Stefan aus der Wiesche

The Effect of the Incidence Angle on the Flow Over a Rotating Disk Subjected to Forced Air Streams

The flow over a stationary and a rotating disk subjected to an outer forced air stream was investigated particular with a look to the effect of incidence angle β. The experiments were carried out with an electrically heated disk placed in a wind tunnel. The measurement of the mean and the local convective heat transfer coefficient enabled a clear study of the flow behavior. The incidence angle β was scanned with high resolution over the entire range, and a large number of crossflow and rotational Reynolds numbers were covered. It was found that a well defined transition incidence angle β existed leading to a symmetry breaking of the flow. The resulting mean heat transfer was in agreement with the phenomenological Landau-de Gennes model in case of small rotational Reynolds numbers. In case of large rotational effects, a different type of transition occurred. Based on the experimental and theoretical results, a discussion of the different phenomena due to the involved flow parameters is given in the paper.© 2013 ASME

Development and Validation of a Three-Dimensional Multiphase Flow CFD Analysis for Journal Bearings in Steam and Heavy Duty Gas Turbines

The traditional method for hydrodynamic journal bearing analysis usually applies the lubrication theory based on the Reynolds equation and suitable empirical modifications to cover turbulence, heat transfer, and cavitation. In cases of complex bearing geometries for steam and heavy-duty gas turbines this approach has its obvious restrictions in regard to detail flow recirculation, mixing, mass balance, and filling level phenomena. These limitations could be circumvented by applying a computational fluid dynamics (CFD) approach resting closer to the fundamental physical laws. The present contribution reports about the state of the art of such a fully three-dimensional multiphase-flow CFD approach including cavitation and air entrainment for high-speed turbo-machinery journal bearings. It has been developed and validated using experimental data. Due to the high ambient shear rates in bearings, the multiphase-flow model for journal bearings requires substantial modifications in comparison to common two-phase flow simulations. Based on experimental data, it is found, that particular cavitation phenomena are essential for the understanding of steam and heavy-duty type gas turbine journal bearings.Copyright

Convective Heat Transfer From a Free Rotating Disk Subjected to a Forced Crossflow

The understanding of the heat transfer and flow field behavior of rotating systems is essential from a fundamental point of view and for turbo machinery design. The majority of the publications considers enclosed rotating disk systems and only little is known about the convective heat transfer of free rotating disk systems in a forced flow. In this paper, a free rotating disk system, with particular look on the angle of incidence was investigated. The convective heat transfer from a rotating disk depends at least on three characteristic variables, namely the crossflow, rotational Reynolds numbers and the angle of incidence which are determining the mean Nusselt number. A clear study of the symmetry behavior of the flow field was conducted based on the measurement of the convective heat transfer coefficients. The angle of incidence was scanned with high angular resolution over the entire range between the both extreme cases of a perpendicular disk and a disk in a parallel forced flow. A large number of crossflow and rotational Reynolds numbers were covered by the experiments, too. Based on the experimental and theoretical results, a discussion of the different phenomena and heat transfer regimes is given in this paper.Copyright

Investigation of Flow and Mixing Phenomena in an Oil Supply Pocket of a Heavy Duty Gas Turbine Journal Bearing

In recent year, bearing designers have focused on reduced-temperature designs; whereby better results have been achieved by applying the cool inlet oil more effectively and reducing the hot oil carryover. The key factor in this improvement is the hot-cool oil mixing process in the oil supply pockets. However, relatively little is know about the details of the underlying flow and mixing phenomena, and the classical one dimensional lubrication theory fails to provide an explanation. A detail experimental investigation of the flow and mixing phenomena in a typical oil supply pocket of a heavy duty gas turbine was performed by employing a large-scale model of the geometry. This model has a similarity in respect to Reynolds number, and, by using laser Doppler anemometry (LDA), it permitted optical flow visualization and measurements. In addition to a thin shear layer close to the moving boundary, flow patterns including recirculation zones were detected in the larger part of the flow domain. Fully three-dimensional computational fluid dynamics (CFD) simulations were performed, too. With regard to the observed flow and mixing phenomena, large eddy simulation (LES) methods offer great potential because this approach can directly resolve the large eddies, and it is the interaction between the shear layer and these large eddies that mainly govern the mixing phenomena. The comparison between experimental data and LES results showed a reasonable agreement. Furthermore, the potential of more conventional turbulence models such as the classical k-e-model were also assessed.Copyright

A one-dimensional analytical calculation method for obtaining normal shock losses in supersonic real gas flows

The calculation of isentropic flow and normal shock waves of real gases are important, especially in the preliminary design of turbo-machinery and test rigs. In an ideal gas, the relations for one-dimensional isentropic flow and normal shock waves are well known and can be found in standard textbooks. However, for fluids exhibiting strong deviations from the ideal gas assumption universal relations do not exist due to complex equations of state. This paper presents a analytical method for the prediction of isentropic real gas flows and normal shock waves, based on the Redlich-Kwong (RK) equation of state. Explicit expressions based on a series expansion for describing isentropic flow of Novec™ 649 are compared to Refprop data and ideal gas equations. For moderate pressures the RK method is in very good agreement with the Refprop data, while the ideal gas equations fail to predict the real gas behaviour. The same observations are made for normal shock calculations, where both real gas methods yield very close results. Especially the predicted stagnation pressure losses across a shock wave are in excellent agreement.

