## 1 Introduction  
Three-dimensional corner separation is an inherent flow feature in axial compressors, the induced flow blockage and loss of which can deteriorate the compressor performance. In particular, when the corner separation turns into a corner stall, large backflow will occur at the corner surrounded by the blade suction surface and the end wall [1; 2], and the compressor performance will decrease rapidly [3; 4]. Hence, to suppress the corner separation, especially to delay the occurrence of the corner stall, many active and passive control techniques has been proposed, like the air aspiration [5; 6; 7; 8], air blowing [1; 9; 10; 11], vortex generators [6; 12], three-dimensional blade stacking [2; 13; 14] and so on, while the cantilevered stator is still an efficient technique to be frequently used in practice. Early in 1954, [PERSON] [15] pointed out that the leakage flow that generates from the clearance between the blade tip and the end wall in the cantilevered stator has the opposite flow direction to the secondary flow in the wall boundary layer, hence, it can suppress the migration of the low energy flow to accumulate in the corner surrounded by the blade suction surface and the end wall and the corner separation is weakened consequently. This conclusion was then verified by [PERSON] and [PERSON] [16; 17; 18], who conducted some experiments in the linear cascade and put forward that there exists an optimum clearance size that makes the flow loss to be the minimum. Since then, many studies about the flow inside the cantilevered stator have been conducted, and the investigated objects mainly focused on two topics. The first one is the flow mechanism of the interaction between the leakage flow and the corner separation. For example, [PERSON] [15], [PERSON] and [PERSON] [18], [PERSON] and [PERSON] [19], [PERSON], et al. [20], and [PERSON], et al. [21] thought it is the suppression of the migration of the low energy flow by the leakage flow that weakens the corner separation, while [PERSON], et al. [22] pointed out that it is the suppression of the horseshoe vortex in the leading edge that causes the removal of the corner separation, and [PERSON], et al. [23] considered that the suppression of corner separation is mainly caused by the mixing of the high energy leakage flow with the low energy flow in the corner. The second one is the effect of the clearance size on the compressor or the cascade performance. For example, [PERSON] and [PERSON] [18] and [PERSON] [24] both found that there exists an optimum clearance size that makes the performance of the compressor cascade to be the best and [PERSON] and [PERSON] [18] pointed out that this only happens when the strength of the leakage flow and the secondary flow are comparable and they can just cancel out; [PERSON], et al. [25] also found that there is an optimum clearance at the rotor tip in an industrial gas turbine, and proposed that this phenomenon is mainly caused by the decrease of clearance flow mixing loss and increase of viscous loss in the tip clearance with the decrease of tip clearance size; in addition, [PERSON], et al. [23] found that the leakage flow can eliminate corner separation at the stator hub, and improve the compressor performance, which has been verified by [PERSON] [26], and [PERSON] [27] at the rotor tip, while [PERSON], et al. [22] revealed that the clearance flow tends to strengthen the corner separation when the clearance size is very small. From the discussion above it can be seen that there are still some controversies on the mechanism of the suppression of the corner separation by the leakage flow, and the effect of stator hub clearance on the compressor performance is not consistent yet. Hence, in this paper, a detailed experimental investigation is conducted in a cantilevered stator of a highly loaded axial compressor, at varies stator hub clearance sizes, i.e., 0% (sealed), 0.25%, 0.5%, and 1% blade height. Both the oil-flow visualization and the five-hole probe are used to measure the complicated flow structures.