Post on 30-Apr-2023
1 Copyright © 2013 by ASME
Proceedings of the ASME 7th International Conference on Energy Sustainability
ES-FuelCell2013 July 14-19, 2013, Minneapolis, MN, USA
ES-FuelCell2013-18145
Enhanced Thermal Performance of Ionic Liquid-Al2O3 Nanofluid as Heat Transfer Fluid
for Solar Collector
Titan C. Paul
Department of Mechanical Engineering
University of South Carolina
Columbia, SC, USA
A.K.M. M. Morshed
Department of Mechanical Engineering
University of South Carolina
Columbia, SC, USA
Elise B. Fox
Savannah River National Laboratory
Aiken, SC, USA
Ann E. Visser
Savannah River National Laboratory
Aiken, SC, USA
Nicholas J. Bridges
Savannah River National Laboratory
Aiken, SC, USA
Jamil A. Khan
Department of Mechanical Engineering
University of South Carolina
Columbia, SC, USA
ABSTRACT
Next generation Concentrating Solar Power (CSP) system
requires high operating temperature and high heat storage
capacity heat transfer fluid (HTF), which can significantly
increase the overall system efficiency for power generation. In
the last decade several research going on the efficacy of ionic
liquids (ILs) as a HTF in CSP system. ILs possesses superior
thermophysical properties compare to currently using HTF such
as Therminol VP-1 (mixture of biphenyl and diphenyl oxide)
and thermal oil. However, advanced thermophysical properties
of ILs can be achieved by dispersing small volume percentage
of nanoparticles forming nanofluids, which is called
Nanoparticle Enhanced Ionic Liquids (NEILs). In the present
study NEILs were prepared by dispersing 0.5% Al2O3
nanoparticles (spherical and whiskers) in N-butyl-N,N,N-
trimetylammonium bis(trifluormethylsulfonyl)imide
([N4111][NTf2]) IL. Viscosity, heat capacity and thermal
conductivity of NEILs were measured experimentally and
compared with the existing theoretical models for liquid–solid
suspensions. Additional, the convective heat transfer
experiment was performed to investigate thermal performance.
The thermal conductivity of NEILs enhanced by ~5%, heat
capacity enhanced by ~20% compared to the base IL, which
also gives 15% enhancement in heat transfer performance.
Keywords: Ionic Liquid; Nanoparticle Enhanced Ionic Liquids
(NEILs); Thermal Conductivity; Heat Capacity; Heat Transfer
Coefficient.
1. INTRODUCTION
Nowadays concentrated solar power (CSP) system become
one of the greatest interest for energy researchers investment
due to the storage and economic concern [1]. In CSP system,
heat transfer fluid (HTF) plays a very important role as a
storage medium. HTF currently used (Therminol VP-1,
Thermal oil) in solar collector have low to moderate thermal
stability, low thermal conductivity, and low heat storage
capacity, which results in high operating costs [2], therefore
researches are working for new energy-efficient HTF.
Fluid with suspended metallic or nonmetallic naoparticles
called nanofluids, which has already shown significant
enhancement of thermal conductivity [3]. Typical base fluids
for nanofluids are water, ethylene glycol, and oil. In the last
decade several research going on the efficacy of ionic liquids
(ILs) (group of salts and liquid at room temperature) as a HTF
in CSP system [4-5]. ILs have excellent thermophysical
properties such as high thermal stability, low melting point,
negligible vapor pressure, high electrical, and chemical
conductivity [6-9]. For their excellent thermophysical
properties ILs was considered as the base fluid of nanoparticle
enhanced ionic liquids (NEILs) for HTF of CSP system. A.
Nieto de Castro et al. and S. M. S. Murshed [10-11] reported
enhanced thermal conductivity and heat capacity of several
imidazolium and pyrrolidinium ILs based nanofluids which
were formed with ILs and carbon nanotubes. N. J. Bridges et al.
[12] have studied the heat capacity and thermal stability of
Al2O3 based imidazolium ILs; and has reported higher heat
capacity values. B. Wang et al. [13-14] also reported high
thermal conductivity and thermal stability of nanofluids
2 Copyright © 2013 by ASME
consisting of ILs and gold nanoparticles. T. C. Paul et al. [15]
have recently reported the ~3% enhancement of thermal
conductivity of NEIL made with N-butyl-N-
methylpyrrolidinium bis{(trifluoromethyl)sulfonyl} imide,
([C4mpyrr][NTf2]) and 0.5% Al2O3 nanoparticles.
