Application of laser-based instrumentation for measurement of time-resolved temperature and velocity fields in the thermoacoustic system

This work aims to develop reliable laser-based measurement techniques to enable fundamental heat transfer and fluid flow studies in thermoacoustic systems. The challenge is to better understand the modes of energy transfer between the key components, such as stacks (or regenerators) and the hot and cold heat exchangers (located on two sides of the stack/regenerator structure), under the oscillatory flow conditions imposed by the acoustic field. The measurement methodologies adopted in this work include combined two-dimensional temperature and velocity field measurements using Planar Laser-Induced Fluorescence (PLIF) and Particle Image Velocimetry (PIV), respectively. These are investigated around the fins of a pair of mock-up heat exchangers placed side by side in a quarter-wavelength standing-wave acoustic resonator, to mimic the working conditions of a thermoacoustic system. The fins are kept at constant temperatures by means of resistive heating and water cooling, respectively. The velocity and temperature field distributions for 20 phases in the acoustic cycle have been obtained. The impact of the inertial, viscous and thermal effects on the time-dependent local temperature and velocity distributions is discussed. Mutual interaction between both fields is also shown. Future work towards obtaining useful heat transfer correlations in oscillatory conditions is outlined.     

Intrinsic instability of flame–acoustic coupling

This paper shows that a flame can be an intrinsically unstable acoustic element. The finding is clarified in the framework of an acoustic network model, where the flame is described by an acoustic scattering matrix. The instability of the flame acoustic coupling is shown to become dominating in the limit of no acoustic reflections. This is in contrast to classical standing-wave thermoacoustic modes, which originate from the positive feedback loop between system acoustics and the flame. These findings imply that the effectiveness of passive thermoacoustic damping devices is limited by the intrinsic stability properties of the flame.     

Combustion instabilities in sudden expansion oxy-fuel flames

An experimental study on combustion instability is presented with focus on oxy-fuel type combustion. Oxidants composed of CO₂/O₂ and methane are the reactants flowing through a premixer-combustor system. The reaction starts downstream a symmetric sudden expansion and is at the origin of different instability patterns depending on oxygen concentration and Reynolds number. The analysis has been conducted through measurement of pressure, CH^∗ chemiluminescence, and velocity. As far as stability is concerned, oxy-fuel combustion with oxygen concentration similar to that found in air combustion cannot be sustained, but requires at least 30% oxygen to perform in a comparable manner. Under these conditions and for the sudden expansion configuration used in this study, the instability is at low frequency and low amplitude, controlled by the flame length inside the combustion chamber. Above a threshold concentration in oxygen dependent on equivalence ratio, the flame becomes organized and concentrated in the near field. Strong thermoacoustic instability is then triggered at characteristic acoustic modes of the system. Different modes can be triggered depending on the ratio of flame speed to inlet velocity, but for all types of instability encountered, the heat release and pressure fluctuations are linked by a variation in mass-flow rate. An acoustic model of the system coupled with a time-lag-based flame model made it possible to elucidate the acoustic mode selection in the system as a function of laminar flame speed and Reynolds number. The overall work brings elements of reflection concerning the potential risk of strong pressure oscillations in future gas turbine combustors for oxy-fuel gas cycles.     

Novel Helmholtz resonator used to focus acoustic energy of thermoacoustic engine

A thermoacoustic engine (TE) converts thermal energy into acoustic power without any mechanical moving parts. It shows several advantages over traditional engines, such as simple configuration, stable operation, and environment-friendly working gas. In order to further improve the performance of a thermoacoustically driven system, methods are needed to focus the acoustic energy of a TE to its load. By theoretical analysis based on linear thermoacoustics, a novel Helmholtz resonator is proposed to increase the transmission ability of a TE, which makes full use of the interaction between inertance and compliance effects. With this configuration, the output pressure amplitude of a TE is amplified and the maximal pressure amplitude can occur at the end of the Helmholtz resonator tube with a length much shorter than 1/4 wavelength. Furthermore, the Helmholtz resonator has shown remarkably increased volume flow rates at both ends. In experiments, a Helmholtz resonator amplifies the pressure ratio from 1.22 to 1.49 and produces pressure amplitude of 0.44 MPa with nitrogen of 2.2 MPa as working gas. Relatively good agreements are obtained between computational and experimental results. This research is instructive for comprehensively understanding the transmission characteristics of acoustic components.     

