Numerical and experimental studies of heat and flow characteristics in a laminar pipe flow of nanofluid

Thermal performance of energy systems can be improved by adding metal or metal-oxide nanoparticles to a base fluid, thereby increasing heat-transfer efficiency. Laminar pipe flow of a Cu–water nanofluid was studied using discrete phase model numerical simulation and experimental methods. The forces including thermophoretic and Brownian forces were considered to solve the particles governing equation. A two-step method was employed in the preparation of the nanofluid. The influences of Reynolds number, fluid temperature, and particle volume fraction on the flow pressure drop and convective heat-transfer coefficient of the nanofluid have been studied. The results demonstrated that adding nanoparticles to a base fluid significantly enhanced convective heat transfer in a pipe and increased energy loss. The pressure drop increased with increasing Reynolds number. A critical nanoparticle volume fraction existed, beyond which the pressure drop changed from increasing to decreasing with increasing nanoparticle volume fraction. This is attributed to competition between slip of particles on the pipe wall and the effect of a drag force on the particles. The deposition efficiency of nanoparticle changing with the particle size and volume fraction also has been illustrated.     

Effects of curvature ratio on forced convection and entropy generation of nanofluid in helical coil using two-phase approach

Applying nanofluid and helical coils are two effective methods for thermal performance enhancement. Combination of these techniques could improve the energy efficiency of thermal equipment dramatically. In this study, a numerical analysis of nanofluid flowing in helical coil with constant wall temperature boundary condition was performed to evaluate nanofluid superiority over the base fluid. Forced convective heat transfer and entropy generation of aqueous Al₂O₃ nanofluid with temperature dependent properties were investigated. Eulerian two-phase mixture model was employed for nanofluid modeling and governing mass, momentum, energy, and volume fraction equations were solved using finite volume method. Simulations covered a range of nanoparticle volume fraction of 1–3%, Reynolds number from 200 to 2000, and curvature ratio of 0.05–0.2. In order to evaluate the heat transfer performance, a parameter referred as thermo-hydrodynamic performance index was applied. Also, entropy generation analysis was performed to examine the efficiency of the helical coil and nanofluid. The results demonstrate that performance index enhances by decreasing the Reynolds number and the increasing nanoparticle concentration. The best thermo-hydrodynamic performance can be obtained at low Reynolds number, high nanoparticle volume fraction, and large curvature ratio. Increasing curvature ratio decreases the ratio of local entropy generation by nanofluid to the base fluid. So, utilization of water based Al₂O₃ nanofluid in higher curvature ratio is more efficient from irreversibility point of view.     

Entropy generation in a heat exchanger working with a biological nanofluid considering heterogeneous particle distribution

Entropy generation rates considering particle migration are evaluated for a biologically produced nanofluid flow in a mini double-pipe heat exchanger. The nanofluid is used in tube side and hot water flows in annulus side. Silver nanoparticles synthesized through plant extract method from green tea leaves are utilized. Particle migration causes non-uniform concentration distribution, and non-uniformity intensifies by increase in Reynolds number and concentration. The results indicate that at high concentrations and Reynolds numbers, particle migration can have a great effect on entropy generation rates. For water inlet temperature of 308 K, the contribution of friction in nanofluid entropy generation is much more than that of heat transfer. However, as the water inlet temperature increases to 360 K, the heat transfer contribution increases such that at low Reynolds numbers, the thermal contribution exceeds the frictional one. For total heat exchanger, Bejan number is smaller than 0.2 at water inlet temperature of 308 K, while Bejan number has a large value at water inlet temperature of 360 K. Furthermore, entropy generation at the wall has an insignificant contribution, such that for Re = 1000 and φ_m = 1%, the total entropy generation rates for the nanofluid, wall, and water are 0.098810, 0.000133, and 0.041851 W/K, respectively.     

What dominates heat transfer performance of hybrid nanofluid in single pass shell and tube heat exchanger?

Influence of nanoparticle volume concentration and proportion on heat transfer performance (HTP) of Al₂O₃ – Cu/water hybrid nanofluid in a single pass shell and tube heat exchanger is analyzed. Multiphase mixture model is adopted to model the flow. Three-dimensional governing equations and associated boundary conditions are solved using finite volume method. The numerical results are validated with the experimental results. Results indicate that optimized nanoparticle volume concentration and proportion dominate HTP of hybrid nanofluid. Heat transfer coefficient and Nusselt number are monotonic increase functions of nanoparticle volume concentration and proportion. The percentage increase in heat transfer coefficient of hybrid nanofluid is 139% than water and 25% than Cu/water nanofluid. At higher Reynolds number, the increment in Number of Transfer Units (NTU) between water and hybrid nanofluid is close to 75%. Maximum enhancement in Nusselt number for hybrid nanofluid exceeds 90% when compared to nanofluid (Al₂O₃/Water nanofluid). Consequently, highest heat transfer performance is attained for hybrid nanofluid systems. Effectiveness of heat exchanger increases almost to 124% when hybrid nanofluid is employed. We show that it is higher than water as well (conventional coolant). Results are expected to be helpful in further industrial-scale deployment of nanofluids, which is an area that is currently relevant for ongoing academia-industry partnership efforts worldwide.     

