# Evaluation of Physicothermal Properties of Solar Thermic Fluids Dispersed with Multiwalled Carbon Nanotubes and Prediction of Data Using Artificial Neural Networks

Authors: K. Ch. Sekhar; Raviteja Surakasi; ilhan Garip; S. Srujana; V. V. Prasanna Kumar; Naziya Begum
Journal: Journal of Nanomaterials
 (2021)

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## Abstract

A review of multiwalled carbon nanotubes as solar thermic fluids and their thermophysical properties is done in this article. The basic fluids were ethylene glycol and water in ratios of 100 : 0, 90 : 10, and 80 : 20. To investigate how surface modification impacts thermophysical properties, three base fluids were combined with surfactant-assisted MWCNTs and oxidized MWCNTs in weight fractions of 0.125, 0.25, and 0.5 percent, respectively. It takes two months to check whether the dispersion stays constant. Thermal conductivity and viscosity measurement were done using heated discs and Anton Paar viscometers. Using oxidized MWCNTs to disperse, the base fluids increased thermal conductivity by 15% to 24%. Surfactant-assisted MWCNTs in nanofluids perform worse than oxidized MWCNTs. The dynamic viscosity of nanofluids is higher than that of basic fluids between 50 and 70°C. During a mathematical computation, all of the MWCNT weight fractions and ethylene glycol volume percentages are included. The correlation may be a good fit for the experimental data within limits. The characteristics are forecasted using feed-forward backpropagation. In this research, buried layer neurons and factors are examined.


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## Body

## 1. Introduction

Studies show suggested nanofluids could be used in a variety of applications, such as heat exchangers, cooling systems for engines but also solar thermal systems, and electronic cooling and tribology. Their use may help industrial equipment transmit heat more efficiently. A nanofluid is a mixture of one or many nanoparticles in a basic fluid. Nanofluid. The dispersing in base fluids of a number of nanomaterials [1–19] is currently being studied. Thermic fluids made of ethylene glycol and water may be used to heat water. These fluids may also be utilised in heat exchangers and industrial applications. The thermophysical characteristics of ethylene glycol may be adjusted by diluting it with water. An ethylene glycol-water combination with thermophysical characteristics is given in Table 1. Several researches assessed the impact of carbon nanotubes on nanofluid thermophysical characteristics. To improve the nanofluids thermal conductivity, viscosity and density researchers discovered that volume percentage increment leads to decreased specific heat while increasing thermal conductivity. Temperature enhanced thermal conductivity and specific heat. The researchers found that raising volume percent reduced specific heat while boosting thermal conductivity in order to enhance nanofluid’s thermal conductivity, density, and viscosity. Thermal conductivity and specific heat increased in direct proportion to temperature. As the temperature rises, so do the viscosity and density. Almost all research experiments generate large amounts of foam as a result of a lack of surface modification techniques and surfactant usage.Table 1
Base fluid coolant configuration.
S.NoComposition1.Monoethylene glycol (100 : 0)2.Monoethylene glycol and distilled water (90 : 10)3.Monoethylene glycol and distilled water (80 : 20)


### 1.1. Present Studies

One of the most significant problems concerning nanofluids is their inappropriate behavior. Excessive foaming was shown to be a problem in almost all of the investigations. MWNTs with acid surface functionalization are used to dissolve ethylene glycol. Two months is an unusually long time for one nanofluid stability research. Comparison of nanofluids with and without surfactants was done for thermal conductivity. Thermic fluids including EG-water mixes are used in this study, as opposed to earlier studies that used other fluids. It is possible to heat ethylene glycol-water mixtures to temperatures up from 100 degrees Celsius to 194 degrees Celsius. In terms of thermophysical properties, they are tested from 50°C up to just below boiling. Prediction of both thermal conductivity and also viscosity to be affected by temperature and MWCNT mass fraction.


