L. S. Fletcher

Recent Developments in Contact Conductance Heat Transfer

The characteristics of thermal contact conductance are increasingly important in a wide range of technologies. As a consequence, the number of experimental and theoretical investigations of contact conductance has increased. This paper reviews and categorizes recent developments in contact conductance heat transfer. Among the topics included are the theoretical/analytical/numerical studies of contact conductance for conforming surfaces and other surface geometries; the thermal conductance in such technological areas as advanced or modern materials, microelectronics, and biomedicine; and selected topics including thermal rectification, gas conductance, cylindrical contacts, periodic and sliding contacts, and conductance measurements. The paper concludes with recommendations for emerging and continuing areas of investigation.

The effect of plate spacing on free convection between heated parallel plates

In the past, considerable attention has been given to free convection between heated vertical parallel plates. This problem is considerable interest to engineers because of its application to electronic equipment cooling and solar energy. Some attempts have been made to optimize the spacing between parallel plates in the past. Bodoia and Osterle analytically derived a criterion for an optimum plate spacing for which the heat dissipation is maximum. The objective of this paper is to predict the optimum plate spacing for single channels by using the governing equations for a continuous system model. No heat transfer coefficient known a priori will be used in these calculations, but will be calculated as part of the solution.

Heat Transfer Through Falling Film Evaporation and Boiling on Horizontal Tubes

Evaporation and boiling heat transfer coefficients are presented for thin, distilled water films flowing over the outside of horizontal, electrically heated brass tubes. Tests were conducted with a thin-slot water distribution system for 2.54- and 5.08-cm-dia smooth tubes. Both local and average heat transfer data were obtained for nonboiling and boiling conditions corresponding to feedwater temperatures ranging from 49 to 127C and heat-flux values ranging from 30 to 80 kW/m{sup 2}. Feedwater flow rates ranged from 0.135 to 0.366 kg/s per meter length per side of the tube. Both nonboiling and boiling correlations of the average heat transfer coefficients were developed and compared.

Review of Models for Thermal Contact Conductance of Metals

Thermal contact conductance is a topic of great relevance to such applications as electronics packaging, satellite thermal control, nuclear reactor cooling, aerodynamic heating of supersonic aircraft and missiles, and turbine and internal combustion engine design. A fundamental problem in this discipline concerns the contact of metals in a vacuum environment for which gap and radiative conductance are negligible and only contact (solid spot) conductance is appreciable. A number of conductance models for metals are compiled and compared to experimental data from the literature. Theoretical models have been developed that accurately predict contact conductance for the two bounding cases of flat, rough surfaces and nonflat (spherical), smooth surfaces. However, these do not agree with most results for arbitrarily nonflat, rough surfaces, those usually obtained from common manufacturing processes. Empirical and semiempirical correlations, although many are developed for nonflat, rough surfaces, also suffer from limited applicability. The few theoretical models for nonflat (spherical), rough surfaces are computationally intensive and are not readily applied to design.

HEAT TRANSFER IN A THREE-DIMENSIONAL CHANNEL WITH BAFFLES

A numerical investigation of laminar forced convective heat transfer was performed in a three-dimensional channel with baffles in which a uniform heat flux was applied to the top and bottom walls, and the sidewalls were considered adiabatic. The trade-off between heat transfer enhancement and pressure drop produced by the baffles was studied for periodically fully developed flow (PDF). The numerical analysis was performed using a finite volume approach. The computer code was validated against the experimental results of Goldstein and Kreid [1] and Beavers et al. [2] for a three-dimensional channel without baffles. Parametric runs were made for Reynolds numbers of 150, 250, 350, and 450 for baffle height to channel width ratios (H/D y ) of 0.5, 0.6, 0.7, and 0.8. Heat transfer behavior was studied for Prandtl numbers of 0.7 and 7.0, and for wall thermal conductivity to fluid thermal conductivity ratios (K) of 1, 10, 100, and 1000.

