Numerical analyses on the prediction of nusselt numbers for upward and downward flows of water in a

Abstract

Keywords: Natural Convection; Single Phase Flow; Buoyancy and Property Variation; Friction Factor; Nusselt Number

Introduction

Mixed convection conditions occur when both forced and free convection have no negligible effects on flow. Mixed convection is nearly always the case in laminar pipe flows. If low external pressure gradient and low flow velocity, which are encountered in laminar flow, combine with the sufficient amount of density difference due to temperature change, mixed convection currents may form that modify the flow pattern significantly. Mixed convection has received remarkable interest over the years from various areas, mainly nuclear engineering and electronic cooling. In vertical pipes, mixed convection is named according to the acting direction of the density-based pressure gradients. If flow is in upward direction and cools as it progresses, density-based pressure gradients are counter current to the flow direction and named as opposing flow. Opposite condition (aiding flow) is true when fluid is flowing upward and heated along the channel because density-based pressure gradients will be in upward direction this time. It also should be noted that aiding and opposing flow conditions will also occur in downward flow (downward cooling and downward heating for aiding and opposing flows respectively). Tanaka et al. [1] found out the buoyancy and acceleration effects resulted in reduced shear stress near wall zone in turbulent forced convection in vertical tubes. These effects were found to be the reason behind the increment of friction; however, they have no remarkable influence over the heat transfer. In his experimental research with nitrogen in down flow condition, Easby [2] observed an increase in Stanton number due to buoyancy and shear stress distribution. His results also showed lower values as compared with previous correlations. Saylor and Joye [3] developed a method to compute the hydrostatic pressure difference that occurred due to temperature difference along the vertical pipe axis in mixed convection flows. Their improved method integrates the This paper was recommended for publication in revised form by Regional Editor Bekir Yilbas 1 Department of Mechanical Engineering, Yildiz Technical University, Istanbul, TURKEY *E-mail address: dalkilic@yildiz.edu.tr Orcid Id: 0000-0002-5743-3937, 0000-0002-8357-5453, 0000-0003-3593-5183 Manuscript Received 27 August 2017, Accepted 28 January 2018

