Turbulent Tube Flow Research Articles

Thermal performance augmentation in round tube with louvered V-winglet vortex generator

The article is concerned with investigating the effect of insertion of louvered V-winglet (LVW) vortex generators on convection heat transfer and pressure loss in a tubular heat exchanger. A metal tape was adopted for supporting the 30° LVW elements placed periodically on the edges of the tape to create two pairs of streamwise counter-rotation vortex flows throughout the tested tube. The use of LVWs could help to promote the fast mixing of fluid flow, to break up the development of boundary layer and to induce the impinging jets resulting in the faster heat transfer rate. Air as a working fluid flowed into the tested tube at Reynolds number from about 4200–25,800. The purpose of employing the louver mounted on the winglet is to reduce the pressure loss with slightly deteriorating the strength of main vortices appearing behind the winglet. The winglet parameters were three different relative winglet pitches, (RP=0.5–1.5), six louver angles (θ = 10°–90°), at a fixed relative height (RB=0.2) and attack angle (α = 30°). Influences of the mentioned parameters on the friction factor and Nusselt number including the thermal performance were explored. The experimental result has shown that among the LVWs the case of RP = 0.5, θ = 10° provides the largest friction factor and Nusselt number owing to impingement flows induced from the vortices onto the heated wall in the rear region of the winglet. The investigation reveals that the highest thermo-hydraulic performance from the LVW is about 2.48 at RP = 1 and θ = 30°. To scrutinize the flow pattern and the mechanism of enhancing convection coefficient, a 3-dimensional simulation of turbulent tube flow through LVWs was performed and the simulated results were verified with available measurements. The empirical correlations for measured Nu and f data of the LVW were also offered.

Heat transfer augmentation for turbulent flow in circular tubes using inserts with multiple curved vortex generator elements

Measurements of local heat transfer coefficients on the circumference of a circular tube, fitted with two modified versions of an already existing axially inline curved Vortex Generator (VG) element insert are reported in this study. Measurements for average pressure drop are also presented and the air was used as the working fluid. The first modified insert is manufactured with a pair of two VG elements spaced symmetrically 180° apart but with the subsequent pairs along the axis staggered at an angle of 90°. The geometric features for the staggered insert were identical to the optimal one for the existing inline one. The staggered insert gives a more uniform circumferential and axial Nusselt number distribution. The ratio of the highest and lowest Nusselt number values for the staggered and inline case is equal to approximately 1.3 and 1.6 respectively. The average Nusselt number ratio at constant Reynolds number (10,000 < Re < 45,000) and constant pumping power however remained the same at about 2.6 and 1.5 respectively. The second modified insert was manufactured with three vortex generators placed symmetrically 120°apart at each axial location with all subsequent locations in line with one another. Two angles of attack 30°and 45° were used for the vortex generator elements and the pitch to projected length ratio of the VGs was varied between 1.6 and 12.2. The average heat transfer coefficient augmentation ratio using this insert at the equal Reynolds number and constant pumping power was obtained in the range of 1.5–5.5 and 1.0 - 1.8, respectively. The experimental data is presented in the form of empirical correlations for Nusselt number, and friction factor as a function of tube flow parameters, and non-dimensional geometrical parameters of VG inserts, which predict the data with an accuracy equal to ±20 %.

Numerical investigation and multi-criteria optimization of the thermal–hydraulic characteristics of turbulent flow in conical tubes fitted with twisted tape insert

Numerical investigation and multi-criteria optimization of the thermal–hydraulic characteristics of turbulent flow in conical tubes fitted with twisted tape insert

Numerical investigation on the influence of particle deposition on nanofluid turbulent flow and heat transfer characteristics

Based on the two-body collision model, a particle collision, deposition and peeling model is established to investigated the influence of nanoparticle deposition process on TiO2 -water nanofluid turbulent flow and heat transfer characteristics in a horizontal circular tube by compiling, linking and executing a program with the Euler-Lagrange multiphase model in Ansys Fluent 19.1. Compared with four different multiphase flow models, this model has a better consistency with the pressure drop of experimental data. And it is applied to study the influence of particle deposition on the flow field and heat transfer. The flow characteristics is analysed from micro-flow perspective to reveal the nanofluid heat transfer enhancement mechanism. Result shows that the nanoparticle deposition layer grows as clusters during flow process, the nanofluid velocity boundary layer becomes thinning, it is easier to deposit at the entrance and the amount can reach more than 7 times of that minimum near the outlet. The velocity difference value between phases is larger as much as 1.23642 m/s, which directly enhances the momentum exchange and energy exchange between phases and wall. That plays a main role of heat transfer enhancement.