The present study focused on the enhanced thermophysical
properties of N-butyl-N,N,N-trimetylammonium
bis(trifluormethylsulfonyl)imide ([N4111][NTf2]) based NEILs
with two different shapes (spherical and whiskers) of Al2O3
nanoparticles. Based on the thermophysical properties heat
transfer performance was studied for whiskers shape Al2O3
based NEILs. Heat transfer performance evaluation was
performed under forced convection experiments in a circular
tube under the laminar flow region.
2. EXPERIMENTAL MEASUREMENTS
2.1. Material and Synthesis of NEILs
Al2O3 nanoparticles were purchased from Sigma-Aldrich,
USA. Spherical shape nanoparticles are γ-phase with particle
size < 50nm (TEM) and surface area >40 m2/g (BET); whiskers
nanoparticles having diam. × L, 2-6 nm × 200-400 nm and
aspect ratio> 100 (TEM). The base IL N-butyl-N,N,N-
trimetylammonium bis(trifluormethylsulfonyl)imide
([N4111][NTf2]) was purchased from IoLiTec Company
(Germany). Molecular weight and molecular formula of
[N4111][NTf2] is 396.37 g/mol and C9H18F6N2O4S2 respectively.
NEILs were prepared by mixing 0.5 wt% Al2O3 with
[N4111][NTf2] as base IL. Al2O3 nanoparticles were dispersed
into the IL using a vortex mixture and agitated for
approximately 90 min to break any possible agglomerated
nanoparticles.
2.2. Thermal conductivity, viscosity, and heat capacity
measurements
Thermal conductivity of the base IL and NEILs were
measured using KD2 Pro thermal property analyzer (Decagon
Device, USA). The measurement principle is based on the
transient hot wire method. The meter has a probe of 60 mm
length and 1.3 mm diameter with a heating element and a
thermoresistor, which is inserted vertically into the test sample.
The probe is connected to a microcontroller for controlling and
conducting the measurements. Before using base IL and NEIL,
the meter was calibrated with distilled water and standard
glycerin. A thermal bath (Thermo NESLAB) was used to
maintain constant temperature of the measuring sample.
Temperature accuracy of the bath was within ±0.01 K.
Viscosity of the base IL and NEILs were measured using a
cone and plate type rotary viscometer (LVDV-II+ProCP, from
Brookfield Engineering Co.). The sample size required for the
cone and plate arrangement is 1mL. The cone and plate
arrangement has a thermal jacket to maintain constant sample
temperature within accuracy of ±0.1oC. The viscometer was
calibrated by using standard oil.
Heat capacity of IL and NEILs were measured using
Differential Scanning Calorimetry (DSC Q2000 from TA
instruments Inc.). The sample was placed in a standard
aluminum hermetic pan covered with lid and the average
sample size was 12.75mg. Nitrogen was used as the cooling
system at a flow rate of 40 mL/min. The DSC run was
performed from 25oC to 345
oC at a heating rate of 10
oC/min.
There were three different runs was performed and the
experimental procedure was the same as described by D. Shin
et al., [16].
2.3. Measurements of thermal performance
2.3.1. Experimental system
Fig. 1 presents schematic of the flow loop used in this
experiment. The loop consisted of a pump, test section, heat
exchanger, collection tank, and pressure transducer. The pump
was connected to a frequency inverter which was calibrated for
the pump using a stopwatch and bucket method. The test
section was a stainless steel tube of 3.86 mm inner diameter
and 990.6 mm length. Uniform heat flux was applied to the test
section using a flexible heating tape (OMEGA Engg. FGS101-
040). Power was supplied to the heater using a DC power
(Agilent Technologies: 6655A) supply. To reduce the heat loss
to the ambient and to ensure constant heat flux condition, the
entire test section was insulated with fiberglass insulation. Five
thermocouples were mounted on the tube surface to measure
surface temperature. Another two thermocouples were inserted
into the tube to measure the inlet and outlet liquid temperatures.
