Flow In Helical Coils Research Articles

Heat transfer and pressure drop characteristics of two phase flow in helical coils

This paper presents a review on heat transfer coefficient and pressure drop during condensation and boiling in helically coiled tubes. The geometric parameters such as coil diameter, coil pitch and operating parameters containing saturation temperature, system pressure, mass flux, heat flux and vapor quality have significant effects on the heat transfer characteristics. The frictional pressure drop during flow boiling increases with increasing vapor quality and mass flux, and decreases with increasing system pressure. Also, the effect of the curvature on the two-phase frictional multiplier is negligible. The frictional pressure drop in condensation increased with increasing average vapor quality and mass flux, and decreased with increasing saturation temperature. According to the results reported, heat flux had little effect on the pressure drop during condensation. Flow boiling heat transfer coefficient in helical coils of large diameter showed the similar characteristics and trend with the straight tube as the system pressure decreased. Lockhart-Martinelli parameter was generally preferred to be used by the authors as a function in the correlations to predict flow boiling heat transfer coefficient. There was a decrease in the average Nusselt number with increasing heat flux for the constant Dean number during condensation. This reduction in the average Nusselt number indicated that the turbulence effect created by the secondary flow in the helically coiled tubes eliminated the effects caused by the difference in density. The correlations used for determination of Nusselt number and pressure drop in helical coils with their major findings are presented in details.

Better thermoregulation of brain temperature using phase change material-mediated head cooling system

Regulation of the brain temperature is critical for managing heat stress related adverse events. It can be achieved by external cooling of the head. Head cooling systems that are lightweight, portable, and suitable for active work scenarios such as firefighting, mining, and construction work are currently unavailable in the market. This study proposes a novel active head cooling system that a) uses phase change material for thermal storage and b) can be designed for portability. The closed-loop bench-top system consists of two heat exchangers: a water cooled heat exchanger with an attached heater, mimicking heat generated from the head, and a helical tube heat exchanger surrounded with ice as heat sink. These heat exchanges are interconnected by tubing for water circulation. The system performance is assessed by cooling duration that depends on mass of ice used and heat transfer rate. The system was evaluated for different a) heat loads varying from rest to exercise conditions (20 W – 40 W) and b) flowrates (0.25 l/min – 0.65 l/min). The results show that the system can handle heat generation rate of 40 W from the head for 100 min (for the flowrate of 0.25 l/min), which is about 3 times the safe duration of firefighting drills. The cooling time increases linearly with decreasing heat load: 138 min for 30 W and 190 min for 20 W, an increase of 38% and 90%, respectively. It also increases with decreasing flowrate. The range of Nusselt number for helical coil flow is about 4.4 – 6.8 times higher when compared to that of a straight pipe flow. The helical design of heat exchanger led to enhanced heat transfer owing to the formation of Dean's vortical flow. The results suggest that the head cooling system, having possible features of being portable, cost-effective, light weight, and easy to use, can assist in the thermoregulation of brain temperature for workers during elevated thermal stress conditions.

Tomographic PIV measurements and RANS simulations of secondary flows inside a horizontally positioned helically coiled tube

Helically coiled tubes are widely used in industry to enhance heat and mass transfer in the laminar flow regime, due to their secondary flow pattern. In this study, tomographic particle image velocimetry (tomo-PIV) is used in a horizontally coiled helical tube to systematically acquire 3C-3D velocity fields for Reynolds numbers ranging from 20 to 1400 and Dean numbers from 8 to 567. The velocity field evaluations are performed using two different approaches: time-averaged velocity field calculation from instantaneous velocity fields and velocity field determination by cross-correlation from an ensemble of instantaneous reconstructed volumes. Equivalent velocity field accuracy is achieved in both velocity approaches when the flow can be considered stationary. Moreover, numerical simulations were carried out in the same geometry at the same flow conditions and were validated against the experimental 3C-3D data sets. The simulation results show good agreement with the measured velocities, offering the possibility of parametric studies and design optimization. To the authors’ best knowledge, this is the first systematic experimental investigation of a helical coil flow by means of 3C-3D velocity measurements, which results can now be used for validation of numerical models in computational fluid dynamics.Graphic abstractMeasured time-averaged velocity visualized by vector-magnitude colour at horizontal and vertical slices (left) and Dean vortices detected by 3D Q-criterion (right) from the time-averaged measurements (purple isosurfaces) and from the simulation (red isosurfaces) inside the helically-coiled tube at Re = 220 and De = 89.

