Thermohydraulic performance evaluation for horizontal tube by using combination of modified coiled wire inserts and graphene nanoplatelet-water nanouids

Abstract

The effects of combined using of water-graphene nanoplatelet nanofluid and three different conical wire coils (converging, converging-diverging and diverging) on thermohydraulic performance of a heat exchanger tube are experimentally studied. Three different weight fractions 0.5, 0.75 and 1% of Graphene-Water nanofluids are used and converging, converging-diverging, diverging conical wire coils with two different pitch ratios 2 and 3 are selected for the experiments. The experiments are conducted under constant heat flux and Reynolds number ranging from 4000 to 27,000. The results indicated that use of conical wire coils cause to increase both the heat transfer rate and fluid friction as expected, in addition, adding the graphene nanoplatelet in pure water leads to strongly increase in heat transfer rate meanwhile cause to slightly increase in friction factor. Experimental results revealed that combination of diverging conical wire coil insert with weight fraction of 1% Graphene-Water nanofluid provides higher heat transfer performance than the other cases. The highest Performance Evaluation Criteria of 1.73 is observed while the diverging conical wire coil, which has the pitch ratio of 2, at Reynolds number of 6128 for weight fraction of 1% Graphene-Water nanofluid.

Introduction

In complex engineering systems, heat control not only increases the energy efficiency but also extends the service life of these systems and provides operational stability. For this reason, ultrahigh performance cooling is significant in thermal applications that need to augment the heat transfer performance with using various methods. These various methods is classified as passive and active techniques. Several models of the passive technique are employed to heat exchangers with the purpose of enhancing the thermal performance by promoting the turbulence with this way swirl motion, secondary flows and vortices [1]. In recent years, suspending the fluid additives to conventionasl fluids such as oil, water and ethylene glycol and using it in the thermal systems to increase the heat transfer rate is the most remarkable topic [[2], [3], [4], [5], [6], [7]]. It is noteworthy that the base fluids containing nanoparticles are used instead conventional fluids. The primary goal in the application of nanoparticles in basic fluids is to increase the average heat transfer coefficient of the fluids. At the same time, solid particles added to the fluid increase the heat transfer coefficient of the fluid, and contribute to the formation of turbulence in the flow, the increase of the surface area and thus the heat transfer rate.

Nanofluids are novel fluid types that have the opportunity to apply in many fields such as air conditioning [8] and automotive industry, energy and defense industry [9], nuclear and process engineering [10] etc. with the development of nanotechnology in the world. It is known that nanofluids, which have a widespread use, have significantly increased heat transfer with the use of heat exchangers as a result of previous studies. Ghozatloo et al. [11] investigated both the increase in the thermal conductivity coefficient and the increase in the amount of heat transfer by convection using 0.05 0.075, 0.1% graphene-water nanofluid with three different weight ratios in a heat exchanger. Sadeghinezhad et al. [12] conducted a thermal performance analysis in a pipe using graphene-water nanofluid with weight ratios ranging from 0.025 to 0.1% in the range of 5000–22,000 Reynolds numbers. With the use of graphene nanoparticles in the results obtained, thermal conductivity was increased in the range of 7.96–25% compared to base fluid water. Akhavan-Zanjani et al. [13] carried out a study based on the improvement of thermal conductivity coefficient and heat transfer using graphene-water nanofluid in laminar flow conditions in a pipe where constant heat flux was applied. They revealed that the heat transfer enhancement by convection was realized as 17.9, 22.5, 26%, respectively, in the concentration ratio of 0.005, 0.01, 0.02% according to the base fluid in case of using graphene-water fluid. Selvam et al. [14] investigated the effect on the improvement in the average heat transfer coefficient by adding graphene nanoparticles with a volume concentration ranging from 0.1 to 0.5% in a car radiator with a water-ethylene glycol mixture (70,30 by volume) at five different mass flows. In another study Yarmand et al. [15] mixed graphene nanoparticle and platinum nanoparticle into the water as a hybrid nanofluid and investigated the heat transfer enhancement.

