Heat exchange mediated by a quantum system

We consider heat transfer between two thermal reservoirs mediated by a quantum system using the generalized quantum Langevin equation. The thermal reservoirs are treated as ensembles of oscillators within the framework of the Drude-Ullersma model. General expressions for the heat current and thermal conductance are obtained for arbitrary coupling strength between the reservoirs and the mediator and for different temperature regimes. As an application of these results we discuss the origin of Fourier's law in a chain of large, but finite subsystems coupled to each other by the quantum mediators. We also address a question of anomalously large heat current between the STM tip and substrate found in a recent experiment. The question of minimum thermal conductivity is revisited in the framework of scaling theory as a potential application of the developed approach.Comment: 16 pages, 6 figure

Engineered Nanostructures for High Thermal Conductivity Substrates

In the DARPA Thermal Ground Plane (TGP) program[1],we are developing a new thermal technology that will enable a monumental thermal technological leap to an entirely new class of electronics, particularly electronics for use in high-tech military systems. The proposed TGP is a planar, thermal expansion matched heat spreader that is capable of moving heat from multiple chips to a remote thermal sink. DARPA’s final goals require the TGP to have an effective conductivity of 20,000 W/mK, operate at 20g, with minimal fluid loss of less than 0.1%/year and in a large ultra-thin planar package of 10cmx20cm, no thicker than 1mm. The proposed TGP is based on a heat pipe architecture[2], whereby the enhanced transport of heat is made possible by applying nanoengineered surfaces to the evaporator, wick, and condenser surfaces. Ultra-low thermal resistances are engineered using superhydrophilic and superhydrophobic nanostructures on the interior surfaces of the TGP envelope. The final TGP design will be easily integrated into existing printed circuit board manufacturing technology. In this paper, we present the transport design, fabrication and packaging techniques, and finally a novel fluorescence imaging technique to visualize the capillary flow in these nanostructured wicks.United States. Defense Advanced Research Projects Agency (SSC SD Contract No. N66001-08-C-2008

High-Performance Thermal Management

Traditionally thermal management systems are designed for steady-state behavior. Air Force systems tend to be highly complex, coupled, and highly-dynamic. The concept of high-performance thermal management offers a paradigm shift in the technical approach to thermal management to one that addresses the inherent need to develop thermal management systems. This seminar will address the conceptual and philosophical approach to addressing the science and engineering needed to evolve thermal management technologies for high-performance and rapidly responding thermal management systems.https://corescholar.libraries.wright.edu/physics_seminars/1008/thumbnail.jp

Convective Flow Boiling of R-134a on Micro-Structured Aluminum Surfaces

In this study, we have examined the convective flow boiling performance of R-134a on various micro-structured aluminum surfaces produced using advanced manufacturing techniques. More specifically, we have calculated the boiling heat transfer coefficient of R-134a on a bare aluminum surface and three micro-structurally enhanced surfaces. Two of these surfaces were produced using photolithography and reactive ion etching techniques, and the third surface was produced by means of laser-ablation. Experiments were performed in a conventional two-phase, single-pass loop which allowed for heat transfer and pressure drop measurements over a range of inlet qualities with only small quality changes occurring in the test section. There was also optical access to the test specimen to permit flow visualization. To begin, both single-phase and two-phase flow experiments were performed on the bare aluminum surface to compare these baseline results with data found in the literature. Once baseline testing and validation were complete, the sample was exchanged and the three micro-structured surfaces were then each subsequently tested. The temperature and pressure of the refrigerant were measured at stations in the flow upstream and downstream of the test section, and the temperature of the test surface was measured using five T-type thermocouples in contact with the sample. The evaporation of the refrigerant was driven by thin ceramic heaters in contact with the underside of the test samples. The pressure, temperature, and quality within the test section were prescribed using an upstream heat exchanger, and the mass flow rate of the refrigerant was controlled using a magnetic gear pump and measured using a positive displacement flow meter. Experiments were performed for mass fluxes between 75 and 600 kg/m2s and for heat fluxes between 5 and 25 kW/m2