Jesús Manuel Fernández Oro

Unsteady Flow and Wake Transport in a Low-Speed Axial Fan With Inlet Guide Vanes

The present study is focused on the analysis of the dynamic and periodic interaction between both fixed and rotating blade rows in a single stage, low-speed axial fan with inlet guide vanes. The main goal is placed on the characterization of the unsteady flow structures involved in an axial flow fan of high reaction degree, relating them to working point variations and axial gap modifications. For that purpose, an experimental openloop facility has been developed to obtain a physical description of the flow across the turbomachine. Using hot-wire anemometry, measurements of axial and tangential velocities were carried out in two transversal sectors: one between the rows and the other downstream of the rotor, covering the whole span of the stage for a complete stator pitch. Ensemble- and time-averaging techniques were introduced to extract deterministic fluctuations from raw data, both of which are essential to understand flow mechanisms related to the blade passing frequency. An exhaustive analysis of the measured wakes has provided a comprehensive description of the underlying mechanisms in both wake-transport phenomena and stator-rotor interaction. In addition, unmixed stator wakes, observed at the rotor exit, have been treated in terms of dispersion and angular displacement to indicate the influence of the blades loading on the transport of the stator wake fluid. The final aim of the paper is to highlight a complete picture of the unsteady flow patterns inside industrial axial fans

Unsteady Flow Patterns for a Double Suction Centrifugal Pump

The flow in a double suction centrifugal pump is presented in this paper. The static performance of the machine has been obtained in a proper test rig, and the results have been compared with equivalent numerical results from an Unsteady Reynolds Averaged Navier―Stokes Equations (URANS) calculation. In a second step, the numerical results have been exploited to get detailed information about the flow inside the turbomachine. The main goal of the study is, on one hand, the validation of the numerical procedure proposed and, on the other hand, the detailed flow-field analysis for the machine, which points out the possibilities and drawbacks of the pump design. For a double suction machine, the inlet flow is characterized by the existence of a particular geometry that tries to force a uniform flow, at least for the nominal flow rate. On the contrary, at off-design conditions the lack of uniformity produces an unsteady incidence that gives rise to strong hydraulic loading variations. Instantaneous and average pressure fields have been analyzed in this paper to study the evolution of such inlet flow unsteadiness throughout the impeller and the volute. The analysis of both static and dynamic effects on the pump shaft has been carried out from the numerical calculation of the radial forces. The results have shown that the performance of the double suction centrifugal pump is suitable for typical design conditions. The best operation point or nominal flow rate is found to be at ϕ = 0.274, which turns out to produce a specific speed ω S = 1.25, well in the range for centrifugal impellers. This operating point is also found to be the one with better efficiency and with better flow characteristics, regarding the axisymmetry of the flow pattern and the fluid forces obtained. However, some particular features produce also interesting results for off-design operating points.

Decomposition of Deterministic Unsteadiness in a Centrifugal Turbomachine: Nonlinear Interactions Between the Impeller Flow and Volute for a Double Suction Pump

Modern designs for centrifugal pumps consider flow unsteadiness as a main concern. The averaged impact of blade to blade structures and impeller-tongue or impeller-diffuser interactions is considered relevant to accurately address the machine performance. In this framework, the search for a comprehensive way to explain unsteady patterns would be particularly desirable. In the present paper, an analysis of the deterministic interaction between the impeller flow and both the inlet and outlet volute tongues in a double suction centrifugal pump is carried out. The zero velocity condition imposed in both tongues gives rise to an interaction that, up to the authors’ knowledge, has not yet been studied for this type of pumps. A well-tested numerical model, developed by the authors for previous studies with the same pump, has been exploited to predict deterministic correlations in the interaction region. Therefore, the main goal of this paper becomes the evaluation of the nonuniformities induced by both the inlet and the outlet tongues over the blade to blade distributions within the impeller. As a consequence, fluctuation levels in the blade loadings, derived from deterministic nonuniformities of the inlet tongue, can be provided in the relative frame of reference. The collected data using the numerical model can be further employed to establish modeling issues for deterministic stresses. Since more and more often, it becomes a widespread practice for pump manufacturers to include the analysis of the unsteady flow patterns in the design process, the availability of methodological tools to characterize pure unsteady terms in the early stages may be extremely useful for designers. Comparison of deterministic fluctuations with unresolved turbulence intensities is also available in this study, showing the importance of unsteady mechanisms in the modeling of pumping turbomachines.

