Experimental investigation on heat transfer effect of conical strip inserts in a circular tube under laminar flow

The aim of this paper is to observe the Nusselt number and friction factor behavior of the circular tube with conical strip inserts as turbulators in a laminar flow condition, using staggered and non-staggered conical strips with three different twist ratios (Y = 2, 3 and 5). The conical strip is inserted in the forward and backward direction individually compared to the flow of water which is the working fluid. The results indicate that the conical strip inserts increases the Nusselt number when compared to the plain surface tube. It is observed that the strip geometry has a major effect on the thermal performance of the circular tube. On examination of different strips for determining the enhancement of Nusselt number, the staggered conical strip with the twist ratio of Y = 3 has given a better result compared to the other two strips. Finally, correlations have been derived using regression analysis for predicting the Nusselt number and friction factor.     

Heat transfer improvement in a tube by inserting perforated conical ring and wire coil as turbulators

In the present study, two various passive methods for heat transfer enhancement, including conical ring and wire coil are placed in a tube as turbulators. Four conical rings with four side holes are utilized with the same distance. The wire coil is employed at the center of the tube. The considered Reynolds numbers are between 4000 and 10,000. The studied geometrical parameters contain the pitch and diameter of a wire coil. Four different pitches of wire coil, including 10, 12, 14, and 16 mm, are evaluated. Furthermore, four values of wire coil diameter such as 2, 4, 6, and 8 mm are certain. The obtained numerical results displayed that by declining the pitch of a wire coil (37.5%), the average Nusselt number increases by about 143%. Also, augmentation in wire coil diameter by 300% leads to a growth in average Nusselt number by about 131%. Moreover, owing to utilizing two various turbulators, the pressure drop is significantly high in comparison with the bare tube. At Re = 10,000, growth in the inner diameter of the wire coil by 300% leads to an increase in thermal performance by about 36.12%. Moreover, as the pitch of the wire coil rises by 60%, the thermal performance declines by about 35.71%.     

Heat transfer and turbulent flow friction in a circular tube fitted with conical-nozzle turbulators

The paper deals with an experimental study of the influence of conical-nozzle turbulator inserts on heat transfer and friction characteristics in a circular tube. In the present work, the turbulators are placed in the test tube section with two different types: (1) diverging nozzle arrangement (D-nozzle turbulator) and (2) converging nozzle arrangement (C-nozzle turbulator). The turbulators are thoroughly inserted inside the tube with various pitch ratios, PR = 2.0, 4.0, and 7.0. The Reynolds number based on the bulk average properties of the air is in a range of 8000 to 18,000 and the experimental data obtained are compared with those obtained from the plain tube and from the literature. The experimental results reveal that increasing the Reynolds number at a given pitch ratio of the turbulators leads to the significant increase in Nusselt number indicating enhanced heat transfer coefficient due to rising convection as the flow increases. However, the friction factor at a given Reynolds number considerably increases with the reduction of pitch ratio and Reynolds number. The D-nozzle arrangement, creating stronger reverse/turbulence flow, provides higher the heat transfer rate and friction factor than the C-nozzle arrangement. The heat transfer rates obtained from using both nozzle-turbulators, in general, are found to be higher than that from the plain tube at a range of 236 to 344%, depending on Reynolds number and the turbulator arrangements. In addition, proposed correlations from the present experimental data for Nusselt number and friction factor are also presented.     

Numerical study of heat transfer and friction factor in a tube with groove and rib on the wall

Heat transfer and friction factor in a tube with turbulator internally grooved and rib are studied using numerical simulation. Two different turbulator geometries, including only groove and both groove and rib, on the tube wall are investigated. The study is developed for different parameters of turbulator including pitch ratio (PR), groove and rib height (DR), and Reynolds number in the range of 10 000 to 38 000. The results revealed that the best Nusselt number is achieved for GR turbulator and for the tube wall with PR = 2 and DR = 1. Furthermore, for a specific turbulator parameter, Nusselt number is improved by 46% compared with the experimental result. The results show that the friction factor decreases by increase of both step and height ratios. For example, based on Reynolds number, 21% to 69% reduction in the friction factor was observed for PR = 7 and DR = 3. In addition, the correlations for Nusselt number and friction factor with acceptable deviation are presented for the studied turbulators. Finally, thermal performance enhancement factor is defined to be a good criterion for selection of the optimum turbulator and their parameters.     

