S R Chakravarthy

Pulsating behaviour is observed in laminar flames of different kinds ranging from edge flames in a mixing layer to premixed flames even in partially premixed triple flame. This study focuses to study the physical mechanisms behind the pulsating behaviour of triple flames. A numerical simulation is performed for a typical case of high Lewis number and low Damköhler number, where pulsating instability is observed. The flame undergoes a limit cycle oscillations. Last cycle of oscillation is analyzed to study the physical mechanisms. Temperature, and concentration filed data is plotted for different phases of the cycle. Vector plots representing the heat and mab diffusive flux gives further insight into the flow of reactants into the flame zone and also the flow of heat away from the reaction zone. Also a closer look at the field variables at the triple point is made. With the help of these plots, some insight is obtained in understanding the underlying mechanism driving the thermo-diffusive oscillations.

In the present work, the proportion of carbon monoxide to hydrogen is widely varied to simulate different compositions of synthesis gas and the potential of the fuel mixture to excite combustion oscillations in a laboratory-scale turbulent bluff body combustor is investigated. The effect of parameters such as the bluff body location and equivalence ratio on the selfexcited acoustic oscillations of the combustor is studied. The flame oscillations are mapped by means of simultaneous highspeed CH and OH chemiluminescence imaging along with dynamic pressure measurement. Mode shifts are observed as the bluff body location or the air flow Reynolds number/overall equivalence ratio are varied for different fuel compositions. It is observed that the fuel mixtures that are hydrogen-rich excite high amplitude pressure oscillations as compared to other fuel composition cases. Higher H2 content in the mixture is also capable of exciting significantly higher natural acoustic modes of the combustor so long as CO is present, but not without the latter. The interchangeability factor Wobbe Index is not entirely sufficient to understand the unsteady flame response to the chemical composition.

The growth of the average size of liquid droplets suspended in a turbulent air flow is of paramount importance in several natural and engineering systems. Here we present an experimental study of the effects of air flow turbulent intensity on the size growth of water droplets in a polydisperse droplet field. For a given initial distribution of droplets in the size range of 0-120 μm diameter, we identify an optimum air turbulent intensity that maximizes the rate at which the average droplet diameter increases with time. The observed trend is understood in terms of droplet collision rate statistics, droplet clustering, and the existence of a crossover diameter, below and above which the number of droplets decreases and increases in time, respectively. We show that the onset of clustering suppresses the intuitive effect of an increase in droplet collision rate with air turbulent intensity, resulting in the existence of an optimum air turbulent intensity that maximizes the average droplet size growth rate due to droplet coalescence. Our results bear consequences for the understanding of warm rain initiation from clouds and the design of engines with improved combustion characteristics.

The leading edge flame (LEF) anchored in the vicinity of the interface between the solid oxidizer and solid fuel of a heterogeneous propellant is subjected to externally imposed oscillations in pressure and velocity normal to the burning surface of the propellant. The problem is investigated by means of numerical simulation. When the flame is anchored close to the surface, its oscillations exhibit a mild hysteresis, but the flame does not undergo any significant changes in its structure during the oscillatory cycle. However, when the flame is anchored close to a blow-off condition, hysteresis behavior is prominently observed, under conditions favorable to the prevalence of stable oscillations. The flame is blown off for large amplitude and/or time period of the imposed oscillations. The flame exhibits a large response, in terms of its total chemical heat release, for large time periods of the imposed oscillations, in a non-linear manner. It appears that these features involve the merger of adjacent LEFs into a premixed flame as they move away from the burning surface, and vice versa, as postulated in some of the earlier studies.

The process of agglomeration of aluminum particles on the burning surface of a ammonium perchlorate (AP)-based composite propellant is modeled using a computer algorithm. A random pack of particulate ingredients of given size and mass specifications is cast on the computer to simulate the propellant microstructure. The aluminum particles are tracked as the burning surface is regressed at an empirically inputted burning rate for a given propellant formulation, as the particles emerge at the burning surface, accumulate into filigrees, and get ignited by the near-surface leading-edge oxidizer-binder diffusion flamelets (LEFs) attached to the exposed areas of certain AP particles. An approximate heat transfer model is incorporated to estimate the ignition delays radially inward and outward from the LEFs into the filigrees accumulated over AP particles and surrounding binder/fine AP matrix layers. The delay influences the number of parent aluminum particles constituting a filigree and consequently the size of the agglomerate that the filigree rolls up into. The implementation of the algorithm is validated against experimental results available in the literature, which were specifically obtained to investigate the relationship between the decrease in the agglomerate size and attachment of LEFs over more and finer AP particles in the propellant with increase in pressure.

This study is focused on the valorization of heterogeneous municipal solid waste collected from the landfill using hydrothermal liquefaction process using glycerol as a co-solvent. The effects of temperature (300–350 °C) and residence time (15–45 min) on the yields and quality of the product fractions were investigated at 8 wt% solid loading. The yield of bio-crude significantly increased from 15.2 wt% with water as the solvent, to 58 wt% with water-glycerol (1:1 v/v) as the solvent possessing an energy content of 35.6 MJ/kg at 350 °C, 30 min. The quality of the bio-crude obtained using glycerol was comparable to that using tetralin as a hydrogen donor co-solvent. Phenolic compounds and cyclooxygenates were the major compounds in the bio-crude, and aliphatic hydrocarbons increased with residence time. Maximum energy recovery of 95% was achieved in the products with an energy consumption ratio of 0.43 for the bio-crude signifying the energetic feasibility of the process.

