Helical Coiled Tube Research Articles

Exergy analysis of laminar forced convection of nanofluids through a helical coiled tube with uniform wall heat flux

In this paper, entropy generation in nanofluids has been studied analytically. For this purpose, the fully developed laminar forced convection of Al2O3–water nanofluid through helical coiled tubes is investigated. The nanofluid has been considered to be a single–phase homogeneous fluid. Entropy generation rates due to heat transfer and friction loss, and the total entropy generation rate have been derived for nanofluids, and the effects of various coil and flow parameters on the entropy generation rates have been analysed. The results showed that increasing the basefluid's Reynolds number, nanoparticles concentration, and curvature ratio causes thermal entropy generation to decrease and frictional entropy generation to increase. Further, the variation of the dimensionless pitch has a minor effect on the total entropy generation distribution of nanofluids, and can be set to a constant value, whereas the effects of the nanoparticles concentration on the total entropy generation rate are major, and cannot be neglected.

Review on Comparative Study between Helical Coil and Straight Tube Heat Exchanger

The purpose of this study is to determine the relative advantage of using a helically coiled heat exchanger against a straight tube heat exchanger. It is found that the heat transfer in helical circular tubes is higher as compared to Straight tube due to their shape. Helical coils offer advantageous over straight tubes due to their compactness and increased heat transfer coefficient. The increased heat transfer coefficients are a consequence of the curvature of the coil, which induces centrifugal forces to act on the moving fluid, resulting in the development of secondary flow. The curvature of the coil governs the centrifugal force while the pitch (or helix angle) influences the torsion to which the fluid is subjected to. The centrifugal force results in the development of secondary flow. Due to the curvature effect, the fluid streams in the outer side of the pipe moves faster than the fluid streams in the inner side of the pipe. The difference in velocity sets-in secondary flows, whose pattern changes with the Dean number of the flow. In current work the fluid to fluid heat exchange is taken into consideration, Most of the investigations on heat transfer coefficients are for constant wall temperature or constant heat flux. The effectiveness, overall heat transfer coefficient, effect of coldwater flow rate on effectiveness of heat exchanger when hot water mass flow rate is kept constant and effect of hot water flow rate on effectiveness when cold water flow rate kept constant studied and compared for parallel flow, counter flow arrangement of Helical coil and Straight tube heat exchangers. The inner heat transfer coefficient calculated from Wilson plot method. Then Nusselt no and correlation obtained on the basis of inner heat transfer coefficient. All readings were taken at steady state condition of heat exchanger. The result shows that the heat transfer coefficient is affected by the geometry of the heat exchanger. Helical coil heat exchanger are superior in all aspect studied here.

An empirical study on the pressure drop characteristics of nanofluid flow inside helically coiled tubes

In this study, pressure drop characteristics of nanofluid flow inside vertical helically coiled tubes are investigated experimentally for the laminar flow regime. The temperature of the tube wall is maintained constant at around 95 °C to have isothermal boundary condition Experiments are implemented for fluid flow inside helically coiled tubes and a straight one. A wide range of various variables is taken into account. Pitch to tube-diameter ratio ranges between 1.6 and 6.1 and coil-to-tube diameter ratio varies from 14.1 to 20.5. Heat transfer oil is used as the base fluid, and Multi-Walled Carbon NanoTubes (MWCNTs) are utilized as the additive to provide the nanofluids. The working fluids are extremely temperature dependent, so rough correlations are proposed to predict their thermo-physical properties. Regarding the experimental data, utilization of helical coiled tubes instead of straight ones increases the pressure drop exponentially. Irrespective of the tube geometry in which the fluid flows, nanofluid flows show higher rate of pressure drop compared to that of the base fluid flow. Finally according to the observations, combination of the two techniques used in this study causes the pressure of the fluid flow to drop significantly along the test section.

Experimental investigation on the convective heat transfer enhancement for highly viscous fluids in helical coiled corrugated tubes

In the present analysis, the forced convective heat transfer in smooth and corrugated helical coiled tubes was experimentally studied in the Reynolds and Dean number ranges 50÷1200 and 12÷295 respectively, by adopting Ethylene Glycol as working fluid. The primary aim of the investigation is to study the combined effect of the wall curvature and of the wall corrugation in the thermal entrance region for highly viscous fluids. Two coiled tubes with a curvature ratio of about 0.06, one with smooth wall and the other with spirally corrugated wall, were investigated under the uniform heat flux boundary condition. The main conclusion is that in the Reynolds number range analyzed, both curvature and corrugation enhance the heat transfer. For Dean number values lower than about 120 the wall curvature effect prevails, and the heat transfer enhancement reflects Nusselt numbers that are approximately 2–3 times higher than the straight smooth section. For greater Dean number values, the wall corrugation instead prevails. In fact the corrugated coiled tube reaches Nusselt number values which are up to 8 times higher than the ones expected for the smooth straight tube. The smooth coiled tube shows instead thermal performances at maximum 3.6 times over the straight section.

