Physical mechanisms of heat transfer during single bubble nucleate boiling of FC-72 under saturation conditions. II: Theoretical analysis

This paper is the second part of a two-part study concerning the dynamics of heat transfer during the nucleation process of FC-72 liquid. The experimental findings on the nature of different heat transfer mechanisms involved in the nucleation process were discussed in part I. In this paper, the experimental results are compared with the existing boiling models. The boiling models based on dominance of a single mechanism of heat transfer did not match the experimental results. However, the Rohsenow model was found to closely predict the heat transfer through the microconvection mechanism that is primarily active outside the bubble/surface contact area. An existing transient conduction model was modified to predict the surface heat transfer during the rewetting process (i.e. transient conduction mechanism). This model takes into account the gradual rewetting of the surface during the transient conduction process rather than a simple sudden surface coverage assumption commonly used in the boiling literature. The initial superheat energy of the microlayer (i.e. microlayer sensible energy) was accurately calculated and found to significantly contribute in microlayer evaporation. This even exceeded the direct wall heat transfer to microlayer at high surface superheat temperatures. A composite model was introduced that closely matches our experimental results. It incorporates models for three mechanisms of heat transfer including microlayer evaporation, transient conduction, microconvection, as well as their influence area and activation time. The significance of this development is that, for the first time, all submodels of the composite correlation were independently verified using experimental results.     

Unified theoretical prediction of fully developed nucleate boiling and critical heat flux based on a dynamic microlayer model

A new dynamic microlayer model has been proposed to predict theoretically the heat flux in fully developed nucleate boiling regions including critical heat flux (CHF). In this model, the heat transfer with boiling is mainly attributed to the evaporation of the microlayers which are periodically formed while the individual bubbles are forming. Since the initial microlayer thickness becomes thinner with the increase of wall superheat, both the local evaporation and the partial dryout speed of the microlayer increase. As a result, the time-averaged heat flux during the period of individual bubble has a maximum point, the CHF, at the predicted continuous boiling curve.     

Physical mechanisms of heat transfer during single bubble nucleate boiling of FC-72 under saturation conditions-I. Experimental investigation

This paper is the first of a two-part study concerning the dynamics of heat transfer during nucleation process of saturated FC-72 liquid. Experimental results discussed in this paper provide new physical insight on the nature of heat transfer events at the nucleation site during the nucleate boiling process. The thermal field underneath a bubble during the boiling of FC-72 was measured with a spatial resolution of 22--40 μm. The time period of activation, area of influence, and magnitude of three different mechanisms of heat transfer active at the nucleation site were determined. These mechanisms consisted of: (1) microlayer evaporation following the rapid bubble expansion, (2) transient conduction due to rewetting of the surface during bubble departure, and (3) microconvection in the region external to the bubble/surface contact area. The area of influence of the transient conduction mechanism was found to be limited to the bubble/surface contact area, with most of the heat transfer occurring prior to the bubble detachment from the surface. The microconvection heat transfer mechanism was localized primarily outside the contact area and was found to be steady in nature. All three mechanisms of heat transfer were found to make significant contributions to the total surface heat transfer. The second part of this study provides the theoretical analysis of the results.     

Modeling of Particulate Fouling on Heat Exchanger Surfaces: Influence of Bubbles on Iron Oxide Deposition

Studies of iron oxide deposition on Alloy-800 heat exchanger tubes have been part of a continuing research program at the University of New Brunswick (UNB); the present work formulates mechanisms for the effect of bubbles on deposition in water under boiling conditions. To supplement results from earlier deposition experiments in a fouling loop at UNB, measurements of bubble frequency and departure diameter as a function of heat flux were performed. High-speed movies of bubbling air/water systems indicated that a pumping action moved particles from adjacent areas at the surface to bubble nucleation sites. To explain the observations, the model considers deposition and concomitant removal. Deposition includes microlayer evaporation and filtration through the porous deposit. The deposit is sparse in the first stage, when the dominant process is microlayer evaporation including particle trapping and pumping, creating spots of deposit. Filtration becomes more important as the deposit thickens to a stage when microlayer evaporation becomes negligible. Chimney effects then control. Turbulence due to detaching and collapsing bubbles affects removal. In subcooled boiling, collapsing bubbles generate enough turbulence to maintain much of the deposit labile, while in bulk boiling bubble detachment from the nucleation site is dominant and a smaller portion of the deposit is labile and subject to removal. Model predictions are presented and shown to agree quite well with experimental data.     

