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2000
INFLUENCE OF CONVECTION ON NUCLEATE BOILING HEAT TRANSFER AROUND HORIZONTAL TUBES
15
10.1615/MultScienTechn.v12.i3-4.10
M.
Mann
Institut fur Technische Thermodynamik und Thermische Verfahrenstechnik, Universitat Stuttgart, Pfaffenwaldring 9, 70550 Stuttgart, Germany
Karl
Stephan
Institut fur Technische Thermodynamik und Thermische Verfahrenstechnik, Universitat Stuttgart, Pfaffenwaldring 9, 70550 Stuttgart, Germany
A detailed micro region model for heat transfer to single vapor bubbles at heated walls is combined with models of bubble population around a horizontal cylinder, and of heat transfer in the thermal boundary layer which is disturbed by the rising bubbles.
The micro region model takes into account the existence of an adsorbed film between wall and vapor and describes heat transfer in the narrow wedge shaped region adjacent to this film. Solution of the micro region model yields the growth rate and the heat transferred to the bubble.
Enhancement of convection by rising vapor bubbles on the lower side of the cylinder is given special attention in a convection model based on bubble population. The area where the thermal boundary layer is disturbed by rising bubbles is estimated. There a portion of the superheated liquid in the thermal boundary layer is removed and replaced by liquid of saturation temperature. The cold liquid warms up again by transient heat conduction, leading to an enhancement of heat transfer.
Nucleate pool boiling heat transfer coefficients from a horizontal cylinder were then modeled by the combination of heat transfer to bubbles adhering at the wall, described by the micro region model, and heat transfer due to convection, described by the convection model. The contribution of convection was found to depend strongly on the heat flux. In fully developed nucleate boiling the contribution of convection undergoes a maximum and decreases again at higher heat fluxes where evaporation becomes the dominating mechanism. The results obtained for i-pentane, propane and R114 at different reduced pressures pr are in good agreement with experimental data of others obtained with horizontal cylinders.
We can thus state that the model permits prediction of nucleate boiling heat transfer coefficients at flat plates and along horizontal cylinders. The only parameters needed are bubble site densities.
3-DIMENSIONAL DESCRIPTION OF THE MICROSTRUCTURE OF HEATED SURFACES IN NUCLEATE POOL BOILING
13
10.1615/MultScienTechn.v12.i3-4.20
Andrea
Luke
Laboratorium f. Warme- und Kaltetechnik, Universitat (GH) Paderborn; University of Kassel, Technical Thermodynamics
Elmar
Baumhogger
Warme- und Kaltetechnik, Universitat (GH) Paderborn Warburger Str. 100, D-33098 Paderborn, Germany
P.
Scheunemann
Warme- und Kaltetechnik, Universitat (GH) Paderborn Warburger Str. 100, D-33098 Paderborn, Germany
The trend for better understanding for fundamental processes connected with nucleate boiling heat transfer and the introduction of characteristic features of the heating surface and bubble formation in calculation methods for the heat transfer in boiling are supported by new developments in computer calculations and in measurement techniques. Because less quantitative information exists on the microstructure especially of the topography of the heated surfaces and on bubble formation than on heat transfer, little progress has been made so far in the development of theoretically based predictive methods, especially for heat transfer conditions relevant in practice.
To improve the situation the surface roughness and the topography of the heated surface is measured before and after the heat transfer measurements, which are studied simultaneously with the bubble formation. The paper focuses on the detailed description of the structure of the heated surface. New three-dimensional roughness measurement techniques without contact of the stylus with the surface open the possibility to get more detailed information for the cavities in their three-dimensional form, as it will be shown by selected examples.
The analysis of the surface roughness profile with an enveloped curve to calculate the size distributions of parameters describing the cavities possibly acting as nucleation sites can be transferred to the third dimension. The local distributions of potential nucleation sites are calculated and compared with the local distributions of the active sites.
