Coiled Flow Inverter Research Articles

Novel three-dimensional microfluidic device for process intensification

Microchannel heat exchangers and chemical reactors have extensive applications in process industries, especially in electronics cooling, air conditioning, and chemical industries. Microchannel devices provide enhanced heat and mass transfer characteristics by controlled flow conditions and high surface-to-volume ratios. The different designs of microchannels are based on planar structures from various fabrication technologies. In the present study a novel three-dimensional micro-structured device, the micro coiled flow inverter (MCFI) with 0.38–0.8mm internal diameter, has been numerically investigated as a micro heat exchanger. In comparison micro helical coil (MHC) and straight tube of same heat transfer area (dt=0.5mm, A=3.9×10−4m2) has been studied, too. The Reynolds and Prandtl numbers are varied from 25 to 1200 and 0.74 to150, respectively. The MCFI offers a four-fold heat transfer enhancement as compared to straight tube of same heat transfer area at NRe=1200 and NPr=7. Furthermore, the heat transfer coefficient in MCFI augments by 38.5% as compared to MHC, with slight increase (5–16%) in friction factor. New design correlations are developed for Nusselt number and friction factor in MCFI. The MCFI device offers 1.15–1.7-folds higher thermal merit as compared to micro straight tube.

Flow Characteristics of Power-Law Fluids in Coiled Flow Inverter

A novel device coiled flow inverter (CFI) has shown great potential as a heat exchanger and inline mixer.(1-3) The device offers negligible thermal and concentration gradient in the radial directio...

Liquid–Liquid Mixing in Coiled Flow Inverter

The mixing of liquids is a common operation in process industries such as refineries and chemical and pharmaceutical industries, etc. However, the problem of mixing of different liquids has not been rigorously characterized. Therefore, the objective of this paper is to investigate liquid–liquid mixing in a novel coiled flow inverter (CFI). The device works on the principle of flow inversion which is achieved by bending a coiled tube to 90° at equidistant length. In the present study, velocity field and scalar concentration distribution of liquids were characterized. The mixing performances and pressure drop in CFI was investigated and compared with that of a straight, coiled tube and helical element mixer (HEM) for a liquid flow range of 98 ≤ Re ≤ 1020. CFI exhibits significant mixing of two liquids with negligible change in pressure drop as compared to a coiled tube as well as a HEM. The present study reveals that CFI is an efficient device for the mixing of two liquids in process industries.

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.

Augmentation of heat transfer performance in coiled flow inverter vis-à-vis conventional heat exchanger

In the present work, primarily two studies were carried out to ascertain the performance of coiled flow inverter (CFI) as heat exchanger at pilot plant scale. In the first study, performance of CFI heat exchanger has been compared with conventional heat exchangers, i.e. shell and tube heat exchanger (SHE) and plate type heat exchanger (PHE) under identical heat transfer area and process conditions. Experiments were conducted with water flowing under laminar flow regime within the flow range of 30–300 kg/h in the tube side of SHE and PHE. Friction factor and Nusselt number calculated from present experimental study in SHE and PHE were compared with the experimental data previously reported for CFI heat exchanger ( Kumar et al., 2007). The Number of Transfer units (NTU) calculated in the present study for CFI was nearly 3.7–7.5 times higher as compared to SHE and 2–2.5 times higher as compared to PHE. In the second part of the study, experiments were performed first time to investigate the pressure drop and heat transfer of compressed air flowing under turbulent flow condition in CFI heat exchanger at pilot plant scale. Hot air at elevated pressures (10–30 kg/cm 2) in the tube side of CFI heat exchanger with flow range 3×10 4< N Re<1.4×10 5 was cooled by either cooling water or ambient air. The friction factor and Nusselt number values for compressed air flowing in the CFI were also compared with the experimental data reported in the literature for coiled tube at ambient conditions. On the basis of experimental results, new correlations for friction factor and Nusselt number of compressed air flowing under turbulent flow condition in CFI heat exchanger have been developed.

