Artificial neural network modeling to examine spring turbulators influence on parabolic solar collector effectiveness with hybrid nanofluids

https://doi.org/10.1016/j.enganabound.2022.06.026Get rights and content

Abstract

Numerical simulation and artificial neural network modeling of turbulent flow inside a pipe equipped with two spring turbulator samples with two different scales and a segmental cross-section have been investigated. Increased heat transfer rate (HTR) due to the use of a spring turbulator is predicted for the TiO2single bondCu-Water hybrid nanofluid based on the single-phase model, feed-forward artificial neural network (ANN) and fitting method. The role of Reynolds number (Re), scale and volume fraction (ϕ) on Nusselt number (Nu), pressure drop (ΔP), performance evaluation coefficient (PEC), solar collector efficiency (η), and the field synergy principle (FSP), compared to simple pipe, is considered using the finite volume method. The results show that increasing the spring turbulator scale increased the contact surface of the working fluid and the spring turbulator. As a result, the flow turbulence is increased, which leads to better mixing of the nanofluid as the operating fluid of the solar collector absorber pipe. Finally, ANN outputs and fitting results are compared, and it has been observed that the obtained ANN could predict the targets accurately.

Introduction

These days, flat plate collectors are used in a wide range of engineering fields. Solar thermal energy can be used directly as heat or indirectly as the driving force of a heat engine to generate useful mechanical energy. Moreover, thermal energy can be used to generate electrical energy and in the process of thermal desalination, low to medium capacities can be used [1]. As a result of increasing thermal efficiency in heating units, it is very useful to conserve energy [2], given that energy demand is increasing rapidly and has caused considerable concern these days. Therefore, it is very important to use new technologies to improve HTR in heat exchangers, which leads to increased efficiency and energy savings. Improved heat exchanger pipes are one essential technique to increase their performance. In general, there are three types of approaches for enhancing HTR in heat exchangers: active [3], passive [4], and mixed methods [5]. Surface alterations, geometric flow channels of materials, specific springs, and the addition of liquids and gasses are all employed in these procedures. The HTR has a significant impact on the efficacy of each of these approaches. The application procedure determines factors such as single-phase current, free or forced convection HTR, welding or forced convection density, and heat exchanger type. A passive approach involves imparting a rotation to the bulk flow and disrupting the boundary layer at the pipe's surface with pipe turbulators to increase the convective HTR.

As a consequence, by stacking turbulence and eddy motion (rotational flow), a thinner boundary layer is created, resulting in a superior HTC and improved HTR [6]. Pipe turbulators have been documented and debated for decades as a way to improve the HTR of sluggish or turbulent flow. Improved heat exchanger pipes are one of the most essential strategies to increase their performance [7], [8], [9], [10]. Increased HTR in the absorber pipe typically has a significant influence on heat exchanger overall performance. As a result, the heat exchanger's size may be lowered, and cost savings can be realized.

Several kinds of turbulators have been employed within the pipe to improve HTR in earlier papers [11], [12], [13], [14], [15]. In this regard, one of the inactive methods in heat exchangers to increase HTR is the use of spring turbulators, which have been studied extensively due to the benefits of this technique over other grades, which include reliable operation, a comparatively simple and low-cost manufacturing methods, and simple design and detachment of such grades.

Spring wire has recently been recommended as a turbulator for coil pipes [16], [17], [18]. The HTR of fluid flow via coiled pipes containing spring wires increased significantly, according to their outcomes. Furthermore, to the coil pipe forming secondary currents (owing to centrifugal forces), the spring wire raises the turbulence level of the fluid flow along the coil pipe, resulting in a rise in Nu and, as a result, a drop in ΔP and exergy. As a result, the advantage of this process is contingent on the relevance of ΔP in heat exchanger applications. For a prolonged period, several investigations have been carried out to assess the influence of coiled wire on HTR and coefficient of friction. Wang and Sunden [19] compared the thermal and hydraulic characteristics of twisted strip turbulators and twisted wire turbulators in both smooth and turbulent flow zones [20]. The twisted coil was shown to be more effective in raising HTR in a turbulent flow zone than the twisted strip, which had a lower overall efficiency. Rahai et al. [21, 22] and Rahai and Wong [23, 24] investigated further uses of coil wires to improve the mixing of a turbulent jet for a Bunsen burner. Promvenge [25] examined the effect of coiled square wire turbulators on HTR in a circular pipe and discovered that coiled square wire works better than circular wire by roughly 10% to 15%, with the same size wire and other parameters. Prasad and Shen [26], Agrawal et al. [27], and Inaba and Ozaki [28] studied the rise in HTR utilizing different coiled turbulators and exergy analyses.

