Research
The research conducted at TDCE cater to fundamental as well as applied and industry-driven challenges in combustion
The research conducted at TDCE cater to fundamental as well as applied and industry-driven challenges in combustion
Addition of oxygenates to diesel fuel has been found to reduce soot emissions and increase its resistance to extinction, which means that a combustion system can be made more stable even at higher strain rates and at the same time operate with reduced emissions. In order to critically evaluate the overall combustion behaviour of DME via numerical simulations, an accurate as well as compact kinetic mechanism consisting of 23 species and 88 elementary reactions is proposed to describe the oxidation of DME in premixed as well as non-premixed systems. This mechanism is further reduced by introducing quasi-steady state assumptions for six intermediate species to finally obtain a 14-step global kinetic scheme. A code is developed in MATLAB to obtain these 14 global steps and their corresponding rate expressions in terms of the elementary reaction rates.
Ignition delay has an important role in combustion. Ignition delay is affected by different parameters such as temperature, pressure, Equivalence ratio, etc. The objective of the topic is to predict ignition delays of fuels based on the molecular structure and the aforementioned parameters.
Biodiesel is a potential alternative to fossil diesel. In combustion simulations, in order to circumvent the difficulty in integrating reaction schemes for biodiesels, which are typically of a large size and not well understood, a surrogate approach to simplify the representation of its long chain methylester components is adopted. In this work, a compact reaction scheme for methyl butanoate, which is a potentially important candidate for biodiesel surrogates, is derived, updated with recent literature, and comprehesively validated. The effect of addition of low-temperature chemistry pathways to the methyl butanoate chemical kinetic mechanism has also been explored.
Biodiesel, a mixture of different methyl esters is one of the potential fuels to replace conventional fossil fuels. There is a need to understand the kinetics of methyl esters. In our recent studies the kinetics of methyl butanoate, a saturated methyl ester has been investigated as it can represent the methyl ester functional group of biodiesel. The composition of biodiesel reveals a significant amount of unsaturated esters and hence unsaturated ester kinetics has to be investigated. In this study, the kinetics of methyl crotonate will be studied leading to the development of a combined biodiesel surrogate formulation comprising of dodecane, methyl butanoate and methyl crotonoate. The importance of unsaturation will be studied and rate rules for larger esters will be formulated.
Poly methyl methacrylate (PMMA) is widely used in transport and construction sectors. In order to analyze the high temperature gas phase oxidation of PMMA, and thereby predict its fire behaviour with less computational effort, a compact kinetic model for the oxidation of its primary decomposition product, methyl methacrylate (its monomer) CH2=C(CH3)-C(=O)-O-CH3 (MMA), is most essential. As a part of this work, a compact reaction model to describe the oxidation of MMA is developed and validated against fundamental experimental datasets obtained in premixed flames. The final `short MMA mechanism' consists of 88 species and 1092 reactions.
Despite the push for renewable energy sources, coal combustion is to play a major role in the energy sector of the world and India in particular. Any increase in the efficiency of the existing coal combustion technologies would lead to a significant reduction in fuel consumption and carbon-dioxide emissions. Motivated by the MILD combustion technique using jets with large velocity differential, efforts are on to combust/gasify high ash Indian coal in an efficient manner. Computational studies which are backed by a single particle coal combustion model, shed light on the combustor dynamics. Insights from the preliminary experiments and computations are used to tailor the composition of the syngas obtained from gasification of coal using mixtures of O2/CO2/H2O.
In this study, co-gasification of rice husk and coal in a lab-scale bubbling bed gasification reactor is reported. Blends of Indian coal with 36% ash and Indian rice husk with 22% ash are used considering proper particle sizes, air flow rates and steam flow rates. Air and steam are used as gasification agents. The reactor is operated at 40 kW (thermal), under atmospheric pressure. Blends are prepared on the basis of power share; rice husk percentage in the blend is varied to contribute to 0–90% of the total power. Results shows that when rice husk is added, all the performance metrics such as total carbon conversion, cold gas efficiency and calorific value of the synthetic gas, show increasing trend. When rice husk contributes from 50% to 75% of the total power, the total carbon conversion is around 89%, cold gas efficiency is around 78% and the calorific value of the synthetic gas is around 5.4 MJ/cubic-meter. Methane yield increases from a volumetric percentage of 1% with 0% rice husk to around 8.37% with 75% rice husk. In summary, a good performance is achieved and blending of rice husk and high ash coal is highly beneficial.
