Radial Turbine Aerodynamics

Introduction

A turbine is a device that transfers energy from a working fluid to a rotating shaft, thereby producing work. In a radial inflow turbine the flow enters the nozzle blades in the radial direction (or in the case of a swirl generating volute in the tangential direction). The flow enters the rotor blades in the radial direction, is turned in the rotor passage and exits along the axis of rotation.

Radial inflow turbines are suitable for many applications in ship, aircraft, space power systems, and other systems where compact power sources are required. They are also used in air liquefaction plants and have been employed in power generation units providing up to 2 megawatts of power for a variety of industrial applications, aiming to replace heavy diesel engines. Radial turbines have several advantages over an axial turbine. They maintain a relatively high efficiency when reduced to very small sizes and can handle an elevated pressure ratio. The pressure ratio for radial turbines used in turbochargers can be up to 4:1, but in some applications, as for example in their use in power generation systems, it can be as high as 6:1.

A Study of Tip Leakage in a Radial Inflow Turbine

The operation of an unshrouded radial turbine requires a minimum of tip clearance between the rotating running blades and the stationary wall casing. This gap gives rise to leakage flow driven by the pressure difference between the pressure side and the suction side. The tip leakage flow is largely unturned, so that no work is done. It exits the tip clearance gap on the suction side in a near perpendicular direction. The "mixing zone" between the tip leakage flow and the main flow near the suction side causes entropy generation.

The general objective of this study was to gain physical insight into tip leakage flow in radial inflow turbines and eventually to model this flow. Performing an extensive experimental programme, three regions of different tip leakage flow behaviour were discovered. These regions are characterised by more or less intensive interaction between the scraping flow and the tip clearance flow. Scraping flow is caused by relative casing motion and opposes the tip leakage flow in a turbine. Based on a new understanding of the scraping mechanism a tip leakage model was developed. The model was successfully compared with integrated hot-wire measurements inside the tip gap.

Research Details

Research aimed at understanding the nature of tip clearance flow in radial inflow turbines has been conducted with the support of IHI of Japan and the Swiss national science foundation. A large scale, low speed radial inflow turbine (rotor inlet diameter 1.2 meter) is employed in this study. An extensive experimental program was performed. The flow features of the rotor tip region were visualised by using ammonia gas and diazo paper. The time-mean, steady state pressure and the unsteady pressure variation at the casing were measured at 20 spanwise locations. The static pressure at the blade tip surface was also measured. A hot-wire was traversed into the tip gap of the rotating turbine at 7 spanwise positions. Measurements were performed for 3 different gap heights, 3 different blade speeds (for a constant mass flow) and 2 different inlet boundary layer thicknesses. In-house codes (3-D steady and unsteady, structured and unstructured, Navier-Stokes) were then used to assess the experimental data and to enhance the understanding of tip clearance flows.

A schematic of the tip leakage flow in radial turbines is presented in Figure 1. The tip leakage flow is opposed by flow adjacent to the casing, which is driven by the relative casing motion.The suction surface, being on the leading side of the blade, "scrapes up" some of this boundary layer fluid (so-called scraping flow) from the casing and causes it to deflect down to the hub and into the main flow direction. The remainder of the scraping flow is "dragged" through the gap from the suction side to the pressure side.  

Figure 1 Schematic of tip leakage flow and opposing scraping flow in the relative frame

Figure 2 presents relative velocity vectors inside the tip gap at 34% S_m. S_m is the distance from the inlet to exit when viewed along a streamline in the axial-radial plane. It is known as the meridional length. It can be observed that the flow adjacent to the casing is moving towards the blade on the suction side due to relative casing motion. Part of this flow is blocked of by the oncoming tip leakage flow and turned into streamwise direction or towards the hub (hence the name scraping flow). The other part is dragged through the gap from the suction side to the pressure side.

Figure 2 Relative velocity vectors in the tip gap region at 34% Sm

Experiments have established that there are three zones of tip leakage flow behaviour in a radial inflow turbine. In the radial inlet portion of the turbine (i.e. the inducer), the so-called scraping effect dominates the flow field and the tip leakage flow is very different to that found in axial turbines. As a result of the scraping effect, almost the entire gap is filled with scraping flow. This can be observed in the upper part of Figure 3, showing blade normal velocity vectors in the tip gap region at 9% S_m. The importance of the scraping effect in the inducer is due to the high blade tip radius, the low blade camber angle and the low over-tip pressure difference. Consequently, less tip leakage mass flow passes through the gap. Hence, the result that efficiency is less sensitive to axial clearances in the inducer than to radial clearances in the exducer.

In the midsection, the scraping effect is less significant and the blade loading near the tip has increased. Hence, the tip leakage flow in this section is nearly perpendicular to the blade camber line. In the exducer, the tip leakage flow is similar to that in axial turbines. This is illustrated in the lower part of Figure 3, showing blade normal velocity vectors in the tip gap region at 58% S_m. The scraping effect in the exducer is relatively unimportant and no scraping flow is seen to be dragged through the gap.

Since most studies of tip clearance flow in axial turbines have been undertaken without considering the relative casing motion, it was found that existing tip leakage models cannot be applied to radial turbines. For design purposes a tip leakage mass-flow and loss model for radial turbines has been developed. The model is based on the new understanding of the scraping mechanism and has been successfully validated against measurements in the tip gap.

Figure 3 Relative velocity vectors in blade normal view for different meridional lengths

Papers by Roger Dambach and Howard Hodson are available in the download section. Press here to down load papers and PhD Theses

Howard Hodson and Roger Dambach

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