Nucleate and transition boiling in narrow horizontal spaces

Nucleate and transition boiling are performed in a horizontal narrow space between a heated upward-facing copper disk and an unheated surface for saturated n-pentane. The heat flux and the wall temperature are determined by mean of an inverse heat conduction method. The influence of the confinement on the boiling curves and the flow patterns are analysed. Characteristic instabilities are observed at low heat flux and during the transition regime.     

Effect of surfactant concentration on saturated flow boiling in vertical narrow annular channels

Saturated flow boiling of environmentally acceptable nonionic surfactant solutions of Alkyl (8–16) was compared to that of pure water. The concentration of surfactant solutions was in the range of 100–1000 ppm. The liquid flowed in an annular gap of 2.5 and 4.4 mm between two vertical tubes. The heat was transferred from the inner heated tube to two-phase flow in the range of mass flux from 5 to 18 kg/m² s and heat flux from 40 to 200 kW/m². Boiling curves of water were found to be heat flux and channel gap size dependent but essentially mass flux independent. An addition of surfactant to the water produced a large number of bubbles of small diameter, which, at high heat fluxes, tend to cover the entire heater surface with a vapor blanket. It was found that the heat transfer increased at low values of relative surfactant concentration C/C₀, reaches a maximum close to the value of C/C₀ = 1 (where C₀ = 300 ppm is the critical micelle concentration) and decreased with further increase in the amount of additive. The dependence of the maximal values of the relative heat transfer enhancement, obtained at the value of relative concentration of C/C₀ = 1, on the boiling number Bo may be presented as single curve for both gap sizes and the whole range of considered concentrations.     

Nucleate boiling and critical heat flux of HFE-7100 in horizontal narrow spaces

The pool boiling heat transfer and critical heat flux CHF of saturated HFE-7100 at atmospheric pressure on a confined smooth copper surface were experimentally studied. The horizontal upward boiling surface was confined by a face-to-face parallel unheated surface. We analysed the effects obtained by changing the diameter of the unheated surface and the gap between the boiling surface and the adiabatic surface. The gap values investigated were s = 0.5, 1.0, 2.0, 3.5 mm. To confine the circular boiling surface (d = 30 mm), two different Plexiglas discs were used: one with a diameter D = 30 mm, equal to that of the copper boiling surface, and the other with a diameter D = 60 mm, equal to that of the overall test section support. For each configuration, boiling curves were obtained up to the thermal crisis. For both configurations, it was observed that, at low wall superheat, the effect of confinement was not significant if Bo > 1, while for Bo ? 1 the heat transfer coefficient increased as the channel width s decreased. By contrast, at high wall superheat, a drastic reduction in both heat transfer and CHF was seen when the channel width s decreased; this reduction was less pronounced when the smaller confinement disc (D = 30 mm) was used. CHF data were also compared with the values predicted by literature correlations.     

Electric field effects during nucleate boiling from an artificial nucleation site

An experimental study of saturated pool boiling from a single artificial nucleation site on a polished copper surface has been performed. Isolated bubbles grow and depart from the artificial cavity and the bubble dynamics are recorded with a high speed camera. Experimental results are obtained for bubble growth, departure and vertical rise both with and without the application of an electric field between an upper electrode and the boiling surface. As detailed in a previous paper from the same research group the high spatial and temporal resolution of the video sequences facilitated the development of a baseline experimental bubble growth law which predicts the bubble volumetric growth characteristics for a range of surface superheats at atmospheric pressure. The presence of an electric field has been found to positively augment the convective heat transfer over that of buoyant natural convection. Further to this, for high electric field strengths, the bubble shape, volumetric growth characteristics and bubble rise are different from that of the baseline cases. These results provide compelling evidence that electric fields can be implemented to alter the bubble dynamics and subsequent heat transfer rates during boiling of dielectric liquids.     

Laminar film boiling on a vertical fin

Laminar film boiling on a vertical fin is formulated as a conjugate phenomenon and investigated for no slip and zero shear conditions at the vapor-liquid interface. The results indicate that the combined effects of thermal leakage at the ends of the fin and radiation from its lateral face have profound influence on the average Nusselt number. Further, from the formulation it can be shown that the isothermal condition can be deduced by suitably changing the boundary conditions of the fin at its extremities. The results of the investigation are rendered into dimensionless functional relationships between the average Nusselt numberNu _m, fin parameterM, radiation parameterN _R and temperature ratio term Ψ. The proposed equation can be made use of in design calculations.     

