Parameter estimation for the characterization of thermoacoustic stacks and regenerators

Abstract

Highlights• A model for the propagation of acoustic waves in a thermoacoustic core is proposed • Geometrical and thermal parameters are adjusted by using an inverse method • Parameters are estimated for a ceramic catalyst, a stack of grids and a carbon foam• The values for geometrical parameters are close to manufacturers' data • The heat diffusion length is higher for the stainless-steel grids stack Abstract This paper deals with the in-situ characterization of open-cell porous materials that might be used as a so-called stack (or regenerator) in a thermoacoustic engine. More precisely, the manuscript presents an inverse method aiming at estimating geometrical and thermal properties of various samples of porous media surrounded by heat exchangers and connected to a thermal buffer tube to form a ThermoAcoustic Core (TAC). This estimation is realized from acoustic measurements, and it is expressed as a minimization problem applied to the squared norm of the difference between experimental and theoretical transfer matrices of the TAC. Experimental data, obtained for different stacks (ceramic catalyst, pile of stainless steel wire meshes, carbon and metallic foams) under various heating conditions, are used in order to fit the theoretical forward model by adjusting geometrical properties of the sample and heat exchange coefficients. Common geometrical properties (porosity and average pore's radius) obtained with the present method are consistent with available data from manufacturers. Moreover, this method allows to estimate the tortuosity of the material which is not given by manufacturers. Estimation of heat coefficients (and their variations with heating) provides global in-formation about anisotropic heat diffusion through the porous material employed as a thermoacoustic stack submitted to a temperature gradient. Among the four characterized samples, it appears that the carbon foam allows to get the highest temperature gradients

Coupling of an acoustic emissions system to a laboratory torrefaction reactor

“…Thermoacoustic is responsible for some phenomena such as combustion instabilities (self-excited oscillation due to coupling between unsteady heat release and acoustic waves) [3,4], Rijke tubes [5] and thermoacoustic heat engines [6,7].…”

Performance optimization of the regenerator of a looped thermoacoustic engine powered by low-grade heat

“…4 Yu et al conducted an experimental study on a small-sized TWTAE to explore the optimal internal dimensions of the regenerator made up of plain-wave metal screens. 7 Wu et al studied how the acoustic field, hot end temperature, and material affect the performance of different regenerators. Yu and Jaworski studied the performance of the parallel-plate and stacked-screen regenerators in a TWTAE.…”

“…6 By investigating the geometrical and thermal characteristics of regenerators with different materials, Guedra et al claimed that the carbon foam was the most efficient material for the regenerator in a TWTAE. 7 Wu et al studied how the acoustic field, hot end temperature, and material affect the performance of different regenerators. They reported a maximum output acoustic power of 715 W and a highest thermoacoustic efficiency of 35.6% with the stack-type and random-fiber-type regenerators.…”

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

“…Among the key points which still require further research are the accurate description of sound propagation and heat transport through stack/regenerators having complex geometries (e.g. metallic foams or stacked meshgrids 9,10 ), the accurate description of both entropic and hydrodynamic entrance eects involved at the ends of the stack and of the heat exchangers 1113 , or the proper account of nonlinear eects like acoustic streaming 14,15 or higher harmonics generation 16 . It is indeed worth pointing out that although various design tools 17,18 based on Rott's linear theory of thermoacoustics 19 are available, the accurate prediction of a limit cycle amplitude (or even its stability) remains, sometimes 20 , an important issue.…”

“…Guedra et al 10,21 as well as Bannwart et al 22 have shown that the measurements of the acoustic transfer matrices of a thermoacoustic core (within a given range of heat supply and frequency) can be used to predict the onset of the thermoacoustic instability for various en-gines equipped with the thermoacoustic core characterized beforehand. A similar approach has been used by Hatori et al 23 who have shown that acoustic impedance measurements can be used to dene the optimum load that should be connected to the thermoacoustic core, so that a minimum temperature gradient is required to trigger self-sustained oscillations.…”