André Günther

Local Measurements of Disk Heat Transfer in Heated Rotating Cavities for Several Flow Regimes

This paper discusses experimental results from a two-cavity test rig representation of the internal air system of a high-pressure compressor. Thermal steady-state measurements of the time-averaged local heat fluxes on both sides of the middle disk are presented for three different flow regimes: pure axial throughflow of cooling air and axial throughflow of cooling air in two directions with a superposed radial inflow of hot air in one cavity. Mass flow ratios between 1/40 < mrad /max < 2/1 are measured. Tests were carried out for a wide range of non-dimensional parameters: Reφ up to 107 , Rez up to 2 × 105 , and Cw up to −2.5 × 104 . In all cases, the shroud is uniformly heated to approximately 100 °C. The local axial heat fluxes are determined separately for both sides of the middle disk from measurements of the surface temperatures with open spot-welded thermo-couples. The method of heat flux determination and an analysis approach calculating the uncertainties and the sensitivity are described and discussed. The local heat flux results of the different flow paths are compared and interpreted by assumed flow structures. The time-averaged heat flux results can be adequately interpreted by flow structures of two toroidal vortices for axial throughflow and a source-sink flow for the radial inflow. The measurements show that the axial heat flux can change direction, i.e., areas exist where the disk is heated and not cooled by the flow. For axial throughflow, a local minimum of heat flux exists on the impinged side in the range of x = 0.65. On the back side, a heating area exists in all tests in the lower half of the disk (x < 0.6) due to recirculated air of higher temperature. This heating area corresponds to the range of the inner vortex and increases with higher axial and rotational Reynolds numbers.

A cell library of scalable neural network classifiers for rapid low-power vision and cognition systems design

The application of neural classifiers in intelligent systems, both as general purpose chips or as dedicated cells in larger VLSI designs, has been an active field of research due to the inherent properties of fault tolerance, graceful degradation, and adaptation capability. Underlying systems for, e.g., biometrics, multimedia, advanced image coding, or automotive tasks, enjoy increasing industrial acceptance and application. Tight application constraints such as, e.g., size, speed, performance, and especially power consumption give increasing incentive to dedicated integrated system implementations, exploiting bio-inspiration and opportunistic design techniques in analog or mixed-signal circuits and systems. In this work, we investigate the major classification techniques with regard to their performance, training properties, and implementation aptitude with special focus on power dissipation and area consumption. A cell library implementing the basic VLSI building blocks for scalable classifier design is progressed in our work. Application examples of this library for system-level VLSI implementation, e.g., for a low-power eye-tracker, are presented. Our work contributes to a research effort with the objective to develop and advance a dedicated top-down design methodology and design flow for integrated vision and cognition systems employing opportunistic and parsimonious design style.

The Effects of Rotation and Mass Flow on Local Heat Transfer in Rotating Cavities With Axial Throughflow

This paper discusses experimental results from a two cavity test rig representative of the internal air system of a high pressure compressor. Thermal steady state measurements of the time-averaged, local heat fluxes based on surface temperatures on both sides of the mid disc are presented for the case of axial throughflow of cooling air. Additionally, measurements of the air temperature and the static pressure inside the cavities are given. Tests were carried out for a wide range of rotational Reynolds numbers up to 107 and axial Reynolds numbers up to 2×105 with a uniformly heated shroud. The method of heat flux determination and the approach to calculating the uncertainties are described and discussed. The local heat flux results from different rotational frequencies, and mass flows are compared and interpreted in terms of assumed flow structures. Using the results of air temperature and static pressure measurements, simple theoretical models of the density gradient and the mixing mass flow which radially enters the cavity, deliver deeper insight into the flow structure and its influence on the heat transfer.The results show that the heat flux increases with increasing mass flow. The influence of rotation on the heat flux is weaker and more complex than the effect of mass flow. The flow can be separated into four parts, the existence and strength of which depend on the test conditions: rotating cavity flow, impinged flow and resultant secondary flow, instabilities due to a negative radial density gradient respective to the buoyancy-induced flow, and instabilities of the incoming jet. For the geometry with a small inlet gap tested here, the flow and heat transfer are dominated by the throughflow or rather by the secondary flow for Rossby numbers Roz > 1.5. The buoyancy-induced flow is negligible. For Rossby numbers Roz < 1.5, the rotation dominates the flow structure. Buoyancy-induced flow can increase radial mixing between the throughflow and the cavity flow as well as the heat transfer to and from the disc. However, at high rotational frequencies, the density gradient becomes positive due to the increase in pressure induced by the centrifugal force which reduces mixing and heat transfer to/from the disc. Therefore, the highest heat transfer at the disc is measured at medium rotational frequencies.Copyright

