Multiple Reference Frame Technique Research Articles

CFD modelling of powder flow in a continuous horizontal mixer

This work presents a continuum model for simulating the flow of powders inside continuous horizontal mixers. The challenge is to adopt a reliable rheological model that allows simulating granular flows accurately. We selected the μ(I)-rheology model. First, we considered a set of granular collapse experiments, showing that the model can successfully reproduce these flows, and also using the experimental results to evaluate the material properties of the powders. Then, we investigated the complex powder flow in continuous horizontal mixers. Here computational cost is a challenge. We showed that the sliding mesh technique is accurate but expensive owing to the rotation of the boundaries, making this method impractical for industrial applications. Therefore, we also employed the multiple reference frame technique, showing that its results are accurate at far lower computational cost. The results section ends with a sensitivity analysis of the mixer solid mass loading to the powder material properties.

Numerical method to predict ice accretion shapes and performance penalties for rotating vertical axis wind turbines under icing conditions

This paper proposes a numerical method to predict the ice accretion shapes and aerodynamic performance of rotating vertical axis wind turbine (VAWTs) under icing conditions. A multiple reference frame (MRF) and sliding mesh technique (SMT) are combined to efficiently reflect the unsteady icing effects on rotating wind turbines. The SMT calculates the flow field considering the rotational and unsteady effects of the VAWTs. The MRF can efficiently clarify the rotational effects of the droplet field and ice accretion. Using the MRF technique, a series of icing simulations is implemented in which the ice shapes are updated at azimuth angle intervals of 36°. Using the proposed method, ice shapes in agreement with those obtained in icing wind tunnel tests can be obtained. Moreover, ice that is evenly distributed over the blade surface under glaze ice conditions can be examined instead of only the forms concentrated on the leading-edge, such as ice horns. The overall output power of an ice-covered VAWT is noted to be significantly reduced. Massive flow separation is induced owing to the increased airfoil thickness at azimuthal angles between 0° and 180°. Nevertheless, the performance of the thickened airfoil is enhanced owing to the delayed flow separation via dynamic stall in azimuthal angles between 180° and 270°.

Computations with the multiple reference frame technique: Flow and temperature fields downstream of an axial fan

ABSTRACTIn numerical computations, axial fans are typically abstracted as two-dimensional surfaces, and this forms the basis for the “Lumped Fan” (LF) model. The LF-model relies on experimentally derived P–Q (fan) curves which should conform to the published test codes. Despite its simplicity, the LF-model’s accuracy depends on the application and acceptable error margin. Therefore, with decreasing error margins in thermal engineering, there has been an interest in accurate fan modeling techniques such as “Multiple Reference Frame” (MRF) model. The current effort provides a two-part validation of the MRF model results for an axial fan against the relevant experiments: comparison of the (i) P–Q curves, and (ii) temperature distribution in an electronic enclosure. In the former, both fan models exhibit good agreement with the experiments, however the flow structures determined by the two models exhibit substantial differences. Given these flow structures’ strong influence on the temperature field, the MRF model demonstrates good correlation and significantly better agreement with the measured temperature distribution, compared to those of the LF-model. In light of these findings, benefits, limitations, and possible applications of both models are also discussed.

Assessment of Interaction between Baffles and Impeller Blades in Stirred Tanks

In this work, the sensitivity of hydrodynamics on stirrer position is considered when the Multiple Reference Frame technique is applied to simulate mixing tanks of single and multiphase systems. A typical stirred reactor equipped with four baffles and a six blade Rushton turbine was chosen for the studies. Operational conditions of constant speed (5 rps) and 2.653·10 −4 m/s gas superficial velocity with single bubble size (1 mm) were set. The reactor was simulated in Fluent 15 with four different impeller blade positions. The results were averaged and validated against experimental data produced by Particle Image Velocimetry. The comparison was made using profiles of velocity components along the sampled lines representing various zones of the vessel. The impeller angle of rotation with respect to baffle position was found to contribute to the overall reactor hydrodynamics especially for multiphase systems. Averaging of the simulation results was found to improve the accuracy of the simulated mixing hydrodynamics.

