Quasi-homogeneous boiling nucleation on a small spherical heater in microgravity

Pool boiling experiments were conducted in the European Space Agency (ESA) multi-user facility, the bubble, drop, particle unit (BDPU) in the microgravity environment of space. A part of the study involved the heating of a small sphere immersed in R-123 to the onset of nucleate boiling. An analysis of the nucleation process is presented, based on a prior work for so-called quasi-homogeneous nucleation with a flat heater surface in microgravity. Reasonably good qualitative agreement exists between the analysis and measurements.     

Bubble nucleation in advanced ODS ferritic steels for fusion applications

Abstract

Oxide dispersion strengthened (ODS) ferritic steels have been suggested as an advanced ferritic steel for service as the structure of a fusion power plant operating at high temperatures (up to 800°C) as well as low swelling fast reactor fuel cladding. Powder metallurgical methods are used to fabricate such steels, which contain a fine dispersoid of nanometric scale strengthening particles. In this paper, the author investigates the possibility that the ODS particles not only strengthen the steel, but also act as centres for the nucleation of bubbles containing transmutation helium atoms, generated over the 5–10 year lifetime in the replaceable first wall blanket by 14 MeV fusion neutrons. Calculations suggest that the lifetime swelling of such materials will be of the order of 2% in a fusion power plant, and the helium atoms are prevented from reaching grain boundaries where embrittlement might otherwise occur.     

Analysis of Micro Vapor Bubble Growing Process in Open Capillary Microgrooves

The growing process of the individual microbubble in an open rectangular capillary microgroove was theoretically analyzed in this study. Several correlations of bubble growth rate for pool boiling were proved not available for microgroove boiling. A theoretical model based on thermal equilibrium and force balance was developed in this article. The growing process of the individual microbubble was divided into three stages: initial growing stage, normal and axial confined ellipsoidal growing stage, and axial subcylindrical growing stage. Growth period and volume increment of the micro vapor bubble were analyzed. The calculation results indicate that the growth of the micro vapor bubble is confined by the geometric structure of the microgroove. Comparison of the results between calculation and experiment shows that the correlation is available to predict the bubble growth rate for boiling in microgrooves.     

Heat transfer from small objects in microgravity: Experiments and analysis

Heat transfer over a sub-millimeter spheroidal solid is of interest in many engineering processes. One important mechanism of heat transfer in the above processes is natural convection which leads to heat transfer rates many times larger than that of pure conduction. Despite the huge literature devoted to natural convection heat transfer rates over spheres (and to a smaller extent over spheroids) there is not a generally accepted correlation especially for small Rayleigh numbers. Existing correlations for external geometries predict a progressively increasing contribution of natural convection to heat transfer with respect to gravity (starting from zero gravity). To test the validity of these correlations, experiments are performed for the estimation of heat transfer rates at low gravity. Heat pulses are given to a miniature thermistor with a nearly spheroidal shape immersed in a liquid and its thermal response is registered during heating in parabolic flights. The contribution of natural convection to heat transfer is undoubtedly estimated from runs in which acceleration varies from 0 to 1.8 g. Surprisingly enough, the experiments showed that the Rayleigh number must take a minimum value before non-negligible effect of natural convection on heat transfer appears (existence of a threshold Rayleigh number). In the absence of natural convection (below Ra_thr) the experimental thermal response curves can be successfully described by approximating solutions of the transient heat conduction equation for the spheroidal geometry of the thermistor. Apparently, additional research is needed regarding the natural convection around sub-millimeter objects for small Rayleigh numbers.     

Bubble dynamics in microchannels. Part I: single microchannel

The present work explores experimentally bubble dynamics in a single trapezoid microchannel with a hydraulic diameter of 41.3 μm. The fabrication process of the microchannel employs a silicon bulk micromachining and anodic bounding process. Bubble nucleation, growth, departure size, and frequency are observed using a high speed digital camera and analyzed by the Image-Pro. The results of the study indicates that the bubble nucleation in the microchannel may be predicted from the classical model with microsized cavities and the bubble typically grows with a constant rate from 0.13 to 7.08 μm/ms. Some cases demonstrate an extraordinarily high growth rate from 72.8 to 95.2 μm/ms. The size of bubble departure from the microchannel wall is found to be governed by surface tension and drag of bulk flow and may be fairly correlated by a modified form of Levy equation. The bubble frequency in the microchannel is comparable to that in an ordinary sized channel. The traditional form of frequency-departure-diameter relationship seems to be inexistent in the microchannel of this study.     

