Heat Transfer In Coiled Tubes Research Articles

A sensitivity analysis of the effect of curvature ratio on the thermal-hydraulic performance in helically coiled tubes with corrugated walls

To enhance thermal-hydraulic performance by rising the heat transfer rate with reasonable pressure loss, heat transfer augmentation methods should be considered in the heat exchanger designs. The use of two distinct passive heat transfer augmentation methods, helically coiled tube structure and helically corrugated tube wall, is considered in this study to investigate numerically the effect of coil diameter on the heat transfer and flow characteristics of single-phase liquid water flowing in the corrugated coiled tubes. Curvature ratio, the ratio of tube-to-coil diameter, and the use of corrugation are one of the most influential factors on heat transfer in coiled tubes due to their contributions to the formation of secondary and swirl flows. The curvature ratios between 0.0958 and 0.2875 are considered for smooth and equivalent corrugated coiled tubes with the same tube diameter of 5.75 mm and length of 390 mm. In all simulations, the water flows through the investigated tubes at Dean numbers between 150 and 3270 under boundary conditions of uniform surface heat flux. The influence of coil diameter on Nusselt number, friction factor, and thermal-hydraulic performance is examined by a commercial software. As a result, the enhancement in the thermal-hydraulic performance of corrugated coiled tubes compared to smooth straight ones is much higher, up to 270%, at Reynolds numbers below 2300 than those at higher Reynolds numbers. Additionally, the Genetic Aggregation Response Surface Methodology for the estimation of output parameters and their sensitivity analysis is employed by evaluating the Reynolds number and curvature ratio as inputs, and Nusselt number, friction factor, and performance evaluation criteria as outputs. Finally, the output parameters are estimated with an error range of ±5%, and the Reynolds number on Nusselt number and performance evaluation criteria, and curvature ratio on friction factor are realized to be more effective.

Influence of thermal boundary conditions on local convective heat transfer in coiled tubes

Helical coil heat exchangers are commonly used in a wide variety of engineering applications because, due to curvature of the tube that generates secondary flows, this kind of geometry can generate an enhancement of the convective heat transfer coefficient. Since the distributions of the fluid velocity and temperature in coiled pipes are strongly asymmetrical over the cross-section of the tube, particularly in laminar flow conditions, it is important to monitor the local heat transfer phenomena, mainly with regard to the applications where the local behaviour plays a leading role, such as in food industry. Indeed, due to the high viscosity of fluid foods or to the low velocity in which they may operate, the flow regime is often laminar in this kind of devices. In laminar flow regime, the thermal boundary conditions considerably affect the convective heat transfer performance. In the present paper, a numerical investigation focused on the evaluation of the effect of the choice of the thermal boundary condition on the local heat transfer phenomena in coiled-tubes, within the laminar regime, is presented. To cover realistic configurations in which the helical coil heat exchangers are commonly used, the conjugate heat transfer problem in the tube was also considered. The momentum and energy equations were solved by means of the open-source CFD software OpenFOAM®, with second-order accurate finite-volume schemes. The here adopted numerical model was validated by comparing the results with local experimental data. The numerical outcomes, obtained for several values of the Prandtl number, revealed that the thermal boundary conditions could significantly change the local distribution of the convective heat transfer coefficient. Finally, the influence of the thermal boundary condition on the average convective heat transfer coefficient were evaluated by considering the two definitions of the circumferentially averaged Nusselt number commonly used in literature.

Experimental validation of numerical model for evaluation of local heat transfer coefficient in coiled tubes

The efficiency of the enhanced heat transfer equipment was widely investigated in literature by adopting both experimental and numerical approaches. Obviously, the numerical tools allow improving the knowledge of the fundamental heat transfer mechanisms in curved geometries, since they enable to evaluate punctually the velocity and temperature distributions. On the other hand, it has to be pointed out that the numerical results can be considered reliable only if they are validated throughout the comparison with either analytical results or experimental data. The validation of numerical models is usually carried out by considering average heat transfer performances, therefore this approach does not guarantee that these models enable to evaluate the local heat transfer phenomena with the same accuracy. In the present paper a promising numerical tool for evaluating local convective heat transfer in coiled-tubes, within the laminar regime, is presented. The Navier-Stokes and energy equations are solved by means of the open-source CFD software OpenFOAM®, with second-order accurate finite-volume schemes, within the framework of the SIMPLE algorithm. The robustness of the numerical model is assessed by comparing the numerical outcomes to the local experimental data available in literature.

