Measurements of void fraction, liquid temperature and velocity under boiling two-phase flows using thermal-anemometry

Abstract

Forced convective boiling is of great interest for several applications in the power and process industry, particularly in nuclear plants. Under certain nominal, incidental or accidental conditions, boiling crisis (considering Departure from Nucleate Boiling as well as Dry-Out) may lead to mechanical damage of the heated surface. An accurate prediction of the conditions leading to the occurrence of this phenomenon is then essential. It is believed that such an objective cannot be reached unless a good and accurate description of the associated two-phase flow is provided. In our work, we propose to use thermal anemometry for measuring the void fraction, the liquid temperature and liquid velocity for high pressure and high temperature Freon R134A boiling flow. Experiments have been conducted in a circular tube whose inner diameter and length are 19.2 mm and 3.5 m, respectively. The tube is heated by Joule effect. The sensor (hot-wire d5μm) has been operated with the Constant Current mode (CCA) and the multiple overheating method, which is classical for gas measurements but seems to be very innovative for liquid flows. This method, has been used to access simultaneously the liquid velocity and temperature. To consider for temperature effect on velocity calibration, a new non-dimensional representation of the calibration curve has been proposed. The frequency response of the probe has also been improved using a digital compensation method. The method has been first checked for single-phase flows and has shown that it was possible to get very accurate measurements of both mean and fluctuating liquid and temperature profiles. For boiling flows, a specific two-steps approach has been developed to first measure the void fraction where it is necessary to set a high overheat ratio leading to boiling on the wire surface and secondly to measure the liquid temperature and velocity for the case where boiling on the wire surface is not acceptable due to the multiple overheating method. An innovative method using probability density functions has been adapted from the pioneering work of Delhaye (1969).Some tests have been conducted for boiling flows and the experimental results have been compared to previous ones using thermocouples and optical probes for respectively the liquid temperature and the void fraction. The experimental uncertainties have been carefully analyzed and they are estimated to be close to 0.5 °C for the liquid temperature, ±2% for the void fraction (absolute uncertainty) and ± 5% for the liquid velocity (relative velocity). Those data aim to be used for NEPTUNE_CFD code validation.