Dynamics of hydrogen powered CAES based gas turbine plant using sodium alanate storage system

Typical compressed air energy storage (CAES) based gas turbine plant operates on natural gas or fuel oils as fuel for its operation. However, the use of hydro-carbon fuels will contribute to carbon emissions leading to pollution of the environment. On the other hand, the use of hydrogen as fuel for the gas turbine will eliminate the carbon emissions leading to a cleaner environment. Hydrogen can be produced using renewable energy sources like wind, solar etc. Storage of hydrogen is a bottleneck for such a system. A high capacity sodium alanate metal hydride bed is used in this study to store the hydrogen. The dynamics of the CAES based gas turbine plant operating with hydrogen fuel is presented along with discharge dynamics of the metal hydride bed. The heat required for desorbing the hydrogen from the metal hydride bed is provided partly by the hot flue gas exiting from the low pressure turbine and partly by external heating. Thus some of the heat from the flue gas is extracted. A novel multiple bed strategy is employed for efficient desorption. Each bed consists of a shell and tube, with alanate in the shell and heating fluid flowing through the helical coiled tube. Hydrogen combustor is modeled using a simplified Continuous Stirred Tank Reactor (CSTR) assumption in CANTERA. The NOx emissions in the low pressure turbine exhaust stream are presented.

Comparison of Critical Heat Flux for R-134a Flow Boiling in a Horizontal Straight Tube and a Helically-Coiled Tube

Experimental studies on critical heat flux (CHF) have been conducted in a uniformly heated horizontal straight tube and helically-coiled tube respectively with R-134a as the working fluid. The helically-coiled tube has the same heated length and inner diameter with the straight tube and experiments were performed under the following conditions: pressure from 0.4 to 2.5 MPa, mass flux values from 80 to 1500 kg m-2 s-1, inlet quality from -0.23 to 0.28 and critical quality from 0.65 to 0.86. The CHF data of the helically-coiled tube have been compared with that of the straight tube. The results show that the helically-coiled tube gets significant improvement in the CHF values vs. the straight tube under the same conditions and the degree of improvement depends on the mass flux, system pressure, inlet quality and critical quality.

Studies on Fluid-to-Fluid Modeling of Critical Heat Flux in a Helically-Coiled Tube

An experimental study on critical heat flux (CHF) in a helically-coiled tube cooled with R-134a has been completed in order to assess present fluid-to-fluid modeling approaches. The investigated range of flow parameters for R-134a was: pressure from 0.2 to 0.5 MPa, mass flux values from 50 to 1500 kg m-2 s-1 and inlet quality from -0.2 to 0.1. The CHF data of R-134a have been compared with that of water by applying the Ahmad and the Katto modeling. The water equivalent CHF data translated from R-134a CHF data by using the two modeling approaches have shown a good agreement with the actual water CHF data from previous studies when mass flux exceeds 600 kg m-2 s-1. The results indicate that both the Ahmad and the Katto modeling can be applied only for the high mass flux conditions in helically-coiled tubes.

Numerical study on pressure drop and heat transfer for designing sodium-to-air heat exchanger tube banks on advanced sodium-cooled fast reactor

A numerical study is performed to investigate the thermal and hydraulic characteristics and build up design model of the AHX (sodium-to-air heat exchanger) unit of a sodium-cooled fast reactor. Helical-coiled tube banks in the AHX are modeled as porous media and simulated heat and momentum transfer by a commercial program. Two-dimensional flow characteristic appears differently at the inlet region of the AHX annulus, and the required length of the inlet region is shorter for an inlet having a 45 degree chamber or a round shape than for one with a perpendicular corner. Pressure drop and heat transfer coefficient for rectangular, parallelogram and staggered tube banks as the main components of the AHX are evaluated and discussed. Pressure drop and heat transfer shows similar trends and underestimated values, respectively, when compared with Zhukauskas empirical correlations. The parallelogram tube bank shows similar results to the rectangular arrangement.

