Inlet distortion is an important consideration in fan performance. Distortion can be generated through flight conditions and airframe-engine interfaces. The focus of this paper is a series of high-fidelity, time-accurate Computational Fluid Dynamics (CFD) simulations of a multistage fan, investigating distortion transfer, distortion generation, and the underlying flow physics under different operating conditions. The simulations are full annulus and include 3 stages and the inlet guide vane (IGV). The code used to carry out these simulations is a modified version of Overflow2.2 that was developed as part of the Computational Research and Engineering Acquisition Tools and Environment (CREATE) program. The inlet boundary condition is a single revolution (sinusoidal pattern with one period over the circumference ) total pressure distortion. Simulations at choke, design, and near stall are analyzed and compared to experimental data. Distortion transfer and generation is analyzed under these different operating conditions. Analysis includes the phase and amplitude of total temperature and pressure distortion through each stage of the fan, level of distortion transfer and generation in each stage, and blade loading. An understanding of the flow physics associated with distorted flows will help fan designers account for unsteady flow physics at design and off-design operating conditions, in order to build more robust fans offering a greater stability margin.

Thesis (MScEng)--Stellenbosch University, 2012. / ENGLISH ABSTRACT: Axial flow fans used in large-scale air-cooled steam condensers (ACSCs) may operate under distorted inflow conditions. These conditions occur due to the prevailing wind conditions, the presence of buildings, and the location of the fan within the ACSC. Fans located on the periphery of the ACSC are affected the most due to their exposure to strong winds and the inner fans drawing in air past them. Distorted inflow conditions cause varying fan blade and gearbox loading conditions. The purpose of the investigation was to simultaneously measure the inlet air flow and the resultant blade and gearbox loading conditions of a single fan located on the periphery of a large-scale ACSC. Inlet and heat exchanger bundle outlet air flow velocities were measured using a combination of ultrasonic and propeller anemometers while blade loading was measured with strain gauges attached at the neck of the specific blade being monitored. Strain gauges were also attached to the low-speed fan shaft to measure gearbox loading. Measurements were recorded over a period of 8 days where it was found that increased wind resulted in increased air flow in the axial direction of the fan, which then caused a reduction in average blade loading. This was due to a decreased static pressure rise over the fan. The fan blade was found to vibrate at its own natural frequency of 6 Hz when excited by the variable aerodynamic loading. The aerodynamic loading was extracted from the measured data and was found to correlate well with previous experimental work performed by Bredell et al. (2006a). Shaft bending stresses and torque were found to oscillate at the fan’s rotational frequency of 2Hz with a large torque exerted on the shaft during fan start-up. / AFRIKAANSE OPSOMMING: Aksiaalvloeiwaaiers wat by groot lugverkoelde stoomkondensors gebruik word, werk dikwels onder verwronge inlaat lugvloei toestande wat geskied as gevolg van heersende winde, die teenwoordigheid van geboue en die posisie van die waaier in die kondensor. Waaiers wat geleë is op die rand van die kondensor word die meeste beïnvloed as gevolg van blootstelling aan die sterk winde en dwarsvloei wat deur die binneste waaiers geïnduseer word. Verwronge inlaat lugvloei veroorsaak gevolglik variërende waaierlem en ratkas belastingstoestande. Die doel van hierdie ondersoek was om terselfdetyd die inlaat lugvloei asook waaierlem en ratkas belastingstoestande van ’n enkele waaier wat op die rand van ’n grootskaalse lugverkoelde stoomkondensor geleë is, te meet. Waaier inlaat en warmteruiler uitlaat lugvloei snelhede is gemeet met ’n kombinasie van ultrasoniese- en skroefwindsnelheidsmeters terwyl die lem en ratkas belastings gemeet is met rekstrokies. Metings is oor ’n tydperk van 8 dae geneem. Die bevindinge toon dat ’n toename in windsnelheid ’n toename in aksiale lugvloei tempo, deur die waaier veroorsaak. ’n Afname in die gemiddelde lembelasting is waargeneem as gevolg van ’n afname in die waaier statiese druk. Daar is ontdek dat die waaierlem teen ’n natuurlike frekwensie van 6 Hz vibreer wanneer dit opgewek word deur die wisselende aerodinamiese belasting. Die aerodinamiese belasting is verkry uit die gemete data en vergelyk goed met die numeriese werk van Bredell et al. (2006a). Daar is ook bevind dat waaier-as buigspannings en wringkragte ossileer teen die waaier se rotasiefrekwensie van 2Hz met ’n groot wringkrag wat op die as uitgeoefen word wanneer die waaier aangeskakel word. iii