Upper limit of enhancement of boiling heat transfer in a narrow vertical rectangular channel due to passing bubbles

An experimental study has been made of saturated boiling heat transfer for water and R113 in a narrow vertical rectangular channel (2 mm space, 20 mm wide, and 200 mm long) at atmospheric pressure, in which the vertical heated surface (10 mm long and 20 mm wide) is located on one side at a position of 150 mm from its entrance and bubbles are forcibly passed through it at a designated period from 0.33 to 1.0 sec. The experiment shows that the heat transfer coefficients are increased by the bubble passing through the heated surface for the value of thermal diffusivity,a, times period, T₀, of the passing bubbles above about 6×10^–9 m² (a T ₀>6×10^–9 m²) while fora T ₀< 6×10^–9 m², the heat transfer coefficients become independent of the period and the effectiveness of the enhancement of the heat transfer owing to the passing bubble disappears.

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Die obere Grenze der Verbesserung des Wärmeübergangs beim Sieden in einem vertikalen, rechteckigen Kanal infolge von aufsteigenden Blasen

Zusammenfassung Es wurden Experimente über den Wärmeübergang beim Sättigungssieden mit Wasser und R113 in einem engen, vertikalen, rechteckigen Kanal (2 mm Abstand, 20 mm Breite und 200 mm Länge) bei Umgebungsdruck durchgeführt, wobei die vertikale, beheizte Oberfläche (10 mm lang und 20 mm breit) auf der einen Seite in einem Abstand von 150 mm vom Eintritt angeordnet ist und die Blasen zwangsweise durch den Kanal sich mit einem Periodenabstand von 0,033 bis 1,0 s bewegen. Das Experiment zeigt, daß die Wärmeübergangskoeffizienten durch das Vorbeistreichen der Blasen an der beheizten Oberfläche verbessert werden, wenn das Produkt aus Temperaturleitfähigkeit,a, mal der Periode, T₀, der vorbeistreichenden Blasen größer als 6×10^–9 m² liegt, während unterhalb dieses Wertes der Wärmeübergangskoeffizient unabhängig von der Blasenperiode ist und die Effektivität der Wärmeübergangsverbesserung infolge der Blasenströmung verschwindet.

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Nomenclature a thermal diffusivity of liquid - ¯h time-averaged heat transfer coefficient - q _w heat flux at wall - T ₀ period of passing bubble - T _w(t) temperature of heated surface - T _w amplitude of heated surface temperature Greek symbols thermal conductivity - thickness of liquid film     

Stability of nucleation sites in pool boiling

A study was carried out to observe bubble nucleation and site deactivation mechanisms under equilibrium pool boiling of liquids on single sites. The experiments were conducted with benzene, ethanol, and their mixtures on single sites made of glass. These mechanisms were filmed using a cine camera as the temperature of the boiling liquid was decreased in programmed steps. The experiments showed that the deactivation of a bubble nucleation site in the surface occurred by progressive condensation of vapor within it until the volume of vapor became equal to or smaller than the volume of a spherical bubble of diameter equal to that of the site. Based upon these experimental observations, a site deactivation mechanism is proposed for heterogeneous pool boiling and stability criteria are developed for single cylindrical cavities. The effect of relevant parameters on depth-to-diameter ratio of the sites is also determined.     

Critical heat flux in pool boiling on a vertical heater

Critical heat flux during pool boiling on a vertical heater of wire or plate has been measured employing water and R113. The experiment was made for a wire of 0.5 to 2 mm in diameter and for a plate of 5, 7 and 30 mm in width and from 20 to 300 mm in height. The pressure was 1 and 2 bar for water and 1, 2, 3 and 4 bar for R113. The experiment shows that for the case of both wire and plate of 5, 7 mm, a large coalesced bubble entirely surrounds the vertical heater and rises surrounding it, while for the case of w = 30 mm, a large bubble cannot surround and rises along its surface. The characteristic of CHF can be divided into two regimes depending on the flow condition when CHF takes place. Correlations are proposed for the CHF of the wire and the plate of w = 5, and 7 mm, yielding good accuracy. The CHF for the plate of w = 30 mm has a similar tendency to that in one side headed surface and can be predicted reasonably by existing correlation for one side heated surface.     

