Thermal performance of helical coils with reversed loops and wire coil inserts

https://doi.org/10.1016/j.ijheatmasstransfer.2019.118723Get rights and content

Highlights

  • Modified helical coils with reversed loops and wire coil inserts exhibit unique flow and heat transfer characteristics.

  • The thermal performance of traditional helical coils can be improved by adding reversed loops with wire coil inserts.

  • New friction factor and heat transfer correlations for helical coils with reversed loops and wire coil inserts have been proposed.

Abstract

Flow and heat transfer characteristics of water in a newly designed helical coil heat transfer device were investigated in this experimental study. The conventional helical coil configuration was structurally modified aiming to improve its heat transfer performance. Specifically, 360° plastic tubing with or without wire coil inserts was added after each 180° of the main helical coil loop to enhance fluid mixing and redistribute the flow, which should have a direct effect on the thermal performance of the helical coil heat transfer device. Experimental results show that the structural modifications of the conventional helical coil configuration led to enhanced heat transfer in the test section while the pressure drop penalty increased slightly. Furthermore, the heat transfer performance of the overall test section improved by using wire coil inserts in the plastic tubing after every 180° of the main helical coil loop. The results reveal that the modified helical coil section offers a good trade-off between heat transfer enhancement and pressure drop penalty.

Introduction

As the demand for thermal energy continues to increase, devices such as heat exchangers should be modified and optimized to meet the needs of energy-transport systems. Over the last century, heat exchangers have been designed and used to transfer thermal energy from one fluid to another in relatively compact configurations. In particular, coil heat exchangers (CHX) are efficient due to their compactness and good heat transfer performance. Secondary flows induced by centrifugal forces occur due to the curvature of the coil, which positively affects thermal performance in a CHX. Researchers have studied hydrodynamic and convective heat transfer performance of flows in conventional helically coiled tubes [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [20], [23]. However, no study has investigated the effect of wire coil inserts on the thermal performance of helical coils with external reversed loops. Specifically, the use of wire coil inserts as passive enhancements only in the external reversed loops have not been considered before, which may enhance the heat transfer performance while avoiding higher pressure drop if used throughout the whole heat transfer loop. The effect of the wire coil pitch (3 mm and 20 mm) on pressure drop, heat transfer coefficient and overall performance enhancement index were investigated as part of this study. As shown in the results section, wire coil inserts in the reversed loops led to greater heat transfer rate and greater performance enhancement index values when compared to the standard helical coil systems.

Accordingly, a new helical coil test configuration for CHX has been designed and tested successfully. Specifically, a 360° reversed plastic tubing with or without wire coil inserts have been integrated to the main helical loop at each 180° of the coil to further enhance heat transfer within the CHX.

In summary, external reversed loops with inserts were used in the study to promote fluid mixing and enhance heat transfer in the CHX.

Dean [1] first discovered the presence of a secondary flow field in coiled tubes and formulated a non-dimensional (Dean) number to account for the effects of curvature ratio and Reynolds number on secondary flows. The Dean number is defined as follows:De=Re·rR=ρ·u·dμ·rRwhere ρ, μ, u, d, r and R are the density, dynamic viscosity of the fluid, fluid mean velocity, inner diameter of the tube, inner radius of the tube, curvature radius of the coil, respectively.

Dravid et al. [2] observed and validated the existence of a secondary flow field in coiled tubes. Numerical studies were conducted under laminar flow regime with Dean Numbers greater than 100. Their results revealed that cyclic oscillations in coil wall temperature could be observed along the tube axis, due to the presence of secondary flows.

Pakdaman et al. [3] characterized the performance of nanofluids flowing inside helically coiled tubes. It was found that the corresponding heat transfer enhancement was significant in a helically coiled tube when compared to a straight tube.

Huttl et al. [4] numerically studied turbulent flows in straight, curved and helically coiled pipes. It was found that a secondary flow was generated due to the curvature of the pipes, which made the corresponding flow structures different from those in straight pipes.

Mishra and Gupta [5] experimentally measured pressure drop of water flowing through helical coils in both laminar and turbulent regimes. For turbulent flows, the following correlation was obtained based on the least-square analysis of the data.fc=fs+0.0075·dDc0.5Dc=2Rc1+p2πR2where 4000 < Re < 100,000,0.00289<r/R<0.1550<p/Dc<25.4d is the inner diameter of the coil tube, Dc is the effective curvature diameter of the coil defined in Eq. (3), p is the pitch of the helical coil, and the Fanning friction factor fs in a smooth straight tube under turbulent flow condition is as follows:fs=0.079Re0.25where 4000 < Re < 100,000.

