Martin Wetzel

Bio: Martin Wetzel is an academic researcher from Johns Hopkins University. The author has contributed to research in topics: Heat transfer & Holographic interferometry. The author has an hindex of 5, co-authored 6 publications receiving 244 citations.

Design optimization of thermoacoustic refrigerators

Abstract: Thermoacoustic refrigeration was developed during the past two decades as anew, environmentally safe refrigeration technology. The operation of thermoacoustic refrigerators employs acoustic power to pump heat. Nowadays, as commercial applications are sought, it is important to be able to obtain fast and simple engineering estimates for the design and optimization of prototypes. This paper provides such estimates by implementing the simplified linear model of thermoacoustics - the short stack boundary layer approximation - into a systematic design and optimization algorithm. The proposed algorithm serves as an easy-to-follow guideline for the design of thermoacoustic refrigerators. Performance calculations applying the algorithm developed in this paper, predict values of 40–50% of Carnot's efficiency for the thermoacoustic core, the heart of a thermoacoustic refrigerator. One reason that these efficiencies have not yet been achieved in devices built to date, is the poor performance of the heat exchangers in thermoacoustic refrigerators. This issue and other remaining challenges for future research are also addressed in the paper. Solving these problems in the near future, we believe, will bring an environmentally safe refrigeration technology a step closer to commercial use.

Experimental study of thermoacoustic effects on a single plate Part I: Temperature fields

Abstract: The thermal interaction between a heated solid plate and the acoustically driven working fluid was investigated by visualizing and quantifying the temperature fields in the neighbourhood of the solid plate A combination of holographic interferometry and high-speed cinematography was applied in the measurements A better knowledge of these temperature fields is essential to develop systematic design methodologies for heat exchangers in oscillatory flows The difference between heat transfer in oscillatory flows with zero mean velocity and steady-state flows is demonstrated in the paper Instead of heat transfer from a heated solid surface to the colder bulk fluid, the visualized temperature fields indicated that heat was transferred from the working fluid into the stack plate at the edge of the plate In the experiments, the thermoacoustic effect was visualized through the temperature measurements A novel evaluation procedure that accounts for the influence of the acoustic pressure variations on the refractive index was applied to accurately reconstruct the high-speed, two-dimensional oscillating temperature distributions

Experimental study of thermoacoustic effects on a single plate. Part II: Heat transfer.

Abstract: Heat fluxes close to the edge of a heated solid plate aligned parallel to the axis of an acoustic standing wave were measured for drive ratios DR≡P A/p m of 1, 2 and 3. It was found that at the highest drive ratio (3), the resulting heat flux vector at the edge of the plate is directed into the plate, opposite to the direction of the heat flux imposed by the resistive heaters within the plate. This observation confirms the thermoacoustic effect previously detected in the visualized temperature fields and discussed in part I of this paper. Through the energy balance the magnitudes of the heat fluxes into the plate, caused by the thermoacoustic effect, were determined. The measured data are in good agreement with numerical and analytical predictions.

Accurate measurement of high-speed, unsteady temperature fields by holographic interferometry in the presence of periodic pressure variations

Abstract: This paper expands the applicability of holographic interferometry to measurements of unsteady temperature distributions for physical situations characterized by periodic pressure variations. For this purpose a new analytical model that accounts for pressure variations in the interpretation of the interferometric fringe pattern was elaborated. This model was then implemented into the newly developed evaluation procedure that is based on the measurement of the interference order as a continuous function of two spatial coordinates and time, by employing digital image processing algorithms. To validate the evaluation procedure, the oscillating temperature fields in a model of a thermoacoustic refrigerator were measured. These measurements proved the validity of the approach by indicating very good agreement with existing theory.

Loudspeaker-driven thermo-acoustic refrigeration

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Performance analysis of the standing wave thermoacoustic refrigerator: A review

Abstract: Concerns over environmental impacts of hazardous refrigerants have spurred much research into alternative technologies as well as more environmentally friendly refrigerants. A thermoacoustic refrigeration system uses no refrigerant but is currently not a feasible solution due to the still immature technology with much still unknown about the theories that explain the thermoacoustc cooling effects and the desired performance. This paper reviews past studies to achieve the desired outputs; lowest temperature, the highest temperature difference generated across the stack, the lowest acoustical work required for cooling, or/and the highest coefficient of performance (COP) of the standing wave thermoacoustic refrigerator and various attempts at optimization in terms of the many parameters that represent the outcomes. The review looked at methods employed to analyze the performance with discussions on the relevant parameters that must and have been be considered by past researchers. To date, most studies have been focused on the stack, the heart of the system. Optimization work has been performed parametrically, experimentally or/and numerically, where discrete variations of the parameters investigated are completed whilst others are held constant. Lately, genetic algorithm, a statistical approach, has been utilized in simultaneous optimization of the parameters of the desired outputs where conflicting objectives are possible. To date, thermoacoustic refrigerator remains an attractive alternative technology towards a global agenda of a more sustainable future.