Gas Turbines - Introduction — Page 212, Lesson 280

Let’s pick up with the turboshaft engine, because it’s the natural bridge from the turboprop we just looked at. Think of a turboshaft as a turboprop where the propeller has been replaced by a shaft. That shaft doesn’t drive a propeller — it drives something else, most commonly a helicopter rotor. But it can also be used where you need a compact supply of electrical power, with the output shaft attached to an alternator. That’s exactly the type of engine used as the Auxiliary Power Unit, the APU, on most modern transport aircraft. Now, the key feature of nearly all turboshaft engines is the free power turbine. Let me define that precisely: a free power turbine is one that is not connected to any of the compressors. Because it isn’t mechanically tied to the compressor spool, it’s freed from the constraint of having to rotate at a speed that suits the compressor. That gives it a much wider operating speed range — the turbine can spin at whatever speed the driven load needs, independent of the compressor’s optimum speed. Look at the single spool turboshaft in Figure 13.9. It uses a reverse flow combustion chamber system. Reverse flow means the air is turned back on itself to enter the combustor, which allows the engine to be much shorter, stiffer, and lighter than it otherwise would be. But there’s a trade-off: this arrangement requires a centrifugal compressor in the high pressure stage. Why? Because the centrifugal compressor throws the air out radially, and that radial discharge lets the air enter the combustion chamber in the correct direction for the reverse flow layout. Apart from that deviation, the airflow follows exactly what we described for the turbojet, right up to the point where it leaves the high and low pressure turbines. By then, enough energy has been converted to drive the two compressors. The gas then passes through the free power turbine, where all of the remaining energy is used to drive whatever is attached to the output shaft. Now let’s move to the low bypass ratio engine, and I want to define bypass ratio carefully because it’s a term you’ll use constantly. The bypass ratio is the ratio of the mass airflow which flows through the fan-duct — that’s the bypass duct — to the mass of air which is directed through the hot core. A low ratio is considered to be in the region of about 1 or 2 to 1, whereas a high ratio would be around 5 to 1. Here’s the worked example from the text. Suppose the fan mass-flow is 1500 pounds, and the core mass-flow is 300 pounds. The bypass ratio is 1200 divided by 300, which gives 4 to 1. Notice where the 1200 comes from — it’s the fan flow minus the core flow, the air that goes around the core through the bypass duct. The engine in Figure 13.10 is a twin spool, low bypass ratio engine. The airflow as far as the end of the low pressure compressor is identical to a pure turbojet. Then the airflow splits into two. An amount depending on the bypass ratio flows down the bypass duct, and the remainder continues into the high pressure compressor — the HP compressor. Now, this figure introduces a station numbering system you need to know cold, because it’s how we talk about pressures and temperatures at each point through the engine. The symbols are P for gas pressure, T for gas temperature, and N for rotating assembly. Each symbol is accompanied by a number identifying its position from the front to the rear of the engine. Rolls Royce have historically used these designations. P0 and T0 are ambient conditions. P1 and T1 are at the inlet. P2 and T2 are at LP compressor delivery. P3 and T3 are at HP compressor delivery. P4 and T4 are at turbine entry. P5 and T5 are at HP turbine exit. P6 and T6 are at LP turbine exit. P7 and T7 are at the exhaust. And P8 and T8 are at the propelling nozzle. For the rotating assemblies, N1 is the LP compressor and turbine, and N2 is the HP compressor and turbine. So when you see P3, you know instantly you’re talking about the pressure at HP compressor delivery, and N1 tells you the speed of the low pressure spool. That numbering is the universal shorthand for station positions through the engine, and it’s worth committing to memory now because every performance calculation and every instrument reading will reference these stations.