Investigation of the effect of the finned coiled wire insert on the heat transfer intensification of circular tube: Energy and exergy analysis

Highlights

• Energy and exergy performance of the tube heat exchangers with finned coiled wire insert were discussed.

• Equations were proposed for calculation of objective variables.

• Multi-objective study was implemented to obtain the optimum conditions.

Abstract

In the current investigation, CFD (computational fluid dynamics) code is applied for the three-dimensional simulation of fluid flow and turbulent transferred thermal energy through circular duct with coiled finned wire inserts. The realizable K-ε model will be used for the obtaining the influence of the turbulence. The validation of the model is performed by comparing the numerical Nusselt number with the experimental one. The impact of the various operating parameters i.e. Prandtl and Reynolds numbers and geometric variables i.e. fin height and thickness, coil pitch and wire diameter on the dimensionless exergy loss, thermal efficiency and the dimensionless working power will be obtained. Furthermore, correlations will be suggested to estimate these objective variables as functions of the geometric and operating variables. A new multi-objective optimization way is introduced and applied to obtain the optimal and critical values or range of the input variables in the studied range. Results also show if the Reynolds number increases by 100 % in the study range, the thermal efficiency, the dimensionless exergy loss and dimensional working power reduces by 15.9 %, 9.4 % and 5.9 %, respectively. Furthermore, with doubling pitch, they reduce 17.2 %, 14.9 % and 48.9 %, respectively.

Introduction

Transferred thermal energy [[1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14]] improvement is always one of the important and interesting topic for the heat exchanger designers. Therefore, many studies and investigations have been performed about the transferred thermal energy augmentation in different tools like heat exchangers. The inserts intensify and improve the thermal energy transfer rate in the heat exchanges which are applied in various industrial applications such as refrigeration, petroleum, solar systems, HVAC, food, etc. Using of the CWIs (Coiled Wire Inserts) is one of the best method for improvement of the transferred thermal energy. These CWIs enhance the fluids contact with the solid regions thus, they can increase the level of the flow turbulence and swirl which causes augmentation of the transferred thermal energy rate. Therefore, studies can be found about the application of the CWI for the transferred thermal energy improvement.

For instance, Garcia et al. [15] applied experimental set-up to investigate the thermo-hydraulic characteristics of circular tubes, which have CWI for all types of the flow regimes. They found, the impact of the geometry of the CWI on the enhancement of the frictional pressure drop and transferred thermal energy. Thus, they realized that for turbulent flow regime the inserts increase the heat transfer rate and the friction, up to four and nine times, respectively with respect to empty tubes. They did not studied the impact of the operational and geometrical parameters on the thermal efficiency and exergy characteristics and also optimization of the operative and geometric parameters was not performed. Hong et al. [16] by application of the experimental method studied a circular tube with CWI in a turbulent regime. The working fluid of their study was air in the range of 6000< Re< 20000. They found the influence of pitch and width of the coil on the transferred thermal energy and frictional pressure drop and realized the CWI improves Nusselt number 2.5 times while, it increases the friction factor up to 18.62 times with respect to the empty ducts. In their research, the exergy loss characteristic was not investigated and also parameters was not optimized. Agrawal et al. [17] applied experimental test to study the improvement of the transferred thermal energy of R-22 through the horizontal tubes due to application of the CWI. They obtained the influence of the CWI diameter and pitch on the enhancement of the transferred thermal energy and realized it can enhance the coefficient of the transferred thermal energy two times. In their research, no correlations was performed for objective parameters like Nusselt number, thermal efficiency and exergy loss. Esparza et al. [18] used numerical code to simulate the laminar flow through circular tubes, which has CWIs and realized with enhancing the non-dimensional pitch, the frictional pressure drop decreases. However, they did not studied the thermal characteristics. Promvonge [19] experimentally perused the influence of circular and square cross-sections CWIs on the improvement of the transferred thermal energy of the air inside tubes for wide ranges of the Reynolds number. They have changed the common geometry. This is the novelty of their work. Similarly, in the present work different changes on the common geometry will be performed. Gunes [20] applied experimental tests to investigate the effect of non-dimensional pitch and length of the inserts with triangular cross section on the transferred thermal energy and frictional pressure drop enhancement for 3500< Re<27000. They realized that, increasing the thickness of the CWI and Reynolds number and decreasing the non-dimensional pitch, improve the Nusselt number. Like other traditional studies, only the effect of simple geometrical and operational parameters on the thermo-hydraulic (not exergic) characteristics were investigated.

