Evaporating characteristics of a microlayer under a bubble

Using the Hallinan‐Ervin model, the flow and evaporation in a bubble microlayer were theoretically analyzed, and the dryout characteristics and Staub's criterion were discussed in detail. It was revealed that the critical dryout radius is associated with the wettability of the heated surface, and that the dominant role for microlayer evaporation is disjoining pressure, not surface tension gradient. © 2002 Wiley Periodicals, Inc. Heat Trans Asian Res, 31(6): 456–462, 2002; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.10052     

电场作用下气泡的行为研究

为进一步揭示EHD强化沸腾换热机理，分析了电场作用下气泡的受力情况，综述了国内外对电场作用下气泡的行为研究成果，系统总结了电场对气泡行为及其动力学特性的影响，并对该领域的进一步研究提出了建议。     

Temperature fluctuation in bubble nucleate pool boiling: A proposed new factor of evaporation

The phase change rate and related physical constants of superheated water at the liquid surface can be determined by the mechanism of hydrogen bonding and by the Maxwell‐Boltzmann principle. In this paper, we introduce a physical constant and attempted to demonstrate the existence and the value of this physical constant. First, we measured temperature fluctuations under the bubbles in a pool of boiling water. Second, a sample analytical model was introduced, to determine the value of the physical constant. Comparisons were made between the experimental data and those of the calculations. © 2002 Wiley Periodicals, Inc. Heat Trans Asian Res, 31(7): 578–594, 2002; Published online in Wiley InterScience ( www.interscience. wiley.com ). DOI 10.1002/htj.10059     

An experimental study on bubble growth within a narrow space under an interference‐piece

An experimental study was performed on the nucleate boiling of water in the narrow space formed between an interference piece and the heating surface of heat‐proof glass. A past study analyzed the bubble growth due to the conduction of heat through the thinner liquid layer under a growing bubble; however, the thickness of the liquid layer was not shown experimentally. This study investigates the thickness of this thinner layer by the interference method. Almost no change was found on the thickness of the liquid layer under both the interference‐piece and on the heating surface. The experimental results indicate the great contribution of superheating of the test liquid to bubble growth. © 2006 Wiley Periodicals, Inc. Heat Trans Asian Res, 35(2): 165–177, 2006; Published online in Wiley InterScience ( www.interscience. wiley.com ). DOI 10.1002/htj.20100     

Dynamics of a vapor bubble on a heated substrate

The dynamics of a vapor bubble between its liquid phase and a heated plate is studied in relation to the breakdown and recovery of the film boiling. By examining the expansion and the contraction of the vapor bubble the film boiling and transition boiling states are predicted. Conservation laws in the vapor, solid, and liquid phases are invoked along with fully nonlinear, coupled, free boundary conditions. These coupled system of equations are reduced to a single evolution equation for the local thickness of the vapor bubble by using a long-wave asymptotics, which is then solved numerically to yield the transient motion of the vapor bubble. Of the numerous parameters involved in this complex phenomenon we focus on the effects of the degree of superheat from the solid plate, that of the supercooling through the liquid, and the wetting/dewetting characteristics of the liquid on the solid plate. A material property of the substrate thus is incorporated into the criteria for the film boiling based on hydrodynamic models.     

Quasi-static bubble formation on submerged orifices

The static formation of air bubbles emanating from a submerged orifice was analyzed based on the principles of force balance. The analysis was corroborated by experiments using a shadow imaging technique. The influence of the Young contact angle of the three material components was investigated by identifying two modes of formation, corresponding to bubble formation at the orifice rim and spreading of the bubble base on the horizontal surface around the bubble. The characteristics of the formation process such as shape, height and volume of the bubble and the non-dimensional pressure difference at the orifice are given for both modes up to the critical equilibrium, defined by the maximum bubble volume, at which the bubble releases dynamically.     

The effects of bubble translation on vapor bubble growth in a superheated liquid

A theoretical analysis of vapor bubble growth in a uniformly superheated liquid has been carried out to determine the effects of translational motion of the bubble on the bubble growth rate. Assuming potential flow in the region surrounding the bubble the appropriate convective diffusion equation is solved by means of a new similarity transformation. The results of the theoretical analysis are compared with available experimental data and with analyses of the limiting cases of no bubble translation and quasi steady state bubble growth. The analysis is shown to reduce to the Plesset and Zwick or Scriven analysis for stationary growing bubbles. The effects of translation are found to be significant when the translational velocity is sufficiently high at moderate Jakob numbers, but for high Jakob numbers radial convection predominates and translation has little effect on the growth rates. The analysis predicts results in good agreement with experimental data available in the literature.     

