Showing papers in "International Journal of Spray and Combustion Dynamics in 2011"
TL;DR: In this paper, it was shown that additive stochastic perturbations can cause triggering before the linear stability limit of a thermo-acoustic system, and the authors explored the mechanism of triggering in a simple Rijke tube, the system is seen to evolve to self-sustained oscillations via an unstable periodic solution of the governing equations.
Abstract: This paper explores the mechanism of triggering in a simple thermoacoustic system, the Rijke tube. It is demonstrated that additive stochastic perturbations can cause triggering before the linear stability limit of a thermoacoustic system. When triggering from low noise amplitudes, the system is seen to evolve to self-sustained oscillations via an unstable periodic solution of the governing equations. Practical stability is introduced as a measure of the stability of a linearly stable state when finite perturbations are present. The concept of a stochastic stability map is used to demonstrate the change in practical stability limits for a system with a subcritical bifurcation, once stochastic terms are included. The practical stability limits are found to be strongly dependent on the strength of noise.
50 citations
TL;DR: In this article, numerical simulations are performed to ascertain reasons for some of the trends of stable combustion in a single cavity TVC with methane as fuel and the range of flow conditions for stable operation.
Abstract: 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 inject...
31 citations
TL;DR: In this paper, a water-cooled, modular, atmospheric pressure Trapped Vortex Combustor (TVC) test rig is designed and fabricated for reacting and non-reacting flow experiments.
Abstract: 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 ...
29 citations
TL;DR: In this article, the authors point out the ability of the multiphase flow DNS (Direct Numerical Simulation) to help to understand basic physics and to interpret some experimental observations, including droplet dispersion, laminar spray flame instability, spray combustion regimes or acoustic modulation effect on a two-phase flow Bunsen burner.
Abstract: The objective of this article is to point out the ability of the multiphase flow DNS (Direct Numerical Simulation) to help to understand basic physics and to interpret some experimental observations. To illustrate the DNS' potential to give access to key phenomena involved in reactive multiphase flows, several recent results obtained by the authors are summed up with a bridge to experimental results. It includes droplet dispersion, laminar spray flame instability, spray combustion regimes or acoustic modulation effect on a two-phase flow Bunsen burner. As a perspective, two-phase flow DNS auto-ignition is considered thanks to a skeletal mechanism for the n-heptane chemistry involving 29 species and 52 reactions. Results highlight evaporating droplet effects on the auto-ignition process that is generally dramatically modified by spray distribution resulting from the turbulent fluid motion. This paper shows that DNS is a powerful tool to understand the intricate coupling between the evaporating spray, the t...
28 citations
TL;DR: In this paper, a model for the nonlinear heat source dynamics is obtained from unsteady computational fluid dynamics in combination with feed-forward neural network identification, where an equivalent representation for the input-output relation in Volterra series form is derived, where VOLTERRA kernels are computed in terms of the weights of the neural network.
Abstract: For prediction of limit cycle oscillations of linearly unstable thermo-acoustic systems, a frequency-domain, low-order system with explicit modal coupling is developed. To this purpose, a model for the nonlinear heat source dynamics is obtained from unsteady computational fluid dynamics in combination with feed-forward neural network identification. From the neural network, an equivalent representation for the input-output relation in Volterra series form is derived, where Volterra kernels are computed in terms of the weights of the neural network. Then the kernels are transformed into the frequency domain to obtain the higher order transfer functions, through which the modes are coupled. In this way nonlinear energy exchange among the modes can be described explicitly. Comparison with a Galerkin time domain simulation shows that deviations from purely sinusoidal behaviour in the limit cycle are captured correctly, while the computational cost is drastically reduced.
26 citations
TL;DR: In this article, a low-order state-space modeling approach for thermoacoustic systems has been developed, which is based on present and past values of nodal characteristic wave amplitudes, allowing to simulate the time evolution of the system state, but also the efficient computation of the (pseudo-)spectrum of the evolution operator.
Abstract: A low-order, state-space modeling approach for thermoacoustic systems has been developed, which is based on present and past values of nodal characteristic wave amplitudes The method allows to simulate the time evolution of the system state, but also the efficient computation of the (pseudo-)spectrum of the evolution operator It is demonstrated by comparison with a frequency-domain "network model" that eigenmodes and asymptotic linear stability are predicted correctly The influence of various model parameters (downstream reflection coefficient, temperature ratio across the heat source, magnitude and spread of heat source time delays) on transient growth of perturbation energy is explored The discussion in the present paper is limited to simple test cases, but the approach can be generalized to systems with non-trivial topology
19 citations
TL;DR: In this paper, a global mode decomposition was applied to high speed jets over a range of jet-to-ambient density ratios and Mach numbers and the maximum transient response was obtained for each system by computing an optimal superposition of non-normal global modes.