Journal of Thermal Engineering, Research Article, Vol. 5, No. 3, pp. 166-180, April, 2019 temperature along the pipe axis to have more reliable density and hydrostatic pressure values. Parlatan et al. [4] investigated friction factor and heat transfer characteristics of aiding and opposing flows in their experimental study. Friction factor was found to deviate from isothermal conditions by about 25% depending on the type of the flow. Depending on whether the flow is opposing or aiding type, alteration of friction factor deviates when viscosity variation is taken into consideration. They also found out that the heat transfer coefficient enhances in opposing flow, while it first decreases and then regains in aiding flow. You et al. [5] analyzed the vertical flow of heated air with direct numerical solution. They observed an increase and later a decrease in friction and Nusselt number with increasing heat flux while in upward flow conditions. In downward flow, no deviation is observed with friction, but with increasing heat flux an increase in Nusselt number is present, which leads to the conclusion that the velocity and temperature may have dissimilarity due to buoyancy. Joye [6] experimentally investigated the laminar mixed flow in vertical pipe and derived an equation to predict the pressure drop as a function of flow rate. The equation can be used for Grashof number up to 108 and Reynolds number up to 11000. Busedra and Soliman [7] carried out a numerical study for mixed convection in inclined semicircular ducts for both aiding and opposing flow conditions under uniform heat flux and uniform wall temperature boundary assumptions. For upward flow conditions, friction factor was found to have an increasing characteristic under both boundary types but to have higher values in uniform heat flux conditions in high Grashof numbers. However, in downward flows, friction factor is always higher in uniform heat flux boundary conditions. Voicu et al. [8] numerically investigated the temperature and velocity profile of aqueous glycol solution in a vertical double pipe parallel heat exchanger under laminar mixed convection condition. They found out the Richardson number has influence over the velocity and temperature profile of the inner flow. Flow reversal in inner flow is observed above Richardson number 1; however, this situation diminishes as the flow progresses on because temperature difference along the radial direction decreases. Friction factor values were found to be related to Richardson number near the inlet region while they approach the forced convection regime values along the pipe length. Kang and Chung [9] made an experimental study to determine the influence of height-diameter ratio of a vertical tube over buoyancy effect. They recommended using the heated length as the characteristic length for the Grashof number because diameter was found to be inappropriate and cannot respond to the variety of the results due to length change. They used fixed diameter with varying pipe heights and fixed pipe height with varying diameters to identify the concept. Tam et al. [10] carried out an ANN analysis on laminar and turbulent heat transfer in horizontal tube to determine the importance of the different variables effecting heat transfer performance. Reynolds and Prandtl numbers are found to have the largest impact on the heat transfer for turbulent flow while Graetz and Rayleigh numbers are the most important parameters for laminar flow. Selimefendigil and Öztop [11] numerically investigated mixed flow characteristics of a square cavity with rotational cylinder in the center. The working fluid is water-Al2O3 nanoparticles mixture. GRNN method is the best procedure to determine the output, which was averaged Nusselt number. Their inputs are Reynolds number, Grashof number, volume fraction, and cylinder rotation speed. Generally all inputs are found to act as an enhancer to heat transfer if increased in certain amounts. In this study, an artificial neural network analysis of Parlatan et al.’s [4] experimental data was carried out as one of rare studies of literature on this subject. New correlations were formed upon acquiring ANN data using dimensionless numbers of Reynolds number, Grashof number, Prandtl number, Bond number, Darcy friction factor, isothermal friction factor in forced convection, ratio of dynamic viscosities, and Parlatan et al.’s friction factor, and dependency analysis has been performed among inputs to determine the parameters’ importance. Apart from other most in-tube forced convection studies, all analyses have the dimensionless numbers regarding buoyancy influence (Gr T) and the effects of temperature-dependent viscosity variations (µw/µb) to benefit from one of the artificial intelligence techniques numerically.

Data Reduction

Input and output parameters were selected from the dimensionless numbers used in Parlatan et al.’s study [4]. Inputs of the ANN model were Reynolds number, Prandtl number, Grashof number, Bond number, Darcy friction factor, isothermal friction factor in forced convection, ratio of dynamic viscosities, and Parlatan et al.’s friction factor, which was obtained from the experiments [4]. Inputs of the ANN model were given as follows: Reynolds, Prandtl, Grashof (ΔT), Buoyancy, and Darcy friction factor numbers are expressed respectively:

Journal of Thermal Engineering, Research Article, Vol. 5, No. 3, pp. 166-180, April, 2019

However, GrΔT was used in ANN model in accordance with Parlatan et al.’s experiments. Outputs of the ANN model were averaged with Nusselt numbers over pipe length and Nusselt number in forced conditions. Nusselt number is defined as:

Since the ANNs are one of the generally used and established models to examine the formula between linear or non-linear input-output patterns, they try to generalize the training group and then estimate the test group. Performance of ANNs is determined with the achievement of the estimation. It should be noted that extensive knowledge of the use of ANN methods on the single- and two-phase flows, method of least squares, error analyses’ calculation procedure including R square error, proportional error, and Mean square error can be seen from authors’ previous publications [12-19].

