Ann Lee

Gradient-guided color image contrast and saturation enhancement

Digital color images are capable of presenting hue, saturation, and brightness perceptions. Therefore, quality improvement of color images should be taken into account to enhance all three stimuli. An effective method is proposed that aims at enriching the colorfulness, vividness, and contrast of color images simultaneously. In this method, color correction based on magnitude stretching is carried out first, image enhancement is then derived from an intensity-guided operation that concurrently improves the contrast and saturation qualities. Furthermore, the proposed methodology mitigates the heavy computational burden arising from the need to transform the source color space into an alternative color space in conventional approaches. Experiments had been conducted using a collection of real-world images captured under various environmental conditions. Image quality improvements were observed both from subjective viewing and quantitative evaluation metrics in colorfulness, saturation, and contrast.

Numerical computation and investigation of the characteristics of microscale synthetic jets

A synthetic jet results from periodic oscillations of a membrane in a cavity. Jet is formed when fluid is alternately sucked into and ejected from a small cavity by the motion of membrane bounding the cavity. A novel moving mesh algorithm to simulate the formation of jet is presented. The governing equations are transformed into the curvilinear coordinate system in which the grid velocities evaluated are then fed into the computation of the flow in the cavity domain thus allowing the conservation equations of mass and momentum to be solved within the stationary computational domain. Numerical solution generated using this moving mesh approach is compared with an experimental result measuring the instantaneous velocity fields obtained by µPIV measurements in the vicinity of synthetic jet orifice 241 µm in diameter issuing into confined geometry. Comparisons between experimental and numerical results on the streamwise component of velocity profiles at the orifice exit and along the centerline of the pulsating jet inmicrochannel as well as the location of vortex core indicate that there is good agreement, thereby demonstrating that the moving mesh algorithm developed is valid.

Heat Transfer Enhancement in Microchannel Using Nanofluids

Numerical investigation of heat transfer enhancement in two-dimensional microchannel heat sink (MCHS) using Al2O3-water, CuO-water and TiO2-water was conducted. The effect of different type of nanoparticles at particle volume concentration of 1%, 2% and 5% on the thermal performance in the MCHS was examined. The thermal performance is increased when nanofluids with high thermal conductivity and low dynamic viscosity was used. As the particle volume concentration increases, the heat transfer performance also improved. The result shows that the heat transfer performance of all the nanofluids used in this study was better than that of pure water. Overall, nanofluids with Al2O3-water at 5% particle volume concentration show the best cooling performance.

Investigation of the effect of magnetic field on ferrofluid in microelectromechanical devices (MEMS)

Considerable efforts have been spent in the development of magnetic nanoparticles (MNPs) in the last decade to understand their behaviour, and the improvement of their applicability in many different areas. Precise control over the synthesis conditions and surface functionalization of MNPs is crucial because it governs their physical properties and their colloidal stability. The magnetic platforms possess very small size and narrow size distribution together with high magnetization values. These nanoparticles (NPs) must combine high magnetic susceptibility for an optimum magnetic enrichment and loss of magnetization after removal of the magnetic field. Computational Fluid Dynamics (CFD) approach has been used to investigate the impact of a magnetic field in ferrofluid flow through a T-microchannel. The microchannel consists of one 400μm wide main branch and two 200μm wide sidebranches. Available experimental data is used to validate the Eulerian-Eulerian approach in simulating the nanoparticles in flow flow under the influence of magnetic field. In general, magnetic nanoparticles are deflected across the suspending ferrofluid by negative magnetophoresis and confined by a water flow to the center of the micro-channel. The effect of ferrofluid flow rate on the particle focusing performance has been examined. It is found that the particle focusing effectiveness increases with decreasing flow rate.

Effects of Operating Frequency of a Synthetic Jet and Cross Flow Velocity on the Heat Transfer Enhancement in a Micro-Channel

Current devices have been reported to approach 1 MW/m2 so that current heat dissipation devices will not be able to cope with increasing heat flux. It has therefore been proposed that in order to manage the ever-increasing heat rejection demands, it will be necessary to have cooling fluid flowing through micro-channels in the microchip itself. Since laminar flow is likely to result for reasonable pressure drops in these micro-channels, the heat transfer rate will need to be enhanced if this approach is to be successfully used. Synthetic jets, which are the main focus of this research, generate vortex structures which disrupt the flow. They have, therefore, been proposed as a means of providing mixing, thereby augmenting the heat transfer potential of the fluid in the micro-channel.A two-dimensional computational model has been developed to investigate the cooling effect of a synthetic jet interacting with a turbulent cross-flow in a micro-channel. Validation of the hydrodynamics feature of the flow was done by comparing numerical results against existing experimental results.A parametric study was performed on a fixed geometry by using a constant wall temperature to investigate the effect of operating frequency of the synthetic jet actuator coupling with different flow rates in the micro-channel. The operating frequencies of the jet were simulated at 1000 Hz, 1500 Hz and 2000 Hz while the cross flows vary from 0 to 10 m/s. In general, the flow structures in the micro-channel were shown to be greatly disrupted when the synthetic jet actuator was turned on. However, the heat transfer enhancement due to the operation of the synthetic jet reduces as the cross flow increases. The frequency of the diaphragm oscillation has a large influence on the distance between the adjacent vortices and therefore on the average flow rate in the micro-channel. The near wall Nusselt Number was calculated in order to compare the effects of operating frequency of the jet and flow rate in the micro-channel. The jet Reynolds number was increased by 50% when the actuator frequency was increased from 1000 Hz to 1500 Hz while the heat transfer enhancement was increased by 21%. Further increment of actuator frequency from 1000 Hz to 2000 Hz resulted in a doubled jet Reynolds number while the heat transfer enhancement was improved by 66%. The heat transfer enhancement showed greater improvement when the actuator operating at 2000 Hz.Copyright

