- Email: [email protected]
Heat transfer enhancement of nanofluids using iron nanoparticles decorated carbon nanotubes
Accepted Manuscript Heat transfer enhancement of nanofluids using iron nanoparticles decorated carbon nanotubes Abdallah D. Manasrah, Usamah A. Al-Mubaiyedh, Tahar Laui, Rached BenMansour, Mohammed J. Al-Marri, Ismail W. Almanassra, Ahmed Abdala, Muataz A. Atieh PII: DOI: Reference:
S1359-4311(16)31155-3 http://dx.doi.org/10.1016/j.applthermaleng.2016.07.026 ATE 8624
To appear in:
Applied Thermal Engineering
Received Date: Revised Date: Accepted Date:
14 October 2015 10 June 2016 4 July 2016
Please cite this article as: A.D. Manasrah, U.A. Al-Mubaiyedh, T. Laui, R. Ben-Mansour, M.J. Al-Marri, I.W. Almanassra, A. Abdala, M.A. Atieh, Heat transfer enhancement of nanofluids using iron nanoparticles decorated carbon nanotubes, Applied Thermal Engineering (2016), doi: http://dx.doi.org/10.1016/j.applthermaleng. 2016.07.026
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Heat transfer enhancement of nanofluids using iron nanoparticles decorated carbon nanotubes Abdallah D. Manasrah a , Usamah A. Al-Mubaiyedh b, Tahar Lauic, Rached Ben-Mansourc, Mohammed J. Al-Marrid, Ismail W. Almanassrab, Ahmed Abdala e,f, Muataz A. Atiehe,f* a
Chemical Engineering Department, University of Calgary, Calgary, Canada Chemcial Engineering Department, King Fahd Univervery of Petroleum & Minerals, Dhahran, Saudi Arabia c Mechanical Engineering Department, King Fahd Univervery of Petroleum & Minerals, Dhahran, Saudi Arabia d Gas Processing Center, College of Engineering, Qatar University, P.O. Box 2713 Doha, Qatar e Qatar Environment and Energy Research Institute, HBKU, Qatar Foundation, PO Box 5825, Doha, Qatar f College of science and Engineering, HBKU, Qatar Foundation, PO Box 5825, Doha, Qatar b
*
Corresponding author: Dr. Muataz Ali Atieh, Email: [email protected]
Abstract Enhancing heat transfer in thermal fluid systems can contribute significantly towards the improvement of thermal efficiency resulting in reducing energy consumption and hence carbon emission. Conventional fluids like water and oil have limited heat transfer potential. The need for the development of new classes of fluids with enhanced heat transfer properties is thus becoming essential. Many studies have developed nanofluids using nanoparticles, however, they showed a limited enhancement in heat transfer. This study investigated the heat capacity, enhancement of heat transfer, viscosity, and pressure drop of nanofluids with carbon nanotubes (CNTs) and CNTs doped with iron oxide nanoparticles (Fe2O3-CNT). The surfaces of carbon nanotubes were doped with 1 wt.% and 10.0 wt. iron oxide nanoparticles. The pristine and doped CNTs were used to prepare heatexchange nanofluids with additive concentrations of 0.01, 0.05, and 0.10 wt.%. A shell and tube heat exchanger was used to evaluate the overall heat transfer coefficient and the associated pressure. The specific heat capacity of the nanofluids was measured by differential scanning calorimetry (DSC). The results showed that the specific heat capacity of the nanofluids with undoped and doped CNTs is significantly higher than that of pure water by about 10 % and 55%, respectively. The heat transfer rate of the nanofluids increased sharply with the CNT dosage the iron nanoparticles loading and reached up to 55% enhancement with doped CNTs. We observed that the power required to exchange 1.8 kW heat using nanofluid containing 0.1 wt.% of 10 wt. % Fe 2O3-CNTs was 20 times lower than the power required to exchange the same amount of heat using water. This is because the iron nanoparticles enhanced the dispersion of the CNTs and increased their heat capacity and thermal
conductivity. Compared with that of pure water, the encountered pressure drop of the nanofluid at the same flow rate was almost unchanged, resulting in no extra pumping energy penalty. Keywords: Nanofluid, Impregnated carbon nanotube, Nanoparticles, Heat capacity, Pressure drop, Shell and tube heat exchanger.
LIST OF ABBREVIATIONS Q
: Heat transfer rate in tube (kW).
mn : Mass flow rate for nanofluids (kg/h). Cpn : Specific heat capacity of nanofluids (J/g.C). Tn,in : Tube inlet temperature (°C). Tn,out : Tube outlet temperature (°C). n
: Subscript for nanofluid.
Re : Reynolds’s number. Nu : Nussle’s number. ∆P : Pressure Drop (mbar). Ƞ : viscosity of nanofluids (cp). k : Thermal conductivity (W/m.K). : Friction factor.
1. Introduction Heat exchangers are an integral component in many industrial applications including automotive, petrochemical, power generation, food processing, cooling and heating units, and electronic thermal management systems. In heat exchanger systems, the cost and size of the system are related to the properties of the heat-transfer fluids. Conventional fluids, such as water and oils, have limited heat transfer potentials. The development of new classes of fluids with enhanced heat transfer capabilities has been the subject of significant research [1, 2]. One area of interest in this field involves the use of additives to improve the properties of heat-transfer fluids. Metal and metal oxide additives are of particular interest in heat-transfer fluid applications, especially when they are manufactured on the nanoscale [1]. In 1995, Choi was the first researcher to report the application of nanoparticles in heattransfer fluids [1]. Since then, the suspension of nanoparticles in conventional heat-transfer fluids has been shown to improve the thermal conductivity and convective heat-transfer performance by an order of magnitude over the traditional base fluids (ethylene glycol, water, oils) [3]. A key consideration in using nanoparticles in heat-transfer fluids to produce nanofluids that yield reliable results is to ensure that the nanofluid is stable. The agglomeration of nanoparticles is a major problem that makes the fluid unstable. One way to create a stable fluid is to disperse the particles in the solution mechanically, by applying sonication to break down the van der Waals interaction forces, but this has a short term effect. Alternatively, chemical modification uses other additives such as dispersants or surfactants to prevent agglomeration of particles in the fluid and produce homogeneous mixtures [4-6]. Recently, great interest has been shown in the development of carbon nanomaterials. Carbon nanomaterials have gained significant attention over the last decade since the discovery of carbon nanotubes (CNTs)[7,8] . The most eye-catching features of these structures are their thermal properties, which can permit future applications in thermal science and engineering. CNTs and graphene nanoparticles have unusual heat transfer properties. In the lengthwise direction, they show excellent heat transfer performance. They also possess remarkable thermal properties with ultra-high thermal conductivity (2000–3000 Wm-1K-1), which is much higher than those of the metallic or their oxide nanomaterial used in nanofluids [9]. CNTs can be dispersed homogenously in conventional heat transfer fluids [10]. Recent research has demonstrated that there is a substantial increase in the thermal conductivities of different CNT nanofluids in comparison to their base fluids. In general, research on CNT nanofluids has blossomed in many different directions and has attracted a great deal of attention. Several studies investigated the thermal conductivity of nanofluids containing small
(0.01–1 vol%) amounts of carbon nanotubes (CNTs), as nanoparticles, in different solvents (oil, water) with or without the use of dispersing agents (sodium dodecyl sulfate, Arabic gum), and reported thermal conductivity enhancements ranging from 11–350%. Other studies investigated the use of alumina nanoparticles dispersed in water at concentrations ranging from 0.15–1 vol%, and observed heat transfer enhancements of up to 30% [11-22]. Anoop et al. [23] studied the effect of nanoparticle size on the heat transfer of water containing alumina additives, and showed that smaller nanoparticles result in a greater enhancement of heat transfer. Mohammed et al. [24] studied the effect of silver, silica, alumina and titania nanoparticles on the heat transfer of water-based nanofluids under laminar flow conditions, and reported that silver had the lowest pressure drop and that alumina the highest heat transfer coefficient. A recent study conducted by Liu et al. [25] investigated the enhancement of thermal conductivity in nanofluids prepared using a two-step method for dispersing copper oxide and carbon nanotubes in water, ethylene glycol and synthetic oil without using surfactants. Their results showed that the nanofluids containing low volume concentrations of nanoparticles had greater thermal conductivity than the original fluids. The researchers concluded that better dispersion of the nanoparticle additives may further enhance system performance. Many experimental studies have been initiated by different research groups to study the transport behavior of nanofluids. Ding et al. [10] studied the effect of MWCNTs on the heat transfer performance of nanofluids using a straight copper tube. The nanofluids were produced by dispersing MWCNTs in distilled water. They found that with increasing concentrations of CNT and nanofluid temperature the thermal conductivity increased as well as the viscosity. Another study has been conducted by Ko et al. [36], a nanolfluid has been prepared with carbon nanotubes in water, the experiment has been conducted in turbulent flow regimes of heat exchanger, their results shown that CNT nanofluids produced low friction factors compared to the conventional fluids. By incorporating carbon nanotubes (CNTs) in fluids, it is expected that such nanofluids would exhibit major improvement in thermal conductivity due to the very high thermal conductivity of CNT. To the best of our knowledge, there is no published work on the thermophysical properties of nanofluids using water as base fluid containing doped CNTs with iron oxide and their behavior as a heat-transfer fluid in turbulent flow regimes, where most of practical flows fall. Therefore, this study focuses on the measurement of the basic thermophysical properties of CNTs impregnated with iron oxide on
their surfaces, such as specific heat capacity (Cp), viscosity, heat transfer rate and pressure drop behavior in turbulent flow regimes in a shell and tube heat exchanger commonly used in industry 2. Experimental 2.1 Materials Ferric nitrate (Fe(NO3)3 9H2O), 99% purity (Sigma Aldrich) was used as the precursor for the iron oxide nanoparticles.
2.2 Preparation of CNTs and their Impregnation with Iron Oxide Nanoparticles The production of carbon nanotubes in the present work was conducted in two vertical tubular reactors. The vertical reactors were quartz tubes of 50 mm in diameter and 1200 mm in length and were heated by silicon carbide heating elements as shown in figure 1. The catalyst with hydrocarbon were injected at the first reaction chamber to evaporate the hydrocarbon and the catalyst while the second reaction chamber was used for the reaction and growth processes. Two types of gases were used in this system, hydrogen as the reacting gas and the argon for flushing the air from the system. After the temperature reached the reaction temperature, the ferrocene catalyst was heated in the first reaction chamber at temperature 120 °C in order to produce the ferrocene vapour. The hydrogen gas was mixed with benzene and ferrocene gases and passed into the first reaction chamber 800 oC as reaction temperature for 1 hr reaction time. The produced CNTs were impregnated with iron oxide nanoparticles via wet impregnation technique using Fe(NO3)3 9H2O as the iron precursor. The Fe(NO3)3 9H2O precursor was dissolved in ethanol at two weight concentrations to impregnate the surface of CNTs with 1 and 10 wt.% loadings as determined by TGA. The impregnation was conducted under ultrasonication using probe sonicator for 30 minutes at room temperature followed by calcination at 350°C for 3.5 hours to fully oxidize the iron nanoparticles.
2.3 Characterization of Pure and Impregnated CNTs The morphology of the CNT and the dispersion of the iron oxide nanoparticles were examined using Field Emission Scanning Electron Microscope (FE-SEM, Tescan Lyra3) and High Resolution Transmission Electron Microscopy (HR-TEM, JOEL-2100F)). The elemental analysis of the pure and
impregnated CNT was measured using energy dispersive X-ray (EDX) spectrometer attached to the FE-SEM. The SEM images were acquired in secondary electron and backscattering modes with accelerating voltage of 20.0 keV. Before SEM imaging, the samples were gold coated using gold sputtering system. The raw and impregnated CNT were also analyzed by Thermal Gravimetric Analysis (TGA, Model SDT Q600 TA Instrument) at the temperature range of 25-700° C under Nitrogen. 2.4 Preparation of Nanofluids Nanofluids were prepared by dispersing the raw and impregnated CNTs in water using probe sonicator for 30 minutes at an amplitude of 100%, a power density of 750 W/m 3, and a frequency of 30 kHz. Nanofluids samples with 0.01, 0.05, and 0.1 wt.% Fe2O3-CNT were prepared without using any surfactant or dispersant.
