Effect of rotating twisted tape on thermo-hydraulic performances of nanofluids in heat-exchanger systems

doi:10.1016/j.enconman.2018.04.086

Energy Conversion and Management

Volume 166, 15 June 2018, Pages 744-757

https://doi.org/10.1016/j.enconman.2018.04.086 Get rights and content

Highlights

•
Exergy-resistance instead of thermo-hydraulic of nanofluids is analyzed.
•
Effects of rotating instead of static twisted tape on nanofluids are studied.
•
Rotating twisted tape with nanofluids can enhance heat transfer by 101.6%.

Abstract

Stable TiO₂-H₂O nanofluids are prepared and their stabilities are studied. An experimental set for studying the heat transfer and flow characteristics of nanofluids is established. Heat transfer and flow characteristics of TiO₂-H₂O nanofluids in a circular tube with rotating and static built-in twisted tapes are experimentally investigated and compared. An innovative performance evaluation plot of exergy efficiency is developed and the exergy efficiency of tube with rotating and static built-in twisted tapes filled with nanofluids is analyzed in this paper. The results indicate that the combination of rotating built-in twisted tape and TiO₂-H₂O nanofluids shows an excellent enhancement in heat transfer, which can increase the heat transfer by 101.6% compared with that of in a circular tube. The effects of nanoparticle mass fractions (ω= 0.1%, 0.3% and 0.5%) and Reynolds numbers (Re = 600–7000) on the heat transfer and flow characteristics of TiO₂-H₂O nanofluids are discussed. It is found that there is a critical Reynolds number (Re = 4500) for the maximum value of relative heat transfer enhancement ratio. The comprehensive performance of the experimental system is analyzed. It can be found that the comprehensive performance index of the experimental system firstly increases and then reduces with Reynolds number, and it can reach 1.519 at best. However, for the performance evaluation of exergy efficiency, the coupling of rotating twisted tape and nanofluids deteriorates the exergy efficiency. Also, it can be found that the exergy efficiency of the circular tube with twisted tape is greater than that of circular tube under the same pumping power and pressure drop, but it shows deterioration under the same mass flow rate.

Graphical abstract

Introduction

With the development of science and technology, the thermal load of the heat exchanger gradually increases. Also, the traditional structure of heat exchanger and working fluid cannot meet the requirement of heat exchanger in a limited heat exchange area. Hence, the heat transfer enhancement technology needs to be improved.

Improving the thermal conductivity of the working medium is one way to enhance the heat transfer. Nanofluids, as a new type of high efficient energy transport medium, have great application values in many fields. Huang et al. [1] added the Au@TiO₂ core-shell nanoparticles into the clean water. It was found that the core-shell structure can improve the photo-thermal conversion efficiency and the evaporation of seawater. Many scholars applied nanofluids to solar photothermal conversion. Chen et al. [2] studied the solar absorption performances of different core-shell nanoparticles. It was found that the core-shell ratios and mixing ratios of nanofluids are two key factors for improving the absorption of solar energy efficiency. Wang et al. [3] applied CNT nanofluids with different concentrations to direct solar steam generation and found that the evaporation efficiency can reach 45% under a solar illumination power of 10 Sun when the concentration of CNT nanofluids is 0.001904 vol%. Liu et al. [4], [5] proposed the principle of photonic nanofluids and studied the solar-thermal conversion efficiencies of different types of nanospheres.

Xuan et al. [6] presented a procedure for preparing nanofluids and proposed a theoretical model to calculate the heat transfer performance of nanofluids. Oztop et al. [7] researched the natural convection of nanofluids in rectangular enclosures by numerical simulation. It was found that the heat transfer enhancement of low aspect ratio is much better than that of high aspect ratio. Heris et al. [8] investigated the heat transfer characteristic of Al₂O₃-water nanofluids in a circular tube and found that the heat transfer coefficient increases with nanoparticle concentration and Peclet number. Li et al. [9], [10] measured the thermophysical properties of nanofluids and found that metal nanoparticles can increase the thermal conductivity and viscosity of the fluid. Fu et al. [11] analyzed the viscosity of Fe₃O₄ ethylene glycol-water nanofluids considering the effect of particle disaggregation. It was found that nanofluids behaved as Newtonian fluid when the nanoparticles were evenly dispersed in the base fluid. Hong et al. [12] investigated the dynamic concentration of nanofluids in laminar low and proposed an empirical equation to calculate the concentration of nanoparticles in a pipe. It was found that the concentration of nanofluids decreases from the wall to centre in the pipe and it has a maximum value near the pipe wall. Sheremet et al. [13] studied the effects of boundary temperature oscillating frequency on the natural convection of a square cavity filled with alumina-water nanofluids and found that Nusselt number increases with the oscillating frequency of boundary temperature. In addition, Sheremet et al. [14] numerically investigated the natural convection of a triangular cavity filled with micropolar fluid. It was found that the average Nusselt number and fluid flow rate all decrease with the vortex viscosity parameter. Also, Sheremet et al. [15] analyzed the natural convection of Cu-water nanofluids in a cavity and found that heat transfer decreases with Hartmann number. Sheikholeslami et al. [16] researched the natural convection of magnetohydrodynamic nanofluids and found that Nusselt number increases with Darcy number, supplied voltage and Rayleigh number. Sheikholeslami et al. [17] also studied the effect of uniform magnetic field on natural convection of nanofluids in a porous media with sinusoidal hot cylinder and found that temperature gradient decreases with Hartmann number. In addition, Sheikholeslami et al. [18] investigated the effect of nanoparticle shape on heat transfer by means of CVFEM. It was found that Platelet shaped nanoparticles has the highest heat transfer performance.

