活塞内冷油腔中纳米流体振荡流动与传热特性影响因素研究

采用计算流体力学的方法,研究了分别含Al₂O₃、Cu O、Si O₂的3种纳米机油在纳米颗粒体积分数为1%、3%、5%时相对于传统机油的振荡传热能力和机油在油腔内流动的规律。结果表明,纳米颗粒的加入改变了流体的物性参数,纳米流体的传热效果比传统机油更好,且内冷油腔的传热系数随着纳米流体体积分数的增加而增加,但对内冷油腔内瞬态机油的瞬态分布和充油率的影响不大;纳米流体的黏度、密度、导热系数、比热容都能影响内冷油腔的传热性能,密度的增加会使流体对壁面的冲击作用更强,从而增强油腔的传热能力;在纳米颗粒体积分数为5%时CuO纳米机油的传热系数比Al₂O₃、SiO₂纳米机油分别高8.2%和14.6%。     

Al2O3-H2O纳米流体池内沸腾强化传热实验的曲面响应优化

为了提高余热回收效率,强化沸腾换热。在池内沸腾强化换热实验中运用Al_2O_3-H_2O纳米流体,研究了Al_2O_3纳米流体浓度、工件壁厚、热流密度对强化率的影响及最优强化条件。单因素分析结果显示,随着Al_2O_3纳米流体浓度、热流密度增大,强化率先增大后减小;随着工件壁厚增大,强化率逐渐减小。在单因素分析结果上,采用响应曲面法中Box-Behnken Design(BBD)模型对池内沸腾传热条件优化,得出三个因素对强化率的影响大小为:Al_2O_3纳米流体浓度工件壁厚热流密度。并且Al_2O_3纳米流体浓度与热流密度交互作用对强化率最为显著。通过曲面响应拟合最佳实验条件为:质量浓度1.2 wt%、热流密度83 543 W/m~2、壁厚0.45 mm,模拟结果强化率为107%,实验测得最优条件下强化率为106%,与预测接近。     

水基纳米流体流动与换热数值模拟

采用两步法制备体积分数φ为0.001%、0.01%、0.1%的Al_2O_3-H_2O纳米流体,运用热力学相关式进行计算,并采用Lattice Boltzmann方法模拟圆管内Al_2O_3-H_2O纳米流体的流动与换热,研究分析不同纳米粒子体积分数和粒径对纳米流体平均Nu数的影响。结果表明,不同体积分数的Al_2O_3-H_2O纳米流体,随着纳米颗粒的运动,边界层发生变化,其流动特性和换热特性也受到影响,对于相同位置的纳米流体,当体积浓度为0.9%、0.5%、0.1%时,平均Nu数分别为21、17.8、16,随着纳米颗粒体积分数越大,其平均Nu数越大,即换热强度越大。当纳米颗粒为20 nm,Re数为1000、3000、5000、7000、9000时,平均Nu数分别为11.5、14.5、18、20、21.5,随着Re数的增加,纳米流体的强化换热效果越好。     

纳米流体在泡沫金属管内强化换热的数值模拟

本研究通过在流体流过的管内核心区插入不同半径的泡沫金属、在基液中添加纳米粒子的方法达到强化换热的目的。通过泡沫金属管与光管内温度场及速度场的比较来分析泡沫金属对强化换热的作用,研究了泡沫金属填充比和纳米流体对流动及换热性能的影响。研究表明:模拟结果与文献实验结果吻合良好,将泡沫金属填充在管内核心区可以提升换热特性,而纳米流体的加入可以使换热效果增强。在低流速的条件下,换热效果随填充比和纳米流体浓度增大而增强,但泡沫金属填充比和纳米流体体积分数之间存在最佳搭配。研究可知,在填充厚度为6 mm、纳米流体体积分数为0.3%时综合换热性能最佳;流速和填充比的增大有利于强化换热,但压降也随之增大。     

纳米流体受限式浸没射流冲击凸台表面的换热研究

为了分析纳米流体受限式浸没射流冲击到凸台表面的换热效果,以及与水射流冲击光滑平板的换热情况对比,详细分析了纳米流体颗粒表面形状、纳米流体体积份额、纳米颗粒材料、射流Re数、喷嘴距换热表面的相对高度H/D对滞止点及整个热表面换热系数的影响。实验发现,表面形状对换热效果影响较大,射流冲击到凸台表面上滞止点换热系数h_0最小,但整个换热表面的局部换热系数h_x及平均换热系数h_(av)均为最大值,且换热系数随Re的增大而增大。纳米流体体积份额对换热效果的影响与喷射的相对高度H/D有关,当H/D为3时,h0及hav随纳米颗粒浓度的增大而增大;当H/D为5时,纳米流体体积份额φ为0.2%时的换热效果最好。     