Thermo-hydraulic Performance of a Circular Tube with Perforated and Slotted Conical Ring Inserts

ABSTRACT

The present work deals with the investigation of heat transfer and friction in a circular tube fitted with solid conical ring inserts, perforated conical ring inserts and slotted conical ring inserts. The experiments have been carried out with different types of conical inserts having twist ratio of 2, 4, 6 and 8 by varying the Reynolds number between 4,000 and 12,000 using water as a working fluid. The experimental data have been collected in terms of outlet temperature, inlet temperature, wall temperature, volume flow rates and pressure drop across the test section. The maximum enhancements in Nusselt number and friction factor for solid conical ring inserts, perforated conical ring inserts, and slotted conical ring inserts are respectively 4.17 and 6.95, 3.86 and 5.48, 3.01 and 3.52 times that of a plain tube. The Nusselt number and friction factor increase with the decrease in pitch ratio, however, an increase in a number of holes as well as slots mitigates the friction factor at all the values of Reynolds number. The maximum value of thermo-hydraulic performance is found to be 2.18 for solid conical ring inserts.     

Thermal Performance Assessment of Turbulent Tube Flow Through Wire Coil Turbulators

This paper presents characteristics of turbulent convective heat transfer in a tube fitted with wire coil turbulators. Two different wire coils are introduced: (1) with typical/uniform coil pitch ratio (CR) and (2) with periodically varying three-coil pitch ratio. Various uniform coil pitch ratios (CR = 4, 6, and 8) and two periodically varying coil pitch ratios, the D-coil (decreasing three-coil pitch ratio arrangement) and DI-coil (decreasing/increasing three-coil pitch ratio arrangement), are experimentally investigated in a uniform heat flux tube. The experiments are performed for turbulent flows with Reynolds numbers ranging between 4500 and 20,000. All of the experimental results are compared with those obtained from using the plain tube, while the thermal performance factor is evaluated under an equal pumping power constraint. The experimental results show that the use of the tube fitted with all wire coils leads to an advantage on the basis of heat transfer enhancement over the plain tube with no insert. It is also observed that the uniform-pitch wire coil with higher coil pitch ratio (CR = 8) gives a higher thermal performance factor compared to ones with lower coil pitch ratios (CR = 4 and 6). In addition, for two periodically varying coil pitch ratios, the DI-coil performs with better heat transfer rate than the uniform-pitch ratio (CR = 6) and the D-coil for all Reynolds number ranges studied. The empirical correlations developed in terms of coil pitch ratios (CR), varying coil pitch ratios (D-coil and DI-coil), and Reynolds number are fitting the experimental data within plus or minus 3% and 5% for Nusselt number (Nu) and friction factor (f), respectively. The results of the thermal performance factor for various CR, D-coil, and DI-coil values are also determined.     

Performance and flow-induced vibration characteristics for conical-ring turbulators

In the present study, the performance characteristics of the conical-ring turbulators are determined by means of the entropy-generation minimization method based on the second law and enhancement efficiency based on the first law of thermodynamics. The relations between these performance and flow-induced vibration characteristics have been examined. The maximum entropy generation, at the same Reynolds number, occurs in sequence by the conical rings with 10, 20 and 30 mm pitches, respectively. The conical rings are thermodynamically advantageous (N_s,a<1) up to 8000 Reynolds number with respect to entropy generation. The enhancement efficiency increases as the pitch decreases and varies between 0.86 and 1.16. The turbulators with 20 and 30 mm pitch, especially, produce vortices having low amplitudes up to a Reynolds number of 12,000.     

Heat transfer augmentation through the use of wire-rod bundles under constant wall heat flux condition

This article reports an experimental investigation on heat transfer, friction factor and thermal performance characteristics of turbulent flow (6000 ≤ Re ≤ 20,000) in heat exchanger tubes with wire-rod bundles as flow turbulators. The experiments were carried out at three different pitch ratios (P/D) of 1.0, 1.5 and 2.0 and three wire-rod number per bundle (N) of 4, 6 and 8. The experimental results show that Nusselt number increases with increasing Reynolds number and wire-rod number per bundle, and decreasing pitch ratio (P/D) of the turbulators. As compared to the results of the tube without wire-rod (the plain tube), heat transfer rate and friction factor are respectively increased in ranges of 3.5 to 68.8% and 156 to 353%, depending on the operating conditions. At the same pumping power, the use of wire-rod turbulators results in thermal performance factor up to 1.02 times of those of the plain tube. In addition, the correlations that developed from the present experimental data for Nusselt number, friction factor and thermal performance factor are also presented.     

