An Accurate Method of Assessing Tube Wall Thickness Based on Flow Boiling and Single Phase Heat Transfer Measurements

Abstract

Fuel bundle simulators used in thermalhydraulic studies typically consists of bundles of directly heated tubes. It is usually assumed that these tubes have a uniform circumferential heat flux distribution. In practice, this heat flux distribution is not uniform because of wall thickness variations. Ignoring the non-uniformity in wall thickness will lead to underestimating the critical heat flux and overestimating the film boiling temperature.It is the objective of this paper to present an accurate method for determining this wall thickness variation. The method is based on the principle that for directly heated tubes, cooled internally, the single phase heat transfer coefficient (or the wall superheat in nucleate boiling or at ONB) is uniform around the circumference and is therefore independent of the variation in wall thickness.Three analytical methods for evaluating the wall thickness variation have been compared: based on single phase heat transfer data, based on nucleate boiling data, based on ONB data. The wall thickness evaluations have subsequently been validated by two direct measurements: (i) based on direct caliper measurements, and (ii) based on photographic measurements. The agreement between these five methods is good. The results show that the proposed analytical approach is a highly accurate method for determining the circumferential wall thickness variation.