Numerical analysis of the laminar forced convective heat transfer in coiled tubes with periodic ring-type corrugation

Wall curvature and wall corrugation represent two of the most used passive techniques to enhance convective heat transfer. The effectiveness of wall curvature is due to the fact that it gives origin to a secondary fluid motion orthogonal to the main flow, while wall corrugation is used to disrupt the development of the boundary layers, by enhancing the convective heat transfer mechanism. The compound use of the two techniques has been investigated in literature, mainly experimentally, but further investigation is still needed. In particular, it has been experimentally observed that this compound enhancement technique brings an additional heat transfer augmentation in the majority of applications whereas in the very low Reynolds number range the surface average performances of corrugated coils are lower than the one shown by smooth wall coils.This paper deepened the knowledge on this phenomenon presenting a numerical investigation of the effect induced by a periodic ring-type corrugation on the laminar convective heat transfer in coiled tubes. The study considered the laminar flow in the Reynolds and Dean number range 25-100 and 6-24 respectively. The investigation was particularly focused on the Dean's vortices destruction mechanism, induced by the wall corrugation and on the consequent breakdown of the average Nusselt number.

Effect of wall corrugation on local convective heat transfer in coiled tubes

The present paper presents the application of an inverse analysis approach to experimental infrared temperature data with the aim of estimating the local convective heat transfer coefficient for forced convection flow in coiled pipe having corrugated wall. The estimation procedure here adopted is based on the solution of the Inverse Heat Conduction Problem within the wall domain, by adopting the temperature distribution on the external coil wall as input data of the inverse problem: the unwanted noise is filtered out from the infrared temperature maps in order to make feasible the direct calculation of its Laplacian, embedded in the formulation of the Inverse Heat Conduction Problem, in which the convective heat transfer coefficient is regarded to be unknown. The results highlighted the local effects of both wall curvature and of wall corrugation on the convection heat transfer augmentation mechanism.

Numerical and experimental study of local heat transfer enhancement in helically coiled pipes. Preliminary results.

In the last years, the attention of heat transfer equipments manufacturers turned toward helically coiled-tube heat exchangers, especially with regards to applications for viscous and/or particulate products. The recent progress achieved in numerical simulation motivated many research groups to develop numerical models for this kind of apparatuses. These models, intended both to improve the knowledge of the fundamental heat transfer mechanisms in curved geometries and to support the industrial design of this kind of apparatuses, are usually validated throughout the comparison with either theoretical or experimental evidences by considering average heat transfer performances. However, this approach doesn't guarantee that the validated models are able to reproduce local effects in details, which are so important in this kind of non-standard geometries. In the present paper a numerical model of convective heat transfer in coiled tubes for laminar flow regime was formulated and discussed. Its goodness was checked throughout the comparison with the latest experimental outcomes of Bozzoli et al. [1] in terms of convective heat flux distribution along the boundary of the duct, by ensuring the effectiveness of the model also in the description of local behaviours. Although the present paper reports only preliminary results of this simulation/validation process, it could be of interest for the research community because it proposes a novel approach that could be useful to validate many numerical models for nonstandard geometries.

Estimation of the local heat-transfer coefficient in the laminar flow regime in coiled tubes by the Tikhonov regularisation method

Wall curvature is a widely used technique to passively enhance convective heat transfer that has proven to also be effective in the thermal processing of highly viscous fluids. These geometries produce a highly uneven convective heat-flux distribution at the wall along the circumferential coordinate, thus affecting the performance of the fluid thermal treatment. Although many authors have investigated the forced convective heat transfer in coiled tubes, most of them have presented the results only in terms of the Nusselt number averaged along the wall circumference. A procedure to estimate the local convective wall heat flux in coiled tubes is presented and tested in this paper: the temperature distribution maps on the external coil wall were employed as input data of the inverse heat conduction problem in the wall under a solution approach based on the Tikhonov regularisation method with the support of the fixed-point iteration technique to determine a proper regularisation parameter. The investigation was focused on the laminar flow regime.

Estimation of local heat transfer coefficient in coiled tubes under inverse heat conduction problem approach

Wall curvature is a widely used technique to passively enhance convective heat transfer and it is particularly effective in the thermal processing of highly viscous fluids. These geometries produce a highly uneven convective heat flux distribution along the circumferential coordinate, impacting on the performance of the fluid thermal treatment. Although many authors have investigated the forced convective heat transfer in coiled tubes, most of them have presented the results only in terms of the Nusselt number averaged along the wall circumference. Moreover, regarding the laminar flow regime, few data about the local heat transfer coefficient are available. In this paper a procedure to estimate the local convective heat flux in coiled tubes is presented and tested: the temperature distribution maps on the external coil wall are employed as input data for the inverse heat conduction problem in the wall under a solution approach based on Tikhonov regularization method. The investigation was particularly focused on the laminar flow regime.

