Investigation on Variations of Void Fraction in a Subcooled Boiling Channel Under Vertical Forced Excitations

Abstract

Experimental tests were carried out to investigate the vertically forced excitation effects on the subcooled boiling flow. The heated circular channel with an inner diameter of 11.9 mm was operated with various heat fluxes (q″ ≈ 14.6–41.1 kW/m2) and inlet flow conditions (vin ≈ 0.21–0.42 m/s) under various vertical forced excitations (f ≈ 0–1.63 Hz), and the time variations of void fraction, near-wall fluid temperature and pressure were recorded during the tests. Fast Fourier transform (FFT) was applied to extract the dominant frequency from the transient signals, and the variations of averages and standard deviations of test data were obtained for analysis. Under lower heat flux, lower flow, and lower void conditions, the time-averaged void fraction may decrease under forced excitations, and the dominant frequencies of void variations were identical to those of forced excitations. However in higher heat flux and higher void conditions, the void fraction can slightly increase under forced excitations, but the excitation frequencies may not be clearly observed in the void FFT plots. In general, the transient and time-averaged void fraction can be affected by forced excitations, and the void variation trends are similar to those of near-wall fluid temperature, which implies the void variations may be related to the changes of thermal boundary layer thickness. Besides, the potential variations of void fraction were estimated by assuming changes of heat transfer coefficient and/or wall superheat, which appear similar trends to the observed void variations in the present tests.