Robert M. Keolian

Measurements of resistance of individual square-mesh screens to oscillating flow at low and intermediate Reynolds numbers

Measurements are reported of pressure losses across single screens subjected to low-frequency oscillating flow for 0.002 ≤Re d ≤400, where Re d is Reynolds number based on wire diameter and peak approach velocity. Several correlation methods are examined. Extensive comparisons are made between present oscillating-flow results and previous reports of the resistance of screens to steady flow

Influence of velocity profile nonuniformity on minor losses for flow exiting thermoacoustic heat exchangers (L)

Nonuniform, time-dependent velocity profiles for laminar oscillating flow are employed in calculating effective minor loss exit coefficients. The results could represent an improvement over the Borda–Carnot formula for calculating flow losses at the exit from heat exchangers in thermoacoustic devices, since the Borda–Carnot formula assumes uniform velocity across a flow cross section. Results are presented for parallel plate and circular tube geometries.

Effectiveness of parallel-plate heat exchangers in thermoacoustic devices

Measurements are made of the heat transferred between two identical parallel-plate heat exchangers under conditions of oscillating flow over a range of frequencies and amplitudes. The results are analyzed and summarized in terms of heat-exchanger effectiveness, the ratio of the actual heat transfer rate to the maximum possible heat transfer rate. Measured results are compared to the DELTAE model that is often used in the design of conventional thermoacoustic devices, and possible improvements to the model are offered.

High-frequency thermoacoustic-Stirling heat engine demonstration device

A small thermoacoustic-Stirling engine demonstration device that can produce sound in excess of 100 dB at 560 Hz has been constructed. The engine consists of a quarter wavelength acoustic resonator with a smaller diameter coaxial regenerator positioned toward the resonator’s closed end, thereby forming an acoustic feedback path around the regenerator. Acoustic oscillations begin spontaneously when the hot heat exchanger adjoining one end of the regenerator is heated to a sufficient temperature. A water stream in a second heat exchanger maintains the opposite end of the regenerator near ambient temperature. This device was inspired by the Backhaus-Swift engine and is a preliminary step in the investigation of regenerator operation at frequencies much higher than may be practical with mechanical or free-piston Stirling engines.

Calculated effects of pressure-driven temperature oscillations on heat exchangers in thermoacoustic devices with and without a stack

A Lagrangian computational method is used to explore the performance of heat exchangers in conditions of oscillating flow and oscillating pressure relevant to thermoacoustic devices. Pressure oscillations cause temperature oscillations within the working gas. Depending on phase, these “pressure-driven temperature oscillations” can enhance or degrade heat transfer within the exchangers of a thermoacoustic device.

Measurements of the resistance of parallel-plate heat exchangers to oscillating flow at high amplitudes

Measurements of the acoustic resistance of parallel-plate heat exchangers are reported. The resistance is measured for two identical exchangers separated by a small gap, and also by a large gap. High amplitude deviations from linear theory are analyzed in terms of a minor loss coefficient. Results are compared to theoretical predictions made in a previous article.

Energy conversion through thermoacoustics and piezoelectricity

Waste or prime heat can be converted into electricity with thermoacoustic-Stirling engines coupled to piezoelectric alternators. An inline arrangement of engines and alternators allows a vibration balanced, multiphase power generator that is compact, light weight and low cost. The engines convert heat into high amplitude ≈400 Hz oscillations in pressurized helium gas. These pressure oscillations cause a thin steel diaphragm to flex like a drumhead. The diaphragm is supported at its perimeter by a ring of piezoelectric elements. As the diaphragm flexes in either direction, it pulls inward on the piezoelectric elements causing a large amplified ≈800 Hz fluctuating compressive stress in the elements which then convert the stress into electricity with high efficiency. The flexible-diaphragm piezoelectric alternator overcomes the large acoustic impedance mismatch between the helium and piezoelectric elements without exceeding the limited fatigue strength of available materials. So far, a prototype generator ha...

Acoustic Measurement of Isotopic Concentration of Gases

Two identical, thermally connected spherical resonators have been constructed to make accurate measurements of the isotopic concentration of 3He‐4He gas mixtures. One sphere is filled with pure 4He, while the unknown mixture is placed in the other, Radial modes are excited in order to minimize viscous wall losses which reduce the quality of the resonance. A small cylindrical PVF2 driver at the center of each sphere selectively excites the radial modes, and electret microphones are used for pick‐up at the walls. Measuring the ratio of the resonant frequencies gives the molecular weight ratio of the two gases. Phase‐locked loop circuits have been developed for tracking the resonances and digital electronics are used for direct readout of the concentration. [Work supported by ONR and NSF.]

High‐efficiency 2‐kW thermoacoustic driver

A moving‐magnet electrodynamic driver will be described which is intended to maintain an acoustic resonance within a thermoacoustic air conditioner containing an inert gas mixture of helium and argon at 30 bar. It is energized by a linear motor with a Bl‐product of 41 N/A and a coil resistance of 0.24 Ω. An auxiliary spring system augments the magnetic stiffness (154 kN/m) to provide a driver mechanical resonance frequency near 60 Hz. An Inconel bellows forms a flexure seal which provides an effective piston area that maximizes electroacoustic conversion efficiency by making the Joule heating losses in the coil equal to the mechanical dissipation in the magnetic suspension. The driver’s electroacoustic conversion efficiency is predicted to be in excess of 83%. [Work supported by the Office of Naval Research.]

Utilization of the k -Space Method in the Design of a Ferroelectric Hyperthermia Phased Array

This research utilizes the k-space computational method and a three-dimensional (3D), inhomogeneous, large scale, and coarse grid human prostate model to design and to fabricate an intracavitary probe for hyperthermia treatment of prostate cancer. A 3D acoustical prostate model was created utilizing photographic data from the Visible Human Project®. Ultrasound wave propagation of the designed phased array was simulated by means of the k-space computational method. Four stacked linear phased arrays composed the 80 elements hyperthermia array. Good agreement between the exposimetry and the k-space simulation results was obtained. As an example, the measured −3 dB distance of the focal volume in the propagation direction deviated by only 9% compared to simulated results.