A noninvasive pulsed microwave phased array for transcranial thermoacoustic ultrasound wave generation in the brain is explored using a phantom. The microwave antenna consists of a 4-element helical ...monopole array transmitting at 2 GHz into a cylindrical head phantom with brain tissue equivalent material. With the microwave array transmitting in pulsed mode, the brain phantom surface is heated, and the thermally induced acoustic wave is measured with an ultrasound medical transducer. In principle, this acoustic wave can be detected and the brain tissue imaged with a noninvasive optical imaging system.
Recent advances in the knowledge of heat transport near the liquid-gas critical point under the influence of the piston effect are reviewed with an emphasis on the different physical mechanisms and ...timescales in regard to thermal and density relaxations. Near the critical point, thermophysical properties exhibit singular behaviors, such as the diverging compressibility and vanishing thermal diffusivity. The resulting fast thermalization leads to the unexpected discovery of the piston effect. We describe the previous theoretical, numerical, and experimental investigations of this unique critical phenomenon and related topics, including its thermoacoustic nature with various nonlinear features on the acoustic timescale. Hydrodynamic and thermovibrational instabilities on the diffusion timescale in near-critical fluids are addressed as well. The review ends with a brief discussion of the merits and limitations of selected research methods in common use.
•The thermoacoustic nature with nonlinear features of the piston effect is emphasized.•The interplay between the piston effect and mechanical instabilities is analyzed.•The merits and limitations of various modeling and experimental techniques are discussed.
The problem of unsteady compressible fluid flow in an enclosure induced by thermoacoustic waves is studied numerically. Full compressible set of Navier–Stokes equations are considered and numerically ...solved by boundary-domain integral equations approach coupled with wavelet compression and domain decomposition to achieve numerical efficiency. The thermal energy equation is written in its most general form including the Rayleigh and reversible expansion rate terms. Both, the classical Fourier heat flux model and wave heat conduction model are investigated.
The velocity–vorticity formulation of the governing Navier–Stokes equations is employed, while the pressure field is evaluated from the corresponding pressure Poisson equation. Material properties are taken to be for the perfect gas, and assumed to be pressure and temperature dependent.
•Transient fluid flows and heat transfer of n-decane at supercritical pressure.•Flow dynamics dictated by thermoacoustic oscillation and transient convection.•Surface heat flux strongly influences ...pressure oscillating magnitude.•Surface heating rate affects both oscillating magnitude and frequency.•Tube length and inlet flow velocity affect total transient responding time.
Turbulent heat transfer of hydrocarbon fuel at supercritical pressure plays a crucial role in regenerative cooling of aerospace propulsion systems. In this paper, flow dynamics in transient heat transfer of n-decane at a supercritical pressure of 5MPahas been numerically investigated, focusing on the effects of a number of key influential parameters, including the surface heat flux, surface heating rate, cooling tube length, and inlet flow velocity, on the transient responding behaviors. Results indicate that the transient responding process is dictated by two fundamental mechanisms: the initial thermoacoustic oscillation, which is caused by strong fluid thermal expansion, and the subsequent transient convection. The thermoacoustic oscillating magnitude increases as the surface heat flux, surface heating rate, and cooling tube length are increased, but it decreases as the inlet flow velocity is increased. The surface heating rate and cooling tube length also exert strong impacts on the oscillating frequency of the thermoacoustic wave. Moreover, the cooling tube length and inlet flow velocity significantly affect the second-stage transient convective process and thus the total transient responding time, which both increase as the cooling tube length is increased and/or the inlet flow velocity is decreased. Results obtained herein are helpful for fundamental understanding of the transient heat transfer mechanisms relevant to regenerative engine cooling processes.
•Inlet boundary condition makes strong impact on thermoacoustic wave frequency.•Oscillation is in first longitudinal mode under a constant stagnation pressure.•Wave frequency proportionally decreases ...as the inlet section length increases.•Wave amplitude reaches around 10% of its steady-state value at the tube outlet.
A numerical study has been conducted to analyze boundary effects, including different inlet boundary conditions and different inlet section lengths (distances from the inlet boundary to the heated section), on flow dynamics in transient supercritical-pressure heat transfer of n-decane in a circular tube. As the inlet boundary condition changes from a constant flow velocity to a constant stagnation pressure, the pressure oscillation frequency is nearly doubled, and the thermoacoustic wave is in the first longitudinal mode. The frequency of mass flow rate oscillation is essentially the same as that of the pressure wave. Different inlet boundary conditions exert no effect on the pressure wave amplitude but make strong influence on the amplitude of mass flow rate oscillation at the outlet of the heated section. As the inlet insulation section length increases under a constant inlet stagnation pressure, frequencies of both pressure and mass flow rate oscillations proportionally decrease, and the thermoacoustic waves remain in the first longitudinal mode. Different inlet section lengths appear to make only minor effect on the amplitudes of both pressure and mass flow rate oscillations at the outlet of the heated section, and the maximum amplitude in each wave is around 10% of its steady-state value.
Acoustic waves can be generated in response to thermal disturbances near the critical point, due to the diverging compressibility. We study the thermomechanical effect in a slab of supercritical ...nitrogen subjected to various forms of boundary heating by numerically solving the governing hydrodynamic equations. The results show that, dependent on the rapidity of the heating, inherently different fluid-dynamical wave behaviors occur on the acoustic timescale with respect to acoustic emission, propagation, and reflection patterns. Specifically, the sudden ramp of the boundary temperature is capable of triggering a strong thermoacoustic pulse in the fluid, whose reflection at the isothermal boundary introduces complex features. In contrast, linear compressive waves dominate under the gradual heating. On a longer timescale, both types of fast processes lapse into slow thermal diffusion coupled by pronounced density inhomogeneities, via different routes nonetheless.
Effects of thermoacoustic wave motion on the developing natural convection process in a compressible gas-filled square enclosure were investigated numerically. In the cases considered, the left wall ...temperature is raised rapidly (impulsively or gradually) while the right wall is held at a specified temperature. The top and the bottom walls of the enclosure considered are thermally insulated. The numerical solutions of the full Navier–Stokes equations were obtained by employing a highly accurate flux-corrected transport algorithm for the convection terms and by a central differencing scheme for the viscous and diffusive terms. The strength of the pressure waves associated with the thermoacoustic effect and resulting flow patterns are found to be strongly correlated to the rapidity of the wall heating process. Fluid thermal diffusivity was found to affect the strength of the thermoacoustic waves and the resulting interaction with the buoyancy-induced flow.
A modified SIMPLEC method which can solve compressible flows at low Mach number is introduced and used to study thermoacoustic waves induced by a rapid change of temperature at a solid wall and ...alternating- direction flows generated by thermoacoustic effects in a ta- pered resonator. The results indicate that the algorithm adopted in this paper can be used for calculating com- pressible flows and thermoacoustic waves. It is found that the pressure and velocity in the resonator behave as stand- ing waves, and the tapered resonator can suppress high- frequency harmonic waves as observed in a cylindrical res- onator.
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