Figure 10 Axial inter-distances
Figure 11 shows the profiles of total pressure at exit from IGV and near
the leading edge of blade for different inter-distances, revealing
distinguishable differences between the small and large gaps. Indeed,
for small inter-distances, positions (P1, P2), there are additional
losses produced between blade span 30 % and 70 % compared to the large
inter-distance such as the position (P3), subsequently the fan stage
produces low aerodynamic efficiency. Another reason is that the IGV
wakes do not completely mix prior to entering the blades passages in
case of a small axial gap. The IGV wakes entering the blades passage
influence the boundary layer to cause transition to turbulent flow
sooner than if the wakes were allowed to mix sufficiently, which results
in greater entropy production. Moreover, the tip vortex does not reach
the blade span below 80 % for the farthest inter-distances, whereas for
the closest inter-distances the tip vortex may reach at around 40 %
span because it drifts to the lower spans. The greater strength of
leakage flow for the closest spacing over the farthest case may be
another reason that the vortex is able to influence the mid-span region.
At the farthest position (P6) the total pressure profile varies greatly
with blade span, with a clear defect in the total pressure below the
span fraction 40 % where there is a noticeable shift along the span due
to more losses attributed to IGV wakes mixing within the rotor passages.
As also noticed the streamlines showed the tendency of IGV wakes to
drift toward the rotor blades pressure side. The present results concur
well with the assessment of Hetherington and Moritz [29] and
Zachcial and Nürnberger [30] who have shown that the axial
inter-distance between the blade rows allows for mixing prior entering
the rotor passage. Due to wake mixing losses, Hetherington and Moritz
[29] have suggested that blades-rows should be spaced far enough
apart to allow for the majority of the mixing to be accomplished. In the
present study the effect of inter-distance on the performance of axial
fan stage in term of total-to-total isentropic efficiency was computed
and assessed at the nominal operating speed. As presented in Fig. 12,
there is a large variation in total-to-total isentropic efficiency with
inter-distance. Accordingly, the maximum efficiency of 78.81%
corresponds to the optimum inter-distance of 40 mm based on 61.5 % of
the IGV chord and 44.5 % of the rotor blade chord. Above this distance
the efficiency decreases noticeably. Indeed, the small axial
inter-distance seems to strengthen the recovery of wake decay and then
reduces the mixing losses of wakes, so did the total pressure loss. The
efficiency obtained in conditions of reduced axial inter-distance is
higher than that obtained corresponding to increased axial gap beyond
the optimum distance.