Michael Heinz

Bio: Michael Heinz is an academic researcher from Technische Universität Darmstadt. The author has contributed to research in topics: Wetting & Imbibition. The author has an hindex of 2, co-authored 3 publications receiving 17 citations.

Influence of System Pressure on Pool Boiling Regimes on a Microstructured Surface Compared to a Smooth Surface

Abstract: This study focuses on the influence of the system pressure on pool boiling regimes on a microstructured surface compared to a smooth surface. The microstructured surface consists of copper wires wi...

Capillary rise and evaporation of a liquid in a corner between a plane and a cylinder: A model of imbibition into a nanofiber mat coating

Abstract: Wetting of surfaces with porous coating is relevant for a wide variety of technical applications, such as printing technologies and heat transfer enhancement. Imbibition and evaporation of liquids on surfaces covered with porous layers are responsible for significant improvement of cooling efficiency during drop impact cooling and flow boiling on such surfaces. Up to now, no reliable model exists which is able to predict the kinetics of imbibition coupled with evaporation on surfaces with porous coatings. In this work, we consider one of possible mechanisms of imbibition on a substrate covered by a nanofiber mat. This is the capillary pressure-driven flow in a corner formed between a flat substrate and a fiber attached to it. The shape and the area of the cross-section occupied by the liquid as well as the capillary pressure change along the flow direction. A theoretical/numerical model of simultaneous imbibition and evaporation is developed, in which viscosity, surface tension and evaporation are taken into account. At the beginning of the process the imbibition length is proportional to the square root of time, in agreement with the Lucas-Washburn law. As the influence of evaporation becomes significant, the imbibition rate decreases. The model predictions are compared with experimental data for imbibition of water-ethanol mixtures into nanofiber mat coatings.

Experimental and semi-analytical investigation of heat transfer in nucleate pool boiling by considering surface structuring methods

Abstract: Pool boiling is the process in which the heating surface is submerged in a large body of stagnant liquid. In the present work, heat transfer in nucleate pool boiling is modeled both experimentally ...

Capillary Flow with Evaporation in Open Rectangular Microchannels

Abstract: Numerous applications rely upon capillary flow in microchannels for successful operation including lab-on-a-chip devices, porous media flows, and printed electronics manufacturing. Open microchannels often appear in these applications, and evaporation of the liquid can significantly affect its flow. In this work, we develop a Lucas-Washburn-type one-dimensional model that incorporates the effects of concentration-dependent viscosity and uniform evaporation on capillary flow in channels of a rectangular cross section. The model yields predictions of the time evolution of the liquid front down the length of the microchannel. For the case where evaporation is absent, prior studies have demonstrated better agreement between model predictions and experimental observations in low-viscosity liquids when using a no-slip rather than a no-stress boundary condition at the upper liquid-air interface. However, flow visualization experiments conducted in this work suggest the absence of a rigidified liquid-air interface. The use of the no-stress condition results in overestimation of the time evolution of the liquid front, which appears to be due to underestimation of the viscous forces from (i) the upper and front meniscus morphology, (ii) dynamic contact angle effects, and (iii) surface roughness, none of which are accounted for in the model. When high-viscosity liquids are considered, the large bulk viscosity is found to suppress these factors, resulting in better agreement between model predictions using the no-stress condition and experiments. Model predictions are also compared to prior experiments involving poly(vinyl alcohol) in the presence of evaporation by using the evaporation rate as a fitting parameter. Scaling relationships obtained from the model for the dependence of the final liquid-front position and total flow time on the channel dimensions and rate of uniform evaporation are found to be in good agreement with experimental observations.

Pool boiling review: Part II – Heat transfer enhancement

Abstract: Heat transfer enhancement by surface modification has been extensively studied in the last twenty years. However, there remains a large discrepancy among researchers on the performance of enhanced surfaces even for the same fluid and surface preparation technique. The reasons of this discrepancy are not understood and are not discussed in past papers, including paper reviews. Part II of this two-part paper aims to present a detailed assessment of pool boiling heat transfer enhancement, relating this to Part I [1], which presented a critical assessment of fundamental concepts of heterogeneous nucleation. Current challenges in evaluating the performance of enhanced surfaces is first discussed. The performance of smooth and roughened surfaces is then discussed and the effect of fluid type is explained. Pool boiling data of two fluids, namely water and FC-72, on two enhanced substrate materials, i.e. copper and silicon were digitized and assessed in order to elucidate the reason for the discrepancy in published works and present future recommendations for heat transfer enhancement. The heat transfer enhancement mechanisms adopted by researchers were presented and critically discussed and compared. The paper contributes to the understanding of the effect of fluid-surface combinations and suggest guidelines for researchers to consider when evaluating the performance of enhanced surfaces. This will help the research community and industry to conclude on the best surface structure and surface manufacturing technique matching particular fluid of interest.

Nucleate pool boiling heat transfer above laser machining heating surfaces with different micro-cavity geometric shape for water-aluminum oxide nanofluid

Abstract: The present paper reports the experimental investigation of nucleate pool boiling heat transfer above heating surface with the creation of new geometric micro-cavities. Cylindrical, cubic, and pent...