Katia María Argüelles Díaz

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

Evaluation of Interaction and Blockage Effects for Multi-fan Units used in Public Transport HVAC Systems

Abstract A specific type of centrifugal fan named the “squirrel-cage” is broadly used in parallel configurations within an evaporator unit of HVAC systems for public transport. In these units, interaction effects arise, both between fans themselves and between fans and the structural elements. Blockage effects, such as the electrical motor, the evaporator gate and some aesthetic covering may also appear when these external elements are located very close to the fans aspiration nozzles. In this work, an experimental study of different configurations enables the determination of the minimum distances between the elements of the multi-fan unit, in order to minimise these interaction effects and, as a consequence, the problems related to them. Also, the quantitative influence of different motor sizes, covering solutions and the evaporator gate on the fans performance have been determined. Appreciable performance losses were found due to these blockage effects, thus recommendations to avoid the location of any external element close to the fans aspiration nozzles are proposed. The practical conclusions obtained in this work could be of interest to manufactures of HVAC systems subjected to installation size restrictions and where fan performance be optimized with low fabrication costs. This is usually the case for many HVAC systems, not only in the public transport applications considered in this paper.

Fault Diagnosis Technique for a Squirrel Cage Fan Using Vibration Analysis Signals

An experimental vibration study for a small centrifugal fan unit is presented in this work. The flow features in this kind of units are always of unsteady nature due to the geometrical constraints. As a consequence of such unsteady features, the vibrations and noise become often a serious design problem and may cause the unit bad functioning. With the use of fast response accelerometers, a fault diagnosis protocol is introduced in order to measure the extent of signal variations for different and expected failures.The study is based on the experimental measurements of two perpendicularly placed sensors which capture the acceleration signals for three different flow rates and three different operations, namely, the normal (or no failure) operation, the obstructed channel operation and the unbalance shaft operation. A squirrel cage or Sirocco fan is used for the study. It is a kind of centrifugal turbomachine with forward curved blades, often characterized by their small size. Its low cost and size make them be ideal for residential heating and cooling of vehicles. One of the important characteristics, the small chord of the rotor blades and the high number of them often used has been previously reported in the bibliography. Typically, most common damages or faults come from the moving blades, bearings, fixed blades, rotor and sealing. In general, the vibration signals indicate malfunction symptoms, one symptom may be due to two different faults, and the same fault can produce two different symptoms.The main goal of the study is the application of a fault diagnosis technique as the purified orbit diagram (POD) to reveal the limit range values for some standard failures. The finding of such limits is determinant to implement a fault diagnosis protocol that will decrease the maintenance costs and increase the operation efficiency of the unit. Such protocol would become straightforward from the presented data. The Purified Orbit Diagram is obtained by harmonic reconstructing corresponding harmonic components of two perpendicular directions in time domain. Three different flow rates, namely nominal and two off-design conditions are tested to be able to compare the different POD components as a function of the flow rate.Copyright

Effect of Rotor-Stator Configuration in the Generation of Vortical Scales and Wake Mixing in Single Stage Axial Fans: Part II — Assessment of Vortex Sound Sources

This second part is devoted to the identification of vortex sound sources in low-speed turbomachinery. As a starting point, the time-resolved evolution of the vortical motions associated to the wake shear layers (reported in the first part of the present study) is employed to obtain vorticity distributions in both blade-to-blade and traverse locations throughout the axial fan stage.Following, the Powell analogy for generation of vortex sound is revisited to obtain the noise sources in the nearfield region of the fan. Both numerical and experimental databases presented previously are now post-processed to achieve a deep understanding of the aeroacoustic behavior of the vortical scales present in the flow.A LES simulation at midspan, using a 2.5D scheme, allows an accurate description of the turn-out time of the shedding vortices, within high-density meshes in the blades and vanes passages, and a correct modeling of the dynamics of turbulence. Besides, thermal anemometry has been employed with a two-wire probe to measure the planar flow in the midspan sections of the fan. Statistical procedures and signal conditioning of velocity traces have confirmed experimentally the unsteady flow patterns devised in the numerical model.The comparison of the rotor-stator and the stator-rotor configurations provides the influence of the wake mixing and the nucleation of turbulent spots in the distribution of the Powell source terms. Moreover, the relation between the turbomachine configuration and the generation of vortex sound can be established, including the impact of the operating conditions and the contributions of the interaction mechanisms.Copyright

Evaluation of design criteria for squirrel-cage fans used in public transport HVAC systems

This article completes a thorough review of the leading squirrel-cage fans, currently used for HVAC urban transport units, in terms of geometry and both aerodynamic and noise performance. Based on a twin-impeller, forward-curved centrifugal configuration, benchmarking with up to 15 different commercial units from several manufacturers has been carried out in this study, ranging the most significant geometrical and operational parameters for comparison. Experimental data concerning best efficiency points, overall efficiency range, noise generation, and performance have been obtained in a test facility built according to standardized industrial normative. The analysis of the obtained results is applied to select the best alternatives among those analyzed, focusing on energy efficiency and minimum noise generation criteria. In this way, the most important design criteria are established in order to get more efficient and less noisy ventilation systems.

Statistical Procedure to Obtain Accurate Time-Resolved Measurements in Turbomachinery Environments Using Fast-Response Probes

Fast-response probes in multistage turbomachinery are typically used to measure unsteady flows and turbulence in a number of traverse locations throughout the machine (rotor-stator inter-regions, inlet and outlet sections, tip clearance gaps…). When used intensively, they provide complete information of time-resolved flow structures, including wake patterns, wake mixing, wake-wake and rotor-wake interactions or turbulence transport in 2D planes and even 3D pictures if the raw signals are post-processed accurately.The segregation between deterministic, unsteady features and turbulent scales is essential to understand the unsteady mechanisms responsible for the energy transfer and requires an accurate selection of the sampling frequencies and the total length of the measured traces to assure a valid statistical reduction. Similar considerations must be made if they are filtered in a frequency basis (for example, filtering low-scale turbulence or extracting only BPF components), employing well-designed periodograms or power spectra with minimum scatter and large periods of time integration.This work presents the general guidelines that any statistical procedure must follow to assure that phase-locked averaging results are consistent when applied to velocity signals in multistage turbomachinery. The procedure is established in terms of convergence (residuals) and coherence (error) between time-resolved traces retrieved using different sampling frequencies and number of total samples. The possible effects of three-dimensionality, the measured regions (hub, tip, midspan) or the sensibility to turbulence levels is also explored.The proposed methodology is applied to a low-speed axial fan, so a concise survey of usual practices employed by other authors in the literature is firstly reviewed, in order to identify fundamental parameters and values typically adopted to guarantee convergence. Finally, recommendations are made as a function of the variable analysed, the wake pattern to be described or the global disorder of the flow structures inside axial flow fans.© 2012 ASME