Abstract-Radiative rectification is one of the newest solutions for thermal building insulation. It is a phenomenon which has good analogy with electronic diode principle. It consists on being blocking for solar radiations in summer and passing radiations in winter. Among materials which have rectification capacity, Vanadium dioxide (VO 2 ) offers good applicability to building insulation with quite good rectification efficiency. VO 2 has a transition temperature around 68°C. Under this temperature, the material has a semiconductor crystallographic structure; therefore it becomes transparent to visible and infrared solar spectra. Above 68°C, the crystallographic structure of VO 2 changes to a metallic state for which it becomes more reflective to the same spectra. This semiconductor/metallic transition influences the optical properties of VO 2 which are highly dependent to temperature and wavelength. These properties are refractive index n(T,λ) and extinction coefficient k(T,λ), they are necessary in the optical study of VO 2 insulation capacities. Determination of these properties is purely experimental using ellipsometry techniques. In this work we suggest a method for numerical determination of these optical properties. This method uses the Particle Swarm Optimization (PSO) algorithm and it is based on the theoretical model of Lorentz oscillators at VO 2 nanoparticles scale. We calculated n(T,λ) and k(T,λ) for 3 temperatures and compared them with experimental results. This comparison showed good agreement between numerical and experimental results.Vanadium-dioxide, radiative rectification, building insulation, optical properties, optimization algorithms. I. INTRODUCTION Vanadium dioxide is a metal oxide that attracts many research teams over the world. It offers thermo-chromic properties depending on its semiconductor/metallic phase transition around 68°C [1,2]. VO 2 has many potential applications as a photonic device. In our case, we are trying to use VO 2 as a smart solution for radiative rectification applied to thermal building insulation, a smart solution because of its thermo-chromic properties highly dependent to temperature. At temperatures less than 68°C, VO 2 has a semiconductor crystallographic structure, with inter-atomic distances which allow high radiative transmission. Above 68°C, VO 2 switches to a metallic structure with small inter-atomic distances, thus VO 2 become more reflective to solar radiations. VO 2 is proposed for application to building glazing as smart window coatings by C.G. Granqvist et al [3]. The transition temperature too high compared with the comfort temperature is one of the limitations for smart windows application. G.V. Jorgenson et al [4] and W. Burkhardt et al [5] proposed a solution to decrease the VO 2 transition temperature by doping with some materials such as tungsten (W), Fluoride (F) and Molybdenum (Mo) with calculated concentrations to adjust the transition temperature around 25°C. The second limitation is related to high absorption in the visible part of sol...
Pool boiling heat transfer is a very efficient mode of heat transfer. It is used in various energy conversion and heat exchange systems and in the cooling of high energy density electronic components. In this work, heat transfer during nucleate boiling for a brass ribbon horizontally immersed in liquid nitrogen is experimentally and numerically studied. An experimental apparatus was built to conduct a pool boiling heat transfer study. The capabilities of Computational fluid dynamics (CFD) boiling model in pool boiling heat transfer is investigated. The computational model used combines the Euler/Euler two-phase flow. The general measurements of the super-heat, the influences of super-heat on the triggering of the boiling, as well as the critical flux which represents an important issue for the security of the systems were analyzed. The results of numerical simulations were compared with experimental data.
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