A universally applicable thermal-hydraulic mixture-fluid coolant channel model based on drift-flux will be provided, providing the basis for a corresponding digital ‘Coolant Channel Module (CCM)’. The ‘Separate-Region Mixture Fluid Solution’ derived for this reason should give the currently dominant ‘Separate-Phase Models’ an alternative platform where each phase is handled separately. Contrary to this, a direct protocol may be generated here. Its objective is to allow the steady state and transient behaviour of characteristic single- and/or (now not generally separated) two-phase parameters of fluids flowing inside any form of heated or non-heated coolant channels to be simulated in the most general way possible. In addition to the benefit of being able to use conservation laws now, the requisite constitutive equations can also be taken directly from the corresponding well-proofed drift flux, single- and two-phase friction and heat transfer correlations. Therefore, it is no longer based on the often very speculative terms of trade between the stages and the wall, but also on the two stages themselves. A potential sudden transition from single to two-phase flow in the case of water/steam flowing through a heated or cooled channel and vice versa has the consequence that mathematical discontinuities must be predicted within the solution procedure. To overcome these difficulties in the here presented approach the basic coolant channel (BC) will be assumed to be subdivided into a number of sub-channels (SC-s), each of them occupied exclusively by only a single or a two-phase flow regime. Hence, after an appropriate spatial discretization and nodalization of the BC (and thus its SC-s) a ‘modified finite volume method’ (together with a special polygon approximation procedure PAX) could be developed. The desired set of non-linear ordinary differential equations of the 1st order (for each SC and, if extended to the entire channel, BC) could be derived in accordance with the conservation laws of thermal-hydraulics and with corresponding constitutive relations. It is clear that the eventually varying SC outlet location (= entrance to the following SC) had to be given special attention, thus defining the movement along the channel BC of boiling boundaries or mixture levels. This also involves the risk of SC-s vanishing or being formed anew. The resulting universally applicable CCM code package can then serve as a fundamental element within a complex physical framework for the simulation of thermal-hydraulic situations over each type of coolant channel (distinguished by its main numbers).
Dr. Alois Hoeld
Retired from Gesellschaft fuer Anlagen- und Reaktorsicherheit (GRS), Garching/Munich, Germany.
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