Experimental and numerical investigation on forced convection heat transfer and pressure drop in helically coiled pipes using TiO2/water nanofluid

In the present article, forced convection heat transfer and pressure drop in helically coiled pipes using TiO₂/water nanofluid as working fluid were investigated experimentally and numerically. The aim is to investigate and provide additional insight about the effects of physical and geometrical properties on heat transfer augmentation and pressure drop in helically coiled tubes. The experiments were conducted in the range of Reynolds number from 3000 to 18,000 and in the nanoparticle concentrations of 0.1, 0.2, and 0.5% for five different curvature ratios. In numerical simulations the thermophysical properties of the working fluid were assumed to be a function of nanofluid temperature and concentration. For turbulent regime the standard k − ε model was used to simulate the turbulent flow characteristics. The numerical results were in good agreement with the experimental data. The results showed that utilization of nanofluid instead of distilled water leads to an enhancement in the Nusselt number up to 30%. Also, four formulas were introduced to obtain the average Nusselt number and friction factor in helically coiled tubes under constant wall temperature condition for both laminar and turbulent flow regimes.     

Experimental hydrothermal characteristics of minichannel heat sink using various types of hybrid nanofluids

The hydrothermal characteristics of minichannel heat sink are analyzed experimentally by using deionized (DI) water based different nanoparticles mixture dispersed hybrid nanofluids. Al₂O₃, MgO, SiC, AlN, MWCNT and Cu nanoparticles are considered for this study. Different nanoparticles combinations (oxide-oxide, oxide-carbide, oxide-nitride, oxide-carbon nanotube and oxide-metal) in 50/50 vol ratio with base fluid (DI water) have been taken as coolants for volume concentration of 0.01%. Effects of volume flow rate (0.1–0.5LPM), fluid inlet temperature (20–40 °C) and Reynolds number (50–500) are studied for heat flux of 50 W/cm². Convective heat transfer coefficient and pressure drop are increased by about 42.24% and 22% for Al₂O₃ + MWCNT hybrid nanofluid. The maximum reduction of 21.36% in thermal resistance is obtained for Al₂O₃ + MWCNT hybrid nanofluid in comparison to DI water. Heat transfer effectiveness and figure of merit are above one for all the hybrid nanofluids which conclude that hybrid nanofluid is better option for electronics cooling over DI water. Al₂O₃ + MWCNT hybrid nanofluid is better in terms of heat transfer effectiveness; whereas, Al₂O₃ + AlN hybrid nanofluid (oxide-nitrite mixture) has maximum heat transfer coefficient to pressure drop ratio and coefficient of performance.     

Numerical study on heat transfer and entropy generation of developing laminar nanofluid flow in helical tube using two-phase mixture model

Nanofluids and helical tubes are among the best methods for heat transfer enhancement. In the present study, laminar, developing nanofluid flow in helical tube at constant wall temperature is investigated. The numerical simulation of Al₂O₃-water nanofluid with temperature dependent properties is performed using the two-phase mixture model by control volume method in order to study convective heat transfer and entropy generation. The numerical results is compared with three test cases including nanofluid forced convection in straight tube, velocity profile in curved tube and Nusselt number in helical tubes that good agreement for all cases is observed. Heat transfer coefficient in developing region inside a straight tube using mixture model shows a better prediction compared to the homogenous model. The effect of Reynolds number and nanoparticle volume fraction on flow and temperature fields, local and overall heat transfer coefficient, local entropy generation due to viscous dissipation and heat transfer, and the Bejan number is discussed in detail and compared with the base fluid. The results show that the nanofluid and the base fluid have almost the same axial velocity profile, but their temperature profile has significant difference in developing and fully developed region. Entropy generation ratio by nanofluid to the base fluid in each axial location along the coil length showed that the entropy generation is reduced by using nanofluid in at most length of the helical tube. Also, better heat transfer enhancement and entropy generation reduction can be achieved at low Reynolds number.     

