EXPERIMENTAL STRATEGY FOR THE DETERMINATION OF HEAT TRANSFER COEFFICIENTS IN PEBBLE-BEDS COOLED BY FLUORIDE SALTS

摘要

It is important to accurately model the heat transfer coefficient between the fuel pebbles and the flibe coolant in order to correctly predict fuel temperatures in the core of the fluoride-salt-cooled, high temperature reactor (PB-FHR). We have been performing experiments using simulant oils that match key non-dimensional parameters expected in PB-FHRs. A 3.5" long test section is filled with 1/4" copper pebbles, some of which are instrumented with thermocouples. Oil is circulated through the test section. The entering temperature of the oil is varied, and the time-varying exit temperature of the oil and temperatures of the instrumented pebbles are recorded. Using these temperatures the interfacial heat transfer coefficient can be extracted as a function of position and time. Correlations for interfacial heat transfer coefficients are available in the literature, but these are derived using experimental data that do not entirely encompass PB-FHR operating conditions. Generally for the PB-FHR the Reynolds and Prandtl numbers during normal operation are higher than reported experimental results. Thus, it is important to perform tests in the appropriate PB-FHR Reynolds and Prandtl number range. Preliminary results indicate that experimentally measured heat transfer coefficients may be at least 20% higher than predicted using Wakao and Funazkiri's correlation for Nusselt number in pebble beds. These preliminary results also reinforce the need to reduce uncertainties in the collected data. This paper will also present experimental techniques that aim to accomplish this. Specifically, we detail how frequency response techniques can be used to extract heat transfer coefficients from a pebble-bed test section and how experiments can be designed using simulant oils to achieve this.