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.