Dynamics of a Vapor Bubble in Film Boiling and the Superheat Effect

FELLA CHOUARFA1, ABIDA BAHLOUL2, M.E.HOCINE BENHAMZA1, SAMIRA BOUFAS1

University May 8, 1945 Guelma,

P.O. Box 401 Guelma, 24000,

ALGERIA

University May 8, 1945 Guelma,

and frequency of detachment under different wall superheats is numerically investigated. The continuity,

momentum, and energy equations are solved for the two immiscible fluids phases using the finite volume

method. The phase change model and the results exhibited a good agreement with the theoretical models. The

obtained results show that the velocity of bubble growth and its frequency of emission promotes heat

exchange. It is found that the shape of a bubble has been influenced by the wall superheat. It is also found that

the high superheat generates a large amount of steam in which the steam bubble takes the shape of a fungus.

So, a clear correlation exists between heat transfer and the frequency of detachment. As long as the frequency

is greater, the heat transfer increases. Most of the heat transfer is induced by the liquid movements associated

with the vapor bubble detachment.

Simulation film boiling is a computational

technique used to model the process of film boiling,

contact with a surface that is much hotter than the

liquid's boiling point. In this process, a layer of

vapor, which is formed between the hot surface and

the liquid, reduces the contact area and slows down

the heat transfer process. To simulate film boiling,

researchers resort to creating a virtual model of the

to study the heat transfer process.

Simulation film boiling can be used to

investigate various aspects of the phenomenon,

such as the heat transfer rate, the formation of

vapor bubbles, and the stability of the system. It

can also be used to design and optimize heat

transfer systems for various applications, such as

nuclear reactors and electronic cooling systems.

enables researchers to gain a deeper understanding

depend on the specific application and the available

computational resources.

simulations can be used to model the formation of

vapor bubbles, the transfer of heat between the

surface and the fluid, and the behavior of the fluid

under different conditions. CFD simulation, which

is recognized today as one of the essential design

tools, is widely used. The choice of using this

numerical method will essentially depend on the

behavior, and the modeling of the medium.

The equations governing the movements of a fluid,

reflecting the conservation of both the energy

(Navier-Stokes equations) as well as the fluid`s

mass and momentum are solved by the numerical

flow simulation codes or CFD codes. It can be

observed that most of these codes use the finite

volume method.

In this paper, avapor–liquid phase change

model is proposed for the volume-of-fluid (VOF)

language that allows it to automate certain

procedures such as boundary conditions, periodic,

or others. This is achieved through the use of UDFs

(User Defined Function) characteristic which will

be compiled by an integrated compiler to be then

executed, [2].

In Fluent, the VOF method solves a transport

equation for one of the fluids volume fractions in

each computational cell of the simulation domain.

The fraction volume is defined as the fluid volume

Film boiling is an important heat transfer

phenomenon, which is characterized by the

formation of a continuous vapor film on a heated

wall. It is a two-phase flow heat transfer regime in

which the bulk liquid is separated from the heating

Film boiling is a major heat transfer mechanism that

occurs when the wall temperature is much higher

than the saturation temperature of the liquid.

Several methods have been developed to study film

predicted using an approach introduced by

Berenson, [8], This approach takes into

consideration the concept of hydrodynamic

instabilities which was later developed by Zuber to

describe the heat flux. This approach, which

interfacial instabilities. This model was notably

developed by Berenson as a continuation of Zuber's

work, considering the case of a flat plate.

The vapor phase is usually generated in the thin

film region on the lower portion of the surface and

is removed upward through the formation and

release of bubbles. The vapor layer of density ρl

is initially flat. The evolution of the vapor-liquid

interface during film boiling on a horizontal surface

When all wavelengths can appear on the interface,

the most likely value is the one with the most rapid

growth. This wavelength is referred to as the

dominant or the most dangerous one, [14], [15].

thermodynamic models, such as the ideal gas law

or the Antoine equation. Also, parameters such as

density, viscosity, thermal conductivity, and

domain, the width along the x-axis will be equal to

the wavelength of the Rayleigh–Taylor instability

thickness δ(x) is formed over a horizontal flat plate

due to the rapid boiling of the liquid film that

initially covers the plate. The thickness of the vapor

film varies along the plate in the x-direction due to

variations in the local heat flux and fluid properties.

The thickness of the vapor film is an important

parameter that affects the heat transfer rate and the

stability of the boiling process. The vapor film acts

as a thermal insulator, reducing the rate of heat

(13)

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Boundary conditions at the exit on the upper

surface of the domain:

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Fig. 2: Evolution of a bubble for different meshes.

Fig.1 presents the mesh and boundary condition.

