C. J. Teo

Microchannel flows with superhydrophobic surfaces: Effects of Reynolds number and pattern width to channel height ratio

Superhydrophobic surfaces are widely adopted for reducing the flow resistance in microfluidic channels. The structures on the superhydrophobic surfaces may consist of longitudinal grooves, transverse grooves, posts, holes, etc. In this paper their effective slip performances are systematically studied and compared in detail through numerical simulations. The numerical results show that channel wall confinement effects have a positive influence on the effective slip length for square posts and longitudinal grooves, and a negative influence for square holes and transverse grooves. Square posts, holes, and transverse grooves all exhibit deteriorating effective slip performances at higher Reynolds numbers, while the effective slip performance of longitudinal grooves remains independent of the Reynolds number. For small pattern width to channel height ratios and at low Reynolds numbers, for low shear-free fractions, the effective slip length of square posts is equivalent of that of transverse grooves, and both...

Dynamic response of a hot-wire anemometer. Part II: A flush-mounted hot-wire and hot-film probes for wall shear stress measurements

The present study continues the work described in Part I of this paper in utilizing a specifically designed apparatus for generating a known near-wall fluctuating flow field for the purpose of quantifying the dynamic response of a flush-mounted hot-element wall shear stress gauge. Two different types of wall shear stress gauges were tested: a flush-mounted hot wire in contact with the wall substrate and commercially available quartz substrate hot-film gauges with different thicknesses of quartz coating. It is found that the flush-mounted hot-wire gauge has a much higher dynamic response than the quartz substrate gauges, although it is still lower than the marginally elevated hot-wire configuration. This may suggest the possible use of a marginally elevated hot wire as a wall shear stress gauge to ensure sufficient responsiveness for time-resolved wall shear stress measurements.

Dynamic response of a hot-wire anemometer. Part I: A marginally elevated hot-wire probe for near-wall velocity measurements

Experiments were carried out to generate a known near-wall fluctuating flow field for the purpose of quantifying the dynamic response of a marginally elevated hot-wire probe. It is found that the dynamic response of the hot wire is dependent on both the wires height (h) above the wall substrate and the convective velocity at the wires location. Both larger values of and smaller values of h improve the dynamic response of a near-wall hot wire. Further experiments were also performed to investigate the effect of the thermal conductivity of the wall substrate on the response characteristics of the hot wire. A thermally more conducting material for the wall yields a better response hot-wire probe for near-wall velocity measurements.

ON THE PROLONG ATTACHMENT OF LEADING EDGE VORTEX ON A FLAPPING WING

In this paper, we take a fundamental approach to investigate the effect of spanwise flow on the prolonged attachment of leading edge vortex (LEV) on a flapping wing. By imposing a constant acceleration-constant velocity flow on elliptic wings of various sweep angles and angles of attack, our experimental and numerical results show that while spanwise flow per se has negligible influence on the prolong attachment of the LEV, vortex stretching can significantly delay detachment of the LEV, even for a small spanwise flow.

Fabrication and Testing of a High-Speed Microscale Turbocharger

A microelectromechanical system (MEMS) turbocharger has been designed, fabricated, and tested as part of a Massachusetts Institute of Technology program aimed at producing a microfabricated gas turbine engine for portable power applications. A gas turbine engine requires high-speed high-efficiency turbomachinery operating at tip speeds of several hundred meters per second. This MEMS turbocharger serves to demonstrate these requirements. The turbochargers silicon rotor, which is supported on hydrostatic gas thrust and journal bearings in a silicon stator housing, was spun to 480 000 rpm, corresponding to a tip speed of 200 m/s. This paper discusses critical fabrication processes that enabled the capabilities of this device. Operational issues and test results are also presented. The turbochargers compressor demonstrated a pressure ratio of 1.21 at a mass flow rate of 0.13 g/s, with a combined compressor-turbine spool efficiency of 0.24. Under these conditions, the turbine produced about 5 W of power. Results from the simultaneous operation of a spinning rotor and burning combustor within the microscale turbocharger are also presented. Experimental results compare well with analytical models and computations.

