Sound waves can chill ice cream, while heat can generate intense whistling tones. Welcome to the bizarre world of thermoacoustics, a promising source of ideas for next generation heat engines.
Whoof. The gas burner spits out a blue flame and fires up the metal maze in a steel pipe that begins to glow. After about ten seconds, professor of energy technology, dr. Bendiks Jan Boersma, of the 3mE faculty’s process and energy department, turns the pipe upright, causing a siren-like tone to emerge from the pipe. Triumphantly, and with a broad smile and boyish pleasure, the professor declares: “This is the Rijke tube, named after a Dutch physicist who demonstrated the link between heat and sound.” The first documentation of this phenomenon dates from 1850. Glassblowers reported the generation of a pure tone when heating the neck of a bottle. So what in fact was going on?
The basis of it all is that compression and expansion of air in sound waves involves exchanges of heat, as well as pressure and volume changes. As early as 1816, Laplace corrected Newton’s earlier calculation of the speed of sound in air by accounting for the slight variations in temperature that occur. He thus arrived at the correct speed of sound, which was 18 percent higher than Newton’s estimate. Still, the changes in pressure and temperature involved in sound are very modest. Even in a deafening 120-decibel blast, the temperature only varies 0.02 degrees.
The thermoacoustical effect can however be amplified to serve in practical applications; for example, in 2004, Ben & Jerry’s, the environmentally orientated ice-cream manufacturers, debuted the thermoacoustic chiller, made by Steven Garrett, who ten years earlier had developed sound-driven refrigerators for the Space Shuttle, which were eventually used on a US Navy destroyer. These refrigerators delivered 400 watts of cooling power for 200 watts of acoustic power. So, thermoacoustics works, but how?
Boersma draws a pipe between a hot and a cold spot (typically heat exchangers) and a loudspeaker that forces the air to oscillate. If, supposing, the speaker compresses a small volume of gas towards the warm side, it will warm up as a result of the compression and release its heat. At the next half of the cycle, the volume of gas travels back to the cold, where it expands and cools, and draws heat from the cold zone. The result is a tiny net heat transport from the cold to the hot side, driven by acoustical resonance.
If the temperature differences over the stack (a porous solid in which heat-exchanges between gas and metal occur) are much higher, the reverse happens: spontaneous oscillating air absorbs heat from the hot side and delivers it to the cold side, where it expands before oscillating back. In the Rijke tube, an initial shock wave of hot air rising kick-starts this process. An ‘acoustic laser’ designed by Reh-lin Chen, from Penn State University (US), turned the same processes into a continuous one. Boersma sets it up for demonstration. His set-up consists of a loose plug supporting an electrical wire at the back end of a glass tube. Boersma switches on the power. As the wire starts glowing, he gently blows along the top of the horizontal tube. Suddenly, the tube takes over the tone and starts producing a loud and pure whistle. A broad grin appears on the professor’s face. “Just make it bigger, more efficient and hook on a generator. That’s what we aim to do,” he says.
Boersma wants to develop a heat engine based on the thermoacoustic principle, which would be like the Sterling engine (a heat engine based on a patent from 1816 by Robert Sterling, a Scottish minister), but without the need of pistons and cranks. In fact, it would be an engine without any moving parts except for the oscillating air driving a generator. If this sounds far-fetched, Boersma is the first to admit that thermoacoustic generators may indeed still be 20 years away from fruition. But at least he now has the budget to hire a post-doctoral researcher to build a demo. “If he’s good enough,” the professor says, “he or she can start tomorrow.”
