Gaëlle Poignand

Capture of instantaneous temperature in oscillating flows: Use of constant-voltage anemometry to correct the thermal lag of cold wires operated by constant-current anemometry

A new procedure for the instantaneous correction of the thermal inertia of cold wires operated by a constant-current anemometer is proposed for oscillating flows. The thermal inertia of cold wires depends both on the wire properties and on the instantaneous incident flow velocity. Its correction is challenging in oscillating flows because no relationship between flow velocity and heat transfer around the wire is available near flow reversal. The present correction procedure requires neither calibration data for velocity nor thermophysical or geometrical properties of the wires. The method relies on the splitting of the time lag of cold wires into two factors, which are obtained using a constant-voltage anemometer in the heated mode. The first factor, which is intrinsic to the wire, is deduced from time-constant measurements performed in a low-turbulence flow. The second factor, which depends on the instantaneous flow velocity, is acquired in situ. In oscillating flows, data acquisition can be synchronized with a reference signal so that the same wire is alternatively operated in the cold mode by a constant-current anemometer and in the heated mode by a constant-voltage anemometer. Validation experiments are conducted in an acoustic standing-wave resonator, for which the fluctuating temperature field along the resonator axis is known independently from acoustic pressure measurements, so that comparisons can be made with cold-wire measurements. It is shown that despite the fact that the wire experiences flow reversal, the new procedure recovers accurately the instantaneous temperature of the flow.

Development of Laser Techniques for Acoustic Boundary Layer Measurements. Part II: Comparison of LDV and PIV Measurements to Analytical Calculation

The adaptation of Laser Doppler Velocimetry (LDV) and Particle Image Velocimetry (PIV) for acoustic boundary layer measurements is considered. The specificities of acoustic boundary layer are presented and the theoretical expression of acoustic particle velocity is reminded. Appropriate parameters of the PIV system for sound measurements are determined. Results of LDV and PIV measurements of particle velocity profiles in acoustic boundary layers are compared with theoretical predictions based on the literature for different phases along the acoustic period. These results are very satisfactory and show that these two techniques are suitable for acoustic boundary layer measurements.

Active control of thermoacoustic amplification in a thermo-acousto-electric engine

In this paper, a new approach is proposed to control the operation of a thermoacoustic Stirling electricity generator. This control basically consists in adding an additional acoustic source to the device, connected through a feedback loop to a reference microphone, a phase-shifter, and an audio amplifier. Experiments are performed to characterize the impact of the feedback loop (and especially that of the controlled phase-shift) on the overall efficiency of the thermal to electric energy conversion performed by the engine. It is demonstrated that this external forcing of thermoacoustic self-sustained oscillations strongly impacts the performance of the engine, and that it is possible under some circumstances to improve the efficiency of the thermo-electric transduction, compared to the one reached without active control. Applicability and further directions of investigation are also discussed.

Investigation of the acoustic field in a standing wave thermoacoustic refrigerator using time-resolved particule image velocimetry

In thermoacoustic devices, the full understanding of the heat transfer between the stack and the heat exchangers is a key issue to improve the global efficiency of these devices. The goal of this paper is to investigate the vortex structures, which appear at the stack plates extremities and may impact the heat transfer. Here, the aerodynamic field between a stack and a heat exchanger is characterised with a time-resolved particle image velocimetry (TR- PIV) set-up. Measurements are performed in a standing wave thermoacoustic refrigerator operating at a frequency of 200 Hz. The employed TR-PIV set-up offers the possibility to acquire 3000 instantaneous velocity fields at a frequency of 3125 Hz (15 instantaneous velocity fields per acoustic period). Measurements show that vortex shedding can occur at high pressure level, when a nonlinear acoustic regime preveals, leading to an additional heating generated by viscous dissipation in the gap between the stack and the heat exchangers and a loss of efficiency.

Thermoacoustic, Small Cavity Excitation to Achieve Optimal Performance

The compactness of thermoacoustic devices is a topic of continuing importance in fundamental thermoacoustics and in its practical applications. The design for a small scale thermoacoustic refrigerator is presented in this paper. Two loudspeakers, set at the opposite ends of a small cavity and fully enclosed within the device, create an appropriate acoustic field inside a stack filling this cavity. A push-pull concept with two cavities placed on either side of one of the loudspeakers could be considered. An analytical approach describing the pressure and velocity fields which can be obtained from these sources is provided, assuming that the wavelength is much greater than the dimensions of the cavity. This new thermoacoustic device simultaneously provides compactness and flexibility compared to the classical standing wave or traveling wave devices which would achieve equivalent performances.

