Experimental investigation on heat transfer and frictional characteristics of spirally corrugated tubes in turbulent flow at different Prandtl numbers

Corrugated tubes have been experimentally studied in order to obtain their heat transfer and isothermal friction characteristics. The use of water and ethylene glycol as test fluids has allowed to cover a wide range of turbulent fluid flow conditions: Reynolds number from 2000 to 90 000 and Prandtl number from 2.5 to 100.The paper presents a comprehensive experimental study on a family of 10 corrugated tubes which were manufactured by cold rolling. Artificial roughness is characterised by rib height h/d ranging from 0.02 to 0.06 and spiral pitch p/d from 0.6 to 1.2. The results show that a unique dimensionless parameter named severity index (φ=h²/pd) can be used to establish roughness influence on flow.The large amount of experimental data has been correlated in order to obtain easy to use expressions for Fanning friction factors and Nusselt numbers as functions of flow and geometry dimensionless parameters. The real benefits which are offered by corrugated tubes have been assessed by calculating one of the performance evaluation criteria commonly used in the enhanced heat transfer literature. Finally an optimisation study shows the guidelines to choose which roughness geometry offers the best performance for specific flow conditions.     

利用流体脉动强化换热的试验研究

通过管内流体脉动的试验研究，分析脉动流体的水力参数和脉动特性对强化换热的影响规律。采用自行研制的自激振荡腔作为流体产生脉动的装置，并通过改变自激振荡腔的腔室长度和后喷嘴长度来达到调节流体脉动特性参数的目的。结果表明：流体的水力参数和自激振荡腔结构对流体脉动强化换热都有显的影响，随着流量或自激振荡腔腔室长度的增加，换热效果将增强；而后喷嘴长度则存在一个最优尺寸，在此处，换热效果最好。     

Momentum and heat transfer phenomena of spheroid particles at moderate Reynolds and Prandtl numbers

The momentum and heat transfer phenomena of spheroid particles in an unbounded Newtonian fluid have been numerically investigated by solving governing conservation equations of the mass, the momentum and the energy. The numerical solution methodology has been benchmarked by performing comparisons between present results with those reported in the literature. Further, extensive new results have been obtained to elucidate effects of pertinent dimensionless parameters such as the Reynolds number (Re), the Prandtl number (Pr) and the aspect ratio (e) on the flow and heat transfer behaviour of spheroid particles in the range of parameters: 1 ? Re ? 200; 1 ? Pr ? 1000 and 0.25 ? e ? 2.5. Regardless of the value of the Reynolds number, the total and individual drag coefficients of oblate spheroids (e < 1) are smaller than those of spheres (e = 1) and opposite trend has been observed for prolate spheroids (e > 1). Irrespective of values of Reynolds and Peclet numbers, the average Nusselt number is large for prolate particles as compared to spheres and opposite trend has been observed for the case of oblate particles. Major contribution of this work is the development of simple correlations for the total drag coefficient and the average Nusselt number of unconfined isolated spheroid particles based on present numerical results which can be used in new applications.     

CFD and experimental studies on heat transfer enhancement in an air cooler equipped with different tube inserts

This paper reports the experimental and Computational Fluid Dynamics (CFD) modeling studies on heat transfer, friction factor and thermal performance of an air cooled heat exchanger equipped with three types of tube insert including butterfly, classic and jagged twisted tape. In the studied range of Reynolds number the maximum thermal performance factor was obtained by the butterfly insert with an inclined angle of 90°. The results have also revealed that the difference between the heat transfer rates obtained from employing the classic and jagged inserts reduces by decreasing the twist ratio. The CFD predicted results were used to explain the observed results in terms of turbulence intensity. In addition, good agreements between the predicted and measured Nu number as well as friction factor values were obtained.     

A study on heat transfer enhancement using straight and twisted internal fin inserts

The present study investigated the effect of internal aluminum fins with a star-shape cross-section on the heat transfer enhancement and pressure drop in a counterflow heat exchanger. A concentric-tube heat exchanger was used with water as the working fluid. The heat transfer rate increased by 12–51% over a plain tube value, depending on internal fin configurations used. However, the pressure drop also increased substantially by 286–399%. The results showed that a straight-fin configuration is the best to produce a heat transfer increase in a counterflow heat exchanger. Twisted fin configurations did not further increase the heat transfer rate.     

