2011 International Conference on Alternative Energy in Developing Countries and Emerging Economies
- 127 -
researchers had used the visualization technique to observe
the phenomena inside [2-8]. However, there is no report
when the oscillating heat pipe is taken as an extended
surface of heat exchanger.
The objective of this study was to observe the two
phase flow pattern of working fluid inside an oscillating
heat pipe by a visualization technique when it was used as
the extended surface of a heat exchanger under natural
convection. The result aimed to find out the suitable
operating condition for this heat exchanger type.
II. M
ATERIAL AND METHODOLOGY
A wire-on-tube heat exchanger using a capillary tube
as an extended surface was installed in a controlled
temperature room and the schematic sketch of the
experimental apparatus is shown in Figure 3. The hot water
was flowing inside the heat exchanger tubes and
exchanged heat with the surrounding air. The hot water
flow rate was kept constant at 1 L/min. The water inlet
temperature was varied between 25 and 85
q
C. Both the
inlet and the outlet water temperatures and the surface
temperature of the heat exchanger were measured by a set
of calibrated K-type thermocouples and the signals were
recorded by a temperature data logger. In this experiment,
the ambient temperature was kept constant at 25
q
C.
Test Section
Pump
Heater&
Temperature
Controller
Hot Water Tank
T
T
T
Data
Logger
Computer
T
Flow Meter
Fig. 3. Schematic sketch of the experimental set-up.
The feature of the wire-on-tube heat exchanger used in
this experiment is shown in Figure 4 and its dimensions are
shown in Table I. The glass capillary tube filled with
working fluid and acts as an oscillating heat pipe was
attached to the heat main tube panel. In this experiment,
R123, acetone and methanol were selected as working
fluids in the heat pipe. The filling ratio was kept constant
at 50% of the total volume inside the glass capillary tube.
In this work, the closed loop oscillating heat pipe was
oriented in vertical direction as also shown in Figure 4.
Fig. 4. Heat exchanger geometry.
TABLE I
D
IMENSIONS OF WIRE
-
ON
-
TUBE HEAT EXCHANGERS
No.
Tube
dia.
(mm)
Tube
pitch
(mm)
Number
of tube
in row
Tube
length
(cm)
Cap.
tube
dia.
(mm)
Cap.
tube
pitch
(mm)
Cap.
Tube
length
(cm)
1
9.53
30
6
30
1
20
400
2
9.53
40
6
30
1
20
400
3
9.53
50
6
30
1
20
400
4
9.53
30
6
30
2
20
400
5
9.53
40
6
30
2
20
400
6
9.53
50
6
30
2
20
400
III. R
ESULTS AND DISCUSSION
A. Effect of surface temperature
When the hot water was flowing inside the main tube
panel the surface temperature of tube was higher than that
of the ambient then there was heat transfer from the tube
panel to the fin body and to the surrounding ambient.
Figure 5 shows the effect of the main tube surface
temperature on the flow pattern of the working fluid inside
the capillary tube. In this case, the diameter of capillary
tube was 2 mm, while the tube pitch was 50 mm. The
working fluid charged in the capillary tube was R123. It
was found that the flow pattern changed with the surface
temperature. When the surface temperature was between
30
q
C and 40
q
C a slow moving slug of bubble occurred.
However, when the surface temperature was increased to
the ranges of 40
q
C to 60
q
C and 60
q
C to 80
q
C, bubbles
with short slug and bubbles with short and long slug were
observed, respectively.
These results agreed with the boiling of working fluid
inside the micro channel [3,5] of which the shape of
bubble depended on the surface temperature.
