l3:thm.cls.p7.01 nucleation site density, bubble - casl . casl-u-2013-0325-000. 1 . nucleation site...

L3:THM.CLS.P7.01 Nucleation Site Density,

Bubble Departure Diameter, Bubble

Departure Frequency, and Local Temperature

Distribution in Subcooled Flow Boiling of Water

B. Phillips, J. Buongiorno and T.

McKrell Massachusetts Institute of

Technology, Cambridge, MA

June 30, 2013

CASL- -2013-0325-000

CASL-U-2013-0325-000

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Nucleation Site Density, Bubble Departure

Diameter, Bubble Departure Frequency, and Local

Temperature Distribution in Subcooled Flow

Boiling of Water

B. Phillips, J. Buongiorno and T. McKrell

Massachusetts Institute of Technology, Cambridge, MA, USA bren@mit.edu, jacopo@mit.edu, tmckrell@mit.edu

(Rev. 0)

Massachusetts Institute of Technology

Cambridge, MA, USA

June 2013

CASL-U-2013-0325-000

mailto:bren@mit.edumailto:jacopo@mit.edumailto:tmckrell@mit.edu

2

ABSTRACT

The physics of subcooled flow boiling of water were explored using high-speed video

(HSV), and infrared (IR) thermography. HSV allowed measurement of the bubble

departure diameter, and IR thermography allowed measurement of wall superheat (both

the local distribution and the surface-averaged value), heat transfer coefficient,

nucleation site density, and bubble departure frequency. The tests were performed at

1.05 and 1.5 bars for subcoolings of 5, 10 and 15C. The mass flux values explored

were 150, 250, 500, 750, 1000, and 1250 kg/m2-s. The heat flux values explored were

100, 200, 400, 600, 800, 1000, 1200, 1400, and 1600 kW/m2. As expected, the heat

transfer coefficients increased with increasing mass flux in the single-phase convection

and partial boiling regions, and they converged to a fully-developed boiling curve for

high heat fluxes. The bubble departure diameter decreased with increasing mass flux,

decreasing heat flux, and increasing subcooling in accord with models by Klausners [1]

and Sugrue [2]. The nucleation site density increased with increasing superheat and

decreasing mass flux, in accord with the model by Kocamustafaogullari and Ishii [3],

which however does not fully account for the effect of subcooling. The departure

frequency increased with the superheat. The frequency correlation of Basu [4] does not

reproduce the data correctly, as it ignores the effects of subcooling and heater thermal

diffusivity, which are expected to be important. At relatively low heat fluxes, bubbles

were observed sliding along the wall after departure from a nucleation site; the velocity

of the sliding bubbles was measured as a function of heat flux, mass flux and

subcooling, along with the intense localized cooling the sliding bubbles produce on the

boiling surface; these effects should be considered in advanced models of subcooled

flow boiling.

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1. INTRODUCTION

Subcooled flow boiling is a complicated physical phenomenon present in many industrial heat transfer

applications including conventional power plants and nuclear power reactors. It includes multiple heat

transfer mechanisms (e.g. single phase convection, nucleate boiling, evaporation and condensation),

two phase flow and thermal non-equilibrium conditions existing between the vapor and the liquid

phases, and is further complicated by the effects of the chemico-physical conditions (e.g. roughness,

hydrophilicity, porosity) present on the boiling surface. Although, it has been studied for many years,

it is difficult to fully understand the underlying physics because of limitations on the quantities and

phenomena that can be accurately measured. A better understanding of subcooled flow boiling would

allow for development of better codes and predictive methods, with consequent benefits for the

aforementioned applications.

1.1 Objective

The objective for the proposed work is to generate a new set of high-resolution data on subcooled flow

boiling characteristics, including bubble departure diameter, bubble departure frequency, and

temperature profiles of the boiling surface. This data may inspire the development of analytical models

of subcooled flow boiling as well as be used to validate numerical models in CFD codes. The

approaches used in CFD include the Eulerian-Eulerian, two-fluid, six-equation model [5], and closure

relations momentum and energy equations can be provided by models such as the heat flux partitioning

model of Kurul and Podowski [6] and Kolevs bubble interaction model [7], which require bubble

departure diameter, wait and growth times, and nucleation site density as input.

The heat flux/mass flux test matrix explored in this study is shown in Table 1; the matrix was repeated

for 1.05 and 1.5 bar, and for subcoolings of 5, 10, and 15C.

