Experimental investigations of flow distribution in coolant system of Helium-Cooled-Pebble-Bed Test Blanket Module

Abstract

This paper deals with investigations of flow distribution in the coolant system of the Helium-Cooled-Pebble-Bed Test Blanket Module (HCPB TBM) for ITER. The investigations have been performed by manufacturing and testing of an experimental facility named GRICAMAN. The facility involves the upper poloidal half of HCPB TBM bounded at outlets of the first wall channels, at outlet of by-pass pipe and at outlets of cooling channels in breeding units. In this way, the focus is placed on the flow distribution in two mid manifolds of the 4-manifold system: (i) manifold 2 to which outlets of the first wall channels and inlet of by-pass pipe are attached and (ii) manifold 3 which supplies channels in breeding units with helium coolant. These two manifolds are connected with cooling channels in vertical/horizontal grids and caps.The experimental facility has been built keeping the internal structure of manifold 2 and manifold 3 exactly as designed in HCPB TBM. The cooling channels in stiffening grids, caps and breeding units are substituted by so-called equivalent channels which provide the same hydraulic resistance and inlet/outlet conditions, but have significantly simpler geometry than the real channels. Using the conditions of flow similarity, the air pressurized at 0.3MPa and at ambient temperature has been used as working fluid instead of HCPB TBM helium coolant at 8MPa and an average temperature of 370°C. The flow distribution has been determined by flow rate measurements at each of 28 equivalent channels, while the pressure distribution has been obtained measuring differential pressure at more than 250 positions.The measured data have shown that critical for flow distribution in manifold 2 are grid/cap channels attached to the manifold regions adjacent to the first wall. In these regions high velocity jets exiting from the first wall induce low pressure at the channel inlets and, therefore, lower flow supply. Making resistance to high velocity streams from the first wall the perforated penetrations of horizontal grid plates which divide the manifold 2 into chambers are crucial for pressure homogenizing and reasonable flow distribution among channels attached to the central part of manifold 2. In the major part of manifold 3 very small differential pressures were measured except for some breeding unit inlets where strong pressure peaks were recorded. Nevertheless, no dramatic differences were found between experimentally determined flow rates in individual breeding unit channels.