Nucleate boiling heat transfer in aqueous solutions with carbon nanotubes up to critical heat fluxes

In this study, pool boiling heat transfer coefficients (HTCs) and critical heat fluxes (CHFs) are measured on a smooth square flat copper heater in a pool of pure water with and without carbon nanotubes (CNTs) dispersed at 60 °C. Tested aqueous nanofluids are prepared using multi-walled CNTs whose volume concentrations are 0.0001%, 0.001%, 0.01%, and 0.05%. For the dispersion of CNTs, polyvinyl pyrrolidone polymer is used in distilled water. Pool boiling HTCs are taken from 10 kW/m² to critical heat flux for all tested fluids. Test results show that the pool boiling HTCs of the aqueous solutions with CNTs are lower than those of pure water in the entire nucleate boiling regime. On the other hand, critical heat flux of the aqueous solution is enhanced greatly showing up to 200% increase at the CNT concentration of 0.001% as compared to that of pure water. This is related to the change in surface characteristics by the deposition of CNTs. This deposition makes a thin CNT layer on the surface and the active nucleation sites of the surface are decreased due to this layer. The thin CNT layer acts as the thermal resistance and also decreases the bubble generation rate resulting in a decrease in pool boiling HTCs. The same layer, however, decreases the contact angle on the test surface and extends the nucleate boiling regime to very high heat fluxes and reduces the formation of large vapor canopy at near CHF. Thus, a significant increase in CHF results in.     

Enhancement of pool boiling heat transfer in a horizontal water layer through surface roughness and screen coverage

An experimental investigation was carried out to study the augmentation of heat transfer in saturated pool boiling of a liquid water layer on a heated horizontal stainless steel plate by roughing the surface and/or covering it with a single layer of stainless steel screen. The results were presented in terms of the boiling curves. Effects of various parameters – the surface roughness, liquid level and size of the stainless steel mesh on the boiling heat transfer were examined in detail. The measured data clearly indicated that a lowering of the liquid level from 60 to 5?mm in water depth causes heat transfer reduction. Roughing the surface was found to sig- nificantly enhance the heat transfer. Use a layer of metal screen to cover the heated surface was shown to substantially augment the heat transfer especially for a shallow water layer if the mesh size is comparable with the bubble departure diameter. Covering the rough surface with the metal mesh, however, reduced the heat transfer.     

Preparation and analysis of different roughness structures for evaporator tubes

The processes of the phase change in boiling occur at the solid–liquid interface by heat transfer from a solid heating surface to the boiling liquid. The characteristic features of the heating surfaces are therefore of great interest to optimize the design of evaporators. The microstructure with all its peaks and cavities influences directly the wetting and rewetting conditions of the heated surface by the boiling liquid and hence bubble formation and heat transfer. The roughness structures of different evaporator copper tubes with 8 or 25 mm diameter are characterized quantitatively with regard to the cavities offered to nucleation. The surfaces of the heating elements are sandblasted by different means resulting in a stochastic microstructure. The surfaces are investigated by a three-dimensional contactless roughness measurement technique combining the stylus technique with the near field acoustic microscopy. The method opens the possibility to obtain results according to standard for practical applications and additionally delivers detailed information about the three-dimensional shape of each cavity within the surface investigated. The analysis of the microstructure implies the total number of cavities, their local and size distribution calculated by the method of the envelope area. The results of the surface analysis are linked to those of heat transfer and bubble formation discussed in a contribution by Kotthoff and Gorenflo.     

Bubble growth in saturated pool boiling in water and surfactant solution

The presence of surfactant additives in water was found to enhance the boiling heat transfer significantly. The objective of the present investigation is to compare the bubble growth in water to that of a surfactant solution with negligible environmental impact. The study was conducted at two values of heat fluxes to clarify the effect of the heat flux on the dynamics of bubble nucleation. The bubble growth under condition of pool boiling in water and non-ionic surfactant solution was studied using high speed video technique. The bubble generation was studied on a horizontal flat surface; and the natural roughness of the surface was used to produce the bubbles.     

