16 results found
Ardron K, Giustini G, 2021, On the wetting behavior of surfaces in boiling, Physics of Fluids, Vol: 33, Pages: 1-10, ISSN: 1070-6631
Nucleate boiling heat transfer is strongly influenced by surface wettability as characterized by the Young's contact angle, 𝜃𝑌. The contact angle is usually obtained from measurements on sessile droplets on horizontal test surfaces, but in the case of water at high temperatures and pressures, 𝜃𝑌 values from droplet experiments appear to be typically 30°–50° higher than values needed to explain bubble departure sizes for similar surfaces and temperatures. We explain the differences between 𝜃𝑌 values for droplets and vapor bubbles by using the surface adsorption theory of Adamson. This theory suggests that in the case of bubble formation in high pressure boiling, as the non-wetted surface inside the bubble is in contact with a saturated vapor, it will be covered by an adsorbed liquid layer of nanoscale thickness. Droplet experiments, on the other hand, generally use autoclaves pressurized by permanent gases in which the vapor pressure is far below saturation: in these relatively dry gases, the adsorbed liquid nanolayer is expected to be absent. We suggest that the presence of the adsorbed layer in the case of vapor bubbles will increase the work of formation of a new wetted surface by an amount comparable to the liquid surface tension, resulting in a significant reduction in 𝜃𝑌. We show that by applying Adamson's model with plausible choices for unknown parameters, it is possible to explain the magnitude of the differences in 𝜃𝑌 in bubble and droplet experiments and to explain why 𝜃𝑌 appears much less sensitive to surface material conditions in the case of departing vapor bubbles than in the case of sessile droplets. We conclude that 𝜃𝑌 measurements for sessile droplets on heated surfaces in pressurized gas rather than saturated vapor environments may not be relevant to vapor bubbles and values should not be used directly in models of nucleate boiling.
Giustini G, Issa R, 2021, A method for simulating interfacial mass transfer on arbitrary meshes, Physics of Fluids, Vol: 33, ISSN: 1070-6631
This paper presents a method for modelling interfacial mass transfer in Interface Capturingsimulations of two-phase flow with phase change. The model enables mechanistic predictionof the local rate of phase change at the vapour-liquid interface on arbitrary computationalmeshes and is applicable to realistic cases involving two-phase mixtures with large densityratios. The simulation methodology is based on the Volume Of Fluid (VOF) representation ofthe flow, whereby an interfacial region in which mass transfer occurs is implicitly identified bya phase indicator, in this case the volume fraction of liquid, which varies from the valuepertaining to the ‘bulk’ liquid to the value of the bulk vapour. The novel methodologyproposed here has been implemented using the Finite Volume framework and solutionmethods typical of ‘industrial’ Computational Fluid Dynamics (CFD) practice. The proposedmethodology for capturing mass transfer is applicable to arbitrary meshes without the need tointroduce elaborate but artificial smearing of the mass transfer term as is often done in othertechniques. The method has been validated via comparison with analytical solutions for planarinterface evaporation and bubble growth test cases, and against experimental observations ofsteam bubble growth.
During boiling at a solid surface, it is often the case that a liquid layer of a few microns of thickness (’microlayer’) is formed beneath a bubble growing on the heated surface. Microlayers have been observed forming beneath bubbles in various transparent fluids, such as water and refrigerants, subsequently depleting due to evaporation, thus contributing significantly to bubble growth and possibly generating the majority of vapor in a bubble. On the other hand, boiling of opaque fluids, such as liquid metals, is not amenable to optical observations, and microlayers have not yet been observed in liquid metals. Among that class of fluids is sodium, suitable as a coolant for nuclear reactors and as the working fluid in phase-change solar power receivers. In order to support these applications, it is necessary to understand the boiling behavior of sodium and identify the parameters that might influence microlayer formation during boiling of this important fluid. This paper presents simulations of the hydrodynamics of sodium vapor bubble growth at a surface. An interface capturing flow solver has been implemented in the OpenFOAM code and used to predict the behavior of a sodium vapor bubble near a solid surface in typical boiling conditions. The methodology has been validated using recently reported direct experimental observations of microlayer formation in water and then applied to sodium boiling cases. Simulations indicate that microlayers are formed in sodium in a similar fashion to water. Comparison of simulation results with an extant algebraic model of microlayer formation showed good agreement, which increases confidence in the current predictions of microlayer formation. Typical values of microlayer thickness thus computed indicate that the microlayer is likely to play an important role during bubble growth in sodium.
