Numerical and experimental study of a looped travelling‐wave thermoacoustic electric generator for low‐grade heat recovery

Thermoacoustic technology has drawn increasing attention due to its advantages such as reliability and environmental benignity. Aiming at low‐grade heat recovery, we developed a travelling‐wave thermoacoustic electric generator consisting of a looped travelling‐wave thermoacoustic engine and a linear alternator. In order to explore the operating characteristics of the electric generator, we numerically analyzed the acoustic field characteristics with a modified model. The analysis shows that high acoustic impedance appears in all three stages, and the travelling‐wave component dominates the acoustic field of the loop, which is significant for both thermoacoustic conversion and acoustic power propagation. Furthermore, we also investigated the effects of external electric compliance, resistance, and hot end temperature on the output electric power, thermal‐electric efficiency, and other related parameters. In the experiments, a thermal‐electric efficiency of 3.7% with an output electric power of 24 W has been achieved, when the hot end temperature is 120°C. The relative Carnot efficiency can exceed 14% when the hot end temperature is between 120°C and 190°C. The promising results demonstrate the significant potential of thermoacoustic electric generation in low‐grade heat recovery.     

Theoretical calculation and experimental investigation on acoustic characteristics of a regenerator in thermoacoustic apparatus

An acoustic‐driven thermoacoustic device, which is used to investigate acoustic characteristics of a regenerator, was designed and manufactured. A model of the acoustic characteristics of the regenerator is discussed. The acoustic characteristics of the regenerator, such as acoustic impedance Ẑ_n, reflection coefficient , transmission loss TL, and phase angle between incident and reflected wave at x=0, were obtained by processing the experimental results with the correlation‐spectra analysis (the auto‐ and cross‐spectra) methods theoretically. Comparisons of acoustic characteristics between two cases, A (regenerator) and B (regenerator and two additional heat exchangers), are discussed. Different heating power influence on acoustic characteristics is also investigated. The results obtained will be helpful in further investigations on the regenerator model. © 2005 Wiley Periodicals, Inc. Heat Trans Asian Res, 34(8): 539–546, 2005; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20093     

Experimental Investigation of Self—sustained Oscillation in a Traveling Wave Thermoacoustic System

A traveling wave thermoacoustic engine consisting of a loop tube with a resonator has been tested. The onset characteristic together with the transition of oscillation mode from traveling wave to standing wave and the periodic shifting between modes in this system are investigated experimentally. The process of self-sustained thermoacoustic oscillation in this heat engine is described and analyzed through phase space distribution reconstructed from the time series of acoustic signal.     

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.     

Design of a traveling wave thermo‐acoustic engine based on genetic algorithm

This paper presents a novel design scheme for traveling wave thermo‐acoustic Stirling engines (TASEs) based on a target frequency using genetic algorithm (GA). First, the effects of engine design parameters on the system performance are studied via root locus method. Accordingly, it is found that the resonator length, the inertance diameter, and the hot gas temperature are the most effective parameters in the engine design procedure. Next, the relation between the closed‐loop poles positions of the thermo‐acoustic system and the effective design variables are described parametrically. Consequently, in order to estimate the optimum values of the effective design parameters as well as the unknown positions of the nondominant poles, an appropriate fitness function based on the desired engine frequency is proposed. Subsequently, a GA is used to achieve the optimal values of design parameters so as to minimize the considered cost function. Finally, the validity of the proposed design technique is verified by comparing the obtained results with the available data of an experimental case study.     

新型回热器结构设计及换热特性研究

回热器作为斯特林热机的关键部件,对于太阳能斯特林热机整机性能有着重要影响。为克服传统金属丝网回热器结构存在的填料单一,制造成本较高,工艺复杂问题,采用实用等温分析法,以回热器的长径比、通流面积、填料种类以及孔隙率各项回热器参数为基础,设计了一种新型斯特林热机回热器,该回热器具有轴向压降小,换热性能高,结构稳定,加工制造简单的特点。开展了新型回热器和传统金属丝网回热器的换热性能对比研究,采用振荡条件下的局部热平衡方法研究回热器的传热过程,对比传统金属丝网回热器和新型回热器的温度变化,速度变化以及压力变化。结果表明:在整体孔隙率相同的条件下,新型回热器和传统金属丝网回热器相比,整体启动速率相似,但新型回热器压降减少0.04 MPa,速度出现分段式变化,有利于回热器的换热和结构稳定。因此,新型回热器不但在结构上优于传统金属丝网回热器,在换热特性上也优于传统金属丝网回热器。     

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.     

