The use of CFD for heliostat wind load analysis

Hariram, Adhikar Vishaykanth (2015-03)

Thesis (MEng)--Stellenbosch University, 2015.

Thesis

ENGLISH ABSTRACT: The capability of computational fluid dynamics (CFD), in particular the FLUENT ™ commercial software suite, to predict wind loadings on heliostats has been investigated. If CFD proves useful in this area then the overall development costs of heliostats and concentrating solar thermal power plants could be reduced. Due to the largest loading on the heliostat originating from wind loads, by using CFD to determine these loads it could be possible to ensure heliostats are not overdesigned. This thesis contains a first study within the Solar Thermal Energy Research Group (STERG) at Stellenbosch University into the use of CFD for determining heliostat wind loads. The relevant theoretical background concerning the turbulence models used in this study, namely, the RNG k-ε, Realisable k-ε and SST k-ω turbulence models is reiterated. The „standard‟ k-ε model and the large eddy simulation (LES) approach, due to their relevance to bluff body flows, are also revisited. Some analysis is also provided around each model to gain insight as to the role of respective modelling sensitivities and their advantages. Previous work done in the area of heliostat wind studies is reviewed. The geometric considerations when dealing with heliostats leads onto the discussion concerning the requirement of modelling boundary layer profiles. Hence some background is provided on boundary layer modelling techniques. Further insight is drawn from more general previous bluff body CFD reported in the literature, from which observations and recommendations regarding the use of variations of the k-ε turbulence model can be inferred. The simulation procedure from geometry creation to results obtained for the flow over a vertical flat plate is reported. This investigation led to the conclusion that the Realisable k-ε should be used for the heliostat simulations on account of its accurate drag prediction under steady state flow conditions. It was also found that for transient simulations for heliostat like geometries, the SST k-ω model appears most suitable. The Realisable k-ε model is then used to model the flow about a heliostat using the same procedures as for the flat plate; both with flat and boundary layer inlet profiles. The overall conclusions drawn from this work are that the Realisable k-ε would not be suitable for predicting wind loads used in the final design of heliostats although it may be used with flat velocity and turbulence profiles to compare differences between early heliostat designs. The conclusion that the Realisable k-ε model should not be used to predict the flow field in the vicinity of a heliostat is also reached. It is recommended that further work should be carried out by using more advanced modelling techniques, such as the LES, to determine wind loads on heliostats. Furthermore, additional studies focused on accurately reproducing the velocity and turbulence profiles should be done. Lastly a larger set of data containing the orientations mentioned in literature should be generated using the methods contained within this study.

AFRIKAANSE OPSOMMING: Die vermoë van Numeriese Vloei Meganika (NVM), spesifiek die van die FLUENT ™ kommersiële sagtewarepakket, om die windlaste op heliostate te voorspel was ondersoek. As daar gevind word dat NVM wel betekinsvolle resultate kan lewer, kan dit die totale ontwikkelingskoste van heliostate en gekonsentreerdesonkragstasies verlaag. Wind plaas die grootste las op heliostate, dus deur gebruik te maak van NVM om die windlaste op heliostate te voorspel, kan dit gebruik word om te verseker dat heliostate nie oorontwerp word nie. Hierdie tesis bevat „n eerste studie binne die Sontermiese Energie Navorsings Groep aan die Universiteit van Stellenbosch, wat die gebruik van NVM om windlaste op heliostate te voorspel ondersoek. Alle relevante teoretiese agtergrond wat turbulensiemodelle aanbetref, naamlik die RNG k-ε, Realiseerbare k-ε en SST k-ω turbulensiemodelle, word bespreek. Hulle relevansie tot stompligaamvloei toegestaan, word die „standaard‟ k-ε model en die groot werwel simulasie (GWS) benaderings ook bespreek. Elke model word bespreek om die leser insig te gee in dié model se sensitiwiteite en voordele. Vorige studies wat betrekking het tot die studie van heliostate en wind word bespreek. Die geometrie van heliostate lei tot „n bespreking oor die noodsaklikheid vir „n model vir die grenslaagprofiel, dus word grenslaagmodelleringstegnieke bespreek. Verdere insig word verkry van vorige NVM studies uit die literatuur met meer algemene stomp liggame, wat waarnemings en voorstelle vir die gebruik van die k-ε turbulensiemodel en variante verskaf. Die simulasieproses, vanaf geometrieskepping tot die resultate vir die vloei oor 'n vertikale vlak, word bespreek. Hierdie ondersoek het tot die gevolgtrekking gelei dat die realiseerbare k-ε model gebruik moet word vir die heliostaat simulasies, as gevolg van die akkurate sleurvoorspellings onder bestendigetoestande. Daar was ook gevind dat vir heliostaatagtige liggame onder oorgangskondisies, die SST k-ω model mees geskik sal wees. Die Realiseerbare k-ε model word dan gebruik om die vloei om 'n heliostaat te modelleer deur gebruik te maak van dieselfde proses wat gebruik word om vloei oor 'n plat plaat te analiseer: albei met plat en grenslaaginlaatprofiele. Die gevolgtrekkings van hierdie studie is dat die Realiseerbare k-ε model nie gebruik kan word tydens die finale ontwerpfase om die windlaste op 'n heliostaat te voorspel nie. Dit kan wel gebruik word met plat snelheids- en turbulensieprofile om die versikille tussen vroeë heliostaatkonsepte te vergelyk. Daar was ook bepaal dat die Realiseerbare k-ε model nie gebruik moet word om die vloeiveld om 'n heliostaat te voorspel nie. Daar word voorgestel dat verdere studies in hierdie vakgebied met meer gevorderde modelleringstegnieke aangepak word. Dit word aanbeveel dat verdere werk uitgevoer moet word deur die gebruik van meer gevorderde modellering tegnieke, soos GWS, om die wind kragte op heliostats te bepaal. Verder, studies wat akkurate snelheid en turbulensieprofiele produseer sal nog bygelas moet word. Laastens 'n groter stel data met oriëntasies soos wat in die literatuur beskryf word, moet deur middel van die metodes van dié studie gegenereer word.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/97048
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