Experimental investigation and prediction of post-dryout heat transfer for steam-water flow in helical coils

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.

Two-phase pressure drop in a helical coil flow boiling system

The objective of the present work is to study the two-phase pressure drop in helical coils. Literature on the two-phase pressure drop in a helical coil suggests the complexity in flow boiling inside a helical coil due to secondary flow. Most of correlations reported in the literature on the two-phase pressure drop in a helical coil are limited to a specific operating range. No general correlation is available for a helical coil which is applicable for all fluids. In the present study, an experimental databank collected containing a total of 832 data points includes the data from the present study and from the literature. The data includes diabatic pressure drop of two fluids namely water and R123. Data covers a range of parameters namely a mass flux of 120–2058 kg/m2 s, a heat flux of 18–2831 kW/m2, an exit quality of 0.03–1, a density ratio of 32–1404 and a coil to tube diameter ratio of 14–58. The databank is compared with eighteen empirical correlations which include well referred correlations of straight tubes and the available correlations of helical coils. The straight tube correlations are not working well for the present data set. The helical coil correlations work reasonably well for the present databank. A correlation is suggested to predict the two-phase pressure drop in helical coils. The present study suggests that the influence of a helical coil is completely included in the single phase pressure drop correlation for helical coils.

Heat transfer distribution in helical coil flow boiling system

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/m2 s, a heat flux of 2–640 kW/m2, 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.

Development of experimental heat transfer correlations using Newtonian fluids in helical coils

The experimental work was carried out using water, 10 and 20 % glycerol–water mixture as Newtonian fluids under isothermal and non-isothermal conditions in helical coils. The experiments were performed in laminar and turbulent flow regimes using four helical coils of coil curvature ratios as; δ = 0.055, 0.064, 0.0685 and 0.0757. For the first time, an innovative heat transfer correlations in terms of new dimensionless number ‘M’ are developed based on 135 and 183 tests conducted under isothermal and non-isothermal conditions. The developed heat transfer correlations were compared with the work of earlier investigators and are found to be in good agreement. Hence, M number could be used for characterization of fluid flow in helical coils for any types of fluids which is significant. Also, comparison of Nusselt numbers for water and glycerol–water mixtures under different experimental conditions is presented in this paper. It was observed that as helix diameter increases, Nusselt number decreases due to decrease in centrifugal force.

Prediction of hydrodynamic entrance length for single and two-phase flow in helical coils

The hydrodynamic entrance length in helical coils for single and two-phase bubbly flow was studied experimentally and numerically. Development region length and the detailed characteristics of fluid flow have been investigated by varying helical coil parameters such as tube diameter, coil diameter and void fraction. For the CFD simulation of the two-phase fluid flow, the Eulerian–Eulerian model was employed. To calculate the turbulent fluctuations, the SSTk−ω turbulence model has been used. The experimental and numerical simulation of the local parameters demonstrates that the hydrodynamic developing length (L/D) increases when Reynolds number increases in the single and two-phase flows. The obtained results show that the development entrance length increases with the increase of pipe diameter and decreases with the increase of coil diameter. Also, entrance length decreases while curvature ratio of helical coil and void fraction increase. A correlational equation has been suggested to predict the hydrodynamic entrance length as functions of various parameters of the helical coil.

Phase distributions of boiling flow in helical coils in high gravity

Abstract Phase distributions of boiling flow in helical coils in high gravity are investigated both experimentally and numerically. The study focuses on the boiling flow in helical coils of inner diameter 8.9 mm and coil diameter 150 mm at mass velocity 134 kg/(m 2 s), heat flux from 452 to 3164 W/m 2 . The simulation uses the volume of fluid (VOF) multiphase model with user defined functions. Its results agree with the experimental visualization. In terrestrial gravity, the flow patterns in ascending and descending half-coils show differences. In high gravity, the phase distributions are different from those in terrestrial gravity and evolve highly depending on the high gravity direction and magnitude. An analysis on the acceleration calculation makes a prediction of the pressure gradient and the phase distribution.

Single and two-phase pressure drop in serpentine mini-channels

The pressure differential of single and two-phase flow in mini-channel serpentine geometries was investigated to determine the effects of flow patterns and radius of curvature of the serpentine on pressure drop. The friction factor for single phase flow through a straight channel was comparable to existing literature, while that in the serpentine geometry fell between conventional theory for straight channels and fully developed flow in helical coils. Extension of the single phase results to two-phase flow using a separated flow model led to the development of empirical correlations for two-phase pressure drop in the straight and serpentine configurations. Five operating regions were identified within the serpentine, each with distinct pressure drop characteristics dependent on the flow pattern and extent of bubble deformation. Two of the operating regions corresponded to bubbly and slug/unstable-annular flow, while the boundaries between the three remaining regions occurred at We LG L C = 2.7 and 15.5; corresponding to the onset of mild cap deformation and continuous bubble breakup, respectively.

