3.1. ANN model using variables of Kbf, Kp, φ, T, and dnp
To address the accuracy and performance of the presented ANN model, the results of experimental data were compared with the results of the predicted thermal conductivity using the ANN model. The variables of Kp, Kbf, φ, T, and dnp were respectively defined as input variables, and thermal conductivity obtained from laboratory data was applied as the objective function in this modeling. As reported in Table 2, the number of neurons gradually increased from 1 to 17 in each step to obtain a proper and acceptable result. Here, increasing the number of neurons up to 13 led to a higher correlation coefficient and lower Mean squared error for all data.
Table 2
The results of neural networks with different numbers of neurons for total data
Number of Neurons | Mean Square Error (MSE) | Correlation Coefficient (R) |
1 | 0.9914 | 0.000811 |
2 | 0.9933 | 0.000627 |
3 | 0.9946 | 0.000513 |
4 | 0.9951 | 0.000471 |
5 | 0.9955 | 0.000422 |
6 | 0.9962 | 0.000358 |
7 | 0.9962 | 0.000359 |
8 | 0.9963 | 0.000349 |
9 | 0.9968 | 0.000295 |
10 | 0.9969 | 0.000296 |
11 | 0.9969 | 0.000281 |
12 | 0.9972 | 0.000262 |
13 | 0.9975 | 0.000238 |
14 | 0.9975 | 0.000270 |
15 | 0.9974 | 0.000289 |
16 | 0.9974 | 0.000293 |
17 | 0.9974 | 0.000292 |
However, it was observed that using a greater number of neurons, up to 17 caused in increasing the MSE, as well no positive change was achieved in R. So, the results of the prediction with 13 neurons were satisfactory, and this neurons number was chosen as an optimum number of neurons. According to the results, the best R and lowest MSE using 5-13-1 topology were founded to be about 0.9975 and 0.000238, respectively, indicating good fitting between predicted results and target points.
Also, the results of the presented network using 13 neurons for all data sets and sub-sets including training, validation, and test data are given in Table 3. As is clear, the best correlation coefficient and the lowest mean square error were related to the network training sub-set, about 0.9976 and 0.000229, respectively.
Table 3
The results of developed ANN model using 13 neurons
Data Set | Correlation Coefficient (R) | Mean Square Error (MSE) | Data Points Number |
training | 0.9976 | 0.000229 | 560 |
Validation | 0.9977 | 0.000240 | 120 |
Testing | 0.9971 | 0.000275 | 120 |
Total data | 0.9975 | 0.000238 | 800 |
Figure 2 shows the comparison between the estimated nanofluid's thermal conductivity using the presented ANN model (output) and actual data (target), for all data and sub-sets. Accordingly, a good fifing was observed for all data and each sub-sets, indicating proper accuracy and performance of the presented ANN model.
Also, the results of the ANN model include MSE and ± maximum of the error (max error) from laboratory data (Kexp,m − KANN,m) for water, oil, ethylene glycol, and RC base nanofluids were reported in Table 4. As is clear, the ANN has a more appropriate ability for predicting the thermal conductivity of nanofluids based on radiator cooling, oil and ethylene glycol. However, the most positive and negative errors from laboratory data and the highest MSE are + 0.077, -0.063, and 0.000359, respectively, which are related to water-based nanofluids.
Table 4
The results of the developed ANN model using 13 neurons for various base fluids
Base fluid | MSE | + Max Error | − Max Error | Data points No. |
Water | 0.000359 | + 0.077 | −0.063 | 472 |
EG | 0.000072 | + 0.027 | −0.018 | 238 |
Oil | 0.000037 | + 0.009 | −0.027 | 65 |
RC | 0.000041 | + 0.003 | −0.010 | 25 |
The error histogram graph was used to further evaluate the amount of error between values predicted by ANN model and experimental data. The results in Fig. 3, illustrated an approximately narrow distribution of error so that most of the error between values predicted by 5- parameter ANN model and used experimental data is less than ± 0.02, confirming the good performane and proper accuracy of the designed 5-input ANN model. According to the results presented in Fig. 3, the values of error maximum are approximately + 0.077 and − 0.063 and include a limited amount of data.
The details of developed 5-13-1 topology in the current ANN model including generated weights and biases in hidden and output layers are given in Table 5. These reported datails is need to predict the thermal conductivity of the nanofluid in similar conditions and systems. Here, parameters of i, j and z represent the number of input variables equal to 5, the number of optimal neurons equal to 13, and the number of elements of the output layer and equals 1.
