Yoav Peles

Nanostructured Copper Interfaces for Enhanced Boiling

Phase change through boiling is used in a variety of heat-transfer and chemical reaction applications. The state of the art in nucleate boiling has focused on increasing the density of bubble nucleation using porous structures and microchannels with characteristic sizes of tens of micrometers. Traditionally, it is thought that nanoscale surfaces will not improve boiling heat transfer, since the bubble nucleation process is not expected to be enhanced by such small cavities. In the experiments reported here, we observed unexpected enhancements in boiling performance for a nanostructured copper (Cu) surface formed by the deposition of Cu nanorods on a Cu substrate. Moreover, we observed striking differences in the dynamics of bubble nucleation and release from the Cu nanorods, including smaller bubble diameters, higher bubble release frequencies, and an approximately 30-fold increase in the density of active bubble nucleation sites. It appears that the ability of the Cu surface with nanorods to generate stable nucleation of bubbles at low superheated temperatures results from a synergistic coupling effect between the nanoscale gas cavities (or nanobubbles) formed within the nanorod interstices and micrometer-scale defects (voids) that form on the film surface during nanorod deposition. For such a coupled system, the interconnected nanoscale gas cavities stabilize (or feed) bubble nucleation at the microscale defect sites. This is distinct from conventional-scale boiling surfaces, since for the nanostructured surface the bubble nucleation stability is provided by features with orders-of-magnitude smaller scales than the cavity-mouth openings.

Suppression of Boiling Flow Oscillations in Parallel Microchannels by Inlet Restrictors

parallel channel and upstream compressible volume instabilities commonly exhibited during flow boiling in parallel microchannels have been investigated. The heat fluxes at the onset of unstable boiling have been obtained over effective heat fluxes ranging from 9t o614 W/cm 2 and mass fluxes from 115 to 389 kg/m 2 s. A dimensionless parameter M, which accounts for the pressure drop increase imposed by the inlet restrictors, has been used to correlate the extent of flow instability suppression. It has been shown that the onset of unstable boiling asymptotically increases with M. At sufficiently high M values, parallel channels and upstream compressible volume instabilities are completely eradicated although it gives way to another instability to develop, namely, the critical heat flux conditions. A correlation has been developed in terms of M to predict the conditions leading to unstable boiling. DOI: 10.1115/1.2150837

Laminar Flow Across a Bank of Low Aspect Ratio Micro Pin Fins

Pressure drops and friction factors associated with the forced flow of de-ionized water over staggered and in-line circular/diamond shaped micro pin-fin bundles 100 μm long with hydraulic diameter of 50 and 100 μm have been experimentally investigated over Reynolds number ranging from 5 to 128. Pin fins were arranged according to two different horizontal and vertical pitch ratios (1.5 and 5). The applicability of conventional scale correlations to evaluate micro flow tests results has been assessed

Superhydrophobic Graphene Foams

The static and dynamic wetting properties of a 3D graphene foam network are reported. The foam is synthesized using template-directed chemical vapor deposition and contains pores several hundred micrometers in dimension while the walls of the foam comprise few-layer graphene sheets that are coated with Teflon. Water contact angle measurements reveal that the foam is superhydrophobic with an advancing contact angle of ∼163 degrees while the receding contact angle is ∼143 degrees. The extremely water repellent nature of the foam is also confirmed when impacting water droplets are able to completely rebound from the surface. Such superhydrophobic graphene foams show potential in a variety of applications ranging from anti-sticking and self-cleaning to anti-corrosion and low-friction coatings.

Heat Transfer in Microchannels—2012 Status and Research Needs

Heat transfer and fluid flow in microchannels have been topics of intense research in the past decade. A critical review of the current state of research is presented with a focus on the future research needs. After providing a brief introduction, the paper addresses six topics related to transport phenomena in microchannels: single-phase gas flow, enhancement in single-phase liquid flow and flow boiling, flow boiling instability, condensation, electronics cooling, and microscale heat exchangers. After reviewing the current status, future research directions are suggested. Concerning gas phase convective heat transfer in microchannels, the antagonist role played by the slip velocity and the temperature jump that appear at the wall are now clearly understood and quantified. It has also been demonstrated that the shear work due to the slipping fluid increases the effect of viscous heating on heat transfer. On the other hand, very few experiments support the theoretical models and a significant effort should be made in this direction, especially for measurement of temperature fields within the gas in microchannels, implementing promising recent techniques such as molecular tagging thermometry (MTT). The single-phase liquid flow in microchannels has been established to behave similar to the macroscale flows. The current need is in the area of further enhancing the performance. Progress on implementation of flow boiling in microchannels is facing challenges due to its lower heat transfer coefficients and critical heat flux (CHF) limits. An immediate need for breakthrough research related to these two areas is identified. Discussion about passive and active methods to suppress flow boiling instabilities is presented. Future research focus on instability research is suggested on developing active closed loop feedback control methods, extending current models to better predict and enable superior control of flow instabilities. Innovative high-speed visualization and measurement techniques have led to microchannel condensation now being studied as a unique process with its own governing influences. Further work is required to develop widely applicable flow regime maps that can address many fluid types and geometries. With this, condensation heat transfer models can progress from primarily annular flow based models with some adjustments using dimensionless parameters to those that can directly account for transport in intermittent and other flows, and the varying influences of tube shape, surface tension and fluid property differences over much larger ranges than currently possible. Electronics cooling continues to be the main driver for improving thermal transport processes in microchannels, while efforts are warranted to develop high performance heat exchangers with microscale passages. Specific areas related to enhancement, novel configurations, nanostructures and practical implementation are expected to be the research focus in the coming years.

