Investigation of the flow field in the vicinity of an axial flow fan during low flow rates

Louw, Francois George (2015-12)

Thesis (PhD)--Stellenbosch University, 2015.

Thesis

ENGLISH ABSTRACT: Large axial flow fans (≈9 m in diameter) are used in air-cooled heat exchangers (ACHEs). Adverse inflow conditions occur in these systems, especially near the periphery of the air-cooled heat exchanger (ACHE) platform, affecting reduced flow rates through the fans located in these regions. The focus of the present study is to investigate the flow field in the vicinity of an axial flow fan during these low flow conditions, by means of experimentation and numerical analyses. A 1.542 m diameter model of an axial flow fan, termed the B2a-fan, is constructed and tested in a British standard 848, type A test facility to obtain its characteristic curves. A wireless system to conduct blade surface pressure measurements (BSPMs) on both the pressure and suction sides of a fan blade is also developed and implemented. An eighth sector of the fan is modelled using the commercial CFD code, ANSYS Fluent 14.0, for a range of flow rates. Pearson correlation factors of RP; Fs = 0:997 and RP; Fs = 0:990 are obtained for the respective characteristic curves of fan static pressure and static efficiency. Factors of RP;BSPM & 0:990 are obtained for more than 90% of the BSPMs along circumferential sections of a blade, at different radii. These correlations validate the numerical model. Flow field visualizations show that the flow field becomes more unstable as the flow rate through the fan is decreased with considerable increases in radial velocity. Instability starts near the blade root and progressively moves towards higher blade spans with a decrease in flow rate. A deviation occurs between the numerically calculated blade sectional lift and drag characteristics and the two-dimensional NASA LS 0413 aerofoil data at low flow rates. The highest blade sectional lift coefficients occur near the hub and decrease towards higher blade spans. An Euler turbo-machinery analysis of the flow field data shows that the component of Coriolis power is close to zero at near-design flow rates, whereas the aerodynamic power component is almost equal to the total power applied by the fan. However, as the ow rate decreases a gradual inversion of this characteristic is observed. Lastly, a new simplified fan model in the form of an actuator-disc model (ADM) is developed, termed the Reverse engineered empirical actuator-disc model (REEADM). This model implements the blade sectional lift, drag and radial force coefficients calculated from the CFD data. The results obtained from this model provides a fair comparison to the experimental fan characteristics at near-design flow rates, but a deviation is observed for low flow rates, although an improvement is made compared to the original ADM.

AFRIKAANSE OPSOMMING: Groot aksiaalvloeiwaaiers (≈ 9 m in deursnee) word geïmplementeer in lugverkoelde hitteruilers (LVHRs). Ongunstige invloeipatrone, wat veral naby die rante van 'n LVHR-platform voorkom, veroorsaak 'n afname in volumevloeitempo's deur die waaiers in hierdie omgewings. Dus is die fokus van die huidige studie om die vloeiveld in die nabye omgewing van 'n aksiaalvloeiwaaier, onderhewig aan lae vloeitoestande, te ontleed deur beide eksperimentele en numeriese ondersoeke. 'n 1.542 m deursnee model van 'n aksiaalvloeiwaaier, genaamd die B2awaaier, is gebou en getoets in 'n toetsfasiliteit wat voldoen aan die Britse standaard 848, tipe A, om die karakteristieke waaierkrommes te bepaal. 'n Koordlose sisteem om lemoppervlak-drukmetings (LODMs) mee uit te voer, op beide die suig- en drukkant van die waaierlem, is ook ontwikkel en geïmplementeer. 'n Agtste sektor van die waaier is gemodelleer deur van die kommersiële program, ANSYS Fluent 14.0, gebruik te maak vir die reeks volumevloeie wat in ag geneem is. Pearson verwantskapsfaktore van RP; Fs = 0:997 en RP; Fs = 0:990 word verkry vir die onderskeie karakteristieke krommes van waaier-statiese druk en statiese effektiwiteit. Verwanskapsfaktore van RP;LODM & 0:990 word verkry vir meer as 90% van die LODMs langs omtreksnitte van die waaierlem by verskillende radiusse. Hierdie verwantskappe dien as bevestiging die numeriese model se akkuraatheid. Visuele uitbeelding wys dat die vloeiveld toenemend onstabiel raak as die vloeitempo deur die waaier verminder. Daarbenewens word 'n aansienlike toename radiale vloei ook opgemerk. Die vloei begin onstabiel raak by die waaiernaaf en beweeg geleidelik uitwaarts na groter lemspanwydtes soos wat die vloeitempo deur die waaier verminder. 'n Afwyking tussen die numeries berekende hef- en sleurkarakteristieke by verskillende lemsnitte en die tweedimensionele NASA LS 0413 vleuelprofiel data, kom voor by lae vloeitempo's. Die hoogste lemsnit-hefkoëffisiënte kom by die naaf voor en verminder soos na hoër spanwydtes beweeg word. 'n Euler turbomasjienerie analise van die vloeivelddata dui aan dat die Coriolis drywingskomponent naby aan nul is vir vloeitempo's naby aan die ontwerpspunt, maar dat die aerodinamiese drywingskomponent naastenby gelyk is aan die totale drywing wat deur die waaier uitgeoefen word. 'n Gelydelike inversie van hierdie karakteristiek vind egter plaas soos die vloeitempo verminder word. Laastens is 'n nuwe vereenvoudigde waaiermodel ontwikkel in die vorm van 'n aksie-skyf model (ASM), genaamd die Teruggewerkte empiriese aksie-skyf model (TEASM). Hierdie model implementeer die lemsnit hef-, sleur- en radiaalkragkoëffisiënte wat uit die numeriese data bereken is. Die karakteristieke, wat deur die laasgenoemde model bereken is, vergelyk goed met die eksperimentele waaierkarakteristieke by vloeitempo's soortgelyk aan die ontwerpstoestand, maar 'n afwyking kom by lae vloeitempo's voor. Die TEASM model bied egter steeds 'n verbeterde antwoord by lae vloeitempo's in vergelyking met die oorspronklike ASM.

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