A STUDY OF 3-DIMENSIONAL NATURAL-CONVECTION IN HIGH-PRESSURE MERCURY LAMPS .1. PARAMETRIC VARIATIONS WITH HORIZONTAL MOUNTING

A three-dimensional numerical model utilizing curvilinear coordinates and an efficient solution method has been used to investigate natural convection in horizontal, high-pressure mercury-vapor arctubes. For horizontal arcs, convection velocities are predominantly transverse to the arc axis. The associated upward bowing of the arc column results in non-uniform heating of the quartz wall which can substantially degrade the performance of the modern horizontal metal halide lamp. This investigation shows how the design parameters of the arctube, such as the mercury pressure, the curvature of the curved arctube, the offset electrode distance, and the electrode insertion length affect the temperature distribution in the arctube. Qualitatively, a downward curved arctube can move the hot arc core downward, and offset electrodes can improve temperatures near the ends of the arctube. The evolution of the design improvement of a 400 W high-pressure mercury lamp with the aim of centering the arc and having isotherms conform to the wall is achieved through better quantitative understanding of the transport characteristics. © 1990.