Keklikcioglu et al. [21] did experimental investigation to study the influence of triangle cross section CWI, on the augmentation of the transferred thermal energy in a circular tube. They found, the transferred thermal energy could enhance by 1.67 times compared to the smooth tube by using a coil with specific geometric parameters. The triangle cross section CWI was a novel type of the inserts which was introduced in their research.

Akhavan-Behabadi et al. [22] perused the effect of MWCNT Nano fluid concentration, on the intensification of the transfer of thermal energy to a horizontal tube with CWI, which is constantly heated using an electrical coil installed on the outer wall of the tube. The results of their study shows the transferred thermal energy and the pressure drop can increase by 85 % and 475 %, respectively. They also suggested two correlations to predict the factor of friction and Nusselt number for these types of tubes with CWIs and Nano fluid as the working fluid. The novelty of their research was using of Nano fluid instead of pure fluids.

Gunes et al. [23] did experimental investigation to obtain the effect of geometric and operating variables of a separated CWI in a tube with various pitch and distance from tube wall for wide range of the Reynolds number from 4000 to 26000. They realized the most enhancement in the transferred thermal energy, which is 50 %, occurs when the dimensionless pitch is equal to unity, the distance from the wall tube is 1 mm and the Reynolds number is equal to 4220. This research was a traditional study like ref [20].

Abdul Hamid et al. [24] perused the transferred thermal energy augmentation in a tube with CWIs and TiO₂-SiO₂ and obtained the effect of Nano fluid particle concentration (like Akhavan-Behabadi et al. [22] work) and the operating as well as geometric variables of the heat exchanger. They found for Reynolds number in the range of 2300–12000, the transferred thermal energy could enhanced up to 254.4 % and the friction factor increases 1.76–6.38 times with respect to the empty smooth tube.

Keklikcioglu et al. [25] conducted and experimental investigation for study the affect off dimensionless pitch and then distance between the tube and CWI. It was found, the transferred thermal energy can increase by 1.82 times when dimensional pitch and the distance between tube and inserts are equal to unity and Reynolds number is equal to 3429. It was a traditional study like Gunes et al. [20] and [23] works.

San et al. [26] applied experimental test to peruse the impact of the dimensionless pitch and wire diameter on the Nusselt number and factor of friction for air flow and water flow through a tube with CWI. It was found, with increase of the dimensionless wire diameter and decrease of the dimensionless pitch, the Nu increases. They also obtained correlations for prediction of these two objective variables and obtained the optimum values for dimensionless pitch and wire diameter. It was a traditional study like Keklikcioglu et al. [25], Gunes et al. [20] and 23] works however, an optimization study is also performed. In this research a wider optimization study will be done.

Sharifi et al. [27] used CFD, GA and ANN methods to investigate transferred thermal energy and pressure drop enhancement for twelve types of CWI. By using these methods, they successfully obtained the best type on of inserts. Their method and finding the best geometry for the inserts was the novelty of their reseach.

Sajadi et al. [28] experimentally tested the boiling transferred thermal energy of R123 refrigerant in a horizontal tube, which has a coiled wire, insert. They found the application of the insert enhances the transferred thermal energy coefficient up to 72 % and it enhances the pressure drop up to 85 %. The novelty of their research is that, unlike the previous works, they studied two phases heat transfer instead of single phase.

Marzouk et al. [29] experimentally studied thermal characteristics of the tube side of shell and tube heat exchangers, which has wired nails circular inserts and obtained the effect of geometric as well as operating variables on the objective variables like effectiveness, number of transfer units, exergy loss and overall transferred thermal energy coefficient. It was found the overall transferred thermal energy coefficient, number of transfer units, thermal effectiveness and exergy efficiency increases up to 218 %, 149 %, 224 % and 210 %, respectively compared to the smooth tube. In this work, the exergy characteristics will be investigated like Marzouk et al. [29] study however, the studied heat exchangers are different.

