Ocean thermal energy conversion (OTEC) has attracted attention as a technique to obtain renewable energy. OTEC systems usually use a plate heat exchanger and ammonia as a working fluid. The materials used for manufacturing the heat exchanger are generally “titanium” or “stainless steel” of which contact surface should not be corroded with ammonia. However, the thermal conductivity of these materials is very low. Consequently, the heat transfer performance of the heat exchanger deteriorates. Therefore, the author proposed an advanced plate heat exchanger material with high thermal conductivity for OTEC systems. This plate is made of an aluminum alloy and the surface is coated with a special high polymer material (PEEKTM polymer), which has high ammonia resistance. In this study, two tolerance experiments were performed using the advanced plate at different coating thicknesses of 300 and 20 μm for approximately one month. These experiments were (1) immersion of the plate in a pressure tank containing liquid ammonia and (2) exposure of the plate in the path of a forced convective flow of liquid ammonia. As a result, although the PEEK coating aluminum plate surface deteriorated slightly, it is found that the base of the plate has not influenced by the ammonia.
Ocean Thermal Energy Conversion (OTEC) usually uses plate type heat exchangers. For the improvements of OTEC efficiency, it is important to improve the heat exchanger efficiency. There are some methods for improving of plate type heat exchanger. As a typical method, there are the method of arranging grooves to a channel, and the method of giving the forced oscillation to a flow. By the former method, since a self-sustained oscillatory flow is generated in high Reynolds number, heat transfer enhancement is expectable. By the later method, since the resonance phenomenon appears between self and forced oscillations in the high Reynolds number flow, heat transfer is improved. However, few investigations were carried out for heat transfer enhancement on the low Reynolds number flow. Therefore, we carried out numerical calculations in grooved channel flow with forced oscillation at the low Reynolds number, and evaluated the influence of heat transfer enhancement and pressure drop. In addition, we carried out visualization experiments in the same model, and compared with the numerical solutions.
We performed transient film boiling experiments of water under the atmospheric pressure on vertical cylinders with various bottom shapes. We photographed the aspect of the vapor film near the film boiling lower limit point using a high-speed video camera to discuss the film collapse phenomenon. We detected the key points and the image feature and analyzed the number, place and time of the detected image feature points using SIFT method. We classified the trends in the image feature points into several categories according to the pattern of the vapor film collapse.
In this study, the effect of channel gap size for boiling heat transfer of the plate-type evaporator for small thermal energy conversion such as OTEC system was investigated. The heat transfer coefficient of ammonia was measured for three different gap sizes of 1, 2 and 5mm. The result shows that the heat transfer coefficient is increasing with a decreasing in gap size. On the other hand, the present data was compared with conventional correlation by Liu et al. The result of that the correlation indicate underprediction heat transfer coefficient. Therefore, Mortada's correlation which is made for mini-channels was adopted. The correlation for the case of δ=1mm is closer to present data than the former one.
Inverse solutions for one- and two-dimensional heat conduction has been explicitly derived using Laplace transform to estimate surface temperature and heat flux. These inverse solutions can be widely applied to transient heat transfer problems, for which it is usually difficult to calculate the surface condition form a measurement of temperatures in a solid. A few examples of the applications of inverse solutions are presented.
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