Tetsushi Biwa 교수

Using thermoacoustic energy conversions, both amplification and damping of acoustic intensity are demonstrated. A differentially heated regenerator is installed near the velocity node of the resonator and thereby a high specific acoustic impedance and a traveling wave phase are obtained. It is shown that the gain of acoustic intensity resulting from the traveling wave energy conversion reaches 1.7 in a positive temperature gradient and 0.3 in a negative gradient. When the regenerator is replaced

Premixed lean combustion is vital for reducing NO${}_{x}$ and CO emissions from gas turbine engines, but is prone to thermally induced gas oscillations that can lead to serious engine damage. In coupling two thermoacoustic oscillators, the authors take advantage of the phenomenon of amplitude death to stop the unwanted oscillations completely. Amplitude death is often associated with frequency detuning, but here a combination of time-delay and dissipative couplings allows even a pair of identica

To what extent can we lower the critical temperature ratio (CTR) necessary to start a thermoacoustic engine? We present an experimental method for predicting the CTR before the temperature ratio arrives at it using quality factor measurements. Based on the experimental quality factors, we tried to decrease the CTR of a thermoacoustic Stirling engine consisting of a looped tube and a branch resonator. Installation of the multiple regenerators at suitable positions can markedly enhance acoustic po

Two-sensor method proposed by Fusco et al. ["Two-sensor power measurements in lossy ducts," J. Acoust. Soc. Am. 91, 2229-2235 (1992)] is a novel technique that determines acoustic intensity of a gas column in a wide duct from measurements of pressure based on the boundary layer approximation. For further development of this method, its validity is experimentally tested through comparison with the direct method measuring the pressure and the velocity simultaneously, and its formulation is modifie

This paper reports on the acoustic streaming in a looped-tube thermoacoustic prime mover equipped with an asymmetric constriction called a jet pump. The time-averaged mass flow velocity was determined using visualization methods and using acoustic field measurements. It was demonstrated that the magnitude and the direction of the velocity were dependent on the orientation of the jet pump. From the observed velocities, we estimated the heat carried away from the hot heat exchanger by the mass flo

A cylindrical acoustic resonator was externally driven at the first resonance frequency by a compression driver. The acoustic energy stored in the resonator and the power dissipated per unit time were evaluated through the simultaneous measurements of acoustic pressure and velocity, in order to determine the Q value of the resonator. The resulting Q value, being employed as a measure of the damping in a resonator, was obtained as 36. However, the Q value determined from a frequency response curv

A gas column in a tube with a temperature gradient spontaneously begins to oscillate when it is driven away from equilibrium.We observed a transition from the standing-to the traveling-acoustic-wave modes through a quasiperiodic state using a looped tube attached to a resonator.Through the measurements of entropy flow, we experimentally propose a thermodynamical mode selection rule, which acts as a guiding principle for a sequence of unstable modes appearing in nonequilibrium systems.

Ceperley proposed a concept of a traveling wave heat engine ["A pistonless Stirling engine-The traveling wave heat engine," J. Acoust. Soc. Am. 66, 1508-1513 (1979).] that provided a starting point of thermoacoustics today. This paper verifies experimentally his idea through observation of amplification and strong damping of a plane acoustic traveling wave as it passes through axial temperature gradients. The acoustic power gain is shown to obey a universal curve specified by a dimensionless par

This paper reports thermally induced shock waves observed in an acoustic resonance tube. Self-sustained oscillations of a gas column were created by imposing an axial temperature gradient on the short stack of plates installed in the resonance tube filled with air at atmospheric pressure. The tube length and axial position of the stack were examined so as to make the acoustic amplitude of the gas oscillations maximum. The periodic shock wave was observed when the acoustic pressure amplitude reac

This study experimentally analyzes the cessation of self-sustained periodic oscillations of gas columns in delay-coupled Rijke tube oscillators. The Rijke tube oscillator comprised an open-ended resonance tube with a Bunsen burner inserted into it. Delay coupling was introduced using acoustic waves propagating through a gas-filled tube with both ends connected to the resonance tubes. Two coupling methods, single- and double-tube coupling, were tested for comparison. A significant reduction in th

Shock waves were explored in the thermoacoustic spontaneous gas oscillations occurring in a gas column with a steep temperature gradient. The results show that a periodic shock occurs in the traveling wave mode in a looped tube but not in the standing wave mode in a resonator. Measurements of the harmonic components of the acoustic intensity reveal a clear difference between them. The temperature gradient acts as an acoustic energy source for the harmonic components of the shock wave in the trav

Nonlinear excitation of periodic shock waves in high-amplitude standing waves was studied from measurements of the acoustic intensity. A gas column of atmospheric air in a cylindrical resonator was driven sinusoidally by an oscillating piston at the fundamental resonance frequency. Acoustic pressure and axial acoustic particle velocity were simultaneously measured and decomposed into the Fourier components, from which the intensity associated with each of the oscillating modes in the resonator w

The use of two pressure sensors [Fusco et al., J. Acoust. Soc. Am. 91, 2229 (1992)] makes it possible to determine the acoustic intensity of a gas column in a duct, but the application of this method was limited to wide ducts. In this letter, the formulation of the method is modified to include narrow ducts where the duct radius is as small as the viscous boundary layer thickness of the gas. The validity of this method is shown by comparison with the direct measurements of the pressure and veloc

To control one-way propagation of pressure waves in air or water, acoustic diodes typically employ a nonlinear medium to double frequency, or specially designed diffraction structures with which power loss is unavoidable. The authors present a three-stage device that instead takes advantage of the thermal interaction between gas particles and the walls of a porous material with a temperature gradient along the wave path. This thermoacoustic diode $a\phantom{\rule{0}{0ex}}m\phantom{\rule{0}{0ex}}