Sebastian Burgmann

Laser-optical methods for transport studies in low temperature fuel cells

Abstract: The flow distribution and structure in a polymer electrolyte membrane fuel cell (PEMFC) or a direct methanol fuel cell (DMFC) strongly affects the performance of the cell. The main functions of the micro-channel flow field are the homogeneous distribution of the reactants, an enhancement of an effective mixing and the expulsion of the reaction products such as liquid water. To properly design the flow field of a cell and the manifold of a stack, detailed experimental velocity measurements are essential. Non-intrusive Laser-optical measurement techniques such as particle-image velocimetry, micro-particle image velocimetry, laser Doppler velocimetry, and molecular-tagging velocimetry qualify for such investigations and are more and more used in fuel cell research. The basic measurement principles of these measurement techniques are explained and the relevant applications in fuel cell research are depicted and critically reviewed. A forecast to the future use of these measurement techniques in fuel cell research is risked and additional sources of information concerning the measurement techniques and fluid mechanical problems are presented.

Micro Particle-Image-Velocimetry für Gasströmungen in Mikrokanälen

Zusammenfassung Es wird ein Messverfahren vorgestellt, mit dem sich die Geschwindigkeitsverteilung einer Mikrokanalströmung in der Gas- oder Flüssigphase bestimmen lässt. Diese Micro Particle-Image-Velocimetry (μPIV) genannte Technik ist ein berührungsloses, laseroptisches Geschwindigkeitsmessverfahren, das auf der Detektion von kleinsten der Strömung zugegebenen Partikeln bzw. deren Verschiebung beruht. Die μPIV ist insbesondere beim Einsatz in Gasströmungen entscheidend von den Partikeleigenschaften wie Fluoreszenz, Partikelgrößenverteilung und Partikelkonzentration abhängig. Das Partikelfolgeverhalten stellt einen kritischen Parameter der μPIV in Gasströmung dar. Die Messtechnik wird hier exemplarisch in einem optisch transparenten Mikrokanal mit einer 90°-Umlenkung angewendet, um die Anwendbarkeit der μPIV sowohl in einer Wasser- als auch einer Gasströmung zu demonstrieren. Abstract We present a measurement technique that allows the determination of the velocity distribution of a micro-channel flow in the gas or liquid phase. Micro Particle-Image Velocimetry (μPIV) is a non-intrusive, laser-optical velocity measurement technique that is based on the determination of the displacement of small particles that are added to the flow. Especially in the gas phase μPIV strongly depends on particle characteristics like fluorescence, particle size distributions, and particle concentrations. The fidelity of the particles to adequately follow the flow is one of the key parameters of μPIV. The measurement technique is exemplarily applied in an optically transparent micro-channel with a 90° elbow to demonstrate the applicability of μPIV in a water flow as well as in a gaseous flow.

Time-Resolved Two- and Three-Dimensional Measurements of Transitional Separation Bubbles

Advanced methods of the Particle-Image Velocimetry (PIV) technique such as Time-Resolved and Stereo-Scanning PIV have been applied to investigate the separation bubbles on top of a finite circular cylinder and on the suction side of an SD7003 airfoil. A kind of three-dimensional vortex-shedding mechanism associated with the inviscid Kelvin-Helmholtz-instability has been identified to govern the flow field in both cases. The time-averaged velocity distribution is strongly affected by these vortex cascades although the unsteadiness of the bubbles due to the band of frequencies in the spectra of the vortex-shedding mechanism is hidden. Several analyzing methods are used and/or developed and the capabilities of more sophisticated experimental investigations of the three-dimensional flow field associated with separation bubbles are outlined.

Partially Vaned Diffuser With Variable Cross-Section for Centrifugal Fans

The present research focuses on the efficiency improvement at part-load of a centrifugal fan for a 30 kW fuel cell combined heat and power (CHP) unit. For this purpose, the fan stage is equipped with a partially vaned diffuser with a variable crosssectional area using a moving backplate. The design and the performance of the partially vaned diffuser with a variable cross-sectional area are described in the first part of this paper. The performance results are compared to measurements of the same centrifugal fan with a vaneless diffuser carried out for the previous investigation. For the second part, the influence of the variable cross-sectional area on the diffuser flow field is investigated using optical PIV (Particle Image Velocimetry) measurements and CFD (Computational Fluid Dynamics) simulations. The combination of a variable cross-section, partially vaned diffuser was able to achieve a 10 percent increase in pressure ratio, a 5 percentage points increase in part-load efficiency while maintaining the whole operating range of the vaneless, constant cross section reference design. INTRODUCTION This paper presents the second part of an ongoing investigation on the performance improvement of a centrifugal fan for the air supply of a 30 kW fuel cell system [1]. The objective of this research is the improvement of the part-load operation of the centrifugal fan by means of the variability of the crosssectional area of the radial diffuser and the volute. Through improving the performance, the parasitic power consumption of the fan can be reduced, resulting in an increased part-load efficiency of the fuel cell system. As a part of the transition process in the energy supply, the demand for decentralized energy conversion is growing. Small energy conversion units have to operate under variable power demand at high efficiencies, but more importantly, they have to be cost-effective in order to achieve customer acceptance. Particularly small fuel cell systems show great potential for the decentralized power and heat co-generation, with regards to the decreasing heat demand of newly constructed and modernized buildings. They achieve high electrical efficiencies and high power-to-heat ratios for the whole operating range. In contrast to automotive fuel cell applications, these stationary systems are operated at moderate pressure levels because of the stack size, and for this reason the power density is less important. Consequently, radial fans are a suitable solution for the cathode air supply. They achieve high peak efficiencies at moderate pressure ratios, but more importantly, they are a low-cost, state-of-the-art technology. One major disadvantage of radial fans is the rapid decreasing efficiency at off-design operation. This operating behavior is a significant penalty for the overall efficiency of the fuel cell system at part-load operation, as described among others by Kulp et al. [2]. For this reason, the development of simple and cost-effective performance stabilizing devices is of increasing importance, not only with regard to stationary fuel cells. There are a number of measures and devices to improve the part-load operation of turbomachines, as already presented in a previous paper [1]. However, most of them are either very Proceedings of ASME Turbo Expo 2017: Turbomachinery Technical Conference and Exposition GT2017 June 26-30, 2017, Charlotte, NC, USA