2.5 Measurement of the specific heat capacity of nanofluids The specific heat capacity of the nanofluids was measured by Differential Scanning Calorimetry (DSC) (DSC Q1000, TA Instruments) connected to a rapid cooling system (RCS 90) with nitrogen gas. After temperature equilibrium and 3-minute isotherm at 25 °C, the heat flux measurements were taken while ramping at a rate of 1.5 °C/min to 50 °C. Three measurements were taken for each sample. The samples were sonicated for 10 minutes and cooled to room temperature in an ice bath immediately before being placed in the DSC. 2.6 Viscosity and thermal conductivity measurement The dynamic viscosity of the nanofluids in the temperature range of 25 to 65 °C was measured using Stormer viscometer where the viscosity was calculated by measuring the time needed for the inner cylinder to perform 100 revolutions in response to a moving weight. The thermal conductivity (k) of the nanofluids was measured via the modified transient plane source technique that uses a one-sided interfacial heat reflectance sensor to apply a constant heat source to the sample using Mathis TCi system (Mathis Instruments Ltd.). The method eliminates the intrusive nature of the hot wire because the heating element is supported on a backing that provides mechanical
support as well as electrical and thermal insulation. It also allows to test smaller volumes of material as the wire is coiled. 2.7 Heat Exchanger Setup A shell and tube heat exchanger was designed and fabricated to investigate the heat transfer characteristics of different nanofluids at different flow rates. A schematic diagram of the heat exchanger is shown in Figure 2. The unit consists of two flow cycles, a heating unit (thermostat) and a cooling unit. One flow loop carries the heated nanofluids and the other transports cooling water (chiller) and both are connected to a controlled pump and instrumented with a flow meter. The heat exchanger shell is constructed from corrosion resistant borosilicate glass 3.3. A 316 stainless steel tube bundle consisting of three tubes, but only one tube with used and the other two tubes were closed from both ends. The tube length (L), inside diameter (di), outside diameter (d o) and heat transfer area (A) are 2.98 m, 0.010 m, 0.014 m, and 0.13 m 2. Four Me-thermocouples (Fe-CuNi) type were inserted in the shell and the tube sides to measure the inlet and outlet temperatures of each stream and an additional thermocouple was inserted in the nanofluid reservoir. Differential pressure transmitters were installed at both ends of the pipes to measure the pressure drop. A chiller with 4.6 kW cooling capacity was used to keep the water temperature constant at 20 °C and a 3.5 kW electric heater was used to keep the temperature of the nanofluid in the 12-liter capacity thermostat tank constant at 35 °C. Experiments were performed at mass flow rates ranging from 200 kg/h up to 640 kg/h. The inlet and outlet temperatures and the pressure drops were measured after achieving steady state conditions (after approximately 30 mins). From these measurements, the heat transfer rate was calculated based on the following equation:
Q = mn Cpnf (Tn,in - Tn,out ) .
where Q is the heat transfer rate in the tube,
(1) where Q is the nanofluid flow rate, Cpnf is the
nanofluid specific heat capacity, Tn, in is the tube inlet temperature and Tn, out is the tube outlet temperature.
3.
Results and Discussion
3.1 Surface characterization of raw and impregnated CNTs A Vertical Reaction Chemical Vapor Deposition (V-CVD) reactor was designed and fabricated in order to produce large amount of Carbon Nanotubes by using continuous injection atomization. Bundles of multi wall carbon nanotubes (MWCNTs) were produced by this system. The V-CVD parameters (hydrogen flow rate, reaction temperature, and reaction time) were studied to optimize the CNTs production and quality. The maximum yield and purity of CNTs was obtained at 2000 ml/min of hydrocarbon injected into the reactor at 800° C as reaction temperature for almost 1 hour reaction time. The synthesized nanotubes were characterized by scanning electron microscopy (SEM), Transmission Electron Microscopy (TEM) and Thermogravimetry (TGA) techniques. The produced CNTs were doped with iron oxide nanoparticles using wet impregnation technique. Figure 3 (a, b and c) displays the SEM images of raw CNTs and CNT doped with 1 and 10 with % Fe 2O3 nanoparticles. The diameter of the raw CNTs varies from 20–40 nm with an average diameter of 24 nm, while their length varied from 10–30 µm (as shown in figure 3 a). The white spots in the SEM image of CNTFe2O3 show the iron oxide particles which confirmed by Energy dispersive x-ray spectroscopy (EDS) analysis (as indicated in table 1). Back Scattering FE-SEM image of CNT-Fe 2O3 was taken in order to verify the presence of nanoparticle ions on the surfaces of the CNTs (as shown in figure 3 b and c). The distribution and agglomeration of Fe 2O3 nanoparticles were also investigated. It was observed that, there are formations of white crystal structures of Fe 2O3 nanoparticles with small sizes and irregular shapes. It can be seen that Fe 2O3 nanoparticles spread widely on the surfaces of carbon nanotubes forming very small crystal particles with diameters varying from 1-5 nm.
3.2
Thermal Degradation Analysis
The study of the thermal stability of materials, which is typically studied using TGA, is of a major importance as it determines the upper usage temperature limit of the material. Figure 4 depicts the TGA-DTG results for the carbon nanotubes with (Fe2O3-CNT) and without iron nanoparticles. In the thermogram, the initial degradation of CNTs in air starts at approximately 550°C, reaches
a
maximum
weight
loss
rate
at
approximately
600
°C
and
reach
a
constant weight at approximately 670 °C, as revealed by the DTG thermograms. The doping of CNT accelerates the pyrolysis of CNT as indicated by the shift of the onset of the rapid degradation step to a lower temperature with the increase of the doping dose. For CNTs impregnated with 1 wt.% iron nanoparticles, the initial oxidation starts at 500 °C, reaches a maximum weight loss rate at approximately 550 °C and completes at approximately 600 °C, as shown in Figure 4. When the loading of Fe2O3 was increased to 10.0 wt.%, the oxidation peak in the thermogram shifts to lower temperatures, so that the oxidation starts at 450 °C with a maximum weight loss rate at 500 °C and completes at 540 °C. We believe that the iron particles doped on CNTs acted as heating accelerator agent, which accelerate the heat transfer to the body of the CNTs as can be seen by the faster combustion of the doped CNTs (oxidization) compared to undopped CNTs. In addition to the effect of doping on CNT thermal stability, TGA provides an accurate estimate of the loading of Fe 2O3 by comparing the final constant weight for CNT and its iron doped derivatives (inset of Figure 4). The final remaining weight of raw CNTs and CNTs doped with 1 wt.% Fe 2O3, and 10 wt. % Fe 2O3 is 0.99, 2.05, and 10.06 wt.%.
3.3 Viscosity of the nanofluids Despite their ability to enhance the heat transfer rate, nanofluids may be associated with a large increase in the solution and consequently the power requirement. Therefore, studying the rheology of the nanofluid is one of the most critical parameters in determining the performance of nanofluids. The viscosity of the nanofluids containing different concentrations of undoped and doped CNT nanofluids as a function of temperature is shown in Figure 5 a and b. It is worth mentioning that the viscosity of the nanofluids is highly dependent on the concentration of the nanofluid, but independent of the iron oxide loading. Regardless of the iron oxide loading, the viscosity of the nanofluids increases significantly with the nanoparticles concentration and decreases as the temperature increases with maximum increase in viscosity of 50% for the highest nanoparticle concentration at the highest measurement temperature. The viscosity of the nanofluid is less sensitive to temperature compared to water as indicated by the increase in the relative viscosity (
=
) with temperature as shown in
Figure 5 b. The effect of the temperature on the viscosity of water can be described by [26]:
( ) = 10
(2)
This model accurately describes the effect of temperature on the viscosity of water and nanofluids as is shown by the solid lines in Figure 5 a. The values of a, b, and c were extracted from figure 5 a . The values are shown in table 2
3.4 Thermal conductivity of nanofluids Figure 6 shows the thermal conductivity of nanofluids with different concentrations (0.01, 0.05 and 0.1 wt.%) of raw and doped CNTs. It is clear that CNT–water nanofluid have noticeably higher thermal conductivity than the base fluid and the thermal conductivity increases with the increase of CNT concentration. Moreover, the thermal conductivity enhancement increases with the Fe 2O3 concentration. For undoped CNT–water suspensions at a weight concentration of 0.01%, a thermal conductivity enhancement of up to 2.5% is observed compared to pure water while for higher concentrations of 0.05 wt.% and 0.1 wt.%, the thermal conductivity enhancements were 3.27% and 5.77%, respectively. However, for CNT doped with 1 wt.% of Fe2O3, a significant enhancement of the thermal conductivity was observed. By adding 0.01 wt.% of Fe 2O3-CNT (1 wt.%), the thermal conductivity of the nanofluid increased by 8%, while upon increasing the concentration of the nanoparticles to 0.05 and 0.1wt.%, the enhancement was increased to 9.0% and 11%, respectively. By increasing the Fe2O3 loading on the surface of CNTs to 10.0 wt.%, the thermal conductivity of the nanofluid increased by 9.0%, 11% and 16.4% for nanoparticle concentration of 0.01, 0.05 and 0.1 wt.%, respectively.
A possible mechanism for the thermal conductivity enhancement with Fe 2O3 loading is that the coating of iron oxide nanoparticles on the surface of carbon nanotubes enhances their separation and reduces the aggregation. Undoped CNTs slightly enhance the heat capacity of water, while CNTs impregnated with iron oxide nanoparticles significantly increase the heat capacity.
The
agglomeration of nanomaterials is a major problem that reduces the heat transfer of the nanofluid. Dispersion of highly conductive nanomaterials in the fluid sharply increases the heat capacity of the fluid. To obtain a homogenized solution, two methods can be applied: mechanical, by applying ultrasonic sonication waves to break down the van der Waals interaction forces; and chemical, by doping
the surfaces of the particles by adding a surfactant or other metal elements that reduce their aggregation and enhanced their dispersion into the solution due to the chemical interaction between the surfactant attached to the surfaces of the particles the solutions. In this study, both techniques were used to increase the dispersion.
Another possible mechanism for the thermal conductivity enhancement with Fe 2O3 loading is the surface phenomenon, where a small layer of CNTs coats the wall of the steel tube and enhances the thermal conductivity of the steel. The last explanation involves the influence of Brownian motion. While the Brownian movement of particles has a molecular timescale, it is manifested through particles that have inertia and respond to much higher timescales. At these higher timescales, the movement of the particles locally agitates the fluid, causing increased heat transfer. Mixing these highly thermally conductive nanoparticles with water will definitely enhance the thermal properties of the nanofluid, as is observed from the results [27-30].