Fire spread phenomena: The role of observation in experiment

Summarized are recent experimental findings of fire spread phenomena. This review covers flame spread over solids (including melting solids and metals), large-scale spread through discrete fuels (such as fire brands and fire whirls), and scale modeling techniques applied to flame spread study. Emphasis is placed on the importance of observation in experiments which is the source of imagination and successful modeling.     

The Influence of System Pressure on Bubble Coalescence in Nucleate Boiling

Boiling is one of the most effective heat transfer mechanisms. In spite of a long time of research, the physical fundamentals are still not sufficiently understood. Pursuing the objective to predict heat transfer based on physical and geometrical properties, experimental and numerical investigations are conducted at the institute of the authors. The focus of the presented research is the coalescence of two single bubbles under varying pressure conditions. In the experiment a thin stainless-steel foil is used as a Joule heater. The experiments were performed in a pressure range of 300–1000 mbar using FC72 as working fluid. Two types of heaters with a distance between two artificial nucleation sites of 300 μm (type 3) and 500 μm (type 5) were used. The experimental results indicate a strong dependence of the occurrence of bubble coalescence on pressure. For the type 5 heater, a Gaussian distribution for the coalescence frequency when plotted over pressure is observed. Experimental results with the type 3 heater show a similar distribution of the frequency with a shifted maximum. Further, it is shown that during bubble coalescence a small droplet can remain inside the bubble and enhance the heat transfer, which is attributed to an additional thin film region. The formation of this remaining droplet is sensitive to system pressure. Numerical investigations of bubble coalescence were conducted with the computational fluid dynamics (CFD) software OpenFOAM. In OpenFOAM, dynamic mesh handling allows high spatial resolution at the phase boundary, which is captured with the volume-of fluid method. Evaporation and a subgrid microscale model were implemented in the flow solver to account for evaporation at the phase boundary and the three-phase contact line. The results show a strong dependence of bubble dynamics and coalescence on contact angle and bubble growth rate. Although it was possible to observe the creation of the residual droplet, more effort needs to be put into finding appropriate initial conditions.     

Bubble dynamics in microchannels. Part II: two parallel microchannels

The present work investigates experimentally the bubble dynamics in two parallel trapezoidal microchannels with a hydraulic diameter of 47.7 μm for both channels. The fabrication process of the two parallel microchannels employs a silicon bulk micromachining and anodic bounding process. The results of this study demonstrate that the bubble growth and departure is generally similar to that in a single microchannel, i.e., bubbles, in general, grow linearly with time and their departure is governed by surface tension and drag due to bulk two-phase flow. For the two low mass flow rates, the growth of bubble in slug flow is also investigated. It is found that the bubble grows in the axial direction both forward and backward with its length increases exponentially due to evaporation of the thin liquid film between the bubble and heating wall. However, the coefficient of exponent is much smaller than that caused by evaporation due to the limitation effect of liquid pressure around the bubble.     

Simulation and experiment of the unsteady heat transport in the onset time of nucleation and crystallization of ice from the subcooled solution

Heat transfer is an unsteady process in the initial period of ice nucleation or phase transition from aqueous solution. During this period the latent heat of freezing increases the temperature in bulk solution monotonously until the system reaches equilibrium. Meanwhile heat can transfer from the solution to the environment or vise versa. The analysis of this unsteady heat transfer process leads to the establishment of a mathematical model, which is represented by two simultaneous differential equations. Using the Laplace transform and inverse transform, and incorporating the initial condition of ice nucleation, we obtained an analytical solution of this model. Further discussion of the model’s fitness by comparing to the experimental data leads to a recognition that ice fouling (or ice adhesion) on the cooler wall should be highlighted in estimating the heat transfer resistance at the very beginning of the ice formation. The model fits to the experimental data satisfactorily.     

The influence of contact line velocity and acceleration on the dynamic contact angle: An experimental study in microgravity

In this study, a novel experimental method by using the short-time microgravity condition created by the drop tower facility is proposed to investigate the dependence of dynamic contact angle on both the velocity and acceleration of contact line. The values of the dynamic contact angle with respect to different velocity and acceleration of contact line were determined under some different parameters. Our experimental results show that the dynamic contact angle depends not only on the velocity of the contact line but also its acceleration.     