## 1.1. Present Studies

One of the most significant problems concerning nanofluids is their inappropriate behavior. Excessive foaming was shown to be a problem in almost all of the investigations. MWNTs with acid surface functionalization are used to dissolve ethylene glycol. Two months is an unusually long time for one nanofluid stability research. Comparison of nanofluids with and without surfactants was done for thermal conductivity. Thermic fluids including EG-water mixes are used in this study, as opposed to earlier studies that used other fluids. It is possible to heat ethylene glycol-water mixtures to temperatures up from 100 degrees Celsius to 194 degrees Celsius. In terms of thermophysical properties, they are tested from 50°C up to just below boiling. Prediction of both thermal conductivity and also viscosity to be affected by temperature and MWCNT mass fraction.


## 2. Materials and Methodology

### 2.1. Materials

CheapTubes Inc., USA, supplied MWCNTs produced via CVD. The MWCNTs are highly entangled, 95% pure, 30–50 nm in diameter, and 3–15 m long. All additional chemicals and surfactants used are analytical grade and obtained from M/s Sigma-Aldrich.


### 2.2. Multiwalled Carbon Nanotube Surface Modification

Many researches have shown that for nanofluid, there is a significant function for stability and dispersion in enhancing the fluid’s thermophysical characteristics, especially its thermal conductivity. Nanofluids were evaluated for their final properties based on their dispersion stability. Multiwalled carbon nanotubes, as previously discovered, are very resistant to a variety of chemicals but do not dissolve in polar solvents. Nanotube clusters form and settle in liquid environments due to their hydrophobicity, losing their properties. To disperse CNTs in base fluids, scientists often use a surfactant. Surfactants reduce heat transfer rates by increasing fluid foaming. Open-ended pure carbon nanotubes have been demonstrated by Hou et al. [2] as well as Chen et al. [3]. Through acid treatment, Rosca et al. [4] and Vaisman et al. [5] as well as Chiang et al. [6] have shown MWCNTs are hydrophilic in nature. Two distinct surface modification methods were used on MWCNTs. There are two methods used: surface modification using surfactants and oxidation to generate carboxyl and carbonyl groups. Ultrasonically dispersing CTAB and MWCNTs for 10 minutes in methanol is the first technique. Following full evaporation of the solvent, the MWCNTs may be reclaimed. It takes three hours for MWCNTs to decompose in a solution of H2SO4 : HNO3. In the morning, the burnt leftovers were washed thoroughly to a pH of 7 before being baked again at 60°C overnight. FESEM was used to characterise the structure of MWCNTs. HRSEM images of unoxidized CNTs are shown in transform infrared spectroscopy for functional group detection (A and B). Pristine multiwalled carbon nanotubes are shown in Figure 1(a). Purification of MWCNTs, as shown in Figure 1(b), allows the tips to be opened.Figure 1
(a) Pristine multiwalled carbon nanotube and (b) oxidized multiwalled carbon nanotube FESEM image.
(a)(b)Figure2 shows the FTIR spectra both for pure and oxidized MWCNTs. In contrary to Figure 2(a)’s pristine MWCNTs, Figure 2(b) displays increases within spectral at 1125 and 1740 cm1, indicating the production of hydroxyl and carboxyl groups. These hydrophilic groups make MWCNTs highly water dispersible. A high dispersibility nanofluid has better characteristics than unsteady nanofluids.Figure 2
(a) Pristine MWCNT and (b) oxidized MWCNT FTIR analysis.
(a)(b)


## 2.1. Materials

CheapTubes Inc., USA, supplied MWCNTs produced via CVD. The MWCNTs are highly entangled, 95% pure, 30–50 nm in diameter, and 3–15 m long. All additional chemicals and surfactants used are analytical grade and obtained from M/s Sigma-Aldrich.


