Evaluation of Centrifugal Compressor Performance With Water Injection Page: 7 of 16
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NACA RM E51E21
with and without water injection, the flow was limited by choking at the
inlet to the vaned diffuser section. With water injection the net effect
of an increase in density and a decrease in through-flow velocity as
effected by the cooling of the air stream is an increase in the maximum
air weight flow. Inasmuch as choking occurs very near the impeller out-
let only that evaporation which occurs within the impeller (and the
short vaneless diffuser section) contributes to an increase in air weight
flow. The water-air ratios at which the maximum equivalent air-weight-
flow and total-pressure-ratio curves tend to level off would not be
expected to coincide inasmuch as they do not represent the same compres-
sor operating points.
Peak adiabatic efficiency. - The maximum rate of decrease in peak
adiabatic efficiency occurred until a water-air ratio of 0.03 was
reached, after which the curve tended to level off. There are several
reasons for the large decrease in adiabatic efficiency which accompanies
the injection of water; the most probable primary reason being the ther-
modynamic losses due to the evaporative cooling which occurs within the
diffuser. The high rate of diffusion, which produces a rapid increase
in static temperature, results in an unstable mixture of water droplets,
superheated vapor, and air. The losses which result from this unstable
condition in addition to those caused by heat transfer from the air to
the droplets across a temperature difference may be quite large for high
water-air ratios. An estimate of the amount of evaporation indicated
that the evaporation within the impeller is very small and as a result
the quantity of-water evaporated in the diffuser becomes greater as the
water-air ratio is increased. It would be difficult to eliminate this
inefficiency due to unstable evaporation in the diffuser because water
droplets were present at the compressor outlet even at the lowest water-
air ratio (w/a = 0.01). The cooling in the impeller and the diffuser
may also lower the aerodynamic efficiency of the compressor. Reference 2
shows that decelerations in flow are quite severe in this type compressQr.
Cooling of the air results in greater decelerations (more severe velocity
gradients), which are highly conducive to boundary-layer build-up and
separation. Experimental data show that the reduced velocities at the
impeller outlet cause larger negative angles of attack at the inlet to
the vaned diffuser section, which also contributes to lower over-all
compressor efficiency.
Effect of Varying Inlet-Air Temperature
Increasing the inlet-air temperature resulted in an increase in
evaporation. The increased evaporation had the same effects upon the
over-all performance characteristics as those resulting from an increase
in the quantity of water injected.6
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Beede, William L.; Hamrick, Joseph T. & Withee, Joseph R., Jr. Evaluation of Centrifugal Compressor Performance With Water Injection, report, July 18, 1951; (https://digital.library.unt.edu/ark:/67531/metadc58894/m1/7/: accessed June 8, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.