POOL BOILING HEAT TRANSFER ON HORIZONTAL STEEL TUBES WITH DIFFERENT DIAMETERS
15
10.1615/MultScienTechn.v12.i3-4.30
Paul
Kaupmann
DBB Fuel Cell Engines, Universitat (GH) Paderbom, Warburger Str. 100, D-33098 Paderborn, Germany
Dieter
Gorenflo
Laboratorium für Wärme- und Kältetechnik, Universität -GH- Paderborn, D-4790 Paderborn, Pohlweg 55, Germany
Andrea
Luke
Laboratorium f. Warme- und Kaltetechnik, Universitat (GH) Paderborn; University of Kassel, Technical Thermodynamics
Compared with the many experimental investigations on nucleate boiling heat transfer in the literature, the papers devoted to the influence of the tube diameter are scarce, and very often the variations of either tube diameter or saturation pressure or heat flux are limited, or other important features of the heating surface, like wall material or roughness, have not been kept constant. Therefore, former pool boiling experiments on horizontal steel tubes with diameters of 7.6 mm and 88.4 mm have been extended to diameters of 4 mm and 30 mm. The tubes were made of mild steel (St35.8 or St37.8), with similar inner structure and their surfaces were emeried in the same manner, resulting in similar values of the mean roughness parameters. Propane and n-Hexane were used as boiling liquids. The saturation pressure varied between approx. 1 and 80 per cent of the critical pressure and the heat flux between 0.1 and 100 kW/m2.
The results show that the heat transfer coefficients α at constant normalized pressure p* = ps /pc (pc = critical pressure) do not vary much with tube diameter at intermediate to high heat fluxes; only at small heat fluxes near beginning nucleation, the differences in a may amount up to approx. ±25%, particularly at intermediate pressures (0.4 < p* < 0.04). The relative increase of the heat transfer coefficient with pressure or heat flux, respectively, is somewhat more pronounced for the small tubes than for the bigger ones. Both tendencies can be explained as effects of the bubbles which are produced on the lower parts of the tube and slide a certain distance along the tube surface on their way upwards. As a whole, the influence of the tube diameter found in these experiments is significantly smaller than according to correlations in the literature.
SINGLE BUBBLE HEAT TRANSFER IN SATURATED POOL BOILING OF FC-72
17
10.1615/MultScienTechn.v12.i3-4.40
Nagaraja
Yaddanapudi
University of Maryland, Department of Mechanical Engineering College Park, MD 20742, USA
Jungho
Kim
Department of Mechanical Engineering, University of Maryland, College Park, Maryland, USA
A study of single bubbles growing on a microscale heater array kept at nominally constant temperature was performed. The heater array consisted of 96 heaters each nominally 0.27 mm × 0.27 mm in size. The heater temperatures were kept constant using electronic feedback loops similar to those used in hot-wire anemometry, and the power required to do this was measured throughout the bubble departure cycle for each heater in the array. Data were obtained at a wall superheat of 22.5 °C during saturated pool boiling of FC-72. Bubbles were observed to nucleate from a single site on the surface at this temperature. Evaluation of various heat transfer models are made.
THE INFLUENCE OF SUBCOOLING ON POOL BOILING HEAT TRANSFER AND ITS ACOUSTIC MANIFESTATION
19
10.1615/MultScienTechn.v12.i3-4.50
Jens J.
Schroder
Institute for Thermodynamics, University of Hannover Hannover, Germany
Andreas
Bode
Institute for Thermodynamics, University of Hannover Hannover, Germany; BASF AG, Process Engineering, GCT/D - L540, D-67056 Ludwigshafen
This paper shows how subcooling influences even fully developed boiling heat transfer. This influence is audibly accompanied by a change of sound emission. Experimental results with water as test fluid reveal that the heat transfer coefficient in first approximation does not only depend on the heat flux and the reduced pressure but also on the Jakob number. The latter dependency is not uniformly directed - it reverses while subcooling is increased. Beyond a critical value, different boiling modes may substitute or supply each other in a spatio-temporal alternation on the heating surface. Consequently, boiling curves with contradicting behavior appear. A calculation method based on a simplified model is proposed and compared with experimental results.
PRESSURE PULSE GENERATION BY SINGLE BUBBLES IN SUBCOOLED POOL BOILING
17
10.1615/MultScienTechn.v12.i3-4.60
Andreas
Bode
Institute for Thermodynamics, University of Hannover Hannover, Germany; BASF AG, Process Engineering, GCT/D - L540, D-67056 Ludwigshafen
Jens J.
