22-06-2012, 05:18 PM
International Communications in Heat and Mass Transfer
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Introduction
Several techniques have been applied to enhance heat transfer
rate in order to reduce the size of heat exchanger and also operating
cost. Among them, the following methods are considered as the effective
ones: (1) utilizing nanofluids (2) inserting fluid turbulators and
(3) roughening heat exchanger surfaces. Nanofluids are introduced
as the fluids containing nanometer-sized particles or nanoparticles.
Typical nanoparticles used in nanofluids are made of metals, oxides
and carbides while common base fluids are water, ethylene glycol
and oil. Nanofluids exhibit their great potentials for heat transfer applications
in many areas, including microelectronics, fuel cells, pharmaceutical
processes, and hybrid-powered engines [1]. As revealed
in several research works [2–5] the effective conductivities of nanofluids
are considerably enhanced as compared to those of their base
fluids, leading to superior convective heat transfer coefficients.
Experimental set-up and procedure
Experimental facility for heat transfer test is shown in Fig. 2. The
facility mainly consisted of: (1) a concentric tube heat exchanger,
(2) cooling water tank and heating water tank, (3) a set of thermocouple,
(4) data logger, (5) manometer, (6) a centrifugal water
pump, (7) rotameter, (8) watt/amp meter, and (9) variac transformer.
The inner tube (corrugated tube) was made from stainless steel
with inner diameter of 10 mm while the annulus (smooth tube)
was made from stainless steel with inner diameter of 25 mm. The
test section with length of 900 mm was well insulated to minimize
heat losses to surroundings. The RTDs were installed at the inlet and
outlet of the test section to monitor the temperatures at the positions.
The pressure drop across the test section was measured under isothermal
flow condition using the manometer connected to the pressure
taps located at the inlet and outlet of the corrugated tube.
Data reduction
Thermophysical properties of nanofluids
The thermophysical properties (density, specific heat, viscosity
and thermal conductivity) of the nanofluid were calculated as a function
of nanoparticle volume concentration (ϕ) together with properties
of base fluid and nanoparticles. The density of nanofluid was
evaluated using the general formula for the mixture:
Conclusions
The experimental results of the heat transfer enhancement by
using CuO/water nanofluid in a corrugated tube fitted with twisted
tape lead to the following conclusions.
1. Convective heat transfer, friction factor as well as thermal performance
factor associated with the simultaneous application of
CuO/water nanofluid and twisted tape are higher than those associated
with the individual techniques.
2. Convective heat transfer, friction factor as well as thermal performance
factor tend to increase with increasing CuO concentration
of nanofluid and twisted ratio of twisted tape.
3. At similar condition, the corrugated tube coupled with twisted
tape in counter arrangement (CA) offer higher heat transfer performance
than the ones in parallel arrangement (PA).