Results And Discussion

In dimensional examination, a dimensionless value is an amount without a related physical dimension. Dimensionless numbers are commonly used in many scientific areas. There are a lot of well-known quantities, such as Reynolds number, which is the ratio of inertial forces to viscous forces in a fluid. It is also defined as the ratio of total momentum transferred to the molecular transferred. The flow is smooth, continues, streamlines at low Re numbers. By contrast, the flow has eddies, vortices, and discontinues at high Re numbers. Prandtl number, which is the ratio between the momentum diffusivity to thermal diffusivity, depends on the state and type of the fluid and is independent from any length dimension. Grashof number, which is the ratio of the buoyancy to viscous force acting on a fluid, is a dominant one for natural convection. Bond number is the ratio of gravitational force to surface tension force. Nusselt number is the ratio of convective heat transfer across to boundary layer of the fluid to the conductive heat transfer. In this study, Nusselt numbers were determined numerically using the ANN analyses including various dimensionless numbers. The experimental data of Parlatan et al. [4] was shown in Tables 1 and 2 for opposing and aiding turbulent mixed convection flow conditions respectively. The range of experiments was 4160 ≤ Re ≤ 9060, 0.30 ≤ GrΔT × 10-6 ≤ 6.93, 4.28 ≤ Pr ≤5.43, and 0.03 ≤ Bo ≤ 1.27 for water in a vertical pipe.

Journal of Thermal Engineering, Research Article, Vol. 5, No. 3, pp. 166-180, April, 2019 Table 1. Inputs and outputs of numerical analyses having the experimental database of upward flow from Parlatan et al. [4]’s study. Input 1 Input 2 Input 3 Input 4 Input 5 Input 6 Input 7 Input 8 Output 1 Output Exp Re GrΔTx10Pr Bo f fo µw/µb fvp Nuavg 2 6 Nuo 1U 7513 4.73 4.97 0.138 0.0287 0.0328 0.51 0.0275 24.3 49.390 2U 7391 2.11 5.06 0.114 0.0306 0.0330 0.7 0.0307 41.6 49.056 3U 7200 0.988 5.22 0.061 0.0317 0.0333 0.84 0.0323 46.8 48.397 4U 7031 0.474 5.36 0.031 0.0328 0.0336 0.91 0.0329 47.1 47.865 5U 6069 0.538 5.38 0.05 0.0344 0.0355 0.9 0.0348 39.7 42.688 6U 5012 0.653 5.4 0.094 0.0409 0.0378 0.87 0.0371 31.5 36.713 7U 4163 1.08 5.36 0.151 16.6 31.499 8U 4143 0.405 5.38 0.089 0.0408 0.0402 0.92 0.0394 24.8 31 9U 4990 0.304 5.43 0.047 0.0388 0.0380 0.94 0.0376 33.8 36.540 10U 5087 1.61 5.31 0.139 0.0328 0.0377 0.75 0.0354 20.1 36.948 11U 4210 2.06 5.29 0.249 14.3 31.567 12U 6218 1.05 5.24 0.088 0.035 0.0352 0.82 0.0338 37.6 43.168 13U 5268 3.13 5.1 0.227 17.8 37.552 14U 4391 3.24 5.04 0.497 19.3 35.873 15U 4578 4.62 4.82 0.771 28.1 33.733 16U 5462 4.48 4.9 0.398 0.0394 0.0368 0.47 0.0302 22.7 38.151 17U 6341 2.82 5.12 0.14 0.0312 0.0349 0.65 0.0317 24 43.557 18U 6689 5.54 4.81 0.27 0.0436 0.0343 0.46 0.0278 24.8 44.604