3.5 Specific heat capacity of nanofluids The heat capacity of nanofluids containing different concentrations of undoped and doped (1% and 10 % of Fe2O3 nanoparticles) CNTs was measured using DSC. Figure 7 shows the enhancement in the heat capacity of the nanofluids (Cpnf/Cpw) as a function of temperature, where Cp nf is the specific heat capacity of the nanofluid and Cp w is the specific heat capacity of water. The heat capacity of nanofluids increases significantly with concentration of both undoped and doped CNTs. The maximum enhancements of the specific heat capacity for undoped CNTs and doped CNTs with 1 wt.% Fe2O3 and 10 wt.% Fe2O3 at a weight concentration of 0.1 wt.% and 35 °C were 8%, 19% and 38%, respectively. It can be noted that the undoped CNTs slightly enhances the heat capacity of the water, while CNTs impregnated with iron oxide nanoparticles dramatically increases the specific heat it. We are not aware of any report on enhancing Cp of water using nanoparticles. There are several possible mechanisms. The first mechanism relates to the dispersed CNTs acts as heat sinking into a network of CNTs which will store the heat between them. The second possible mechanism is related to the interfacial interactions between the water layer and the surfaces of CNTs which may change the characteristic of the water and consequently changes the properties of the water. The last mechanism stems from the high thermal conductivity of CNT, which makes the nanofluid water stores heat faster
than water and reach the maximum capacity within shorter time and evenly distribute the heat into the water [23,24]. Donghyun and Debjyoti [31], proposed three different thermal transport mechanisms to explain the unusual enhancement of the specific heat capacity of the nanofluids. In the first Mode, they refer to the enhancement of the specific heat capacity of the nanofluids due to the higher specific heat capacity of the single nanoparticles than the bulk value of the same materials. Literature [32-34] reports show that, the specific heat capacity of nanoparticles can be enhanced up to 25% compared to bulk values of the same materials due to the high surface energy per unit mass. In the second Mode II, they postulate that, the interfacial interactions (e.g., such as interfacial thermal resistance and capacitance) playing a major role to increase thermal storage between the nanoparticles and the adhering liquid molecules due to the high surface area of the nanoparticles. In the last Mode III, they hypothesize that, the liquid molecules forms a layers behave like a semi-solid layer on the surfaces of nanoparticles. The thickness of these semi-solid layers or adhesion layeres of liquid molecules would depend on the surface energy of the nanoparticles. These semi-solid layers usually have higher thermal properties than the bulk liquid and therefore contribute to increasing the effective specific heat capacity of nanofluid. Based on these mechanisms, we believe that, the large enhancement of the specific heat capacity of the iron doped CNTs nanofluids due the same reasons, which is high surfaces area of modified CNTs with F2O3 compared to agglomerated raw CNTs, and that’s why the doped CNTs shows better heat capacity enhancement compared to undoped CNTs. The iron oxide nanoparticles works as spacer and enhancer of the surface area of CNTs and therefore contribute to increase the heat capacity of the nanofluids.
In addition to that, the enhancement in the heat capacity of nanofluids also due to the
surface morphology and hydrophobicity of the modified CNTs that is playing a major role to increase the semi-liquid layer of the liquid molecules adhere to surfaces of CNTs. Increasing the hydrophilicity of the CNTs would increase the thickness of the semi-liquid layer adhere to the surfaces of CNTs. CNTs doped with F2O3 increases the hydrophilicity of the CNTs that will lead to increase the liquid layer and hence increase the thermal properties of the nanofluids.
3.6 Pressure drop of nanofluids A differential pressure transmitter was used to measure the pressure drop between the inlet and outlet tubes. The pressure drop was measured for the turbulent flow regime with Reynolds numbers varying from 5000 to 25000. Figure 8 and 9 (a, b and c) shows the variation of the pressure drop as a function of mass flowrates and Reynolds numbers for undoped and doped CNTs nanofluids. No major change in the pressure drop was observed with changes in iron oxide nanoparticles doping or in the concentration of CNTs in water as shown in figure 8. In figure 9, the variation of pressure drop as a function of Reynolds numbers is due to the changing of the viscosity of the fluids which is a function of the temperature of the fluid. While it is well-known that, the presence of nanoparticles increases the pressure drop of the system, in this study, the nanoparticles were not observed to affect the pressure drop of the system owing to the very low surface roughness of the nanoparticles.
The surface rougness of the pipe were calculated from pressure drop of the nanofluids. The pressure drop (∆ ) of the fluid flow along of the pipe line (L) as a function of friction factor ( ), fluid flow (
) and diameter of the pipe (D) is given below: ∆
=
2
The following formula, known as Haaland Equation [35], gives a good representation of the experimentally determined friction factor in the turbulent region: 1
= −1.8
/ 3.7
.
+
6.9
Where is the surface roughness and Re is the Reynolds number (
=
) a dimensionless number
that gives a measure of the ratio of inertial forces to viscous forces for given flow conditions. The Reynolds number is an important parameter that describes whether flow conditions lead to laminar or turbulent flow.
By substituting Haaland equation into the pressure drop formula, the final pressure drop equation in the turbulent region as a function of the pipe length (L), pipe diameter (D), fluid density (r), fluid viscosity (m), roughness ratio ( / ) and Reynolds number is given below:
∆ = 0.154
/ .
.
+
.
(3)
Equation 3 provides an excellent fit to the experimental data as shown by the solid line in Figure 7. From the fit of the experimental using equation 3, the roughness ratio (e/D) and viscosity (m) are calculated and provided in in table 3. The calculated viscosity value for each concentration represents the average value for samples with different iron oxide loading and averaged over the entire tube length. These values are consistent with the experimentally measured values within the same temperature range. On the other hand, there is a slight increase in the roughness as the concentration of the nanoparticle increases. The increase may be caused by the deposition of the nanoparticles on the inner surface of the tube, which is expected to become significant as the nanoparticle concentration increases. 3.7 Heat Transfer of Nanofluids A shell and tube heat exchanger was used to measure the heat transfer of the nanofluid. Undoped CNTs, 1 wt.% Fe2O3-CNTs and 10.0 wt.% Fe2O3-CNTs were tested. The inlet temperature of the nanofluids was fixed at 35 °C by a controlled heating bath, while their flow rate was regulated by a digital mass flow controller to remain in the turbulent regime and ranged between 200–640 kg/h. Figure 10 shows the effect of different weight concentrations of undoped CNTs (0.01, 0.05 and 0.1 wt.%) on the enhancement of the heat transfer of the nanofluids at different flow rates. It is observed that the heat transfer ratio (Qnf/Qw) increases with the increase of the concentration of undoped CNTs. The maximum enhancement of heat transfer was 15% at 0.1 wt.% at a mass flow rate of 400 kg/h. This result is consistent with the Cp results, which show an enhancement of the heat capacity and thermal conductivity of water by approximately 8% and 7%, respectively after adding the CNTs. The other 7% enhancement could be from the increase of the dispersion of the highly thermal conductive materials (CNTs) into the solution due to the high motion of the fluid, which reduces agglomeration and increases the dispersion.
The same phenomena were observed when CNTs doped with iron nanoparticles were used. The heat transfer of the nanofluid increases with an increase in the concentration of doped CNTs, as shown in Figure 11 and table 4. Due to the large enhancement of the heat capacity and thermal conductivity of the CNT/iron oxide composite, the heat transfer of the nanofluid containing CNTs/iron oxide at 0.1 wt.% increases by almost 30% compared to water and 15% compared to undoped CNTs. Increasing the loading of the iron oxide nanoparticles to 10 wt.% increased the heat transfer of the nanofluid up to 60% at the concentration of 0.1 wt.%. 3.8 Pumping Power versus Heat Flow Although the benefits of using the nanofluid to enhance heat transfer are well established in the heat transfer community, their overall performance has only been discussed by a small number of research groups. We will therefore attempt to quantify the benefits versus the disadvantages of using nanofluids by looking at one of the major factors such as the pumping power. The optimum design of heat exchanger for minimum pumping power (i.e., minimum pressure drop) and efficient heat transfer is a great challenge from the energy savings point of view. Figure 12 (a, b, and c) compares results pertaining to heat transfer enhancement and the required pumping power.
As one may expect, the addition of nanoparticles into a base fluid will enhance heat transfer, but will also increase the pressure drop in the system. However, in the present study interesting results were obtained. A remarkable increase in the heat transfer with increasing nanoparticles concentration associated with a very slight increase in pumping power is obtained, as illustrated in Figure 12. As an example, the power required to exchange 1.8 kW of heat using nanofluids with 0, 0.01, 0.05 and 0.1 wt. % of raw CNTs is 4.4, 3.1, 2.7, and 1.4 kW, respectively. That’s mean the power required to exchange 1.8 kW of heat using CNT nanofluid with 0.1 wt.% CNT is three times lower than the power required to exchange the same amount of heat using water. This is due to the fact that the presence of CNTs nanoparticles sharply increases the heat transfer with minimal pumping penalty as discussed earlier. A more significant difference in the required pumping power for nanofluid and water is observed when doped CNTs are used. For example, the required pumping power to exchange 1.8 kW of heat using 10 wt. % Fe2O3-CNTs nanofluid with 0, 0.01, 0.05 and 0.1 wt.% nanoparticle concentration is 4.4, 1.5, 1.0 and 0.2 kW, respectively. This means the power required to exchange 1.8 kW of heat at using 0.1 wt.% of CNT doped with 10 wt.% Fe 2O3 is more than 20
times lower than the power required to exchange the same amount of heat using water (Figure 12 c). These very promising results would increase the interest on such materials leading to a high and immediate impact on thermal management devices and equipment. 4. Conclusions This paper is concerned with the heat transfer of nanofluids, composed of water as a base fluid and undoped or iron oxide-doped carbon nanotubes (Fe2O3-CNT) as suspended highly thermally conductive nanomaterials, under the turbulent flow regime. The prepared nanofluid shows a great potential in enhancing the heat transfer characteristic. One reason is that the suspended nanoparticles have a remarkable effect on the thermal conductivity of the nanofluid. The volume fraction, shape, dimension and properties of the nanoparticles affect the thermal conductivity of nanofluids. The use of CNTs and Fe2O3-CNT nanoparticles as the dispersed phase in water significantly enhanced the convective heat transfer and heat capacity of the nanofluid. The maximum enhancement of heat transfer obtained was 60% at 0.1 wt.% loading of doped CNTs with 10 wt.% Fe 2O3 nanoparticles and a 400 kg/h mass flow rate with no noticeable increase in the pressure drop of the system when these nanoparticles were added owing to the low roughness of the surface of the nanomaterials. 5. Acknowledgment The authors would like to acknowledge the support provided by King Abdulaziz City for Science and Technology (KACST) through Science and Technology Unit at King Fahd University of petroleum and Minerals (KFUPM) for funding this work through project No: 09-NAN758-04 titled "Experimental Investigation of Heat Transfer Characterization for CNT-Nanofluid in Heat Exchangers" as part of the National Science Technology and Innovation Plan (NSTIP).
6. References [1] Chol S., Enhancing thermal conductivity of fluids with nanoparticles. ASME-Publications-Fed, 1995, 231: p. 99-106. [2] Xuan Y. and Q. Li, Heat transfer enhancement of nanofluids, International Journal of Heat and Fluid Flow, 2000, 21(1): p. 58-64. [3] Yu W., Review and assessment of nanofluid technology for transportation and other applications, 2007, Argonne National Laboratory (ANL). [4] Keblinski, P., J.A. Eastman, and D.G. Cahill, Nanofluids for thermal transport. Materials today, 2005. 8(6): p. 36-44.
