Indian Institute of Technology Palakkad&nbsp;(IIT Palakkad or IIT PKD) is a public autonomous&nbsp;engineering&nbsp;and&nbsp;research&nbsp;institute located in&nbsp;Palakkad,&nbsp;Kerala. It was&nbsp;one of the five new&nbsp;IITs&nbsp;proposed in the&nbsp;2014 Union budget of India. The campus was inaugurated on 3 August 2015. The institute has 94 faculty, 978 students, and 63 non-teaching staffs.

IT Palakkad currently offers four-year Bachelor of Technology (B.Tech) and two years MS programmes in 8 different engineering disciplines including&nbsp;Computer Science, Electrical, Mechanical, Civil among others.

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IIT Palakkad

Soot volume fraction predictions through simulations carried out on OpenFOAM® are reported in diffusion flames with ethylene fuel. A single-step global reaction mechanism for gas-phase species with an infinitely fast chemistry assumption is employed. Traditionally soot formation includes inception, nucleation, agglomeration, growth, and oxidation processes, and the individual rates are solved to determine soot levels. However, in the present work, the detailed model is replaced with the soot formation and oxidation rates, defined as analytical functions of mixture fraction and temperature, where the net soot formation rate can be defined as the sum of individual soot formation and oxidation rates. The soot formation/oxidation rates are modelled as surface area-independent processes. The flame is modelled by solving conservation equations for continuity, momentum, total energy, and species mass fractions. Additionally, separate conservation equations are solved to compute the mixture fraction and soot mass fraction consisting of source terms that are identical and account for the mixture fraction consumption/production due to soot. As a consequence, computational time can be reduced drastically. This is a quantitative approach that gives the principal soot formation regions depending on the combination of local mixture fraction and temperature. The implemented model is based on the smoke point height, an empirical method to predict the sooting propensity based on fuel stoichiometry. The model predicts better soot volume fraction in buoyant diffusion flames. It was also observed that the optimal fuel constants to evaluate soot formation rates for different fuels change with fuel stoichiometry. However, soot oxidation strictly occurs in a particular region in the flame; hence, they are independent of fuel. The numerical results are compared with the experimental measurements, showing an excellent agreement for the velocity and temperature. Qualitative agreements are observed for the soot volume fraction predictions. A close agreement was obtained in smoke point prediction for the overventilated flame. An established theory through simulations was also observed, which states that the amount of soot production is proportional to the fuel flow rate. Further validations underscore the predictive capabilities. Model improvements are also reported with better predictions of soot volume fractions through modifications to the model constants based on mixture fraction range. © 2021 American Society of Mechanical Engineers (ASME). All rights reserved.

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Department of Mechanical Engineering

Proceedings of the ASME Turbo Expo

Analytical formulation-based soot modelling in ethylene laminar jet diffusion flames

The present work showcases simulation of a turbulent diffusion flame for validation of the turbulencechemistry interaction approach. The turbulence model employed was k-ε with the model constants determined from simulation of a non-reacting turbulent jet. The jet decay rate and spread rate were determined with excellent accuracy when compared with experimental measurements. The non-dimensional velocity profiles also show a significant agreement except at radial locations far away from the axial centreline. Simulations of the DLR-A flame using a single step infinite rate chemistry assumption were carried out with the model constants thus determined. The reaction rates are determined based on the mixed-is-combusted approach where the mixture fraction, a conserved scalar quantity is obtained by the solution of a governing equation. In addition the mean velocity, species mass fractions and temperature profiles in the axial and radial direction are compared with experimental data. The axial profiles show very good agreement while the radial profiles show deviations due to the assumption of equal binary mass diffusivities for all species and unity Lewis number, which is a suitable approximation for turbulent gaseous flows. In addition, the temperatures are overpredicted by close to 15% due to the single-step infinite rate chemistry approach adopted in this study. The validation serves to justify the choice of the combustion model for the simulation of turbulent reacting flows. This is especially significant due to the lesser computational effort involved in obtaining the rates of the reaction for a turbulent diffusion flame. © 2021, Begell House Inc. All rights reserved.

