Arganthaël Berson

Nonperiodicity of the flow within the gap of a thermoacoustic couple at high amplitudes

The flow inside a thermoacoustic couple is investigated experimentally using particle image velocimetry. Measurements show the oscillation of the shear layers flowing out of a single stack, thus forming an asymmetric vortex street at high driving amplitudes. Development of vortices is also observed within the gap of a thermoacoustic couple. It causes the flow not to repeat from one acoustic period to another. The nonperiodicity of the flow will lead to unsteady heat transfer between the stack and heat exchangers and to the oscillation of the cooling load.

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.

A strategy to eliminate all nonlinear effects in constant-voltage hot-wire anemometry

A constant-voltage anemometer is subject to nonlinear effects when the operating hot wire is exposed to large velocity fluctuations in the incident flow. This results in the generation of undesirable higher harmonics, just as in the two classic systems, constant-current and constant-temperature anemometers, for which no attempts are normally made to correct the nonlinearities. The present investigation shows that these undesirable higher harmonics can be suppressed in the case of a constant-voltage anemometer. A new approach to process experimental data is proposed. It is based on three explicit equations established and solved with all terms included, i.e., without linearization. These are (1) the first-order differential equation that describes the electronic circuit of a constant-voltage anemometer-this equation permits to deduce the instantaneous resistance of the hot wire from the output voltage of the anemometer; (2) the first-order differential equation that expresses the thermal lag behavior of the hot wire when used in a constant-voltage mode-this equation permits to restore the instantaneous resistance that an ideal wire would have without thermal inertia in the same flow conditions; and (3) the algebraic relation that expresses the heat-transfer law of an ideal wire, according to Kings law, a look-up table, or a polynomial fit-this relation permits to deduce the instantaneous flow velocity from the instantaneous resistance of the ideal wire. The proposed method is easily implemented on a personal computer and permits odd turbulence moments, such as skewness factors, to be obtained satisfactorily.

On the use of hot-wire anemometry in pulsating flows. A comment on ‘A critical review on advanced velocity measurement techniques in pulsating flows’

In their recent topical review, Nabavi and Siddiqui (Meas. Sci. Technol. 2010 21 042002) recommended the use of hot-wire anemometry for velocity measurements in pulsating flows, especially at high frequency. This recommendation is misleading. The procedures invoked by these authors are valid only for small-amplitude fluctuations, which are of little interest for pulsating flows. When large-amplitude velocity changes occur without flow reversal, new procedures for the correction of the nonlinearities of the hot wire are required. This case was thoroughly investigated for a constant-voltage anemometer by Berson et al (Rev. Sci. Instrum. 2009 80 045102). In addition, we show that when flow reversal occurs—a case most relevant to pulsating flows—accurate hot-wire velocity measurements are not possible because heat transfer is not well defined when velocity passes through zero and changes direction.

Hot wire anemometry for the measurement of temperature and velocity fluctuations inside a thermoacoustic cooler

Hot wire anemometry was used to measure velocity and temperature fluctuations inside a thermoacoustic refrigerator. In an oscillating flow with zero‐mean velocity, such as in an acoustic standing wave, heat transfer is different from heat transfer in a steady flow. A dynamic calibration of the system is required. This calibration was done inside the resonator of the cooler, with no stack and no heat exchangers. Two thermoacoustic systems were studied to calibrate the anemometer in air at atmospheric pressure and in He/Ar mixtures at a mean pressure of 6 bars. Measurements of temperature and velocity were done at the end of the stack and heat exchangers. Correlations between temperature and velocity fluctuations were obtained, and the enthalpy convective flux was computed. These results were compared with results obtained from numerical simulations recently done at Ecole Centrale de Lyon [D. Marx, AIAA J. 42(7) (2004)].

Advanced flow measurements in thermoacoustic systems

One of the bottlenecks for the conception of highly efficient thermoacoustic systems is the poor performances of heat exchangers. A better understanding of both aerodynamical and thermal phenomena that occur at the interface between the stack and the heat exchangers is necessary for the improvement of heat transport between these components, especially at high acoustic amplitudes that are required for industrial applications. To this end, a specific Particle Image Velocimetry (PIV) method has been developed and measurements are performed within a standing‐wave thermoacoustic refrigerator model driven at high drive ratios (up to 5%). Vortex streets are observed behind the plates of a single stack at high acoustic level. The flow is characterized using advanced vortex analysis tools and dimensionless numbers. Vortices also appear within the gap between the stack and the heat exchangers. They will influence heat transport as was previously shown in numerical simulations from the literature. Moreover, as a fi...

High‐resolution particle image velocimetry (PIV) measurements of the flow inside the stack‐heat exchanger couple of a thermoacoustic refrigerator driven at high amplitude

The design of efficient heat‐exchangers for thermoacoustic applications is one of the major challenges in the development of high‐performance thermoacoustic coolers. With this prospect, an important step has been taken in this study with the characterization of the flow field around both stack and heat‐exchangers, especially at high drive ratio when acoustics is nonlinear and edge effects become more important. High‐resolution particle image velocimetry measurements are performed in a standing wave thermoacoustic refrigerator using a technique that has already been validated and successfully compared to numerical simulations Blanc‐Benon et al., C.R. Mecanique 331, 17–24 (2003). High spatial resolution is achieved with an optical zoom mounted on a digital camera. Measurement areas down to 3×2.5 mm2 are obtained. The resonator is driven at drive ratios up to 5%, with oscillating velocity amplitudes up to 15 m s−1 within the stack. As oscillating velocity amplitude increases, the vorticity layer developin...