Prediction of velocity profile of water based copper nanofluid in a heated porous tube using CFD and genetic algorithm

The heat transfer improvements by simultaneous usage of the nanofluids and metallic porous foams are still an attractive research area. The Computational fluid dynamics (CFD) methods are widely used for thermal and hydrodynamic investigations of the nanofluids flow inside the porous media. Almost all studies dedicated to the accurate prediction of the CFD approach. However, there are not sufficient investigations on the CFD approach optimization. The mesh increment in the CFD approach is one of the challenging concepts especially in turbulent flows and complex geometries. This study, for the first time, introduces a type of artificial intelligence algorithm (AIA) as a supplementary tool for helping the CFD. According to the idea of this study, the CFD simulation is done for a case with low mesh density. The artificial intelligence algorithm uses learns the CFD driven data. After the intelligence achievement, the AIA could predict the fluid parameters for the infinite number of nodes or dense mesh without any limitations. So, there is no need to solve the CFD models for further nodes. This study is specifically focused on the genetic algorithm-based fuzzy inference system (GAFIS) to predict the velocity profile of the water-based copper nanofluid turbulent flow in a porous tube. The most intelligent GAFIS could perform the most accurate prediction of the velocity. Hence, the intelligence of GAFIS is tested for different values of cluster influence range (CIR), squash factor(SF), accept ratio (AR) and reject ratio (RR), the population size (PS), and the percentage of crossover (PC). The maximum coefficient of determination (~ 0.97) was related to the PS of 30, the AR of 0.6, the PC of 0.4, CIR of 0.15, the SF 1.15, and the RR of 0.05. The GAFIS prediction of the fluid velocity was in great agreement with the CFD. In the most intelligent condition, the velocity profile predicted by GAFIS was similar to the CFD. The nodes increment from 537 to 7671 was made by the GAFIS. The new predictions of the GAFIS covered all CFD results.

Study of Flow and Heat Transfer Characteristics of Lead-Based Liquid Metals in a Turbulent Tube Flow and the Impacts of Roughness

Lead-based liquid metals (LLMs) such as lead–bismuth eutectic (LBE) and lead, are currently the most interesting candidate coolants for fast reactors because of their excellent physical properties, which can improve safety and reduce costs. However, in comparison to other liquid metals, previous research on the flow and heat-transfer characteristics of LLMs has been limited. Therefore, this work carried out flow and heat-transfer experiments in LBE flowing through a circular tube in the Natural Circulation Capability Loop (NCCL) facility. The results show a significantly higher friction factor than that of water flowing in a smooth pipe. Furthermore, the Nusselt numbers were found to be lower than those found in data in the literature for experiments carried out in a smooth tube at low Péclet numbers, while they were higher at high Péclet numbers. Therefore, theoretical analyses were performed for LLMs flowing in both smooth and rough pipes, and the impacts of roughness on the heat transfer of an LLM were examined. The theoretical relations for a smooth pipe and a rough pipe were validated using experimental data from the literature and the results of the NCCL experiments, respectively. The results of the theoretical relation for a smooth pipe fitted the literature data well. The derived theoretical relation for a rough pipe with a relative roughness of 0.004 fitted the NCCL data best. Moreover, it was established from the theoretical analysis that roughness has two competitive impacts on the heat transfer of an LLM: it reduces conductive heat transfer while enhancing convective heat transfer. Because conductive heat transfer is important for liquid metals, even with turbulent flow, a small roughness will lead to heat-transfer deterioration at low Péclet numbers, and it may even deteriorate across the whole typical Péclet-number range. This discovery has important implications for the thermal–hydraulic design of LLM reactors, because corrosion and erosion by an LLM will lead to a rough surface after long operating times.

