J.M. Fernández Oro

Numerical simulation of the unsteady stator‐rotor interaction in a low‐speed axial fan including experimental validation

Purpose – The purpose of this paper is to focus on the analysis of the dynamic and periodic interaction between both fixed and rotating blade rows in a single‐stage turbomachine.Design/methodology/approach – A numerical three‐dimensional (3D) simulation of the complete stage is carried out, using a commercial code, FLUENT, that resolves the 3D, unsteady turbulent flow inside the passages of a low‐speed axial flow fan. For the closure of turbulence, both Reynolds‐averaged Navier‐Stokes modeling and large eddy simulation (LES) techniques are used and compared. LES schemes are shown to be more accurate due to their good description of the largest eddy structures of the flow, but require careful near‐wall treatment.Findings – The main goal is placed on the characterization of the unsteady flow structures involved in an axial flow blower of high reaction degree, relating them to working point variations and axial gap modifications.Research limitations/implications – Complementarily, an experimental facility wa...

Impact of the tip vortex on the passage flow structures of a jet fan with symmetric blades

Abstract The goal of this study is the simulation of the flow inside a rotor with elliptic airfoils, where the Kutta condition cannot be satisfied. This work develops a three-dimensional numerical modelling of a monoplane axial jet fan with symmetric blades. The three-dimensional model includes tip clearance gridding and turbulence modelling based on high-order Reynolds-averaged Navier—Stokes (RANS) schemes. The flow patterns inside the blade passage and the wake-core structure will be studied at design operating conditions. Also, the interaction of the tip leakage flow with blade-to-blade structures will be analysed in detail. The investigation shows how the tip leakage vortex modifies the blade loading on the suction surface. The leakage flow rolls up in a vortical structure at the suction side, establishing a mixing mechanism that produces a low-axial velocity region. As a result, the adverse pressure gradient is enhanced and a major flow separation overcomes. This feature is especially critical in the case of a rotor with symmetric blades, where the flow is always detached at the trailing edge. The simulation is carried out using a commercial code, FLUENT, which resolves the Navier—Stokes set of equations. A high dense mesh is introduced in the model, so tip leakage is expected to be well captured. Different turbulence models have been tested in order to determine the most accurate choice. It is shown that a linear Reynolds stress model provides velocity distributions more adjusted to experimental data. This suitable prediction for rotating flow passages is a consequence of the characteristics of the model: consideration of anisotropic turbulence and direct inclusion of curvature and rotation effects in the transport equations. Therefore, swirl effects of the tip vortex can be modelled correctly. The numerical results are compared with previous experimental data of velocity fields to validate the simulation. In particular, the instantaneous wake flow structure was measured with a two hot-wire anemometer. Axial and tangential velocity profiles were obtained after pitch averaging the time-resolved flow patterns.

Non-deterministic kinetic energy within the rotor wakes and boundary layers of low-speed axial fans: frequency-based decomposition of unforced unsteadiness and turbulence

A detailed analysis of the non-deterministic scales of the flow in low-speed axial turbomachinery has revealed the presence of significant large-scale unsteadiness, with periodic features different to the blade passing frequency (BPF), which are superimposed to the turbulent structures. Introducing a frequency-based decomposition, this additional component has been segregated from turbulent phenomena, so the total unsteadiness has been found to be contributed by three components: forced unsteadiness (deterministic scales), unforced unsteadiness (large-scale unsteadiness) and turbulence (small-scale randomness). Dual hot-wire anemometry has been employed intensively within the stage of a low-speed axial fan to provide a valuable experimental database, where the phase-locked averaging technique has been applied to retrieve time-resolved fluctuations and isolate the non-deterministic contribution. The present investigation shows that the presence of the unforced component is mainly related to instabilities of the rotor wakes and tip vortex structures, as well as wake–wake interactions. Moreover, typical eddies size of this component distribute their energy within the frequency range that contains 80% of the total unsteady kinetic energy. As a consequence, it is expected that large eddy simulation (LES) schemes with accurate spatial discretizations may address this component within the resolved scales of the filter, while unsteady Reynolds averaged Navier-Stokes (RANS) modelling could require additional modelling of the unforced mechanisms. In addition, maps and radial distributions of every component illustrate that major flow patterns are identifiable in all of them due to the redistribution of all-range scales throughout the energy cascade. Turbulent and forced mechanisms present important variations with the operating conditions, while the unforced component is barely affected by flow rate variations. It is shown that typical values of unforced unsteadiness reach up to 20% of the total unsteady energy, even for nominal conditions at midspan of the rotor passage. Higher levels of all components are found towards tip and hub boundary layers, as the total unsteadiness is reinforced by massive flow separations, tip blockages and major flow disturbances.

