Orthotropic Thermal Conductivity Effect on Cylindrical Pin Fin Heat Transfer,”

ABSTRACT There is growing interest in the use of polymer composites with enhanced thermal conductivity for high performance fin arrays and heat sinks. However, the thermal conductivity of these materials is relatively low compared to conventional fin metals, and strongly orthotropic. Therefore, the design and optimization of such polymer pin fins requires extension of the one dimensional classical fin analysis to include twodimensional orthotropic heat conduction effects. An analytical equation for heat transfer from a cylindrical pin fin with orthotropic thermal conductivity is derived and validated using detailed finite-element results. The thermal performance of such fins was found to be dominated by the axial thermal conductivity, but to depart from the classical fin solution with increasing values of a radius-and radial conductivity-based Biot number. Using these relations, it is determined that fin orthotropy does not materially affect the behavior of typical air-cooled fins. Alternatively, for heat transfer coefficients achievable with water cooling and conductivity ratios below 0.1, the fin heat transfer rate can fall more than 25% below the "classical" heat transfer rates. Detailed orthotropic fin temperature distributions are used to explain this discrepancy. Simplified orthotropic pin fin heat transfer equations are derived and validated over a wide range of orthotropic conditions

Orientation Effects in Two-Phase Microgap Flow

The high power density of emerging electronic devices is driving the transition from remote cooling, which relies on con-duction and spreading, to embedded cooling, which extracts dissipated heat on-site. Two-phase microgap coolers employ the forced flow of dielectric fluids undergoing phase change in a heated channel within or between devices. Such coolers must work reliably in all orientations for a variety of applications (e.g., vehicle-based equipment), as well as in microgravity and high-g for other applications (e.g., spacecraft and aircraft). The lack of acceptable models and correlations for orientation- and gravity-independent operation has limited the use of two-phase coolers in such applications. Previous research has revealed that gravita-tional acceleration plays a diminishing role in establishing flow regimes and transport rates as the channel size shrinks, but there is considerable variation among the proposed microscale criteria and limited research on two-phase flows in low aspect ratio mi-crogap channels. Reliable criteria for achieving orientation- and gravity-independent flow boiling would enable emerging sys-tems to exploit this thermal management technique and streamline the technology development process. As a first step toward understanding the effect of gravity on two-phase microgap flow and transport, in the present effort the authors have studied the effect of evaporator orientation and mass flux on near-saturated flow boiling of HFE7100 in a 1.01 mm tall by 13.0 mm wide by 12.7 mm long microgap channel. Orientation-independence, defined as achieving similar critical heat fluxes, heat transfer coefficients, and flow regimes across evaporator orientations, was achieved for mass fluxes of 400 kg/m2-s and greater. The present results are compared to pub-lished criteria for achieving gravity-independence

Modeling and Prediction of Two-Phase Refrigerant Flow Regimes and Heat Transfer Characteristics in

ABSTRACT This keynote lecture will open with a brief review of the primary two-phase flow regimes and their impact on thermal transport phenomena in tubes and channels. The Taitel and Dukler flow regime mapping methodology will then be described and applied to the two-phase flow of refrigerants and dielectric liquids in microgap channels

Boiling and condensation in a liquid-filled enclosure

A combined experimental and analytical investigation of boiling and condensation in a liquid-filled enclosure, with water and Freon- 113 as the working fluids, is described. The operating characteristics of a boiling system, utilizing a condenser submerged in the fluid, are presented and related to specific operational modes and thermal transport mechanisms. A lower bound of operation, corresponding to natural convection heat transfer at both the heated and condenser surfaces, is identified. Similarly, for the commonly encountered range of system operation, a condensive upper bound is identified and shown to correspond to vapor space condensation. A nondimensional vapor bubble collapse length, L c/W, is found to govern the rate and mechanism of heat transfer at the submerged condenser surface. LValues of wC > 1 condensation is found to dominate thermal transport at the condenser surface. 4 possible technique for augmenting condensation heat transfer on horizontal surfaces is examined in an attempt to raise the condensive upper bound of submerged condenser operation. A doubly-rippled surface with small, constant radius of curvature undulations is shown to yield a factor of two increases in the rate of vapor space condensation based on the projected area of the condenser surface.(cont.) A systematic design procedure for submerged condenser systems utilizing the proposed models and correlations is described and related to typical design considerations

Orientation Effects in Two-Phase Microgap Flow

The high power density of emerging electronic devices is driving the transition from remote cooling, which relies onconduction and spreading, to embedded cooling, which extracts dissipated heat on-site. Two-phase microgap coolersemploy the forced flow of dielectric fluids undergoing phase change in a heated channel within or between devices. Such coolers must work reliably in all orientations for a variety of applications (e.g., vehicle-based equipment), as well as in microgravity and high-g for other applications (e.g., spacecraft and aircraft). The lack of acceptable models andcorrelations for orientation- and gravity-independent operation has limited the use of two-phase coolers in suchapplications. Previous research has revealed that gravitational acceleration plays a diminishing role in establishing flow regimes and transport rates as the channel size shrinks, but there is considerable variation among the proposed microscale criteria and limited research on two-phase flows in low aspect ratio microgap channels. Reliable criteria for achieving orientation- and gravity-independent flow boiling would enable emerging systems to exploit this thermal management technique and streamline the technology development process

Boiling and condensation in a liquid-filled enclosure,

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 1971.Vita.Bibliography: leaves 97-99.by Avram Markowitz.Ph.D