Jingxuan Li

Beihang University (BUAA) | BUAA · School of Astronautics

Heat release rate fluctuations cause recurrent problems in many steady operating combustors. Direct measurements are difficult and these fluctuations are generally inferred from optical diagnostics. An alternative acoustic method was recently developed and is validated here in the case of unconfined laminar premixed flames submitted to harmonic flo...

where is the normalised heat release rate fluctuation and the normalised inlet velocity perturbation impinging on the flame. The FDF F(ω, |u′|) is generally expressed in the frequency domain as gain (amplitude) G(ω, |u′|) and phase φ(ω, |u′|) which are functions of both forcing frequency ω and amplitude |u′|. This approach makes the assumption of w...

Laser interferometric vibrometry(LIV) has recently been proposed as an alternative mean to obtain time-resolved density and heat release rate measurements at relatively low cost and experimental effort. This technique is sensitive to fluctuations of the refractive index of gases resulting from density and composition changes along the laser beam in...

Abstract

Lean premixed combustion chambers are susceptible to combustion instabilities arising from the coupling between the heat release rate perturbations and the acoustic disturbances. These instabilities are not desirable and knowledge of this complex mechanism is necessary in order to prevent or at least suppress them. A low order model is developed co...

The two-way interaction between the unsteady flame heat release rate and acoustic waves can lead to combustion instability within combustors. To understand and quantify the flame response to oncoming acoustic waves, previous studies have typically considered the flame dynamic response to pure tone forcing and assumed a dynamically linear or weakly...

A practical problem of applying background-oriented schlieren (BOS) to "nearly axisymmetric" flames is that the flames may be sufficiently asymmetric, while the available optical access is not always considerable to reconstruct the asymmetrical flame. This paper extends two asymmetrical Abel inversion algorithms into BOS tomography to show three-di...

In aerospace engine delivery systems, “one-in-two-out” bifurcation structures are commonly used for flow distribution to downstream pipelines. There are two common “one-in-two-out” bifurcation structures in aircraft engines: the planar orthogonal bifurcation and the spatial orthogonal bifurcation. By adjusting the flow supply upstream and the cross...

Effects of swirl divergence cup and the central bluff body on premixed flame response with external excitation are experimentally investigated. Flame transfer functions (FTFs) associated with different swirlers are measured in 50–450 Hz. The corresponding flame and flow responses are examined with the help of chemiluminescence images and Particle I...

The use of gas energy includes a wide range of applications to directly accelerate the liquid in a pipeline without the aid of mechanical equipment, such as marine gas-liquid jet propulsion. To clarify the characteristics of energy transfer by interphase forces for gas-liquid flows in variable cross-section tubes, two-fluid models of annular flow,...

Bluff-body swirling flows have been widely employed in gas-turbine combustors to achieve flame stabilization. Meanwhile, considerable efforts have been made to understand swirling flow dynamics, the effects of swirl number and bluff body on flow structure and dynamics are still not well understood. To this end, a series of direct numerical simulati...

In gas turbines, aero-engines and rocket engines, flames are always disturbed by perturbations of dual or multiple harmonic frequencies, resulting in corresponding combustion instability. This paper analyses the nonlinear response of laminar premixed flames to dual-input harmonic disturbances to further understand those associated combustion instab...

Experimental results showed that configuring different baffles (a regular flat, a single- or double-layer perforated plate and a Helmholtz resonator) in the plenum can well suppress the self-excited oscillation in annular combustor. In this paper, a 2D low-order network model is proposed to describe this phenomenon and predict the modal frequencies...

This paper demonstrates the ability of the neural network trained on frequency-sweeping signals with different amplitudes to reconstruct the flame nonlinear response. The neural network architecture consists of a decreasing sequence increasing dimension (DSID) model and a sequence model; the latter one uses the Long short-term memory (LSTM) and enc...

Predictions of thermoacoustic instabilities in annular combustors are essential but difficult. Axial variations of flow and thermal parameters increase the cost of numerical simulations and restrict the application of analytical solutions. This work aims to find approximate analytical solutions for the acoustic field in annular ducts with linear gr...

In the analysis of combustion instabilities in the annular combustors, especially for the azimuthal combustion modes, the flame motion is usually neglected and the flame is typically regarded as a discontinuity at rest. However, this simplicity may cause severe prediction errors for certain conditions. In this paper, the influence of the flame moti...

