THERMOACOUSTICS Energy and Work from Sound By Joe Olivares ME 258 Fall 2012 Multiple Regenerator Thermoacoustic Generator

Getting Work from Sound I. Fundamentals Getting Work from Sound • What is Sound?- Its mechanical radiant energy that is transmitted by longitudinal pressure waves in a material medium (as air) and is the objective cause of hearing • How are Heat and Sound/Pressure Related? –Thermodynamics, Fluid Mechanics, Heat Transfer How do I make WORK?- Drive a Speaker in Reverse and Make Electricity OR Transfer the Heat! Drive a Stirling Engine? Thermoacoustic Generators are a Basis of a Bigger Thermodynamic Cycle.

Waves The two types of waves dealt with in thermoacoustics are the standing wave and the traveling wave. The standing wave looks like it’s standing still because it is vibrating at the resonant frequency, or mode, of the tube. The traveling wave is a moving wave, when it hits one end it is reflected back. This reflection can either weaken or strengthen its amplitude. Air travels in a compression wave and as air is heated it wants to move in accordance with natural laws.

Waves Standing waves have nodes and anti nodes. In the case of air these nodes are representative of the pressure they have along the dimensional axis. With the variations in pressure there is a variation in temperature and normally it is negligible, unless you talk very loudly and right in-front of someone. With resonance the temperature and pressure effects are no longer dismissed and you get a great effect.

Reijke and Sondhauss Tubes Making Noise • The Sondhauss effect was noted by glassblowers and explained and replicated in 1850 by the German physicist Karl Friedrich Julius Sondhauss The effect is a standing sound wave in a tube with one open end and one closed end • In 1859 physicist PL Rijke discovered a way to sustain a sound in a cylindrical tube open at both ends using heat by moving the heat source around in the tube. The sound comes from a standing wave whose wavelength is about twice the length of the tube, giving the fundamental frequency. • 1877 John William Strutt, “Lord Rayleigh” writes the Theory of Sound the definitive book on acoustics relating pressure and sound waves, and creating the basis for thermoacoustics. The field is silent for almost 90 years • Robert Carter (1960’s) of Atomics International Division of North American Aviation, Inc., Gregory Swift (1980’s) at LANLab, Thermal Physics Group, Nicolas Rott (1980) lay the mathematical and modern basis using a system of linearized Navier-Stokes equations coupled with acoustical energy balances to describe the dynamics of a modern system •

Reijke Tube Basics Making Noise

Reijke and Sondhauss Tube Power of Resonance 1850-1880 Open Ends-Reijke Closed End-Sondhauss

Building on the Noisy Tube Why such a long time? The Modern System Building on the Noisy Tube Why such a long time? Modern methods of controlling the waveform have led to increases in pressure and heat transfer rates that can no longer be ignored but harvested as energy sources. Modern computing has led to numerical analysis that can solve the multiphysics problem

Building on the Noisy Tube The Modern System Building on the Noisy Tube The major improvement to modern systems was how do you get the heat exchangers to work like efficiently to get closer to Carnot

Building on the Noisy Tube The Modern System Building on the Noisy Tube • Each ‘simple’ system is comprised of a Resonator, Stack, Working Fluid/gas • The resonator is designed to at a ¼, ½ wavelength of the standing frequency to maximize heat transfer into the stack elements • The stack is a porous plug. It’s design and placement along the tube is the fundamental design issue in all systems. Some stacks are parallel plates of thermally conducting material or ceramic insulators. The distance between the plates and or holes is defined as 𝛿 𝑘 = 𝑘 𝜋𝑓𝜌 𝑐 𝑝 , called the thermal penetration depth and is roughly the distance heat can diffuse through the gas during the time 1/f𝜋 • The working fluid is chosen for its dynamic property values. Air and other inert gasses are most common. Most working mean pressures in ‘simple’ systems are 3atm or less.

A typical system consists of a resonator, a “Stack” of plates or fins connected to the outside world, and a driver/driven object like a speaker that can create sound or electricity from vibrations. The two main versions of engines are the Standing Wave and Traveling Wave type. The multiple combinations of regenerators and heat exchangers creates combined cycle analysis and a much more complex analysis

The Generator Vs Refrigerator To Pump or to Push • In 1980 Gregory Swift (LANLab) wrote the fundamental paper explaining to other researchers the simplicity of a complex problem. • In his paper he stressed that not only could you use a TA device as a generator but if you drove the engine in reverse you could create a refrigerator • Q in, Sound Out (Heat Engine) OR Sound In, Q out (Refrigeration) was the result and two branches of research were developed. The major difference is that you can’t get as large a gradient from cooling as heat pumping but you can still get very cool.

The Half Wave Generator Getting Work from Sound

Getting Work from Sound I. Fundamentals Getting Work from Sound The Refrigerator Here the speaker is the driving force and the heat pump is turned into a refrigerator. Once the gas reaches the “cold end” it readily absorbs heat. Some researchers have modeled this as a reverse Stirling Cycle.

Solar Powered Electricity Generation II. Current Research Solar Powered Electricity Generation In May 2012 alternative researchers in China designed a Traveling Wave TA engine that used a parabolic array of mirrors as it’s heating source. They focused the mirrors on a “Sodium” based liquid base that conducted heat into the TA’s stack. This method allows for a high heat input. It produced a thermal to electric efficiency of 15% and an output of 481 Watts, using 3.5MPa pressurized helium. Although the effectiveness was small this research allowed others to see that higher pressure systems are realizable but more research needs to be used into the “stack” transfer. The problem is in funneling of heat and overcoming the lag associated with conductive heating.

Experimentally Identifying the Faults in Efficiency II. Current Research Experimentally Identifying the Faults in Efficiency Because the simulated flow of such a system is extremely complex some asian researchers feel that the system inefficiencies are characterized by “incomplete” data and have begun to take thermal time lapse studies of the TA’s temperature gradients as a function of frequency of oscillation and physical characteristics like diameter, fluid density, etc. Their conclusions were that the first resonant frequency is the most effective working frequency for refrigerators and that the velocity distributions become non-uniform the larger the temperature difference at the stack ends. Lastly, they saw that the addition of sound automatically shifted the problem to that of a forced convection one.

Power Refrigeration With No Moving parts II. Current Research Power Refrigeration With No Moving parts Although not as recent, 1980-Current, power refrigeration has been at the forefront of US research at the Los Alamos National Laboratory under the guidance of G. Swift. The use of refrigeration without moving parts is of particular interest to the US with regards to HE physics. they easily been able to reach temperatures of -70 Celsius Older Refrigeration Charts

References Backhaus, Scott., Swift, Greg. “New Varieties of Thermoacoustic Engines” 9th International Congress on Sound and Vibration (2002): LA-UR-02-2721. Swift, Gregory. “Thermoacoustic Engines and Refrigerators” Physics Today (July 1995): Pg. 22-28. Wu, Z., Dai, Wei., Man, Man., Luo, Ercang “A Solar Powered Traveling Wave Thermoacoustic Electricity Generator ” Solar Energy #86-Elsevier (May 2012): Pg. 2376-2382. Pan, Na., Wang, S., Shen, Chao “Visualization Investigation of the Flow and Heat Transfer in Thermoacoustic Engine Driven by Loudspeaker” International Journal of Heat and Mass Transfer #55(July 2012): Pg. 7737-7746