This work presents and analyses the results of an experimental activity aimed at the characterization of stratified air–water flow conditions, which have been poorly analyzed in previous studies although they are significant for industrial applications. Tests were performed in a 24 m long, 60 mm inner diameter PMMA pipe; the superficial velocities ranged between 0.03 m/s and 0.06 m/s for the water and between 0.41 m/s and 2.31 m/s for air. The pressure gradient along the pipeline was determined and compared to the one obtained implementing two-fluid models available in the literature. Fair agreement with the models was found only at high values of the superficial gas velocities, i.e., above 1.31 m/s. Moreover, the void fraction was measured through a resistive probe and compared with the values predicted by available models. Since none of them was able to satisfactorily predict the void fraction in the whole range of superficial velocities, a drift flux model was successfully implemented. Eventually, with both the measured pressure gradient and the void fraction, a two-fluid model was implemented in order to determine the interfacial shear stress and to compare the outcome with the literature, emphasizing the influence of the operating conditions on the prediction performance.

Aqueous foams are useful in several applications, especially to reduce liquid loading in the oil and gas industry. The rheology of these foams evolves rapidly, and suitable constitutive models are required to describe the resulting multiphase flow. We describe a new experimental setup for advanced rheometry involving 4-arm and 12-arm vane-in-textured-cup toolsets. The cup was designed to provide in situ foaming to minimize injection times and flow-history artifacts before measurement, while the 12-arm vane was selected to eliminate slip and generate a homogeneous stress field in a weak foam. Using these tools, we measure the decay of linear viscoelasticity and yield stress and link the rheological evolution to optical measurements of the bubble size distribution. Time-resolved rheological measurements of the full flow curve of an aging foam are performed and used to construct a rheological master curve. Measurements of the transient linear viscoelastic response and observations of the bubble size distribution show that foams, after an initial induction period, experience an increase in the Sauter mean bubble radius that scales as t^1/2. Using the well-known Princen and Kiss model as a framework, we define a single unique time-dependent shift factor that varies with the Sauter mean bubble radius and enables us to use the rheological master curve to predict the temporal evolution of the foam's elastic and steady-state viscoplastic properties.

Environmental concerns are forcing the replacement of commonly used refrigerants, and finding new fluids is a top priority. Soon the R134a will be banned, and the hydro-fluoro-olefin (HFO) R1234ze(E) has been indicated as an alternative due to its smaller global warming potential (GWP) and shorter atmospheric lifetime. Nevertheless, for an optimal replacement, its thermo-fluid-dynamic characteristics have to be assessed. Flow boiling experiments (saturation temperature T_sat = 5 °C, mass flux G = 65 ÷ 222 kg·m⁻²·s⁻¹, mean quality x_m = 0.15 ÷ 0.95, quality changes ∆x = 0.06 ÷ 0.6) inside a microfin tube were performed to compare the pressure drop per unit length and the heat transfer coefficient provided by the two fluids. The results were benchmarked for some correlations. In commonly adopted operating conditions, the two fluids show a very similar behavior, while benchmark showed that some correlations are available to properly predict the pressure drop for both fluids. However, only one is satisfactory for the heat transfer coefficient. In conclusion, R1234ze(E) proved to be a suitable drop-in replacement for the R134a, whereas further efforts are recommended to refine and adapt the available predictive models.