Influence of drag and turbulence modelling on CFD predictions of solid liquid suspensions in stirred vessels

Suspensions of solid particles into liquids within industrial stirred tanks are frequently carried out at an impeller speed lower than the minimum required for complete suspension conditions. This choice allows power savings which usually overcome the drawback of a smaller particle-liquid interfacial area. Despite this attractive economical perspective, only limited attention has been paid so far to the modelling of the partial suspension regime.In the present work two different baffled tanks stirred by Rushton turbines were simulated by employing the Eulerian-Eulerian Multi Fluid Model (MFM) along with either the Sliding Grid algorithm (transient simulations) or the Multiple Reference Frame technique (steady state simulations). In particular, a comparison of alternative modelling approaches for inter-phase drag force and turbulence closure is presented. The results are evaluated against a number of experimental data concerning sediment features (amount and shape) and local axial profiles of solids concentration, with emphasis on the partial suspension regime.Results show that some of the approaches commonly adopted to account for dense particle effects or turbulent fluctuations of the volumetric fractions may actually lead to substantial discrepancies from the experimental data. Conversely simpler models which do not include such additional effects give the best overall predictions in the whole range of partial to complete suspension conditions.

VLSI Oriented Fast Motion Estimation Algorithm Based on Pixel Difference, Block Overlapping and Motion Feature Analysis

One VLSI friendly fast motion estimation (ME) algorithm is proposed in this paper. Firstly, theoretical analysis shows that image rich of sharp edges and texture is regarded as high frequency abundant image and macroblocks (MBs) in such image will express large pixel difference. In our paper, we apply adaptive subsampling method during ME process based on pixel difference analysis, so the computation complexity of full pixel pattern can be reduced. Secondly, statistic analysis shows that for MBs with static feature, the ratio of selecting previous reference frame as best one is very high and multiple reference frame technique is not required for these MBs. Based on this analysis, we give out a block overlapping method to pick out static MBs and apply MRF elimination process. Thirdly, since many redundant search positions exist in MB with small motion trend and large search range is only contributive to MB with big motion, we extract motion feature after ME on first reference frame and use it to adjust search range for rest ME process. So, the computation complexity of redundant search positions is eliminated. Experimental results show that, compared with hardware friendly full search algorithm, our proposed algorithm can reduce 71.09% to 95.26% ME time with negligible video quality degradation. Moreover, our fast algorithm can be combined with existing fast ME algorithms like UMHexagon method for further reduction in complexity and it is friendly to hardware implementation.

Multiple Reference Frame-Based Control of Three-Phase PWM Boost Rectifiers under Unbalanced and Distorted Input Conditions

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> Many control algorithms and circuits for three-phase pulse width modulation active rectifiers have been proposed in the past decades. In most of the research, it is often assumed that the input voltages are balanced or contain only fundamental frequency components. In this paper, a selective harmonic compensation method is proposed based on an improved multiple reference frame algorithm, which decouples signals of different frequencies before reference frame transformation. This technique eliminates interactions between the fundamental-frequency positive-sequence components and harmonic and/or negative-sequence components in the input currents, so that fast and accurate regulation of harmonic and unbalanced currents can be achieved. A decoupled phase-locked loop algorithm is used for proper synchronization with the utility voltage, which also benefits from the multiple reference frame technique. The proposed control method leads to considerable reduction in low-order harmonic contents in the rectifier input current and achieves almost zero steady-state error through feedback loops. Extensive experimental tests based on a fixed-point digital signal processor controlled 2 kW prototype are used to verify the effectiveness of the proposed ideas. </para>

CFD Modelling of Liquid Homogenization in Stirred Tanks with One and Two Impellers Using Large Eddy Simulation