Schroder
Institute for Thermodynamics, University of Hannover Hannover, Germany
Noise generation in nucleate boiling systems is a widely known phenomenon. Nevertheless, the physics connecting bubble dynamics and pressure pulse generation does not seem to be fully understood. This paper reviews literature on boiling sound emission and focuses on pressure pulses generated by bubbles at a single nucleation site. Experimental results are presented for water vapor bubbles in subcooled boiling on a vertical copper surface at subatmospheric pressure. Signals of two hydrophones at different positions in the evaporator have been recorded. These signals are synchronized with high speed video recordings. Bubbles at a single nucleation site and at two simultaneously activated sites have been observed at a pressure of 40 kPa.
The experimental findings confirm results from a momentum balance for potential flow which yields a direct proportionality between pressure pulse amplitude and liquid level. Also, the influence of the free surface and the dependence of pulse shape on bubble volume acceleration and deceleration is shown.
ASSESSMENT OF CONVECTIVE POOL BOILING ON HORIZONTAL TUBES
26
10.1615/MultScienTechn.v12.i3-4.70
Keith
Cornwell
Department of Mechanical Engineering, Heriot-Watt University, Edinburgh EH14 4AS Scotland, UK
S. D.
Houston
Heriot-Watt University, Edinburgh EH14 4AS Scotland, UK
For boiling on horizontal tubes, understanding of the mechanisms and consequent development of correlations for heat transfer are important owing to the wide use of this geometrical arrangement in industrial systems. These mechanisms are reviewed and the importance of the bubbly flow layer and hence the tube diameter is demonstrated.
A correlation based on the convective situation, developed earlier by the authors, is examined and further evidence is presented that corroborates the approach. This general correlation may be used for any fluid and conditions within specified constraints. The nature of these constraints and the influence of some other factors are explored.
LOCAL FLOW BOILING HEAT TRANSFER COEFFICIENTS IN NARROW CONDUITS
16
10.1615/MultScienTechn.v12.i3-4.80
J. R.
Baird
Department of Chemical Engineering, University of Sydney
Z. Y.
Bao
Department of Chemical Engineering, University of Sydney
David F.
Fletcher
School of Chemical and Biomolecular Engineering, The University of Sydney, Australia
Brian S.
Haynes
School of Chemical and Biomolecular Engineering, The University of Sydney, NSW 2006, Australia
This paper presents local saturated and subcooled boiling heat transfer coefficients for flow in narrow conduits. Flow of Freon R11, HCFC123 and CO2 in circular pipes with diameters of 0.92 and 1.95 mm over a wide range of heat fluxes, mass fluxes, qualities and system pressures is examined. In all cases the saturated boiling heat transfer coefficient is found to be a function of the wall heat flux and not of the mass flux or quality, indicating the dominant heat transfer mechanism is nucleate boiling. The saturated data are fitted well by the nucleate pool boiling correlation of Cooper. The subcooled data suggest that heat transfer is also controlled by nucleation phenomena and a model based on the mixing of subcooled fluid into the superheated layer adjacent to the wall is proposed.
EXPERIMENTAL STUDY OF HEAT TRANSFER IN LAMINAR FALLING FILMS AT HIGH PRANDTL NUMBERS
16
10.1615/MultScienTechn.v12.i3-4.90
Kemal
Tuzla
Lehigh University, Department of Chemical and Biomolecular Engineering, 111 Research Drive, Room # B-323, Bethlehem PA 18015
Thomas R.
Palmer
Department of Chemical Engineering, Lehigh University Bethlehem, PA 18015, USA
John C.
Chen
Department of Chemical Engineering, Lehigh University, Bethlehem, PA 18015, USA
Heat transfer for laminar falling films that undergo surface evaporation is experimentally studied. The special test fluid used in this study provided single-component Prandtl numbers between 500 - 6,200, and viscosities of 86 - 650 cP. The new experimental data extended the ranges in Prandtl number and viscosity over previously existing studies by more than two orders of magnitude. Experimental results on heat transfer coefficients were compared to available predictive models. Among these, predictions of recently developed ATC model showed good agreement with the experimental data.
ESTIMATION OF MAJOR CORRELATIONS FOR FRICTIONAL PRESSURE DROP IN GAS-LIQUID TWO-PHASE FLOW IN HORIZONTAL PIPES USING PREDICTED FLOW REGIME INFORMATION
16
10.1615/MultScienTechn.v12.i3-4.100
Satoru
Momoki
Department of Mechanical Systems Engineering, Nagasaki University, Nagasaki, Japan
Avram
Bar-Cohen
Laboratory of the Thermal Management of Electronics, Department of Mechanical Engineering, University of Minnesota, Minneapolis, MN 55455, USA; Defense Advanced Research Projects Agency (DARPA), Microsystems Technology Office, University of Maryland, College Park, MD, USA
Arthur E.