Limit load analyses of helical coiled steam generator tubes with a volumetric flaw

ABSTRACTMaintaining structural integrity of piping has always been an important effort in the nuclear power industry. To resolve piping issues such as unanticipated failures caused by diverse planar and volumetric flaws, several guidelines were developed and used especially for assessment of steam generator tubes. However, because the major components of new reactors have dissimilar geometric features and loading conditions compared with those of conventional operating reactors, most of existing assessment methods are not expected to be applicable; therefore, new alternative assessment guidelines are required. In this paper, a systematic structural integrity assessment of helical coiled steam generator tubes for a small and medium modular reactor, which is currently being designed, is introduced. Three‐dimensional detailed finite element (FE) limit analyses have been carried out to simulate the behaviours of the tube containing a volumetric flaw such as elliptical wear‐type, rectangular wear‐type and tapered wear‐type defects subjected to external pressure. Failure pressures were calculated from the FE analyses by changing defect depth, defect length and defect angle affecting the load‐carrying capacity of the tube. Thereby, engineering equations were developed as a function of these key parameters to predict structural failures and system reliability to enable more reasonable design and manufacturing decisions.

Experimental Study of Forced Convection over Equilateral Triangle Helical Coiled Tubes

This study presents an experimental investigation of an equilateral triangular cross-sectioned helical tube under uniform heat flux boundary condition. The experiments are carried out for nine helical coiled-tubes of different parameters. Different diameter ratio (D/a) ranged from 6.77 to 15.43 and pitch ratio (P/a) ranged from 1.127 to 3.062 are employed in the present study, The experiments covered a range of Reynolds number from 5.3X10 2 to 2.2X10 3 . Uniform heat flux is applied to the inside surface of the helical coil and air is selected as tested fluid. The experimental results obtained from the equilateral triangular cross-sectioned helical tube indicated that the parameters of the coil diameter and pitch of helical coil have important effects on the heat transfer coefficient. The Nusselt number increases with the increase of Reynolds number and coil diameter at constant pitch of the helical coil. Also, Nusselt number increases with the increase of Reynolds number and Pitch of helical coil at constant coil diameter tube. A comparison between the present experimental data with a previous work with circular cross-sectioned helical tubes have the same test conditions was achieved. From this comparison, it is clear that the average enhancement of Nusselt number for equilateral triangular cross-sectioned helical is about 1.12 ~ 1.25 times the circular cross-sectioned helical for all tested conditions. A general correlation of the average Nusselt number as a function in Re, D/a and P/a ratios is obtained to describe the forced convection from the equilateral triangle cross sectioned coiled tube. Key words : Forced convection; Helical coiled tubes; Coil diameter ratio; Pitch ratio

Experimental investigation on the convective heat transfer of nanofluid flow inside vertical helically coiled tubes under uniform wall temperature condition

In this study, heat transfer enhancement of a nanofluid flow inside vertical helically coiled tubes has been investigated experimentally in the thermal entrance region. The temperature of the tube wall was kept constant at around 95°C to have isothermal boundary condition. Experiments were conducted for fluid flow inside straight and helical tubes. In these experiments, the effects of a wide range of different parameters such as Reynolds and Dean numbers, geometrical parameters and nanofluid weight fractions have been studied. In order to investigate the effect of the fluid type on the heat transfer, pure heat transfer oil and nanofluids with weight concentrations of 0.1, 0.2 and 0.4% were utilized as the working fluid. The thermo-physical properties of the working fluids were extremely temperature dependent; therefore, rough correlations were proposed to predict their properties. Based on the experimental data, utilizing helical coiled tubes instead of straight ones enhances the heat transfer rate remarkably. Besides, nanofluid flows showed much higher Nusselt numbers compared to the base fluid flow. Finally, it was observed that combination of the two enhancing methods has a noticeably high capability to the heat transfer rate.

Experimental Study of Free Convection in Coiled Tube Heat Exchanger with Vertical Orientation

An experimental study has been conducted on steady-state natural convection heat transfer from helical coil tubes in vertical orientation. Water was used as a bath liquid without any mixing and cold water was used as a coolant fluid. A straight copper tube of 6 mm ID, 8 mm OD and 3 m length was bend to fabricate the helical coil. Four coils are used in this experiment has different curvature ratios and pitches. The data were correlated using tube diameter as the characteristic length. The results show that the overall heat transfer coefficient and Nusselt number increase when the flow rate of coolant and curvature ratio increase. The effect of coil pitch was investigated and the results show that when of the coil pitch (angle of inclination) increases Nusselt number increase. A correlation was presented to calculate the outside average Nusselt number of coil.