Experimental investigation of twisted tape inserts performance on condensation heat transfer enhancement and pressure drop

A comprehensive experimental investigation is conducted on the augmentation of heat transfer coefficients and pressure drop during condensation of HFC-134a in a horizontal tube at the presence of different twisted tape inserts. The test section is a 1.04 m long double-tube counter-flow heat exchanger. The refrigerant flows in the inner copper and the cooling water flows in annulus. The experiments are performed for a plain tube and four tubes with twisted tapes inserts of 6, 9, 12 and 15 twist ratios. The pressure drop is directly measured by a differential pressure transducer. It is found that the twisted tape with twist ratio of 6 gives the highest enhancement in the heat transfer coefficient and the maximum pressure drop compared to the plain tube on a nominal area basis. For this case the enhancement in heat transfer and the pressure drop are increased by 40 and 240% in comparison with to the plain tube. It is observed that the twisted tape with the twist ratio of 9 has the best performance enhancing the heat transfer with the minimum pressure drop. Also empirical correlations are developed to predict smooth tube and swirl flow pressure drop. Predicted results are compared to experimental data and it is found that these correlations are reliable for pressure drop estimation.     

Numerical study on heat transfer enhancement for laminar flow in a tube with mesh conical frustum inserts

Enhanced heat transfer tubes (EHTT) with segmented mesh-conical frustums are considered. Tube diameter and frustum apex angle are fixed as 20?mm and 60o, respectively. The height ratio of frustum and sliced part are set as a golden ratio (1.618). Laminar thermal-hydraulic performance and effects of some parameters, e.g., bottom frustum diameter and pitch, are numerically simulated. The equal equivalent diameter and total flow area criteria are adopted to simplify 3D mesh pores to 2D ones. Flow and temperature fields show large velocities and gradients close to the wall and smaller velocities in the bulk region. This enhances heat transfer with a limited pressure drop. EHTTs obtain 1.4 - 3.3 times higher heat transfer than bare tubes and the performance evaluation criterion (PEC) varies from 1.3 to 1.8. Nusselt number (Nu) and friction factor (f) correlations are proposed. New insights into heat transfer enhancement and tube configuration are provided.     

Experimental investigation of heat transfer in a triple tube heat exchanger with coiled spring inserts

Experiments have been performed to investigate the effect of coiled spring inserts on heat transfer, pressure drop, and performance parameters of a triple tube heat exchanger (TTHX). Three different spring inserts having a pitch of 5, 10, and 15 mm are used and the diameter of the spring wire is taken as 1 mm. The experiments were carried out under a turbulent flow regime, with water as a working medium in parallel and counter flow configurations. The variation in different performance characteristics like heat transfer coefficient, Nusselt number, and effectiveness have been compared at various Reynolds numbers ranging between 4000 and 16,000 in the considered flow patterns. The Nusselt number of TTHX with the lowest pitch spring is found to be higher than that of the plain TTHX by 57.27% at Re = 4000 for the counter flow configuration. Both the thermal performance factor and effectiveness increased as the pitch of the spring insert was decreased. The effectiveness of TTHX with the lowest pitch spring insert is found higher than that of the plain TTHX by 43.84% in the counter flow pattern.     

Flows and heat transfer of the transition to an unsteady state in a finned cavity for different Prandtl numbers

Flows and heat transfer of the transition to an unsteady state in a finned cavity are studied for Prandtl numbers (Pr) from 0.1 to 100 and Rayleigh numbers (Ra) from 10⁷ to 10¹⁰. Transient flows are described in the finned cavity. Critical Rayleigh numbers of the transition to an unsteady state are obtained for different Prandtl numbers and the relation between two dimensionless parameters is given. The spectral analysis is applied for the oscillations of unsteady flows and the dominant frequency dependent on governing parameters is presented. Heat transfer of the transition to an unsteady flow is quantified and the corresponding relations dependent on the Prandtl number and Rayleigh number are gained. It is demonstrated that the flow rate and the Nusselt number of the finned cavity significantly increase due to the presentence of the fin, which depend on the Prandtl number and the Rayleigh number.     

Experimental investigation on heat transfer enhancement of a tube with coiled-wire inserts installed with a separation from the tube wall

An experimental investigation was conducted for the thermo-hydraulic performance of a circular tube with coiled-wire inserts which were installed with a small separation from the inner wall of the tube. The wire inserts had an equilateral triangular cross-sections with a constant side length of e = 6 mm and they were coiled with three different pitch-to-diameter ratios: P/D = 1, P/D = 2, P/D = 3. A specific method was employed to coil the wires so that an edge of the triangle was oriented to face the incoming air flow. The coiled-wire inserts were installed with 1 and 2 mm separation from the inner tube wall so that the heat transfer enhancement due to the laminar boundary layer disturbance could be investigated. Experiments were performed for Reynolds numbers from 3429 to 26,663. The coiled-wire inserts led to a significant increase in both the heat transfer rate and friction factor over the smooth tube based on coil pitches and clearance. The maximum thermal performance was observed around 1.82 for the P/D = 1, s = 1 type at a Reynolds number of 3429. In conclusion, the laminar boundary layer disturbance can be effectively enhanced by using these types of coiled-wire inserts.     

