Fig. 5 shows the changes of density and mass flow rate with the
variation of nanoparticles volume fraction at constant volumetric
flow rate 3 L/min. Density was calculated from Eq. (1). As shown
in Fig. 5, density and mass flow rate of nanofluids climbed up with
the enhancement of nanoparticles volume percentage. Mass flow
rate was calculated from the following relation, m_ ¼ Vq where, q
represents the density of fluid in kg/m3. For a particular geometrical
dimension, density and nanoparticles volume fraction is
responsible for varying mass flow rate as well as Reynolds number.
Using nanofluids properties Table 2 and Eq. (1), mass flow rate was
calculated and it was found that for same volume flow rate, density
and mass flow rate of CuO/water nanofluids become higher compare
to Al2O3/water and ZnO/water nanofluids. Results are agreed
with Sohel et al. [30].
It is expected that entropy generation number should be decreased
to gain exergy. When nanofluid is used as agent fluid,
the entropy generation number is reduced. In fact, it is known that,
by adding different nanoparticles to the water entropy generation
can be reduced [16].The scaled non-dimensional entropy generation
rate W had been computed using Tables 1 and 2 and Eq.
(22), after assigning the values of B0, Re, Nu and f. Assuming the
temperature difference was constant at 40 K.
Fig. 6 shows the changes of entropy generation rate of coiled
tube heat exchanger between the volumetric flow rate 3–6 L/min.
From the Fig. 6, the entropy generation rate of helical coil heat exchanger
significantly decreases with rise of volume flow rate and
nanoparticles volume concentration respectively. Findings are
completely matched with the results of Leong et al. [31] and Mahmoudi
et al. [32].
CuO/water nanofluids with higher volume fraction of nanoparticles
may be a good choice as a working fluid because of their entropy
generation rate is lower than the other considering
nanofluids. Maximum 6.14% decreased in entropy generation rate
is found for CuO/water nanofluids. From Hamilton and Crosser
model [33], it is concluded that the nanofluid’s thermal conductivity
is related to the shape and volume fraction of nanoparticles. It
can be stated that more particles addition contributes to increased
effective surface area for heat transfer. As a result, the inherently
greater thermal conductivity of nanoparticles will enhance the
thermal conductivity of the nanofluids. This may reason for minimization
of entropy generation rate i.e. improvement in exergy
efficiency. CuO/water nanofluids might provide better heat transfer
coefficient and entropy generation rate with comparing to
Al2O3/water and ZnO/water nanofluids. Thus, the analytical results
indicate that, in helical coil heat exchanger, there is a definite probability
to get maximum exergy by using CuO/water nanofluids as
working fluid.
5. Conclusion
In the present study, we have focused on the benefits of using
different nanofluids in a helically coiled tube heat exchanger. We
have studied the effects of volume flow rate, nanoparticles volume
fraction, mass flow rate, density, thermal conductivity, Reynolds
number and Nusselt number on heat transfer coefficient and entropy
generation rate of the heat exchanger. Analytical outcomes revealed
that, CuO/water nanofluids could increase the heat
transfer coefficient and decrease the entropy generation about
7.14% and 6.14% respectively. Study also remarked that the increment
of particle volume fraction and volume flow rate of nanofluids
could enhance heat transfer coefficient and reduce the
entropy generation rate i.e. higher exergy efficiency may be found.
For equal volume flow rate, mass flow rate could be increased by
injecting nanoparticles in base fluid only and represented higher
efficiency. Density and thermal conductivity are the most important
parameters for efficiency improvement. From this study, it
may be concluded that the performance of a heat exchanger can
be enhanced by converting the working fluid with nanofluids. On
the basis of this study, CuO/water nanofluids may be a good option.
Acknowledgments
The authors are indebted to the UM High Impact Research Grant
UM-MOHE UM.C/HIR/MOHE/ENG/40 for financial support from the
Ministry of Higher Education Malaysia to carry out this research.
The authors are very grateful to the reviewers due to their appropriate
and constructive suggestions as well as their proposed corrections,
which have been utilized in improving the quality of
the paper.
