The detailed description of the experimental apparatus has
already been done previously [13]. The main features are recalled hereafter. Flow loop used for pressure drop and convective heat
transfer coefficient measurements with fixed wall temperature
boundary conditions is shown schematically in Fig. 5. From
a reservoir tank, the nanofluid, with specified concentration, was
circulated using a gear pump (Micropump, Ismatec, 0e200 l h1).
Assuming that nanofluids are considered as homogeneous fluids,
the flow rate was measured by a Coriolis flow meter (Micro Motion
ELITE, CFM10) that was calibrated with 0.1% accuracy over the
range of 0e80 kg h1. The pressure drop was measured directly by
three differential strain-gauge pressure transducers operating over
a range of 0e1620 kPa with uncertainty within 0.075% f.s., as calibrated
by the manufacturer (Rosemount). A pH meter (Eutech
Instruments) was inserted downstream of the test section to follow
the nanofluid pH change with a maximum accuracy of 0.01.
The test section (Fig. 6) consisted of a 0.5 m long tube-in-tube
heat exchanger, the tested nanofluid flowing into the 4 mm diameter
and 1 mm thick inner copper tube (CuA1) and heating or
cooling water flowing into a 10 mm diameter and 1 mm thick
stainless steel annular tube. The test section was preceded by
a 0.5 m (125 diameters) adiabatic section.
The nanofluid was circulated inside the inner tube (primary
loop) with a temperature varying between 15 and 90 C. To observe
the potential influence of the transverse temperature gradient, the
water temperaturewas varied within the same range allowing us to
change the temperature difference between the fluid and the wall.
The fluid could be heated or cooled thanks to various valves, and
then the gradient direction could be modified. After passing
through the test section, the nanofluid entered a heat exchanger in
which water was used as a cooling or heating fluid depending on
nanofluid heating or cooling tests. For both primary and secondary
loops, temperature was controlled using two thermostatic baths
(Polystat 37, Fischer Scientific) and a second heat exchanger.
The entire test section was insulated with polyurethane foam
(Armaflex) in order to minimize heat losses