Bubble Dynamics During Boiling in Enhanced Surface Microchannels

An experimental investigation is presented on flow boiling of deionized water in 227-mum hydraulic diameter microchannels with reentrant type cavities. Key features of nucleate boiling are discussed. Active nucleation site density, bubble frequency and departure diameter, and flow patterns over mass velocities ranging from 41 kg/m2-s to 302 kg/m2-s and heat fluxes ranging from 28 to 445 W/cm2 are studied. Similarities and differences with results obtained on large-scale systems and unenhanced microchannels are discussed

Reduced Pressure Boiling Heat Transfer in Rectangular Microchannels With Interconnected Reentrant Cavities

Boiling flow of deionized water through 227 μm hydraulic diameter microchannels with 7.5 μm wide interconnected reentrant cavities at 47 kPa exit pressure has been investigated. Average two-phase heat transfer coefficients have been obtained over effective heat fluxes ranging from 28 to 445 W/cm 2 and mass fluxes from 41 to 302 kg/m 2 s. A map is developed that divides the data into two regions where the heat transfer mechanisms are nucleation or convective boiling dominant. The map is compared to similar atmospheric exit pressure data developed in a previous study. A boiling mechanism transition criterion based on the Reynolds number and the Kandlikar k 1 number is proposed.

Flow Boiling Heat Transfer on Micro Pin Fins Entrenched in a Microchannel

Flow boiling of 1-methoxyheptafluoropropane (HFE 7000) in 222 μm hydraulic diameter channels containing a single row of 24 inline 100 μm pin fins was studied for mass fluxes from 350 kg/m2 s to 827 kg/m2 s and wall heat fluxes from 10 W/cm2 to 110 W/cm2. Flow visualization revealed the existence of isolated bubbles, bubbles interacting, multiple flow, and annular flow. The observed flow patterns were mapped as a function of the boiling number and the normalized axial distance. The local heat transfer coefficient during subcooled boiling was measured and found to be considerably higher than the corresponding single-phase flow. Furthermore, a thermal performance evaluation comparison with a plain microchannel revealed that the presence of pin fins considerably enhanced the heat transfer coefficient.

Critical Heat Flux of R-123 in Silicon-Based Microchannels

Critical heat flux (CHF) of R-123 in a silicon-based microchannel heat sink was investigated at exit pressures ranging from 227 kPa to 520 kPa. Critical heat flux data were obtained over effective heat fluxes ranging from 53 W/cm 2 to 196 W/cm 2 and mass fluxes from 291 kg/m 2 s to 1118 kg/m 2 s. Flow images and high exit qualities suggest that dryout is the leading CHF mechanism. The effect of mass velocity, exit quality, and system pressure were also examined, and a new correlation is presented to represent the effect of these parameters.

TCPT-2006-096.R2: Micro Scale pin fin Heat Sinks —Parametric Performance Evaluation Study

A parametric study of heat transfer and pressure drop associated with forced flow of deionized water over five micro pin fin heat sinks of different spacing, arrangements, and shapes was conducted experimentally. Nusselt numbers and friction factors were obtained over Reynolds numbers ranging from 14 to 720. The thermal and hydraulic results were obtained to evaluate and compare the heat sinks performances at fixed mass flow rate, fixed pressure drop, and fixed pumping power. Two distinct regions of the Nusselt number dependency on the Reynolds number separated by a critical Reynolds number have been identified for unstreamlined pin fin devices while the streamlined device showed no slope change. The effects of spacing, shape of pin fins, and arrangement on friction factor and heat transfer were in agreement with existing literature. The results indicate that utilizing streamlined pin fin heat sinks can significantly enhance the thermal-hydraulic performance of the heat sink, but only at moderate Reynolds numbers.