Repeatability of heat transfer tests in a 5 × 5 bundle geometry

This paper describes the repeatability and reliability of bundle heat transfer data obtained in a 5 × 5 PWR-type bundle subassembly operating at PWR conditions of interest. The 5 × 5 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 by means of three different tests: (i) single phase heat transfer tests, (ii) boiling tests, and (iii) onset of nucleate boiling (ONB) tests. 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.