Srinivasan et al. [6] conducted an experimental study comparing twelve coils with curvature ratios d/D from 0.0097 to 0.135. The experiments were carried out with water and oil under both laminar and turbulent conditions. A correlation for friction factor of laminar flows was defined based on Seban and McLaughlin’s approach [7], as follows:fc=6.7dDDe0.5where D is the coil diameter, 30 < De < 300, 0.0097 < d/D < 0.135. Seban and McLaughlin [7], also provided a correlation for the friction factor in the transitional region, as follows:fc=1.8ReDd0.5where 300 < De < Decri, 0.0097 < d/D < 0.135, Decri is the critical Dean number. The critical Reynolds number was defined by Ito et al. [11], as follows:Recri=21001+12rR0.5

The friction factor for turbulent flow conditions was predicted based on Huttl et al. [4], as follows:fc=0.084·dD0.2De0.2where Decri < De < 1400, 0.0097 < d/D < 0.135.

Seban et al. [7] carried out heat transfer experiments for laminar flow of oil and turbulent flow of water in coiled tubes. For turbulent cases, the local heat transfer coefficient on the outer wall surface was found to be two to four times greater than those of the inner wall. Based on these results, it was found that the peak mean velocity in the coil shifted to the outer side of the tube, which led to higher fluid velocities near the outer side than near the inner side of the coil. An empirical correlation for the local Nusselt number of laminar flows in coiled tubes was proposed based on their experimental results, as follows:Nuc=Afc8·Re2·dx1/3·Pr1/3where fc, Re, d, x, Pr are the friction factor of the coil, Reynolds number of the fluid, inner diameter of the coil tube, longitudinal distance along the coil and Prandtl number of the fluid, respectively.

Rogers and Mayhew [8] conducted heat transfer experiments for water flowing through steam heated coils under turbulent conditions, and modified the Nusselt number correlations from Seban et al. [7] and Kirpikov et al. [10] to better predict their heat transfer results in coiled tubes, as follows:Nuc=0.0456·Re0.8·Pr0.4·rR0.21

where 0.056 < r/R < 0.1, 10,000 < Re < 45,000.Nuc=0.021·Re0.85·Pr0.4·rR0.1where 0.05 < r/R < 0.0093, 10,000 < Re < 45,000.

Mori et al. [9] pointed out that the Nusselt number inside a curved pipe was found to be remarkably influenced by the presence of secondary flows induced by the pipe curvature. An empirical Nusselt number correlation based on the analytical study was postulated, as follows:Nuc=Pr0.441Re5/6rR1/121+0.061RerR2.51/6where (Re∙(r/R) 2.5) > 4.0, Pr > 1. The Nusselt number values calculated from the above theoretical Eq. (12) matched well with their experimental Nusselt number values for R/r equal to 18.7 and 40.

Acharya et al. [12] numerically characterized the heat transfer enhancement of flows in coil heat exchangers due to chaotic mixing. Chaotic mixing was achieved by periodically rotating the coil axis in contrast with the common helical coil heat exchanger. Correspondingly, periodically hydrodynamic flow development occurred after each coil axis switch. The periodic breaking of the interior boundary layer led to increased convective mixing, which was suggested to be the main factor for heat transfer enhancement.

Sreejith et al. [13] compared heat transfer performance of a helical coil heat exchanger to a straight tube heat exchanger under the same experimental conditions. It was claimed that the secondary flow induced by the curvature of the helical coil heat exchanger was the main factor that enhanced heat transfer rate in relation to a straight tube heat exchanger. Results also showed increased heat exchanger effectiveness and greater overall heat transfer coefficient in helical coil heat exchangers than in straight tube heat exchangers for all mass flow rates and operating conditions.

Kong et al. [14] experimentally evaluated the heat transfer characteristics of a Newtonian-like slurry flowing through a helical coil. It was indicated that the curvature ratio of the coil had a significant influence on heat transfer performance. A later study of Kong et al. [15] showed that the local heat transfer coefficients oscillated along the coil due to the potential prevalence of secondary flows. It was found that the secondary flow could enhance fluid mixing along the radial direction of a helically coiled tube, which in turn would result in better heat transfer of helically coiled tubes than in straight pipes. A new Nusselt number correlation as a function of Reynolds number and Prandtl number was postulated for water in helically coiled tubes, as follows:Nuc=0.03·Re0.84·Pr0.4

where 6300 < Re < 27,000, 1000 < De < 4000.