Numerical computation for parallel plate thermoacoustic heat exchangers in standing wave oscillatory flow

A simplified computational method for studying the heat transfer characteristics of parallel plate thermoacoustic heat exchangers is presented. The model integrates the thermoacoustic equations of the standard linear theory into an energy balance-based numerical calculus scheme. Details of the time-averaged temperature and heat flux density distributions within a representative domain of the heat exchangers and adjoining stack are given. The effect of operation conditions and geometrical parameters on the heat exchanger performance is investigated and main conclusions relevant for HX design are drawn as far as fin length, fin spacing, blockage ratio, gas and secondary fluid-side heat transfer coefficients are concerned. Most relevant is that the fin length and spacing affect in conjunction the heat exchanger behavior and have to be simultaneously optimized to minimize thermal losses localized at the HX-stack junctions. Model predictions fit experimental data found in literature within 36% and 49% respectively at moderate and high acoustic Reynolds numbers.     

Performance of an air-cooled looped thermoacoustic engine capable of recovering low-grade thermal energy

An air-cooled looped thermoacoustic engine is designed and constructed, where an air-cooled cold heat exchanger (consisting of copper heat transfer block, aluminum flange, and aluminum fin plate) is adopted to extract heat and the resonant tube is spiraled and shaped to fit to the available space. Experiments have been conducted to observe how onset temperature difference and resonant frequency are affected by mean pressure, working fluid, and diameter of compliance tube. Besides, the influences of temperature difference, mean pressure, working fluid and diameter of compliance tube on pressure amplitude, output acoustic power, and thermal efficiency of the system have been investigated. The air-cooled looped thermoacoustic engine can start to oscillate at a lowest temperature difference of 46°C, with the working fluid of carbon dioxide at 2.34 MPa. A highest output acoustic power obtained is 6.65 W at a temperature difference of 199°C, with the working gas of helium at 2.58 MPa, and the thermal efficiency is 2.21%. This work verifies the feasibility of utilizing low-grade thermal energy to drive an air-cooled looped thermoacoustic engine and extends its application in the water deficient areas.     

A miniature thermoacoustic stirling engine

A miniature thermoacoustic stirling engine was simulated and designed, having overall size of length 0.65 m and height of 0.22 m. The acoustic field generated in this miniature system has been described and analyzed. Some efforts had been paid to coupling and matching, and a miniature thermoacoustic engine and some extra experimental components have been constructed. Analysis and experimental results showed that to obtain better performance of the engine, the diameter of the resonance tube must be chosen appropriately according to the looped tube dimension and the input heating power. It provided an effective way to miniaturize the thermoacoustic stirling heat engine. The experimental results showed that the engine had low onset temperature and high pressure amplitude and ratio. With the filling helium gas of 2 MPa and heating power of 637 W, the maximal peak to peak pressure amplitude and pressure ratio reached 2.2 bar and 1.116, respectively, which was able to drive a refrigerator, a heat pump or a linear electrical generator. The operating frequency of the engine was steady at 282 Hz.     

非对称双级环路行波热声热机的实验研究*

对于双级环路行波热声热机,两个热声核的相对位置直接影响到其起振温度,而热声热机的起振温差决定了其可利用的热源品位。基于线性热声理论分析,通过改变两个热声核的相对位置,研究了两个热声核的相对位置改变对其起振温差、压力振幅和压比等的影响。结果表明,双级环路行波热声热机的起振温度随着两个热声核从中心对称位置逐步靠近时先下降再上升,当两个热声核之间的谐振管长度比例为1:3.5时,系统获得最小的起振温差为59.6℃（工质为N₂,充气压力为2.5 MPa）。在相同温差下,该系统在谐振管长度比例为1:3.5的位置相较于其他位置具有较大的压力振幅和压比。     

