Evaluation of Heat Transfer Shear of Different Mechanisms in Flow Subcooled Nucleate Boiling

Abstract

Since the mean characteristic of nucleate boiling is bubble formation and collapse near the heating surface, in this study attempts have been made to investigate heat transfer effects due to the bubbles ebullition and collapse cycle. All possible heat transfer mechanisms are studied qualitatively and quantitatively. For quantitative calculation of the heat transfer portion by each of the introduced mechanisms, a series of bubble parameters was selected from the experimental data provided by the high-speed photography of a heated rod containing nucleate boiling regime on its surface. According to the present results, the portion of the mechanisms such as latent heat transfer, super heated layer mixing and single-phase heat transfer are between 6~15%, 12~17%, and 3~5%, respectively. The results also show that the two mechanisms of the turbulence induced by the bubble formation and collapse and quenching have the main performance in transferring heat from the heating surface to the bulk flow. The shared percentage of these two mechanisms is estimated between 23~58.5% and 20.5~40% respectively.

Full text

Page 1

Mohammadreza Nematollahi, Mohammad Nazififard
International Journal of Engineering (IJE), Volume (4): Issue(1)
37
Evaluation of Heat Transfer Shear of Different Mechanisms in
Subcooled Nucleate Boiling



Mohammadreza Nematollahi
School of Mechanical Engineering
Shiraz University
Shiraz, 71348-51154, Iran

Mohammad Nazififard mnazifi@shirazu.ac.ir
School of Mechanical Engineering
Shiraz University
Shiraz, 71348-51154, Iran
nema@shirazu.ac.ir

Abstract

Since the mean characteristic of nucleate boiling is bubble formation and
collapse near the heating surface, in this study attempts have been made to
investigate heat transfer effects due to the bubbles ebullition and collapse cycle.
All possible heat transfer mechanisms are studied qualitatively and quantitatively.
For quantitative calculation of the heat transfer portion by each of the introduced
mechanisms, a series of bubble parameters was selected from the experimental
data provided by the high-speed photography of a heated rod containing
nucleate boiling regime on its surface. According to the present results, the
portion of the mechanisms such as latent heat transfer, super heated layer
mixing and single-phase heat transfer are between 6~15%, 12~17%, and 3~5%,
respectively. The results also show that the two mechanisms of the turbulence
induced by the bubble formation and collapse and quenching have the main
performance in transferring heat from the heating surface to the bulk flow. The
shared percentage of these two mechanisms is estimated between 23~58.5%
and 20.5~40% respectively.

Keywords: Bubble, Heat Transfer, Nucleate Boiling, Quenching & Turbulence


1. INTRODUCTION
The ability of subcooled boiling flow for transferring high heat loading in compact area without a
significant void fraction and heating surface temperature increase has caused it to be widely used
in industry. Because of the special characteristics of subcooled boiling flow, this technique is used
for making compact heat exchangers, or transferring high heat loadings in nuclear plants. Using
the subcooled boiling technique, transferring heat fluxes of up to 108 W/m2 are reported to be
attainable through high velocities, large subcooling, small diameter channels and short heated
lengths [1].
Vandervort et al. (in 1992) have identified all possible mechanisms for subcooled boiling [2]. Their
explanation is divided into two different parts of; 1) the bubble growing on the heated surface and
2) the bubble detachment from the heated surface.

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Mohammadreza Nematollahi, Mohammad Nazififard
International Journal of Engineering (IJE), Volume (4): Issue(1)
38
In recent studies Nematollahi et al. [3] ,according to their experimental study, addressed the nine
mechanisms as the following:
(1) Single-phase forced convection
(2) Local dispersion of the super heated layer around the active cavity due to implosive bubble
formation that is referred to as super heated layer
(3) Latent heat transport due to bubble collapse
(4) Vapor/liquid interchange due to the bubble departure that is addressed as quenching
(5) induced micro-convection due to evaporation of the entrapped water from the cavity
(6) induced micro-convection due to the boundary layer motion around the bubble
(7) Micro-convection due to the special manner of the bubble collapse
(8) Transferring of heat and kinetic energy by the stable micro-bubble to the bulk flow
(9) Marangoni-force induced micro-convection

