K.M. Argüelles Díaz

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.

Integrators of several orders in time to study the evolution of an aerosol by coagulation

We have obtained some numerical methods to integrate the coagulation equation for an aerosol. They are semi-implicit and stable regarding the time integration step, which can be freely chosen (a very important matter in numerical solution of differential equations). The methods are of two types: extrapolative (based on a previously known first-order semi-implicit formula) and purely semi-implicit, both mass-conservative. The same methodology used here to develop these new methods can be applied to improve the well-known sectional ones. The extrapolative and the semi-implicit methods are really of the order we had deduced from their analysis. However, as the order of the method increases, for small time steps, the roundoff causes the error no longer to behave as expected. The extrapolative methods are self-starting but the semi-implicit ones are not, so we need the first ones to start the others. If we take into account both the error and the CPU time, the second-order methods are comparable, but the third-order semi-implicit one is better than the extrapolative one. The comparison of higher order methods is disturbed by the roundoff error. Both methods can be used with fixed and moving bins with respect to the discretization of the size in the particle size distribution. These methods are valid to complement the specific ones developed to solve the growth and other phenomena in the time-splitting method which is used to analyse the evolution of an aerosol in the general case.