Radial-Flow Gas Turbines
Introduction
The radial-flow turbine has had a long history of development being first conceived for the purpose of producing hydraulic power over 180 years ago. A French engineer, Fourneyron, developed the first commercially successful hydraulic turbine (circa 1830) and this was of the radial-outflow type. A radial-inflow type of hydraulic turbine was built by Francis and Boyden in the United States (circa 1847), which gave excellent results and was highly regarded. This type of machine is now known as the Francis turbine, a simplified arrangement of it being shown in Figure 1.1. It will be observed that the flow path followed is from the radial direction to what is substantially an axial direction. A flow path in the reverse direction (radial outflow), for a single-stage turbine anyway, creates several problems, one of which (discussed later) is low specific work. However, as pointed out by Shepherd (1956) radial-outflow steam turbines comprising many stages have received considerable acceptance in Europe. Figure 8.1, from Kearton (1951), shows diagrammatically the Ljungstro¨m steam turbine, which, because of the tremendous increase in the specific volume of steam, makes the radial-outflow flow path virtually imperative. A unique feature of the Ljungstro¨m turbine is that it does not have any stationary blade rows. The two rows of blades constituting each of the stages rotate in opposite directions so that they can both be regarded as rotors.
The inward-flow radial (IFR) turbine covers tremendous ranges of power, rates of mass flow, and rotational speeds, from very large Francis turbines used in hydroelectric power generation and developing hundreds of megawatts (see Figures 9.12 and 9.13) down to tiny closed cycle gas turbines for space power generation of a few kilowatts.
The IFR gas turbine has been, and continues to be, used extensively for powering automotive turbocharges, aircraft auxiliary power units, expansion units in gas liquefaction, and other cryogenic systems and as a component of the small (10 kW) gas turbines used for space power generation (Anon, 1971). It has been considered for primary power use in automobiles and in helicopters. According to Huntsman, Hodson, and Hill (1992), studies at Rolls-Royce have shown that a cooled, high efficiency IFR turbine could offer significant improvement in performance as the gas generator turbine of a high technology turboshaft engine. What is needed to enable this type of application are some small improvements in current technology levels. However, designers of this
new generation of IFR turbines face considerable problems, particularly in the development of advanced techniques of rotor cooling or of ceramic, shock-resistant rotors.
As indicated later in this chapter, over a limited range of specific speed, IFR turbines provide an efficiency about equal to that of the best axial-flow turbines. The significant advantages offered by the IFR turbine compared with the axial-flow turbine are the greater amount of work that can be obtained per stage, the ease of manufacture, and its superior ruggedness.