Thermoacoustic combustion instability of propane-oxy-combustion with CO2 dilution: Experimental analysis

In this study, the effect of CO₂ dilution on the thermoacoustic stability of propane-oxyfuel flames is studied in a non-premixed, swirl-stabilized combustor. The results, obtained at a fixed combustor power density (4 MW/m³ bar) and global stoichiometric equivalence ratio (Φ = 1.0), show that the oxy-flame is stable at 0% and low CO₂ concentrations in the oxidizer. A self-amplifying coupling between heat release and pressure fluctuations was observed to occur at the CO₂ concentration of 45%, which matches the point of flame transition from a jet-like to a V-shaped flame resulting from the formation of inner recirculation zone. The observed frequency for both the pressure and heat release oscillations is 465 Hz and the ensuing thermoacoustic instability is believed to have been resulted from vortexes and flame interactions. Subsequent to the coupling of the oscillations at the CO₂ concentration of 45%, their amplitudes grew at 50% to 60% CO₂ dilution levels. The maximum amplitude was observed at 60% CO₂ concentration after which, as CO₂ dilution level increases, the acoustic amplitude and that of its counterpart in the heat release spectrum decreased due to damping (energy dissipation) arising from heat loss and viscous dissipation. An increase in hydrogen concentration in the fuel and a decrease in the combustor power density were observed to lower the acoustic amplitude. Furthermore, a frequency shift is observed with a change in the combustor firing rate, which shows that the mode scales with the flow velocity, and therefore, unlikely to be a natural acoustic mode of the combustor. This study, therefore, reveals thermoacoustic instability in non-premixed oxy-combustion driven by changes in flame dynamics and macrostructures as the CO₂ concentration in the oxidizer mixture varies.     

Study on energy flows in thermoacoustic engines utilizing two-temperature heat sources

The proposal of a novel thermoacoustic regenerator using multi-temperature heat sources (MTHS) makes it possible to utilize lower-grade energy and keep relatively high efficiency in a thermoacoustic engine (TE) simultaneously. Based on thermodynamic laws combined with linear thermoacoustic theory, the time-averaged total power, enthalpy flux, acoustic power, entropy flux, and exergy flux in each component are derived and calculated to further understand the mechanism of a TE with the regenerator using two-temperature heat sources (TTHS). The comparison of the energy flows between the traditional TEs and those utilizing TTHS shows that the improvement of the temperature gradient in the regenerator by adding a mid-heater with appropriate heating power can increase the acoustic power and efficiency of a TE.     

浓相粉煤灰气力输送差压波动ARMA功率谱分析

在料罐体积均为0．6m^3,输送管内径为20mm,输送距离为20m的试验台上进行浓相气力输送试验研究。采用ARMA功率谱分析输送过程中固气比对气体表观速度、压力损失和能量在频域分布的影响。结果显示：随着固气比的增加,表观速度降低,单位长度的压力损失逐渐增大,差压波动逐渐减小;功率在频域内分布随着固气比增大逐渐向低频移动,主峰峰值也随之增大。这些均与粉煤灰流动形态有关。它揭示了功率在频域内的分布情况,对研究管内运动和能量交换提供了新的研究途径。     

Thermoacoustic limit cycles in a premixed laboratory combustor with open and choked exits

This paper presents an experimental and theoretical investigation of the response of a turbulent premixed flame during thermoacoustic limit cycle in a simple, laboratory combustor. The flame dynamics are examined using high-speed pressure transducers and CH∗ chemiluminescence. The so-called ‘interaction index’ and time delay between the acoustic velocity fluctuations at the flame holder and the flame’s overall heat release fluctuations are then determined. A wide range of operating conditions, traversing the combustor’s flammability limits in Mach number and equivalence ratio, are studied for four different combustor exits, including one where the exit is choked. In all cases the time delay correlates very well with the amplitude of the velocity fluctuations. There is also some correlation between the interaction index and these velocity fluctuations, but this is less clear. These results suggest a novel, nonlinear flame model, derived entirely empirically. An existing low-order thermoacoustic model is then extended to include convection and dispersion of entropy fluctuations downstream of the flame, enabling the effect of the choked nozzle to be examined. The novel nonlinear flame model is integrated into the low-order thermoacoustic model, and used to investigate the experimentally observed thermoacoustic limit cycles. The model correctly simulates the observed switch to a low-frequency, entropically driven instability observed when the combustor exit is choked.     

