Figure
5 illustrates the time-lapse of melting temperature. Observing the
temperature contours over time, it’s evident that the temperature
distribution closely mirrors the behavior of the liquid fraction. In the
initial stage, the temperature in the boundary layer gradually rises
until it reaches the PCM’s liquidous temperature, signifying the onset
of PCM melting. Subsequently, the temperature experiences a gradual
increase over time.
In the second stage, the temperature of the melting PCM steadily rose
until it entered the third stage, marked by a notable increase until it
reached the heat transfer fluid (H.T.F.) temperature. During this
temperature increase, the primary mode of heat transfer to the PCM was
sensible heat in the upper half, with a minor contribution of latent
heat in the lower half. As the upper half reached the H.T.F.
temperature, natural convection reached its minimal level. Subsequently,
heat transfer predominantly occurred through conduction as latent heat
to the lower solid PCM.
These stages appeared in three models, although at different times, as
illustrated in figures 3 and 5. Whereas model A-2 melts faster than
model A-3, model A-1 melts the slowest. The merging of regions (1, 2,
and 3) is slower in models A-2 and A-3 than in model A-1 because the
side fins (fins 2 and 5) are shorter, and those fins contributed to
generation region (2 and 3), so the merging will be delayed with region
(1), as discussed in heat transfer rate graphs. While region (4) is
created faster in models A-2 and A-3 and contains more melted PCM, this
is due to longer lower fins (fin number 3 and 4) that contribute to
region (4) generation.
Figure (6) displays the results of the volume-averaged temperature of
the PCM over time. The temperature behavior over time reveals distinct
patterns among the three models. Model A-2 and Model A-3 maintain a
consistent temperature over time, whereas Model A-1 exhibits a higher
overall temperature. The temperature graph exhibits three distinct
regions for all three models. The first region corresponds to stage (1),
marked by a rapid temperature increase over time. The second region
aligns with stage (2), where the temperature increase slows due to the
influence of latent heat as opposed to sensible heat. In the third
region, a temporary, rapid temperature increase over time is observed.
This increase results from the rising temperature of the upper half due
to natural convection until it reaches the temperature of the heat
transfer fluid. At this point, the heat transfer rate initiates the
melting of the solid PCM in the lower half, maintaining a constant
temperature. The fresh melted PCM gradually heats to match the
temperature of the upper half, contributing to a gradual increase in the
volume-averaged temperature over time.