CFD simulation on the performance of twin thermoacoustic prime mover for various resonator lengths and operating pressures

The main objective of the present investigation is to analyze the effect of resonator length and different working pressures on the performance of a twin thermoacoustic prime mover, which was measured in terms of frequency and amplitude of the oscillations using the computational fluid dynamics tool FLUENT 6.3. The simulation was carried out for different resonator lengths such as 0.5, 0.6, 0.8, 1.1, and 1.4 m using nitrogen as a working fluid for various operating pressures such as 1, 2, 3, 4, and 5 bar. It was observed from the results that the pressure amplitude increases with an increase in resonator length and that the output frequency decreases with an increase in resonator length for a fixed operating pressure. For a constant resonator length, the increase in operating pressure has little influence on output frequency, and the pressure amplitude also increases. The results obtained were found to be in accordance with experimental works published by Hariharan and colleagues. 1     

Experimental study on the performance of the thermoacoustic refrigerating system

This study is on the performance of the thermoacoustic refrigerating system with respect to some critical operating parameters. Experiments were performed on the system under various operating conditions. The experimental setup consists of the thermoacoustic refrigerating system with appropriate valves for the desired controls, instrumentation and the electronic data acquisition system. The resonator was constructed from aluminum tubing but it had plastic tube lining on the inside to reduce heat loss by conduction. Significant factors that influence the performance of the system were identified. The cooling produced increases with the temperature difference between the two ends of the stack. High pressure in the system does not necessarily result in a higher cooling load. There exists an optimum pressure and an optimum frequency for which the system should be operated in order to obtain maximum cooling load. Consequently, for the thermoacoustic refrigeration system, there should be a related compromise between cooling load, pressure and frequency for best performance.     

Theoretical Evaluation of a 10‐Watt Cooling Power Thermoacoustic Refrigerator

This article deals with the design, simulation, and analysis of a 10‐watt capacity thermoacoustic refrigerator using short‐stack boundary layer approximation assumptions and dimensional normalization technique. The variation of stack diameter with average gas pressure and cooling power is studied. The theoretical evaluation of quarter‐wavelength and one‐fifth‐wavelength resonator using helium gas is discussed for an operating frequency ranging from 300–500 Hz in the steps of 50 Hz. For the optimized stack, a 38.5% improvement in the stack performance for a 10‐watt cooling power quarter‐wavelength hemispherical‐ended resonator operating at 350 Hz frequency resulted in a 13.5% improvement in the coefficient of performance (COP) compared to published results. The resonator design is tested with DeltaEC software with 2% and 3% drive ratios using helium, hydrogen, neon, air, and carbon dioxide as working gases and the results are discussed. © 2013 Wiley Periodicals, Inc. Heat Trans Asian Res, 43(7): 577–591, 2014; Published online 3 October 2013 in Wiley Online Library ( wileyonlinelibrary.com/journal/htj ). DOI 10.1002/htj.21094     

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。     

Experimental study on the excitation of thermoacoustic instability of hydrogen-methane/air premixed flames under atmospheric and elevated pressure conditions

The current work examines the excitation of thermoacoustic instability of lean premixed hydrogen-methane/air low swirl flames under both atmospheric and elevated pressure conditions (up to 0.3 MPa). Under a given pressure condition, The tests were conducted at different bulk velocities (U), hydrogen proportions (η_H), and equivalence ratios (Ф). Results show that thermoacoustic instability can be excited by increasing one of these variables while keeping others the same. It was found that pressure elevation has a minor effect on the oscillation frequency. Moreover, it was demonstrated that the current instability is induced by large coherent structures. The effect of pressure elevation on the excitation of thermoacoustic instability is found to be Φ dependent. As indicators of the flame response to impinging vortices, the curvature and local flame surface area features were calculated with images captured with the planar laser-induced fluorescence of the OH radical (OH-PLIF) method. Results demonstrate a great similarity between the flame front evolution and the instability trend, implying that the effect of the chamber pressure on the instability trend can be indicated by the change in the flame front curvature and local flame surface area.     

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.     

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.     

Hot Dates

For this article, an analytical study has been conducted on the flow and energy transfer of unsteady compressible oscillating flow through channels filled with porous medium representing stack in thermoacoustic engines/refrigerators. The flow in the porous material is described by the Darcy momentum equation. Analytical expressions for oscillating velocity, temperature in the porous layer, complex Nusselt number, and energy flux density are obtained after simplifying and solving the governing differential equations with reasonable approximations (such as long wave, short stack, small amplitude oscillation, etc.). The result for heat transfer between the porous medium and the channel wall is expressed as a dimensionless Nusselt number. For the limiting case of nonporous medium, the Nusselt number obtained in the present study matches quantitatively with the expression available in the existing literature. The results reveal that the Nusselt number in oscillating flow is significantly enhanced (almost an order of magnitude) by employing sufficiently large thermal conductivity of porous media in a channel. The system of equations developed in the present study is a helpful tool for thermal engineers to design porous stacks for thermoacoustic devices.     

