Michael Schroll

PIV Measurement of Secondary Flow in a Rotating Two-Pass Cooling System With an Improved Sequencer Technique

The flow field characteristics of a two-pass cooling system with an engine-similar lay-out have been investigated experimentally using the non-intrusive Particle Image Velocimetry (PIV). It consists of a trapezoidal inlet duct, a nearly rectangular outlet duct, and a sharp 180 degree turn. The system has been investigated with smooth and ribbed walls. Ribs are applied on two opposite walls in a symmetric orientation inclined with an angle of 45 degrees to the main flow direction. The configuration was analyzed with the planar two component PIV technique (2C PIV), which is capable of obtaining complete maps of the instantaneous as well as the averaged flow field even at high levels of turbulence. In the past, slip between motor and channel rotation causes additional not negligible uncertainties during PIV measurements due to unstable image position. These were caused by the working principle of the standard programmable sequencer unit used in combination with unsteady variations of the rotation speed. Therefore, a new sequencer was developed using FPGA-based hardware and software components from National Instruments which revealed a significant increase of the stability of the image position. Furthermore, general enhancements of the operability of the PIV system were achieved. The presented investigations of the secondary flow were conducted in stationary and, with the new sequencer technique applied, in rotating mode. Especially in the bend region vortices with high local turbulence were found. The ribs also change the fluid motion as desired by generating additional vortices impinging the leading edge of the first pass. The flow is turbulent and isothermal, no buoyancy forces are active. The flow was investigated at Reynolds number of Re = 50,000, based on the reference length d (see Fig. 3). The rotation number is Ro = 0 (non-rotating) and 0.1. This investigation is aimed to analyze the complex flow phenomena caused by the interaction of several vortices, generated by rotation, flow turning or inclined wall ribs. 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 flow simulation tools with special regard to prediction of flow turbulence under rotational flow regime as typical for turbomachinery. Future work will include the investigation of buoyancy effects to the rotational flow. This implicates wall heating which result from the heater glass in order to provide transparent models.

Characterization of Lean Burn Module Air Blast Pilot Injector With Laser Techniques

For lean burn combustor development in low emission aero-engines, the pilot stage of the fuel injector plays a key role with respect to stability, operability, NOx emissions, and smoke production. Therefore it is of considerable interest to characterize the pilot module in terms of pilot zone mixing, fuel placement, flow field and interaction with the main stage.This contribution focusses on the investigation of soot formation during pilot-only operation. Optical test methods were applied in an optically accessible single sector rig at engine idle conditions. Using planar laser-induced incandescence (LII), the distribution of soot and its dependence on air/fuel ratio, as well as geometric injector parameters, was studied. The data shows that below a certain air/fuel ratio, an increase of soot production occurs. This is in agreement with smoke number measurements in a standard single sector flame tube rig without optical access. Reaction zones were identified using chemiluminescence of OH radicals. In addition, the injector flow field was investigated with PIV. A hypothesis regarding the mechanism of pilot smoke formation was made based on these findings. This along with further investigations will form the basis for developing strategies for smoke improvement at elevated pilot only conditions.Copyright

Flow Field Characterization at the Outlet of a Lean Burn Single Sector Combustor by Laser-Optical Methods

High OPR engine cycles for reduced NOx emissions will generate new aggravated requirements and boundary conditions by implementing low emission combustion technologies into advanced engine architectures. Lean burn combustion systems will have a significant impact on the temperature and velocity traverse at the combustor exit. Lean burn fuel injectors dominate the combustor exit conditions. This is due to the fact that they pass a majority of the total combustor flow, and to the lack of mixing jets like in a conventional combustor. With the transition to high pressure engines it is essential to fully understand and determine the high energetic interface between combustor and turbine to avoid excessive cooling, which has a detrimental impact on turbine and overall engine efficiency. Velocity distributions and their fluctuations at the combustor exit for lean burn are of special interest as they can influence the efficiency and capacity of the turbine. Within the EU project LEMCOTEC, a lean burn single sector combustor was designed and built at DLR, providing optical access to its rectangular exit section. The sector was operated with a fuel staged lean burn injector from Rolls-Royce Deutschland. Measurements were performed under various operating conditions, covering idle and cruise operation. Two techniques were used to perform velocity measurements at the combustor exit in the demanding environment of highly luminous flames under elevated pressures: Particle Image Velocimetry (PIV) and Filtered Rayleigh Scattering (FRS).The latter was used for the first time in an aero-engine combustor environment. In addition to a conventional signal detection arrangement, FRS was also applied with an endoscope for signal collection, to assess its practicality for a potential future application in a full annular combustor with restricted optical access. Both measurement techniques are complementary in several respects, which justified their respective application and comparative assessment. PIV is able to record instantaneous velocity distributions and is therefore capable to deliver higher velocity moments, in addition to temporal averages. Applied in two orthogonal traversable light sheet arrangements, it could be used to map all three velocity components across the entire combustor cross section, and obtain data on velocity variances, cross-correlations and turbulence intensities. FRS is limited to measurements of average velocities, as long sampling times are required due to the weak physical process of Rayleigh scattering. However, FRS has two advantages: It requires no particle seeding, because it is based on the measurement of a molecular Doppler shift, and it can provide temperature information simultaneously. This contribution complements a second paper (GT2016-56370) focusing on the measurement of temperature distributions at the same combustor exit section by laser-based optical methods.

