2011 International Conference on Alternative Energy in Developing Countries and Emerging Economies
- 126 -
Visualization of Two-Phase Flow Pattern inside a
Closed Loop Oscillating Heat Pipe Used as a Wire
Fin: A Case Study of Natural Convection
T. Samana*, T. Kiatsiriroat** and A. Nuntaphan***
*Graduate School, Chiang Mai University,
(Thailand)
** Department of Mechanical Engineering, Chiang Mai University,
(Thailand)
*** Thermal Technology Research Laboratory, Mae Moh Training Center, Electricity Generating Authority of Thailand, (
Thailand
)
Abstract--
The purpose of this research is to study two
phase flow phenomena inside a closed loop oscillating heat
pipe (CLOHP) by visualization technique. The CLOHP was
acted as an extended surface of a wire-on-tube heat
exchanger. The effects of surface temperature, the capillary
tube diameter, the tube pitch and the working fluid type were
examined. The results showed that
the two phase flow pattern
observed by visualization technique depended on the surface
temperature, the capillary tube diameter and the tube pitch.
Moreover, it was found that R123 was the best working fluid
compared with acetone and methanol.
Index Terms--
Visualization, Wire-on-tube, Closed loop
oscillating heat pipe, Heat exchanger, Natural convection
I. I
NTRODUCTION
A wire-on-tube heat exchanger consists of a tube panel
attached with solid steel wires as its extended surface. It
has been used in the refrigerators and the air-conditioning
systems for many decades. Figure 1 shows the sketch of a
wire-on-tube
heat exchanger. It should be noted that
normally the heat transfer from the external surface to the
ambient air is taken by natural convection.
The conventional design of wire-on-tube heat
exchanger is performed by taking a solid wire as extended
surface. Therefore, the heat transfer from main tube
surface to fin is performed only by heat conduction thus
there is a limitation due to the thermal resistance of the
wire material. To overcome this problem, a closed-loop
oscillating heat pipe (CLOHP) which is a closed-loop
capillary tube filled with working fluid as shown in Figure
2 is used instead of the solid metal wire. The heat transfer
from the tube surface to its fin is the combination of
conduction through the capillary tube material and
convective heat transfer through the boiling and the
condensation of the working fluid inside the capillary tube.
___________________________
This work was supported by the Electricity Generating Authority of
Thailand for facilitating the testing equipments and the Commission on
Higher Education, Thailand.
Fluid in
Fluid out
Metal wire
Tube
g
Fig. 1 Conventional wire-on-tube heat exchanger.
Fluid in
Fluid out
Cap Tube
Tube
g
Fig. 2. Wire-on-tube heat exchanger using capillary tube as extended
surface material.
The air-side performance of wire-on-tube heat
exchanger of which the wire is an oscillating heat pipe was
reported by Nuntaphan et al. [1]. They found that in case
of forced convection, the performance of heat exchanger
increased around 10% compared to that of the
conventional wire-on-tube heat exchanger. However, the
result in case of natural convection was not included in this
report.
To understand the boiling and condensation of the
working fluid inside the oscillating heat pipe, many
