Investigation of Heat Transfer in Serpentine Shaped Microchannel Using Al2O3/Water Nanofluid

An experimental investigation was conducted to explore the maximum heat transfer in a serpentine shaped microchannel by varying the hydraulic diameter, flow rates and with influence of Al₂O₃ nanofluid. Microconvection is an important area in heat transport phenomena. Surface area is one of the important factors in high heat transfer in a microchannel heat exchanger. In this study, serpentine shaped microchannels of hydraulic diameters 810, 830, 860, and 890 μm are analyzed for the optimizing the hydraulic diameter to get enhanced thermal performance of the microchannel. A copper material microchannel having length a of 70 mm is used. Flow rate also varied from 1 lpm (Litres per minute) to 3.5 lpm for optimization with nanofluid as a medium. From numerical study it is observed that as the hydraulic diameter decreases from 890 μm to 810 μm the pressure drop increases with a decrease in hydraulic diameter. Also as heat input to the microchannel increases from 5 watts to 70 watts. From analysis it is observed that the hydraulic diameter of the microchannel is a major factor in microchannel heat transfer which is dependent on flow rate of fluid in the microchannel. The results also show that suspended Al₂O₃ nanoparticles in fluids have enhanced heat transfer when compared to the base fluid.     

Experimental and Numerical Study of Turbulent Fluid Flow and Heat Transfer of Al2O3/Water Nanofluid in a Spiral-Coil Tube

Enhancement of heat transfer by nanofluids is reported by a large number of researchers. In this study, numerical and experimental investigation of heat transfer and flow characteristics of Al₂O₃/water nanofluid flowing in a spiral-coil tube is performed for various flow conditions. The spiral-coil tube is immersed horizontally in a hot water bath maintained at 60°C. Experiments are conducted in a turbulent flow regime using distilled water and nanofluid with 0.5%, 1%, and 1.5% particle volume concentrations. Also, a computational fluid dynamics methodology is used to simulate heat transfer and flow characteristics corresponding to the experimental measurements and for further flow conditions. Simulation results are compared with the experimental measurements, and 85% agreement between the results is observed. The results showed that convective heat transfer coefficient of nanofluid is enhanced up to 61% compared with that of the base fluid. Based on the experimental measurements, a new correlation is developed to predict convection heat transfer from nanofluids in spiral-coil tubes.     

Experimental Investigation of Al2O3/Water Nanofluid Through Equilateral Triangular Duct With Constant Wall Heat Flux in Laminar Flow

This paper experimentally investigates the heat transfer of an equilateral triangular duct by employing an Al₂O₃/water nanofluid in laminar flow and under constant heat flux conditions to improve the heat transfer performance of this type of duct. The Nusselt numbers were obtained for different nanoparticle concentrations of the nanofluid at various Peclet numbers. The results show that the experimental heat transfer coefficient of Al₂O₃/water nanofluid is higher than that of distillated water. Also, the experimental heat transfer coefficient of Al₂O₃/water nanofluid is higher than the theoretical one. The experimental results also indicate that the heat transfer enhancement increases with increases in the nanofluid volume concentration and Peclet number.     

Entropy Generation of Natural Convection Heat Transfer in a Square Cavity Using Al2O3–Water Nanofluid

Entropy generation of an Al₂O₃–water nanofluid due to heat transfer and fluid friction irreversibility has been investigated in a square cavity subject to different side‐wall temperatures using a nanofluid for natural convection flow. This study has been carried out for the pertinent parameters in the following ranges: Rayleigh number between 10⁴ and 10⁷ and volume fraction between 0 and 0.05. Based on the obtained dimensionless velocity and temperature values, the distributions of local entropy generation, average entropy generation, and average Bejan number are determined. The results are compared for a pure fluid and a nanofluid. It is totally found that the heat transfer, and entropy generation of the nanofluid is more than the pure fluid and minimum entropy generation and Nusselt number occur in the pure fluid at any Rayleigh number. Results depict that the addition of nanoparticles to the pure fluid has more effect on the entropy generation as the Rayleigh number goes up.     

Convective Heat Transfer and Fluid Dynamic Characteristics of SiO2 Ethylene Glycol/Water Nanofluid

Nanofluids comprised of silicon dioxide (SiO₂) nanoparticles suspended in a 60:40 (% by weight) ethylene glycol and water (EG/water) mixture were investigated for their heat transfer and fluid dynamic performance. First, the rheological properties of different volume percents of SiO₂ nanofluids were investigated at varying temperatures. The effect of particle diameter (20 nm, 50 nm, 100 nm) on the viscosity of the fluid was investigated. Subsequent experiments were performed to investigate the convective heat transfer enhancement of nanofluids in the turbulent regime by using the viscosity values measured. The experimental system was first tested with EG/water mixture to establish agreement with the Dittus-Boelter equation for Nusselt number and with Blasius equation for friction factor. The increase in heat transfer coefficient due to nanofluids for various volume concentrations has been presented. Pressure loss was observed to increase with nanoparticle volume concentration. It was observed that an increase in particle diameter increased the heat transfer coefficient. Typical percentage increases of heat transfer coefficient and pressure loss at fixed Reynolds number are presented.     