Microscope activation near the wall during explosive boiling

The inception process of nucleation in explosive boiling systems is theoretically investigated. With the effect of pulse heating or sudden cooling, the temperature distribution near the surface during explosive boiling is calculated. The liquid near the wall can maintain a stable layer induced by strong attractive force, and there exists maximum supersaturation beyond this stable layer. As the surface temperature and temperature gradient are high enough, the critical distance of maximum supersaturation can be larger than the radius of critical bubble, and the homogeneous nucleation will dominate the inception boiling process. For explosive boiling induced by pulse heating, homogeneous nucleation forms after a short time; while homogeneous nucleation can dominate the initial explosive boiling induced by sudden cooling.     

单空泡与自由液面相互作用规律研究进展

空化与空泡溃灭现象普遍存在于自然界、标识码械和生物医学等领域.空泡与自由面相互作用会产生瞬态强烈耦合,涉及到空泡非球形溃灭、自由面非线性变形及失标识码象,是流体力学领域重要的前沿与基础问题. 本文围绕这一热点,从空泡非球形演化和自由面变形规律角度出发,概标识码纳近年该领域的研究进展与成果. 对于近自由面空泡的非球形演化,基于表征开尔文冲量的无量纲参数,重点关注了体积振荡、射流生成、水锤效应及溃灭标识码生成等关键过程,介绍了关键参数的理论建模方法,获得了空泡溃灭过程中能量分配机制. 针对自由液面变形演化,根据细射流和粗射流生成和发展,归纳了 4 种典型现象及特点:透明水层及水柱生成、不稳定与稳标识码水裙结构. 进一步总结了开尔文冲量理论、界面凹陷奇点概念和泰勒不稳定性等理论模型的建立和应用,讨论了气泡溃灭过程、液面标识码界面稳定性等主要机制. 此外,本文也概述了空泡脉动对球状、圆柱状等非平面液面变形行为的影响,归纳了曲率对于液面变形的影响机制. 最后,针对目前研究状况提出该领域研究中尚未解决的问题,期望对将来的空泡及空泡群与自由液面相互作用深入研究提供借鉴.      

Visualization of oxygen concentration fields and measurement of concentration gradients at bubble surfaces in surfactant-contaminated water

A laser-induced fluorescence (LIF) technique is employed for visualizing a thin two-dimensional (2D) dissolved oxygen concentration field and measuring local oxygen concentration gradients near the surface of an oxygen bubble in water containing surfactant (Triton X-100, SigmaAldrich, St Louis, MO, USA)). The fluorescence of pyrene butyric acid (PBA) is induced by a planar pulse of nitrogen laser light. Oxygen transferring from the bubble to the deoxygenated water quenches the fluorescence of the PBA. Images of the fluorescence fields are captured by a UV-intensified CCD camera. The intensity of fluorescence quenching at each image pixel is used to measure dissolved oxygen concentration in a 2D field. Images of bubbles are obtained at 200 ppm, 100 ppm, and 50 ppm Triton X-100-containing water and in ultra clean deionized water. Higher surfactant concentrations decrease local and average concentration gradients of oxygen at the bubble surface. The ensemble means of dissolved oxygen concentration boundary layer thicknesses of 0.160 mm, 0.130 mm, and 0.072 mm, for the images of bubbles obtained at 200 ppm, 100 ppm, and 50 ppm Triton X-100-containing water, respectively. Local concentration boundary layer thickness increases from the top to the bottom along the bubble surface. A series of images of the bubble flow fields are analyzed to measure the oxygen concentration gradients in water in the presence of surfactant. The images captured in clean water are not fully resolvable because of their poor resolution. The formation of the attached wake in the fluorescence field images at the bottom of the bubbles in clean water tends to be promoted by increasing oblateness owing to the presence of surfactant at the surface.     

The bubble fossil record: insight into boiling nucleation using nanofluid pool-boiling

Subcooled pool boiling of Al₂O₃/water nanofluid (0.1 vol%) was investigated. Scanning electron microscopy and energy dispersive X-ray spectroscopy were used to observe surface features of the wire heater where nanoparticles had deposited. A layer of aggregated alumina particles collected on the heated surface, where evidence of fluid shear associated with bubble nucleation and departure was “fossilized” in the fluidized nano-porous surface coating. These structures contain evidence of the fluid forces present in the microlayer prior to departure and provide a unique understanding of boiling phenomena. A unique mode of heat transfer was identified in nanofluid pool boiling.     

