Programming turbulence models in FORTRAN

The two-equation turbulence models Wilcox k-omega and Menter k-omega SST are programmed in FORTRAN and tested on the cases channel flow and backward-facing step. A short time step and a good initial field is required to obtain a solution. Results are adequate for most engineering purposes with a 15 \% error in predicted reattachment length. Gain from parallel programming is only found on the elliptic equation solver

Experimental Investigations of Single Oil Droplet Breakage in a Turbulent Water Flow

Multiphase flow and the transient behavior of the dispersed phase is important to numerous industrial applications. Predictive modeling of the dispersed phase would be beneficial in designing, for example, phase separation equipment. A possible framework for modeling of the dispersed phase is the population balance equation. As the dispersed phase may undergo breakage, the breakage phenomena must be sufficiently understood to formulate universally predictive models describing the breakage processes. However, additional data from experimental investigations is required as the current understanding of the breakage phenomena is not at the required level. In this work, the breakage phenomena are investigated experimentally by high-speed imaging of single octanol droplets in a turbulent water flow. A new experimental facility has been designed and constructed to perform the investigation. To determine the design criteria of the experimental facility the derivation of available turbulent breakage models was examined. In addition, a review of previous experimental setups and a review of isotropic turbulence facilities were performed. Four criteria were identified as critical for experimental investigation of turbulent droplet breakage. One, single droplets should be considered. Two, the entire breakage event must be observed by high-speed imaging and the procedure for extracting data must be transparent and well defined. Three, the experiments must be repeatable and reproducible as several experiments under the same conditions are required. Four, the region of breakage should be defined by known local flow conditions exhibiting low gradients in the turbulence level. To fulfill the determined criteria, a facility utilizing channel flow was constructed. A LDV investigation was performed to characterize the continuous flow conditions. The resulting instantaneous velocity measurements were used to obtain the turbulent kinetic energy. Taylor's frozen hypothesis was used for estimating two-point correlations, which were used to obtain the turbulent kinetic energy dissipation rate. A well-defined image analysis procedure was defined, elucidating the procedure of interpreting individual videos of breakage. Two interpretations of the breakage event, the initial breakage event definition and the cascade breakage event definition, were considered in the analysis. To combine the information obtained from several videos, a clearly defined statistical analysis procedure was provided. In the procedure a quantitative precision of the measured quantities were obtained using 95% confidence intervals. Based on the statistical procedure it was determined that the number of investigations required for statistically relevant results were ~30. Single octanol droplet experiments were performed and investigated to elucidate the breakage phenomena. The impact of both the mother drop size and the turbulence characteristics could be investigated, as each breakage event was associated with known local flow conditions from the LDV investigation. Known model concepts could be fitted to the data for the breakage time and the breakage probability with reasonable accuracy. However, the model coefficients were different from previous investigations, thus the models can not be considered to be universal. The average number of daughters and the daughter size distribution function exhibits behaviors which are not in agreement with available model concepts

Progress in Applied CFD. Selected papers from 10th International Conference on Computational Fluid Dynamics in the Oil & Gas, Metallurgical and Process Industries

Droplet size distributions in liquid-liquid turbulent flow are determined by droplet breakage and coalescence. The current understanding of these processes are not sufficient. An experimental study on single droplet breakage in turbulent flow, where coalescence can be neglected, is presented in this paper to study the droplet breakage mechanism. A rectangular channel consisting of a pair of opposite steel walls having a series of stationary protuberances to enhance turbulence level, and a pair of opposite glass walls that are smooth to facilitate image capture of the droplet breakage process is used as the droplet breakage channel. The commercial CFD code FLUENT is utilised to simulate the continuous single-phase flow in the droplet breakage channel with interest particularly in the turbulent characteristics such as the turbulent kinetic energy and turbulent energy dissipation rate, as these parameters are closely related to the droplet breakage process. The large eddy simulation (LES) method was used to provide detailed features of the flow. Results from LES were also compared with those from a RANS model (SST k-ω). The simulation results demonstrated that the turbulence level is enhanced across the pair of walls with protuberances. There are more coherent strong vortices in the region close to the wall with protuberances. Some preliminary experimental results on droplet breakage are also presented.publishedVersio