Heat Transfer in a Loop Heat Pipe using Diamond-H2O Nanofluid

The main aim of this study is to enhance the thermal performance of loop heat pipe (LHP) charged with nanofluid as the working fluid. Thus, experiments are conducted to investigate heat transfer characteristics of using diamond-H₂O nanofluid with nanoparticle mass concentration ranged from 0% to 3% in a LHP as a working medium for heat input range from 20 W to 60 W. The three-dimensional model, laminar flow and heat transfer governing equations are solved using the finite volume method. The simulations are carried out with three-dimensional model based on the characterization of the working fluid inside the LHP to give an insight into the heat transfer and fluid flow mechanism. The LHP performance is evaluated in terms of temperature distributions and total thermal resistance of LHP. It is inferred that the temperatures obtained at all points in evaporator side of LHP charged with diamond-H₂O nanofluid are lower and reach their steady state faster than LHP charged with pure water. At the constant heat input, test results showed the average decrease of 5.7%?10.8% at nanoparticle mass concentrations ranging from 0.5% to 3% in R_th of LHP as compared with pure water (0%).     

Fluid Flow Distribution and Heat Transfer in Plate-Fin Heat Exchangers

Single-phase and two-phase flow distribution in plate-fin heat exchangers and the influence of nonuniform fluid flow distribution on the thermal performance of such heat exchangers were experimentally investigated. The experimental results show that flow maldistribution can be a serious problem in plate-fin heat exchangers because of nonoptimized header configurations. The uneven distribution of two-phase flow in plate-fin heat exchangers is more pronounced than that of single-phase flow. It is shown that the uneven distributions result in a significant deterioration of the heat transfer performance. The relationship between the flow maldistribution characteristics and the resulting loss in heat exchanger effectiveness has been studied in this work. Certain improved header configurations with perforated plates were proposed in order to solve the maldistribution problem. It was found that the new header configurations could effectively improve the thermal performance of plate-fin heat exchangers. By changing the header configuration, the degree of flow and temperature nonuniformity in the plate-fin heat exchanger was reduced to 16.8% and 74.8%, respectively, under the main test condition.     

Nanofluid Flow and Heat Transfer in Boundary Layers: The Influence of Concentration Diffusion on Heat Transfer Enhancement

ABSTRACT

The present work uses a perturbation procedure to deduce the small perturbation differential equations for velocity, temperature, and the diffusion equation for nanoparticle volume concentration. Thermophysical variables are obtained from conventional means (e.g., mixture and field theory estimates) for nanofluids consisting of alumina nanoparticles dispersed in water (alumina–water nanofluid) and gold nanoparticles dispersed in water (gold–water nanofluid), and, in the case of gold–water nanofluid, molecular dynamics results are used to estimate such properties, including the transport coefficients. The very thin diffusion layer, at large Schmidt numbers, is found to have a great impact on the velocity and temperature profiles, owing to the transport property dependency and has a profound influence on surface conduction heat transfer rate enhancement and skin friction suppression for the case of nanofluid concentration withdrawal at the wall. In this case, the diffusional heat transfer rate is negligible, again, owing to the large Schmidt numbers encountered. Possible experiments directed at this interesting phenomenon are discussed.     

A Single-Component Nonhomogeneous Lattice Boltzmann Model for Natural Convection in Al2O3/Water Nanofluid

Natural convection heat transfer in Al₂O₃/water nanofluid is analyzed using the single-component nonhomogeneous lattice Boltzmann method (SCNHLBM). There exists a contradictory observation between the numerical and experimental works in the literature with respect to the heat transfer of nanofluids in natural convection. Nanofluid is treated as a single component with nonhomogeneous particle distribution introduced by a concentration transport equation of nanoparticles by considering the Brownian and thermophoretic diffusions. The average Nusselt number is found to deteriorate with increasing nanoparticle volume fraction; thus the trend of the experimental results is captured using SCNHLBM. Addition of Brownian and thermophoretic diffusion results in additional thermal diffusion and hence reduces the convective transport of heat. The contribution of Brownian and thermophoretic diffusions in heat transfer deterioration is revealed.     

