Experimental Investigation of the Thermal-hydraulic Performance of Conically Coiled Tubes using Al 2 O 3 /Water Nanofluid

-The thermal-hydraulic performance of conically coiled tubes ( CCT s) was investigated experimentally under constant heat flux boundary conditions in this study. The effect of several factors, such as the flow Dean number, coil torsion, and varied nanoparticle weight concentrations, on the flow's heat transfer coefficient and pressure drop was studied. Thermo-hydraulic performance was studied at 0.3%, 0.6%, and 0.9% volume concentrations ( ϕ ) of Al 2 O 3 /water nanofluid, a Dean number ( De ) of 1148–2983, and coil torsions (λ) ranging from 0.02 to 0.052. Results indicated that the heat transfer rates ( HTR s) of CCT s increased when the coil torsion was decreased. According to the findings, the average heat transfer coefficient ( h avg ) rises as De increases, while friction factor (ƒ) tends to decrease. The average increase in h avg is 32% at lower De values and 26% at higher De values as the nanofluid concentration increased from 0.3% to 0.9%. The thermal performance factor ( TPF ) improved when λ lowered from 0.052 to 0.02.


I. INTRODUCTION
Numerous scientific studies have demonstrated that heat transfer rates (HTRs) in coiled tubes surpass those of straight tubes.Due to its compact design and efficient HTR, this type of coil is able to deliver a substantial heat transfer area in a little space.In the current era, adding nanoparticles to base fluids can improve heat transfer capability.These nanotechnology-based fluids, termed as nanofluids, have outstanding properties such as high thermal conductivity.Today, nanofluids are used to improve thermal performance in a wide variety of applications [1][2][3][4][5][6].Recent research has proven that the use of nanofluids and coiled tubes concurrently may considerably increase the HTR [7][8][9].
Heyhat et al. [10] examined the effect of nanofluids and CCT geometrical variables on pressure drop and heat transfer.With studies performed under laminar conditions, various SiO2/water concentrations were utilised.As geometric variables, different cone angles and coil pitches were utilised.According to the data, increasing the coil pitch enhances the HTR.Moreover, cone angle variation enhances heat transfer more effectively than coil pitch variation.Sheeba et al. [11] studied the experimental and numerical heat transfer properties of a CCT heat exchanger.According to their observations, the inner Nusselt number was substantially affected by the inner and annulus Dean number.Moreover, according to their results, the employment of a CCT heat exchanger as opposed to an HCT type may result in an increase in the overall heat transfer coefficient.
Mansouri and Zamzamian [12] investigated experimentally the HTR and pressure drop properties of Al2O3/water nanofluid within a horizontally helically coiled tube (HCT).Results demonstrated that the h increased by 6.4%, 19.0%, and 23.7%, respectively, for heat fluxes of 2283 and 4975 W/m 2 at a 1 wt% concentration of Al2O3/water nanofluid compared to pure water.Hasan and Bhuiyan [13] investigated the thermal performance and the entropy generation of a HCT heat exchanger with different coil revolutions utilizing a 5% concentration of Al2O3/water nanofluid.Results demonstrated that when coil revolutions increase, the total HTR and friction factor increase.Maximum entropy generation increased by 19.5% when the coil revolution was varied but the rib profile remained unchanged.
Sundar and Shaik [14] studied experimentally the TPF at varied volume fraction of water and EG mixture based ND nanofluids flow in a shell and HCT heat exchanger.According to observations, at a Reynolds number of 2702 and a volume fraction of 1%, h and Nu are enhanced by 36.05 and 27.47 %, respectively, and the TPF is enhanced by 1.2094 compared to the base fluid.Hasan et al. [15] studied numerically the heat transfer performance of an HCT heat exchanger using various nanofluids, taking into account diverse head-ribbed geometries with varied coil revolutions.The HTR was increased by 20% to 80% using a 2-rib head geometry and by 17% to 66% using a 30-turn coil.It has been determined that Al2O3 has the maximum HTR, whereas SiO2 has the lowest.