Electro-Thermal Measurement of Heat Transfer Coefficients

Heat transfer coefficients are very important for the design of the various flow paths found in turbomachinery. Therefore, the measurement of heat transfer coefficients plays an important role in the field of turbomachinery research. An accurate measurement of heat transfer is not a simple task considering gaseous flow in combination with good thermal conductivity of the boundaries along the flow path. The majority of the measurement methods applied has at least one of the following problems. The measurement setup as for instance a heat flux sensor is a thermal barrier in the object of interest or the sensor introduces for measurement reasons a lot of heat into the object of interest. In both cases the main error results from the modification of the system, which is critical for the investigation of any kind of flow influenced by buoyancy. Furthermore, insufficient fluid reference temperature and/or heat flux with changing sign corrupts any attempt to calculate reasonably heat transfer coefficients.The measurement of heat transfer coefficients becomes even more complicated if the flow path of interest rotates at some thousand rpm as for instance in gas turbines or any other fast rotating machine with fluid flow. This contribution presents an experimental investigation of a setup for the direct measurement of heat transfer coefficients in gaseous flow with metallic boundaries. The calibration of a test sample probe is presented for the standard case forced convection on a flat plate. The sensor setup provides low influence of the measurement on the object investigated. It overcomes the problem of a reference temperature and delivers always positive heat transfer coefficients. Furthermore, the measurement setup fulfils the requirements of telemetric application in the rotating system of a machine. The test of the sensor for its strength against centrifugal acceleration is part of the continuation of the work.Copyright

First Results of a New Test Rig for the Research on the Internal Air System of an Industrial Gas Turbine

Improvement of the internal air system has great impact on the efficiency and power of gas turbines. This paper describes a new two-stage test rig for research on the cooling air supply of industrial gas turbines. The design is modeled on a simplified geometry of the internal cavities of the high pressure turbine with receiver holes simulating the restriction imposed by internal blade cooling flow circuits. The test rig consists of a rotor-stator cavity and a full rotating cavity. The Stage One supply and the Stage Two supply are separated inside the rotorstator cavity. The intended aim of the research is the branched cooling air supply. The rim seal flow, which effect on cavity flows is known to be non-trivial, is outside the scope of this area of interest.This paper concentrates on the flow path supplying the Stage Two. Variations of the axial gap size and the radial location of the connecting holes respectively the outlets of the rotor-stator cavity are described here. The air enters axially without pre-swirl at the outer radius of the stator and leaves the rotor-stator cavity through three rotating, axially directed connecting holes at a radius depending on the investigated case, which causes axial throughflow in Case 1 and radial inflow in Case 2.The experimental results show that the net cavity mass flow, presented in terms of a reduced mass flow parameter, increases with increasing pressure ratio, rotational Reynolds number and gap size. The increase due to a larger gap size depends on the rotation and is less prominent at higher rotational Reynolds numbers. An axial throughflow at the outer radius results in higher values of the reduced mass flow parameter, as compared to the case with radial inflow. The difference between the two cases increases with increasing rotational Reynolds number. Measured static pressure fluctuations inside the rotor-stator cavity due to the rotating nozzles can be raised up to ± 4% of the mean in the case with the small gap and the outlet at outer radius. The Pitot probe measurements show a low swirl ratio, radial outflow near the rotor and radial inflow close to the stator, which is consistent with Batchelor-type flow.Copyright

Kleine Thermistoren zur Messung von Wärmeübergangskoeffizienten

Zusammenfassung Die Bestimmung von Wärmeübergangskoeffizienten an rotierenden Maschinenbauteilen ist ein messtechnisches Problem, das für gasumströmte schnell drehende Teile mit guter Wärmeleitfähigkeit bisher ungenügend beherrscht wird. Über die erfolgreiche Erprobung einer Messanordnung, die mit kleinen Thermistoren eine Kompatibilität zu telemetrischer Messtechnik erreicht, berichtet dieser Beitrag. Abstract The measurement of heat transfer coefficients at rotating machine parts is a difficult measurement task and is particularily challenging for fast rotating parts with good thermal conductivity in combination with gaseous flow. This contribution reports about the test of a setup with small thermistors employed to achieve compatibility with the demands of telemetry.