Abstract: Global mode decomposition is applied to high speed jets over a range of jet-to-ambient density ratios and Mach numbers. Maximal transient response was obtained for each system by computing an optimal superposition of non-normal global modes. The non-normality of the global modes increased with decreasing density ratio as well as decreasing Mach number.
18 citations
TL;DR: It is demonstrated that correlation image velocimetry techniques are generally superior in precision and accuracy as compared to edge detection techniques for the application of spray velocIMetry within a reasonable parameter space of noise and defocus.
Abstract: Optical diagnostic techniques are commonly used to observe the breakup of dense sprays. In order to extract quantitative data from such images, edge detection algorithms have commonly been used. However, correlation image velocimetry techniques are now also becoming available for such applications. An empirical comparison between these two techniques is demonstrated for the high-speed velocimetry of the breakup of an annular air-assisted spray. A threshold based sub-pixel interpolating edge detection algorithm is employed. Both real and synthetic images are used to determine the sensitivity of the error in these techniques to changes in both image noise and defocus, the two leading causes of information loss. It is demonstrated that correlation image velocimetry techniques are generally superior in precision and accuracy as compared to edge detection techniques for the application of spray velocimetry within a reasonable parameter space of noise and defocus.
15 citations
TL;DR: In this article, the authors developed a gated picosecond digital holocamera, using optical Kerr cell gating, to demonstrate features of gated digital holography that make it an exceptional candidate for this application.
Abstract: This paper describes work that demonstrated the feasibility of producing a gated digital holography system that is capable of producing high-resolution images of three-dimensional particle and structure details deep within dense particle fields of a spray. We developed a gated picosecond digital holocamera, using optical Kerr cell gating, to demonstrate features of gated digital holography that make it an exceptional candidate for this application. The Kerr cell gate shuttered the camera after the initial burst of ballistic and snake photons had been recorded, suppressing longer path, multiple scattered illumination. By starting with a CW laser without gating and then incorporating a picosecond laser and an optical Kerr gate, we were able to assess the imaging quality of the gated holograms, and determine improvement gained by gating. We produced high quality images of 50-200 μm diameter particles, hairs and USAF resolution charts from digital holograms recorded through turbid media where more than 98% of...
14 citations
TL;DR: In this article, a non-normal and non-linear model of a horizontal Rijke tube was examined and two types of nonlinear optimal initial states were found, which maximize acoustic energy growth over a given time from a given energy.
Abstract: This theoretical paper examines a non-normal and non-linear model of a horizontal Rijke tube. Linear and non-linear optimal initial states, which maximize acoustic energy growth over a given time from a given energy, are calculated. It is found that non-linearity and non-normality both contribute to transient growth and that, for this model, linear optimal states are only a good predictor of non-linear optimal states for low initial energies. Two types of non-linear optimal initial state are found. The first has strong energy growth during the first period of the fundamental mode but loses energy thereafter. The second has weaker energy growth during the first period but retains high energy for longer. The second type causes triggering to self-sustained oscillations from lower energy than the first and has higher energy in the fundamental mode. This suggests, for instance, that low frequency noise will be more effective at causing triggering than high frequency noise.
11 citations
TL;DR: In this paper, the phase averaged axial velocity variation of the droplets is high compared to that of phase averaged radial and tangential velocities with in an acoustic cycle.
Abstract: Combustors with fuel-spray atomizers are particularly susceptible to pressure or velocity oscillations resulting from the occurrence of thermoacoustic oscillations. Experimental investigations of distilled water spray - swirl - acoustic interactions are presented. The presence of a swirl flow field alters the behavior of water spray drastically. Investigations showed complex behavior of water sprays in an acoustic cycle. In the presence of acoustic oscillations the phase averaged droplet diameter variation with in an acoustic cycle is more at the axial locations compared with off axis locations. The phase averaged axial velocity variation of the droplets is high compared to that of phase averaged radial and tangential velocities with in an acoustic cycle. Periodic clustering of droplets in the spray field is observed in the presence of acoustic field. A maximum particle count of 14 times the minimum count in a phase angle bin is observed within an acoustic cycle for the acoustic pressure amplitude of 3000...
TL;DR: In this article, a numerical model with a multi-step chemical kinetics mechanism, variable thermo-physical properties, multi-component species diffusion and a radiation sub-model is employed for this purpose.
Abstract: Numerical investigation of laminar diffusion flames established on a flat plate in a parallel air stream is presented. A numerical model with a multi-step chemical kinetics mechanism, variable thermo-physical properties, multi-component species diffusion and a radiation sub-model is employed for this purpose. Both upward and downward injection of fuel has been considered in a normal gravity environment. The thermal and aerodynamic structure of the flame has been explained with the help of temperature and species contours, net reaction rate of fuel and streamlines. Flame characteristics and stability aspects for several air and fuel velocity combinations have been studied. An important characteristic of a laminar boundary layer diffusion flame with upward injection of fuel is the velocity overshoot that occurs near the flame zone. This is not observed when the fuel is injected in the downward direction. The flame standoff distance is slightly higher for the downward injection of fuel due to increase in dis...
