Effect of lateral fin profiles on turbulent flow and heat transfer performance of internally finned tubes

Heat transfer performance of internally finned tubes with blocked core-tube was numerically investigated by the realizable k–ε turbulence model with wall function method using a commercial software FLUENT. Three kinds of lateral fin profiles, that is, S-shape, Z-shape and V-shape, were studied and compared. The corresponding correlations of Nusselt number and friction factor for different-shape internally finned tubes were obtained. The comprehensive performances of the studied tubes were compared under identical mass flow rate, identical pumping power and identical pressure drop conditions. It was found that tubes with S-shape fins and Z-shape fins were superior to that with V-shape fins, and moreover, tube with Z-shape fins had the best performance. The fin outer curvature radius R near the inner surface of out-tube for the S-shape finned tube had appreciable effect on heat transfer, whereas the fin inner curvature radius r near the outer surface of blocked core-tube had little impact on heat transfer. Hence, when manufacturing the internally finned tube with S-shape fins, it is better to select the outer curvature radius R as smaller as possible.     

Constructal design of longitudinally finned tubes cooled by forced convection

The constructal design method is used in the present study to find the configuration of longitudinally finned tubes cooled by forced convection. The finned tubes are arranged in parallel inside a fixed two‐dimensional domain. Two degrees of freedom inside the domain are considered for the design. The first degree of freedom is the tube‐to‐tube spacing, and the second is the length of the longitudinal fin. For both these degrees of freedom, a three‐fin position inside the domain is considered. The fin is placed in the front, back, and front and back of the tube in the first, second, and third positions, respectively. Maximization of the heat flow density (heat transfer/volume) from the finned tubes to the cold cross flow is the objective function of the present study. For the three fin positions, the constant pressure difference between the upstream and the downstream drives the cross flow. The dimensionless continuity, momentum, and energy equations for two dimensional, steady, and incompressible flows are solved by discretizing it according to the finite volume method. The thermal condition of the fins and the tubes is constant surface temperature. The dimensionless pressure drop known as Bejan number is varied in the range of 10³ ≤ Be ≤ 10⁵. The fin length is changed from L_f = 0 (unfinned tube) to L_f = 0.2, 0.4, and 0.4. The tubes are cooled by air (Prandtl number = 0.71). The results illustrated that for the considered Bejan numbers and fin positions, the spacing between the unfinned and the finned tubes can be adjusted to optimal spacing such that the heat flow from the tubes to the coolant is maximum.     

Numerical investigation on the dryout point of annular flow in the upward narrow annuli with bilateral heating

Based on the fundamental conservation principles —the mass, momentum, and energy conservation equations of liquid films and the momentum conservation equation of vapor core, a theoretical three-fluid model has been developed to predict the dryout point of upward annular flow in vertical narrow annuli with bilateral heating. The range of the parameters are: pressure from 0.5 to 5.0 MPa; mass flow rate from 30 to 150 kg/(m²·s); gap size from 1.2 to 2.0 mm. Through numerically solving the model, the relationships among the parameters of the critical quality (X _C), critical heat flux (Q _CHF), mass flow rate, system pressure, and the ratio of heat flux on the inner wall of the outer tube to that on the outer wall of the inner tube (q _o/q _i) are obtained and analyzed. The predicted results accurately match with the experimental data. For a fixed q _o, X _C will increase with the decreases in the gap size and the tube curvature when the dryout point occurs on the outer wall of the inner tube. However, for a fixed q _i, when the dryout point occurs on the inner wall of the outer tube, the parametric trend is reverse. When the dryout point on the inner and outer walls occur simultaneously, X _C reaches a peak value, and the ratio of q _o/q _i at this position changes with the gap size and the tube curvature.     

