Peter Ott

École Polytechnique Fédérale de Lausanne | EPFL · Institute of Mechanical Engineering

Experimental heat transfer measurements are used in a wide range of fields, for example to validate new cooling concepts in turbomachinery, to assess the performances of heat exchangers, and to provide data for numerical simulations. A particular challenge is posed by complex geometries, where the heat transfer coefficients cannot be determined wit...

Heat transfer measurements are required in a wide range of fields, for example to validate new cooling concepts in turbomachinery, to assess the performances of heat exchangers, and to provide data for numerical simulations. Thereby transient methods are often applied for local heat transfer resolution. A particular challenge is posed by complex fl...

Sequential impingement channels can reduce the adverse effect of crossflow in narrow impingement channels, as well as increase the cooling efficiency. In this work, sequential impingement channels are experimentally investigated using the transient liquid crystal technique to assess their thermal performances. A low heat transfer region is identifi...

The mechanisms that should be considered for separation flow control applications of nanosecond pulsed dielectric barrier discharge (DBD) actuators were investigated on a NACA 0015 profile for velocities of 10 m / s ( R e = 100,000 ) and 20 m / s ( R e = 200,000 ) in ambient wind tunnel conditions. Near and post-stall angles of attack were consider...

The characterization of non-thermal homogeneous plasmas is possible using optical emission spectroscopy (OES), notably by estimating the reduced electric field. This method was applied to characterize streamers generated by a nanosecond pulsed surface dielectric barrier discharge (SDBD) operated in quiescent air at atmospheric pressure and also at...

Integrally cast turbine airfoils with wall-integrated cooling cavities are greatly applicable in modern turbines providing enhanced heat exchange capabilities compared to conventional cooling passages. In such arrangements, narrow impingement channels can be formed where the generated crossflow is an important design parameter for the achievement o...

This paper examines the applicability of a triple layer of thermochromic liquid crystals (TLCs) for the determination of local heat transfer coefficients using the transient liquid crystal (LC) technique. The experiments were carried out in a narrow impingement channel , typically used for turbine blade cooling applications. Three types of narrow b...

Transient heat transfer experiments typically employ thermochromic liquid crystals to temporally map surface temperatures. The desired heat transfer coefficient is then calculated from the solution of Fourier’s 1D transient heat conduction equation which is set to model the wall temperature at the solid–fluid interface. However, the experimental co...

In the present study the interaction of nanosecond pulsed dielectric barrier discharge (ns-DBD) actuators with aerodynamic flow up to transonic velocities was investigated. The primary focus was on the influence of the flow on the discharge and the effects of the discharge itself. In addition, the influence of the ns-DBD on a shock-wave was studied...

The present study describes an experimental investigation into narrow impingement cooling channels with divergent geometries. The following analysis aims to clarify the influence that this particular form factor exerts on the heat transfer distribution, as well as on the traverse crossflow of the cooling passages. The research is based on the outco...

This paper demonstrates that the impact of nanosecond pulsed dielectric barrier discharge (ns-DBD) actuators on the structure of the boundary layer can be investigated using quantitative convective heat transfer measurements. For the experiments, the flow over a flat plate with a C4 leading edge thickness distribution was examined at low speed inco...

This paper examines the applicability of a triple layer of thermochromic liquid crystals on the evaluation of local heat transfer coefficients using the transient liquid crystal measurement technique. The experiments were carried out in a narrow impingement cooling channel typically found in turbine airfoils. Narrow-band liquid crystals of 35°C, 38...

In modern turbine airfoils, narrow impingement cooling channels can be formed in a double-wall configuration. In these wall-integrated cooling cavities, the generated crossflow is one of the most important design factors, and hence, the number of impingement holes included in a channel. This study examines experimentally the influence of the number...

A reliable determination of the unsteady aerodynamic loads acting on the blades is essential to predict the aeroelastic stability of vibrating compressor cascades with accuracy. At transonic flow conditions, the vibration of the shock may change the blade aeroelastic behavior. Numerical tools still have difficulties to capture the physics associate...

