Date of Award

Fall 2002

Document Type

Dissertation

Degree Name

Doctor of Philosophy in Mechanical Engineering - (Ph.D.)

Department

Mechanical Engineering

First Advisor

Chao Zhu

Second Advisor

Robert Pfeffer

Third Advisor

Rajesh N. Dave

Fourth Advisor

Edward L. Dreyzin

Fifth Advisor

Teh C. Ho

Abstract

Injection of evaporating sprays into gas-solid flows is encountered in many engineering processes such as energy production industry and chemical industry. The phenomenon involves phase change, three-phase interactions, heat and mass transfer. All of these characteristics control the process efficiency, pollutant production and product quality. However, very limited studies are available on the evaporating spray jets in gas-solid flows, especially on the spray evaporation rate within a gas-solid medium. A combined study of experiments and theoretical analysis has been carried out here to investigate the fundamental mechanism of evaporating Crossflow spray jets in gas-solid flows.

In this study, in addition to a laboratory scale circulating fluidized bed to provide a continuous gas-solid flows, a laser/lamp-light assisted visualization and image analysis system and a computer aided temperature measurement system have been developed which enables measurement of spray trajectories and temperature distributions of mixture phases in dilute/dense gas-solid flows. All the experiments have been performed in the circulating fluidized bed with a simple rectangular column, controllable solids load and flow conditions, and well-defined liquid nitrogen sprays. The spray trajectory, spray penetration length, and flow pattern are investigated. The geometric and operating parameters, such as nozzle size, nozzle type, injection angle, jetting velocity, and solids loading are studied in the experiments. The experimental study shows that the loading of solid particles in mainstream can significantly shorten the penetration length and alter the spray structure. It is also shown that the quick evaporation of spray droplets leads to the dilution of solids concentration in the evaporation region as well as the reduction of phase temperatures. In this study, a fundamental parametric model for applications of an evaporating liquid jet into a gas-solid flow, which takes into account the three-phase interactions as well as phase changes. The model is focused on the study of the effects of spray parameters on the mixing characteristics such as spray penetration length, temperature and velocity of each phase, trajectories, and the phase volume fraction distributions. The governing equations are based on the conservation principles of mass, momentum and energy of all three phases. The model predictions have also been found in good agreement with the experimental results.

Droplet evaporation rate is the most important factor to affect the phase interactions of the spray in gas-solid flows. The spray evaporation is dominated by the heat transfer through collisions between droplets and solid particles. Part of this study is focused on the Leidenfrost collisions between evaporating droplets and solid particles, which are involved in many multiphase flow applications, e.g., petroleum refinery, surface coating, and fire quenching. In this study, an analytical model has been developed to describe the Leidenfrost collision between a droplet and a hot solid sphere. The whole collision process, the maximum collision time, the maximum deformation area, and the evaporation rate are simulated. Effects of solid curvature and Weber number on the collision time and droplet evaporation rate are illustrated. Modeling predictions are validated by the available experimental results.

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