Syngas generation using air as a gasifying medium with biomass/coal as fuel is a well established technique used currently in small and medium power and heating applications. It is also possible to tailor the syngas compostion in a single step process by replacing the inert nitrogen component in air with reactive components like CO2 and steam. The resulting syngas, rich in CO and hydrogen can be used in catalytic polymerization reactors (say FT for instance) to produce liquid fuels and fine chemicals. Objective of the work is explore the fundamental characteristics gasification/combustion of biomass in a canonical counter current reactor with mixtures of O2/CO2/steam as oxidizers. Syngas quality improvement, understanding the dynamics of packed beds and framing a theoretical model to predict the propagation rate and gas compostion are the main outcomes of the work.
Screech is a high frequency tonal noise generated from afterburners (AB, for short and used for thrust augmentation in gas turbine engines) as a result of coupling of combustion heat release fluctuations with the acoustic modes of the afterburner. Liner, which is a perforated annular chamber enclosing the flame holder and extending through the length of the AB acts as passive damping device to suppress amplification of acoustic oscillations. The damping characteristics of the liner are dependent on the size, number and more importantly the Mach numbers of the bias and grazing flow. We are developing a CFD based tool for design of acoustic liners for ABs.
A new and novel approach to modeling composite propellant burn rate behavior is proposed based on the fact that composite propellant combustion is largely boxed between the premixed limits – of pure AP and fine AP-binder (HTPB, here) whose burn behaviors are taken as known. The current strategy accounts for particle size distribution using the burn time averaging approach. The specialty of the present approach is that it invokes local extinction for fuel rich conditions for specific particle sizes when the heat balance causes the surface temperature to drop below the low pressure deflagration limit of AP; this feature allows for the prediction of extinction of propellant combustion. Comparisons of burn rate data over nearly thirty compositions from different sources appear excellent to good.
Reducing supersonic jet noise by active and passive control techniques has been of great interest to researchers and scientists. Its application in commercial and military aviation is vital for both safety and human comfort. In the present study, the passive method is used to control the under-expanded free jet noise by using passive grids. The main advantage of passive control method over the active control method is that the method does not require any external power source for noise control. The presence of grids successfully eliminate the dominant tones (screech) by disrupting the feedback loop and attenuate the Broadband Shock Associated Noise by converting strong shock cells into weaker multiple shock-lets.
The current work investigates the effect of Hartmann cavity acoustics on the atomization of droplet sprays. Initially, the experiments are conducted on a single droplet to understand its behavior in the sound field of a Hartmann whistle. The atomization studies on single droplet reveal that the existence of sound field causes the droplet to undergo large deformation and become irregular in shape. The degree of droplet deformation is quantified based on smaller circularity and larger Feret's diameter. The increase in cone angle of spray to a higher value in the presence of acoustics in comparison to its absence shows that the acoustics enhances the atomization. The stroboscopic visualization of sprays in the presence of acoustics further reveals the breakup of ligaments, large scatter as well as the formation of more number of droplets, indicating atomization enhancement.
Cavities placed in high speed flows can serve as high intensity, narrow band acoustic sources with selective directivity. Such sources can be gainfully used to control the flow, mixing and combustion processes in various propulsion applications.The flow diversion around the cavity explains the observed shift in directivity towards higher angles for the whistle, as compared to the free jet flow. The acoustic power and efficiency are high for small values of stand-off distances and larger cavity lengths.
An experimental investigation of acoustic radiation from underexpanded air jets of different shear layer thicknesses has been performed. The initial shear layer thickness variation is achieved by allowing the jet to exit through pipes of various lengths. Acoustic radiation is characterized in terms of overall sound pressure level, directivity, tonal, and broadband shock associated noise. Increase in initial shear layer thickness in pipe jets results in the decrease of screech tone amplitude and increase in broadband shock associated noise level. Turbulent mixing noise levels are higher for shorter pipe jets compared to longer ones. Longer pipe jets exhibit more number of screech modes while the shorter pipe jets show only one or two screech modes. The screech frequency and the peak frequency of broadband shock associated noise do not show much variation with increase in initial shear layer thickness.