Fig. 5 shows the changes of density and mass flow rate with the
variation of nanoparticles volume fraction at constant volumetric
flow rate 3 L/min. Density was calculated from Eq. (1). As shown
in Fig. 5, density and mass flow rate of nanofluids climbed up with
the enhancement of nanoparticles volume percentage. Mass flow
rate was calculated from the following relation, m_ ¼ Vq where, q
represents the density of fluid in kg/m3. For a particular geometrical
dimension, density and nanoparticles volume fraction is
responsible for varying mass flow rate as well as Reynolds number.
Using nanofluids properties Table 2 and Eq. (1), mass flow rate was
calculated and it was found that for same volume flow rate, density
and mass flow rate of CuO/water nanofluids become higher compare
to Al2O3/water and ZnO/water nanofluids. Results are agreed
with Sohel et al. [30].
It is expected that entropy generation number should be decreased
to gain exergy. When nanofluid is used as agent fluid,
the entropy generation number is reduced. In fact, it is known that,
by adding different nanoparticles to the water entropy generation
can be reduced [16].The scaled non-dimensional entropy generation
rate W had been computed using Tables 1 and 2 and Eq.
(22), after assigning the values of B0, Re, Nu and f. Assuming the
temperature difference was constant at 40 K.
Fig. 6 shows the changes of entropy generation rate of coiled
tube heat exchanger between the volumetric flow rate 3–6 L/min.
From the Fig. 6, the entropy generation rate of helical coil heat exchanger
significantly decreases with rise of volume flow rate and
nanoparticles volume concentration respectively. Findings are
completely matched with the results of Leong et al. [31] and Mahmoudi
et al. [32].
CuO/water nanofluids with higher volume fraction of nanoparticles
may be a good choice as a working fluid because of their entropy
generation rate is lower than the other considering
nanofluids. Maximum 6.14% decreased in entropy generation rate
is found for CuO/water nanofluids. From Hamilton and Crosser
model [33], it is concluded that the nanofluid’s thermal conductivity
is related to the shape and volume fraction of nanoparticles. It
can be stated that more particles addition contributes to increased
effective surface area for heat transfer. As a result, the inherently
greater thermal conductivity of nanoparticles will enhance the
thermal conductivity of the nanofluids. This may reason for minimization
of entropy generation rate i.e. improvement in exergy
efficiency. CuO/water nanofluids might provide better heat transfer
coefficient and entropy generation rate with comparing to
Al2O3/water and ZnO/water nanofluids. Thus, the analytical results
indicate that, in helical coil heat exchanger, there is a definite probability
to get maximum exergy by using CuO/water nanofluids as
working fluid.
5. Conclusion
In the present study, we have focused on the benefits of using
different nanofluids in a helically coiled tube heat exchanger. We
have studied the effects of volume flow rate, nanoparticles volume
fraction, mass flow rate, density, thermal conductivity, Reynolds
number and Nusselt number on heat transfer coefficient and entropy
generation rate of the heat exchanger. Analytical outcomes revealed
that, CuO/water nanofluids could increase the heat
transfer coefficient and decrease the entropy generation about
7.14% and 6.14% respectively. Study also remarked that the increment
of particle volume fraction and volume flow rate of nanofluids
could enhance heat transfer coefficient and reduce the
entropy generation rate i.e. higher exergy efficiency may be found.
For equal volume flow rate, mass flow rate could be increased by
injecting nanoparticles in base fluid only and represented higher
efficiency. Density and thermal conductivity are the most important
parameters for efficiency improvement. From this study, it
may be concluded that the performance of a heat exchanger can
be enhanced by converting the working fluid with nanofluids. On
the basis of this study, CuO/water nanofluids may be a good option.
Acknowledgments
The authors are indebted to the UM High Impact Research Grant
UM-MOHE UM.C/HIR/MOHE/ENG/40 for financial support from the
Ministry of Higher Education Malaysia to carry out this research.
The authors are very grateful to the reviewers due to their appropriate
and constructive suggestions as well as their proposed corrections,
which have been utilized in improving the quality of
the paper.
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