(237g) Flow Visualization of Closed Loop Pulsating Heat Pipe (CLPHP) Charged with Olive Oil for High Temperature Applications

           
Pulsating heat pipe (PHP) is a passive, two-phase heat transfer device. It is a
new category of heat pipes which is invented by Akachi in 1990 (Akachi 1990).
It consists meandering turns of a capillary tube in between an evaporator and a
condenser section. It does not require any internal wick structure; therefore
it is simple in structure compared to the conventional heat pipes. After the
evacuation of the tube, it is filled with a suitable working fluid which
distributes itself inside the PHP in the form of liquid slugs and vapour plugs.
When a heat input is provided at the evaporator the liquid slugs and vapour
plugs oscillate due to pressure fluctuations inside the PHP. Majorly four types
of flow phenomena have been observed inside the PHP depending on heat flux,
filling ratio and inclination angle. These are oscillation, circulation,
annular flow and dryout (Yang et al. 2008).

           
Based on the applications of conventional heat pipes, a large number of fluids (ammonia
to liquid sodium) have been tested and used (Faghri 2014). However, present
literature shows that PHP is tested with ethanol, methanol, acetone and water
which are low boiling point temperature fluids (Han et al. 2016; Shi & Pan
2017). Only one study performed by Mahapatra et al., (2016) shows the
application of high temperature fluids in the PHP. Therefore, these factors
limit its application to medium temperature applications. However, the PHP has
a great potential for high temperature applications such as waste heat recovery
and nuclear power plants. This leads to the search for new working fluids.

           
With this motivation, an experimental study is performed to
enhance the operating temperature of the PHP by using
olive oil as a working fluid. A single
turn closed loop PHP (CLPHP) made up of quartz
tubes is employed in the present study. The quartz CLPHP gives a rough
estimation of temperature data. Visualization of the working fluid gives a much
deeper understanding of the internal phenomena. Therefore, our work is focused
on the visualization rather than temperature measurement. Distilled water is also tested as a comparative fluid in
the CLPHP. Internal hydrodynamics of the CLPHP charged with olive oil
and distilled water is visualized. A high speed camera is employed to capture
the internal flow phenomena. The major flow phenomena observed in CLPHP when
tested with olive oil with increasing input heating
power are shown in Fig. 1.

           
From this figure, different flow phenomena starting from no bubble to large slug-plug flow can be
observed. During this study, it is
observed that olive oil moves very slowly due to its high viscosity and it
circulates unidirectionally with a little flow reversal. However, a high flow
reversal is observed with distilled water due to its low viscosity. Cavitation is
also observed with distilled water in high pressure drop regions such as at
tube bends. This phenomenon is not reported in literature till now. A dryout
condition is observed in CLPHP when charged with distilled water. However, this
condition is not observed with olive oil at applied input heating powers during
this study. This study confirms that the CLPHP with olive oil could
transfer heat at high heat fluxes compared to when charged with distilled
water. However, there are some issues with chemical stability of the olive oil
during its working at high temperatures
which needs immediate attention and
further investigation to make olive oil as a suitable working fluid for
existing PHPs. Further, details will be provided in the full paper.

References

Akachi, H., 1990. Structure of a
heat pipe. US Patent 4,921,041.

Faghri, A., 2014. Heat pipes:
review, opportunities and challenges. Frontiers in Heat Pipes, 5(1).

Han, X. et al., 2016. Review of the
development of pulsating heat pipe for heat dissipation. Renewable and
Sustainable Energy Reviews, 59, pp.692-709.

Mahapatra, B., Das, P. & Sahoo,
S., 2016. Scaling analysis and experimental investigation of pulsating loop
heat pipes. Applied Thermal Engineering, 108, pp.358-367.

Shi, W. & Pan, L., 2017. Influence
of filling ratio and working fluid thermal properties on starting up and heat
transferring performance of closed loop plate oscillating heat pipe with
parallel channels. Journal of Thermal Science, 26(1), pp.73-81.

Yang, H., Khandekar, S. &
Groll, M., 2008. Operational limit of closed loop pulsating heat pipes. Applied
Thermal Engineering, 28(1), pp.49-59.