Void fraction measurement in cryogenic flows. Part I: Design and validation of a void fraction capacitive sensor

Abstract

This manuscript presents the design of a capacitive void fraction sensor for cryogenic LN2 two phase flows and its validation at room conditions. The capacitive void fraction sensor is first designed by means of Electric Field Analysis (EFA) simulations taking into account specific technical constraints coming from the test section in which it should be accommodated. Then it is manufactured and validated using a proper combination of fluids (Polydimethylsiloxane (PDMS) and air) having a dielectric constant ratio similar to the one encountered in LN2/GN2 two phase flows. The validation is performed through comparison with void fraction measured by means of optical visualizations and shows how the capacitive measurement technique robustness allows obtaining reasonable accurate values of void fraction also for the substitute fluid case. The sensor presented in this manuscript was used to evaluate the void fraction during LN2 chilldown of a rectangular cooling channel and the results are presented in the second part of this work.

Void fraction measurement in cryogenic flows. Part II: Void fraction capacitive sensor performances in chilldown experiments

“…As described in Part I [1] and in [2,12,13], the numerical results obtained with the so-called EFA, has been proved a trustful tool in order to predict the capacitive sensor performances. In this work, this methodology is again applied according to the procedure summarized in [3,9] to estimate the capacitive sensor reliability in cryogenic conditions.…”

“…4, the EFA computations resulting capacitance and void fraction computed according to Eq. (1) [1] are provided for all the different flow configurations. The first positive remark is that the computed gas content never exceeds the imposed boundaries of 0% and 100%, results that may occur in less precise sensors [2].…”

“…Indeed, the linear relationship (Eq. ( 1) [1]), theoretically valid to translate the measured capacitance C M to the void fraction is improved. The modified capacitance conversion formula expressed by Eq.…”

“…The void fraction commonly intended as the fraction of a pipe volume that is occupied by the gas phase, is one of the most important parameters required to characterize a two-phase flow. As addressed in [1][2][3] several void fraction measurement techniques have been and are currently investigated. Various measuring principles are exploited, for example optical fibers [4], wire meshes [5], impedance changes [6] or wave propagations (such as x-rays [7] or ultrasounds [8]).…”

“…In the present study, a capacitance-based measurement technique for cryogenic fluid is developed. In Part I [1], the void fraction sensor was numerically and experimentally characterized by means of a bubbly flow obtained injecting air in Polydimethylsiloxane at room temperature. Although such a configuration differs from the design application of the sensor (i.e.…”