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The influence of system pressure on bubble coalescence in nucleate boiling

Sielaff, Axel and Dietl, J. and Herbert, S. and Stephan, Peter (2012):
The influence of system pressure on bubble coalescence in nucleate boiling.
In: Proceedings ECI 8th Int. Conference on Boiling and Condensation Heat Transfer, [Conference or Workshop Item]

Abstract

Boiling is one of the most effective heat transfer mechanisms. In spite of a long time of research, the physical fundamentals are still not sufficiently understood. Pursuing the objective to predict heat transfer based on physical and geometrical properties, experimental and numerical investigations are conducted at the institute of the authors. The focus of the presented research is the coalescence of two single bubbles under varying pressure conditions. In the experiment a thin stainless-steel foil is used as a Joule heater. The experiments were performed in a pressure range of 300--1000 mbar using FC72 as working fluid. Two types of heaters with a distance between two artificial nucleation sites of 300 \textgreekmm (type 3) and 500 \textgreekmm (type 5) were used. The experimental results indicate a strong dependence of the occurrence of bubble coalescence on pressure. For the type 5 heater, a Gaussian distribution for the coalescence frequency when plotted over pressure is observed. Experimental results with the type 3 heater show a similar distribution of the frequency with a shifted maximum. Further, it is shown that during bubble coalescence a small droplet can remain inside the bubble and enhance the heat transfer, which is attributed to an additional thin film region. The formation of this remaining droplet is sensitive to system pressure. Numerical investigations of bubble coalescence were conducted with the computational fluid dynamics (CFD) software OpenFOAM. In OpenFOAM, dynamic mesh handling allows high spatial resolution at the phase boundary, which is captured with the volume-of fluid method. Evaporation and a subgrid microscale model were implemented in the flow solver to account for evaporation at the phase boundary and the three-phase contact line. The results show a strong dependence of bubble dynamics and coalescence on contact angle and bubble growth rate. Although it was possible to observe the creation of the residual droplet, more effort needs to be put into finding appropriate initial conditions.

Item Type: Conference or Workshop Item
Erschienen: 2012
Creators: Sielaff, Axel and Dietl, J. and Herbert, S. and Stephan, Peter
Title: The influence of system pressure on bubble coalescence in nucleate boiling
Language: English
Abstract:

Boiling is one of the most effective heat transfer mechanisms. In spite of a long time of research, the physical fundamentals are still not sufficiently understood. Pursuing the objective to predict heat transfer based on physical and geometrical properties, experimental and numerical investigations are conducted at the institute of the authors. The focus of the presented research is the coalescence of two single bubbles under varying pressure conditions. In the experiment a thin stainless-steel foil is used as a Joule heater. The experiments were performed in a pressure range of 300--1000 mbar using FC72 as working fluid. Two types of heaters with a distance between two artificial nucleation sites of 300 \textgreekmm (type 3) and 500 \textgreekmm (type 5) were used. The experimental results indicate a strong dependence of the occurrence of bubble coalescence on pressure. For the type 5 heater, a Gaussian distribution for the coalescence frequency when plotted over pressure is observed. Experimental results with the type 3 heater show a similar distribution of the frequency with a shifted maximum. Further, it is shown that during bubble coalescence a small droplet can remain inside the bubble and enhance the heat transfer, which is attributed to an additional thin film region. The formation of this remaining droplet is sensitive to system pressure. Numerical investigations of bubble coalescence were conducted with the computational fluid dynamics (CFD) software OpenFOAM. In OpenFOAM, dynamic mesh handling allows high spatial resolution at the phase boundary, which is captured with the volume-of fluid method. Evaporation and a subgrid microscale model were implemented in the flow solver to account for evaporation at the phase boundary and the three-phase contact line. The results show a strong dependence of bubble dynamics and coalescence on contact angle and bubble growth rate. Although it was possible to observe the creation of the residual droplet, more effort needs to be put into finding appropriate initial conditions.

Divisions: 16 Department of Mechanical Engineering
16 Department of Mechanical Engineering > Institute for Technical Thermodynamics (TTD)
Exzellenzinitiative
Exzellenzinitiative > Clusters of Excellence
Zentrale Einrichtungen
Exzellenzinitiative > Clusters of Excellence > Center of Smart Interfaces (CSI)
Event Title: Proceedings ECI 8th Int. Conference on Boiling and Condensation Heat Transfer
Date Deposited: 17 Mar 2015 14:57
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