[5] Ho C. J., L. Wei, and Z. Li, An experimental investigation of forced convective cooling performance of a microchannel heat sink with Al2O 3/water nanofluid. Applied Thermal Engineering, 2010, 30(2): p. 96-103. [6] Zhou D., Heat transfer enhancement of copper nanofluid with acoustic cavitation. International Journal of Heat and Mass Transfer, 2004, 47(14): p. 3109-3117. [7] Iijima S., Helical microtubules of graphitic carbon. Nature, 1991, 354: p. 56-58. [8] Dresselhaus M., G. Dresselhaus and P. Avouris, Carbon Nanotubes: Synthesis, Structure, Properties, and Applications, Germany: Springer-Verlag Berlin Heidleberg, 2001. [9] Rashmi W., Mohammad K. S., Seiksan O., Ahmad F. I., Application of CNT nanofluids in a turbulent flow heat exchanger, Journal of Experimental Nanoscience, 2016, 11:1. [10] Ding Y., Hajar A., Dongsheng W., Richard W., Heat transfer of aqueous suspensions of carbon nanotubes (CNT nanofluids). International Journal of Heat and Mass Transfer, 2006, 49(1): p. 240-250. [11] Ghozatloo A., A. Rashidi, and M. Shariaty-Niasar, Effects of surface modification on the dispersion and thermal conductivity of CNT/water nanofluids, International Communications in Heat and Mass Transfer, 2014, 54: p. 1-7. [12] Amiri A., M. Shanbedi, H. Amiri, S. Zeinali, S. Kazi, B. Chew, H. Eshghi, Pool boiling heat transfer of CNT/water nanofluids, Applied Thermal Engineering, 2014, 71(1): p 450-459 [13] Meibo X., Y. Jianlin, W. Ruixiang, Thermo-physical properties of water-based single-walled carbon nanotube nanofluid as advanced coolant, Applied Thermal Engineering, 2015, 87( 5 ): p 344-351 [14] Choi, S., Z. Zhang, W. Yu, F. Lockwood and E. Grulke, Anomalous thermal conductivity enhancement in nanotube suspensions, Applied Physics Letters, 2001, 79(14): p. 2252-2254. [15] Marquis F., and L. Chibante, Improving the heat transfer of nanofluids and nanolubricants with carbon nanotubes, JOM, 2005, p. 32-43. [16] Xua Y. and Q. Li, Heat transfer enhancement of nanofluids. International Journal of Heat and Fluid Flow, 2000, 21(1): p. 58-64. [17] Liu M., M.C. Lin, and C. Wang, Enhancements of thermal conductivities with Cu, CuO, and carbon nanotube nanofluids and application of MWNT/water nanofluid on a water chiller system. Nanoscale Research Letters, 2011, 6(1): p. 1-13. [18] Choi S., Z. G. Zhang, W. Yu, F. E. Lockwood and E. A. Grulke, Anomalous thermal conductivity enhancement in nanotube suspensions. Applied physics letters, 2001, 79(14): p. 2252-2254. [19] Assael M., C. Chen, I. Metaxa, W.Wakeham, Thermal conductivity of suspensions of carbon nanotubes in water. International Journal of Thermophysics, 2004. 25(4): p. 971-985. [20]Assael M., I. Metaxa, J. Arvanitidis, D. Christofilos, C. Lioutas, Thermal conductivity enhancement in aqueous suspensions of carbon multi-walled and double-walled nanotubes in the presence of two different dispersants. International Journal of Thermophysics, 2005, 26(3): p. 647-664. [21] Liu M. S., Mark C. L., I-Te H., C. Wang, Enhancement of thermal conductivity with carbon nanotube for nanofluids. International Communications in Heat and Mass Transfer, 2005, 32(9): p. 1202-1210. [22] Ko G.H., K. Heo, K. Lee, D. Kim, C. Kim, Y. Sohn, M. Choi, An experimental study on the pressure drop of nanofluids containing carbon nanotubes in a horizontal tube. International Journal of Heat and Mass Transfer, 2007, 50(23): p. 4749-4753.
[23] Anoop K., T. Sundararajan, and S. K. Das, Effect of particle size on the convective heat transfer in nanofluid in the developing region. International Journal of Heat and Mass Transfer, 2009, 52(9): p. 2189-2195. [24] Mohammed H., G. Bhaskaran, N.H. Shuaib, H.I. Abu-Mulaweh, Influence of nanofluids on parallel flow square microchannel heat exchanger performance. International Communications in Heat and Mass Transfer, 2011, 38(1): p. 1-9. [25] Liu, M., M.C. Lin, and C. Wang, Enhancements of thermal conductivities with Cu, CuO, and carbon nanotube nanofluids and application of MWNT/water nanofluid on a water chiller system. Nanoscale research letters, 2011. 6(1): p. 1-13. [26] Al-Shemmeri T., Engineering Fluid Mechnanics. Ventus Publishing ApS. 2012. 17–18. ISBN 978-87-403-0114-4 [27] Jang S.P., S. Choi, Role of Brownian motion in the enhanced thermal conductivity of nanofluids, Applied Physics Lete., 2004, 84, p. 4316-4318. [28] Wei W., A Comprehensive Model for the Enhanced Thermal Conductivity of Nanofluids. Journal of Advanced Research in Physics , 2013, 3.2. [29] Pradhan N. R., H. Duan, J. Liang, and G. S. Iannacchione, Specific heat and thermal conductivity measurements for anisotropic and random macroscopic composites of cobalt nanowires, Nanotechnology, 2008, 19. [30] Jang S. P. and S. U. Choi, Cooling performance of a microchannel heat sink with nanofluids, Applied Thermal Engineering, 2006, 26: 17-18, p. 2457–2463,. [31] Donghyun S., and B., Debjyoti , Enhancement of specific heat capacity of high-temperature silica-nanofluids synthesized in alkali chloride salt eutectics for solar thermal-energy storage applications, International Journal of Heat and Mass Transfer , 2011, 54, p.1064–1070 [32] Kim K. M., Comparison of thermal performances of water-filled, SiC nanofluid-filled and SiC nanoparticles-coated heat pipes. International Journal of Heat and Mass Transfer, 2015, 88, p. 862-871. [33] Wang B. X., L.P. Zhou, X.F. Peng, Surface and size effects on the specific heat capacity of nanoparticles, Int. J. Thermophys, 2006, 27, p.139–151. [34] Wang L., Z. Tan, S. Meng, D. Liang, G. Li, Enhancement of molar heat capacity of nanostructured Al2 O3, J. Nanoparticle Res, 2004, 3 p. 483–487. [35] Haaland S. E. Simple and explicit formulas for the friction factor in turbulent pipe flow. Journal of Fluids Engineering , 1983, 105:1, p.89-90. [36] Ko, Gwon Hyun, et al. "An experimental study on the pressure drop of nanofluids containing carbon nanotubes in a horizontal tube." International journal of heat and mass transfer 50.23 (2007): 4749-4753.
50 ml/hr
Gas Flowmeters
Syringe Pump 1500 ML/MIN
200 ML/MIN
Two Tube Furnaces CNTs Collector
Figure 1: Schematic Diagram of CVD Reactor
Argon Gas
Hydrogen Gas
Gas outlet
Figure 2. Schematic diagram of the experimental Shell and tube heat exchanger used to measure the thermal conductivity, inlet and outlet temperature of the two fluids and the tube side pressure drop.
Figure 3. FE-SEM images of a) CNTs and CNT doped with iron oxide nanoparticles at a loading of b) 1 wt.% and c) 10 wt.%.
1 µm
100
Derivative weight, % / ° C
80
0% Fe2O3
% Weight
60
40 1% Fe2O3 9.0% Fe2O3 20
0 100
200
300
400 Temperature, °C
500
600
700
Figure 4. Thermogravimetric analysis of pure and impregnated CNT shown the residual mass (solid lines) and mass loss derivatives (dashed lines).
1.25
, cP
1.00
0.75
0.50
0.25 295
310 325 Temperature, K
340
295
310 325 Temperature, K
340
1.5
1.4
rel
1.3
1.2
1.1
1.0
Figure 5. Effect of temperature on the viscosity (h) (a) and relative viscosity (hrel) (b) of water and nanofluids containing different concentrations of the CNT. The solid lines in (a) is the fit using equation (1).
0.75
K, W/m.K
0.70
0.65
0.60 0
0.02
0.04 0.06 0.08 Concentration, wt.%
0.1
0.12
Figure 6. Thermal conductivity of nanofluids containing undoped and doped CNTs.
7.5
1% Fe2O3
0% Fe2O3
7.0 Cp, J/g.°C
6.5 6.0 5.5 5.0
45 35 25
4.5
45 35 25
4.0 0
0.05 0.1 Nanoparticle conc., wt.%
10% Fe2O3
0.00 0.05 0.10 Nanoparticle conc., wt.%
45
35 25
0
0.05 0.1 Nanoparticle conc., wt.%
Figure 7. Enhancement of heat capacity of (a) CNT (b) 1 wt.% Fe 2O3-CNT (c) 10 wt.% Fe2O3-CNT nanofluids with respect to temperature ( 25, 35 and 45 oC).
250
0% Fe2O3
200
P, mbar
0% 0.01%
150
0.05% 0.10%
100
50
0 0
100
200 300 400 500 Flow Rate, kg/h
600
700
250
1% Fe2O3 200
0% 0.01% 0.05%
P, mbar
150
0.10%
100
50
0 0
100
200 300 400 500 Flow Rate, kg/h
600
700
250
10% Fe2O3 200
0%
P, mbar
0.01% 150 0.05% 0.10%
100
50
0 0
100
200 300 400 500 Flow rate, kg/h
600
700
Figure 8. Variation in the pressure drop with respect to the mass flowrate for (a) undopded CNT nanofluids, (b) 1% Fe2O3-CNT nanofluids and (c) 10% Fe2O3-CNT nanofluids in a steel tube.
250
0% Fe2O3
P, mbar
200 0%
150
0.01% 0.05%
100
0.10%
50
0 0
8000
16000 Re
24000
32000
250
1% Fe2O3
P, mbar
200
150
100
50
0 0
8000
16000 Re
24000
32000
250
10% Fe2O3
P, mbar
200 150 100 50 0 0
8000
16000 Re
24000
32000
Figure 9. Variation in the pressure drop with respect to Reynolds Number for (a) undopded CNT nanofluids, (b) 1% Fe2O3-CNT nanofluids and (c) 10% Fe2O3-CNT nanofluids.
3.0
1% Fe2O3
0% Fe2O3 Heat Flow, kW
2.5 0.10% 0.05% 0.01% 0.00%
2.0
0.10% 0.05% 0.01% 0.00%
1.5
1.0 100 200 300 400 500 600 700 Flow Rate, kg/h
100 200 300 400 500 600 700 Flow rate, kg/h
10% Fe2O3 0.10%
0.05% 0.01% 0.00%
100 200 300 400 500 600 700 Flow rate, kg/h
Figure 10. Heat transfer rate of nanofluids containing different concentrations of undoped and doped CNTs as a function of flow rate.
1.2
1.3
0% Fe2O3
1% Fe2O3
0.10 %
1.2 Qnf/Qw
1.1 0.10 %
1.0
0.05 %
1.1 0.01 % 0.05 %
0.01 %
1.0
0.9
0.9 100
250 400 550 Flow rate, kg/h 1.6
700
100
250 400 550 Flow rate, kg/h
700
10% Fe2O3
1.5 0.10 %
1.4 0.05 %
1.3 1.2
0.01 %
1.1 1.0 0.9 100
250 400 550 Flow rate, kg/h
700
Figure 11. The enhanced heat transfer of (a) 1 wt.% Fe 2O3-CNT and (b) 10 wt.% Fe2O3CNT nanofluids as a function of flow rate.
a
b
Figure 12.Power required as a function of the heat generated for (a) CNTs (b) 1 wt.% Fe2O3-CNT and (c) 10 wt.% Fe 2O3-CNT.
c
Table 1. EDS analysis of CNTs and CNT- Fe 2O3
Element C O Fe Pt Ni Total %
Composition, wt.% 0 wt.% 1 wt.% 10 wt.% Fe2O3 Fe2O3 Fe2O3 85.42 80.01 98.50 3.64 4.52 1.50 - 1.44 10.03 9.50 5.09 - 0.35 100 100 100
Table 2: Constant values of Eq.2 at different concentration of nanofluids Constant a b c
Water
0.01 wt% CNTs
0.05 wt% CNTs
0.1 wt% CNTs
1.86E-03 881.8 -30.5
2.70E-02 510.1 -37.3
1.06E-02 770.6 -94.8
5.40E-02 278.8 86.6
Table 3. Viscosity and roughness ratio of nanofluids flowing through the stainless steel pipe Nanofluid conc, % 0 0.01 0.05 0.10
m 0.63 0.73 0.76 0.82
e/D 6.72 x10-4 7.50 x10-4 9.01 x10-4 9.34 x10-4
Table 4. Heat flow ratio
Heat Flow ratio, QNF/Qw
Conc, wt.%
0% Fe2O3
0.01
1.00 ±0.04 1.05 ±0.02 1.14 ±0.03
0.05
0.99 ±0.06 1.13 ±0.03 1.19 ±0.02
0.10
1.03 ±0.02 1.23 ±0.04 1.50 ±0.02
1% Fe2O3
10% Fe2O3
Research highlights The thermal performance of shell and tube heat exchanger is investigated. Water based nanofluids of CNTs doped with Fe2O3 nanoparticles at different weight loading and concentration are considered. The effects of volume concentration and Reynolds number on viscosity, heat capacity, thermal conductivity, enhancement of heat transfer, and pressure drop of nanofluids are evaluated. 0.1wt% concentration of CNTs doped with 10 wt% of Fe2O3 nanoparticles exhibits the best heat transfer performance.
