Artur J. Jaworski PhD MSc(Eng) DIC CEng FRAeS CMgr FCMI FHEA
Professor in Engineering and Head of School
School of Engineering and Design
College of Technology and Environment, London South Bank University
103 Borough Road, London SE1 0AA, United Kingdom
E: artur.jaworski@lsbu.ac.uk; T: +44(0) 207 815 7502
W: https://researchportal.lsbu.ac.uk/en/persons/artur-jaworski
Thermoacoustics
Introduction to Thermoacoustic Technologies
In thermoacoustic devices, an acoustic wave interacts with a porous solid material either to produce acoustic power, induced by a temperature gradient imposed on the solid, or to obtain a temperature gradient along the solid, induced by an imposed acoustic wave. The technology is based on the thermoacoustic effect whereby appropriately phased pressure and velocity oscillations enable the compressible fluid to undergo a thermodynamic cycle (similar to a Stirling cycle) in the vicinity of the solid material. These processes have been utilised in thermoacoustic engines and coolers. Their advantages are the lack of moving parts and construction simplicity: the conversion between thermal and acoustic energies is realised by an oscillating gas, without the need for piston-cylinder arrangements, sliding seals or harmful working fluids. Figure below shows a schematic of a quarter-wavelength, standing-wave device working as a refrigerator thanks to the acoustic power input, Wac.
Figure 1: Schematic of a thermoacoustic standing-wave cooler arrangement and explanation of heat pumping effect.
Central to its operation is a thermoacoustic “stack”. This can be imagined as a series of plates forming a set of parallel channels. The gas pressure in the resonator oscillates acoustically at a frequency set by the resonance between the gas in the duct and the moving mass of the transducer. The distribution of pressure amplitude, |p| and velocity amplitude, |u| illustrates the standing wave present in the resonator. The oscillating gas, within a short distance of thermal penetration depth, δν from the adjacent solid boundaries, communicates heat with the stack (as shown on the right) and heat exchangers, and the acoustics of the system ensure that the timing between the pressure and gas displacement is such that heat (the cooling load, Qc) is pumped out of the cold heat exchanger towards the hot heat exchanger (and removed to the surroundings as denoted by Qh ), using a hydrodynamic energy transfer “cascade” enabled by compressing and expanding gas parcels. It is also possible to reverse the operation of such systems to form an engine: a high temperature gradient along the stack leads to a spontaneous generation of acoustic power which can be converted to electricity by a linear alternator.
Modern thermoacoustic devices are typically built using travelling-wave topologies. Figure below shows a torus-shaped thermoacoustic engine, whose operation can be likened to a power amplifier in an electrical circuit analogy. The main part of the engine comprises of a regenerator (REG, a porous medium, which maintains the longitudinal temperature gradient), a hot heat exchanger (HHX) which adds heat to the gas, and a cold heat exchanger (CHX) which removes heat to the surroundings. The hot and cold heat exchangers are placed on each side of the regenerator – thus a temperature gradient exists between the heated and cooled ends of the regenerator, leading to a spontaneous generation of an acoustic wave. The circulating acoustic power is amplified in the engine’s “core” (CHX-REG-HHX), while the travelling wave phasing is maintained by the compliance, inertance and resistance components (the former two being synonymous with capacitance and inductance in electrical circuit analogies). Part of the energy can be removed as an electrical output by a linear alternator placed within the loop.
Figure 2: Schematic of a travelling-wave thermoacoustic engine.
The intensities of the acoustic waves used in thermoacoustics are enormous compared to the conventional audio applications. They are usually described by a “drive ratio”: the ratio of the maximum pressure amplitude of the acoustic wave to the mean pressure in the resonator. It is not uncommon to have mean pressures in the resonator in the range of 30-60 bar, which at drive ratios of the order of 10% corresponds to acoustic amplitudes 3-6 bar. This is needed to ensure the sufficient power densities, and is not dissimilar to pressures/pressure changes in more conventional thermodynamic machines. Fortunately, the thermoacoustic processes are contained in closed steel resonators and do not pose any noise discomfort to the user.
