# Influence of Aluminum Silicate and Cerium (IV) Oxide Nanofluid on Pool Boiling Characteristics

Authors: Yoganand Kumaravelu; Vasanthkumar Periyathambi; Poongundran Udhayanan; Mohanraj Murugesan; R. Jothi Ramalingam; Hamad Al-Lohedan; Dhaifallah M. Al-dhayan; Ishwarya Komalnu Raghavan
Journal: International Journal of Photoenergy
 (2022)

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## Abstract

A study into pool boiling heat transfer with nanofluids particularly aluminum silicate and cerium (IV) oxide was used to prepare nanofluids. A review of existing nanofluid implementations done previously in multiple literature and research journals was taken into consideration while determining their effects as nanoparticles in necessary base fluids. The nanofluids were prepared with two-step method by dispersing Al2SiO5 and CeO2 nanopowders in water and were analyzed at base temperatures of 50-75°C and peak flux readings taken at saturation temperature. An inference between these and surface modifications due to settlement of nanoparticles on heater surface was studied by SEM imaging, and dispersion was studied with TEM imaging. The volume concentrations of Al2SiO5 and CeO2 nanofluids are varied from 0.1%≤φ≤0.3%. Readings taken at temperatures varied by 5°C between 50°C to 75°C and at 100°C. The improvement of q″ for Al2SiO5/H2O and CeO2/H2O nanofluids is about 120.5±0.6% in PHF over water as base fluids for 0.3% volume concentration solutions.


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## Body

## 1. Introduction

Boiling heat transfer has always been a powerful energy extradition technology which persuades the way in achieving milestones in the advancement of the futuristic technologies. The ameliorations in the field of nanotechnology have gone hand-in-hand with almost all the fields, and the enhancement by implementing it in the boiling heat transfer has exhibited enormous increase in the efficiency of the process and has played a role in the elevation of the reliability of the process by increasing various factors that are need of the hour. Our experiments had us spectate that the PHF has inclined to a steep level upon addition of minimal quantity of nanoparticles. Nanofluids were prepared with the aid of dispersion of the nanoparticles of the order 10-50 nm which gave the increase of density of fluid and also high specific surface-area that paves the way to higher heat transfer between surface and fluids. It also grants acclimation of properties such as thermal conductivity and wettability by varying the concentrations. Our experiments were conducted based on pool boiling, and the results were focused mainly on uplifting of the PHF using nanofluids as the base fluid [1]. All these property enrichments are noticed because the high surface area of the nanoparticle and their enhanced thermal properties. Pool boiling is mode of the boiling phenomenon in which fluid is stationary with respect to heating surface in the beginning; vapor is spawned at a superheated wall that is tiny compared to the dimensions of the pool of ostensibly dormant liquid in which it is intent. In our experiments as cited in several papers, we use Ni-Chrome (80-20% composition) wire as the heating element through a set of platinum wire which has minimal resistance and anticorrosive in nature, and electricity is supplied. The bubbles formed by the cause are captured, and the certain parameters were calculated on the nucleation site, and the corresponding density with the diameter of the bubble that is formed when different nanofluid is applied [2, 3]. The existence of a vermicular structure in CuO nanoparticles contributes to the particles’ wear resistance and improved lubrication [4].The literature demonstrated that a variety of parameters could affect the performance of pool boiling of nanofluids as represented by the pool boiling heat transfer coefficient including the amount of nanoparticle loading, the size of nanomaterials, the roughness of the heater, the thickness of the thermally insulating nanolayer, and the time of boiling. A number of parameters may directly affect the behavior of bubbles, including the density of nucleation sites, bubble diameter, bubble frequency, bubble waiting time, and growth time, considered as essential boiling parameters that might influence the boiling performance of pools.It has been determined that cerium oxide nanoparticles can be considered as an attractive nanomaterial based on the data from all published experimental studies. They are inexpensive, have excellent chemical and physical properties, are nontoxic compared to other metal oxide powders, and are available as a readily available material. As acclaimed in various papers that the alumina [5] and silica nanoparticles [6] tend to increase the peak heat flux of the heating element, we have proposed to use a complex aluminum silicate which has the properties, and in addition to that, cerium was also employed for exhibiting high electron conductivity and also has an increased thermal conductivity due to its valence of electrons.


## 2. Materials and Methods

### 2.1. Chemical and Reagents

Aluminum silicate (Al2SiO5, 99.9% purity) and cerium (IV) oxide (CeO2, 99.9% purity), which was purchased from Platonic nanotech, were used to prepare nanofluids with DI water used as base fluid. All chemicals were of analytical grade. The average sizes were found to be 50 nm and 40 nm for aluminum silicate and cerium (IV) oxide, respectively.