The nanofluids at different flow regimes have minor effects on the pressure drop.
S1359-4311(16)31155-3 http://dx.doi.org/10.1016/j.applthermaleng.2016.07.026 ATE 8624
To appear in:
Applied Thermal Engineering
Received Date: Revised Date: Accepted Date:
14 October 2015 10 June 2016 4 July 2016
Please cite this article as: A.D. Manasrah, U.A. Al-Mubaiyedh, T. Laui, R. Ben-Mansour, M.J. Al-Marri, I.W. Almanassra, A. Abdala, M.A. Atieh, Heat transfer enhancement of nanofluids using iron nanoparticles decorated carbon nanotubes, Applied Thermal Engineering (2016), doi: http://dx.doi.org/10.1016/j.applthermaleng. 2016.07.026
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Heat transfer enhancement of nanofluids using iron nanoparticles decorated carbon nanotubes Abdallah D. Manasrah a , Usamah A. Al-Mubaiyedh b, Tahar Lauic, Rached Ben-Mansourc, Mohammed J. Al-Marrid, Ismail W. Almanassrab, Ahmed Abdala e,f, Muataz A. Atiehe,f* a
Chemical Engineering Department, University of Calgary, Calgary, Canada Chemcial Engineering Department, King Fahd Univervery of Petroleum & Minerals, Dhahran, Saudi Arabia c Mechanical Engineering Department, King Fahd Univervery of Petroleum & Minerals, Dhahran, Saudi Arabia d Gas Processing Center, College of Engineering, Qatar University, P.O. Box 2713 Doha, Qatar e Qatar Environment and Energy Research Institute, HBKU, Qatar Foundation, PO Box 5825, Doha, Qatar f College of science and Engineering, HBKU, Qatar Foundation, PO Box 5825, Doha, Qatar b
*
Corresponding author: Dr. Muataz Ali Atieh, Email: [email protected]
Abstract Enhancing heat transfer in thermal fluid systems can contribute significantly towards the improvement of thermal efficiency resulting in reducing energy consumption and hence carbon emission. Conventional fluids like water and oil have limited heat transfer potential. The need for the development of new classes of fluids with enhanced heat transfer properties is thus becoming essential. Many studies have developed nanofluids using nanoparticles, however, they showed a limited enhancement in heat transfer. This study investigated the heat capacity, enhancement of heat transfer, viscosity, and pressure drop of nanofluids with carbon nanotubes (CNTs) and CNTs doped with iron oxide nanoparticles (Fe2O3-CNT). The surfaces of carbon nanotubes were doped with 1 wt.% and 10.0 wt. iron oxide nanoparticles. The pristine and doped CNTs were used to prepare heatexchange nanofluids with additive concentrations of 0.01, 0.05, and 0.10 wt.%. A shell and tube heat exchanger was used to evaluate the overall heat transfer coefficient and the associated pressure. The specific heat capacity of the nanofluids was measured by differential scanning calorimetry (DSC). The results showed that the specific heat capacity of the nanofluids with undoped and doped CNTs is significantly higher than that of pure water by about 10 % and 55%, respectively. The heat transfer rate of the nanofluids increased sharply with the CNT dosage the iron nanoparticles loading and reached up to 55% enhancement with doped CNTs. We observed that the power required to exchange 1.8 kW heat using nanofluid containing 0.1 wt.% of 10 wt. % Fe 2O3-CNTs was 20 times lower than the power required to exchange the same amount of heat using water. This is because the iron nanoparticles enhanced the dispersion of the CNTs and increased their heat capacity and thermal
conductivity. Compared with that of pure water, the encountered pressure drop of the nanofluid at the same flow rate was almost unchanged, resulting in no extra pumping energy penalty. Keywords: Nanofluid, Impregnated carbon nanotube, Nanoparticles, Heat capacity, Pressure drop, Shell and tube heat exchanger.
LIST OF ABBREVIATIONS Q
: Heat transfer rate in tube (kW).
mn : Mass flow rate for nanofluids (kg/h). Cpn : Specific heat capacity of nanofluids (J/g.C). Tn,in : Tube inlet temperature (°C). Tn,out : Tube outlet temperature (°C). n
: Subscript for nanofluid.
Re : Reynolds’s number. Nu : Nussle’s number. ∆P : Pressure Drop (mbar). Ƞ : viscosity of nanofluids (cp). k : Thermal conductivity (W/m.K). : Friction factor.
1. Introduction Heat exchangers are an integral component in many industrial applications including automotive, petrochemical, power generation, food processing, cooling and heating units, and electronic thermal management systems. In heat exchanger systems, the cost and size of the system are related to the properties of the heat-transfer fluids. Conventional fluids, such as water and oils, have limited heat transfer potentials. The development of new classes of fluids with enhanced heat transfer capabilities has been the subject of significant research [1, 2]. One area of interest in this field involves the use of additives to improve the properties of heat-transfer fluids. Metal and metal oxide additives are of particular interest in heat-transfer fluid applications, especially when they are manufactured on the nanoscale [1]. In 1995, Choi was the first researcher to report the application of nanoparticles in heattransfer fluids [1]. Since then, the suspension of nanoparticles in conventional heat-transfer fluids has been shown to improve the thermal conductivity and convective heat-transfer performance by an order of magnitude over the traditional base fluids (ethylene glycol, water, oils) [3]. A key consideration in using nanoparticles in heat-transfer fluids to produce nanofluids that yield reliable results is to ensure that the nanofluid is stable. The agglomeration of nanoparticles is a major problem that makes the fluid unstable. One way to create a stable fluid is to disperse the particles in the solution mechanically, by applying sonication to break down the van der Waals interaction forces, but this has a short term effect. Alternatively, chemical modification uses other additives such as dispersants or surfactants to prevent agglomeration of particles in the fluid and produce homogeneous mixtures [4-6]. Recently, great interest has been shown in the development of carbon nanomaterials. Carbon nanomaterials have gained significant attention over the last decade since the discovery of carbon nanotubes (CNTs)[7,8] . The most eye-catching features of these structures are their thermal properties, which can permit future applications in thermal science and engineering. CNTs and graphene nanoparticles have unusual heat transfer properties. In the lengthwise direction, they show excellent heat transfer performance. They also possess remarkable thermal properties with ultra-high thermal conductivity (2000–3000 Wm-1K-1), which is much higher than those of the metallic or their oxide nanomaterial used in nanofluids [9]. CNTs can be dispersed homogenously in conventional heat transfer fluids [10]. Recent research has demonstrated that there is a substantial increase in the thermal conductivities of different CNT nanofluids in comparison to their base fluids. In general, research on CNT nanofluids has blossomed in many different directions and has attracted a great deal of attention. Several studies investigated the thermal conductivity of nanofluids containing small
(0.01–1 vol%) amounts of carbon nanotubes (CNTs), as nanoparticles, in different solvents (oil, water) with or without the use of dispersing agents (sodium dodecyl sulfate, Arabic gum), and reported thermal conductivity enhancements ranging from 11–350%. Other studies investigated the use of alumina nanoparticles dispersed in water at concentrations ranging from 0.15–1 vol%, and observed heat transfer enhancements of up to 30% [11-22]. Anoop et al. [23] studied the effect of nanoparticle size on the heat transfer of water containing alumina additives, and showed that smaller nanoparticles result in a greater enhancement of heat transfer. Mohammed et al. [24] studied the effect of silver, silica, alumina and titania nanoparticles on the heat transfer of water-based nanofluids under laminar flow conditions, and reported that silver had the lowest pressure drop and that alumina the highest heat transfer coefficient. A recent study conducted by Liu et al. [25] investigated the enhancement of thermal conductivity in nanofluids prepared using a two-step method for dispersing copper oxide and carbon nanotubes in water, ethylene glycol and synthetic oil without using surfactants. Their results showed that the nanofluids containing low volume concentrations of nanoparticles had greater thermal conductivity than the original fluids. The researchers concluded that better dispersion of the nanoparticle additives may further enhance system performance. Many experimental studies have been initiated by different research groups to study the transport behavior of nanofluids. Ding et al. [10] studied the effect of MWCNTs on the heat transfer performance of nanofluids using a straight copper tube. The nanofluids were produced by dispersing MWCNTs in distilled water. They found that with increasing concentrations of CNT and nanofluid temperature the thermal conductivity increased as well as the viscosity. Another study has been conducted by Ko et al. [36], a nanolfluid has been prepared with carbon nanotubes in water, the experiment has been conducted in turbulent flow regimes of heat exchanger, their results shown that CNT nanofluids produced low friction factors compared to the conventional fluids. By incorporating carbon nanotubes (CNTs) in fluids, it is expected that such nanofluids would exhibit major improvement in thermal conductivity due to the very high thermal conductivity of CNT. To the best of our knowledge, there is no published work on the thermophysical properties of nanofluids using water as base fluid containing doped CNTs with iron oxide and their behavior as a heat-transfer fluid in turbulent flow regimes, where most of practical flows fall. Therefore, this study focuses on the measurement of the basic thermophysical properties of CNTs impregnated with iron oxide on
their surfaces, such as specific heat capacity (Cp), viscosity, heat transfer rate and pressure drop behavior in turbulent flow regimes in a shell and tube heat exchanger commonly used in industry 2. Experimental 2.1 Materials Ferric nitrate (Fe(NO3)3 9H2O), 99% purity (Sigma Aldrich) was used as the precursor for the iron oxide nanoparticles.
2.2 Preparation of CNTs and their Impregnation with Iron Oxide Nanoparticles The production of carbon nanotubes in the present work was conducted in two vertical tubular reactors. The vertical reactors were quartz tubes of 50 mm in diameter and 1200 mm in length and were heated by silicon carbide heating elements as shown in figure 1. The catalyst with hydrocarbon were injected at the first reaction chamber to evaporate the hydrocarbon and the catalyst while the second reaction chamber was used for the reaction and growth processes. Two types of gases were used in this system, hydrogen as the reacting gas and the argon for flushing the air from the system. After the temperature reached the reaction temperature, the ferrocene catalyst was heated in the first reaction chamber at temperature 120 °C in order to produce the ferrocene vapour. The hydrogen gas was mixed with benzene and ferrocene gases and passed into the first reaction chamber 800 oC as reaction temperature for 1 hr reaction time. The produced CNTs were impregnated with iron oxide nanoparticles via wet impregnation technique using Fe(NO3)3 9H2O as the iron precursor. The Fe(NO3)3 9H2O precursor was dissolved in ethanol at two weight concentrations to impregnate the surface of CNTs with 1 and 10 wt.% loadings as determined by TGA. The impregnation was conducted under ultrasonication using probe sonicator for 30 minutes at room temperature followed by calcination at 350°C for 3.5 hours to fully oxidize the iron nanoparticles.
2.3 Characterization of Pure and Impregnated CNTs The morphology of the CNT and the dispersion of the iron oxide nanoparticles were examined using Field Emission Scanning Electron Microscope (FE-SEM, Tescan Lyra3) and High Resolution Transmission Electron Microscopy (HR-TEM, JOEL-2100F)). The elemental analysis of the pure and
impregnated CNT was measured using energy dispersive X-ray (EDX) spectrometer attached to the FE-SEM. The SEM images were acquired in secondary electron and backscattering modes with accelerating voltage of 20.0 keV. Before SEM imaging, the samples were gold coated using gold sputtering system. The raw and impregnated CNT were also analyzed by Thermal Gravimetric Analysis (TGA, Model SDT Q600 TA Instrument) at the temperature range of 25-700° C under Nitrogen. 2.4 Preparation of Nanofluids Nanofluids were prepared by dispersing the raw and impregnated CNTs in water using probe sonicator for 30 minutes at an amplitude of 100%, a power density of 750 W/m 3, and a frequency of 30 kHz. Nanofluids samples with 0.01, 0.05, and 0.1 wt.% Fe2O3-CNT were prepared without using any surfactant or dispersant.
