Reid A. Berdanier

The Effects of Tip Leakage Flow on the Performance of Multistage Compressors Used in Small Core Engine Applications

Large rotor tip clearances and the associated tip leakage flows are known to have a significant effect on overall compressor performance. However, detailed experimental data reflecting these effects for a multistage compressor are limited in the open literature. As design trends lead to increased overall compressor pressure ratio for thermal efficiency benefits and increased bypass ratios for propulsive benefits, the rear stages of the high-pressure compressor will become physically small. Because rotor tip clearances cannot scale exactly with blade size due to the margin needed for thermal growth considerations, relatively large tip clearances will be a reality for these rear stages. Experimental data have been collected from a three-stage axial compressor to assess performance with three-tip clearance heights representative of current and future small core machines. Trends of overall pressure rise, stall margin, and efficiency are evaluated using clearance derivatives, and the summarized data presented here begin to narrow the margin of tip clearance sensitivities outlined by previous studies in an effort to inform future compressor designs. Furthermore, interstage measurements show stage matching changes and highlight specific differences in the performance of rotor 1 and stator 2 compared to other blade rows in the machine.

Experimental Investigation of Factors Influencing Operating Rotor Tip Clearance in Multistage Compressors

An analysis of compressor rotor tip clearance measurements using capacitance probe instrumentation is discussed for a three-stage axial compressor. Thermal variations and centrifugal effects related to rotational speed changes affect clearance heights relative to the assembled configuration. These two primary contributions to measured changes are discussed both independently and in combination. Emphasis is given to tip clearance changes due to changing loading condition and at several compressor operating speeds. Measurements show a tip clearance change approaching 0.1 mm (0.2% rotor span) when comparing a near-choke operating condition to a near-stall operating condition for the third stage. Additional consideration is given to environmental contributions such as ambient temperature, for which changes in tip clearance height on the order of 0.05 mm (0.1% rotor span) were noted for temperature variations of 15°C. Experimental compressor operating clearances are presented for several temperatures, operating speeds, and loading conditions, and comparisons are drawn between these measured variations and predicted changes under the same conditions.

Experimental Characterization of Tip Leakage Flow Trajectories in a Multistage Compressor

Fast-response pressure measurements collected in a three-stage axial compressor highlight the development of the rotor tip leakage flow. Data collected from an array of high-frequency-response pressure transducers measure time-resolved static pressure over the rotors at several loading conditions for three tip clearance heights. The influence of surrounding vane rows on the rotor tip leakage flow is investigated by adjusting the position of the vanes with respect to fixed sensor positions. One key result is that the wake from the upstream vane creates a modulation of the leakage flow. However, the upstream propagating potential field from the downstream vane row has no measureable impact on the leakage flow trajectory. In some cases, variations of the leakage flow trajectory angle due to these blade row interactions are greater than the differences due to a doubling of the rotor tip clearance height, an important finding not previously reported in the literature. Differences of the leakage flow trajectory...

Reconciling Compressor Performance Differences for Varying Ambient Inlet Conditions

Careful experimental measurements can capture small changes in compressor total pressure ratio (TPR), which arise with subtle changes in an experiments configuration. Research facilities that use unconditioned atmospheric air must account for changes in ambient compressor inlet conditions to establish repeatable performance maps. A unique dataset from a three-stage axial compressor has been acquired over the duration of 12 months in the Midwest U.S., where ambient conditions change significantly. The trends show a difference in compressor TPR measured on a cold day versus a warm day despite correcting inlet conditions to sea level standard day. To reconcile these differences, this paper explores correcting the compressor exit thermodynamic state, Reynolds number effects, and variations in rotor tip clearance (TC) as a result of differences in thermal growth.

Effects of Tip Clearance on Stall Inception in a Multistage Compressor

Rotor tip clearance height and the associated tip leakage flow have a significant effect on the performance and stability of compressors. Existing studies considering tip clearance effects on stability have been primarily limited to low-speed compressors, and many of these evaluated single-stage machines, which may not adequately represent stall trends for engine-scale compressors. Furthermore, test campaigns for engine-scale compressors cannot provide instrumentation accessibility required for detailed stage performance and stall investigations. Using results collected from a three-stage intermediate-speed axial compressor with appreciable density rise, this study addresses these needs. In this paper, three rotor tip clearances are tested, ranging from 1.5 to 4% span (1 to 3% chord). Previous studies have primarily shown a transition from short-length-scale spikes to long-length-scale modes as the clearance is increased, whereas the present study shows the opposite: a transition from modes to spikes with...

Quantifying Blockage in a Multistage Compressor for Different Tip Clearances using Steady and Unsteady Pressure Measurements

With future gas turbine designs seeking increased overall pressure ratios, blade heights in the rear stages of high-pressure compressors are expected to decrease. As this will lead to an increase of relative tip clearance height, a detailed knowledge of the associated rotor tip leakage flows becomes increasingly important. In particular, the development of blockage due to large tip leakage flows can be an key component leading to an understanding of stage matching for multistage machines. To assess blockage related to increased tip clearance, a series of measurements are presented here from a three-stage compressor at two loading conditions on the 100% corrected speedline. These data correspond to three tip clearance configurations, up to 4% span. Initially, three-component velocities measured using hot-wire anemometry are analyzed. Following these initial calculations, a blockage definition technique related to gradient identification is introduced, by which the tip leakage flow size downstream of the rotors is directly related to the blockage in the tip region of the compressor. Further, steady pressures are used to calculate an approximate blockage downstream of the stators. Ultimately, these simplified pressure-based blockage calculation techniques reveal results which compare well with the velocity-based results, within three percent or better.

Demonstrating Multistage Compressor Blockage Calculations Using Pressure Measurements for Large Tip Clearances

With future gas turbine designs seeking increased overall pressure ratios, blade heights in the rear stages of high-pressure compressors are expected to decrease. As this will lead to an increase o...

Humidity Effects on Experimental Compressor Performance: Corrected Conditions for Real Gases

The effects of humid air on the performance of a multistage research compressor and new methods of humidity accounting to ensure appropriate representation of performance parameters are investigated in this paper. Turbomachinery textbooks present methods of correcting speed and mass flow rate using perfect gas assumptions, but these methods can reduce the ability to achieve repeatable compressor performance when using unconditioned air in a climate where absolute humidity may vary. Instead, a new method is introduced which models humid air as a real gas and circumvents the need for assumptions in the correction process. In the area of compressor research, the ability to measure small changes in performance parameters and ensure repeatable results is essential. Errors of more than 0.5% can result from using perfect gas assumptions to calculate corrected speed, which can lead to misrepresented performance parameters beyond the uncertainty of the measurements. Multiplicative correction factors based on analytical data are also introduced as an alternate method of applying the new real-gas method, and these correction factors are compared to those derived by previous authors applying ideal gas methods for humidity accounting. This is the first time in open literature that experimental results for a component of a gas turbine engine are presented comparing a humid air correction method with traditional correction methods.Copyright