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。     

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.     

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.     

以氦气为工质的行波热声发动机研究

随着对热声热机研究的深入，特别是行波热声发动机概念的提出，热声发动机效率得到了质的提高。为了实现热声发动机与制冷机的良好匹配，以氦气为工质时热声发动机需具有较低的起振温度、较大的压力波强度、较好的单频率特性。本文对自行研制的新型热声发动机进行了深入研究，以氦气为工质，在充气压力为2．0MPa时获得了1．19的压比，系统频率稳定在约73Hz，为利用新型热声发动机驱动脉管制冷机或其它热声制冷机创造了有利条件。此外，该热声发动机起振温度较低，初步具备了利用工业废热等低品位能源驱动的条件。     

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.     

Theoretical thermodynamic analysis of Rankine power cycle with thermal driven pump

A new approach to improve the performance of supercritical carbon dioxide Rankine cycle which uses low temperature heat source is presented. The mechanical pump in conventional supercritical carbon dioxide Rankine cycle is replaced by thermal driven pump. The concept of thermal driven pump is to increase the pressure of a fluid in a closed container by supplying heat. A low grade heat source is used to increase the pressure of the fluid instead of a mechanical pump, this increase the net power output and avoid the need for mechanical pump which requires regular maintenance and operational cost. The thermal driven pump considered is a shell and tube heat exchanger where the working fluid is contained in the tube, a tube diameter of 5 mm is chosen to reduce the heating time. The net power output of the Rankine cycle with thermal driven pump is compared to that of Rankine cycle with mechanical pump and it is observed that the net power output is higher when low grade thermal energy is used to pressurize the working fluid. The thermal driven pump consumes additional heat at low temperature (60 °C) to pressurize the working fluid.     

Temperatures near the interface between an ideal heat exchanger and a thermal buffer tube or pulse tube

A thermal buffer tube (or pulse tube) thermally isolates two heat exchangers at different temperatures in a thermoacoustic engine or refrigerator while allowing the flow of acoustic power. For many heat transport mechanisms, the quality of the thermal isolation depends on the time-averaged mean temperature distribution in the thermal buffer tube, which is determined by boundary conditions set up by the heat exchangers. However, finite-amplitude effects within one peak-to-peak gas displacement of the heat exchangers can lead to significant modification of the thermal boundary conditions and thus the heat transport. To explore these effects, measured mean temperature profiles in the vicinity of the interface between a heat exchanger and thermal buffer tube are reported for a broad range of acoustic and thermal conditions. A one-dimensional Lagrangian model is developed to predict the mean temperature distribution, and reasonable agreement between experimental data and model results is found for the majority of the acoustic conditions considered.     

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.     

Performance comparison of looped thermoacoustic electric generators with various thermoacoustic stages

Thermoacoustic engine is a kind of novel heat engine based on thermoacoustic effect, with the merits of environmental benignity, simplicity, and reliability. In this work, looped travelling-wave thermoacoustic electric generators (LTTEGs) with one to four thermoacoustic stages have been developed and experimentally studied. It is observed that adding thermoacoustic stages can improve the thermal-electric efficiency of LTTEGs, while whether the extra stages lead to efficiency gain depends on the number of existing stages and other operating parameters (hot temperature, for instance). One main reason is that the Gedeon streaming, which might cause severe heat loss, can be enhanced by adding thermoacoustic stages and increasing hot temperature. The results suggest that the suppression of streaming in the looped thermoacoustic engine with multiple stages is even more urgent than in the traditional travelling-wave engine with only one stage.     

不同加热功率下充液率对无芯环路热管性能的影响

设计了以铝为管材、丙酮为传热工质的无芯环路热管。其蒸发段采用加热带加热,冷凝段用风冷降温。热管依靠蒸发压头使工质循环,并依靠重力作用,使冷凝液回流到蒸发段。搭建试验台并研究了不同加热功率下充液率对无芯环路热管的传热温差、传热量、热效率、热阻和当量导热系数的影响。结果表明:加热功率为150.00 W、充液率为30%时,无芯环路热管的均温性最好;传热温差和热阻均最小,分别为6.75℃、0.045 K/W。传热量132.00 W、热效率0.88、当量导热系数168 125 W/(m·K),均达到最大值。所以,该无芯环路热管在本实验研究范围内的最佳工作条件为加热功率150.00 W、充液率30%。     

124K热声驱动的脉管制冷机

热声压缩机是一种利用热能（如太阳能集热、废热等）进行驱动的新型驱动器。自行研制的驻波型热声压缩机驱动脉管经过改进后，以氦为工质，取得了124.3K的制冷温度。此外，该文还讨论了热声压机系统中的水冷却存在的问题以及它对系统整体性能的影响，并对热声机的应用前景进行了展望。     

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.     

