Performance investigation of turning diffusers at various geometrical and operating parameters

Nordin, Normayati (2016) Performance investigation of turning diffusers at various geometrical and operating parameters. PhD thesis, Universiti Teknologi PETRONAS (UTP).

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Abstract

The performance of the turning diffuser regardless of its expansion type, i.e., two-dimensional (2-D) or three-dimensional (3-D), has been traditionally rated using the guidelines established specifically for a 2-D turning diffuser. This has provided merely an approximation and has often led to an inaccurate prediction of 3-D turning diffuser performance. On top of that, the existing guidelines have just integrated the geometrical effect by discounting the effect of operating condition on the turning diffuser performance. Therefore, the current work aims to experimentally and numerically investigate the performance of 2-D and 3-D turning diffusers for various geometrical and operating parameters. The performance indexes (pressure recovery coefficient, flow uniformity index) as a function of geometrical (inner wall length to inlet throat width ratio, outlet-inlet configurations) and operating (inflow Reynolds number) parameters are correlated by means of Asymptotic Computational Fluid Dynamics technique. Stereoscopic particle image velocimetry was used to examine the flow characteristics, and a manometer provided the inlet and outlet wall static pressures. Among all the models tried, the best results were obtained with the standard k- and enhanced wall treatment of y+  1.1 – 1.8 was applied for the intensive simulation. Results showed that there was a potential performance of applying 3-D turning diffuser relative to 2-D turning diffuser. The 3-D turning diffuser provided higher pressure recovery at low inflow Reynolds number, Rein = 5.786 x 104 - 6.382 x 104 and better flow uniformity at high inflow Reynolds number, Rein = 1.027 x 105 – 1.775 x 105 than the 2-D turning diffuser. Minimal flow separation occured within the 3-D turning diffuser that was close to the outlet edge, 0.9Lin/W1. While flow separation within the 2-D turning diffuser took place earlier on half of the inner wall length, 0.5Lin/W1. Secondary flow vortices initially emerged at Rein = 1.027 x 105 (3-D turning diffuser) and Rein = 1.397 x 105 (2-D turning diffuser). The pressure recovery was affected mainly by the existence of flow separation and vortices, whereas the flow uniformity was affected by the dispersion of core and secondary flows. A high free-stream turbulent intensity imposed on the flow favoured the overall performance of the turning diffuser by suppressing the separation of the inner wall boundary layer and mixing to give better uniformity of the flow. Excessive elongations, Lin/W1 ≥ 20 (2-D turning diffuser) and Lin/W1 ≥ 9 (3-D turning diffuser) inherently impaired the pressure recovery. The performance correlations as a function of geometrical and operating parameters for 2-D and 3-D turning diffusers were successfully developed to satisfy both the CFD and experimental results within ±8%. In conclusion, the physics of flow particularly within the 3-D turning diffuser have been grasped with credible performance data have been established as benchmark. The developed correlations can be used henceforth by one to evaluate the performance of turning diffusers without necessarily running the full simulation or experiment. For future work, the same outlined methods particularly via Asymptotic Computational Fluid Dynamics can be applied to develop performance correlations of other diffuser types. The current work can be further extended by considering the variation of turning angles and installation of flow control devices to improve the performance of turning diffusers. The effects of skin friction and turbulent intensity should be also looked into more details.

Item Type:Thesis (PhD)
Uncontrolled Keywords:diffuser; computational fluid dynamics; particle image velocimetry; asymptotic method; pressure recovery; flow uniformity
Subjects:T Technology > TJ Mechanical engineering and machinery > TJ1-162 Mechanical engineering and machinery
Divisions:Faculty of Mechanical and Manufacturing Engineering > Department of Energy and Thermofluid Engineering
ID Code:8389
Deposited By:Mrs. Normayati Nordin
Deposited On:05 Oct 2016 11:47
Last Modified:05 Oct 2016 11:47

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