International Symposium on Advances in Computational Heat Transfer

Rans based numerical simulations of turbulent diffusion flame using openfoam®

The use of crude oil as a backup fuel in gas turbines remains an economical option for power plants in petroleum-rich countries like Saudi Arabia. However, operating gas turbines with crude oil can corrode their Hot-Gas-Path (HGP) components due to the formation of vanadium oxides and alkali metal vanadates, when preventative measures are not employed. While the pre-combustion treatment of crude oil fuel is effective at reducing the content of sodium (Na) and potassium (K) in the fuel and, therefore, their resulting combustion products, any pre-combustion removal of vanadium has remained unviable on a commercial scale. Typically, a best practice limit of 0.5 ppmw vanadium is imposed on liquid fuels when inhibitors are not employed, and the total alkali content is limited to 1.0 ppmw. To evaluate the effects of incremental exceedances of such limits on the structural materials and coatings of gas turbines, a high-pressure hot-corrosion test rig was built at King Abdullah University of Science and Technology (KAUST). Arabian Extra Light (AXL) crude oil with a vanadium content of up to 2 ppm was chosen as the test fuel. This burner rig was designed to accommodate high temperatures (~1173 K/ 900 °C) and the combustion of unrefined crude oils at an elevated pressure of up to 15 bar. Highly reduced flow velocities (~1 m/s) were employed to deconvolute erosional effects from corrosion. This experiment exposed the tested materials to combustion products under these conditions for extended periods (up to 500 hours). The development of the rig included several design iterations and changes to the configuration of various sub-systems, owing to some challenges in maintaining stable combustion at low flow rates, management of coke/ash formation from the burning of crude oil, and adequate control over the pressure and temperature during extended experimental campaigns. The present publication summarizes the design and commissioning of this high-pressure hot-corrosion rig facility, and the successful demonstration of several long duration (400+ hour) experimental runs with AXL crude oil, where the environment of the sample section with test coupons was maintained at 5.5 bar and 1173 K. The pressure and temperature inside the sample section of the rig were continuously monitored. The batch of material coupons was removed from the sample section after the runs and sent for corrosion analysis. Deposits resulting from the AXL combustion and fouling were collected from several sections of the rig for analytical analysis. The fuel wash procedure, the design of components in the burner rig, and best practices of operation were also documented. Copyright © 2019 ASME.

Development of a high-pressure hot-corrosion burner rig for testing structural materials following long exposures to Arabian extra light crude oil combustion products

This work presents the first non-intrusive quantitative measurements of [OH] and temperature in H2/CO syngas/air counterflow diffusion flames as a function of H2:CO ratio and local strain rates. The H2:CO ratio was varied as 8:32, 20:20 and 32:8 volumetrically maintaining a constant dilution of 58% N2and 2% CH4. The local strain rates ranged from 35 s−1to 1180 s−1. Peak temperature and [OH] were observed to increase with increasing H2:CO ratio in the syngas fuel. Peak [OH] displayed a non-monotonous function of strain rate for all compositions of syngas. The measurements were used for rigorous assessment of five H2/CO chemical kinetic mechanisms from the literature designated as M1 to M5. The M1 mechanism was found to provide the best overall match with experiments, however, significant discrepancies were observed in comparison between measurements and [OH] predictions for low H2:CO ratio at low strain rates and for higher H2:CO ratio at high strain rates. While sensitivity analyses and reaction rate analyses identified H + O2+ M ⇔ HO2+ M as one of the key reactions affecting the [OH] at low strain rates. However, at higher strain rates, chain-branching reactions such as H + O2⇔ OH + O and O + H2⇔ OH + H were found to be important at higher strain rates. The reaction sensitivity towards [OH] was found to change from negative to positive as the strain was increased for medium and high H2:CO ratios. A reduction in the Arrhenius co-efficient for the third body reactions H + O2+ M ⇔ HO2+ M and H + OH + M ⇔ H2O + M showed significant improvements in the predictions of [OH] and extinction limits for non-premixed flames while showing good agreement with the laminar flame speed data available in the literature.

