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%。     

Numerical Study of Nanofluid Convective Heat Transfer in Sinusoidal Tubes

ABSTRACT

The waviness of tube wall and adding nanoparticles to fluid as two passive enhanced heat-transfer techniques are dully accepted; however, the combined effect of their simultaneous usage has not been dealt with, yet. Therefore in the present study, the convective heat transfer of nanofluid laminar flow inside straight tube and sinusoidal tubes under constant heat flux boundary condition was documented. The nanofluid used in this study was Al₂O₃/water with volume fractions from 0 to 4%. The effects of Reynolds number, volume fractions of nanoparticles, and the geometry of sinusoidal tubes upon the heat-transfer coefficient were investigated. The results showed that using sinusoidal tubes enhances heat-transfer coefficients. Also, it was observed that increasing Reynolds number leads to higher heat-transfer coefficients in the convergent section. Moreover, it was observed that increasing the sinusoidal wave amplitude augments the convective heat-transfer coefficients; however, the increase in Nusselt number was slight. Furthermore, adding nanoparticles enhances heat transfer especially in large wave amplitude sinusoidal tubes.     

Heat Transfer Performance of Heat Pipe Radiator with SiO2/Water Nanofluids

The effect of nanofluids on thermal performance of the miniature heat pipe radiator which was assembled by two heat pipes containing 0.6 vol.% SiO₂/water nanofluids and 30 pieces of rectangular aluminum fins was investigated experimentally and theoretically. The wall temperatures of the miniature heat pipe and fin surface temperatures were measured. Results showed that the utilization of SiO₂/water nanofluids as a working fluid in the heat pipe enhanced the heat performance by reducing wall temperature differences. Compared with Deionized water (DI water), the thermal resistance of the miniature heat pipe with SiO₂/water nanofluids decreased by about 23% to 40%. Furthermore, the theoretical calculation on a basis of one dimension found that the fin heat dissipation in the miniature heat pipe radiator charged SiO₂/water nanofluids was about 1.17 times of that of the DI water radiator.     

Heat Transfer and Flow Characteristics of Al2O3/Water Nanofluid in Various Heat Exchangers: Experiments on Counter Flow

Abstract

On account of nanofluids influence on heat exchangers (HEs), a vigorous discussion can be made to concurrently contrast HEs to one another under the same conditions to detect maximum efficacy. Based on an extensive experimental study, this research is established to examine the effect of nanofluids on the performance of heterogeneous HEs with the same heat transfer surface area considering counter flow arrangement. A double pipe HE, a shell and tube HE and a plate HE are intended to accomplish the experiments. The experiments are executed under turbulent flow conditions using distilled water and Al₂O₃/water nanofluid with 0.2, 0.5, and 1% particle volume concentrations. From the results shown in the article, the double pipe HE revealed the best outcome for the heat transfer coefficient with a maximum enhancement of 60% while a maximum enhancement in the heat transfer coefficient of 11% was reported for the plate HE. Utilizing a nanofluid represented the lowest penalty in the pressure drop with a maximum enhancement of 27% for the plate HE while the highest penalty in the pressure drop with a maximum enhancement of 85% was observed in the double pipe and shell and tube HEs.     

Effects of Surfactant on Flow Boiling Heat Transfer of Ethylene Glycol/Water Mixtures in a Minitube

This study investigated effects of surfactant sodium dodecylbenzene sulfonate (SDBS) on flow boiling heat transfer of ethylene glycol/water mixtures in a vertical minitube. Experiments were performed using solutions containing 300 ppm by weight of surfactant and the results were compared with those of pure mixture. Local heat transfer coefficient was measured and found to be dependent on the mass quality. Although the surfactant intensifies the vaporization process, it doesn't necessarily enhance the heat transfer coefficient. Heat transfer coefficients were compared at two different mass fluxes, and the results might be explained based on the local flow pattern and the heat transfer mechanism. After a critical quality, higher quality tends to deteriorate heat transfer due to intermittent dryout, and therefore adding a surfactant to generate more vapor may be of negative effect on the flow boiling heat transfer in a minitube, which is contrary to the experience of enhancing nucleate pool boiling heat transfer with trace surfactants.     

Numerical Investigation of Nanofluid Laminar Convective Heat Transfer through a Circular Tube

Laminar-flow convective heat transfer of nanofluid in a circular tube with constant wall temperature boundary condition is investigated numerically. A dispersion model is used to account for the presence of nanoparticles. Numerical predictions are in agreement with experimental results obtained in our laboratory for different particles in different sizes. Results clearly show that addition of nanoparticles to base liquid produces considerable enhancement of heat transfer. Heat transfer coefficients increase with nanoparticle concentration. Decreasing nanoparticles size at a specific concentration increases heat transfer coefficients.     

