Work Accomplished
The existing flow-through facility has been modified to accommodate
the micro-PCM slurry. The existing gear pump was replaced with a centrifugal
pump to prevent damage to the micro-slurry. The filter and the quick disconnect
couplings were removed from the system because small passages in the flow
loop would impede the movement of the slurry. The reservoir in the system
was eliminated to minimize the total volume of the system. The resulting
system now has a volume of 0.45 gallons, as opposed to 5.5 gallons in the
previous system. Preliminary tests have been conducted with a PAO/CTFE
(both synthetic oils) mixture without the PCM.
A literature survey of heat transfer enhancement in phase change
material slurries was conducted. It has been reported that heat transfer
rates some two to four times higher than in single phase flow may be achieved
with a slurry system. Surface-to-fluid temperature difference, mass flow,
pumping power, and storage volume requirements were also found to be significantly
reduced. It was also found that the effective thermal conductivity of the
moving slurry is as much as three times the thermal conductivity of stationary
suspensions.
Experimental Effort - Experiments were conducted
with a 2-pass parallel flow SEM-E module, with both the carrier fluid alone
as well as with the slurry. Tests were conducted for a range of fluid flow
rates and module powers. Care was taken to ensure that the PCM was uniformly
entrained in the carrier fluid and did not coagulate. The turbine flow
meter was calibrated individually for both the carrier fluid and the slurry.
The maximum surface temperature rise of the module with the carrier fluid
(PAO/CTFE mixture) was very close to the results obtained previously with
PAO. In the tests conducted with the slurry, it was noticed that the maximum
surface temperature rise was strongly controlled by the melting point of
the PCM, and was independent of the slurry flow rate. Thus, superior thermal
performance was achieved even at low flow rates with the micro-PCM slurry.
This enhancement is due to the infinite heat capacity associated with the
melting of the PCM and to enhanced micro-convection resulting from the
carrier fluid/pellet interaction. The surface temperatures were essentially
independent of the slurry flow rates. Even at the lowest flow rates tested,
the latent heat capacity of the PCM was greater than the total heat supplied
to the module. Therefore, the PCM in the slurry was at the melting point
of the PCM. This was verified by a fluid bulk temperature measurement at
the outlet of the module. A series of tests as conducted at higher slurry
inlet temperatures to test the effect of inlet temperature on the thermal
performance. The minimum temperature in the system was still below the
freezing point of the PCM. The increased inlet temperature did not have
a significant effect on module surface temperatures. Thus, the lower fluid
temperature rise before melting is compensated for by the sensible heat
absorbed. A key feature of the modified flow loop is the incorporation
of a chiller to cool the slurry below the freezing temperature of the PCM.