Numerical Model for the Unsteady Flow Features of a Squirrel Cage Fan

This paper shows a numerical research on the unsteady flow field inside a squirrel cage fan. The studied features are both the instantaneous flow fields and the average fluid flow associated to the blade passage frequency. The squirrel cage fan studied is a small centrifugal fan with a twin impeller configuration, each with 23 forward curved blades. The blades chord is 0.013 m and each impeller has a diameter of 0.08 m and a width of 0.094 m. The impellers operate inside an external spiral casing with a rectangular exit, followed by the outlet duct. A first series of experimental tests were performed in order to characterize the unit. The performance curves (head, power and efficiency versus flow rate) were measured. These tests show a nominal flow rate at around 0.098 m3 /s and a specific speed ωs = 1.9. From there on, three different flow rates were considered to study different flow behaviours in the impeller. In parallel with the mentioned experimental study, the unsteady 3D flow field inside the fan equipped with the same impeller was modelled for the referred flow rates, by means of the commercial CFD code FLUENT. To facilitate the modification of any geometrical feature, the mesh of the modelled fan was divided in several regions: inlet duct, impeller, volute and diffuser with outlet duct. The main goal of the paper is to show the numerical results obtained on the absolute and relative frames. Three main flow features will be analysed: the inlet flow distribution, the blade to blade field and the impeller exit flow. At the fan inlet, special care will be taken to detect possible recirculation or separation zones. On the other hand, and for each studied flow rate, the distribution of outlet flow field is also analysed. Conclusions on flow uniformity are drawn.© 2009 ASME

Numerical Simulation of the Fuel Oil Cooling Process in a Wrecked Ship

This work deals with a numerical simulation developed to predict the characteristic cooling times of a low-thermal diffusivity fuel oil confined in the tanks of a wrecked ship. A typical scenario has been introduced through the definition of tank geometries, physical boundary conditions (deep sea temperatures), and rheological properties of the fuel oil. The fluid dynamic behavior of the oil (free convection) inside the tanks, as well as the heat exchange with surrounding sea water has been simulated using a commercial code, FLUENT, which directly solves the Navier-Stokes set of equations, including energy. The purpose is focused on the prediction of both spatial and temporal evolution of the fuel oil characteristic temperature inside the tanks. The objective is to determine the deadline in which the asymptotic temperature curve of the fuel oil converges with deep sea thermal conditions. Inspectional analysis is also outlined, as a powerful tool to predict an order of magnitude in the cooling process.

Extended Angular Range of a Three-Hole Cobra Pressure Probe for Incompressible Flow

This paper analyzes the operative characteristics of a three-hole cobra type probe especially designed to attain an angular range higher than 180 deg for planar turbulent flows. A new calibration and data reduction method are also introduced, discriminating three different zones inside the angular range of the calibration. This methodology improves the probe performance, extending its operative angular range from the typical ±30 deg to ±105 deg. In addition, the transmission of the uncertainty-from the pressure measurements to the flow variables-is estimated, showing reasonably low levels for the whole angular range. Furthermore, the sensibility of the probe calibration to the Reynolds number and the pitch angle is considered, and the influence of the turbulence level is outlined. Regarding these factors, the probe precision in the extended angular range is found to be similar to that of the traditional range. Finally, the probe is tested in a flow field with large variations of the incidence angle, and the results obtained with the new method are compared to those given by the traditional calibration.

Multiphase Modelling of the Steel Grade Transition in a Continuous Casting Tundish

Nowadays, continuous casting is extremely conditioned by sequences of different steel grades that produce a large amount of intermixed steel. Due to customer requirements, steel producers are forced to deliver a few slabs of high-specialized steels, so the number of castings handling steels of dissimilar grades has been significantly increased in recent years. As a consequence, manufacturers are particularly concerned with the development of practical methods to know exactly where the mixed regions begin and end, in order to make a precise classification of the steel grade that has been produced and avoid further downgrading. Pioneering works by Huang and Thomas introduced a 1-D model to estimate the intermixed region during a grade transition. This model reached a notable popularity because of its ability to provide on-line predictions, though it is assumed that mixing inside the tundish is globally determined with a number of fixed parameters. Recently, Cho and Kim have introduced a modification reducing the number of parameters required, but with the full unsteady description of the tundish flow still unresolved. Moreover, all these models require experimental calibration, using the results from full-scale water models. Additionally, other researchers have been focused on the development of numerical simulations to analyze the flow structures and mixing features of the tundish, mainly during stable operation, but using limiting simplifications and/or steady schemes. In the present investigation, to the author’s knowledge, a 3D, unsteady numerical simulation using a volume-of-fluid formulation is carried out for the first time. With this technique, the transient behavior of the tundish during the ladle change can be fully modelled, tracking the free surface and extending the computations towards the steady state. A transport equation is resolved for a non-reactive scalar, representing a dimensionless concentration, so it is possible to predict the mixing degree of the steel at the tundish exit for different operating conditions. The final objective is the development of an off-line methodology to estimate precise intermixing periods during grade transition in continuous casting.Copyright