Nucleate and film boiling on a microheater under pulse heating in a microchannel

Using MEMS technology, a Pt microheater (60 × 100 µm²) fabricated on a glass wafer is placed in a silicon-based microchannel of trapezoidal cross section. With the aid of a high-speed CCD and based on Pt's linear temperature-resistance characteristic, flow boiling phenomena and temperature response on the surface of the microheater in the microchannel under pulse heating are observed and recorded. At a given mass flux, nucleate boiling and film boiling begin to appear on the microheater with increasing heat flux. A flow boiling map, showing the effects of heat and mass flux on nucleate and film boiling regimes on the microheater at a pulse heating width of 2 ms, is presented. It is found that nucleate boiling is changed to film boiling as the heat flux supplied to the microheater is increased. Furthermore, increasing mass flux increases the heat flux required for the incipience of nucleate boiling and film boiling on the microheater in the microchannel.     

A model for vapor bubble growth under variable liquid pressure fields

An explicit bubble growth formula which applies both for constant and variable liquid pressure fields is presented. The present model is based in part on the previous variable pressure bubble growth correlations and in part on the asymptotic solution offered by Plesset and Zwick. When the present analytical quasi-asymptotic bubble growth curves are compared with those of actual experimental radius versus time curves and with the model presented by Plesset and Zwick, the present variable pressure theoretical bubble radius predictions agree with the experimental data much better than do the predictions of the constant pressure theories.     

Temperature fluctuation under bubbles of pool boiling in decompression

Experiments on pool boiling of water were performed under decompression. Temperature fluctuation was measured with three thermocouples which were equipped on the copper block surface by plating. Rapid temperature drops in this fluctuation arise at the same times. This shows the existence of evaporation of the microlayer under bubbles. Evaluation of this evaporation of the microlayer and rapid temperature drops revealed the existence of a new evaporation factor in obedience to the liquid surface overheating. Temperature drops are of two types at decompression. One is rapid drop due to cavity existence and the other is slow drop due to generation in the superheated bulk liquid on the mirror surface. This rapid temperature drop is gradual at the first stage. For verification of gradual drops, we introduced an equation concerning micro film thickness under the bubble on the basis of the viscosity and the preservation of momentum. This equation has an exponential factor for microfilms. Numerical calculations showed good agreement with the experimental data. © 2003 Wiley Periodicals, Inc. Heat Trans Asian Res, 32(7): 567–581, 2003; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.10102     

基于连续超声波测量气泡参数的实验研究

研究了一种基于声阻原理的测量液体中毫米级气泡粒度、速度并能同时计数的方法。超声波在含气泡液体中传播时,由于气泡和液体间声阻抗差极大,致使其在气泡表面发生强烈反射/散射,阻碍了声波通过。利用频率为200 kHz的超声连续波,采用一发一收式测量,对某润滑油中的气泡进行测量,实验中气泡大小为2～6 mm,速度在0.10～0.30 m·s~(-1)之间,气泡通过测量区的频率为5～10 Hz。通过分析实验数据的波形幅值与气泡粒度、波形转变时间与气泡速度、通过数之间的对应关系,并利用图像法进行了标定和校验。实验结果表明,利用连续超声波可以测得油中连续通过的气泡,其原理和测量装置简单,测量结果稳定。     

Theoretical analysis of the stability of vapor film in subcooled film boiling on a horizontal wire

Linear stability analysis of a thin vapor film in subcooled film boiling on a horizontal cylinder is reported. The effects of liquid inertia, vapor viscosity and compressibility, and heat transfer were taken into account. Theoretical predictions of the heat transfer coefficient at the neutral stability point were compared with experimental data at the minimum-heat-flux point that was obtained during rapid quenching of thin horizontal wires in water and ethanol. At high liquid subcooling, the experimental value was 60% of the theoretical prediction irrespective of the wire diameter and quenching liquid. This difference was considered to be due to the nonuniformity of the vapor film which was neglected in the theoretical analysis. © 1998 Scripta Technica. Heat Trans Jpn Res, 26(4): 219–235, 1997     

The effect of inlet constriction on bubble growth during flow boiling in microchannels

Flow boiling through microchannels is characterized by nucleation of vapor bubbles on the channel walls. In parallel microchannels connected through a common header, formation of vapor bubbles often results in flow mal-distribution that leads to reversed flow in certain channels. One way of eliminating the reversed flow is to incorporate flow restrictions at the channel inlet. In the present study, a nucleating vapor bubble placed near the restricted end of a single microchannel is numerically simulated. Placing restrictions at channel inlet increased the incoming liquid velocity for the same flow rate that prevented explosive bubble growth and reversed flow. It is proposed that channels with increasing cross-sectional area may be used to promote unidirectional growth of the vapor plugs and prevent reversed flow.     