19. U 4658

2.59 5.19 0.259 16.3 34.100 20U 4837 3.54 4.97 0.441 0.0471 0.0384 0.54 0.0330 21 34.710 21U 5220 6.93 4.55 0.92 27.8 36.010 ANN analyses have 8 inputs mainly as dimensionless values of Reynolds number, Grashof number, Prandtl number, Bond number, Darcy friction factor, isothermal friction factor in forced convection, ratio of dynamic viscosities, and Parlatan et al.’s friction factor, while Nuavg and Nuo were the outputs of ANN analyses. Fig. 1 shows ANN results for the pipe length averaged and forced convection’s Nusselt numbers within the deviation band of ±20% and ±10% respectively. Table 3 shows the error rates of ANN analyses for the predicted outputs. According to this table, it is possible to derive many results. The effect of input numbers after first 4 inputs on the outputs were found to have almost negligible effect. For instance, the first 4 inputs have prediction capacity of output one as 0.965 whereas all inputs have increased these values to 0.973 in this table. This result shows the importance of first 4 inputs in the analyses. All inputs of analyses were found to have significant importance with their individual error values in this table. Generally, Table 3 shows the possibility of the prediction by ANN analyses on the investigated subject using proper inputs clearly. Tables 4 and 5 include all proposed correlations by means of the method of least squares for the prediction of outputs 1 and 2 respectively. Number of inputs in these correlations decreases with increasing their labels. Correlation 1 has all inputs and correlation 6 has 3 inputs. As their success can be seen from Figs. 2 to 5, correlation 1 is the most predictive one due to its large number of inputs as expected. Figs. 2 and 3 illustrate the results of proposed correlations of 1, 2, 5, and 6 for output 1 while Figs. 4 and 5 depict the results of proposed correlations of 1, 2, 5, and 6 for the output 2, which was predicted more accurately than another one within the deviation band of ±5%. Table 6 shows the results of correlations 3 and 4 numerically. Accuracy of the proposed correlations increase with increasing input numbers as shown in related tables and figures.

Journal of Thermal Engineering, Research Article, Vol. 5, No. 3, pp. 166-180, April, 2019 Table 2. Inputs and outputs of numerical analyses (Downward flow data from Parlatan et al. [4]) Input 1 Input 2 Input 3 Input 4 Input 5 Input 6 Input 7 Input 8 Output 1 Output 2 Exp Re GrΔTx10-6 Pr Bo f fo µw/µb fvp Nuavg Nuo 22D 23D 24D 25D 26D 27D 28D 29D 30D 31D 32D 33D 34D 35D 36D 37D 38D 39D 40D 41D 42D 43D 44D 45D 46D 47D 48D 49D 50D 51D

Table 3. Error rates of ANN analyses for the predicted outputs Output 1 (Nuavg,exp) Input Input 2 Input Input Input Input ANN inc. all ANN inc. inputs 1 3 4 5 6 inputs from 1 to 4 (GrT -6 (Re) (Pr) (Bo) (f) (fo) .10 ) Root Mean Squared Error R-Squared Error

2.284 2.34 2.404 0.957 0.955 0.953 Output 2 (Nuo,exp) Input Input Input 3 4 5 (Pr) (Bo) (f)

Journal of Thermal Engineering, Research Article, Vol. 5, No. 3, pp. 166-180, April, 2019

60 ANN result including all inputs ANN result including 4 inputs 55

b) Figure 1. ANN results for the pipe length averaged (a) and forced convection’s (b) Nusselt numbers

Journal of Thermal Engineering, Research Article, Vol. 5, No. 3, pp. 166-180, April, 2019

Nuavg,exp Figure 2. Comparison of proposed correlations with output 1 experimental data

Journal of Thermal Engineering, Research Article, Vol. 5, No. 3, pp. 166-180, April, 2019

Nuavg,exp Figure 3. Comparison of proposed correlations with output 1 experimental data

Journal of Thermal Engineering, Research Article, Vol. 5, No. 3, pp. 166-180, April, 2019

Nuo,exp Figure 4. Comparison of proposed correlations with output 2 experimental data

Journal of Thermal Engineering, Research Article, Vol. 5, No. 3, pp. 166-180, April, 2019

Nuo,exp Figure 5. Comparison of proposed correlations with output 2 experimental data

