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Inductive power transfer (IPT) systems often encounter limitations in efficiency and transmission distance due to near-field magnetic coupling characteristics. Increasing the inductance can enhance the transmission distance, but it also raises the system's Q factor, leading to several issues. This study aimed to optimize the magnetic core design of helical coils to enhance magnetic coupling in IPT systems while minimizing the increase in self-inductance. Through finite element analysis, various core placements were evaluated, leading to a proposed core design process that integrates inner and lower cores at optimal angles. The proposed design was compared with conventional cores, and its performance was validated in an IPT system. The results demonstrate that the proposed core design significantly enhances the coupling coefficient (k) and extends power transmission distance compared with conventional planar and U-shaped core structures without substantially increasing self-inductance (L). This design effectively balances the trade-off between increasing inductance and maintaining system stability, thereby improving transmission efficiency while minimizing frequency instability and voltage stress. [ABSTRACT FROM AUTHOR]

Helical coil tubes are essential in a variety of engineering applications due to their improved heat transfer and compact shape. The current study presents an experimental and numerical investigation of geometrical optimization for helical coil elliptic tubes. Four distinct coil pitch values are examined with a constant perimeter for all geometries and three aspect ratio values (1, 0.75, 0.5) utilizing water as the working fluid across varying the volume flow rates. The helical coil has the same surface area as the tube for each coil. The numerical simulation is conducted using the ANSYS FLUENT CFD software (Version 19.2). The experimental and numerical results indicate that the increase in heat transfer due to an increase in mass flow rate is significantly greater than the improvement gained by changing the aspect ratios. Increasing the volume flow rate to 3 L/min significantly improves the average heat transfer coefficient, with a 66.6 % increase found at a maximum coil pitch of 0.08 m and an aspect ratio of 0.5. Furthermore, a helical coil with an aspect ratio of one (circular cross-section) exhibits the best compromise between pressure drop penalty and heat transfer enhancement, resulting in the greatest hydraulic thermal performance parameter.

Embolization of the middle meningeal artery (MMA) is a safe and effective adjunct in the treatment of chronic subdural hematoma. While prior authors describe the use of coils to assist embolization by preventing reflux through eloquent collaterals, we de- scribe the use of coils to further open the MMA, allowing the administration of greater amounts of embolisate for a more robust embolization. The objective of this study was to demonstrate that helical coils can safely open the MMA following the administration of polyvinyl alcohol (PVA) particles. This allows for more embolisate to be administered into the MMA for more effective treatment. A retrospective review was conducted at our institution including intraoperative images and postoperative clinical and radiographic follow up. Failure rates using MMA embolization with PVA and helical coil augmentation were compared to failure rates in the literature of MMA embolization with PVA or ethylene vinyl-alcohol copolymer alone. A total of 8 cases were reviewed in which this technique was implemented. There were no immediate complications after treatment. All patients that underwent helical coil embolization following the administration of PVA had increased amount of embolisate delivered into the MMA. All patients at follow up had resolution of the subdural hematoma on outpatient imaging. Helical coil embolization allows for more embolisate administration into the MMA and provides a technical advantage for patients that fail traditional techniques of embolization. Case series are taking place to further test this hypothesis and identify the ideal patient population that may gain maximal yield from this novel technique.

Helically coiled pipes are widely used in many industrial and engineering applications because of their compactness, larger heat transfer area per unit volume and higher efficiency in heat and mass transfer compared to other pipe geometries. They are commonly encountered in heat exchangers, steam generators in power plants and chemical reactors. The most notable feature of flow in helical pipes is the secondary flow (i.e., the cross-sectional circulatory motion) caused by centrifugal forces due to the curvature. Other important features are the stabilization effects of turbulent flow and the higher Reynolds number at which the transition from a laminar to a turbulent state occurs compared to straight pipes. A survey of the open literature on helical pipe flows shows that a good deal of experimental and theoretical work has been conducted to derive appropriate correlations to predict frictional pressure losses under laminar and turbulent conditions as well as to study the dependence of the flow characteristics and heat transfer capabilities on the Reynolds number, the Nusselt number and the geometrical parameters of the helical pipe. Despite the progress made so far in understanding the flow and heat transfer characteristics of helical pipe flow, there is still much work to be completed to address the more complex problem of multiphase flows and the impact of pipe deformation and corrugation on single- and multiphase flow. The aim of this paper is to provide a review on the state-of-the-art experimental and theoretical research concerning the flow in helically coiled pipes. [ABSTRACT FROM AUTHOR]

Inductive power transfer (IPT) systems often encounter limitations in efficiency and transmission distance due to near-field magnetic coupling characteristics. Increasing the inductance can enhance the transmission distance, but it also raises the system’s Q factor, leading to several issues. This study aimed to optimize the magnetic core design of helical coils to enhance magnetic coupling in IPT systems while minimizing the increase in self-inductance. Through finite element analysis, various core placements were evaluated, leading to a proposed core design process that integrates inner and lower cores at optimal angles. The proposed design was compared with conventional cores, and its performance was validated in an IPT system. The results demonstrate that the proposed core design significantly enhances the coupling coefficient (k) and extends power transmission distance compared with conventional planar and U-shaped core structures without substantially increasing self-inductance (L). This design effectively balances the trade-off between increasing inductance and maintaining system stability, thereby improving transmission efficiency while minimizing frequency instability and voltage stress.