换热器内纳米流体换热与压降特性的分析

以人字形板式换热器翅片的单元流道二维截面为几何模型,采用Fluent软件对换热介质分别为水、水基Al_2O_3纳米流体和水基CuO纳米流体进行换热、压降特性的数值分析,结果表明:在水中添加纳米颗粒可以提高流体的换热能力,且随着纳米颗粒体积分数的增加,努塞尔数随之增大,同时,纳米颗粒体积分数为3.0%时,流体对应的努塞尔数增幅明显高于1.0%～2.0%的水基Al_2O_3纳米流体。但结果还表明,纳米颗粒体积分数为3.0%的水基Al_2O_3纳米流体对应的压降最高,不利于流体流动,因而在将纳米流体作为流动换热介质时,应该综合考虑换热与压降的影响。另外,在相同纳米颗粒体积分数以及入口雷诺数时,水基Al_2O_3纳米流体换热能力高于水基CuO纳米流体,但两者的流动压降几乎相同,因而应优先选用Al_2O_3纳米颗粒。由于目前将纳米流体与换热器相结合的相关研究较少,因而得出数值结论对纳米流体在换热器中的应用设计具有一定的参考意义。     

纳米流体在内置扭带外螺纹管内的流动和换热特性

为了研究纳米流体在内置扭带外螺纹管内的流动与传热特性,在Re(雷诺数)为2 000~12 000的范围内,分别对质量分数为0.1%、0.2%、0.3%、0.4%、0.5%和0.6%的Cu、Al、A1_2O3、Fe_2O_3、多壁碳纳米管和石墨纳米流体在内置扭带外螺纹管内的流动与对流换热特性进行了实验研究。实验结果表明:在相同Re下不同纳米流体都存在最佳浓度比0.5%,其中Cu-水纳米流体的换热性能最好但是摩擦阻力较大,石墨的换热性能和摩擦阻力方面的综合性能最好。内置扭带外螺纹管较光管在换热性能方面提高了50.32%,但摩擦阻力系数也相应增加。根据实验数据对热性能系数进行了综合分析,得到了石墨纳米流体内置扭带外螺纹管对流换热以及摩擦阻力系数关联式,其计算值和实验值有较好的吻合度。     

立管内椭球颗粒群强化传热特性模拟研究

为提高液固两相流传热效率,通过添加椭球形颗粒改善立式换热管内液固两相流分布,采用CFD-DEM数值模拟颗粒长径比β、颗粒进口体积分数φ以及流体进口速度v与管内椭球形颗粒群传热的关系,并根据PEC指数评价颗粒强化传热能力。模拟结果表明：在1.50≤β≤2.50,2.00%≤φ≤4.00%,1.0 m/s≤v≤1.5 m/s时,椭球形颗粒PEC指数总高于球形颗粒,即前者强化换热效果优于后者。     

纳米流体强化活塞组-气缸套传热的数值模拟研究

将活塞组-气缸套作为一个耦合体,利用数值模拟法研究了只在润滑油中与只在冷却水中加入纳米Cu颗粒两种情况下的强化传热效果,并与原机传热情况进行比较。研究结果表明:无论是以Cu-润滑油纳米流体还是以Cu-水纳米流体作为传热工质,都可以显著提高活塞组-气缸套的散热性能,且散热量随着纳米粒子浓度的增高而增大。对于活塞侧,Cu-润滑油纳米流体的改善效果强于Cu-水纳米流体,其中Cu体积分数为5%的Cu-润滑油纳米流体可使燃烧室中心点a、燃烧室喉口点b、顶面外边缘点c和第一环外侧上边缘点d的温度平均降低约28.4、21.7、22.8和27.7K;对于气缸套侧,Cu-水纳米流体强化换热效果更理想,Cu体积分数为5%的Cu-水纳米流体可使气缸套内侧上边缘点e的温度平均降低约10.4K。     

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 investigation of effective parameters in convective heat transfer of nanofluids flowing under a laminar flow regime

This article presents a numerical investigation on heat transfer performance and pressure drop of nanofluids flows through a straight circular pipe in a laminar flow regime and constant heat flux boundary condition. Al₂O₃, CuO, carbon nanotube (CNT) and titanate nanotube (TNT) nanoparticles dispersed in water and ethylene glycol/water with particle concentrations ranging between 0 and 6 vol.% were used as working fluids for simulating the heat transfer and flow behaviours of nanofluids. The proposed model has been validated with the available experimental data and correlations. The effects of particle concentrations, particle diameter, particles Brownian motions, Reynolds number, type of the nanoparticles and base fluid on the heat transfer coefficient and pressure drop of nanofluids were determined and discussed in details. The results indicated that the particle volume concentration, Brownian motion and aspect ratio of nanoparticles similar to flow Reynolds number increase the heat transfer coefficient, while the nanoparticle diameter has an opposite effect on the heat transfer coefficient. Finally, the present study provides some considerations for the appropriate choice of the nanofluids for practical applications.     