#### 2.1.1. Characterization of Material

The nanoparticles were studied with FTIR, XRD, HRSEM, and HRTEM to get better understanding of material properties and compositions. HRSEM was taken with F E I Quanta FEG 200, HRTEM was taken with JEOL Japan, JEM-2100 Plus, and XRD was done using BRUKER USA D8 Advance, Davinci, with nanoparticles in powder form. HRTEM imaging of nanoparticles was taken after drying then once dispersed in DI water as shown in Figures1(a) and 1(b).Figure 1
HRTEM image of dried nanoparticles after dispersion in water: (a) CeO2 nanoparticle and (b) Al2SiO5 nanoparticle.
(a)(b)


#### 2.1.2. Preparation of Nanofluids

Two methods are used for preparing nanofluids—one-step and two-step methods. Many researchers have reported that two-step method is most economical way to synthesize nanofluids. The advantage of two-step approach is it is cost-effective and able to produce large volumes of nanofluids. In this work, two-step method is adopted for preparing aluminum silicate (Al2SiO5) and cerium oxide (CeO2) nanofluids. In this process, nanoparticles are prepared and dispersed in the base fluid (distilled water) with or without the help of surfactants by mechanical stirring or ultrasonication methods. The two-step method is widely chosen because the nanoparticles have high surface area and surface energy interactions which tend to aggregate the nanoparticles which are available in dried form.The following steps are involved in preparation of Al2SiO5 and CeO2 nanofluids:Determination of volume concentration (φ) for adding to base fluid was done by using the following equation [7]:
(1)φ=Wn/ρnWn/ρn+Wbf/ρbf∗100.The above equation can be simplified as(2)Wn=φ100−φρnρbfWbf.By using the above formula, the quantities of nanoparticles to be added are determined as follows in Table1.
(i)
The measured nanoparticle is added little by little into the base fluid and mechanically stirred for 2 hours. Despite of having extremely small sizes and relatively high kinetic energies resulting from Brownian motion, they do not remain in suspension. With time, nanoparticles settle out of solution under the influence of gravity [8](ii)
Hence, after stirring for 2 hours, the nanofluid is subjected to ultrasonication process for 4 hours to ensure uniform dispersion of nanoparticles as well as to prevent the nanoparticles from agglomerating in the base fluid. Many studies show that increase in sonication and stirring time reduces particle clustering(iii)
Nanofluids are considered stable only if no sedimentation takes place over time. Various sedimentation techniques are used to determine long-term stability of wide variety of nanofluids, and we considered simple 24-hour settlement under natural conditions for this studyTable 1
Volume concentration values with respective weights.
Vol. Conc.,φ (%)MaterialDensity,ρn (g/cc)Weight,Wn (g/L)Molecular weight (g/mol)0.1Al2SiO51.50.15162.050.2Al2SiO51.50.3162.050.3Al2SiO51.50.465162.050.1CeO26.50.656172.1150.2CeO26.51.325172.1150.3CeO26.52.01172.115


## 2.1. Chemical and Reagents

Aluminum silicate (Al2SiO5, 99.9% purity) and cerium (IV) oxide (CeO2, 99.9% purity), which was purchased from Platonic nanotech, were used to prepare nanofluids with DI water used as base fluid. All chemicals were of analytical grade. The average sizes were found to be 50 nm and 40 nm for aluminum silicate and cerium (IV) oxide, respectively.


### 2.1.1. Characterization of Material

The nanoparticles were studied with FTIR, XRD, HRSEM, and HRTEM to get better understanding of material properties and compositions. HRSEM was taken with F E I Quanta FEG 200, HRTEM was taken with JEOL Japan, JEM-2100 Plus, and XRD was done using BRUKER USA D8 Advance, Davinci, with nanoparticles in powder form. HRTEM imaging of nanoparticles was taken after drying then once dispersed in DI water as shown in Figures1(a) and 1(b).Figure 1
HRTEM image of dried nanoparticles after dispersion in water: (a) CeO2 nanoparticle and (b) Al2SiO5 nanoparticle.
(a)(b)


