Haiwang Li

Two-dimensional heat transfer distribution in a rotating smooth rectangular channel with four surface heating boundary condition:

In this study, two-dimensional heat transfer distribution on the leading surface and trailing surface in a rotating smooth channel with typical boundary condition of four surface heating was investigated experimentally for the first time. The Reynolds number, based on channel hydraulic diameter (80 mm) and the bulk mean velocity, ranges from 15,000 to 30,000, and the highest rotation number is 0.359 with the Reynolds number of 15,000. The mean density ratio is about 0.11 in this work. The obtained result showed that rotation has an important effect on the heat transfer distribution along the span-wise direction. On the leading side, rotation-induced secondary flow makes the Nu/Nu0 at the edge area of the wall surface (Z/D = 0 and Z/D = 1) is higher than that on the middle area (Z/D = 0.5). On the trailing surface, the trend of Nu/Nu0 ratio along the span-wise direction is reversed. Along the stream-wise direction, the Nu/Nus ratio increases with the rotation number monotonously on the trailing side. However, on the leading side, with the increase in rotation number, the Nu/Nus does not decrease monotonously along the stream-wise direction, there is a slight enhancement along the stream-wise direction. The secondary flow induced by rotation enhances Nu ¯ / N u s ratio up to 1.35 on the trailing side and weakens the Nu ¯ / N u s ratio close to 0.75 on the leading side with the Re = 15,000, and Ro = 0.359. More details of two-dimensional distribution of temperature and Nu on the leading and trailing side are shown in this work.

Manufacturing of a Compact Micro Air Bearing Device for Power Micro Electro Mechanical System (MEMS) Applications Using Silica Film Assisted Processing

The focus of this study is on the manufacturing of micro air bearings (MABs) using silica film assisted processing. Structure of the three-layer micro air bearing is described in detail and the salient process flow of etching and bonding is illustrated. The main manufacturing challenges and the methods adopted to overcome them are also presented. The uniformity of wet etching for nozzles with 20 μm in diameter to silica film is improved by adopting an ultrasound assisted method. Particular attention is given to the novel fabrication procedures for the second layer of MAB (with three depths on aft side). This paper develops new applications of silica film in Micro Electro Mechanical System (MEMS) processing for MABs to realize the etching of multi-depth on the same side and efficient three-layer bonding with increased bonding areas. A silica etch mask is proven to achieve a higher accuracy in surface topography when compared to a photoresist mask for multi-depth etching, resulting in precise depth and vertical control. The bonding rate of three-layer direct bonding for MAB is increased by 50% from 0.05 to 0.1 with the novel silica film protection method.

Anti-Reflectance Optimization of Secondary Nanostructured Black Silicon Grown on Micro-Structured Arrays

Owing to its extremely low light absorption, black silicon has been widely investigated and reported in recent years, and simultaneously applied to various disciplines. Black silicon is, in general, fabricated on flat surfaces based on the silicon substrate. However, with three normal fabrication methods—plasma dry etching, metal-assisted wet etching, and femtosecond laser pulse etching—black silicon cannot perform easily due to its lowest absorption and thus some studies remained in the laboratory stage. This paper puts forward a novel secondary nanostructured black silicon, which uses the dry-wet hybrid fabrication method to achieve secondary nanostructures. In consideration of the influence of the structure’s size, this paper fabricated different sizes of secondary nanostructured black silicon and compared their absorptions with each other. A total of 0.5% reflectance and 98% absorption efficiency of the pit sample were achieved with a diameter of 117.1 μm and a depth of 72.6 μm. In addition, the variation tendency of the absorption efficiency is not solely monotone increasing or monotone decreasing, but firstly increasing and then decreasing. By using a statistical image processing method, nanostructures with diameters between 20 and 30 nm are the majority and nanostructures with a diameter between 10 and 40 nm account for 81% of the diameters.