Coiled flow inverter as an inline mixer

Helical coils are widely used in the process industries to improve the mixing efficiency under laminar flow conditions. It was further observed that in the regular helical coils, there exists a confined region in the tube cross-section where fluids are entrapped and can escape only by diffusion. In the present work, an attempt has been made to further enhance the mixing in the coiled tube at low Dean number using the phenomenon of flow inversion. The study is performed in coiled flow inverter (CFI) [Saxena, A.K., Nigam, K.D.P., 1984. Coiled configuration for flow inversion and its effect on residence time distribution. A.I.Ch.E. Journal 30, 363–368] which was developed using the concept of inverting the direction of fluid by 90 ∘ . It comprises coils with equidistant 90 ∘ bends. The scalar mixing of two miscible fluids has been quantified for different process conditions (Dean number, Schmidt number and number of bends) by using scalar transport technique. There was a significant increase in mixing performance of CFI as compared to regular helical coils at low Dean number. The mixing efficiency increased with the increase in Dean number and number of bends. It was also observed that the mixing performance was enhanced with increase in Schmidt number. A new correlation has been proposed for unmixedness coefficient of CFI as a function of Dean number, Schmidt number and number of bends. The proposed correlation has maximum error of ± 20 % with the numerical predictions.

Liquid-Phase Residence Time Distribution for Two-Phase Flow in Coiled Flow Inverter

The coiled flow inverter (CFI) is an innovative device, which has potential for the intensification of processes currently carried out in conventional mixers. Step response experiments were carried out in a CFI to study liquid-phase residence time distribution (RTD) for gas-liquid flow under the conditions of both negligible and significant molecular diffusion using Newtonian fluids. A total of 16 CFIs of different curvature ratios ranging from 6.7 to 20, dimensionless pitch from 1 to 2.5, and number of bends from 1 to 15 were investigated. The range of Dean numbers for the gas and the liquid phase was varied from 235 to 1180 and 3.16 to 1075, respectively. The reduction in dispersion number is nearly 2.6 times for two-phase flow in CFI with 15 number of bends as compared to a straight helix under identical process conditions. A modified axial dispersion model is proposed to describe the RTD. The efficiency of the device is characterized by a mixing criterion.

Experimental Investigation of Pressure Drop during Two-Phase Flow in a Coiled Flow Inverter

Experimental investigation of the pressure drop for a gas−liquid two-phase flow system was conducted in an innovative device known as a coiled flow inverter (CFI). This configuration consists of helical coils with equispaced 90° bends introduced at specific intervals in the coils. The idea is to create random mixing in a cross-sectional plane because of helical coils and complete flow inversion via the insertion of bends. The coils were prepared using a tube for which the following parameters were varied: internal diameter (0.005−0.015 m); curvature ratio, which is defined as coil diameter/tube diameter (6.7−20); pitch (1−2.5); and number of bends (1−15). The liquid flow rate was varied from 3.33 × 10-6 m3/s to 1 × 10-3 m3/s, and the gas flow rate was changed from 8.33 × 10-5 m3/s to 1 × 10-3 m3/s. Sixteen CFIs of different geometric configurations were tested. Comparison of the two-phase friction factor with that of different geometries, such as straight tube and straight helix for single-phase and two-...

Experimental investigation of void fraction and flow patterns in coiled flow inverter

Coiled flow inverter is an innovative device, which works on the principle of complete flow inversion and centrifugal force. It was fabricated by introducing 90° bends in helical coils. It has two-fold advantage of intensifying the convective transfer processes (i.e. increase heat and mass transfer coefficients) and also provide increased transfer area per unit volume of space. It is very compact in size as compared to straight tubes and straight helix. Hence, it is a step towards process intensification. An experimental study was conducted for estimating the gas void fraction and flow patterns for gas–liquid two-phase flowing through this device. The coils were prepared using transparent PVC tubing of 0.005–0.015 m internal diameter with coil diameter varying from 0.08 to 0.2 m. A wide range of gas and liquid flow rates, 8.33 × 10 −5 to 0.001 and 3.33 × 10 −6 to 0.001 m 3/s respectively were investigated. Sixteen coiled flow inverters of different geometric configurations were tested. Various flow patterns observed were stratified, slug, plug, wavy and churn flow. An empirical correlation for void fraction for laminar, transition and turbulent regimes were developed. Data from 7582 gas–liquid flow experiments in coiled flow inverter have been processed for power law and composite power law friction factor correlations to flow regime determination. Separate power law correlations for laminar and turbulent flows were obtained for all flow regimes in the database and also for different flow patterns. The results were compared with Lockhart–Martinelli correlation and Hughmark's correlation.

