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COMPUTATIONAL HEAT TRANSFER MODELLING OF RICE - WATER SUSPENSION IN TUBE

Kanishka Bhunia
Department of Biological Systems Engineering, Washington State University, Pullman, WA 99164-6120

A. K. Datta
Agricultural and Food Engineering Department, Indian Institute of Technology Kharagpur, Paschimbanga, India - 721 302

Résumé

In this study of solid-liquid flow, rice was used as the dispersed medium and water was used as the carrier fluid. Experiments were carried out on slurries with solid concentrations of 5%, 10%, and 15% w/w which flowed in a 13 mm ID and 3 m long tube- in- tube heat exchanger. Steam was used as the heating medium. The effects of flow rate, particle surface area and particle concentration were investigated. Calculated convective film to particle heat transfer coefficient (hfp) values ranged from 11 to 32 kW m−2 K−1 for rice with uncertainty of ±2 kW m−2 K−1. A decrease in heat transfer coefficient values was found as a result of short residence time at the higher flow rates. To investigate the solid-liquid two-phase flow Eulerian k − ε multiphase model was adopted in simple axisymmetric geometry. Velocity profiles of the liquid and solid phases with different particle fractions were estimated from the simulated results. The respective velocities of both phases were higher in the upper part of the tube than in the lower portion because of settling caused by gravity. The slip velocity of the particles was estimated from the simulations and it ranged from 13.83 cm s−1 to 19.38 cm s−1 for the rice particles. The rice grains always lagged the liquid phase. The particle volume concentration profile was also investigated and it was observed that a high particle concentration formed a core around tube centre line. Dimensionless correlations were developed to predict liquid-to-particle heat transfer coefficients from combining experimental results and simulations. The combinations were able to predict hfp values within particular range of slip Reynolds number (Res) obtained from slip velocities as predicted by the simulations obtained from FLUENT. The exponents of Res and the solid volume fraction were negative and it was positive for the Prandtl number. The correlations predicted hfp and Nusselt number values with a maximum error value of 10%. The correlations developed in this work gave better fitting lines with R2 value of 0.98 and a Rd (relative deviation) value of 1.08 for rice particulate flow. Further relative to experimental values a correlation was able to represent the relationship between dependent and independent parameters at more than 95% level of significance.

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