Transportphenomena in subcooled flow boiling

Subcooled boiling has been subject of extensive research studies in the literature. So far, the approach of studying the phenomena of subcooled boiling has rather been an integral method by measuring the heat transport from the wall to the liquid and by following the growth and collapse of the bubbles. However, little is known about the heat transport at the phase interface between the surface of the bubbles and the subcooled liquid. Experimental attempts to study the transport phenomena around the bubble surface quantitatively have been performed by using the holographic interferometry, an optical method, which works in an inertialess and non-invasive way. The conventional holographic interferometry has somewhat been modified by applying the finite fringe method. With this technique interesting insights could be gained and precise quantitative data could be evaluated.     

A fractal analysis of subcooled flow boiling heat transfer

A fractal model for the subcooled flow boiling heat transfer is proposed in this paper. The analytical expressions for the subcooled flow boiling heat transfer are derived based on the fractal distribution of nucleation sites on boiling surfaces. The proposed fractal model for the subcooled flow boiling heat transfer is found to be a function of wall superheat, liquid subcooling, bulk velocity of fluid (or Reynolds number), fractal dimension, the minimum and maximum active cavity size, the contact angle and physical properties of fluid. No additional/new empirical constant is introduced, and the proposed model contains less empirical constants than the conventional models. The proposed model takes into account all the possible mechanisms for subcooled flow boiling heat transfer. The model predictions are compared with the existing experimental data, and fair agreement between the model predictions and experimental data is found for different bulk flow rates.     

Interface oscillation of subcooled flow boiling in locally heated microchannels

An investigation was conducted to understand flow boiling of subcooled de-ionized water in locally heated parallel microchannels. High-speed visualization technology was employed to visually observe the transient phase change process in an individual microchannel. Signal analysis method was employed in studying the interface movement and phase change process. The phase change at locally heated condition was different from those at entirely heated condition where elongated bubble(s) stayed quasi-stable for a long time without venting out. Diversified and intensive interface oscillation was observed occurring on both of the upstream and downstream bubble caps. Evaporation and condensation modes were characterized with distinguished oscillation frequencies. The film-driven oscillations of both evaporating and condensing interfaces generally operated at higher frequencies than the oscillations driven by nucleation or dropwise condensation.     

Axial development of subcooled boiling flow in an internally heated annulus

For the main purpose of database construction in order to develop the interfacial area transport equation, axial developments of local void fraction, interfacial area concentration, bubble Sauter mean diameter, interfacial velocity, and bubble number density were measured in boiling water bubbly flows in a vertical-upward internally heated annulus using a double-sensor conductivity probe. The annulus channel consisted of an inner rod with a diameter of 19.1 mm and an outer round tube with an inner diameter of 38.1 mm, and the hydraulic equivalent diameter was 19.1 mm. A total of 11 data sets were acquired consisting of four inlet liquid velocities, 0.500, 0.664, 0.987 and 1.22 m/s, two heat fluxes, 100 and 150 kW/m², and two inlet liquid temperatures, 95.0 and 98.0°C. The axial developments of the flow parameters were discussed based on the measured data in detail. In addition to the database construction, the measured data validated recently proposed constitutive equations for the distribution parameter, drift velocity, and bubble Sauter mean diameter, which will improve the accuracy of the drift-flux model in subcooled bubbly flow.     

Effects of turbulence and secondary flows on subcooled flow boiling

Experiments are conducted on the influence of turbulence and longitudinal vortices on subcooled flow boiling in a vertical, rectangular channel. Different flow inserts are used to create turbulence and vortices in the channel. Studied boiling regimes range from the onset of nucleate boiling over the critical heat flux up to fully developed film boiling. A wide range of measuring techniques is applied: time averaged particle image velocimetry (PIV) is used in cold flows for the evaluation of the effects the inserts have on the flow, high speed PIV and photography are used to determine the effects on the fluid and vapor movement in boiling experiments. Digital Holographic Interferometry is used for the evaluation of temperature distributions in the boiling flow. Furthermore, optical microprobes are used to obtain pointwise measurements in areas inaccessible to the imaging techniques. The experiments show that the flow inserts can have considerable impact on the heat fluxes and the distribution of vapor and temperature along the channel. All used inserts lead to an increase in critical heat flux, which is more pronounced for stronger turbulence and higher flow rates and fluid subcoolings. The measuring techniques reveal both a better transport of vapor from the heater surface as well as an increase in mixing in the liquid phase with flow inserts.     

