Characterization of R134a two-phase flow regimes in horizontal and vertical smooth tubes with capacitive sensors

This paper presents the identification of slug/stratified-wavy, stratified-wavy and annular regimes for horizontal flow (in the range of mass fluxes 40 – 150 kg m⁻² s⁻¹), and slug, churn, and annular flow regimes for vertical upward flow (in the range 65 – 115 kg m⁻² s⁻¹) for R134a flow through 7 mm ID tube. Flow regimes are characterized based on time plot of normalized capacitive signals, kernel density estimation (KDE), power spectral density (PSD), and visualization results from a high-speed camera. Sensors with different axial lengths (D, 2D/3, and D/2) are also tested to study the adequacy of shorter sensors for the characterization of flow regimes. Results show that all three sensors have a similar capability of characterizing flow regimes, justifying the use of the shorter sensors in many applications with limited space.