An experimental investigation on casing treatments in a one-stage transonic compressor is presented. The reference case consists of a radially staggered blisk and six circumferential grooves. Speedlines show that this axisymmetric treatment already provided a substantial increase in operating range with relatively small losses in efficiency. Since the onset of rotating stall in tip-critical high-speed compressors is always linked to the tip-leakage flow and the build-up of blockage within the blade passage. High-resolution measurement techniques have been employed to investigate the corresponding effects. Results with Particle Image Velocimetry show that the interaction between the tip leakage vortex and the shock front cause a blockage area. When throttled further, the blockage increases. The shock structure changes similar to the phenomena of vortex breakdown described by different researchers in the past, but a stagnation point is not present. Before reaching the stability limit, the interface line between the incoming flow and the blocked area moves towards the inlet plane of the rotor indicating spike-type stall inception. Wall pressure measurements confirmed this theory for the smooth wall, but with circumferential grooves applied, a part span stall cell develops prior to the stability limit. In order to assess the performance of circumferential grooves, two additional configurations are presented. The corresponding measurements addressed the questions whether circumferential grooves also provide an operating range extension when applied to an optimized rotor design with higher initial stall margin. Therefore, an identical casing treatment is applied to a forward swept rotor. The second question is, how circumferential grooves perform in direct comparison to a non-axisymmetric endwall structure. Axial slots have been applied to the radially staggered rotor. While the stall margin exceeds all other configurations, detrimential effects in efficiency are observed. A detailed anaylsis of probe data shows the changes of the radial profile at the rotor outlet which allows recommendations for more efficient CT designs. Parameters allowing to evaluate the CT influence are presented.

In the present study, unsteady flow phenomena due to tip clearance flow instability in a modern transonic axial compressor rotor are studied in detail. First, unsteady flow characteristics due the oscillating tip clearance vortex measured with the particle image velocimetry (PIV) and casing-mounted unsteady pressure transducers are analyzed and compared to numerical results with a large eddy simulation (LES). Then, measured characteristic frequencies of the unsteady flow near stall operation are investigated. The overall purpose of the study is to advance the current understanding of the unsteady flow field near the blade tip in an axial transonic compressor rotor near the stall operating condition. Flow interaction between the tip leakage vortex and the passage shock is inherently unsteady in a transonic compressor. The currently applied PIV measurements indicate that the flow near the tip region is unsteady even at the design condition. This self-induced unsteadiness increases significantly as the compressor operates toward the stall condition. PIV data show that the tip clearance vortex oscillates substantially near stall. The calculated unsteady characteristics from LES agree well with the PIV measurements. Calculated unsteady flow fields show that the formation of the tip clearance vortex is intermittent and the concept of vortex breakdown from steady flow analysis does not seem to apply in the current flow field. Fluid with low momentum near the pressure side of the blade close to the leading edge periodically spills over into the adjacent blade passage. The spectral analysis of measured end wall and blade surface pressure shows that there are two dominant frequencies near stall. One frequency is about 40–60% of the rotor rotation and the other dominant frequency is about 40–60% of the blade passing frequency (BPF). The first frequency represents the movement of a large blockage over several consecutive blade passages against the rotor rotation. The second frequency represents traditional tip flow instability, which has been widely observed in subsonic compressors. The LES simulations show that the second frequency is due to movement of the instability vortex.

