Papers by Author: Tzong Shyng Leu

Abstract: Micro catalytic combustors are studied experimentally. Microchannels, coated with Pt catalytic walls and columns were fabricated to investigate microscale catalytic reaction. This microscale reaction enhancement by Pt catalytic surface area is characterized by increasing outlet gas temperature with the increase of surface-to-volume ratio. It is found that the reaction efficiency improvement by Pt catalytic columns will extend the operation conditions especially for smaller microchannel size.

Abstract: The unsteady flow fields above NACA 0015 airfoil pitching with/without upstream turbulence generator are investigated in a water tunnel by mean of particle image velocimetry (PIV). The turbulence was generated by a square bar mesh situated at the inlet of the test section. The airfoil pitching waveform is performed under the condition calculated from the angle of attack histogram of a vertical axis wind turbine (VAWT). By using PIV, the instantaneous vortex structures above the pitching airfoil can be revealed. It allows us to study the free stream turbulence effects on dynamic stall over an airfoil at pitching waveform the same as VAWT. It is found that the free stream turbulence intensity has significant impacts on the dynamic stall process. The dynamic stall process is delayed to higher incidence angles on increasing the turbulence intensity.

Abstract: In this paper, a novel MEMS-fabricated thermal bubble pump is proposed. By using thermocapillary effect in a microchannel loop, bubbles are designed to move only in one direction of the loop. Driving heater with a square-wave signal, bubble nucleates and grows asymmetrically above the heater. As soon as the heating pulse is turned off, an interesting phenomenon is found. Instead of bubble collapse, bubble moves toward growing direction. Continuous bubbles grow and move periodically when square-wave pulsing signals are applied. Thermocapillary pumping phenomena are achieved successfully in the experiments. By adjusting background temperature field, thermocapillary pumping phenomena can be sustained. Detailed mechanism for pumping force will be discussed in this paper.

Abstract: Meniscus dynamic behaviors, as well as its optimal waveform of driving signal, in a squeeze mode piezoelectric inkjet device are studied in this paper. To use the squeeze mode piezoelectric inkjet device as a drop-on-demand dispenser, the driving signal is trapezium waveform. The parameters in a trapezium waveform of the driving signal include rise time (trise), dwelling time (tdwell), fall time (tfall) and voltage (Vp). Among these parameters, the most important parameter of driving signal is the dwelling time. To demonstrate the importance of dwelling time, a LED signal synchronized with the driving signal is used to visualize the instantaneous meniscus shape under a microscope. Experimental results show the meniscus oscillation at beginning and droplet ejection at the later phase. It is found there is an optimal dwelling time. At the optimal dwelling time, droplet ejects in a minimum time and its ejection velocity reaches the maximum. The optimal dwelling time can be further identified it relates to a pressure wave oscillation within the device. The optimal dwelling time can be approximated as the length (L) of the piezoelectric inkjet device divide by the speed of sound (c) in the dispensed fluid.

Abstract: This paper described design, fabrication and study of micro electrospray chips. The micro electrospray chips are fabricated by using micro-electro-mechanical system (MEMS) technology. The micro electrospray techniques can be used in many applications, including high viscous fluid dispenses for 3D rapid prototyping and biological sample analysis in mass spectrometer. In this study, micro electrospray chips with different wedge angels are investigated by using microscopic visualization techniques. Four spray modes are identified at various operation voltages and currents. It is found that the voltage for stable cone-jet mode decrease as wedge angle becomes smaller. On the other hand, it is also found that stable cone-jet mode represent a constant current range in current (I)-voltage(Ф) curve. It allows us to easily identify if the micro electrospray chips operate at cone-jet mode.