Hydraulic feature investigation on supercritical water natural and forced circulation loops

A comparative investigation on steady-state hydraulic feature of both natural and forced water circulations under supercritical pressure is carried out. To extract the hydraulic essence and related features for these two types of water circulation under supercritical pressures, two simple yet typical loops of natural and forced circulation with almost the same geometries (except that the forced circulation loop is equipped with a pump) are considered for comparison. A unified hydraulic model for both natural and forced circulation is established, considering the unique variation of thermo-physical properties at supercritical pressures, along with the nonlinear momentum transport and system coupling characteristics. Related frictional pressure drop correlation, which has been developed under supercritical pressure conditions by other authors, is also included within the model to account for the special momentum transport characteristics in supercritical water flow. Furthermore, appropriate nonlinear numerical computation, which is based on continuation concept, is applied for numerical calculation for solving the model equations. Steady-state solutions of the model both for natural and for forced circulation present the complex but typical hydraulic features of both loop systems. A comparative study on static hydraulic features of both types of supercritical water circulations reveals the special non- monotonic relationships of mass flow-rate vs. heating power. Meanwhile, effects of such factors as inlet bulk temperature of heating section, local resistances and/or pump characteristics on system hydraulic and heat transmission features are delivered. Furthermore, characteristics of forced circulation driven by different real pumps and a ideal pump are compared. Through the above-mentioned investigation, following conclusions are drawn, which deliver rather interesting and useful insights for future engineering application of supercritical water circulations and, for which further experimental validation is preferable: (1) A specific feature of flow-rate - heat load relation of natural circulation under supercritical pressure is revealed. Flow-rate in the loop first increases and then decreases rapidly with heating power and reaches it maximum Gm at certain heating power Qm. Favorable operating region for local heat transfer and loop heat transport is found to be within range of Q < Qm. In contrast, mass flow-rate in forced circulation experiences first a nearly flat region followed with an abrupt drop one with heating power increases. Adverse condition in abrupt dropping region of mass flow-rate may possibly impair loop heat transport and operation safety. (2) For natural circulation, both higher inlet temperature and local loss lead to flow-rate decreasing and make outlet temperature increase. Among the influences of inlet and exit local loss, the latter tends to be larger. For forced circulation, trends of inlet temperature and local loss effects on system hydraulic features are similar. (3) In forced circulation, pump characteristics have obvious influences on system hydraulics and heat transport. Higher circulation flow-rate might be obtained through either increasing pump effective power or decreasing the slope of G - H curve. These strategies are also beneficial for loop heat transport. (author)