Rudyak et al. [19] conducted an experiment on aluminum lithium-liquid argon nanofluids with different nanoparticle sizes. It was found that the viscosity of nanofluids increases with the decreasing nanoparticle size. Pendyala et al. [20] and Ilyas et al. [21] applied nanofluids to transformers and obtained that adding CNTs and graphite nanoparticles with different sizes can significantly improve the thermal conductivity of fluid. Kouloulias et al. [22] studied the precipitation of Al₂O₃-H₂O nanofluids and analyzed the natural convection heat transfer characteristics of nanofluids. It was found that Nusselt number decreases with the nanoparticle concentration. Qi et al. [23] conducted an experiment on different rotation angles of enclosure filled with TiO₂-water nanofluids. It was found that the enclosure with rotation angle α = 0° has the highest Nusselt number. Qi et al. [24], [25] studied the effects of nanoparticle radius on the natural convection heat transfer by numerical simulation and found that Nusselt number decreases with the increasing nanoparticle radius. Also, Qi et al. [26] investigated the natural convection heat transfer of enclosures with different aspect ratios and found that Nusselt number increases with the aspect ratio of the enclosure. Qi et al. [27] also researched the boiling heat transfer of TiO₂-water nanofluids. The results showed that TiO₂-water nanofluids enhance the heat transfer coefficient by 77.7% at best compared with water. In addition, Qi et al. [28] introduced nanofluids as a working medium to cool the CPU. It was found that Al₂O₃-H₂O and TiO₂-H₂O nanofluids can reduce the temperature of CPU by 23.2% and 14.9% at best compared with based fluid (water) respectively.

Above studies show that nanofluids with a certain mass fraction can play a role in enhancing heat transfer. In order to improve the heat transfer of heat exchanger, enhanced tubes are used instead of smooth tube. In addition, researchers have done some work on the heat transfer of nanofluids in enhanced tubes.

Shahril et al. [29] studied the heat transfer performance of Cu-H₂O nanofluids in a concentric tube. It was found that the thermal conductivity can be improved by 60% when the volume fraction of nanoparticles reaches 2%. Sun et al. [30], [31] researched the flow and heat transfer of different types of nanofluids in the built-in twisted belt external thread tubes. The results presented that the coupled heat transfer between Cu-H₂O nanofluids and the built-in belt can improve the heat transfer by 50.32%. Naphon et al. [32] experimentally studied the flow and heat transfer characteristics of TiO₂-water nanofluids in a horizontal spirally coiled pipe. The results presented that the heat transfer can be improved by 34.07% when the volume fraction of nanofluids is 0.05%. Qi et al. investigated the heat transfer characteristics of nanofluids in a corrugated tube [33], a spirally fluted tube [34] and a horizontal elliptical tube [35] respectively. It was found that the heat transfer of enhanced heat tubes can be greatly improved at the cost of little increase in flow resistance compared with that of conventional tubes. Sundar et al. [36] experimentally studied the heat transfer of CNT-Fe₃O₄/water hybrid nanofluids in a built-in twisted tape tube. The study found that the built-in twisted tape tube can enhance the Nusselt number by 42.51%.

The first law of thermodynamics is about the quantity of energy, but the second law of thermodynamics is about the quality of energy. Therefore, the second law of thermodynamics is more suitable for evaluation of the heat exchanger heat transfer process under certain conditions. Based on the second law of thermodynamics, scholars conducted many researches on entropy and exergy.

