George Mozurkewich

Time-average temperature distribution in a thermoacoustic stack

The three-dimensional time-average temperature distribution in a pore of a thermally isolated thermoacoustic stack is calculated. A boundary-value problem is formulated in the acoustic and short-stack approximations from the equation of conservation of energy using literature results for the time-average energy flux. In the central region of the pore, the solution for the time-average temperatures of the wall, Tw, and of the gas along its center line, Tg, share a common profile, linear in the axial coordinate, z. Near the pore ends, where the energy flux approaches zero, the axial gradient of Tg approaches the critical temperature gradient over a distance of order the acoustic displacement amplitude. The axial gradient of Tw approaches zero over a much smaller distance, provided the wall has small thermal conductivity. The transverse heat-flux density, q, is nonzero only near pore ends. Under certain conditions, q=h1(Tg−Tw), where h1 is proportional to the thermal conductivity of the gas divided by the th...

Heat transfer from transverse tubes adjacent to a thermoacoustic stack

The goal of this work was to test the usefulness of the time-average steady-flow equivalent (TASFE) approximation for the analysis of heat exchangers for thermoacoustic engines. The TASFE approximation assumes that the heat transfer in an acoustic standing wave with zero mean flow is equivalent to averaging the corresponding steady-flow correlation over a sinusoidal distribution of gas speeds. The experiment tested three simple heat-exchanger configurations located immediately adjacent to the hot end of a thermoacoustic stack. Two configurations consisted of bare, parallel, water-carrying tubes oriented transverse to the mean acoustic flow. A third configuration consisted of a single layer of woven copper screen soldered to transverse tubes. Experiments with pure helium and with a mixture of helium and argon found qualitative, and sometimes quantitative, agreement with expectations of the TASFE approximation at both large and small acoustic amplitudes. In addition, large heat transfer with zero temperatur...

A model for transverse heat transfer in thermoacoustics

A model for transverse heat transfer in parallel thermoacoustic pores is developed. The model is one-dimensional, in the sense that the temperatures in the gas, Tg, and in the solid walls, Tw, are assumed to be fully represented by their dependences on the pore’s axial coordinate, z. All effects of transverse variation across the cross section of the pore are subsumed into a transverse heat-transfer coefficient, h1, that couples the temperatures according to q(z)=h1[Tg(z)−Tw(z)], where q is the transverse heat transfer per unit area. First the model is applied to thermally isolated pores, and results are compared to a recent three-dimensional analysis of this case [G. Mozurkewich, J. Acoust. Soc. Am. 103, 380–388 (1998)]. Then it is extended to the case of heat-exchanger pores immediately adjacent to the end of a long stack. The heat-transfer results are cast in nondimensional form as the product of two factors. One factor depends on gas properties and the geometry of the heat exchanger, the other on the ...

HEAT TRANSFER FROM A CYLINDER IN AN ACOUSTIC STANDING WAVE

Heat transfer was measured from wires of various diameters located at a velocity antinode in an acoustic standing wave. A transient method was used, in which the rate of heat transfer was deduced from the rate of change of temperature after heating was turned off. For fixed wire diameter and acoustic frequency, the dimensionless heat‐transfer coefficient (Nusselt number) Nu shows a distinctive variation with acoustic amplitude. At high amplitude, Nu follows the well‐known, steady‐flow, forced‐convection correlation, time averaged over an acoustic cycle, while at low amplitude, Nu has a constant value determined by natural convection. The transition between these regimes, which occurs rather abruptly when the streaming Reynolds number (based on wire diameter and acoustic velocity amplitude) equals 88, is discussed in terms of a heat‐transfer bottleneck that is opened by acoustic streaming. An empirical correlation is presented. Applicability to heat exchangers for thermoacoustic heat engines is considered.

Spatially resolved ultrasonic attenuation in resistance spot welds: implications for nondestructive testing.