Dissipation rate estimation in a rectangular shaped test section with periodic structure at the walls

An experimental study of turbulent flow concerning the characterization of turbulence by two-point correlation and estimations of the turbulent dissipation rate is presented. The fluid used is deionized water and the test section used was a square shaped channel of 24mm by 30mm on the cross section with a length of 1m. The test section also presented periodic baffle structure at two of the walls for enhancing and maintaining turbulence. The study consisted in the measurement of the velocities at different positions of the channel using Laser Doppler Velocimetry instrument (LDV), the velocity measurement obtained were used for estimating two point correlations using the Taylor's frozen hypothesis. Finally, the results from the two-point correlation were used for estimating the turbulent dissipation rate. Considering the difference in the methods for calculating the dissipation rate, a comparison of the accuracy of each method is presented. It was found that the methods presented in this work showed similar results and trends of the dissipation rate with respect to changes in flow condition and distance to the channel walls. However, better accuracy was obtained by estimating the dissipation rate using the second order structure function and the Kolmogorov's two-third law

Experimental and numerical study on single droplet breakage in turbulent flow

Droplet size distributions in liquid-liquid turbulent flow are determined by droplet breakage and coalescence. The current understanding of these processes are not sufficient. An experimental study on single droplet breakage in turbulent flow, where coalescence can be neglected, is presented in this paper to study the droplet breakage mechanism. A rectangular channel consisting of a pair of opposite steel walls having a series of stationary protuberances to enhance turbulence level, and a pair of opposite glass walls that are smooth to facilitate image capture of the droplet breakage process is used as the droplet breakage channel. The commercial CFD code FLUENT is utilised to simulate the continuous single-phase flow in the droplet breakage channel with interest particularly in the turbulent characteristics such as the turbulent kinetic energy and turbulent energy dissipation rate, as these parameters are closely related to the droplet breakage process. The large eddy simulation (LES) method was used to provide detailed features of the flow. Results from LES were also compared with those from a RANS model (SST k-ω). The simulation results demonstrated that the turbulence level is enhanced across the pair of walls with protuberances. There are more coherent strong vortices in the region close to the wall with protuberances. Some preliminary experimental results on droplet breakage are also presented

Single Drop Breakage in Turbulent Flow: Statistical Data Analysis

To improve breakage models in the population balance framework, single octanol droplet experiments have been performed in a channel flow and recorded by high-speed camera. The study investigates impact of mother drop size on the breakage time, breakage probability, average number of daughters and the daughter size distribution for known turbulence characteristics. Each breakage event is associated with an individual turbulence level, based on the local flow characteristics. A clearly defined statistical analysis is presented. Using 95% confidence intervals, the precision of each of the determined properties is described quantitatively. Furthermore, the confidence intervals are a tool for determining whether an increased number of experiments will yield a significant increase in the precision, considered against the sources of error. It is found that 35–50 breakage events are sufficient to obtain confidence intervals of desired precision

Determination of Breakage Parameters in Turbulent Fluid-Fluid Breakage

Numerous sets of single‐particle breakage experiments are required in order to provide a sufficient database for improving the modeling of fluid particle breakage mechanisms. This work focuses on the interpretation of the physical breakage events captured on video. In order to extract the necessary information required for modeling the mechanisms of the fluid particle breakage events in turbulent flows, a well‐defined image analysis procedure is necessary. Two breakage event definitions are considered, namely, initial breakup and cascade breakup. The reported breakage time, the number of daughter particles created, and the daughter size distribution are significantly affected by the definition used. For each breakage event definition, an image analysis procedure is presented