Analysis of the influence of different heat transfer correlations for HTR helical coil tube bundle steam generators with the system code TRACE

For next high temperature reactor projects, e.g. for electricity production or nuclear process heat applications, the steam generator is a crucial component. The typical design consists of a helical coil tube bundle steam generator with several tubes in different tube cylinders connected in parallel. This type of steam generator was a significant component used in the THTR-300. The influence of different heat transfer correlations for the primary side on the steam outlet temperature is being analyzed by the nodal thermal hydraulics code TRACE for the THTR-300 steam generator and compared to experimental data. The results of the standard TRACE code are compared to results with modified Nusselt numbers for the primary side and they will be elaborated in this paper. These modified Nusselt numbers were used for the heat transfer calculations of the THTR-300 steam generator design.

SOLAR WATER HEATER WITH SHELL AND HELICAL COILED TUBE HEAT EXCHANGER AS A STORAGE TANK

In this work an experimental study is performed to evaluate the thermal performanceof locally made closed loop solar hot water system using a shell and helical coiled tubeheat exchanger as a storage tank. Several measurements are taken include inlet and outlettemperatures of both collectors and supply water and temperature distribution within thestorage tank. This is beside the water flow rate in both collectors and load cycle. Themain parameters of the system are obtained.

Investigation of potential of improvement of helical coils based on avoidable and unavoidable exergy destruction concepts

An inevitable problem challenges heat exchanger designers is that the heat transfer augmentation in a thermal system is always achieved at the expense of an increase in pressure loss. Thus, the trade-off by choosing the most proper configuration and best flow condition has become the critical problem for design work. The brief survey on literature shows that optimal Reynolds number of laminar forced convection in a helical tube, was specified based on minimum entropy generation. Therefore, the present study analyzes the thermodynamic potential of improvement for steady, laminar, fully developed, forced convection in a helical coiled tube subjected to uniform wall temperature based on the concept of avoidable and unavoidable exergy destruction. The influence of various parameters such as coil curvature ratio, dimensionless inlet temperature difference, dimensionless passage length of the coil, and fluid properties on avoidable exergy destruction have been investigated for water as working fluid. Results show considerable potential of thermodynamic optimization of helical coil tubes. In addition, a relation for determining the amount of optimum Dean Number is proposed for the range considered in the present study.

ICONE19-43935 Development of a Helical-Coil Double Wall Tube Steam Generator for 4S Reactor

The 4S, Super-Safe Small and Simple, is a small-sized sodium-cooled fast reactor. A fast reactor usually uses sodium as a coolant to transfer heat from core to turbine/generator system. The heat of the intermediate heat transport system and that of the water steam systems are exchanged by the steam generator (SG) tubes. If the tube failure occurs, a sodium/water reaction could be occurred. To prevent the reaction and enhance safety, a helical-coil-type double wall tube with wire mesh interlayer and continuous monitoring systems of tube failure are applied to the SG of the 4S. The development and general features of this type double wall tube were described in Ref.1) and Ref.2). Those paper summarized following results; The tubes studied in these references were straight type. To establish this SG, development of manufacturing method of helical-coil-type double wall tube and validation of the tube failure monitoring system are needed. In this study, three demonstration tests have been performed; welding test of the double wall tube to manufacture the tubes with 70-80m length, assembling test of the helical-coil tube, and confirmation test of the tube processing system using the fabricated helical-coil tubes. As a result, following technologies have been successfully established. (1) Development of the welding techniques for manufacturing of the helical-coil-type double wall tube with wire mesh interlayer. (2) The confirmation test for manufacturing the helical coil tube of the SG.

Experimental study on critical heat flux characteristics of R134a flow boiling in horizontal helically-coiled tubes

Critical heat flux (CHF) experiments were performed to study the R134a CHF characteristics in horizontal helically-coiled tubes. The stainless steel test sections were heated uniformly, with tube inner diameters of 3.8–11 mm, coil diameters of 135–370 mm, helical pitches of 40–105 mm and heated lengths of 0.85–7.54 m. The experimental conditions are pressures of 0.30–1.10 MPa, mass fluxes of 60–480 kg m −2 s −1, inlet qualities of −0.32–0.36 and heat fluxes of 6.0 × 10 3–9.0 × 10 4 W m −2. It was found that the wall temperatures jumped abruptly once the CHF occurred. The CHF values decrease with increasing heated lengths, coil diameters and inner diameters, but the DNB (departure from nucleate boiling) CHF seems independent when length-to-diameter L/ d i > 200. The coil-to-diameter ratios are more important than length-to-diameter ratios for CHF in helically-coiled tubes, while the helical pitches have little effect on CHF. The CHF value increases greatly with increasing mass flux and decreases smoothly with increasing pressure. It decreases nearly linearly with increasing inlet and critical qualities, but it varies more acutely at x cr < 0.5 than higher critical qualities. New correlations for R134a flow boiling CHF in horizontal helically-coiled tubes were developed for applications.

