The paper deals with the impact of inlet conditions on bubble to slug flow transition in mini systems. A new experimental test loop with a glass mini-tube (D=1.2 mm ID) has been constructed to assess the effects of inlet conditions on the two-phase flow pattern development in the spatial and temporal domains. The interchangeable inlet part of the test section allowed different geometrical combinations for the mixing of gas and liquid prior to it entering the mini-tube. Porous media mixer and cross-junction mixer were considered. High speed video recordings were taken of 70 combinations of flow rates, corresponding to superficial velocities ranging from 0.2 to 11 m/s and 0.25 to 3 m/s for air and water, respectively. The following discernible flow patterns are considered: bubbly, slug and semi-annular flow. No significant differences were found when comparing flow pattern maps for each mixer. Digital image post processing of high speed video recordings was used to estimate the equivalent diameter for every gas structure. A comparison of the distribution of bubbles with equivalent diameter revealed the inlet mixer's strong impact on bubble size and bubble distribution along the mini-tube and thus, the bubble to slug flow transition. The VOF method, implemented in ANSYS Fluent was used as the numerical tool to predict the flow patterns in a mini-tube of 1.2 mm ID. A novel approach to mimic continuous mixing is presented. By changing only the prescribed flow rates, different flow patterns can be simulated. Similar interfacial structures were obtained by numerical simulation and experiment for both mixers. Reasonable quantitative agreement was also achieved when analyzing bubble to slug flow transition.
COBISS.SI-ID: 13081371
The main objective of this study was to develop a piezoelectric probe for measuring the impact rate of particles to enable characterization of the shape of particle number density profiles and characteristic particle impact frequencies inside the draft tube of a Wurster coating chamber. Pellets from 800microm to 900 m were subjected to fluidization in a conventional and swirl Wurster coating chambers. Fast Fourier Transform (FFT) analysis of the raw signal obtained at four heights and three different radial distances from the draft tube centerline was performed for experiments in a conventional and a swirl Wurster coating chamber at different process parameters. Power spectra revealed three characteristic frequency bands for both chambers: 1-5 Hz, 5-7 Hz, and 12-14 Hz. Particle number density profiles demonstrated an evident difference between both chambers. In the conventional Wurster coater, the location of the maximum number of particle impacts was close to the tube center, whereas in the swirl Wurster coater the particle impacts were distributed rather uniformly. The difference in particle number density profiles for both process chambers can help to understand the difference in performance of both Wurster coater designs.
COBISS.SI-ID: 3357553
To better understand the underlying two-phase phenomena and thus better predict transitions between the various two-phase flow patterns, it is necessary to update our way of thinking from one-of-a-kind flow pattern maps of limited applicability to a generalized approach based on first principles, mechanistic analysis and multi-scale characterization and representation of the important features of these complex flows. While in macro-sized channels and pipes this need is typically addressed by the use of empirically validated flow regime maps, there is - as yet - no consensus on two-phase flow regime maps for microchannels and miniature pipes. This study presents a set of recommendations for the development of a new comprehensive type of flow pattern map that not only covers adiabatic, evaporating and condensing flows in one seamless flow pattern identification tool, but also includes multiscale information about the flow itself, and furthermore contains embedded mechanistic methods for the principal two-phase phenomena for use in developing unified models for pressure gradients, heat transfer, void fraction, CHF, etc., all in one coherent global method.
COBISS.SI-ID: 12534811