Effect of moving walls on heat transfer and entropy generation in a nanofluid-filled enclosure

In this work, a numerical investigation of mixed convection has been carried out in a two-sided lid-driven enclosure filled with copper–water nanofluid. Three different cases have been discussed depending on the direction of moving vertical walls to analyze the behavior of fluid flow and heat transfer in nanofluid. The buoyancy effects are incorporated using two discrete heat sources placed on the bottom wall maintaining a fixed distance from both the side walls. The stationary part of the bottom wall is kept insulated while other walls are maintained at constant low temperature. A two-dimensional computational visualization technique has been employed to demonstrate the main findings of the presented work. The effect of higher nanoparticle volume fraction (up to 20%) with variations of Reynolds number and Richardson number is studied to find the rate of heat transfer. The results are presented using streamlines, isotherms, and energy flux vectors. The thermodynamic optimization of the system is analyzed by using Nusselt number and entropy generation.     

Numerical comparison for thermo-hydraulic performance of pin fin heat sink with micro channel pin fin heat sink

Cooling of miniature size electronic components has become a challenge for designer in the development of integrated circuits. Micro pin fin heat sink and Micro channel pin fin heat sink are thermal management techniques for effective cooling. The paper presents comparison of fluid flow and heat transfer characteristics for micro pin fin heat sink and micro channel pin fin heat sink with unfinned micro channel heat sink. A three-dimensional heat sink with water as coolant subjected to constant heat flux 10 W/cm², for Reynolds number ranging between 100 and 900 was considered for the study. Extended surfaces of different shapes namely, square and circular with staggered arrangement was considered for both micro pin fin heat sink and micro channel pin fin heat sink. Two non-dimensional parameters namely Nusselt number and thermal performance index were employed to access the performance of heat sink. Results indicate that the microchannel pin fin heat sink has highest nusselt number and friction factor over the whole Reynolds number range. Results also revealed that formation of secondary vortices enhances heat transfer in micro channel heat sink with square pin fin compared to micro channel heat sink with circular pin fin. However, pin fin heat sink has better thermal performance index compared to Micro channel pin fin heat sink and is more preferable when heat dissipation is compared with pressure drop penalty. The Governing equations for fluid and solid domain were solved using FLUENT to study flow and heat transfer characteristics.     

Investigation of turbulent heat transfer and nanofluid flow in a double pipe heat exchanger

In present study, heat transfer and turbulent flow of water/alumina nanofluid in a parallel as well as counter flow double pipe heat exchanger have been investigated. The governing equations have been solved using an in-house FORTRAN code, based on finite volume method. Single-phase and standard k-ε models have been used for nanofluid and turbulent modeling, respectively. The internal fluid has been considered as hot fluid (nanofluid) and the external fluid, cold fluid (base fluid). The effects of nanoparticles volume fraction, flow direction and Reynolds number on base fluid, nanofluid and wall temperatures, thermal efficiency, Nusselt number and convection heat transfer coefficient have been studied. The results indicated that increasing the nanoparticles volume fraction or Reynolds number causes enhancement of Nusselt number and convection heat transfer coefficient. Maximum rate of average Nusselt number and thermal efficiency enhancement are 32.7% and 30%, respectively. Also, by nanoparticles volume fraction increment, the outlet temperature of fluid and wall temperature increase. Study the minimum temperature in the solid wall of heat exchangers, it can be observed that the minimum temperature in counter flow has significantly reduced, compared to parallel flow. However, by increasing Reynolds number, the slope of thermal efficiency enhancement of heat exchanger gradually tends to a constant amount. This behavior is more obvious in parallel flow heat exchangers. Therefore, using of counter flow heat exchangers is recommended in higher Reynolds numbers.     

Numerical investigation of heat transfer enhancement in a square ventilated cavity with discrete heat sources using nanofluid

A numerical study of a laminar mixed convection problem in a ventilated square cavity partially heated from bellow is carried out. The fluid in the cavity is a water-based nanofluid containing Cu nanoparticles. The effects of monitoring parameters, namely, Richardson number, Reynolds number, and solid volume fraction on the streamline and isotherm contours as well as average Nusselt number along the two heat sources are analyzed. The computation is performed for Richardson number ranging from 0.1 to 10, Reynolds number from 10 to 500, and the solid volume fraction from 0 to 0.1. The results show that by adding nanoparticles to the base fluid and increasing both Reynolds and Richardson numbers the heat transfer rate is enhanced. It is also found, regardless of the Richardson and Reynolds numbers, and the volume fraction of nanoparticles, the highest heat transfer enhancement occurs at the left heat source surface.     