Optically Accessible Multisector Combustor: Application and Challenges of Laser Techniques at Realistic Operating Conditions

For their application in a multisector combustor, several laser-based measurement techniques underwent further development to generate useful results in the demanding environment of highly luminous flames under elevated pressures. The techniques were applied to two burner configurations and the results were used to explain their respective behavior.Multisector combustors at elevated pressure present formidable difficulties to the operation of laser based techniques, as the optical path length is longer than for a single sector while the optical density of the flowing medium can be quite high. Hence, the techniques have to be set up to perform under low signal to noise levels. Nevertheless for a validation exercise geared at multidimensional simulation, quantitative results are requested. Here the modification of standard Laser Induced Incandescence as a means to measure soot concentrations with higher dynamic range is described. For situations where the optical density is too high for the application of imaging techniques, laser absorption was used and its application in the multisector combustor is presented.Since combustion and soot formation is closely coupled to flowfield and mixing, velocity measurements are highly desired for comparison with computed flowfields. Although with Laser-Doppler Anemometry a well-established technique is at hand, the high operating costs of a multisector combustor cannot be supported for the needed time of operation. Therefore an effort was made to make the Particle Imaging Velocimetry technique operable in highly luminous flames by using a second camera. The two-camera system and its operation are described in the paper.Finally the application on two different burner configurations is reported together with chemiluminescence as a tracer for heat release, and differences in soot production are related to the measured flow field.Copyright

Investigation of Unsteady Compressor Flow Structure With Tip Injection Using Particle Image Velocimetry

Fluid injection at the tip of highly loaded compressor rotors is known to be very effective in suppressing the onset of rotating stall and eventually compressor instability. To understand the effects of tip injection, the flow field at the tip region of a transonic compressor rotor with and without fluid injection was investigated in this paper. Using results acquired by phase-locked PIV measurements as well as the static pressure field obtained by fast response pressure transducers, the unsteady interaction between the injection jet and the rotor could be described thoroughly. Both, an influence of the rotor’s flow field on the jet as well of the jet on the rotor was clearly visible. Since unsteady inflow conditions to the front rotor in the relative frame of reference were imposed by the injection jets, the rotor’s unsteady response was investigated by inspection of the position of the tip leakage vortex trajectory. It could be shown that due to a short time for the flow to adapt at the rotor’s leading edge, its position didn’t change distinctly. Because a significantly longer time was needed for the overall passage flow to adapt, it was concluded that this causes the beneficial effect of tip injection.Copyright

Investigation of the Effect of Rotation on the Flow in a Two-Pass Cooling System with Smooth and Ribbed Walls using PIV