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.     

Thermodynamic analysis of onset characteristics in a miniature thermoacoustic Stirling engine

This paper analyzes the onset characteristics of a miniature thermoacoustic Stirling heat engine using the ther-modynamic analysis method. The governing equations of components are reduced from the basic thermodynamic relations and the linear thermoacoustic theory. By solving the governing equation group numerically, the oscillation frequencies and onset temperatures are obtained. The dependences of the kinds of working gas, the length of resonator tube, the diameter of resonator tube, on the oscillation frequency are calculated. Meanwhile, the influences of hydraulic radius and mean pressure on the onset temperature for different working gas are also presented. The calculation results indicate that there exists an optimal dimensionless hydraulic radius to obtain the lowest onset temperature, whose value lies in the range of 0.30 0.35 for different working gases. Furthermore, the amplitude and phase relationship of pressures and volume flows are analyzed in the time-domain. Some experiments have been performed to validate the calculations. The calculation results agree well with the experimental values. Finally, an error analysis is made, giving the reasons that cause the errors of theoretical calculations.     

Numerical simulation of the onset characteristics in a standing wave thermoacoustic engine based on thermodynamic analysis

In this paper, a simplified physical model of standing wave thermoacoustic engines (SWTE) is developed based on thermodynamic analysis. Transient pressure drop and heat transfer data are first calculated based on linear thermoacoustic theory. The effects of stack spacing, charge pressure, and resonator length on onset temperature were investigated and compared with experimental results. The calculations agree well with the experimental results, which validates the model for calculating the onset conditions.     

Effects of variation in working fluids and operating conditions on the performance of a thermoacoustic refrigerator

This study reports on a numerical investigation of the effects of variation in working fluids and operating conditions on the performance of a thermoacoustic refrigerator. The performance of a thermoacoustic refrigerator is evaluated based on the cooling power, coefficient of performance (COP), and the entropy generation rate within the device. The effect of the variation of the working fluid is observed by changing the Prandtl number (Pr) between 0.7 and 0.28. The operating conditions investigated are drive ratio (DR), stack plate spacing (y₀), and mean pressure (p_m). The present research shows that lowering the Pr of the working fluid does not improve the performance of a thermoacoustic refrigerator for all of the selected operating conditions. COP increases 78% by reducing the Pr from 0.7 to 0.28 at y₀ = 3.33δ_k, at atmospheric pressure and a DR of 1.7%. While the COP decreases by reducing the Pr from 0.7 to 0.28 at y₀ = 1.0δ_k, at atmospheric pressure, and a DR of 1.7%. The results are compared with the available experimental data and found good agreement.     

Influence of acoustic pressure amplifier dimensions on the performance of a standing-wave thermoacoustic system

A standing-wave thermoacoustic engine, employing an acoustic pressure amplifier (APA), is simulated with linear thermoacoustics to study the influence of APA’s dimensions on performance of the thermoacoustic system. Variations of operating parameters, including pressure ratio, acoustic power, hot end temperature of stack etc., versus length and diameter of APA are presented and discussed based on an analysis of pressure and velocity distribution in APA. Simulation results indicate that a largest amplification effect of both pressure ratio and acoustic power output is achieved at a critical length for the occurrence of pressure node and velocity antinode in APA, close to but less than one fourth of the wavelength. The distribution characteristics of pressure and velocity in APA are similar to a standing-wave acoustic field, which is the reason for the amplification effect. From the viewpoint of energy, the amplification effect results from the changed distribution of acoustic energy and acoustic power loss in the thermoacoustic system by APA. Experiments have been carried out to validate the simulation, and experimental data are presented.     

多孔介质稳焰器对旋流火焰振荡燃烧特性影响的试验研究

为研究多孔介质稳焰器孔密度变化对贫预混旋流火焰振荡燃烧特性的影响,通过光电倍增管测量全局火焰热释放率,采用双麦克风方法测量旋流器入口速度脉动,获得不同孔密度多孔介质稳焰器火焰传递函数;并通过高速相机测量不同孔密度多孔介质稳焰器振荡火焰结构的变化。试验结果表明：多孔介质能够改变燃烧室声模态,有效抑制燃烧振荡,但孔密度对受迫燃烧火焰热释放率和压力脉动影响具有非线性;高频入口扰动对火焰响应特性影响较弱,火焰受迫响应呈现低通滤波特性;火焰传递函数增益峰值对应入口激励频率存在差异,但相位分布斜率基本一致;多孔介质导致火焰向稳焰器中心轴线聚拢,相干结构更加明显;宽频扰动范围内的火焰张角分布趋势与火焰传递函数增益曲线的分布趋势相反。     

