Copyright (c) 2009 Purdue Libraries All rights reserved. http://docs.lib.purdue.edu/coolingpubs Recent documents in Research Publications en-us Fri, 02 Oct 2009 12:54:38 PDT 3600 http://docs.lib.purdue.edu/coolingpubs/120 http://docs.lib.purdue.edu/coolingpubs/120 Thu, 11 Jun 2009 12:16:48 PDT This paper presents a microfluidic pumping approach using traveling-wave dielectrophoresis (twDEP) of microparticles. With this approach, the flow is generated directly in the microfluidic devices by inducing strong electromechanical effects in the fluid using integrated microelectrodes. The fluidic driving mechanisms due to the particle-fluid and particle-particle interactions under twDEP are analyzed, and the induced flow field is obtained from numerical simulations. Experimental measurements of the flow velocity in a prototype DEP micropumping device show satisfactory agreement with the numerical predications. Results from this work indicate that the DEP-induced micropumping scheme holds great promise for devising versatile, self-contained microscale fluidic delivery systems. D Liu http://docs.lib.purdue.edu/coolingpubs/119 http://docs.lib.purdue.edu/coolingpubs/119 Thu, 11 Jun 2009 11:57:47 PDT lPIV is a widely accepted tool for making measured. This method allows simultaneous non-intrusive temperature and velocity measurements in integrated accurate measurements in microscale flows. The particles cooling systems and lab-on-a-chip devices. that are used to seed the flow, due to their small size, undergo Brownian motion which adds a random noise component to the measurements. Brownian motion intro- duces an undesirable error in the velocity measurements, but also contains valuable temperature information. A PIV algorithm which detects both the location and broadening of the correlation peak can measure velocity as well as temperature simultaneously using the same set of images. The approach presented in this work eliminates the use of the calibration constant used in the literature (Hohreiter et al. in Meas Sci Technol 13(7):1072-1078, 2002), mak- ing the method system-independent, and reducing the uncertainty involved in the technique. The temperature in a stationary fluid was experimentally measured using this technique and compared to that obtained using the particle tracking thermometry method and a novel method, low image density PIV. The method of cross-correlation PIV was modified to measure the temperature of a moving fluid. A standard epi-fluorescence lPIV system was used for all the measurements. The experiments were conducted using spherical fluorescent polystyrene-latex particles suspended in water. Temperatures ranging from 20 to 80°C were Pramod Chamarthy http://docs.lib.purdue.edu/coolingpubs/118 http://docs.lib.purdue.edu/coolingpubs/118 Thu, 11 Jun 2009 11:57:45 PDT Piezoelectric fans are thin elastic beams whose vibratory motion is actuated by means of a piezoelectric material bonded to the beam. These fans have found use as a means to enhance convective heat transfer while requiring only small amounts of power. The objective of the present work is to quantify the influence of each operational parameter and its relative impact on thermal performance. Of particular interest are the vibration frequency and amplitude as well as the geometry of the vibrating cantilever beam. The experimental setup consists of a piezoelectric fan mounted normal to a constant heat flux surface. Temperature contours on this surface captured via an infrared camera are used to extract the forced convection coefficient due to the fluid motion generated from the fan. Different fans, with fundamental resonance frequencies ranging from 60 to 250 Hz, are considered. Results show that the performance of the fans is maximized at a particular value of the gap between the fan tip and the heated surface. It is found that when a fan operates at this optimum gap, the heat transfer rate is dependent only on the frequency and amplitude of oscillation. Correlations based on appropriately defined dimensionless parameters are developed and found to successfully predict the thermal performance across the entire range of fan dimensions, vibration frequency and amplitude. An understanding of the dependence of thermal performance on the governing variables allows for improved design of piezoelectric fans as a method of enhancing heat transfer. Mark L. Kimber http://docs.lib.purdue.edu/coolingpubs/117 http://docs.lib.purdue.edu/coolingpubs/117 Thu, 11 Jun 2009 11:57:44 PDT Microscale fluid flow using traveling-wave induction electrohydrodynamics is demonstrated. A three-phase traveling-wave device fabricated for the experiments provides a temporally and spatially varying electric field which helps induce ions in a fluid that is subjected to a temperature gradient. These ions are moved as the traveling wave propagates, resulting in a drag force being exerted on the surrounding fluid. Repulsion-type electrohydrodynamic flow is visualized in a microchannel of depth 50 μm, and results are presented in terms of velocity measurements using particle image velocimetry. The effects of voltage, traveling-wave frequency and the addition of externally applied heat are demonstrated and heat transfer capabilities of the micropump are discussed. Brian D. Iverson http://docs.lib.purdue.edu/coolingpubs/116 http://docs.lib.purdue.edu/coolingpubs/116 Thu, 11 Jun 2009 11:57:42 PDT The melting of a particle-laden slurry in a cylinder is investigated. The slurry consists of neutrally buoy- ant ceramic hollow spheres suspended in a paraffin wax. Melt front propagation and heat transfer pro- cesses during phase change in this particle-laden material is studied. The numerical analysis employs a particle-diffusive model and the enthalpy method. Experiments are carried out to validate the numer- ical model. The experimental boundary conditions are adequately represented with a constant and uni- form side-wall temperature, a constant and uniform lower-surface temperature, and an adiabatic top wall. Reasonable agreement is obtained between the experiments and numerical predictions. It