Chihyung Huang

Microchannel Pressure Measurements Using Molecular Sensors

Fluid mechanics on the microscale is an important subject for researchers who are interested in studying microdevices since physical phenomena change from macroscale to microscale. Channel flow is a fundamental topic for fluid mechanics. By using a molecular sensor known as pressure-sensitive paint (PSP), detailed pressure data can be obtained inside the microchannel and at the channel entrance. The achievable spatial resolution of the acquired pressure map can be as high as 5 mum. PSP measurements are obtained for various pressure ratios from 1.76 to 20, with Knudsen number (K n) varying from 0.003 to 0.4. Compressibility and rarefaction effects can be seen in the pressure data inside the microchannel and at the channel entrance.

Numerical and experimental study of gas flows in 2D and 3D microchannels

In the experiments conducted at Purdue, the air flow in rectangular cross-section microchannels was investigated using pressure sensitive paint. The high resolution pressure measurements were obtained for inlet-to-outlet pressure ratios from 1.76 to 20 with the outlet Knudsen numbers in the range from 0.003 to 0.4 based on the hydraulic diameter of 151.7 ?m and the length-to-height ratio of about 50. In the slip flow regime, the air flow was simulated by the 2D and 3D Navier?Stokes equations with no-slip and slip boundary conditions. For various pressure ratios, the entrance flow development, compressibility and rarefaction effects were observed in both experiments and numerical simulations. It was found that the accurate modeling of gas flows in finite-length channels requires the inlet and outlet reservoirs to be included in computations. Effects of entrance geometry on the friction factor were studied for 3D cases. In both experiments and numerical modeling, significant pressure drop was found starting at the inlet chamber. The numerical modeling also predicted an apparent temperature drop at the channel exit.

Flow Visualization and Pressure Measurement in Micronozzles

Micro devices have been widely used in aerospace engineering for years. Engineers are interested in applications of micro devices such as microjets, micro actuators, and micronozzles. The small size nozzles can be used for attitude adjustment and propulsion of micro-satellites or mini-spacecraft. In this paper, convergent-divergent micronozzles have been investigated at supersonic speed with various total pressures and Reynolds numbers. The throat of the micronozzle is 250 micron wide and the nozzle is designed as de Laval type. For the measurements, the Reynolds number at the throat varies from 1200 to 11000 and total pressure varies from 6 psia to 55 psia. Experimental results are obtained with pressure-sensitive paint for pressure measurement and schlieren imaging for flow visualization. Flow visualization is a challenge for conventional techniques due to the small length scales and small depth of the density gradient. A modified schlieren technique is used to increase the sensitivity by taking the ratio of wind-on and wind-off images. Pressure-sensitive paint is also used to obtain global pressure measurement of the flow field and to compare with the schlieren results.

Protein micro arrays immobilized by μ-stamps and -protein wells on PhastGel® pad

This paper reports a novel stamping system, employing μ-stamps and -protein wells to simultaneously transfer proteins onto an array without de-naturalization, cross-contamination, and de-attachment of the proteins. The μ-stamps and -protein wells were successfully fabricated by micro machining and micro molding process. The effect of surface properties of μ-stamp on micro printing has been studied, and results demonstrated the feasibility of printing protein arrays with spot-size of 350 m square and pitch of 100 m. Testing results show that each stamped protein sample can be clearly identified with uniform deposition, and lasts for 6 h under water washing without appreciable de-attachment. This method may be used to transfer numerous different protein samples with the help of pre-filled μ-protein wells.

A modified schlieren technique for micro flow visualization

A modified schlieren technique has been developed for studies of supersonic flow through a micronozzle setup. After taking the ratio between reference and schlieren images, the noise has been reduced and the contrast in the image has been improved. The shock wave pattern in the micronozzle has been clearly identified at the total pressure of 41.4 kPa.

