Muenster University of Applied science

Steinfurt, Germany

Muenster University of Applied science

Steinfurt, Germany
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Helcig C.,Muenster University of Applied science | Aus Der Wiesche S.,Muenster University of Applied science
American Society of Mechanical Engineers, Fluids Engineering Division (Publication) FEDSM | Year: 2016

The understanding of the heat transfer in separated and reattached flows is essential for a wide range of engineering applications. The majority of the literature uses air as a working fluid for experimental and numerical investigations. The blunt flat plate configuration offers a generic starting point for flow and heat transfer investigations in separated and reattached flow regions. The present contribution is dealing with experimental measurements of the flow and convective heat transfer within the separated and reattached flow on a blunt flat plate. The blunt plate was placed inside a closed circuit water tunnel. By this approach, in comparison with reliable literature data for air, it was possible to investigate the effect of the Prandtl number on the convective heat transfer of blunt flat plate. Copyright © 2016 by ASME.


Helcig C.,Muenster University of Applied science | Aus Der Wiesche S.,Muenster University of Applied science | Shevchuk I.V.,MBtech Group
Journal of Heat Transfer | Year: 2017

Convective heat transfer in rotating disk systems is of great importance in many engineering applications. Despite the high practical relevance, there have been only a small number of experimental investigations regarding the influence of the Prandtl number larger than unity. Ever since Dorfman's pioneering work more than 50 years ago, various analytical works about the heat transfer of a rotating disk have been published. However, this study is a novelty because measurements of the laminar convective heat transfer over a free rotating disk for a wide range of Prandtl number up to Pr=5000 are presented. The accuracy of the employed experimental apparatus was assessed by heat transfer measurements in air, for which reliable literature data are widely available. Natural convection effects and temperature-dependent physical properties have been taken into consideration using the property-ratio method. The experimental results are in excellent agreement with analytical self-similar solutions and the theoretical correlation of Lin and Lin. The applicability of frequently used heat transfer correlations is assessed by the means of the new experimental data. Copyright © 2017 by ASME.


Aus Der Wiesche S.,Muenster University of Applied science
Proceedings of the ASME Turbo Expo | Year: 2012

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 turbo-shaft 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 with a commercial micro gas turbine; good agreement was obtained between the experimental results and the theoretical process data. Copyright © 2012 by ASME.


Albers K.,Muenster University of Applied science | Wittrock U.,Muenster University of Applied science
Applied Physics B: Lasers and Optics | Year: 2011

Efficient quasi-three-level laser operation requires the generation of the highest possible pump rate from a given pump source. We derive the fundamental scaling laws for the pump rate and we extract optimization criteria for pump concepts from these laws. The analysis is then applied to the thin-disk laser. Based on the results, a novel pump concept for thin-disk lasers is proposed, which allows for several tens of pump beam passes and reduces the optical complexity of conventional pump concepts. Furthermore, the beam quality of the pump source is preserved almost completely, facilitating the highest possible pump rate. © 2011 Springer-Verlag.


Perchermeier J.,Muenster University of Applied science | Wittrock U.,Muenster University of Applied science
Optics Letters | Year: 2013

We report on interferometric measurements of the thermo-optical aberrations of the laser medium of an Yb:YAG thin-disk laser in pumped and cw lasing conditions at several pump-power levels with a mean repeatability of 5 nm. These measurements build the basis for future intracavity compensation of the aberrations with our deformable mirror in order to improve the fundamental-mode efficiency. © 2013 Optical Society of America.


Vorholt C.,Muenster University of Applied science | Wittrock U.,Muenster University of Applied science
Optics Letters | Year: 2015

We present, to the best of our knowledge, the first intracavity pumped Yb:YAG thin-disk laser. It operates at 1050.7 nm with a quantum defect of just 1.74% due to pumping at 1032.4 nm. Low absorption of the pump light at the pump wavelength of 1032.4 nm is compensated for by placing the disk inside the resonator of another Yb:YAG thin-disk laser which is diode-pumped at 940 nm. The intra-cavity pumped laser has an output power of 10.3 W and a slope efficiency of 8.3%. © 2015 Optical Society of America.


