Operational Optimization of GaN Thin Film Growth Employing Numerical Simulation in a Showerhead MOCVD Reactor

A detailed reaction-tran sport model was studied in a showerhead reactor for metal organic chemical vapor deposition of GaN film by using computational fluid dynamics simulation. It was found that flat flow lines without swirl are crucial to improve the uniformity of the film growth, and thin temperature gradient above the suscptor can increase the film deposition rate. By above-mentioned research, we can employ higher h (the distance from the susceptor to the inlet), P (operational pressure) and the rate of susceptor rotation to improve the film growth.

Synthesis of biodiesel from waste cooking oil using immobilized lipase in fixed bed reactor

Waste cooking oil (WCO) is the residue from the kitchen, restaurants, food factories and even human and animal waste which not only harm people's health but also causes environmental pollution. The production of biodiesel from waste cooking oil to partially substitute petroleum diesel is one of the measures for solving the twin problems of environment pollution and energy shortage. In this project, synthesis of biodiesel was catalyzed by immobilized Candida lipase in a three-step fixed bed reactor. The reaction solution was a mixture of WCO, water, methanol and solvent (hexane). The main product was biodiesel consisted of fatty acid methyl ester (FAME), of which methyl oleate was the main component. Effects of lipase, solvent, water, and temperature and flow of the reaction mixture on the synthesis of biodiesel were analyzed. The results indicate that a 91.08% of FAME can be achieved in the end product under optimal conditions. Most of the chemical and physical characters of the biodiesel were superior to the standards for 0(#)diesel (GB/T 19147) and biodiesel (DIN V51606 and ASTM D-6751).

Lipase-catalyzed synthesis of biodiesel from acid oil in fixed bed reactor

Acid oil, which is a by-product in vegetable oil refining, mainly contains free fatty acids (FFAs) and acylglycerols and is a feedstock for production of biodiesel fuel now. The transesterification of acid oil and methanol to biodiesel was catalyzed by immobilized Candida lipase in fixed bed reactors. The reactant solution was a mixture of acid oil, water, methanol and solvent (hexane) and the main product was biodiesel composed of fatty acid methyl ester (FAME) of which the main component was methyl oleate. The effects of lipase content, solvent content, water content temperature and flow velocity of the reactant on the reaction were analyzed. The experimental results indicate that a maximum FAME content of 90.18% can be obtained in the end product under optimum conditions. Most of the chemical and physical properties of the biodiesel were superior to the standards for 0(#) diesel (GB/T 19147) and biodiesel (DIN V51606 and ASTM D6751).

A three-dimensional analytical solution of the microheater temperature before bubble nucleation

A three-dimensional analytical solution of the microheater temperature based on heat diffusion equation is developed and compared with experimental results. Dimensionless parameters are introduced to analyze the temperature rise time and the distribution under steady state. To study the microheater temperatures before bubble nucleation, a set of working fluids and microheaters are considered. It is shown that the dimensionless time xi(-)(0) required for the temperature rise from room to 95% of the steady state temperature is about 75, not dependent on working fluids and microheaters. Heat transfer to the surrounding liquid is mainly caused by conduction, not by convection and radiation mechanisms. The microheater length affects the surface temperature uniformity, while its width influences the steady temperatures significantly, yielding the transition from heterogeneous to homogeneous nucleation mechanism from square microheaters to narrow line microheaters. 

Effect of pulse heating parameters on the microscale bubble dynamics at a microheater surface

We study the effects of pulse heating parameters on the micro bubble behavior of a platinum microheater (100 mu m x 20 mu m) immersed in a methanol pool. The experiment covers the heat fluxes of 10-37 MW/m(2) and pulse frequencies of 25-500 Hz. The boiling incipience is initiated at the superheat limit of methanol, corresponding to the homogeneous nucleation. Three types of micro boiling patterns are identified. The first type is named as the bubble explosion and regrowth, consisting of a violent explosive boiling and shrinking, followed by a slower bubble regrowth and subsequent shrinking, occurring at lower heat fluxes. The second type, named as the bubble breakup and attraction, consists of the violent explosive boiling, bubble breakup and emission, bubble attraction and coalescence process, occurring at higher heat fluxes than those of the first type. The third type, named as the bubble size oscillation and large bubble formation, involves the initial explosive boiling, followed by a short periodic bubble growth and shrinking. Then the bubble continues to increase its size, until a constant bubble size is reached which is larger than the microheater length. 