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.     

Performance of solar powered thermoacoustic engine at different tilted angles

In this study, the thermodynamic performance of a thermoacoustic engine charged with different working fluids were examined at different tilted angles ranging from ?90° to 90° with 45° angular interval. The results suggest that the influence of the tilted angle on the onset temperature of the engine depends on the viscidity of the working gas. The lower the viscidity is, the more obvious the influence is. The difference between the maximum and the minimum onset temperature of the engine charged with nitrogen could be as high as 52 °C, but the difference for system charged with helium is only about 1.5 °C. The tilted angle has little or no effect on the pressure oscillation amplitude, pressure ratio, resonance frequency and the relation of the temperature versus heat power. They are mainly affected by the properties of the working gas. Furthermore, the interactions of the oscillatory motion and the natural convection of the working gas within the thermoacoustic core were also examined. The properties are of importance for the thermoacoustic engine driven by two-axis solar collector, for the tilted angle of the engine varies with the sun position.     

Visualization investigation of the flow and heat transfer in thermoacoustic engine driven by loudspeaker

Heat transfer process in thermoacoustic engine is affected by acoustic oscillation which makes it different from the heat transfer in steady flow. This study pays attention to the flow and heat transfer characteristics of thermoacoustic engine driven by loudspeaker. Thermal infrared imager and particle image velocimetry (PIV) were used to investigate the temperature and flow fields under two heat levels (150 °C and 200 °C). The radial and axial temperature distribution was analyzed through dimensionless temperature. To explore the appropriate working frequency, resonance characteristic was discussed. The experimental results illustrated that the first resonance frequency is the most effective driving frequency where thermoacoustic system shows the best performance. Heat transfer mode changed from natural convection to forced convection with the addition of acoustic oscillation. Original temperature field induced by heat convection was destroyed and temperature gradient redistributed as parabolic after sound addition.     

Oscillatory Flow in Thermoacoustic Sound Wave Generator

Oscillatory flow in a thermoacoustic sound wave generator is described. The thermoacoustic sound wave generator plays an important role in thermoacoustic equipment. The heat exchange between the working fluid and the stack, the acceleration and deceleration of the working fluid and viscous friction loss both in the stack and in the resonance tube influence the performance of the thermoacoustic sound wave generator. Particularly, oscillatory flow significantly influences the heat exchange mechanism between the working fluid and the stack. Temporal changes in pressure and velocity are sinusoidal inside the resonance tube. Flow forms an oscillatory jet just behind the tube outlet, and becomes intermittent far downstream outside the resonance tube. The open-end corrections of 0.63R, that is, the region where oscillatory flow characteristics are maintained downstream in spite of being outside the tube outlet, are confirmed by velocity measurements and flow visualization. Also, they are almost equal to acoustical theoretical results.     

海上油气平台余热朗肯循环发电系统热力学分析

海上油气平台存在大量的燃气轮机余热。通过建立海上平台余热朗肯循环发电系统仿真模型,开展平台余热发电热力学及热经济性分析。选取工质泵功率、发电机输出功率、系统热效率、换热面积和单位面积发电量等参数作为优化目标,研究不同冷凝温度下优化目标函数随蒸发器烟气进出口温差的变化规律。结果表明:随着蒸发器烟气进出口温差的增加,工质泵功率、发电机输出功率和系统APR先增大后减小。冷凝温度越高,工质泵功率越大,发电机输出功率和系统热效率越小。当冷凝温度为65℃时,系统APR最大。受透平出口蒸汽干度的限制,所研究工况下,系统发电机最大输出功率为7 496 kW,系统最大热效率和APR分别为14.16%和5 kW·m~(-2)。研究结果可为撬装化、集成化海上油气平台余热发电系统研制提供理论参考。     

Numerical evaluation and the effects of geometrical and operational parameters on thermal performance of the shell and double coil tube heat exchanger

Heat exchangers are extensively used in various industries. In this study, the impact of geometric and flow parameters on the performance of a shell and double helical coil heat exchanger is studied numerically. The investigated geometric parameters include external coil pitch, internal coil pitch, internal coil diameter, and coil diameter. The influences of considered geometrical parameters are analyzed on the output temperature of the hot and cold fluid, convective heat transfer coefficient, pressure drop, and average Nusselt number. Water is considered as working fluid in both shell and tube. As an innovation, double helical coils are used instead of one in the heat exchanger. To compare the obtained results accurately, in each section, the heat transfer area (coil outer surface) is kept constant in all models. The results show that the geometrical parameters of double helical coils significantly affect the heat transfer rate.