Table 5
details of developed 5-13-1 topology including generated weights and biases in hidden and output layers
Neurons | Hidden layer | Output layer |
Weights (j,i) | Bias (j) | Weights (z, j) | Bias (z) |
wj,1 | wj,2 | wj,3 | wj,4 | wj,5 | bj | Wz,j | b2 |
1 | -2.373 | -0.750 | -2.868 | -2.1324 | − .63109 | 3.4503 | -1.3477 | -0.0666 |
2 | 2.6033 | -2.6548 | 1.0633 | -0.6431 | -0.00685 | -5.8058 | 1.1825 |
3 | 5.3041 | 6.2036 | − .8895 | 11.5857 | -3.1982 | 6.6723 | -0.1936 |
4 | -0.07695 | -1.9939 | 0.11738 | -1.0823 | -1.8938 | 0.2621 | -1.6987 |
5 | -1.7939 | 0.5535 | 0.39141 | 0.3496 | -1.4243 | 0.3815 | -2.3237 |
6 | 3.9334 | 1.2779 | 2.0192 | -8.7285 | -0.12049 | -5.4235 | 0.04595 |
7 | 4.2268 | -4.819 | -0.33342 | 2.703 | -1.722 | -1.9611 | -0.11795 |
8 | 1.5564 | -0.6616 | -0.46827 | -0.49512 | 1.3717 | -0.6102 | -1.8786 |
9 | 5.399 | 0.6448 | -0.07069 | 11.1908 | -1.6698 | 2.7623 | 0.2034 |
10 | 1.5471 | 4.5512 | -0.6625 | -0.93448 | 4.9159 | -3.4479 | -0.7357 |
11 | 3.5399 | 0.0980 | -0.37568 | 1.2641 | 0.71327 | 4.2202 | 0.3323 |
12 | 1.6815 | -9.2135 | 1.2068 | 0.012136 | -0.29878 | -11.479 | -1.4107 |
13 | -0.7935 | 01.9723 | -0.3867 | 2.5227 | 0.034926 | -1.6416 | -1.8418 |
3.2. Validation of Presented ANN model
To evaluate the accuracy of the ANN model to estimate the thermal conductivity of nanofluids, the validation of presented model was done with the test of 104 other experimental data points, which were collected from some previous research. The details of the used database for validation of the 5-input ANN model were reported in Table 6.
Table 6
details of the used database to validat the 5-input ANN model
nanoparticle | Base fluid | dnp (nm) | Φ (%) | T (°C) | Kp (w/mk) | reference |
ZnO | water | 30 | 1–3 | 22–75 | 29 | [67] |
Al2O3 | 75 | 1.5-6 | 20–50 | 27 | [68] |
CuO | 31 | 1–3 | 20–50 | 18 | [40] |
Fe | 25 | 1–5 | 20 | 80 | [69] |
CuO | EG | 31 | 1–3 | 20–50 | 18 | [40] |
Al2O3 | 12 | 2–3 | 25 | 27 | [70] |
ZnO | 50 | 0.7, 1 | 15–55 | 18 | [37] |
Cu | Oil | 40 | 0.11-2 | 60 | 383 | [65] |
CuO | 31 | 1–3 | 20–50 | 18 | [40] |
Al2O3 | 45 | 1–3 | 20–50 | 27 | [40] |
Ag | 20 | 0.012–0.180 | 40–70 | 430 | [71] |
The results of the comparison between predicted values of thermal conductivity (output) versus corresponding actual laboratory data (target) were represented in Fig. 4. As is clear, the results of the developed ANN model indicated a proper agreement with the experimental data, confirming the proper ability of the presented model. The maximum deviation between values predicted by ANN model and corresponded experimental data for nanofluids based on oils and ethylene glycol is less than + 1.2% and − 0.5%, respectively. However, the maximum error for nanofluids based on water is about + 3.7%, which shows a acceptable and satisfactory amount. The outcomes proved the accuracy and proper strength of the developed 5-input ANN model to predict the thermal conductivity of nanofluids that are in the limitation of reported operating conditions in the current study.
Also, the results of the semi-experimental model, presented by Ahmadlou et al. were compared with the proposed 5-input ANN model [27]. They developed an ANN model to predict the thermal conductivity of nanofluids using 800 different experimental data points related to different nanoparticles and main fluids of water, ethylene glycol, and oil in the range of 10–71°C, volume fractions of 0.1-1%, and nanoparticles diameter of 5-150 nm. According to their results, the R and MSE were found to be 0.993 and 0.000527. Ali Aminian [23] also investigated and predicted the thermal conductivity of nanofluids using an ANN model over various nanofluids. They defined the parameters of T, ϕ, dnp, Kbf and Kp as input variables of ANN. The presented model by them was performed in the range of 0-200°C and a nanoparticle diameter of 9-150 nm and a nanoparticle volume fraction of 0.005-14%. The calculated correlation coefficient was about 0.9309 and the mean squared error was 3.06% for all data. While the 5-input ANN model presented in the current study, showed a better mean squared error of 0.000238 and a further correlation coefficient of 0.9975, confirming the accuracy and appropriate performance of the proposed 5-input ANN model in predicting the thermal conductivity of nanofluids.