2.5 Measurement of the specific heat capacity of nanofluids The specific heat capacity of the nanofluids was measured by Differential Scanning Calorimetry (DSC) (DSC Q1000, TA Instruments) connected to a rapid cooling system (RCS 90) with nitrogen gas. After temperature equilibrium and 3-minute isotherm at 25 °C, the heat flux measurements were taken while ramping at a rate of 1.5 °C/min to 50 °C. Three measurements were taken for each sample. The samples were sonicated for 10 minutes and cooled to room temperature in an ice bath immediately before being placed in the DSC. 2.6 Viscosity and thermal conductivity measurement The dynamic viscosity of the nanofluids in the temperature range of 25 to 65 °C was measured using Stormer viscometer where the viscosity was calculated by measuring the time needed for the inner cylinder to perform 100 revolutions in response to a moving weight. The thermal conductivity (k) of the nanofluids was measured via the modified transient plane source technique that uses a one-sided interfacial heat reflectance sensor to apply a constant heat source to the sample using Mathis TCi system (Mathis Instruments Ltd.). The method eliminates the intrusive nature of the hot wire because the heating element is supported on a backing that provides mechanical
support as well as electrical and thermal insulation. It also allows to test smaller volumes of material as the wire is coiled. 2.7 Heat Exchanger Setup A shell and tube heat exchanger was designed and fabricated to investigate the heat transfer characteristics of different nanofluids at different flow rates. A schematic diagram of the heat exchanger is shown in Figure 2. The unit consists of two flow cycles, a heating unit (thermostat) and a cooling unit. One flow loop carries the heated nanofluids and the other transports cooling water (chiller) and both are connected to a controlled pump and instrumented with a flow meter. The heat exchanger shell is constructed from corrosion resistant borosilicate glass 3.3. A 316 stainless steel tube bundle consisting of three tubes, but only one tube with used and the other two tubes were closed from both ends. The tube length (L), inside diameter (di), outside diameter (d o) and heat transfer area (A) are 2.98 m, 0.010 m, 0.014 m, and 0.13 m 2. Four Me-thermocouples (Fe-CuNi) type were inserted in the shell and the tube sides to measure the inlet and outlet temperatures of each stream and an additional thermocouple was inserted in the nanofluid reservoir. Differential pressure transmitters were installed at both ends of the pipes to measure the pressure drop. A chiller with 4.6 kW cooling capacity was used to keep the water temperature constant at 20 °C and a 3.5 kW electric heater was used to keep the temperature of the nanofluid in the 12-liter capacity thermostat tank constant at 35 °C. Experiments were performed at mass flow rates ranging from 200 kg/h up to 640 kg/h. The inlet and outlet temperatures and the pressure drops were measured after achieving steady state conditions (after approximately 30 mins). From these measurements, the heat transfer rate was calculated based on the following equation:
Q = mn Cpnf (Tn,in - Tn,out ) .
where Q is the heat transfer rate in the tube,
(1) where Q is the nanofluid flow rate, Cpnf is the
nanofluid specific heat capacity, Tn, in is the tube inlet temperature and Tn, out is the tube outlet temperature.
3.
Results and Discussion
3.1 Surface characterization of raw and impregnated CNTs A Vertical Reaction Chemical Vapor Deposition (V-CVD) reactor was designed and fabricated in order to produce large amount of Carbon Nanotubes by using continuous injection atomization. Bundles of multi wall carbon nanotubes (MWCNTs) were produced by this system. The V-CVD parameters (hydrogen flow rate, reaction temperature, and reaction time) were studied to optimize the CNTs production and quality. The maximum yield and purity of CNTs was obtained at 2000 ml/min of hydrocarbon injected into the reactor at 800° C as reaction temperature for almost 1 hour reaction time. The synthesized nanotubes were characterized by scanning electron microscopy (SEM), Transmission Electron Microscopy (TEM) and Thermogravimetry (TGA) techniques. The produced CNTs were doped with iron oxide nanoparticles using wet impregnation technique. Figure 3 (a, b and c) displays the SEM images of raw CNTs and CNT doped with 1 and 10 with % Fe 2O3 nanoparticles. The diameter of the raw CNTs varies from 20–40 nm with an average diameter of 24 nm, while their length varied from 10–30 µm (as shown in figure 3 a). The white spots in the SEM image of CNTFe2O3 show the iron oxide particles which confirmed by Energy dispersive x-ray spectroscopy (EDS) analysis (as indicated in table 1). Back Scattering FE-SEM image of CNT-Fe 2O3 was taken in order to verify the presence of nanoparticle ions on the surfaces of the CNTs (as shown in figure 3 b and c). The distribution and agglomeration of Fe 2O3 nanoparticles were also investigated. It was observed that, there are formations of white crystal structures of Fe 2O3 nanoparticles with small sizes and irregular shapes. It can be seen that Fe 2O3 nanoparticles spread widely on the surfaces of carbon nanotubes forming very small crystal particles with diameters varying from 1-5 nm.
3.2
Thermal Degradation Analysis
The study of the thermal stability of materials, which is typically studied using TGA, is of a major importance as it determines the upper usage temperature limit of the material. Figure 4 depicts the TGA-DTG results for the carbon nanotubes with (Fe2O3-CNT) and without iron nanoparticles. In the thermogram, the initial degradation of CNTs in air starts at approximately 550°C, reaches
a
maximum
weight
loss
rate
at
approximately
600
°C
and
reach
a
constant weight at approximately 670 °C, as revealed by the DTG thermograms. The doping of CNT accelerates the pyrolysis of CNT as indicated by the shift of the onset of the rapid degradation step to a lower temperature with the increase of the doping dose. For CNTs impregnated with 1 wt.% iron nanoparticles, the initial oxidation starts at 500 °C, reaches a maximum weight loss rate at approximately 550 °C and completes at approximately 600 °C, as shown in Figure 4. When the loading of Fe2O3 was increased to 10.0 wt.%, the oxidation peak in the thermogram shifts to lower temperatures, so that the oxidation starts at 450 °C with a maximum weight loss rate at 500 °C and completes at 540 °C. We believe that the iron particles doped on CNTs acted as heating accelerator agent, which accelerate the heat transfer to the body of the CNTs as can be seen by the faster combustion of the doped CNTs (oxidization) compared to undopped CNTs. In addition to the effect of doping on CNT thermal stability, TGA provides an accurate estimate of the loading of Fe 2O3 by comparing the final constant weight for CNT and its iron doped derivatives (inset of Figure 4). The final remaining weight of raw CNTs and CNTs doped with 1 wt.% Fe 2O3, and 10 wt. % Fe 2O3 is 0.99, 2.05, and 10.06 wt.%.
3.3 Viscosity of the nanofluids Despite their ability to enhance the heat transfer rate, nanofluids may be associated with a large increase in the solution and consequently the power requirement. Therefore, studying the rheology of the nanofluid is one of the most critical parameters in determining the performance of nanofluids. The viscosity of the nanofluids containing different concentrations of undoped and doped CNT nanofluids as a function of temperature is shown in Figure 5 a and b. It is worth mentioning that the viscosity of the nanofluids is highly dependent on the concentration of the nanofluid, but independent of the iron oxide loading. Regardless of the iron oxide loading, the viscosity of the nanofluids increases significantly with the nanoparticles concentration and decreases as the temperature increases with maximum increase in viscosity of 50% for the highest nanoparticle concentration at the highest measurement temperature. The viscosity of the nanofluid is less sensitive to temperature compared to water as indicated by the increase in the relative viscosity (
=
) with temperature as shown in
Figure 5 b. The effect of the temperature on the viscosity of water can be described by [26]:
( ) = 10
(2)
This model accurately describes the effect of temperature on the viscosity of water and nanofluids as is shown by the solid lines in Figure 5 a. The values of a, b, and c were extracted from figure 5 a . The values are shown in table 2
3.4 Thermal conductivity of nanofluids Figure 6 shows the thermal conductivity of nanofluids with different concentrations (0.01, 0.05 and 0.1 wt.%) of raw and doped CNTs. It is clear that CNT–water nanofluid have noticeably higher thermal conductivity than the base fluid and the thermal conductivity increases with the increase of CNT concentration. Moreover, the thermal conductivity enhancement increases with the Fe 2O3 concentration. For undoped CNT–water suspensions at a weight concentration of 0.01%, a thermal conductivity enhancement of up to 2.5% is observed compared to pure water while for higher concentrations of 0.05 wt.% and 0.1 wt.%, the thermal conductivity enhancements were 3.27% and 5.77%, respectively. However, for CNT doped with 1 wt.% of Fe2O3, a significant enhancement of the thermal conductivity was observed. By adding 0.01 wt.% of Fe 2O3-CNT (1 wt.%), the thermal conductivity of the nanofluid increased by 8%, while upon increasing the concentration of the nanoparticles to 0.05 and 0.1wt.%, the enhancement was increased to 9.0% and 11%, respectively. By increasing the Fe2O3 loading on the surface of CNTs to 10.0 wt.%, the thermal conductivity of the nanofluid increased by 9.0%, 11% and 16.4% for nanoparticle concentration of 0.01, 0.05 and 0.1 wt.%, respectively.
A possible mechanism for the thermal conductivity enhancement with Fe 2O3 loading is that the coating of iron oxide nanoparticles on the surface of carbon nanotubes enhances their separation and reduces the aggregation. Undoped CNTs slightly enhance the heat capacity of water, while CNTs impregnated with iron oxide nanoparticles significantly increase the heat capacity.
The
agglomeration of nanomaterials is a major problem that reduces the heat transfer of the nanofluid. Dispersion of highly conductive nanomaterials in the fluid sharply increases the heat capacity of the fluid. To obtain a homogenized solution, two methods can be applied: mechanical, by applying ultrasonic sonication waves to break down the van der Waals interaction forces; and chemical, by doping
the surfaces of the particles by adding a surfactant or other metal elements that reduce their aggregation and enhanced their dispersion into the solution due to the chemical interaction between the surfactant attached to the surfaces of the particles the solutions. In this study, both techniques were used to increase the dispersion.
Another possible mechanism for the thermal conductivity enhancement with Fe 2O3 loading is the surface phenomenon, where a small layer of CNTs coats the wall of the steel tube and enhances the thermal conductivity of the steel. The last explanation involves the influence of Brownian motion. While the Brownian movement of particles has a molecular timescale, it is manifested through particles that have inertia and respond to much higher timescales. At these higher timescales, the movement of the particles locally agitates the fluid, causing increased heat transfer. Mixing these highly thermally conductive nanoparticles with water will definitely enhance the thermal properties of the nanofluid, as is observed from the results [27-30].
