This chapter is mainly focused on illustrating some introductory progress on thermal hydraulic issues of supercritical water, including heat transfer characteristics, pressure loss characteristics, flow stability issues and numerical method. These works are mainly performed in Nuclear Power Institute of China (NPIC) these years, to give a basic idea of elementary but important topics in this area. An analytical method was proposed up to predict the heat transfer coefficient and friction coefficient based on the two-layer wall function. Flow instability experiments have been carried out in a two-parallel-channel system with supercritical water, aiming to provide an up-to-date knowledge of supercritical flow instability phenomena and initial validation data for numerical analysis. An in-house code has been developed in NPIC in order to better utilize and further expand the experimental results on supercritical flow instability. At last, some future research directions are suggested for reference.
TopIntroduction
The Supercritical Water-Cooled Reactor (SCWR) is a high temperature, high pressure water cooled reactor that operates above the thermodynamic critical point (374°C, 22.1 MPa) of water. The reactor outlet temperature can be as high as 500°C which can increase the thermal efficiency up to 40% or even more. Supercritical water is a single phase fluid without two-phase interfaces, leading to simplification of the primary system, no need for steam generator, pressurizer and main circulation pump of the PWRs, and no need for the inner circulation pumps, steam separator and dryer of BWRs. As for the safety advantages, due to the characteristics of the supercritical water, no phase change would occur in the SCWR core under nominal conditions, therefore there are no DNB risks like in the PWRs (See 2015 GIF Annual Report).
Gaps exist in applying supercritical fluid into nuclear reactors. Among them, the heat transfer, pressure loss and flow stability are some typical and key thermal hydraulic issues to be solved. The significant changes in the thermal physical properties may have big influence on the flow resistance characteristics. When the wall temperature is greater than the pseudocritical point and bulk fluid temperature is below, large variation of fluid property will happen across the boundary layer. In such situation, the wall heat transfer coefficient will be dependant on the heat flux. As the increase of heat flux, the wall heat transfer coefficient is decreased. The friction coefficient will not depend solely on the bulk Reynolds number, but also bonds tightly with the physical property of the boundary layer. Experiments of supercritical water needed to be performed and data needed to be collected and compiled to develop the suitable empirical or semi-empirical correlations. Meanwhile, the theoretical analysis is relatively few, attributed to the fact of the difficulty in dealing with the complex variation of the physical properties.
The large density difference between the core inlet and outlet raises doubts about the possible occurrences of flow instabilities similar to those observed in BWR. As the licensing of SCWR will probably require, at a minimum, demonstration of the ability to predict the onset of instabilities, it is necessary to understand the instability phenomena in SCWR, to identify the important variables affecting these phenomena, and ultimately to generate the maps identifying the stable operating conditions of different SCWR designs.
Numerical methods are now playing an important part in investigating the flow and heat transfer behaviors of SCW under different flowing conditions. Experimental data obtained in the corresponding tests could be used to verify the validity of different numerical tools, including the system analysis code and CFD code. These methods could be used to investigate the flow instability characteristics under different working conditions. Detailed distributions of both the flow parameters and the fluid properties in regions near the heated wall can be obtained on the basis of numerical results to help understand the complexity and distinct features of the flow and heat transfer of the SCW, especially in the case with deteriorated heat transfer.
In this chapter, four sections would be organized to illustrate some introductory progress on thermal hydraulic issues of supercritical water. These sections are:
Section I: The heat transfer characteristics of supercritical water.
Section II: The pressure loss characteristics of supercritical water.
Section III: The flow stability issues of supercritical water in parallel channels.
Section IV: Modeling and analysis of supercritical flow instability in parallel channels.