Nonlinear thermoacoustics of ducted premixed flames: The influence of perturbation convection speed

When a premixed flame is placed within a duct, acoustic waves induce velocity perturbations at the flame’s base. These travel down the flame, distorting its surface and modulating its heat release. This can induce self-sustained thermoacoustic oscillations. Although the phase speed of these perturbations is often assumed to equal the mean flow speed, experiments conducted in other studies and Direct Numerical Simulation (DNS) conducted in this study show that it varies with the acoustic frequency. In this paper, we examine how these variations affect the nonlinear thermoacoustic behaviour. We model the heat release with a nonlinear kinematic G-equation, in which the velocity perturbation is modelled on DNS results. The acoustics are governed by linearised momentum and energy equations. We calculate the flame describing function (FDF) using harmonic forcing at several frequencies and amplitudes. Then we calculate thermoacoustic limit cycles and explain their existence and stability by examining the amplitude-dependence of the gain and phase of the FDF. We find that, when the phase speed equals the mean flow speed, the system has only one stable state. When the phase speed does not equal the mean flow speed, however, the system supports multiple limit cycles because the phase of the FDF changes significantly with oscillation amplitude. This shows that the phase speed of velocity perturbations has a strong influence on the nonlinear thermoacoustic behaviour of ducted premixed flames.     

An electret-based thermoacoustic-electrostatic power generator

This study reports a new concept for power generation from thermal energy, which integrates a thermoacoustic engine (TAE) with a contact-free electret-based electrostatic transducer. The TAE converts thermal energy into high-intensity acoustic energy, while the electret-based electrostatic transducer converts the generated acoustic energy into electricity. The experiments demonstrate the feasibility and potential of the proposed electret-based thermoacoustic-electrostatic power generator (TAEPG). The dynamic response of the electrostatic transducer and energy conversion inside the TAE are further investigated using a lumped element model and a frequency-domain reduced-order network model. Good agreement is achieved between experimental measurements and theoretical predictions. Furthermore, a parametric study is performed to study the effect of key parameters including the external heating power, air gap, and resistive load on the performance of the TAEPG. Results show that an open-circuit voltage amplitude of 4.7 V is produced at a closed-end pressure amplitude of 480 Pa in the experiment, and it is estimated that 25.2% of the acoustic power generated by the TAE has been extracted by the electret-based electrostatic transducer. In this case, the maximum electric power output is measured to be 2.91 μW at a resistive load of around 2.2 MΩ. By increasing the external heating power, the TAEPG can produce a maximum voltage amplitude of 8 V. This work shows that the proposed concept has great potential for developing miniature heat-driven power generators.     

Design and experimental validation of looped-tube thermoacoustic engine

The aim of this paper is to present the design and experimental validation process for a thermoacoustic looped-tube engine.The design procedure consists of numerical modelling of the system using DELTA EC tool,Design Environment for Low-amplitude ThermoAcoustic Energy Conversion,in particular the effects of mean pressure and regenerator configuration on the pressure amplitude and acoustic power generated.This is followed by the construction of a practical engine system equipped with a ceramic regenerator-a ...     