Numerical investigation of static flow instability in a low-pressure subcooled boiling channel

A three-dimensional two-fluid model to predict subcooled boiling flow at low pressure is presented. The model is adopted to investigate the two-phase flow and heat transfer characteristics in a heated channel. The presence of bubbles as a consequence of heating flow through a vertical rectangular channel has a significant effect on the overall pressure drop along the channel. Numerical results were compared against a series experimental data performed at various conditions – mass flux, heat flux, inlet temperature and exit pressure. Good agreement on the overall pressure drop was achieved. The onset of flow instability velocity was also accurately determined when compared against measurements. Predicted results of void fraction provided useful information towards a more fundamental understanding of the occurrence of onset of nucleate boiling, onset of significant voiding and onset of flow instability. The phenomenon of boiling onset oscillations was also predicted through the use of the two-fluid model.     

PTV experiments of subcooled boiling flow through a vertical rectangular channel

Time resolved Particle Tracking Velocimetry (PTV) experiments were carried out to investigate turbulent, subcooled boiling flow of refrigerant HFE-301 through a vertical rectangular channel with one heated wall. Measurements were performed with liquid Reynolds numbers (based on the hydraulic diameter) of Re = 3309, 9929 and 16,549 over a wall heat flux range of 0.0–64.0 kW/m². Turbulence statistics are inferred from PTV full-field velocity measurements. Quantities such as: instantaneous 2D velocity fields, time-averaged axial and normal velocities, axial and normal turbulence intensities, and Reynolds stresses are obtained. The present results agree well with previous studies and provides new information due to the full-field nature of the technique. This work is an attempt to provide turbulent subcooled boiling flow data for validation and improvement of two-phase flow computational models.     

Experimental and numerical study on nucleate bubble deformation in subcooled flow boiling

Experimental and numerical study was conducted to investigate the bubble behaviors in subcooled flow nucleate boiling. The bubble behaviors in subcooled flow boiling in an upward annular channel were investigated in the range of subcooling degree 5–30 K by visualization with high spatial and temporal resolutions using a high speed video camera and Cassegrain tele-microscope. Obvious deformation on the upstream side surface of the bubble during its growth process was frequently observed. This deformation phenomenon was caused by the condensation occurring at the upstream side bottom of the bubble, which results from the Marangoni flow along the bubble surface from the bubble bottom to the top. Since the Marangoni flow cannot be directly observed by the current experiments because it occurs in a very thin interface along the bubble surface, the numerical simulations of bubble growth and departure behaviors in subcooled flow boiling were carried out. As a result, it was confirmed that the bubble deformation was caused by the Marangoni flow along the bubble surface. Moreover, the phenomenon of wave propagation on the bubble surface during the condensation process was observed, and it can enhance the heat transfer between the bubble and the surrounding subcooled liquid.     

A theoretical and experimental study on subcooled flow boiling at high heat flux

In a subcooled flow boiling system at high heat flux, the major heat transfer mechanism places emphasis on a very thin liquid layer, known as the sublayer which is trapped between the heated surface and the vapor blankets. Base on the convective boiling heat transfer dominated by the heat conduction through the liquid sublayer, a theoretical model for subcooled flow boiling heat transfer has been developed. To provide useful data in the simulation of Light Water Reactors (LWRs) conditions, heat transfer experiments for up-flow boiling water through a vertical tube at the pressure ranging from 6.9 to 15.5 MPa have been conducted. The experimental results are compared with the predictions of the present model and other five famous correlations. For the LWRs subcooled flow boiling, the comparison reveals that the present model show the best agreement with the measured data.