Periodic bubble emission and appearance of an ordered bubble sequence (train) during condensation in a single microchannel

Condensation of steam in a single microchannel, silicon test section was investigated visually at low flow rates. The microchannel was rectangular in cross-section with a depth of 30 pm, a width of 800 mu m and a length of 5.0 mm, covered with a Pyrex glass to allow for visualization of the bubble formation process. By varying the cooling rate during condensation of the saturated water vapor, it was possible to control the shape, size and frequency of the bubbles formed. At low cooling rates using only natural air convection from the ambient environment, the flow pattern in the microchannel consisted of a nearly stable elongated bubble attached upstream (near the inlet) that pinched off into a train of elliptical bubbles downstream of the elongated bubble. It was observed that these elliptical bubbles were emitted periodically from the tip of the elongated bubble at a high frequency, with smaller size than the channel width. The shape of the emitted bubbles underwent modifications shortly after their generation until finally becoming a stable vertical ellipse, maintaining its shape and size as it flowed downstream at a constant speed. These periodically emitted elliptical bubbles thus formed an ordered bubble sequence (train). At higher cooling rates using chilled water in a copper heat sink attached to the test section, the bubble formation frequency increased significantly while the bubble size decreased, all the while forming a perfect bubble train flowing downstream of the microchannel. The emitted bubbles in this case immediately formed into a circular shape without any further modification after their separation from the elongated bubble upstream. The present study suggests that a method for controlling the size and generation frequency of microbubbles could be so developed, which may be of interest for microfluidic applications. The breakup of the elongated bubble is caused by the large Weber number at the tip of the elongated bubble induced by the maximum vapor velocity at the centerline of the microchannel inside the elongated bubble and the smaller surface tension force of water at the tip of the elongated bubble.

Study on methane hydration process in a semi-continuous stirred tank reactor

The methane hydration process is investigated in a semi-continuous stirred tank reactor. Liquid temperatures and reaction rates without stirrer are compared with those occurring with stirrer, while at the same time better stirring conditions of the methane hydration process are given by the experiments. Some basic data of fluid mechanics, for example, stirring Reynolds number, Froucle number and stirrer power, are calculated during the methane hydration process, which can be applied to evaluate stirrer capacity and provide some basic data for a scaled up reactor. Based on experiment and calculations in this work, some conclusions are drawn. First, the stirrer has great influence on the methane hydration process. Batch stirring is helpful to improve the mass transfer and heat transfer performances of the methane hydration process. Second, induction time can be shortened effectively by use of the stirrer. Third, in this paper, the appropriate stirring velocity and stirring time were 320 rpm and 30 min, respectively, at 5.0 MPa, for which the storage capacity and reaction time were 159.1 V/V and 370 min, respectively. Under the condition of the on-flow state, the initial stirring Reynolds number of the fluid and the stirring power were 12,150 and 0.54 W, respectively. Fourth, some suggestions, for example, the use of another type of stirrer or some baffles, are proposed to accelerate the methane hydration process. Comparing with literature data, higher storage capacity and hydration rate are achieved in this work. Moreover, some fluid mechanics parameters are calculated, which can provide some references to engineering application.

Deterministic Column-Based Matrix Decomposition

In this paper, we propose a deterministic column-based matrix decomposition method. Conventional column-based matrix decomposition (CX) computes the columns by randomly sampling columns of the data matrix. Instead, the newly proposed method (termed as CX_D) selects columns in a deterministic manner, which well approximates singular value decomposition. The experimental results well demonstrate the power and the advantages of the proposed method upon three real-world data sets.