Heat Transfer and Flow Resistance Characteristics of Helical Baffle Heat Exchangers with Twisted Oval Tube

To overcome the defect of the significant increase in pressure drop when the heat transfer performance of helical baffle heat exchanger is improved,a novel heli...     

Heat Transfer and Flow Characteristics in Rib-/Deflector-Roughened Cooling Channels with Various Configuration Parameters

The present paper deals with a detailed numerical investigation of the turbulent flow inside a stationary rib-/deflector–roughened cooling channel. Various downstream-shaped deflectors including sloping-board deflectors [Cases A1, A2], guide-shaped deflectors [Cases B1, B2], and drop-shaped deflectors [Cases C1, C2] and configuration parameters such as channel aspect ratio (AR = 0.5, 1 and 2), and rib-pitch-to-rib-height ratio (P/e = 5, 8, and 10) are investigated. The main objective is to design an appropriate deflector to improve the flow characteristics in the wake of the deflectors and guide the flow between two neighboring rib turbulators to enhance the heat transfer performance. A quasi-three-dimensional flow structure, supported by the stream tracer field in some planes, is established to improve and deepen the understanding as well as the analysis of the complex flow field in the rib-/deflector–roughened channels. In addition, the thermal performance corresponding to various rib-pitch-to-rib-height ratios and aspect ratios emphasizes the role of the configuration parameters in the heat transfer and flow resistance performance. The results demonstrate that the deflectors trip the boundary layer and blend the fluid flow, and that the sloping board deflectors contribute to enlarging the turbulence level of the whole cooling channel, more than in the wake region. It is found that Cases A1 and A2 provide the best heat transfer performance, while Case C1 presents the largest thermal enhancement factor Nu/Nu₀/(f/f₀)^1/3 at high Reynolds number. The wide-aspect-ratio channel with deflectors and large pitch-to–height ratio ribs exhibits much better heat transfer performance.     

单相逆流换热器温度分布及(火用)传递特性的研究

借鉴无相变逆流换热器对数平均温度的推导方法,导出了无相变换热器中流体及换热管内、外壁的温度分布,并基于热力学第一、第二定律及非平衡热力学理论,以某高压加热器过热段为例,分别求出了换热过程中蒸汽至管外壁和管内壁至给水的传(火用)系数,从(火用)传递的角度分析了无相变管壳式换热器的换热性能,为优化换热器结构提供了理论参考依据.     

Air-Side Flow and Heat Transfer in Compact Heat Exchangers: A Discussion of Enhancement Mechanisms

Abstract

The behavior of air flows in complex Heat exchanger passages is reviewed with a focus on the heat transfer effects of boundary-layer development, turbulence, spanwise and streamwise vortices, and wake management. Each of these flow features is discussed for the plain, wavy, and interrupted passages found in contemporary compact Heat exchanger designs. Results from the literature are used to help explain the role of these mechanisms in Heat transfer enhancement strategies.     

Fluid Flow and Heat Transfer in Plate-Fin and Tube Heat Exchangers in a Transitional Flow Regime

The unsteady behaviors of fluid flow and heat transfer in plain plate-fin and tube heat exchangers with a wide range of fin spacings from 2.06 mm to 16.48 mm and tube diameter 8.28 mm are studied by a large eddy simulation technique (LES). Velocity fluctuations and vortex sheddings induced by the tubes in the channel are modeled. The results found that the flow in passages of large spacings is quite different from that of small spacings. The flow is co-determined by two effects: the duct effect and the tube bank effect. The tube bank effect is more dominant with increasing fin spacings.     

Experimental Study on Laminar Forced Convection of Al2O3/Water and Multiwall Carbon Nanotubes/Water Nanofluid of Varied Particle Concentration with Helical Twisted Tape Inserts in Pipe Flow

In this study an experimental investigation has been carried out to analyze the laminar forced convection of Al₂O₃/water and multiwall carbon nanotubes (MWCNT)/water nanofluids through uniformly heated horizontal circular pipe with helical twisted tape inserts. Tests were conducted for varied range of nanoparticle volume concentration (0.15%, 0.45%, 0.60%, and 1%) and helical tape inserts of twist ratios of 1.5, 2.5, and 3. The heat transfer enhancement and the increase of friction factor of nanofluids with helical inserts are compared with that of pure water results with plain tube without inserts. The Nusselt number is found to increase with the increase in Peclet number and nanofluid concentration. The MWCNT/water nanofluids with helical screw tape inserts exhibits higher thermal performance compared to Al₂O₃/water nanofluid. The maximum thermal performance factor was found to be 1.79 and 1.99 for Al₂O₃/water and MWCNT/water nanofluids with helical twisted tape inserts, respectively. The pressure drop for Al₂O₃ nanofluid is found to be higher compared to the MWCNT nanofluid for all the twist ratio of helical screw tape inserts.     