Kia et al. [16] conducted a computational and experimental investigation of the HTR and pressure drop characteristics of Al2O3 and SiO2/Base oil nanofluid flow in a HCT subject to uniform heat fluxes.The results demonstrated that using nanofluid in place of the base fluid boosted the heat transfer factor and pressure drop.The greatest HTR is associated with Al2O3 and SiO2 nanofluid volume concentrations of 0.5%, which are 41.4% and 27.3% greater than base oil, respectively.Güngor et al. [17] investigated the thermal performance of a new shell and HCT heat exchanger design with integrated rings and discs both numerically and experimentally.The results showed a 22.1% average rise in the overall coefficient of heat transfer for a 3 l/min hot fluid flow rate as well as a 19.6% average growth for a 4 l/min.Fuxi et al. [18] investigated the effects of employing hybrid nanofluids in a shell and HCT heat exchanger.One of the effective variables in boosting the rate of heat transfer is the impact of increasing the Re.Thus, at a pitch of 20 mm and a volume fraction of 4%, a 66.7% rise in Nu was seen when the Re number increased from 10,000 to 20,000.Tuncer et al. [19] utilized single and hybrid type nanofluids to investigate the thermal performance of a shell and HCT heat exchanger with and without fins.It was discovered that the effect of utilizing TiO2/water single and CuO-TiO2/water nanofluids at high Re is comparatively greater than at lower Re.Also, It was discovered that using a singletype TiO2/water nanofluid on the coil side of shell and HCT heat exchanger with and without fins resulted in an average 9.2% and 14% improvement in the coefficient of heat transfer, respectively.
Kumar and Chandrasekar [20] investigated the HTR and friction factor of MWCNT nanofluids in a double HCT heat exchanger.It is believed that MWCNT/water nanofluids resulted in a greater convective heat transfer than water.It is also believed that heat transfer rises with increasing MWCNT/water nanofluid volume fraction.At a 0.6% volume concentration of MWCNT/water nanofluids at a flow rate of 140 L/ h and a Dean number of 1400, the greatest convective heat transfer of 35% was observed.Al-Abbas et al. [21] conducted experimental research and exergy analyses on shell and HCT heat exchangers for free convection heat transfer.It has been demonstrated that exergy efficiency increases linearly with increasing De and decreasing volume flow rates of cold water.Moreover, as the diameter of the coil increases, the pressure drop decreases noticeably.
Shafiq et al. [22] investigated numerically the thermal performance enhancement of shell and HCT heat exchangers utilizing MWCNTs/water nanofluid.The findings indicate that the Nu of the fluid flowing through the coil improved as the Re of the coil and nanofluid volume concentration rose.When a 0.5% volume concentration of MWCNT/water nanofluid was employed at a coil Re of 15,000, the maximum improvement in Nu, i.e., 31.5%, and the highest-pressure decrease, 47.35 %, were found under the same situations.Khalil et al. [40] studied experimentally the impact of placing coil wire in the shell side of a doublepipe heat exchanger on pressure drop and heat transfer.As comparison to the smooth pipe, the heat transfer coefficient rose up to 1.59 while the pressure drop increased 10 times.Findings also shown that the increase in heat gain is far greater than the reduction in pumping power.
Khosravi-Bizhaem et al. [41] studied experimentally the HTR and the entropy generation for nanofluid flow through HCTs.The findings demonstrated that Ag nanoparticles increase thermal conductivity and h by 8 to 25%, MWCNT/water increases the HTR by up to 10%, whilst GO nanoplates reduce the HTR.Recent experimental and computational investigations into heat transfer in coiled tubes are represented in Table 1.
Regardless of the fact that substantial study has been performed on nanofluids and HTCs independently, only a few studies have been completed on the case of merging the CCTs and nanofluids approaches, and more research is required.Experimentally investigating the thermal-hydraulic performance of Al2O3/water nanofluid flow within CCTs with a uniform heat flux.All experiments were conducted using a broad range of Dean number (De).The authors look at how the nanofluid concentration, De, and coil torsion affect heat transfer and pressure drop along the CCTs.

II. EXPERIMENTAL SETUP AND PROCEDURE
The experimental test rig is depicted in Figure 1.Conically coiled tubes (test section), storage tank, cooling system, pump, pipes, control system, flowmeter, and pressure sensors comprise the experimental setup.Table 2 shows the geometric parameters of the three CCTs with the same curvature ratio but different coil torsions.The nanofluid tank is filled with nanofluid with the required volume fraction of nanoparticles.The reservoir's external wall was covered to avoid losing heat and was made of stainless steel.
The thermocouples are first attached to the appropriate positions.Afterwards, nanofluid was cycled through into the test section (CCT) and the experiment was conducted with the proper equipment to regulate and measure the factors that affect the heat transfer and pressure drop.Included in these factors are flow rate, temperature, and pressure.
The instruments used in the experimental setting are detailed in Table 3.The uniform heat flux on the tubes' surfaces was provided using an electrical heater that was wound around a CCT and adequately insulated.Twelve Ktype thermocouples were used to determine the surface temperatures of the tube, and all temperatures were collected using a data acquisition device.All parameters are collected and stored when the system enters a steady state.