Effect of Rotor-Stator Configuration in the Generation of Vortical Scales and Wake Mixing in Single Stage Axial Fans: Part I — LES Modelling and Experimental Validation

The unsteadiness and vortical structures associated to impinging wakes convected through rotor and stator passages have been studied in detail in a single-stage low-speed axial fan, with 9 blades and 13 inlet/outlet guide vanes. In particular, in this first part, the effect of complementary rotor-stator (RS) and stator-rotor (SR) configurations has been addressed in terms of wake mixing and generation of vortical structures in both absolute and relative frames of reference.A LES simulation of the midspan section (in a 2.5D model) is introduced to resolved the largest scales of the vortical motion within the wakes, related to vortex shedding, especially at off-design conditions. Chopping mechanisms and periodic interactions of the coherent turbulent structures are described and the presence of turbulent spots due to wake-wake interactions is revealed. Another relevant flow pattern, like the advection of leading edge separation through the downstream passage, is also identified and linked to the periodic potential interaction of upstream vanes (SR) or blades (RS).Additionally, complete experimental databases of the time-resolved and the turbulent scales of the flow are available for both configurations by means of hot-wire anemometry measurements. Wake transport and viscous mixing are identified in corresponding measuring windows, and primary flow structures at midspan are also recovered and compared with the numerical results for validation.From the comparison of experimental and numerical results it can be concluded that the numerical modeling is able to reproduce accurately the unsteady phenomena that occur inside the axial fan, and shows the potentiality of LES techniques to resolve with high fidelity the main turbulent structures present in the flow, especially at off-design flow rates.Copyright

Application of Deterministic Correlations in the Analysis of Rotor-Stator Interactions in Axial Flow Fans

Deterministic stresses account for the time-averaged contribution of the periodic unsteadiness in single-stage turbomachinery. An accurate modelling of these stresses allows for the development of steady simulations with full contribution of deterministic unsteady sources. Moreover, introducing both relative and absolute reference frames, the deterministic tensor can be split into a set of correlations which addresses the influence of the stator and rotor non-uniformities separately. As a consequence, a pure unsteady term showing rotor-stator interaction is rediscovered and can be established as an essential indicator of the strength of viscous, non-linear effects in multistage turbomachinery environments.This work provides a comprehensive description of these correlations in the interrow region of a single stage axial flow fan. Two configurations are analyzed here: the upstream stator acting as a non-uniform inflow for the rotor and the complementary stator placed downstream of the rotor for pressure recovery at the exit. To illustrate the driven mechanisms in the tensor, a three-dimensional numerical simulation at the midspan section of a low-speed axial fan is employed in the present investigation. A deep analysis of the interaction between the rows using the deterministic framework is shown as a useful tool to devise deterministic models for steady computations.The commercial code FLUENT is used to resolve the full unsteady 3-D Navier-Stokes equations using LES schemes. Nominal and off-design flow conditions have been considered to observe the relevance of the different deterministic correlations in the establishment of the unsteady sources.The final objective pursues a deep understanding of the correlations behavior under variable flow conditions and different stage configurations. Thus, physical insight will be gained and more efficient and reliable deterministic models could be proposed, available for researchers and experts in the field.Copyright

Flow Analysis and Deterministic Decoupling in a Squirrel Cage Fan

Squirrel cage fans are often used as blowers for automobile applications or for small industrial equipment. The flow in this kind of fans happens to be quite complex and with unsteady features, that makes it quite difficult to be studied. In particular, unsteady flow separation at the machine inlet or at the impeller blades and a variety of flow induced vibrations is found for most of the operation conditions. The deterministic stress analysis becomes an interesting tool in analyzing the main flow features from an existing numerical model. In this paper, the analysis of the internal flow and the periodic phenomena resulting from a deterministic study is intended. To accomplish this goal, a series of numerical routines were performed for different flow rates, including also failing operating modes of the fans (blocked inlets as a typical problem for this type of arrangements). The squirrel cage fan studied is a small centrifugal fan with a twin impeller configuration, each with 23 forward curved blades. The blades chord is 0.013 m and each impeller has a diameter of 0.08 m and a width of 0.09 m. The performance curves (head, power and efficiency versus flow rate) was obtained numerically in previous works and confirmed experimentally in normalized test campaigns. These tests have shown a nominal flow rate at around 352 m3 /h and a specific speed ns = 1.9. The main goal of the paper consists on the evaluation of the non-uniformities induced by the volute tongue over the blade to blade distributions within the impeller. As a consequence, fluctuation levels in the blade loadings, derived from deterministic non-uniformities can be provided in the relative frame of reference. The practical applications of the conclusions do imply a progress in the knowledge of the working parameters for machines that affect in a direct way to the passengers comfort.© 2011 ASME