Turbulent convective heat transfer in helical tube with twisted tape insert

Enhancement of heat transfer has been a main focus in thermal-fluid engineering field among the other aspects in industrial for decades. Despite numerous studies have been done on turbulent flows, helical tubes, and twisted tapes related to enhancement, no study has been done on the combination of all three components in augmentation of heat transfer. The objective of this study is to numerically investigate heat transfer of turbulent flow in helical tube with twisted tape insert subjected to constant wall temperature. A three-dimensional computational fluid dynamic model was developed and validated against experimental measured data and correlations. The model was then used to evaluate the turbulent convective heat transfer in straight and helical tube with and without twisted tape insert. To obtain comprehensive view on the heat transfer performance, the effect of twist ratio and inlet Reynolds number are evaluated as well. The insertion of twisted tape is found to increase the intensity of secondary flow in the radial direction. Helical tube with twisted tape insert was found to have higher heat transfer performance. The results show that for every case, the higher the inlet Reynolds number, the heat transfer increases proportionally while friction coefficient drops with decreasing amount of decrement. It is found that helical coiled tube with twisted tape at twist ratio of 7.86 has the highest performance among the configuration studied after taking consideration of heat transfer, friction coefficient and entropy generation. Finally, correlations of Nu and f are developed to predict the heat transfer and pressure drop within considered range of parameters in this study. This study serves as a foundation on the development of a high-performance heat exchanger for industrial application.

Analytical study of heat transfer for liquid metals in a turbulent tube flow

Research on turbulent heat transfer of liquid metals has been lasting for decades since 1940s. Although it had developed various heat transfer relations of liquid metals, most of them are based on the structure of semi-empirical correlation proposed by Lyon in 1949 and fitted according to their respective experimental data. Due to the differences between the experimental data, there are inevitably obvious differences between the empirical correlations, especially in the range of high Peclet numbers. Therefore, reliable relation for turbulent heat transfer of liquid metals is still missing. Focused on this problem, a theoretical analysis of heat transfer for liquid metals in a fully developed turbulent tube flow has been performed following the method of Lyon, but use the velocity and vortex flow diffusivity profile obtained by theoretical analysis. Based on a reasonably determined turbulent Prandtl number, a theoretical heat transfer relation for liquid metals is obtained and validated with experimental data of various liquid metals. The most difference between the theoretical relation and existing relations is the “thermal conductive term” of theoretical relation has a relationship with the Peclet number, while for most of the existing relations this term is constant. Further, it reveals the turbulent Prandtl number has significant impacts on turbulent heat transfer of liquid metals while the molecular Prandtl number has little impact. Therefore, the turbulent heat transfer of Heavy Liquid Metals (HLM) is different from other liquid metals because their turbulent Prandtl number is found to be larger than other liquid metals by existing research.

CFD Investigation of transfer of the heat and turbulent flow in circular copper tube with perforated conical rings of aluminum material

Today industries, societies surrounded by hybrid devices like hybrid solar collector, hybrid micro solar collectors, CNC machines etc, and these devices requires effective heat relocation systems for boosting up the life span efficiency, effectiveness and reliability of such devices. Present work based on comparative and simulation study of thermal attributes and fluid flow attributes for conical perforated ring placed inside copper tune of heat exchanger. Experimentation carried out for acknowledging the effect of diverse Pitch Ratio having range 4, 6, 12 and number of holes range 2, 3, 4 with diverse range of Reynolds number (Re number) 4000, 8000, 12000, 16000, 20,000 on thermal attributes and fluid flow attributes for conical perforated rings placed inside copper tube. Results predicted that the copper tube with different conical perforated-rings have excellent results as compared to corresponding smooth tube. So, this research offers an improved conical perforated-rings design for internal fluid flow for enhancing the heat relocation rate. The highest values of Nusselt number (Nu number) is obtain at PR = 4, conical holes N = 2 for Re number Re = 20000.

