Numerical Modeling and Performance Evaluation of Standing Wave Thermoacoustic Refrigerators with a Multi-Layered Stack

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.

Analysis of AE characteristics of tool wear in drilling CFRP/Ti stacked material

Based on the signal data acquired in drilling process of carbon fiber reinforced polymer/titanium alloy (CFRP/Ti) stacked materials, the acoustic emission (AE) characteristic values were carefully studied, by using the method of statistical analysis, spectrum analysis and wavelet packet. The results show that the root mean square(RMS) value of the AE signals and the energy of the wavelet packet are closely related to the tool wear. Meanwhile, experiments indicate that different materials, chips and tool tipping will cause instantaneous signal mutation, which has different forms in time domain and in time-frequency domain. These mutations may increase the difficulty of identifying the tool wear. Fortunately, with repeated experiments and comparison, some identifiable mutations were recognized. When a tool is processed from CFRP to Ti, the signal intensity decreases generally, the high-frequency component of signal increases gradually, and the signal has a tendency to show in high frequencies.

Multilayer test method for water vapor transmission testing of construction materials

Water vapor transmission (WVT) measurements conducted with highly permeable materials are complicated by the instability of boundary conditions, more specifically by changes in vapor pressure and air velocity on either or both sides of the specimen. Such effects pose a greater challenge in determining the water vapor transport characteristics of thin, flexible, and highly permeable construction materials. This article presents a novel approach to WVT testing that improves some of these issues. The approach denoted as ‘multilayer test’ involves testing simultaneously several vertically stacked material layers each separated by air gap with equal thickness. Wireless relative humidity and temperature sensors mounted on the opposing surfaces of each layer provide continuous temperatures and relative humidity monitoring near the specimen surfaces. The test is conducted in a controlled environment with the set-up kept on an analytical balance. Change in the weight of the system is determined automatically at predetermined time intervals, and fluxes are calculated. Two approaches are used in determining the permeance of each material layer. The results of both approaches are compared. The multilayer WVT tests provide several benefits over traditional ‘Dry’ or ‘Wet’ cup WVT test method in that; the boundary layer effects due to moving air above the specimen surface can be accounted for, and simultaneous testing of multiple specimens provides greater statistical confidence. The method is particularly advantageous in cases when the transport coefficient has a strong dependence on moisture content. Using data from a single multilayer test, a continuous transport function can be derived. This article highlights that the multilayer test approach leads to significant reduction in experimental effort, resources required for testing, and test durations, and improves the precision of the material property data.

Grain Boundary Dislocations and Stacking Defects in the 9R Phase at an Incoherent Twin Boundary in Copper

Experiment and modeling show that there is a general mode of grain boundary dissociation, common in low stacking fault energy FCC metals, that can be well understood in terms of the emission of arrays of stacking faults from the grain boundary plane. Most extensively studied of such dissociated interfaces are the Σ=3 incoherent twin boundaries. Numerous observations now exist of grain boundary dissociation at such interfaces showing a layer that is well described as a narrow, several nanometer wide slab of 9R stacked material. The 9R stacking sequence is equivalent to a close-packed stacking of FCC ﹛111﹜ planes with an intrinsic stacking fault inserted every three planes (i.e., a stacking sequence of ABC/BCA/CBA …). The width of the 9R layer is sensitive to the local stress state. Figure 1 shows results of an atomistic calculation simulating the effect of an applied shear strain parallel to the boundary.