On the convective flow in Hg high pressure discharge lamps: influence of tube geometry

Summary form only given, as follows. Transport phenomena in electric arcs are responsible for effects of considerable importance for a stable operation of the device. In fact if the convection flow is unstable, undesirable phenomena appear and several discharge characteristics are affected. In the case of an atmospheric pressure discharge this flow is largely dependent of arc pressure and current as well as arc tube dimensions. In this paper we limit the discussion on the influence of the arc tube dimensions by using numerical simulation results. This simulation of arc properties is performed with a two-dimensional code. Herewith, mass, energy and momentum continuity time dependent equations are solved in order to calculate transport flows. Numerical resolution is done with a finite element semi-implicit scheme. It was found that stable laminar mono-cellular convection flow occurs at low values of tube diameter for a cylindrical burner. Furthermore, in the case of a wall stabilised discharge these convection losses are a small fraction of the total energy amount in the discharge. However, it was recognised that convection in some cases could affect lamp performances. For example, in vertically operating lamp, our calculations clearly show that the arc is more constructed in the lower electrode region because of the cold gas from the arc periphery penetrating into the hot central channel. At sufficiently high diameter this constriction is so important that are instability occurs.