Investigation of heat transfer and bubble dynamics in a boiling thin liquid film flowing over a rotating disk

Nucleate boiling heat transfer and bubble dynamics in a thin liquid film on a horizontal rotating disk were studied. A series of experiments were conducted to determine the heat transfer coefficient on the disk. At low rotation and flow rates, vigorous boiling increased the heat transfer coefficients above those without boiling. Higher rotational speeds and higher flow rates increased the heat transfer coefficient and suppressed boiling by decreasing the superheat in the liquid film. The flow field on the disk, which included supercritical (thin film) flow upstream of a hydraulic jump, and subcritical (thick film) flow downstream of a hydraulic jump, affected the type of bubble growth. Three types of bubble growth were identified. Vigorous boiling with large, stationary bubbles were observed in the subcritical flow. Supercritical flow produced small bubbles that remained attached to the disk and acted as local obstacles to the flow. At low rotational rates, the hydraulic jump that separated the supercritical and subcritical regions produced hemispherical bubbles that protruded out of the water film surface and detached from the disk, allowing them to slide radially outward. A model of the velocity and temperature of the microlayer of water underneath these sliding bubbles indicated that the microlayer thickness was approximately 1/25th of that of the surrounding water film. This microlayer is believed to greatly enhance the heat transfer rate underneath the sliding bubbles.     

Theoretical studies on transient pool boiling based on microlayer model

An analytical model for transient pool boiling heat transfer was developed in this study. The boiling curves of the transient boiling were obtained based on the microlayer model proposed by the authors and the mechanism of transition from the non-boiling regime to film boiling, i.e., direct transition was theoretically examined. Since the nucleate boiling heat flux is mainly due to the evaporation of the microlayer and its initial thickness decreases rapidly with increasing superheat, the duration of nucleate boiling is markedly decreased as the incipient boiling superheat is increased. It is found that the direct transition is closely connected to the rapid dryout of the microlayer which occupies almost the whole surface at high wall superheat.     

Applicability of a Mechanistic Model for the Prediction of Boiling Heat Transfer for Refrigerants

Among the different mechanisms of heat transfer during nucleate pool boiling, evaporation from the micro layer, evaporation from the macro layer, and transient heat transfer from the fluid mass above the macro layer play very crucial roles. Based on these three mechanisms, a model made earlier by the authors can predict the pool boiling heat transfer of water from flat and tubular surfaces quite accurately. To the best of our knowledge no model has so far been constructed for refrigerants considering evaporation from micro and macro layers. This leaves an opportunity for extending the generic micro–macro layer-based boiling heat transfer model for refrigerants. In the present work the authors' earlier model has been used to predict boiling heat transfer for a number of refrigerants. Experimental data for both flat and tubular surfaces have been considered for the validation of the model. Good agreements have been observed in most of the cases. Although the model is developed for nucleate boiling, it has been extended for a higher degree of superheat, and a matching trend can be seen beyond the critical heat flux for various heat input rates. Values of critical heat flux are also well predicted by the present model.     

Convection and evaporation of microlayer underneath bubbles under microgravity

The multidimensional heat transfer and fluid flow in the microlayer region below a vapor bubble formed during boiling in microgravity are investigated by numerically solving the Navier–Stokes equations with the energy equation. The flow is driven by Marangoni flow due to the surface tension gradient along the bubble surface that results from the temperature gradient. The model also includes condensation and evaporation at the bubble surface. The flow field and heat transfer are calculated for microlayer thicknesses from 0.01 mm to 10 mm to investigate the effect of microlayer thickness. The results show that the velocities are small and have only a small effect on the temperature distribution as compared to the solution for pure conduction in the liquid. Natural convection is shown to have a negligible effect on heat transfer. For less than ideal evaporative heat transfer at the bubble interface, Marangoni convection caused the heat transfer to increase several percent. The flow in the microlayer is shown to agree with the lubrication analogy only for thin, relatively flat interfaces. © 2000 Scripta Technica, Heat Trans Asian Res, 30(1): 1–10, 2001     

Contribution of thin-film evaporation during flow boiling inside microchannels

Flow boiling through microchannels is characterized by nucleation and growth of vapor bubbles that fill the entire channel cross-sectional area. As the bubbles nucleate and grow inside the microchannel, a thin film of liquid or a microlayer gets trapped between the bubbles and the channel walls. The heat transfer mechanism present at the channel walls during flow boiling is studied numerically. It is then compared to the heat transfer mechanisms present during nucleate pool boiling and in a moving evaporating meniscus. Increasing contact angle improved wall heat transfer in case of nucleate boiling and moving evaporating meniscus but not in the case of flow boiling inside a microchannel. It is shown that the thermal and the flow fields present inside the microchannel around a bubble are fundamentally different as compared to nucleate pool boiling or in a moving evaporating meniscus. It is explained why thin-film evaporation is the dominant heat transfer mechanism and is responsible for creating an apparent nucleate boiling effect inside a microchannel.     