中深层地热地埋管实际运行影响因素研究

为研究中深层地热地埋管运行的影响因素,分析西咸新区中深层地热地埋管供暖系统的长期运行结果,并结合关中地区地质数据,建立深度为2510 m的中深层地埋管换热器全尺寸模型,采用数值模拟法研究实际岩层分布下地埋管的运行、结构和材料因素对其取热能力的影响。结果表明,西咸新区某项目1号地埋管和2号地埋管两个地埋管,其平均取热功率均在310 kW以上,具有优良的取热能力。地埋管进水温度随季节变化明显,并引起用户侧负荷及热泵回水温度的波动。在结构方面,随内管径由63 mm增至125 mm,平均出口水温和换热功率分别降低1.9%和4.8%,但内管径过小将影响内管运行的安全性,综合安全和换热两方面因素,最佳内管径应选取ϕ110 × 10mm规格;随外管径由168.3 mm增至244.5 mm,平均出口水温和换热功率分别增加3.5%和9%,综合成本和换热两方面因素,最佳外管径应选取ϕ 177.8 × 19 mm规格;在运行方面,地埋管出口水温随着流量的增加而减小,换热功率随着流量增加而增加;出口水温随着进水温度的升高而上升,换热功率也随之减小。在材料方面,减小内管导热系数和增加固井材料导热系数均能增加地埋管出口水温和换热功率,考虑换热功率变化和成本因素,在工程中导热系数为0.42 W/（m∙K）的内管和导热系数为3 W/（m∙K）左右的固井材料。     

Numerical and Experimental Investigation of Heat Transfer of α-Al2O3/Water Nanofluid in Double Pipe and Shell and Tube Heat Exchangers

This research presents an experimental and numerical study on the heat transfer of α-Al₂O₃/water nanofluid flowing through the double pipe and shell and tube heat exchangers, under laminar flow conditions. Effects of important parameters such as hot and cold volume flow rates, nanofluid temperature, and nanoparticles concentration on the heat transfer characteristics are investigated. The results indicated that the heat transfer performance of both double pipe and shell and tube heat exchangers increases with increasing the hot and cold volume flow rates, as well as the particle concentrations and nanofluid inlet temperature. Compared with pure water, the results indicated that the heat transfer coefficients of nanofluid in the double pipe and shell and tube heat exchangers are higher than those of water by 13.2% and 21.3%, respectively. Also, the heat transfer performance of nanofluid in a shell and tube heat exchanger is 26.2% higher than the double pipe heat exchanger. A computational fluid dynamics (CFD) technique was used for heat transfer simulation in the previously mentioned heat exchangers. Computed overall heat transfer coefficients of the nanofluids are in good agreement with the experimental data.     

Experimental investigation of titanium nanofluids on the heat pipe thermal efficiency

The enhancement heat transfer of the heat transfer devices can be done by changing the fluid transport properties and flow features of working fluids. In the present study, therefore, the enhancement of heat pipe thermal efficiency with nanofluids is presented. The heat pipe is fabricated from the straight copper tube with the outer diameter and length of 15, 600 mm, respectively. The heat pipe with the de-ionic water, alcohol, and nanofluids (alcohol and nanoparticles) are tested. The titanium nanoparticles with diameter of 21 nm are used in the present study which the mixtures of alcohol and nanoparticles are prepared using an ultrasonic homogenizer. Effects of %charge amount of working fluid, heat pipe tilt angle and %nanoparticles volume concentrations on the thermal efficiency of heat pipe are considered. The nanoparticles have a significant effect on the enhancement of thermal efficiency of heat pipe. The thermal efficiency of heat pipe with the nanofluids is compared with that the based fluid.     

Investigation of thermohydraulic performance of triple concentric-tube heat exchanger with CuO/water nanofluid: Numerical approach

In this study, numerical investigation of CuO/water nanofluids in a triple concentric-tube heat exchanger has been carried out using a commercial CFD software. The primary objective of this study is to conduct a heat transfer and pressure drop characteristics of water-based CuO nanofluids under turbulent flow regime. Reynolds number for the nanofluid has also been considered in the range of 2500 to 10,000 with a nanoparticle volume concentration of 0% to 3%. The effects of flow rate, volume concentration of nanoparticles, and flow arrangement on heat transfer performance of nanofluid have been studied for four flow arrangements. The comparison of the performance with and without nanofluid has been done. It was found that thermal performance and overall effectiveness increased with the increase in Reynolds number and volume concentration of nanoparticles in all the four flow arrangements for the considered range of operating parameters.     