Heat transfer analysis of fin-and-tube heat exchangers with flat and louvered fin geometries

This paper analyzes the fluid flow and heat exchange on the air side of a multi-row fin-and-tube heat exchanger. A comparison is given between fin-and-tube heat exchanger characteristics with flat and louvered fins in a wider range of operating conditions defined by Reynolds number (based on fin spacing and air frontal velocities). The detailed representation of calculated data for the louvered heat exchanger shows significantly better heat transfer characteristics and a slightly higher pressure drop. The CFD procedure was validated by comparing the numerical simulation results with the experimental results showing the minimal average Nusselt number deviation and an almost perfectly corresponding pressure drop.     

Heat transfer and pressure drop characteristics of nanofluids in a plate heat exchanger

In this paper, the heat transfer characteristics and pressure drop of the ZnO and Al2O3 nanofluids in a plate heat exchanger were studied. The experimental conditions were 100-500 Reynolds number and the respective volumetric flow rates. The working temperature of the heat exchanger was within 20-40 degrees C. The measured thermophysical properties, such as thermal conductivity and kinematic viscosity, were applied to the calculation of the convective heat transfer coefficient of the plate heat exchanger employing the ZnO and Al2O3 nanofluids made through a two-step method. According to the Reynolds number, the overall heat transfer coefficient for 6 vol% Al2O3 increased to 30% because at the given viscosity and density of the nanofluids, they did not have the same flow rates. At a given volumetric flow rate, however, the performance did not improve. After the nanofluids were placed in the plate heat exchanger, the experimental results pertaining to nanofluid efficiency seemed inauspicious.     

多壁碳纳米管水基纳米流体的对流换热特性

实验研究了纳米粉体浓度、雷诺数Re和热流密度对多壁碳纳米管水基纳米流体(MWNTs/H2O)对流换热性能的影响。纳米粉体浓度分别为0.05 g/L、0.1 g/L、0.2 g/L和0.4 g/L,雷诺数Re为500~900,热流密度为10~20 k W/m2。结果表明:1)纳米流体对流换热系数随着纳米粉体浓度、Re、热流密度的增加而增加。如在Re为631且纳米粉体浓度为0.4 g/L时,纳米流体对流换热系数比基液增大了17.6%;2)纳米流体对流换热系数的提高率明显大于对应的导热系数提高率,当纳米粉体浓度为0.05g/L时,其对流换热系数和导热系数的提高率分别为7.4%和0.15%;3)在Eubank-Proctor方程的基础上,建立了适合于低Re条件下的混和对流换热的实验关联式。     

基于Al2O3纳米流体的槽式太阳能热发电集热器传热建模及性能分析

基于计算流体动力学中的有限体积法,研究了Al₂O₃/Syltherm800导热油纳米流体作为传热介质时槽式太阳能热发电集热器的性能,建立了真空管集热器的三维模型,进行了光学模拟和传热数值模拟,并通过实验进行了验证。在非均匀热流密度分布的情况下,研究了进口温度、进口流速等运行参数对采用纳米流体的槽式集热器传热性能的影响规律。结果表明:随着Al₂O₃体积分数的增加,槽式集热器的换热性能及热效率均有所提高;进口温度、进口流速等运行参数对集热器的传热性能影响很大,随着进口温度的上升和进口流速的减小,纳米流体对传热性能的影响程度逐渐增大。     

Investigation of natural convection heat transfer performance of the QFN-PCB electronic module by using nanofluid for power electronics cooling applications

Natural convection heat transfer of Cu-water nanofluid in a 3?×?3 array of a typical isothermal quad flat non-lead (QFN) embedded printed circuit board (PCB) module which can be bounded with respect to the sealed non-Darcy porous enclosure is numerically investigated. The Darcy-Brinkman-Forchheimer model is adopted to model the fluid flow in the porous medium under the presence of an external magnetic field. The transport governing equations are solved by the finite volume method based on the SIMPLE algorithm and the power law scheme. Main efforts focus on the effects of the parameters such as a nanoparticle volume fraction, Hartmann number, Darcy number and the enclosure side aspect ratio on the fluid flow and heat transfer characteristics inside the enclosure. The obtained results indicate that the influence of nanofluid of the overall Nusselt number increases with increasing the Darcy number, but decreases with increasing the Hartmann number. The overall Nusselt number attains its maximum value in the range of enclosure side aspect ratio from 1.5 to 3 with respect to the Darcy number. Moreover, the proposed correlations improve the efficiency, reliability and stability of the electronic device encountered in various applications in electronic industries.     

