Heat transfer from a flat plate under impinging acoustic oscillations
Robinson Nick*, Lawn ChrisIn a particular design of thermoacoustic engine for electricity generation, heat transfer to the hot end of the regenerator is through radiation from a plate heated by a fire underneath it. The travelling acous-tic waves impinge on the plate and cool it substantially, thus degrading the heat transfer to the regener-ator. The need to quantify this process uncovered the absence of data for heat transfer to impinging acoustic waves, and led to this experimental study. A 300mm circular duct was positioned vertically over a flat aluminium plate, which was electrically heated from below to about 58 degC by tempera-ture-controlled pads. Two sub-woofers fed sound into the duct, and the acoustic impingement velocity was determined by extrapolating two-microphone measurements, for three frequencies: 30 Hz, 50 Hz and 90 Hz. The temperature difference between the plate and the impinging air was recorded by ther-mocouples, and was typically 30 degC. The heat flux distribution from the plate to the air was meas-ured by traversing a calibrated conduction meter and the Nusselt numbers were evaluated as a function of the amplitude of the acoustic velocity and the frequency. As expected from the measurement of acoustic velocity profiles close to the plate, Nu increased radially from a base level on the axis of the duct to a position near the lip, and then decreased rapidly as the acoustic waves spread out into the surroundings. For given radial positions, provided the enhancement in heat transfer over the natural convection background exceeded 50%, a linear correlation of Nu with the ratio of stroke length to thermal penetration depth was found. Enhancements in heat transfer by up to a factor of five were recorded. However, suppression of the natural convection was observed when the stroke lengths were short.
Recurrence analysis of forced synchronization in a self-excited thermoacoustic system
Murugesan Meenatchidevi*, Balusamy Saravanan, Hochgreb Simone, Li Larry K BWe use recurrence analysis to investigate the forced synchronization of a self-excited thermoacoustic system. The system consists of a swirl-stabilized turbulent premixed flame in an open-ended duct. We apply periodic acoustic forcing to this system at different amplitudes and frequencies around its natural self-excited frequency, and examine its response via unsteady pres-sure measurements. On increasing the forcing amplitude, we observe two bifurcations: from a periodic limit cycle (unforced) to quasiperiodicity (weak forcing) and then to lock-in (strong forcing). To analyse these bifurcations, we use cross-recurrence plots (CRPs) of the unsteady pressure and acoustic forcing. We find that the different time scales characterizing the quasiperiodicity and the transition to lock-in appear as distinct structures in the CRPs. We then examine those structures using cross recurrence quantification analysis (CRQA) and find that their recurrence quantities change even before the system transitions to lock-in. This shows that CRPs and CRQA can be used as alternative nonlinear tools to study forced synchronization in thermoacoustic systems, complementing classical linear tools such as spectral analysis.
Sensitivity analysis of thermoacoustic instabilities
Sogaro Francesca*, Morgans Aimee, Schmid PeterThermoacoustic instability is a phenomenon that occurs in numerous combustion systems, from rockets to land based gas turbines. The resulting acoustic oscillations can result in severe vibrations, thrust oscillations, thermal stresses and mechanical loads that lead to fatigue or even failure. This propensity to instability has been found to occur much more frequently in lean premixed combustion, one of the recent methods used in the gas turbine industry of aeroengines and power gas turbines to reduce NOx emissions.
In this work we consider a simplified combustion system, and analyse the sensitivity of its thermoacoustic modes to small changes in the flame and combustor geometry parameters. Such a sensitivity analysis offers insights on how best to change the combustion system so as to "design-out" instability.
The simplified combustor is modelled using a low order network representation: linear plane acoustic waves are combined with the appropriate acoustic boundary and flame jump conditions and a linear n-tau flame model.
A sensitivity analysis is then performed using adjoint methods, with special focus on the sensitivity of the modes to parameters, such as reflection coefficients and flame model gain and time delay. The gradient information obtained reveals how the thermoacoustic modes of the system respond to changes to the various parameters. The results offer key insights into the behaviour and coupling of different types of modes – for example acoustic modes and so-called "intrinsic" modes associated with the flame model. They also provide insights into the optimal configuration for the design of such combustors.
