دانشگاه: Louisiana State University and Agricultural & Mechanical College
: 7.25 MB
اندازه گیری و محاسبات جریان در مقیاس کامل قند تبخیری-قالب و در مدل های مقیاس آزمایشگاهی
Abstract: The circulation of massecuite is a key factor in achieving efficient heat transfer and crystallization in sugar evaporative crystallizers, and should be as high as practically possible for recovery, quality, and capacity reasons. This research report presents results on the circulation obtained applying modern experimental and numerical techniques. The main goals are contributing to expand the understanding of the process in sugar crystallizers; developing realistic models for the simulation of the circulation; and studying the effect of different design parameters. The circulation of massecuite is driven by buoyancy force due to density difference between the vapor generated and the surrounding liquid in calandria tubes, where the momentum exchange models normally used for flow simulation cannot predict correctly the complex interfacial interactions. To address this problem, the exchange of momentum in buoyancy-driven gas-liquid vertical channel flows has been investigated experimentally from a fundamental perspective, focusing particularly on the complex regimes associated with high void fractions and with highly viscous media. A drag model has been developed from the experimental results, which represents the transfer of momentum in gas-liquid multiphase flows under adiabatic conditions. A flow boiling instability has been identified in the calandria tubes, causing intermittent vaporization and pulsating circulation. This boiling instability leads to higher frictional resistance than in corresponding continuous adiabatic gas-liquid flows, and affects the transfer of momentum to the liquid phase. Experimental results on the flow in sugar evaporative crystallizers have been obtained using a lab-scale model, where the major features of the fluid flow were replicated and studied applying Particle Image Velocimetry. Field measurements of the flow in a full-scale continuous crystallizer have also been performed, where hot anemometers were used to determine the massecuite velocity and circulation. Numerical results obtained applying Computational Fluid Dynamics (CFD) are presented and compared with the measurements performed in the lab-scale and full-scale crystallizers, confirming that the CFD solutions developed represent reasonably the flows studied. The CFD model developed has been applied to investigate numerically the effect of different design parameters on circulation, identifying potential alternatives for improving the hydraulic design and performance of sugar crystallizers through enhanced circulation.