Design Improvement of a Vertically Oriented Thermal …
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of the liquid fraction of each case has been mentioned. At 60 min of melting, the
base case obtains a liquid fraction value of 0.76. For case 1, area-weighted liquid
fraction is found to be 0.68 (with the inner zone liquid fraction value of 0.91 and
the outer zone liquid fraction value 0.45), whereas for case 2, area-weighted liquid
fraction is found to be 0.79 (with the upper zone liquid fraction value of 0.83 and the
lower zone liquid fraction value 0.75). It is observed that in the case 1, inner zone
has a relatively higher liquid fraction value than the outer zone. Similarly, in case 2,
the upper zone has a higher liquid fraction value than the lower zone. Therefore, if
the same value of liquid fraction value is attained in each zone (same melting time of
the PCM kept in the zone), performance of the TES system can be improved further.
The factor that results in the improvement of the TES system due to the introduction of partition is the establishment of separate melting front and circulation in
each zone. Domination of natural convection during melting and occurrence of more
than one melting front results in melting rate enhancement. Further, case 1 showed
lower melting time than case 2 because of the greater height of each zone. It can be
noted that for all the cases shown in Fig. 3, the melting front propagates downward
(irrespective of the partition). For case 1, the top region of the inner zone is melted
initially (almost complete melting is achieved at 60 min). This helps to start melting
the outer zone. In fact, with this arrangement melting in the outer zone cannot start
until the inner zone is melted. Therefore, achieving the same total melting time at
each zone that can reduce the melting time substantially due to the modification of
the system is limited in this case although case 1 showed better improvement than
case 2. While in case 2, the PCM in both zones begins to melt since the start. Due
to the presence of the partition that is attached between the inner tube of HTF and
outer tube of PCM, it acts as an extended surface. Therefore, the bottom region of the
upper zone also melts. This is not present in the base case or the lower zone of case
2. Consequently, upper zone in case 2 has lower total melting time of PCM. Another
factor that assists faster melting of PCM situated at the top region is the presence of
higher energy content of HTF near the entry. Thus, case 2 can be modified to have
the partition introduced closer to the bottom region to achieve approximately same
melting time in each zone and overall faster melting of PCM in the TES system.
It is observed that the introduction of partition in the PCM domain can significantly
enhance the melting rate of PCM and improves the thermal performance of the TES
system. It is also noticed that there can be an optimum proportion of PCM mass in
each zone. Melting should be completed at the same time in each zone to further
improve the performance of TES system.
5.2 Effect of PCM Mass Proportion in Each Zone
The provision of partition in the PCM domain improves the performance of the TES
system due to improved natural convection current and the partition that itself acts
as an extended surface. However, PCM mass proportion in each zone also affects the
total melting time of PCM. The total melting time of PCM is an important factor for
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of the liquid fraction of each case has been mentioned. At 60 min of melting, the
base case obtains a liquid fraction value of 0.76. For case 1, area-weighted liquid
fraction is found to be 0.68 (with the inner zone liquid fraction value of 0.91 and
the outer zone liquid fraction value 0.45), whereas for case 2, area-weighted liquid
fraction is found to be 0.79 (with the upper zone liquid fraction value of 0.83 and the
lower zone liquid fraction value 0.75). It is observed that in the case 1, inner zone
has a relatively higher liquid fraction value than the outer zone. Similarly, in case 2,
the upper zone has a higher liquid fraction value than the lower zone. Therefore, if
the same value of liquid fraction value is attained in each zone (same melting time of
the PCM kept in the zone), performance of the TES system can be improved further.
The factor that results in the improvement of the TES system due to the introduction of partition is the establishment of separate melting front and circulation in
each zone. Domination of natural convection during melting and occurrence of more
than one melting front results in melting rate enhancement. Further, case 1 showed
lower melting time than case 2 because of the greater height of each zone. It can be
noted that for all the cases shown in Fig. 3, the melting front propagates downward
(irrespective of the partition). For case 1, the top region of the inner zone is melted
initially (almost complete melting is achieved at 60 min). This helps to start melting
the outer zone. In fact, with this arrangement melting in the outer zone cannot start
until the inner zone is melted. Therefore, achieving the same total melting time at
each zone that can reduce the melting time substantially due to the modification of
the system is limited in this case although case 1 showed better improvement than
case 2. While in case 2, the PCM in both zones begins to melt since the start. Due
to the presence of the partition that is attached between the inner tube of HTF and
outer tube of PCM, it acts as an extended surface. Therefore, the bottom region of the
upper zone also melts. This is not present in the base case or the lower zone of case
2. Consequently, upper zone in case 2 has lower total melting time of PCM. Another
factor that assists faster melting of PCM situated at the top region is the presence of
higher energy content of HTF near the entry. Thus, case 2 can be modified to have
the partition introduced closer to the bottom region to achieve approximately same
melting time in each zone and overall faster melting of PCM in the TES system.
It is observed that the introduction of partition in the PCM domain can significantly
enhance the melting rate of PCM and improves the thermal performance of the TES
system. It is also noticed that there can be an optimum proportion of PCM mass in
each zone. Melting should be completed at the same time in each zone to further
improve the performance of TES system.
5.2 Effect of PCM Mass Proportion in Each Zone
The provision of partition in the PCM domain improves the performance of the TES
system due to improved natural convection current and the partition that itself acts
as an extended surface. However, PCM mass proportion in each zone also affects the
total melting time of PCM. The total melting time of PCM is an important factor for