Stabilization of self-sustained acoustic oscillation using helmholtz and quarter wave resonators
Bourquard Claire*, Noiray NicolasThe robust reduction of thermoacoustic instabilities can be achieved by the implementation of acoustic dampers on the combustion chamber walls. This work provides a simple analytical model to analyse the acoustic coupling between the dampers and the combustion chamber. The damper is continuously purged and modeled as a harmonic oscillator with the assumption that vortex shedding at the resonator mouth is the main dissipation mechanism. The combustion chamber is modeled as a cavity featuring an linearly unstable thermoacoustic mode. The eventual detuning of the damper is taken into account by the model. It is shown that using the analytical model can support the choice for the best damper geometry depending on the constraints on the available damping volume and purge mass ﬂow. The analytical model is then experimentally validated on a simple rectangular cavity, where the thermoacoustic instability resulting for the interaction between heat release and acoustic pressure is mimicked by a feedback loop using a loudspeaker and microphones. The stabilization capabilities of a Helmholtz and a Quarter-Wave damper are compared and the experimental results are in good agreement with the analytical predictions.
Coupled cfd-green's function approach for prediction of combustion instabilities in gas turbines
Alessandra Bigongiari*, Maria Heckl, Dmytro IurashevMany combustion systems, such as industrial gas turbine engines, are prone to suffering thermo-acoustic instabilities. This is a phenomenon where a feedback occurs between the acoustic waves and the heat release rate from the flame in a combustion chamber, generating pressure oscillations of high amplitudes, which in turn can cause serious damage to the combustor hardware. It is very important to predict under what conditions such instabilities occur and what oscillation amplitudes are reached. The aim of this paper is to present a fast prediction tool based on a one-dimensional Green's function approach that can be used to bypass numerically expensive computational fluid dynamics (CFD) simulations. We will demonstrate this tool by applying it to the case of a laboratory swirl burner. The flame will be modeled using a Flame Describing Function derived from full three-dimensional CFD simulations. Stability predictions will be compared with results obtained from three-dimensional simulations for selected operating conditions (limited by the high computational cost).
Optimising the acoustics of short circular holes with mean flow
Johnson Holly G.*, Morgans Aimee S.Short circular holes with a high Reynolds mean flow passing through them are a common
occurrence in applications such as Helmholtz resonators, perforated plates or liners and fuel injectors.
The acoustic response of such holes has been shown to be strongly dependent on the
path followed by the vorticity which is shed at the hole inlet and convected downstream to form
a vortex sheet. Coupling between this vorticity and the acoustic waves has the potential either
to absorb or to generate acoustic energy in the low frequency region. A semi-analytical model
based on Green's function method (The acoustics of short circular holes opening to confined and
unconfined spaces, Yang & Morgans, Journal of Sound and Vibration, 2017) is combined with a
gradient-based optimisation technique to determine the optimal vortex sheet shapes for absorption
or amplification of noise. As the shape of the vortex sheet depends directly on the geometry of
the hole inlet, finding the optimal shape provides information on the geometry required to achieve
the desired acoustic effect.
Phase space dynamics of thermoacoustic interactions during vortex acoustic lock-on
Singh Gurpreet, Mariappan Sathesh*, Singaravelu BalasubramanianWe investigate the dynamics of thermoacoustic interactions associated during vortex acoustic lock-on in the phase space. Experiments are performed in a premixed, gas fueled bluff body stabilized combustor. The geometry of the burner is designed such that vortex shedding from the bluff body is strong. Air flow rate is varied in a quasi-steady manner, for a given fuel flow rate. It is observed that at low air flow rates, the dominant frequency increases linearly with the air flow rate. The obtained Strouhal number matches with the mode associated with vortex shedding from the bluff body. On the other hand, for higher flow rates the dominant frequency remains almost constant with the air flow rate, indicating the acoustic mode of the combustor. It is also found that at these flow rates, vortex shedding process locks on to the frequency of the acoustic mode. Tools from nonlinear time series analysis are applied to study this transition. The attractor is reconstructed in the phase space and its properties are monitored.
