Heat transfer using a correlation by neural network for natural convection from vertical helical coil in oil and glycerol/water solution

A physical-empirical model is designed to describe heat transfer of helical coil in oil and glycerol/water solution. It includes an artificial neural network (ANN) model working with equations of continuity, momentum and energy in each flow. The discretized equations are coupled using an implicit step by step method. The natural convection heat transfer correlation based on ANN is developed and evaluated. This ANN considers Prandtl number, Rayleigh number, helical diameter and coils turns number as input parameters; and Nusselt number as output parameter. The best ANN model was obtained with four neurons in the hidden layer with good agreement (R > 0.98). Helical coil uses hot water for the inlet flow; heat transfer by conduction in the internal tube wall is also considered. The simulated outlet temperature is carried out and compared with the experimental database in steady-state. The numerical results for the simulations of the heat flux, for these 91 tests in steady-state, have a R ≥ 0.98 with regard to experimental results. One important outcome is that this ANN correlation is proposed to predict natural convection heat transfer coefficient from helical coil for both fluids: oil and glycerol/water solution, thus saving time and improving general system performance.     

Numerical evaluation and the effects of geometrical and operational parameters on thermal performance of the shell and double coil tube heat exchanger

Heat exchangers are extensively used in various industries. In this study, the impact of geometric and flow parameters on the performance of a shell and double helical coil heat exchanger is studied numerically. The investigated geometric parameters include external coil pitch, internal coil pitch, internal coil diameter, and coil diameter. The influences of considered geometrical parameters are analyzed on the output temperature of the hot and cold fluid, convective heat transfer coefficient, pressure drop, and average Nusselt number. Water is considered as working fluid in both shell and tube. As an innovation, double helical coils are used instead of one in the heat exchanger. To compare the obtained results accurately, in each section, the heat transfer area (coil outer surface) is kept constant in all models. The results show that the geometrical parameters of double helical coils significantly affect the heat transfer rate.     

Experimental Study on Cu/Oil Nanofluids through Concentric Annular Tube: A Correlation

This study deals with an empirical investigation on the convective heat transfer of Cu/oil nanofluid flow inside a concentric annular tube with constant heat flux boundary condition and suggests a correlation to predict the Nusselt number. The average size of particles was 20 nm and the applied nanofluid was prepared by Electrical Explosion of Wire technique with no nanoparticle agglomeration during nanofluid preparation process and experiments. The nanofluid flowing between the tubes is heated by an electrical heating coil wrapped around it. The effects of different parameters such as the flow Reynolds number, tube diameter ratio, and nanofluid particle concentration on heat transfer coefficient are studied. Using the acquired experimental data, a correlation is developed for the estimation of the Nusselt number of nanofluid flow inside the annular tube. This correlation has been presented by using the exponential regression analysis and least‐squares method. The correlation is valid for Cu/base oil nanofluid flow with weight concentrations of 0.12, 0.36, and 0.72 in the hydrodynamically full‐developed laminar flow regime with Re <140, which is applicable in mini‐ and microchannel heat exchangers, and it is in good agreement with the experimental data.     

Convective heat transfer in helical coils for constant-property and variable-property flows with high Reynolds numbers

Forced convection heat transfer of single-phase water in helical coils was experimentally studied. The testing section was constructed from a stainless steel round tube with an inner diameter of 10 mm, coil diameter of 300 mm, and pitch of 50 mm. The experiments were conducted over a wide Reynolds number range of 40000 to 500000. Both constant-property flows at normal pressure and variable-property flows at supercritical pressure were investigated. The contribution of secondary flow in the helical coil to heat transfer was gradually suppressed with increasing Reynolds number. Hence, heat transfer coefficients of the helical tube were close to those of the straight tube under the same flow conditions when the Reynolds number is large enough. Based on the experimental data, heat transfer correlations for both incompressible flows and supercritical fluid flows through helical coils were proposed.     

Experimental Investigation on Flow Boiling Heat Transfer and Pressure Drop of HFC-134a inside a Vertical Helically Coiled Tube

An investigation on flow boiling heat transfer and pressure drop of HFC-134a inside a vertical helically coiled concentric tube-in-tube heat exchanger has been experimentally carried out. The test section is a six-turn helically coiled tube with 5.786-m length, in which refrigerant HFC-134a flowing inside the inner tube is heated by the water flowing in the annulus. The diameter and the pitch of the coil are 305 mm and 45 mm, respectively. The outer diameter of the inner tube and its thickness are respectively 9.52 and 0.62 mm. The inner diameter of the outer tube is 29 mm. The average vapor qualities in test section were varied from 0.1 to 0.8. The tests were conducted with three different mass velocities of 112, 132, and 152 kg/m²-s. Analysis of obtained data showed that increasing of both the vapor qualities and the mass fluxes leads to higher heat transfer coefficients and pressure drops. Also, it was observed that the heat transfer coefficient is enhanced and also the pressure drop is increased when a helically coiled tube is used instead of a straight tube. Based on the present experimental results, a correlation was developed to predict the flow boiling heat transfer coefficient in vertical helically coiled tubes.     

