A novel fluidic oscillator incorporating step-shaped attachment walls

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Abstract

A novel fluidic oscillator incorporating step-shaped attachment walls and acute-angle splitters has been designed and verified experimentally to stabilize the oscillation of circulation flow, to modify the flow patterns and to improve the performance. Focusing on various oscillators of plane-wall and step-wall designs, we systematically analyzed the pressure spectra, pressure loss, and flow structure, via particle-imaging velocimetry, flow visualization, and pressure sensors. The results reveal that the novel design promotes conditions to initiate the oscillator and makes the recirculation vortices oscillate more effectively and stably. The operating range of this step-wall oscillator is broadened and the ratio of signal to noise of the step oscillator is 17 times as great as that with plane walls at a greater flow rate, 65 L/min. Comparison of the wave form of the spectral analysis and the ratio of signal to noise over the entire experimental range further corroborates the superior features of the novel design.

Introduction

A fluidic oscillator is widely applied to measure flow rates [1], [2], [3] because of a characteristic that its oscillating frequency is linearly proportional to the inlet flow rate for Reynolds numbers over a broad range. The basis of operation is that a jet entering a divergent cross-section or a sudden-expansion channel generally diverts toward either side, due to the Coanda effect, then develops to become a periodically oscillating flow at a fixed frequency [4], [5], [6]. Beyond this application in a flow meter, the oscillating features are widely used for actuators [7], [8], mixers [9], and memory and control devices [10].

Previous researchers reported specific correlations among the oscillation characteristics and geometric parameters. Based on the channel structure and the operating principle, previous investigations of fluidic oscillators are classified into three types: feedback oscillator [1], [2], [3], [7], [8], [9], Karman vortex oscillator [11], [12], and concave-type oscillator (Vee-gutter or U concavity) [13], [14], [15], [16], [17], [18]. Tippetts et al. [19], [20] deduced four major parameters for feedback fluidic oscillator of relaxation type, namely its Strouhal number (Sr), Reynolds number (Re), Euler number (Eu), and a dimensionless control loop inductance (L′), which serve also for dimensionless analysis in discussion, defined as follows:Sr=fhuRe=uhνEu=2Δpρu2L=4Inπd2in which h is the characteristic length of the oscillator, f the frequency of pressure fluctuation, u the inlet velocity of flow, ν the viscosity, Δp the pressure loss, and ρ is the fluid density. Furthermore, I = I/dn and d = d/dn, with I and d being the length and diameter of the control loop, respectively; dn and n are the width and aspect ratio of the inlet port, respectively. Tippetts et al. further reported that for a Reynolds number smaller than a critical value no fluctuation occurred.

For a Reynolds number in a certain range, the Strouhal number remained constant; the oscillation frequency thus became linearly proportional to the flow rate and independent of fluid properties. The dimensionless control loop inductance was found to correlate linearly with the Strouhal number. Wang et al. [4], [5], [6] combined a vortex amplifier with the oscillator and significantly improved both pressure loss and oscillation spectra of an oscillatory flow meter. This design was used for remote monitoring of crude-oil pipes.

For a miniature design, Gebhard et al. used a LIGA microfabrication technique to produce a micro-oscillator of length 720 μm, width 500 μm and depth 250 μm, and successfully combined this micro-oscillator with a micro-actuator to become a dynamic microsystem [7], [8]. Teseř et al. [21] suggested replacing the Reynolds number with a pressure drop and derived a Teser number to characterize the micro-oscillator.

Most previous work on a fluidic oscillator emphasized more the design than an analysis of the hydrodynamic structure. In our work, we therefore undertook a quantitative analysis of the dynamic behavior of these oscillators. Based on flow phenomena, we present a novel fluidic oscillator, called a step-wall oscillator (Fig. 1) [22], and several new design concepts to improve the features of the oscillator, and seek to illuminate the design concept of fluidic oscillators.

Section snippets

Experimental design

The first objectives of the work are to explore the hydrodynamics in the oscillator and to investigate the function of the major fluidic elements. Then the similar and distinct features of the step-wall oscillator and a contrasting oscillator named a plane-wall oscillator (Fig. 2) are analyzed systematically through frequency detection and laser diagnosis. The plane-wall oscillator has been tested in our laboratory to have a superior performance among existing feedback-type fluidic oscillators.

Results and discussion

We begin with the overall flow pattern, then contrast the performance of two geometries, and finally present our investigation of mainstream and circulation flow via PIV.

Conclusions

In this work we investigated experimentally the flow patterns and performances of two fluidic oscillators, of plane-wall and step-wall designs, via particle-imaging velocimetry (PIV) and analysis of pressure spectra. The experimental results reveal that the dynamic behavior of an oscillator is modulated by interactions among flow fluctuations in the inlet region, the development of the recirculation flow, and the flow structure near step-walls and splitters. Enhancing the feedback effect and

Acknowledgement

The National Science Council of the Republic of China partially supported this work under contract numbers NSC 91-2212-E-007-063 and NSC 91-2218-E-007-042.

Jing-Tang Yang is a Professor at the Department of Power Mechanical Engineering and the Institute of Microelectromechanical Systems at National Tsing Hua University. His current research topics include microfluidics, biomimetic engineering, energy and combustion, jet propulsion, and laser diagnostics. His PhD degree was earned at the University of Wisconsin at Madison in August 1983. Currently, he serves as the chairman of the steering committee of Taiwan government research and development on

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    Jing-Tang Yang is a Professor at the Department of Power Mechanical Engineering and the Institute of Microelectromechanical Systems at National Tsing Hua University. His current research topics include microfluidics, biomimetic engineering, energy and combustion, jet propulsion, and laser diagnostics. His PhD degree was earned at the University of Wisconsin at Madison in August 1983. Currently, he serves as the chairman of the steering committee of Taiwan government research and development on energy, the coordinator of advanced energy research program of the National Science Council, and the director of mid-north regional center of K-12 nanotechnology education in Taiwan.

    Chi-Ko Chen was born in Taiwan in 1979. He received the BS degree from Yuan-Ze University in 2001 and the MS degree in the Department of Power Mechanical Engineering from National Tsing-Hua University, Hsinchu, Taiwan, in 2003, where he is currently pursuing the PhD degree. His research interests include microfluidics system, μ–LIF and FRET method development.

    Kun-Jyh Tsai received his BS degree in mechanical engineering from National Cheng Kung University in 2001, Taiwan and MS degree in the Power Mechanical Engineering from Nation Tsing-Hua University in 2003, where he is presently a PhD student.

    Wei-Zhih Lin received his BS and MS degree in the Power Mechanical Engineering from Nation Tsing-Hua University in 2000 and 2002.

    Horn-Jiunn Sheen is currently a professor at the Institute of Applied Mechanics, National Taiwan University. He is also the director of the Nano-Electro-Mechanical-System (NEMS) Research Center, National Taiwan University. His research interests include micro-fluidics, bio-sensors, and flow measurements.

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