### 2.1.2. Preparation of Nanofluids

Two methods are used for preparing nanofluids—one-step and two-step methods. Many researchers have reported that two-step method is most economical way to synthesize nanofluids. The advantage of two-step approach is it is cost-effective and able to produce large volumes of nanofluids. In this work, two-step method is adopted for preparing aluminum silicate (Al2SiO5) and cerium oxide (CeO2) nanofluids. In this process, nanoparticles are prepared and dispersed in the base fluid (distilled water) with or without the help of surfactants by mechanical stirring or ultrasonication methods. The two-step method is widely chosen because the nanoparticles have high surface area and surface energy interactions which tend to aggregate the nanoparticles which are available in dried form.The following steps are involved in preparation of Al2SiO5 and CeO2 nanofluids:Determination of volume concentration (φ) for adding to base fluid was done by using the following equation [7]:
(1)φ=Wn/ρnWn/ρn+Wbf/ρbf∗100.The above equation can be simplified as(2)Wn=φ100−φρnρbfWbf.By using the above formula, the quantities of nanoparticles to be added are determined as follows in Table1.
(i)
The measured nanoparticle is added little by little into the base fluid and mechanically stirred for 2 hours. Despite of having extremely small sizes and relatively high kinetic energies resulting from Brownian motion, they do not remain in suspension. With time, nanoparticles settle out of solution under the influence of gravity [8](ii)
Hence, after stirring for 2 hours, the nanofluid is subjected to ultrasonication process for 4 hours to ensure uniform dispersion of nanoparticles as well as to prevent the nanoparticles from agglomerating in the base fluid. Many studies show that increase in sonication and stirring time reduces particle clustering(iii)
Nanofluids are considered stable only if no sedimentation takes place over time. Various sedimentation techniques are used to determine long-term stability of wide variety of nanofluids, and we considered simple 24-hour settlement under natural conditions for this studyTable 1
Volume concentration values with respective weights.
Vol. Conc.,φ (%)MaterialDensity,ρn (g/cc)Weight,Wn (g/L)Molecular weight (g/mol)0.1Al2SiO51.50.15162.050.2Al2SiO51.50.3162.050.3Al2SiO51.50.465162.050.1CeO26.50.656172.1150.2CeO26.51.325172.1150.3CeO26.52.01172.115


## 2.1.1. Characterization of Material

The nanoparticles were studied with FTIR, XRD, HRSEM, and HRTEM to get better understanding of material properties and compositions. HRSEM was taken with F E I Quanta FEG 200, HRTEM was taken with JEOL Japan, JEM-2100 Plus, and XRD was done using BRUKER USA D8 Advance, Davinci, with nanoparticles in powder form. HRTEM imaging of nanoparticles was taken after drying then once dispersed in DI water as shown in Figures1(a) and 1(b).Figure 1
HRTEM image of dried nanoparticles after dispersion in water: (a) CeO2 nanoparticle and (b) Al2SiO5 nanoparticle.
(a)(b)


## 2.1.2. Preparation of Nanofluids

Two methods are used for preparing nanofluids—one-step and two-step methods. Many researchers have reported that two-step method is most economical way to synthesize nanofluids. The advantage of two-step approach is it is cost-effective and able to produce large volumes of nanofluids. In this work, two-step method is adopted for preparing aluminum silicate (Al2SiO5) and cerium oxide (CeO2) nanofluids. In this process, nanoparticles are prepared and dispersed in the base fluid (distilled water) with or without the help of surfactants by mechanical stirring or ultrasonication methods. The two-step method is widely chosen because the nanoparticles have high surface area and surface energy interactions which tend to aggregate the nanoparticles which are available in dried form.The following steps are involved in preparation of Al2SiO5 and CeO2 nanofluids:Determination of volume concentration (φ) for adding to base fluid was done by using the following equation [7]:
(1)φ=Wn/ρnWn/ρn+Wbf/ρbf∗100.The above equation can be simplified as(2)Wn=φ100−φρnρbfWbf.By using the above formula, the quantities of nanoparticles to be added are determined as follows in Table1.
(i)
The measured nanoparticle is added little by little into the base fluid and mechanically stirred for 2 hours. Despite of having extremely small sizes and relatively high kinetic energies resulting from Brownian motion, they do not remain in suspension. With time, nanoparticles settle out of solution under the influence of gravity [8](ii)
Hence, after stirring for 2 hours, the nanofluid is subjected to ultrasonication process for 4 hours to ensure uniform dispersion of nanoparticles as well as to prevent the nanoparticles from agglomerating in the base fluid. Many studies show that increase in sonication and stirring time reduces particle clustering(iii)
Nanofluids are considered stable only if no sedimentation takes place over time. Various sedimentation techniques are used to determine long-term stability of wide variety of nanofluids, and we considered simple 24-hour settlement under natural conditions for this studyTable 1
Volume concentration values with respective weights.
Vol. Conc.,φ (%)MaterialDensity,ρn (g/cc)Weight,Wn (g/L)Molecular weight (g/mol)0.1Al2SiO51.50.15162.050.2Al2SiO51.50.3162.050.3Al2SiO51.50.465162.050.1CeO26.50.656172.1150.2CeO26.51.325172.1150.3CeO26.52.01172.115