Heat transfer in a circular tube fitted with free-spacing snail entry and conical-nozzle turbulators

The paper presents the effect of a free-spacing snail entry together with conical-nozzle turbulators on turbulent heat transfer and friction characteristics in a uniform heat-flux tube. The insertions of the conical or converging nozzle (C-nozzle) with different pitch ratios (PR) in common with the free-space snail entry are examined in a Reynolds number range from 8000 to 18000. A substantial augmentation of heat transfer for using the C-nozzles and snail entrance is expected by a strong influence from nozzle-induced reverse/re-circulation motion and snail-produced vortex/swirl motion for high Reynolds number. The experimental result shows a considerable increase in friction factor and heat transfer over the plain tube under the same operation conditions. Over the range investigated, the Nusselt numbers for employing both the enhancement devices with PR = 2.0, 4.0 and 7.0 are found to be higher than that for the plain tube around 315%, 300% and 285% respectively. The results obtained are correlated in the form of Nusselt number as a function of Reynolds number, Prandtl number and pitch ratio. For performance comparison at equal pumping power, both the enhancement devices with the smallest pitch ratio perform the best, especially at low Reynolds number. The present results are also compared with correlations obtained from similar enhancement devices but without free-spacing entry.     

Investigation of Inclined Turbulators for Heat Transfer Enhancement in a Solar Air Heater

ABSTRACT

In the present study, the effect of forward inclined turbulators on the heat transfer enhancement in a duct is investigated, for forced convection. Turbulator configurations with three different pitch ratios and three different inclination angles are investigated for seven Reynolds numbers within the range 500–50,000. Investigations are performed experimentally as well as computationally, within a computational fluid dynamics framework. A distinguishing feature of the latter has been the employment of a turbulence model, the transitional shear stress transport model that is applicable throughout the presently considered range of Reynolds numbers containing laminar, transitional, and turbulent regions. At the beginning of the study, measurements and predictions are validated against analytical and empirical expressions known for a plain duct. The results obtained for turbulators configurations indicate that Nusselt number increases with the inclination angle but decreases with the pitch ratio. The influence of the inclination angle on the Nusselt number and thermal enhancement factor is found to be stronger than that of the pitch ratio. For all Reynolds numbers and for all configurations, the thermohydraulic performance is observed to increase, leading to thermal enhancement factors within the range 2–5. In all cases, a quite good agreement of the predictions and experiments is observed, which increases the confidence in the accuracy of both approaches.     

Application of soft computing techniques to optimize thermal parameters in a double heat exchanger with cylindrical turbulators

In the present study, the dimensions of the cylindrical turbulators have been optimized to achieve the highest thermal performance. A double pipe heat exchanger with cylindrical turbulators placed in the annulus side is modeled and a numerical simulation is carried out for different operating conditions. The simulation is conducted for different diameters of the turbulators for various Re for the annulus fluid. The Nusselt number (Nu), friction factor (f), and thermohydraulic performance index (THPI) are the responses simulated for the above different cases of input conditions. Response surface methodology has been used to study the influence of operating parameters on the responses. It is observed that Nu, f, and THPI increased as the Re and turbulator diameter increased. Response optimizer is used to optimize the turbulator diameter to obtain the highest thermal performance in terms of highest Nu and THPI and lowest f. The results indicated that maximum performance was obtained for a diameter of 4.45 mm and for a Re of 5530. The Nu and THPI corresponding to the above combinations are 68.4 and 2, respectively.     

Analysis on flow structure and improvement of heat transfer in 3D circular tube with varying axial groove turbulator configurations

Using passive devices are an efficient method to enhance streamline behavior when liquid flows through the circular pipe. The interrupted structure groove is usually used to change the flow patterns. In this analysis, a heat performance numerical technique is applied to study the characteristics of fluid flow and heat transfer of the circular pipe using different axial groove geometrical configurations with different axial groove numbers, including 2, 3, and 4, under different conditions. The number of annular grooves and circumferential positions are the important parameters to analyse with varying operating conditions, with the Reynolds number (Re) range from 1500 to 23,000. A three-dimensional coordinate pipe system is applied using tetrahedron grids. The discretization equations are obtained by deriving algebraic approximations to integral conservation equations. Results observed that using this type of passive method has a low effect on pressure dope compared to the normal one (smooth pipe). The flow change occurs near and closed to the axial groove parameters. Moreover, the Nusselt number (Nu) value for the groove turbulators was higher than the normal one, about 14.5%–21%. The friction factor (f) value for the groove turbulators was higher than the normal one, were about 7.5%–24%. Most friction losses are caused by dynamical pressure dissipation owing to more viscous losses closed to the wall surfaces. The improvement of heat performance using this type of passing method was more than 1.2%.     