Stability analysis of a matrix burner flame using a generalised heat release rate law featuring multiple time-lags
Gopinathan Sreenath Malamal*, Heckl Maria A.In the present work, we perform a stability analysis of a matrix burner configuration consisting of a semi-infinite 1D duct. It is closed with a rigid piston at the upstream end. A cavity is formed by a perforated plate placed a finite distance downstream of the piston; this perforated plate also acts as a flame holder, in that it stabilises a matrix flame. The distance between the closed end and the perforated plate, i.e. the cavity length, can be varied by varying the piston location. Our model for the flame is based on the FDF (amplitude-dependent flame transfer function) measured by Noiray (Ph.D. Thesis, École Centrale Paris, 2007). It is an analytical expression and features two prominent time-lags, i.e. we write the heat release rate in terms of the time-delayed velocity as a superposition of two Gaussians, each characterised by three amplitude-dependent quantities: central time-lag, peak value and standard deviation. The parameters of our time-lag model are deduced from the experimental FDF using error minimisation and nonlinear optimisation techniques. We then analyse the stability behaviour of the combustion system using a tailored Green's function approach. The main parameters of interest are the amplitude and the cavity length.
Linear and nonlinear stability predictions for a domestic boiler with a heat exchanger for passive instability control
Surendran Aswathy*, Heckl Maria A.In this study, we investigate the stability behaviour of a domestic boiler system, when the heat absorption rate at the heat exchanger (hex) varies with the mean flow velocity and with the amplitude of velocity fluctuations upstream of the hex. The domestic boiler system is modelled as a 1D quarter-wave resonator (open at one end and closed at the other). The hex is placed near the closed end, and the flame is upstream of the hex. The flame is modelled as a compact heat source, assumed to follow a basic n - τ law. The heat absorption rates at the hex are treated as amplitude-dependent and are calculated in two ways: (1) from numerical simulations and (2) from an empirical correlation based in King's law for the heat release rate of a hot wire. The stability analysis is done in the frequency domain, and the stability predictions are carried out using the classical eigenvalue method.
The marginal condition for the onset of thermoacoustic oscillations of a gas in a tube having a wet wall porous medium
Yuki Ueda*, Kenichiro TsudaPropagation of an acoustic wave in a tube causes the oscillations of pressure, temperature, and specific volume of a gas. When these oscillations are coupled with the thermal interaction between the gas and the wall of the tube, a rich variety of thermoacoustic phenomena is produced. As one of the thermoacoustic phenomena, a self-sustained gas oscillation is known: when a porous medium located in a tube is differentially heated, a gas in the tube can spontaneously oscillate. In this study, we experimentally and numerically investigated the marginal conditions for the onset of the self-sustained gas oscillation. The important point in this study is that we consider the effect of the phase change of working fluid.
The experimental device which was composed of a straight tube and a porous medium sandwiched by two heat exchangers was constructed, and the temperature difference of the heat exchangers at the onset of the gas oscillation, DT_onset, was measured. Two experimental conditions were tested; in one condition, water was contained in a porous media and in the other condition, water was not contained. It was found that the presence of water dramatically decreases DT_onset: when the porous media contained water, DT_onset was 49 degree C, whereas it did not contain water, DT_onset was 138 degree C. We also numerically calculated DT_onset under both the conditions by using the theory proposed by Raspet et al.[JASA Vo. 112, 1414 (2002)] and obtained a good agreement between the experimentally and numerically obtained DT_onset.
Experimental investigation of acoustic streaming in a simple thermoacoustic engine
Ramadan Islam*, Bailliet Helene, Valiere Jean-ChristopheThermoacoustic engines suffer from many non-linearities that deteriorate the overall performance. Streaming phenomenon is one of these non-linearities which can affect the performance by convecting a certain amount of heat. In this study, the axial mean velocity distribution inside a basic standing-wave thermoacoustic engine is measured using Laser Doppler Velocimetry (LDV). A stack is positioned in a λ/2 resonator; the left end of the stack is electrically heated and the rest of the resonator is left uncontrolled. LDV measurements are performed from the right end of the stack up to the guide termination together with thermocouple measurements. Three different regions are distinguished and so called the "end-effects" region (very close to the stack), the hot streaming region (further away from the stack) and the cold streaming region (even further). In the cold streaming region at low acoustic level, there is a good agreement between the measured mean velocity and the Rayleigh streaming theoretical expectation. As the acoustic level is increased, results start to deviate from theoretical values, which agrees with the literature. In addition, the size of the cold region decreases. In the hot streaming region, the measured mean velocity distribution at all acoustic levels differs from Rayleigh streaming due to the effect of convection originated by the temperature distribution. Finally, the mean flow close to the stack is disturbed due to vortex shedding and this disturbance ex-tends along a distance which defines the size of the last region (end-effects region). The balance between the three phenomena associated with the three regions is discussed for different experimental conditions.