A differential pressure transducer was connected between the
inlet and outlet of the test section to measure the pressure drop
within the test section. All thermocouples and pressure
transducer were connected to a National Instrument (NI) data
acquisition system cDAQ-9178 via a temperature card NI 9211
and pressure card NI 9203 which were interfaced with a
computer. Labview software was used for recording all data.
2.3.2. Data reduction
The test section was allowed to reach steady state before the
temperature data were recorded. Heat flux (𝑞˶) was calculated
from input power (𝑄) of the heater and heating surface area
(𝐴ℎ) using the following equation:
𝑞˶ =𝑄
𝐴 =
𝑉𝐼
𝜋𝐷 𝐿 (1)
where, 𝑉 and 𝐼 are the input voltage and current respectively,
𝐷𝑜 is the outer diameter of tube, and 𝐿 is the heating length.
Local heat transfer coefficient at an axial distance 𝑥, along the
test section, ℎ(𝑥), was calculated using the equation:
ℎ(𝑥) =𝑞˶
𝑇 (𝑥)−𝑇 (𝑥)
(2)
3 Copyright © 2013 by ASME
where, 𝑇𝑤′ (𝑥) and 𝑇𝑓(𝑥) are the local temperatures of the inner
surface and liquid respectively.
The inner surface temperature was calculated using steady
state one-dimensional heat conduction equation with constant
heat flux boundary condition for which the governing equation
is: 1
𝑟
𝑑
𝑑𝑟(𝑟
𝑑𝑇
𝑑𝑟) = 0 (3)
The solution for the inner surface temperature becomes
𝑇𝑤′ (𝑥) = 𝑇𝑤(𝑥) −
𝑄.ln (𝑟 /𝑟 )
2𝜋𝐿𝑘 (4)
where, 𝑇𝑤(𝑥) is the local temperature of the outer surface as
measured by the thermocouples, 𝑟𝑜 and 𝑟𝑖 are the outer and
inner radius of the test tube respectively, and 𝑘𝑠 is the thermal
conductivity of stainless steel.
3. RESULTS AND DISCUSSION
3.1. Thermal conductivity of NEILs
Thermal conductivity of base IL and NEILs as a function of
temperature are presented in Fig. 2. Thermal conductivity of
NEILs enhanced by ~3% for spherical and ~5% for whiskers
Al2O3 nanoparticles over the measured temperature range,
which is clearer in the Fig. 3, where thermal conductivity of
NEILs are normalized with respect to the corresponding
thermal conductivity of base IL. The experimental results were
compared with the predicted thermal conductivity using the
well established Maxwell’s [17] equation:
𝑘
𝑘 =
𝑘 +2𝑘 −2𝜙(𝑘 −𝑘 )
𝑘 +2𝑘 +𝜙(𝑘 −𝑘 ) (5)
where 𝑘𝑁𝐸𝐼𝐿, 𝑘𝐵𝐿, 𝑘𝑠 = 36 𝑊 𝑚𝐾⁄ are the thermal conductivity
of NEIL, base IL, and Al2O3 nanoparticles respectively. 𝛷 is the
nanoparticles volume fraction.
Figure 2: Thermal conductivity of NEILs and base IL as a
function of temperature
0.1
0.11
0.12
0.13
0.14
280 300 320 340
The
rmal
co
nd
uct
ivit
y, W
/m.K
Temperature, K
Base IL
0.5 wt% spherical AL2O3
0.5 wt% whiskers AL2O3
Pump
Tin T1 T2 T3 T4 T5 Tout
Data Acquisition System
Test section Insulation
Ionic liquid
Collection tank
Cooling
water in
Cooling
water out
Pin Pout
Figure 1: Schematic of the experimental system
Base IL
0.5wt% spherical Al2O3
0.5wt% whiskers Al2O3
4 Copyright © 2013 by ASME
Maxwell’s model predicts lower thermal conductivity of NEILs
compare to the experimental results, which is clearer in Fig.3.
This may be the reason of Maxwell’s model only considers the
thermal conductivity of nanoparticles and nanoparticles volume
fraction.
Figure 3: Normalized effective thermal conductivity of NEILs
as a function of temperature
3.2. Rheological behavior of NEILs
Fig. 4 shows the shear viscosity 0.5 wt% NEIL as a function
of shear rate at different temperature. The shear thinning
behavior occurs at all the measured temperatures and the shear
thinning increases with temperature.