Predictions of thermoacoustic instabilities in annular combustors are essential but difficult. Axial variations of flow and thermal parameters increase the cost of numerical simulations and restrict the application of analytical solutions. This work aims to find approximate analytical solutions for the acoustic field in annular ducts with linear gr...

The bifurcation process of self-sustained combustion instability pressure perturbations in a liquid rocket combustor is investigated based on the Helmholtz equations and a pressure dependent flame describing function. The modal frequency and growth rates are numerically resolved by the commercial software COMSOL multiphysics. Validation of the nume...

Cylindrical ducts with axial mean temperature gradient and mean flows are typical elements in rocket engines, can combustors, and afterburners. Accurate analytical solutions for the acoustic waves of the longitudinal and transverse modes within these ducts can significantly improve the performance of low order acoustic network models for analyses o...

Combustion instability is the biggest threat to the reliability of liquid rocket engines, whose prediction and suppression are of great significance for engineering applications. To predict the stability of a combustion chamber with a hypergolic propellant, this work used the method of decoupling unsteady combustion and acoustic system. The turbule...

Baffled injectors are typical passive control devices to restrain combustion instabilities in liquid rocket engines. However, there still has been no appropriate model to describe the acoustic characteristics of baffled injectors. In this work, the baffled injector gap is modelled as a narrow passageway with a cross-sectional surface area gradient....

The two-way interaction between the flame and acoustic waves leads to combustion instability. To understand and quantify the response of the flame to acoustic waves, studies have typically considered single-harmonic forcing of the flame, which assumes a dynamically linear or weakly nonlinear flame response. This study extends this approach by intro...

Acoustic and entropy waves interacting in a duct with a steady heat source and mean flow are analysed using an asymptotic expansion (AE) for low frequencies. The analytical AE solutions are obtained by taking advantage of flow invariants and applying a multi-step strategy. The proposed solutions provide first-order corrections to the compact model...

Acoustic and entropy waves interacting in a duct with a steady heat source and mean flow are analysed using an asymptotic expansion (AE) for low frequencies. The analytical AE solutions are obtained by taking advantage of flow invariants and applying a multi-step strategy. The proposed solutions provide first-order corrections to the compact model...

Thermoacoustic instability in the combustion chamber of gas turbines, aero and rocket engines has become a topic concern. In this paper, a new instability phenomenon in the field of the thermoacoustic instability - Intrinsic ThermoAcoustic instabilities (ITA) is studied. The classic thermoacoustic instability is caused by the coupling among acousti...

The time-averaged heat release rates of two-dimensional laminar premixed flames subjected to axial oncoming flow perturbations are theoretically studied in this paper. The proposed models of both the two-dimensional conical flame and V-shape flame are derived from the G-equation method. The weakly nonlinear perturbation analysis has been applied in...

In the analysis of combustion instabilities, especially for the longitudinal modes, the flame typically is regarded as a discontinuity at rest and the flame motion is usually ignored. However, this may lead to errors in the prediction of combustion instabilities for certain conditions. This paper investigates the effect of flame motion on the longi...

Transversal and longitudinal combustion instabilities typically occur in gas turbines and rocket engines. Quantifying the nonlinear flame dynamic response of turbulent premixed flames subjected to the two-way disturbances and mean bulk flows is significant to understand the complex nonlinear combustion instability phenomena. A Two-way Nonlinear Unf...

In this work, experimental measurements of the heat conduction transfer function (HCTF) within the limit cycle of thermoacoustic instabilities were conducted associated with a feedback controller. The responses of the heat conduction from the heater to the air in the vicinity to the oncoming flow disturbance can be described as the HCTF. When self-...

Transversal and longitudinal combustion instabilities typically present in gas turbines and rocket engines. It is thus important to investigate the flame responses of the two directional disturbances. When the disturbance level remains weak, the flame response can be considered linear. The flame transfer function (FTF) is typically used, and the gl...

Weakly nonlinear flame (or heater) dynamic models, only accounting for the terms of heat release rate disturbances from the flame (or heater) at the forcing frequencies and omitting harmonic terms due to nonlinear mechanisms, are widely used in low-order tools for the analysis and prediction of thermoacoustic instabilities, because they have a nume...

