Novel concept for the control and operation of radial turbine

Abstract

Radial turbines have proven their capability in many applications, due to their ability to operate at high-pressure ratios, structure robustness, and inherent cost advantages compared to the axial turbine. Controlling their performance allows efficient operation, even when operating at off-design conditions. Various control concepts are used for this purpose. These concepts depend mainly on movable parts and complicated control mechanisms, which limits radial turbines usability in some applications. Hence the need rises for a new control concept based entirely on fixed parts. Responding to these requirements, the Institute of Thermal Turbomachinery and Machinery Laboratory (ITSM) at the University of Stuttgart started a research project which aims to build and test a new control concept for radial turbines. The idea behind this concept is to replace the traditional spiral casing with a new casing. This casing consists of multiple channels which divided the rotor inlet circumferentially to many sectors. Each channel is connected to control valves. Opening and closing these valves will control the turbine inlet area and the operating mass flow by applying different partial admission configurations. The main advantage of this new concept is that it is based only on fixed geometry, and the control valves could be placed away upstream of the turbine. Therefore the turbine can operate under control in any application requiring higher temperature ranges and smaller turbine size. Among various radial turbine applications, turbocharger has been chosen to apply and test the new control concept. In turbocharger application, matching between the radial turbine and the Internal Combustion Engine (ICE) is a complicated balance of many design parameters. Therefore it requires a control mechanism to adjust the turbine mass flow for different ICE operating regimes and achieve an efficient operation. Thus, it was chosen for this task due to its sensitivity toward the control aspects. The first assignment of this study is to build a design tool in order to apply the new control concept by designing Multi-channel Casing (MC) for the radial turbine. This casing will replace the traditional spiral casing and provide a mass flow characteristic comparable to that of the original turbine design at acceptable operating efficiency. Moreover, this design tool will be tested experimentally for a selected case to ensure the design requirements. After achieving the design requirements, different partial admission configurations will be applied to control the turbine performance. The second assignment is to study the effect of the casing replacement on many radial turbine design aspects such as performance, blade excitation and aerodynamic damping at full and different partial admissions conditions. The thesis delivers a MC design model that is capable of replacing the spiral casing with an MC and ensuring comparable mass flow characteristics. It proves also that using the MC design can enhance the radial turbine operating efficiency by 1 up to 3 percent compared to the traditional spiral casing under some design considerations. The results illustrate also the effect of using the MC on the turbine performance and blade vibration during full and partial admission. They explain the reason behind the efficiency drop during the partial admission which is attributed mainly to the flow deviation due to the pressure difference between open and closed channels. They show also how selecting the channel count and building the excitation map for different admission configurations is crucial to avoid high blade vibration amplitude and High Cyclic Fatigue (HCF) during the turbine operation.