Giustini G, 2020, Modelling of boiling flows for nuclear thermal hydraulics applications—a brief review, Inventions, Vol: 5, Pages: 1-18, ISSN: 2411-5134
The boiling process is utterly fundamental to the design and safety of water-cooled fission reactors. Both boiling water reactors and pressurised water reactors use boiling under high-pressure subcooled liquid flow conditions to achieve high surface heat fluxes required for their operation. Liquid water is an excellent coolant, which is why water-cooled reactors can have such small sizes and high-power densities, yet also have relatively low component temperatures. Steam is in contrast a very poor coolant. A good understanding of how liquid water coolant turns into steam is correspondingly vital. This need is particularly pressing because heat transfer by water when it is only partially steam (‘nucleate boiling’ regime) is particularly effective, providing a great incentive to operate a plant in this regime. Computational modelling of boiling, using computational fluid dynamics (CFD) simulation at the ‘component scale’ typical of nuclear subchannel analysis and at the scale of the single bubbles, is a core activity of current nuclear thermal hydraulics research. This paper gives an overview of recent literature on computational modelling of boiling. The knowledge and capabilities embodied in the surveyed literature entail theoretical, experimental and modelling work, and enabled the scientific community to improve its current understanding of the fundamental heat transfer phenomena in boiling fluids and to develop more accurate tools for the prediction of two-phase cooling in nuclear systems. Data and insights gathered on the fundamental heat transfer processes associated with the behaviour of single bubbles enabled us to develop and apply more capable modelling tools for engineering simulation and to obtain reliable estimates of the heat transfer rates associated with the growth and departure of steam bubbles from heated surfaces. While results so far are promising, much work is still needed in terms of development of fundamental understandi
Giustini G, Kim I, Kim H, 2020, Comparison between modelled and measured heat transfer rates during the departure of a steam bubble from a solid surface, International Journal of Heat and Mass Transfer, Vol: 148, ISSN: 0017-9310
This paper presents analysis of the heat transfer attendant upon the departure of a single steam bubble during pool boiling of water at atmospheric pressure. The flow of heat from a solid surface to liquid water during and immediately after bubble lift-off has been extracted from micro-scale measurements of the spatial and temporal variation of the temperature at the solid surface beneath the bubble. The numerical procedure used to extract the heat flux from the temperature variations at the solid surface has been assessed and verified, and applied to investigate the heat transfer during the bubble departure phase, and after the eventual bubble lift-off. Results confirm that fluid motion activated by a departing bubble is the cause of heat transfer enhancement. The phenomenon can be characterised as the process of rewetting, by an advancing liquid front, of a dry portion of wall area at the base of the bubble. The portion of wall area that is affected by the observed heat transfer augmentation mechanism has been found to be that of a circle of diameter roughly equal to half the bubble departure diameter. The current measurements enabled validation of interface-capturing numerical simulation of the hydrodynamics and heat transfer of single bubble formation and departure from a surface, including conjugate heat transfer in the solid substrate. From simulation results, the spatial and temporal variation of the heat flux at the solid surface beneath the bubble has been computed and monitored during bubble departure and after the eventual bubble lift-off. Heat transfer rates at bubble departure extracted from simulation have been found in good agreement with measurements. The simulation correctly captured experimental trends and was found to give an accurate estimate of the magnitude of the flows of heat to the liquid due to the bringing of cold fluid in the vicinity of the wall caused by the bubble departure process.
Giustini G, Walker SP, Sato Y, et al., 2019, CFD analysis of the transient cooling of the boiling surface at bubble departure, Journal of Heat Transfer: Transactions of the ASME, Vol: 139, ISSN: 0022-1481
Component-scale computational fluid dynamics (CFD) modeling of boiling via heat flux partitioning relies upon empirical and semimechanistic representations of the modes of heat transfer believed to be important. One such mode, “quenching,” refers to the bringing of cool water to the vicinity of the heated wall to refill the volume occupied by a departing vapor bubble. This is modeled in classical heat flux partitioning approaches using a semimechanistic treatment based on idealized transient heat conduction into liquid from a perfectly conducting substrate. In this paper, we apply a modern interface tracking CFD approach to simulate steam bubble growth and departure, in an attempt to assess mechanistically (within the limitations of the CFD model) the single-phase heat transfer associated with bubble departure. This is in the spirit of one of the main motivations for such mechanistic modeling, the development of insight, and the provision of quantification, to improve the necessarily more empirical component scale modeling. The computations indicate that the long-standing “quench” model used in essentially all heat flux partitioning treatments embodies a significant overestimate of this part of the heat transfer, by a factor of perhaps ∼30. It is of course the case that the collection of individual models in heat flux partitioning treatments has been refined and tuned in aggregate, and it is not particularly surprising that an individual submodel is not numerically correct. In practice, there is much cancelation between inaccuracies in the various submodels, which in aggregate perform surprisingly well. We suggest ways in which this more soundly based quantification of “quenching heat transfer” might be taken into account in component scale modeling.
Giustini G, Ardron KH, Walker SP, 2018, Modelling of bubble departure in flow boiling using equilibrium thermodynamics, International Journal of Heat and Mass Transfer, Vol: 122, Pages: 1085-1092, ISSN: 0017-9310
To improve the closure relations employed for component-scale Computational Fluid Dynamics simulation of boiling flows, a first-principles method for predicting bubble departure diameters in flow boiling has been developed. The proposed method uses minimisation of the free energy of a system in thermodynamic equilibrium to predict the contact angle and the resistance to sliding of a vapour bubble attached to a surface in the presence of a forced liquid flow. Predictions of the new method are compared with measurements from existing experimental databases, and agreement with data is shown to be comparable or superior to that obtained with previous bubble departure models that have generally used a force-balance approach. The main advantages of the energy-based method over the previous force-based methods are that its formulation is simpler, and that the new model does not require the use of ad hoc tunable parameters to define force terms, or geometrical characteristics of the attached bubble such as its base area, which cannot be confirmed experimentally. This increases confidence in the validity of the new approach when applied outside the rather limited range of current test data on bubble departure in flow boiling.