Numerical and Experimental Studies on Heat Transfer Enhancement in a Circular Tube Inserted with Twisted Tape Inserts

Numerical simulations and experiments are carried out in this paper to research about characteristics of heat transfer, frictional resistance and comprehensive performance of turbulent water flow (3000 < Re < 10,000) in a circular tube with twisted tape inserts. A new type of twisted tape is proposed and it shows excellent performance on heat transfer enhancement. Effects of three parameters including twist ratio (p/w=3, 4, 5), geometrical structure of alternation of clockwise and counterclockwise, existence of semicircular cuts are researched in simulations. The results show that lower twist ratio and the geometrical structure of alternation of clockwise and counterclockwise can enhance heat transfer while the existence of semicircular cuts can significantly reduce flow resistance. Experiments are conducted to validate the actual effect of the twisted tapes and the experimental results are basically consistent with the simulation results. Nusselt number and friction factor with the new type of twisted tape inserts are respectively 1.66-2.42 times and 4.36-4.97 times to that of plain tube while the maximum value of PEC (performance evaluation criteria) reaches 1.42 in experiments. The results of simulations and experiments indicate that the new type of twisted tape has a distinct advantage on heat transfer enhancement. Verification between simulations and experiments are carried out. The result shows that all deviations of corresponding values between simulations and experiments are within 15%, which reflects that the numerical predications are basically in agreement with the experiment results. Finally, a comparative analysis was carried out with the new type of twisted tape in this paper and the different twisted tapes in the previous research. The new type of twisted tape proposed in this paper has higher Nusselt number and friction coefficient.

Numerical analysis of thermal-hydraulic performance of fully developed turbulent flow in heat exchanger tube with Half Moon inserts

This paper presents the numerical analysis of heat transfer characteristics of working fluid in a circular tube with inserted Half-Moon shape roughness. The Half-Moon (HM) shape roughness is specially used to upgrade the heat transfer rate in circular tubes along the length. In the present paper, we have a circular tube of length L = 1500 mm, Inner diameter Din = 30 mm and Outer diameter Do = 35 mm. The Half-Moon roughness varied with an interpolated number of holes in the range from 2 to 3 and pitch space between the HM roughness. In a circular tube, the working fluid is arranged for the Reynolds number in the category of 4000 to 32,000 and solved by the Renormalization model in Ansys fluent 14.0. In this study, we found that the heat transfer rate enhanced by a sufficient amount complained about the thermal performance of the circular tube.

Numerical Simulation for Comparative Thermo-Hydraulic Performance of Turbulent Flow in Tubes with Twisted Tape Inserts

This article focused on investigating numerically the effects of key design parameters including tape thickness, tube wall to tape edge clearance and three different perforated shapes cut (circular, triangular and pentagon) on the heat transfer coefficient enhancement in tube with twisted tape inserts. Water was considered as the working fluid under the turbulent flow of Reynolds number (Re) between 12,000 and 22,000, with constant wall temperature of the tubes. It is found that due to the swirling flow and efficient mixing of the fluid, all the tape inserts considered in this study produced better thermalhydraulic performance than the plain tube without an insert. At the same time, tape inserts with small tube wall to tape edge clearance and considerable thickness value is beneficial to reduce the friction factor, consequently enhancing the heat transfer rate. Through the thermal performance factor (TPF) evaluation, the twisted tape with perforated triangular shape cut has an advantage over the pentagon and circular shapes cut with increasing Re. A maximum TPF as high as 1.68 can be obtained for the twisted tape with perforated triangular shape cut under low turbulent Re.

The hydraulic resistance paradox in rapid narrow pipe waterflow

In this work we present experimental results of cross-sectional speed of water flow in narrow cylindrical metal tubes at high pressure gradients up to 1.1 GPacdotm−1. The measurement draws attention to the paradoxical behaviour of flowing water in internal diameters less than 250 upmum. At constant pressure gradient, its cross-section speed decreases with decreasing diameter in accordance with the classical hydrodynamic prediction for turbulent flow in rough cylindrical tube. However for very low diameters below 250 upmum, the cross-section speed rises again and reaches almost the maximum theoretical value of the outflow speed for the appropriate pressure without energy loss caused by contraction or hydraulic friction. Our contribution describes mainly experimental character of the new phenomenon and its motivation is to promptly provide the material for further study to the professional public.