III. PREPARATION AND STABILITY OF NANOFLUID
In this study, a stable Al2O3/water nanofluid was used as a heat transfer fluid.Al2O3 nanoparticles with a notional average diameter of 50 nm were suspended in distilled water using a twostep procedure.The nanofluid was created with three concentrations by volume: 0.3%, 0.6%, and 0.9%.
Utilizing the appropriate surfactants, nanoparticles are suspended in nanofluid, and an ultrasonic bath is then used to separate the nanofluid clusters.During a 6-day visual examination of the stability, no sedimentation was seen.As demonstrated in Fig. 2, the free surface did not appear until 7 days had passed.Table 4 shows the thermo-physical properties of pure water and Al2O3 nanoparticles.Fig. 3 shows transmission electron microscopy (TEM) analysis of Al2O3 nanoparticles to identify their size.The average size of nanoparticles is 50 nm.

After preparation
After 7 days Assuming that the nanoparticles are uniformly dispersed throughout the base fluid, the relevant physical properties of the composites have been computed via well formulas/models for single-phase fluids, as given in Table 5.

B. Heat Transfer Calculations
For the estimation of Nu and ƒ, the bulk temperature of a fluid was determined using the following formula.
The following formula is used to calculate the total rate of heat transfer in the CCTs: ˚= ̇   (  −   ) (12) Because it was possible to measure the temperature at multiple sites along the tube, the heat transfer coefficient was computed locally in this study.The local h was computed as follows [27].
Ȧ ( , −  , ) (13) where  , is the wall surface temperature and  , is the fluid bulk temperature at each location.
Using the trapezoidal approach, the average h was calculated numerically by integrating the local values along the length of the tube as follows [28]: The following formulas were used to get the average Nu and ƒ: V. UNCERTAINTY ANALYSIS The accuracy of experimental results is contingent upon the accuracy of specific measuring devices and methodologies.Kline and McClintock [29] showed that a root-sum-square combination of the effects of the different inputs can be used to find the uncertainty in a calculated result with a high degree of accuracy.The partial derivatives of each variable are assessed in order to weight the overall uncertainty based on the uncertainty associated with each variable.The major parameters, h avg and ƒ, employed in this work to interpret experimental results are dependent of several factors, including laboratory measurement data and physical attributes.The uncertainty of the preceding experiment was evaluated using Equation (18).
R is a function of J independent measurable variables.  and   are the result and independent variable measurement uncertainties, respectively.Table 6 summarises, for all experimental runs, the average uncertainty of key parameters.

VI. RESULTS AND DISCUSSION
In this section, the major outcomes of the experimental testing for water and Al2O3/water are presented and described in depth.Experiments were performed on CCTs with De ranging from 1148 to 2983, concentrations of nanoparticles varying from 0.3% to 0.9%, and λ ranging between 0.02 and 0.052 using the same curvature ratio.The flow rate ranged from 0.03 to 0.09 kg/s, and the fluid temperature at the entry of the CCTs was kept at 25 °C.

A. Heat Transfer Results
According to earlier research [7,8,12], the addition of nanoparticles to pure water enhances the thermal conductivity and, therefore, the HTRs.Increasing the volume concentration of Al2O3 nanoparticles will further enhance heat conductivity and the disruptive effect of the nanoparticles.Figure 4 illustrates the variation in ℎ  versus De.A decrease in the coil's torsion led to an increase in ℎ  , according to the results.This is a consequence of the amplification and expansion of secondary flow, which leads to enhanced HTR.CCTs produce secondary flow generation to facilitate the mixing of nanoparticles with low-viscosity fluids while preserving the nanofluids' stability.In addition, according to the data, boosting the Al2O3/water nanofluid concentration from 0.3% to 0.9% increased ℎ  .The average increase in ℎ  at lower and higher De values is 32% and 26%, respectively.The incorporation of nanoparticles may improve the HTR owing to the higher thermal conductivity of Al2O3/water nanofluid as well as the randomized motion of nanoparticles, which promotes heat transfer.In other words, the processes of heat transmission get more intense as the concentration of nanoparticles increases.
Figure 5 depicts the experimental average h of Al2O3/water nanofluid to that of pure water ( h nf h w ) within CCTs at all nanofluid concentrations tested.As predicted, this ratio rises with increasing volume fraction of nanoparticles.For instance, the ratio of the average h of nanofluids to that of distilled water ranges between 1.28 and 1.5 for 0.9% nanofluids within CCTs.