To investigate the influence of spring wire on the thermal characteristics of their heat exchanger, Mashoofi et al. [16] and Panahi et al. [29] employed spring wire as a turbulator in the heat exchanger of shell pipes and heat exchanger of two helical pipes. In their investigation, the shape of the spring wire was set, and no experimental correlations were proposed. The HTR and ΔP properties of a horizontal pipe with a twin coil turbulator were examined by Nafon [30]. Eren et al. [31] tested a pipe with inclined helical springs as a turbulator. It was discovered that the slope angle has a significant impact on HTR and friction decrease, although the spring number has a minimal impact. Guns et al. [32] investigated HTR and ΔP in a turbulator-assisted coiled pipe. According to their findings, the Nu increases with rising Re and wire thickness and decreases with lowering step ratio.

Only the use of spring turbulators with changing cross-sectional geometry and the usage of dual spring grades were adequate in prior investigations. Gunes et al. [33] made a significant breakthrough in this area by removing the turbulator from the pipe wall for the experimental research of HTR and ΔP. According to their findings, adhering the turbulators to the pipe wall may generate contamination over time as well as increased resistance to HTR. As a result, separating turbulators from the wall is more cost effective. Choosing the appropriate volume percentage is critical for maximizing efficiency. Nano-based studies are utilized in many studies [34], [35], [36], [37], [38], [39], [40], [41], [42], [43], [44]. The nanofluid and volume fraction that can contribute to an increase in efficiency are critical [45], [46], [47], [48], [49], [50].

A number of studies have shown enhancements the traditional coil spring to reach the projected HTR of the greatest efficiency heat exchanger possible, in order to enhance the thermal hydraulic performance of a coil spring pipe. Nevertheless, it is obvious that the properties of HTR and ΔP in pipes containing TiO2single bondCu-Water hybrid nanofluid as an operating fluid in heating systems have received less attention. Almost all prior practical and computational research on spring turbulators with a segmental cross section have focused on mono nanofluids. The purpose of this work is to conduct a computational analysis of HTR features in order to improve the efficiency of solar collectors. The following impacts of various factors have been examined for this goal.

  • 1-Increasing the volume fraction of nanoparticles (2% −4% −6%)

  • 2-Increase the input speed (Re = 10,000, 14,000, 18,000)

  • 3-Increase the scale (scales 1, 2)

As characteristic parameters in samples 1 and 2 are also examined and compared with the results obtained from simple pipe. Also, explain the mechanism of increasing HTR for pipes equipped with spring grades, evaluation criteria Performance (PEC) and Field Synergy Principle (FSP) are also defined and reviewed.

Section snippets

Physical model

Fig. 1 depicts a generic diagram of the proposed system. A parabolic concentrator concentrates solar radiation onto the absorber pipe. HTR then transfers the concentrated radiation to the working fluid (TiO2single bondCu-Water). Table 1 lists the properties of the parameters utilized in the modeled geometry (1).

Equations

The HTR and efficiency of solar collectors in the pipe with spring turbulators have been investigated numerically in the range of Re= 18,000–10,000. The fluid inlet temperature (T) is 300 Kelvin.

