Internal Fluid Mechanics of an Effervescent Atomizer PDF Download

Author: Miguel A. Balzán
Publisher:
ISBN:
Category : Atomizers
Languages : en
Pages : 314

Get Book Here

Book Description
An experimental investigation was conducted with the purpose of studying the effects of selected operating and design variables in the flow inside an effervescent atomizer. A series of tests were performed in a horizontally oriented, square conduit of 12.7 mm in diameter which constituted the mixing chamber of the effervescent atomizer. The operating fluids were water and air. The air was injected perpendicularly into a fully developed, turbulent water flow, whose bulk water velocity values ranged between 1.1 and 4.3 m/s. The gas mass flow rate values were in the range between 8 and 60 × 10-3 g/s. Three different gas injectors, with diameters of 0.27 mm, 0.52 mm and 1.59 mm were used. The combination of variables allowed the operation of the atomizer within the limits of what constitutes a bubbly flow in pipes. High-speed shadowgraphy was the technique used to investigate thoroughly the dynamics between the gas and liquid phases near the gas injection region as well as upstream the discharge nozzle. A set of original, empirical expressions used to estimate the incipient centerline and borderline trajectories of the gas phase, during its initial interaction with the liquid and based on dimensionless parameters, were introduced. The assessment of the correlations gave a strong prediction of the initial centerline and borderline trajectories of the gas jet in the flowing liquid. The effects that the gas injection velocity, liquid mean velocity and injection gas injection diameter have on the process of bubble generation were investigated. Four distinct regimes were identified: Single Bubbling (SB), Pulse (P), Elongated Jetting (EJ) and Atomizing Jetting (AJ). It was observed that the shift between regimes occurs gradually, producing the need to identify transitional regions: SBP and PTJ. Sets of independent dimensionless variables were used to categorize the proposed regimes in bubble formation maps. Empirical correlations that delimit the boundaries between ordered and chaotic bubble generation were determined. An introductory description of the forces involved in the bubbling process was conducted. The results indicated that the form-induced drag and added mass force were dominant detaching and cohesive force respectively. While there was agreement with previous works regarding the dominant breakup effect, the results obtained for the main attaching force were unique. Also, a novel methodology for the estimation of dynamic shape based drag and added mass coefficient was included. The morphological features of the gas jet were described through empirical correlations based on relevant dimensionless numbers associated to the variations of three fundamental design parameters: liquid cross-flow velocity, gas mass flow rate and the nozzle dimensions. The gas jet features were compared with representative statistical diameters from the population distribution, resulting in an estimation of the averaged Sauter mean diameter and maximum bubble diameter as a function of the gas jet dimensions. It was determined that the gas injection conditions play a fundamental role in the internal flow characteristics for an effervescent atomizer.