Abstract
The paper presents the experimental analysis and the characterization of an electrical capacitance probe (ECP) that has been developed at the SIET Italian Company, for the measurement of two-phase flow parameters during the experimental simulation of nuclear accidents, as LOCA. The ECP is used to investigate a vertical air/water flow, characterized by void fraction higher than 95%, with mass flow rates ranging from 0.094 to 0.15 kg/s for air and from 0.002 to 0.021 kg/s for water, corresponding to an annular flow pattern. From the ECP signals, the electrode shape functions (i.e., the signals as a function of electrode distances) in single- and two-phase flows are obtained. The dependence of the signal on the void fraction is derived and the liquid film thickness and the phase’s velocity are evaluated by means of rather simple models. The experimental analysis allows one to characterize the ECP, showing the advantages and the drawbacks of this technique for the two-phase flow characterization at high void fraction.
Highlights
The design of new nuclear reactors requires carrying out integral and separate effect tests on simulation facilities, as well as performing safety systems verification and safety code validation
The paper presents the characterization of an electrical capacitance probe (ECP) that has been developed by the SIET Company and that consists of ten measurement electrodes, nine external and an internal one
The central electrode allows the evaluation of the average cross section void fraction in annular flow, where the liquid film is the principal cause of the sensor low sensitivity
Summary
The design of new nuclear reactors requires carrying out integral and separate effect tests on simulation facilities, as well as performing safety systems verification and safety code validation. The advantage is its excellent time resolution arising from the very fast measurement of electrical resistances, while the drawback is the relatively low spatial resolution, since the phases distribution reconstruction is based on measurements at the periphery of the sensor This problem is stressed in presence of an annular flow as the liquid film at the wall creates a preferential path for the electric field lines that shields the core region and makes the sensor poorly sensitive to the void fraction. The range of interest of the flow parameters, in steady state and transient conditions, has been investigated by means of the RELAP5 code [10] and, for each line of the SPES3 facility, the range of measurement of the different parameters has been identified; the performed simulations show that, in a relative long period of the transients, the two-phase flow corresponds to an annular flow pattern, characterized by very high void fraction and high flow velocity, so that the selected instruments of the Spool Piece must be sensitive to the flow in such conditions. Each external electrode is connected, at the upper and lower extremity, to two reed relays to activate, in a predefined sequence, the excited electrode and the measuring one; the internal one is connected only in the upper extremity and it is always used as a measuring electrode, when the corresponding reed relay is activated
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