Numerical study of turbulent forced convection in coiled flow inverter

A numerical study is done to investigate turbulent forced convection in a new device of coiled flow inverter. The proposed device works on the technique based on flow inversion by changing the direction of centrifugal force in helically coiled tubes thus enabling rotation of the plane of vortex. The objective of the present study is to characterize the flow development and temperature fields in coiled flow inverter (CFI) under turbulent flow for the range of 10,000 < N Re < 30,000. The flow pattern obtained for the curved tubes are in agreement to those observed by the previous investigators [1,2]. However, a slight rotation of contours was observed in case of helical coil with finite pitch. Similar rotation of contours was found even with fluids of different Prandtl number (0.7 ≤ N Pr ≤ 150) in helical coil with constant pitch. The study was also carried out for different fluids (air, water, kerosene and ethylene glycol). The coiled flow inverter shows 4–13% enhancement in the heat transfer as compared to the straight helical coil while relative pressure drop is 2–9%. The gain in heat transfer in coiled flow inverter for turbulent flow condition as compared to the straight tube for same flow rate and boundary conditions is 35–45% while the increase in pressure drop is 29–30%.

Coiled flow inverter as a heat exchanger

In the present work attempts are made to investigate the hydrodynamics and heat-transfer characteristics of a coiled flow inverter (CFI) as heat exchanger at the pilot plant scale. The experiments are carried out in counter-current mode operation with hot fluid in the tube side and cold fluid in the shell side. Experimental study is made over a range of Reynolds numbers from 1000 to 16,000 using water in the tube side of the heat exchanger. The shell side fluids used are either cooling water or ambient air. The coiled flow inverter is made up of coils and 90 ∘ bends and inserted in a closed shell. The shell side is fitted with three types of baffles to provide high turbulence and avoid channeling in the shell side. The bulk mean temperatures at various downstream positions are reported for different flow rate on tube side, as well as the heat transfer efficiency of the heat exchanger is also reported. Pressure drop and overall heat-transfer coefficient is calculated at various tube and shell side process conditions. The outer and inner heat-transfer coefficients are determined using Wilson plot technique. The results show that at low Reynolds numbers, heat-transfer is 25% higher as compared to coiled tubes. At high Reynolds numbers, the configuration has less influence on heat transfer. New empirical correlations are developed for hydrodynamic and heat-transfer predictions in the coiled flow inverter.

Numerical simulation of steady flow fields in coiled flow inverter

Flatter velocity profiles and more uniform thermal environments are extremely desirous factors for improved performance in flow reactors and heat exchangers. One means of achieving it in laminar flow systems is to use mixers and flow inverters. In the present study a new device is introduced based on the flow inversion by changing the direction of centrifugal force in helically coiled tubes. The objective of the present study is to characterize flow development and temperature fields in the proposed device made up from the configurations of bent coils. The main mechanism generating the flow is the production of spatially chaotic path by changing the direction of flow using a 90° bend in helical coils (alternating Dean flow). If the direction of centrifugal force is rotated by any angle, the plane of vortex formation also rotates with the same angle. Thus in helical flow a 90° shift in the direction of centrifugal force cause a complete flow inversion. Complete flow fields and thermal fields in helical coil and bent coil configuration were studied using computational fluid dynamics software (FLUENT 6.0). The three-dimensional governing equations for momentum and energy under the laminar flow conditions were solved with a control-volume finite difference method (CVFDM) with second-order accuracy. The flow pattern obtained for the helical coil was in good agreement to those observed by the previous investigators [S.W. Jones, O.M. Thomas, H. Aref, Chaotic advection by laminar flow in twisted pipe. J. Fluid Mech. 209 (1989) 335–357; Ch. Duchene, H. Peerhossaini, P.J. Michard, On the velocity field and tracer patterns in a twisted duct flow. Phys. Fluids 7 (1995) 1307–1317]. The comparison of the flow fields and temperature fields in the helical tube and bent coil configuration are discussed. The bent coil configuration shows a 20–30% enhancement in the heat transfer due to chaotic mixing while relative pressure drop is 5–6%. The results of the present study can be used to model transport processes for developing flows in curved tubes such as chromatographic columns (less axial dispersion [A.K. Saxena, K.D.P. Nigam, Coiled configuration for flow inversion and its effect on residence time distribution. AIChE J. 30 (1984) 363–368]), Chemical reactors (narrower RTD), heat transfer devices, and some biomedical devices.