Spatial variation of ultrasonic attenuation and velocity has been measured in plane parallel specimens extracted from resistance spot welds. In a strong weld, attenuation is larger in the nugget than in the parent material, and the region of increased attenuation is surrounded by a ring of decreased attenuation. In the center of a stick weld, attenuation is even larger than in a strong weld, and the low-attenuation ring is absent. These spatial variations are interpreted in terms of differences in grain size and martensite formation. Measured frequency dependences indicate the presence of an additional attenuation mechanism besides grain scattering. The observed attenuations do not vary as commonly presumed with weld quality, suggesting that the common practice of using ultrasonic attenuation to indicate weld quality is not a reliable methodology.

Heat transport by acoustic streaming within a cylindrical resonator

Abstract Several experiments on heat transport within a cylindrical resonance tube, mediated by acoustic streaming, are described. The amplitude dependence of the heat transfer coefficient, h, from a hot object located inside the tube depends on the size of the object. For an object short compared to the acoustic displacement amplitude, h is proportional to the square root of amplitude; for a long object, h is linear in amplitude. For an empty resonator with a heated wall segment, the radial heat flux varies with position in a manner consistent with the global streaming pattern within the tube. The magnitude of heat transport from the heated wall segment is increased by inserting an object into the tube because the localized streaming velocity induced by the object is larger than the global streaming velocity in the empty tube. These effects could find application in the cooling of hot objects like electronic components or in thermoacoustic engines.

Vibrating beam accelerometer

A device for measuring acceleration based on the shift in resonant frequency of a vibrating cantilevered beam in response to an acceleration force having a component along its major axis is disclosed. Such accelerometers may be manufactured by micromachining in silicon, and comprise one or more beams, each provided with drive means for keeping the beam in vibration at the resonant frequency, and detection means for sensing changes in the resonant frequency. The sensitivity of accelerometers according to the invention is determined by the length of the cantilevered beam, a factor easily controlled during manufacture, and by the resonant frequency of the beam, which is continuously measured in use. Accelerometers constructed according to the invention are self-calibrating, self-testing and susceptible to large-scale batch manufacture.

Analysis of Young's modulus anomaly at the Peierls transition in TaS3

Abstract The temperature dependence of Youngs modulus in o -TaS 3 in the vicinity of the charge-density-wave phase transition is analyzed. It is assumed that strain enters the free energy through a strain-dependent transition temperature T p . Successful fitting to data requires the inclusion of a quadratic term in the strain dependence, in addition to the usual linear one. This is equivalent to introducing a term in the modulus proportional to the charge-density-wave gap. The ratio between linear and quadratic terms is consistent with a previous measurement of the stress dependence of T p .

Simulated performance and cofluid dependence of a CO2-cofluid refrigeration cycle with wet compression

Abstract Recent experiments demonstrate the viability of a low-pressure CO 2 -cofluid compression refrigeration cycle in which CO 2 and a non-volatile cofluid are circulated in tandem and co-compressed in a compliant scroll compressor. This work explores the theoretical performance limitations of such a cycle operating under environmental conditions representative of automotive air conditioning and studies the dependence of this performance on the properties of the CO 2 -cofluid mixture. The vapor–liquid equilibrium and thermodynamic properties of the mixture are described using a previously reported activity-coefficient model. A coupled system of physically based equations that allows for consideration of both ideal and real hardware components is used to represent the system hardware and its interaction with the environment. The system efficiency is analyzed in terms of entropy generation rates in the various hardware components; entropy generation in the internal heat exchanger—a component required to achieve sufficiently low cooling temperatures—strongly influences overall system efficiency. The vapor pressure of the CO 2 -cofluid mixture and the heat of solution of CO 2 in cofluid have large and somewhat independent contributions to the system performance: lower saturation pressure lowers the optimal operating pressures at fixed CO 2 loading, while increasingly negative heat of solution contributes to higher specific refrigeration capacity and efficiency.

Thermodynamic anomalies at the peierls transition in blue bronze

Abstract Thermodynamic response functions have been measured on pieces of a single crystal of blue bronze at its 180K charge-density-wave (CDW) transition and are analyzed using a free energy appropriate for a 3D-XY model.