Studies on pool boiling heat transfer: Macrolayer thickness in transition boiling

Macrolayer thicknesses in transition boiling were determined from the energy balance relation q_tr = ρ_lH_fgδ_l·f , based on measurements of q_tr (the time-averaged heat flux in transition boiling) and f (the detachment frequency of vapor masses) for water and ethanol boiling on vertical and horizontal 15-mm-diameter surfaces under atmospheric pressure. The macrolayer thickness for the vertical surface, designed to prevent liquid contact with the periphery of the surface during the vapor mass hovering, agreed well with the correlation proposed previously by the present authors, when the heat flux at macrolayer formation is obtained from a nucleate boiling curve extrapolated to the superheat of transition boiling. The macrolayer on the horizontal surface was apparently thickened due to the inflow of bulk liquid beneath the growing vapor masses. © 1999 Scripta Technica, Heat Trans Jpn Res, 27(8): 568–583, 1998     

Interface Shape and Marangoni Effect Around aBubble Within the Thermal Boundary Layer

INTRoDUCTIoNDuetohighheattransferperformancecharacter-izedbysmalltemPeraturedifferencesandhighheatfluxes,transportprocesseswithphasechange,espe-ciallyboilingandcondensationprocessesarewidelyemployedinnumerousenergyconversionandtrans-portsystems,heatingand/orcoolingdevices,andaerospaceaPplications.Priortotheutilizationofboil-ingprocessesinspaceapplications,suchasspacecraftthermalcontrol,additionalunderstandingofboilingheattransferbehaviorisneeded.Becauselargedmer-encesekistinthefiuiddensiti…     

Holographic interferometric study of heat transfer to a sliding vapor bubble

Heat transfer associated with a vapor bubble sliding along a downward-facing inclined heater surface was studied experimentally using holographic interferometry. Volume growth rate of the bubbles as well as the rate of heat transfer along the bubble interface were measured to understand the mechanisms contributing to the enhancement of heat transfer during sliding motion. The heater surface was made of polished silicon wafer (length 185 mm and width 49.5 mm). Experiments were conducted with PF-5060 as test liquid, for liquid subcoolings ranging from 0.2 to 1.2 °C and wall superheats from 0.2 to 0.8 °C. The heater surface had an inclination of 75° to the vertical. Individual vapor bubbles were generated in an artificial cavity at the lower end of the heater surface. High-speed digital photography was used to measure the bubble growth rate. The temperature field around the sliding bubble was measured using holographic interferometry. Heat transfer at the bubble interface was calculated from the measured temperature field. Results show that for the range of parameters considered the bubbles continued to grow, with bubble growth rates decreasing with increasing liquid subcooling. Heat transfer measurements show that condensation occurs on most of the bubble interface away from the wall. For the parameters considered condensation accounted for less than 12% of the rate heat transfer from the bubble base. In this study the heater surface showed no drop in temperature as a result of heat transfer enhancement during bubbles sliding.     

An experimental investigation of bubble nucleation of a refrigerant in pressurized boiling flows

An experimental investigation is performed to determine the effect of system pressure and heat flux on flow boiling and associated bubble characteristics of a refrigerant in a narrow vertical duct. A high-pressure flow boiling test loop was built and TLC (thermo-chromic liquid crystal) was applied to the back of the heater foil for high resolution and accurate measurement of heater surface temperature. Refrigerant R-134a is used as the test fluid at different pressures ranging from 690 to 827 kPa and different heat fluxes to quantify their influence in bubble characteristics such as bubble nucleation, growth, departure, and coalescence. Two synchronized high resolution and high-speed cameras are used to simultaneously capture TLC images as well as bubbling activities at high frame rates. By varying flow rate and system pressure, TLC and bubble images were captured and analyzed. Results show that the bubble generation frequency and size increase with heat flux. An increase in pressure from 690 to 827 kPa increased the bubble frequency and size by about 32 Hz and 20 μm, respectively. Bubble coalescence was also observed after departure from the nucleation site.     

Zero‐flux zone model analysis of a single bubble in boiling liquid

We start from a bubble, the basic unit of boiling, to explore liquid boiling. Research indicates that the heat and mass transfer between liquid and vapor is the determinant factor of boiling heat transfer. We have analyzed interfacial vaporization and condensation of a single bubble in boiling liquid based on a zero‐flux zone model. We have deduced the expression of zero‐flux angle and discussed the relationship between zero‐flux zone and the other parameters in order to comprehend the mechanism of boiling heat transfer. © 2002 Wiley Periodicals, Inc. Heat Trans Asian Res, 31(4): 249–256, 2002; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.10026