Non-synchronous excitation under low volume operation is a major risk to the mechanical integrity of last stage moving blades (LSMBs) in low-pressure (LP) steam turbines. These vibrations are often induced by a rotating aerodynamic instability similar to rotating stall in compressors. Unsteady computational fluid dynamics (CFD) has been applied to...

The development of integrally cast turbine airfoils allows the production of narrow impingement channels in a double-wall configuration, where the coolant is practically injected within the wall of the airfoil providing increased heat transfer capabilities. This study examines the cooling performance of narrow impingement channels with varying jet...

The present study demonstrates an alternative approach for describing fluid flow characteristics very close to the wall, using locally resolved convective heat transfer experiments. Heat transfer coefficients on the base surface and around a surface mounted vortex generator of delta-wing shape design, are evaluated with the transient liquid crystal...

The ability of nanosecond pulsed DBD actuators to control flow separation was investigated on a NACA 0015 profile for velocities reaching Re = 230,000 (24 m/s). The optimal location for the actuator was determined from oil flow experiments. Moderate voltage levels were applied (3 and 6 kV) and the actuator was operated at frequencies ranging from 0...

The current capabilities of the foundry industry allow the production of integrally cast turbine airfoils. Impingement cooling effectiveness can be then further increased due to the manufacturing feasibility of narrow impingement cavities in a double-wall configuration. This study examines experimentally, using the transient liquid crystal techniqu...

The stabilizing effect of grouping rotor blades in pairs has been assessed both, numerically and experimentally. The bending and torsion modes of a low aspect ratio high speed turbine cascade tested in the non-rotating test facility at EPFL (Ecole Polytechnique Fédérale de Lausanne) have been chosen as the case study. The controlled vibration of 20...

In many engineering applications, heat transfer enhancement techniques are of vital importance in order to ensure reliable thermal designs of convective heat transfer applications. This study examines experimentally the heat transfer characteristics on the base plate around various surface mounted obstacles. Local convection coefficients are evalua...

This study examines experimentally the cooling performance of narrow impingement channels as could be cast-in in modern turbine airfoils. Full surface heat transfer coefficients are evaluated for the target plate and the sidewalls of the channels using the transient liquid crystal technique. Several narrow impingement channel geometries, consisting...

The accuracy of flutter or forced response analyses of turbomachinery blade assemblies strongly depends on the correct prediction of the unsteady aerodynamic loads acting on the vibrating blades. For unsteady linearized CFD solvers, the quality of the steady-state flow solution constitutes the basis for efficient and accurate CFD unsteady computati...

The accuracy of flutter or forced response analyses of turbomachinery blade assemblies strongly depends on the correct prediction of the unsteady aerodynamic loads acting on the vibrating blades. For unsteady linearized CFD solvers, the quality of the steady-state flow solution constitutes the basis for efficient and accurate CFD unsteady computati...

The research scope of GTT includes efforts to increase reliability, efficiency and power output of gas turbines and aero engines. The most important areas of research are the following: fluid-structure interaction at forced response and flutter for turbine and compressor blades, cooling techniques for gas turbine blades, flow control using plasma.

The presented work consists of the presentation and discussion of unsteady experimental results of controlled vibration measurements for two single-blade and two cluster test cases subjected to the same subsonic flow conditions. The experiments were performed in a non-rotating annular test facility with the aim of investigating the influence of clu...

Nonsynchronous excitation under low volume operation is a major risk to the mechanical integrity of last stage moving blades (LSMBs) in low-pressure (LP) steam turbines. These vibrations are often induced by a rotating aerodynamic instability similar to rotating stall in compressors. Currently extensive validation of new blade designs is required t...

Impingement cooling effectiveness of gas turbine vanes and blades can be further increased due to the manufacturing feasibility of integrally cast airfoils which can provide narrow impingement cooling cavities. This study examines experimentally using the transient liquid crystal technique, and numerically using a commercial CFD package, the heat t...

