NAVAL ENGINEERING PLANTS (1955 - 1990)

The gas turbine engine bears some resemblance to an internal combustion engine of the reciprocating type and some resemblance to a steam turbine. However, a brief consideration of the basic principles of a gas turbine engine reveals several ways in which the gas turbine engine is quite unlike either the reciprocating internal combustion engine or the steam turbine. Let us look first at the thermodynamic cycles of the three engine types. The reciprocating internal combustion engine has an open, heated-engine cycle and the steam turbine has a closed, unheated-engine cycle. In contrast, the gas turbine has an open, unheated-engine cycle—a combination we have not previously encountered in our study of naval machinery. The gas turbine cycle is open because it includes the atmosphere; it is an unheated-engine cycle because the working substance is heated in a device, which is separate from the engine. Another way in which the three types of engines differ is in the working substance. The working fluid in a steam turbine installation is steam. In both the reciprocating internal combustion engine and the gas turbine engine, the working fluid may be considered as being the hot gases of combustion that result from the burning of fuel in air. However, there are very important differences in the way the working fluid is used in the reciprocating internal combustion engine and in the gas turbine engine. Still other differences in the three types of engines become apparent when we consider the arrangement and relationship of component parts and the processes that occur during the cycle. From our study of previous chapters of this text, we are already familiar with the functional arrangement of parts in steam turbine installations and in reciprocating internal combustion engines. Now let us discuss the relationship of the major components in a basic gas turbine engine. In the steam turbine installation, the processes of combustion and steam generation take place in the boiler, while the process by which the thermal energy of the steam is converted into mechanical work takes place in the turbine. In the reciprocating internal combustion engine, three processes—the compression of atmospheric air, the combustion of a fuel-air mixture, and the conversion of heat to work—all take place in one unit, the cylinder. The gas turbine engine is similar to the reciprocating internal combustion engine in that the same three processes—compression, combustion, and conversion of heat to work—occur; but it is unlike the reciprocating internal combustion engine in that these three processes take place in three separate units rather than in one unit. In the gas turbine engine, the compression of atmospheric air is accomplished in the compressor; the combustion of fuel is accomplished in the combustion chamber; and the conversion of heat to work is accomplished in the turbine. Many different types and models of earlier gas turbine engines were in use: a single-shaft type because one shaft from the turbine rotor drives the compressor and an extension of this same shaft drives the load; and a split-shaft type. The split-shaft type was considered to be split into two sections: the gas-producing section, or gas generator, and the power turbine section. The gas-generator section, in which a stream of expanding gases is created as a result of continuous combustion, includes the compressor, the combustion chamber (or chambers), and the gas-generator turbine. The power turbine section consists of a power turbine and the power output shaft. In this type of gas turbine engine, there is no mechanical connection between the gas-generator turbine and the power turbine. When the engine is operating, the two turbines produce the same effect as that produced by a hydraulic torque converter. The split-shaft gas turbine engine is well suited for use as a propulsion unit where loads vary, since the gas-generator section can be operated at a steady and continuous speed while the power turbine section is free to vary with the load. Starting effort required for a split-shaft gas turbine engine is far less than that required for a single-shaft gas turbine engine connected to the reduction gear, propulsion shaft, and propeller. In the twin-spool gas turbine engine, the air compressor is split into two sections or stages and each stage is driven by a separate turbine element. The low-pressure turbine element drives the low-pressure compressor element and the high-pressure turbine element drives the high-pressure compressor element. Like the split-shaft type, the twin-spool gas turbine engine is usually divided into a gas-generator section and a power turbine section. However, some twin-spool gas turbine engines are so arranged that the low-pressure turbine element drives the low-pressure compressor element and the power output shaft. The basic cycle of the gas turbine engine is one of isentropic compression, constant-pressure heat addition, isentropic expansion, and constant-pressure heat rejection. As the hot combustion gases are expanded through the turbine, converting thermal energy into mechanical work, some of the turbine work is used to drive the compressor and the remainder is used to drive the load. The power output from the turbine is steady and continuous and, after the initial start, self-sustaining. Although this chapter deals only with earlier gas turbine engines, which operated on the simple open cycle, it should be mentioned that other cycles are also of interest to designers of gas turbine engines. Among the cycles that have been considered (and to some extent used) are the closed cycle, the semi-open cycle, and various modifications of the simple open cycle. In one such modification, known as the regenerated open cycle, the hot exhaust gases from the turbine are passed through a heat exchanger in which they give up some heat to the air between the compressor discharge and the inlet to the combustion chamber. The utilization of this heat decreases the amount of fuel required and there-by increases the efficiency of the cycle.