Konrad Bamberger

Optimization of axial fans with highly swept blades with respect to losses and noise reduction

A low pressure axial fan with swept blades is optimized with respect to sound emission and efficiency. Noise is addressed by a modified sweep strategy. Regarding aerodynamics, geometrical parameters describing variations of the blade section and the hub contour are defined. The optimum in terms of maximal total-to-total fan efficiency at the design point is achieved by numerous CFD simulations embedded in the Simplex optimization method. Besides a moderate increase in efficiency at the design point, a remarkable extension of operating range is observed. The numerical results are successfully validated against experiment measurements. Acoustic measurements furthermore show a decrease in sound emission over the complete operating range.

Order of experimentation for metamodeling tasks.

The order in which measurements are carried out, determines the accuracy of models in early stages of the measurement process, i.e. while measurements are still in progress. Reliable models in early stages of the data acquisition phase allow for model-based investigations like optimization runs or an earlier switching to an active learning phase. This paper compares different methods to determine the order of experimentation for regression problems in metamodeling tasks. The data distribution and the data density in the input space are varied for several randomly generated synthetic functions in order to find the most promising determination strategy for the order of experimentation. As an application example, all strategies are also applied to a computational fluid dynamics (CFD) metamodel. The order of experimentation based on the intelligent k-means clustering algorithm turns out to be the best overall order-determination strategy.

Analysis of the Flow Field in Optimized Axial Fans

Three design strategies for low pressure axial fans are compared. Benchmark fans are designed with the blade element momentum (BEM) method. All common validity limits are respected. Optimized fans are designed according to design guidelines obtained in earlier studies by the authors of this work. Here, “optimal” always means maximum possible total-to-static efficiency at exact fulfillment of a specific design point. One typical difference between the benchmark and optimized fans is that the optimization yields considerably smaller hub-to-tip ratios. The two design methodologies are combined for a third series of fans. These fans are also designed with the BEM method, but with the same hub size and blade number as obtained from the optimization. As a consequence of the smaller hub size, the aforementioned validity limits are violated. All three design strategies are applied at three distinct design points which are supposed to outbid the bounds of axial fans according to Cordier’s diagram. The nine resulting fans are simulated by means of steady CFD (Reynolds-averaged Navier Stokes method, RANS). Quality assurance is considered by a grid independence study and a comparison with transient simulations (SAS method).The aerodynamic comparison reveals the weaknesses of the BEM designs which suffer from high exit losses (if designed with common validity limits) or from high hydraulic losses (if designed with small hub). Additionally, BEM designs with small hubs still have unnecessary exit losses which originate from an uneven distribution of flow velocity downstream of the fan. In contrast, the optimized designs enable a small hub without increased losses resulting in a total-to-static efficiency improvement by 2–14 percentage points depending on the design point. The flow fields of all fans are analyzed in detail to find reasons for the superiority of the optimized designs. It is found that optimized fans benefit from an evenly distributed meridional velocity profile downstream of the fan. Reasons are given why such a flow field is hardly achievable by BEM designs. Further advantages of optimized fans are found in the blade shape near the hub which strictly avoids flow separations which in standard designs often compromise the efficiency in the complete blade channel.© 2015 ASME

Performance Prediction of Axial Fans by CFD-Trained Meta-Models

The work presented in this paper is concerned with a methodology for substituting time consuming CFD investigations of the operational characteristics of axial fans by CFD-trained meta-models. For that, the fan geometry is parameterized by 25 physically interpretable quantities allowing for a huge variety of potential fan designs. The parameters are varied by Design of Experiment (DoE) and characteristic curves of approximately 10,000 fan designs are produced using the Reynolds-averaged Navier Stokes (RANS) method. Pressure rise, efficiency, and circumferentially averaged flow profiles upstream and downstream of the rotor are extracted from the RANS results and used to train the meta-models which are Artificial Neural Networks (ANN) or, more specifically, Multilayer Perceptrons (MLP). Special care is taken to mitigate extrapolation weaknesses of the MLPs which could compromise their suitability to compute the target function in optimization algorithms. With these extra efforts, it is possible to aerodynamically optimize axial fans for arbitrary design points within the range typical for axial or even mixed-flow fans according to Cordier’s diagram of turbo machinery. On top of that, designs with good efficiency are also found outside the well known Cordier range. In particular, an extension of feasible operating points towards untypically high specific fan diameters is observed. These findings are relevant for designs aiming at high total-to-static efficiency and make optimized axial fans compete with other fan types, especially with mixed-flow fans.Copyright

A Novel Optimization Based Design Method for Centrifugal Fans

A novel design method for impellers of centrifugal fans with backward curved blades is proposed. The overall geometry of the impellers considered is kept close to customary industrial designs (e.g. circular arc blades). The design method is based on seven simple correlations which determine the geometrical impeller parameters as a function of the targeted aerodynamic duty (design) point. Unlike classic design methods, these correlations are derived from aerodynamic optimization. The objective function of the optimization is the maximization of total-to-static efficiency while precisely matching the targeted design points. The range of design points studied covers the full typical realm of centrifugal fans according to Cordiers diagram. A comparison to the classic slip theory as e.g. by Pfleiderer reveals that the novel method is superior regarding both fulfillment of the targeted design point and energy efficient performance.