Low-temperature stratification, high-volumetric storage capacity, and less-complicated material processing make phase-changing materials (PCMs) very suitable candidates for solar energy storage applications. However, their poor heat diffusivities and suboptimal containment designs severely limit their decent storage capabilities. In these systems, the arrangement of tubes conveying the heat transport fluid (HTF) plays a crucial role in heat communication between the PCM and HTF during phase transition. This study investigates a helical coil tube-and-shell thermal storage system integrated with a novel central return tube to enhance heat transfer effectiveness. Three-dimensional computational fluid dynamics simulations compare the proposed design against a baseline helical coil system without a return tube under equivalent conditions. Outcomes quantify the return tube's efficacy in augmenting heat transfer uniformity and accelerating phase transition. Adding the return tube markedly boosts heat storage and recovery rates, increasing charging by 88% and discharging by 56% versus the baseline. Moreover, total phase transition time reduces by 48% for melting and 36% for solidification with the return tube. The accelerated charging stems from sustained convective heat transfer inside the return tube even as the molten layer thickens. Meanwhile, enhanced solidification results from ongoing cooling of inner regions. Isotherm analysis visualizes the return tube's efficacy in maintaining thermal uniformity throughout the phase transition process. Overall, the return tube significantly improves PCM thermal response, demonstrating a novel but straightforward approach to address heat transfer limitations in latent thermal storage systems.

Due to the intermittent and hazardous nature of lightning impulse, researchers rely on artificial impulse generators to study it. The Tesla coil is a commonly used device to generate artificial lightning. This paper presents a parametric analysis of the primary side coil of the Tesla transformer. The analysis employs seven key electrical properties: skin depth, current density, voltage distribution, Ohmic loss, electric field strength, charge density, and energy content. These parameters are used to evaluate the response of various coil types to high voltage impulses. The Ansys high-frequency electromagnetic solver is used to perform a comparative analysis of essential parameters. The results can aid lightning researchers and Tesla coil designers in determining the optimal designs for Tesla primary coils in the future. The analyzed data is validated using Tesla coil hardware, which is used as an artificial lightning producer to test the feasibility of a lightning energy harvester.

Helically coiled pipes are widely used in many industrial and engineering applications because of their compactness, larger heat transfer area per unit volume and higher efficiency in heat and mass transfer compared to other pipe geometries. They are commonly encountered in heat exchangers, steam generators in power plants and chemical reactors. The most notable feature of flow in helical pipes is the secondary flow (i.e., the cross-sectional circulatory motion) caused by centrifugal forces due to the curvature. Other important features are the stabilization effects of turbulent flow and the higher Reynolds number at which the transition from a laminar to a turbulent state occurs compared to straight pipes. A survey of the open literature on helical pipe flows shows that a good deal of experimental and theoretical work has been conducted to derive appropriate correlations to predict frictional pressure losses under laminar and turbulent conditions as well as to study the dependence of the flow characteristics and heat transfer capabilities on the Reynolds number, the Nusselt number and the geometrical parameters of the helical pipe. Despite the progress made so far in understanding the flow and heat transfer characteristics of helical pipe flow, there is still much work to be completed to address the more complex problem of multiphase flows and the impact of pipe deformation and corrugation on single- and multiphase flow. The aim of this paper is to provide a review on the state-of-the-art experimental and theoretical research concerning the flow in helically coiled pipes.

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Latent heat thermal energy storage (LHTES) using phase change materials (PCM) is a promising way to store energy. The primary problem with PCMs is their low heat conductivity, which causes the charge and discharge processes to take a long time. This study presents an experimental study using fins on helical coils to overcome this issue. The effect of different numbers of fin, changing heat transfer fluid (HTF) inlet temperature and flow rate, are investigated in the charging and discharging processes. Temperature variations in different regions are measured by thermocouples and imaged by a thermal camera to illustrate the PCM melting and solidification. Temperature fluctuation, mean temperature response (MTR), Stefan number (STE), thermal imaging, and the discharge process's heat power supply are used as metrics to examine the impact of different parameters. Results indicate that using 15, 25, and 35 fins can decrease the charging by about 11.2, 18.2, and 29.5%, respectively, and the discharging process by about 6.31, 11.9, 20.9%, respectively. Also, An increase in temperature and flow rate of HTF in the charging remarkably increases the full charge time. Nonetheless, a drop in the flow rate and temperature of the HTF intake has lengthened the discharge. [ABSTRACT FROM AUTHOR]