The single-use of passive techniques due to its high flow blockage in the tube is prone to increase the pressure drop and therefore result in decreasing the overall performance of the heat exchangers which encourages the researchers to investigate the effect of the combination used of passive techniques. Combined use of a passive technique and nanofluids becomes a significant part of heat transfer enhancement subject. This combination ensures higher thermohydraulic performance in heat exchangers due to high flow mixing rate, turbulance and better thermophysical properties. Moreover, the mixture of nanofluid is supported and the stability is maintained by using an inserts in a tube as a promoter. Hence,in recent studies, the combined use of nanofluids with an inserts has been remarkable and researchers have focused on this field, in recent studies. Adibi et al. [16] numerically analyzed the effects of perforated anchors on heat transfer enhancement in a nanofluid flow region. The results showed that the usage of anchors with Al₂O₃/water nanofuid considerably increase the thermal enhancement factor up to 12.64%. Sundar et al. [3] analyzed energy, economic, heat transfer and pressure drop characteristics of solar flat plate collector with Al₂O₃ nanofluids and wire coil with core-rod inserts. It was concluded that an enhancement of 64.15% for collector efficiency achieved with wire coil with core-rod insert of p/d = 1.79 and 0.3% nanofluid particle loading. In another work, Sundar et al. [17] experimented on the thermal performance of combination of Co₃O₄ deposited rGO hybrid nanofluids and longitudinal strip inserts. For the dilution of 0.2% concentration of hybrid nanoparticles in water and with a straight strip insert of aspect ratio 1, the Nusselt number was enhanced by 25.65% and 110.56%, respectively. Pandya et al. [18] performed a numerical analysis on the effect of the geometrical parameters and particle concentration levels of hybrid nanofluid on the thermal performance of axial grooved heat pipe. The study revealed that the highest heat flow was obtained at 1.25% volume concentration of hybrid nanofluid. Hamid et al. [19] studied the heat transfer performance of wire coil inserted tube with using water/EG as a working fluid. In the study, the combined effect of the wire coil and TiO₂ − SiO₂ nanofluids also investigated. It was reported that the average heat transfer enhancement was recorded between 24 and 142%, respectively for the particular pitch ratios between 4.17 and 0.83. Thermal performance in a tube with wire coils in two different pitches experimentally carried out by Akyurek et al. [20]. According to the obtained results, wire coil with 25 mm pitch showed better heat transfer performance than the 39 mm. Goudarzi and Jamali [21] experimentally studied the heat transfer enhancement of a car radiator with Al₂O₃-Ethylene Glycol nanofluid together with wire coils. They reported that the use of wire coils in the left and right side of the car radiator increased the Nusselt number about 7%. Due to its compatibility in the dual phase flow, the helically coiled wire has been also used in many heat transfer enhancement investigations [[22], [23], [24], [25]] include nanofluid as a working fluid.

Recently, Graphene nanoparticles has remarkable results due to their specific properties, such as higher thermal conductivity, better stability and larger surface area [5]. The enhancement of thermophysical properties of base fluids with using Graphene nanoplatelet has also been reported in literature and it has been concluded in previous studies that the thermohydraulic performance of a heat exchanger tube achieves strong values by using wire coils. It was also reported that wire coil inserts can be used to create secondary flow causing enhancement in turbulent fluctuations and show significant effect on the thermal performance of combined system and graphene has good potential to improve the performance of all these systems [26,27]. Although the mono use of the passive methods is effective in enhancing of heat transfer, it is an undesirable fact that it also increases the pressure drop [28,29]. Therefore, by combining Graphene nanoparticles, which have unique thermophysical properties, and specially designed wire coil inserts that are relatively low in increasing pressure drop, the potential improvement resulting from the use of these two methods together has been revealed for the first time in this study. In our previous study [30], the entropy generation analyses of converging, converging-diverging and diverging wire coils in a heat exchanger tube experimentally analyzed. It was determined that the diverging conical wire coils caused the lowest entropy generation number. Unlike the previous study, in this study, in addition to the mono use of the conical wire coils, the graphene nanoparticle is added and the thermohydraulic performance of the heat exchanger tube is examined. Thus, the following critical issues are addressed in this study.

• Effects of modified inserts on flow characteristics, flow blockage and ability to destruct boundary layer depending on the use with graphene/water nanofluid.

• Effect of the combined use of graphene/water nanofluid with modified wire coils on forced convective heat transfer and pressure drop.

• Effect of combined use on overall performance depending on nanofluid weight fraction, placement of wire coils in the tube and Reynolds number.