Rotating Disk in Air Stream

The flow and heat transfer behavior for a stationary disk subjected to a uniform stream of air was discussed in Chap. 5. The various phenomena could be explained on the basis of a critical point and bifurcation theory and fundamental boundary layer considerations because mainly the translational Reynolds number Re u and the incidence β govern the flow field. The extension to a rotating disk requires substantial efforts because a third major parameter, the rotational Reynolds number Re ω , is now involved. Correspondingly, the flow behavior becomes much more complicated in comparison to the stationary disk. However, to a large extent it is possible to systematically discuss the new phenomena within the framework given by the limited case of a stationary disk. The rotational effects are then considered as perturbations. This approach is chosen in the present chapter as a start into the complex field of the flow over an inclined rotating disk.

Influence of Thermophysical Wall Properties During Pool Boiling on Large Diamond and SiC Heaters

The results of pool boiling experiments with synthetic diamond and silicon carbide (SiC) heaters are presented for water as the boiling liquid. The diamond and SiC heaters varied considerably in thermal conductivity, but they had smooth, nearly identical surfaces, which was also the case in regard to their contact angles for water. Temperature sensors and electric heating wires were directly vapor-deposited underneath the surfaces. The experiments were carried out with comparable large heaters (15 mm × 15 mm) for pure water under atmospheric pressure (1 bar) in a pool boiling cell. The heat transfer characteristics including the corresponding boiling curves were obtained. In prior work, it was found that the influence of the thermophysical wall properties might be substantial in the case of special heater geometries, leading to trapped bubbles, but no significant differences between both materials were observed in the case of conventional heater configurations.

Boiling Under Hele-Shaw Flow Conditions: The Occurrence of Viscous Fingering

Boiling and bubble dynamics were experimentally investigated in a Hele-Shaw flow cell using pure water at atmospheric pressure as working fluid. The resulting vapor bubble shapes were recorded by means of a high-speed camera for several plate spacings and heating power levels. It was found that viscous fingering phenomena of vapor bubbles occurred only under very special boiling conditions and cell parameters. The evaporation front velocity was identified as a major parameter for the onset of viscous fingering. The observed basic viscous fingering dynamics was in reasonable agreement with theoretical analyses. In addition to that classical viscous large fingering, small-scale evaporation instability was observed leading to microscopic roughening of accelerating evaporation fronts. This instability might be explicitly related to evaporative heat and mass transfer effects across the fast-moving phase interface.Copyright

A Mobile Test Rig for Micro Gas Turbines Based on a Thermal Power Measurement Approach

It is widely considered that micro gas turbines are potential devices for future energy needs. However, many micro gas turbine development projects have failed, particularly those with a very low power level below 10 kW. The financial and experimental capabilities for micro gas turbine development projects are typically extremely limited; hence, there is a need for low cost mobile test rigs. To close this gap, a robust mobile test rig for turboshaft micro gas turbines was developed and validated. The shaft power can be determined using a thermal measurement approach. This circumvents any issues associated with high voltage and electric current levels. The shaft power can be obtained by a straightforward parameter identification procedure based on simple temperature measurements. This approach is feasible because an analytical expression for the transient temperature field can be obtained. The test rig and thermal power measurement concept were fully validated by a commercial micro gas turbine; good agreement was obtained between the experimental results and the theoretical process data.

Kondensatoren und Rückkühlwerke

Zur Schliesung des Dampfkraftprozesses ist thermodynamisch die Niederschlagung des aus der Turbine stromenden Abdampfes in einer Kondensationsanlage erforderlich. Das niedergeschlagene Kondensat wird uber eine Speisewasserpumpe wieder dem Dampferzeuger zugefuhrt, was den dauernden Ersatz und die teure Aufbereitung des Arbeitsfluides erspart. Der Kondensator stellt physikalisch die Umkehrung des Verdampfungsprozesses dar, bei dem eine grose Anderung des spezifischen Volumens des Arbeitsfluides auftritt. Durch den Einsatz eines Kondensators konnen im Gegensatz zur offenen Dampfkraftanlage sehr niedrige Abdampfdrucke ermoglicht werden, was thermodynamisch fur den Warmekraftprozess gunstig ist. In diesem Fall kann die im Kondensator ubertragene Abwarme aufgrund ihres geringen Temperaturniveaus nicht mehr weiter sinnvoll genutzt werden. Bei einem Heizkondensator, der mit hoheren Gegendrucken arbeitet, wird die Abwarme hingegen nach dem Prinzip der Kraft-Warme-Kopplung energetisch verwertet.