Dimensionless paraments

The following dimensionless parameters have been selected to evaluate the rate of HTR and friction loss of turbulent airflow inside pipes equipped with spring turbulators. For this purpose, the numbers Re and Nu, the coefficients of friction f and the thermal efficiency η are defined as follows [57]:Re=ρVDhμ.Dh=4APf=2ΔPDh/ρu2LNux=hxDhkη=m˙cp(ToutTin)GAcPECold=(Nu/Nus)/(f/fs)1/3

In the above equations, ∆P is the pressure drop across the whole pipe, Dh is the hydraulic diameter of the pipe and u

Computational field

Fig. 2 shows grids for the computational field. In order to study the grid independence, the average HTR and friction loss of turbulent flow inside the pipe containing the second sample turbulator at Re = 18,000, φ =% 2 for three different elements Calculated. Details are provided in Table (3). The results show that HTR increases with increasing Re while friction loss decreases. The trend of changes in the coefficient of friction with the turbulent flow inlet (V) is similar to the Modi diagram.

Comparing simulation data with experimental data

To validate the model, the numerical results of the simple pipe Nu, are compared with the Gnielinski and Filonenko [59].

Gnielinski's experimental formula is calculated as follows:Nu=(f/8)(Re1000)Pr1+12.7(f/8)(Pr2/31)[1+(deL)2/3](Tf/Tw)0.45

As displayed in Fig. 3, the present results of the Nu of both the simple pipe and the pipe with a helical spring are compared with the results of the experimental formula and the experimental results, respectively. Fig. 3 depicts that the simulation results

The field synergy principle

Guo [60, 61] proposed the concept of field synergy principle (FSP) to express the physical mechanism of single-phase convective HTR. By considering the two-dimensional flow of the steady-state boundary layer on a cold flat plate with a zero angle of collision, the energy equation is obtained as follows:

  • ρCp(uTx+vTy)=Ty(λTy)

Expansion of Equation (19) across the thermal efficiency boundary layer:

  • ρCp0t(U¯0gradT)dy=λ(Ty)y=0(20)

The local synergistic angle among (V) and (T) gradient is

Flow contour analysis

Fig. 4 and 0.5, for φ =% 2 and Re = 14,000 show the (V) contour and the (V) of the longitudinal section Z = 225 mm, respectively. In CFD analysis, it was understood that the (V) of the pipe with spring turbulator was significantly improved compared to the simple pipe, which indicates the distribution of more intense turbulence intensity in the pipe containing spring turbulator. Fig. 5 shows that only axial current is detected in a simple pipe, while both rotational and axial currents are

Artificial neural network

In recent years, numerous numerical and mathematical methods are of great interest to researchers because of their accuracy, speed, lower cost, and reliability [62], [63], [64], [65], [66], [67], [68], [69], [70], [71], [72], [73], [74]. In fact, these advantages are due to the advancement of computer science. In the meantime, artificial neural networks (ANNs) are used to predict the behavior of systems. These networks can predict the behavior of systems with nonlinear or complicated behavior [

Fitting method

Another famous method in prediction the Reynolds number based on the material type, ϕ and Nusselt number is the fitting method. For each material type a fitting method is applied separately. In Eq.27, the fitted surface is presented.f(x,y)=p00+p10x+p01y+p20x2+p11xy+p02y2

In Eq.27, x represents ϕ and y represents Nusselt number and f is the Reynolds number. As there are there types of materials including case1, case2 and the Plain pipe, three fitted surface are obtained. The coefficients of these

Conclusion

In the present experimental study, the increase of HTR and heat efficiency of the solar collector through the absorber pipe containing spring inserts with two different scales were numerically investigated.

The presence of a coil with a circular cross-section significantly increases the ΔP and HTR. The value of the Nu in the pipe containing the spring turbulator with two scales (scale 1.2) for the Re= 18,000 and the volume fraction of 2% increases 3.38 and 2.88 times compared to the pipe without

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work is supported by Key Industry Innovation Chain (Group) Project of Shaanxi Province (2020ZDLNY07–05), and Key Research and Development Program of Shaanxi (Program No. 2021NY-193).

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