Convective Heat Transfer in a Vertical Rectangular Duct Filled with a Nanofluid

An analysis is performed to study natural convective heat transfer in a vertical rectangular duct filled with a nanofluid. One of the vertical walls of the duct is cooled by a constant temperature, while the other wall is heated by a constant temperature. The other two sides of the duct are thermally insulated. The transport equations for a Newtonian fluid are solved numerically with a finite volume method of second‐order accuracy. The influence of pertinent parameters such as Grashof number, Brinkman number, aspect ratio and solid volume fraction on the heat transfer characteristics of natural convection is studied. Results for the volumetric flow rate and skin friction for Copper and Diamond nanoparticles are also drawn. The Nusselt number for various types of nanoparticle such as silver, copper, diamond and titanium oxide are also tabulated. The results indicate that inclusion of nanoparticles into pure water improves its heat transfer performance; however, there is an optimum solid volume fraction which maximizes the heat transfer rate.     

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.     

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.     

缸内对流换热与气体流动的计算分析

摘要本文将内燃机燃烧室简化成轴对称的二维空间,将计算缸内流动的二维模型与边界层模型相结合,分析了缸内气体的迁移特性与对流换热.文中介绍了在内燃机工作过程中缸内气体边界层的分布与变化、边界层对对流换热的影响,给出了对流挟热系数沿燃烧室表面的分布与变化.与实测结果的比较表明,本文的模型具有较高的精度.     

Heat Transfer and Pressure Drop Characteristics of Nanofluid Flows Inside Corrugated Tubes

An experimental investigation is carried out to study the heat transfer and pressure drop characteristics of multiwalled carbon nanotubes (MWCNTs)/heat transfer oil nanofluid flows inside horizontal corrugated tubes under uniform wall temperature condition. To provide the applied nanafluids, MWCNTs are dispersed in heat transfer oil with mass concentrations of 0.05, 0.1, and 0.2 wt%. The Reynolds number varies between 100 and 4,000. Three tubes with hydraulic diameters of 11.9, 13.2, and 15.5 mm are applied as the test section in the experimental setup. Tubes are corrugated four times on the cross section; that is, there are four different helices around the tube. Depths of the corrugations are chosen as 0.9, 1.1, and 1.3 mm, and pitch of corrugation is 14 mm. The acquired data confirm the increase of heat transfer rate as a result of utilizing nanofluids in comparison with the base fluid flow. However, corrugating the tubes decreases the heat transfer rate at low Reynolds numbers. The highest increase in heat transfer rate is observed for the Reynolds numbers for which the smooth tube is in the transition regime and the corrugated tube reaches the turbulent flow, that is, Reynolds number in the range of 1,000 to 3,000. Rough correlations are proposed to predict the Nusselt number and friction factor.     

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%).     

Rheology and Convective Heat Transfer Properties of Borate Cross-Linked Nitrogen Foam Fracturing Fluid

This article considers the rheology and convective heat transfer characteristics of borate cross-linked guar and borate cross-linked foam fracturing fluid by conducting experiments on a large-scale test loop at 30 MPa. The results show that severe chemical degradation of borate cross-linked guar occurred as temperature increased. When temperature was higher than the threshold value, the cross-linker was almost disabled and the guar was no longer cross-linked. It was also found that the viscosity of borate-cross-linked foam fracturing fluid was proportional to the increment of foam quality, which was inversely proportional to the increase of temperature. The influence of fluid behavior index on the velocity gradient of non-Newtonian fluid at the wall is tremendous, resulting in a negative temperature-dependent convective heat transfer coefficient. Moreover, to calculate the convective heat transfer coefficients of these two fluids and also to match the experimental data, a multiplier should be utilized to account for the shear-dependent thermal conductivity and heat transfer enhancement. The contribution of shear-induced bubble-scale microconvection was significant for the heat transfer enhancement of foam fluid. The correlations between the viscosity and the convective heat transfer coefficient of borate cross-linked guar and borate cross-linked foam fracturing fluid deduced from the experimental data were also obtained.     

Investigation of Electrohydrodynamically-Enhanced Convective Heat and Mass Transfer from Water Surface

Enhancement of forced flow evaporation rate by applying electric field (corona wind) has been experimentally evaluated in this study. Corona wind produced by a fine wire electrode which was charged with positive high DC voltage impinges to water surface and leads to evaporation enhancement by disturbing the saturated air layer over the water surface. The study was focused on the effects of corona wind velocity, electrode spacing and air flow velocity on the level of evaporation enhancement. Two sets of experiments, i.e., with and without electric field, have been conducted. Data obtained from the first experiment were used as reference for evaluation of evaporation enhancement at the presence of electric field. Applied voltages ranged from corona threshold voltage to spark over voltage at 1 kV increments. The results showed that corona wind has great enhancement effect on the water evaporation rate, but its effectiveness gradually diminishes by increasing air flow velocity. Maximum enhancement ratios were 7.3 and 3.6 for air velocities of 0.125 and 1.75 m/s, respectively. Finally two empirical correlations were obtained for prediction of electrohydrodynamic evaporation enhancement and its coefficient of thermal performance.     