Unsteady 3D Simulation of a Jet Fan With Symmetric Blades

This work develops the numerical modeling of a monoplane axial jet fan with symmetric blades. The goal of the study is the simulation of the flow inside a rotor with elliptic airfoils, where the Kutta condition cannot be satisfied. The unsteady 3D model includes tip clearance gridding and a sliding mesh technique to simulate transient effects. The flow patterns inside the blade passage and the wake-core structure will be studied at design operating conditions. Also, the interaction of the tip leakage flow with time-averaged structures will be analyzed in detail. Therefore, the impact of the tip vortex in the mean time performance of the jet fan will be introduced. The investigation shows how the tip leakage vortex modifies the blade loading on the suction surface. The leakage flow rolls-up in a vortical structure at the suction side, establishing a mixing mechanism that produces a low axial velocity region. As a result, the adverse pressure gradient is enhanced and a major flow separation overcomes. This feature is especially critical in case of a rotor with symmetric blades, where the flow is always detached at the trailing edge. The simulation is carried out using a commercial code, FLUENT, which resolves the Navier-Stokes set of equations. An extremely high dense mesh is introduced in the model, so tip leakage is expected to be well-captured. In addition, fully-developed detachment of the boundary layer requires superior discretizations and high quality meshes, so restrictive y+ criteria have been employed for both endwall boundaries and blade surfaces. Turbulence modeling is closed using URANS models. The Reynolds Stress Model (RSM) has been employed because of its suitable predictions for rotating flow passages. In addition, this model considers anisotropic turbulence, and effects of curvature and rotation are directly addressed in the transport equations. Therefore, swirl effects of the tip vortex are expected to be well-captured. The numerical results are compared with previous experimental data of velocity fields to validate the simulation. Axial and tangential velocity profiles were obtained using a five-hole probe. Complementary, the instantaneous wake flow structure was measured with a dual hot wire anemometer.Copyright

Tonal Noise Generation in an Inlet Vaned Axial Blower at Several Axial Gaps

The main objective is the numerical study of the noise generation in a single stage axial flow blower. For the present work, just the discrete noise generation (tonal noise) is dealt with. A numerical methodology based on a generalization of Lighthill’s aero-acoustic analogy, the so-called Ffowcs Williams and Hawkings aero-acoustic analogy (FFWH), is developed. The numerical methodology has been tested with two different operational configurations of the blower: a first one, with a reduced axial gap between the rows of the stage (i.e. 50 mm) and a second one with a higher axial distance (i.e. 70 mm). The specific contribution of every tonal noise source term will be analyzed for both configurations.© 2006 ASME

Upstream Potential Propagation Effects of Unsteady Stator-Rotor Interaction in an Axial Flow Blower: Numerical Analysis and Experimental Validation