Table 1- Test matrix performed for 1.05 and 1.5 bar, and for subcoolings of 5, 10, and 15C.

150 250 500 750 100 1250

ONB x x x x x x

100 x x x x x x

200 x x x x x x

300 x x x x x x

400 x x x x x x

500 x x x x x x

600 x x x x x x

700 x x x x x x

800 x x x x x x

1000 x x x x x x

1200 x x x x x x

1400 x x x x

1600 x x

Mass Flux (kg/m/s)

Heat Flux (kW/m)

A comparison of the range of conditions of the experiment to those of a PWR is shown Table 2. The

Reactor Prototypicality Parameter (RPP) proposed by Dinh [8] is used to judge the magnitude of the

distortions introduced by the differences between the experiment and the actual application. The RPP

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is defined for dimensionless quantities and is a simple ratio of their values at PWR conditions and

experimental conditions, respectively:

APPMod

EXPMod

k

k

SC

SCRPP

][

][ (10)

where EXPModkSC ][ is the parameter value from the experiment and APPModkSC ][ is the parameter value

from the application (PWR conditions). If the RPP is within one order of magnitude (a value of 0.1 to

10), the scaling of the experiment is considered adequate. All the parameters scale well except for

pressure and density ratio. These are difficult parameters to match experimentally due to the

component high cost and complexity associated with operation at PWR pressure (155 bar). High

pressures also make visualization of the boiling process a lot more challenging from a practical point of

view because of the need for windows capable of withstanding high pressure and temperature, and the

small spatial scale of the vapor bubbles at such pressures.

Table 2 - Comparison of potential experiment conditions to PWR conditions and associated RPP.

Parameter Experiment Range PWR Range Typical RPP

Reynolds Number 0-1105 2105-8105 0.20

Prandtl Number 1.1-6.2 0.9-1.2 1.00

Froude Number 0-27 0-217 0.12

Boiling Number 0-10 0-0.25 1.00

Jakob Number 0-100 ~20 1.00

Equilibrium Quality at Outlet -0.24 to 0 -0.38 to 0.10 1.00

Water velocity (m/s) 0-2 2-6 0.20

Mass Flux (kg/m2s) 0-1800 3000-5000 0.25

Temperature (C) 25-150 286-324 --

Pressure (MPa) 0.1-0.4 15.5 0.03

Subcooling (C) 0-75 21-58 1.00

Hydraulic Diameter (mm) 15 12 1.33

Wetted Perimeter (mm) 80 30 3.67

Heated Perimeter (mm) 10 or 15 30 0.33

Heat Flux (MW/m2) 0-2 0.0-1.2 1.00

Channel Area (mm2) 300 88 5.11

Density Ratio ( 440-1620 6 73

1.2 Previous Work

Subcooled flow boiling has been extensively investigated. Here we limit the literature survey to those

studies that have utilized techniques similar to the ones adopted in this study.

Del Valle and Kenning [9] measured surface temperature and used high speed photography in

subcooled flow boiling with water to examine nucleation site density in a rectangular channel. They

calculated the heat removed through various processes including microlayer evaporation, quenching of

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the surface after bubble departure, and convection and compared the total values to the experimental

heat flux. They found that for their 84 K subcooled flow, quenching of the surface was the most

important component of the heat transfer and the microlayer evaporation effect was negligible.

Basu et al. [10] measured the temperature distribution with thermocouples in a rod bundle. They

developed a correlation for the onset of subcooled nucleate boiling based on the contact angle of the

fluid with the surface, the local subcooling, and the axial location. Their correlation is for water and

valid for contact angles from 1-85, pressures from 1-137.5 bars, local liquid subcooling of 1.7-80C,

and velocities from 0 m/s-17 m/s. They found that the nucleation site density depended only on contact

angle and wall superheat.

Situ et al. [11] conducted subcooled flow boiling experiments with water in an annular channel. They

measured bubble parameters such as departure diameter, nucleation site density, departure frequency,

and velocity after departure with high speed photography. Situ et al. [12] went on to develop a

correlation for bubble departure frequency. Hong et. al [13] measured departure diameters in

stationary and heaving conditions and developed a model to predict departure diameter.

Euh et al. [14] examined bubble departure in an annular channel during subcooled boiling. They varied

the test conditions