Acoustic Optimization of Rotor-Stator Interaction Noise by Trailing-Edge Blowing

The paper deals with a novel approach for minimizing tonal rotor-stator interaction noise. A stage of a low-pressure axial compressor with a axial rotor and a downstream stator was designed and investigated experimentally. Seven discrete orifices at the rear end of each rotor blade are fed with pressurized air via separate internal passages and pressure control valves and allow the generation of arbitrary spanwise trailing edge blowing profiles from hub to tip. 3D hot wire measurement revealed that the upwash velocity fluctuations downstream of the rotor are caused by (i) the wakes from the rotor blades, (ii) vortex structures at hub and tip, (iii) the potential flow field of the rotor. Former studies proposed that perfect blade wake filling will lead to maximum reduction of rotor/stator interaction noise. In this study some reduction could be achieved by this strategy, but the utillization of an evolutionary optimiziation algorithm in the experiments proved to be much more effective: Targeting directly the far field sound pressure level we identified a spanwise trailing edge flowing distribution that eliminates the wake related tone completely. Spectral analysis proved that the blade wakes cause the acoustic rotor/stator interaction tone at twice the blade passing frequency whereas the fundamental tone is predominantely due to the vortex structures at hub and tip and the potential flow field interacting with the rotor. This explains, why with trailing edge blowing only the second harmonic of the blade passing frequency could be eliminated but not the fundamental tone.

Impact of Different Aerodynamic Optimization Strategies on the Sound Emitted by Axial Fans

The correlation between the aerodynamic design strategy and the sound emission of low pressure axial fans is investigated by a case study. The four fans investigated are equal in terms of total-to-static design point, rotational speed, diameter, and tip clearance, but different regarding hub size and blade geometry. One fan (the baseline) is designed using the analytical blade element momentum method whereas the other fans are optimized by stationary CFD simulations embedded in an evolutionary optimization algorithm. Three different target functions are applied: Maximization of total-to-static and total-to-total efficiency at design point, and maximization of operating range. The distinction between the two efficiencies addresses two competing effects: While the fan optimized with respect to total-to-total efficiency has the lowest secondary flow effects, its aerodynamic load required to fulfill the same total-to-static pressure rise is higher. The characteristic curves of each fan are measured on a chamber test rig to validate the CFD results. The acoustic investigations are conducted in a semi-anechoic chamber. It is found that optimization with respect to total-to-total efficiency, i.e. minimization of losses, leads to the lowest sound power levels around the design point at low Mach numbers while the design optimized with respect to total-to-static efficiency is superior at high Mach numbers. The fan optimized with respect to operating range is loudest at the design point, but this observation inverses when moving to off-design flow rates. It is concluded that none of the design strategies can be recommended purely from an acoustical point of view and that there always is a trade-off between noise, power consumption, operating range, and development cost.

Aerodynamic and Acoustic Optimization of Axial Fans for Locomotive Cooling Systems

The aerodynamic and acoustic performance of a locomotive cooling module is optimized by a new design strategy for the fan unit. The fan selection or development of the cooling modules is usually based on the specific design point but without consideration of installation effects.The new approach considers the fan development in a more integrated manner. A simplified 1:4 model of the cooling system is constructed and used for system analysis, numerical flow simulations, and experimental validation. The subsequent fan and guide vane optimization is based on numerical simulations embedded in an optimization algorithm taking installation effects into account. The noise emitted by the fan is addressed by a smoothed inflow velocity profile, a new blade sweep strategy, the reduction of secondary flows, and the reduction of fan size and rotational speed.The optimized fan unit is then integrated into the non-simplified full-scale system. Experiments reveal an energy saving of 20% and an overall sound power level reduction of 6 dB with even higher reduction of the tone at blade passing frequency.All results are discussed with respect to transferability to other cooling modules. It is found that a set of general design recommendations originates from this work, whereas the optimization loops would need to be repeated for other cooling module types.Copyright

Forced-Air Diesel Locomotive Cooling: Prediction of Noise and Energy Consumption Under Realistic Operational Conditions

An important subsystem in most surface transport vehicles is the forced-air cooling module. Under specific operational conditions of the vehicle the cooling system is the major noise source and the component with the largest consumption of energy. A comprehensive time domain simulation model was developed for simulation of the cooling module in a Diesel locomotive under realistic operational conditions. It includes the components that produce waste heat such as the engine, the turbo transmission, the brake, etc. and the cooling module with its fans. Given the operation of the locomotive e.g. in terms of speed vs. time along a track and its load, data from experimental full scale tests agree well with predictions from the time domain model. The onset of cooling fan operation is predicted well, with it their instantaneous energy consumption and sound radiation. Three optimized cooling unit assemblies for the new locomotive Voith Gravita 15L had been developed and pre-assessed utilizing the model and eventually tested in the locomotive under realistic operational conditions. A new thermodynamically advanced cooling unit with aerodynamically and acoustically optimized fans was found superior by approx. 2 dB (A) less sound power radiation and some 30% less energy consumption as compared to the benchmark. It is anticipated that those advantages are even more distinct as the ambient temperature decreases. The work is part of the European FP7 transport research project ECOQUEST.