## 2.2. Multiwalled Carbon Nanotube Surface Modification

Many researches have shown that for nanofluid, there is a significant function for stability and dispersion in enhancing the fluid’s thermophysical characteristics, especially its thermal conductivity. Nanofluids were evaluated for their final properties based on their dispersion stability. Multiwalled carbon nanotubes, as previously discovered, are very resistant to a variety of chemicals but do not dissolve in polar solvents. Nanotube clusters form and settle in liquid environments due to their hydrophobicity, losing their properties. To disperse CNTs in base fluids, scientists often use a surfactant. Surfactants reduce heat transfer rates by increasing fluid foaming. Open-ended pure carbon nanotubes have been demonstrated by Hou et al. [2] as well as Chen et al. [3]. Through acid treatment, Rosca et al. [4] and Vaisman et al. [5] as well as Chiang et al. [6] have shown MWCNTs are hydrophilic in nature. Two distinct surface modification methods were used on MWCNTs. There are two methods used: surface modification using surfactants and oxidation to generate carboxyl and carbonyl groups. Ultrasonically dispersing CTAB and MWCNTs for 10 minutes in methanol is the first technique. Following full evaporation of the solvent, the MWCNTs may be reclaimed. It takes three hours for MWCNTs to decompose in a solution of H2SO4 : HNO3. In the morning, the burnt leftovers were washed thoroughly to a pH of 7 before being baked again at 60°C overnight. FESEM was used to characterise the structure of MWCNTs. HRSEM images of unoxidized CNTs are shown in transform infrared spectroscopy for functional group detection (A and B). Pristine multiwalled carbon nanotubes are shown in Figure 1(a). Purification of MWCNTs, as shown in Figure 1(b), allows the tips to be opened.Figure 1
(a) Pristine multiwalled carbon nanotube and (b) oxidized multiwalled carbon nanotube FESEM image.
(a)(b)Figure2 shows the FTIR spectra both for pure and oxidized MWCNTs. In contrary to Figure 2(a)’s pristine MWCNTs, Figure 2(b) displays increases within spectral at 1125 and 1740 cm1, indicating the production of hydroxyl and carboxyl groups. These hydrophilic groups make MWCNTs highly water dispersible. A high dispersibility nanofluid has better characteristics than unsteady nanofluids.Figure 2
(a) Pristine MWCNT and (b) oxidized MWCNT FTIR analysis.
(a)(b)


## 3. Base Fluid Preparation

In the present research, three different kinds of thermic fluids are being used. In the coolant configuration shown in Table1, oxidized multiwalled CNTs distributed in three different weight-percentage ranges utilise an ultraprobe sonicator; the base fluids were mixed with the coolants.


## 4. Physicothermal Property Evaluation

Thermal conductivity of liquids is difficult to determine during a test because of the significant convective heat transfer in fluids. Thermal conductivity is now measured using the Hot Disk method, which reduces the possibility of making errors while evaluating liquid thermal conductivity. In order to minimise convection, the Kapton sensor 7577 is selected for the testing. We ran three separate sets of tests with varying measurement durations on the samples, and the averages are shown in the following.The Anton Paar MCR 302 Rheometer is fluid strain that can be adjusted rapidly and accurately, resulting in extremely accurate findings for evaluating dynamic viscosity. The samples are put through three rounds of testing, each with a different temperature, and the average result is calculated. It is important to make sure the sample is air-free before placing it in a rheometer, where the results were recorded at steady-state temperatures. Dynamic viscosity and thermal conductivity are determined for all samples at temperatures ranging from 50 to just below boiling.