Heat transfer enhancement of nanofluids in a double pipe heat exchanger with louvered strip inserts

The effect of using louvered strip inserts placed in a circular double pipe heat exchanger on the thermal and flow fields utilizing various types of nanofluids is studied numerically. The continuity, momentum and energy equations are solved by means of a finite volume method (FVM). The top and the bottom walls of the pipe are heated with a uniform heat flux boundary condition. Two different louvered strip insert arrangements (forward and backward) are used in this study with a Reynolds number range of 10,000 to 50,000. The effects of various louvered strip slant angles and pitches are also investigated. Four different types of nanoparticles, Al₂O₃, CuO, SiO₂, and ZnO with different volume fractions in the range of 1% to 4% and different nanoparticle diameters in the range of 20 nm to 50 nm, dispersed in a base fluid (water) are used. The numerical results indicate that the forward louvered strip arrangement can promote the heat transfer by approximately 367% to 411% at the highest slant angle of α = 30° and lowest pitch of S = 30 mm. The maximal skin friction coefficient of the enhanced tube is around 10 times than that of the smooth tube and the value of performance evaluation criterion (PEC) lies in the range of 1.28–1.56. It is found that SiO₂ nanofluid has the highest Nusselt number value, followed by Al₂O₃, ZnO, and CuO while pure water has the lowest Nusselt number. The results show that the Nusselt number increases with decreasing the nanoparticle diameter and it increases slightly with increasing the volume fraction of nanoparticles. The results reveal that there is a slight change in the skin friction coefficient when nanoparticle diameters of SiO₂ nanofluid are varied.     

扭带强化传热性能的数值与实验研究

通过模拟和实验的方法研究在湍流工况下(3 000Re10 000)换热器管内插入不同扭带模型后的传热特性和阻力特性。区别于传统螺旋扭带,提出一种顺时针与逆时针交替扭转的正反扭带。对不同扭率的传统扭带以及扭率为3的无缺口和半圆缺口正反转扭带进行模拟计算,并将模拟结果与实验结果进行对比验证。结果表明:在湍流流态下,雷诺数越小,扭带的强化换热效果表现越好;对不同扭率的扭带,其努塞尔数、摩擦系数和综合性能指标随扭率的减小而增大;扭率为3时,两种正反扭带的强化换热效果均优于传统扭带,无缺口正反扭带的换热效果最好;模拟计算的结果数据与实验结果数据比较,最大误差不超过8%。     

Experimental investigation on heat transfer and pressure drop characteristics in a smooth tube with different twisted tape inserts

Experiments are performed to investigate the impact of inserts (TTI, TBI, and TBHI) accompanied by different twist ratios (ie, y/w = 3.69, 4.39, and 5.25) with uniform heat flux condition to study the performance characteristics of pressure drop, rate of heat flow, and heat transfer enhancement. Experiments were carried out on different twisted tape inserts in a turbulent flow regime by choosing suitable Reynolds number between 3100 and 39 000. A plain tube is tested and compared with the empirical correlations and are found to be in good agreement with the experimental data. In the case of twisted tape inserts stronger swirl flow is observed along the length of the tube. The variation of reduction in pressure along the length of tube and heat flux in the form of the friction factor and Nusselt number are represented graphically. Thermal performance factor tends to increase with a decrease in the taper twist ratio. The maximum enhancement in Nusselt number and friction factor was found to be in the case of TBI and TBHI. Experimental results are justified and are found to be reliable and accurate with the analytical results, with ±5% and ±4.2% deviation for Dittus‐Boelter and Petukhov correlation in the case of Nusselt number and ±7.2% deviation, respectively, for loss in the friction.     

Experimental study of heat transfer enhancement in a drag-reducing two-dimensional channel flow

In this paper, drag reduction and heat transfer enhancement were studied in a fully developed two-dimensional water channel flow. Surfactant solutions at different concentrations, namely 30, 70, 80 and 90 ppm, were used to examine the influence of surfactant additives on the skin friction drag and heat transfer coefficient. The magnitudes of the maximum achievable drag reduction at the above four different surfactant concentrations are about 7%, 30%, 50% and 55%, respectively. The present results show that there is no heat transfer reduction when 30 ppm of surfactant is added to the flow. With the increase of surfactant concentration to 90 ppm, heat transfer rate was reduced by about 55%. The critical Reynolds number for loss of heat transfer reduction increases with the increase of surfactant concentration. The effect of the low-profile vortex generators on heat transfer rate was examined for the surfactant concentration of 90 ppm. The results show that the averaged Nusselt number is enhanced by 180%, 160% and 150% for the Reynolds numbers of 7000, 12,000 and 16,200, respectively, as compared with that obtained in the surfactant solution without the use of vortex generators and yet the pressure drop penalty for heat transfer enhancement is rather small.     