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Eine theoretische und experimentelle Studie über unterkühltes Sieden bei hoher Wärmestromdichte

Zusammenfassung In einem Fluidsystem, das bei hohem Wärmefluß den Effekt des unterkühlten Siedens zeigt, findet der wesentliche Wärmetransportmechanismus in einer sehr dünnen Schicht statt, die zwischen der beheizten Oberfläche und den Dampfpolstern liegt und als Unterschicht bekannt ist. Basierend auf den Gesetzmäßigkeiten des konvektiven Siedens unter dominierendem Einfluß der Wärmeleitung durch die Flüssigkeits-Unterschicht wurde ein theoretisches Modell zu Beschreibung des Wärmeübergangs bei unterkühltem Sieden entwickelt. Um nützliche Daten für die Simulation der in Leichtwasserreaktoren (LWR) herrschenden Bedingungen zu gewinnen, erfolgten die Experimente bei Aufwärtsströmung siedenden Wassers in einem senkrechten Rohr im Druckbereich 6,9 bis 15,5 MPa. Diese experimentellen Ergebnisse werden mit Vorausberechnungen nach dem erstellten Theoriemodell, sowie jenen nach fünf der bekanntesten Korrelationen verglichen. Für unterkühltes Sieden in Leichtwasserreaktoren zeigte sich hierbei, daß die Experimente am besten durch das neuentwickelte Modell wiedergegeben werden.

     

Studies on heat transfer and flow characteristics in subcooled flow boiling—Part 1. Boiling characteristics

Measurement of wall temperature profile and photographic observation are performed for R-113 subcooled boiling flow in a channel with heat fluxes up to the CHF. The incipient boiling superheats measured are little affected by mass velocity and liquid subcooling. Hysteresis in boiling observed by increasing and decreasing heat flux seems to be ascribed to variation in size of active nucleation cavities on the wall. Increasing heat flux up to the CHF, the bubble density on the heated surface increases and remarkably large coalescent bubbles appear periodically near the heating section outlet.     

A mechanistic critical heat flux model for subcooled flow boiling based on local bulk flow conditions

The critical heat flux (CHF) mechanisms for subcooled flow boiling are reviewed. Based on experimental observations reported by previous investigators, the authors have developed a new mechanistic CHF model for vertical subcooled flow at high pressure and high mass velocity. This model is based on the dryout of a thin liquid layer (sublayer) beneath an intermittent vapor blanket due to a Helmholtz instability at the sublayer-vapor interface. The parametric trends of CHF have been explored qualitatively and quantitatively with respect to variations in pressure, mass velocity, subcooling and tube diameter. Comparisons of the model predictions with experimental data for water show good agreement in the simulation of subcooled flow conditions of pressurized water reactors (PWRs).     

Studies on heat transfer and flow characteristics in subcooled flow boiling—Part 2. Flow characteristics

Distributions of fluid temperature and its fluctuation are measured across a R-113 subcooled boiling flow channel with heat fluxes up to the CHF. A microthermocouple probe associated with an electric compensation circuit for the time constant is used for this purpose. Applying statistical treatments to the recorded temperature fluctuation, the heat transfer process in the flow and the characteristics of the bubbles flowing close to the heated surface are investigated. For high heat fluxes nearby the CHF, some bubbles adjacent to the heated surface show a clear trend to coalesce to large volume bubbles with relatively long passing periods, suggesting a mechanism of departure from nucleate boiling by periodical wall temperature rise due to momentary liquid film dryout underneath the large bubbles.     

Bubble jet flow interaction during pool boiling of subcooled water on horizontal thin wires

Both visual experiments and numerical analyses were conducted to investigate the interaction between bubble jet flows during pool boiling of subcooled water on horizontal thin wires. The bubble jet flows nearby attracted each other, and they can combine into one jet flow under strong interaction. As the adjacent bubble departs, the bubble jet flow would experience an unsteady evolution process with the jet flow interaction weakening. Since the unsymmetrical thermocapillary force at the bubble interface was induced by the adjacent bubble as a cold source, the bubble jet flow would trend to the adjacent bubble, and the mechanism based on thermocapillary force and cold source can explain the bubble jet flow interaction very well. The steady bubble jet flow interaction phenomena were further simulated by laminar model, and the calculated jet flow interaction phenomena were in a good agreement with the experimental results.     

Heat transfer at the phase interface of bubbles collapsing in subcooled liquids and during subcooled boiling

Heat transfer phenomena with bubble condensation in subcooled liquids and during subcooled boiling are discussed, which were studied by using holographic interferometry. This inertialess and non-intrusive measuring technique allows interesting insights into fluid dynamics and thermodynamics of phase change processes. The resistance for the heat transport through the phase boundaries is mainly on the liquid side and therefore the heat transfer coefficient can expressed by simple correlations following the procedure in forced convective flow. For the sake of simple boundary conditions first bubble condensation in subcooled liquids with constant and homogenous temperature are studied and than the more complex situation with large temperature gradients in the wall near boundary during subcooled boiling are discussed.Dedicated to Prof. Dieter Mewes on the occasion of his 65th birthday.     