## 3. Experimental Details and Data Analysis

### 3.1. Experimental Setup

The setup as shown in Figure2 consists of a 230 V AC power supply, hot plate, MI type K thermocouple (4 Nos.), temperature indicators (4 Nos.), jacketed condenser, and Ni-Chrome wire of diameter 40 SWG and 60 mm length which serves as the heater element. To demonstrate this concept, a custom-made glass beaker is made for experimenting peak heat flux enhancement in nanofluid. Through the lid of the beaker, voltage sensing wires are connected to two platinum wires which are suspended at ends of the beaker in which the Ni-Chrome wire is connected and current is passed through platinum wire to avoid contact resistance with test specimen. Hot plate is used to heat up to the saturation temperature Tsat (100°C for water) of fluid, at 1 atm, while bulk temperature was being monitored with thermocouples. Similarly, the four thermocouples are equipped in beaker through the lid and all are placed at equal distance to determine the average wire temperature. Since thermocouples are placed too close to the surface, along with the minimal contact resistance of the tiny gap between the thermocouple tip and the wire surface, it is assumed that the measured temperature represents the boiling surface temperature.Figure 2
Experimental setup.


### 3.2. Experimental Procedure

Initially, the experiment is conducted in distilled water. The distilled water is heated on hot plate till its saturation temperatureTsat is 100°C, and temperature was being monitored by thermocouple. Current is applied to Ni-Chrome wire through platinum wires by regulating the variac. Voltage and ammeter readings are noted during burnout [9] of Ni-Chrome wire for heat flux measurement and for comparing with various nanofluids. Similarly, the nanofluid is heated on a hot plate to saturation temperature Tsat (100°C) of base fluid (distilled water) at atmospheric pressure 1 atm, while bulk temperature is monitored by thermocouples. Current is applied to Ni-Chrome wire that is regulated by variac for constant heat flux control, with increments of 0.5 A. The voltage measurements are noted at steady rate just after the current increase, and intervals are given after each increment to changes in system. Heat flux at any point is calculated by the following equation:
(3)q"=VIπDL.


## 3.1. Experimental Setup

The setup as shown in Figure2 consists of a 230 V AC power supply, hot plate, MI type K thermocouple (4 Nos.), temperature indicators (4 Nos.), jacketed condenser, and Ni-Chrome wire of diameter 40 SWG and 60 mm length which serves as the heater element. To demonstrate this concept, a custom-made glass beaker is made for experimenting peak heat flux enhancement in nanofluid. Through the lid of the beaker, voltage sensing wires are connected to two platinum wires which are suspended at ends of the beaker in which the Ni-Chrome wire is connected and current is passed through platinum wire to avoid contact resistance with test specimen. Hot plate is used to heat up to the saturation temperature Tsat (100°C for water) of fluid, at 1 atm, while bulk temperature was being monitored with thermocouples. Similarly, the four thermocouples are equipped in beaker through the lid and all are placed at equal distance to determine the average wire temperature. Since thermocouples are placed too close to the surface, along with the minimal contact resistance of the tiny gap between the thermocouple tip and the wire surface, it is assumed that the measured temperature represents the boiling surface temperature.Figure 2
Experimental setup.


## 3.2. Experimental Procedure

Initially, the experiment is conducted in distilled water. The distilled water is heated on hot plate till its saturation temperatureTsat is 100°C, and temperature was being monitored by thermocouple. Current is applied to Ni-Chrome wire through platinum wires by regulating the variac. Voltage and ammeter readings are noted during burnout [9] of Ni-Chrome wire for heat flux measurement and for comparing with various nanofluids. Similarly, the nanofluid is heated on a hot plate to saturation temperature Tsat (100°C) of base fluid (distilled water) at atmospheric pressure 1 atm, while bulk temperature is monitored by thermocouples. Current is applied to Ni-Chrome wire that is regulated by variac for constant heat flux control, with increments of 0.5 A. The voltage measurements are noted at steady rate just after the current increase, and intervals are given after each increment to changes in system. Heat flux at any point is calculated by the following equation:
(3)q"=VIπDL.


## 4. Results and Discussion

Figure3(a) indicates presence of OH functional groups which might be due to the presence of moisture in the sample in the range around 3500 cm-1. The peak at 1100 cm-1 indicates the Si-O-Si antisymmetry stretching. Figure 3(b) indicates no peaks which evidently shows the absence of organic compounds in the CeO2 nanoparticle.Figure 3
FTIR spectroscopy: (a) Al2SiO5 nanoparticle and (b) CeO2 nanoparticle.
(a)(b)