Heat Transfer and Hydraulic Resistance in Turbulent Flow in Vertical Round Tubes at Supercritical Pressures—Part 1: Results From Numerical Simulations Using Algebraic Turbulent Viscosity Models

Abstract The review of numerical studies on supercritical pressure (SCP) coolants heat transfer and hydraulic resistance in turbulent flow in vertical round tubes based on Reynolds-averaged Navier–Stokes (RANS) equations and different models for turbulent viscosity is presented. The paper is the first part of the general analysis, the works based on using algebraic turbulence models of different complexity are considered in it. The main attention is paid to Petukhov–Medvetskaya and Popov et al. models. They were developed especially for simulating heat transfer in tubes of the coolants with significantly variable properties (droplet liquids, gases, and SCP fluids) under heating and cooling conditions. These predictions were verified on the entire reliable experimental database known at that time (the data for SCP H2O, CO2, N2, He, etc.). It is shown that in the case of turbulent flow in vertical round tubes, these models make it possible predicting heat transfer and hydraulic resistance characteristics of SCP flows that agree well with the existed reliable experimental data on normal and certain modes of deteriorated heat transfer, if significant influence of buoyancy and radical flow restructuring are absent. In addition, due to their relative simplicity and clearness, these methods made it possible recognizing certain important physical effects, which are useful for constructing advanced models of SCP-liquid thermohydraulics. For the more complicated cases than a flow in round vertical tubes, as well as for the case of rather strong buoyancy effect, more sophisticated prediction techniques must be applied.

Heat Transfer and Hydraulic Resistance in Turbulent Flow in Vertical Round Tubes at Supercritical Pressures—Part II: Results From Numerical Simulation With Differential Turbulent Viscosity Models

Abstract The review of numerical studies on the turbulent flow and heat transfer of supercritical pressure (SCP) coolants in heated vertical round tubes, which were conducted using different differential turbulent viscosity models, is presented. The results of predictions are compared with the experimental data on wall temperature and heat transfer rate. It is shown that most often the turbulent viscosity models only qualitatively (but not quantitatively) predict the deteriorated heat transfer effects, which appear due do buoyancy forces and thermal acceleration effects at strongly variable physical properties of a coolant. At the same time, the regimes of normal heat transfer are successfully reproduced by “standard” k–ε and RNG models with wall functions (WFs), as well as by two-layer models. The conclusion is made that none of the presently known turbulent viscosity models can be confidently recommended for predicting any flow regimes and heat transfer of SCP coolants. Strongly variable properties of SCP coolant stipulate more strict demands for validating mesh independence of the obtained results and for an accuracy of approximation of the tabulated values of the coolant properties. It was ascertained that using more and more numerous calculation codes and the results from their application requires certain caution and circumspection. Sometimes, the energy transport equations were erroneously written for temperature or temperature variance rather than for enthalpy. Crying discrepancy in the predictions of different authors conducted using the same CFD codes and turbulence models and possible reasons for such a discrepancy are not analyzed.

URANS of turbulent flow and heat transfer in divergent swirl tubes using the k-ω SST turbulence model with curvature correction

In the current work, the turbulent flow and heat transfer in divergent swirl cooling tubes is studied numerically by using the open source CFD code OpenFOAM. The k-ω SST turbulence model with curvature correction, i.e., the k-ω SSTCC model, is used to compute the strong turbulent swirling flow. The unsteady solver “rhoPimpleFoam” is adopted in the computation and a time-averaging operation is implemented for the pressure, velocity and temperature fields to cover the effect of large-scale, low-frequency deterministic fluctuations of the vortex breakdown phenomenon. To generate high-quality grids, different parts of the swirl tubes are meshed with different block structures in ICEM and the Arbitrarily Coupled Mesh Interface (ACMI) boundary condition is used to couple the neighboring segments with non-conformal grids. The current model is validated by comparing the numerical results of a swirl tube with constant diameter to experimental data and numerical results from literature. In this study we investigated the turbulent heat transfer enhancement in a divergent swirl tube that features two 180°-displaced tangential inlets and a divergence angle of 0.596°. Moreover, the influence of axially multiple tangential inlets is studied for the same divergence angle. The current work demonstrates that the computationally cheap k-ω SSTCC model can predict the flow and heat transfer performances in a swirl tube with reasonable accuracy. Additionally, it is found that the Nu number of the divergent swirl tube with 180°-displaced inlets (S2D), is about 3.57 times higher than the one in a smooth tube with axial flow, and it is slightly lower (by 13%) than that of constant-diameter swirl tube (S2C), while its decrement of pressure drop is notable (by40%) compared with S2C. Axially-displaced multiple inlets can result in a tremendous reduction of pressure drop for swirl tubes by over 90% at the cost of a limited decrease in the Nu number (by ˜10%) compared to the swirl tube S2D.

