Description
A typical turbine map is shown on the right. In this particular case, lines of percent corrected speed (based on a reference value) are plotted against the x-axis which is pressure ratio, but deltaH/T (roughly proportional to temperature drop across the unit/component entry temperature) is also often used. The y-axis is some measure of flow, usually non-dimensional flow or, as in this case, corrected flow, but not actual flow. Sometimes the axes of a turbine map are transposed, to be consistent with those of a compressor map. As in this case, a companion plot, showing the variation of isentropic (i.e. adiabatic) or polytropic efficiency, is often also included.
In this example the turbine is a transonic unit, where the throat Mach number reaches sonic conditions and the turbine becomes truly choked. Consequently, there is virtually no variation in flow between the corrected speed 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 percent corrected speed lines in the 'choked' 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 corrected speed. This separates the speed lines, enabling working lines (and efficiency contours) to be cross-plotted and clearly seen.
Read more about this topic: Turbine Map
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