Compressor map

Introduction

[[image:hpcompressorchic.750pix.gif|thumb|right|250px|Each compressor (or fan) in a gas turbine engine has an operating map. Complete maps are either based on compressor rig test results or are predicted by a special computer program. A typical compressor map (or characteristic) for a high pressure (HP) compressor is detailed. A low pressure ratio fan map is also discussed.

Flow axis

The x-axis is usually some function of compressor entry mass flow, either in absolute or relative form. This axis is a measure of the axial Mach number of the flow through the device.

Pressure ratio axis

Normally the y-axis is pressure ratio, although deltaT/T (or similar) is also used.

Surge line

The slightly kinked diagonal line on the main (i.e. lower) part of the map is known as the surge (or stall) line. Above this line is a region of unstable flow, which is an area best avoided.

A compressor surge, typically, causes an abrupt reversal of the airflow through the unit, as the pumping action of the aerofoils stalls (this is akin to an aircraft wing stalling).

Speed lines

The slightly curved, near vertical, lines on the main part of the map are the (constant rotational) speed lines. They are a measure of rotor blade tip Mach number.

Note on the illustration that the speed lines are not distributed linearly with flow. This is because this particular compressor is fitted with variable stators, which open progressively as speed increases causing an exaggerated increase in flow in the medium to high speed region. At low speed, the variable stators are locked, causing a more linear relationship between speed and flow.

Also note that beyond 100% flow, the speed lines close up rapidly, due to choking. Beyond choke, any further increase in speed will generate no further increase in airflow.

Efficiency axis

A sub-plot shows the variation of isentropic (i.e. adiabatic) efficiency with flow, at constant speed. Some maps use polytropic efficiency. Alternatively, for illustrative purposes, efficiency contours are sometimes cross-plotted onto the main map.

Note that the locus of peak efficiency exhibits a slight kink in its upward trend when variable stators are closed-off. This due to the choking-up of the compressor as speed increases. The trend line resumes, once the variables start to move.

Working line

Also shown on the map, is a typical steady state working (or operating/running) line. This is a locus of the operating points of the engine, as it is throttled.

Being a high pressure ratio device, the working line is relatively shallow. This would cause handling problems if the unit had no variable geometry, because the surge line would be very steep and cross the working line.

During a slam-acceleration from, say, mid-throttle, the compressor working line will move rapidly towards surge and then slowly approach the steady state operating point, further up the map. The reverse effect occurs during a slam-deceleration. These effects are caused by the sluggish response of the spool (i.e. inertia effects) to rapid changes in engine fuel flow. Compressor surge is particular problem during slam-accelerations and can be overcome by suitable adjustments to the fuelling schedule and/or use of blow-off (bleeding air off the compressor, for handling purposes).

Because an HP compressor 'sees' the choked flow capacity of the HP turbine, the compressor working line is almost unique, hardly being affected by flight condition. The slope of the working line approximates to a constant corrected outlet flow.

Fan map

[[image:fanchic.750pix.gif|thumb|right|250px|As the second illustration shows, a low pressure ratio fan (such as that used on a high bypass ratio turbofan) has a range of working lines. At high flight speeds, the ram pressure ratio factors up the fan pressure ratio, causing the cold propelling nozzle to choke. Above the choking condition, the working lines tend to coalesce into a unique steep straight line. At lower flight speeds, where the nozzle unchokes, the working lines become more curved and gradually fan-out, upwards, towards surge. The lowest surge margin working line occurs at static conditions. The curvature of the working line reflects the shape of nozzle characteristic.

Owing to the nature the constraints involved, the fan working lines of an mixed, high bypass ratio, turbofan are somewhat steeper than those of the equivalent unmixed engine.