Due to the compact structure, high heat transfer efficiency and strong reliability, helical coil once-through steam generator (HCSG) is widely used in advanced small reactors. In HCSG, the liquid in the tube side is heated by the helical coils so that subcooled water in the tube side is transferred into superheated steam, and the entire heat transfer regime is covered. Moreover, the evolution mechanism of thermalhydraulic behaviors is complex because it is affected by the helical geometry, the centrifugal force and the secondary flow. Hence, the development of advanced HCSG was limited due to the lack of more accurate models of heat transfer and regime transition. With the financial support of the three major nuclear power groups in China, experimental and theoretical research on heat and mass transfer behaviors in helical coils was carried out systematically. Different helical coils were investigated and comprehensive parametric analysis of heat transfer was conducted to provide a better understanding of different heat transfer regimes. According to our research, a fundamental database of heat transfer and flow resistance in helical coils with a wide range of geometric dimensions and thermalhydraulic parameters was established. In addition, the transition pattern of the circumferential nonuniform heat transfer mechanism was clarified. Due to the secondary flow, it consequently leaded to the nonuniform distribution of flow field. The heat transfer and flow resistance characteristics were notably affected by the special behaviors of liquid in the helical coils. Hence, based on mechanism analysis, several models were proposed to construct a high precision library of allregime prediction model, such as a threeregion dryout quality model, a postdryout heat transfer model based on dryness gradient and a widerange model for twophase frictional multiplier factor. For the model of dryout quality, it takes the relative intensity of gravity, buoyancy and centrifugal force into consideration. Since different dominant forces lead to different heat transfer behaviors, a piecewise correlation is established to give more accurate predictions. As for the postdryout heat transfer model, it considers the influence of dryness gradient and achieves a more accurate prediction. For the twophase pressure drop model, the modified frictional multiplier factor is adjusted according to our database. Then, a prediction model was proposed which was also validated by other researchers and had great prediction accuracy in a wide range. In all, this work includes part of our investigation on the heat transfer of helical coils and provides a highprecision prediction in the entire heat transfer regime, which meets the design requirement of HCSG for advanced reactors.

The radius of the coil, the number of turns of the windings, and the parasitic resistances of energy-storing elements affect the performances of wireless power transfer systems. We aimed to study the effects of coil parameters on a wireless power transfer system with the capacitor–capacitor–coupling coil (CCL)-based circuit using numerical simulations. The power transfer system topologies, including series–CCL (S–CCL), CCL–S, and CCL–CCL, were studied vis-à-vis coil parameters. The helical coil and the system topologies were modeled using MATLAB, and the performances of the topologies were examined in comparison to the series–series (S–S) topology. The variables used in the simulations included the radius and number of turns, the parasitic resistance that was merged in the impedance, and the reactance of energy-storing elements. Subsequently, the performances of the topologies were estimated by numerical simulations under several circumstances. The simulation results showed that the parasitic resistance of the coupling coil affects the performances of the topologies directly. The coupling coils with smaller geometries are beneficial to power transfer in the wireless power transfer system and may further contribute to miniaturization. [ABSTRACT FROM AUTHOR]

This paper investigates the cryogenic heat transfer phenomena of nitrogen flowing in helically coiled tubes under the combined effects of pseudocritical conditions, buoyancy, and coil curvature. The ultimate goal was to design optimum heat exchangers for liquid air energy storage. Local heat transfer coefficients were evaluated peripherally across tube cross sections. The pressure, mass flux, and heat flux effects on the heat transfer were examined. The dual effect of buoyancy and coil curvature on heat transfer coefficients was interpreted via a dimensionless number Ψ, which denotes a ratio between the two effects. Results reveal that the heat transfer coefficients increase with increasing mass flux but decreasing pressure and heat flux. The buoyancy effect dominates the heat transfer at fluid temperatures below the pseudocritical temperature (e.g., −146.3 °C at 35 bar), while the coil curvature-induced centrifugal effect dominates at higher temperatures. The heat transfer coefficients for the helical coil were approximately 13% lower compared with those in straight tube at fluid temperatures below the pseudocritical temperature, but their difference shrinks (<±6%) at higher temperatures. The reason is that the benefits of coil curvature and improved turbulent mixing on heat transfer are counteracted by the thermophysical property variation and buoyancy effect. [ABSTRACT FROM AUTHOR]

The forming of helical coils using a rolling process results in geometrical irregularities (wrinkles and ovality) that are likely to influence the hydrodynamic behaviour of the flow field inside the coil in applications such as air generators. In this study, the above behaviour was investigated by experimental and numerical analyses considering the heat exchanger used in dry air generators. In experimental analysis, a three-turn copper helical coil with wrinkles and ovality was investigated to estimate the global hydrodynamic characteristics inside the helical coil. The results were compared with that of the ideal geometry of a coil without wrinkles and ovality. The effect of wrinkles was assessed through friction factor, and the corresponding equivalent surface roughness was found to increase by 5.7 times, owing to the presence of wrinkles in the helical coil. Numerical simulation was conducted to determine the pressure distribution, velocity distribution, and secondary flow inside the helical coil; the results were validated with experimental data. A critical portion of the helical coil with multiple wrinkles was considered for numerical simulation to investigate the localized effects of wrinkles on the flow field behaviour. The analysis in the vicinity of wrinkles revealed negative pressure development during flow, which in turn would cause re-circulation and cavitation that are undesirable. [ABSTRACT FROM AUTHOR]