Performance of a beta-configuration heat engine having a regenerative displacer

This paper investigates the performance of a beta-configuration heat engine having a regenerative displacer. In the conventional beta-engine; the displacer and the power piston are incorporated in one cylinder. The displacer transfers the working fluid between expansion and compression spaces via the heater, the regenerator, and the cooler. In the present work, successive homogeneous layers of square wire meshes occupy the displacer space of a beta-engine that make the displacer to be a displacer and a regenerator simultaneously. The theoretical analysis of the engine is based mainly on Schmidt theory. The optimum dimensions of the heater, cooler, regenerator, piston stroke and displacer stroke as dimensionless ratios of the bore were found. The optimum phase angle between the piston and the displacer and the optimum ranges of the speed for each working gas were also found. In a comparison between the proposed engine which has a regenerative displacer and the GPU-3 engine which has a stationary regenerator and a solid displacer; it was found that; the proposed one delivers 20% more power with 10% more efficiency than the GPU-3 engine.     

Performance investigation of a novel Franchot engine design

The double‐acting Franchot engine is inferior to the double‐acting Siemens engine under configurations limited by the Siemens engine. In this contribution, the performance of a novel Franchot engine design without the Siemens engine limitations is investigated with a new mathematical definition of the regenerator end temperatures, and the initial statement is challenged. The main advantages of the Franchot engine compared with the Siemens engine are the free control of the phase angle and the thermal separation of the cylinders. Here, the performance of a cylinder‐heated/cooled air‐filled Franchot engine is investigated at medium temperature under variations of engine speed, phase angle, geometry, dead volume, and gas density. A second‐order thermodynamic model with nonconstant, polytropic heat transfer is developed and implemented in Matlab/Simulink for this investigation. The nonconstant heat transfer is crucial to accurately model the behaviour of the direct cylinder heating and cooling. The results show that the phase angle and air charge density have the largest effect on the engine performance. An increase of the phase angle from 90^o to 150^o at a speed of 1000 RPM led to an increased output power of 58 W compared with a maximum power less than 20 W for a phase angle of 90^o. The efficiency at a phase angle of 150^o is approximately 25% which is slightly lower than the ideal Curzon and Ahlborn efficiency of 29.3%. This discrepancy can be explained by the nonconstant, polytropic heat transfer. In addition to the increase in engine power, the operation at higher phase angles reduces the pressure difference across the power piston by a factor larger than 4 which leads to a significant reduction in gas leakage across the power pistons. This shows that at higher phase angles, the 2 main disadvantages compared with the Siemens engine are at least reduced and arguably completely removed. Thus, the Franchot engine has the potential to be superior to the Siemens engine if freed from the operational restrictions of the Siemens engine.     

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 substrate used in automotive catalytic converters with fine square channels. The preliminary testing results are obtained and compared with the simulations in detail. The measurement results agree very well on the qualitative level and are reasonably close in the quantitative sense.     

Study the Heat Recovery Performance of Micro and Nano Metfoam Regenerators in Alpha Type Stirling Engine Conditions

This paper experimentally investigates the performance of micro and nano metfoam regenerators in alpha-type Stirling engine conditions. The thermal efficiency of this engine depends on performance of regenerator. Therefore, increase the heat recovery of regenerator raises the total efficiency. Accordingly, two types of regenerators from porous media are designed and simulated with different materials. Three-dimensional regenerator CFD simulation shows that randomize porous open cell metfoam made of silver as high conductivity and high heat capacity material is the best structure to fabricate metfoam regenerator. The porosity and matrix element diameter are optimized. The nano coating methodology enhances the activated surface. The regenerators are fabricated using casting polymer mold layer deposition. The nano silver particles are coated on the metfoam by sol-gel coating method. Experimental results show the improvement in regenerator percentage of heat recovery by 3.40% and 5.93% for micro metfoam and nano metfoam, respectively. The maximum improvement is achieved up to 8.65% in case of using the nano metfoam regenerator at 543 K.     