Computational fluid dynamics studies on the induction period of crude oil fouling in a heat exchanger tube

Fouling impairs the hydrodynamic and thermodynamic performance of crude oil pre-heat train exchangers in refineries. The very early stage of fouling, so-called induction period, containing inappreciable levels of fouling is poorly understood due to a lack of experimental data. To better understand this period and seek potential fouling mitigation strategies, a model is implemented in a computational fluid dynamics (CFD) code, which accounts for chemical reaction- and precipitation-driven fouling, turbulent flow, and ageing in a heat exchanger tube. Three-dimensional CFD simulations are carried out under various operating conditions to gauge the relative importance of the two fouling mechanisms, and provide direct predictions of deposition rates. The interaction between chemical and precipitation fouling processes is characterized using an “interference factor”, introduced to evaluate the extent of suppression of the chemical mechanism and enhancement of precipitation. The results provide insights into the interaction of key mechanisms during the induction period (up to three seconds flow time) of fouling in crude oil heat exchangers and possible strategies for prolonging this near nil-fouling period.

Structure of a turbulent bubbly flow and heat transfer in a vertical tube

The results of numerical simulation of the flow structure and heat transfer in a vertical polydispersed bubbly flow are presented. The mathematical model is based on the Euler approach taking into account the effect of bubbles on the mean characteristics and turbulence of the carrier phase. The polydispersed distribution of bubbles size in a two-phase flow is modeled by the method of delta approximation considering the process of bubble break-up and coalescence. Carrier phase (fluid) turbulence is predicted using the Reynolds stress transport model. The simulation results showed good agreement with the experimental data presented in the literature. The measured and predicted thermal-hydraulic parameter distribution indicates that in a turbulent bubbly flow, the wall friction increase is greater than heat transfer enhancement.

Prediction of the precessing vortex core in the Francis-99 draft tube under off-design conditions by using Liutex/Rortex method

The turbulent flow in the draft tube of a Francis turbine is very complicated while working under off-design conditions. Although the off-design conditions were widely studied, the vortex core line in the draft tube of a Francis turbine with splitter blades is not well understood, especially the vortex rope property. This letter presents a prediction of the behavior of the vortex rope in the draft tube of the Francis-99 turbine obtained by the computational fluid dynamics (CFD), where the Liutex/Rortex method, as the most recent vortex definition, is applied to analyze the periodical precession of the vortex rope in the draft tube cone. The advantage of this Liutex/Rortex method is shown by its enhanced ability to represent the vortex rope structurewith the vortex-core lines. Furthermore, since it seems to be very hard to define a sharp boundary surface for the whole vortex structure, it is advantageousfocusing only on the vortex core line,by which different vortex structures can be clearly differentiated. The evolution of the vortex core and the process of the vortex breakdown in the draft tube are revealed, which might help to comprehend the development of the turbulent flow in the draft tube.

Entropy Generation Minimization Analysis of Solar Salt, Sodium, and Lead–Bismuth Eutectic as High Temperature Heat Transfer Fluids

Abstract The amount of entropy generation in heat transfer devices impacts their operation economy and should therefore be minimized during the design phase. Entropy generation also depends on the individual thermophysical properties of the heat transfer fluid (HTF). An entropy generation minimization analysis of three different liquid coolants, namely, solar salt (SS), sodium, and lead-bismuth eutectic (LBE) is thus performed for fully turbulent flow in a circular tube under circumferentially uniform heat flux by considering a heat rate, inlet and outlet temperatures and heat flux densities typical of a concentrated solar tower plant. The Reynolds number is determined at which the proper combination of Nusselt number and friction factor minimizing the entropy generation, and consequently the exergy loss, is obtained as well as the best thermodynamically performing coolant fluid over the operating range of Reynolds numbers. Sodium can operate at 60% lower entropy generation than solar salt while providing a smaller wall-to-bulk temperature difference. Despite its high thermal conductivity, LBE performs similarly to solar salt. However, it can be advantageous compared to solar salt if operated at higher temperatures that cannot be achieved by the latter due its thermal stability limit.