FIELD OF INVENTION
The present invention relates to a process for measuring the concentration of
nanofluids by ultrasonic technique. More particularly, the invention relates to a process
for measuring the concentration of nanofluids by ultrasonic velocity measurement
process.
BACKGROUND OF THE INVENTION
Dispersions wherein nanometer sized particles are suspended in base fluid (e.g. water,
ethylene glycol) are commonly known as nanofluids. Nanofluids have become important
because of their enhanced thermal property. Concentration of nanoparticle is one of the
most important parameter which governs the nanofluid thermal property. Although the
concentration of nanofluid is initially known when nanofluid was synthesized, however,
it may not remain same as the initial value during heat transfer experiments and
applications. This is due to settling and sedimentation of nanoparticles. Hence it is
critically important to be able to measure the concentration of nanoparticle in the fluid
at different stages of the process without disturbing it. Ultrasonic velocity measurement
technique is used to measure the relative concentration of colloidal dispersion. Due to
its non-destructive nature, ultrasonic technique is a suitable technique in the present
case. In typical ultrasonic velocity measurement equipment, a pulse is created by an
electrically stimulating source of transducer, a lead-zirconium titanate or other
piezoelectric ceramic disc. A timer is also started simultaneously. This pulse travels
through the liquid filled sample cell to the far wall and gets reflected from there. The
returning pulse is detected and the timer is stopped instantaneously. The acoustic time
of flight (T) is used to calculate the sound velocity and is given by

where sis the length of liquid column in the sample cell, v is the ultrasound velocity and
T is the acoustic time of flight. Now, Uricks equation is used which relates the velocity
of sound to the concentration of dispersed solid in liquid media. Now being a colloidal
dispersion it is expected the nanofluid concentration can be determined using the same
technique. Since very little was known about the physical property of nanofluid, this
technique was not used for determining the concentration of nanofluid. In the present
invention, ultrasonic technique is used to determine the concentration of nanofluid and
based on the agreement with the calculated theoretical value, it is proposed as a new
technology for in-situ nanofluid concentration measurement.
OBJECTS OF THE INVENTION
Therefore it is an object of the invention to propose a process for measuring the
concentration of nanofluids by ultrasonic technique which is capable of in-situ nanofluid
concentration measurement.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Fig.1 - shows experimental setup of ultrasound testing
Fig.2 - shows an ultrasonic reflector in the form of a nickel coated steel angle
Fig.3 - shows an SEM image of titanium oxide (TiO2) nano-particle
Fig.4 - shows velocity of sound (m/s) and time (µs) required to travel the gap of 18.09
mm with weight fraction of titanium oxide nano-particles
DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE
INVENTION
Nanofluid means a new class of nanotechnology-based heat transfer fluids that are
engineered by stably suspending particle, fibres or tubes with dimensions in the order
of 1-50 nm in traditional (nanometer) heat transfer fluids (HTF). HTF means water,
ethylene glycol and light oils. These HTFs are called base fluids. Particles making the
nanofluid can be metallic, metal-oxide, carbon based or polymer based. The nano-
particles are self-stabilized or stabilized using a surfactant of polymeric origin.
Concentration means the volume fraction of particles, fibres or tubes suspended in the
base fluids, which can vary from 0-1%.
It is a novel method to detect the concentration of nanoparticles in water in-situ using
ultrasound. The apparatus comprises of a transmitter continuously supplying electrical
high-frequency power at a suitable frequency for generating ultrasound waves and a
receiver to receive it. It also measures the acoustic time of flight and from this, the
velocity of sound is calculated.
The underlying principle is that the velocity of sound through a medium is a function of
its density and compressibility, and in case of dispersions, both the factors depend on
the concentration (volume fraction) of solid. Urick equation gives the dependence of
sound velocity on nanofluid concentration, and using that equation the concentration of
nanofluid is calculated.
Recent research and development of nanofluid technology have demonstrated the
excellent potential of nanofluids for heat transfer applications. The concentration of
nanofluid is one of the most important governing parameters in nanofluid heat transfer.
Hence, for all practical applications, it is crucial to measure the concentration of
nanofluid in-situ at different stages of the process. The present invention is to address
this requirement.
Figure 1 shows the schematic diagram of experimental setup of ultrasound machine (1).
The experimental setup consists of an ultrasonic flow detector and an ultrasonic
reflector (4). A datalogger (2) is disposed in the machine (1). The ultrasound velocity of
sound in m/s in water and nano-fluid is measured by a digital ultrasonic flow detector.
The ultrasonic probe (3) of 10 MHz frequency is immersed into the solution kept in a
beaker (5). The measurement accuracy of ultrasound probe is 0.001 us. An ultrasonic
reflector (4) in the form of a nickel coated steel angle as shown in Fig.2 is used to
reflect the sound wave in the solution. The distance between the probe (3) and the
horizontal surface of angle (4) is 18.09 mm. The system is calibrated with water as
medium. The ambient temperature is kept at 25°C. Spherical titanium oxide (TiO2) is
used as the nano-particle in the present experimental study. The concentrations of
nano-fluid used in the experiments are 0.1% wt, 0.5% wt and 2% wt.
Results and Discussion
Figure 3 shows the SEM images of the titanium oxide nano-particles. The size of
particles used in the present study is in the range of 20-50 nanometer (nm). Figure 4
shows the velocity of sound wave in different concentration of nano-fluids. It is
observed from the graph that the presence of nano-particles in water alters the
ultrasonic velocity and the ultrasonic velocity decreases with the concentration of nano-
fluid. The ultrasound velocity in a medium depends on two factors, viz. i) density and ii)
elasticity of the medium. The sound velocity (v) in a homogeneous fluid medium is
given by,