Influence of swirl number on jet noise reduction has been studied. In this work, jet noise reduction using a swirling flow surrounding a circular free jet has been demonstrated and the flow visualization also done to confirm the noise reduction. The co-axial swirl jets always reduce the low frequency noise, irrespective of the nozzle pressure ratio. The screech tone is entirely eliminated and broadband shock associated noise mitigated by the co-axial swirl jets This work proposes swirl as an excellent passive tool for jet noise suppression.
The emphasis is on understanding the effect of higher ambient pressure and temperature as well as the initial fuel temperature on the spray dispersion and evaporation processes. The spray characteristics like penetration length, SMD, droplet and gas-phase velocities, and the mixture distribution are investigated.
In this work, the effects of orifice divergence on spray characteristics have been reported. Parameters such as spray cone angle, liquid sheet thickness, coefficient of discharge, break-up length, and Sauter mean diameter are greatly affected by the half divergence angle at orifice exit. An experimental investigation is carried out in which water sprays from five atomizers having half divergence angle values of 0 degrees, 5 degrees, 10 degrees, 15 degrees, and 20 degrees are studied at different injection pressures. Image processing techniques are used to measure spray cone angle and break-up length from spray images, whereas the sheet thickness outside the orifice exit is obtained using the scattered light from a thin Nd-YAG Laser beam. Phase Doppler interferometry is also used to obtain the Sauter mean diameter at different axial locations. A few numerical simulations based on the volume of fluid method are included to obtain physical insight of the liquid film development and air core flow inside the atomizer. It is observed that the liquid sheet thickness as well as tangential and radial components of velocity at orifice exit are modified drastically with a change in half divergence angle. As a consequence, the droplet size distribution is also altered by variation in the nozzle divergence angle. The mechanism responsible for such variations in the spray behavior is identified as the formation of an air core or air cone inside the liquid injector as a result of the swirl imparted to the liquid flow.
Spray breakup processes of small-scale simplex atomizers have been characterized. With tangential entry of fuel, the cases of no air flow and concentric co-swirl air flow are considered. Ten simplex atomizers with different values of dimensionless tangential port area K have been employed. The breakup mode changes from film breakup to jet breakup, with increase in K value. For small K values, small port area causes high liquid swirl, which aids in the formation of an air core and a hollow cone spray with large cone angle. At high K values, jet breakup gives rise to a full cone spray with a small cone angle. The SauterMean Diameter (SMD) for the spray exhibits bimodal variation in the radial direction close to the atomizer. Based on a large volume of data for kerosene and water sprays, accurate correlations (covering wide operating conditions) have been developed for the spray cone angle, Cd, and axial variation of SMD for small-scale simplex atomizers. Swirl air interaction with the liquid film causes a transition from closed tulip shape to an open spray at a critical air flow rate. The critical air flow rate for transition is initial condition-dependent and it exhibits hysteresis.
The paper presents a two-phase numerical model to simulate transient vaporization of a spherical two component liquid fuel droplet. The model considers variation of thermo-physical properties in both liquid and vapor-phases, as functions of temperature and species concentrations. Multi-component diffusion and surface tension effects are also considered. The model has been validated using the experimental data available in literature. The validated model is used to study the vaporization characteristics of both suspended and moving methanol–ethanol blended droplets in an atmospheric pressure environment. Relative strengths of forced convection and Marangoni convection are studied and compared for both suspended and moving droplets. Results in terms of streamlines, isotherms and isopleths at different time instants are reported and discussed. For low relative velocities, solutal Marangoni effects are seen to be important.