3.5 Specific heat capacity of nanofluids The heat capacity of nanofluids containing different concentrations of undoped and doped (1% and 10 % of Fe2O3 nanoparticles) CNTs was measured using DSC. Figure 7 shows the enhancement in the heat capacity of the nanofluids (Cpnf/Cpw) as a function of temperature, where Cp nf is the specific heat capacity of the nanofluid and Cp w is the specific heat capacity of water. The heat capacity of nanofluids increases significantly with concentration of both undoped and doped CNTs. The maximum enhancements of the specific heat capacity for undoped CNTs and doped CNTs with 1 wt.% Fe2O3 and 10 wt.% Fe2O3 at a weight concentration of 0.1 wt.% and 35 °C were 8%, 19% and 38%, respectively. It can be noted that the undoped CNTs slightly enhances the heat capacity of the water, while CNTs impregnated with iron oxide nanoparticles dramatically increases the specific heat it. We are not aware of any report on enhancing Cp of water using nanoparticles. There are several possible mechanisms. The first mechanism relates to the dispersed CNTs acts as heat sinking into a network of CNTs which will store the heat between them. The second possible mechanism is related to the interfacial interactions between the water layer and the surfaces of CNTs which may change the characteristic of the water and consequently changes the properties of the water. The last mechanism stems from the high thermal conductivity of CNT, which makes the nanofluid water stores heat faster
than water and reach the maximum capacity within shorter time and evenly distribute the heat into the water [23,24]. Donghyun and Debjyoti [31], proposed three different thermal transport mechanisms to explain the unusual enhancement of the specific heat capacity of the nanofluids. In the first Mode, they refer to the enhancement of the specific heat capacity of the nanofluids due to the higher specific heat capacity of the single nanoparticles than the bulk value of the same materials. Literature [32-34] reports show that, the specific heat capacity of nanoparticles can be enhanced up to 25% compared to bulk values of the same materials due to the high surface energy per unit mass. In the second Mode II, they postulate that, the interfacial interactions (e.g., such as interfacial thermal resistance and capacitance) playing a major role to increase thermal storage between the nanoparticles and the adhering liquid molecules due to the high surface area of the nanoparticles. In the last Mode III, they hypothesize that, the liquid molecules forms a layers behave like a semi-solid layer on the surfaces of nanoparticles. The thickness of these semi-solid layers or adhesion layeres of liquid molecules would depend on the surface energy of the nanoparticles. These semi-solid layers usually have higher thermal properties than the bulk liquid and therefore contribute to increasing the effective specific heat capacity of nanofluid. Based on these mechanisms, we believe that, the large enhancement of the specific heat capacity of the iron doped CNTs nanofluids due the same reasons, which is high surfaces area of modified CNTs with F2O3 compared to agglomerated raw CNTs, and that’s why the doped CNTs shows better heat capacity enhancement compared to undoped CNTs. The iron oxide nanoparticles works as spacer and enhancer of the surface area of CNTs and therefore contribute to increase the heat capacity of the nanofluids.
In addition to that, the enhancement in the heat capacity of nanofluids also due to the
surface morphology and hydrophobicity of the modified CNTs that is playing a major role to increase the semi-liquid layer of the liquid molecules adhere to surfaces of CNTs. Increasing the hydrophilicity of the CNTs would increase the thickness of the semi-liquid layer adhere to the surfaces of CNTs. CNTs doped with F2O3 increases the hydrophilicity of the CNTs that will lead to increase the liquid layer and hence increase the thermal properties of the nanofluids.
3.6 Pressure drop of nanofluids A differential pressure transmitter was used to measure the pressure drop between the inlet and outlet tubes. The pressure drop was measured for the turbulent flow regime with Reynolds numbers varying from 5000 to 25000. Figure 8 and 9 (a, b and c) shows the variation of the pressure drop as a function of mass flowrates and Reynolds numbers for undoped and doped CNTs nanofluids. No major change in the pressure drop was observed with changes in iron oxide nanoparticles doping or in the concentration of CNTs in water as shown in figure 8. In figure 9, the variation of pressure drop as a function of Reynolds numbers is due to the changing of the viscosity of the fluids which is a function of the temperature of the fluid. While it is well-known that, the presence of nanoparticles increases the pressure drop of the system, in this study, the nanoparticles were not observed to affect the pressure drop of the system owing to the very low surface roughness of the nanoparticles.
The surface rougness of the pipe were calculated from pressure drop of the nanofluids. The pressure drop (∆ ) of the fluid flow along of the pipe line (L) as a function of friction factor ( ), fluid flow (
) and diameter of the pipe (D) is given below: ∆
=
2
The following formula, known as Haaland Equation [35], gives a good representation of the experimentally determined friction factor in the turbulent region: 1
= −1.8
/ 3.7
.
+
6.9
Where is the surface roughness and Re is the Reynolds number (
=
) a dimensionless number
that gives a measure of the ratio of inertial forces to viscous forces for given flow conditions. The Reynolds number is an important parameter that describes whether flow conditions lead to laminar or turbulent flow.
By substituting Haaland equation into the pressure drop formula, the final pressure drop equation in the turbulent region as a function of the pipe length (L), pipe diameter (D), fluid density (r), fluid viscosity (m), roughness ratio ( / ) and Reynolds number is given below:
∆ = 0.154
/ .
.
+
.
(3)
Equation 3 provides an excellent fit to the experimental data as shown by the solid line in Figure 7. From the fit of the experimental using equation 3, the roughness ratio (e/D) and viscosity (m) are calculated and provided in in table 3. The calculated viscosity value for each concentration represents the average value for samples with different iron oxide loading and averaged over the entire tube length. These values are consistent with the experimentally measured values within the same temperature range. On the other hand, there is a slight increase in the roughness as the concentration of the nanoparticle increases. The increase may be caused by the deposition of the nanoparticles on the inner surface of the tube, which is expected to become significant as the nanoparticle concentration increases. 3.7 Heat Transfer of Nanofluids A shell and tube heat exchanger was used to measure the heat transfer of the nanofluid. Undoped CNTs, 1 wt.% Fe2O3-CNTs and 10.0 wt.% Fe2O3-CNTs were tested. The inlet temperature of the nanofluids was fixed at 35 °C by a controlled heating bath, while their flow rate was regulated by a digital mass flow controller to remain in the turbulent regime and ranged between 200–640 kg/h. Figure 10 shows the effect of different weight concentrations of undoped CNTs (0.01, 0.05 and 0.1 wt.%) on the enhancement of the heat transfer of the nanofluids at different flow rates. It is observed that the heat transfer ratio (Qnf/Qw) increases with the increase of the concentration of undoped CNTs. The maximum enhancement of heat transfer was 15% at 0.1 wt.% at a mass flow rate of 400 kg/h. This result is consistent with the Cp results, which show an enhancement of the heat capacity and thermal conductivity of water by approximately 8% and 7%, respectively after adding the CNTs. The other 7% enhancement could be from the increase of the dispersion of the highly thermal conductive materials (CNTs) into the solution due to the high motion of the fluid, which reduces agglomeration and increases the dispersion.
The same phenomena were observed when CNTs doped with iron nanoparticles were used. The heat transfer of the nanofluid increases with an increase in the concentration of doped CNTs, as shown in Figure 11 and table 4. Due to the large enhancement of the heat capacity and thermal conductivity of the CNT/iron oxide composite, the heat transfer of the nanofluid containing CNTs/iron oxide at 0.1 wt.% increases by almost 30% compared to water and 15% compared to undoped CNTs. Increasing the loading of the iron oxide nanoparticles to 10 wt.% increased the heat transfer of the nanofluid up to 60% at the concentration of 0.1 wt.%. 3.8 Pumping Power versus Heat Flow Although the benefits of using the nanofluid to enhance heat transfer are well established in the heat transfer community, their overall performance has only been discussed by a small number of research groups. We will therefore attempt to quantify the benefits versus the disadvantages of using nanofluids by looking at one of the major factors such as the pumping power. The optimum design of heat exchanger for minimum pumping power (i.e., minimum pressure drop) and efficient heat transfer is a great challenge from the energy savings point of view. Figure 12 (a, b, and c) compares results pertaining to heat transfer enhancement and the required pumping power.
As one may expect, the addition of nanoparticles into a base fluid will enhance heat transfer, but will also increase the pressure drop in the system. However, in the present study interesting results were obtained. A remarkable increase in the heat transfer with increasing nanoparticles concentration associated with a very slight increase in pumping power is obtained, as illustrated in Figure 12. As an example, the power required to exchange 1.8 kW of heat using nanofluids with 0, 0.01, 0.05 and 0.1 wt. % of raw CNTs is 4.4, 3.1, 2.7, and 1.4 kW, respectively. That’s mean the power required to exchange 1.8 kW of heat using CNT nanofluid with 0.1 wt.% CNT is three times lower than the power required to exchange the same amount of heat using water. This is due to the fact that the presence of CNTs nanoparticles sharply increases the heat transfer with minimal pumping penalty as discussed earlier. A more significant difference in the required pumping power for nanofluid and water is observed when doped CNTs are used. For example, the required pumping power to exchange 1.8 kW of heat using 10 wt. % Fe2O3-CNTs nanofluid with 0, 0.01, 0.05 and 0.1 wt.% nanoparticle concentration is 4.4, 1.5, 1.0 and 0.2 kW, respectively. This means the power required to exchange 1.8 kW of heat at using 0.1 wt.% of CNT doped with 10 wt.% Fe 2O3 is more than 20
times lower than the power required to exchange the same amount of heat using water (Figure 12 c). These very promising results would increase the interest on such materials leading to a high and immediate impact on thermal management devices and equipment. 4. Conclusions This paper is concerned with the heat transfer of nanofluids, composed of water as a base fluid and undoped or iron oxide-doped carbon nanotubes (Fe2O3-CNT) as suspended highly thermally conductive nanomaterials, under the turbulent flow regime. The prepared nanofluid shows a great potential in enhancing the heat transfer characteristic. One reason is that the suspended nanoparticles have a remarkable effect on the thermal conductivity of the nanofluid. The volume fraction, shape, dimension and properties of the nanoparticles affect the thermal conductivity of nanofluids. The use of CNTs and Fe2O3-CNT nanoparticles as the dispersed phase in water significantly enhanced the convective heat transfer and heat capacity of the nanofluid. The maximum enhancement of heat transfer obtained was 60% at 0.1 wt.% loading of doped CNTs with 10 wt.% Fe 2O3 nanoparticles and a 400 kg/h mass flow rate with no noticeable increase in the pressure drop of the system when these nanoparticles were added owing to the low roughness of the surface of the nanomaterials. 5. Acknowledgment The authors would like to acknowledge the support provided by King Abdulaziz City for Science and Technology (KACST) through Science and Technology Unit at King Fahd University of petroleum and Minerals (KFUPM) for funding this work through project No: 09-NAN758-04 titled "Experimental Investigation of Heat Transfer Characterization for CNT-Nanofluid in Heat Exchangers" as part of the National Science Technology and Innovation Plan (NSTIP).
6. References [1] Chol S., Enhancing thermal conductivity of fluids with nanoparticles. ASME-Publications-Fed, 1995, 231: p. 99-106. [2] Xuan Y. and Q. Li, Heat transfer enhancement of nanofluids, International Journal of Heat and Fluid Flow, 2000, 21(1): p. 58-64. [3] Yu W., Review and assessment of nanofluid technology for transportation and other applications, 2007, Argonne National Laboratory (ANL). [4] Keblinski, P., J.A. Eastman, and D.G. Cahill, Nanofluids for thermal transport. Materials today, 2005. 8(6): p. 36-44.
