Experimental and numerical investigation of flow structure and heat transfer in gas turbine HP compressor secondary air systems

Puttock-Brown, Mark Richard (2018) Experimental and numerical investigation of flow structure and heat transfer in gas turbine HP compressor secondary air systems. Doctoral thesis (PhD), University of Sussex.

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Abstract

With the continuing growth of the air traffic sector and a drive towards increasinglyefficient aero engines the overall pressure ratio of such engines is set to climb. As a consequence, blade tip clearances will become proportionally larger as blade size decreases. Accurate sizing of the tip clearance is dependent on knowledge of the radial growth of the compressor discs, which in turn is dependent on their radial temperature gradient. Currently, 2D thermo-mechanical models based upon empirical correlations and scaling laws are used to predict this radial growth and the temperature increase in the secondary air system. These require knowledge of the buoyancy-induced flow that occurs in heated cavities between adjacent co-rotating discs.
This thesis presents experimental and numerical results of the heat transfer and flow structure of a buoyancy-induced rotating cavity flow field undertaken using the University of Sussex TFMRC Multiple Cavity Rig. This rig simulates the rotating components of a gas turbine secondary air system of a high pressure compressor. The objective is to gain a deeper understanding of the flow mechanisms operating within a rotating cavity at low Rossby numbers, representative of non-dimensional engine conditions and demonstrate that the shroud is the dominant source of heat transfer to the axial throughflow. The working conditions cover the range: 1.1×105<Rez<5.1×105,1.7×106<Reθ<3.2×106,0.1<Ro<0.6,0.32< β∆T <0.40 and 3.1×1011<Gr<1.3×1012 in Phase A and 1.2×104<Rez<5.2×104,1.5×106<Reθ<3.2×106,0.05<Ro<1.34,0.33< β∆T <0.51 and 2.4×1011<Gr<1.6×1012 in Phase B. The numerical study uses the working conditions: Gr= 8.94×1011, Rez= 4.4×104, Reθ= 2.83×106,β∆T= 0.35,β∆Tav= 0.14 and Ro = 0.29.
Using experimental measurements of surface temperature, the Nusselt numbers onthe rotating surfaces have been derived using a finite-element conduction solution. MonteCarlo simulation is used to give confidence intervals based on experimental uncertainty. New correlations have been derived for both the shroud and diaphragm and compared to existing literature. The shroud surface is shown to exhibit a magnitude of heat transfersimilar to turbulent levels but with a trend - correlated to Grashof number - indicative of laminar behaviour. This is thought due to unsteady laminar free convection. Also discussed is the shroud corner, the interface between the disc diaphragm and shroud, which has received little previous attention, yet shows a high level of heat transfer. The diaphragm Nusselt number correlation is based on a modified form of the Grashof number that acknowledges the effects of both free and forced convection inside the rotatingcavity.
An accompanying numerical simulation has been conducted using Computational Fluid Dynamics to assess the complex nature of the buoyancy-induced cavity flow field.The 3D Unsteady Reynolds-Averaged Navier-Stokes equations with the SST k-ω turbulence model and experimentally measured boundary conditions are solved on a mesh ofapproximately 16 million elements. Validation of the numerical results is presented and includes comparison to measured high-frequency pressures on the static central shaft andair temperatures inside the rotating cavity. A type of Rayleigh-Bénard convection manifesting as a series of propagating streaks along the shroud periphery is identified and shown to modify the local Nusselt number, however their existence cannot be ratified without experimental evidence. Assessment of the cavity surface heat transfer shows that the shroud contributes 36%, the shroud corners 30%, the diaphragms 31% and the cobs 2%. Illustrating the shroud surface as the dominant heat transfer feature in the cavity system.

Item Type: Thesis (Doctoral)
Schools and Departments: School of Engineering and Informatics > Engineering and Design
Subjects: T Technology > TJ Mechanical engineering and machinery > TJ0266 Turbines. Turbomachines (General)
Depositing User: Library Cataloguing
Date Deposited: 22 May 2018 11:07
Last Modified: 25 Nov 2020 13:39
URI: http://sro.sussex.ac.uk/id/eprint/75214

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