Moreover, regarding the choice of mesh, Fig.2

shows the different meshes tested (mesh (64,192) is

the most suitable).

film boiling simulation is highly dependent on the

local flow conditions and heat transfer mechanisms,

as well as the properties of the fluid and the heated

surface. Accurately capturing these phenomena in

simulation requires careful modeling of the relevant

physical processes and a high-quality mesh that can

resolve the steep gradients in temperature, pressure,

and velocity.

from birth to disappearance is presented in Fig.3

increase of heat flow. For a superheat of 5°C and

time equal to 1.6s, there has been a single life cycle

of a vapor bubble. For 10°C, two life cycles have

been obtained, whereas, three cycles have been

obtained for 15°C. During the life cycle of a steam

bubble, it is found that the shape of the bubble is

not always spherical; however, there is a change in

superheat of the wall. In addition, the bubble

detachment from the hot surface affects the steam

film, thus improving the heat transfer.

bubble

which were taken at various times and are presented

in Fig.4, Fig.5, and Fig.6.

noted that the velocity of the fluid is higher at the

heart of the bubble in addition to the formation of

symmetrical counter-rotations (vortexes) to mix the

hot and cold steam located on both lateral sides of

the wake of the bubble. Minimum velocities are

located at the heart of these cells, at the lower

antinodes, and at the point of birth of the bubble

(node). During its growth, the hemispherical bubble

attracts all the liquid around it thus increasing its

speed (the hot vapor is transported to the lower parts

of the bubbles, whereas, the cold vapor is

The direction of the velocities is practically parallel

to the bubble direction movement, except in the

lateral regions.

The velocity of the fluid is almost zero in the

detachment. For a time, equal to 1 second, the steam

bubble is ascending and the maximum velocity of

the fluid decreases reaching a value of 0.25 m/s, this

is due to the expansion effect of the bubble and the

maximum velocity vectors, which are located at the

heart and around the bubble.

For the time of 1.2 seconds, when the bubble

has touched the free surface and reached the value

of 0.63m/s, the velocity of the fluid around the

bubble increases. It can be seen that all the fluid is

influenced by the movement of the ascending

3.2 Steam Bubble Velocity Movement

Fig.7 shows the maximum velocity at the heart of

the bubble as a function of time. For each life cycle

of a vapor bubble, two successive peaks represent

the formation, detachment, ascent, and

disappearance of the bubble. The velocity of the

detachment. It is low when the bubble is rising with

values between 0.25-0.35 m/s. The maximum

velocity of the first bubble is lower than that of the

subsequent bubbles.

These findings are also observed in the

experiments of [28], who noticed that the second

bubble is formed when the first is released. This

second bubble is driven and lengthened by the

circulatory movement created behind the first bubble

which thus pulls it faster from the heating surface.

It is defined as:

(14)

proportional. This explains the behavior of the vapor

Nusselt number as a function of time for different

different superheats of 5°C, 10°C, and 15°C, shows

values (Nuavg) of our simulation with those of the

According to Fig.8, there is a similarity between the

simulation results and those of the two correlations,

with low errors between (6% - 12%).

Fig. 8: Variation of the average Nusselt number

indicating the formation of four vapor bubbles. For

superheat, 10°c and 15°C seven bubbles and eleven

bubbles were formed respectively. The study of the

frequency of vapor bubble detachment, which is

shows that for a superheat equal to 5°C, every

second there is an average of one bubble with an

initial detachment time equal to td=0.9s. For

superheating at 10°C and with a lower detachment

15°C the initial detachment time decreases again

vapor film thickness. Nu and q are high when the

vapor film thickness is thin and vice versa.

Therefore, the average heat flux increases as more

vapor rushes to fill the bubble, and the film becomes

vapor deficient which leads to the reduction of its

thickness resulting in a higher Nu number.

3.4 Effect of the High Superheat

the superheat at the heated surface can affect the

shape and behavior of the vapor bubbles that form

in the liquid. High superheat conditions can lead to

can be more difficult to control than smaller

designing and optimizing film boiling systems for

various applications. The shape of the vapor bubbles

characteristics of the system and can also affect the

Fig.10 shows the influence of high superheat on the

steam film for the same conditions considered

30°C, and 100°C.

At high superheat, after the detachment of the first

steam bubble a second bubble of small size is

of the first bubbles, the third in turn detaches from

the steam. Then, another bubble of the same

dimension is formed as the third detaches with a

more or less rapid time. This phenomenon continues,

with a detachment of the bubble being done at a

lower position for steam of lower height. The

diameters of the latter bubbles are small and then

pass to medium dimensions.

It is found that at high superheat, a large amount of

steam is generated in which the steam bubble takes