Flow characteristics and phase interactions of evaporating sprays in gas-solid suspensions
Department of Mechanical Engineering
Doctor of Philosophy
Dreizin, Edward L.
Geskin, E. S.
Ho, Teh C.
Cross flow gas-solid
Evaporating spray jets have a wide range of applications in many industrial processes such as fluidized catalytic cracking of crude oil in petroleum refinery, coal gasification, fluidized coking to make synthetic crude oil, and condensed mode gas polymerization. The rapid evaporation of spray jets significantly affects the gas-solids mixing, solids concentration, gas/solids temperature and gas/solids velocity acceleration. The complex interactions mechanism of phase change, three phase interaction, heat and mass transfer of gas, solids and evaporating droplets dominate the process efficiency and product quality. In such three-phase flows, the phase transport characteristics are influenced by both droplet-gas interaction and droplet-solid interactions. In dense phase flow, the droplet-solid interaction becomes to play a dominant role in the transport of both spray and gas-solid flow. In very hot environments of gas-solid suspensions, the rapid vaporization of sprays results from the droplet-solids collisions. With the preheated temperature of solids beyond the Leidenfrost temperature, the heat transfer into spray droplets and resulted vaporization can be greatly restricted by the film boiling characteristics that are in turn coupled with collision dynamics. So far few studies on these complex yet important interaction mechanisms have been reported.
This doctoral dissertation study is aimed to gain an in-depth understanding on characteristic interactions among spray, gas and solids. The study is focused on cases where the rapid vaporization concurs with the spray injection into a gas-solid suspension that is preheated beyond the Leidenfrost temperature. A combined study of experiments, mechanistic modeling and numerical simulations is thus conducted to investigate three fundamental effects or mechanisms: (1) effect of vaporization on gas entrainment in a submerge spray jet; (2) jet characteristics of sprays from rectangular nozzles with a fan angle; and (3) dynamic transport in a type-IT Leidenfrost collision (between a small hot solid particle and a large droplet).
In the gas entrainment study, a direct measurement method is developed to study the air entrainment induced by a liquid nitrogen spray jet into an unbounded and stagnant room air. The air entrainment is determined by measurements of local oxygen concentration using an in-situ oxygen concentration analyzer, gas temperature using a thermocouple system, and droplet velocity using a Laser Doppler Velocimeter. The results not only support the approximation of flow similarities in evaporating sprays with round nozzles but also suggest that the evaporation of spray markedly weakens the gas entrainment. In addition, a parametric model is developed to provide a theoretical basis of the data analysis for the cross-section averaged spray evaporation rate within the spray jet region.
A three-dimensional simulation study is performed to investigate the hydrodynamic behaviors of a cross-flow liquid nitrogen spray injected into an air-FCC riser of rectangular cross-section. The gas-solid (air-FCC) flow is simulated using the multi-fluid method while the evaporating sprays (liquid nitrogen) are calculated using the Lagrangian trajectory method, with a strong two-way coupling between the two. Two parametric effects are studied in details here: (1) effect of aspect ratio of rectangular nozzles on flow characteristics and (2) effect of nozzle fan angle on spray coverage as well as vapor flux distribution. The study concludes that there exists an optimized value of aspect ratio for which the spray coverage area is maximized. The simulation also shows the formation of a dense layer of solids around the spray from the compression effect of vapor expansion and rigid wall of gas-solids flow chamber. It is also found that the spray coverage is basically dominated by the spray fan angle. The spray vaporization flux per unit area of spray coverage is highly non-linearly distributed along the spray penetration.
Interaction mechanism of large spray droplets and small solid particles is the key factor to govern the phase interactions of the spray in gas-solid flows as most of the evaporation of spray occurs in the initial part of spray jet as concluded by the fist two parts of this thesis. A careful analysis of this system reveals that this interaction mechanism is basically different than that of Leidenfrost collision between a small droplet and a large solid particle (defined as Type I Leidenfrost collision here). Portion of this study is thus focused on the heat and mass transfer during the Leidenfrost collisions between large evaporating droplets and small solid particles (defined as Type II Leidenfrost collision). In this study, experiments are conducted first to identify various modes of Type II Leidenfrost collision, and then basic mechanistic models have been developed to describe these modes. Parametric effects of different particle temperatures and velocities on the heat and mass transfer during the collision process have been illustrated using these models.
njit-etd2007-027 (118 pages ~ 10,326 KB pdf)
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Created September 9, 2008