Turbulent film boiling from a vertical non-isothermal surface

The turbulent film boiling from a vertical non-isothermal surface is formulated with due consideration to thermal radiation from its lateral face. It is observed that the application of Reynolds analogy together with thermal conduction in the test surface has yielded a conjugate solution from which the case of an isothermal condition can be generated as a special case. The analysis has further paved the way in establishing a functional relation between the Nusselt numberNu, radiation parameterN _R , fin parameterM, temperature ratio termT _s /(T _w,0?T _s ), and a product of characteristic modified Grashof, Prandtl and superheating parameter defined as (Gr ² Pr S). In a fully developed turbulent film boiling i.e., modified Grashof number being greater than 10¹⁰, the temperature ratio term accounts for the non-linearities arising due to the inclusion of radiation from the lateral face of the fin. The results are in good agreement with experimental data over a wide range of system conditions.     

Bubble nucleation characteristics in pool boiling of a wetting liquid on smooth and rough surfaces

Quantitative measurements are obtained from high-speed visualizations of pool boiling at atmospheric pressure from smooth and roughened surfaces, using a perfluorinated hydrocarbon (FC-77) as the working fluid. The boiling surfaces are fabricated from aluminum and prepared by mechanical polishing in the case of the smooth surface, and by electrical discharge machining (EDM) in the case of the roughened surface. The roughness values (R_a) are 0.03 and 5.89 μm for the polished and roughened surfaces, respectively. The bubble diameter at departure, bubble departure frequency, active nucleation site density, and bubble terminal velocity are measured from the monochrome movies, which have been recorded at 8000 frames per second with a digital CCD camera and magnifying lens. Results are compared to predictions from existing models of bubble nucleation behavior in the literature. Wall superheat, heat flux, and heat transfer coefficient are also reported.     

Effect of inlet subcooling on two-phase flow oscillations in a vertical boiling channel

In this work, results of experimental research to investigate the effects of heat transfer augmentation and inlet subcooling on two-phase flow instabilities are presented. For this purpose, a simple set-up was designed and built. The effect of inlet subcooling was investigated using different heat transfer surfaces and inlet temperatures at constant heat input of 415 W. Freon-11 has been used as the test fluid, and the experiments were carried out for six heater tubes having different heat transfer surfaces. Inlet temperatures were in the range of –9.8°C to 38°C. The results indicate that, in the range of present experiments, the system becomes more stable, that it's instability boundary moves into lower mass flow rates, with increase in the inlet subcooling. However, the amplitudes and the periods of the oscillations increase with increase in the inlet subcooling. For some of the tested surfaces there was a particular inlet subcooling, above and below which the system's stability decreased.

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Der Einfluß der Eintrittsunterkühlung auf die Oszillationen einer Zwei-Phasen-Strömung in einem senkrechten Siede-Kanal

Zusammenfassung In dieser Arbeit werden die Ergebnisse einer experimentellen Untersuchung des Einflusses von Verbesserungen der Wärmeübertragung und der Eintrittsunterkühlung auf Instabilitäten der Zweiphasenströmung dargestellt. Zu diesem Zweck wurde ein einfacher Aufbau konstruiert und aufgebaut. Der Einfluß der Eintrittsunterkühlung wurde unter Benutzung verschiedener Wärmeübertragungsoberflächen und Eintrittstemperaturen bei einer konstanten Wärmezufuhr von 415 W untersucht. Als Testfluid wurde Freon-11 benutzt. Die Experimente wurden für sechs Heizrohre mit verschiedenen Wärmeübertragungsoberflächen durchgeführt. Die Eintrittstemperaturen lagen in diesem Bereich von –9.8°C bis 38°C. Die Ergebnisse zeigen, daß in dem Bereich der vorliegenden Untersuchungen das System mit einem Ansteigen der Eintrittsunterkühlung stabiler wird und daß dessen Instabilitätsgrenze sich zu niedrigeren Massenstromdichten bewegt. Die Amplituden und Perioden der Schwingungen steigen mit Zunahme der Eintrittsunterkühlung an. Für einige der getesteten Oberflächen existierte eine bestimmte Eintrittsunterkühlung, über bzw. unter welcher die Stabilität des Systems abnahm.