www.americanscientist.org/issues
,The Rijke tube,An acoustic laser,
Recommended reading in American Scientist The power of sound
Boo-hoo for the poor international students stuck in Delft over the Christmas holiday, especially on December 25th and 26th, when the TU’s Sports & Cultural Centers are closed, all the Dutch have gone home to celebrate with their families, and the TU campus is nothing but a depressing ghost town, with empty plastic bags blowing down deserted streets like tumbleweeds. So is that you sad and lonely foreigner biking through Delft all alone on Christmas Eve? Well come on, pull yourself together, don’t start feeling sorry for yourself yet! I’m here to help! A man with a plan! Listen, all you got to do is find out where some Polish foreign students live, or even some Polish laborers, and you’re in luck. You see, the Polish have this special ‘Wigilia’ Christmas dinner tradition that involves one extra place being set at the table in front of an empty chair, in case some unexpected guest or stranger happens to show up, which of course means all you’ve got to do, stranger, is knock on the right door in Delft and a delicious Polish Christmas feast is yours! – or else it’s just some b.s. Christmas tradition in theory, in which case they’ll probably slam the door in your face or, worse, if they’re drunken illegal workers, kick your butt off their balcony. But so what, at least you’ll have had a Christmas in Delft to remember. (Photo/caption: Babak Nikkhah Bahrami, from Iran/Netherlands, BSc mechanical engineering)
Whoof. The gas burner spits out a blue flame and fires up the metal maze in a steel pipe that begins to glow. After about ten seconds, professor of energy technology, dr. Bendiks Jan Boersma, of the 3mE faculty’s process and energy department, turns the pipe upright, causing a siren-like tone to emerge from the pipe. Triumphantly, and with a broad smile and boyish pleasure, the professor declares: “This is the Rijke tube, named after a Dutch physicist who demonstrated the link between heat and sound.” The first documentation of this phenomenon dates from 1850. Glassblowers reported the generation of a pure tone when heating the neck of a bottle. So what in fact was going on?
The basis of it all is that compression and expansion of air in sound waves involves exchanges of heat, as well as pressure and volume changes. As early as 1816, Laplace corrected Newton’s earlier calculation of the speed of sound in air by accounting for the slight variations in temperature that occur. He thus arrived at the correct speed of sound, which was 18 percent higher than Newton’s estimate. Still, the changes in pressure and temperature involved in sound are very modest. Even in a deafening 120-decibel blast, the temperature only varies 0.02 degrees.
The thermoacoustical effect can however be amplified to serve in practical applications; for example, in 2004, Ben & Jerry’s, the environmentally orientated ice-cream manufacturers, debuted the thermoacoustic chiller, made by Steven Garrett, who ten years earlier had developed sound-driven refrigerators for the Space Shuttle, which were eventually used on a US Navy destroyer. These refrigerators delivered 400 watts of cooling power for 200 watts of acoustic power. So, thermoacoustics works, but how?
Boersma draws a pipe between a hot and a cold spot (typically heat exchangers) and a loudspeaker that forces the air to oscillate. If, supposing, the speaker compresses a small volume of gas towards the warm side, it will warm up as a result of the compression and release its heat. At the next half of the cycle, the volume of gas travels back to the cold, where it expands and cools, and draws heat from the cold zone. The result is a tiny net heat transport from the cold to the hot side, driven by acoustical resonance.
If the temperature differences over the stack (a porous solid in which heat-exchanges between gas and metal occur) are much higher, the reverse happens: spontaneous oscillating air absorbs heat from the hot side and delivers it to the cold side, where it expands before oscillating back. In the Rijke tube, an initial shock wave of hot air rising kick-starts this process. An ‘acoustic laser’ designed by Reh-lin Chen, from Penn State University (US), turned the same processes into a continuous one. Boersma sets it up for demonstration. His set-up consists of a loose plug supporting an electrical wire at the back end of a glass tube. Boersma switches on the power. As the wire starts glowing, he gently blows along the top of the horizontal tube. Suddenly, the tube takes over the tone and starts producing a loud and pure whistle. A broad grin appears on the professor’s face. “Just make it bigger, more efficient and hook on a generator. That’s what we aim to do,” he says.
Boersma wants to develop a heat engine based on the thermoacoustic principle, which would be like the Sterling engine (a heat engine based on a patent from 1816 by Robert Sterling, a Scottish minister), but without the need of pistons and cranks. In fact, it would be an engine without any moving parts except for the oscillating air driving a generator. If this sounds far-fetched, Boersma is the first to admit that thermoacoustic generators may indeed still be 20 years away from fruition. But at least he now has the budget to hire a post-doctoral researcher to build a demo. “If he’s good enough,” the professor says, “he or she can start tomorrow.”
The Rijke tube
An acoustic laser
Recommended reading in American Scientist The power of sound
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