This paper presents the numerical simulation of evaporation of a moving two-component liquid fuel spherical droplet under atmospheric pressure. The transient two-phase numerical model includes variations of thermo-physical properties as functions of temperature and species concentration in liquid and vapor phases, multi-component diffusion, and surface tension effects. The model has been validated using the experimental data available in the literature for suspended heptane-decane-blended droplets evaporating under a forced convective air environment. The validated model is used to study the vaporization characteristics of moving binary droplets. The blends considered in this study are isooctane blended with ethanol and decane blended with methyl-butyrate. The temporal variations of the evaporation constant, droplet Reynolds number, and drag coefficients are presented. Variations of integrated quantities, such as the time-averaged evaporation constant, droplet lifetime, and droplet final penetration distance as a function of blend composition, are also presented. The behavior of isooctane-ethanol blends is seen to be quite different from that of methyl-butyrate-decane blends.
Numerical simulations of the evaporation of stationary, spherical, two-component liquid droplets in a laminar, atmospheric pressure, forced convective hot-air environment are presented. The transient two-phase numerical model includes multi-component diffusion, a comprehensive method to deal with the interface including the surface tension effects and variation of thermo-physical properties as a function of temperature and species concentration in both liquid- and vapor-phases. The model has been validated using the experimental data available in literature for suspended heptane-decane blended droplets evaporating under a forced convective air environment. The validated model is used to study the vaporization characteristics of heptane-decane droplets under different convective conditions. For an initial composition having 75% by volume of more volatile fuel component, the evaporation transients are presented in terms of variations in interface quantities. Flow, species and temperature fields are presented at several time instants to show the relative strengths of forced convection and Marangoni convection. Results show that at low initial Reynolds numbers, the solutal Marangoni effects induce a flow-field within the liquid droplet, which opposes the flow of the external convective field. The strength of this liquid-phase flow field increases with the consumption of the more volatile fuel component.
This paper presents comprehensive numerical simulations of evaporation of droplets constituted of two liquid fuels in high pressure nitrogen ambient under normal gravity condition. A transient, two-phase and axisymmetric numerical model has been used for the simulations. Transport processes in liquid- and vapor-phases have been solved along with interface coupling conditions. Gas-phase non-idealities, solubility of ambient gas in liquid-phase, and pressure and temperature based variable thermo-physical properties in both liquid- and vapor-phases are considered in the numerical model. Phase equilibrium has been estimated using fugacity coefficients of all species in both phases. The range of Weber number has been chosen such that droplet remains almost spherical throughout its lifetime. Simulations have been carried out until the droplet surface regresses to one-tenth of its initial value or when the critical state for the mixture is reached. The numerical model has been quantitatively validated against the experimental data available in literature. The validated model is used to systematically study the evaporation characteristics of suspended n-heptane-hexadecane droplets in nitrogen ambient. The effects of the pressure, temperature, initial liquid-phase composition and forced convection velocity on evaporation characteristics have been discussed in detail.
The paper has two goals. First, to experimentally characterize the droplet clusters in air-assist sprays, and second, to study the consequence of droplet clustering on the local spray unsteadiness by measurement of turbulent number flux of droplets. Unlike clustering of droplets in spray-laden turbulent flows, the entrained air flow around a spray plays a very important role in the dispersion of droplets in the spray. The present work also examines the influence of local liquid mass fraction on clustering of droplet in the spray, which has not been considered in detail in the earlier works.
Water splitting is the chemical reaction in which water is broken down to oxygen and hydrogen. In this fuel-hungry world, hydrogen can prove as an alternative, clean source of energy. The generation of hydrogen has been in research since 1960s, but the efficiency and feasibility have to be improved to reach a practically viable state. Production of Hydrogen using Solar concentrated power was found to be the most efficient.
Biogas is an alternative fuel that typically contains around 45% carbon-dioxide by volume, besides methane. Due to the inherent content of carbon-dioxide, it is necessary to study the flame characteristics and stability limits in cross-flow non-premixed burners. In this study, cross-flow non-premixed flames, where biogas is injected through a horizontal porous plate and air is blown parallel to the fuel injector, are studied systematically. In order to increase the stable operating regime, devices such as backward facing steps and cylindrical bluff-bodies are commonly employed. Different step-heights and locations from leading edge of the fuel injector are considered for the cases with backward facing steps. A rectangular cylindrical bluff-body is also used as a flame stabilizing obstacle. Baseline cases are studied without any backward facing step or cylindrical bluff-body. Volume flow rate of biogas is varied from 36 liter per hour to 360 liter per hour. Air velocity is varied in the range of 0.2 m/s to 3.0 m/s. For a given fuel velocity, air velocity is gradually increased in order to record the transition of flame from one regime to another. Flame stabilization is carefully assessed by monitoring the high definition direct flame photographs captured from front and top views, for all the cases. The cases are repeated at least three times to ensure repeatability. Stability maps are plotted as a function of fuel velocity and air velocity for all the cases. For cases with backward facing steps, both step height and its location play an important role in delineating the boundaries of the flame regimes. Parametric variations show interesting features. Bluff-body flames become quite oscillatory and three dimensional at higher air velocities. For this case, stability maps of flames from biogas and pure methane are compared.