     

Mixed convection flow in narrow vertical ducts

The mixed convection flow in a vertical duct is analysed under the assumption that , the ratio of the duct width to the length over which the wall is heated, is small. It is assumed that a fully developed Poiseuille flow has already been set up in the duct before heat from the wall causes this to be changed by the action of the buoyancy forces, as measured by a buoyancy parameter . An analytical solution is derived for the case when the Reynolds numberRe, based on the duct width, is of 0 (1). This is extended to the case whenRe is 0 (^–1) by numerical integrations of the governing equations for a range of values of representing both aiding and opposing flows. The limiting cases, || 1 andR=Re of 0 (1), andR and both large, with of 0 (R ^1/3) are considered further. Finally, the free convection limit, large with R of 0 (1), is discussed.

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Mischkonvektion in engen senkrechten Rohren

Zusammenfassung Mischkonvektion in einem senkrechten Rohr wird unter der Voraussetzung untersucht, daß das Verhältnis der Rohrbreite zur Länge, über welche die Wand beheizt wird, klein ist. Es wird angenommen, daß sich bereits eine voll entwickelte Poiseuille-Strömung in dem Rohr eingestellt hat, bevor Antriebskräfte, gemessen mit dem Auftriebsparameter , aufgrund der Wandbeheizung die Strömung verändern. Es wird eine analytische Lösung für den Fall erhalten, daß die mit der Rohrbreite als charakteristische Länge gebildete Reynolds-ZahlRe konstant ist. Dies wird mittels einer numerischen Integration der wichtigsten Gleichungen auf den FallRe =f (^–1) sowohl für Gleich- als auch für Gegenstrom ausgedehnt. Weiterhin werden die beiden Grenzfälle betrachtet, wenn || 1 undR=Re konstant ist, sowieR und beide groß mit proportionalR ^1/3. Schließlich wird der Grenzfall der freien Konvektion, großes mit konstantem R, diskutiert.

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Nomenclature g acceleration due to gravity - Gr Grashof number - G modified Grashof number - h duct width - l length of the heated section of the duct wall - p pressure - Pr Prandtl number - Q flow rate through the duct - Q ₀ heat transfer on the wally=0 - Q ₁ heat transfer on the wally=1 - Re Reynolds number - R modified Reynolds number - T temperature of the fluid - T ₀ ambient temperature - T applied temperature difference - u, velocity component in thex-direction - v, velocity component in they-direction - x, co-ordinate measuring distance along the duct - y, co-ordinate measuring distance across the duct - buoyancy parameter - ₀ modified buoyancy parameter, ₀=R ^–1/3 - coefficient of thermal expansion - ratio of duct width to heated length, =h/l - (non-dimensional) temperature - _w applied temperature on the wally=0 - kinematic viscosity - density of the fluid - ₀ shear stress on the wally=0 - ₁ shear stress on the wally=1 - stream function     

Flow boiling heat transfer in a vertical spirally internally ribbed tube

 Experiments of flow boiling heat transfer and two-phase flow frictional pressure drop in a spirally internally ribbed tube (φ22×5.5 mm) and a smooth tube (φ19×2 mm) were conducted, respectively, under the condition of 6×10⁵ Pa (absolute atmosphere pressure). The available heated length of the test sections was 2500 mm. The mass fluxes were selected, respectively, at 410, 610 and 810 kg/m² s. The maximum heat flux was controlled according to exit quality, which was no more than 0.3 in each test run. The experimental results in the spirally internally ribbed tube were compared with that in the smooth tube. It shows that flow boiling heat transfer coefficients in the spirally internally ribbed tube are 1.4–2 times that in the smooth tube, and the flow boiling heat transfer under the condition of smaller temperature differences can be achieved in the spirally internally ribbed tube. Also, the two-phase flow frictional pressure drop in the spirally internally ribbed tube increases a factor of 1.6–2 as compared with that in the smooth tube. The effects of mass flux and pressure on the flow boiling heat transfer were presented. The effect of diameters on flow boiling heat transfer in smooth tubes was analyzed. Based on the fits of the experimental data, correlations of flow boiling heat transfer coefficient and two-phase flow frictional factor were proposed, respectively. The mechanisms of enhanced flow boiling heat transfer in the spirally internally ribbed tube were analyzed. Received on 1 December 1999     