Brinkman–Forchheimer modeling for porous media thermoacoustic system

In this paper, analytical studies have been conducted on the flow and thermal fields of unsteady compressible viscous oscillating flow through channels filled with porous media representing stacks in thermoacoustic systems. The flow in the porous material is described by the Brinkman–Forchheimer–extended Darcy model. Analytical expressions for oscillating velocity, temperature, and energy flux density are obtained after linearizing and solving the governing differential equations with long wave, short stack, and small amplitude oscillation approximations. Experimental work is also conducted to verify the temperature difference obtained across the porous stack ends. To produce the experimental results, a thermoacoustic heat pump is designed and constructed where reticulated vitreous carbon (RVC) is used as the stack material. A very good agreement is obtained between the modeling and the experimental results. The expression of temperature difference across the stack ends obtained in the present study is also compared with the existing thermoacoustic literature. The proposed expression surpasses the existing expression available in the literature. The system of equations developed in the present study is a helpful tool for thermal engineers and physicist to design porous stacks for thermoacoustic devices.     

多孔介质发动机工作过程单区模型模拟

采用单区燃烧模型模拟多孔介质（PM）发动机的压缩、燃烧和膨胀过程。以热力学第一定律为基础,引入多孔介质换热模型,建立了多孔介质发动机的能量方程。计算了多种工况参数下PM发动机缸内温度、压强变化规律,分别讨论了压缩比、过量空气系数、多孔介质温度、多孔介质体换热系数等参数对多孔介质发动机燃烧过程的影响。将PM发动机与传统发动机加以比较,结果表明PM使缸内温度和压强的变化趋于平缓,这有利于混合气着火并可降低NO,排放。     

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.     

Numerical analysis of microwave melting of ice-saturated porous medium filled in a rectangular waveguide with resonator using a combined transfinite interpolation and PDE methods

A numerical study is performed for the melting process of ice-saturated porous medium filled in a rectangular waveguide with a resonator subjected to electromagnetic energy. A microwave system supplies a monochromatic wave in a fundamental mode (TE₁₀ mode) with operating frequency of 2.45 GHz. Focus is placed on establishing a computationally efficient approach for solving moving boundary heat transfer problem in a two-dimensional structured grid. Numerically, preliminary grids are first generated by an algebraic method, based on a transfinite interpolation method, with subsequent refinement using a PDE mapping method. A preliminary case study indicates successful implementation of the numerical procedure. The predicted results from two-dimensional melting model are then validated against available experimental results and subsequently used as a tool for efficient computational prototyping. Based on the numerical results are performed illustrating the influence of resonator and layered configuration, in case of the installed resonator has strongly affected on the microwave power absorbed, temperature distribution, and the melting front during microwave melting process.     

Effects of Tabular Stratified CO_2/O_2 Jets on Dynamic and NO_x Emission Characteristics of a Model Gas Turbine Combustor

The effects of tabular stratified CO_2/O_2 jet in cross flow on thermoacoustic instability and NO_(x )emission were experimentally studied.To explore the dependence of injection positions on flame stability,two factors were taken:the injection height and the injection direction of CO_2/O_2 gas.Results show that the injection positions seriously affect the control effectiveness.The optimum acoustic amplitude-damped ratio of thermoacoustic instability can reach 76.61%with the first layer of horizontal direction.The sound pressure amplitude declined from 56 Pa to 13.1 Pa.The concentration-damped ratio of NO_x emission can achieve 66.67%with the first layer of vertical direction.The concentration of NO_x emission declined from 50.4 mg/m~3 to 16.8mg/m~3 as the jet in cross flow rate increased.Higher oxygen ratio of stratified CO_2/O_2 jets can produce lower NO_(x )emission but higher combustion instability.The descending gradient of NO_(x )emissions is different among different injection positions.Frequency shifting of the sound pressure and flame CH*chemiluminescence emerged.The oscillation frequency declined as the flow rate of CO_2/O_2 jets increased.The unsteady long and compact flame was dispersed after CO_2/O_2 injection.The macrostructure of flame was characterized as flatter and short under jet in cross flow.The variation curves of the flame length and top view area are similar to the shape of half saddle lines.This research proved the optimal control of thermoacoustic instability and NO_x emissions with a passive method,which could be conducive to the realization of clean and secure combustion in industrial lean premixed combustors.     

An investigation of convective transport in micro proton-exchange membrane fuel cells

The flow phenomena in a serpentine microchannel segment attached to a porous transport layer in a micro proton-exchange membrane fuel cell is investigated. Due to the presence of a porous transport layer, the fluid flow for this configuration exhibits different characteristics compared with that through a simple serpentine channel. The pressure drop and friction factor variation in the channel is examined for various values of Reynolds number and radii of curvature. Also, the effect of variation of permeability is investigated. There are two modes of fluid transport in this geometry—one through the serpentine channel and other via the porous media. With increasing permeability, more fluid is convected through the porous transport layer.