Khalkhali et al. [37] studied the entropy production of heat pipes, and found that the entropy production is caused by the temperature difference of the hot and cold fluids, the flow friction and the evaporation temperature/pressure drop along the heat pipe. Haddad et al. [38] obtained the distribution of entropy production based on the entropy production equation and studied the effects of different thermal boundary conditions on heat, viscosity and total entropy production. It was found that the entropy production and the Reynolds number are inversely proportional to the dimensionless inlet temperature and proportional to the radius ratio. Ploumen et al. [39] studied the exergy efficiency of three different types of turbines and pointed out the main components of the exergy loss. The results showed that the exergy loss of the combustion chamber accounted for 22%. Replacing the combustion chamber with a fuel tank can reduce the exergy loss by 10%. Gutowski et al. [40] analyzed the energy conversion process in manufacturing, and summarized the thermodynamic data of the thermal efficiency and exergy efficiency of materials in the manufacturing process by energy analysis and exergy analysis. Modarresi [41] studied the process of producing bio-ethanol, bio-methane, heat and power from wheat straw using exergy analysis. It was found that the bio-ethanol process has the highest exergy efficiency.

It can be seen from above studies that researchers have made great contributions to the heat transfer enhancement of nanofluids. However, there is little research on the effects of the rotating built-in twisted tape on heat transfer and flow characteristics of tube filled with TiO₂-H₂O nanofluids, also, there is no an exergy efficiency evaluation criteria. In this paper, heat transfer and flow characteristics of TiO₂-H₂O nanofluids in a circular tube with rotating and static built-in twisted tapes are experimentally investigated and compared. The influences of nanoparticle mass fraction and Reynolds number on the comprehensive thermo-hydraulic performances are analyzed. The main innovations are as follows: (1) Unlike the thermo-hydraulic comprehensive evaluation frequently adopted by researchers, exergy-resistance comprehensive evaluation instead of it is analyzed, and an innovative performance evaluation plot for exergy efficiency is developed; (2) Unlike the studies of the effects of static built-in thermo-hydraulic performance, the effects of rotating instead of static twisted tapes on exergy-resistance performance are investigated.

Section snippets

Nanofluids preparation and stability study

In this paper, TiO₂-H₂O nanofluids with different mass fractions (ω = 0.1%, 0.3% and 0.5%) are prepared by a two-step method. Firstly, nanoparticles are added into the base fluid (deionized water), then some dispersant and NaOH are added to prevent nanoparticles from gathering or precipitating, finally, the nanofluids are oscillated by ultrasonic about 40 min to make the nanoparticles distribute uniformly in the base fluid. The preparation process is shown in Fig. 1. Table 1 shows the

Experimental system validation

Before testing the heat transfer and flow characteristics of the experimental system, experimental system verification is carried out to ensure its correctness and reliability.

The heat transfer and flow characteristics of deionized water at different Reynolds numbers in a circular tube have been researched in this section. Fig. 14 shows the comparisons between experimental results and the results calculated by Sieder-Tate formula [44], Gnielinski formula [44] and the results of Pak [45]. It can

Conclusions

Heat transfer and flow characteristics of TiO₂-H₂O nanofluids in a circular tube with rotating and static built-in twisted tapes are experimentally investigated and analyzed by exergy efficiency in this paper. Some conclusions are obtained as follows:

(1)
An innovative performance evaluation plot for exergy efficiency is developed in this paper, and it is shown that Region 4 (the highest slope) has the largest exergy efficiency, which can provide some help in exergy efficiency analysis for future

Acknowledgements

This work is financially supported by “National Natural Science Foundation of China” (Grant No. 51606214).

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Effect of rotating twisted tape on thermo-hydraulic performances of nanofluids in heat-exchanger systems

Highlights

Abstract

Graphical abstract

Introduction

Section snippets

Nanofluids preparation and stability study

Experimental system validation

Conclusions

Acknowledgements

Energy Convers Manage

Appl Energ

Energy Convers Manage

Sol Energy

Int J Heat Fluid Flow

Int J Heat Fluid Flow

Int J Heat Mass Transf

Exp Therm Fluid Sci

Int J Heat Mass Transf

Int J Heat Mass Transf

Int J Heat Mass Transf

Int J Heat Mass Transf

Int J Heat Mass Transf

Int J Heat Mass Transf

Phys Lett A

Procedia Eng

Int J Heat Mass Transf

Int J Heat Mass Transf

Int J Heat Mass Transf

Energy Convers Manage

Int J Heat Mass Transf

Int J Heat Mass Transf

Int J Heat Mass Transf