A numerical investigation on the vortex formation and flow separation of the oscillatory flow in jet pumps

A two-dimensional computational fluid dynamics model is used to predict the oscillatory flow through a tapered cylindrical tube section (jet pump) placed in a larger outer tube. Due to the shape of the jet pump, there will exist an asymmetry in the hydrodynamic end effects which will cause a time-averaged pressure drop to occur that can be used to cancel Gedeon streaming in a closed-loop thermoacoustic device. The performance of two jet pump geometries with different taper angles is investigated. A specific time-domain impedance boundary condition is implemented in order to simulate traveling acoustic wave conditions. It is shown that by scaling the acoustic displacement amplitude to the jet pump dimensions, similar minor losses are observed independent of the jet pump geometry. Four different flow regimes are distinguished and the observed flow phenomena are related to the jet pump performance. The simulated jet pump performance is compared to an existing quasi-steady approximation which is shown to only be valid for small displacement amplitudes compared to the jet pump length.Comment: The following article has been accepted by the Journal of the Acoustical Society of America. After it is published, it will be found at: http://scitation.aip.org/JAS

Jet pumps for thermoacoustic applications: design guidelines based on a numerical parameter study

The oscillatory flow through tapered cylindrical tube sections (jet pumps) is characterized by a numerical parameter study. The shape of a jet pump results in asymmetric hydrodynamic end effects which cause a time-averaged pressure drop to occur under oscillatory flow conditions. Hence, jet pumps are used as streaming suppressors in closed-loop thermoacoustic devices. A two-dimensional axisymmetric computational fluid dynamics model is used to calculate the performance of a large number of conical jet pump geometries in terms of time-averaged pressure drop and acoustic power dissipation. The investigated geometrical parameters include the jet pump length, taper angle, waist diameter and waist curvature. In correspondence with previous work, four flow regimes are observed which characterize the jet pump performance and dimensionless parameters are introduced to scale the performance of the various jet pump geometries. The simulation results are compared to an existing quasi-steady theory and it is shown that this theory is only applicable in a small operation region. Based on the scaling parameters, an optimum operation region is defined and design guidelines are proposed which can be directly used for future jet pump design.Comment: The following article has been accepted by the Journal of the Acoustical Society of America. After it is published, it will be found at http://scitation.aip.org/JAS

Accounting for convective effects in zero-Mach-number thermoacoustic models

This paper presents a methodology to account for some mean-flow effects on thermo-acoustic instabilities when using the zero-Mach-number assumption. It is shown that when a computational domain is represented under the M=0 assumption, a nonzero-Mach-number element can simply be taken into account by imposing a proper acoustic impedance at the boundaries so as to mimic the mean flow effects in the outer, not computed flow domain. A model that accounts for the coupling between acoustic and entropy waves is presented. It relies on a “delayed entropy coupled boundary condition” (DECBC) for the Helmholtz equation satisfied by the acoustic pressure. The model proves able to capture low-frequency entropic modes even without mean-flow terms in the fluctuating pressure equation

Identification and Attenuation of Losses in Thermoacoustics: Issues Arising in the Miniaturization of Thermoacoustic Devices

Thermoacoustic energy conversion is based on the Stirling cycle and uses sound waves to displace and compress the working gas. When this process occurs inside a porous medium that is subject to a temperature gradient, a thermoacoustic engine creates intense sound. Conversely, when strong sound waves interact with a porous medium, a temperature gradient can be imposed through the attenuation of the pressure amplitude, creating a thermoacoustic refrigerator. The device size is a limiting factor to widespread use. This work investigates issues arising in their miniaturization in three separate ways. To date, the thermal properties of the driving components are largely ignored during the design phase, partially because the traditional design ``works,' and partially because of a lack of understanding of the thermal energy fluxes that occur during operation. First, a direct quantification of the influence of the thermal conductivity of the driving components on the performance of a thermoacoustic engine and refrigerator is performed. It is shown that materials with low thermal conductivity yield the highest sound output and cooling performance, respectively. As a second approach to decreasing the footprint of a thermoacoustic system, the introduction of curvature to the resonator tube was investigated. A CFD analysis of a whole thermoacoustic engine was performed, and the influence of the stack assembly on the flow behavior was investigated. Nonlinearities in the temperature behavior and vortices in the flow close to the stack ends were identified. Resonator curvature prompts a decrease in the amplitude of the pressure, velocity, and temperature oscillations. Furthermore, the total energy transfer from the stack to the fluid is also reduced. Finally, through combining the aforementioned investigations, an optimization scheme is applied to a standing wave engine. A black box solver was used to find the optimal combination of the design parameters subject to four objectives. When focusing solely on acoustic power, for example, the device should be designed to be as large as possible. On the other hand, when attempting to minimize thermal losses, the stack should be designed as small as possible

Numerical simulation on onset characteristics of traveling-wave thermoacoustic engines based on a time-domain network model

