VGB POWERTECH 11 (2019)

VGB PowerTech 11 l 2019
Sub-cooled boiling of natural circulation in narrow rectangular channels
as the flow pattern in experiment channel
by increasing the heating power. The inlet
sub-cooling remains constant at an atmospheric
pressure, and a gap size of 3 mm experiment
channel is used. Figure 7a shows
the trend of volume flow rate with the
heating power and F i g u r e 7 b shows the
flow pattern in the experiment channel.
As seen from F i g u r e 7 a , an increase in
the heating power leads to a corresponding
increase in the average volume flow rate,
accompanied with oscillation. In the beginning,
the volume flow rate increases with
slight oscillation. This is a direct result of
small bubbles adhering to the heating surface.
As the power is further increased, the
bubbles combine together and detach from
the heating surface. The bubbles are quickly
compensated by the main fluid which is
in a sub-cooled state. At this time, the volume
flow tends to grow with drastic oscillation.
When the heating power reaches a
certain range, the main fluid becomes saturated.
At this time, it is difficult to make
bubbles condensation. Hence, the volume
flow rate grows slowly with slight oscillation.
As observed from F i g u r e 7 b , the bubbles
are mostly generated on side of the
rectangular channel. It reveals that the
heat transfer coefficient is higher near the
edges of the channel, than in the middle of
experiment channel. The bubbles attach
themselves to the heating wall, and then
slip along with the flow direction. During
this process, the bubbles are gradually compensated.
The main fluid is highly subcooled
near the entrance of the experiment
channel, where the bubbles adhere themselves
to the heating wall and initiate a
slight disturbance in the thermal boundary
layer. Hence, there is a lower heat transfer
coefficient at this location. The main fluid is
sub-cooled to a lower level in the upper section
of the rectangular channel. At this location,
the bubbles begin to polymerization
and break away from the boundary layer,
creating a drastic disturbance on thermal
boundary layer. This results in a higher
value of heat transfer coefficient.
Mechanism analysis
By analyzing the effects of inlet sub-cooling
and heating power, observing motion characteristic
of bubbles in experiment channel,
this paper proposes three stages about subcooled
boiling of natural circulation in a
narrow rectangular channel. F i g u r e 8
( a ) ( b ) ( c ) show the characteristics of
bubbles in different stages about subcooled
boiling of natural circulation in a
narrow rectangular channel
First stage: As shown in F i g u r e 8 ( a ) ,
the main fluid is in the early sub-cooled
boiling stage, where the small bubbles are
adhered to the heating surface. These bubbles
remain stationary when the power is
kept constant. On one hand, the bubbles
are condensed by the main fluid which is in
Heat Wall
Figure 8a Figure 8b Figure 8c
Fig. 8 (a) (b) (c). Characteristics of bubbles in different stages about sub-cooled boiling of natural
circulation in a narrow rectangular channel.
a sub-cooled state, hence decreasing their
size. On the other hand, the external heat
tends to increase the size of bubbles. The
actual size of the bubbles is determined by
the balance of such opposing effects. There
is a relatively small heat transfer coefficient
at this stage, and the volume flow presents
an increase with slight oscillation.
Second stage: As shown in F i g u r e 8 ( b ) ,
the main fluid is sub-cooled to a lesser degree
along the flow direction as the heating
power is increased. Bubbles begin to grow
and break away from the heating wall.
Some bubbles begin to coalesce, due to the
squeezing effect of rectangular narrow
channel. However, the bubbles gradually
become smaller as they slip along the flow
direction since the main fluid is in a subcooled
state. The bubbles are periodically
generated, separated and then condensed.
In this stage, there is a relatively high heat
transfer coefficient due to a higher disturbance
effect. The volume flow increases
with dramatic oscillation.
Third stage: As shown in F i g u r e 8 ( c ) , it
presents a reduction in the single phase
and sub-cooled boiling sections, as the
heating power is increased in the experiment
channel. The upper part of the channel
shows saturated boiling with the occurrence
of mixing flow. In this stage, the
main fluid has a lower degree of sub-cooling,
and the generation rate of bubbles is
much higher than the rate of their compensation.
The disturbance caused by the bubbles
generation and detachment enhances
the turbulent kinetic energy of the
boundary layer, which increases the heat
transfer coefficient. At this time, the subcooled
boiling begins to exhibit a transition
towards the saturated boiling phenomenon.
Empirical correlation
At present, Rohsenow correlation [23]
shown as equation (4) is usually used to
calculate heat transfer coefficient of subcooled
boiling at small flow rate. As for
natural circulation, Cao correlation [24]
shown as equation (5) and Hong correlation
[25] shown as equation (6) are used to
Heat Wall
Heat Wall
calculate heat transfer coefficient of subcooled
boiling.
(4)
In the above equations, C pl is specific heat
measured in J/(kg K). ∆t is the wall superheat
measured in o C. r is latent heat of vaporization
measured in J/kg. C wl is Rohsenow’s
constant. q is the heat flux measured
in kW/m 2 . l is the kinetic viscosity of saturated
liquid measured in Pa˙s. is the surface
tension measured in N˙m. g is the
gravitational acceleration measured in m/
s2. l is the density of saturated liquid
measured in kg/m 3 . v is the density of saturated
steam measured in kg/m 3 . Pr l is the
Prandtl number of saturated liquid.
(5)
(6)
In the above equations, h is the heat transfer
coefficient of sub-cooled boiling measured
in kW/(m K). q is the effective heating
power of the experiment channel measured
in kW/m 2 . ∆T sub is the sub-cooled degree
measured in o C. b is the narrow gap of
experiment channel measured in m.
These equations (Eq. 4-6) are used to obtain
the theoretical calculation results.
F i g u r e 9 shows a comparison between
theoretical calculations and the experiment
results of the natural circulation system.
As depicted in F i g u r e 9 , the calculations
of Cao and Hong correlations fit well with
the experiment results. The relative errors
between theoretical calculations and the
experiment results are less than 30 %.
However, the theoretical calculations of
Rohsenow correlation exhibit a large error.
This is because the Rohsenow correlation
only considers the effect of heat flux
whereas the experiment considers the influence
of channel size as well as inlet subcooling
on the heat transfer coefficient,
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