Thermal performance characteristics of a triple spiral tube heat exchanger

https://doi.org/10.1016/j.cep.2019.107707Get rights and content

Highlights

  • A new design of triple spiral tube heat exchanger is presented.

  • Three dimensional CFD study is achieved.

  • The effect of hot fluid source inlet temperature, flow arrangements and Alumina nanofluid and energy saving are the main points of interest.

  • Nusselt number, friction factor, Effectiveness, heat transfer per unit pumping power and performance index are presented.

Abstract

In the current paper, the thermo fluid characteristics of a new design of curved tube called a triple spiral tube heat exchanger (TSTHE) was achieved with a brave comparative with a double spiral tube heat exchanger (DSTHE) as particular reference. A three-dimensional computational fluid dynamic model was developed using Ansys 14.5 software package. The Realize k–ε model had been applied with enhanced wall treatment to simulate the thermo-fluid characteristics. The governing equations were solved by a finite volume discretization method. The new design was created by adding a third tube to a double concentric spiral tube heat exchanger. The study carried out under consideration of turbulent fluid to fluid heat transfer condition. The influence of different key design parameter such as; inlet hot fluid temperatures, flow arrangements, concentration of Alumina (Al2O3) nanoparticles, and Dean number were the main point of interest. The effect of Alumina-water nanofluid was considered in three different concentrations of 0.5%, 1% and 2% by volume. The results carried out at Dean number ranged from 1100 to 10,500 in the turbulent region corresponding to Reynolds number from 6400 to 57,400. The results presented that the Nusselt number obtained from a TSTHE was higher than that value of DSTHE at the same flow conditions. The results also presented that a significant effect of hot water inlet temperature on the Nusselt number while the increase in pressure drop can be approximately considered neglect. Also, the counter flow arrangement gives better effect on hot fluid Nusselt number compared with the three other flow arrangements. A notice augmentation of heat transfer enhancement occurs due to use Alumina–water nanofluid in TSTHE by 20.8 %. New correlations for inner annulus hot fluid Nusselt number, effectiveness, and friction factor of heat exchanger were presented.

