Hiroto Sakashita

Pool boiling heat transfer—II. Thickness of liquid macrolayer formed beneath vapor masses

Abstract The macrolayer thickness was determined on the basis of the energy balance relation qCHF = δ1ϱ1Hfgf, which was derived from the dryout model for liquids, proposed by Katto and co-workers. The critical heat flux (CHF) and the detachment frequency were measured on 20 mm diameter horizontal and vertical heated disks at pressures of 0.03–0.4 MPa with water, ethanol, methanol and acetone. Two semi-empirical correlations for the macrolayer thickness were derived by dimensional analysis of a model in which primary or coalesced bubbles form a macrolayer. The proposed correlations arrange the data of macrolayer thickness obtained from CHF measurements well.

Method for predicting boiling curves of saturated nucleate boiling

Abstract The present paper proposes a new correlation for nucleate boiling consisting of heat flux, superheat, density of nucleation sites, and a group of physical properties. The correlation is based on a model in which heat transfer occurs primarily by heat conduction through the conduction layers formed under primary bubbles. Nucleation site densities were calculated by combining the correlation with experimental data of q −Δ T sat . This procedure yielded a semi-empirical correlation of the densities of nucleation sites that takes into account the effects of growth of neighboring bubbles and changes in wettability with pressure. Combining these two correlations results in a correlation that predicts boiling curves.

Bubble Growth Rates and Nucleation Site Densities in Saturated Pool Boiling of Water at High Pressures

Experiments were carried out to observe boiling behaviors of water on horizontal and vertical surfaces at pressures from 0.35 to 5 MPa. The growth curves of the primary bubbles are well described by the t1/2 variation over the whole range of pressures. The growth rates of primary bubbles are proportional to the square root of the Jakob number, and agree with the correlation by Labuntsov with the arbitrary constant β = 3. The conventional correlations of bubble growth rates, which are directly proportional to the Jakob number, predict slower growth rates at higher pressures. The coalescence behaviors of the primary bubbles were also measured on the vertical surface at 3.66 MPa. The coalesced bubbles, which were formed by the coalescence of two primary bubbles, grow at rates similar to the rates of the primary bubbles. The nucleation site densities measured on the vertical surface at pressures up to 5 MPa increase in proportion to about the 1.5th power of the pressure under equivalent heat flux conditions. The dependence of the nucleation site densities on the heat flux is very similar to the results obtained near atmospheric pressure where the nucleation site density is proportional to the 1.5th power of the heat flux. The nucleation site densities measured in the range of pressures of 0.35 to 5 MPa and at heat fluxes of 0.05 to 0.35MW/m2 agree fairly well with the available correlations.

Critical Heat Flux and Near-Wall Boiling Behaviors in Saturated and Subcooled Pool Boiling on Vertical and Inclined Surfaces

The mechanism of the critical heat flux (CHF) where the departure from nucleate boiling (DNB)-type boiling transition takes place has not been fully elucidated. In this paper, we examine the trigger mechanism of the CHF for saturated and subcooled pool boiling on vertical and inclined surfaces based on measurements of the liquid-vapor behaviors near heating surfaces by using a conductance probe. The angle of inclination was varied from 90° (vertical) to 170° (facing almost horizontally downwards). The probe signals and the void fraction distributions showed that a liquid layer remains beneath the vapor masses moving upward along the heating surface at high heat fluxes near the CHF. The thickness of the liquid layer was determined from the location where the probe signals corresponding to the vapor masses disappeared. The thickness of the liquid layer formed on the vertical surface increased with increasing degree of subcooling, which may be the cause of the increases in CHF with increasing degree of subcooling. The measurements of saturated boiling on the inclined surface confirmed that the orientation of the heating surface greatly affects the period it takes for vapor masses to pass, but it negligibly affects the liquid layer thickness. This suggests that the decrease in CHF with increasing angle of inclination is primarily caused by the lengthening of the duration of vapor mass passage.