Investigation of Phasing on Synthetic Jet Actuator Arrays

Unsteady computations of laminar flow have been performed for two-dimensional configurations of micro-channel equipped with two synthetic jets. The effect of phasing has been investigated at in-phase and 180̊ out-of-phase of the synthetic jet actuators at a fixed operating frequency and oscillating amplitudes. It was shown that the 180̊ out-of-phase configuration of the synthetic jets promotes better and more continuous flow mixing within the channel during the oscillation. This was due to the discrete pattern of vortex forming which disrupts the main channel flow. The 180̊ out-of-phase jet configuration exhibits higher cooling performance compared to the in-phase jet configuration in terms of the reduction in the maximum temperature in the silicon wafer.

An Assessment of Turbulence Models in Simulating a Synthetic Jet

This paper presents a two-dimensional numerical study on the interaction of synthetic jet and the cross flow inside a microchannel. Three different turbulence models namely the standard k-, Shear Stress Transport (SST) and Scale Adaptive Simulation Shear Stress Transport (SAS SST) were tested for their ability to predict the flow structure generated by a synthetic jet. The results are validated against existing experimental data. The SAS SST model was found to give the most realistic prediction of the fluid flow based on the good agreement with experimental data.

Experimental development and control of magnetorheological damper towards smart energy absorption of composite structures

Experimental investigation and efficient control of magnetorheological (MR) damper towards smart energy absorption of composite structures are presented in this paper. The evaluation of an existing MR damper based on the damping force presented in our earlier work is limited by the experiment configuration setup. Using two arms configuration, an experimental test rig is designed to overcome this limitation and enabled the MR damper to be investigated throughout its full velocity range capability. A controller is then developed based on the MR damper investigation to provide automated variable control of induced current with a set crushing force and available data of composite tube crushing force. The controller is assessed numerically and shows that MR damper is controlled to provide consistent crushing force despite oscillation from the composite tube crushing force. This, thus, shows promise of MR damper integration towards smart energy absorption of composite structures.

Forced convection in micro-channel with synthetic jet: Effect of operating frequency

A three-dimensional computational model has been developed to investigate the cooling effect on the microchip of synthetic jet interacting with a cross-flow in a micro-channel. The conjugate problem is solved by determining the temperature distributions in a heated solid and the fluid flowing in the micro-channel which cools it, thereby simulating the application to a microchip. A parametric study was performed on a fixed geometry by using 1 MWm−2 heat flux at the surface of the silicon wafer to investigate the effect of frequency of the jet at a constant Reynolds number, that is the amplitude is reduced in proportion to the increase in frequency. The hot region in the silicon wafer resulting from the use fluid flowing undisturbed in a micro-channel, are removed when the synthetic jet is switched on thereby significantly lowering the maximum temperature in the wafer. Contrary to the two-dimensional case, there is little difference in the cooling performance when the jet was driven at different frequencies in three-dimensional configuration. This is illustrated by the fact in the end of the simulations at a jet Reynolds number of 40, the maximum temperature in the substrate was 0.5 K lower at 1120 Hz than at 560 Hz and 1 K lower than at 280 Hz.Copyright

Three-dimensional modelling of heat transfer in micro-channels with synthetic jet

An in-house computer code is developed and applied to investigate the effect of a synthetic jet on heat transfer rates in forced convection of water in silicon micro-channels etched in the rear side of the silicon substrate. To account for the deflection of the membrane located at the bottom of the actuator cavity, a moving mesh technique to solve the flow and heat transfer is purposefully adopted. The governing equations are transformed into the curvilinear coordinate system in which the grid velocities evaluated are then fed into the computation of the flow in the cavity domain thus allowing the conservation equations of mass, momentum and energy to be solved within the stationary computational domain. The fully three-dimensional model considers the SIMPLE method to link the pressure and velocity. A heat flux of 1 MW/m2 is applied at the surface of the top of the silicon wafer and the resulting complex, conjugate heat transfer through the silicon substrate is included. The hydrodynamics feature of the flow is validated against existing experimental results and verified against numerical results from commercial package ANSYS CFX 11.0. Good agreement has been achieved. To track the development of the flow and heat transfer when the actuator is switched on, numerical results of 20 full cycles of the actuator are simulated. When the actuator is switched on, noticeable temperature drop is observed at all points in the substrate from those which existed when there has been a steady water flow in the channel. At the end of 20th cycle of actuation, the maximum temperature in the wafer has reduced by 5.4 K in comparison with the steady flow values. In comparison with the two-dimensional study which account for 17K reduction, it indicates that synthetic jet has only smaller beneficial cooling and has been over-estimated in the previous two-dimensional study.Copyright