The effect of tip clearance and tip gap geometry on the performance of a one and a half stage axial gas turbine.
In a previous work of a similar nature, the performance of a low speed axial turbine with a second stage nozzle was examined with respect to the effect of the variation of tip clearance for various tip shapes. Present findings suggest some interesting phenomena, including the effect of tip clearance on the flow within the rotor and show that poor resolution from a transducer and insufficient data points in the critical tip region, where a high velocity peak was found, were responsible for a number of incorrect conclusions in the original study. In terms of blade tip geometry, a standard flat tip shape was found to deliver only a marginally better performance when compared to a double squealer tip and the two streamlined shapes previously investigated. Although contemporary opinion suggests that a streamlined tip should increase the leakage flow and hence cause greater mixing losses, the machine efficiency was not significantly reduced. This is an exciting result since it suggests that a streamlined tip shape can be used to alleviate the problem of blade tip burnout without significantly reducing machine efficiency. When the single stage performance in the absence of a second nozzle was examined, slightly different trends were obtained. The low entropy tips produced slightly lower mixing loss, suggesting that the internal gap loss is an important parameter in determining the rate at which the leakage jet mixes downstream of the rotor. The flow behind the rotor (ie time averaged) was found to be in remarkable agreement with linear cascade data when time averaged even though the latter did not include any effects of relative motion. An increase in clearance was seen to reduce the Euler work and also to cause a deficit of mass flow across the remainder of the blade right down to the hub. The leakage flow was also seen to induce a flow blockage which resulted in a higher driving pressure across the rotor for the same mass flow rate. As in the previous study, the second stage nozzle efficiency was seen to be independent of tip clearance or tip shape and was moderately better than that of the first nozzle. However, the improvement was not found to be as large, due to a previously undetected very thin ring of high energy leakage fluid. When this is taken into account, the efficiency of the second stage nozzle is comparable to the first. The second nozzle was seen to have a flow straightening effect on the poorly deflected, high energy leakage flow, causing a rapid mixing process within these downstream blade passages. The growth of secondary flow was reduced at both the hub and the tip and this is believed to result in a slight decrease in loss. The outlet flow was closer to design conditions than that of the first stage nozzle.