Onset characteristics of thermoacoustic engines are of great importance for understanding the internal working mechanisms of thermoacoustic conversion. A one-dimensional time-domain network model for predicting the onset characteristics of traveling-wave thermoacoustic engines with helium as working gas is built. The acoustic resistance, inertance, compliance, and thermal-relaxation effects of all the acoustic components are included. The viscous and heat transfer terms in the time-domain governing equations of the acoustic tubes and the heat exchangers are deduced from the frequency-domain linear thermoacoustic theory. Combining the time-domain governing equations of the regenerator, numerical simulations of the whole onset process are then conducted in a wide operating condition range. The complete dynamic pressure wave evolution processes are simulated successfully. It is shown that a steady standing-wave acoustic field forms in almost all parts of the traveling-wave thermoacoustic engine except for the regenerator area. Onset temperature, operating frequency, and quality factor are calculated with a relatively high accuracy. The thermal relaxation effects in the regenerator are found to have a remarkable impact on the onset characteristics, especially at high mean pressures. It is also shown that the experimental damping temperature is closer to the calculated onset temperature than the experimental onset temperature. Furthermore, the reasonable distributions of the pressure and volume flow rate and the phase relationship between them in the whole system are obtained and analyzed

Computational fluid dynamics analysis of the oscillatory flow in a jet pump: the influence of taper angle

A two-dimensional CFD model for predicting the oscillating flow through a jet pump is developed. Various taper angles are investigated and total minor loss coefficients are derived. A good correspondence is achieved with experimental results from the literature. However, at higher taper angles a dramatic decay in the jet pump pressure drop is observed, which serves as a starting point for the improvement of jet pump design criteria for compact thermoacoustic application

Simulation and experimental design of thermoacoustic heat engine

Renewable energy is an important field in providing reliable and sustainable energy to the world. Wasted heat is found to be a good source of renewable energy. This wasted energy can be found almost in all types of production processes, including the heat exchanger. The heat energy dissipated from these processes is unutilized leading to inefficiency in the system. The need to harvest the wasted heat is essential in making sure the energy can be further utilized for other applications. Previous research works conducted on harvesting heat into sound in the system is still lacking and there is no specific standard can be employed. This research focused on analysing and developing a reference method of harvesting sound from a thermoacoustic heat engine system. A simulation approach was employed to investigate the performance of heat flow on the heat exchanger and related components. A standard test rig was designed to evaluate the performance of heat transfer experimentally. A comprehensive laboratory work was set-up to collect ample data to obtain the correlation of acoustic sound pressure-volume due to heat transfer performance by the oscillatory flow on the thermoacoustic system. The design of the developed thermoacoustic engine was able to produce waste heat in the range between 200°C and 700°C, and the harvested sound frequency ranged from 20Hz to 2kHz. From the experimental study, the sound level started at 4s to 8s and reaches a steady-state at 10s. The temperature gradient on stack performance was 8.45°C/mm with a temperature difference at the steady-state point of 300°C. The spectrum analysis amplitude reached 133.5dB with the frequency value of 397.5 Hz. The pressure volume analysis has proved the existence of both isochoric and isothermal process through the gas bucket brigade phenomenon as the lead compression and expansion happened at the stack wall between the sound pressures of 12.94Pa and 20.15Pa. The finding confirmed that the sound energy from the heat oscillation can be harvested and a standard method has been developed. This study also confirmed the presence of a thermoacoustic cycle on the stack wall. This finding is significant as it provides a new standard in harvesting sound from the thermoacoustic heat engine. The efficiency of the system was successfully improved by 40% and the wasted energy was successfully harvested for further applications

Large eddy simulation of thermally induced oscillatory flow in a thermoacoustic engine

In this paper, a comprehensive high-fidelity three-dimensional computational fluid dynamic research using large eddy simulation has been conducted to investigate a standing-wave quarter-wavelength thermoacoustic engine that consists of a hot buffer, a stack and a resonator. The performance of the thermoacoustic engine has been analysed in four aspects. Firstly, the dynamic characteristics of the engine during the initial start-up process are investigated when changing the temperature gradient imposed on the stack. Numerical results are compared with those from a system-wide reduced-order network model based on linear thermoacoustic theory. Secondly, the acoustic behaviour of the engine operating at steady state is studied. Fourier Series Model is utilized to decompose the steady-state acoustic pressure oscillations which reveals the unstable longitudinal acoustic modes excited in the engine. The stack serves as an energy source for the fundamental mode while it extracts acoustic power from the second harmonic. Thirdly, the hydrodynamic performances of the engine are inspected, and the obtained three-dimensional flow fields inside the engine enable us to probe into rich nonlinear phenomena including minor losses, mass streaming, etc. Finally, the heat transfer characteristics have been analysed by examining the mean temperature field and transversal heat fluxes along the engine. This research demonstrates that the large eddy simulation framework is effective in simulating the thermally induced oscillatory flow inside thermoacoustic engines. The multi-perspective analytical methodologies are valuable in comprehending the engine performance and provide guidelines for the design and optimization of efficient thermoacoustic engines for recovering waste thermal energy from various sources