Journal of Thermal Engineering, Research Article, Vol. 5, No. 3, pp. 166-180, April, 2019 Table 4. Proposed correlations, their coefficients and p-values for output 1 Correlation 1: b1/(1 + exp(b2*input1 + b3*input2 + b4*input3 + b5*input4 + b6*input5 + b7*input6 + b8*input7 + b9*input8)) b1 b2 b3 b4 b5 b6 b7 b8 b9 Coefficient 76.6042 -0.0005 0.0683 1.6986 -3.2181 9.8277 -85.3757 -6.6665 78.2192 p-value 1.22E-05 0.0005 0.6859 0.0007 0.0012 0.312 0.6205 0.0071 0.6478 Correlation 2: b1/(1 + exp(b2*input1 + b3*input3 + b4*input4 + b5*input5 + b6*input6 + b7*input7 + b8*input8)) b1 b2 b3 b4 b5 b6 b7 b8 b9 Coefficient 78.2551 -0.0005 1.6871 -3.1923 8.375 -33.5803 -6.6123 26.6497 p-value 1.34E-05 0.0003 0.0005 0.0011 0.3277 0.768 0.006 0.8136 Correlation 3: b1/(1 + exp(b2*input1 + b3*input3 + b4*input4 + b5*input5 + b6*input6 + b7*input7)) b1 b2 b3 b4 b5 b6 b7 b8 b9 Coefficient 79.1742 -0.0004 1.6126 -3.1975 7.6753 -6.8591 -6.181 p-value 1.05E-05 0.0002 6.11E-06 0.0008 0.313 0.2837 4.35E-05 Correlation 4: b1/(1 + exp(b2*input1 + b3*input3 + b4*input4 + b5*input6 + b6*input7)) b1 b2 b3 b4 b5 b6 b7 b8 b9 Coefficient 75.2515 -0.0005 1.7637 -3.6054 -0.4375 -6.8368 p-value 6.49E-07 5.70E-05 9.20E-07 0.0001 0.2918 5.51E-06 Correlation 5: b1/(1 + exp(b2*input1 + b3*input3 + b4*input4 + b5*input7)) b1 b2 b3 b4 b5 b6 b7 b8 b9 Coefficient 74.4857 -0.0005 1.7732 -3.6349 -6.9243 p-value 3.50E-07 5.27E-05 8.32E-07 0.0001 4.61E-06 Coefficient 70.7088 -0.0004 0.6426 3.3675 -0.8131 -15.8385 p-value 1.81E-12 3.67E-05 1.34E-08 1.65E-06 0.0916 2.14E-05 Table 5. Proposed correlations, their coefficients and p-values for output 2 Correlation 1: b1/(1 + exp(b2*input1 + b3*input2 + b4*input3 + b5*input4 + b6*input5 + b7*input6 + b8*input7 + b9*input8)) b1 b2 b3 b4 b5 b6 b7 b8 b9 Coefficient 73.4234 -0.0003 0.079 0.1861 0.3411 -1.3435 -9.6328 0.5148 10.9657 p-value 9.79E-16 7.12E-12 0.0004 4.19E-05 9.42E-05 0.2305 0.61 0.017 0.5594 Correlation 2: b1/(1 + exp(b2*input1 + b3*input2 + b4*input3 + b5*input4 + b6*input5 + b7*input7 + b8*input8)) b1 b2 b3 b4 b5 b6 b7 b8 b9 Coefficient 73.5949 -0.0003 0.0724 0.1778 0.3241 -1.6517 0.5688 1.4057 p-value 1.67E-16 1.68E-12 2.13E-05 9.60E-06 2.69E-05 0.0786 0.0022 0.0805 Correlation 3: b1/(1 + exp(b2*input1 + b3*input2 + b4*input3 + b5*input4 + b6*input5 + b7*input7)) b1 b2 b3 b4 b5 b6 b7 b8 b9 Coefficient 73.3652 -0.0003 0.0715 0.1612 0.3578 -0.0147 0.6424 p-value 9.54E-17 1.68E-12 4.33E-05 2.88E-05 6.65E-06 0.8222 0.0009 Correlation 4: b1/(1 + exp(b2*input1 + b3*input2 + b4*input3 + b5*input4 + b6*input7)) b1 b2 b3 b4 b5 b6 b7 b8 b9 Coefficient 73.3305 -0.0003 0.0714 0.1612 0.3584 0.6414 p-value 1.55E-17 4.71E-13 2.85E-05 1.86E-05 3.83E-06 0.0007 Correlation 5: b1/(1 + exp(b2*input1 + b3*input2 + b4*input3 + b5*input4)) b1 b2 b3 b4 b5 b6 b7 b8 b9 Coefficient 76.677 -0.0002 0.0152 0.2564 0.2842 p-value 1.24E-16 6.82E-11 0.0677 5.11E-36 6.83E-08 Coefficient 75.8406 -0.0002 0.251 0.3574 p-value 8.06E-17 1.02E-10 3.11E-37 4.11E-18 -