Dry-out CHF correlation for R134a flow boiling in a horizontal helically-coiled tube

An experimental study was carried out to investigate the R134a dry-out critical heat flux (CHF) characteristics in a horizontal helically-coiled tube. The test section was heated uniformly by DC high-power source, and its geometrical parameters are the outer diameter of 10 mm, inner diameter of 8.4 mm, coil diameter of 300 mm, helical pitch of 75 mm and valid heated length of 1.89 m. The experimental parameters are the outlet pressures of 0.30–0.95 MPa, mass fluxes of 60–500 kg m −2 s −1, inlet qualities of −0.36–0.35 and heat fluxes of 7.0 × 10 3–5.0 × 10 4 W m −2. A method based on Agilent BenchLink Data Logger Pro was developed to determine the occurrence of CHF with a total of 68 T-type thermocouples (0.2 mm) set along the tube for accurate temperature measurement. The characteristics of wall temperatures and the parametric effect on dry-out CHF showed that temperature would jump abruptly at the point of CHF, which usually started to form at the front and offside (270° and 90°) of the outlet cross-section. The CHF values decrease nearly linearly with increasing inlet qualities, while they decrease more acutely with increasing critical qualities, especially under larger mass flux conditions. The mass flux has a positive effect on CHF enhancement, but the pressure has negative one. A new dimensionless correlation was developed to estimate dry-out CHF of R134a flow boiling in horizontal helically-coiled tubes under current experimental conditions and compared to calculated results from Bowring and Shah correlations.

Experimental study of forced convection from helical coiled tubes with different parameters

The forced convection from outside surfaces of helical coiled tubes is studied experimentally with a constant wall heat flux. Ten helical coiled-tubes of different parameters with a range of diameter ratio ( D/ d o ) ranged from 7.086 to 16.142 and pitch ratio ( P/ d o ) ranged from 1.81 to 3.205 are employed in the present study. The experiments are performed in an open-circuit airflow wind tunnel system operated in suction mode. The experiments covered a range of Reynolds number (Re) of 6.6 × 10 2 to 2.3 × 10 3. The experimental results indicated that these parameters ( D/ d o and P/ d o ) have important effects on the average heat transfer coefficient. The average Nusselt number (Nu m) increases with the increase of D/ d o at constant Re and P/ d o . Also, Nu m increases with the increase of P/ d o at constant Re and D/ d o . A considerable agreement between the present experimental data and previous work is achieved. The average Nusselt numbers is correlated with Re, D/ d o and P/ d o .

Numerical modeling of polystyrene synthesis in coiled flow inverter

Recent studies show that microfluidic devices are gaining importance as micromixer in chemical and bio-chemical analysis systems. However, little attention has been paid to investigate chemical reactions such as polymerization reaction process in microreactors. In the present study, numerical modeling of the free-radical polymerization of styrene was carried out in novel coiled flow inverter (CFI) microreactor. The concept of CFI is based on the technique developed by Saxena and Nigam (AlChE J 30:363–368, 1984). This device is made up of helical coiled tube which is bent periodically to 90° at equidistant length. The CFD modeling for polymerization reaction taking place in coiled tube reactor was also performed in order to understand the influence of secondary flows on reactor performance for fluid flowing with very low flow rate. Its performance was compared with CFD results obtained in a straight tube reactor having identical length and operating under the same process conditions. The results showed that monomer conversion in the coiled tube reactor was higher than that of the straight tube reactor. Further work was carried out in the novel CFI reactor to study the effect of diffusion coefficient and number of bends on different parameters such as monomer conversion, number-average degree of polymerization (DP n ), and polydispersity indexes (PDI). It was found that the performance of CFI as reactor increased when the diffusion coefficients of reactants was decreased. Thus, CFI was found to be an efficient microfluidic device for controlling the free-radical polymerization.