Experimental and Numerical Analysis of a PCM-Supported ...
Experimental and Numerical Analysis of a PCM-Supported ...
Experimental and Numerical Analysis of a PCM-Supported ...
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7.2.7 Concluding remarks<br />
It has been demonstrated that the productivity <strong>of</strong> an evaporator can be noticeably<br />
enhanced by the use <strong>of</strong> a conductive packing media. The thermal conductivity<br />
represents an influencing parameter that controls local heat <strong>and</strong> mass transfer rates.<br />
The effect <strong>of</strong> thermal conductivity on the evaporator effectiveness <strong>and</strong> distillate rate<br />
has been analyzed through utilization <strong>of</strong> different packing media. It was found that<br />
the proportionality <strong>of</strong> evaporation rate is neither uniform with the thermal conductivity<br />
k s nor linear with the hot water mass flow rate. It is also important to point out that<br />
the maximum evaporation rate was obtained when k s is around 1.14 W.m -1 .K -1 as for<br />
the case <strong>of</strong> Pyrex glass <strong>and</strong> fired clay brick. The study concluded that, using smaller<br />
size <strong>of</strong> spherical fired clay bricks as a conductive media with optimum thermal<br />
conductivity, <strong>and</strong> due to its lower cost would be the ideal c<strong>and</strong>idate for the present<br />
application, rather than <strong>PCM</strong> media that usually have poor thermal conductivity <strong>and</strong><br />
high cost. The present analysis indicated that air to water mass flow ratio is one <strong>of</strong><br />
the most crucial operational parameters <strong>and</strong> its optimum value lies around 1.0.<br />
7.3 Condenser Performance<br />
Due to similarity between the evaporator <strong>and</strong> condenser, the previous discussions<br />
<strong>and</strong> interpretations <strong>of</strong> the effect <strong>of</strong> various influencing parameters on the evaporator<br />
performance can serve as a common background to underst<strong>and</strong> the condenser<br />
behaviour.<br />
7.3.1 Effect <strong>of</strong> inlet cooling water temperature<br />
As pointed out earlier, under steady state conditions the energy <strong>and</strong> mass balances<br />
imply that the liquid phase will be the final destination (i.e. the sole sink) <strong>of</strong> all the<br />
energy <strong>and</strong> mass transfer from gas to both solid <strong>and</strong> liquid phases. Effect <strong>of</strong> cooling<br />
water temperature <strong>and</strong> mass flow rate or the heat sink capacity can be considered<br />
both <strong>of</strong> self-evident <strong>and</strong> fundamental importance on the energy flow between all<br />
phases in the system under each specific boundary <strong>and</strong> geometrical conditions. As<br />
shown in figure (7.12), the lower inlet cooling water temperature <strong>and</strong> higher cooling<br />
water mass flow rate, the more fresh water that can be produced under given<br />
conditions.<br />
Following the same trend, the condenser performance has more pronounced<br />
response to the inlet cooling water temperature under higher cooling water mass<br />
flow rates. This appears clear from the steeper slopes for higher cooling water mass<br />
flow rate in figure (7.12). Given certain boundary conditions, cooling water mass flow<br />
rate can be adjusted with the inlet cooling water temperature to attain a specific<br />
cooling effect (i.e. condensation rate). For example, to get a condensation rate <strong>of</strong><br />
40l/h, three different combinations <strong>of</strong> cooling water mass flow rates <strong>and</strong> inlet<br />
temperatures can be used; 750l/h with 20°C, 1000l/h with 30°C, <strong>and</strong> 1500l/h with<br />
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