From the previous studies, it is evident that greater fluid mixing and better heat transfer can be achieved with helical coil heat exchangers.

Inserts have been used in coiled tubes to enhance fluid mixing and improve heat transfer performance. Webb [16] pointed out that wire coil inserts could enhance heat transfer inside tubes by inducing flow separation at the wire which in turn would disrupt the boundary layer growth. However, under those circumtances, it was found that the boundary layer mixing effect dissipated rapidly. Therefore, the spring wire pitch should be taken into consideration in order to optimize the heat transfer performance of flowing fluids in coiled tubes.

Ravigururajan and Bergles [17] conducted a flow visualization study of water in a straight pipe that showed that inserted coil wires could induce flow rotation which in turn would improve heat transfer performance. The numerical experiments were conducted for different coil wire diameters and Reynolds number range between 150 and 2600. It was shown that the developing hydrodynamic length for tubes with coil wire inserts was much smaller than for plain tubes.

Neal and Stephen [18] stated that most studies of internal flow with wire coil inserts showed heat transfer enhancement from 50% to 400%. It was claimed that one crucial feature of internal flow in tubes with wire coil inserts is the lack of a significant hydrodynamic entry length since the boundary layer breaks down and reforms continuously. In summary, the heat transfer coefficient is usually greater when wire coil inserts are used than in tubes without wire coil inserts.

Chiou [19] investigated the effects of wire coil inserts inside tubes on heat transfer performance. It was proposed that the principal mechanism for heat transfer enhancement in tubes with compression spring wire inserts was the induced fluid mixing, which was capable of disrupting the laminar sub-layer and increasing the level of fluid turbulence.

Yildiz et al. [20] carried out an experimental study of heat transfer and pressure drop of air in a helical pipe containing wire coil inserts with different pitch values. Their results showed that heat transfer in a helical pipe with wire coil inserts could be enhanced by as much as 5 times when compared to the heat transfer performance of a plain helical pipe. However, the pressure drop increment could be 10 times higher while the Nusselt number increased with decreasing pitch-to-wire diameter ratio.

Ali et al. [21] experimentally investigated convective heat transfer in a straight tube with different wire coil inserts in the turbulent regime. The effects of wire coil inserts on Nusselt number and friction factor were evaluated. It was found that the performance enhancement index was greater than one for all the experimental cases. The performance enhancement index, η was defined as follows:ηoverall=henhancedtubehplainstraigttube/ΔPenhancedtubeΔPplainstraighttube1/3which in this study, is a function of the heat transfer coefficient ratio and pressure drop ratio between the enhanced tube with wire coil inserts and the plain tube without wire coil inserts for the same pumping power [22], [23], [24], [25].

As it can be seen, better convective heat transfer can be achieved in coiled tubes than in straight pipes. In addition, periodically switching coil axis or using wire coil inserts in tubes should disturb the boundary layer formation and increase the level of turbulence mixing. Therefore, a modified helical coil test configuration consisting of well-spaced reversed plastic coils for enhanced fluid mixing was designed and tested in the study. Wire coil inserts were also used in the reversed plastic coils to further promote turbulence mixing and enhance heat transfer performance.

Section snippets

Description of the pump-driven flow loop

In order to experimentally characterize the heat transfer coefficient and pressure drop of water flowing through helical coils with and without (w/o) reversed loops and wire coil inserts, a pump-driven flow loop was designed and constructed as shown in Fig. 1. The heat transfer and pressure drop characteristics were investigated independently with instrumented heat transfer and pressure drop test sections, respectively, with the same geometry and dimensions.

As Fig. 1 shows, a progressive cavity

Pressure drop results

Pressure drop in the plain helical coils (without reversed loops and wire coil inserts) was measured for different flowrates. The experimental friction factor was calculated and compared with the correlations by Mishra et al. [5] and Srinivasan et al. [6]. Experimental conditions including curvature ratio, Reynolds and Dean Number were compared to the established correlations [5], [6] as seen in Table 2.

The friction factor is defined as follows:f=d·ΔP2·ρ·L·u2where ρ, d, L and u are the fluid

Conclusions

In this study, a commonly used helical coil heat transfer device has been modified by adding a 360° reversed plastic tubing with or without wire coil inserts after each 180° of the main helical coil loop. The experimental heat transfer and pressure drop results showed that the modified helical coils exhibited better thermal performance. Wire coils in the reversed loops enhanced heat transfer in the main helical coil loop. Overall, a unique combination of reversed loop curvature ratio, wire coil

Declaration of Competing Interest

The authors declared that there is no conflict of interest.

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