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Title: A one-dimensional study of unsteady wave propagation in turbocharger turbines
Author: Costall, Aaron
ISNI:       0000 0001 3389 8679
Awarding Body: Imperial College London
Current Institution: Imperial College London
Date of Award: 2008
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Flow in a turbocharger turbine is highly unsteady in nature as it responds to the exhaust manifold of an internal combustion (IC) engine. Despite this it is conventional to use quasi-steady turbine models in one-dimensional turbocharged engine simulations, even though they cannot reproduce the known hysteresis of turbine mass flow and performance characteristics recorded under pulsating flow conditions. Using filling-and-emptying models improves the situation by permitting mass accumulation in the turbine volute. Depending on the unsteadiness level, this approach may still be insufficient to capture true turbine operation since neither method can resolve unsteady effects due to pressure wave action in the flow. It is unclear when transition occurs between filling-and- emptying and wave action modes. To this end, a proprietary computational gas dynamics code in C++ is presented to simulate the unsteady, compressible flow inherent to IC engine exhaust manifolds. The Euler equations for one-dimensional inviscid flow are discretized to provide second-order, conservative, shock-capturing finite difference schemes able to resolve wave propagation in ducts with area variation, wall friction and heat transfer. A wave action turbine volute model is constructed using bespoke boundary conditions. Validation against experimental data shows satisfactory agreement for pulse frequencies up to 40 Hz, and improved instantaneous swallowing capacity prediction at all tested frequencies compared to quasi-steady calculations. Fourier series characterization of on-engine pulse waveforms reveals multiple harmonic components, causing significant regions of divergence between filling-and-emptying and wave action predicted hystereses. Comparison of concurrent wave action and filling-and-emptying simulations applying simpler sinusoidal waveforms allows development of the unsteadiness measures FSt and FSt(p). An approximate guideline to ensure a filling-and-emptying mode stipulates FSt [Symbol appears here. To view, please open pdf attachment]0.15 and FSt(p) [Symbol appears here. To view, please open pdf attachment]0.02. Evaluation of FSt and FSt(p) for an example on-engine case indicates certain wave action already by 1600 rev/min, borne out by subsequent inspection of the swallowing capacity traces.
Supervisor: Martinez-Botas, Ricardo Sponsor: Engineering and Physical Sciences Research Council
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral