Investigation on Optimal Design of Radial and Mixed flow turbines

Abstract

Micro Turbine Technology BV (MTT) is a company that develops Combined Heat and Power generation (CHP) systems using turbocharger technology. They are developing a 3 kW recuperated microturbine for a micro-CHP application and are constantly looking at options to improve the performance of the microCHP system. A suitable modification in the turbocharger technology can be achieve improvement in microCHP’s performance. MTT currently uses an off-the-shelf turbocharger system modified to accommodate a generator to generate power. As they use an off-the-shelf product, one way to improve the system efficiency is to opt for turbine/compressor design modifications to optimise its performance for the particular working condition of MTT’s CHP unit.

MTT is continuously looking at optimising their radial compressors and turbine. Literature and various researches over the past few decades suggest mixed flow turbines as an alternative with advantages for the conventional radial turbines. Thus, this thesis is oriented towards the design and analysis of new mixed flow turbines for MTT’s microCHP system. One of the main goals is to investigate the performance of the turbine stage by adopting new turbine rotors and their design impact on the current turbine stage. The relevant design process, loss models for the turbine, design parameters considered and the procedures followed in few past pieces of research are discussed. A 1D mean-line design procedure has been developed for MTT to design a new rotor considering the upstream and downstream components and the packaging constraints.

Four different turbine configurations are chosen for comparison. The existing radial turbine in use by MTT is considered, and two mixed flow turbines for the same dimensions of the existing radial turbine are analysed for its performance to compare. While designing new rotors, optimum inlet conditions are assumed to neglect the effect of incidence and its loss. Considering the optimum inlet conditions for the turbine rotor, new radial and mixed flow rotors are designed and compared for their performance and sizing. In comparison with the existing rotors, the turbine rotors designed for optimal performance meets the required exit pressure. Mixed flow turbines designed with inlet optimal conditions perform significantly (about 3- 5 %) better than the new radial turbines designed
with the same optimal conditions. A reduction in the inlet diameter (by 5-6 mm) of the rotors is observed for the newly designed mixed flow turbines. Relieving the packaging restrictions, the rotors designed with optimum inlet conditions show an increase in the rotor and stage dimensions with reduced specific power and efficiency due to positive swirl at the rotor exit.

As a first step towards the more detailed aerodynamics and structural analysis, a 2D gas flow-path design procedure has been adapted and programmed to obtain the end wall contours of the flow path within the rotor. Thus, a basic two-dimensional flow path design for the rotors are created. 3D models of the turbine rotors are built using the commercial analysis and modelling software. The three-dimensional models of the newly designed radial and mixed flow turbine rotors are assembled in the current turbine stage to study the geometrical impact of the rotors on the turbine stage. The new rotors have larger exit diameter compared to the existing rotors which make the existing turbine volute to be modified to accommodate the new rotors. The design modifications for MTT’s micro CHP turbine stage have been suggested.