Numerical study of the intensified heat transfer of an internally longitudinal ridged finned tube under pulsating flow

The turbulent pulsating flow and heat transfer in an internally longitudinal protuberant finned tube was numerically investigated by solving unsteady three‐dimensional elliptical Navier–Stokes equations. The realized k–? turbulent model was adopted. The dynamic behaviors of velocity field, average Nusselt number, and friction number of the internally longitudinal protuberant finned tube were numerically analyzed in a pulsating period, and it was further investigated by changing the frequency of the pulsating flow. It was found that the intensity of heat transfer enhancement increases with an increase of pulsating frequency, while the pressure drop will be increased simultaneously, the intensification of heat transfer in internally longitudinal protuberant finned tubes are gradually better than the pressure drop with an increase of pulsating frequency. © 2009 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20253     

Experimental study on the effect of porous medium on performance of a single tube heat exchanger: A CO2 case study

An experimental study on single‐phase laminar forced convection in a single porous tube heat exchanger is presented. Parametric studies are conducted for different inlet pressures, different mass flow rates, and different porosities to evaluate the effects of particle diameter and Reynolds number on the heat transfer and friction factor. The Nusselt number and friction factor are developed for efficient design of a porous heat exchanger based on the present configuration. Heat is transferred to the walls of the heat exchanger by natural convection mode. Gravel sand with different porosities is used as a porous medium during the tests. The flow of carbon dioxide as a working fluid in the porous medium is modeled using the Brinkman–Forchheimer‐extended Darcy model. A dimensionless performance parameter is developed in order to be used in evaluating the porous tube heat exchanger based on both the heat transfer enhancement and the associated pressure drop. The study covers a wide range of inlet pressures (P_i), mass flow rates ( ), porosity of gravel sand (ε), and particle diameters (d_m) which ranged 34.5 ≤ P_i ≤ 43 bars, 8 ?? 10^?5 ≤ ≤ 16 ?? 10^?5 kg/s, 34.9% ≤ ε ≤ 44.5%, 1.25 ≤ d_m ≤ 5.15 mm, respectively. This study revealed that a smaller particle diameter can be used to achieve higher heat transfer enhancement, but a larger particle diameter leads to a more efficient performance based on heat transfer enhancement. The average heat transfer coefficient of carbon dioxide decreases when the porosity increases. © 2013 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley Online Library (wileyonlinelibrary.com/journal/htj). DOI 10.1002/htj.21059     

Effect of tube alignment on the heat/mass transfer from a plate fin and two-tube assembly: naphthalene sublimation results

Flow and heat/mass transfer in a plate fin and two-tube assembly is examined using the naphthalene sublimation technique. The examined parameter is the offset of two-tube centers to tube diameter ratio s/D for two Reynolds numbers Re_D: 1770 and 2660.The local heat/mass transfer coefficient is always large at the leading edge of the plate and also in front of the tube. The horseshoe vortex formed in front of the first tube increases the local heat/mass transfer rate not only around the first tube itself but also around the second tube. The total heat/mass transfer rate from the plate increases with s/D and it reaches a saturation beyond s/D=0.5. Similar behavior is observed for the pressure drop, showing that s/D≅0.5 is the optimal offset for the tested range. When the Reynolds number is as high as 2660, the effect of tube offset is significant, so that the total heat/mass transfer rate from the plate for the two-row tube case is larger than that for a single tube case.     

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In this study, fully developed laminar flow and convective heat transfer in an internally finned tube heat exchanger are investigated numerically. The flow is assumed to be both hydrodynamically and thermally developed with uniform outside wall temperature. Parameters of the thickness, length, and number of fins and thermal conductivity ratio between fin and working fluid are varied to obtain the friction factor as well as Nusselt number. The results show that the heat transfer improves significantly if more fins are used; however, the pressure drop turns out to be large in this heat exchanger. In addition, it is found that the emergence of closed-loop isotherms between the areas of two neighboring fins leads to heat transfer enhancement in the internally finned tube. When the fin number is smaller than 14, there appears a maximum Nusselt number at about 0.8 of the dimensionless fin length. Finally, an experiment is conducted to verify the numerical results.     

内展翅片换热器应用在压缩空气系统中的节能效益分析

文中通过实验数据分析以及实际工程案例,就内展翅片换热器在压缩空气系统的相同工况下,采用光管换热器和外翅片换热器的换热效果及阻力特性进行了比较,结果表明,内展翅片换热器不仅具有较好的强化传热效果,而且单位压降损耗低,节能效果显著。     