At present, solenoid (helical) coils are widely used in various researches, especially electromagnetic forming (EMF), due to their advantages of simple manufacture and high magnetic field strength. However, in the study of EMF, there are still some defects in the high precision calculation of solenoid coils in the magnetic field the main reason is that the solenoid coil is of non-axisymmetric structure, which is generally equivalent to the 2-D or 3-D axisymmetric structure in practical research. Not only can it ignore the effect of the pitch effect of the solenoid coil, but also causes a certain deviation in the calculation of the magnetic field. Hence, this paper establishes 5 helical coils and 3-D axisymmetric finite element coils models with different layers for comparative analysis. The simulation results show that the magnetic flux density distribution deviation of helical coil and the 3-D axisymmetric coil is small and can be ignored when the coil layers are 1 or 2 layers. However, the number of linear circles increasing, the deviation between the two increases obviously. Therefore, in the study of EMF, the effect of solenoid coil pitch effect on magnetic field calculation can be ignored when the layers, radius and pitch of helical coils are used less whereas the effect of solenoid coil pitch effect on magnetic field calculation must be considered when the layers, radius and pitch of helical coils are used more.

The radius of the coil, the number of turns of the windings, and the parasitic resistances of energy-storing elements affect the performances of wireless power transfer systems. We aimed to study the effects of coil parameters on a wireless power transfer system with the capacitor–capacitor–coupling coil (CCL)-based circuit using numerical simulations. The power transfer system topologies, including series–CCL (S–CCL), CCL–S, and CCL–CCL, were studied vis-à-vis coil parameters. The helical coil and the system topologies were modeled using MATLAB, and the performances of the topologies were examined in comparison to the series–series (S–S) topology. The variables used in the simulations included the radius and number of turns, the parasitic resistance that was merged in the impedance, and the reactance of energy-storing elements. Subsequently, the performances of the topologies were estimated by numerical simulations under several circumstances. The simulation results showed that the parasitic resistance of the coupling coil affects the performances of the topologies directly. The coupling coils with smaller geometries are beneficial to power transfer in the wireless power transfer system and may further contribute to miniaturization.

This paper investigates the cryogenic heat transfer phenomena of nitrogen flowing in helically coiled tubes under the combined effects of pseudocritical conditions, buoyancy, and coil curvature. The ultimate goal was to design optimum heat exchangers for liquid air energy storage. Local heat transfer coefficients were evaluated peripherally across tube cross sections. The pressure, mass flux, and heat flux effects on the heat transfer were examined. The dual effect of buoyancy and coil curvature on heat transfer coefficients was interpreted via a dimensionless number Ψ, which denotes a ratio between the two effects. Results reveal that the heat transfer coefficients increase with increasing mass flux but decreasing pressure and heat flux. The buoyancy effect dominates the heat transfer at fluid temperatures below the pseudocritical temperature (e.g., −146.3 °C at 35 bar), while the coil curvature-induced centrifugal effect dominates at higher temperatures. The heat transfer coefficients for the helical coil were approximately 13% lower compared with those in straight tube at fluid temperatures below the pseudocritical temperature, but their difference shrinks (

A computational fluid dynamics (CFD) study was conducted to analyse the flow structure and the effect of varying the coil pitch on the coil friction factor and wall shear stress, through utilising different models' configurations. Three coils were tested, all of them having the same diameter and coil diameter: 0.005m and 0.04m respectively. Pitch variations began with 0.01, 0.05, 0.25 m for the first, second and third model respectively. Two turbulence models, STD(k-ε) and STD(k-w), were utilised in this simulation in order to determine the turbulence model which could capture most of the flow characteristics. A comparison was made between the STD(k-ε) and STD(k-w) models in order to analyse the pros and cons of each model. The results were validated with Ito's equation for turbulent flow and compared with Filonenko's equation for a straight pipe. The governing equations were discretized using finite volumes method and the SIMPLE algorithm was used to solve the equations iteratively. All the models were simulated using the ANSYS Fluent solver CFD commercial code. The results showed that in turbulent flows, Dean number had a stronger effect on reducing coil friction factor than the increment in pitch dimension. [ABSTRACT FROM AUTHOR]

The behavior of two-phase flow and corresponding flow regimes in helical tubes significantly differ when compared to two-phase flows in straight tubes due to centrifugal and torsion effects. In order to gain physical insight and gather data for validating computational models, a large number of experiments were performed on a helical coil experimental setup operated with a mixture of water and air. The experimental data were used to assess the predictive capabilities of current two-phase Computational Fluid Dynamics (CFD) models based on the Volume of Fluid (VOF) approach. In the present paper, a comparison of the CFD simulation results with the high-resolution experimental data is discussed, with special emphasis on two-phase pressure drops and void fraction distributions. It is shown that the CFD VOF model is able to correctly capture the occurrence of five flow regimes observed in the experiments, namely bubbly flow, plug flow, slug flow, slug-annular flow, and annular flow. However, a good quantitative agreement for pressure drops and void fraction distributions is found in slug flow and slug-annular flow regimes only. The good agreement found only in a limited range of flow regimes demonstrates that there is not a single set of best-practice guidelines for CFD VOF models that can be applied across a wide range of two-phase flow regimes. Also, there is not a single mesh that can be used to simulate all of the flow regimes and a case-specific mesh and time-step convergence study is needed for each individual flow regime. In the current study, optimal mesh size and time step were obtained for a slug flow test case. Hence, good agreement was obtained only for similar flow regimes, leading to significant disagreement with experimental data for test cases with substantially different flow patterns.