## 5. Prediction of Data Using ANN

Physicothermal properties of thermic fluids are predicted using an artificial neural network. Also studied is the impact of variables and hidden layer neurons on prediction accuracy. For example, in medical diagnosis, financial forecasting, and many engineering applications, artificial neural networks are a powerful and effective tool. Input data signals are analysed by ANNs to predict output signals. This technique can predict, categorise, and estimate scientific data very accurately. ANNs are a kind of AI technique. It may be used for nonlinear prediction, optimization, pattern recognition, and other purposes. It may also be used to model and simulate data from many sources. Unknown data relationships may be discovered by ANNs via training. It can also deal with nonlinear, complicated, and noisy data sources. It is known for its accuracy and speed. Notably, even with little knowledge, the ANN technique is able to accurately anticipate a dataset’s outcome.A neural network comprises of three layers: an “input layer” of neurons, an “input hidden layer” that handles inputs, and an “output layer” that predicts the model’s output. Experts advise increasing the number of hidden layers in neural networks to increase their accuracy. The input layer has input neurons for the ANNs’ hidden layer. The “hidden layer” transmits data generated as from the input layer to the output layer. Synapses are “weighted inputs” that define the output of a neuron when given an input. In an adaptive system, synapses convert a neural network.The TC of alumina–water nanofluid was predicted using an ANN model and experimental data correlation. Nanofluid TC (25-60°C) is utilised to train ANNs. The experimental findings suggest a relationship between the nanofluid thermal conductivity, volume percentage, and temperature. The correlation predicts the TC of nanofluids. The ANN model also agreed well with the experimental findings which were studied by Esfe et al. [20]. To compare empirical and ANN-based models for prediction of experimental TC of MgO-water nanofluid, as used by Afrand and Esfe [21], the seven-neuron network predicted the results best. Last but not the least, ANN predicted the nanofluid TC improvement better than the curve-fit model. Yousefi et al. [22] developed a DNN to mimic nanofluid TC. Adding alumina, titania, and silica nanoparticles to nanofluids predicted relative viscosity. A base liquid comprising propylene glycol, ethylene glycol, and water was utilised. Other models and results were compared [23–25]. The DNN estimated relative viscosities of dispersions match the published data. In the current study, backpropagation ANNs are used to compute the contribution of error by each neuron after a batch of data is processed. The experiments are performed with pure ethylene glycol-water and ethylene glycol-water (90 : 10 and 80 : 20) suspended with MWCNTs in 0.125%, 0.25%, and 0.5% weight fractions of MWCNTs. The data from the experimentation is taken and modeled using ANNs. Figure 3 shows the ANN architecture of the proposed model.Figure 3
ANN architecture of the proposed model.An ANN model with three inputs, two outputs, and 10 hidden neurons was constructed. Heat transfer and viscosity are calculated using temperature, MWCNT percentage, and ethylene glycol volume percent. To train the ANN, all neuron weights were modified for expected and measured outputs. Performance-wise, we selected pure linear and tangent sigmoid activation functions. The parameterR2 predicts the performance of ANNs. R2 compares the experimental and anticipated values. It improves evaluation precision. The following equation computes the observed and expected results.
(1)R2=1−∑jtj−oj2∑joj2.To determine the best fit model, the hidden layer’s number of neurons is changed from 2 to 10. The experimental values are learned using MATLAB using input and output values to forecast the error. The constructed network has input values of ethylene glycol volume %, MWCNT mass fraction, temperature, and output values of TC and viscosity.