Heat Transfer and Flow Characteristics Inside Droplet Formed on Water Surface

Abstract

Dynamics and heat transfer of a silicone oil droplet formed on a water surface are investigated. The silicone oil droplet is heated by the water in a constant temperature container. Temperature and velocity fields inside the droplet are simulated in line with the experimental conditions. The influence of the droplet volume on the flow and heat transfer characteristics is also incorporated in the analysis. The oil droplet pinning and its geometric features for different droplet volumes are examined. Temperature predictions are validated with a thermal camera data. It is found that temperature predictions agree well with the thermal camera data. The constant temperature heating of the water container wall gives rise to two counter rotating circulation cells inside the water, which in turn modifies temperature and flow fields in the water. The flow direction occurs from the droplet top region towards the droplet–water interface. The heated fluid in the region close to the droplet–water interface is carried by the flow current to the droplet sides giving rise to temperature increase in these regions. The values of the Bond number attains greater than unity. The Nusselt and the Bond numbers increase with the droplet volume.     

两种用于HCPV/T的水冷换热器传热特性的对比实验研究

本文对两种适用于高倍聚光发电供热（HCPV/T）系统的多槽道和微通道水冷换热器进行了实验研究。利用模拟热源模拟了HCPV/T系统中光伏电池工作时的热流密度,分别研究了流量、壁面温度和输入电压对两种换热器传热特性的影响,并利用传热学理论对两种换热器的特点进行分析,获得了两种换热器努赛尔数Nu与雷诺数Re的拟合经验公式。实验结果表明,微通道换热器在低流量下有较强的换热能力,但在高流量下,换热能力无法随流量增大继续提高;多槽道换热器在低流量下换热能力不佳,但在高流量下仍可随流量增大继续提高。     

Numerical Investigation of Nanofluid Flow and Heat Transfer Around a Calabash-Shaped Body

Numerical simulation on unsteady flow and heat transfer of alumina–water nanofluids around a calabash-shaped body was performed in the present study. Improved models of drag force and Brownian force were introduced. As the reaction time of the particle perturbation is short, fluctuation in vorticity is more intense than that in temperature, and many extreme values are found. The streamline is uplifted near the separation point due to the contribution of the particle inertia, which increases the recirculation zone of the quasi-steady vortex. Fewer particles enter the vortex near the waist portion from the separation region, and relatively more particles enter the recirculation region from the reattachment zone. The local streamline is straightened and flow heat transfer is enhanced. It is shown that the variation in the Nusselt number is strongly related to the critical points along the wall.     

Three-Dimensional Numerical Study on Thermal-Hydraulic Performance of Twisted Mini-Channel Using Al2O3-H2O Nanofluid

Abstract

Effects of design parameters of the twisted mini-channel, including twist pitch, width, height, and length, are investigated numerically at the Reynolds numbers between 300 and 1500. The Al₂O₃-H₂O nanofluid is used as working fluid, and it is simulated using the mixture two-phase model. The volume fraction of Al₂O₃ nanoparticles is 1, 2, 3, and 4%. The results show that decreasing of twist pitch and channel length leads to higher values of the Nusselt number. As the Reynolds number is increased, the influence of these factors enhances. The increase of the channel width and height decreases the convective thermal resistance and thus improves the Nusselt number along with a certain penalty in the pressure drops. For all models, by increasing the volume fraction of nanoparticles, the convective thermal resistance is reduced leading to the thermal performance improvement. Finally, two correlations are proposed for the Nusselt number and friction factor, which are useful as a tool to predict the heat transfer and pressure drop characteristics of the twisted mini-channel.     

SiO2纳米流体强化换热的实验和仿真研究

为研究纳米流体稳定性并增强换热机理,在乙二醇/去离子水基液中,采用原液化学生长法制备了不同质量浓度（1%,2%,3%,4%和5%）的氧化硅-乙二醇/水纳米流体,通过Zeta电位测量和透射扫描电镜实验表征纳米流体的稳定性。实验测量并研究了温度和质量浓度对纳米流体的导热系数和粘度的影响。依据实测结果,利用格子玻尔兹曼方法对圆管内纳米流体的流动与换热特性进行数值模拟研究。结果表明：二氧化硅颗粒在基液中具有良好的稳定性;纳米流体的导热系数随温度和质量浓度的提高而增大;纳米流体的加入可以显著提高基液的对流换热系数,当质量浓度为5%时对流换热系数的提高幅度可达到25.5%。