Experimental investigations of the flows inside helically coiled pipe are difficult and may also be expensive, particularly for small diameters. Computational fluid dynamics (CFD) packages, which can easily construct the geometry and change the dimensions with 100% of accuracy, provide an alternative solution for the experimental difficulties and uncertainties. Therefore, a computational fluid dynamics (CFD) study was conducted to analyse the flow structure and the effect of varying the coil pitch on the coil friction factor and wall shear stress, through utilising different models’ configurations. Three coils were tested, all of them sharing the same pipe and coil diameter: 0.005m and 0.04m respectively. Pitch variations began with 0.01, 0.05, 0.25 m for the first, second and third model respectively. In this study, the pressure drop was analysed, and the effects of this reduction on wall shear stress and coil friction factor were also examined using incompressible viscous laminar flow to elucidate the effect of varying coil pitch on the friction factor and wall shear stress. The results were validated by Ito’s equation for the laminar flow and compared with the Hagen-Poiseuille resistance formula for a straight pipe. [ABSTRACT FROM AUTHOR]

The effect of shell side and coil side volume flow rate on overall heat transfer coefficient, effectiveness, pressure drop and exergy loss of shell and helical coil heat exchanger were studied experimentally under steady state conditions. The working fluid, i.e., water was allowed to flow at three different flow rates of 1, 2, and 3 l/min on shell side (cold water) and at 1, 1.5, 2, 2.5, and 3 l/min on coil side (hot water) for each shell side flow rate at the temperatures of 298±0.4K and 323±0.4K, respectively. The results found that the overall heat transfer coefficient increased with increasing both shell side and coil side volume flow rates. The inner Nusselt number significantly increased with the coil side Dean number. [ABSTRACT FROM AUTHOR]

The following article presents experimental comparison research on a hexagonal shelland-tube latent thermal energy storage (TES). Such shape of a shell was deliberately chosen instead of a cylindrical one due to its high modularity and with intent for future applications in automobiles (EV and PHEV) air conditioning systems (HVAC). Two geometries of helical coils, acting as tubes, were studied in this article. One was a simple helical coil with a small pitch. The other one was a helical coil with a larger pitch but enhanced with six longitudinal fins for each corner of the hexagon. Fins were added to study the effects of higher heat transfer area as well as effects of better thermal penetration of the Phase Change Material (PCM). Geometries of coils were matched with intent to occupy the same volume in the storage tank for a better comparison of the results. Results were also compared with previous studies carried out by authors for a copper pipe placed in a cylindrical shell. PCM chosen for this study was RT18HC due to its high value of latent heat and the melting temperature close to 18°C. Melting temperature value is crucial for applications in vehicle thermal control systems. [ABSTRACT FROM AUTHOR]

This study presents exergy analysis for forced convection heat transfer in shell and helically coiled tube heat exchangers. The effect of operational and geometrical parameters on the exergy efficiency was investigated. Water is selected as the working fluid of both sides. Results show that, the efficiency decreases linearly with the increase of the fluids dimensionless inlet temperature difference. Based on the results, a correlation was developed to predict the efficiency for wide range of mass flow rates ratio (0.1

The study aims at quantifying the effect of fluid elasticity on frictional pressure drop for the flow of viscoelastic fluid through a helical coil. Aqueous solutions of polyacrylamide (PAA) in the concentration range 0.25 %–0.5 % (wt/vol.) were used as test fluids. The fluids were characterized using dynamic rheometer under rotational and oscillatory modes. The rheological study depicts that the fluids exhibit elasticity and shear thinning behavior. Experimental data on pressure drop for the flow of non-Newtonian viscoelastic fluid through a helical coil have been generated. Drag enhancement was observed for the fluids with significant elastic character. An existing correlation applicable over a wide range of Germano number (NGn) for the flow of Newtonian fluids through helical coils (11.7≤Dc/dt≤105.48, 8.3≤p/Dt≤66.7), proposed by Gupta et al.22, has been extended to non-Newtonian viscoelastic fluids by incorporating the effect of fluid elasticity in terms of Weissenberg number (We). The proposed model’s uncertainty has been reported for 95 % confidence to assess the significance of newly reported results. The observed data were compared with the available models in the literature statistically using mean relative quadratic error (MRQE) and arithmetic relative error (%ARE). Lower values of MRQE and %ARE depict the acceptability of the proposed model.

In the present study, the effect of operational and geometrical parameters on the thermal effectiveness of shell and helically coiled tube heat exchangers was investigated. Analysis was performed for the steady state. The working fluid of both sides is water, that its viscosity and thermal conductivity were assumed to be dependent on temperature. Based on the results, two correlations have been developed to predict the thermal effectiveness, for wide ranges of mass flow rates ratio, dimensionless geometrical parameters and product of Reynolds numbers. Furthermore, it was found for same values of NTU and Cr, the effectiveness is averagely 12.6% less than the effectiveness of parallel flow heat exchangers and this difference is approximately constant.