Optimal regenerator performance in Stirling engines

The key component of a Stirling engine is its regenerative heat exchanger. This device is subject to losses due to dissipation arising from the flow through the regenerator as well as due to imperfect heat transfer between the regenerator material and the gas. The magnitudes of these losses are characterized by the Stanton number St and the Fanning friction factor f, respectively. Using available data for the ratio St/f, results are found for the Carnot efficiency and the power output of the regenerator. They depend on the conductance and on the ratio of pressures at the two sides of the regenerator. Optimum results for efficiency and power output of the regenerator are derived in the limit of zero Mach number. The results are applied to the Stirling engine. The efficiency and the power output of the engine are found for given amplitude of the compression piston. Optimization with respect to regenerator conductance and piston phase angle leads to a maximum possible value of the power output. Under optimal conditions, the Carnot efficiency just below this maximum is close to 100%. Copyright © 2009 John Wiley & Sons, Ltd.     

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.     

Geometric optimization of a thermoacoustic regenerator

This work illustrates the optimization of thermoacoustic systems, while taking into account thermal losses to the surroundings that are typically disregarded. A simple thermoacoustic engine is used as an example for the methodology. Its driving component, the thermoacoustic regenerator (also referred to as the stack), is modeled with a finite element method and its dimensions are varied to find an optimal design with regard to thermal losses. Thermoacoustic phenomena are included by considering acoustic power, and viscous and capacitive losses that are characteristic for the regenerator. The optimization considers four weighted objectives and is conducted with the Nelder–Mead Simplex method. When trying to minimize thermal losses, the presented results show that the regenerator should be designed to be as short as possible. It was found that there is an optimal regenerator diameter for a given length. The results are presented for a variety of materials and weights for each objective.     

Thermodynamic analysis of a Stirling engine including dead volumes of hot space, cold space and regenerator

This paper provides a theoretical investigation on the thermodynamic analysis of a Stirling engine. An isothermal model is developed for an imperfect regeneration Stirling engine with dead volumes of hot space, cold space and regenerator that the regenerator effective temperature is an arithmetic mean of the heater and cooler temperature. Numerical simulation is performed and the effects of the regenerator effectiveness and dead volumes are studied. Results from this study indicate that the engine net work is affected by only the dead volumes while the heat input and engine efficiency are affected by both the regenerator effectiveness and dead volumes. The engine net work decreases with increasing dead volume. The heat input increases with increasing dead volume and decreasing regenerator effectiveness. The engine efficiency decreases with increasing dead volume and decreasing regenerator effectiveness.     

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 ...     

Thermodynamic analysis of a Stirling engine including regenerator dead volume

This paper provides a theoretical investigation on the thermodynamic analysis of a Stirling engine with linear and sinusoidal variations of the volume. The regenerator in a Stirling engine is an internal heat exchanger allowing to reach high efficiency. We used an isothermal model to analyse the net work and the heat stored in the regenerator during a complete cycle. We show that the engine efficiency with perfect regeneration doesn’t depend on the regenerator dead volume but this dead volume strongly amplifies the imperfect regeneration effect. An analytical expression to estimate the improvement due to the regenerator has been proposed including the combined effects of dead volume and imperfect regeneration. This could be used at the very preliminary stage of the engine design process.     

用声响应法测量直喷式柴油机燃烧压力振荡共振频率的研究

本文根据声波动理论推导出声压波动方程，证明了在密闭空间中声压波动具有与柴油机燃烧室内气体压力振荡相类似的规律性。实验还表明，通过声响应法测量的声压波动共振频率与燃烧压力振荡共振频率是基本相等的。     

催化裂化再生器能量和(火用)分析

收集整理某炼油厂催化裂化再生器生产操作数据,通过对再生器能量平衡和火用平衡的计算,确定了再生器的能量利用效率和火用效率,并利用石油化工工艺过程用能三环节模式对再生器的用能状况进行了分析。对该装置节能增效进行了探讨。得出了该再生器的能量利用情况,为下一步装置的能量使用提出了改进的措施。