Numerical Investigation of Laminar Forced Convection and Entropy Generation in a Helical Coil with Constant Wall Heat Flux

ABSTRACT In this article, laminar forced convection and entropy generation in a helical coil with constant wall heat flux are investigated numerically. Both the entrance and fully developed regions are included. Water (Prandtl number 5.98) has been selected as working fluid in the present study. The cases studied cover a Reynolds number range from 1,000 to 7,500, and wall heat fluxes of 160, 320, and 640 W/m2. The development of flow fields, including secondary flow motion, distributions of temperature, Nusselt number, and friction factor, are given and discussed. In particular, the distributions of entropy generation rate in the helical coil are highlighted. According to the minimum entropy generation principle and thermodynamic second law, the analysis of optimal Reynolds number for the helical coil flow with constant wall heat flux is carried out, through which the optimal Reynolds numbers are found to be related to the wall heat flux. The optimal Reynolds number is suggested to be chosen as the flow operating condition so that the thermal system can have the least irreversibility and best exergy utilization.

Entropy generation and thermodynamic optimization of fully developed laminar convection in a helical coil

The present paper analyses the entropy generation of the fully developed laminar convection in a helical coil with constant wall heat flux and presents the optimal design based on the minimum entropy generation principal. The important design parameters, including Reynolds number ( Re), coil-to-tube radius ratio ( δ) and nondimensional coil pitch ( λ) are varied to investigate their influences on the entropy generation. The results presented in this paper cover Re range of 100–10,000, δ and λ range from 0.01 to 0.3. Compared with Re and δ, the coil pitch λ is found to have minor influence on the entropy generation. For a demonstrated case, the minimum entropy generation occurs in the range bounded by Re from 2271 to 4277 and δ from 0.17 to 0.3, within which the irreversibility of the system is lowest and the system performance would be optimum. The details show that there is an optimal Re for a helical coil with a fixed δ; meanwhile for a helical coil flow with a specified Re, the smaller δ should be selected when the Re is larger than 5000, and the larger δ should be selected when the Re is less than 5000. These results provide worthwhile information for heat exchanger designers to find the optimal helical coil design from the viewpoint of the thermodynamic second law.

Design and evaluation of a continuous flow microwave pasteurization system for apple cider

The purpose of this project was to design a continuous flow microwave pasteurization system and to evaluate the following process parameters: volume load size (0.5 and 1.38l), input power (900–2000W), and inlet temperature (3°C, 21°C, and 40°C). Water and two apple ciders, one from a cold press and the other from a hot press extraction, were the fluids used to study the heating characteristics. Volumetric flow rate and absorbed power were criteria in the evaluation. The microwave pasteurization system consisted of helical coils throughout a large cavity oven, which was shown to produce uniform and reproducible heating throughout the cavity. Fluid viscosity of water and cider was measured at temperatures between 20°C and 70°C to characterize the flow in helical coils based on the Dean number. Process lethality was verified based on inoculation of Escherichia coli 25922 in apple cider, in which the pasteurization process resulted in a 5-log10 reduction.

An experimental investigation of the frictional pressure drop of steam–water two-phase flow in helical coils

Fractional pressure drops of single-phase water and steam–water two-phase flows were studied in a pressure range of 3.0–3.5 Mpa. Two helical coiled tubes were employed as test sections and their four different helix axial inclinations were examined. It is found that helix axial angles have little influence on the single-phase frictional pressure drop, while variation of the steam–water two-phase flow frictional pressure drop is enlarged to 70%. For single-phase flow, some previous correlations were quite accurate in predicting the frictional pressure drop for lower Reynolds number conditions, and a modified correlation was obtained from an enlarged Reynolds number range of the present test. For boiling two-phase flow, great deviation was found among the published few empirical equations and their capability of practical utilization is rather poor on account of their complication in structure. A new easily stipulated correlation is deducted from our present data.