Time-correlated sampling of quantities in transient combustion processes requires high-speed imaging at repetition rates in the order of typical flame-inherent frequencies. The present study demonstrates the feasibility of temperature measurements in turbulent flames along a line at 10 kHz using Rayleigh scattering. High signal intensities were gained using an 80 W Nd:YAG laser for excitation in combination with an optimized combination of an achromatic lens, an objective lens and a CMOS camera. This allowed achieving signal-to-noise ratios up to 140 at 10 kHz repetition rates. The experimental setup and data processing aspects are described as well as details on the system characteristics are given. Temperature measurements of the DLR-A jet flame with a Reynolds number of 15.200 were compared with high-quality conventional 10 Hz simultaneous Raman/Rayleigh data. The data showed excellent agreement highlighting the reliability of the here demonstrated technique. © 2010 Springer-Verlag.

This paper presents burning rates as a function of pressure of several propellant formulations based on ammonium perchlorate (AP) and hydroxyl-terminated polybutadiene cured by isophorone diisocyanate, many of which exhibit significantly low (nearly zero or negative) values of the pressure exponent of the burning rate in distinct pressure ranges, termed as plateau burning rate trends. The propellants contain a bimodal distribution of AP particles with the size of the coarse and fine particles within narrow ranges whose mean values are widely separated. Two mean sizes of fine particles were considered for the propellant formulations in the present work, namely, 5 and 20 μm. These choices are based on the mid-pressure extinction behavior exhibited by the matrix of fine AP and binder contained in the propellants but when tested alone over a wide range of fine AP size and pressure. The propellants that include the fine AP/binder matrixes exhibiting a mid-pressure extinction, in turn, exhibit the plateau burning rate trends within the corresponding pressure ranges. A plateau is also observed at elevated pressures in the burning rates of some formulations, which is related to the diminishing relative importance of the near-surface leading-edge region of the oxidizer/fuel diffusion flame in the gas-phase combustion zone. The choice of the coarse AP size influences the exact pressure range within the mid-pressure extinction domain of the matrix where the propellant exhibits the plateau burning rate trends. © 2007 Springer Science+Business Media, Inc.

The present work focuses on the early stages of flame-kernel development in laser-induced spark ignited mixtures issuing out of a Bunsen burner. The time-scale of 3 μs to 1 ms associated with the flame-kernel evolution stage of an ignition event is targeted in this work. A CH4/air mixture (equivalence ratio φ = 0.6) is studied as a base case, and compared with CH4/CO2/air (mole fractions = 0.059/0.029/0.912, respectively) and CH4/H2/air (mole fractions = 0.053/0.016/0.931, respectively) mixtures for nearly the same adiabatic flame temperature of 1649 K. The spatio-temporal flame-kernel evolution is imaged using planar laser induced fluorescence of the OH radical (OH-PLIF), simultaneously with H-alpha emission from the plasma. The H-alpha emission suggests that the plasma time-scale is well below 1 μs. The PLIF images indicate all the stages of kernel development from the elongated kernel to the toroidal formations and the subsequent appearance of a front-lobe. The different time-scales associated with these stages are identified from the rate of change of the kernel perimeter. The plasma is followed by a supersonic kernel-perimeter growth. Larger flame-kernel spread is found in the case of CH4/H2 mixtures. A distinct shift in the trends of evolution of LIF intensity and kernel perimeter is observed as the fuel concentration is varied near the lean flammability limit in CH4/air (φ = 0.35-0.65) and H2/air (φ = 0.05-0.31) mixtures. The flow velocity (Reynolds number, Re) effect in both laminar and turbulent flow regimes (Re = ~600-6000) indicates that the shape of the flame-kernel changes at higher velocities, but the size of the kernel does not change significantly for a given time from the moment of ignition. This could be due to a balance between two competing effects, namely, increase in the strain rate that causes local extinction and thus decreases the flame-kernel growth, and increase in the turbulence levels that facilitates increased flame-kernel surface area through wrinkling, which in turn increases the flame-kernel growth.

The dynamical behaviour of a radially outward moving vertical liquid sheet is examined experimentally in the present work. When a cylindrical liquid jet impinges orthogonally on a single slotted cone-disc deflector, two liquid sheets form simultaneously- a circular liquid sheet issues at the solid part of horizontal deflector and a vertical sheet forms from the slot of the deflector. Since the radially expanding sheet has been studied more extensively, the present work focuses on the radially outward moving vertical sheet. The goal is to correlate the breakup events at leading and lower edges of the vertical sheet to the dimensions of ligaments and droplets that are formed. High-speed shadowgraphy at 5.4 kHz framing rate is carried out simultaneously to observe the front, side, and top views of the vertical liquid sheet at different jet Weber numbers over the range 523 < Wejet < 3786. It is found that the breakup distance of the leading edge increases with the Wejet. Further, in the vertical sheet, the surface tension force is stronger at the lower edge as compare to leading edge, which causes ligaments and droplets larger in size as compared to those produced from the leading edge. The droplet D32 or Sauter mean diameter (SMD) from the leading and lower edges is measured and found to follow Wejet −1/3 powerlaw. Correlations are obtained between the D32 at the leading and lower edges with Wejet, ligament size, sheet thickness and breakup distance.