For annular combustors, the interactions between different modal structures (longitudinal, azimuthal spinning, standing, and slanted) remain a challenge in ongoing studies of thermoacoustic instabilities. The present work develops an annular combustor named TurboCombo, similar to the multiple-injector annular combustor named MICCA, but with a conve...

Baffled injectors are widely used to restrain transverse combustion instabilities in liquid rocket engines. However, there is still a lack of theoretical research on the attenuation mechanisms. In this work, a theoretical model accounting for the visco-thermal effect has been constructed upon the narrow gap between two neighbouring injectors. The a...

Hydrogen-enrichment of conventional natural gas mixtures is an actively-explored strategy for reducing pollutant emissions from combustion. This study investigates the effect of hydrogen enrichment on the unsteady flame response to perturbations, with a view to understanding the implications for thermoacoustic stability. The Level Set Approach for...

The velocity fields of the laminar premixed flames with external acoustic excitation are studied in this work. The velocity fields in both burnt and unburnt regions have been experimentally investigated by PIV methodology. Different acoustic modulations have been applied to the flames. Dynamical mode decomposition (DMD) is adopted to elaborate the...

In this work, a weakly nonlinear analytical model of the flame describing function (FDF) of a two-dimensional laminar premixed slit flame subjected to both transverse perturbations and two-dimensional mean flows has been proposed. Two oncoming velocity convection models are used to investigate their effects on the flame tip movements and the nonlinear...

Hydrogen-enrichment of conventional natural gas mixtures is an actively-explored strategy for reducing pollutant emissions from combustion. This study investigates the effect of hydrogen enrichment on the unsteady flame response to perturbations, with a view to understanding the implications for thermoacoustic stability. The Level Set Approach for...

Low-order acoustic network models, treating the complex combustor geometry as a network of simple geometry elements, are typically used to analyse circumferential combustion instabilities in annular combustion chambers. These elements are typically assumed to have uniform cross-sectional surface area and average radius so that the analytical soluti...

Combustion instability is one significant issue in the development of aero-engines and gas turbines. In practical combustors, the exit boundary of the combustion chamber is complex and the mechanisms of thermoacoustic coupling become more complicated. In order to study the effect of outlet boundary on combustion instabilities, the premixed swirling...

Annular ducts with axial cross-sectional surface area gradient are typical elements in aero-engines and land-use gas turbines. Acoustic, entropy and vorticity waves propagating in these regions are generally coupled and may affect the thermoacoustic instabilities. However, due to the highly coupled mechanism, there has been no analytical solutions;...

Rectangular ducts with mean temperature distribution and bulk flow are typical industrial elements sustained to high-frequency transverse thermoacoustic instabilities. However, there has been no analytical solution for the three-dimensional transverse acoustic field within these types of ducts, and the present work aims to develop it. A second-orde...

This paper makes an effort to establish an explicit expression of Sauter Mean Diameter (SMD) based on the stability analysis theory and images of liquid jet surfaces observed in the experiments. It is believed that waves in a certain wavelength range will form liquid tongues on the surface of the liquid jet. Under the action of aerodynamic force, t...

Heat release rate perturbations of two-dimensional asymmetric premixed slot flames subjected to transverse perturbations, so called flame transfer functions (FTFs), are theoretically studied in this paper. The proposed models of both the two-dimensional conical flame and V-flame are studied based upon the linear analysis. The flame front dynamic re...

This paper presents analytical solutions for the three dimensional acoustic field in cylindrical and annular ducts with dependence of mean temperature on axial position. A wave equation for the pressure perturbation is constructed in cylindrical coordinates, applying a zero mean flow condition. Separation of variables is used to express the pressur...

Steady state burning and thermodiffusive instability of spherical premixed flames stabilized on a spherical porous-plug burner are analyzed using large-activation-energy asymptotic approach. The steady-state solution and linear stability solution are validated by comparing them to the published theoretical studies since there is limited experiment...

The thermoacoustic instability produced in a simple heat-driven-acoustic-oscillation device (so-called Rijke tube) has been controlled by the periodic plasma discharge. A perforated plate is placed at the upstream of the tube as the heater. Due to the interaction between the heat and acoustic waves, thermoacoustic instabilities occur. The high-volt...

