Jia-Kun Chen

Assessment of three AC electroosmotic flow protocols for mixing in microfluidic channel

This study performs an experimental investigation into the micromixer capabilities of three different protocols of AC electroosmotic flow (AC EOF), namely capacitive charging (CC), Faradaic charging (FC) and asymmetric polarization (AP). The results reveal that the vortices generated by the FC protocol (the frequency is around 50-350 Hz) are stronger than those induced by the CC protocol (the frequency is higher than 350 Hz), and therefore provide an improved mixing effect. However, in the FC protocol, the frequency of the external AC voltage must be carefully controlled to avoid damaging electrodes as a result of Faradaic reactions. The experimental results indicate that the AP polarization effect (the applied voltage and frequency are V(1) = 1 V(pp) and V(2) = 20 V(pp)/5 kHz) induces more powerful vortices than either the CC protocol or the FC protocol, and therefore yields a better mixing performance. Two AP-based micromixers are fabricated with symmetric and asymmetric electrode configurations, respectively. The mixing indices achieved by the two devices after an elapsed time of 60 seconds are found to be 56.49 % and 71.77 %, respectively. This result shows that of the two devices, an asymmetric electrode configuration represents a more suitable choice for micromixer in microfluidic devices.

Electroosmotic Flow Driven by DC and AC Electric Fields in Curved Microchannels

The purpose of this study is to investigate electroosmotic flows driven by externally applied DC and AC electric fields in curved microchannels. For the DC electric driving field, the velocity distribution and secondary flow patterns are investigated in microchannels with various curvature ratios. We use the Dean number to describe the curvature effect of the flow field in DC electric field. The result implies that the effect of curvatures and the strength of the secondary flows become get stronger when the curvature ratio of C/A (where C is the radius of curvature of the microchannel and A is the half-height of rectangular curved tube.) is smaller. For the AC electric field, the velocity distribution and secondary flow patterns are investigated for driving frequencies in the range of 2.0 kHz (Wo=0.71) to 11 kHz (Wo=1.66). The numerical results reveal that the velocity at the center of the microchannel becomes lower at higher frequencies of the AC electric field and the strength of the secondary flow decreases. When the applied frequency exceeds 3.0 kHz (Wo=0.87), vortices are no longer observed at the corners of the microchannel. Therefore, it can be concluded that the secondary flow induced at higher AC electric field frequencies has virtually no effect on the axial flow field in the microchannel.

Effects of Mannequin and Walk-by Motion on Flow and Spillage Characteristics of Wall-Mounted and Jet-Isolated Range Hoods

Laser-assisted flow-visualization experiments and tracer gas concentration tests were conducted for the wall-mounted and jet-isolated range hoods to examine the physical mechanisms and relative magnitudes of hood spillages. The effects of a mannequin standing in front of the test rig and walk-by motions (which are situations always encountered in kitchens) were emphasized. The results showed that a mannequin (or a cook) standing in front of the counter would attract oil fumes toward the mannequins body, induce large turbulent flows, and cause a significant dispersion of oil fumes into the environment through the front edge of the hood. Very high tracer gas concentrations were detected around the breathing zone of the mannequin. Increasing the suction flow rate did not reduce the spillage levels of the wall-mounted range hood but could moderately lower those of the jet-isolated hood. Serious spillages from both the wall-mounted and jet-isolated range hoods were detected as the simulated walk-by motion was performed. The jet-isolated range hood presented a much lower robustness in resisting the influence of peoples walk-bys than did the wall-mounted range hood. In summary, both the wall-mounted and jet-isolated range hoods were vulnerable to the influences of a cooks presence and a cooks walk-by motions. Increasing the suction flow rate might not obtain satisfactorily low spillages of pollutants but might increase noise level and energy consumption.

Flow and Containment Characteristics of a Sash-less, Variable-Height Inclined Air-Curtain Fume Hood

To increase containment efficiency and reduce energy consumption, a sash-less, variable-height inclined air-curtain fume hood (sIAC hood) was developed and tested by a laser-assisted flow visualization technique and tracer-gas detection method. This novel design requires neither sash nor baffle. The sIAC hood employed the inclined push-pull air-curtain technique and two deflection plates installed on the side walls of the hood to induce a tetra-vortex flow structure. The results of flow visualization showed that the slot for suction flow, offset from the slot for the up-blowing jet, caused the air curtain to incline towards the rear wall, thus enhancing the robustness of the tetra-vortex flow structure. Such a flow structure could reduce the influence of draught and human walk-by across the hood face. The containment around the central area of the hood was isolated by the inclined push-pull air curtain. The pollutants carried by the reverse flow induced by the flow separation were guided by the deflection plates from the side walls towards the rear, thus contributing to the formation of the tetra-vortex flow structure. The up/down movable ceiling positioned the suction slot close to the devices pollutant emission opening, but left room (less than 50 cm) for unrestricted hand movement. Testing was carried out based on the methodology described in EN14175. The results of a static test showed that small face velocities of 0.25 and 0.16 m s(-1) were enough to obtain nearly null leakage levels for low and tall pollutant sources. The results of a traversing plate test showed that the face velocity, 0.32 m s(-1), would cause negligibly small leakage levels. The sIAC hood could obtain significantly higher containment efficiency than a conventional hood by operating at a face velocity significantly lower than that of conventional hoods.