Bergles
Department of Mechanical, Aerospace, and Nuclear Engineering, Rensselaer Polytechnic Institute, Troy, NY, USA; University of Maryland, College Park, MD, USA; Massachusetts Institute of Technology, Cambridge, MA, USA
While a large number of correlations have been proposed for the pressure drop encountered in gas-liquid two-phase flow in pipes, insufficient attention has been paid to the flow regimes associated with specific datasets used in developing these correlations. This may reduce the extrapolative accuracy of these correlations, and limit their utility in the design of future steam generation plants, air-conditioners with new alternative refrigerants, and other emerging applications of two-phase flow. In this study, five commonly used correlations for the frictional pressure drop component in gas-liquid two-phase flow in a horizontal smooth tube were examined for the experimental dataset sorted by the predicted flow regime using the Taitel et al method in order to explore the potential for using flow regime information to improve the predictive accuracy. The simple homogeneous model correlation was found to work very well for all of the entire annular data, with an average accuracy of ±27%, and the Chisholm correlation worked best for intermittent data, with agreement within ±24%, where the simple homogeneous correlation offers ±36%. These results may support the premise that consideration of some flow pattern indicator may prove effective in establishing better two-phase flow correlations.
HEAT TRANSFER DURING TRANSIENT COOLING OF HIGH TEMPERATURE SURFACE WITH AN IMPINGING JET
18
10.1615/MultScienTechn.v12.i3-4.110
Yuichi
Mitsutake
Department of Mechanical Engineering, Saga University, 1 Honjo Saga City, Saga 840-8502, Japan
Masanori
Monde
Department of Mechanical Engineering, Saga University, 1 Honjo Saga City, Saga 840-8502, Japan
An experimental study of transient boiling heat transfer during a cooling of a hot cylindrical block with an impinging water jet has been made at atmospheric pressure. The experimental data were taken for the following conditions: a degree of subcooling of ΔTsub = 20 − 80 K, a jet velocity of uj = 5 − 15 m/s, a nozzle diameter of dj = 2 mm and three materials of copper, brass and carbon steel. The block was initially and uniformly heated to about 250 °C and the transient temperatures in the block were measured at 8 locations in r-direction at two different depths from the surface during the cooling of hot block. The surface heat flux distribution with time was evaluated using a numerical analysis of 2D-heat conduction. Behavior of the wetting front, which is extending the nucleate boiling region outward, is observed with a high-speed video camera.
A position of wetting region is measured and it is correlated well with a power function of time. The changes in estimated heat flux and temperature were compared with the position of wetting region to clarify the effects of subcooling, jet velocity and thermal properties of block on the transient cooling.
NUMERICAL SIMULATION OF BUBBLE GROWTH IN NUCLEATE BOILING - EFFECTS OF SYSTEM PARAMETER
20
10.1615/MultScienTechn.v12.i3-4.120
Qiang
Bai
Department of Mechanical Engineering, Kyushu University, Hakozaki 6-10-1, Fukuoka, 812-8581, Japan
Yasunobu
Fujita
Department of Mechanical Engineering, Kyushu University, Hakozaki 6-10-1, Fukuoka, 812-8581, Japan
A numerical simulation scheme was developed in this paper to analyze the bubble growth process in nucleate boiling. A single bubble was assumed to grow axisymmetrically on an isothermal wall with heat being supplied through the microlayer beneath the growing bubble and from the superheated liquid outside the bubble cap. In the simulation no assumption was made on the bubble shape during its growth. Stephan and Hammer model was used for the evaporation in the micro-region beneath the bubble. In the macro-region around the bubble cap, transient Navier-Stokes and energy equations were solved using a two-dimensional arbitrary Lagrangian-Eulerian finite element method (ALE-FEM) based on quadrilateral elements. Numerical calculation was carried out in nucleate boiling under saturated, subcooled, terrestrial- and reduced-gravity conditions. The present simulation scheme offers a useful means to consistently investigate the growth process of an isolated vapor bubble in various conditions with a minimum number of assumptions and simplifications. It enables to describe quantitatively the flow and temperature fields around the growing bubble from the inception through departure to subsequent rising in stagnant fluid.