Numerical heat transfer analysis of encapsulated ice thermal energy storage system with variable heat transfer coefficient in downstream

During charging and discharging processes, the heat transfer behavior of the encapsulated ice thermal energy storage (TES) system changes during downstream case and this should be taken into account since the temperature of heat transfer fluid (HTF) and especially the heat transfer coefficient varies considerably around each capsule. This requires a careful study of the problem with variable heat transfer coefficient to contribute to the state-of-the-art. This has been the primary motivation behind the present study. Here, we first develop a new heat transfer coefficient correlation by simulating a series of 120 numerical experiments for different capsule diameters, mass flow rates and temperatures of HTF and second undertake a comprehensive numerical analysis using the temperature based fixed grid solution with control volume approach for studying the heat transfer behavior of an encapsulated ice TES system. Thirdly, we validate the present numerical model and the new correlation with some experimental data obtained from the literature, and hence a good agreement is obtained between the model results and experimental data. The results indicate that the heat transfer coefficient varies greatly during downstream and highly affects the heat transfer taking place during the process. So, the solutions with constant heat transfer coefficient appear to be unreliable for analysis and system optimization. The results also show that the solidification process is chiefly governed by the magnitude of Stefan number, capsule diameter and capsule row number.     

Performance Enhancement of Shell and Helical Coil Water Coolers Using Different Geometric and Fins Conditions

Experimental investigations of heat transfer characteristics and performance enhancement of shell and helical coil water coolers using external radial fins and different shells diameters were conducted. The study aims to enhance the water coolers performance in a trial to improve coil compactness. Two helical coils; one with a plain tube and the other with external radial fins, were tested in four shells of different tube diameters. Refrigerant passing inside the helical coils was used to cool water that enclose/passes in the space between the helical coil and the shell. Tests were conducted under mixed convection heat transfer regimes. Results showed performance and compactness enhancement with the insertion of external radial fins and increasing the shell diameter to helical coil diameter ratio. For nonfinned and finned coils, Nusselt number increased with increasing Reynolds number, Grashof number, and shell diameter. Correlations were predicted to give the Nusselt number in terms of Reynolds number, Grashof number, and shell diameters for finned and nonfinned helical coils. Correlations predictions were compared with present and previous experimental results and good agreements were obtained.     

Turbulent heat transfer in a horizontal helically coiled tube

An experiment has been conducted in detail to study the turbulent heat transfer in horizontal helically coiled tubes over a wide range of experimental parameters. We found that the enhancement of heat transfer in the coils results from the effects of turbulent and secondary flows. With Reynolds number increasing to a high level, the contribution of the secondary flow becomes less to enhance heat transfer, and the average heat transfer coefficient of the coil is closer to that in straight tubes under the same conditions. The local heat transfer coefficients are not evenly distributed along both the tube axis and the periphery on the cross section. The local heat transfer coefficients on the outside are three or four times those on the inside, which is half of the average heat transfer. A correlation is proposed to describe the distribution of the heat transfer coefficients at a cross section. The average cross-section heat transfer coefficients are distributed along the tube axis. The average value at the outlet section should not be taken as the average heat transfer coefficient. © 1999 Scripta Technica, Heat Trans Asian Res, 28(5): 395–403, 1999     

Condensation heat transfer and pressure drop of HFC-134a in a helically coiled concentric tube-in-tube heat exchanger

The two-phase heat transfer coefficient and pressure drop of pure HFC-134a condensing inside a smooth helically coiled concentric tube-in-tube heat exchanger are experimentally investigated. The test section is a 5.786 m long helically coiled double tube with refrigerant flowing in the inner tube and cooling water flowing in the annulus. The inner tube is made from smooth copper tubing of 9.52 mm outer diameter and 8.3 mm inner diameter. The outer tube is made from smooth copper tubing of 23.2 mm outer diameter and 21.2 mm inner diameter. The heat exchanger is fabricated by bending a straight copper double-concentric tube into a helical coil of six turns. The diameter of coil is 305 mm. The pitch of coil is 35 mm. The test runs are done at average saturation condensing temperatures ranging between 40 and 50 °C. The mass fluxes are between 400 and 800 kg m⁻² s⁻¹ and the heat fluxes are between 5 and 10 kW m⁻². The pressure drop across the test section is directly measured by a differential pressure transducer. The quality of the refrigerant in the test section is calculated using the temperature and pressure obtained from the experiment. The average heat transfer coefficient of the refrigerant is determined by applying an energy balance based on the energy rejected from the test section. The effects of heat flux, mass flux and, condensation temperature on the heat transfer coefficients and pressure drop are also discussed. It is found that the percentage increase of the average heat transfer coefficient and the pressure drop of the helically coiled concentric tube-in-tube heat exchanger, compared with that of the straight tube-in-tube heat exchanger, are in the range of 33–53% and 29–46%, respectively. New correlations for the condensation heat transfer coefficient and pressure drop are proposed for practical applications.     