Manipulating Flow to Reduce Drag of a Square Cylinder by Using a Self-Sustained Vibrating Rod

The flow, vortex shedding, and surface pressure of a square cylinder at incidence were manipulated by means of a self-sustained vibrating rod placed around the leading edge of the upwind-facing lateral face of the square cylinder. The flow patterns on the cylinder surface were studied by using the surface-oil flow method for a Reynolds number between 4.5 10 4 and 1.1 x 10 5 as the incidence angle varied from 0° to 45°. Vortex-shedding characteristics were measured by means of a single-component hot-wire anemometer, and surface-pressure distributions were detected by using a linear-pressure scanner. The results show that owing to the influence of the rod vibration, the flow pattern on the agitated face changed from its natural state of a dual-ring bubble to the mode of boundary-layer separation. The critical incidence angle separating the dual-ring bubble and single-ring bubble modes was advanced to 11° from its natural state of 15°. The locations of the characteristic points on the cylinder surface were altered by the rod vibration, implying that the whole flow field surrounding the square cylinder was modified by the vibrating rod installed around the leading edge of the upwind-facing lateral face. The Strouhal numbers of wake instability of the controlled and uncontrolled cylinders did not present significant difference. The variations of the pressure coefficients induced by the rod vibration were closely related with the modification of the flow field on the cylinder surface. The decreases in the pressure coefficients on the upwind-facing faces and on the leeward-facing faces lead to drag reduction of the controlled cylinder by ∼25% when compared with the uncontrolled cylinder.

Flow Characteristics and Spillage Mechanisms of Wall-Mounted and Jet-Isolated Range Hoods

The flow characteristics and oil mist spillages of wall-mounted and jet-isolated range hoods were studied experimentally. Flow patterns were examined using a laser-light, sheet-assisted, smoke flow visualization technique. Spillages were diagnosed by the locally averaged tracer gas concentration test method. Tracer gas concentration test results correlated well with those of flow visualizations. For the wall-mounted hood, primary leakages occur around the region near the front edge of a countertop due to boundary layer separation, as well as the region just below the lower edge of the side panels of the hood due to the expansion effect of plumes. Increasing the suction flow rate above some critical values may help to reduce leakages out of the lateral planes but would increase spillages around the front edge of the countertop. For the jet-isolated range hood, oil mists spread widely and present unsteady motions with a high degree of turbulence because insufficient free air is allowed to enter the space enclosed by the jets and rear wall. Spillages across the jets into the environment due to turbulent dispersion become significant. Increasing the suction flow rate above some critical values may help to reduce spillages, while increasing the jet velocity would increase turbulent dispersion and thus lead to larger leakages.

Improving Flow Patterns and Spillage Characteristics of a Box-Type Commercial Kitchen Hood

A conventional box-type commercial kitchen hood and its improved version (termed the “IQV commercial kitchen hood”) were studied using the laser-assisted smoke flow visualization technique and tracer-gas (sulfur hexafluoride) detection methods. The laser-assisted smoke flow visualization technique qualitatively revealed the flow field of the hood and the areas apt for leakages of hood containment. The tracer-gas concentration detection method measured the quantitative leakage levels of the hood containment. The oil mists that were generated in the conventional box-type commercial kitchen hood leaked significantly into the environment from the areas near the front edges of ceiling and side walls. Around these areas, the boundary-layer separation occurred, inducing highly unsteady and turbulent recirculating flow, and leading to spillages of hood containment due to inappropriate aerodynamic design at the front edges of the ceiling and side walls. The tracer-gas concentration measurements on the conventional box-type commercial kitchen hood showed that the sulfur hexafluoride concentrations detected at the hood face attained very large values on an order of magnitude about 103–104 ppb. By combining the backward-offset narrow suction slot, deflection plates, and quarter-circular arcs at the hood entrance, the IQV commercial kitchen hood presented a flow field containing four backward-inclined cyclone flow structures. The oil mists generated by cooking were coherently confined in these upward-rising cyclone flow structures and finally exhausted through the narrow suction slot. The tracer-gas concentration measurements on the IQV commercial kitchen hood showed that the order of magnitude of the sulfur hexafluoride concentrations detected at the hood face is negligibly small—only about 100 ppb across the whole hood face.