The introduced mechanisms of 5,6,7,8 and 9 could count as a comprehensive mechanism of the
turbulence induced by the bubble formation and collapse because of their similarity in creation of
turbulence by inducing micro-convection.
In the present study an attempt was made to investigate the mechanisms of heat transfer in a
subcooled boiling condition by analyzing bubble behavior in subcooled boiling flow on a heated
rod using the technique of high-speed photography. The bubble behavior at three locations 1, 25
and 45 cm from the beginning of a heated rod were analyzed at different conditions of subcooling
temperature, linear power density and flow rate. For the evaluation of the heat transfer shear of
different mechanisms in nucleate boiling, and the quantitative calculation of partial heat transfer
by each of the introduced mechanisms, a series of bubble parameters was selected from the
experimental data provided by high-speed photography of a heated rod having a nucleate boiling
regime on its surface.


2. Experimental apparatus and procedure of the High-Speed Photography
Fig. 1 shows the schematic view of the test loop. The subcooled boiling was performed by Joule
heating on the middle part of the stainless steel rod, which has high electrical resistance
compared to the rest of the rod made by copper. Flow of distilled water was used as the
recirculating coolant in the loop. A power between 0~60 kW was supplied from a voltage
regulator and a transformer connected to an electrical power line with 200V and 300A electricity
current.
The rod diameter and length of the heated surface of the rod are 10 mm 50 cm, respectively. The
outer tube of the test section was made of glass having 30mm inner diameter, which was
permitted visual observation. In the present experiment, several different conditions were chosen
such as different incoming coolant subcooling temperature 25, 50 and 75K, coolant flow velocities
16, 32, 53 cm/sec and imposed linear power densities 100~600, W/cm for the interracial high-
speed photography from the heating surface.
High-speed photography was performed using a KODAK EKTAPRO High-speed Motion Analyzer
(model 4540), which contains an imager, a processor and a keypad. By using this system
photographic pictures were able to be taken from 30 to 40500 frames per second. This system
was connected with three other accessories, a computer, a monitor and a video tape recorder.
Fig. 2 shows the experimental setup of the high-speed photography.
The high-speed photography was operated at 13500 frames per second in different conditions of
subcooled boiling. The taken pictures would cover the cross-section 2.5mm X 2.5mm of the
visible test section. This was achieved by using a 100mm telephoto lens (SMC PENTAX-M
MACRO 1:4 100mm) with a 180mm extended tube. The slow motion behaviors (10 frames per
second) of the bubble for five minutes were recorded on videotape by the video tape recorder for
each condition For each condition a minimum of 200 consequential images of the bubble
behaviors have been saved in the computer memory. The bubble behaviors were visualized for
four different conditions which were in different heights, inlet subcooled temperatures, linear
power densities and flow rates.

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Mohammadreza Nematollahi, Mohammad Nazififard
International Journal of Engineering (IJE), Volume (4): Issue(1)
39


Fig.1: Schematic diagram of the experimental setup




Fig. 4: Experimental setup of high-speed photography

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Mohammadreza Nematollahi, Mohammad Nazififard
International Journal of Engineering (IJE), Volume (4): Issue(1)
40
3. Experimental Results

The behavior of bubbles in different conditions of subcooling temperature, linear power density
and flow rate was analyzed using high-speed photography. The observed behaviors can be
divided into two main categories:
a) General aspects of interfacial behavior of the coolant in subcooled boiling flow
b) Specific aspects of bubble behavior at different subcooled boiling flow conditions.
A brief explanation on the results is presented here.


3.1. General aspects

In all cases in which the bubble formed on the heating surface, a thin shear flow, which looks
similar to a dark fluid layer in the photographs, covers the heating surface. The darkness of the
thin layer can be explained by the difference in the reflection angle of the back light due to the
temperature difference between the subcooled water and the saturated or superheated water.
This thin layer disappears in the linear power densities less than that required for the onset of
nucleate subcooled boiling flow, and in the low subcooling or in the saturated conditions.
High-speed photography of the bubble ebullition cycle shows that vapor blown from a cavity
forms as a bubble with a hemispheric or an elongated hemispheric shape. This process occurs
during a short interval between less than 74 µs to around 222 µs, depending on parameters such
as linear power density, subcooling, and cavity characteristics. The motion of water in the layer
around the bubble causes the bubble to separate from the heating surface. The contact area of
the bubble with the wall starts to shrink. This continues until the bubble adheres to the wall at a
single point, at which time the bubble becomes similar to a balloon. This process is followed by
the ejection of the bubble from the surface. Most of the bubble volume condenses in bulk flow
near the surface. The collapse starts from the bottom of the bubble. It appears that the
condensation rate from the bottom is much more notable than the bubble top.
Condensation of the bubble and the work due to surface tension cause a balloon type bubble to
change into a micro-bubble with a diameter around one fiftieth of that of the original bubble.
Immediately after its formation, the micro-bubble escapes very rapidly from its location towards
the bulk flow. A micro-bubble has a relatively high kinetic energy. In addition to this, a simple
theoretical calculation shows that the inside pressure and temperature of the micro-bubble are
both relatively much higher than the original bubbles height. These states can keep micro-
bubbles much more stable. Due to this stable characteristic of micro-bubbles, the authors call it
"stable micro-bubble" or in brief "SMB". A general bubble behavior in subcooled boiling flow is
shown in Fig. 3.

