A rotating cooling system with a 180 deg turn is investigated experimentally using the 2C PIV technique to measure the flow inside. This cooling configuration consists of two ducts of arbitrary cross-sections representing a two-pass front part of an idealized but nevertheless engine relevant turbine blade cooling design. The system has been investigated with ribbed walls in both passages for cooling enhancement as well as with smooth walls as a reference version in order to identify the effects induced by ribs. The rib orientation on the walls is 45 deg. With a rib height of 0.1 of hydraulic duct diameter and a pitch of 10 times rib height, a representative well-established rib lay-out was selected. This paper presents measurements of the axial flow during rotation of this two-pass system for rotation numbers up to 0.1. Together with previously obtained stationary results, this data completes the investigation of the secondary flow field with rotational results acquired with a two-component PIV measuring technique with improved sequencer technique. The Two-Pass Cooling System was analyzed on the rotating test rig using two-component Particle Image Velocimetry (2C PIV) a non-intrusive optical planar measurement technique. PIV is capable of obtaining complete flow maps of the instantaneous as well as averaged flow field even at high turbulence levels, which are typical for the narrow serpentine-shaped ribbed cooling systems. An in-house developed synchronization device enables very accurate control of the laser flashes and image acquisition with regard to the angular position of the measurement plane (light sheet) and thereby very accurately stabilizes the position of the channel within the image during PIV recording which then leads to very accurate mean velocities. The presented investigations were conducted in stationary and rotating mode. The results demonstrate the combined interaction of different vortices induced by several effects such as the inclination of ribs, Coriolis forces due to rotation and inertial forces within the bend. Additionally, a flow separation was observed at the divider wall downstream of the bend (in the second pass) that has a strong impact on the flow field depending on the rotational speed. The axial flow maps presented in this paper in combination with the secondary flow maps published previously are of sufficient high quality and spatial resolution to serve as a benchmark test case for the validation of flow solvers. The turbulent channel flow was investigated at a Reynolds number of 50,000 and at rotation numbers of 0.0 and 0.1.

Investigation of the Reacting Flow Field of a Lean Burn Injector With Varying Degree of Swirl at Elevated Pressure Condition

Two RR Lean Direct Injection (LDI) injector versions with different amounts of pilot swirl were investigated. Experiments, performed at elevated pressure and temperature, corresponding to engine conditions at idle include Mie scattering. LII and absorption measurements are used for soot concentration within the primary zone. The soot emission at the outlet is measured by an SMPS instrument. These experimental studies are complemented with PIV measurements. The acquired data allows evaluation of the combustion process from the liquid phase, followed by evaporation, reaction and finally soot production with high spatial resolution. The change of swirl produced rather moderate changes in the flow field, nevertheless qualitative changes in the fuel placement were observed. Starting from there, differences in heat release and soot formation can be explained, which lead to larger changes of soot emission. These observations show that a good knowledge of the interaction of gas and liquid phase is necessary to predict the occurrence of behavioral changes in the operating regime.

Validation of Flow Field and Heat Transfer in a Two-Pass Internal Cooling Channel Using Different Turbulence Models

In this work the flow regime within a generic turbine cooling system is investigated numerically. The main objective is to validate the performance of various turbulence models with different complexity by comparing the numerical results with experimental data. To maximize surface heat transfer rates, present-day cooling systems of high pressure turbines have highly complex shapes generating high turbulence levels and flow separations. These flow structures lead to higher requirements of CFD-techniques for sufficient prediction. To simulate complex flows in the industrial design process, Reynolds averaged Navier-Stokes (RANS) techniques are applied instead of computationally expensive LES and DNS simulations. Therefore, higher order turbulence models are necessary to predict flow field and heat transfer performance in such complex motion. The DLR standard flow solver for turbomachinery flows, TRACE, is used to solve the RANS equations. Four turbulence models have been analysed: the one equation model of Spalart and Allmaras, the two equation k – ω model of Wilcox, the two equation k – ω SST model of Menter and the anisotropy resolving Explicit Algebraic Reynolds Stress model (EARSM) of Hellsten. The investigated cooling geometry consists of a two-pass smooth channel with a 180 degree bend. At the DLR institute of propulsion technology PIV measurements in a rotating cooling channel test bed for Rotation numbers up to 0.1 have been performed. This work uses the experimental data for Re = 50,000 and Ro = 0 without rotation for comparison. For all models adiabatic and diabatic calculations have been performed. In order to accurately apply the turbulence models, a study concerning the turbulent boundary conditions has been performed prior to the calculations. The results obtained through RANS simulations are presented in comparison with the experiments along planes in the flow direction and in the orthogonal direction to study the velocity field, the shape and size of the separation bubbles and the wall shear stress. The EARSM predicts the flow field overall more accurately with improved agreement between all relevant parameters compared to the other models. The diabatic simulations reflect the adiabatic results. However, it can be noticed that higher complexity in turbulence modelling is related to increased heat transfer. Our work confirms the EARSMs ability to predict complex flow structures better than the more elementary approaches.Copyright