Vorholt C.,Muenster University of Applied science | Wittrock U.,Muenster University of Applied science
Applied Physics B: Lasers and Optics | Year: 2015

The spatially varying intensity in a standing wave resonator leads to spatial hole burning in the gain medium of a laser. The spatial hole burning changes the gain of different longitudinal modes and can thus determine the optical spectrum of the laser. We simulate this longitudinal mode competition in standing wave resonators of thin-disk lasers. The resulting optical spectra of the laser are compared to measured optical spectra. We examine two types of resonators: I-resonators and V-resonators with different angles of incidence. In V-resonators, the non-normal incidence of the laser beam on the disk lifts the degeneracy of the polarization. Experiments show that the slight gain advantage for the $$p$$p-polarization does not lead to polarized emission. For both types of resonators, the measured spectra are in good agreement with the simulated ones. The simulations allow to study the influence of spectral intra-cavity losses on the optical spectrum of a thin-disk laser. © 2015, Springer-Verlag Berlin Heidelberg.


Aus Der Wiesche S.,Muenster University of Applied science
Proceedings of the ASME Turbo Expo | Year: 2014

Based on a voice-of-the-industry survey covering major turbine manufactures as well as power plant owners and operators an undergraduate course on gas and steam turbines was developed at Muenster University of Applied Sciences. This course is also supported by cost-efficient experiments. The experimental investigations on laboratory test rigs are making the students more familiar with turbomachinery phenomena like gas turbine cycle performance, fundamental rotordynamics, blade vibrations, and flow through turbine cascades and loss correlations. The experiments and test rigs were developed in great part by students as part of their Bachelor or Master theses. Furthermore, the experiments did not require tremendous efforts or an expensive infrastructure; they were operated in typical University laboratory environments. Copyright © 2014 by ASME.


Scholz T.,Muenster University of Applied science | Dickmann K.,Muenster University of Applied science | Ostendorf A.,Ruhr University Bochum
Physics Procedia | Year: 2013

New developments and characteristics of high brilliant laser sources have led to new applications in the field of laser remote processing. Due to high particle formation rates within the vapor plume, a significant influence of the interaction between laser radiation and nanoparticles on the process may occur. The presented work shows results of the investigation of the dynamical formation of nanoparticles within the vapor plume during the welding of stainless steel with a 2 kW Multi-Mode fiber laser under laser remote conditions. The particle size distribution is measured by the evaluation of TEM-images, whereas, the plasma temperature and particle density are analyzed in dependence of the irradiation time. © 2013 The Authors.


Scholz T.,Muenster University of Applied science | Dickmann K.,Muenster University of Applied science | Uphoff H.,Muenster University of Applied science | Lammers L.,Muenster University of Applied science
Optics and Lasers in Engineering | Year: 2012

A characteristic of the laser ablation process of metals with high brilliant radiation is a significant formation of nanoparticles within the vapour plume. Due to the interaction between the incoming radiation and the particles, the intensity is attenuated at the workpiece. This leads to a decrease of the ablation rate and the related ablation velocity. In order to verify the influence of the interaction between nanoparticles and high brilliant radiation on the ablation process, experimental and theoretical results of the laser ablation process of stainless steel with a single mode fiber laser are presented. For an ablation process with an intensity of 1.9×10 8 W/cm, it is shown that the average particle size is 9 nm. Within a further time-dependent analysis of the correlation between particle formation and ablation velocity, the beam of a test laser is directed through the vapour plume and the angle-dependent scattered radiation is detected. The results point out that a decrease of the ablation velocity corresponds with an increasing of the particle density within the ablation plume. © 2011 Elsevier Ltd. All rights reserved.

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