Journal of Thermal Engineering, Research Article, Vol. 5, No. 3, pp. 166-180, April, 2019

Ex p 1U 2U 3U 4U 5U 6U 7U 8U 9U 10 U 11 U 12 U 13 U 14 U 15 U 16 U 17 U 18 U 19 U 20 U 21 U 22 D 23 D 24 D 25 D 26 D 27 D 28 D 29 D

Table 6. Comparison of proposed correlations of 3 and 4 for prediction of outputs Input 2 Input Input Input Input Input Input Corr. Corr. Output GrΔTx103 4 5 6 7 8 3 4 1 6 Pr Bo f fo µw/µb fvp Nuav Nuav Nuav

Journal of Thermal Engineering, Research Article, Vol. 5, No. 3, pp. 166-180, April, 2019

Table 6. (Cont.). Comparison of proposed correlations of 3 and 4 for prediction of outputs Input Input 2 Input Input Input Input Input Input Corr. Corr. Output 1 GrΔTx103 4 5 6 7 8 3 4 1 6 Re Pr Bo f fo µw/µb fvp Nuav Nuav Nuav

32D 4527 33D 4999 34D 6333 35D 7357 36D 8468 37D 8640 38D 7582 39D 6606 40D 5048 41D 4868 42D 7702 43D 6710 44D 5293 45D 5020 46D 8740 47D 6993 48D 7930 49D 9059 50D 5518 51D 5251

It should be noted that this paper should be evaluated as an extension of authors’ previous study [18]. Detailed information on ANN analyses can be obtained from that study [18].

Conclusion

This investigation studied some single-phase flow characteristics of water during transition and turbulent flow in a vertical tube. Parlatan et al.’s [4] experimental data were used in the analyses. The purpose of this numerical study was to illustrate ANN method’s capability to determine Nusselt numbers, effect of dimensionless numbers as inputs on Nusselt numbers as outputs, and propose various empirical correlations using the inputs. The validation process of the results revealed the importance of dimensionless numbers on the measured pipe length averaged Nusselt numbers and forced convection’s Nusselt numbers according to aiding and opposing flow types. There are few research studies on such cases in the literature. For this reason, the results of this study are expected to fill a gap in the literature. Twelve different correlations were proposed using inputs of the numerical model for the outputs separately. The best correlations’ figures were plotted within the deviation of ±5%. Some dimensionless numbers regarding buoyancy influence (Gr T) and the effects of temperature-dependent viscosity variations (µw/µb) were added into both ANN analyses.

Acknowledgement

The second author would like to thank King Mongkut’s University of Technology Thonburi for the support during his research in Thailand. The fifth author would like to express his appreciation to the National Science and Technology for Development (NSTDA 2013) and the National Research University (NRU 2014) for the support.

Nomenclature

Journal of Thermal Engineering, Research Article, Vol. 5, No. 3, pp. 166-180, April, 2019 Bo D f fo g Gr L Nu Nuavg Nuo Pr Re q” Tb Tw ΔP β υ α ρ

Buoyancy number Inside diameter of tube, m Darcy friction factor Isothermal friction factor in forced convection Gravitational acceleration, m s-2 Grashof number Test section length Nusselt number Tube length averaged Nusselt number Nusselt number in forced convection Prandtl number Reynolds number Net wall heat flux, W m-2 Bulk temperature, oC Wall temperature, oC Pressure drop, Pa Thermal expansion coefficient, 1 K-1 Kinematic viscosity, m2 s-1 Thermal diffusivity Density, kg m-3

Share and Cite