Fuel

Quantitative OH measurements and numerical investigation of H2/CO kinetics in syngas-air counterflow diffusion flames

Laser-based Diagnostics and Numerical Simulations of Syngas Combustion in a Trapped Vortex Combustor

Simulations and exhaust measurements of temperature and pollutants in a syngas-fired model trapped vortex combustor for stationary power generation applications are reported. Numerical simulations employing Reynolds-averaged Navier-Stokes (RANS) and large eddy simulations (LES) with presumed probability distribution function (PPDF) model were also carried out. Mixture fraction profiles in the trapped vortex combustor (TVC) cavity for nonreacting conditions show that LES simulations are able to capture the mean mixing field better than the RANS-based approach. This is attributed to the prediction of the jet decay rate and is reflected on the mean velocity magnitude fields, which reinforce this observation at different sections in the cavity. Both RANS and LES simulations show close agreement with the experimentally measured OH concentration; however, the RANS approach does not perform satisfactorily in capturing the trend of velocity magnitude. LES simulations satisfactorily capture the trend observed in exhaust measurements which is primarily attributed to the flame stabilization mechanism. In the exhaust measurements, mixing enhancement struts were employed and their effect was evaluated. The exhaust temperature pattern factor was found to be poor for baseline cases, but improved with the introduction of struts. NO emissions were steadily below 3 ppm across various flow conditions, whereas CO emissions tended to increase with increasing momentum flux ratios (MFRs) and mainstream fuel addition. Combustion efficiencies ∼96% were observed for all conditions. The performance characteristics were found to be favorable at higher MFRs with low pattern factors and high combustion efficiencies.

Fulltext

Journal of Engineering for Gas Turbines and Power

Numerical and Experimental Studies on a Syngas-Fired Ultra Low NOx Combustor

Flame quenching by heat loss to a surface continues to be an active area of combustion research. Close wall temperature measurements in an isothermal wall-stabilized flame are reported in this work. Conventional N2-vibrational Coherent Anti-Stokes Raman Scattering (CARS) thermometry as close as 275 $\mu$m to a convex wall cooled with water has been carried out. The standard deviation of mean temperatures is observed to be $\sim$6.5% for high temperatures (>2000K) and $\sim$14% in the lower range (<500K). Methane/air and ethylene/air stoichiometric flames for various global strain rates based on exit bulk velocities are plotted and compared. CH* chemiluminescence is employed to determine the flame location relative to the wall. Flame locations are shown to move closer to the wall with increasing strain rates in addition to higher near-wall temperatures. Peak temperatures for ethylene are considerably higher ($\sim$250-300K) than peak temperatures for methane. Preheat zone profiles are similar for different strain rates across fuels. This work demonstrates close wall precise temperature measurments using CARS.

AIAA SciTech Forum - 55th AIAA Aerospace Sciences Meeting

Temperature measurements in a wall stabilized steady flame using CARS

This article reports on the quantification of the heating induced by nanosecond repetitively pulsed (NRP) glow discharges on a lean premixed methane-air flame. The flame, obtained at room temperature and atmospheric pressure, has an M-shape morphology. The equivalence ratio is 0.95 and the thermal power released by the flame is 113 W. The NRP glow discharges are produced by high voltage pulses of 10 ns duration, 7 kV amplitude, applied at a repetition frequency of 10 kHz. The average power of the plasma, determined from current and voltage measurements, is 1 W, i.e., about 0.9% of the thermal power of the flame. Broadband vibrational coherent anti-Stokes Raman spectroscopy of nitrogen is used to determine the temperature of the flame with and without plasma enhancement. The temperature evolution in the flame area shows that the thermal impact of NRP glow discharges is in the uncertainty range of the technique, i.e., ±40 K.

Combustion Science and Technology

Investigation of Gas Heating by Nanosecond Repetitively Pulsed Glow Discharges Used for Actuation of a Laminar Methane-Air Flame

