Design procedure of a compact crossover diffuser for micro gas turbine application

Burger, Christiaan Johannes (2016-03)

Thesis (MEng)--Stellenbosch University, 2016.

Thesis

ENGLISH ABSTRACT: The design of a compact crossover diffuser to replace the size-limited radial diffuser and axial de-swirl cascade of the BMT120KS MGT is discussed. A crossover diffuser strives to combine the performance of a channel diffuser with the operating range and efficiency of a vaneless diffuser. Continuous aerodynamic vanes are designed to match the centrifugal impeller discharge flow and gradually diffuse the air towards the axial combustor inlet where the effect of swirling flow on combustor performance is experimentally investigated. Swirling combustor flow promotes combustion intensity, reduces combustion length, and enhances combustion stability. The study includes the development of a one-dimensional Mean-Line Code (MLC) which aids the designer in preliminary design and performance evaluation. A three-dimensional multiple operating point optimisation is conducted thereafter using Numeca FineTM/Turbo). The final design is manufactured using a five-axis CNC milling machine and experimentally tested by modifying the existing BMT120KS engine. The optimised crossover diffuser and modified impeller yielded a numerical increase in the total-to-static compressor stage pressure ratio from 2:93 to 3:83, increase in the static pressure recovery coefficient from 50:92% to 68:49%, and relaxed the compressor stage discharge flow angle from 84:14° to 34:28° (compared to the modified BMT120KS engine). Experimental results are recorded up to a rotational speed of 106 000RPM and extrapolated to the design speed of 120 000 RPM. The extrapolated results indicate a thrust increase from 107:3N to 220N and increase in total-to-static compressor stage pressure ratio from 2:62 to 3:65 at 120 000RPM (compared to the original BMT120KS engine).

AFRIKAANSE OPSOMMING: Die ontwerp van n kompakte oorgangsdiffusor om die radiale lem diffusor en aksiale rig-lemme van die bestaande BMT120KS mikrogasturbine te vervang word bespreek. ‘n Kompakte oorgangsdiffusor beoog om die werksverigting van ‘n kanaaldiffusor met die bedryfsgebied van n lemlose diffusor te kombineer. Deurlopende aerodinamiese lemme is ontwerp om die sentrifigaalkompressor rotor uitlaatvloei geleidelik te diffundeer tot waar dit die verbrandingsruim binne gaan met ‘n beduidende tangensiale vloeikomponent. Die studie sluit in die ontwikkeling van ‘n een-dimensionele kode wat die ontwerper instaat stel om ‘n kompakte oorgangsdiffusor te ontwerp en te evalueer . Daarna is optimering gedoen in Numeca FineTM/Turbo (BVM sagteware) en die finale ontwerp is vervaardig met behulp van ‘n vyf-as numeries beheerde freesmasjien. Die geoptimeerde oorgangsdiffusor en rotor lewer ‘n numeriese toename in die totale-tot-statiese kompressor stadium drukverhouding vanaf 2:93 na 3:83, ‘n toename in die statiese druk herwinningskoëffisiënt vanaf 50:92% tot 68:49%, en verminder die uitlaat-vloeihoek vanaf 84:14°na 34:28° (vergeleke met die gewysigde BMT120KS enjin). Eksperimentele resultate is verkry tot by ‘n rotasie spoed van 106 000RPM en geëkstrapoleer tot die ontwerpspoed van 120 000 RPM. Die resultate dui op ‘n stukrag toename vanaf 107:3N na 220N en ‘n toename in die totale-tot-statiese kompressorstadium drukverhouding vanaf 2:62 na 3:65 (vergeleke met die oorspronklike BMT120KS enjin).

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