Graphical abstract Highlights • Oscillatory baffled reactors enhance micromixing via engulfment using vortices. • Helical coils produce the smallest micromixing times because of additional swirling. • Micromixing times decrease with increasing oscillation intensity. • Central/integral baffles can't achieve the lowest mixing times and good plug flow. • Helical baffles, whereas, can achieve the lowest mixing times and good plug flow. Abstract The mesoscale oscillatory baffled reactor (meso-OBR) is a novel screening platform that does not require significant optimisation of the operating conditions, making it ubiquitous. Although it is known that micromixing performance influences the observed reaction kinetics, the micromixing of meso-OBRs has not previously been characterised. Therefore, in this study we have measured the micromixing times in three meso-OBR configurations across a broad range of oscillatory (Re o = 50–1000) and net flow Reynolds numbers (Re n = 5–40) for the first time using the Villermaux-Dushman competing reaction scheme. Helical baffles exhibited the lowest and most consistent micromixing times, followed by the central axial baffle design and the integral baffle design. Using the micromixing times, design equations were developed that showed the ratio of the micromixing time to mean residence time was dependent only on the velocity ratio (ratio of oscillatory to net flow Reynolds numbers). In all baffle designs, micromixing times decreased with increasing velocity ratios (ψ > 25). Therefore, for the central baffle and integral baffle designs, the lowest micromixing times would not be accessible for flow chemistry applications, because the optimal velocity ratios for plug flow in these designs are only around 4–10. Whereas, the helical baffle can achieve plug flow across a much broader range of velocity ratios (up to ψ = 250), because of the additional swirling component to the flow, enabling the lowest micromixing times of the helical baffle to be exploitable for reaction engineering applications. The lowest micromixing time produced was 0.03 s, using the helical baffle with Re n = 10 and Re o = 750. [ABSTRACT FROM AUTHOR]

Highlights • Measurements of post-dryout heat transfer in helical coils are obtained. • Curvature significantly enhances post-dryout heat transfer in helical coils. • The post-dryout heat transfer coefficient of helical coils decreases in quality. • Heat transfer correlation for post-dryout regime of helical coils is developed. Abstract Post-dryout heat transfer for steam-water flow in helical coils is experimentally investigated. The inner diameters of the helical coils are 12.5 mm and 14.5 mm, and the coil diameters range from 180 mm to 380 mm. The experimental system pressure covers the range 2–8 MP. The results indicate that the centrifugal force induced secondary flow has a great influence on post-dryout heat transfer in helical coils. In the low quality post-dryout region, because of the high redeposition rate induced by secondary flow, the heat transfer coefficient of helical coils is significantly higher than that of straight tubes. As the total droplet mass decreases with increasing quality, the heat transfer performance of helical coils decreases and continuously approaches that of straight tubes. In high quality post-dryout region, the bulk fluid in helical coils is closer to thermodynamic equilibrium state due to the secondary flow, and the heat transfer coefficient may be exceeded by straight tubes. For the effects of flow parameters, the post-dryout heat transfer performance decreases in heat flux and increases in mass flux. For system pressure, due to the change of the dominant heat transfer mechanism, the post-dryout heat transfer decreases in the low quality region but increases in the high quality region with increasing system pressure. Based on a detailed analysis of the effects of various dimensionless parameters, a new correlation containing a quality correction is proposed and provides good prediction accuracy. [ABSTRACT FROM AUTHOR]

Highlights • A helical coil with reversed loops show unique pressure drop and heat transfer characteristics. • The thermal effectiveness of a common helical coil can be improved by adding reversed loops. • Adding reversed loops may enhance turbulence mixing in helical coil heat exchanger. • New friction factor and heat transfer correlations for a helical coil with reversed loops have been postulated. Abstract In this experimental study, flow and heat transfer characteristics of water through a newly designed helical coil heat transfer test section were investigated. A structural modification of the traditional helical coil has been implemented to improve its thermal effectiveness. Specifically, a 360° plastic reversed loop was added after each 180° of the main helical coil loop to redistribute the flow’s velocity profile, which should have a direct effect on the thermal performance of the helical coil heat transfer test section. Results reveal that reversed loops do enhance heat transfer rates by more than 20%, while pressure drop penalty also increases but not at the same rate. The curvature of the reversed loops with respect to the main coil is a critical factor in terms of thermal performance. Furthermore, as the size of the reversed loop decreases, the heat transfer enhancement and pressure drop penalty increment for the modified helical coil test section increase as well. Accordingly, it is suggested that there may exist an optimum helical coil structural modification for which the best tradeoff between pressure drop penalty and heat transfer enhancement can be found. [ABSTRACT FROM AUTHOR]