MHD convective heat transfer of nanofluids through a flexible tube with buoyancy: A study of nano-particle shape effects

This paper presents an analytical study of magnetohydrodynamics and convective heat transfer of nanofluids synthesized by three different shaped (brick, platelet and cylinder) silver (Ag) nanoparticles in water. A two-phase nanoscale formulation is adopted which is more appropriate for biophysical systems. The flow is induced by metachronal beating of cilia and the flow geometry is considered as a cylindrical tube. The analysis is carried out under the low Reynolds number and long wavelength approximations and the fluid and cilia dynamics is of the creeping type. A Lorentzian magnetic body force model is employed and magnetic induction effects are neglected. Solutions to the transformed boundary value problem are obtained via numerical integration. The influence of cilia length parameter, Hartmann (magnetic) number, heat absorption parameter, Grashof number (free convection), solid nanoparticle volume fraction, and cilia eccentricity parameter on the flow and heat transfer characteristics (including effective thermal conductivity of the nanofluid) are examined in detail. Furthermore a comparative study for different nanoparticle geometries (i.e. bricks, platelets and cylinders) is conducted. The computations show that pressure increases with enhancing the heat absorption, buoyancy force (i.e. Grashof number) and nanoparticle fraction however it reduces with increasing the magnetic field. The computations also reveal that pressure enhancement is a maximum for the platelet nano-particle case compared with the brick and cylinder nanoparticle cases. Furthermore the quantity of trapped streamlines for cylinder type nanoparticles exceeds substantially that computed for brick and platelet nanoparticles, whereas the bolus magnitude (trapped zone) for brick nanoparticles is demonstrably greater than that obtained for cylinder and platelet nanoparticles. The present model is applicable in biological and biomimetic transport phenomena exploiting magnetic nanofluids and ciliated inner tube surfaces.     

Conjugated Effect of Joule Heating and Magnetohydrodynamic on Laminar Convective Heat Transfer of Nanofluids Inside a Concentric Annulus in the Presence of Slip Condition

In the current study, the conjugated effect of Joule heating and magnetohydrodynamics (MHD) on the forced convective heat transfer of fully developed laminar nanofluid flows inside annular pipes, under the influence of MHD field, has been investigated. The temperature and nanoparticle distributions at both the inner and outer walls are assumed to vary in the direction of the fluid. Furthermore, owing to the nanoparticle migrations in the fluid, a slip condition becomes far more important than the no-slip condition of the fluid–solid interface, which appropriately represents the non-equilibrium region near the interface. The governing equations—obtained by employing the Buongiorno’s model for nanofluid in cylindrical coordinates—are converted into two-point ordinary boundary value differential equations and solved numerically. The effects of various controlling parameters on the flow characteristics, the average Nusselt number and the average Sherwood number have been assessed in detail. Additionally, the effect of the inner to outer diameter ratio on the heat and mass transfer rate has been studied. The results obtained indicate that, in the presence of a magnetic field when the fluid is electrically conductive, heat transfer will be reduced significantly due to the influences of Joule heating, while the average mass transfer rate experiences an opposite trend. Moreover, the increase in the slip velocity on both the walls causes the average heat transfer to rise and the average mass transfer to decrease.     

超临界二氧化碳微细管内冷却换热研究

对超临界二氧化碳在微细竖直圆管内冷却条件下的对流换热进行了数值模拟研究,分析了不同管径、进口雷诺数及不同的热流率对超临界二氧化碳对流换热的影响,考察管内局部流体温度、湍动能、湍流雷诺数的变化。湍流模型采用低雷诺数YS模型。研究表明,在临界温度区域比较大的截面,超临界二氧化碳局部传热系数达到最大值,同时管内传热受湍流雷诺数影响较大。     

A numerical study on heat transfer of the nanofluid flow inside helical tube through the two-phase method

In this study, a new numerical investigation was carried out to study the heat transfer characteristics of nanofluid flow inside a copper helical tube under constant heat flux. A nanofluid with different particle weight concentrations of 0.5%, 1.0%, and 2.0% was used. The effects of different parameters such as Reynolds number, nanofluid particle concentration, and constant heat fluxes (1500 and 3800?W/m²) on heat transfer coefficient were studied. For validation, Nusselt number and convection heat transfer coefficient obtained from the numerical model was compared with the experimental results. Also, to verify the accuracy of the method, grid independency was studied for each heat flux. The observations showed that the heat transfer coefficient increased by using nanofluid instead of base fluid. In addition, the convection heat transfer coefficient performance improved by increasing the nanoparticles’ concentration. The results from the numerical simulation compared with the experimental data showed that this new numerical method has high accuracy and could correctly predict the heat transfer behavior of nanofluids with different weight particle concentrations under constant heat flux.