Aerodynamic and Thermal Measurements in a Standing Wave Thermoacoustic Refrigerator

Thermoacoustic refrigerators produce a cooling power from an acoustic energy. Over the last decades, these devices have been extensively studied since they are environment-friendly, robust and miniaturizable. Despite all these advantages, their commercialization is limited by their low efficiency. One reason for this limitation comes from the complex thermo-fluid process between the stack and the two heat exchangers which is still not sufficiently understood to allow for optimization. In particular, at high acoustic pressure level, vortex shedding can occur behind the stack as highlight by [Berson & al., Heat Mass Trans, 44, 10151023 (2008)]. The created vortex can affect heat transfer between the stack and the heat exchangers and thus, they can reduce the system performance. In this work, aerodynamic and thermal measurements are both conducted in a standing wave thermoacoustic refrigerator allowing investigation of vortex influence on the system performance. The proposed device consists on a resonator operated at frequency of 200 Hz, with hot and cold heat exchangers placed at the stack extremities. The working fluid is air at ambient temperature and atmospheric pressure. The aerodynamic field behind the stack is described using high-speed Particle Image Velocimetry. This technique allows the acoustic velocity field measurement at a frequency up to 3000 Hz. Thermal measurements consist on the acquisition of both the temperature evolution along the stack and the heat fluxes extracted at the cold heat exchanger. These measurements are performed by specific micro-sensors developed by MEMS technology. The combination of these two measurements should be helpful for the further understanding of the heat transfer between the stack and the heat exchangers.Copyright

Prediction of limit cycle amplitudes in thermoacoustic engines by means of impedance measurements

This paper deals with the prediction of the frequency and the amplitude of self-sustained oscillations generated in thermoacoustic prime movers, which are compared to measurements. A specially designed, high amplitude, acoustic impedance sensor was developed to perform measurements of the input impedance of a thermoacoustic core, as a function of the heating power supplied to the device, of the frequency, and of the amplitude of acoustic forcing. Those measurements can then be used to predict the spontaneous generation of acoustic oscillations and their saturation up to a steady-state. Those predictions were successful for various acoustic loads connected to the thermoacoustic core. Moreover, the measurements of acoustic impedance as a function of the amplitude of acoustic oscillations are compared to a model based on the linear thermoacoustic theory, and this comparison provides insights into the processes controlling the saturation of acoustic oscillations. The experimental procedure described in this p...

To predict a thermoacoustic engine's limit cycle from its impedance measurement

Thermoacoustic engines are self-oscillating systems converting thermal energy into acoustic waves. Recent studies on such engines highlight much nonlinear effects responsible for the engine’s saturation, leading to a limit cycle, which can be stable or not. Those effects however, are not sufficiently known, even with today knowledges, to accurately predict a limit cycle oscillations amplitude. This work suggests a new approach, based on acoustic impedance measurement at large forcing amplitudes, to predict the limit amplitude in steady state for a given engine. This method allows one to predict an engine’s saturation amplitude without studying in detail its intern geometry. In the case of a quarter wave length engine, its input impedance can easily be obtained from an impedance sensor for example. Increasing the speaker’s forcing leads to a nonlinear impedance depending on the acoustic field amplitude. Once measured, this function contains information such as the limit cycle amplitude, its stability and t...

Simplified modeling of a thermo-acousto-electric engine forced by an external sound source

Thermoacoustic wave-generators are usually designed and optimized using software tools based on the linear thermoacoustic theory. These tools enable to predict the operating point of a thermoacoustic engine from the balance between the thermoacoustic amplification process and the numerous nonlinear effects saturating wave amplitude growth, the latter effects being very difficult to describe properly. A non exhaustive list of these effects includes (apart from the thermoacoustic heat pumping accompanying wave amplification) different kinds of streaming, nonlinear acoustic propagation or aerodynamical and thermal entrance effects. These nonlinear effects are responsible for both dissipation of acoustic power and perturbations of the temperature field in the thermoacoustic core, that work together to limit the overall performances of thermal-to-acoustic conversion. Therefore, the development of adequate simulation tools is still needed to describe the high level of complexity of the processes involved above the onset of thermoacoustic instability. Direct numerical simulation seems to be the only way to reproduce quantitatively the effects mentioned above, but it is still limited by large computation times inherent to the complicated physics and multiple time and space scales involved in the description of thermoacoustic engines. Simplified analytical models can help in getting a deeper physical insight about the processes involved, but they are also based on substantial approximations. In this study, we use a simplified modeling of thermoacous-tic engines to help understanding recent experimental observations dealing with the external forcing of thermoacoustic oscillations.

Theoretical study of a thermo-acousto-electric generator equipped with an electroacoustic feedback loop

A simplified model of a Stirling-type thermoacoustic engine coupled to a resonant mechanical system is presented. The acoustic network is presented as its temperature-dependent lumped element equivalent, and the nonlinear effects involved in such engines are accounted for in a nonlinear heat equation governing the temperature distribution through the thermoacoustic core. The low-order model is sufficient to capture the behavior of the engine, both in terms of stability and dynamic behavior.