Influence of Torsion on the Laminar Flow and Convective Heat Transfer in Coiled Tubes Arranged in a Rectangular Pattern

Influence of Torsion on the Laminar Flow and Convective Heat Transfer in Coiled Tubes Arranged in a Rectangular Pattern

Experimental Study on Pressure Drop and Heat Transfer of Turbulent Flow in Tube in Tube Helical Heat Exchanger

In the present study, experiments were conducted for the first time in tube in tube helical heat exchanger at the pilot plant scale to investigate the fluid flow and heat transfer under turbulent flow conditions. The experiments were carried out with hot compressed air in the inner tube and cooling water in the outer tube in the countercurrent mode of operation. The flow rate of compressed air flowing in the inner tube was varied for Reynolds numbers from 14 000 to 86 000. The pressure of compressed air was varied from 10 to 30 kgf/cm2. The friction factor and Nusselt number values for compressed air flowing in the inner tube were calculated for elevated pressure and compared with the experimental data reported in the literature at ambient conditions. Most of the works reported in the literature on heat transfer in coiled tubes have been carried out considering wall boundary conditions of either constant heat flux or constant wall temperature. In the present work, heat transfer calculations were based on physically realistic condition of fluid to fluid heat transfer. The inner tube Nusselt number of compressed air in the present study was found to be slightly higher than the data reported in the literature for ambient conditions while the friction factor values are within the range reported in the literature for ambient conditions. The friction factor as well as Nusselt number calculated for cooling water flowing in the outer tube was higher than the existing literature data. On the basis of the experimental results, new correlations for friction factor and Nusselt number in the inner as well as outer tubes have been developed.

AN EFFICIENT NUMERICAL PROCEDURE FOR THE SOLUTION OF LAMINAR FLOW AND HEAT TRANSFER IN COILED TUBES

Effects of curvature on the fluid flow and heat transfer with axially uniform wall heat flux and peripherally uniform wall temperature in a tube are analyzed numerically. Solutions are available over a wide range of Prandtl numbers and Dean numbers up to 1305. It is found that the secondary flow circulations become more complex as the Dean number increases. Nusselt number increases are most evident at high Prandtl and Dean numbers. The introduction of a convergence parameter into the alternating direction implicit solution method results in substantially reduced computation time. A method of finding the optimum convergence parameter is described.

An Efficient Numerical Procedure For the Solution Of Laminar Flow and heat Transfer in Coiled Tubes

Effects of curvature on the fluid flow and heat transfer with axially uniform wall heat flux and peripherally uniform wall temperature in a tube are analyzed numerically. Solutions are available over a wide range of Prandtl numbers and Dean numbers up to 1305. It is found that the secondary flow circulations become more complex as the Dean number increases. Nusselt number increases are most evident at high Prandtl and Dean numbers. The introduction of a convergence parameter into the alternating direction implicit solution method results in substantially reduced computation time. A method of finding the optimum convergence parameter is described.

Laminar convective heat transfer in helical coiled tubes

An experimental and numerical study has been made on convective heat transfer in coiled tubes. The experiments have been carried out for tube diameter/coil diameter ratios from 1 100 to 1 10 , Prandtl numbers from 10 to 500 and Reynolds numbers from 20 to 4000. The heat transfer has been studied for two boundary conditions: for a uniform peripherally averaged heat flux and for a constant wall temperature. Attention has been paid to the heat transfer in the thermal entry region as well as in the fully developed thermal region. The results obtained and the relations proposed could be explained from and are based on the flow behaviour.

Local Heat-Transfer Measurements during Forced-Convection Boiling in a Helically Coiled Tube

Forced-convection boiling heat transfer to water at atmospheric pressure was studied in two helically coiled tubes. Temperature measurements were made at four positions round the tube at each of nine stations along the tube, permitting calculation of the local heat-transfer coefficient at each point. The local coefficients are correlated by the Lockhart-Martinelli parameters for two-phase flow, using the Seban-McLaughlin correlation for liquid phase heat transfer in coiled tubes. The curves for each of the four peripheral positions, compared to that previously obtained for the peripheral mean coefficient, are consistent with a flow structure having a vapour core with a strong secondary flow serving to distribute the liquid over the entire surface of the tube. The correlation and its interpretation in terms of flow structure may fail at high pressures as a result of the much smaller change in the radial acceleration in systems with small density difference between liquid and vapour.