Thermoacoustic refrigerators have huge potential to replace conventional refrigeration systems as an alternative clean refrigeration technology. These devices utilize conversion of acoustic power and heat energy to generate the desired cooling. The stack plays a pivotal role in the performance of Standing Wave Thermoacoustic Refrigerators (SWTARs), as the heat transfer takes place across it. Performance of stacks can be significantly improved by making an arrangement of different materials inside the stack, resulting in anisotropic thermal properties along the length. In the present numerical study, the effect of multi-layered stack on the refrigeration performance of a SWTAR has been evaluated in terms of temperature drop across the stack, acoustic power consumed and device Coefficient of Performance (COP). Two different aspects of multi-layered stack, namely, different material combinations and different lengths of stacked layers, have been investigated. The combinations of four stack materials and length ratios have been investigated. The numerical results showed that multi-layered stacks produce lower refrigeration temperatures, consume less energy and have higher COP value than their homogeneous counterparts. Among all the material combinations of multi-layered stack investigated, stacks composed of a material layer with low thermal conductivity at the ends, i.e., RVC, produced the best performance with an increase of 26.14% in temperature drop value, reduction in the acoustic power consumption by 4.55% and COP enhancement of 5.12%. The results also showed that, for a constant overall length, an increase in length of side stacked material layer results in an increase in values of both temperature drop and COP.
In this work, an FEA based methodology is proposed for the accurate prediction of stiffness properties of top plate of special application 6-DOF platform. The effect of joint contact conditions on the static stiffness and natural frequency of the top plate is analysed and compared with experimental results. Top plate is first designed using a simplified finite element model where the joint contact is considered to be rigid. The design failed to meet the actual operating conditions when tested experimentally. The design is revised using actual loading conditions and introducing realistic joint contact formulation. The results obtained are in close agreement with experimental data. Based on the developed methodology, a Multi-Objective Genetic Algorithm (MOGA) based optimization study has been carried out to reach an optimum design meeting the desired performance with a minimum possible weight. Both size and shape of the top plate has been optimized and weight reduction of 5.08 % has been achieved using the shape optimization. The performance parameters of final design of top plate differ by only 1.43 % for static stiffness and 0.44 % for modal response when compared with the experimental results.
Sensitivity analysis using normalized sensitivity coefficient (NSC) can be used to identify important model parameters affecting the device performance by allowing one to one comparison. The results are highlighted in the form of order of magnitudes change in output for a unit change in the input variable. In this work, sensitivity analysis of a small capacity standing wave thermoacoustic refrigerator (SWTAR) has been performed using NSC. Specialized tool DeltaEC has been used to generate the results for subsequent sensitivity analysis. Three key performance parameters i.e. temperature difference achieved (ΔT), coefficient of performance (COP) and relative coefficient of performance (COPR) have been analyzed for perturbations in number of device variables, namely oscillating pressure amplitude, two different stack material properties and four stack geometric parameters i.e. stack length, stack center position, stack plate thickness and half stack spacing. Sensitivity results are obtained for a wide range of mean operating pressures (Pm), mean operating temperature (Tm) and drive ratios (DR). It has been found that performance parameters are most sensitive to the perturbations in oscillating pressure amplitude and least affected by the perturbations in stack length. With respect to oscillating pressure amplitude, maximum NSC of 24.12 has been reported for ΔT at mean pressure of 5 bar.
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