Heat transfer augmentation in 3D internally finned and microfinned helical tube

Experiments are performed to investigate the single-phase flow and flow-boiling heat transfer augmentation in 3D internally finned and micro-finned helical tubes. The tests for single-phase flow heat transfer augmentation are carried out in helical tubes with a curvature of 0.0663 and a length of 1.15 m, and the examined range of the Reynolds number varies from 1000 to 8500. Within the applied range of Reynolds number, compared with the smooth helical tube, the average heat transfer augmentation ratio for the two finned tubes is 71% and 103%, but associated with a flow resistance increase of 90% and 140%, respectively. A higher fin height gives a higher heat transfer rate and a larger friction flow resistance. The tests for flow-boiling heat transfer are carried out in 3D internally micro-finned helical tube with a curvature of 0.0605 and a length of 0.668 m. Compared with that in the smooth helical tube, the boiling heat transfer coefficient in the 3D internally micro-finned helical tube is increased by 40-120% under varied mass flow rate and wall heat flux conditions, meanwhile, the flow resistance is increased by 18-119%, respectively.     

内展翅片换热器在压缩空气系统中的应用分析

通过实验数据分析以及实际工程案例,对内展翅片换热器在压缩空气系统的相同工况下采用光管换热器和外翅片换热器的换热效果及阻力特性进行了比较。结果表明,内展翅片换热器不仅具有较好的强化传热效果,而且单位压降损耗低,节能效果显著。     

Investigation of heat transfer augmentation in a horizontal tube using different rectangular cut ring inserts

Experimental and computational investigations have studied the heat transfer, friction factor, and enhancement of heat transfer in a horizontal tube equipped with rectangular cut ring inserts and different diameter ratios (D/d) and pitch-to-tube diameter ratios (p/d_t). In the present study, air having a Reynolds no. range of 6700–20,100 was used as a working fluid. Three diameter ratios (D/d) were considered experimentally and numerically as 1.2, 1.25, and 1.3, and the pitch-to-tube diameter ratio (p/d_t) was (1, 0.625, and 0.5). Air was forced as working fluid through the tube and a uniform heat flux of 2000, 3500, and 5000 W/m² was applied through the tube's exterior surface. On the basis of the turbulence model k–ɛ with various parameters, three-dimensional numerical simulations using the ANSYS Fluent software 17.2 were investigated. Under the same working conditions, the results manifested a higher heat transfer rate and friction factor as compared to the plain tube. The results evinced that the Nusselt number for a horizontal tube equipped with rectangular cut ring inserts having various pitch ratios and diameter ratios is discovered to be higher than that for the plain tube. With the increased ring spacing, the overall improvement in heat transfer occurred. And, with a rise in Re, the total enhancement ratio decreased. Consequently, the greatest overall improvement attained was 38% at Reynolds number (Re = 12,860) with the pitch ratio (p/d_t = 1). The three diameter ratios (D/d) of 1.3, 1.25, and 1.2 gives in this study the average thermal performance factor in the value of 1.6, 1.5, and 1.4, respectively. Using the Nusselt number and friction factor, the results are correlated as a function of the Reynolds number, diameter ratio, and pitch ratio.     

Optimal design and operation of ammonia decomposition reactors

The design and steady-state operation of a packed bed reactor with tubular geometry is optimized. Direct optimal control methods are used. Two objective functions are considered: (i) minimization of the ammonia mass fraction at reactor outlet and (ii) minimization of the heat flux necessary to reach a predefined value of the ammonia mass fraction at reactor outlet. The optimization process is performed by using different controls, that is, the space distributions of (1) tube wall temperature T_w , (2) circular tube diameter D_tube , and (3) diameter d_p of the catalyst spherical particles. Results for the first objective function are as follows. The optimal distribution of T_w along the reactor consists of a constant temperature or a U-shaped space temperature distribution, respectively, depending on the allowed range of variation of T_w . The optimal space distribution of D_tube (or, in other words, the shape of the reactor tube) depends of T_w . For smaller values of T_w the tube is narrower at inlet and larger at outlet while the reverse situation happens for larger values of T_w . For lower T_w values, particles with smaller diameter d_p are placed at reactor inlet while when higher values of T_w are considered, particles with larger d_p are placed at reactor inlet. When both D_tube and d_p are used as controls, the optimization results are generally different from the results obtained from one-control optimization. Results for the second objective function are as follows. The optimal space distribution of T_w starts with high values at reactor inlet. Next, the temperature decreases abruptly towards a minimum (which is lower for longer tubes). Finally, the temperature increases smoothly towards a maximum near the reactor outlet. The required heat flux slightly decreases by increasing the tube length. The optimal D_tube ranges between its maximum allowed value (at reactor inlet) and its minimum allowed value (at reactor outlet). The best performance is obtained for catalyst particles of the smallest allowed diameter.     