The analytical solution for the one-dimensional acoustic field in a straight duct with arbitrary mean temperature gradient and mean flow has been recently derived based on an adapted WKB approximation (J. Li, A.S. Morgans, J. Sound Vib. 400 (2017) 248–269 [12]). These solutions reproduce the acoustic field very accurately across a wide range of flo...

Lean combustion technique and annular geometries are preferred in aero-engines and gas turbines, which however may lead to azimuthal combustion instabilities. Active control can be used to stabilize combustion instabilities. Due to its easy use, linear feedback controllers embedded with linear flame response models under weak perturbation amplitude...

Baffled injectors play an important role in suppressing the high frequency instability e-merged in liquid propellant rocket engines． However，the detailed mechanism of acoustic absorption enhanced by baffled injectors has not been explained clearly． To reveal the acoustic absorption mech-anism of baffled injectors，a numerical model was developed in...

Baffled injectors, have played an important role in suppressing high-frequency combustion instability in liquid propellant rocket engines by changing acoustic characteristics of a combustion chamber. However, no appropriate model can be successful predicting effects of baffled injectors on acoustic characteristics of a combustion chamber. In order...

This work performs a theoretical and numerical analysis of the communication between one-dimensional acoustic and entropy waves in a duct with a mean temperature gradient. Such a situation is highly relevant to combustor flows where the mean temperature drops axially due to heat losses. A duct containing a compact heating element followed by an axi...

Wave-based models for one-dimensional duct acoustics are widely used in thermoacoustic network models. However, they currently assume a constant mean temperature and mean flow within each duct module, while in practice many ducts of relevance sustain a significant axial temperature gradient or mean flow gradient. This paper presents an analytical s...

This paper presents an analytical solution for the one-dimensional acoustic field in a duct with arbitrary mean temperature gradient and mean flow. A wave equation for the pressure perturbation is derived which relies on very few assumptions. An analytical solution for this is derived using an adapted WKB approximation. The solution is a superposit...

This work performs a numerical study of the ORACLES combustor which contains a 1-meter-long turbulent premixed flame and sustains low frequency thermoacoustic instabilities. The flame heat release rate response to acoustic disturbances is characterized, but because the flame is long, the axial variation of the response is accounted for using an axi...

Computation of local flame describing functions for thermoacoustic oscillations in a combustor with a long flame

Combustion instabilities arise owing to a two-way coupling between acoustic waves and unsteady heat release. Oscillation amplitudes successively grow, until nonlinear effects cause saturation into limit cycle oscillations. Feedback control, in which an actuator modifies some combustor input in response to a sensor measurement, can suppress combusti...

Accurately predicting the thermoacoustic modes of a combustor depends upon knowledge of the thermodynamic properties within the combustor; flame temperature, heat release rate, speed of sound and ratio of specific heats all have a strong effect. Calculating the global equilibrium properties resulting from fuel combustion is not straightforward due...

A low-order network modelling accounting for the local flame transfer function is proposed in this work. The long flame is axially split to NOS elements with an equal width. In each segment, the acoustic network is coupled with the local flame transfer function via the acoustic jump conditions. The heat release response to acoustic forcing is deter...

When a flame burns unsteadily, two types of flow perturbation are generated. The first are acoustic waves, which propagate away from the flame at the speed of sound (relative to the flow), and are known as " direct combustion noise ". The second are temperature fluctuations, also known as hot/cold spots or entropy waves, which advect away from the...

Lean premixed combustion chambers are susceptible to combustion instabilities arising from the coupling between the heat release rate perturbations and the acoustic disturbances. These instabilities are generally harmful. A second heat source equipped downstream of the unsteady flame can be used to interrupt the coupling between the acoustic waves...

Lean premixed combustion chambers are susceptible to combustion instabilities arising from the coupling between the heat release rate perturbations and the acoustic disturbances. These instabilities are generally harmful. Active feedback control can be used to interrupt the coupling between the acoustic waves and unsteady heat release and prevent o...

Une technique optique, utilisant un vibromètre laser (LIV), est développée pour reconstruire des perturbations du taux de dégagement de chaleur à partir de mesures de fluctuations de densité intégrées le long du trajet optique sondé par le faisceau laser. Dans la zone de réaction de flammes perturbées par un forçage acoustique, la contribution acou...