Cooling is a primal issue for future disk array systems using 2.5-inch hard disks because of high packing density of the disks. Accordingly, measurement of thermal characteristics of the hard disks arrayed in rows along a flow direction is conducted. Temperature distribution in the main body of the hard disks is almost uniform. And, the circuit board temperature of the hard disks is higher than the main body temperature by nearly 20 °C to 30 °C. The temperature rise of the hard disks relates with air velocity to the power of -0.8, thus the flow near the hard disks is considered as fully turbulent. And, the temperature rise is almost proportional to the heat generation in the hard disks. The temperature rise is less than 15 °C at air velocity of over 2.3 m⁄s.
Because of high packing density of hard disks, cooling of future disk array systems using 2.5-inch hard disks is extremely difficult than conventional systems using 3.5-inch hard disks. Accordingly, cooling study of 2.5-inch hard disk array systems is conducted using a mockup model. The mockup has tandem fans for redundant cooling. Heat transfer experiment is performed for the standard type where hard disks are arrayed in rows without any modification of flow channel, and for the frame type where plate with large holes is set between the rows of disks. Heat generation of dummy hard disks is set at 7 W that corresponds heat dissipation in an actual 2.5-inch hard disk for high reliance disk array systems. Maximum temperature increase of dummy hard disks at normal fan operation is less than 15 degree at fan voltage of 9 V(3⁄4 of standard voltage) that meets the cooling specification of hard disks. The maximum temperature increase for the frame type is lower than the standard type by 2 to 5 degree. The cooling specification is also satisfied even when operation of one fan is stopped. The maximum cooling capacity is about 9 W for the mockup model at fan voltage of 9 V, and sound pressure level is less than 60 dBA at the fan voltage, those meet all cooling specification.
Sub-cooled flow boiling heat transfer experiments were performed for narrow-flat flow passages of 2 mm wide and 0.2 mm high. A heat transfer surface of 2 mm × 2 mm was placed at the just downstream of the flow channel outlet. A fast wall plane-jet was formed on the heat transfer surface and space for vapor generated on the heat transfer surface to leave freely form the plane jet was provided The experiments covered the flow rate from 5 m⁄s through 20 m⁄s and the inlet sub-cooling from 30 K through 70 K. Critical heat fluxes were greatly augmented about twice compared with those in the previous experiments where the heat transfer surface was located at the outlet end of the same flow channel as that in the present experiments. This has indicated that the present idea of the flow system is effective to enhance the critical heat flux. When the flow velocity was slower than 10 m⁄s, a large secondary bubble that was formed as a result of coalescence of many primary bubbles on the heat transfer surface covered the heat transfer surface. The large-coalesced bubble triggered the occurrence of the critical heat flux. When the flow velocity became faster than 10 m⁄s, the heat transfer surface was covered with many tiny-primary bubbles even at the critical heat flux condition. The critical heat fluxes in the present experiments were much larger than predictions of correlations. The triggering mechanism of the critical heat flux condition was proposed based on the observation mentioned above. It has two parts; for low flow velocity and for high flow velocity. The boundary is 10 m⁄s. In both cases, disappearance of a liquid film under the bubble due to evaporation is related to the appearance of the critical heat flux condition. The predicted critical heat fluxes were larger than that measured, however, qualitatively agreed well.
In polymer electrolyte fuel cell (PEFC), water management is one of the most important issues. In the case that water concentration near the membrane is low, the protonic conductivity of membrane decrease. Whereas, if humidity reaches saturation level, fuel and oxidant gases supply is disturbed by liquid water. Optimization of the water concentration on the surface of the membrane is one of key points for further improvement of PEFCs. However many experiments are conducted with gas diffusion layer (GDL) which has random structure like carbon paper. Due to their complexity, it is difficult to reveal the inside phenomena. Also due to the random structure, it is impossible to do a detailed control of the GDL. In this study, we proposed a new method to control water by using surface energy gradient. Firstly, through theoretical study, we prove the principle of force caused by surface energy differences with triangle shape. And then, we demonstrate the effect in cell performance and the movement of water with Ti gas diffusion layer (Ti-GDL). We compare cell performances with two types of GDLs: one is GDL with circular type of hole, in which the water concentration cannot be managed, another is GDL with triangular type of hole, in which the water concentration can be managed by surface energy gradient. As a result, the cell with GDL with triangular type of hole showed a good performance compared with GDL with circular type of hole. Further we observed Ti-GDL with triangular type of hole keeps wettability with maintaining the gas supply path.
Pool nucleate boiling heat transfer experiments were performed for water using heat transfer surfaces having unified cavities. Cylindrical holes of 10 μm in diameter and 40 μm in depth were formed on a mirror-finished silicon wafer of 0.2 mm in thickness using Micro-Electro Mechanical Systems (MEMS) technology. This silicon plate was used as the heat transfer surface. The test heat transfer surface was heated by a semiconductor laser beam. Experiments were conducted in the range of up to 1.35 × 105 W⁄m2. When a single cavity was formed, the vertical coalescence of bubbles above the cavity was 60 % and no coalescence was 40 %. The ratios of the convection and the phase change were 80 % and 20 %, respectively. When the number of cavities were increased to three, the coalescence of bubbles on the heat transfer surface became important. When the role of the convection and the phase change in nucleate boiling is considered, it is appropriate to examine the bubble departure from the vapor mass on the heat transfer surface not from cavities.