Research over the past three decades in understanding micro ⁄ nanoscale heat transfer phenomena and mechanisms has significantly broadened our knowledge base and created larger intersections with other disciplines. This paper gives a high level reflection of the increased intersections of heat transfer with other disciplines through some examples of progress made in micro ⁄ nanoscale heat transfer research. At the fundamental level, we witnessed the blurring of the boundaries among heat transfer, physics and chemistry. New insights on heat transfer mechanisms led to opportunities in developing materials for heat transfer and energy applications such as high thermal conductivity plastics and improved thermoelectric energy conversion materials. Great opportunities exist in taking the fundamental understandings and advanced materials to develop innovative systems.
Thermal conductivity of air-bridge silicon nanowires with various widths between 60 and 150 nm and a one-dimensional phononic crystal nanostructure are investigated at room temperature. Large reduction in thermal conductivity is observed for thinner nanowires due to more frequent surface scattering of phonons. This result indicates that the phonon transport is ballistic in the investigated systems. The 1D phononic crystal showed smaller thermal conductivity compared with a nanowire with the same width. Phononic band diagrams of various 1D phononic crystals with periods between 100 and 300 nm are calculated. The simulated phononic band gap of the cascaded architecture covers wide frequency area and ensures the proposed cascaded architecture enables the coherent control of heat conduction at cryogenic temperature.
For the bubbly flow, the bubble deformation and size are of great importance to calculate the drag and lift forces, which are the key terms to the two-fluid model. Currently, the bubble deformation was mainly considered based on the correlation obtained from the single bubble or droplet experiments. However, for the bubbly flow, the existence of the many bubbles may also affect the bubble deformation. In the present paper, it was regarded that the bubble deformation for the bubbly flow could be statistically treated by the Sauter mean diameter and the chord length measured by a four-sensor optical probe simultaneously. Based on the experimental measurement of the upward turbulent bubbly flow in a large square duct, the statistical bubble deformation was analyzed and the void fraction effect was shown. In addition, the new correlation for the statistical bubble deformation was proposed by the bubble size and the local void fraction, and then validated by the latest experimental data.
As an application of a thermal management device for a high power microprocessor used in server computers, a loop heat pipe (LHP) containing a new design evaporator with a pin array conduction structure to enhance the evaporation area was developed. It was experimentally proven that the LHP can transfer a heat load of up to 360 W and that the thermal resistance of the LHP decreases to 0.138 °C ⁄ W in a heat load range of 295 to 334 W. The entire LHP is a flat structure in which the evaporator and a condenser are aligned in horizontal orientation. And the total height of the LHP is less than 40 mm, so it can be installed in a 1U (44.5 mm) server. The wick built into the evaporator consists of porous PTFE resin, and its mean pore radius of 10 um provides sufficient capillary pressure for continuous circulation of the working fluid around the LHP (with total piping length of 1.3 m). A cooling module using conventional heat pipes of the same shape as the LHP was fabricated and compared with the developed LHP in term of heat transfer characteristics over a distance of 200 mm. The comparison indicated that the LHP can stably transfer heat when it is subjected to three times or more power than that applied to a conventional heat pipes cooling module.