The current work reports the thermophysical
and flow measurements of novel thermal solvents based on deep eutectic
solvents (DESs) and alumina-based nanoparticle-dispersed deep eutectic
solvents (NDDESs) for its use as a potential solar energy storage
medium. The DESs were synthesized using a hydrogen bond donor (i.e.,
oleic acid) and a hydrogen bond acceptor (i.e.,
dl
-menthol)
by using the COSMO-SAC-predicted equimolar ratio at a temperature
of 350.15 K. Thereafter, NDDESs or nanofluids were formed by dispersing
different volume fractions (0.001, 0.005, 0.0075, and 0.01) of Al
2
O
3
nanoparticles in the DESs. The optimum volume
fraction (0.005) of Al
2
O
3
nanoparticles was
selected through their thermophysical properties (density, viscosity,
thermal conductivity, and specific heat capacity) and its agglomeration
or stability behavior. As expected, NDDESs with a 0.005 volume fraction
gave a higher enhancement in thermal conductivity, viscosity, heat
capacity, and density as compared to DESs. To evaluate the heat transfer
coefficient, forced convection experiments were conducted in a circular
test section for both DESs and NDDESs under laminar conditions (
Re
= 124, 186, and 250). The enhancement of the local heat
transfer coefficient was found to be higher when compared to their
thermophysical properties. This was due to the nanoparticle migration
resulting in a non-uniform distribution of both thermal conductivity
and viscosity fields, which was inherently found to reduce the thermal
boundary layer thickness. In the final section, the heat transfer
coefficient and the Nusselt number were also validated with COMSOL
Multiphysics simulations.
Concentrated
solar power (CSP) is one of the emerging renewable
energy technology, where sunlight is concentrated from a large area
and is stored in a collector filled with heat-transfer fluid (HTF).
The current work reports the synthesis of novel HTF based on menthol-based
deep eutectic solvents (DESs). DES consisting of a hydrogen bond acceptor,
namely, dl-menthol and a hydrogen bond donor (oleyl alcohol),
has been prepared and explored to evaluate its effectiveness as a
thermal fluid. The thermal properties, namely, viscosity, density,
thermal conductivity (TC), and heat capacity, are measured and compared
with both conventional and commercial solvents within the temperature
range T = 298.15–353.15 K. The density of
DES was found to decrease with increase in temperature, while the
rheological measurements suggest a Newtonian fluid with its shear
viscosity decreasing exponentially with increasing temperature. The
TC of the DES was found to be ∼0.161 W/m K, which decreases
linearly with temperature, while a contrary was observed for heat
capacity. Further, the performance of these solvents has been evaluated
in a forced convective heat-transfer configuration under laminar flow
conditions with N
Re = 105, 155, and 260.
The convective heat-transfer experiments demonstrated the fact that
the thermal entrance length of the DES is very large because of its
high viscosity and low TC. The convective heat-transfer data, namely,
heat-transfer coefficient and Nusselt number, were compared with the
in-house AnuPravaha CFD simulator under laminar conditions. From the
obtained thermophysical properties, it is confirmed that the synthesized
DES can be used as a next-generation heat storage media in the CSP
plant.
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