[5] Ho C. J., L. Wei, and Z. Li, An experimental investigation of forced convective cooling performance of a microchannel heat sink with Al2O 3/water nanofluid. Applied Thermal Engineering, 2010, 30(2): p. 96-103. [6] Zhou D., Heat transfer enhancement of copper nanofluid with acoustic cavitation. International Journal of Heat and Mass Transfer, 2004, 47(14): p. 3109-3117. [7] Iijima S., Helical microtubules of graphitic carbon. Nature, 1991, 354: p. 56-58. [8] Dresselhaus M., G. Dresselhaus and P. Avouris, Carbon Nanotubes: Synthesis, Structure, Properties, and Applications, Germany: Springer-Verlag Berlin Heidleberg, 2001. [9] Rashmi W., Mohammad K. S., Seiksan O., Ahmad F. I., Application of CNT nanofluids in a turbulent flow heat exchanger, Journal of Experimental Nanoscience, 2016, 11:1. [10] Ding Y., Hajar A., Dongsheng W., Richard W., Heat transfer of aqueous suspensions of carbon nanotubes (CNT nanofluids). International Journal of Heat and Mass Transfer, 2006, 49(1): p. 240-250. [11] Ghozatloo A., A. Rashidi, and M. Shariaty-Niasar, Effects of surface modification on the dispersion and thermal conductivity of CNT/water nanofluids, International Communications in Heat and Mass Transfer, 2014, 54: p. 1-7. [12] Amiri A., M. Shanbedi, H. Amiri, S. Zeinali, S. Kazi, B. Chew, H. Eshghi, Pool boiling heat transfer of CNT/water nanofluids, Applied Thermal Engineering, 2014, 71(1): p 450-459 [13] Meibo X., Y. Jianlin, W. Ruixiang, Thermo-physical properties of water-based single-walled carbon nanotube nanofluid as advanced coolant, Applied Thermal Engineering, 2015, 87( 5 ): p 344-351 [14] Choi, S., Z. Zhang, W. Yu, F. Lockwood and E. Grulke, Anomalous thermal conductivity enhancement in nanotube suspensions, Applied Physics Letters, 2001, 79(14): p. 2252-2254. [15] Marquis F., and L. Chibante, Improving the heat transfer of nanofluids and nanolubricants with carbon nanotubes, JOM, 2005, p. 32-43. [16] Xua Y. and Q. Li, Heat transfer enhancement of nanofluids. International Journal of Heat and Fluid Flow, 2000, 21(1): p. 58-64. [17] Liu M., M.C. Lin, and C. Wang, Enhancements of thermal conductivities with Cu, CuO, and carbon nanotube nanofluids and application of MWNT/water nanofluid on a water chiller system. Nanoscale Research Letters, 2011, 6(1): p. 1-13. [18] Choi S., Z. G. Zhang, W. Yu, F. E. Lockwood and E. A. Grulke, Anomalous thermal conductivity enhancement in nanotube suspensions. Applied physics letters, 2001, 79(14): p. 2252-2254. [19] Assael M., C. Chen, I. Metaxa, W.Wakeham, Thermal conductivity of suspensions of carbon nanotubes in water. International Journal of Thermophysics, 2004. 25(4): p. 971-985. [20]Assael M., I. Metaxa, J. Arvanitidis, D. Christofilos, C. Lioutas, Thermal conductivity enhancement in aqueous suspensions of carbon multi-walled and double-walled nanotubes in the presence of two different dispersants. International Journal of Thermophysics, 2005, 26(3): p. 647-664. [21] Liu M. S., Mark C. L., I-Te H., C. Wang, Enhancement of thermal conductivity with carbon nanotube for nanofluids. International Communications in Heat and Mass Transfer, 2005, 32(9): p. 1202-1210. [22] Ko G.H., K. Heo, K. Lee, D. Kim, C. Kim, Y. Sohn, M. Choi, An experimental study on the pressure drop of nanofluids containing carbon nanotubes in a horizontal tube. International Journal of Heat and Mass Transfer, 2007, 50(23): p. 4749-4753.
[23] Anoop K., T. Sundararajan, and S. K. Das, Effect of particle size on the convective heat transfer in nanofluid in the developing region. International Journal of Heat and Mass Transfer, 2009, 52(9): p. 2189-2195. [24] Mohammed H., G. Bhaskaran, N.H. Shuaib, H.I. Abu-Mulaweh, Influence of nanofluids on parallel flow square microchannel heat exchanger performance. International Communications in Heat and Mass Transfer, 2011, 38(1): p. 1-9. [25] Liu, M., M.C. Lin, and C. Wang, Enhancements of thermal conductivities with Cu, CuO, and carbon nanotube nanofluids and application of MWNT/water nanofluid on a water chiller system. Nanoscale research letters, 2011. 6(1): p. 1-13. [26] Al-Shemmeri T., Engineering Fluid Mechnanics. Ventus Publishing ApS. 2012. 17–18. ISBN 978-87-403-0114-4 [27] Jang S.P., S. Choi, Role of Brownian motion in the enhanced thermal conductivity of nanofluids, Applied Physics Lete., 2004, 84, p. 4316-4318. [28] Wei W., A Comprehensive Model for the Enhanced Thermal Conductivity of Nanofluids. Journal of Advanced Research in Physics , 2013, 3.2. [29] Pradhan N. R., H. Duan, J. Liang, and G. S. Iannacchione, Specific heat and thermal conductivity measurements for anisotropic and random macroscopic composites of cobalt nanowires, Nanotechnology, 2008, 19. [30] Jang S. P. and S. U. Choi, Cooling performance of a microchannel heat sink with nanofluids, Applied Thermal Engineering, 2006, 26: 17-18, p. 2457–2463,. [31] Donghyun S., and B., Debjyoti , Enhancement of specific heat capacity of high-temperature silica-nanofluids synthesized in alkali chloride salt eutectics for solar thermal-energy storage applications, International Journal of Heat and Mass Transfer , 2011, 54, p.1064–1070 [32] Kim K. M., Comparison of thermal performances of water-filled, SiC nanofluid-filled and SiC nanoparticles-coated heat pipes. International Journal of Heat and Mass Transfer, 2015, 88, p. 862-871. [33] Wang B. X., L.P. Zhou, X.F. Peng, Surface and size effects on the specific heat capacity of nanoparticles, Int. J. Thermophys, 2006, 27, p.139–151. [34] Wang L., Z. Tan, S. Meng, D. Liang, G. Li, Enhancement of molar heat capacity of nanostructured Al2 O3, J. Nanoparticle Res, 2004, 3 p. 483–487. [35] Haaland S. E. Simple and explicit formulas for the friction factor in turbulent pipe flow. Journal of Fluids Engineering , 1983, 105:1, p.89-90. [36] Ko, Gwon Hyun, et al. "An experimental study on the pressure drop of nanofluids containing carbon nanotubes in a horizontal tube." International journal of heat and mass transfer 50.23 (2007): 4749-4753.
50 ml/hr
Gas Flowmeters
Syringe Pump 1500 ML/MIN
200 ML/MIN
Two Tube Furnaces CNTs Collector
Figure 1: Schematic Diagram of CVD Reactor
Argon Gas
Hydrogen Gas
Gas outlet
Figure 2. Schematic diagram of the experimental Shell and tube heat exchanger used to measure the thermal conductivity, inlet and outlet temperature of the two fluids and the tube side pressure drop.
Figure 3. FE-SEM images of a) CNTs and CNT doped with iron oxide nanoparticles at a loading of b) 1 wt.% and c) 10 wt.%.
1 µm
100
Derivative weight, % / ° C
80
0% Fe2O3
% Weight
60
40 1% Fe2O3 9.0% Fe2O3 20
0 100
200
300
400 Temperature, °C
500
600
700
Figure 4. Thermogravimetric analysis of pure and impregnated CNT shown the residual mass (solid lines) and mass loss derivatives (dashed lines).
1.25
, cP
1.00
0.75
0.50
0.25 295
310 325 Temperature, K
340
295
310 325 Temperature, K
340
1.5
1.4
rel
1.3
1.2
1.1
1.0
Figure 5. Effect of temperature on the viscosity (h) (a) and relative viscosity (hrel) (b) of water and nanofluids containing different concentrations of the CNT. The solid lines in (a) is the fit using equation (1).
0.75
K, W/m.K
0.70
0.65
0.60 0
0.02
0.04 0.06 0.08 Concentration, wt.%
0.1
0.12
Figure 6. Thermal conductivity of nanofluids containing undoped and doped CNTs.
7.5
1% Fe2O3
0% Fe2O3
7.0 Cp, J/g.°C
6.5 6.0 5.5 5.0
45 35 25
4.5
45 35 25
4.0 0
0.05 0.1 Nanoparticle conc., wt.%
10% Fe2O3
0.00 0.05 0.10 Nanoparticle conc., wt.%
45
35 25
0
0.05 0.1 Nanoparticle conc., wt.%
Figure 7. Enhancement of heat capacity of (a) CNT (b) 1 wt.% Fe 2O3-CNT (c) 10 wt.% Fe2O3-CNT nanofluids with respect to temperature ( 25, 35 and 45 oC).
250
0% Fe2O3
200
P, mbar
0% 0.01%
150
0.05% 0.10%
100
50
0 0
100
200 300 400 500 Flow Rate, kg/h
600
700
250
1% Fe2O3 200
0% 0.01% 0.05%
P, mbar
150
0.10%
100
50
0 0
100
200 300 400 500 Flow Rate, kg/h
600
700
250
10% Fe2O3 200
0%
P, mbar
0.01% 150 0.05% 0.10%
100
50
0 0
100
200 300 400 500 Flow rate, kg/h
600
700
Figure 8. Variation in the pressure drop with respect to the mass flowrate for (a) undopded CNT nanofluids, (b) 1% Fe2O3-CNT nanofluids and (c) 10% Fe2O3-CNT nanofluids in a steel tube.
250
0% Fe2O3
P, mbar
200 0%
150
0.01% 0.05%
100
0.10%
50
0 0
8000
16000 Re
24000
32000
250
1% Fe2O3
P, mbar
200
150
100
50
0 0
8000
16000 Re
24000
32000
250
10% Fe2O3
P, mbar
200 150 100 50 0 0
8000
16000 Re
24000
32000
Figure 9. Variation in the pressure drop with respect to Reynolds Number for (a) undopded CNT nanofluids, (b) 1% Fe2O3-CNT nanofluids and (c) 10% Fe2O3-CNT nanofluids.
3.0
1% Fe2O3
0% Fe2O3 Heat Flow, kW
2.5 0.10% 0.05% 0.01% 0.00%
2.0
0.10% 0.05% 0.01% 0.00%
1.5
1.0 100 200 300 400 500 600 700 Flow Rate, kg/h
100 200 300 400 500 600 700 Flow rate, kg/h
10% Fe2O3 0.10%
0.05% 0.01% 0.00%
100 200 300 400 500 600 700 Flow rate, kg/h
Figure 10. Heat transfer rate of nanofluids containing different concentrations of undoped and doped CNTs as a function of flow rate.
1.2
1.3
0% Fe2O3
1% Fe2O3
0.10 %
1.2 Qnf/Qw
1.1 0.10 %
1.0
0.05 %
1.1 0.01 % 0.05 %
0.01 %
1.0
0.9
0.9 100
250 400 550 Flow rate, kg/h 1.6
700
100
250 400 550 Flow rate, kg/h
700
10% Fe2O3
1.5 0.10 %
1.4 0.05 %
1.3 1.2
0.01 %
1.1 1.0 0.9 100
250 400 550 Flow rate, kg/h
700
Figure 11. The enhanced heat transfer of (a) 1 wt.% Fe 2O3-CNT and (b) 10 wt.% Fe2O3CNT nanofluids as a function of flow rate.
a
b
Figure 12.Power required as a function of the heat generated for (a) CNTs (b) 1 wt.% Fe2O3-CNT and (c) 10 wt.% Fe 2O3-CNT.
c
Table 1. EDS analysis of CNTs and CNT- Fe 2O3
Element C O Fe Pt Ni Total %
Composition, wt.% 0 wt.% 1 wt.% 10 wt.% Fe2O3 Fe2O3 Fe2O3 85.42 80.01 98.50 3.64 4.52 1.50 - 1.44 10.03 9.50 5.09 - 0.35 100 100 100
Table 2: Constant values of Eq.2 at different concentration of nanofluids Constant a b c
Water
0.01 wt% CNTs
0.05 wt% CNTs
0.1 wt% CNTs
1.86E-03 881.8 -30.5
2.70E-02 510.1 -37.3
1.06E-02 770.6 -94.8
5.40E-02 278.8 86.6
Table 3. Viscosity and roughness ratio of nanofluids flowing through the stainless steel pipe Nanofluid conc, % 0 0.01 0.05 0.10
m 0.63 0.73 0.76 0.82
e/D 6.72 x10-4 7.50 x10-4 9.01 x10-4 9.34 x10-4
Table 4. Heat flow ratio
Heat Flow ratio, QNF/Qw
Conc, wt.%
0% Fe2O3
0.01
1.00 ±0.04 1.05 ±0.02 1.14 ±0.03
0.05
0.99 ±0.06 1.13 ±0.03 1.19 ±0.02
0.10
1.03 ±0.02 1.23 ±0.04 1.50 ±0.02
1% Fe2O3
10% Fe2O3
Research highlights The thermal performance of shell and tube heat exchanger is investigated. Water based nanofluids of CNTs doped with Fe2O3 nanoparticles at different weight loading and concentration are considered. The effects of volume concentration and Reynolds number on viscosity, heat capacity, thermal conductivity, enhancement of heat transfer, and pressure drop of nanofluids are evaluated. 0.1wt% concentration of CNTs doped with 10 wt% of Fe2O3 nanoparticles exhibits the best heat transfer performance.
The nanofluids at different flow regimes have minor effects on the pressure drop.