Department of Mechanical Engineering

We are collaborating with Dr. Qilin Cao from University of Texas Health Science Center on developing a novel technique to fabricate virtual scaffolds for axonal regeneration after spinal cord injury (SCI). This research capitalizes on the ability of using magnetic field to manipulate neural stem cells, which are labeled with nanoengineered cationic magnetoliposomes, remotely and noninvasively, thereby guiding the directional growth of axons to repair the injured spinal cord. This work will not only benefit hundreds of thousands of Americans who are paralyzed or have severely limited mobility incurring from SCI, but it will also shift the paradigm for applications in regenerative medicine and tissue engineering which require directional guidance of the cells or tissues. This work is supported by NSF and DoD (through Methodist Hospital Research Institute).

Tunable adaptive nucleate boiling heat transfer surfaces with electrowetting

Together with Dr. Paul Ruchhoeft from Electrical and Computer Engineering at UH, we are exploiting the complimentary roles of hydrophobicity and hydrophilicity played in nucleate boiling and taking advantage of the ability of electrowetting to alter the surface wettability reversibly and robustly. This work will lead to a fundamental understanding of the effect of dynamically tunable surface wettability on nucleate boiling, and provide a previously unexplored and powerful tool to actively control and optimize the key nucleate boiling processes. This work is supported by NSF.

We have systematically investigated single-phase and two-phase flow physics of nanofluids, which are engineered colloids containing dispersed nanoparticles, and elucidated the crucial role of particle-fluid and particle-particle interactions in determining the thermophysical property and transport characteristics of nanofluids with both spherical and non-spherical nanoparticles. Our results revealed the critical aspects of the long-standing myth of nanofluids and their effectiveness as a potential candidate for advanced heat transfer fluids. This work will be very useful in the design of liquid cooling systems employing nanofluids for thermal management of next-generation electronics.

Friction factor vs. Re

Fig 13

Nusselt number vs. Re

Addition of surfactants to liquids helps to eliminate intermittent two-phase flow patterns and alleviate flow instability. These features are very desirable for two-phase microfluidic applications. However, very little information is available on the two-phase flow pattern of surfactant solution in microchannels. The present paper reports a study of adiabatic two-phase flow with surfactants in a circular microchannel of a 180-µm diameter. Air-water mixtures with trace quantities of sodium dodecyl sulfate (SDS) were used in the experiments. The maximum superficial velocities measured were 4 m/s for the liquid and 65 m/s for the gas. High-speed microphotographic technique was employed to visualize various two-phase flow patterns and identify the transition boundaries between flow regimes. The results were compared to data obtained from air-water flow without surfactants. It was found that addition of surfactants brings in significant modification to the two-phase flow regimes and their transition characteristics in microchannels, in particular, slug flow is effectively suppressed.

Study of Two-Phase Flow Patterns in a Microfluidic ChipAccurate determination of the two-phase flow patterns in microchannels is crucial to select appropriate predictive tools for pressure drop, heat and mass transfer in the microfluidic devices. Most of the prevailing two-phase flow maps were developed using visualization techniques and fail to reveal the fundamental mechanisms that are responsible for the formation of specific flow pattern under given flow conditions. In the present work, the high-speed photographic method is employed to study the liquid-gas two-phase flow in a cross-junction microfluidic chip with a rectangular cross section of 300 mm by 100 mm. The dynamics of bubbly, slug and annular flows are measured. Numerical models using the VOF approach are developed to simulate the two-phase mixing and flow pattern development in the cross-junction device. The roles of the inertia, viscous shear and surface tension forces in various flow patterns are discussed. The experimental results and the simulation data together provide a comprehensive phenomenological description of the key parameters and processes that govern the two-phase flow pattern formation and development in microfluidic devices.

Primary flow patterns	Numerical simulation vs. Visualization
Bubbly flow simulation	Slug flow simulation

Infrared Micro-Particle Image Velocimetry
A non-intrusive diagnostic technique, infrared micro-particle image velocimetry (IR-PIV), is developed for measuring flow fields within MEMS devices with micron-scale resolution. This technique capitalizes on the transparency of silicon in the infrared region, and overcomes the limitation posed by the lack of optical access with visible light to sub-surface flow in silicon-based micro- structures. Experiments with laminar flow of water in a circular micro-capillary tube of hydraulic diameter 255 µm demonstrate the efficacy of this technique. The experimental measurements agree very well with velocity profiles predicted from laminar theory. Cross-correlation and auto-correlation algorithms are employed to measure very-low and moderate-to-high velocities, respectively; the former approach is suitable for biomedical applications while the latter would be needed for measurements in electronics cooling. The results indicate that the IR-PIV technique effectively extends the application of regular micro-PIV techniques, and has great potential for flow measurements in silicon-based microdevices.

Flow boiling heat transfer to water in microchannels is experimentally investigated. The dimensions of the microchannels considered are 275 X 636 um and 406 X1063 um. The experiments are conducted at inlet water temperatures in the range of 67 to 95 C and mass fluxes of 221 to 1283 kg/m2s. The maximum heat flux investigated in the tests is 129 W/cm2 and the maximum exit quality is 0.2. Convective boiling heat transfer coefficients are measured and compared to predictions from existing correlations for larger channels. While an existing correlation was found to provide satisfactory prediction of the heat transfer coefficient in subcooled boiling in microchannels, saturated boiling was not well predicted by the correlations for macrochannels. A new superposition model is developed to correlate the heat transfer data in the saturated boiling regime in microchannel flows. In this model, specific features of flow boiling in microchannels are incorporated while deriving analytical solutions for the convection enhancement factor and nucleate boiling suppression factor. Good agreement with the experimental measurements indicates that this model is suitable for use in analyzing boiling heat transfer in microchannel flows.

Single-phase fluid flow and heat transport of water in microchannels is experimentally studied. Pressure drop measurement with two alternative approaches is presented to account for cases in which the influence of inlet/outlet plenums is pertinent or not, and the results are compared to laminar and turbulent predictions. Transition to turbulence and estimation of local pressure loss are also explored with flow visualization and numerical analysis. Heat transfer measurements are performed with an emphasis on correctly matching the entrance and thermal conditions when comparing the results to conventional heat transfer correlations developed for large-sized channels. The single-phase flow and heat transfer measurements confirm that carefully applied conventional theory and correlations are adequate in predicting transport characteristics in microchannels.