Telemetric Measurement of Heat Transfer Coefficients in Gaseous Flow: First Test of a Recent Sensor Concept in a Rotating Application

Heat transfer coefficients are very important for the design of the various flow paths found in turbomachinery. An accurate measurement of heat transfer is difficult for circumstances of gaseous flow in combination with good thermal conductivity of the boundaries along the flow path. The majority of the measurement methods applied frequently have at least one of the following problems: (1) the measurement system as for instance a heat flux sensor is a thermal barrier in the object of interest, and (2) the sensor introduces for measurement reasons a lot of heat into the object of interest. In both cases the main error results from the modification of the system, which is critical for the investigation of any kind of flow influenced by buoyancy. Furthermore, insufficient fluid reference temperature and/or heat flux with changing sign corrupts any attempt to calculate reliable heat transfer coefficients.The measurement of heat transfer coefficients becomes even more complicated if the flow path of interest rotates at some thousand rpm as for instance in gas turbines or any other fast rotating machine with fluid flow. This contribution presents a new test rig and an experimental investigation of a setup for the direct telemetric measurement of local heat transfer coefficients in gaseous flow with metallic boundaries. The test rig has a complex instrumentation and the measurements are transferred from the rotating to the stationary frame via newly in house developed telemetry system. The measurements presented are based on a recent measurement/sensor concept tested for the first time in the rotating frame. The measurement setup features miniaturized sensor dimensions and low energy consumption. Therefore, the sensor concept is very well suited for use with telemetry system as necessary for many turbomachinery research applications.Furthermore, measurements of the radial distribution of the heat transfer coefficient of a rotating free disc are presented. Additionally a comparison with correlations found in literature as well as a discussion of the results is included.Copyright

Quantitative Measures for Systematic Optimization, Validation, and Imperfection Compensation in the Holistic Modeling and Parsimonious Design of Application-Specific Vision and Cognition Systems

Intelligent systems, e.g., for vision and cognition tasks, enjoy increasing industrial acceptance and application. Application domains range from optical character and handwriting recognition to biometric systems. The joint exploitation of advanced microelectronics, sensor technology, and intelligent systems provides a tremendous economic potential. Tight application constraints such as, e.g., size, speed, performance, and power consumption give increasing attraction to dedicated integrated system implementations exploiting bio-inspiration and opportunistic design techniques in analog or mixed-signal circuits and systems. However, to achieve a viable design at reasonable cost and time-to-market, an appropriate design methodology is required. This paper presents quantitative measures for system-oriented imperfection compensation, optimization, and validation of dedicated application-specific intelligent systems. These measures serve for the fast and consistent behavioral modeling of an aspired intelligent system, and support the rapid and consistent transformation into a physical design. Further, they provide the basis for ensuing design automation of the process, and contribute to the ongoing vivid activities of method and tool development for system simulation and evaluation.

Time-Resolved Pressure Measurements at a Dual-Cavity Test Rig for Research on the Internal Air System of an Industrial Gas Turbine

This paper reports about time-resolved examination of the pressure in a dual-cavity test rig for research on the cooling air supply of industrial gas turbines. The test rig has stationary and telemetric instrumentation. Both systems are capable of time-resolved pressure measurement. The design of the test rig is based on a simplified geometry of the internal cavities of the high pressure turbine with receiver holes and simulates the restriction imposed by internal blade cooling flow circuits. The test rig consists of a rotor-stator cavity and a rotor-rotor cavity. The Stage One and Stage Two supplies are separated inside the rotor-stator cavity. The air enters axially without pre-swirl at the outer radius of the stator and leaves the rotor-stator cavity through three rotating, axially directed connecting holes at a radius that varies among the investigated cases. Therefore, different flow paths in the cavities are studied. The research is focused on the branched cooling air supply system, but the flow path can also be analyzed separately. The rim seal flow is not examined in the research work presented here.Pressure fluctuations in the main gas path caused, for instance, by blade passing and combustor noise, are a well-known phenomenon and therefore the subject of current research, whereas experimental examinations of the pressure fluctuations in the internal air system of gas turbines are very rare. A detailed examination of the pressure in the internal air system is significant in light of the pressure difference between the main gas path and internal air system, which is the driving force for hot gas ingestion. In that sense, the difference between the average pressure on the main gas side to the average pressure in the internal air system is not enough to avoid hot gas ingestion.Therefore, this paper focuses on pressure fluctuations in the internal cavities. The measurements of the pressure fluctuations in the rotor-stator cavity are presented for different operating conditions. The influence of the rotational speed, the mass flow rate, the flow path and the sensor position in the cavity on the time-resolved pressure is examined. Furthermore, time-resolved pressure measurements from the rotor-rotor cavity are presented. Variations of the axial gap size and the radial location of the connecting holes respective to the outlets of the rotor-stator cavity are described.Copyright