24th National Conference on IC Engines and Combustion

Numerical studies on Trapped Vortex Combustor with Syngas fuel

The current work reports quantitative OH species concentration in the cavity of a trapped vortex combustor (TVC) in the context of mixing and flame stabilization studies using both syngas and methane fuels. Planar laser induced fluorescence (PLIF) measurements of OH radical obtained using a Nd:YAG pumped dye laser are quantified using a flat flame McKenna burner. The momentum flux ratio (MFR), defined as the ratio of the cavity fuel jet momentum to that of the guide vane air stream, is observed to be a key governing parameter. At high MFRs ∼4.5, the flame front is observed to form at the interface of the fuel jet and the air jet stream. This is substantiated by velocity vector field measurements. For syngas, as the MFR is lowered to ∼0.3, the fuel-air mixing increases and a flame front is formed at the bottom and downstream edge of the cavity where a stratified charge is present. This trend is observed for different velocities at similar equivalence ratios. In case of methane combustion in the cavity, where the MFRs employed are extremely low at ∼0.01, a different mechanism is observed. A fuel-rich mixture is now observed at the center of the cavity and this mixture undergoes combustion. On further increase of the cavity equivalence ratio, the rich mixture exceeds the flammability limit and forms a thin reaction zone at the interface with air stream. As a consequence, a shear layer flame at the top of the cavity interface with the mainstream is also observed. The equivalence ratio in the cavity also determines the combustion characteristics in the case of fuel-air mixtures that are formed as a result of the mixing. Overall, flame stabilization mechanisms have been proposed, which account for the wide range of MFRs and premixing in the mainstream as well.

Quantitative OH Planar Laser Induced Fluorescence Diagnostics of Syngas and Methane Combustion in a Cavity Combustor

The current work reports optical diagnostic measurements of fuel-air mixing and vortex structure in a single cavity trapped vortex combustor (TVC). Specifically, the mixture fraction using acetone PLIF technique in the non-reacting flow and PIV measurements in the reacting flow are reported for the first time in trapped vortex combustors. The fuel-air momentum flux ratio, where the air momentum corresponds to that entering the cavity through a specially-incorporated flow guide vane, is used to characterize the mixing. The acetone PLIF experiments show that at high momentum flux ratios, the fuel-air mixing in the cavity is very minimal and is enhanced as the momentum flux ratio reduces, due to a favourable vortex formation in the cavity. Stoichiometric mixture fraction surfaces show that the mixing causes the reaction surfaces to shift from non-premixed to partially-premixed stratified mixtures. PIV measurements conducted in the non-reacting flow in the cavity further reinforce this observation. The scalar dissipation rates of mixture fraction were compared with the contours of RMS of fluctuating velocity and showed very good agreement. The regions of maximum mixing are observed to be along the fuel air interface. Reacting flow PIV measurements which differ substantially from the non-reacting cases primarily because of the heat release from combustion and the resulting gas expansion show that the vortex is displaced from the centre of the cavity towards the guide vane. Overall, the measurements show interesting features of the flow including the presence of the dual cavity structure and lead to a clear understanding of the underlying physics of the cavity flow highlighting the importance of the fuel-air momentum ratio parameter.

Experimental Thermal and Fluid Science

Optical diagnostics of fuel-air mixing and vortex formation in a cavity combustor

Syngas is being studied extensively as an effective alternate fuel for various applications in recent times. Syngas combustion offers significant challenges because of varying compositions and considerably low calorific values. Trapped vortex combustor (TVC) is an established design in gas turbine engine combustors utilizing the RQL (rich-burn quick-quench lean-burn) strategy for achieving stable combustion under various operating conditions and meeting stringent pollution norms at the same time. TVC design consists of a cavity in which a vortex is established where rich combustion takes place. The products then mix quickly with the main flow and then the ultra lean mixture undergoes combustion. Previous studies on methane combustion in TVC have shown that this concept helps to achieve a very wide operating range of equivalence ratios with low NOx and particulate emissions. A single cavity TVC rig is used to study the combustion and mixing phenomena. Preliminary experiments on syngas combustion in TVC have shown stable combustion even for very lean equivalence ratios ($\Phi$$\sim$0.1). However, the dynamics in the cavity seem to differ considerably from previous methane combustion studies. The flame is characterised by equivalence ratio which might mean that mixing in the cavity is minimal. To understand the cavity mixing, acetone PLIF has been employed. These results show that the fuel jet momentum plays a very important role in determining the mixing within the cavity. Fuel mixing increases with the increase in fuel jet momentum as also with the guide vane air momentum. Flame luminosity images do not show a flame at the center of the cavity possibly indicating a lack of cavity recirculation vortex. Further comparison between methane and syngas can be carried out by employing parameters like jet momentum ratio.

11th International Energy Conversion Engineering Conference

Experiments on stability of syngas combustion in a 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. {\textcopyright} 2013 Copyright Taylor & Francis Group, LLC.