Begell House Inc.
Multiphase Science and Technology
MST
0276-1459
13
3&4
2001
EXPERIMENTAL METHODS: LOOKING CLOSELY AT BUBBLE NUCLEATION
33
10.1615/MultScienTechn.v13.i3-4.10
D. B. R.
Kenning
School of Engineering and Design, Brunel University, Uxbridge, West London, United Kingdom ; Department of Engineering Science Oxford University, Oxford, OX1 3PJ, UK
This paper notes the limited range of conditions for which detailed examinations of vapour bubble nucleation have been made. It discusses three methods of identifying nucleation sites from surface microgeometry and the use of liquid crystal thermography to examine the thermal conditions around sites and to track the interactions between them.
BUBBLE FORCES AND DETACHMENT MODELS
42
10.1615/MultScienTechn.v13.i3-4.20
G. E.
Thorncroft
Department of Mechanical Engineering, California Polytechnic State University, San Luis Obispo, California 93407, USA
James F.
Klausner
Department of Mechanical Engineering, Michigan State University, East Lansing, Michigan
48824, USA
The forces acting on growing and sliding vapor bubbles associated with heterogeneous boiling are identified. Closed form expressions have been suggested which approximate the magnitude of these forces. Beginning with Newton's second law, a rational basis is proposed for predicting vapor bubble detachment diameters and sliding trajectories in heterogeneous boiling. Model closure is achieved without introducing arbitrary constants. The predicted detachment diameters and sliding trajectories are compared with extensive data for pool and flow boiling in both horizontal and vertical orientations. Good agreement is observed for all cases considered.
POOL BOILING IN REDUCED GRAVITY
28
10.1615/MultScienTechn.v13.i3-4.30
P. Di
Marco
Dipartimento di Energetica, Universita di Pisa, via Diotisalvi 2,5 6126 Pisa. Italy
Walter
Grassi
Lo.Th.A.R. (Low gravity and Thermal Advanced Research Laboratory), DESTEC (Department of Energy, Systems, Territory and Constructions Engineering), University of Pisa−Largo Lucio Lazzarino, 56122 Pisa, Italy
The main outcomes of the worldwide experimental activity dealing with pool boiling in reduced gravity are summarized. The currently available experimental facilities and experimental opportunities are examined, the main results obtained by the various experimental teams are reviewed, and highlights of current and future applications of boiling in space systems are given. The work initiated by several groups around the world seems to indicate that pool boiling (especially the subcooled one) may be safely sustained in micro-g conditions with appropriate measures and that improvements in performances (e.g. by application of other force fields) are possible. However, due to the high cost and low availability of flight opportunities, and to their limitations in space and time, a final assessment has yet to be completed, and some results are still controversial.
In the second part of the paper, a review of the main pool boiling features and of the related models is carried out. The main effects of gravity and other force fields are stressed and compared with the above mentioned experimental results on earth and under reduced gravity conditions. The most commonly accepted viewpoints are reported for each aspect. The empirical correlations developed for boiling heat transfer in terrestrial conditions do not trivially extend their validity outside their range of application. Thorough experimentation in microgravity is thus needed to assess the performance of boiling heat transfer in such conditions.
It is believed that, after further experimental activity, it will be possible to design efficient boiling systems for future spacecrafts. The research in microgravity, by eliminating the dominant effect of the buoyancy forces, may also help clarify the role played by the various mechanisms in the boiling phenomenon.
CRITICAL HEAT FLUX IN SUBCOOLED FLOW BOILING- AN ASSESSMENT OF CURRENT UNDERSTANDING AND FUTURE DIRECTIONS FOR RESEARCH
26
10.1615/MultScienTechn.v13.i3-4.40
Satish G.
Kandlikar
Mechanical Engineering Department, Rochester Institute of Technology, Rochester, New York 14623, USA
Critical Heat Flux, or CHF, is an important condition that defines the upper limit of safe operation of heat transfer equipment employing boiling heat transfer in heat flux controlled systems. Although significant research has been conducted in this field, a clear understanding of the basic mechanisms leading to the CHF condition is still lacking. The present article covers the subcooled flow boiling CHF and reviews the parametric trends and photographic studies reported by earlier investigators. An in-depth review of the existing models is presented in light of these studies, and further research needs are identified.
FLOW REGIME BASED MODELLING OF TWO-PHASE HEAT TRANSFER
30
10.1615/MultScienTechn.v13.i3-4.50
Old-style models for evaporation (and condensation) ignore flow regime effects on heat transfer, concentrating only on the heat transfer mechanisms involved. This approach greatly limits their range of validity and reliability, resulting in prediction errors often surpassing 100% at typical design conditions. In the past few years, general thermal design methods have begun to emerge that are based on local two-phase flow patterns and the flow structure of the two-phases. In fact, the current trend is towards a completely global approach, that is one that provides a unified model of two-phase heat transfer, two-phase flow patterns (and prediction of their transitions as a map), void fraction and two-phase pressure drops. Thus, a new paradigm for modelling of two-phase flow and heat transfer is emerging in which there is no more justification for proposing new two-phase heat transfer correlations that are blind with respect to flow regime. The status of these new developments is discussed with particular emphasis on intube evaporation where most of the progress has been made, but shell-side boiling, falling film evaporation, intube condensation and two-phase pressure drops are also addressed.
DEVIATIONS FROM CLASSICAL BEHAVIOUR IN VERTICAL CHANNEL CONVECTIVE BOILING
30
10.1615/MultScienTechn.v13.i3-4.60
Geoffrey F.
Hewitt
Department of Chemical Engineering & Chemical Technology, Imperial College of Science, Technology & Medicine, Prince Consort Road, London SW7 2B Y, England, UK
Various aspects of forced convective boiling in vertical channels are discussed, with emphasis on deviations from the classical behaviour. These deviations arise from influence of relaminarisation, from instability effects at near-zero quality and from the influence of multi-component mixtures, hi high quality forced convective boiling, me annular flow regime is reached and, here, droplets are entrained from the liquid film and flow with the gas core. Thus, any prediction methodology for this region must, per force, take account of the behaviour of these droplets. Here, two aspects of droplet behaviour are reviewed, namely the magnitude of the fraction of liquid entrained as droplets at the onset annular flow and the influence of heat flux on entrainment and deposition of droplets. In all these areas, there are significant challenges to the present and future research investigator.