Experimental and numerical study on heat transfer enhancement in a channel with Z-shaped baffles

The influence of baffle turbulators on heat transfer augmentation in a rectangular channel has been investigated experimentally and numerically. In the experiment, the baffles are placed in a zigzag shape (Z-shaped baffle) aligned in series on the isothermal-fluxed top wall, similar to the absorber plate of a solar air heater channel. The aim at using the Z-baffles is to create co-rotating vortex flows having a significant influence on the flow turbulence intensity leading to higher heat transfer enhancement in the tested channel. Effects of the Z-baffle height and pitch spacing length are examined to find the optimum thermal performance for the Reynolds number from 4400 to 20,400. The Z-baffles inclined to 45° relative to the main flow direction are characterized at three baffle- to channel-height ratios (e/H = 0.1, 0.2 and 0.3) and baffle pitch ratios (P/H = 1.5, 2 and 3). The experimental results show a significant effect of the presence of the Z-baffle on the heat transfer rate and friction loss over the smooth channel with no baffle. The Nusselt number, friction factor and thermal performance enhancement factor for the in-phase 45° Z-baffles are found to be considerably higher than those for the out-phase 45° Z-baffle at a similar operating condition. The in-phase 45° Z-baffle with larger e/H provides higher heat transfer and friction loss than the one with smaller e/H while the shorter pitch length yields the higher Nu, f and TEF than the larger one. The numerical work is also conducted to investigate the flow friction and heat transfer behaviors in the channel mounted with the 45° Z-baffles, and the numerical results are found in good agreement with experimental data.     

Numerical study of heat transfer enhancement using perforated dual twisted tape inserts in converging‐diverging tubes

In this article, a 3D numerical simulation for investigating friction factor and heat transfer enhancements in case of inserting normal or perforated dual twisted tapes (TTs) in converging‐diverging tubes (CDTs) is presented. The effects of Reynolds number changing from 10,000 to 20,000 and a various number of holes (N = 0, 1, 2, and 3) in TTs, under constant uniform heat flux on tube outer wall and using water as working fluid circumstances, were examined. It was found that generally using dual TTs in CDTs improves the Nusselt number up to 9% compared to bare CDTs. Perforating TTs do not have a noticeable effect on heat transfer, but decrease the friction factor significantly, up to 396% compared to normal dual TT implantation, in higher Reynolds numbers. Following that, thermal performance factor considerably increases up to 57%, compared to CDTs with normal dual TTs. To show the mechanism of these effects, velocity streamlines are presented and will be discussed in this paper.     

Experimental study of heat transfer enhancement in an asymmetrically heated rectangular duct with perforated baffles

The paper presents results of an experimental study of heat transfer and friction in rectangular ducts with baffles (solid or perforated) attached to one of the broad walls. The duct has width-to-height ratio of 7.77; the baffle pitch-to-height ratio is 29; the baffle height-to-duct height ratio is 0.495. The Reynolds number of the study ranges from 2850 to 11500. The baffled wall of the duct is uniformly heated while the remaining three walls are insulated. These boundary conditions correspond closely to those found in solar air heaters. Over the range of the study, the Nusselt number for the solid baffles is 73.7–82.7% higher than that for the smooth duct, while for the perforated baffles, it ranges from 60.6–62.9% to 45.0–49.7%; decreasing with the increase in the open area ratio of these baffles from 18.4% to 46.8%. The friction factor for the solid baffles is found to be 9.6–11.1 times of the smooth duct, which decreased significantly for the perforated baffles with the increase in the open area ratio; it is only 2.3–3.0 times for the perforated baffles with open area ratio of 46.8%. Performance comparison with the smooth duct at equal pumping power shows that the baffles with the highest open area ratio give the best performance.     

Experimental study of heat and mass transfer on an absorber with several enhancement tubes

An experimental study of the absorption process of water vapor into a lithium bromide solution was performed. For the purpose of the development of a high-performance absorption chiller/heater utilizing a lithium bromide solution as the working fluid, it is the most effective way to improve the performance of the absorber with the largest heat transfer area of the four heat exchangers. This paper considers a bare tube, bumping bare tube, floral tube, and twisted floral tube for the absorber of an absorption chiller/heater. The floral and twisted floral tubes have about 40% higher heat and mass transfer performance than the bare tube conventionally used in an absorber. Therefore, floral and twisted floral tubes are expected to realize high heat and mass transfer performance. © 1999 Scripta Technica, Heat Trans Asian Res, 28(8): 664–674, 1999