The heat transfer characteristics of a fully confined impingement jet are experimentally and numerically evaluated. Full surface heat transfer coefficient distributions are obtained for the target and impingement plate of the model using the transient liquid crystal technique and a commercial CFD solver. The confined box consists of a single round...

Experiments were conducted in order to investigate the ability of nanosecond pulsed dielectric barrier discharge (ns-DBD) actuators to control flow on airfoils in subsonic and transonic flow (up to Mα∞ = 0.75, Re = 1.35 · 106). A NACA 0015 profile equipped with a leading edge mounted ns-DBD actuator was investigated up to Re = 2.3 · 105 (u∞ = 24 m/...

This paper presents and discusses unsteady experimental results of controlled vibration measurements of blade-cluster and single-blade test cases, all subjected to the same subsonic flow conditions. The experiments were performed in a non-rotating annular test facility with the aim to investigate the influence of cluster vibrations on the aerodynam...

Non-synchronous excitation under low volume operation is a major risk to the mechanical integrity of last stage moving blades (LSMBs) in low-pressure (LP) steam turbines. These vibrations are often induced by a rotating aerodynamic instability similar to rotating stall in compressors. Currently extensive validation of new blade designs is required...

While the advancement of computing hardware now enables accurate predictions of flutter and forced response at normal flow conditions during the compressor design phase, aeroelastic computations at off-design or reverse flow conditions remain a challenging task. During the flow reversal sequence of a surge cycle, complex aerodynamics occur which ma...

A major part of the experimental work on surface dielectric barrier discharge (DBD) actuators was carried out in flow speeds up to 30 m/s, mostly with actuators driven by AC-voltage. With increasing flow speed, however, the actuator’s control capacity decreases because the ionic wind generated by the AC-driven DBD is up to now limited to a few mete...

The transient liquid crystal technique is widely used for impingement heat transfer experiments. Additionally, due to the difficulty of producing pure temperature steps in the flow, many authors assumed the fluid temperature evolution as a series of step changes using Duhamel's superposition theorem. However, for small impingement configurations wh...

Progress in materials and manufacturing techniques allows an increase of the mean turbine inlet temperature resulting in improved efficiency and specific power of the turbine. Due to the combustor characteristics, thermal and aero-dynamics non-uniformities can be present at the turbine inlet section. Then, a further increase in the inlet temperatur...

This study examines experimentally the cooling performance of integrally cast impingement cooling channels which provide increased heat transfer area compared to traditional impingement configurations. For the evaluation of the heat transfer coefficient, the transient liquid crystal method was used. Full surface heat transfer coefficient distributi...

The diversification of power generation methods within existing power networks has increased the requirement for operational flexibility of plants employing steam turbines. This has led to the situation where steam turbines may operate at very low volume flow conditions for extended periods of time. Under operating conditions where the volume flow...

This study examines experimentally the effect of stators on the performance and heat transfer characteristics of small axial cooling fans. A single fan impeller, followed by nine stator blades in the case of a complete stage, was used for all the experimental configurations. Performance measurements were carried out in a constant speed stage perfor...

While the advancement of computing hardware now enables accurate predictions of flutter and forced response at normal flow conditions during the compressor design phase, aeroelastic computations at off-design or reverse flow conditions remain a challenging task. During the flow reversal sequence of a surge cycle, complex aerodynamics occur which ma...

This investigation examines experimentally the behavior of swirled jets produced by axial flow fans blowing into a crossflow at low velocity ratios. The main difference with non-swirl cases is an asymmetry of the dominant kidney vortex and a slight distortion of the jet trace downstream of the injection hole. The effect of jet rotation at relativel...

The present paper shows the comparison between steady-state and unsteady results from two versions of a flow solver and measurements in a linear cascade at transonic flow conditions for forced vibration of a single blade. Experiments have been conducted in a linear test facility with a cascade composed of five turbine blades. The center blade was f...