Influence of a magnetic field on the energy,work, and heat flux of a multi-plate thermoacoustic system

Research on clean and efficient energy conversion is extremely important to mitigate the high price of fossil fuel and its adverse effects on the environment. Thermoacoustic is a clean energy conversion technology that uses the conversion of acoustic to thermal energy and vice versa. However, the efficient conversion of acoustic to thermal energy using thermoacoustic systems (e.g., engine, refrigerator, or heat pump) demands research on working fluids, operational, and geometric parameters. The present study is a contribution to improve the efficiency of a thermoacoustic heat system by introducing a magnetic field perpendicular to the direction of the oscillating fluid. The major focus of this study is to examine the effect of a magnetic field on three important performance parameters: energy, heat, and work fluxes of a multi-plate thermoacoustic system. Initially, analytical expressions for the fluctuating velocity and temperature are derived from the governing continuity, momentum, and energy equations by applying the first order perturbation technique and solving these equations. The derived first order analytical equations for the fluctuating velocity and temperature enable us to calculate the energy, heat, and work fluxes and are expressed in terms of dimensionless Hartmann number (Ha_δ), temperature gradient ratio (Γ₀), Swift number (S_w), Prandlt number (Pr), and modified Rott's and Swift's parameters (f_v and f_k). It is observed that the normalized energy flux density increases with increasing Ha_δ and Γ₀ when S_w < 1.5. The heat flux and work flux densities also increase with increasing Ha_δ and Γ₀ when S_w < 1.5 and decrease when Ha_δ > 1.5. The findings of this research will provide useful information to thermoacoustic system's designers for the devloepment of effieicnt magnetic thermoacoustic heat pumps.     

A thermoacoustic engine capable of utilizing multi-temperature heat sources

Low-grade energy is widespread. However, it cannot be utilized with high thermal efficiency directly. Following the principle of thermal energy cascade utilization, a thermoacoustic engine (TE) with a new regenerator that can be driven by multiple heat sources at different temperature levels is proposed. Taking a regenerator that utilizes heat sources at two temperatures as an example, theoretical research has been conducted on a traveling-wave TE with the new regenerator to predict its performance. Experimental verification is also done to demonstrate the benefits of the new regenerator. Results indicate that a TE with the new regenerator utilizing additional heat at a lower temperature experiences an increase in pressure ratio, acoustic power, efficiency, and exergy efficiency with proper heat input at an appropriate temperature at the mid-heater. A regenerator that uses multi-temperature heat sources can provide a means of recovering lower grade heat.     

Effect of the Darcy number on the energy flow and operating conditions of a thermoacoustic porous-medium system

In this paper, a simplified porous medium thermoacoustic system is modeled to observe its energy interaction characteristics and identify its operating conditions mainly as a function of porous medium Darcy number. The governing Darcy–Brinkman momentum equation and energy equation are simplified and linearized by using a first order perturbation analysis. Similar perturbation analysis is usually used to solve the linear thermoacoustic problem in the low Mach number limit. Simplified momentum and energy equations are solved, in the frequency domain, in order to obtain the expressions of the fluctuating velocity (u₁) and temperature (T₁). Time averaged and space averaged heat fluxes and work fluxes are calculated using the expressions of fluctuating velocity and temperature. The effects of the drive ratio (DR), Darcy number (Da_δ), temperature gradient (?T_m), and frequency (f) on the heat flux, work flux, and operating conditions are discussed and graphically presented.     

Optimization of the Stack Unit in a Thermoacoustic Refrigerator

ABSTRACT

A thermoacoustic refrigerator is a device that uses acoustic power to pump heat in the absence of harmful refrigerants with no or few moving parts. However, the performance of the thermoacoustic refrigerator, particularly the standing wave types, is currently not competitive compared to its counterpart, the conventional vapor-compression refrigerator. Presently, thermoacoustic refrigeration prototypes only achieved 0.1–0.2 relative coefficient of performance, compared with that of 0.33–0.5 for the conventional vapor-compression refrigerators. Past optimization efforts had been completed based on parametric studies where individual parameters are discretely varied and the final optimized outcome was based on the limited series of numerical/experimental tests. This paper discusses the initial investigation of the optimization of the thermoacoustic refrigerator stack parameters using a multi-objective genetic algorithm. The desired outputs, the maximization of the cooling load and the minimization of the acoustic power at the stack, are obtained with the parameters to be optimized set within some range of values. The stack length and center position are then optimized simultaneously. The optimized results showed that the coefficient of performance of the thermoacoustic refrigerator improves from the published value of 1.3 to 1.37.