Heat Transfer Coefficients for Turbulent Flow in Concentric Annular Ducts

On the basis of a large number of experimental data from the literature, correlations were developed for the heat transfer coefficient for turbulent flow in concentric annular ducts. A proven correlation for heat transfer in circular tubes was extended by factors that take into consideration the effect of the diameter ratio of the annulus and the different boundary conditions for heating or cooling.     

Saturated flow boiling heat transfer and associated bubble characteristics of R-134a in a narrow annular duct

Experiments are conducted here to investigate how the channel size affects the saturated flow boiling heat transfer and associated bubble characteristics of refrigerant R-134a in a horizontal narrow annular duct. The gap of the duct is fixed at 1.0 and 2.0 mm in this study. The measured heat transfer data indicate that the saturated flow boiling heat transfer coefficient increases with a decrease in the gap of the duct. Besides, raising the imposed heat flux can cause a significant increase in the boiling heat transfer coefficients. However, the effects of the refrigerant mass flux and saturated temperature on the boiling heat transfer coefficient are milder. The results from the flow visualization show that the mean diameter of the bubbles departing from the heating surface decreases slightly at increasing R-134a mass flux. Moreover, the bubble departure frequency increases at reducing duct size mainly due to the rising shear stress of the liquid flow, and at a high imposed heat flux many bubbles generated from the cavities in the heating surface tend to merge together to form big bubbles. Correlation for the present saturated flow boiling heat transfer data of R-134a in the narrow annular duct is proposed. Additionally, data for some quantitative bubble characteristics such as the mean bubble departure diameter and frequency and the active nucleation site density are also correlated.     

Enhanced heat transfer with full circumferential ribs in helical pipe

This paper describes an experimental study of heat transfers in the smooth-walled and rib-roughened helical pipes with reference to the design of enhanced cooling passages in the cylinder head and liner of a marine propulsive diesel engine. The manner in which the repeated ribs modify the forced heat convection in the helical pipe is considered for the case where the flow is turbulent upon entering the coil but laminar in further downstream. A selection of experimental results illustrates the individual and interactive effects of Dean vortices and rib-flows on heat transfer along the inner and outer helixes of coils. The experimental-based observations reveal that the centrifugal force modifies the heat transfer in a manner to generate circumferential heat transfer variation with better cooling performance on the outer edge relative to its inner counterpart even with the agitated flow field caused by the repeated ribs. Heat transfer augmentation factor in the range of 1.3 ~ 3 times of the smooth-walled l     

An experimental study of thermal performance of shell-and-coil heat exchangers

In the present study an experimental investigation of the mixed convection heat transfer in a coil-in-shell heat exchanger is reported for various Reynolds and Rayleigh numbers, various tube-to-coil diameter ratios and dimensionless coil pitch. The purpose of this article is to assess the influence of the tube diameter, coil pitch, shell-side and tube-side mass flow rate over the performance coefficient and modified effectiveness of vertical helical coiled tube heat exchangers. The calculations have been performed for the steady-state and the experiments were conducted for both laminar and turbulent flow inside coil. It was found that the mass flow rate of tube-side to shell-side ratio was effective on the axial temperature profiles of heat exchanger. The results also indicate that the ? − NTU relation of the mixed convection heat exchangers was the same as that of a pure counter-flow heat exchanger.     

Numerical analysis of natural convection heat transfer from rectangular shrouded fin arrays on a horizontal surface

The main aim of this investigation is to discover the effects of clearance parameters on the steady-state heat transfer. In order to solve the three-dimensional elliptic governing equations, a finite volume based CFD code was used. The clearance gap between fin tips and shroud, the base and fin temperatures and the size and configuration of the finned surfaces were varied during the parametric study. The numerical results have been compared to existing experimental values from the literature and the comparison shows a good agreement. It is found that the heat transfer coefficient increases with the increase in the clearance parameter and it approaches to the value of heat transfer coefficient obtained for unshrouded fin arrays.     