The application of nanosecond pulse-driven surface dielectric barrier discharge (DBD) actuators as control devices on airfoils and turbomachinery blades in transonic flow is inves- tigated experimentally. Images acquired with a short-exposure ICCD camera document the spatial-temporal discharge development in both absence and presence of flow, as a...

Dielectric Barrier Discharge in air at atmospheric pressure

The prediction of flutter and forced response at normal flow conditions has become a standard procedure during the design of compressor airfoils. But at severe off-design conditions, the flow field becomes very complex, especially during the surge blow-down phase where reversed flow conditions occur. The correct prediction of the unsteady pressures...

The application of nanosecond pulse-driven surface dielectric barrier discharge (DBD) actuators as control devices on airfoils and turbomachinery blades in transonic flow is investigated experimentally. Images acquired with a short-exposure ICCD camera document the spatial-temporal discharge development in both absence and presence of flow, as a me...

Dielectric Barrier Discharge in air at atmospheric pressure

The prediction of flutter and forced response at normal flow conditions has become a standard procedure during the design of compressor airfoils. But at severe off-design conditions, the flow field becomes very complex, especially during the surge blow-down phase where reversed flow conditions occur. The correct prediction of the unsteady pressures...

Annular cascades are particularly useful to study the three dimensional phenomena encountered in turbomachines. The Non-Rotating Annular Test Facility of the Swiss Federal Institute of Technology in Lausanne (EPFL) consists in a radial-axial inlet nozzle and allows the investigation of steady and unsteady flows in compressor and turbine geometries....

Experimental and numerical investigations were performed in order to develop a technique of active flow control by aspiration applied to an annular axial compressor cascade. Continuous aspiration is applied through shaped slots located on the suction side of the blades and close to the suction side on the hub. Test cases with aspiration are compare...

Swirl jets in crosflow are investigated experimentally and numerically. The main difference with non-swirl cases is an asymmetry of the dominant kidney vortex and a slight distortion of the jet trace downstream of the injection hole. Jet rotation, at relatively low swirl numbers (S=0.1-0.4) is investigated by a computational tool while the numerica...

Compressor unsafe operating regimes yield unsteady high speed turbulent flows in which complex aeroelastic phenomena occur. If the blade flutter and forced response behavior (i.e. aeroelastic stability) can be predicted reliably for normal flow conditions, its assessment at severe-off design conditions remains a critical task for compressor develop...

An end-wall cooling geometry with both fan-shaped and cylindrical holes is tested. The test case is composed by a linear cascade with cylindrical blades, the cooling holes disposed over eight rows. Each row is disposed along iso-Mach lines to ensure same blowing conditions for each hole of the row. Fan-shaped and cylindrical holes groups are fed by...

Film cooling experiments are commonly performed with Pressure Sensitive Paint (PSP). The oxygen sensitive probe molecules allow for measurement of the adiabatic film cooling effectiveness by detecting the concentration of a tracer-gas with different oxygen content. This is made possible by the analogy between transport of heat and mass. The techniq...

The present paper is devoted to the study of plasma-flow interaction in a case where the plasma is generated on the surface of an airfoil and interacts with the transonic flow field around that airfoil. The study focuses on the effects of the surface dielectric barrier discharge (DBD) on the shock structure outside the boundary layer and its far-fi...

Convective heat transfer measurements are performed on the platform of a turbine vane model in the context of film cooling studies. The surface temperature response of the wall due to step heating generated with a thin metallic foil glued on top of the surface is monitored with narrow banded liquid crystal coatings for accurate temperature indicati...

Detailed surface measurements of the thermal performance of a film cooling system have been performed on the endwall of a nozzle guide vane (NGV) mounted in a linear cascade facility at EPFL. An external cooling scheme including several rows of fan-shaped and cylindrical cooling holes has been designed. By testing different cooling flow rates at a...