Introduction

Curved tube heat exchangers are widely used in many specific engineering applications such as chemical processes, refrigeration and air conditioning, power station, food processing, and recovery processes of waste heat systems. The spiral tube heat exchanger consists of one (or more) tube/s that wounded around a center to form a spiral shape with an excellent compact volume. A double spiral tube heat exchanger can be converted into a triple tube one by adding an intermediate tube. The third tube attempted to improve the rate of heat transfer as a result of adding a new flow passage and increasing a heat transfer surface area per unit length. The main advantage of using curved tubes is to provide more surface areas for a given volume (compactness), moreover use the benefits of centrifugal force which created as a result of fluid motion in curved tubes. This centrifugal force induce a secondary flow which depending on coil curvature ratio and attempted to enhance the heat transfer characteristics compared with that of a straight tube. Wang et al. [1] study the heat transfer and fluid flow in a trilobal helically tube experimentally and numerically. The study presents the influence of coil structure parameters on the thermo fluid characteristics. The results indicate that the heat transfer augmentation in helical trilobal tube is higher than that of helically plain tube by 1.16–1.36 times on the change of increasing the friction factor by 0.96–1.10 times. Also, the performance criteria of helical trilobal tube is higher than helical elliptical tube by 1.32 times at the same conditions. Wang et al. [2] achieved the heat transfer characteristics in helical tubes with respect to single phase flow. The study investigated the influence of both centrifugal and buoyance forced on heat transfer coefficients. A non dimensionless number between the centrifugal force and buoyancy effect were used to predict a heat transfer characteristics in helical coiled tube. Zhou et al. [3] achieved the thermal performance of multi-row helical heat exchanger. The Nusselt number inside and outside the multi raw helical tubes were considered with the fluid to fluid heat transfer boundary condition. Moya-Rico et al. [4] using an artificial neural network to characterize a triple concentric tube heat exchanger with corrugated tubes. Elattar et al. [5] present numerically the effects of operating and geometric parameters on the novel multi tubes in tube helically coiled heat exchangers performance for turbulent flow regime and numerical correlations were correlated. The effect of tube curvature on spirally coiled tube heat transfer enhancement in a horizontal direction was studied by Naphon and Suwagrai [6]. The results founded that the centrifugal force present a significant influence on fluid flow developments and temperature distributions in the spirally coiled tube. Sahoo et al. [7] use a triple tube helical heat exchanger for achieves the Milk fouling. The study noticed that the complex fouling phenomenon was a function of the interface temperature between the fouling deposit, the bulk fluid and the shear stress of the fluid on the heat exchanger surface. Patil [8] achieve the heat transfer of Newtonian fluids in spiral tubes in the laminar flow region. The results developed new correlations for designing the spiral tubes heat exchangers. Nema and Datta [9] describe an improved simulation model for the accurate estimation of fouling thickness and milk outlet temperature. The results showed that is an increase was occurred in fouling, while the outlet milk temperature decreases with the time. JI et al. [10] studied numerically the performances characteristics of spiral tubes with circular and elliptical cross-sections. The effect of the structural parameters, flow rate, pitch and the radius ratio were presented. The results indicated that with increasing water mass flow rate or the helical pitch, the heat transfer augmentation occurs. Bahiraei et al. [11] present the thermo-fluid characteristics of a triple tube with inserted ribs and with a grapheme nanoplatelet–platinum composite powder hybrid nanofluid. The obtained results indicated that the rib height, rib pitch, and nanoparticle concentration had a significant effect on the performance index. Naphon [12] tested both Al2O3 and CuO nanofluids as working fluids in spiral tube at different curvature ratios. The results found that heat transfer augmentation increase with increasing nanofluids concentration and the curvature ratio decreases. The results also showed that the friction factor increases when curvature ratio decreases. Mohapatra et al. [13] developed an analytical model to predict the effects of different design parameters on the thermal performances of a Three-fluid heat exchanger. The results were compared and validated with literature and good conformity was observed. Naphon and Wiriyasart [14] combined four techniques to achieve the heat transfer enhancement. The study includes curved tube, pulsating flow, nanofluids, and magnetic field techniques in spiral tubes. The results obtained that the pulsating flow gives higher Nusselt number compared to the continuous flow, whereas the magnetic and pulsating flow has a slight increase of the friction factor. Li et al. [15] investigate the thermo-fluid characteristics of propane condensation in a spiral tubes. The computational model developed based on two fluid multiphase models. The results provide some instructions to understand the thermo-fluid characteristics of condensation in the spiral pipe. The behavior of triple tube heat exchangers was developed by Valladares [16]. Different numerical aspects and comparisons with literature were presented to verify and validate the model. Gomaa et al. [17,18] derived an experimental and numerical study of a triple straight concentric tube heat exchanger with and without inserted ribs. The results indicated that the triple tube heat exchanger contributes higher thermal performance and energy saving compared to double tube heat exchanger per unit length. The results also revealed that, the insertion of ribs to the inner annulus fluid flow of the triple tube heat exchanger participate a significant convective heat transfer augmentation. Thermo fluid flow characteristics of a uniformly fluxed circular tube integrated circular ring with inserts mesh were investigated by Bartwal et al. [19] experimentally and numerically new Empirical correlations of Nusselt numberand friction factor were observed in good agreement with the experimental data. Heat transfer and fluid flow characteristics of heat exchanger tube with multiple twisted tapes and solid rings inserts were presented by Singh et al. [20]. The effect of circular ring with twisted tape on heat transfer is tested and new Correlations were developed for the Nusselt number and friction factor of the tubes with combined devices. Eiamsa-ard et al. [21] use of TiO2-water nanofluid for improving the Heat transfer flow in dimpled tube with twisted tape insert. The results showed that the highest thermo-hydraulic performance reach to 1.26 by using the nanofluid with concentration of 0.15 vol. % in the dimple angled of 45°. Verma et al. [22] enhanced the heat transfer and frictional losses in heat exchanger tube with modified helical coiled inserts. The experiments carried out for a smooth tube without insert, tube fitted with helical coiled inserts, and modified helical coiled inserts, and empirical correlations of Nusselt number and friction factor for helical coiled inserts were correlated. Kumar et al. [23] developed a comparative study for thermal-hydraulic performance of circular tube with different inserts geometries Includes twisted tape, twisted tape with ring, circular band, multiple twisted tape, twisted tape with conical rings, using air under turbulent flow regime as working fluid. The results showed that the single twisted tape insert present a maximum thermal performance factor and in the event of twisted tape with circular ring the overall heat transfer rate is maximum. Abdelmagied [24] derive a numerical study on thermofluid characteristics of double spirally coiled tube. The results showed that the curvature ratio and Alumina-water nanofluid concentration presence a notice effect on heat transfer enhancement. The present study aims to achieve numerically the thermo-fluid characteristics of a new design called a triple spiral tube heat exchanger (TSTHE). The design is developed by adding a third tube to double spiral tube heat exchanger and expect to enhance the heat exchanger thermo fluid characteristics and its performance. The study will be examined under fluid to fluid heat transfer boundary condition. The effect of hot water inlet temperature, flow arrangements, and Alumina-water nanofluid concentrations, were the main point of interest.

Section snippets

Numerical model

To achieve the behavior of TSTHE Ansys software package 14.5 was used. A 3D CFD model was developed to study the thermo-fluid characteristics of TSTHE. The governing equations for fluid motion, temperature, and pressure values were solved at every cell. The geometry creation of the three dimensions triple spiral tube is the first step of the problem solving and it was done using Ansys drawing module (design modeler). The second step accomplished by using ICEM CFD for mesh generation. The third

Results and discussion

To achieve the performance characteristics of a TSTHE. The effect of flow arrangements, the hot fluid inlet temperatures, the Al2O3 nanofluid concentration, and the performance criteria are presented. The cooling characteristics of the hot fluid in the in ner annulus side of TSTHE were the point of interest. The Nusselt number of the TSTHE are compared with the Nusselt number of the double spiral tube heat exchanger (DSTHE) at the same conditions (Fig. 6) for both counter and parallel flow

Thermo-hydraulic performance criteria

The thermo-hydraulic performance criteria of the triple spiral tube are used to recognize the heat transfer enhancement that perhaps leads to increase the pumping power compared to the double spiral tube. Three metric parameters had identified the performance. The parameters are the effectiveness, heat transfer per unit pumping power and thermo-hydraulic performance index. The effectiveness of the TSTHE at different concentration of Al2O3 ranged from 0 % to 2 % with respect to water was shown

Conclusions

The heat transfer and pressure drop characteristics of a TSTHE was achieved in turbulent flow region under fluid to fluid heat transfer condition. A brave comparative between DSTHE and TSTHE were covered. The effect of hot water inlet temperature, flow arrangements, and the effect of Al2O3-water nanofluid for a wide range of Dean number were examined. The The convection heat transfer coefficient, effectiveness, the heat transfer per unit pumping power and thermo-hydraulic performance index of

References (36)

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