## 6. Results and Discussions

Figure4 demonstrates that the thermal conductivity of ethylene glycol-water which is solar thermic fluid mixes increases with rising water content and multiwalled carbon nanotube weight fraction as the water content and weight fraction increase. The thermal conductivity of a material decreases as the temperature rises. If you heat ethylene glycol at 10% or 20% concentration, the thermal conductivity increases initially but subsequently decreases as the temperature increases. Using MWCNTs to disperse fluids, we found that they performed effectively for all weight and water percentages that we tested, including 100 percent water.Figure 4
Effect of ethylene glycol and water combination concentration on thermal conductivity.
(a)(b)(c)Figure5 shows temperature-dependent dynamic viscosity variation of nanofluids. The temperature connection of dynamic viscosity for EG/water is projected. Ethylene glycol with 10% water changes dynamic viscosity from 50 to 150°C, while ethylene glycol alone changes from 50 to 175°C. The variation in temperature increase is attributed to the reason that water influences the substance’s boiling point. Warm fluids decrease viscosity. It is difficult to tell MWCNT dispersion from base fluid. The difference in dynamic viscosity between nanofluids and base fluids at lower temperatures is significant.Figure 5
Ethylene glycol-water combination concentrations vary with respect to dynamic viscosity with respect to temperature.
(a)(b)(c)Thermal conductivity results of predicted values using ANNs are plotted against experimental values as shown in the graphs from Figures6–8.Figure 6
Thermal conductivity for monoethylene glycol (100 : 0)-based fluid validation using a variety of different hidden layer neurons.Figure 7
Thermal conductivity for monoethylene glycol (90 : 10)-based fluid validation using a variety of hidden layer neurons.Figure 8
Thermal conductivity for monoethylene glycol (80 : 20)-based fluid validation using a variety of different hidden layer neurons.The influence of number of “hidden layer neurons” on the error or the network is shown in the following figures. The optimum number of neurons is found to be 4. Because as number of hidden layer neurons increases beyond 6, the correlation coefficient decreases somewhat, indicating that prediction noise is strong. Figure6 shows the thermal conductivity for monoethylene glycol (100 : 0)-based fluid validation using a variety of different hidden layer neurons. Figure 7 shows the thermal conductivity for monoethylene glycol (90 : 10)-based fluid validation using a variety of different hidden layer neurons. Figure 8 shows the thermal conductivity for monoethylene glycol (80 : 20)-based fluid validation using a variety of different hidden layer neurons.The results of values of dynamic viscosity forecasted using ANNs are plotted against experimental values as shown in the graphs from 9 to 11. The influence of “number of hidden layer neurons” on the error or the network is shown in the following figures. The effect of number of hidden layers on the error I found to be negligible with the network predicting the values even with minimum number of 2 hidden layer neurons. This suggests the accuracy of experimentation. Figure9 shows the dynamic viscosity for monoethylene glycol (100 : 0)-based fluid validation using a variety of different hidden layer neurons.Figure 9
Validation of dynamic viscosity data for monoethylene glycol (100 : 0)-based fluids with varying hidden layer neurons.Figure10 shows the dynamic viscosity for monoethylene glycol (90 : 10)-based fluid validation using a variety of different hidden layer neurons.Figure 10
Validation of dynamic viscosity data of monoethylene glycol (90 : 10)-based fluid with varying hidden layer neurons.Figure11 indicates the dynamic viscosity for monoethylene glycol (80 : 20)-based fluid validation using a variety of different hidden layer neurons.Figure 11
Validation of dynamic viscosity data of monoethylene glycol (80 : 20)-based fluids with varying hidden layer neurons.


## 7. Conclusions

According to the findings of the research, the following conclusions may be drawn:(1)
By adding functional groups to a surface of MWCNTs, after a period of time, they become more stable in ethylene glycol-water combinations than pure MWCNTs(2)
Nanofluid zeta potential study showed that oxidized MWCNTs were more stable in ethylene glycol-water combinations than pure MWCNTs(3)
Oxidized MWCNT dispersion for all ethylene glycol-to-water ratios, the heat conductivity rose by 10 percent to 20 percent on average(4)
A greater dynamic viscosity is seen in nanofluids than in base fluids when the temperature range is between 50 and 70 degrees Celsius. At higher temperatures, the viscosity of the base and nanofluids, on the other hand, does not change much. Thermal conductivity and dynamic viscosity may be accurately predicted using artificial neural networks. Best results are observed when predicted using the backpropagation method with 2 or 4 number of hidden layer neurons(5)
In every analysis, it has been discovered that as the hidden layer neurons increase, the overall value ofR2 decreases, and it is found that in all the cases, the value of R2 is maximum at 2 hidden neurons. The optimum number of hidden layer neurons is thus found out as 2 or 4 from the pruning method(6)
The values ofR2 at different weight percentages are as follows: for 100, the value is 0.978, for 90 : 10, the value is 0.964, and at 80 : 20, the value is 0.976



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