Boiling characteristics in small vertical tubes with closed bottom for nanofluids and nanoparticle-suspensions

An experimental study was carried out to understand the nucleate boiling characteristics and the critical heat flux (CHF) of water, the water based nanofluids and the water based nanoparticle-suspensions in vertical small heated tubes with a closed bottom. Here, the nanofluids consisted of the base liquid, the CuO nanoparticles and the surfactant. The nanoparticle-suspensions consisted of the base liquid and CuO nanoparticles. The surfactant was sodium dodecyl benzene sulfate. The study focused on the influence of the nanoparticles and surfactant on the nucleate boiling characteristics and the CHF. The experimental results indicated that the nanoparticle concentrations of the nanofluids and nanoparticle-suspensions in the tubes do not change during the boiling processes; the nanoparticles in the evaporated liquid are totally carried away by the steam. The boiling heat transfer rates of nanofluids are poorer than that of the base liquid. However, the boiling heat transfer rates of nanoparticle-suspensions are better than that of the base liquid. Comparing with the base liquid, the CHF of the nanofluids and the nanoparticle-suspensions is higher. The CHF is only related to nanoparticle mass concentration when the tube length and the tube diameter are fixed. The experiment confirm that there is a thin nanoparticle coating layer on the heated surface after the nanofluids boiling test but there is no coating layer on the heated surface after the nanoparticle-suspensions boiling test. This coating layer is the main reason that deteriorates the boiling heat transfer rates of nanofluids. An empirical correlation was proposed for predicting the CHF of nanofluids boiling in the vertical tubes with closed bottom.     

Bubble dynamics in microchannel: An overview of the state-of-the-art

The inception of the boiling, in a pool or flow boiling, is the formation of the vapor bubble at an active nucleation site that plays a crucial role in the boiling process and it becomes critical and unfolds many facets when channel size reduces to submicron. The detailed knowledge of the bubble dynamics is helpful in establishing the thermal and hydraulic flow behavior in the microchannel. In the current paper, bubble dynamics that include bubble nucleation at the nucleation site, its growth, departure, and motion along the flow in a microchannel(s) are discussed in detail. Different models developed for critical cavity radius favorable for bubble nucleation are compiled and observe that models exhibit large deviation. The bubble growth models are compiled and concluded that the development of a more generalize bubble growth model is necessary that would be capable of accounting for inertia controlled and thermal diffusion controlled regions. Bubbles at nucleation sites in a microchannel grow under the influence of various forces such as surface tension, inertia, shear, gravitational and evaporation momentum. Parametric analysis of these forces reckoned that the threshold between macro- to microchannel could be identify through critical analysis of such forces. Eventually, the possible impact of the various factors such as operating conditions, geometrical parameters, thermophysical properties of fluid on bubble dynamics in microchannel has been reported.

     

Study on heat transfer performance of spray cooling: model and analysis

In consideration of droplet–film impaction, film formation, film motion, bubble boiling (both wall nucleation bubbles and secondary nucleation bubbles), droplet–bubble interaction, bulk air convection and radiation, a model to predict the heat and mass transfer in spray cooling was presented in this paper. The droplet–film impaction was modeled based on an empirical correlation related with droplet Weber number. The film formation, film motion, bubble growth, and bubble motion were modeled based on dynamics fundamentals. The model was validated by the experimental results provided in this paper, and a favorable comparison was demonstrated with a deviation below 10%. The film thickness, film velocity, and non-uniform surface temperature distribution were obtained numerically, and then analyzed. A parameters sensitivity analysis was made to obtain the influence of spray angle, surface heat flux density, and spray flow rate on the surface temperature distribution, respectively. It can be concluded that the heat transfer induced by droplet–film impaction and film-surface convection is dominant in spray cooling under conditions that the heated surface is not superheated. However, the effect of boiling bubbles increases rapidly while the heated surface becomes superheated.     

Modes of bubble growth in the slow-formation regime of nucleate pool boiling

The shape and size of a bubble formed slowly on a sharp- or round-edged orifice are derived with the help of a new analytical solution for the bubble profile. Two modes of formation are distinguished, depending on the natural contact angle, ?₀: bubble confined to the orifice (?₀ small); bubble spreading beyond the orifice (?₀ large: Fritz mode). The limits of the slow-formation regime in mucleate pool boiling are estimated, involving an assessment of the influences of liquid inertia, viscosity and surface-tension gradients.“Slow” formation is predicted for large cavities or high pressures and this is borne out by data for water. The Fritz mode of growth, however, is seen to be suppressed.     

