a b s t r a c tThe effect of tool geometry on the plastic flow and material mixing during friction stir spot welding (FSSW) is investigated using the particle method approach. For spot welds made with a cylindrical pin tool with flat shoulder, the model predicts the material flow at the pin periphery to be in the upward direction and the material is pushed downward beneath the shoulder giving rise to the resultant hook geometry. Other pin geometries evaluated include tapered pin, inverse tapered pin, triangular pin, convex shoulder, and concave shoulder. With good correlation with experimental trials, this model is then used to predict the material flow for spot welds. The material flow, and thereby the resultant hook formation, is quantified using numerical methods and is expressed as standard deviation of the particle movement. A triangular pin with a concave shoulder is the preferred tool geometry from the current study that results in high strength spot welds.

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The electrocaloric effect in thin films of an electrocaloric material has the potential to be used for efficient cooling systems for high power electronic devices. We numerically calculated the effect of parameters in electrocaloric refrigeration with a thermal switch of fluid motion on the thermal performance. The system of changing air and water flow with the pulse generation of cold energy increased the heat transfer efficiency to 67% at a frequency of 5 Hz. The optimum time delay of water flow to increase the heat transfer efficiency was zero at low frequency and became half of the time period to change heat for a high frequency of 100 Hz. When the heat transfer efficiency was high, the final temperature change in water flow was not the maximum temperature change.

The purpose of this study is to understand the optimum operating condition of magnetic refrigerator at room temperature for direct air-cooling. The basic components of the target system are a magnetic circuit including two permanent magnets, a test section, an air blower, and an associated instrumentation. The test section consists of ten test cells which enclose gadolinium chips as a magnetic working substance in a prescribed packing rate. In order to change the applied magnetic field from 0 to 0.9T, the magnetic circuit is installed on an electric slider which generates reciprocating motion. The system performances are widely investigated both experimentally and analytically for the variety of conditions such as a volumetric flow rate of air, a packing length of magnetic working substance, and a heat exchange cycle. The results reveal that the present magnetic refrigerator has a maximum value of the cooling rate in an appropriate operating condition. Keywords IntroductionMagnetic refrigerators are capable of maintaining high efficiency at a range of extremely low temperatures at which operation is difficult for a conventional gas compression refrigerator. Such refrigeration devices are used to generate extremely low temperatures in the milli-Kelvin range. Accordingly because realization of a compact, low noise and low vibration refrigerator system is possible, it is expected that a magnetic refrigerator can be developed for a compact residential refrigeration system that can be operated in the room-temperature range.However, at room temperature, the heat capacity of the magnetic working substances -the refrigerants -becomes markedly larger than at the extremely low temperatures at which they are typically used. The temperature distribution of the magnetic working substance obtained by changing the magnetic field becomes smaller. Therefore, efficient heat transfer control is important for constructing a useful magnetic refrigeration system. Improvement of the refrigeration cycle and system miniaturization have been studied [1][2][3][4] to solve this problem and to put room-temperature magnetic refrigerators to practical use, but no technology has been established. Urgent studies are necessary to obtain a practical system. The performance of the magnetic refrigeration system depends on the magnetocaloric effect of the magnetic working substance. Therefore, research and development of magnetic substances [5,6] that can provide markedly higher magnetocaloric effects have been the subjects of great effort. Nevertheless, no examples exist of research and development from the perspective of promoting heat transfer of magnetic substances.Accordingly, the primary objective of the present study is to elucidate heat transfer characteristics between magnetic working substances used for magnetic refrigeration systems and heat transfer media under a variety of conditions. In this study, a magnetic room-temperature refrigeration system is constructed, and an experimental study is executed to explain the influences o...