Experimental Investigation of Single Bubbles and Bubble Interactions in Nucleate Boiling.
Technische Universität, Darmstadt
[Ph.D. Thesis], (2014)
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|Item Type:||Ph.D. Thesis|
|Title:||Experimental Investigation of Single Bubbles and Bubble Interactions in Nucleate Boiling|
Boiling processes are widely used in technical applications. The chemical industry, power plant technology, and refrigeration engineering are just a few examples. Intensive research on pool boiling processes has been undertaken for decades. Nevertheless, the physical phenomena are still not sufficiently understood. This is mainly caused by the large number of influencing factors and the wide range of length and time scales boiling processes act on.
Especially through the research on small length scales a better understanding of the underlying physical phenomena should be achieved. The subject of the present thesis is the experimental investigation of boiling processes with single bubbles and bubble interactions. In a further step, the reliability of the results obtained for a few interacting bubbles is tested on a technical length scale.
Within the framework of this thesis an experimental setup is designed and fabricated. By use of optical measurement methods (black/white and infrared) the shape of the bubbles and the temperature close to the heater surface are measured simultaneously. Within the experimental setup and method emphasis is put on the validity of the obtained data (especially the infrared measurements). The test setup is based on a thin stainless steel foil used as a Joule heater. The working fluid is FC-72. Almost all experiments are performed at subatmospheric pressure.
In this thesis general results for boiling experiments with single bubbles, for example concerning the departure diameter and frequency, are presented. It is observed that for single bubbles a thin liquid layer can remain underneath a bubble after nucleation. By comparing bubble growth with and without this layer a first statement of the validity of the contact line and microlayer model is achieved. Experiments with bubbles coalescing show a clear dependency of the coalescence frequency on the system pressure, partly following a probability distribution. In comparison of a case with and without bubble coalescence an increased heat transfer is found for the coalescence case. This effect is mainly caused by hydrodynamic phenomena and sometimes also by the formation of residual droplets inside large bubbles. It is shown that the effects observed for the coalescence of two bubbles is not transferable to global boiling processes. With the help of a Monte-Carlo simulation these phenomena can be explained by different coalescence behavior. The experimental results furthermore give a possible explanation for the deviations between experimental and analytic bubble departure diameters due to the dynamic effects during bubble nucleation.
|Place of Publication:||Darmstadt|
|Uncontrolled Keywords:||pool boiling, bubble coalescence, boiling model, contactline evaporation, microlayer evaporation|
|Classification DDC:||600 Technik, Medizin, angewandte Wissenschaften > 620 Ingenieurwissenschaften|
|Divisions:||16 Department of Mechanical Engineering > Institute for Technical Thermodynamics (TTD)|
|Date Deposited:||06 Jun 2014 08:23|
|Last Modified:||06 Jun 2014 08:23|
|Referees:||Stephan, Prof. Dr. Peter and Luke, Prof. Dr. Andrea|
|Refereed:||12 February 2014|