Bubble小波在高温熔体图像分割中的应用

在基于CCD图像传感器构成的高温测量系统中，为了计算高温目标的温度场，必须将高温目标图像与其背景图像准确分割。把通常用于图像边缘检测的小波模极大值取阈值法直接应用于目标图像的分割，设计一种基于Bubble小波变换的高温熔体图像分割算法，实验结果表明能快速准确地分割出目标图像，有很强的实用性。     

串级模糊控制器在过热蒸汽温度控制中的应用

过热器是一个典型的非线性、时变、大迟延、大惯性和多随机干扰扰动的设备,数学模型难以建立。利用模糊控制不依赖被控对象的数学模型,将串级控制和模糊控制优势相结合,设计出过热蒸汽温度串级模糊控制方案。仿真结果表明,本控制策略比过热蒸汽温度串级控制具有更强的适应能力和良好的控制品质。     

Surface wettability control by nanocoating: The effects on pool boiling heat transfer and nucleation mechanism

Experiments were performed to highlight the influence of surface wettability on nucleate boiling heat transfer. Nanocoating techniques were used to vary the water contact angle from 20° to 110° by modifying nanoscale surface topography and chemistry. The bubble growth was recorded by a high speed video camera to enable a better understanding of the surface wettability effects on nucleation mechanism. For hydrophilic (wetted) surfaces, it was found that a greater surface wettability increases the vapour bubble departure radius and reduces the bubble emission frequency. Moreover, lower superheat is required for the initial growth of bubbles on hydrophobic (unwetted) surfaces. However, the bubble in contact with the hydrophobic surface cannot detach from the wall and have a curvature radius increasing with time. At higher heat flux, the bubble spreads over the surface and coalesces with bubbles formed at other sites, causing a large area of the surface to become vapour blanketed. The best heat transfer coefficient is obtained with the surface which had a water contact angle close to either 0° or 90°. A new approach of nucleation mechanism is established to clarify the nexus between the surface wettability and the nucleate boiling heat transfer.     

Explosive vapor bubble growth in uniformly superheated liquids: R-113 and mercury

Based on experimental and analytical work conducted previously with R-113, the results of an analytical study of the vapor bubble dynamics for mercury associated with nucleation and growth are presented here. The simulations show that a growing mercury vapor bubble can be substantially unstable under sufficiently high superheat, as with a superheat level of 100 °C, and that surface tension tends to stabilize the process. At a superheat level of 30 °C, the growth is marginally stable due to the high surface tension, and quite stable in the very early stages of the growth. For most cases, the wavelength of growing perturbations appear to be relatively large compared to that for water, again due to the large surface tension of mercury. As a result of the high liquid-to-vapor density ratio, the growth rates of mercury vapor bubbles can be quite high, reaching a diameter of 1 m in 1 s, with an initial superheat of 100 °C and pressure of 0.1 atm. The effects of system pressure are also considered here.     

A comparative analysis of time-of-use electricity rate effects: The Arizona experiment

To date, analyses of the data from the Arizona time-of-use (TOU) electricity pricing experiment have failed to find a significant relationship between electricity usage and electricity price. Results indicated that if the TOU rates had an effect on usage, the effect was virtually identical for each TOU rate. In this study, usage in 1976 under the TOU rates is compared to usage in 1975 under the existing (declining block) rate to determine whether or not there was an effect.It is found that peak period usage was reduced under TOU rates by 7–16% after adjusting for differences in weather between the two years. To a lesser extent, customers reduced their usage during the intermediate period and shifted some consumption to the base period. Total consumption appears to have decreased slightly. It is emphasized that these effects of the TOU rates cannot be discovered in demand analyses that do not utilize the baseline year data.     