Figure 4: Rheological behavior of 0.5 wt% NEIL with different
temperature
Fig. 5 shows the viscosity of 0.5% NEILs and base IL as a
function of temperature and viscosity decreases sharply with
temperature. NEILs shows higher viscosity compare to the base
IL over the measured temperature range. Measured viscosity of
NEILs were compared with the predicted values using
Batchelor equation [18]:
µ
µ = 6.2𝜙2 + 2.5𝜙 + 1 (6)
where, µ𝑁𝐸𝐼𝐿 and µ𝐵𝐿 are the viscosity of NEIL and base IL
respectively, 𝜙 is the nanoparticles volume fraction. Predicted
viscosity shows lower value compared to the experimental
measurements, this is because the model only considers the
particle volume fraction. Nanoparticle clustering, which is very
common for nanofluids was not considered in the model, which
may be the result of higher viscosity of NEILs. The
experimental and predicted viscosity values were consistent
with the previous studies by M. J. Pastoriza-Gallego et al. [19]
for ethylene glycol-based Al2O3 nanofluids and S. Torii [20] for
Al2O3/water nanofluids.
Figure 5: Viscosity of 0.5% NEILs as a function of temperature
3.3. Heat capacity of NEILs
Fig.6 shows the heat capacity of NEILs as a function of
temperature. The heat capacity of NEILs enhanced by ~14%
(spherical) and by ~20% (whiskers) over the measured
temperature range and heat capacity increases linearly with
temperature.
Figure 6: Heat capacity of NEILs and base IL as a function of
temperature
The enhancement of heat capacity was observed by the
previous researcher [21], where silica nanofluids synthesized
0.95
1
1.05
1.1
280 290 300 310 320 330 340
k NEI
L/k B
L
Temperature, K
0.5 wt% whiskers Al2O3Maxwell model0.5 wt% spherical Al2O3
0
20
40
60
80
100
0 5 10 15 20 25
Vis
cosi
ty,
cP
Shear rate, 1/s
30oC 40oC 50oC
0
30
60
90
120
150
290 310 330 350
Vis
cosi
ty,
cP
Temperature, K
Base IL0.5 wt% sperical Al2O30.5 wt% whiskers Al2O3
1
1.5
2
2.5
3
0 50 100 150 200 250 300 350
He
at c
apac
ity,
J/g
.K
Temperature, oC
Base IL0.5 wt% spherical Al2O30.5 wt% whiskers Al2O3
Base IL
0.5wt% spherical Al2O3
0.5wt% whiskers Al2O3
Base IL
0.5wt% spherical Al2O3
0.5wt% whiskers Al2O3
30oC 40oC 50oC
0.5wt% whiskers Al2O3
Maxwell model
0.5wt% spherical Al2O3
5 Copyright © 2013 by ASME
by lithium carbonate and potassium carbonate (62:38 ratio) and
alkali chloride salt eutectic with SiO2 nanoparticles (1%by wt.).
This enhancement in heat capacity of NEILs is important for
their potential applications as a HTF for CSP system.
3.4. Thermal performance of NEILs
Thermal performance of NEIL was performed under forced
convection experiment in the laminar flow region. Before
conducting experiments with NEIL, experiments were carried
out with De-Ionized (DI) water to evaluate the experimental
setup’s reliability. Furthermore experimental results using DI
water were compared with well-known Shah’s correlation [22]
for forced convection heat transfer under laminar flow
conditions:
𝑁𝑢(𝑥) = {1.953 (𝑅𝑒𝑃𝑟
𝐷
𝑥)
(𝑅𝑒𝑃𝑟
𝐷
𝑥) ≥ 33.3
4.364 + 0.0722𝑅𝑒𝑃𝑟𝐷
𝑥 (𝑅𝑒𝑃𝑟
𝐷
𝑥) < 33.3
(7)
where, 𝑁𝑢(𝑥), 𝑅𝑒, 𝑃𝑟 are the local Nusselt number, Reynolds
number, and Prandtl number respectively and defined as
𝑁𝑢(𝑥) =ℎ(𝑥)𝐷
𝑘 ,𝑅𝑒 =
𝜌𝑢𝐷
µ, 𝑃𝑟 =
𝜈
𝛼 (8)
where, 𝐷 is the inner diameter of the test section, 𝑘𝑓 is the
thermal conductivity of fluid, 𝜌 is the fluid density, µ is the
fluid viscosity, 𝑢 is the fluid velocity, 𝜈𝑓(=µ
𝜌) is the fluid
kinematic viscosity, and 𝛼(=𝑘
𝜌𝐶 ) is the fluid thermal
diffusivity. Fig. 7 shows a comparison between the
experimental results of DI water and Shah’s predicted results at
Reynolds number, 𝑅𝑒 = 493. It is clear from Fig. 7 that there
are reasonably good agreements between predicted and
measured heat transfer coefficient of DI water over the
Reynolds number range studied.
