Wang, P; (2017) Multi-objective design of a transonic turbocharger compressor with reduced noise and increased efficiency. Doctoral thesis , UCL (University College London).

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Abstract

In marine diesel engines, transonic centrifugal compressors are widely used due to their capabilities to: 1) downsize engines by increasing output power; 2) cause less fuel consumption; 3) enhance the combus¬tion efficiency. The design of a high performance transonic compressor still poses a great challenge to the turbomachinery community. Apart from the traditional requirements such as good choke and stall mar¬gin, high boosting pressure ratio, and high stage efficiency, it is also necessary to reduce the turbocharger noise as much as possible for a more comfortable working environment. The most effective way is to reduce the compressor noise at the source, i.e. the compressor im¬peller itself must produce less aerodynamic noise rather than using any silencers. This thesis is dedicated to present a design methodol¬ogy for such multi-disciplinary and multi-objective tasks, and propose a new impeller design for a marine turbocharger. The conventional turbomachinery design method has almost reached its bottleneck even if it is coupled with advanced optimisation al¬gorithms. A different design philosophy, which is known as inverse design theory, suggests an alternative solution for such design chal¬lenges. It provides a different perspective to look at the problem. Rather than starting with the blade profiling, this approach prescribes the flow field on the blade surface and robustly finds the correspond¬ing blade geometry that would result in the given flow field. Since the flow field plays a predominant role in determining both aerodynamic and aeroacoustic performances, it gives designers more direct control for getting an optimal design. Thus, this method can significantly strengthen the performances in a much shortened design time. An in-house aeroacoustic code has been developed based upon the well-known Ffowcs-Williams and Hawkings equation to predict the rotor induced tonal noise at the blade passing frequency and its har¬monics. It can also be easily expanded to predict the transient flow induced broadband noise if the very fine unsteady calculations are available. A new centrifugal compressor impeller was designed in this disserta¬tion using the inverse design code TURBOdesignl. Its performances are evaluated from aerodynamic, aeroacoustic and mechanical aspects. The CFD simulations for the compressor characteristics and inter¬nal flow field details, the numerical predictions of aeroacoustic sound emissions, and FEA analysis for the structure integrity have all been attempted to thoroughly assess the performances of baseline and op¬timised impellers. The computational results found that the new im¬peller can lead to better performances in all three aspects, which are supported by the experimental measurements conducted for both impellers using the same test configurations. The experimental data confirmed that the inversely redesigned impeller wheel provide a wider compressor operating range, higher efficiency at large rotating speed, and a few dB(A)s lower noise emissions in the upstream. The present work suggests that the synergistic use of the inverse design approach, aeroacoustic solver, CFD simulations and FEA analysis can allow designers to effectively carry out the multi-disciplinary and multi-objective designs for turbomachines.

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