Visualization of flow boiling of liquid nitrogen in a vertical mini-tube

The flow boiling patterns of liquid nitrogen in a vertical mini-tube with an inner diameter of 1.931 mm are visualized with a high-speed digital camera. The superficial gas and liquid velocities are in the ranges of 0.01–26.5 m/s and 0.01–1.2 m/s, respectively. Four typical flow patterns, namely, bubbly, slug, churn and annular flow are observed. Some interesting scenes about the entrainment and liquid droplet deposition in the churn and annular flow, and the flow reversal with the indication of negative pressure drop, are also presented. Based on the visualization, the two-phase flow regime maps are obtained. Compared with the flow regime maps for gas–water flow in tubes with similar hydraulic diameters, the region of slug flow in the present study reduces significantly. Correspondingly, the transition boundary from the bubbly flow to slug flow shifts to higher superficial gas velocity, and that of churn to annular flow moves to lower superficial gas velocity. Moreover, time-averaged void fraction is calculated by quantitative image-digitizing technique and compared with various prediction models. Finally, three kinds of oscillations with long-period and large-amplitude are found, possible explanation for the oscillations is given by comparing the instantaneous flow images with the data of pressure, mass flux and temperature recorded synchronously.     

Two-phase flow instabilities in a horizontal single boiling channel

A study of the stability of an electrically heated single channel, forced convection horizontal system was conducted by using Freon-11 as the test fluid. Two major modes of oscillations, namely, density-wave type (high frequency) and pressure-drop type (low frequency) oscillations have been observed. The steady-state operating characteristics and stable and unstable regions are determined as a function of heat flux, exit orifice diameter and mass flow rate. Different modes of oscillations and their characteristics have been investigated. The effect of the exit restriction on the system stability has also been studied.A mathematical model has been developed to predict the transient behavior of boiling two-phase systems. The model is based on homogenous flow assumption and thermodynamic equilibrium between the liquid and vapor phases. The transient characteristics of boiling two-phase flow horizontal system are obtained for various heat inputs, flow rates and exit orifice diameters by perturbing the governing equations around a steady state. Theoretical and experimental results have been compared.     

FC72 and FC87 nucleate boiling inside a narrow horizontal space

This paper presents new experimental results for saturated nucleate boiling of FC72 and FC87 on a horizontal copper disc, at atmospheric pressure, for different degrees of confinement, s, in the range of 0.1-13 mm, and with two kinds of confining element, for low and moderated heat fluxes (?40 kW/m²), on both a downward and an upward facing heating surface. For low heat flux a decrease of the confinement gap causes an enhancement of the boiling and a decrease in the dryout heat flux. A visualization of the boiling phenomenon shows the effect of confinement and heat flux on the liquid-vapor configuration.     

Heating-up,nucleation and boiling of a critical solution of fissile material

Applied to a homogeneous solution of uranyl nitrate or plutonium nitrate, the physical mechanisms are discussed which control the mean nuclear power of a solution reactor. The short-time averaged power during the heating-up of a nonboiling solution is related to the mean flow rate of radiolytic gas which is necessary to yield a steady-state neutron flux. If the solution is boiling a similar relation to the flow rate of vapor is derived. This relation, however, can only be applied if the superheat of the solution permits vapor bubble growth. Otherwise, the minimum power for the required superheat will be obtained instead. These power relations are combined into a quasi-steady-state model which is discussed and verified by applying it to two experiments of the CRAC solution reactor.     

Effect of heat transfer augmentation on two-phase flow instabilities in a vertical boiling channel

The effect of different heater surface configurations on two-phase flow instabilities has been investigated in a single channel, forced convection, open loop, up-flow system. Freon-11 is used as the test fluid, and six different heater tubes with various inside surface configurations have been tested at five different heat inputs. In addition to temperature and pressure recordings, high speed motion pictures of the two-phase flow were taken for some of the experiments to study the two-phase flow behavior at different operating points. Experimental results are shown on system pressure drop versus mass flow rate curves, and stability boundaries are also indicated on these curves. Comparison of different heater tubes is made by the use of the stability boundary maps and the plots of inlet throttling necessary to stabilize the system versus mass flow rate. Tubes with internal springs were found to be more stable than the other tubes.