### 4.1. XRD Analysis of CeO2

The crystal phase and the lattice constant of the nanocrystals were evaluated using XRD studies. Figures4 and 5 show the XRD spectrum of the KBr-mediated synthesis of CeO2 and Al2SiO5. The patterns in the figure CeO2, comprising peaks at the 2 theta values of 28.46, 32.96, 47.39, 56.23, 59.03, 69.27, 76.71, and 79.04, respectively, correspond to the (1 1 1), (2 0 0), (2 2 0), (3 1 1), (2 2 2), (4 0 0), (3 3 1), and (4 2 0) planes, as depicted in Figure 4. All the diffraction patterns of CeO2 harmonized with the JCPDS data card No. 898436, and this shows the cubic crystal structure of CeO2 with a space group of Fm3m (225). In XRD patterns for Al2SiO5, the presence of moisture content of the sample showed a slight level of impurities in the prepared sample. No impurity peaks were observed indicating the high purity of the prepared samples. The existence of CeO2 was further identified by SEM and EDAX studies.Figure 4
XRD of CeO2 nanoparticle (JCPDS 898436).Figure 5
XRD of Al2SiO5 nanoparticle.


### 4.2. SEM and EDX Analysis

Scanning electron microscopy (SEM) is a unique technology to characterize the morphology, defects, and structure of materials. The typical SEM images in Figure6 show the morphology of pure Ni-Chrome wire, Figures 7(a), 8(a), and 9(a) show 0.1%-0.3% of Al2SiO5-doped Ni-Chrome, and Figures 7(b), 8(b), and 9(b) show 0.1%-0.3% of CeO2-doped Ni-Chrome, respectively. It can be inferred from the SEM image in Figure 6 that pure Ni-Chrome wire has a smooth surface morphology as obtained while Figure 7(a) shows the SEM images of 0.1% of Al2SiO5-doped Ni-Chrome; Al2SiO5 deposits are found on the Ni-Chrome wire, and with increasing concentration, there is a significant improvement observed. Figure 10 shows the elemental analysis of Ni-Chrome doped with Al2SiO5, in these compositions of Ni, Cr, Si, Al, and O that were present.Figure 6
SEM image of Ni-Chrome wire (uncoated).Figure 7
Surface morphology of heater surface: (a) 0.1% volume concentration Al2SiO5 and (b) 0.1% volume concentration CeO2.
(a)(b)Figure 8
Surface morphology of heater surface: (a) 0.2% volume concentration Al2SiO5 and (b) 0.2% volume concentration CeO2.
(a)(b)Figure 9
Surface morphology of heater surface: (a) 0.3% volume concentration Al2SiO5 and (b) 0.3% volume concentration CeO2.
(a)(b)Figure 10
EDS for 0.3% volume concentration Al2SiO5.SEM imaging of Ni-Chrome wire-untreated (uncoated) was taken initially to study surface morphology and provide for a base to compare with wires used in experimentation. Figure6 showcases a plain Ni-Chrome wire and can be seen to have no coatings and a simple plain structured morphology.As noticed in Figures7–9, a certain layer of coating tends to form on the wire during experimentation which clearly enhances the heat flux of wire as the excess heat dissipation due to this nanocoating [10] is enhanced when compared to uncoated wire and also becomes more efficient with increase in volume concentration of nanofluids; this is also evident in the respective EDS data as shown in Figure 10, which generated that with increase in volume concentration, the amount deposited onto the wire also increases.The aluminum silicate and cerium oxide nanofluids were tested with the base-fluid water, and enhancements were observed that varied with change in volume fraction. The varying bubble diameter at increasing heat flux is portrayed in Figures11 and 12 which show that the bubble diameter increases as the heat flux applied to the wire increases.Figure 11
Bubble diameter variation with increase in heat flux for water.Figure 12
Bubble diameter variation with increase in heat flux for Al2SiO5.It is observed that starting at lower base temperature, a higher value for peak heat flux can be achieved, as illustrated by the graph above. Theoretically, bubble diameter was calculated using Haramura relation.Bubble departure diameter,Db(4)σgρl−ρg=2.77mmforwater.Experimentally, bubble diameter was calculated visually by using images captured during the experimentation process and using the Octave GNU software to attain exact values as shown in Figure13.Figure 13
Bubble departure diameter for Al2SiO5 (0.3% vol) measured in Octave GNU.Macrolayer thickness,δ, was calculated by the following formula, δt=δo2−2λΔT/ρlhfgt.From the above formula, values where calculated using Octave GNU and it was inferred that the macrolayer thickness value decreases from the initial layer thickness as suggested by other literatures [11, 12], while an increase in initial macrolayer thickness is observed with increase in base temperatures for fluids. Graph in Figure 14 shows the values of macrolayer thickness varying with increase in wall superheat of fluid.Figure 14
Heat flux variation with microlayer thickness.Use preestablished models where comparative study was done to determine variations with these and our experimental model [13]. Table 2 shows clear comparison of existing models for pool boiling.Table 2
Heat flux comparison to acclaimed studies.