A prediction of liquid homogenization in stirred tanks using a CFD code FLUENT is presented. The study was performed for tanks agitated by one and two impellers on a centrally-located shaft. Two types of impellers were used: a six-blade 45° pitched blade turbine and a standard Rushton turbine. Different methods were employed for simulations of fluid flow in the stirred tanks—the multiple reference frames technique together with the standard k– ɛ turbulence model, the sliding mesh technique with the standard k– ɛ turbulence model, and the sliding mesh technique with the large eddy simulation (LES) model and a non-iterative time-advancement algorithm. The dynamic Smagorinsky–Lilly model using a locally calculated subgrid scale dynamic viscosity constant was used in the LES calculations. Tracer mass fractions in the tanks were recorded during time-dependent species transport simulations. The calculated flow field results—velocity profiles, power and pumping numbers were compared with the experimental results from literature. Time traces of normalized concentrations obtained from the simulations were compared with our experiments and the resulting mixing times were compared to literature correlations. The LES approach was the most time-consuming method; however, it described the real flow in stirred tanks better and we obtained more realistic courses of the liquid homogenization and the best agreement of the computed and experimental homogenization times of all the used models.

Diagnosis of Stator Inter-Turn Short Circuits in DTC Induction Motor Drives

This paper addresses the subject of stator fault diagnosis in direct torque control (DTC) induction motor drives. The particularities of the behavior of a DTC drive, when in the presence of inter-turn short circuits, are exploited, thus allowing the proposal of two different approaches for the diagnosis of this type of fault. One of the proposed diagnostic techniques is the well known motor current spectrum analysis. It is demonstrated that in a DTC induction motor drive, with stator inter-turn short circuits, the action of the torque and flux controllers introduce a strong third harmonic in the motor supply currents, which can be used to detect this type of fault. The other diagnostic technique is based on the multiple reference frames theory, recently proposed by the authors for the diagnosis of stator faults in line-connected motors. Here, its use is exploited in the diagnosis of stator faults in DTC induction motor drives. Simulation and experimental results demonstrate the effectiveness of the two proposed techniques for the diagnosis of stator faults but emphasize the fact that the multiple reference frames technique is the preferred one when the residual asymmetries of the motor are not negligible.

CFD modelling of continuous precipitation of barium sulphate in a stirred tank

By a combination of computational fluid dynamics (CFD), population balance and kinetic modelling, model solutions have been obtained for the precipitation of BaSO 4 in a dual-feed pipe, 0.3 m diameter continuous reactor agitated by a Rushton turbine. The conditions simulated include four agitator speeds from 200 to 950 rpm, two mean residence times (100 and 1180 s) and two shape factors ( k v=58 and π/6). The flow field was solved using the multiple reference frame technique to give residuals <10 −4 and this solution was then transferred to a stationary reference frame and the iterations for other differential equations were carried out until the residuals were <10 −7. The results showed that >95% BaSO 4 precipitated in the reactor, which intuitively seems correct but this finding is in contrast to much lower values in earlier work, probably because of the more stringent convergence requirements used here. Agitation speed had little effect on the results, as has been recently shown in the literature, justifying the decision not to include a micromixing model in the analysis. However, mean residence time and shape factor did have a very substantial effect on the precipitate crystal size.

Experimental Visualization and CFD Simulation of Flow Patterns Induced by a Novel Energy-Saving Dual-Configuration Impeller in Stirred Vessels

The flow characteristics of a novel agitator, termed ‘Narcissus’ (NS), have been investigated by dye-tracer visualization and laser Doppler velocimetry; the experimental velocity data were also used to initiate and then validate the CFD simulation of the flow induced by the NS in a cylindrical stirred vessel, using a multiple reference-frame technique and an unstructured inner-volume mesh. This novel impeller is shown to have the characteristics of a radial agitator, with a double configuration: the flow loop structure in the vessel depends upon its normal or inverted insertion, with liquid either being drawn radially towards the NS or ejected outwards from it. This dual configuration, in conjunction with its very low power consumption, constitute interesting and important impeller features, especially for gas–liquid applications.