In the current paper, a numerical model was designed using COMSOL Multiphysics software based on experimental results to predict Nusselt number of TiO 2 , ZnO and Ag water-based nanofluids in helical coil under isothermal boundary conditions. All Nusselt number data extracted from the model were compared with experimental data to validate the model and the results showed that the model gives good accuracy and the model could be used to predict non-experimental data. Root Mean Square Error (RMSE) was calculated to evaluate model accuracy and the results were 4.35, 2.40 and 2.53 for TiO 2 /water, Ag/water and ZnO/water respectively. The model was used to predict non-experimental data and the results were logically predicted compared with experimental data and the model was proven to be sufficient for predicting non-experimental data. Model results were compared with two types of artificial neural networks predicting Nusselt number Feed Forward Neural Network (FFNN) and Generalized Regression Neural Network (GRNN). The deviation between predicted data and experimental data was calculated and the results indicated that GRNN network has the highest accuracy with maximum deviations of +0.02% & −0.3% while the COMSOL model has a maximum deviation of +4.5% & −5.9% and finally FFNN network +8.9% & −5.7%. [ABSTRACT FROM AUTHOR]

• Natural convection heat transfers of helical coils were measured varying pitches and turns. • Ring bundles were proved enough to simulate helical coils for ring-by-ring heat transfer. • Heated plumes from lower rings impaired heat transfer of upper ones for small pitches. • Increased pitches and turns enhanced heat transfers by initial velocities and superpositions. • Optimal pitch range for the helical coil design was estimated. A helical coil is gaining interest as the part of a compact heat exchanger since it allows large heat transfer area. Many small modular reactor designs adopt the helical coil steam generator. We measured the local and total heat transfers of the helical coil using ring bundles to confirm the influences of pitches and turns. The pitch to coil diameter ratio was varied from 1.08 to 3.97 and the turns from 2 to 12, with the Ra of 3.63 × 107. Based on the analogy between heat and mass transfers, mass transfer experiments were performed replacing heat transfer ones. For a small pitch, the heated plume from the lower ring provides preheated plume to the upper ring, resulting in an impaired heat transfer. However, for a large pitch, initial velocity is provided, enhancing the heat transfer. The comprehensive heat transfer influences also depend on the coil turns. The measurements of total heat transfer showed the peak at around the pitch to coil diameter ratio range from 1.50 to 2.02 regardless of the number of coil turns. [ABSTRACT FROM AUTHOR]

A helically coiled tube enhances heat transfer rate due to development of secondary flows. Use of helical tube as heat exchanger will enhance heat transfer coefficient. Correlations available in the literature for calculating condensation heat transfer coefficient for straight tube are used to evaluate the performance of helically coiled tube-in-shell heat exchanger. Experiments were performed on helically coiled tube of 175 mm coil diameter. Measurements were taken in 10 steps at inlet rate of mass flow of steam. Mean value of the coefficient of heat transfer at inside surface of the tube for condensation was evaluated by experimental methods and results were compared with three correlations available in literature and the deviations are reported. A new empirical correlation is proposed which is based on the experimental investigations. [ABSTRACT FROM AUTHOR]

Compactness with high performance heat exchangers are main challenges in a lot of engineering applications. Thus, this research reports CFD simulation of a novel MTTHC (multi tubes in tube helically coiled) heat exchanger using ANSYS-FLUENT 14.5. The aim of the work is to investigate the thermal and hydraulic performance of the MTTHC for turbulent flow. The effects of the operating and geometrical parameters of the coil on the cold/hot water Nusselt numbers, heat transfer coefficients, pumping power, effectiveness, and thermal-hydraulic index are studied and presented. The results show that, the largest heat transfer coefficient is found at N = 3 and β = 0° & 90° , and the pumping power (P) rises with ≅ 20 times if N changed from 1 to 5 at any β . Moreover, the effectiveness of the coil ( ε ) has the largest values at β = 0° & 90° and N = 3, and it enhances with 8.5%, 9% and 7% if N increased from 1 to 3 at β  = 0°, 45° and 90°, respectively. In addition, thermal-hydraulic index (ξ) improves with 5%, 8% and 6% if N increased from 1, to 3 at β  = 0°, 45° and 90°, respectively. Finally, Numerical correlations for P, ε and ξ are correlated and presented within reasonable errors. [ABSTRACT FROM AUTHOR]

The objective of the present work is to study the heat transfer distribution in the helical coil flow boiling system. Infrared thermal imaging technique is used to measure the local wall temperature. The wall temperature is measured across the circumference and in axial direction along the length of a helical coil. The literature showing circumferential heat transfer distribution in a helical coil during flow boiling process is scarce. The present work compares the wall temperature and heat transfer coefficient distribution on the inner side and the outer side of helical coils at different diameter ratios and density ratios. In the present study, the experimental databank (containing total 400 heat transfer coefficient data points) includes the data not only from the present experimental study but also from the literature. The data bank includes the subcooled and the saturated flow boiling data. The data includes two fluids namely water and R123. Data covers the range of parameters namely a density ratio of 30–1600, a mass flux of 100–1300 kg/m 2  s, a heat flux of 2–640 kW/m 2 , an exit quality of −0.5 to 1 and a coil to tube diameter ratio of 14–58. Ten empirical correlations which include well referred correlations of straight tubes and available correlations of helical coils are evaluated with the databank. The study concluded that the circumferential wall temperature variation in helical coils decreases with increase in a diameter ratio and with decrease in a density ratio. The circumferential averaged heat transfer coefficient during a saturated flow boiling in a helical coil is same as a straight tube. A correlation is suggested to measure the circumferential average heat transfer coefficient in helical coils. [ABSTRACT FROM AUTHOR]