ADSORPTION OF NANO-PARTICLES ON BUBBLE SURFACE IN NANO-PARTICLE SUSPENSION

The adsorption of nano-particles on bubble surface is discussed for saturated boiling on thin wire of nano-particle suspensions. Owing to the decrease of surface tension for suspensions, the nano-particles tend to adsorb on the bubble surface to decrease the Gibbs free energy for stability, and meanwhile the velocity of nano-particles would be smaller than that of bubble growth. The long-range van der Waals force existing between “water particles” and nano-particles is considered the attractive force between the nano-particles and the bubble surface. Thus, the nano-particles would attach on the bubble surface if the particle-surface distance is smaller than its critical value. The distribution of nano-particles on the bubble surface and in the adjacent region is also investigated.     

High-pressure foaming properties of carbon dioxide-saturated emulsions

The aeration of emulsions with tailored properties and structure is of widespread importance in processing of foods and cosmetics. This report addresses the micro-cellular foam formation of carbon dioxide-saturated oil-in-water emulsions triggered by the application of a controlled pressure drop. The experimental setup combines a stirred pressure vessel with a pressure cell-equipped rheometer and pneumatic expansion valves. This allows to systematically study the process of gas dissolution, bubble nucleation, and growth under defined pressure, temperature, and flow conditions. Investigations on the impact of relevant process parameters show that dissolved gas fraction, emulsion viscosity, and shear rate have a major influence on foam formation. Dissolution of carbon dioxide leads to a viscosity reduction of the emulsion which and is described by a viscosity reduction factor. The point of bubble nucleation is derived from rheological patterns during depressurization. Experiments show that lower emulsion viscosity and higher shear rate favor bubble nucleation upon pressure release. Rheological results are supported by video analysis as the setup allows capturing nucleation, growth, and destabilization of bubbles as a function of pressure, supersaturation, and time. The results of this work yield the understanding of the high-pressure foaming mechanism from a rheological perspective and foster the design of such processes.     

2D lattice Boltzmann investigation of saturated pool boiling using a tunable surface tension model: Prandtl number effects on film boiling

In this paper, the saturated pool boiling is investigated using lattice Boltzmann method. The written FORTRAN code is validated in two aspects: For flow, the thermodynamic consistency test and Laplace law are applied and for heat transfer, the space- and time- averaged Nusselt number is compared with Berenson analytical solution in film boiling regime. In addition, the results of bubble generation and departure are compared with some well-known analytical solutions to show the accuracy of the code. It is confirmed that bubble departure diameter and the departure frequency are related to the gravity acceleration with powers of ? 0.505 and 0.709, respectively, which is in a very good agreement with the existing analytical expressions. The present model has the ability to tune different surface tensions independent of liquid/vapor density ratio, which was unreachable using other existing numerical models of boiling. Thus, the sole effects of surface tension on boiling can also be taken into consideration using the present model. It is also shown that the departure diameter is related to the surface tension with a power of 0.485, which is in good agreement with the analytical expressions. Temperature contours are shown together with flow lines to have a better viewpoint for studying the bubble’s behavior. An intensive temperature gradient is observed in the necking area at the departure time. All the four boiling regimes in the boiling curve are simulated under constant temperature boundary condition. The Prandtl number effects on vapor bubble dynamics in the film boiling regime are investigated using the improved Shan and Chen model for the first time. Results revealed that bubbles are more resistant to depart from the vapor blanket with increasing the Prandtl number.     

Mass transfer behavior at bubble surface during nucleate boiling

 Interfacial mass transfer mechanisms played an essential role to the high heat transfer efficiency noted for nucleate boiling. There existed a zone around the bubble surface that exhibited zero net mass flux, termed herein as the “zero-flux zone”. This work investigated analytically the interfacial vaporization and condensation processes around a boiling bubble, based on which the positional dependence of zero-flux zone was derived. For a stationary bubble the zero-flux zone shifted to the upper hemisphere with decreasing wall superheat and/or with increasing contact angle. Moreover, the bubble growth (shrinkage) largely enhanced (retarded) such a trend. At the extreme condition where the bubble grew at a very fast speed the entire bubble surface would be subject to liquid evaporation only. Experiments observed a “thermal jet” emerging from the bubble cap, which was attributed to the interfacial vapor condensation flux at the bubble cap. Received on 11 December 2000 / Published online: 29 November 2001     

