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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