Molecular Sensors in Microturbine Measurement

Microturbines have been studied for use in micro size engines as a fuel cell for power generation. These micro engines are designed to offer power output around a couple watts with a very small, compact size. However, there are a lot of difficulties for these small size turbines such as high friction, unstable rotor movement etc, that make the microturbines break down much earlier in their life cycle. These problems needed to be solved in order to improve the efficiency of microturbines. However, it is difficult to install micro sensors in the microturbine device for measurement. A novel technique of molecular sensor, known as pressure-sensitive paint and temperature-sensitive paint (PSP/TSP) has been developed to obtain the pressure and temperature field inside the microturbine device. The PSP sensors used in the experiments are PtTFPP/PolyTMSP for time-averaged pressure measurement, and PtTFPP/TMSP for phase-averaged pressure measurement. The TSP sensor of Ru(phen)3+/Polyacrylic is used for the temperature measurement. The PSP/TSP sensor is coated on glass slides and the glass slide is used as cover glass for the microturbine device. Pressure and temperature distributions inside the microturbine device have been successfully obtained with PSP and TSP sensors. The rotation speed for the microturbine varies from 1300 to 4000 rpm for different flow rates from 5 to 15 L/min for the time-averaged experiments. Phase-averaged results have been obtained with a laser triggering system at a rotation speed of 1400 rpm and volume flow rate of 17 L/min. The temperature map inside the microturbine has been acquired with the TSP sensor with a rotation speed from 1300 to 4000 rpm and flow rates from 5 to 15 L/min. The PSP/TSP sensor has been demonstrated with the feasibility of measuring the pressure and temperature field inside the microturbine device with high spatial resolution, both in time-averaged and phase-averaged measurements.Copyright

Temperature Field in Severe Plastic Deformation at Small Strain Rates

The temperature and strain rate fields in severe plastic deformation (SPD) are measured using infra-red thermography and Particle Image Velocimetry (PIV), respectively. Plane strain machining is used as the method of SPD to impose controlled strains and strain rates. For metals such as titanium, the temperature rise is small at small strain rates and SPD occurs at near-ambient temperature. The possibility of exploring dynamic recovery/recrystallization phenomena using the Zener-Hollomon parameter in this SPD framework is briefly discussed.

Modeling and Preliminary Experiment for Rarefied Gas Flows in Constricted Microchannels

Gaseous slip flows in 3D rectangular microchannels with constrictions have been study numerically, and the experiment using pressure-sensitive-paints (PSP) for polymer microchannel pressure measurements are proposed. Constrictions inside microchannels, either being manufacturing defects or functional design features such as micro-orifices or micro-nozzles, will change the flow pattern because of additional frictional resistance and flow separation. In current research, mass-flowrate reduction due to constrictions has been investigated numerically for air flows in the slip regime, where Knudsen number ranges from 0.003 to 0.07. The results have been compared with both straight microchannels and with 3D analytical solutions. Similar to nozzle cases at macroscale, chocked flows has been observed at the critical pressure ratio of about 1.89. A numerical model including finite inlet and outlet chambers has been used in simulations to evaluate effects of reflection waves. Slip effects have been studied for different accommodation coefficients in presence of constrictions. By implementing multi-species numerical models, thermal induced mass transport has also been studied. Preliminary experiment based on PSP measurement for polymer microchannels has able to generate high spatial resolution pressure data, which are comparable with numerical simulations. Finally, further improvement of experimental setup is discussed.Copyright

PSP Measurement in Microchannel Flow

Fluid mechanics on the micro scale is an important subject for researchers who are interested in studying micro devices since physical phenomena change from macro to micro scale. Channel flow is a fundamental topic for fluid mechanics. By using pressure-sensitive paint (PSP), detailed pressure data can be obtained inside the microchannel and at the channel entrance. PSP measurements are obtained for various pressure ratios from 1.76 to 20 with Knudsen (Kn) number changes from 0.006 to 0.8. Compressibility and rarefaction effects can be seen in the pressure data inside the microchannel and at the channel entrance.

Numerical and Experimental Study of Gas Flows in 2D and 3D Microchannels

In the experiments conducted at Purdue, the air flow in rectangular cross-section microchannels was investigated using the pressure sensitive paints. The high resolution pressure measurements were obtained for inlet-to-outlet pressure ratios from 1.76 to 20 with the outlet Knudsen numbers in the range from 0.002 to 0.06 based on hydraulic diameter of 157.9 micron and the length-to-height ratio of about 50. In the slip flow regime, air flow was simulated by the 2D and 3D Navier-Stokes equations with no-slip and slip boundary conditions. For various pressure ratios, the entrance flow development, compressibility and rarefaction effects were observed in both experiments and numerical simulations. It was found that accurate modeling of gas flows infinite-length channels requires that inlet and outlet reservoirs to be included in computations. Effects of entrance geometry on the friction factor were studied for 3D cases. In both experiments and numerical modelings, significant pressure drop was found starting at the inlet chamber. The numerical modeling also predicted an apparent temperature drop especially at the channel exit.© 2007 ASME