Feng et al. [30] used numerical method for simulation of the laminar flow and transferred thermal energy in a heat sink, which has wire coiled inserts. They realized the transferred thermal energy performance increases between 1.4–1.8 times compared to the smooth micro channel. Fangyong et al. [31] conducted an experimental investigation to study the transferred thermal energy of gas liquid in a horizontal tube, which has CWI. They found the inserts could improve the thermal effectiveness. It was concluded the transferred thermal energy coefficient can increases up to 3.5 times compared to the smooth tube. These two works (i.e. refs [30] and [31]) are also traditional studies like Keklikcioglu et al. [25], Gunes et al. [20] and [25].

Recently, some researchers used exergy concept to optimize the energy systems like heat exchangers. For example,

Prasad and Shen [32] did experimental test to study thermal and exergy performance of the tube with CWIs. He obtained the effect of dimensionless pitch and dimensionless wire diameter on the transferred thermal energy coefficient and exergy loss of the heat exchanger. In the present study, the effect of these mentioned parameters on the exergy characteristics is obtained like Prasad and Shen [32] however the optimization study will be performed and some predictive correlations will be proposed which were not done in Prasad and Shen [32] study.

Liu et al. [33] introduced a new type of inserts for plain tube heat exchangers named fluid exchanger inserts and investigated the thermal characteristics of these heat exchangers for laminar flow of the oil. They obtained the effect of operating as well as geometric parameters of the insert on the friction factor and the Nusselt number and found the Nusselt number can enhance up to 2.69 times, the friction factor enhances up to 7.01 times and exergy efficiency of the tube can improve up to 1.9 % compared to the smooth tube. In the present study the introduced insert is new like Liu et al. [33] work.

Keklikcioglu and Ozceyhan [34] applied experimental method to study the impact of different geometric parameters of the triangular cross sections CWI like side length, dimensionless pitch and the distance between the insert and the tube, on the rate of the entropy generation of the plane tube heat exchangers. The results of their study show that, with increase of the dimensionless pitch and decrease of the Reynolds number, the entropy generation rate decreases. Al-Salem et al. [35] did numerical simulation for investigations of the impact of various geometric and operating parameters on the characteristics of the annulus side of a tube in tube conical coiled tube heat exchangers. They proposed some equations for prediction of exergetic characteristics and obtained the optimum values of the operating and geometric variables. Abu-Hamdeh et al. [36] performed CFD analysis on the pin finned tube heat exchangers. They analysed the dimensionless exergy loss, dimensionless work and the thermal effectiveness of the heat exchangers and found the optimum values for the operating and geometric parameters, which optimize these objective variables simultaneously. Wang et al. [37] simulated the turbulent transferred thermal energy of air flow through a shell, coiled finned tube heat exchangers, and performed the energy and exergy analysis on the data to obtain correlations, which predict the optimal condition. They also found there is a linear relationship between the exergy loss and the transferred thermal energy rate. Alimoradi [38] simulated the turbulent heat transfer and fluid flow through a shell and helically coiled tube heat exchangers. He proposed a correlation for estimation of the exergy efficiency of the heat exchanger as a function of the operating and geometric variables. Furthermore some studies can be found about the local exergy destruction in the heat exchangers. For instance, Liu et al. [39] studied the heat exchangers containing vortex generator and introduced a concept called available potential to investigate the local exergy destruction. They obtained the optimized conditions by using of this concept and minimization of the exergy destruction.

In the present study the impact of the geometrical and operational variables on the exergy characteristics will be studied like [[34], [35], [36], [37], [38]] however the studied inserts geometry, range and method are different with those mentioned works.

The objective of the present study are as follows:

• By applying numerical method which is experimentally validated, new types of the CWIs which have longitudinal fin on their outer surface are introduced. They have not been studied in the previous researches (according to the literature).

• The impact of various operating and geometric variables and the thermal and exergy characteristics will be obtained. While, the previous studies mainly focused on the thermal and hydraulic characteristics.

• Correlations will be proposed for prediction of the dimensionless work, thermal efficiency and the dimensionless exergy loss.

• A multi-objective method is introduced and used to obtain the optimum magnitudes of operating and geometric variables.

According to the literature, these aims and novelty cannot be seen in the previous studies.