B. Pressure Drop Results
The hydraulic performance may be evaluated by measuring the pressure drop over the investigated CCTs.Three volume concentrations of Al2O3 nanoparticles are considered: 0.3%, 0.6%, and 0.9% at coil torsion (λ) varied from 0.02 to 0.052.The relationship between friction factor (ƒ) and De is shown in Fig. 6.
The friction factor (ƒ) tends to decrease as De increases.This is brought on by an increase in centrifugal force and the consequent formation of vortices brought on by a reduction in torsion or rotational impact.According to the data, increasing the Al2O3/water nanofluid's concentration from 0.3% to 0.9% also decreased ƒ.For low De and high De, respectively, the average increment in ƒ is 40% and 22%.When nanoparticles are included, the HTR may be increased due to the greater thermal conductivity of nanofluid and the randomized motion of nanoparticles, both of which contribute to enhancing heat transfer.Furthermore, the dynamic viscosity has a significant impact on the drop in pressure across the CCTs, rather than the nanoparticles' density, velocity, and concentration.This impact increases significantly with increasing nanoparticle concentration [30].

C. Thermal-Hydraulic Performance
The thermal performance factor (TPF) is situated between the augmentation of heat transfer and the pressure drop of the CCTs.It is used to evaluate the heat transfer improvement of CCTs under the same pumping power and may be computed using the following formula: In this study, the TPF is utilized to assess the performance of CCTs using Al2O3/water nanofluid and compared to the comparable value for pure water.The average h and ƒ are insufficient only for determining the performance CCTs.
Consequently, the thermal performance factor is shown in this section to demonstrate the superior performance of CCTs.The relationship between the TPF and ϕ for varying particle volume fractions is depicted in Fig. 7.In general, the TPF increases with increasing Al2O3/water nanofluid nanoparticle concentrations.It indicates that in all conditions under consideration, heat gain exceeds pressure loss.The capacity of the nanopowder to increase the thermal conductivity of the base fluid is given as the first explanation, while the mobility of nanoparticles allowing energy transfer is proposed as the second explanation.
According to Fig. 7, TPF values larger than unity show that the heat transfer augmentation process exceeds the pressure drop penalty advance, demonstrating the efficiency of CCTs and nanofluids in enhancing system performance.The TPF increased by 26% when the ϕ was increased from 0.3% to 0.9% for coil #1 with the same De value.For coil #2, the TPF increased by 22%.Furthermore, for coil #3, the TPF increased by 19%.

VII. CONCLUSIONS
This work was conducted to investigate the effects of Al2O3/water nanofluid on the pressure drop and heat transfer of conically coiled tubes (CCTs).The thermohydraulic properties of CCTs with Dean numbers (De) ranging from 1148 to 2983, coil torsions ranging from 0.02 to 0.052, and particle volume concentration values ranging from 0.3% to 0.9% were investigated.In addition to that, the outcomes that were achieved are as follows:  A reduction in CCT's coil torsion resulted in an increase in the average heat transfer coefficient.This is related to the development and enhancement of secondary flow, which contributes to the improvement of heat transfer rates. The average heat transfer coefficient increased as the nanofluid concentration went from 0.3% to 0.9%.The average increase is 32% at lower values of De and 26% at higher values of De.Due to their higher thermal conductivity and the random motion of the nanoparticles, Al2O3/water nanofluids are better at transferring heat than pure water. The Dean number has a tendency to increase, which results in a reduction in the friction factor. The thermal performance factor (TPF) increased when the coil torsion decreased from 0.052 to 0.02.Due to the fact that the TPF for testing nanofluids is more than unity, the employment of nanofluids and CCTs is an energy-efficient way for increasing the thermal capacity of equipment.

VIII. FUTURE RECOMMANDATIONS
Hyper nanofluids may be used to conduct future studies in this area.In addition, the geometry of the coil may be altered in conjunction with a thorough examination of other operating conditions.A numerical model may also be used, and its results may be compared to the experimental findings of this study.The use of nanofluids has become an urgent need to improve the heat transfer process of thermal equipment, particularly those that operate at high temperatures under complex working conditions.
Funding: This research has not received any type of funding.

Conflicts of Interest:
The authors declare that there is no conflict of interest.

Fig. 5 .
Fig. 5.The variations of relative   with ϕ at different coil torsions