Quantifying interfacial parameters of upward and downward annular flow condensation from high-speed visualization

Abstract

Condensation of R-134a in upward and downward annular flow was experimentally studied in a 5-mm ID transparent test section using a LED-based image processing technique. In addition to the flow orientation, the frequency of the film structures was quantified in terms of the following parameters: total mass flow rate, condensation heat transfer rates (outlet vapor quality) and test section inclination with respect to the horizontal. The image processing technique combined Fourier analysis and the Kernel density estimation technique to calculate the two-dimensional (amplitude-frequency) probability density function of the film flow structure as a function of the independent parameters. Although the tube inclination and the condensation rate showed some influence on the frequency of the interfacial waves, the most significant effect was due to the flow orientation, which resulted in abrupt frequency changes apparently linked to flow regime transitions dictated by shear and gravity forces. Keywords Condensation • Annular flow • Quantitative visualization • PDF • KDE List of symbols A Wave amplitude (px, mm) C Constant in Eq. (4) D Tube diameter (mm) E Expected value f Wave frequency (Hz) f c Characteristic frequency (Hz) g Acceleration of gravity (m s −2) G Refrigerant mass velocity (kg m −2 s −1) j Superficial velocity (m s −1) j * Dimensionless superficial velocity m Constant in Eq. (4) p Probability density function u Uncertainty U Expanded uncertainty x Vapor mass quality Greek Tube inclination (rad, •) Mass density (kg m −3) Subscripts g Vapor l Liquid max Maximum o Outlet