Convective Heat Transfer and Resistance Characteristics of Nanofluids with Cylindrical Particles

Numerical research on convective heat transfer and resistance characteristics of TiO₂/water nanofluids with cylindrical particles in laminar channel flow are performed by solving the governing equations of fluid flow with the additional term of cylindrical nanoparticles, the equation of probability density functions for cylindrical nanoparticle orientation, and general dynamics equation for nanoparticle volume concentration. The nonuniformity of nanoparticle distribution is considered and the effects of both particle volume concentration and Reynolds number on friction factor and local Nusselt number are mainly analyzed. The results show that the friction factor of nanofluid flow increases with an increase in particle volume concentration. And the friction factor decreases with increasing Reynolds number and is not dependent on the volume concentration at high Reynolds numbers. The Nusselt number declines when the Reynolds number decreases, and finally approaches an asymptotic value after the Reynolds number falls to a certain value. The Nusselt number is higher in the entrance region than at the downstream locations, and will become steady at somewhere downstream when the flow is thermally and hydraulically developed.     

A Comparative Analysis of Single and Two-Phase Models of Turbulent Convective Heat Transfer in a Tube for TiO2 Nanofluid with CFD

In this article, turbulent forced convection flow of dilute water/TiO₂ nanofluid with a particle diameter equal to 30 nm in a horizontal circular tube exposed to convection with saturated steam at the wall, is numerically analyzed. Two different approaches are taken into consideration: single and two-phase mixture models. It is comprehended that the convective heat transfer coefficient increases with the particle volume concentration and Reynolds number. On the other hand, the mixture model was in better agreement with experimental data than the single phase approach.     

Heat Transfer and Thermal Characteristics Effects on Moving Plate Impinging from Cu-Water Nanofluid Jet

The present article is focused on modelling of flow and heat transfer behaviour of Cu-water nanofluid in a confined slot jet impingement on hot moving plate. Different parameters such as various moving plate velocities, nanoparticles at various concentrations, variation in turbulent Reynolds number and jet nozzle to plate distance have been considered to study the flow field and convective heat transfer performance of the system. Results of distribution of local and average Nusselt number and sk...     

Experimental Investigation of Overall Heat Transfer Coefficient of Al2O3/Water–Mono Ethylene Glycol Nanofluids in an Automotive Radiator

In this research, the overall heat transfer coefficient of Al₂O₃/water–mono ethylene glycol (MEG) nanofluids is investigated experimentally in a car radiator under laminar flow conditions. The experimental rig developed is similar to the automotive cooling system. The stable nanofluid used is prepared by a two‐step method. Ultrasonication is done for proper dispersion of 20 nm Al₂O₃ nanoparticle in carrier fluid water and MEG mixture with 50:50 proportions by volume. The experimental study showed that use of a nanofluid enhances the overall heat transfer coefficient as compared to the base fluid. In this study as the nanoparticle volume fraction increases from 0% to 0.8%, the overall heat transfer coefficient also increases. It was observed that as the nanofluid inlet temperature increased from 65 °C to 85 °C, the overall heat transfer coefficient decreased. It was found that using a 0.2% volume fraction Al₂O₃/water–MEG nanofluid can enable a 36.69 % reduction in surface area of the radiator.     

Review and Comparison of Nanofluid Thermal Conductivity and Heat Transfer Enhancements

This study provides a detailed literature review and an assessment of results of the research and development work forming the current status of nanofluid technology for heat transfer applications. Nanofluid technology is a relatively new field, and as such, the supporting studies are not extensive. Specifically, experimental results were reviewed in this study regarding the enhancement of the thermal conductivity and convective heat transfer of nanofluids relative to conventional heat transfer fluids, and assessments were made as to the state-of-the-art of verified parametric trends and magnitudes. Pertinent parameters of particle volume concentration, particle material, particle size, particle shape, base fluid material, temperature, additive, and acidity were considered individually, and experimental results from multiple research groups were used together when assessing results. To this end, published research results from many studies were recast using a common parameter to facilitate comparisons of data among research groups and to identify thermal property and heat transfer trends. The current state of knowledge is presented as well as areas where the data are presently inconclusive or conflicting. Heat transfer enhancement for available nanofluids is shown to be in the 15–40% range, with a few situations resulting in orders of magnitude enhancement.