Ardron K, Giustini G, Walker SP, 2017, Prediction of dynamic contact angles and bubble departure diameters in pool boiling using equilibrium thermodynamics, International Journal of Heat and Mass Transfer, ISSN: 0017-9310
Murallidharan J, Giustini G, Sato Y, et al., 2016, Computational Fluid Dynamic Simulation of Single Bubble Growth under High-Pressure Pool Boiling Conditions, Nuclear Engineering and Technology, Vol: 48, Pages: 859-869, ISSN: 1738-5733
Component-scale modeling of boiling is predominantly based on the Eulerian–Eulerian two-fluid approach. Within this framework, wall boiling is accounted for via the Rensselaer Polytechnic Institute (RPI) model and, within this model, the bubble is characterized using three main parameters: departure diameter (D), nucleation site density (N), and departure frequency (f). Typically, the magnitudes of these three parameters are obtained from empirical correlations. However, in recent years, efforts have been directed toward mechanistic modeling of the boiling process. Of the three parameters mentioned above, the departure diameter (D) is least affected by the intrinsic uncertainties of the nucleate boiling process. This feature, along with its prominence within the RPI boiling model, has made it the primary candidate for mechanistic modeling ventures. Mechanistic modeling of D is mostly carried out through solving of force balance equations on the bubble. Forces incorporated in these equations are formulated as functions of the radius of the bubble and have been developed for, and applied to, low-pressure conditions only. Conversely, for high-pressure conditions, no mechanistic information is available regarding the growth rates of bubbles and the forces acting on them. In this study, we use direct numerical simulation coupled with an interface tracking method to simulate bubble growth under high (up to 45 bar) pressure, to obtain the kind of mechanistic information required for an RPI-type approach. In this study, we compare the resulting bubble growth rate curves with predictions made with existing experimental data.
Giustini G, Walker SP, 2016, Evaporative thermal resistance and its influence on microlayer evaporation, 3rd International Topical Meeting on Advances in Thermal Hydraulics 2016, ATH 2016
Giustini G, Jung S, Kim H, et al., 2016, Evaporative thermal resistance and its influence on microscopic bubble growth, International Journal of Heat and Mass Transfer, Vol: 101, Pages: 733-741, ISSN: 0017-9310
Simulations of the formation of small steam bubbles indicate that the rate of growth of bubbles is very sensitive to the rate of evaporation of the micro-layer of liquid beneath the bubble. Such evaporation is rapid, and is modelled as being driven by the large heat flux through the thin liquid layer caused by the difference in temperature between the solid–liquid interface, and the saturation temperature in the interior of the bubble. However, application of this approach to recent experimental measurements of Jung and Kim generated anomalous results. In this paper we demonstrate that a model of the micro-layer heat flux that includes an allowance for the finite evaporative thermal resistance is able to eliminate these anomalies. This evaporative thermal resistance is a consequence of near-interface molecular dynamics, characterised by a quantity termed ‘evaporation coefficient’. Whilst in most engineering applications evaporative thermal resistance is small compared to conductive resistance, here, with the micro-layer thickness ranging from a few microns down to zero, it becomes of considerable importance. Selection of a molecular ‘evaporation coefficient’ to restore consistency to the anomalous measurements allows a plausible numerical value to be inferred. For the several times and multiple locations studied, a fairly consistent value of between 0.02 and 0.1 is indicated, (for saturated water in laboratory conditions), which itself is consistent with earlier literature values of this rather difficult quantity. It is shown that the evaporative resistance always represents a large fraction of the conductive resistance, and for important phases of the process dominates it. The need for inclusion of this phenomenon in the micro-layer models used in bubble analysis is clear.
Giustini G, Badalassi V, Walker SP, 2016, Analysis of the liquid film formed beneath a vapour bubble growing at a heated wall without neglect of evaporative thermal resistance, 2016 International Congress on Advances in Nuclear Power Plants, ICAPP 2016
Hansch S, Giustini G, Narayanan C, et al., 2015, Microlayer models for nucleate boiling simulations: The significance of conjugate heat transfer, 16th International Topical Meeting on Nuclear Reactor Thermal Hydraulics, NURETH 2015
Murallidharan, Giustini G, Sato Y, et al., 2015, Interface tracking based evaluation of bubble growth rates in high pressure pool boiling condition, 16th International Topical Meeting on Nuclear Reactor Thermal Hydraulics, NURETH 2015
Giustini G, Murallidharan, Sato Y, et al., 2015, Numerical study of heat diffusion controlled bubble growth in A pressurized liquid, 16th International Topical Meeting on Nuclear Reactor Thermal Hydraulics, NURETH 2015
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