Reiner Hohl

Bio: Reiner Hohl is an academic researcher from Technical University of Berlin. The author has contributed to research in topics: Boiling & Transient (oscillation). The author has an hindex of 4, co-authored 4 publications receiving 51 citations.

Evaporation/Boiling in Thin Capillary Wicks (l)—Wick Thickness Effects

Abstract: Presented here is the first of a two-part investigation designed to systematically identify and investigate the parameters affecting the evaporation/boiling and critical heat flux (CHF) from thin capillary wicking structures. The evaporation/boiling heat transfer coefficient, characteristics, and CHF were investigated under steady-state conditions for a variety of capillary structures with a range of wick thicknesses, volumetric porosities, and mesh sizes. In Part I of the investigation we describe the wicking fabrication process and experimental test facility and focus on the effects of the capillary wick thickness. In Part II we examine the effects of variations in the volumetric porosity and the mesh size as well as presenting detailed discussions of the evaporation/boiling phenomena from thin capillary wicking structures. An optimal sintering process was developed and employed to fabricate the test articles, which were fabricated using multiple, uniform layers of sintered isotropic copper mesh. This process minimized the interface thermal contact resistance between the heated wall and the capillary wick, as well as enhancing the contact conditions between the layers of copper mesh. Due to the effective reduction in the thermal contact resistance between the wall and capillary wick, both the evaporation/ boiling heat transfer coefficient and the critical heat flux (CHF) demonstrated dramatic improvements, with heat transfer coefficients up to 245.5 kW/m 2 K and CHF values in excess of 367.9 W/cm 2 , observed. The experimental results indicate that while the evaporation/boiling heat transfer coefficient, which increases with increasing heat flux, is only related to the exposed surface area and is not affected by the capillary wick thickness, the CHF for steady-state operation is strongly dependent on the capillary wick thickness and increases proportionally with increase in the wick thickness. In addition to these observations, the experimental tests and subsequent analysis have resulted in the development of a new evaporation/boiling curve for capillary wicking structures, which provides new physical insights into the unique nature of the evaporation/boiling process in these capillary wicking structures. Sample structures and fabrication processes, as well as the test procedures are described in detail and the experimental results and observations are systematically presented and analyzed.

Evaporation/Boiling in Thin Capillary Wicks (II)—Effects of Volumetric Porosity and Mesh Size

Abstract: Presented here is the second of a two-part investigation, designed to systematically identify and investigate the parameters affecting the evaporation from and boiling within, thin capillary wicking structures with a range of volumetric porosities and mesh sizes. The experimental studies were investigated under steady-state conditions at atmospheric pressure. Part I of the investigation described the wicking fabrication process and experimental test facility, and focused on the effects of the capillary wick thickness (ASME J. Heat Transfer, 128, pp. 1312-1319). In Part II, we examine the effects of variations in the volumetric porosity and the mesh size. The experimental results presented here indicate that the critical heat flux (CHF) was strongly dependent on both the mesh size and the volumetric porosity; while the evaporation/boiling heat transfer coefficient was significantly affected by mesh size, but not strongly dependent on the volumetric porosity. The experimental results further illustrate that the menisci at the CHF are located in the corners, formed by the wire and the heated wall and between the wires in both the vertical and horizontal directions. The minimum value of these three menisci determined the maximum capillary pressure generated through the capillary wick. The experimental results and observations are systematically presented and analyzed, and the local bubble and liquid vapor interface dynamics are examined theoretically. Based on the relative relationship between the heat flux and superheat, classic nucleate boiling theory, and the visual observations of the phase-change phenomena, as well as by combining the results obtained here with those obtained in Part I of the investigation, the evaporation/boiling heat transfer regimes in these capillary wicking structures are identified and discussed.

Model-Based Experimental Analysis of Kinetic Phenomena in Multi-Phase Reactive Systems

Abstract: This contribution will introduce a novel concept for mechanistic modelling of complex kinetic phenomena and will explore its potential for multi-phase reaction systems modelling. The approach aims at the integration of high resolution measurements, modelling on multiple scales and the formulation and solution of inverse problems in a unifying framework. An incremental and iterative approach based on gradual refinement of experimental techniques, mathematical models and identification problems forms the core of the suggested methodology. The foundations of the approach developed in a collaborative interdisciplinary research centre at RWTH Aachen will be reviewed. Illustrative examples including chemical reaction kinetics, diffusion in liquids and heat transfer at a falling liquid film are presented.