is found that the flow and heat transfer characteristics of the melt are greatly altered due to the presence of the solid particles and that the particle-diffusive model is insufficient to describe the particle migration during melting. Dawei Sun http://docs.lib.purdue.edu/coolingpubs/115 http://docs.lib.purdue.edu/coolingpubs/115 Thu, 11 Jun 2009 11:57:41 PDT Experiments are conducted with a perfluorinated dielectric fluid, Fluorinert FC-77, to investigate the effects of channel size and mass flux (225-1420 kg/m2s) on microchannel flow boiling regimes by means of high-speed photography. Seven different silicon test pieces with parallel microchannels of widths ranging from 100 to 5850 lm, all with a depth of 400 lm, are considered. Flow visualizations are per- formed with a high-speed digital video camera while local measurements of the heat transfer coefficient are simultaneously obtained. The visualizations and the heat transfer data show that flow regimes in the microchannels of width 400 lm and larger are similar, with nucleate boiling being dominant in these channels over a wide range of heat flux. In contrast, flow regimes in the smaller microchannels are dif- ferent and bubble nucleation at the walls is suppressed at a relatively low heat flux for these sizes. Two types of flow regime maps are developed and the effects of channel width on the flow regime tran- sitions are discussed. Tannaz Harirchian http://docs.lib.purdue.edu/coolingpubs/114 http://docs.lib.purdue.edu/coolingpubs/114 Thu, 11 Jun 2009 11:57:39 PDT The theoretical performance of a planar microscale ion generation device is analysed using the direct simulation Monte Carlo (DSMC) technique. The discrete motion and interactions of electrons and ions are modelled for atmospheric air as represented by N2 and O2 . The ionization threshold of the device in air is found to be 70 V because of the effects of molecular excitations that reduce the energy of the free electrons and the nature of the collision cross sections. Additionally, microscale planar ionization devices are revealed to be inherently inefficient because of the loss of electrons and ions to the dielectric boundary. A multiscale simulation of the electrohydrodynamics is also conducted by extracting the ion-neutral interactions from the DSMC calculations and integrating them into a continuum-scale fluid dynamics model. The multiscale simulations show that the ion-neutral body force distribution for the planar devices is concentrated on the face of the cathode and therefore limits the impact of the force on the flow. A scale analysis confirms that the body force distribution is insufficient to induce high flow rates at this scale. D. B. Go http://docs.lib.purdue.edu/coolingpubs/113 http://docs.lib.purdue.edu/coolingpubs/113 Thu, 11 Jun 2009 11:57:38 PDT Recent reviews of flow boiling heat transfer in small tubes and channels have highlighted the need for predictive correlations that are applicable over a wide range of parameters and across different studies. A composite correlation is developed in the present work which includes nucleate boiling and convective heat transfer terms while accounting for the effect of bubble confinement in small channels. The correlation is developed from a database of 3899 data points from 14 studies in the literature covering 12 different wetting and non-wetting fluids, hydraulic diameters ranging from 0.16 to 2.92 mm, and confinement numbers from 0.3 to 4.0. The mass fluxes included in the database range from 20 to 3000 kg mÀ2 sÀ1, the heat fluxes from 0.4 to 115 W cmÀ2, the vapor qualities from 0 to 1, and the saturation temperatures from À194 to 97 °C. While some of the data sets show opposing trends with respect to some parameters, a mean absolute error of less than 30% is achieved with the proposed correlation. S S. Bertsch http://docs.lib.purdue.edu/coolingpubs/112 http://docs.lib.purdue.edu/coolingpubs/112 Thu, 11 Jun 2009 11:57:36 PDT Traveling-wave electrohydrodynamic (EHD) micropumps can be incorporated into the package of an integrated circuit chip to provide active cooling. They can also be used for fluid delivery in microdevices. The pump operates in the presence of a thermal gradient through the fluid layer such that a gradient in electrical conductivity is established allowing ions to be induced. These ions are driven by a traveling electric field. Such a traveling electric field can be realized in practice only via discrete electrodes upon which the required voltages are imposed. The impact of using discrete electrodes to create the traveling wave on the flow rates generated is explored through numerical modeling. The change in performance from an ideal sinu- soidal voltage boundary condition is quantified. The model is used to explore the widths of electrodes and the inter- vening isolation regions that lead to optimized pumping. The influence of the choice of working fluid on the per- formance of the pump is determined using an analytical model. B D. Iverson http://docs.lib.purdue.edu/coolingpubs/111 http://docs.lib.purdue.edu/coolingpubs/111 Thu, 11 Jun 2009 11:57:25 PDT Flow boiling heat transfer with the refrigerants R-134a and R-245fa in copper microchannel cold plate evaporators is investigated. Arrays of microchannels of hydraulic diameter 1.09 and 0.54 mm are consid- ered. The aspect ratio of the rectangular cross section of the channels in both test sections is 2.5. The heat transfer coefficient is measured as a function of local thermodynamic vapor quality in the range À0.2 to 0.9, at saturation temperatures ranging from 8 to 30 °C, mass flux from 20 to 350 kg mÀ2 sÀ1, and heat flux from 0 to 22 W cmÀ2. The heat transfer coefficient is found to vary significantly with heat flux and vapor quality, but only slightly with saturation pressure and mass flux for the range of values investi- gated. It was found that nucleate boiling dominates the heat transfer. In addition to discussing measure- ment results, several flow boiling heat transfer correlations are also assessed for applicability to the present experiments. S S. Bertsch