Investigation of the Heat-Insulation Properties of the Steam-Water Gap in the Downcomer Pipes of a Steam Generator

The results of an experimental investigation of heat transfer through a gap filled with a steam‐water mixture are presented. It is shown that heat transfer through such a gap is considerably different from heat transfer through a solid wall with similar geometry. The heat flux through a gap where steam condenses is almost independent of the temperature of the warming medium, but the proximity of the ‘cold’ wall temperature to the saturation temperature does affect it. The pool arrangement of the coolant circulation loop of the reactor facility presupposes top-down delivery of the feed water to the steam generator and, in consequence, the need for heat insulation of the downcomer pipes in order to keep the water from boiling in them and thereby narrowing the region of thermohydraulic stability. The gap between the two pipes (inner and outer), filled with a low thermal conductivity medium (gas or steam), can serve as heat insulation for the downcomer (Fig. 1). The temperature of the inner and outer downcomer pipes changes considerably along the height, which creates the conditions for condensation/evaporation and, as a result, substantial heat and mass transfer. In other words, instead of pure heat conduction there is convective heat transfer together with evaporation and condensation, which in terms of the intensity of heat exchange greatly surpass heat conduction. Ordinarily, the heat flow through layers of solid or liquid (gaseous) material is calculated by means of Fourier’s equation with the appropriate thermal conductivities and heat transfer through the gaps, where the influence of the convective component is considerable, is supposed to be determined by means of an equivalent thermal conductivity. The method for calculating the equivalent thermal conductivity is based on an empirical relation, using the Rayleigh number. For example, the equivalent thermal conductivity can be determined from the relation [1] λ eq = λARa m ,