Characterization and Optimization of Fluid Flow in a High Biot Number System
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Characterization and Optimization of Fluid Flow in a High Biot Number System Richard Wlezien1, Jason Prapas1, Sarah Briggs, Marc Hodes1, Vincent Manno1, Douglas Matson1, and Luisa Chiesa1 Department of Mechanical Engineering, Tufts University, 200 College Avenue, Medford, MA 02155. ABSTRACT Experiments and analysis have been conducted to characterize flow separators used in applications where heated fluid passes between layers of solid material such as in the manufacturing of gelatinous materials. The Biot number of the configuration is the key parameter, and must be taken into account when optimizing performance. It is shown that most prior work was for low Biot number systems, and the particular configurations under consideration operate at high Biot number. Existing designs developed for lower Biot number (such as membrane filter spacers) are shown to perform poorly for this application. An experimental apparatus was designed and fabricated to quantitatively assess pressure drop through the system using different separation strategies. These results were compared with a simplified two-term model based on the physics of viscous drag in these devices. Channels without separators behave like classical Poiseuille flow. Channels with separators can be modeled with a two-term equation: a baseline Poiseuille term and a form drag term. A variety of separator designs are compared and their overall performance is discussed. We also illustrate the high sensitivity to gap height in all configurations. INTRODUCTION The curing of gels is a process by which the structure of the gel is cross-linked. This process is accomplished by circulating liquid at elevated temperatures between layers of the gel. A combination of thermal and convective processes is used to achieve the desired gel structure. The goal of this work is to optimize the cure time of the gel by maximizing convective heat transfer. In a simplified system, this can be achieved by maximizing the flow rate of fluid between the gel layers. If one fixes the longitudinal pressure drop along the layers, heat transfer can be increased by increasing the fluid flow rate between the layers, for example by increasing the height of the gap between the layers through which fluid can flow. In a real system, this gap height cannot be easily increased for two reasons. The first is that large gap heights would reduce the volume of the gel, and thereby reduce production rates. The second is that the uncured gel is not rigid, and a mesh spacer must be used between the layers to separate them and allow for the flow of the curing liquid. The key parameter used to describe this system is the Biot number, which is the ratio of the convective heat transfer rate from the liquid flowing between the solid layers relative to the conduction heat coefficient into the gel. For the vast majority of systems found in the literature, the solid layers are thin and the system is convection dominated – these are low Biot number configurations [1-8]. Most layer separation techniques have focused on low Bi
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