Premixed flames in confined channels give rise to some of the interesting physical phenomena. The evolution path of the flame kernel and flow field after ignition are very important in determining final flame and flow features. The problem of stability of the premixed flames in meso-channels is studied widely in the literature due to its widespread applications. The study concerns about this aspect of flame holding in meso-channels and associated flame and flow instabilities.
Experimental investigation of the effect of flow turbulence on the steady state burning of methanol is reported. A vertical air tunnel has been mounted with a grid at its exit plane in order to generate turbulence in the free jet stream. The flow field has been characterized using a hot-wire anemometer. Mean and fluctuating flow velocities and integral scale have been measured at an axial location of around 2 times its exit diameter (D). Three types of grids have been used. Classical porous sphere experiments have been carried out to analyze the steady-state burning rate of methanol over the surface of an inert sphere having constant diameter. Experiments have been done at atmospheric pressure under ambient temperature and normal gravity conditions. A porous sphere is positioned at an axial location of 2D, where the approaching flow has been characterized in detail. Results show that the burning rate as well as the flame stability are greatly influenced by the free stream turbulence. The ratio of turbulent time scales and the chemical time scales for grid mounted cases have been estimated from the integral scale, root mean square velocity fluctuation, flame stand-off distance and the vapor blowing velocity. Empirical expression relating the normalized burning rates to diffusion scale based Damköhlar number has been presented. A correlation for Sherwood number as a function of Reynolds number and turbulent intensity has also been proposed.
Experimental and numerical investigations of upward and downward flame spread over flat polymethyl methacrylate (PMMA) slabs are presented here. Experiments have been carried out using PMMA slabs of different thickness in the range of 1.6 mm–5.4 mm. Downward and upward flame spread processes have been recorded under atmospheric pressure and normal gravity conditions. Careful repeatable high resolution measurements of temperature and species fields have also been carried out, to fill the scarcity of such data in literature. A simple numerical model, used widely to simulate flame spread over condensed surfaces, called Fire Dynamics Simulator (FDS), has been employed to numerically simulate the experimental cases. Results from FDS have been validated against numerical and experimental data from literature, by comparing quantities such as mass loss rate, flame spread velocity and flame structure. FDS is seen to capture essential transport processes of a spreading diffusion flame. The overall comparison of the trends has been quite reasonable. Numerical model is capable of predicting the unsteady and steady features of downward spread as well as transient rapid upward flame spread, as observed in the experimental results.
With the need for sustainable energy, demand for clean and renewable energy is increasing. Hydrogen fuel cells is hence, gaining popularity for its application in the Automotive sector. However, there are safety concerns with regard to handling and storing of hydrogen. Therefore, the idea is to produce hydrogen on the spot and feed it directly to the Fuel cells. This can be done through a process called steam reforming, where hydrocarbons can be broken down in the presence of heat. Study of such a process is done using meso and micro-channel systems in view of space and process optimization.
A jet of fluid intermittently ejects from a nozzle causes the roll-up of boundary layers, there by it leads to the formation of vortex rings. A vortex ring consists of rotating core fluid at its top portion, followed by a tail at its bottom. During the upward motion of the vortex ring, the rotating vortex core drags the ambient fluid along with it through viscous action. This will improve the mixing between vortex fluid and the surrounding ambient fluid. In the current research work, mixing between the fuel and the oxidizer are studied for different vortex generating conditions. The fuel-air mixture inside the vortex rings is ignited and the resulting reacting (combusting) vortex rings are also studied for the different fuel-air mixture compositions of vortex fluid.
