Joule effect

Abstract: First-principle calculations based on density functional and non-equilibrium Green's functions are used to compute the power emitted in conducting molecular systems due to scattering with localized vibrations The balance between the rate of phonons emitted and dissipated into the contacts allows the computation of the steady-state distribution of phonon quanta localized in the junction, from which we extract the local temperature reached by the molecule The model includes two critical quantities;(i) the rate of phonon emitted in the junction due to electron-phonon scattering and (ii) a microscopic approach for the computation of the phonon decay rate, accounting for the dynamical coupling between the vibrational modes localized on the molecule and the contact phonons The method is applied to the discussion of several limiting conditions and trends, depending on electron-phonon coupling, incoherent transmission and phonon dissipation rates, using both analytical results and numerical calculations

Abstract: An experimental study of subcooled boiling in a Freon-113 forced flow is presented. The test section is a short tube (length 50 mm, inner diameter 8 mm, and wall thickness 0.3 mm) heated by the Joule effect. Wall temperature profiles along the tube are presented for different operating points and discussed in terms of the upstream propagation of a temperature front separating regions of nucleate and film boiling. This study emphasizes the effect of axial heat conduction on the boiling processes.

Abstract: According to the classical treatments given in standard textbooks, the presence of an electric field in an insulating fluid should favor natural convection. The effect of an electric field on thermal convection was reinvestigated with an experimental setup consisting essentially of a thin platinum wire stretched in the axis of a metal ring. Both electrodes were immersed in a cell containing the liquid under investigation. The wire was heated by Joule effect, and a dc voltage up to 5 kV was applied between the wire and the ring. The cell, fitted with optical windows, was placed in the beam of a striograph (Schlieren optics bench) by which the heat front rising above the wire was observed. The heat exchange between the wire and the liquid could also be monitored by estimating the steady temperature of the wire, through the measurement of its resistance. With these techniques, it was found that the electric field may actually impede thermal convection if the liquid contains a sufficient amount of ionizable impurities. The interpretation is based on the space charge which appears wherever a gradient of temperature—hence of conductivity—and an electric field simultaneously exist. A mathematical analysis, made possible by a few simplifying assumptions, shows that an electrostatic attractive force opposes the upward gravitational pull acting on the heated liquid, and satisfactorily predicts the stable position of the heat front above the wire. Other manifestations of this space charge are briefly described.

Abstract: The capability of a moving-coil loudspeaker to sustaining a certain power is related to its capability of dissipating heat produced by the Joule effect in the moving coil and, due to eddy currents, in the pole piece. Different heat transmission phenomena were analyzed in order to determine a lumped-element thermal model realistically related to the physics of the process. The model is based on the Zuccatti-Button approach. However, a more detailed model has been developed in order to consider all the heat transmission processes involved. In particular, forced-air convection, induced by the cone movement at low frequencies (30-150 Hz), has been taken into account.