Traveling wave instability in helical coil flow

Complementary flow visualization photographs and numerical calculations are presented for the transitional state between the laminar and turbulent flow regimes in a helically coiled pipe. The flow visualization covers a Reynolds number range from 3800 to 8650 (890<De<2030, where De is the Dean number). Estimates of the wavelength and wave speed of a traveling wave instability are made from photographs and video recordings at Re=5060 and 5480 (De=1190 and 1280). The unsteady three-dimensional finite difference approximations of the Navier–Stokes equations formulated for the toroidal coordinate system are solved numerically. The calculations are performed in a curved pipe with a radius of curvature to pipe radius ratio equal to 18.2 and Re=5480 (De=1280). These test conditions match the flow visualization and previously reported laser Doppler velocimetry measurements. The calculations reveal a complex interaction between the centrifugal force and the cross-stream velocity, hence explaining the mechanism for maintaining the traveling wave. An analogy is made with known centrifugal instabilities to explain the character of the motion observed in the inner half of the pipe along planes defined by the radial and streamwise coordinate directions. Simple considerations show that the cross-stream flow has the potential for a centrifugal instability.

Heat Transfer of a Bolling Two-Phase Flow in Helical Coils. Effects of Coil Diameter and Inclined Angle.

The wall temperature distributions of a boiling two-phase flow of R113 in helically coiled tubes were measured, and the effects of a coil diameter and inclined angle on the heat transfer characteristics were investigated. There were large differences in the temperature distributions between the vertically placed coil and the horizontally placed coil at the lower quality region. When the coil was vertically placed, the ratio of the heat transfer coefficient of a boiling two-phase flow to that of a liquid single-phase flow was not affected by coil diameter. When the coil was horizontally placed or inclined by 45 degrees, the heat transfer coefficient at the upper part of the coil was larger than that at the lower part in the lower quality region but smaller in the higher quality region. The overall average heat transfer coefficient of a horizontally placed coil was larger than that of a vertically placed coil.

Flow regimes, hold–up and pressure drop for two phase flowin helical coils

AbstractExperimental results on flow pattern, hold–up and pressure drop are presented for cocurrent upward and downward air water flow in helical coils. A tube of 0.01 m internal diameter was used and the ratio of coil to tube diameter was varied from 11 to 156.5. Water flow rate was varied from 4.9 × 10‐6 m3/s to 92 × 10‐6 m3/s while the range of gas flow rate covered was 83 × 10‐6 m3/s to 610 × 10‐6 m3/s.A new mechanistic approach is proposed to correlate pressure drop data in coils. The proposed model retains the identity of each phase and separately accounts for the effects of curvature and tube inclination resulting from the torsion of the tube. This makes it possible to use a single model to predict pressure drop for both upward and downward two–phase flow in coiled tubes. Required correlations for hold–up, interfacial friction factor and friction factors for individual phases are provided.

Heat transfer of a gas and liquid two-phase flow in helical coils. Effects of coil diameter and set angle.

The distributions of local void fraction and heat-transfer coefficient of an air-water two-phase flow in helical coils were measured, and the effects of coil diameter and set angle on the heat-transfer characteristics were investigated. When the coil axis is horizontally placed, it is found that, at a high gas-phase velocity which forms the annular flow, there are still fairly large differences in the void fraction and the local heat-transfer coefficient between the upper and lower positions on the coil. It is shown that the average heat-transfer coefficients can be divided into three regions by Martinelli's parameter, 1/Xtt, for coils of any diameter. Among these regions, the effects of the set angle are significant at the second and third region. Finally, correlations between the average heat-transfer coefficient and 1/Xtt are proposed.

Flow and heat transfer of a gas and liquid two-phase flow in helical coils (For the case when the coil axis is horizontally placed)

The flow and heat transfer of an air-water two-phase flow in helical coils, whose axis was horizontally placed, were experimentally studied. First, the flow pattern, average void fraction, frictional pressure drop and characteristics of the distribution of the local void fraction at upper and lower pressure drop and characteristics of the distribution of the local void fraction at upper and lower positions of the coil were investigated. Second, variations of the local Nusselt number along the axial and circumferential direction of the coiled tube were examined. At the inside of the bend, the local Nusselt number varied extremely between the upper and lower position of the coil. While, on the outside, there were little differences between them. The relation of the average Nusselt number and volumetric flow fraction was also mentiond.

Pressure drop and holdup studies of air—liquid flow in helical coils

Experimental studies on the pressure drop and holdup for cocurrent upwards (turbulent—turbulent) flow of air—liquid systems through copper coils are presented. A new correlation for the two-phase frictional pressure drop is proposed based on the modified Lockhart—Martinelli parameter φ G 2—1 in terms of dimensionless groups.