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

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

Design of a Microgravity Spray Cooling Experiment

An analytical and experimental study was conducted for the application of spray cooling in a microgravity and high-g environment. Experiments were carried out aboard the NASA KC-135 reduced gravity aircraft, which provided the microgravity and high-g environments. In reduced gravity, surface tension flow was observed around the spray nozzle, due to unconstrained liquid in the test chamber and flow reversal at the heat source. A transient analytical model was developed to predict the temperature and the spray heat transfer coefficient within the heated region. Comparison of the experimental transient temperature variation with analytical results showed good agreement for low heat input values. The transient analysis also verified that thermal equilibrium within the heated region could be reached during the 20-25s reduced gravity portion of the flight profile