Fig. 3: Typical bubble ebullition cycle in subcooled boiling flow

2.5 mm

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Mohammadreza Nematollahi, Mohammad Nazififard
International Journal of Engineering (IJE), Volume (4): Issue(1)
41

Table 2: The summery results of the bubble behavior in the LPDs of the 182, 290, 430 and 600W/cm in fixed
subcooled temperature 75K at the 1, 25 and 45cm

Table 3: The summery results of the bubble behavior in the flow velocities of the 16, 32 and 53cm/s in fixed LPD
of the 182W/cm and subcooling 75K at the 1, 25cm

Flow Rate
Attitude
(cm)
Bubble sites
number in
2.5 mm
Bubble
growing-time
(µs )
Bubble
lifetime
(µs)
Bubble max-
diameter
(µm)
Bubble emission
frequency (Hz)
Super-
heated-
layer
thickness
(µm)
60
100
85
100
100
115
1
25
1
25
1
25
3
4
4
74~148
74~148
~148
74~148
~148
~148
148~370
148~370
222~593
~370
296~740
~296
125~435
125~280
~250
80~315
59~780
80~280
135~1555
135~1350
135~745
~540
135~745
205~475
53
cm/s
cm/s
cm/s
32
6(1)
4
7(1)
16


1. The density of active nucleation sites increased with increasing linear power densities and
decreasing subcooled temperature.
2. The average bubble maximum diameter decreased with increasing linear power density
and subcooled temperature.
3. The bubble emission frequency depends upon the sites.
However, it increased with increasing linear power density and subcooling.
Table 1: The summery results of the bubble behavior in three subcooled temperatures of the 75, 50 and 25K in fixed
LPD 182W/cm at the 1, 25 and 45cm
Degree
of Sub-
cooling
number in
2.5 mm
1 3 74~148 148~370
25 4 74~148 148~370
45 5 ~148 148~444
1 4 ~148 148~444
25 6 ~148 148~519
45 8 148~222 296~519
1 6 ~148 296~1111
25 8 ~148 444~889
45 9 148~222 740~1481
Attitude

Bubble
sites
Bubble
growing-
time (µs)
Bubble life
time (µs)
Bubble max-
diameter
(µm )
Bubble
emission
frequency
(H z )
135~1555
135~1350
205~540
205~945
270~340
135~475
135~880
205~475
405~610
Super-Heated-
Layer Thickness
(µm)
125~435
125~280
156~625
94~375
218~500
155~685
250~685
315~685
440~500
60
100
130
85
115
135
55
?no
no
75K
50K
25K

LPDW/cm
Attitude
(cm)
Bubble sites
number in
2.5 mm
many
Bubble
growing-time
(µs)
? (<<74)
Bubble
lifetime (µs )
Bubble max-
diameter (µm)
Bubble
emission
frequency (Hz)
-
Super-heated-
layer thickness
(µm)
115
230~315
285~360
85
200~285
215~300
70
140~170
140~200
60
100
130
1
25
45
1
25
45
1
25
45
1
25
45
?(<74)
Film Boiling
-
600
20 <74 148~222
Film Boiling
185 405~880
430
6 74~148
74~148
~74
74~148
74~148
~148
148~222
148~296
148~222
148~370
148~370
148~370
125~280
95~280
65~185
125~435
125~280
156~625
270~1015
270~945
205~610
135~1555
135~1350
205~540
10(4)
12(2)
3
4
5
290
182