Water pool boiling heat transfer enhancement for modified surface tubes

This paper deals with the method of decreasing the size of heat exchanger surfaces by increasing the heat transfer coefficients and the importance of heat transfer enhancement for vaporization. We report an experimental study on surfaces modified by passive methods applied to heat transfer surfaces mechanically processed, covered with sleeves made by metallic tissues or covered with metallic porous layers performed using welding procedures. Experiments are made to investigate the heat transfer coefficient on copper tubes with a 22 mm external diameter using heat from inner source to outer vaporizing liquid. There are developed specific heat transfer correlations for each group of enhanced surfaces. The experimental data and new proposed correlations are compared with well known correlations. The results are in best agreement with the Cornwell–Houston correlation.     

Experimental and numerical investigation of thermal characteristics of a novel concentric type tube heat exchanger with turbulators

In this study, the heat transfer performance and friction characteristics of a novel concentric tube heat exchanger with different pitches of helical turbulators were investigated experimentally and numerically for a Reynolds number range from 3000 to 14 000. An experimental system was established to obtain experimental data. The numerical simulations were performed by using a three dimensional numerical computation technique, a commercial CFD computer code. Then, the heat transfer performance and friction characteristics of several helical turbulators were compared. The experimental, numerical and empirical correlation results were in a good agreement with each others. As a result, the heat transfer enhancements using turbulators were 2.91, 2.41, 2.18 and 1.99 times better than the smooth tube for pitch distances of p = 20, 40, 60 and 80 mm, respectively. Copyright © 2012 John Wiley & Sons, Ltd.     

Natural convection heat transfer of water in a horizontal gap with downward-facing circular heated surface

An experimental study of natural convection heat transfer from a downward-facing horizontal circular heated surface in a water gap has been carried out. The results were correlated in different forms of Nusselt number vs Rayleigh number according to different independent variables. The effects of different characteristic length and temperature were discussed and the gap size is the preferred characteristic length, the average fluid temperature between bulk temperature and the saturated temperature is the preferred film temperature. For the estimation of the natural convection heat transfer under the present conditions, empirical correlations in which Nusselt number is expressed as a function of Rayleigh number, or Rayleigh and Prandtl numbers both, may be used. However, the best accuracy is provided by an empirical correlation which expresses the Nusselt number as a function of the Rayleigh and Prandtl numbers, as well as the gap width-to-heated surface diameter ratio, the dimensionless temperature. Artificial neural networks have been trained successfully for analyzing the influences of the gap width-to-heated surface diameter ratio and the wall temperature difference between the temperature of wall and ambient fluid on natural convection heat transfer based on the experimental data in the present study. The results show that the Nusselt number will increase by increasing the gap ratio and decrease by increasing the wall temperature difference.     

On the performance of a vertical helical coil heat exchanger. Numerical model and experimental validation

A numerical model was developed in order to predict the heat transfer process and pressure drop in a vertical helical coil heat exchanger (HCHE) located inside a fluid storage tank in which water is used as inner and outer fluid. Natural convection was considered as boundary condition for the HCHE outer surface. The model was validated with experimental data obtained from an own facility with two HCHEs tested under several operating conditions. The model developed was used to evaluate the main HCHE representative geometrical parameter's influence on the overall heat transfer coefficient and pressure drop. The results show that by increasing the tube diameter causes an increase of the Nusselt number and a larger heat transfer rate to pressure drop ratio is obtained.     

Mathematical Model and Its Application of Radial Effective Thermal Conductivity for Coil Heat Transfer in HPH Furnace

Temperature uniformity of steel coils in High Performance Hydrogen bell-type annealing furnace has a significant effect on their quality and production. The hot rolled coil can be considered as a periodically laminated material composed of steel layers and interface layers in radial direction. A new formula for the radial effective thermal conductivity has been proposed, which is based on surface characteristic, strip thickness and compressive stress of the rolled coil. Furthermore, it has been used to develop a heat transfer mathematical model for steel coils in the HPH furnace. The calculated annealing curves using this mathematical model are in good agreement with the experimental data.     

微通道内流动沸腾特性研究

对国内外微通道流动和换热的研究实验作了总结,阐述了影响微通道换热系数的因素,如热流密度、过热度和干度等.对去离子水在内径为0.65 mm、长为102 mm的圆形管道内流动沸腾换热进行了实验研究,得到了局部换热系数随干度的变化关系,进而根据换热系数的变化趋势讨论了饱和流动沸腾区微通道内主导的换热机制.结果表明:从换热系数随干度的变化关系很难判定主导的换热机制;将实验数据与已发表的预测关联式进行了比较,发现大多关联式都失效,说明基于常规理论的模型不再适用于微通道.