Thermoacoustics is an environmentally friendly technology: the working medium is usually an inert gas such as helium, neon, argon or xenon, or their mixtures; cheaper systems can just use air; and it is potentially low-cost due to the simplicity of constructing thermoacoustic devices (engines, coolers or heat pumps), low maintenance and the use of inexpensive parts, materials and working media. Potential applications of thermoacoustics include heat pumps for domestic applications or upgrading industrial waste heat; direct conversion of waste or geothermal heat or solar power into electricity; liquefaction and re-gasification of natural gas; combined heat and power systems; tri-generation systems (power, heating and cooling); solar driven cooling and air conditioning; localised cooling for microfluidic and microelectronic applications; cooling of radar arrays in navigation equipment; separation of gaseous isotopes used in medical imaging, tumour detection and radiotherapy; cheap electricity generators driven by gas/biomass combustion or satellite power systems based on heat generating radioisotope decay, to name but a few.
Selected Research Rigs
1. Standing Wave Resonator for Fundamental Fluid Flow and Heat Transfer Studies in Oscillatory Flow. The resonator can be filled with atmospheric air or nitrogen. Both can be seeded with smoke for PIV measurements or acetone/toluene for PLIF temperature measurements. The cross-section of the resonator is 136 x 136 mm; the total length in excess of 8 m. The operating frequency in 1/4-wavelength mode is 13.1 Hz. Extraction hoods provide a safe working environment for use of smoke, acetone and toluene.
2. Standing Wave Resonator for Macroscopic Heat Exchanger Analysis in Oscillatory Flow. This is a half-wavelength resonator designed to handle helium at pressures up to 40 bar. Acoustic excitation is provided by a high power linear motor (Q-Drive) which enables achieving drive ratios of up to 10%. The test section contains two heat exchangers side by side. Their heat transfer performance can be measured by calorimetric methods. The rig can be used for similarity studies of various heat exchangers used in thermoacoustic applications. The rig is around 9 m long, but since it is made of pipe sections, it can be made both shorter and longer to fit different projects.
3. Co-Axial Travelling-Wave Thermoacoustic Cooler Driven by a Standing-Wave Thermoacoustic Engine. This rig is a demonstrator for an EPSRC funded project referred to as SCORE. The rig consists of a standing-wave engine (left) and a co-axial travelling-wave cooler (right). In the demonstrator the engine is supplied with thermal input through resistive heating - in reality this heat would come from biomass combustion. The cooler achieves temperatures down to -5°C - in reality a storage compartment of about 1 litre volume will be used for storing medical supplies (vaccines) in remote areas with no access to electricity. Current demonstrator is built out of standard stainless steel pipes with 6 inch diameter and is just over 4 m long. Air at 10 bar is used as a working medium. The challenge of the design is to make it low-cost for developing countries.
4. Travelling-Wave Thermoacoustic Electricity Generator for Rural Areas of Developing Countries. This rig is a demonstrator for an EPSRC funded project referred to as SCORE. It is a travelling wave engine coupled to a linear alternator. The device generates electricity from waste heat used in cooking processes (here this waste heat is simulated by a gas burner. The working medium is air at atmospheric pressure.Construction of the device is driven by the low-cost requirements for developing countries - see for example the use of 2 inch PVC piping for the acoustic resonator.
5. Testing Equipmant for Characterisation of Thermoacoustic Regenerators. A number of experimental setups to characterise the performance of regenerators for the travelling wave thermoacoustic devices exist. The rig shown on the left has been designed to study selected "figures of merit" of regenerators. It consists of a linear alternator (compressor) to produce the acoustic excitation and an RLC network to ensure the travelling wave conditions within the regenerator samples. Appropriately designed heat exchangers ensure the preferred thermal conditions during the tests.The rig is designed to handle helium up to 40 bar pressure (other working media can be used). The rig on the right is designed to measure the thermal conductivity of regenerators. The setup can be enclosed in a pressure vessel which enables measurements in vacuum to eliminate the effects of thermal convection. Other rigs (not shown) can be used to measure quantities such as the pressure drop across the regenerator in steady flows.
