Perumal Kumar

CFD modelling and mixing in stirred tanks

In order to develop rational design procedures for stirred vessels, several important points have to be addressed. Firstly, it is necessary to relate the impeller-vessel geometry to the flow field produced. Secondly, it is necessary to establish energy balance in stirred vessels. Thirdly, it is necessary to understand the linkage between the flow field and the design objective. All the three aspects have been addressed here. Five different designs of axial flow impellers have been studied. The zonal modelling concept used by Sahu et al. (1998 Ind. Engng Chem. Res. 37, 2116–2130.) has been extended to predict the flow field generated by these impellers. It is observed that the predictions of turbulent kinetic energy (k) were significantly improved by using zonal modelling. A new method has been proposed to estimate the values of turbulent energy dissipation rate (e). This new method gave excellent comparison between the estimated values and CFD simulations. The CFD simulations have been extended to predict the mixing time for different impellers. The predicted mixing times are in excellent agreement with the experimental measurements of Rewatkar and Joshi (1991, Chem. Engng commun. 91, 322–353).

Reviews on drag reducing polymers

Polymers are effective drag reducers owing to their ability to suppress the formation of turbulent eddies at low concentrations. Existing drag reduction methods can be generally classified into additive and non-additive techniques. The polymer additive based method is categorized under additive techniques. Other drag reducing additives are fibers and surfactants. Non-additive techniques are associated with the applications of different types of surfaces: riblets, dimples, oscillating walls, compliant surfaces and microbubbles. This review focuses on experimental and computational fluid dynamics (CFD) modeling studies on polymer-induced drag reduction in turbulent regimes. Other drag reduction methods are briefly addressed and compared to polymer-induced drag reduction. This paper also reports on the effects of polymer additives on the heat transfer performances in laminar regime. Knowledge gaps and potential research areas are identified. It is envisaged that polymer additives may be a promising solution in addressing the current limitations of nanofluid heat transfer applications.

Computational Fluid Dynamics-Based Hydrodynamics Studies in Packed Bed Columns: Current Status and Future Directions

Abstract A careful review of the literature reveals that extensive research has been done on the hydrodynamics in packed bed columns using turbulence models. It can be noted that the choice of turbulence model is influenced by the number of phases, type of fluid, Reynolds number range and the type of packing. Thus, comparison of turbulence models for the selection of a suitable model assumes great importance for the better prediction of flow pattern. This is due to the fact that poor prediction of the flow pattern can lead to a limited heat and mass transfer model as the rate of transfer processes in packed bed is governed by the hydrodynamics of the packed bed. The aim of this paper is to give a review of the computational fluid dynamics (CFD)-based hydrodynamics studies of packed bed columns with the primary interest of studying pressure drop and drag coefficient in packed beds. From the literature survey in Science Direct database, more than 48,000 papers related to packed bed columns have been published with more than 3,000 papers focused on the hydrodynamic studies of the bed to date. Unfortunately, there are only a few studies reported on the hydrodynamics of packed columns under supercritical fluid condition. Therefore, it is imperative that the future work has to focus on the hydrodynamics of supercritical packed column and particularly on the selection of suitable turbulence model.

LDA measurements and CFD simulations of flow generated by impellers in mechanically agitated reactors

The turbulent flow produced by various designs of axial flow impellers in a stirred vessel was measured using a laser Doppler anemometer (LDA). Flat-bottomed cylindrical vessels of diameters 0.3 m and 0.5 m provided with 4 baffles ofT/10 width were used as the reactors. The standard two-equation (k-ε) turbulence model was used to numerically simulate the flow (both 2D and 3D). Three numerical schemes, namely upwind scheme, hybrid scheme and power-law scheme, were used to evaluate the competitiveness of the various schemes. The effects of the initial guess values of the flow variables, the under-relaxation parameters and internal iterations etc. on the rate of convergence were analysed for both 2D and 3D models. The effect of grid size was also studied in both the cases. The 2D and 3D predictions were compared and it was observed that the 2D predictions were not good enough to give the details of the flow. So, the sensitivity of the model parameters on the flow characteristics was investigated thoroughly in the case of 3D models. It was observed that no single set of model parameters could yield even reasonable agreement between the model predictions and the experimental observations throughout the vessel. Therefore, the concept of zonal modelling was introduced for ther-z plane. It was observed that with the introduction of zonal modelling the predicted values of flow variables were in good agreement with experimental data even close to the top surface of the tank. The same could not be obtained with a standard set of parameters and single zone.

