"ZOOM" SEMINAR

School of Mechanical Engineering Seminar
Wednesday, December 2, 2020 at 14:00

Free-surface jet heat transfer reexamined: from indifferent regimes to the re-emergence of off-center peaks

Ron Harnik

Msc of Dr. Herman Haustein

Laminar jet impingement is an efficient method for heat transfer processes, though much of its hydrodynamics and the resulting convection are still not fully understood. In this work, it is shown that, stagnation-point heat transfer depends directly on the near-axis radial acceleration, varying strongly with nozzle diameter, normalized nozzle length, normalized nozzle-to-plate spacing, and flow rate. Studying the hydrodynamics of all stages in the jet’s history up-to plate impingement, therefore, led to the development of a universal heat transfer model.

In this study experiments and two-phase flow simulations were conducted to determine the flow regime of a jet inside a nozzle, during free flight, upon impingement and subsequent wall flow. From the results it was seen that existing theory deals only with two extremes, and therefore a general description is required. Relying on previous pipe-flow analysis by our group, the crucial dynamics of the jet’s centerline velocity decay and evolution of the jet width were dealt with under varying levels of flow rate, issuing velocity-profile development and relaxation, surface tension and gravity. In other words, the dependence of the arriving jet’s key characteristics on all relevant crucial dimensionless numbers (Re, L/d, H/d, We & Fr, accordingly) was addressed. For the jet’s impingement, a previous streamline-bending analysis was adapted to the present case of a free-surface jet, thereby tying the characteristics of the velocity profile arriving at the wall and transitioning from free flight to stagnation flow. The present analysis recovers the radial acceleration’s dictation of stagnation heat transfer, as well as the downstream radial distribution. The model is seen to be valid up to the point of boundary layer emergence from the wall-jet film, beyond which Watson’s classical solution is seen to be valid if appropriately scaled.

Finally, it is seen that the present analysis is physically sound and is able to continuously capture the occurrence or re-emergence of an off-center peak in the heat transfer which occurs for under-developed or over-relaxed arrival profiles – a phenomena previously dealt with only in a case-specific manner. The study has also led to the clarification and definition of a criterion for the occurrence of the “indifferent jet regime” – wherein gravity’s acceleration is countered by the profile relaxation to generate a constant-centerline velocity jet. Such a jet’s stagnation heat transfer is indifferent to the nozzle-to-plate spacing, and has multiple useful applications.

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https://zoom.us/j/96584758181?pwd=WC9PMXdsYzJ3NFdEN2Q5ZUtOZEVjdz09 The meeting will be recorded and made available on the School’s site.

"ZOOM" SEMINAR

School of Mechanical Engineering Seminar
Monday, December 21, 2020 at 14:00

Understanding and controlling icing on surfaces: not really a piece of cake

Dr. Carlo Antonini

Department of Materials Science

University of Milano-Bicocca

Abstract presentation

Icing has long been recognized as a serious hazard for safety and for functioning of systems in diverse areas such as transportation, power and communication systems, infrastructures, and even domestic or commercial appliances. The control of water behavior at the interface through the tailoring of surface wetting properties represents a great scientific and technological challenge and opportunity, which can be used as a strategy to control and mitigate ice formation on surfaces. During the lecture, I will present his research activities focused on understanding the interaction mechanism between liquid and solid surfaces in freezing conditions. Focus will be given to the effects of surface wetting and topography on ice nucleation, to the wetting behavior of superhydrophobic surfaces in icing conditions, and to ice adhesion, highlighting the role of environmental conditions where relevant. A brief discussion of practical issues, challenges, and innovations in using non-wetting coatings will also presented.

Short Bio

I received the BSc in Aerospace Engineering (2004) and the MSc in Aeronautical Engineering (2007), from Politecnico di Milano, Italy, and the PhD in Technologies for Energy and Environment from University of Bergamo (2011), Italy, with a thesis titled “Superhydrophobicity as a strategy against icing”. In 2012, I received support from the European Research Council (ERC) to join ETH Zurich, Switzerland, as a Marie Curie Fellow; I worked in the Laboratory of Thermodynamics in Emerging Technologies on the project “ICE2: ICEphobicity for severe ICing Environments”. In March 2015 I joined EMPA – Swiss Federal Laboratory for Material Science and Technology – as scientist, focusing on the control of surface wetting properties of cellulose-based materials for various engineering applications, ranging from liquid separation (oil remediation) to thermal insulation. Since 2017 I am the scientific advisor of ApiTech, an italian startup supporting innovation in SMEs. In September 2018 I joined the Department of Materials Science at the University of Bicocca-Milano (Italy), as tenure-track senior assistant professor, with the support of a Rita Levi Montalcini Fellowship. At UNIMIB I have founded the Surface Engineering and Fluid Interfaces Laboratory (SEFI Lab). I am coordinating the MSCA-ITN project SURFICE - Smart surface design for efficient ice protection and control (2021-2024), involving 13 PhD students, 7 universities and 4 companies.

Affliliation and contact info

Carlo Antonini, PhD

Department of Materials Science

University of Milano-Bicocca

U5 - Via Roberto Cozzi, 55, 20125 Milano MI

phone: +39 02 6448 5188

carlo.antonini@unimib.it

Links

https://en.unimib.it/carlo-antonini

SEFI Lab https://sefilab.mater.unimib.it/

SURFICE https://cordis.europa.eu/project/id/956703 

https://zoom.us/j/96584758181?pwd=WC9PMXdsYzJ3NFdEN2Q5ZUtOZEVjdz09 The meeting will be recorded and made available on the School’s site.