The main selection criterion for pumps in reprocessing applications is the absence of any moving parts or the moving parts not having any direct contact with the process fluid. There are different types of fluidic pumps such as vortex diode, reverse flow diode, etc. In this work, a new design of helical coil fluidic pump has been developed and demonstrated. Helical coils are widely used in process industries as heat exchangers, reactors, motionless mixers, etc. It is for the first time that the authors are proposing the use of helical coil as a fluidic pump based on its inherent property of high pressure drop as compared to a straight tube. The main objective of this work is to develop an alternate pumping system for reprocessing applications to replace the conventional air lift pump. The helical coil based fluidic diode pump was made up of SS 304 seamless pipe (pipe inner diameter is 9.5 mm) with a 36 mm diameter of helical coil and pitch of 14 mm. In this work, friction factor for the given helical coil was measured from pressure drop experiments. This information was also useful for understanding the transition of the laminar to turbulent region. In order to propose the operating region, the pumping capacity was measured for the helical coil based fluidic pump at different operating conditions. Experimental results show that the proposed hardware is able to pump up to 54 dm3/h. Recommendations have been made for proposing an optimum design. [ABSTRACT FROM AUTHOR]

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In this work, the heat transfer intensification in shell and helically coiled tube heat exchangers via installing annular fins on the outer surface of the helical coil, has been numerically investigated. Thirteen heat exchangers were designed for this purpose. All geometrical parameters of the heat exchangers are same except, fin's number or height. All of the heat exchangers have been studied at three different shell side Reynolds number (Re sh = 7500, 15,000 and 30,000). In order to validation of the numerical model two method has been used. In the first method, the calculated heat transfer has been compared with the approximate method which is based on the experimental correlations for both coil and shell side Nusselt numbers and consideration of the E-NTU relations of these types of heat exchangers. In the second method, the heat transfer coefficient of the shell side has been compared with the experimental heat transfer coefficients of the previous works. An acceptable agreement has been observed in these comparisons. Furthermore, the optimum cases and some correlations have been obtained for prediction of the heat transfer coefficient of the shell side. Results indicate that, in the range of 7500 ≤ Re sh ≤ 30,000, the heat transfer rate can increase up to 44.11%. [ABSTRACT FROM AUTHOR]

Oscillatory baffled reactors (OBRs) are able to generate plug flow at laminar net flow conditions, providing appropriate oscillation conditions are selected. Mesoscale OBRs containing helical baffles exhibit wider “operating windows” (i.e. a broader oscillation intensity range) for plug flow than other baffle designs. It has been hypothesised that additional swirling in the flow provides another mechanism to limit axial dispersion. These swirling flows have previously been qualitatively identified, but in this study these flows were investigated both numerically and experimentally using CFD and PIV for the first time. The flow structures obtained via simulation (laminar solver) were visualised using isosurfaces of the Q-criterion and 3D streamlines. The characteristic feature of the flow is a helically shaped vortex that forms behind the baffle. Streamlines move both radially (wrapping around the vortex structure) and tangentially. Using the swirl number and analogous ‘radial’ number, a transition between vortex-dominated and swirl-dominated mixing was observed providing evidence that the hypothesis is valid. It was found that when the oscillation intensity is increased, the tangential motion of the flow increases faster than the increase in radial flow because the vortex sizes are bounded by the column diameter. [ABSTRACT FROM AUTHOR]

This paper presents a numerical analysis of a horizontally oriented constant surface area isothermal helical coil under natural convection heat transfer and surface radiation. Numerical computations are carried out in the laminar regime of Rayleigh number (104 ≤ Ra ≤ 108), surface emissivity (0 ≤ ε ≤ 1), and geometrical parameters of the helical-coil, like diameter of the coil (8 ≤ D / d ≤ 24), pitch (3 ≤ p / d ≤ 7.5), and coil height (40 ≤ H / d ≤ 60) to study the heat transfer characteristics. This study is used for effectively designing the helical coils, which are typically manufactured at an extremely high temperature. The numerical computation has been validated by comparing the average Nusselt number (Nu) with the previous experimental results. Temperature-dependent fluid properties are incorporated to achieve precise results over a wide range of temperatures. The effect of the geometrical parameters on the heat transfer characteristics has been depicted graphically in terms of average Nu , relative convection and radiation heat transfer rate, and the temperature profile. It is found that the relative contribution of radiative heat transfer is 87.8% at an emissivity of 0.9 and Ra of 10(Ali, 1994 4), whereas, for an emissivity of 0.1 and Ra of 10(Moawed, 2005 8), the relative contribution of radiation heat transfer is only 6.4%. Lastly, a correlation has been developed for the average Nu based on all the pertinent parameters which can be beneficial to the engineers in the industry and also academic researchers. The developed correlation has been used to predict the cooling curve of the helical coil in certain situations. [ABSTRACT FROM AUTHOR]