Heat transfer performance for turbulent flow through a tube using double helical tape inserts

The augmentation of heat transfer for turbulent fluid flow through a tube by using double helical tape inserts was investigated experimentally in the present work. The effects of insertion of the helical tape turbulators with different helix angles (9°, 15°, 21° and 28°) on heat transfer and pressure drop in the tube for Reynolds number ranging from 22,000 to 51,000 were examined. Experimental results showed that the heat transfer and thermal performance of the inserted tube were significantly increased compared to those of the plain tube. The study showed the Nussselt number, friction factor as well as thermal enhancement efficiency were increased with decreasing helix angles under the same operating conditions. The results indicated that the Nusselt number and friction factor were increased up to 305% and 170%, respectively, than those over the plain tube while the maximum thermal performance was found to be 215% for using the double helical tape insert with helix angle 9° at high Reynolds number. Furthermore, correlations of the Nusselt number and friction factor were developed in terms of the helix angle (α), Reynolds number (Re) and Prandtl number (Pr) based on the experimental data.     

Thermal characterization of turbulent tube flows over diamond-shaped elements in tandem

Experiments have been conducted to investigate the heat transfer and friction factor characteristics of the fully developed turbulent airflow through a uniform heat flux tube fitted with diamond-shaped turbulators in tandem arrangements. In the experiments, strong turbulence and recirculation flow is expected by using tandem diamond-shaped turbulators (D-shape turbulator) connected to each other by a small rod and placed inside the test tube. The parameters for this study are consisted of Reynolds number (Re) from 3500 to 16,500, the included cone angle (θ = 15°, 30° and 45°), and the tail length ratio (TR = l_t/l_h = 1.0, 1.5 and 2.0) defined as the ratio of the tail length (l_t) to the head length of turbulator (l_h). The variation of Nusselt number and friction factor with Reynolds number under the effect of those parameters are determined and presented. The experimental result reveals that the heat transfer rate increases with increasing Reynolds number and the included cone angle (θ) but decreases with the rise of the tail length ratio (TR). This is because of the mixing of the fluid in the boundary layer thereby enhancing the convective heat transfer and increasing pressure loss. For the tube with the turbulator of θ = 45°, the heat transfer enhancement is found to be 67%, 57% and 46% for tail length ratio, TR = 1.0, 1.5 and 2.0, respectively. Correlations of the Nusselt number (Nu) and friction factor (f) are developed for the evaluation of interactive effects of using the turbulators on the heat transfer and pressure loss. The good agreement between the experimental and the correlated results is obtained within 5–7% deviation. In addition, the heat transfer enhancement efficiency determined under constant pumping power is also provided.     

Effects of the Random Distributions on the Laminar Flow and Heat Transfer in a Quasi-Counter Flow Parallel-Plate Membrane Channel

ABSTRACT

An internally cooled parallel-plate membrane contactor has been employed for liquid desiccant air dehumidification. The contactor is comprised of a series of quasi-counter flow parallel-plate membrane channels (QCPMC). The processing air and the liquid desiccant (solution) streams are separated by the membranes. Cooling tubes are installed in the solution channel to take away the absorption heat. The laminar flow and heat transfer in the QCPMC with the cooling tubes in the solution side (QCPMCC) are studied based on a unit cell containing the sandwiched domain outside the cooling tubes between two neighboring membranes. This paper is focused on the effects of the random distributions in the height direction on the transport phenomena in the QCPMCC. The governing equations of the momentum and thermal transports are built up together with a uniform wall temperature boundary condition. The product of the mean friction factor and Reynolds number and the mean Nusselt number are then calculated. Effects of the various random distributions in the height direction on the product of the mean friction factor and Reynolds number and the mean Nusselt number are analyzed. Compared to the regular arrangement, the mean Nusselt number for the random distribution is larger than that for the regular one when the Reynolds number is less than 68.5. However when the Reynolds number is larger than 68.5, the mean Nusselt number for the random distribution is smaller than that for the regular one.     

