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A comprehensive computational fluid dynamics CFD model was developed in the present study to gain insight into the solid suspension in a stirred slurry reactor. The preliminary simulations highlighted the need for the correct modeling of the interphase drag force. A two-dimensional model problem was then developed using CFD to understand the influence of free stream turbulence on the particle drag coefficient. The proposed correlation was then incorporated in a two-fluid model (Euler−Euler) along with the standard k−ε turbulence model with mixture properties to simulate the turbulent solid−liquid flow in a stirred reactor. A multiple reference frame approach was used to simulate the impeller rotation in a fully baffled reactor. A computational model was mapped on to a commercial CFD solver FLUENT6.2 (of Fluent Inc., USA). The model predictions were compared with the published experimental data of Yamazaki et al. [Powder Technol.1986, 48, 205] and Godfrey and Zhu [AIChE Symp. Ser.1994, 299, 181]. The predicted results show reasonably good agreement with the experimental data. The computational model and results discussed in this work would be useful for extending the applications of CFD models for simulating large stirred slurry reactors.

Rotary desiccant wheels are widely used in dehumidification and energy recovery applications. In this work, we have developed a 2D, steady state model of a rotary desiccant wheel. Mass and energy balance equations for the air streams and the desiccant wheels were developed. The hydraulic diameter and surface area for heat and mass transfer were calculated based on knowledge of the flute geometry. Appropriate correlations for the Sherwood number and Nusselt number were used to estimate heat and mass transfer coefficients. The model is capable of predicting steady state behavior of desiccant wheels having at the most three sections (process, purge, and regeneration). The mathematical model was validated using a real desiccant wheel, and the calculation results are in reasonable agreement with the experimental data. Based on this model, the temperature and humidity profiles in the wheel during both the dehumidification and the regeneration processes are analyzed. The simulated results were used to gain an insight into the operation of desiccant wheels. The model and the presented results will be useful for optimizing dehumidification and energy recovery applications.

Gas−liquid flows in a stirred vessel exhibit different flow regimes and demonstrate complex interaction of transport processes with varying spatio-temporal scales. The knowledge of key space and time scales of fluid dynamics is important for designing and enhancing the performance of gas−liquid stirred reactor. The present study uses simple, robust, and nonintrusive experimental techniques (torque and pressure sensors) to characterize the fluid dynamics in a gas−liquid stirred vessel. Time series obtained from the pressure and torque sensors were analyzed to develop criteria for flow regime identification. Further analyses of torque and pressure time series were done to extract valuable information of different time scales of fluid dynamics. The detailed analysis of torque and wall pressure fluctuations provided not only an insight into the fluid dynamics but also a possible opportunity to on-line monitoring of gas−liquid flows in stirred vessel.

Axial-flow impellers, like pitched-blade impellers, are being increasingly used for gas−liquid systems in stirred vessels. In this work we have used particle image velocimetry (PIV) and computational fluid dynamics (CFD) models to investigate gas−liquid flow generated by a downflow pitched-blade turbine. PIV measurements were carried out in a fully baffled stirred vessel (of 0.19 m diameter) with a dished bottom. Angle-resolved measurements of the flow field with and without gas dispersion were carried out. An attempt was made to capture key details of the trailing vortex, the accumulation of gas, and the flow around the impeller blades. A two-fluid model along with the standard k−ε turbulence model was used to simulate dispersed gas−liquid flow in a stirred vessel. The computational snapshot approach was used to simulate impeller rotation and was implemented in the commercial CFD code, FLUENT 4.5 (of Fluent Inc., USA). The model predictions were verified by comparison with the PIV measurements and other available experimental data. The computational model and results discussed in this work are useful for better understanding and simulation of gas−liquid flow generated by axial impellers in stirred vessels.

The computer automated radioactive particle tracking (CARPT) and positron emission particle tracking (PEPT) techniques have been developed to characterize opaque multiphase flows. In CARPT and PEPT, the Eulerian flow field is inferred from the knowledge of reconstructed tracer particle trajectories. The present study was undertaken to assess the strengths and limitations of the process of estimating the Eulerian flow field from particle trajectories. A two-dimensional problem, which mimics the characteristics of flow in a stirred tank reactor equipped with a standard Rushton turbine, was considered. The Eulerian flow field was numerically simulated. Care was taken to minimize effects of numerical issues on the computed flow field, which was then used to calculate particle trajectories. Standard CARPT data processing was carried out on the simulated particle trajectories to estimate the Eulerian flow field. This estimated flow field was compared with the original flow field used for trajectory simulations to evaluate possible errors associated with the CARPT data processing and flow follow-ability of the particles. Influence of grid used for data processing, sampling frequency, and particle size and particle density on the estimated flow field was examined. The study highlights several issues pertaining to the estimation of the Eulerian flow field from Lagrangian information. The results provide guidelines for selecting appropriate parameters in processing of CARPT or PEPT data.

Chemical Engineers generally model reactive mixing problems using the concepts of macro- and micro-mixing. Various simplified models have been published using these concepts and they are briefly reviewed in this paper to identify their ranges of applicability. A new model is then proposed to compensate for some inadequacies of the previous ones. An attempt has been made to develop a coherent methodology for the calculation of reactive mixing. A new computer code RIAT (for reactions in agitated tanks) is developed to simulate reactive mixing in three dimensional turbulent flows. Comparison of predicted results with experimental data in stirred vessel reactors for the case of fast consecutive-competitive reactions is presented.

The turbulent flow of mildly viscoelastic liquids in curved lubes is analyzed using boundary layer approximations. The momentum integral approach is used and the governing equations are solved numerically by the Runge-Kutta-Merson technique. The influence of Deborah number on the velocity distribution and the boundary layer thickness has been studied for the first time. The theoretically developed expression for frictional losses was checked with available experimental data and sound agreement was found. The results of this analysis are likely to find application in the biomedical fluid mechanics and in the analysis of the flow with so-called "drag reducing" additives.

The residence time distribution in a helically coiled tubular reactor is computed for laminar flow of inelastic non-Newtonian liquid at small Dean number. Effect of pseudoplasticity on the residence time distribution is explored. It was found that pseudoplaslicity of the liquid makes the residence time distribution narrower. The effect of dialalant fluid behavior is opposite but very small in magnitude. The results of numerical integrations are correlated with power law index, allowing predictions over a wide range of the index. The results could be used to predict conversion of reactions in a coiled reactor, such as continuous polymerization or fermentation reactors, where the reaction mixtures are likely to be non-Newtonian in nature. Results will be also useful in characterization of coiled chromatographic columns.

This paper reviews the available information on numerical simulation of dispersed gas-liquid flows. Emphasis is on informing the reader about various aspects of constructing simulation models rather than giving an exhaustive literature review. The information is organised in a way so as to provide answers to the following questions: how to formulate model equations? how to select suitable algorithms and numerical techniques to solve these model equations? how to translate these into workable computer codes? how to use such codes for simulating flows in industrial equipment? Though greater emphasis is given to dispersed gas-liquid flows, the methodology can be applied to any multi-phase problem. Special features of multi-phase flow simulation over single-phase simulation are highlighted. A case of gas-liquid flow in a bubble column is presented as an illustration for the general methodology. The simulated mean flow field agrees reasonably with the experimental data. Properly validatedcfdcodes thus can serve as a useful tool for design engineers of multi-phase systems. Some of the common pitfalls in using simulation codes for design are also discussed. This review is expected to give an overall idea about the present state-of-art of two-phase simulation in industrial equipment.