Temperature fluctuations in a two-phase superheated layer in the boiling of a liquid

One of the basic parameters defining the intensity of heat transfer on the boundary separating liquid and vapor phases is the superheating of the liquid during boiling. In a two-phase boundary layer superheating constitutes a variable (fluctuating) quantity, which depends on several parameters of the system and is of a statistical nature. This paper is devoted to a study of the statistical nature of the fluctuations in the superheating of a liquid. Starting from experimental data, obtained in measuring temperature fluctuations in a two-phase boundary layer during the boiling of water in contact with a heated surface, we carry out a statistical analysis of the amplitude of the fluctuations. Based on this analysis, we determine the average and the maximum superheating as a function of the distance to the heated wall. To determine the microstructure of the temperature fluctuations and to study their origin, we took high-speed pictures of the head of a thermocouple in contact with the two-phase medium. We established that the presence of various size amplitudes is associated, in the main, with two effects: the existence of a superheated layer on a bubble in the course of its growth and the convection of the liquid close to the bubble.Translated from Zhurnal Prikladnoi Mekhaniki i Tekhnicheskoi Fiziki, No. 5, pp. 62–69, September–October, 1973.     

Boiling heat transfer characteristics of nanofluids jet impingement on a plate surface

The jet boiling heat transfer of a bar water–CuO particle suspensions (nanofluids) jet impingement on a large flat surface was experimentally investigated. The experimental results were compared with those from water. The quantificational effects of the nanoparticles concentration and the flow conditions on the nucleate boiling heat transfer and the critical heat flux (CHF) were investigated. The experimental data showed that the jet boiling heat transfer for the water–CuO nanofluid is significantly different from those for water. The nanofluids have poor nucleate boiling heat transfer compared with the base fluid due to that a very thin nanoparticle sorption layer was formed on the heated surface. The CHF for the nanofluid increased compared with that of water. The reasons were that the solid–liquid contact angle decreased due to a very thin sorption layer on the heated surface and the jet and agitating effect of the nanoparticles on the subfilm layer enhance supply of liquid to the surface.     

Influence of surface topography in the boiling mechanisms

The present paper addresses the qualitative and quantitative analysis of the pool boiling heat transfer over micro-structured surfaces. The surfaces are made from silicon chips, in the context of pool boiling heat transfer enhancement of immersion liquid cooling schemes for electronic components. The first part of the analysis deals with the effect of the liquid properties. Then the effect of surface micro-structuring is discussed, covering different configurations, from cavities to pillars being the latter used to infer on the potential profit of a fin-like configuration. The use of rough surfaces to enhance pool boiling mainly stands on the arguments that the surface roughness will increase the liquid–solid contact area, thus enhancing the convection heat transfer coefficient and will promote the generation of nucleation sites. However, one should not disregard bubble dynamics. Indeed, the results show a strong effect of bubble dynamics and particularly of the interaction mechanisms in the overall cooling performance of the pair liquid–surface. The inaccurate control of these mechanisms leads to the formation of large bubbles and strong vertical and horizontal coalescence effects promote the very fast formation of a vapor blanket, which causes a steep decrease of the heat transfer coefficient. This effect can be strong enough to prevail over the benefit of increasing the contact area by roughening the surface. For the micro-patterns used in the present work, the results evidence that one can reasonably determine guiding pattern characteristics to evaluate the intensity of the interaction mechanisms and take out the most of the patterning to enhance pool boiling heat transfer, when using micro-cavities. Instead, it is far more difficult to control the appearance of active nucleation sites and the optimization of the patterns allowing a reasonable control of the interaction mechanisms and in particular of horizontal coalescence, when dealing with the patterns based on micro-pillars. Hence, providing an increase of the liquid contact area by an effective increase of the roughness ratio is not enough to assure a good performance of the micro-structured surface. Despite it was not possible to clearly evidence a pin–fin effect or of an additional cooling effect due to liquid circulation between the pillars, the results show a significant increase of the heat transfer coefficient of about 10 times for water and 8 times for the dielectric fluid, in comparison to the smooth surface, when the micro-patterning based on pillars is used.