Reproducibility of heat transfer tests in a 5X5 bundle geometry

This paper describes the repeatability and reliability of bundle heat transfer data obtained in a 5X5 PWR-type bundle subassembly operating at PWR conditions of interest. The 5X5 fuel bundle simulator, installed in the OMEGA-2 loop, is equipped with simple support grids, designed to have a low impact on the flow and heat transfer. The nine central heaters were equipped with the novel sliding thermocouple technique, capable of measuring the detailed axial and circumferential temperature distributions during single-phase and boiling heat transfer tests. In order to obtain highly accurate bundle heat transfer measurements, appropriate experimental procedures and in-situ calibrations of all essential instrumentation were employed. This includes (i) the employment of calibrated reference fluid temperature measurement devices, (ii) in-situ calibrations of fluid and heater-sheath thermocouples, (iii) calibration of heater wall thickness based on in-situ measurements, and (iv) selection of data that satisfy strict acceptance criteria. After applying these corrections and data screening criteria, the measurement accuracy and repeatability was assessed. This was done by means of three different tests: Single Phase Heat Transfer: The repeatability of heat transfer were assessed by comparing the measurements of two separate 5X5 bundles against the predictions from a Dittus-Boelter-type heat transfer correlation which provided very similar results. Also the single-phase heat transfer repeatability was assessed by performing several repeat runs and comparing results obtained on heaters in symmetric locations. Excellent repeatability was noted and the results for symmetric angular locations are almost identical; Boiling Tests: During the boiling heat transfer tests excellent repeatability and symmetry was observed. The saturation temperature (corresponding to the measured outlet temperature) was found to be in very good agreement with (i) the outlet temperature measured by the platinum resistance temperature detector (RTD) (average difference 0.1 K, range 0 to - 0.2 K) and (ii) the average of the 36 subchannel outlet-coolant temperature measurements (average difference 0.1 K, range +0.5 to -0.6 K); and, Onset of Nucleate Boiling (ONB) Tests: for all tests performed at a constant pressure, heat flux and mass velocity, the ONB for the same symmetric position occurs at the same thermodynamic quality irrespective of location of the nearest grid. Repeatability was again excellent The results demonstrate the capability of the OMEGA-2 loop in obtaining highly accurate and repeatable single-phase and boiling heat transfer measurements in PWR-type subassemblies. (author)