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Sharar D.J.,General Technical Services LLC | Bar-Cohen A.,University of Maryland University College
Journal of Enhanced Heat Transfer

The limited accuracy and parametric range of two-phase heat transfer correlations for internallygrooved tubes have impeded the widespread application of this most promising technology. The success of regime-based analyses and correlations in providing improved predictive accuracy for heat transfer coefficients in smooth tubes has motivated this effort to explore the relationship between two-phase flow regimes and heat transfer rates in internally-grooved tubes. Following a brief introduction to the geometries and manufacturing techniques of internally-grooved tubes and a description of the state-of-The-Art smooth tube flow regime maps, fundamental studies of thermofluid performance in internally-grooved tubes are reviewed and analyzed to demonstrate the relationship between flow regime and evaporative heat transfer rates. Then, the current state of two-phase flow regime maps and heat transfer coefficient correlations for internallygrooved tubes are summarized. Finally, recommendations for future internally-grooved tube research directions are given. The majority of the studies herein deal with halogenated fluids in conventional-sized tubes at standard temperature and pressure. However, studies of small diameter tubes, as well as alternative refrigerants and reduced pressure, are also considered. © 2014 by Begell House, Inc. Source

Oshman C.,University of Colorado at Boulder | Li Q.,University of Colorado at Boulder | Liew L.-A.,University of Colorado at Boulder | Yang R.,University of Colorado at Boulder | And 5 more authors.
Journal of Micromechanics and Microengineering

This paper presents the fabrication and application of a micro-scale hybrid wicking structure in a flat polymer-based heat pipe heat spreader, which improves the heat transfer performance under high adverse acceleration. The hybrid wicking structure which enhances evaporation and condensation heat transfer under adverse acceleration consists of 100 m high, 200 m wide square electroplated copper micro-pillars with 31 m wide grooves for liquid flow and a woven copper mesh with 51 m diameter wires and 76 m spacing. The interior vapor chamber of the heat pipe heat spreader was 30×30×1.0 mm3. The casing of the heat spreader is a 100 m thick liquid crystal polymer which contains a two-dimensional array of copper-filled vias to reduce the overall thermal resistance. The device performance was assessed under 0-10 g acceleration with 20, 30 and 40 W power input on an evaporator area of 8×8 mm2. The effective thermal conductivity of the device was determined to range from 1653 W (m K)-1 at 0 g to 541 W (m K)-1 at 10 g using finite element analysis in conjunction with a copper reference sample. In all cases, the effective thermal conductivity remained higher than that of the copper reference sample. This work illustrates the possibility of fabricating flexible, polymer-based heat pipe heat spreaders compatible with standardized printed circuit board technologies that are capable of efficiently extracting heat at relatively high dynamic acceleration levels. © 2012 IOP Publishing Ltd. Source

Nochetto H.C.,General Technical Services LLC | Jankowski N.R.,U.S. Army | Bar-Cohen A.,Darpa
ASME 2011 International Mechanical Engineering Congress and Exposition, IMECE 2011

The present work uses finite element thermal simulations of Gallium Nitride High Electron Mobility Transistors (GaN HEMTs) to evaluate the impact of device design parameters on the junction temperature. In particular the effects of substrate thickness, substrate thermal conductivity, GaN thickness, and GaN-to-substrate thermal boundary resistance (TBR) on device temperature rise are quantified. In all cases examined, the TBR was a dominant factor in overall device temperature rise. It is shown that a TBR increase can offset any benefits offered through a more conductive substrate and that there exists a substrate thickness independent of TBR which results in a minimum junction temperature. Additionally, the decrease of GaN thickness only provides a thermal benefit at small TBRs. For TBRs on the order of 10-4 cm2K/W or greater, decreasing the GaN thickness can actually increase the temperature as the heat from the highly localized source is not sufficiently spread out before crossing the GaN-substrate boundary. The tradeoff between GaN heat spreading, substrate heat spreading, and temperature rise across the TBR results in a GaN thickness with minimum total temperature rise. For the TBR values of 10 -4 cm2K/W and 10-3 cm2K/W these GaN thicknesses are 0.8 νm and 9 νm respectively. Copyright © 2011 by ASME. Source

Sharar D.J.,General Technical Services LLC | Jankowski N.R.,U.S. Army | Bar-Cohen A.,University of Maryland University College
ASME 2013 International Technical Conference and Exhibition on Packaging and Integration of Electronic and Photonic Microsystems, InterPACK 2013

The absence of phenomenological insights and accurate flow regime models makes it difficult to predict the improved effectiveness of internally-grooved tubes for two-phase heat transfer. A re-interpretation of available data and flow regime maps is used to suggest that performance improvement is a result of early transition to Annular flow. A modified flow regime map, with a newly-developed Stratified-Wavy to Annular transition criteria for internally-grooved tubes, is shown to increase regime prediction accuracy by 27% relative to the traditional, smooth tube flow regime prediction. © 2014 ASME. Source

Sharar D.J.,General Technical Services LLC | Bergles A.E.,University of Maryland University College | Jankowski N.R.,U.S. Army | Bar-Cohen A.,University of Maryland University College
ASME 2014 12th International Conference on Nanochannels, Microchannels, and Minichannels, ICNMM 2014, Collocated with the ASME 2014 4th Joint US-European Fluids Engineering Division Summer Meeting

A non-intrusive optical method for two-phase flow pattern identification was developed to validate flow regime maps for two-phase adiabatic flow in a small diameter tube. Empirical measurements of film thickness have been shown to provide objective identification of the dominant two-phase flow regimes, representing a significant improvement over the traditional use of exclusively visual and verbal descriptions. Use of this technique has shown the Taitel-Dukler, Ullmann-Brauner, and Wojtan et al. phenomenological flow regime mapping methodologies to be applicable, with varying accuracy, to small diameter two-phase flow. Copyright © 2014 by ASME. Source

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