On the transition from partial to fully developed subcooled flow boiling

The subject of the present study is to relate the boiling heat transfer process with experimentally observed bubble behaviour during subcooled flow boiling of water in a vertical heated annulus. It presents an attempt to explain the transition from partial to fully developed flow boiling with regard to bubble growth rates and to the time that individual bubbles spend attached to the heater surface.Within the partial nucleate boiling region bubbles barely change in size and shape while sliding a long distance on the heater surface. Such behaviour indicates an important contribution of the microlayer evaporation mechanism in the overall heat transfer rate. With increasing heat flux, or reducing flow rate at constant heat flux, bubble growth rates increase significantly. Bubbles grow while sliding, detach from the heater, and subsequently collapse in the bulk fluid within a distance of 1-2 diameters parallel to the heater surface. This confirms that bubble agitation becomes a leading heat transfer mode with increasing heat flux. There is however, a sharp transition between the two observed bubble behaviours that can be taken as the transition from partial to fully developed boiling. Hence, this information is used to develop a new model for the transition from partial to fully developed subcooled flow boiling.     

Bubble growth rates in pure and binary systems: Combined effect of relaxation and evaporation microlayers

Pohlhausen's equation has been used to determine the initial thickness of the evaporating microlayer beneath a hemispherical vapour bubble on a superheated horizontal wall. Microlayer thickness is proportional to the square root of the distance to the nucleation site during early bubble growth, while a linear relationship exists during advanced growth.A (heat and mass) diffusion-type solution is derived for advanced bubble growth, which accounts for the interaction of the mutually dependent contributions due to the relaxation microlayer (around the bubble dome) and the evaporation microlayer. The entire bubble behaviour during adherence is determined by a combination of this asymptotic solution and the Rayleigh solution, which governs early growth. Also, expressions are derived for both the radius of the dry area and the radius of the maximum contact area between bubble and wall.At low concentrations of the more volatile component in binary systems, the dominating influence of mass diffusion is demonstrated by the following effects: (i) asymptotic bubble growth is slowed down substantially; (ii) the formation of dry areas beneath bubbles is prevented, even at subatmospheric pressures; (iii) the lower part of the bubble is contracted; (iv) the evaporation microlayer contribution to bubble growth is negligible at atmospheric and at elevated pressures.     

Bubble induced heat transfer in two phase gas-liquid flow

The influence of gas bubbles on heat transfer in two phase gas-liquid systems has been investigated. Platinum wires have been used as heat-transfer probes and the two phase flow has been simulated by generating a single continuous stream of discrete gas bubbles into a stationary liquid. The contribution of various modes of heat transfer has been determined. It has been found that transient conduction into the liquid is the predominant mode of the bubble induced heat transfer and is responsible for about 75 per cent of heat transfer. Convection contributes the remainder. A theoretical model of the bubble induced heat transfer based on the surface renewal and penetration theory has been developed.     

Bubble growth rates in nucleate boiling of water at subatmospheric pressures

The growth rate of vapour bubbles has been investigated experimentally up to departure in water boiling at pressures varying from 26·7 to 2·0 kPa (the corresponding Jakob number increasing from 108 to 2689).Comparison of the data with existing theory shows the substantial influence of liquid inertia during initial growth, in agreement with previous results of Stewart and Cole [1]on water boiling at 4·9 kPa, the Jakob number varying from 955 to 1112. As an extreme case, at a pressure of 2·0 kPa, large “Rayleigh” bubbles are observed during the entire adherence time. During advanced growth, bubble behaviour is gradually governed by heat diffusion, especially at relatively high (subatmospheric) pressures.Experimental bubble growth in the investigated pressure range is in quantitative agreement with the van Stralen, Sohal, Cole and Sluyter theory [10]. This model combines the Rayleigh solution with a diffusion-type solution, which accounts for the contributions to bubble growth due to both the relaxation microlayer (around the bubble dome) and the evaporation microlayer (beneath the bubble).Finally, a curious bubble cycle is observed at the lowest investigated pressures, which is attributed to the combined action of a high-velocity liquid jet (originating in the wake following a large primary bubble) and a succeeding secondary vapour column (generated at the adjacent dry spot at the heating wall beneath the primary bubble).     

The influence of bubbly flow on boiling from a tube in a bundle

The forms of bubbly flow occurring within a tube bundle are discussed and the boiling process in the bundle is notionally divided into mechanisms due to liquid forced convection, sliding bubbles and nucleation. A novel experimental analysis of heat transfer from a tube in a bundle indicates the predominance of the sliding bubble part. There is a virtual absence of nucleation in a bundle except at the lowest tubes indicating that, once enough bubbles have been produced, the other mechanisms are sufficient to transfer the heat from the tubes.     