TL;DR: In this article, an Eulerian/Eulerian multiphase flow model coupled with a population balance model is used as the basis for numerical simulation of atomization in swirling flows.
Abstract: An Eulerian/Eulerian multiphase flow model coupled with a population balance model is used as the basis for numerical simulation of atomization in swirling flows. The objective of this exercise is to develop a methodology capable of predicting the local point-wise drop size distribution in a spray, such as would be measured by the Phase Doppler Particle Analyzer (PDA). Model predictions are compared to experimental measurements of particle size distributions in an airblast atomizer spray to demonstrate good qualitative and quantitative agreement. It is observed that the dependence of velocity on drop size inherent in a multiphase description of the drop cloud appears necessary to capture some features of the experimental data. Using this model, we demonstrate the relative contributions of secondary atomization and transport to the variation observed in the downstream spray drop size distribution.
TL;DR: In this paper, a theoretical examination of the thermo-acoustic properties of a Rijke burner of large aspect ratio rectangular cross-section is made, and an aeroacoustic Green's function is derived which permits the sound pressure produced by arbitrary thermal and vortex sources within the burner to be calculated by convolution.
Abstract: A theoretical examination is made of the thermo-acoustic properties of a Rijke burner of large aspect ratio rectangular cross-section. Such a generic device has been proposed by Kok et al. (2009 paper presented at the 16th International Congress on Sound & Vibration) to make canonical studies of combustion instabilities. An aeroacoustic Green's function is derived which permits the sound pressure produced by arbitrary thermal and vortex sources within the burner to be calculated by convolution. The Green's function corresponds to the potential flow sound field produced by an impulsive point source; its calculation taking account of flame-holder geometry is facilitated by use of the Schwarz-Christoffel transformation. The transformation is performed numerically to accommodate complex burner geometry and validated by comparison with an alternative procedure involving the direct numerical integration of Laplace's equation.
TL;DR: In this paper, the effect of thermal radiation on the dynamics of a thermal explosion of polydisperse fuel spray with a complete description of the chemistry via a single-step two-reactant model of general order is investigated.
Abstract: We investigate the effect of thermal radiation on the dynamics of a thermal explosion of polydisperse fuel spray with a complete description of the chemistry via a single-step two-reactant model of general order. The polydisperse spray is modeled using a Probability Density Function (PDF). The thermal radiation energy exchange between the evaporation surface of the fuel droplets and the burning gas is described using the Marshak boundary conditions. An explicit expression of the critical condition for thermal explosion limit is derived analytically and represents a generalization of the critical parameter of the classical Semenov theory. Because we investigated the model in the range where the temperature is very high, the effect of the thermal radiation is significant.
TL;DR: In this article, the effect of non-linearities inside a matrix burner on the flame response and control of instabilities inside the matrix burner was discussed, in particular, the response of the flame to high inlet velocity modulation level and frequency of the forcing signal.
Abstract: This paper discusses the effect of non-linearities inside a matrix burner on the flame response and control of instabilities inside the matrix burner. In particular, the response of the flame to high inlet velocity modulation level and frequency of the forcing signal are discussed. The importance of the choice of the combustion model used in the numerical simulation of combustion instabilities is highlighted. The effect of non-linearities on the choice of the primary fuel actuator used in active control is discussed. Finally, it is shown that the occurrence of limit cycle behavior inside the combustor of the matrix burner led to hysteresis. The analysis begins with the discussion of the stability map of the matrix burner obtained from measurements.
TL;DR: In this article, a new mechanism for the generation of a self-exited oscillation of combustion in a generic combustion chamber typical for aeroengine combustors is described, where the phase change from liquid to vapour occurs when the preheat temperature of the air flowing through the burner exceeds the boiling temperature at operating pressure and the fuel flow is so low that heat transfer to the liquid fuel causes evaporation within the fuel channels of the burner.
Abstract: A new mechanism for the generation of a self-exited oscillation of combustion in a generic combustion chamber typical for aeroengine combustors is described. The cause of the oscillation is the phase change from liquid to vapour which happens when the preheat temperature of the air flowing through the burner exceeds the boiling temperature at the operating pressure and the fuel flow is so low that heat transfer to the liquid fuel causes evaporation within the fuel channels of the burner. Liquid fuel and vapour alternatively enter the airstream of the burner. This leads to an unstable situation for the flame. Measurements of chemiluminescence and liquid fuel show nearly complete extinction and re-ignition for the limit cycle. Prevention of the oscillation is possible by better thermal management of the fuel path.