Tip Gap Flow Research Articles

Effect of Tip Gap and Squealer Geometry on Detailed Heat Transfer Measurements Over a High Pressure Turbine Rotor Blade Tip

The present study explores the effects of gap height and tip geometry on heat transfer distribution over the tip surface of a HPT first-stage rotor blade. The pressure ratio (inlet total pressure to exit static pressure for the cascade) used was 1.2, and the experiments were run in a blow-down test rig with a four-blade linear cascade. A transient liquid crystal technique was used to obtain the tip heat transfer distributions. Pressure measurements were made on the blade surface and on the shroud for different tip geometries and tip gaps to characterize the leakage flow and understand the heat transfer distributions. Two different tip gap-to-blade span ratios of 1% and 2.6% are investigated for a plane tip, and a deep squealer with depth-to-blade span ratio of 0.0416. For a shallow squealer with depth-to-blade span ratio of 0.0104, only 1% gap-to-span ratio is considered. The presence of the squealer alters the tip gap flow field significantly and produces lower overall heat transfer coefficients. The effects of different partial squealer arrangements are also investigated for the shallow squealer depth. These simulate partial burning off of the squealer in real turbine blades. Results show that some partial burning of squealers may be beneficial in terms of overall reduction in heat transfer coefficients over the tip surface.

AN INTEGRATED NONLINEAR APPROACH FOR TURBOMACHINERY FORCED RESPONSE PREDICTION. PART II: CASE STUDIES

This paper discusses the application of an advanced turbomachinery forced prediction model to two representative turbomachinery cases: an HP turbine and a rig fan. The approach is based on an integrated nonlinear multi-passage analysis that includes both the stator and the rotor blade-rows. The numerical model has advanced features such as nonlinear friction damping for turbine blades, tip gap flows and blade vibratory motion. A series of inviscid and viscous computations were performed for an HP turbine with 36 stator and 92 rotor blades. The peak-to-peak maximum displacements were predicted with and without root friction dampers and the findings were compared with available experimental data. Good agreement was observed in most cases. It was found that most of the unsteady forcing was due to the potential effects. In a second phase of the turbine work, the response to low engine-order excitation was predicted using a multi-row whole-annulus model. The stator assembly was assumed to have blades with varying throat widths and the magnitude of the unsteady aerodynamic forcing was found to increase with increasing scatter in throat width variation. A second forced response study was conducted for a rig fan with 15 variable inlet guide vanes (VIGVs) and 20 rotor blades. For a 30° VIGV opening, a good match was observed between the predicted and measured wake profiles. Similarly, the measured and predicted rotor blade vibration levels were also found to be in good agreement. It is concluded that the proposed methodology can be applied, with reasonable confidence, to the study of industrial cases.

HIGH FREQUENCY NOISE GENERATION IN SMALL AXIAL FLOW FANS

This paper presents results from an investigation of the broadband sources of acoustic noise in small axial flow fans. Observations drawn from flow visualization experiments and fluid dynamic measurements indicate that secondary flows are primary contributors to the broadband noise generated by small axial flow fans. More specifically, flow unsteadiness associated with tip gap flows is identified as a primary source of high frequency noise. As air is forced through the tip gap (i.e. the space between the rotating blade tip and the stationary housing), the flow rolls up forming vortices at the blade tip. These vortices convect into the blade passage and become the dominant source of unsteadiness in the blade passage and at the fan exit plane. The data presented indicates that this turbulence is the dominant source of noise above 1.5 kHz for the fan tested. The likely radiation mechanisms are trailing edge scattering, and radiation from free turbulence and/or boundary layers. Three types of experiments were performed as part of the study. First, flow visualization tests were run in an attempt to obtain a subjective evaluation of the flowfield. Then, stationary and rotating hot-wires were used to provide mean velocity and turbulence intensity data for non-radial flow components. Using the results from the flow visualization and hot-wire tests, possible noise generation mechanisms were postulated. Fan modifications were then made to test the viability of the proposed noise contributors. The addition of flanges to the blade tip and of fabric near the blade trailing edges provided up to a 9 dB decrease in sound power above 1 kHz. These results will need to be coupled to an analogous study of low frequency noise generation (i.e. below 1 kHz) for a significant reduction in perceived noise level to be achieved.

The use of frequency-dependent velocity scaling as a diagnostic tool

This paper will discuss a method for investigating broadband aeroacoustic noise sources. The method is based upon the use of frequency-dependent velocity scaling. The dependence of sound power level upon a characteristic velocity is determined experimentally, and that dependence is used as an indicator of primary aeroacoustic processes. The procedure yields an identification of distinct frequency bands within which source trends are observable. Each band is presumed to be controlled by different processes, and the set of likely processes is fixed according to the average velocity exponent value obtained. In principal, the method is applicable to low-speed aeroacoustic source identification problems where a characteristic flow speed can be measured and systematically varied. Measurements of sound radiated by flow over a flat plate were used to evaluate the performance of the method. The procedure was then applied to a small axial fan typical of those used to cool electronic systems. For the fan, the frequency-dependent velocity scaling demonstrated that shifts in dominant aeroacoustic processes occur near 2 kHz and above 4 kHz. The results added insight to a related study which demonstrated that tip gap flows were the primary source of broadband noise above 2 kHz.

Measurements of the Flow in an Idealized Turbine Tip Gap

To gain further insights into the details of the tip-gap flow in axial turbines, a test section has been constructed with a single idealized, large-scale tip gap. The single “blade” forms a circular arc with 90 deg of turning and has a constant thickness of 78 mm. For a plain, flat tip four clearances have been examined, varying from 0.292 to 0.667 of the blade thickness (corresponding to physical gap heights of 22.8 to 52.1 mm). The large proportions made it possible to obtain very detailed measurements inside the gap. The paper discusses the structure of the gap flow in some detail. One new feature, involving multiple vortices on the tip, probably helps to explain the “burnout” that sometimes occurs on turbine tips near the pressure side. Quantitative results are presented for the static pressures, total pressures, and velocity vectors through the gap. In addition, contraction coefficients for the flow at the separation bubble, discharge coefficients for the gap, and the gap losses have been extracted for comparison with the assumptions made in recent gap–flow models.

Effects of Simulated Rotation on Tip Leakage in a Planar Cascade of Turbine Blades: Part I—Tip Gap Flow

The paper presents further results from a continuing study on tip leakage in axial turbines. Rotation has been simulated in a linear cascade test section by using a moving-belt tip wall. Measurements were made inside the tip gap with a three-hole pressure probe for a clearance size of 3.8 percent of the blade chord. Two wall speeds are considered and the results are compared with the case of no rotation. As in other experiments, significant reduction in the gap mass flow rate is observed due to the relative motion. The detailed nature of the measurements allows the dominant physical mechanism by which wall motion affects the tip gap flow to be identified. Based on the experimental observations, an earlier model for predicting the tip gap flow field is extended to the case of relative wall motion. Part II of the paper examines the effect of the relative motion on the downstream flow field and the blade loading.