Experimental verification of convective heat transfer computations for gas insulated switchgear

Temperature rise of gas insulated switchgear (GIS) has to be limited according to the demands of the IEC 62271 standard. Computational methods for temperature rise are essential during the design of GIS. The heat in GIS is mainly transferred by convection, as the used insulating gas sulfur hexafluoride (SF
₆
) has a high cooling capacity that further increases as the gas is pressurized. The dimensionless Rayleigh number indicates the intensity in which the flow is driven. If a critical Rayleigh number is exceeded, the flow transits from the laminar to the turbulent regime and the heat transfer coefficient increases steeply. Computational fluid dynamics (CFD) is a useful tool to compute heat transfer and temperature rise of a power device while considering the actual geometry, but modeling assumptions have to be made to cope for turbulence. Experimental investigations are carried out to verify the computed heat transfer of natural convection flows. Also, a Rayleigh number region for the transition of the flow regime in an annulus is determined. The investigations are performed for air and the Rayleigh number is varied by using different filling pressures. This practice allows the investigation of flows for Rayleigh numbers varying over three orders of magnitude. An oil mist is used to show the flow patterns, which allows the flow regime to be assessed by visual investigations. The influence of several modeling assumptions on the accuracy of computed heat transfer coefficients is compiled.