Investigation of bubble breakup and coalescence in a packed-bed reactor – Part 2: Development of a new bubble breakup and coalescence model

A mechanistic model of bubble breakup and coalescence has been developed for a packed bed. Bubble breakup and coalescence models are developed for two coalescence and three breakup mechanisms by taking account of geometry effects and local flow conditions. The bubble size distribution estimated with the present bubble breakup and coalescence models are compared with the experimental data. Change of bubble size distributions along the axial direction is studied with the median bubble size. Median bubble size as a function of the axial location is estimated under two inlet flow conditions: (1) bubble breakup dominated flow and (2) bubble coalescence dominated flow. The predictions of the median bubble size with the present model result in the best among other existing bubble breakup and coalescence models. However, the prediction of the median bubble size for the bubble coalescence dominated flow is still significantly larger than the experimental data. Breakup and coalescence coefficients need to be adjusted in order to predict more accurate bubble size distributions and median bubble size for both flow conditions. For the bubble breakup dominated flow, the breakup and coalescence coefficients are found to be 0.35 and 0.4, respectively. For the bubble coalescence dominated flow, the breakup and coalescence coefficients are found to be 0.35 and 0.01, respectively.     

液滴在气体介质中剪切破碎的数值模拟研究

液滴变形和破碎是燃料抛撒问题的重要过程.本文将VOF方法和标准k-ε湍流模型组合,建立了计算液滴在气流中变形破碎过程的数值方法.数值模拟了相关的实验,计算得到的液滴破碎过程与实验结果符合较好.在此基础上,分析了几个关键参数(Weber数、Ohnesorge数、液气密度比)对液滴破碎过程的影响.计算结果表明,Weber数...     

Breakup excitation function at backward angles from α-spectra in the Li + Sm system

Breakup cross sections were obtained for the ⁶Li + ¹⁴⁴Sm system at energies above and below the Coulomb barrier from a detailed analysis of the data recorded at backward angles. These cross sections are compared with inelastic target excitations previously reported revealing a similar behavior as a function of the bombarding energy but a large absolute difference between them. Using kinematical considerations we have analyzed possible contributions from different breakup channels and we have extracted information on magnitudes such as the relative kinetic energies of the corresponding breakup fragments.     

Finite Range Effects on Fusion and/or Breakup of ^6He^＋238U and ^11Li＋^208pb Systems

We have studied the effects of the finite range of the interaction between the fragments of the projectile on the fusion and/or breakup of ⁶He+²³⁸U and ¹¹Li+²⁰⁸Pb systems at near barrier energies within the framework of dynamic polarization potential approach. It has been found that at near barrier energies the maximum flux is lost to the breakup channel and at energies well above the Coulomb barrier the fusion coupled with the breakup channel opens up, initially with sharp rise and later becoming saturated at energy nearly twice of theCoulomb barrier. Further, it is found that the breakup cross section increases with the increasing range of the interaction between the fragments of the projectile while the fusion coupled with the breakup channel cross section decreases with the increasing range.     

Motion of non-wetting drop in constricted geometry

This work is a contribution to the study of deformation of a non-wetting drop transported under the combined effect of gravity and permanent fluid motion in a vertical channel. The deformation being caused during passage of the drop through a constriction formed by two spherical obstacles placed opposite in a vertical channel. For this purpose a three-dimensional computation is conducted in order to illustrate the behavior of the drop in the condition of non-wettability. The flow based on Navier–Stokes equation is solved numerically with volume of fluid (VOF) method. The corresponding simulations are carried out in view to analyse the behavior of the drop when it is forced to move between the obstacles for different values gap size until the breakup is obtained.     

Bubble–wall interaction and two-phase flow parameters on a full-scale boat boundary layer

Full scale bubbly flow experiments were performed on a 6 m flat bottom survey boat, measuring the void fraction, bubble velocity and size distributions as the bubbles naturally entrained at the bow of the boat interact with the boat’s boundary layer. Double-tip sapphire optical probes capable of measuring bubbles down to 50 μm in diameter were specifically designed and built for this experiment. The probes were positioned under the hull at the bow near the bubble entrainment region and at the stern at the exit of the bottom flat plate. Motorized positioners were used to vary the probe distance to the wall from 0 to 50 mm. The experiments were performed in fresh water (Coralville Lake, IA) and salt water (Panama City Beach, FL), at varying velocities with most data analysis performed at 10, 14 and 18 knots. The results indicate that the bubbles interact significantly with the boundary layer. At low velocity in fresh water, bubble accumulation under the hull and coalescence are evident by the presence of large bubbles at the stern. At high speeds bubble breakup dominates and very small bubbles are produced near the wall. It is also observed that salt water inhibits coalescence, even at low boat speeds. The void fraction increases with speed beyond 10 knots and peaks near the wall. Bubble velocities show slip with the wall at all speeds and exhibit large RMS fluctuations, increasing near the wall.     

Simulation of liquid solvent atomization in compressed CO2

The work investigates numerically the atomization regime of a liquid injected into compressed CO₂ under subcritical conditions, i.e. below the CO₂-solvent critical pressure. To vary the conditions within the atomization regime whilst keeping up with realistic experimental background, ethanol and methylene chloride were selected as injected fluid and pressure was modified as well. Results first show that the jet indeed breaks up by atomization, which confirms the validity at high pressure of the breakup classification diagram. Aiming at evaluating the size distribution of the droplets formed by the jet atomization, two methods of interface tracking were investigated. Compared to the VOF-PLIC classical method, the novel sub mesh (VOF-SM) approach allows for determining smaller sized droplets without digital broadcasting.     

Ballistic imaging of liquid breakup processes in dense sprays

Ballistic imaging is the name applied to a category of optical techniques that were originally developed for medical applications. Recently, ballistic imaging was adapted to acquire instantaneous images of the liquid core inside atomizing sprays; a region that has been heretofore inaccessible to spray researchers. An important difference between spray research and the medical imaging problem is the need for high fidelity single-shot (within 10 μs) imaging in a spray whereas stationary tissue images can be averaged. Transient ballistic imaging diagnostics have been used to reveal details of the primary breakup process in a LOX injector, a turbulent water jet, a water jet in cross-flow, a transient diesel fuel spray, a rocket fuel injector, and an aerated spray. This paper briefly discusses various methods for imaging the liquid core, it introduces ballistic imaging and provides specific examples, it describes detailed studies of photon transmission through dense media, and it then discusses incorporation of those results into a model for a ballistic imaging instrument that can evaluate and optimize various concepts.