Flow and Leakage Characteristics of a Sashless Inclined Air-Curtain (sIAC) Fume Hood Containing Tall Pollutant-Generation Tanks

In many fume hood applications, pollutant-generation devices are tall. Human operators of a fume hood must stand close to the front of the hood and lift up their hands to reach the top opening of the tall tank. In this situation, it is inconvenient to access the conventional hood because the sash acts as a barrier. Also, the bluff-body wake in front of the operators chest causes a problem. By using laser-assisted smoke flow visualization and tracer-gas test methods, the present study examines a sashless inclined air-curtain (sIAC) fume hood for tall pollutant-generation tanks, with a mannequin standing in front of the hood face. The configuration of the sIAC fume hood, which had the important element of a backward-inclined push-pull air curtain, was different from conventional configurations. Depending on suction velocity, the backward-inclined air curtain had three characteristic modes: straight, concave, and attachment. A large recirculation bubble covering the area—from the hood ceiling to the work surface—was formed behind the inclined air curtain in the straight and concave modes. In the attachment mode, the inclined air curtain was attached to the rear wall of the hood, about 50 cm from the hood ceiling, and bifurcated into up and down streams. Releasing the pollutants at an altitude above where the inclined air curtain was attached caused the suction slot to directly draw up the pollutants. Releasing pollutants in the rear recirculation bubble created a risk of pollutants’ leaking from the hood face. The tracer-gas (SF6) test results showed that operating the sIAC hood in the attachment mode, with the pollutants being released high above the critical altitude, could guarantee almost no leakage, even though a mannequin was standing in front of the sashless hood face.

Flow Characteristics and Spillage Mechanisms of Wall-Mounted and Jet-Isolated Range Hoods Subject to Influence from Cross Draft

The effects of draft on the flow and spillage characteristics of wall-mounted and jet-isolated range hoods were investigated. A specially designed draft generator that could supply low-swirl air current was used to provide “cross draft” from three directions, lateral (θ = 0o), oblique (θ = 45o), and front (θ = 90o), with respect to the center point of the range hoods. Flow characteristics of oil mist were inspected through visualization of smoke flows with light scattering (laser light sheet-assisted visualization of smoke flow). The leakage mechanisms, which were closely related to the flow features, were studied by examining both movies and still pictures showing smoke-flow evolution. The sulfur hexafluoride tracer gas concentration detection method was employed to measure the capture indices. The results showed that the lateral draft pushed the pollutants generated under the hood in the opposite direction and induced serious spillage. The oblique draft pushed the pollutants toward both the rear wall and opposite side and induced more serious spillage than did the lateral draft. The frontal draft forced the pollutants to bifurcate into streams moving toward the left and the right, and induced the most serious pollutant spillage among the three tested drafts. Pollutant spillage became critically significant as the cross draft velocity was increased to greater than 0.2 m/sec. Spillage of pollutants increased as the velocity of the cross draft was increased. Increasing the suction flow rate of the range hood may increase resistance to the draft, but the benefits were limited at draft velocities greater than 0.2 m/sec. Both range hoods had a similarly low capture index under the influence of the lateral draft. For the oblique and frontal drafts, the jet-isolated range hood demonstrated a higher capture index than did the wall-mounted range hood.

Flow characteristics and robustness of an inclined quad-vortex range hood.

A novel design of range hood, which was termed the inclined quad-vortex (IQV) range hood, was examined for its flow and containment leakage characteristics under the influence of a plate sweeping across the hood face. A flow visualization technique was used to unveil the flow behavior. Three characteristic flow modes were observed: convex, straight, and concave modes. A tracer gas detection method using sulfur hexafluoride (SF6) was employed to measure the containment leakage levels. The results were compared with the test data reported previously in the literature for a conventional range hood and an inclined air curtain (IAC) range hood. The leakage SF6 concentration of the IQV range hood under the influence of the plate sweeping was 0.039 ppm at a suction flow rate of 9.4 m3/min. The leakage concentration of the conventional range hood was 0.768 ppm at a suction flow rate of 15.0 m3/min. For the IAC range hood, the leakage concentration was 0.326 ppm at a suction flow rate of 10.9 m3/min. The IQV range hood presented a significantly lower leakage level at a smaller suction flow rate than the conventional and IAC range hoods due to its aerodynamic design for flow behavior.