Pool boiling heat transfer—I. Measurement and semi-empirical relations of detachment frequencies of coalesced bubbles

Abstract Measurements were made of the detachment frequency of coalesced bubbles from circular, horizontal 2–30 mm diameter disks and thin, horizontal 0.2–3 mm diameter wires at high heat fluxes and atmospheric pressure. The disks area heated copper cylinder and a sintered brass plate through which nitrogen gas is blown. Detachment frequencies from disks and thin wires were measured with water, ethanol and Freon-113. The bubble frequency was determined with high speed video. Semi-empirical correlations of bubble frequencies are proposed for both disks and thin wires. The correlations were derived by the dimensional analysis of force balance equations relevant to the behavior of bubbles.

Pressure effect on CHF enhancement in pool boiling of nanofluids

This paper investigates critical heat flux (CHF) in saturated pool boiling for water and TiO2 nanofluid on a 7-mm-diameter vertical copper surface at pressures of 0.1–0.8 MPa. The nanofluid was prepared by dispersing 0.002 wt% TiO2 nanoparticles in deionized water. The CHF of the nanofluid was enhanced about two times over that of water boiling at atmospheric pressure. With the increasing pressure, however, the CHF enhancement with the nanofluid decreases, and almost disappears at 0.8 MPa.

Simultaneous Determination of Thermophysical Properties Using a Thermistor, Part 2: Experiment

A newly proposed thermistor technique for the simultaneous estimation of thermal conductivity and thermal diffusivity is checked for its accuracy and applicability. Ethanol, toluene, pure water, and mercury, all at 298.15 ± 0.1 K, were selected as target materials. In addition, thermal diffusivity (or thermal capacity) of a powder (bentonite kunigel V1) was also estimated, whereas its thermal conductivity was taken from previously published research. It was found that for both liquids and powder the present method can estimate thermophysical properties with high accuracy. In general, it can determine thermal conductivity and thermal diffusivity within 2 and 6%, respectively. Comparison of thermal properties of liquids measured here with referenced and recently updated data show very good agreement with an accuracy of 2% for thermal conductivity and thermal diffusivity, with the exception of the thermal conductivity of the mercury, found to be accurate to 3.4%.

Simultaneous determination of thermophysical properties using a thermistor, Part 1: Numerical model

A new thermistor-based technique for the measurement of thermophysical properties of liquids and porous materials is proposed. To simulate the real situation, a two-dimensional numerical thermal model of the thermistor, surrounded by a sample of cylindrical shape, is presented. The simultaneous estimation of thermal conductivity and diffusivity by the use of an inverse analysis is detailed. If the measured volume-averaged temperature rise of the thermistor is available, then thermal diffusivity of the sample is evaluated first from the plot of these measured temperatures vs the logarithm of the Fourier number. Once thermal diffusivity is evaluated, thermal conductivity is obtained through the minimization of standard deviation between calculated and experimental temperatures of the thermistor in the least-square sense. The time range used to determine the thermophysical properties is selected within the transient-state part of time-dependent temperature rise of thermistor, expressed in dimensionless form, and is situated between Fourier number Fo = 1 and Fo = 10. In addition, and to get accurate results, an analysis is performed to show the set of parameters to be adjusted before actual measurements. The parameters are the dimensions of the sample, time step, and the mesh sizes used in the numerical analysis.

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

Macrolayer thicknesses in transition boiling were determined from the energy balance relation qtr = ρlHfgδl·f , based on measurements of qtr (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.

Macrolayer formation and mechanisms of nucleate boiling, critical heat flux, and transition boiling

The drying process of a macrolayer on a 15 mm diameter boiling surface was observed with high speed video in the region of nucleate and of transition boiling close to the critical heat flux (CHF). It was found that the macrolayer rests beneath a large vapor mass. It partially dries in nucleate boiling and completely dries in transition boiling at the detachment of the vapor mass. The macrolayer thickness at CHF and in transition boiling was determined on the basis of the energy balance relation proposed by Katto and Yokoya. The macrolayer thickness at low heat flux was obtained by decreasing CHF with downward-facing heating surfaces and agreed well with the correlation proposed previously by the present authors. The macrolayer thickness in transition boiling with a vertical surface also agrees fairly well with the correlation, when the heat flux at macrolayer formation, given on the nucleate boiling curve, is extrapolated to surface superheat of transition boiling and when the surface temperature at macrolayer formation is equal to a time-averaged value.