A NOTE ON THE BUBBLE GROWTH FORCE
18
10.1615/MultScienTechn.v12.i3-4.130
Cees W. M.
van der Geld
Department of Mechanical Engineering, Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven, The Netherlands
A boiling bubble at the verge of detaching from a plane, infinite wall experiences a hydrodynamic force due to its expansion, Fx. Such a bubble is modelled as a pair of spheres with radius R and growth rate R' in inviscid liquid, and Fx is computed using the Lagally theorem. The form of the Lagally theorem used differs in one term from an expression found in the literature. The exact expression for Fx is simplified in order to derive a simple expression that facilitates the prediction of detachment. In the case of diffusion controlled bubble growth, Fx is shown to be approximately given by c2πρLR2R'2, where ρL is the liquid mass density and c2 ~ 1.11. This prediction is compared with experimental fits of the literature.
INFLUENCE OF MICROSCALE CONCENTRATION GRADIENTS IN NUCLEATE BOILING HEAT TRANSFER OF BINARY MIXTURES
15
10.1615/MultScienTechn.v12.i3-4.140
Jurgen
Kern
Darmstadt University of Technology, Chair of Technical Thermodynamics, Petersenstr. 30, 64287 Darmstadt, Germany
Peter
Stephan
Institute for Technical Thermodynamics, Technische Universität Darmstadt,
Alarich-Weiss-Str. 10, 64287 Darmstadt, Germany
Nucleate boiling heat transfer coefficients of binary mixtures can be calculated using a micro region model. The model is described and assessed by comparing calculated heat transfer coefficients with experimental values. The model includes the governing physical phenomena, such as the meniscus curvature, the adhesion pressure, and the interfacial thermal resistance as well as the local variation of composition and liquid-vapor equilibrium and the Marangoni effect. Due to the preferential evaporation of one component of the mixture, strong concentration gradients occur close to the heated wall. The influence of these microscale concentration gradients is investigated. By means of various calculations the physical phenomena are quantified and their relevance in the nucleate boiling process is evaluated.
POOL BOILING HEAT TRANFSER OF NEW ENVIRONMENTALLY HARMLESS REFRIGERANT MIXTURES R32 / R125
16
10.1615/MultScienTechn.v12.i3-4.150
Erich
Hahne
Institut fur Thermodynamik und Warmetechnik, Universitat Stuttgart, 70550 Stuttgart, Germany
Jianya
Shen
Institut fur Thermodynamik und Warmetechnik, Universitat Stuttgart, 70550 Stuttgart, Germany
Klaus K.
Spindler
Pfaffenwaldring 6 D-70550 Stuttgart
A STUDY OF DRYOUT IN ANNULAR FLOW OF SINGLE COMPONENT HYDROCARBONS AND THEIR MIXTURES
19
10.1615/MultScienTechn.v12.i3-4.160
Jader R.
Barbosa, Jr.
Polo Research Laboratories for Emerging Technologies in Cooling and Thermophysics, Department of Mechanical Engineering, Federal University of Santa Catarina 88040-900, Trindade, Florianópolis, SC, Brazil
T.
Kandlbinder
Department of Chemical Engineering and Chemical Technology, Imperial College of Science, Technology and Medicine, London, SW7 2BY
Geoffrey F.
Hewitt
Department of Chemical Engineering & Chemical Technology, Imperial College of Science, Technology & Medicine, Prince Consort Road, London SW7 2B Y, England, UK
Experimental and modelling results on critical heat flux in upward flow in round tubes are presented. The data were collected at a high pressure boiling facility using single component hydrocarbons (pentane and iso-octane), binary and ternary hydrocarbon mixtures (pentane/iso-octane and pentane/hexane/iso-octane). The system pressure ranged from 2 to 11 bar, the total mass fluxes from 100 to 550 kg/m2s and the wall heat flux from 30 to 100 kW/m2. The 8.68 m long vertical test section is made of 321 stainless steel. The inner and outer diameters are 25.4 and 38.0 mm, respectively. When the results are compared with predictions from an annular flow modelling code, an over-prediction (i.e., non-conservative) of the dryout quality for both single component and multicomponent cases is observed. Reasons for such discrepancies are explored and corrections to the calculation methodology are proposed in the context of droplet entrainment and deposition calculations.