Heat Transfer between Duct Wall Element and its Holder for Semi-hot MHD Generator

Abstract

Semi-hot generating duct wall of magnetohydrodynamic generator, being developed as one of national projects in Japan, will be constructed with settling many elements made of dense refractory ceramics by their holders (usually metals). In addition, these holders are also used to cool these elements in order to keep temperatures of operating conditions of materials in running. The usualest settling technique is soldering between a wall element and its holder. It has been reported in recent papers that thermal resistance has been neglected in this soldered part. However, there are many unsolderable materials among them. If unsolderable materials must be used for these wall elements, non-soldered holding techniques will have to be used, and there will cause great thermal resistances between these wall elements and their holders. For this reason, the heat transfer phenomenon between this element and its holder must be investigated for designing the duct wall. The author has studied the heat transfer phenomenon by model tests. The results of experiments are as follows:
Equivalent distances between test pieces and holders, calculated from their heat transfer characteristics, are 76μm for the couple of alumina test piece and stainless steel holder, and 85μm for the couple of magnesia and stainless steel. On the other hand, distances calculated from their surface roughnesses and undulations, measured by touching probe type surface roughness meter, are 65μm for the former and 76μm for the latter. Results are summarized as follows:
(1) Heat transfer between the MHD generator duct wall element and its holder is mainly caused by the thermal conduction through gas layer between them.
(2) The equivalent thickness of this gas layer is comparable to the distance determined from the surface roughness and undulation of each surface of both elements.