Gas Turbines - The Turbine Assembly — Page 272, Lesson 355

We’re starting a new section of the powerplant book: the turbine assembly, and I want to begin with the free turbine, because it sets up everything else. When a turbine is bolted to a compressor to form a single rotating assembly—what we call a spool—it is forced to spin at whatever speed the compressor demands. And the compressor’s speed is set at the point of best compression efficiency. So the turbine has no freedom; it’s a slave to the compressor’s needs. A free turbine breaks that link. It is not connected to the compressor at all. It connects only to the propeller or the rotor reduction gearbox. Because it’s independent, it can seek its own optimum design speed—the speed where it runs most efficiently, regardless of what the compressor is doing. There are practical advantages to this, and I want you to remember three of them. First, the propeller can be held at low rpm during taxiing. That reduces noise pollution and reduces wear on the brakes. Second, less starting torque is required—the starter doesn’t have to spin up the whole compressor train through the propeller. Third, a rotor parking brake can be fitted. That eliminates the danger of propellers rotating in windy conditions on the ground, which is a real safety hazard. Now, let’s move to multi-spool engines, because this is where the turbine design gets interesting. The power output of a turbine can be increased by increasing its diameter. But that brings two penalties: a larger engine means a larger drag factor, and the greater radius means greater centrifugal forces, which impose higher stresses on the blades. A simpler method is to add more stages—that’s what Figure 17.1 shows, a three-stage turbine assembly mounted on one shaft. More stages give more power output while actually allowing a reduction in turbine diameter. Here’s the core physics, and I want you to hold onto this. The efficiency of a turbine blade increases as its rotational speed increases, because the losses reduce in proportion to the square of the mean blade speed. But here’s the trap: the stresses on the blade increase in proportion to the square of the blade speed as well. So the designer is locked in a vicious circle. To increase efficiency, you raise speed. That demands stronger blades. Stronger blades are heavier. Heavier blades create greater stresses. And greater stresses demand even stronger blades. Round and round. The way out came with the high bypass ratio engine. Because it has much greater propulsive efficiency, for a given thrust it can use a smaller turbine. That smaller turbine circumvents the vicious circle to some extent. This type of engine features three spools. Let me walk you through them from front to back. The high pressure turbine—the HP turbine—drives the high pressure compressor at relatively high speeds. To the rear of that is the intermediate pressure turbine, the IP turbine, which drives the intermediate pressure compressor through a shaft that runs inside the shaft of the high pressure turbine. And the rearmost is the low pressure turbine, the LP turbine. In the illustration, it has two stages, and it drives the fan, which is also the low pressure compressor. The LP turbine rotates at the lowest speed of all. Now, the power developed by this LP turbine produces almost all the thrust of the engine. It does this through the reaction of the bypass air—a high mass flow moving at a speed that is relatively slow compared to a pure turbojet. And the shaft connecting the LP turbine to the LP compressor runs inside the shafts that connect the HP and IP compressors to their turbines. So you have three concentric shafts, one inside the other, each spinning at its own speed. That’s the free turbine and the three-spool architecture. When you’re ready, we’ll move on to the next part of the turbine assembly.