An electrically heated foil glued on top of an acrylic wall is employed to measure convective heat transfer on turbine vane platforms. The use of a heater foil requires specific filtering techniques to obtain the searched quantities – heat transfer coefficients and the heat release from the foil. In this paper, limitations of the transient measurem...

The paper presents an application of the transient heater foil measurement technique using thermochromic liquid crystals (TLC) to endwall heat transfer and film cooling investigations in a transonic turbine vane cascade. The film cooling configuration consists of an upstream slot, representing the leakage flow area between the interface of the comb...

Detailed surface measurements of the heat transfer coefficient and the film cooling effectiveness by application of the transient liquid crystal method were carried out on a heavily film cooled nozzle guide vane (NGV) in a linear cascade wind tunnel at the EPFL as part of the European Research Project TATEF2 (Turbine Aero-Thermal External Flows 2)....

A detailed study of the interaction between high-speed gas flows and surface dielectric barrier discharges (DBD) is presented. In the present paper, it is demonstrated that a DBD can be sustained in transonic airflows, up to isentropic Mach numbers of 1.1. The plasma is characterized electrically, as well as optically with a CCD camera and a photom...

The transient liquid crystal technique has been extensively used for measuring the heat transfer characteristics in gas turbine applications. Thereby, the time evolution of the surface temperature is usually evaluated using the model of a semi-infinite flat plate. For experiments on cylinders, Wagner et al. [G. Wagner, M. Kotulla, P. Ott, B. Weigan...

Film cooling experiments usually determine the heat transfer coefficient ratio for film cooling compared to the non-film cooled case hf/h0 and the adiabatic film cooling effectiveness η. For an overall assessment of the obtained cooling performance, a heat flux ratio qf/q0 is commonly calculated. This is generally done assuming an isothermal wall a...

The film cooling performance of a turbine nozzle guide vane (NGV) is tested in a linear cascade. The advanced film cooling arrangement includes multiple rows of fan-shaped holes on both suction and pressure sides, as well as injection on the leading edge (showerhead). The study focuses on the mid-span region of the NGV where the flow is predominant...

Transient liquid crystal experiments have been carried out to measure the effectiveness and heat transfer characteristics of leading-edge film cooling for three different film cooling holes configurations at design and off-design incidence angle. The three configurations are based on the same representative leading edge model of a turbine blade, co...

In modern gas turbines, the turbine airfoil leading edge is currently protected from the hot gas by specific film cooling schemes, so called showerhead cooling. The present paper shows a numerical study of different showerhead cooling geometries. The 3D finite element program ABAQUS as well as a 2D finite element program have been employed to predi...

In modern gas turbines, the turbine airfoil leading edge is currently protected from the hot gases by specific film cooling schemes, so-called showerhead cooling. The present paper shows an experimental study of different showerhead cooling geometries on a blunt body. For these tests, TLC (thermochromatic liquid crystals) have been used for measuri...

This paper presents, for the first time, a detailed experimental comparison between combined and simultaneous motion- and gust-induced unsteady aerodynamic response of a turbine cascade. Experiments are performed in a non-rotating annular test facility to investigate the local validity of the linearised treatment of the forced response problem. The...

The transient liquid crystal technique is nowadays widely used for measuring the heat transfer characteristics in gas turbine applications. Usually, the assumption is made that the wall of the test model can be treated as a flat and semi-infinite solid. This assumption is correct as long as the penetration depth of the heat compared to the thicknes...

In the present paper a numerical and experimental study aiming at the enhancement of the working range of a transonic compressor via boundary layer suction (BLS) is presented. The main objective of the investigation is to study the influence of BLS on the interference between shock wave and boundary layer and to identify the possible benefit of BLS...

An experimental investigation was conducted in which the unsteady aerodynamic response of a single airfoil was measured in the presence of downstream-generated aerodynamic gusts, a forced plunging motion, and a combined vibration-gust excitation. In addition, the results of these experiments were compared with 2-D, Euler computations. These efforts...