Enhancement of rotordynamic performance of high-speed micro-rotors for power MEMS applications by precision deep reactive ion etching

High-precision fabrication is indispensable for high-speed silicon micro-rotors for power MEMS applications so as to minimize the rotor imbalance that deteriorates the rotor performance. Etch variation of deep reactive ion etch (DRIE) process results in differences in rotor blade heights and thus rotor imbalance. A Fourier transform of the etch non-uniformity along the rotor circumference revealed the global etch variation across the wafer and local variations in etch rates depending on the concentration or proximity of the patterned geometry. Rotor imbalance arising from the global etch variation of DRIE process was estimated, which compared favorably to results obtained from spinning experiments. The global etch non-uniformity which culminates in rotor imbalance could be alleviated to 0.25% across a rotor of 4.2 mm diameter by optimizing the plasma chamber pressure. The developed DRIE recipe successfully reduced the rotor imbalance and thus enhanced the rotordynamic performance. The manufacturing processes presented herein are readily applicable to the constructions of other microstructures containing intricate geometries and large etched areas.

The dynamic response of a hot-wire anemometer : III. Voltage-perturbation versus velocity-perturbation testing for near-wall hot-wire/film probes

Experiments were performed for the first time using the electronic square-wave voltage-perturbation test to systematically quantify the frequency response of near-wall hot-wire probes subjected in turn to varying magnitudes of convective velocity, different substrate materials and changes in wall-substrate temperature. In addition, quartz-substrate hot-film gauges with various thicknesses of quartz coating were also tested. Results of were compared against the dynamic frequency response previously obtained in parts I and II using a known near-wall fluctuating flow field. Although the observed trends for and were similar, their magnitudes were vastly different, notably for the commercially available hot-film gauges, or which was up to five orders of magnitude greater than . This signifies that there are possibly inherent differences between square-wave voltage-perturbation and velocity-perturbation tests for quantifying the frequency response of a hot-wire/hot-film system. These differences are then analysed in relation to the equation of a CTA unit put forth by Freymuth.

Unsteady Flow and Dynamic Behavior of Ultrashort Lomakin Gas Bearings

Ultrashort microscale high-speed gas bearings exhibit a whirl instability limit and dynamic behavior much different from conventional hydrostatic gas bearings. In particular, the design space for a stable high-speed operation is confined to a narrow region and involves a singular behavior. The previously developed ultrashort gas bearing theory (Liu et al. (2005, Hydrostatic Gas Journal Bearings for Micro-Turbomachinery, ASME J. Vibr. Acoust., 127(2), pp. 157-164)) assumed fully developed flow in the journal bearing gap. There is experimental evidence that this assumption might not be fully applicable for the relatively short flow-through times in such bearings. This has an impact on the estimation of whirl instability onset, bearing operability and power requirements. In this paper, unsteady flow effects in the bearing gap are investigated with the goal to quantify their impact on the bearing dynamic behavior. It is shown that although three-dimensional flow calculations in the ultrashort journal bearing are necessary to quantify the onset of whirl instability, the underlying mechanisms can be qualitatively described by the impulsive starting of a Couette flow. Using this description, two time scales are identified that govern the journal bearing dynamic behavior: the viscous diffusion time and the axial flow-through time. Based on this, a reduced frequency parameter is introduced that determines the development of the flow field in the journal bearing and, together with bearing force models, yields a criterion for whirl instability onset. Detailed three-dimensional computational fluid dynamics calculations of the journal bearing flow have been conducted to assess the criterion. A singular behavior in whirl ratio as a function of the reduced frequency parameter is observed, verifying the refined stability criterion. Using high-fidelity flow calculations, the effects of unsteady journal bearing flow on whirl instability limit and bearing power loss are quantified, and design guidelines and implications on gas bearing modeling are discussed. The stability criterion is experimentally validated demonstrating repeatable, stable high-speed operation of a novel microbearing test device at whirl ratios of 35.