ModelFormValueZuber modelq"max=0.131hlvρvσρl−ρvgρv21/41.138×103 W/m2Rohsenow and Griffith bubble interface modelq"max=0.012ρghfgρf−ρgρg0.61.353×103 W/m2Haramura and Katto modelq"max=ρfhfgδ1−0.0584ρgρf1/5f1.286×103 W/m2This studyExperimental1.781×103 W/m2Hence, from experimental data as shown in Figures15 and 16, it is observed that both Al2SiO5 and CeO2 show enhancements of about 120.5±0.6% in PHF over water as base fluids for 0.3% volume concentration solutions. The reason for increase in PHF is also contributed by the macrolayer increase at higher temperatures [14]. Nucleation sites were determined using images captured from video taken during the experimentation process and were then fed into Octave GNU with algorithm created by author to determine number of sites as showcased in Figures 17(a) and 17(b).Figure 15
Heat flux comparison.Figure 16
Heat flux variations based on base temperature.Figure 17
Nucleation sites on heater surface: (a) water and (b) water at higher PHF.
(a)(b)Boiling has developed a great interest during the past few decades, as a method to increase the heat transfer rates with modest temperature differences. At high heat fluxes, heat transfer [15] by boiling is highly effective. Hence, find applications such coolants for nuclear reactors and also rocket engines where heat transfer rates are in high values (106 to 107 W/m2). The probability to attain higher thermal energy conversion efficiency through boiling working fluid with heated surface is possible by these methodologies and has a large area of applications.These coating of nanofluids over surface of wire or heater/heating surfaces tend to influence resistance which could be directly tracked to thermal conductance by inverse proportionality as shown in Figure18 that when the coating increases, a decrease in resistance offered by wire is observed, and the values of resistance are for 60 mm of wire lengths as used in experimentation process for testing and generating direct relations. Nanofluids having high specific surface area serve to be better than other commercial fluids for heat transfer applications carrying more torridity between particles being the most important feature of nanofluids in heat carrying applications.Figure 18
Influence of coating over Ni-Chrome wire on resistance of wire.Using metallic fluids in heat carrying applications denotes significant difference when compared to commercial fluids for the same application. The hitch here being clogging of micrometer-sized particles in small passages and nanofluids being nanometer-sized can overcome this obstacle. Adding as low as 1-6 vol% of nanoparticle in a base fluid can enhance the heat carrying ability to a significant change of 20%. Usage of nanomaterials like Cu, Al, Si, Zn, Mg, and Se is advised on this prospect, and so a combination of aluminum and silicate forming aluminum silicate (Al2SiO5) and cerium oxide is tested upon with 0.1%, 0.2%, and 0.3% of both the nanofluids. The stability of oxide-based nanofluids is higher, and therefore, both are chosen to be oxide-based nanofluids. Table 3 clearly depicts the improvement in peak heat flux (PHF) in comparison with other reported results.Table 3
Comparison with earlier literatures.
LiteratureNanofluidsPHFKarima et al. [16], Liu et al. [17]CuO in H2OEnhanced by 25–50%Shi et al. [8]Al2O3 & Fe in H2OEnhanced to ≈60%Wen et al. [5]γ-Al2O3 in H2OEnhanced to ≈40%Milanova et al. [18]Al2O3, SiO2, & CeO2 in H2ODeterioratedVassallo et al. [19]SiO2 in H2OSlight changeThis studyAl2SiO5, CeO2 in H2OEnhanced by ≈20%Alumina particles have the tendency to decrease the pool boiling heat transfer but it has improved the CHF by 20% [20]. The CHF enhancement is due to the surface modifications that happen in the wire surface due to deposition of nanoparticles [21, 22]. A main cost attributing factor for nanofluids is the use of stabilizers which increases the production cost of these nanofluids without which nanoparticles tent to settle over a period of time. So cheaper stabilizers or nanofluids which are more substantial without the use of stabilizers need to be invented. The stabilizers that are commercially used in nanofluids are proven to be unstable at high temperatures being a drawback for stability in heat carrying applications. Most nanofluids upon testing results have lower specific heat at their respective temperatures when compared to its base fluids. On production of nanofluids, the particles tend to agglomerate into larger particles resulting in limiting the main asset of nanoparticles’ high surface area.


## 4.1. XRD Analysis of CeO2

The crystal phase and the lattice constant of the nanocrystals were evaluated using XRD studies. Figures4 and 5 show the XRD spectrum of the KBr-mediated synthesis of CeO2 and Al2SiO5. The patterns in the figure CeO2, comprising peaks at the 2 theta values of 28.46, 32.96, 47.39, 56.23, 59.03, 69.27, 76.71, and 79.04, respectively, correspond to the (1 1 1), (2 0 0), (2 2 0), (3 1 1), (2 2 2), (4 0 0), (3 3 1), and (4 2 0) planes, as depicted in Figure 4. All the diffraction patterns of CeO2 harmonized with the JCPDS data card No. 898436, and this shows the cubic crystal structure of CeO2 with a space group of Fm3m (225). In XRD patterns for Al2SiO5, the presence of moisture content of the sample showed a slight level of impurities in the prepared sample. No impurity peaks were observed indicating the high purity of the prepared samples. The existence of CeO2 was further identified by SEM and EDAX studies.Figure 4
XRD of CeO2 nanoparticle (JCPDS 898436).Figure 5
XRD of Al2SiO5 nanoparticle.