Heat transfer during pool boiling based on evaporation from micro and macrolayer

An analytical model of heat transfer based on evaporation from the micro and macrolayers to the vapor bubble during pool boiling is developed. Evaporation of microlayer and macrolayer during the growth of individual bubbles is taken care of by using temporal and spatial variation of temperature in the liquid layer. Change of bubble shape during the entire cycle of bubble growth and departure is meticulously considered to find out the rate of heat transfer from the solid surface to the boiling liquid. Continuous boiling curve is developed by considering the bubble dynamics and decreasing thickness of liquid layer along with the increase of dry spot radius. Transient variation of macrolayer and microlayer thickness is predicted along with their effect on CHF. Present model exhibits a good agreement with reported experimental data as well as theories.     

High-Speed Visualization of Bubble Behaviors for Pool Boiling of R-141b

A visualization study on the behavior of bubbles has been carried out for pool boiling of R141b on a horizontal transparent heater at pressure 0.1 MPa. The behaviors of bubbles were recorded by a high-speed camera placed beneath the heater surface. The departure diameter, departure time of bubbles and nucleation site density at different heat flux were obtained. The visualization results show that bubble departure diameter and departure time decrease , while the nucleation site density increases as the heat flux increases. It is also observed that there is no liquid recruited into the microlayer in the experiment. Based on the experimental results, boiling curve for R141b was predicted by using the dynamic microlayer model. As a result, the agreement between the predictive result based on the dynamic microlayer model and the experiment data for boiling curve of R141b is good at high heat flux.     

Bubble induced heat transfer in gas fluidized beds

The influence of gas bubbles on heat transfer in gas fluidized beds has been investigated. A platinum wire has been used as a heat-transfer probe and the aggregative gas fluidized bed has been simplified by generating a single continuous stream of gas bubbles into an incipiently fluidized bed. It has been found that in the case of aggregative gas fluidized beds of small particles operating below the radiative temperature level, transient conduction into the emulsion phase is responsible for at least 90% of heat transfer and that the remainder is contributed by the superimposed gas convection. A theoretical model of the bubble induced heat transfer has been developed. Finally, experimental justification for the concept of the property boundary layer introduced in [2] is presented.     

Time and space resolved wall temperature and heat flux measurements during nucleate boiling with constant heat flux boundary conditions

The lack of time and space resolved measurements under nucleating bubbles has complicated efforts to fully explain pool-boiling phenomena. In this work, time and space resolved temperature and heat flux distributions under nucleating bubbles on a constant heat flux surface were obtained using a 10 × 10 microheater array with 100 μm resolution along with high-speed images. A numerical simulation was used to compute the substrate conduction, which was then subtracted from the heater power to obtain the wall-to-liquid heat transfer. The data indicated that most of the energy required for bubble growth came from the superheated layer around the bubble. Microlayer evaporation and contact line heat transfer accounted for not more than 23% of the total heat transferred from the surface. The dominant heat transfer mechanism was transient conduction into the liquid during bubble departure. Bubble coalescence was not observed to transfer a significant amount of heat.     

碳钢_水热虹隙管内部强化传热机理研究

在微层蒸发模型的理论基础上,对热虹吸管内部设置分流管结构强化沸腾传热者机理分析。建立分流管强化热哐吸管内部沸腾传热模型,同时选择七种不同的分流管开孔结构,与光滑管进行了对比实验研究,寻得最佳分流管结构,并综合大量实验数据建立强化沸腾传热准数方程式。     

Scale effects on flow boiling heat transfer in microchannels: A fundamental perspective

Flow boiling in microchannels has received considerable attention from researchers worldwide in the last decade. A scaling analysis is presented to identify the relative effects of different forces on the boiling process at microscale. Based on this scaling analysis, the flow pattern transitions and stability for flow boiling of water and FC-77 are evaluated. From the insight gained through the careful visualization and thermal measurements by previous investigators, similarities between heat transfer around a nucleating bubble in pool boiling and in the elongated bubble/slug flow pattern in flow boiling are brought out. The roles of microlayer evaporation and transient conduction/microconvection are discussed. Furthermore, it is pointed out that the convective contribution cannot be ruled out on the basis of experimental data which shows no dependence of heat transfer coefficient on mass flow rate, since the low liquid flow rate during flow boiling in microchannels at low qualities leads to laminar flow, where heat transfer coefficient is essentially independent of the mass flow rate. Specific suggestions for future research to enhance the boiling heat transfer in microchannels are also provided.