Comparison of experimental and calculated HTC values for short vertical 7-rod bundle cooled with SCW

Current Nuclear Power Plants (NPPs) equipped with light- and heavy-water-cooled reactors (the vast majority of all NPPs) have relatively low thermal efficiencies within the range of 30‒36% (up to 38% for Generation III+) compared to those of modern advanced thermal power plants (SuperCritical Pressure (SCP) coal-fired ‒ up to 55% thermal efficiency and combined cycle ‒ up to 62%). Therefore, next generation reactors / NPPs should have higher thermal efficiencies close to those of current thermal power plants. Based on the proven for more than 60 years experience in thermal-power industry to operate SCP power plants it was proposed to design SuperCritical Water-cooled Reactors (SCWRs), which are one of the six Generation-IV nuclear-reactor concepts under development in selected countries. These days, there are discussions on developing even Small Modular Reactors (SMRs) of SCPs. In spite of a large number of experiments in bare tubes (pipes)cooled with SCW, developing SCWRs requires experimental data in bundle geometries. However, such experiments are extremely complicated and expensive due to that, as of today, very few experimental data in bundles are available for analysis. Therefore, current paper presents experimental data obtained in a short vertical 7-rod bundle of a small hydraulic-equivalent diameter equipped with 4 small helical ribs cooled with upward flow of SCW.Main emphasis of this research is on "liquid-like" cooling within the following operating conditions: SCW, pressures of 22.6, 24.5, and 27.5 MPa, mass flux – 800 and 1000 kg/m²s, bulk-fluid inlet temperature from 178 to 246ºC, outlet temperature up to 350ºC, and heat flux from 0.7 to 1.3 MW/m².Three Heat-Transfer (HT) regimes were encountered at these conditions: 1) Normal HT (NHT); 2) Deteriorated HT (DHT); and 3) Improved HT (IHT). Heat Transfer Coefficients (HTCs) calculated through the Dittus-Boelter and Mokry et al. correlations were compere with the experimental data. Analysis of this comparison is included. (author)