Experimental study on bubble departure characteristics in subcooled nucleate pool boiling

The boiling models use departure diameter and frequency in closure relations for the calculation of nucleate boiling heat flux. These parameters are normally derived from empirical correlations which depend heavily on experiments. While these parameters are studied mostly for saturated conditions, there is not sufficient data for the values of departure diameter and frequency in subcooled boiling. In this work, the bubble departure characteristics, i.e. the departure diameters and frequency have been measured using high speed visualization experiments with subcooled demineralized water at atmospheric pressure for nucleate pool boiling conditions. The water pool dimensions were 300 mm × 135 mm × 250 mm with four different heating elements to carry out the parametric studies of bubble departure behavior. The considered parameters were heater surface roughness, heater geometry and heater inclination along with the experimental conditions like degree of subcooling (ΔT_sub = 5−20 K), superheat (ΔT_sat = 1−10 K) and the heat flux. The departure diameters and frequencies were directly measured from the images captured. It was intended to generate the subcooled nucleate pool boiling data under a wide range of conditions which are not present in the literature. The departure diameter was found to increase with the wall superheat, heater size and the inclination angle while the liquid subcooling and surface roughness produced a damping effect on the diameter. The departure frequency was found to increase with the wall superheat and the inclination angle, but decreases with an increase in the heater size. The frequency increases with the degree of subcooling except very close to the saturation, and is unaffected by the surface roughness beyond a certain superheat value.     

Simulation of void fraction profile evolution in subcooled nucleate boiling flow in a vertical annulus using a bubble-tracking approach

A three-dimensional bubble-tracking model of subcooled nucleate boiling flow in a vertical channel at low-pressure conditions is proposed with specific application to the case of boiling in an annulus with a central heating rod. Vapour is distributed in the liquid in the form of individually tracked bubbles. The overall behaviour of the liquid–vapour system results from motion, interaction, coalescence and boiling mechanisms prescribed mostly at the level of bubbles. Bubbles are nucleated at nucleation sites randomly distributed over the heated surface. After nucleation, bubbles slide on the heated wall, detach and then migrate into the lower-temperature region away from the heated surface, where they condense. The proposed model was applied to experiments on subcooled boiling from Purdue University (USA). Experimental and calculated void fraction radial profiles at different axial locations are compared.     

Effects of ultrasonic waves on the heat transfer enhancement in subcooled boiling

This work represents an experimental basic research aimed to investigate the influence on the heat transfer rate of the ultrasounds, in free convection and in presence of liquid. In fact the ultrasonic waves induce, thanks to vibrations, turbulence on the dynamic field, and so an increase of the convection coefficient. The heater is a circular cylinder, immersed in distilled water, and warmed up by Joule effect. This study has carried on for 1 year at Energetics Department “L. Poggi”. The effect was observed since 1960s: different authors had studied the cooling effect due to the ultrasonic waves at different heat transfer regimes, especially from a thin platinum wire to water. We have chosen to investigate the subcooled boiling regime, because this one is the best condition for the heat transfer enhancement, according to the scientific literature. We have carried out a wide experimental study, varying the different water subcooling degrees, the ultrasonic generator power, the ultrasound frequency and the placement of the heater inside the ultrasonic tank, in function of the range of the values of heat flux per unit surface needed dissipating. These values were supplied us by a possible practical application of the ultrasonic streaming: the cooling of 3D highly integrated electronic components. These packaging systems should have to provide all future devices, such as electronics, actuators, sensors and antenna. In fact, for these systems the thermal problem is a critical challenge, because they do not have to overtake critical temperature, after that they could damage irreversibly. Moreover, the traditional cooling systems used in electronic do not seem to be useful for them. On the contrary, the results obtained with ultrasounds, allow heat transfer coefficient enhancement of about 50% to be reached.The purpose is to find out the set of optimal conditions, in order to apply successively all the results to a real packaging system.