Parametric study and multiple correlations of an H-type finned tube bank in a fully developed region

ABSTRACT

The air-side heat transfer and pressure drop characteristics of an H-type finned tube bank in the turbulent periodically fully developed region are investigated numerically. The effects of seven geometric parameters and Reynolds number are examined. It is found that when the number of tube rows is equal to and larger than 25, the heat transfer and fluid flow approach the fully developed state. Among the seven geometric parameters spanwise tube pitch has the most important effect. Based on the numerical results, the correlations of heat transfer and pressure drop of the H-type finned tube bank for the fully developed region are presented.     

Mass transfer at the confining wall of helically coiled circular tubes in the absence and presence of packed solids

Limiting current measurements were made at the electrode surfaces fixed flush with the inner wall of a helically coiled circular tube. The measured limiting current facilitated the computation of wall-liquid mass transfer coefficients. The mass transfer data were obtained for the flow of electrolyte through the helical coil in the absence and presence of packed solids. The mass transfer coefficient was found to decrease with increasing p/D ratio and approached minimum for straight tube case. The mass transfer data were correlated in terms of Colburn factor j_D, expressed as a function of Reynolds number Re and torsion number Tn obtained as jD=0.34Re−0.58Tn^0.17. Analysis of the experimental data obtained in case of packed coiled tubes of circular cros- section revealed that the effect of p/D ratio on pressure drop was observed to be insignificant. The effect of pitch on mass transfer coefficient in case of packed helical coils was found to be marginal.     

Optimization of a finned heat sink array based on thermoeconomic analysis

The design and specification of heat sinks for electronic systems is not easily accomplished through the use of standard thermal design and analysis tools since geometric and boundary conditions are not typically known in advance. A second-law based thermoeconomic optimization procedure is presented for a finned heat sink array. This involves including costs associated with material, and irreversible losses due to heat transfer and pressure drop. The influence of important physical, geometrical and unit cost parameters on the overall finned array are optimized for some typical operating conditions that are representative of electronic cooling applications. The optimized cost results are presented in terms of Re_D, Re_L, λ_P / λ_H, and q for a finned system in a graphical form. In addition the methodology of obtaining optimum parameters for a finned heat sink system which will result in minimum operating cost is demonstrated. Copyright © 2006 John Wiley & Sons, Ltd.     

Simulation of compressible flow in high pressure buried gas pipelines

The aim of this work is to analyze the gas flow in high pressure buried pipelines subjected to wall friction and heat transfer. The governing equations for one-dimensional compressible pipe flow are derived and solved numerically. The effects of friction, heat transfer from the wall and inlet temperature on various parameters such as pressure, temperature, Mach number and mass flow rate of the gas are investigated. The numerical scheme and numerical solution was confirmed by some previous numerical studies and available experimental data. The results show that the rate of heat transfer has not a considerable effect on inflow Mach number, but it can reduce the choking length in larger f_DL/D values. The temperature loss will also increase in this case, if smaller pressure drop is desired along the pipe. The results also indicate that for f_DL/D = 150, decreasing the rate of heat transfer from the pipe wall, indicated here by Biot number from 100 to 0.001, will cause an increase of about 7% in the rate of mass flow carried by the pipeline, while for f_DL/D = 50, the change in the rate of mass flow has not a considerable effect. Furthermore, the mass flow rate of choked flow could be increased if the gas flow is cooled before entrance to the pipe.     

Frictional performance of R-22 and R-410A inside a 5.0 mm wavy diameter tube

The influence of return bend on the frictional performance of R-410A and R-22 in a 5-mm diameter tube is examined with a curvature ratio of 6.63. The existing single-phase correlations give fairly good agreements with the present single-phase data, but the existing two-phase correlations of the return bend fail to predict the present two-phase data. For test results of the two-phase flow at G?200 kg m⁻² s⁻¹, ((dP/dz)_c/(dP/dz)_s) is approximately equal to 1.8 and is relatively independent of the vapor quality x. However, at a smaller mass flux of 100 kg m⁻² s⁻¹, ((dP/dz)_c/(dP/dz)_s) decreases with x, reaching approximately 5 for x=0.1. The significant increase of this ratio for G increased from 100 to 200 kg m⁻² s⁻¹ may be attributed to the change of the two-phase flow pattern.     

