A review of the diagnostical methods of the flow patterns which largely determine the performance of gas—liquid systems encountered widely in chemical engineering practice is presented. The need for objective and reliable diagnostical criteria has been confirmed by the analysis of published studies; confidence in the flow pattern maps used in design and operation is limited. The methods of stochastic signal analysis as applied to the fluctuations of some structural (voidage) and energetic (pressure) parameters are found to be promising. The increasing availability of suitable hardware facilitates on-line diagnostics aimed at the optimization and safety control of industrial equipment.

Diagnostics of gas—liquid flow patterns in chemical engineering systems

Measurement of annular liquid film thickness in an open-end swirl injector

Local flow regime analysis in vertical co-current downward two-phase flow

Flow regime, void fraction and interfacial area transport and characteristics of co-current downward two-phase flow

In the upper part of boiling water reactors (BWR) the flow regime is dominated by a steam-water droplet flow with liquid films on the nuclear fuel rod, the so called (wispy) annular flow regime. The film thickness and liquid flow rate distribution around the fuel rod play an important role especially in regard to so called dryout, which is the main phenomenon limiting the thermal power of a fuel assembly. The deposition of droplets in the liquid film is important, because this process sustains the liquid film and delays dryout. Functional spacers with different vane shapes have been used in recent decades to enhance droplet deposition and thus create more favorable conditions for heat removal. In this thesis the behavior of liquid films and droplet deposition in the annular flow regime in BWR bundles is addressed by experiments in an adiabatic flow at nearly ambient pressure. The experimental setup consists of a vertical channel with the cross-section resembling a pair of neighboring subchannels of a fuel rod bundle. Within this double subchannel an annular flow is established with a gaswater mixture. The impact of functional spacers on the annular flow behavior is studied closely. Parameter variations comprise gas and liquid flow rates, gas density and spacer shape. The setup is instrumented with a newly developed liquid film sensor that measures the electrical conductance between electrodes flush to the wall with high temporal and spatial resolution. Advanced post-processing methods are used to investigate the dynamic behavior of liquid films and droplet deposition. The topic is also assessed numerically by means of single-phase Reynolds-Averaged-Navier-Stokes CFD simulations of the flow in the gas core. For this the commercial code STAR-CCM+ is used coupled with additional models for the liquid film distribution and droplet motion. The results of the experiments show that the liquid film is quite evenly distributed around the circumference of the fuel rods. The time-averaged liquid film thickness decreases with decreasing liquid flow rate, increasing gas flow rate and increasing gas density. By the introduction of spacers, the liquid film is strongly structured into streaks downstream of the spacer in a distinct pattern depending on the spacer shape. The streaks are mainly generated by the effect of circumferential vector components of the gas shear stresses on the gas-liquid interface, which are generated by the spacer. The resulting local decrease of liquid mass flow rate can be interpreted as a deficiency of a given spacer geometry, since it may promote dryout under BWR conditions in close proximity to the spacer. Furthermore spacers increase the droplet deposition compared to an open channel flow, thus increasing the mass flow rate in the liquid film and the dryout margin. Spacer vanes enhance the droplet deposition by an enhanced turbulence of the gas core, a deflection of the gas streamlines and the direct impact of droplets on the spacer vanes. The contribution of each phenomenon to the deposition

V.A. Maljusov

Papers

Pressure drop, mean film thickness and entrainment in downward two-phase flow