this regime. To this end, the characteristics of a combustor capable of operating in the Flameless regime are studied. Particle Image Velocimetry and thermocouple measurements were performed to obtain the velocity field and gas temperatures respectively, in the combustor under reacting conditions. Results from experiments were used to generate an "informed" chemical reactor network (CRN) model from which, temperature and species distributions were obtained. As such, this paper presents measured data and a methodology to combine it with CRN modelling to obtain gas composition and temperature. The temperature, NOx, CO and O2 mole fractions obtained at three different operating conditions shall be validated with gas composition measurements in the future.