Mixing and Heat Transfer Enhancement in Microchannels Containing Converging-Diverging Passages

Fully-developed flow and heat transfer in periodic converging-diverging channels with rectangular cross sections are studied using computational fluid dynamics (CFD) simulations for Reynolds numbers ranging from 50 to 200. Experimental laser sheet flow visualizations have also been utilized with the aid of an enlarged transparent Perspex model, which serves as a form of secondary verification of the CFD results. The CFD investigations focus on two principal configurations of converging-diverging channels, namely the constant curvature and sinusoidal converging-diverging channel. Heat transfer simulations have been carried out under constant wall temperature conditions using liquid water as the coolant. It is found that due to the fluid mixing arising from a pair of recirculating vortices in the converging-diverging channels, the heat transfer performance is always significantly more superior to that of straight channels with the same average cross sections; at the same time the pressure drop penalty of the converging-diverging channels can be much smaller than the heat transfer enhancement. The effects of channel aspect ratio and amplitude of the converging-diverging profiles have been systematically investigated. The results show that for a steady flow, the flow pattern is generally characterized by the formation of a pair of symmetrical recirculating vortices in the two furrows of the converging-diverging channel. Both the optimal aspect ratio and channel amplitude are being presented with the support of CFD analyses. Experimental flow visualizations have also been utilized and it was found that the experimental results agrees favorably with the CFD results. The present study shows that these converging-diverging channels have prominent advantages over straight channels. The most superior configuration considered in this paper has been found to yield an improvement of up to 60% in terms of the overall thermal-hydraulic performance compared to microchannels with straight walls, thus serving as promising candidates for incorporation into efficient heat transfer devices.

High-Speed Operation of a Gas-Bearing Supported MEMS-Air Turbine

Silicon based power MEMS applications require the high-speed micro-rotating machinery to operate stably over a large range of operating conditions. The technical barriers to achieve stable high-speed operation using micro-gas-bearings are governed by: (1) stringent fabrication tolerance requirements and manufacturing repeatability, (2) structural integrity of the silicon rotors, (3) rotordynamic coupling effects due to leakage flows, (4) bearing losses and power requirements, and (5) transcritical operation and whirl instability issues. Over the past few years, a large body of research was conducted at MIT to address these technical challenges; many lessons were learned and new theories were developed related to the dynamic behavior of micro-gas journal and thrust bearings. Based on the above mentioned experience, a gas-bearing supported micro-air turbine was developed with the objectives of demonstrating repeatable, stable high-speed gas-bearing operation and verifying the new micro-gas-bearing analytical models. The key challenge in this endeavor involved the synthesis and integration of the newly-developed gas-bearing theories and insight gained from extensive experimental work. The focus of this paper is on the process and the outcomes of this synthesis, rather than the details and results of the underlying theoretical models which have been previously published. The characteristics of the new micro-air turbine include a four-chamber journal bearing feed system to introduce stiffness anisotropy, labyrinth seals to avoid rotordynamic coupling effects of leakage flows, a reinforced thrust bearing structural design, a redesigned turbine rotor to increase power, a symmetric feed system to avoid flow and force non-uniformity, and a new rotor micro-fabrication methodology. A large number of test devices were successfully manufactured demonstrating repeatable bearing geometry. More specifically, three sets of devices with different journal bearing clearances were produced to investigate the dynamic behavior as a function of bearing geometry. Experiments were conducted to characterize the “as fabricated” bearing geometry, the damping ratio, and the natural frequencies. Repeatable high-speed bearing operation was demonstrated using isotropic and anisotropic bearing settings reaching whirl ratios between 20 and 40. A rotor speed of 1.7 million rpm (equivalent to 370 m/s blade tip speed or a bearing DN of 7 million mm-rpm) was achieved demonstrating the feasibility of MEMS based micro-scale rotating machinery and validating key aspects of the micro-gas-bearing theory.Copyright