The potential effect of the inlet guide vanes blockage is predominant in an axial one-stage configuration when the upstream flow field is considered. In the same way, rotor downstream, the main unsteadiness is provoked by the rotor wakes mixing-out at the machine discharge. Nevertheless, if the gap between the rows is significantly reduced, the stator wakes are not allowed to be mixed out before impinging the rotor blades, so a chopping effect overcomes, stretching and tilting them, and generating wake-wake interactions and new loss sources at the exit. On a similar trend, it is expected that a reduced axial gap allows the potential unsteadiness of the rotor blockage between the blades to be propagated upstream, modulating the flow conditions at the stator passages, and even at vanes leading edge locations. In this paper, the evolution of the rotor potential interaction within the stator passages and up to the vanes leading edge is analyzed. The main goal is placed on the analysis of the propagation, relating the axial distance with the attenuation of those potential mechanisms. A numerical 3D simulation of a complete single stage axial flow blower has been developed and executed using a commercial code that resolves the URANS set of equations. The axial gap between the 13-IGVs stator and the 9-blade rotor has been modified in order to evaluate its influence on the potential distortion propagated upstream of the stator. For the closing of turbulence, a LES scheme with a Smagorinsky-Lilly model is used in the computations. Finally, due to the LES characteristics, a phase-averaged procedure has to be introduced for the simulation post-processing. Complementary, experimental measurements have been carried out over a test rig with modifiable axial gap between the fixed and rotating blade rows. As a matter of fact, pressure transducers were placed all along the machine shroud to capture pressure fluctuations related to potential sources radiated from the rotor blades. These measurements have been analyzed using frequential analysis, which is essential to identify the origin of the flow inlet distortions. The final objective is to complete the rotor-stator interaction scenario both downstream and upstream the stage. Previous works were focused on the downstream conditions and now the upstream potential propagation is studied in detail.Copyright

Numerical Simulation of the Fuel-Oil Cooling Process in 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 tanks geometries, physical boundary conditions (deep sea temperatures) and reological properties of the fuel-oil. The fluidynamic behaviour of the oil (forced convection) inside the tanks, as well as the heat exchange with surrounding sea water has been simulated throughout a commercial code, FLUENT, that solves directly the Navier-Stokes set of equations, including energy one. The purpose is focused on the prediction of both spatial and temporal evolution of the fuel-oil characteristic temperature inside the tanks. The final objective is placed on the determination of the deadline in which asymptotic temperature curve of the fuel-oil converges to deep sea thermal conditions. Inspectional analysis is also outlined, as a powerful tool to predict an order of magnitude in the cooling process.Copyright

UNSTEADY ROTOR-STATOR INTERACTION IN AN AXIAL FLOW BLOWER. PART I: NUMERICAL AND EXPERIMENTAL FLOW FIELD CHARACTERIZATION

The present study focuses on the analysis of the dynamic and periodic interaction between both fixed and rotating blade rows in a single stage turbomachine. The main goal is placed on the characterization of the unsteady flow structures involved in an axial flow blower of high reaction degree, relating them to working point variations and axial gap modifications. A numerical 3D simulation of the complete stage is carried out, using a commercial code that resolves the URANS set of equations. For the modeling of turbulence, a LES scheme is used in order to achieve a good description of the largest eddy structures of the flow. In the other hand, an experimental facility was developed to get a physical description of the flow inside the machine. Both static and dynamic measurements were used in order to describe the interaction phenomena. A directional probe with pressure transducers was employed for the static characterization, and hot wire anemometry techniques were used for the instantaneous response of the interaction. The scope is based on the development of a methodology to understand the flow mechanisms related to the blade passing frequency in a single rotor-stator interaction.Copyright

Unsteady Rotor-Stator Interaction in an Axial Flow Blower: Part II — Deterministic Stresses Analysis

Since the instantaneous phase-averaged flow field has been characterized and properly described through both numerical and experimental studies in a single stage axial flow blower, the next step is to identify the sources of unsteadiness related to blade passing frequencies. The construction of the deterministic stress tensor in both rotating and fixed frames of reference, through a sequential averaging technique, is extremely useful for this purpose. This kind of tensor shows the location and intensity of the main interaction characteristics of the flow, allowing a perfect segregation of the effects of every blade row onto the total unsteady scenario of the complete stage. The steady signature of the stator wakes, still present at the exit plane, downstream the forward rotor, will be related to the residual deterministic kinetic energy of the mixed-out flow. The migration of these unsteady features onto the averaged flow field will be used to understand the recovery process that occurs when the flow passes through the rotor of the stage, linking flow physical phenomena with the transport and convection of the deterministic stresses.© 2005 ASME