## 4.2. SEM and EDX Analysis

Scanning electron microscopy (SEM) is a unique technology to characterize the morphology, defects, and structure of materials. The typical SEM images in Figure6 show the morphology of pure Ni-Chrome wire, Figures 7(a), 8(a), and 9(a) show 0.1%-0.3% of Al2SiO5-doped Ni-Chrome, and Figures 7(b), 8(b), and 9(b) show 0.1%-0.3% of CeO2-doped Ni-Chrome, respectively. It can be inferred from the SEM image in Figure 6 that pure Ni-Chrome wire has a smooth surface morphology as obtained while Figure 7(a) shows the SEM images of 0.1% of Al2SiO5-doped Ni-Chrome; Al2SiO5 deposits are found on the Ni-Chrome wire, and with increasing concentration, there is a significant improvement observed. Figure 10 shows the elemental analysis of Ni-Chrome doped with Al2SiO5, in these compositions of Ni, Cr, Si, Al, and O that were present.Figure 6
SEM image of Ni-Chrome wire (uncoated).Figure 7
Surface morphology of heater surface: (a) 0.1% volume concentration Al2SiO5 and (b) 0.1% volume concentration CeO2.
(a)(b)Figure 8
Surface morphology of heater surface: (a) 0.2% volume concentration Al2SiO5 and (b) 0.2% volume concentration CeO2.
(a)(b)Figure 9
Surface morphology of heater surface: (a) 0.3% volume concentration Al2SiO5 and (b) 0.3% volume concentration CeO2.
(a)(b)Figure 10
EDS for 0.3% volume concentration Al2SiO5.SEM imaging of Ni-Chrome wire-untreated (uncoated) was taken initially to study surface morphology and provide for a base to compare with wires used in experimentation. Figure6 showcases a plain Ni-Chrome wire and can be seen to have no coatings and a simple plain structured morphology.As noticed in Figures7–9, a certain layer of coating tends to form on the wire during experimentation which clearly enhances the heat flux of wire as the excess heat dissipation due to this nanocoating [10] is enhanced when compared to uncoated wire and also becomes more efficient with increase in volume concentration of nanofluids; this is also evident in the respective EDS data as shown in Figure 10, which generated that with increase in volume concentration, the amount deposited onto the wire also increases.The aluminum silicate and cerium oxide nanofluids were tested with the base-fluid water, and enhancements were observed that varied with change in volume fraction. The varying bubble diameter at increasing heat flux is portrayed in Figures11 and 12 which show that the bubble diameter increases as the heat flux applied to the wire increases.Figure 11
Bubble diameter variation with increase in heat flux for water.Figure 12
Bubble diameter variation with increase in heat flux for Al2SiO5.It is observed that starting at lower base temperature, a higher value for peak heat flux can be achieved, as illustrated by the graph above. Theoretically, bubble diameter was calculated using Haramura relation.Bubble departure diameter,Db(4)σgρl−ρg=2.77mmforwater.Experimentally, bubble diameter was calculated visually by using images captured during the experimentation process and using the Octave GNU software to attain exact values as shown in Figure13.Figure 13
Bubble departure diameter for Al2SiO5 (0.3% vol) measured in Octave GNU.Macrolayer thickness,δ, was calculated by the following formula, δt=δo2−2λΔT/ρlhfgt.From the above formula, values where calculated using Octave GNU and it was inferred that the macrolayer thickness value decreases from the initial layer thickness as suggested by other literatures [11, 12], while an increase in initial macrolayer thickness is observed with increase in base temperatures for fluids. Graph in Figure 14 shows the values of macrolayer thickness varying with increase in wall superheat of fluid.Figure 14
Heat flux variation with microlayer thickness.Use preestablished models where comparative study was done to determine variations with these and our experimental model [13]. Table 2 shows clear comparison of existing models for pool boiling.Table 2
Heat flux comparison to acclaimed studies.
ModelFormValueZuber modelq"max=0.131hlvρvσρl−ρvgρv21/41.138×103 W/m2Rohsenow and Griffith bubble interface modelq"max=0.012ρghfgρf−ρgρg0.61.353×103 W/m2Haramura and Katto modelq"max=ρfhfgδ1−0.0584ρgρf1/5f1.286×103 W/m2This studyExperimental1.781×103 W/m2Hence, from experimental data as shown in Figures15 and 16, it is observed that both Al2SiO5 and CeO2 show enhancements of about 120.5±0.6% in PHF over water as base fluids for 0.3% volume concentration solutions. The reason for increase in PHF is also contributed by the macrolayer increase at higher temperatures [14]. Nucleation sites were determined using images captured from video taken during the experimentation process and were then fed into Octave GNU with algorithm created by author to determine number of sites as showcased in Figures 17(a) and 17(b).Figure 15
