Each
turbineA turbine is a rotary engine that extracts energy from a fluid or air flow and converts it into useful work.The simplest turbines have one moving part, a rotor assembly, which is a shaft or drum, with blades attached. Moving fluid acts on the blades, or the blades react to the flow, so that they...
in a
gas turbineA gas turbine, also called a combustion turbine, is a rotary engine that extracts energy from a flow of combustion gas. It has an upstream compressor coupled to a downstream turbine, and a combustion chamber in-between. Energy is added to the gas stream in the combustor, where air is mixed with...
engine has an operating map. Complete maps are either based on turbine rig test results or are predicted by a special computer program. Alternatively, the map of a similar turbine can be suitably scaled.
A typical turbine map is shown on the right. In this particular case, the x-axis is pressure ratio, but deltaH/T (roughly proportional to temperature drop across the unit/component entry temperature) is also often used.
Each
turbineA turbine is a rotary engine that extracts energy from a fluid or air flow and converts it into useful work.The simplest turbines have one moving part, a rotor assembly, which is a shaft or drum, with blades attached. Moving fluid acts on the blades, or the blades react to the flow, so that they...
in a
gas turbineA gas turbine, also called a combustion turbine, is a rotary engine that extracts energy from a flow of combustion gas. It has an upstream compressor coupled to a downstream turbine, and a combustion chamber in-between. Energy is added to the gas stream in the combustor, where air is mixed with...
engine has an operating map. Complete maps are either based on turbine rig test results or are predicted by a special computer program. Alternatively, the map of a similar turbine can be suitably scaled.
Description
A typical turbine map is shown on the right. In this particular case, the x-axis is pressure ratio, but deltaH/T (roughly proportional to temperature drop across the unit/component entry temperature) is also often used. The other axis is some measure of flow, usually non-dimensional flow or, as in this case,
corrected flowCorrected Flow is the mass flow that would pass through a device if the inlet pressure and temperature corresponded to ambient conditions at Sea Level, on a Standard Day Corrected Flow is the mass flow that would pass through a device (e.g. compressor, bypass duct, etc) if the inlet pressure and...
, but not real flow. Sometimes the axes of a turbine map are transposed, to be consistent with those of a
compressor mapEach compressor 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...
. As in this case, a companion plot, showing the variation of isentropic (i.e.
adiabaticIn thermodynamics, an adiabatic process or an isocaloric process is a thermodynamic process in which no heat is transferred to or from the working fluid. The term "adiabatic" literally means impassable, coming from the Greek roots ἀ- , διὰ- , and βαῖνειν ; this etymology corresponds here to an...
) or polytropic efficiency, is often also included.
In this example the turbine is a transonic unit, where the throat Mach number reaches
sonicSound is a vibration that travels through an elastic medium as a wave. The speed of sound describes how far this wave travels in a given amount of time. In dry air at , the speed of sound is . This equates to , or about one mile in five seconds...
conditions and the turbine becomes truly
chokedChoked flow of a fluid is a fluid dynamic condition caused by the Venturi effect. When a flowing fluid at a certain pressure and temperature flows through a restriction into a lower pressure environment, under the conservation of mass the fluid velocity must increase for initially subsonic...
. Consequently, there is virtually no variation in flow between the
corrected speedCorrected Speed is the speed a component would rotate at if the inlet temperature corresponded to ambient conditions at Sea Level, on a Standard Day Corrected Speed is the speed a component would rotate at if the inlet temperature corresponded to ambient conditions at Sea Level, on a Standard Day...
lines at high pressure ratios.
Most turbines however, are subsonic devices, the highest Mach number at the NGV throat being about 0.85. Under these conditions, there is a slight scatter in flow between the speed lines in the
'choked'Choked flow of a fluid is a fluid dynamic condition caused by the Venturi effect. When a flowing fluid at a certain pressure and temperature flows through a restriction into a lower pressure environment, under the conservation of mass the fluid velocity must increase for initially subsonic...
region of the map, where the flow for a given speed reaches a plateau.
Unlike a compressor (or fan), surge (or stall) does not occur in a turbine. This is because the flow through the unit is all 'downhill', from high to low pressure. Consequently there is no surge line marked on a turbine map.
Working lines are difficult to see on a conventional turbine map, because the speed lines bunch-up. One trick is to replot the map, with the y-axis being the multiple of flow and speed. This separates the speed lines, enabling working lines (and efficiency contours) to be cross-plotted and clearly seen.
Progressive unchoking of the expansion system
The following discussion relates to the expansion system of a 2 spool, high bypass ratio, unmixed, turbofan.
On the RHS is a typical primary (i.e. hot) nozzle map (or characteristic). Its appearance is similar to that of a turbine map, but it lacks any (rotational) speed lines. Note that at high flight speeds (ignoring the change in altitude), the hot nozzle is usually in, or close to, a choking condition. This is because the ram rise in the air intake factors-up the nozzle pressure ratio. At static (e.g. SLS) conditions there is no ram rise, so the nozzle tends to operate unchoked (LHS of plot).
The low pressure turbine 'sees' the variation in flow capacity of the primary nozzle. A falling nozzle flow capacity tends to reduce the LP turbine pressure ratio (and deltaH/T). As the left hand map shows, initially the reduction in LP turbine deltaH/T has little effect upon the entry flow of the unit. Eventually, however, the LP turbine unchokes, causing the flow capacity of the LP turbine to start to decrease.
As long as the LP turbine remains choked, there is no significant change in HP turbine pressure ratio (or deltaH/T) and flow. Once, however, the LP turbine unchokes, the HP turbine deltaH/T starts to decrease. Eventually the HP turbine unchokes, causing its flow capacity to start to fall. Ground Idle is often reached shortly after HPT unchoke.