Flow structure and local Nusselt number variations in a channel with angled crossed-rib turbulators

Spatially-resolved, local flow structure and surface Nusselt numbers are presented for a stationary channel with an aspect ratio of 4 and angled rib turbulators inclined at 45° with perpendicular orientations on two opposite surfaces. Instantaneous flow visualizations and time-averaged flow structural data show a variety of flow phenomena, including: (i) the development of increased numbers of multiple, smaller vortex pairs as the Reynolds number increases, and (ii) strong spanwise secondary flow components, which move in opposite directions in the top and bottom halves of the channel, and result in the formation of other secondary flows and vortical motions. The resulting increases in three-dimensional turbulence transport, as well as other flow phenomena, are then related to local Nusselt numbers, which vary significantly over the surface of the ribbed channel.     

Heat transfer behaviors in a tube with combined conical-ring and twisted-tape insert

Heat transfer, friction factor and enhancement efficiency characteristics in a circular tube fitted with conical-ring turbulators and a twisted-tape swirl generator have been investigated experimentally. The heat transfer test section is heated electrically imposing axially and circumferentially constant wall heat flux boundary conditions. In the experiments, two enhancement heat transfer devices are applied. One is the conical-ring used as a turbulator and placed in the tested tube and the other is the twisted-tape swirl generator placed at the core of the conical-ring. Air as the tested fluid is passed both enhancement devices in a Reynolds number range of 6000 to 26,000. Two twisted-tapes of different twist ratios, Y = 3.75, and 7.5, are introduced in each run. The experimental results reveal that the tube fitted with the conical-ring and twisted-tape provides Nusselt number values of around 4 to 10% and enhancement efficiency of 4 to 8% higher than that with the conical-ring alone. A maximum heat transfer rate of 367% and enhancement efficiency of around 1.96 is found for using the conical-ring and the twisted-tape of Y = 3.75. For all the devices used, the enhancement efficiency tends to decrease with the rise of Reynolds number and to be nearly uniform for Reynolds number over 16,000. In addition, correlations for Nusselt number, friction factor and performance evaluation criteria to assess the real benefits in using the turbulator and swirl generator of the enhanced tube are determined.     

阵列射流冲击冷却传热特性的数值研究

以涡轮叶片冷却技术为背景,采用带转捩的剪切应力输运(Transition SST)模型对阵列射流冲击冷却的传热特性进行数值模拟,分析了冲击Re、冲击间距、初始横向流和冲击孔排列方式的影响规律。结果表明:冲击间距对靶面平均Nu的影响存在最优值,在所计算的范围内,Zn/d=2时平均Nu最大;在冲击孔排列方式影响中,当冲击间距Zn/d≤2时,顺排孔冲击冷却传热效果优于错排,而当Zn/d≥3时,错排孔冷却传热效果优于顺排。     

A numerical investigation on effects of ceiling and floor surface temperatures and room dimensions on the Nusselt number for a floor heating system

In this study, the effect of ceiling and floor surface temperatures and room dimensions on the Nusselt number over the floor of a floor heating system has been investigated numerically. The variation of the Nusselt number with Rayleigh number has been analyzed under constant wall temperature condition for different ceiling temperatures (10–25 °C) and room dimensions. It has been seen that when the room dimensions and temperature difference between the ceiling and interior air are increased, the Nusselt number over the floor increases as well. The numerical results have been compared with the correlations given in the literature. It has been seen that the correlations available in the literature are valid only for given thermal conditions and room dimensions. The results calculated from the correlations which do not consider the effects of ceiling and floor surface temperatures deviate up to 35% than the results of this numerical study carried out for different ceiling and floor surface temperatures and room dimensions. Therefore, a new correlation for Nusselt number over the floor, which contain the influence of thermal conditions and all of room dimensions must be discovered.     

Investigation of nanofluids heat transfer in a ribbed microchannel heat sink using single-phase and multiphase CFD models

In this paper, laminar forced convective heat transfer of water/alumina nanofluids in a straight microchannel is studied numerically using CFD modeling. In the first part, single-phase and two-phase mixture models have been used for prediction of hydrodynamics and heat transfer parameters of nanofluids in a simple microchannel heat sink. The CFD predictions were compared to the experimental data and it was concluded that the two-phase approach gives better predictions compared to the single-phase model. The effects of ribs turbulators on the fluid flow and heat transfer performance of microchannel were investigated in the second part. The effects of geometrical characteristics of the ribs were studied, and the results showed that the Nusselt number and friction coefficient of nanofluids in the ribbed microchannel are higher than those of simple microchannel, and this enhancement increased with increasing the width of the ribs.