Heat flux comparison.Figure 16
Heat flux variations based on base temperature.Figure 17
Nucleation sites on heater surface: (a) water and (b) water at higher PHF.
(a)(b)Boiling has developed a great interest during the past few decades, as a method to increase the heat transfer rates with modest temperature differences. At high heat fluxes, heat transfer [15] by boiling is highly effective. Hence, find applications such coolants for nuclear reactors and also rocket engines where heat transfer rates are in high values (106 to 107 W/m2). The probability to attain higher thermal energy conversion efficiency through boiling working fluid with heated surface is possible by these methodologies and has a large area of applications.These coating of nanofluids over surface of wire or heater/heating surfaces tend to influence resistance which could be directly tracked to thermal conductance by inverse proportionality as shown in Figure18 that when the coating increases, a decrease in resistance offered by wire is observed, and the values of resistance are for 60 mm of wire lengths as used in experimentation process for testing and generating direct relations. Nanofluids having high specific surface area serve to be better than other commercial fluids for heat transfer applications carrying more torridity between particles being the most important feature of nanofluids in heat carrying applications.Figure 18
Influence of coating over Ni-Chrome wire on resistance of wire.Using metallic fluids in heat carrying applications denotes significant difference when compared to commercial fluids for the same application. The hitch here being clogging of micrometer-sized particles in small passages and nanofluids being nanometer-sized can overcome this obstacle. Adding as low as 1-6 vol% of nanoparticle in a base fluid can enhance the heat carrying ability to a significant change of 20%. Usage of nanomaterials like Cu, Al, Si, Zn, Mg, and Se is advised on this prospect, and so a combination of aluminum and silicate forming aluminum silicate (Al2SiO5) and cerium oxide is tested upon with 0.1%, 0.2%, and 0.3% of both the nanofluids. The stability of oxide-based nanofluids is higher, and therefore, both are chosen to be oxide-based nanofluids. Table 3 clearly depicts the improvement in peak heat flux (PHF) in comparison with other reported results.Table 3
Comparison with earlier literatures.
LiteratureNanofluidsPHFKarima et al. [16], Liu et al. [17]CuO in H2OEnhanced by 25–50%Shi et al. [8]Al2O3 & Fe in H2OEnhanced to ≈60%Wen et al. [5]γ-Al2O3 in H2OEnhanced to ≈40%Milanova et al. [18]Al2O3, SiO2, & CeO2 in H2ODeterioratedVassallo et al. [19]SiO2 in H2OSlight changeThis studyAl2SiO5, CeO2 in H2OEnhanced by ≈20%Alumina particles have the tendency to decrease the pool boiling heat transfer but it has improved the CHF by 20% [20]. The CHF enhancement is due to the surface modifications that happen in the wire surface due to deposition of nanoparticles [21, 22]. A main cost attributing factor for nanofluids is the use of stabilizers which increases the production cost of these nanofluids without which nanoparticles tent to settle over a period of time. So cheaper stabilizers or nanofluids which are more substantial without the use of stabilizers need to be invented. The stabilizers that are commercially used in nanofluids are proven to be unstable at high temperatures being a drawback for stability in heat carrying applications. Most nanofluids upon testing results have lower specific heat at their respective temperatures when compared to its base fluids. On production of nanofluids, the particles tend to agglomerate into larger particles resulting in limiting the main asset of nanoparticles’ high surface area.


## 5. Conclusion

In this study, the effects of nanoparticles in base-fluid water on heat transfer characteristics are conducted using a critical heat flux apparatus. The volume concentrations of Al2SiO5 and CeO2 nanofluids are varied from 0.1%≤φ≤0.3%. Readings taken at temperatures varied by 5°C between 50°C to 75°C and at 100°C. The improvement of q″ for Al2SiO5/H2O and CeO2/H2O nanofluids is about 1.781×103 W/m2, which displays a lower improvement in heat transfer for lower volume fraction; however, the significant enhancement of q″ is observed for higher volume fraction. From the results arrived, for enhancement of PHF and HTC nanofluids and nanosurface modifications showcase as dominant tools, modifications to working fluid and/or heater surface high thermal energy conversion efficiency can be achieved for boiling.



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