In a 2-pass cooling system the pressure driven air flow distribution is investigated experimentally using the non-intrusive PIV Technique. The generic model as part of a complex and sophisticated cooling system consists of two square-sectioned ducts with a length of 20 diameters and an inherent 180 degree bend. The system has been investigated basically with smooth walls (case 0) and, later on, with two different kinds of ribbed walls in both legs. Ribs are applied to enhance the cooling performance; they are placed on two opposite walls of both legs in a symmetric (case A) and an asymmetric manner (case B), respectively. The ribs are inclined with an angle of 45 degrees versus the duct axis (i.e. main flow direction). The applied rib lay-out is well-proved and optimized with respect to heat transfer improvement and the inherent pressure drop increase. The system rotates about an axis orthogonal to the centreline of the straight passes. The configuration was analyzed with the planar the two-component Particle Image Velocimetry (2C PIV), which is capable of obtaining complete maps of the instantaneous as well as the averaged flow field even at high turbulence levels, which are typically present within duct turns, near ribs and, above all, during rotation. The presented investigations were conducted in stationary and rotating mode. Especially in the bend region separation phenomena and vortices with high local turbulence are apparent. The presence of ribs changes the fluid motion by generating additional vortices impinging the side walls. Flow visualization with injected oil smoke using the laser light sheet visualization technique was helpful to detect vortex structures and separations. Especially in the bend area separation regions and vortices with high local turbulence are apparent. The results shown in this paper demonstrate the effect of the 180 degree bend in combination with the two rib turbulator geometries for isothermal flow conditions excluding any buoyancy with and without rotation. Turbulent channel flow was investigated at a Reynolds number of 50,000, derived with the hydraulic diameter of the pass, non-rotating and at a rotation number of 0.02 which was chosen still moderate. Engine relevant rotation numbers are in order of .1 or higher. A reconstruction of model mountings will allow higher values for the next tests. Future work will expand to higher rotational speed and, also, will include buoyancy effects. This investigation shall help to clarify the complex flow phenomena due to the interaction of several vortices, present in two-pass cooling systems. The flow maps obtained with PIV are of good quality and high spatial resolution and therefore provide a test case for the development and validation of numerical simulation tools like the DLR flow solver TRACE which is not a topic of this paper.

This paper provides an overview of flow field measurements on a pressurized generic combustor that shares typical features of realistic gas turbine combustors. Both Doppler global velocimetry (DGV) and particle image velocimetry (PIV) were applied in parallel to achieve volumetric, three-component velocity data sets of the reacting flow field at pressures of 2 and 10 bar with 700 K pre-heating. Limited optical access to the mixing zone required a combination of PIV and DGV to obtain averaged three-component velocity data from a single viewing direction. The acquired volume data sets of the time-averaged flow in the mixing zone contain about 40 parallel planes spaced at 2 mm with a spatial resolution of 1.2 × 1.2 mm 2 each. Difficulties encountered in the application of stereoscopic PIV to a simple atmospheric generic combustor illustrate the advantage of the combined PIV-DGV technique.

A single-stage transonic axial compressor was equipped with a casing treatment (CT), consisting of 3.5 axial slots per rotor pitch in order to investigate the predicted extension of the stall margin characteristics both numerically and experimentally. Contrary to most other studies, the CT was designed especially accounting for an optimized optical access in the immediate vicinity of the CT, rather than giving maximum benefit in terms of stall margin extension. Part I of this two-part contribution describes the experimental investigation of the blade tip interaction with casing treatment using particle image velocimetry (PIV). The nearly rectangular geometry of the CT cavities allowed a portion of it to be made of quartz glass with curvatures matching the casing. Thus, the flow phenomena could be observed with essentially no disturbance caused by the optical access. Two periscope light sheet probes were specifically designed for this application to allow for precise alignment of the laser light sheet at three different radial positions in the rotor passage (87.5%, 95%, and 99%). For the outermost radial position, the light sheet probe was placed behind the rotor and aligned to pass the light sheet through the blade tip clearance. It was demonstrated that the PIV technique is capable of providing velocity information of high quality even in the tip clearance region of the rotor blades. The chosen type of smoke-based seeding with very small particles (about 0.5 μm in diameter) supported data evaluation with high spatial resolution, resulting in a final grid size of 0.5×0.5 mm2. The PIV database established in this project forms the basis for further detailed evaluations of the flow phenomena present in the transonic compressor stage with CT and allows validation of accompanying computational fluid dynamics (CFD) calculations using the TRACE code. Based on the combined results of PIV measurements and CFD calculations of the same compressor and CT geometry, a better understanding of the complex flow characteristics can be achieved, as detailed in Part II of this paper.