Figure 7: Comparison between measured Nusselt number of DI
water with predicted values using Shah’s equation
After getting confidence with DI-water, forced convection
experiments were performed for 0.5 wt% Al2O3 whiskers
nanoparticle enhanced IL under laminar flow region. Local
heat transfer coefficient of base IL and NEIL as a function of
axial distance was presented in Fig. 8. The Fig. 8 shows that
there was a significant enhancement of heat transfer coefficient.
Higher heat transfer coefficient enhancement was observed for
higher Reynolds number. The enhancement of heat transfer
coefficient may be the reason of mixing effect of particles near
the wall, thermal conductivity enhancement, particle migration,
and reduction of boundary layer thickness [23].
Figure 8: Local heat transfer coefficient along the axial distance
Fig. 9 shows the Nusselt number of NEIL at different
Reynolds number as a function of axial distance. It is clear
from Fig. 9 that the Nusselt number as well as heat transfer
coefficient increases with Reynolds number, which is because
of at high Reynolds number boundary layer becomes thinner
and shear stress increases within the boundary layer that
increases the heat transfer coefficient. The enhanced heat
transfer performance of NEIL is an important finding for their
potential application of next generation solar thermal system.
Figure 9: Nusselt number along the axial distance for different
Reynolds numbers
0
10
20
30
0 50 100 150 200 250
Nu
sse
lt n
um
be
r, N
u
x/D
Shah's equationmeasured data
200
400
600
800
1000
1200
0 50 100 150 200 250H
eat
tar
nsf
er
coe
ffic
ien
t,
W/m
2 .K
x/D
Base IL, Re=583
NEIL, Re=583
Base IL, Re=936
NEIL, Re=936
5
10
15
20
25
30
35
0 50 100 150 200 250
Nu
sse
lt n
um
be
r, N
u
x/D
Re=767
Re=1248
Re=1763
Re=2193
6 Copyright © 2013 by ASME
4. CONCLUSIONS
Thermophysical properties of N-butyl-N,N,N-
trimetylammonium bis(trifluormethylsulfonyl)imide
([N4111][NTf2]) based NEILs with two different shapes
(spherical and whiskers) of Al2O3 nanoparticles were measured
experimentally. In addition for thermal performance of NEILs,
forced convection experiments were performed for the Al2O3
whiskers nanoparticle enhanced (0.5 wt%) [N4111][NTf2] IL.
From the experimental results the following conclusions can be
drawn:
-Thermal conductivity of the NEILs are enhanced by ~3% for
spherical and ~5% for whiskers Al2O3 nanoparticles over the
measured temperature range.
-Viscosity of the NEILs shows higher values compared to the
base IL within the investigated temperature range.
-Heat capacity of NEILs enhanced by ~14% (spherical) and by
~20% (whiskers) compare to the base IL over the measured
temperature range.
-Thermal performance of NEIL shows enhanced heat transfer
coefficient compare to the base IL. The enhancement in heat
transfer coefficient is due to the enhanced thermal conductivity
and particle migration behavior in the boundary layer. These
preliminary findings are favorable to consider NEILs as a
potential HTF for CSP system.
ACKNOWLEDGEMENTS
The financial support for this research is from Department of
Energy (DOE) Solar Energy Technology Program. Savannah
River National Laboratory is operated by Savannah River
Nuclear Solutions. This document was prepared in conjunction
with work accomplished under Contract No. DEAC09-
08SR22470 with the U.S. Department of Energy.
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