Extinction of premixed methane/air flames in microgravity by diluents: Effects of radiation and Lewis number

Laminar burning velocities and flammability limits of premixed methane/air flames in the presence of various diluents were investigated by combined use of experiments and numerical simulations. The experiments used a 1-m free-fall spherical combustion chamber to eliminate the effect of buoyancy, enabling accurate measurements of near-limit burning velocities and flammability limits. Burning velocities were measured for CH₄/air flames with varying concentrations of He, Ar, N₂ and CO₂ at NTP. The limiting concentration of each diluent was measured by systematically varying the composition and ignition energy and finding the limiting condition through successive experiment trials. The corresponding freely-propagating, planar 1-D flames were simulated using PREMIX. The transient spherically-expanding flames were simulated using the 1-D Spherical Flame & Reactor Module of COSILAB considering detailed radiation models. The results show that helium exhibits more complex limit behavior than the other diluents due to the large Lewis number of helium mixtures. The near-limit helium-diluted flames require much higher ignition energy than the other flames. In addition, for the spherically expanding helium-diluted flames studied here (Le > 1), stretch suppresses flame propagation and may cause flame extinction. For the CO₂-diluted flames, the flame speed predicted by the optically-thick model based on the Discrete Transfer Method (DTW) and a modified wide band model has better agreement with measurements in the near-limit region. A significant amount of heat is absorbed by the dilution gas CO₂, resulting in elevation of temperature of the ambient gases. The optically-thick model, however, still overpredicts flame speed, indicating a more sophisticated radiation property model may be needed. Finally, the chemical effect of CO₂ on flame suppression was quantified by a numerical analysis. The results show that the chemical effect of CO₂ is more important than the other diluents due to its active participation in the reaction CO₂ + H = CO + OH, which competes for H radicals with the chain-branching reactions and thus reduces flame speed.     

Vapor compression multifunctional heat pumps in China: A review of configurations and operational modes

With the growing concerns about worldwide energy and environmental sustainability, heat pump water heaters and solar water heaters became popular in China after 2000. The combinations between the heat pump air conditioner, heat pump water heater and solar water heater brought about more energy saving operational modes besides those included in the above three appliances, which promoted the development of multifunctional heat pumps. The combinations also resulted in the higher utilization ratio and lower operational cost of the heat pump, then the payback time can be shortened greatly. The rapid development of multifunctional heat pumps at the beginning of the 21st century in China indicates its promising application prospect.The heat pump air conditioner was the fundamental component of a multifunctional heat pump. Versatile configurations of multifunctional heat pumps were evolved from the integration of the domestic water heat exchanger with the refrigerant loop of the heat pump air conditioner by various approaches. This paper reviewed the development of multifunctional heat pumps in China, mainly focusing on configuration features and operational modes of the heat pump. The configuration of a multifunctional heat pump fundamentally determines the initial cost, operating cost and operating reliability. Therefore, it is reasonable to make a compromise between the simplicity of the configuration and versatile operational modes in the design of the multifunctional heat pump under different application conditions.     

Valuing energy-saving measures in residential buildings: A choice experiment study

Air-conditioning and heating energy-saving measures can cut back the usage of energy. This paper attempts to apply a choice experiment in evaluating the consumer's willingness to pay (WTP) for air-conditioning and heating energy-saving measures in Korea's residential buildings. We consider the trade-offs between price and three attributes of energy-saving (window, facade, and ventilation) for selecting a preferred alternative and derive the marginal WTP (MWTP) estimate for each attribute. We also try to test irrelevant alternatives property for the estimation model holds and compare the estimation results of the multinomial logit (MNL) and the nested logit (NL) models. The NL model outperforms the MNL model. The NL model show that MWTPs for increasing the number of glasses and their variety, for increasing the thickness of facade for 1 mm, and for establishing a ventilation system are KRW 17,392 (USD 18.2), 1,112 (1.2), and 11,827 (12.4), respectively. Overall, the potential consumers have significant amount of WTP.     

A computational study of spherical diffusion flames in microgravity with gas radiation Part I: Model development and validation

The overarching goal of this study is to improve our understanding of the extinction characteristics of spherical diffusion flames in microgravity. In particular, one of the key objectives is to assess the effects of gas radiation as a means to promote flame extinction. To investigate these phenomena, a one-dimensional computational model was developed to simulate the evolution of a spherical diffusion flame with consideration of detailed chemistry and transport properties. The model formulation was described along with the detailed numerical method. Radiation model was discussed with two aspects: radiation property model and radiative transfer model. Various levels of radiation models were implemented and the results were compared with experimental measurements of flame radius and temperature profiles. It was shown that the statistical narrow band model (SNB) combined with the discrete ordinate method (DOM) reproduced the experimental results with highest accuracy, and this combination of the radiation models were adopted in the subsequent parametric studies in Part II. Computational issues to optimize numerical accuracy and efficiency are also discussed.