Measurement of various flow parameters are essential in two phase flow systems such as primary and secondary nuclear coolant system. The response of the volumetric concentration of the liquid and vapor phases due to perturbations in heat flux and inlet flow is of particular interest, because of its influence on the neutron dynamics in nuclear reactors.
The purpose of this work is to develop a void fraction measurement and flow regime characterization technique in a two phase flow system by capacitance transducer. This technique is based on the fact that dielectric constant of the liquid water is a well defined parameter and the value of the dielectric constant of the liquid (or solid phase) is very large compared to that of the gaseous phase. Depending on the flow pattern and size of bubbles, the total dielectric constant of a two-phase mixture can be related to the relative capacitance.
For example, in the case of annular flow regime it is possible to derive a functional relationship between the void fraction and the relative capacitnace, by considering the liquid and vapor phase as capacitors in series.
Two basic electrode geometries, "ring" and "strip", were examined in the present tests for sensitivity and flow regime dependency. The ring electrodes consisted of two 10 mm wide aluminum strip attached circumferentially to the tube (outside diameter = 30mm) and separated by 10mm. On the other hand, the strip electrodes were made up of two 60mm long and 46mm wide strip attached longitudinally and separated by 4mm.
The void fraction was controlled by varying the diameter of a solid polyacrylate cylinder or the amount of water which is inserted in the pyrex tube. The polyacrylate and paraffin wax were used to simulate annular flows, whereas liquid water and polyacrylate were used to simulate stratified flows. From the test results a relationship between the measured output capacitance of the strip electrodes and void fraction in annular and stratified systems are obtained and the result is compared with the theoretical predictions.
In addition, the flow regime dependence of the relative capacitance is also obtained for both ring and strip electrodes.