A review of thermal enhancement techniques for electronic systems

The demand for electronic components which will satisfy performance standards over a wide range of environmental conditions requires the use of thermal enhancement techniques. Recent thermal enhancement techniques for maximizing the thermal contact conductance, including greases, metallic foils and screens, composite materials and cements, and surface treatments are reviewed. The relative merits of the various enhancement techniques are summarized, and comparisons are made for selected thermal enhancement materials. For most electronic component uses, the use of greases poses a significant problem because of the potential for vaporization and/or migration to other surfaces in the component. The use of metallic foils and screens can provide thermal enhancement at an interface, depending upon the material properties as well as the contact temperature and applied load on the interface. There are a number of synthetic materials which can be used for thermal enhancement, and new materials are continuously being developed. Surface preparation or selected surface finishes can be used to enhance the heat transfer at an interface. Thin layers of vapor-deposited soft metals have been shown to improve the overall heat transfer at an interface significantly. >

Free Convection Between Series of Vertical Parallel Plates With Embedded Line Heat Sources

Laminar free convective heat transfer in channels formed between series of vertical parallel plates with an embedded line heat source was studied numerically. These channels resemble cooling passages in electronic equipment. The effect of a repeated boundary condition and wall conduction on mass flow rate (M), maximum surface temperature ({theta}{sub h,max} and {theta}{sub c,max}), an average surface Nusselt number (Nu{sub h} and Nu{sub c}) is discussed. Calculations were made for Gr* = 10 to 10{sub 6}, K = 0.1, 1, 10, and 100, and t/B = 0.1 and 0.3. The effect of a repeated boundary condition decreases the maximum hot surface temperature and increases the maximum cold surface temperature. The effect of a repeated boundary condition with wall conduction increases the mass flow rate. The maximum increase in mass flow rate due to wall conduction is found to be 155%. The maximum decrease in average hot surface Nusselt number due to wall conduction (t/B and K) occurs at Gr* = 10{sup 6} and is 18%. Channels subjected to a repeated boundary condition approach that of a symmetrically heated channel subjected to uniform wall temperature conditions at K {ge} 100.

Flow Structure and Enhanced Heat Transfer in Channel Flow With Dimpled Surfaces: Application to Heat Sinks in Microelectronic Cooling

The use of dimple technology for improvement in friction factors and enhancement of heat transfer has been attracting the attention of many scientists and engineers. Numerical and experimental studies have shown there is a positive improvement (two-fold on average) in Nusselt number when dimpled surfaces are compared to flat plates, and this improvement is achieved with pressure drop penalties that are small when compared to other more intrusive types of turbulence promoters. When arrays of specific dimple geometry are used, pressure drop penalties are roughly equivalent to the heat transfer improvement. This, at least theoretically, will enable the design of smaller heat transfer devices such as heat sinks, which are especially appealing in those applications where size is an important design factor. A literature review of numerical modeling and experiments on flow over dimpled surfaces was performed, and key parameters and flow structure were identified and summarized. With these premises, a numerical model was developed. The model was validated with published experimental data from selected papers and fine tuned for channel flow within the laminar flow regime. Subsequently, the model was employed for a specific application to heat sinks for microelectronic cooling. This paper, then, provides a comparative evaluation of dimple technology for improving heat transfer in microelectronic systems.

Investigation of Laminar Flow in a Porous Pipe with Variable Wall Suction

An analytical model has been developed to provide an analogy for and predict flow property distributions in the condensing regions of a heat pipe. The model incorporates the phenomenon of incompressibl e, steady, laminar flow through a uniformly-poro us-wall pipe with a closed end-wall. A Karman-Pohlhau sen momentum integral technique was employed to reduce the axisymmetric Navier-Stokes equation to a nonlinear second-order ordinary differential equation. The axial pressure gradient, suction velocity at the porous wall, and wall friction coefficient were computed over a range of Reynolds numbers and wall permeabilities. These flow property distributions were not constrained a priori, but were allowed to vary in accordance with the conservation laws. The calculated flow property distributions for the condenser region of the heat pipe model are compared with published experimental data and several other theoretical models cited in the literature.

Effect of metallic coatings on the thermal contact conductance of turned surfaces

An experimental investigation was conducted to determine the degree to which the thermal contact conductance at the interface of contacting Aluminum 6061 T6 surfaces could be enhances through the use of vapor-deposited metallic coatings