Heat transfer enhancement of round impinging jet by orifice nozzle (Effects of contraction area ratio)

The effects of the nozzle contraction ratio on the flow and heat transfer characteristics of an orifice impinging jet were investigated in this experiment. The nozzle diameter was d_o=10.0 mm=const., and the contraction area ratio CR=(d_o/d_i)², where d_i is the inner pipe diameter was varied from CR=1.00 to 0.11 and the nozzle‐plate distance was varied from H/d_o=2.0 to 5.0. The nozzle Reynolds number was Re=1.5×10⁴=const. The flow characteristics were clarified by measuring the pressure and velocity distributions on the plate and flow visualization. The Nusselt number obtained from measuring the temperature distribution on the plate of an orifice impinging jet with a CR of 0.11 and 0.69 were respectively larger by 19% and 9% than those of a pipe impinging jet (CR=1.00), because the centerline velocity of the orifice jet was larger than that of the pipe jet. Under the same operation power, an orifice impinging jet has improved heat transfer characteristics. © 2008 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20225     

Maximization of heat transfer density from a vertical array of flat tubes in cross flow under fixed pressure drop using constructal design

The density of heat transfer rate from a vertical array of flat tubes in cross flow is maximized under fixed pressure drop using constructal design. With the constructal design, the tube arrangement is found such that the heat currents from the tubes to the coolant flow easily. The constraint in the present constructal design is the volume where the tubes are arranged inside it. The two degrees of freedom available inside the volume are the tube‐to‐tube spacing and the length of the flat part of the tubes (tube flatness). The tubes are heated with constant surface temperature. The equations of continuity, momentums, and energy for steady, two‐dimensional, and laminar forced convection are solved by means of a finite‐volume method. The ranges of the present study are Bejan number (dimensionless pressure drop) (10³ ≤ Be ≤ 10⁵) and tube flatness (dimensionless length of the tube flat part) (0 ≤ F ≤ 0.8). The coolant used is air with Prandtl number (Pr = 0.72). The results reveal that the maximum heat transfer density decreases when the tube flatness decreases at constant Bejan number. At constant tube flatness, the heat transfer density increases as the dimensionless pressure drop (Bejan number) increases. Also, the optimal tube‐to‐tube spacing is constant, irrespective of the tube flatness at constant Bejan number.     

Heat transfer and flow characteristics of flow boiling of CO2‐oil mixtures in horizontal smooth and micro‐fin tubes

Experiments were carried out on the flow pattern, heat transfer, and pressure drop of flow boiling of pure CO₂ and CO₂‐oil mixtures in horizontal smooth and micro‐fin tubes. The smooth tube is a stainless steel tube with an inner diameter of 3.76 mm. The micro‐fin tube is a copper tube with a mean inner diameter of 3.75 mm. The experiments were carried out at mass velocities from 100 to 500 kg/(m²·s), saturation temperature of 10 °C, and the circulation ratio of lubricating oil (PAG) was from 0 to 1.0 mass%. Flow pattern observations mainly showed slug and wavy flow for the smooth tube, but annular flow for the micro‐fin tube. Compared with the flow patterns in the case of pure CO₂, an increase in frequency of slug occurrence in the slug flow region, and a decrease in the quantity of liquid at the top of the tube in the annular flow region were observed in the case of CO₂‐oil mixtures. With pure CO₂, the flow boiling heat transfer was dominated by nucleate boiling in the low vapor quality region, and the heat transfer coefficients for the micro‐fin tube were higher than those of the smooth tube. With CO₂‐oil mixtures, the flow boiling heat transfer was dominated by convective evaporation, especially in the high vapor quality region. In addition, the heat transfer coefficient decreased significantly when the oil circulation ratio was larger than 0.1 mass%. For the pressure drop characteristics, in the case of pure CO₂, the homogeneous flow model agreed with the experimental results within ±30% for the smooth tube. The pressure drops of the micro‐fin tube were 0–70% higher than those predicted with the homogeneous flow model, and the pressure drops increased for the high oil circulation ratio and high vapor quality conditions. The increases in the pressure drops were considered to be due to the increase in the thickness of the oil film and the decrease in the effective flow cross‐sectional area. © 2010 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20287