Analysis of solar water heater with parabolic dish concentrator and conical absorber

This research focuses on developing novel technique for a solar water heating system. The novel solar system comprises a parabolic dish concentrator, conical absorber and water heater. In this system, the conical absorber tube directly absorbs solar radiation from the sun and the parabolic dish concentrator reflects the solar radiations towards the conical absorber tube from all directions, therefore both radiations would significantly improve the thermal collector efficiency. The working fluid water is stored at the bottom of the absorber tubes. The absorber tubes get heated and increases the temperature of the working fluid inside of the absorber tube and causes the working fluid to partially evaporate. The partially vaporized working fluid moves in the upward direction due to buoyancy effect and enters the heat exchanger. When fresh water passes through the heat exchanger, temperature of the vapour decreases through heat exchange. This leads to condensation of the vapour and forms liquid phase. The working fluid returns to the bottom of the collector absorber tube by gravity. Hence, this will continue as a cyclic process inside the system. The proposed investigation shows an improvement of collector efficiency, enhanced heat transfer and a quality water heating system.

Numerical Studies on the Laminar Thermal-Hydraulic Efficiency of Water-Based Al2O3 Nanofluid in Circular and Non-Circular Ducts

Abstract This research presents the numerical results of laminar forced convective heat transfer performance and the flow behaviour for Al2O3-water nanofluid in circular, 2:1 rectangular, 4:1 rectangular and square ducts. The nanoparticles concentration studied were 0.01%, 0.09%, 0.13%, 0.25%, 0.51%, 1.00% and 4.00%. Single phase constant and temperature-dependent properties were employed. For the case of constant properties, the thermal performance and pressure drop increase with the increase of nanofluid concentration and Reynolds number. For the temperature-dependent properties, the Nusselt number and pressure drop also increase when the Reynolds number increases. However, there is a slight decrement in the Nusselt number and no significant pressure drop increment when the nanofluid concentration is increased from 0.01% to 1.00%. When the concentration is further increased to 4.00%, the Nusselt number and pressure drop increase. For the temperature-dependent model, lower thermal performance and pressure drop are identified when compared to those of the constant properties. The maximum Nusselt number enhancement and pressure drop increment occur at the concentration of 4.00% and Reynolds number of 2000. They are 25.43% and 945.69% as well as 4.86% and 117.01% for constant and temperature-dependent properties, respectively. The thermal-hydraulic efficiency of nanofluid is found to be not as good as the pure water.

Development of Adaptive Soft Sensor Using Locally Weighted Kernel Partial Least Square Model

Abstract Locally weighted partial least square (LW-PLS) model has been commonly used to develop adaptive soft sensors and process monitoring for numerous industries which include pharmaceutical, petrochemical, semiconductor, wastewater treatment system and biochemical. The advantages of LW-PLS model are its ability to deal with a large number of input variables, collinearity among the variables and outliers. Nevertheless, since most industrial processes are highly nonlinear, a traditional LW-PLS which is based on a linear model becomes incapable of handling nonlinear processes. Hence, an improved LW-PLS model is required to enhance the adaptive soft sensors in dealing with data nonlinearity. In this work, Kernel function which has nonlinear features was incorporated into LW-PLS model and this proposed model is named locally weighted kernel partial least square (LW-KPLS). Comparisons between LW-PLS and LW-KPLS models in terms of predictive performance and their computational loads were carried out by evaluating both models using data generated from a simulated plant. From the results, it is apparent that in terms of predictive performance LW-KPLS is superior compared to LW-PLS. However, it is found that computational load of LW-KPLS is higher than LW-PLS. After adapting ensemble method with LW-KPLS, computational loads of both models were found to be comparable. These indicate that LW-KPLS performs better than LW-PLS in nonlinear process applications. In addition, evaluation on localization parameter in both LW-PLS and LW-KPLS is also carried out.

Comparison of Turbulence Models for Single Sphere Simulation Study Under Supercritical Fluid Condition

Abstract In this paper, the comparison of turbulence models for fluid flow past single sphere under supercritical conditions is reported. Firstly, Dixon et al.’s models [1], which are under non-supercritical conditions, were used as benchmarks to validate the simulated results. Two turbulence models namely RNG k-ε and SST k-ω models parameters were fine-tuned accordingly in order to obtain almost comparable results generated by Dixon et al.’s models [1]. The simulation works were then extended to simulate flow of supercritical carbon dioxide. The second part of this paper, therefore, presents a comparative study of the turbulence models i. e. standard k-ε, RNG k-ε, realizable k-ε and SST k-ω models. This study emphasises on the predictions and evaluations of the velocity profiles at different flow regimes namely recirculation, recovery and near-wake. Simulations were carried out to determine the velocity profiles at subcritical and supercritical conditions by varying Reynolds numbers (2000 and 20,000), pressures (65 and 80 bar) and temperatures (283.15 and 308.15K). Simulation results indicate that the predicted results are consistent with the literature data. Interesting flow features were identified for all the simulations. The results of this study also reveal that the SST k-ω turbulence model was able to better capture the flow characteristics near-wake of the sphere.