where v is the sound velocity in m/s, B is bulk modulus in newton permeter square
(N/m2), k is adiabatic compressibility in meter square per newton (m2/N), ? is density
(kg/m3) of the media. Equation 1 is also known as the Newton-Laplace equation.
In the present study, the fluid medium is not a homogeneous, but a colloidal dispersion
of nano-particle and water. Wood pointed out that the propagation of sound is
described very well by Newton's second law, whereby a force acting on an element of
the material accelerates the material. Since the displacements are very small, they may
be assumed to be harmonic and is known as linear approximation. The magnitude of
fluctuations (dialation) and density (condensation) associated with the sound wave is
controlled by the medium and the applied forces. Hence, the velocity of sound in
suspension is controlled by the mean density and the mean compressibility of the
medium. Based on this fundamental observation, Wood developed an equation for
sound velocity for a fluid mixture which was later modified by Urick. The modified
equation is known as Urick's equation. The velocity of sound (vm) equation for multi-
component system is given by,

where fj is volume fraction of the j'th component in the system, kj and ?j are
compressibility and density of the j'th component of the system. Volume fraction (f)
can be found from weight fraction (w) by the following equation:
In the present study, the sound velocity (vm) in different volume fractions (or
concentration) of nanofluid is measured and compared with that for the same volume
fraction of nano-particle calculated using Urick's equation for two components system.
The properties of water and the nao-particle are tabulated in Table 1. In the present
calculation, the bulk density of titanium oxide is taken as the bulk density of nano-
particle.

The time required to travel the gap of 18.09 mm is plotted with different concentration
of nano-fluids as shown in Fig.4. It can be observed from the figure that the calculated
and the measured values are following the same trend and agree well. The results
clearly show the fact that the Urick's equation which is applicable for homogeneous
dispersions can also be applied to nano-fluids without much error and no additional
anomalous effect for nano-particles is observed. This is evident from the basic theory
that the ultrasound propagation in fluid depends on the average bulk property of the
medium; hence the particle size will not have an effect on the sound velocity.
Furthermore, the average bulk compressibility and the average bulk density of the
components are used during the calculation of ultrasound velocity in nano-fluid.
However Chen at al in their recent work showed that the compressibility changes with
the size of particle. This may account for the difference between theoretical and
measured value of ultrasound velocities in nano-fluids.
It is apparent from the experiments that there is no anomalous effect, such as ballistic
phonon diffusion in case of acoustic phonons unlike thermal phonons while explaining
the enhanced thermal conductivity of the nanofluid. All of these pointing towards the
fact that the Urick's equation may be a valid one as far as the propagation of ultrasound
in nanofluid is concerned, so there is an ample opportunity of the fact that this equation
can be used for calculating the dispersed volume and sedimentation rate in the case of
nanofluid as it is normally done for the emulsions, which may help immensely in
characterizing heat transfer and colloidal stability of nano-fluid.
The ultrasonic technique means a typical ultrasonic velocity measurement process
where a pulse is created by an electrically stimulating source of transducer, a lead-
zirconium titanate or other piezoelectric ceramic disc. This pulse travels through the
liquid filled sample cell to the far wall and gets reflected from there. The returning pulse
is detected by the receiver and the time of flight is measured. A timer is also started
simultaneously. The acoustic time of flight (T) is used to calculate the sound velocity
and the measured velocity is used in calculating the particle concentration in the base
fluid.
WE CLAIM
1. A process for measuring the concentration of nanofluids by ultrasonic
technique, the process comprising:
preparing a sample cell by filling it with nanofluids;
creating an ultrasonic pulse by an electrically stimulating source of
transducer;
making the pulse to travel through the liquid filled sample cell to the
far wall and getting reflected by the ultrasonic reflector (4);
detecting the returning pulse by the receiver;
measuring the acoustic time of flight (T);
measuring the distance travelled between an ultrasound probe (3)
and the horizontal surface of the angle (4);
characterised in that,
the sound velocity is measured and the concentration of nanofluid is
determined from velocity of sound in the said nanofluid.
2. A process as claimed in claim 1, wherein the transducer may be a lead-
zirconium titanate or other piezoelectric ceramic disc.
3. A process as claimed in claim 1, wherein the ultrasound velocity of sound in
water and nanofluid is measured by a digital ultrasonic probe.
4. A process as claimed in claim 1, wherein the ultrasonic probe of 10 MHz
frequency is immersed into the solution.
5. A process as claimed in claim 1, wherein the measurement accuracy of
ultrasonic probe is within 2%.
6. A process as claimed in claim 1, wherein an ultrasonic reflector in the form of
a nickel coated steel angle is disposed to reflect the sound wave in the
solution.
7. A process as claimed in claim 1, wherein the distance between the probe (3)
and the horizontal surface of angle (4) is 18.9 mm.
8. A process as claimed in claim 1, wherein spherical titanium oxide (TiO2) is
used as the nano-particle in the experiment.
9. A process as claimed in claim 1, wherein the concentration of nano-fluid
used in the experiments are 0.1% wt, 0.5% wt; 1% wt and 2% wt.
10. A process as claimed in claim 1, wherein the size of particles used in the
experiment is in the range of 20-50 nm.

The present invention relates to a process for measuring the concentration of
nanofluids by ultrasonic technique which consists of preparing a sample cell by filling it
with nanofluids, creating an ultrasonic pulse by an electrically stimulating source of
transducer, making the pulse to travel through the liquid filled sample cell to the far
wall and getting reflected by the ultrasonic reflector (4). The returning pulse is detected
by the receiver. The acoustic time of flight is measured. The distance traveled by the
pulse between the ultrasound probe (3) and the horizontal surface of the angle (4) is
found and then the sound velocity is established by implementing acoustic time (T) and
the distance travelled by the pulse to find the concentration of nanofluid.