Effect of Vibration on Forced Convection Heat Transfer for SiO2–Water Nanofluids

In the process of heat transfer, the fluid type and external parameters have a significant impact on heat transfer performance. For this reason, the physical properties, pressure differences, and heat transfer rates of SiO₂–water nanofluids have been experimentally investigated in a straight circular pipe. Experimental results revealed a great difference in physical properties between SiO₂–water nanofluids and purified water. The friction factor of low-volume-concentration nanofluids was slightly increased for laminar flow and tended to be almost independent of the Reynolds number for turbulent flow. The heat transfer coefficient can be enhanced either by adding nanoparticles to purified water or by imposing a transverse vibration on the heat transfer surface. Using these two methods at the same time (compound heat transfer enhancement), heat transfer performance is much better than that with either method alone. The largest increase of about 182% was observed under conditions of compound heat transfer enhancement.     

Experimental and analytical investigation on an automobile radiator with CuO/EG‐water based nanofluid as coolant

In the present study, experimental and analytical thermal performance of automobile radiator using nanofluids is investigated and compared with performance obtained with conventional coolants. Effect of operating parameters and nanoparticle concentration on heat transfer rate are studied for water as well as CuO/EG‐water based nanofluid analytically. The results are presented in the form of graphs showing variations of net heat transfer rate for various coolant flow rate, air velocity, and source temperature for various CuO/EG‐water based nanofluids. Experimental results indicate that with the increase in coolant flow rate and air velocity, heat transfer rate increases, reaches maximum and then decreases. Experimental investigation of a radiator is carried out using CuO/EG‐water based nanofluids. Results obtained by experimental work and analytical MATLAB code are almost the same. Maximum absolute error in water and air side is within 12% for all flow condition and coolant fluids. Nusselt number of nanofluid is calculated using equation number 33[9]. The results obtained from experimental work using 0.2% volume CuO/EG‐water based nanofluids are compared with the results obtained from MATLAB code. The results show that the maximum error in the outlet temperature of the coolant and air is 12% in each case. Thus MATLAB code can be used for different concentration of nanofluids to study the effect of operating parameters on heat transfer rate. Thus MATLAB code developed is valid for given heat exchanger applications. From the results obtained by already validated MATLAB code, it is concluded that increase in coolant flow rate, air velocity, and source temperature increases the heat transfer rate. Addition of nanoparticles in the base fluid increases the heat transfer rate for all kind of base fluids. Among all the nanofluid analyzed in this study, water‐based nanofluid gives highest value of heat transfer rate and is recommended for the heat exchanger applications under normal operating conditions. Maximum enhancement is observed for ethylene glycol‐water (4:6) mixture for 1% volume concentration of CuO is almost equal to 20%. As heat transfer rate increases with the use of nanofluids, the heat transfer area of the radiator can be minimized.     

Natural convection heat transfer enhancement in horizontal concentric annuli using nanofluids

Heat transfer enhancement in horizontal annuli using nanofluids is investigated. Water-based nanofluid containing various volume fractions of Cu, Ag, Al₂O₃ and TiO₂ nanoparticles is used. The addition of the different types and different volume fractions of nanoparticles were found to have adverse effects on heat transfer characteristics. For high values of Rayleigh number and high L/D ratio, nanoparticles with high thermal conductivity cause significant enhancement of heat transfer characteristics. On the other hand, for intermediate values of Rayleigh number, nanoparticles with low thermal conductivity cause a reduction in heat transfer. For Ra = 10³ and Ra = 10⁵ the addition of Al₂O₃ nanoparticles improves heat transfer. However, for Ra = 10⁴, the addition of nanoparticles has a very minor effect on heat transfer characteristics.     

Heat transfer characteristics in double tube helical heat exchangers using nanofluids

In the present work a three-dimensional analysis is used to study the heat transfer characteristics of a double-tube helical heat exchangers using nanofluids under laminar flow conditions. CuO and TiO₂ nanoparticles with diameters of 24 nm dispersed in water with volume concentrations of 0.5–3 vol.% are used as the working fluid. The mass flow rate of the nanofluid from the inner tube was kept and the mass flow rate of the water from the annulus was set at either half, full, or double the value. The variations of the nanofluids and water temperatures, heat transfer rates and heat transfer coefficients along inner and outer tubes are shown in the paper. Effects of nanoparticles concentration level and of the Dean number on the heat transfer rates and heat transfer coefficients are presented. The results show that for 2% CuO nanoparticles in water and same mass flow rate in inner tube and annulus, the heat transfer rate of the nanofluid was approximately 14% greater than of pure water and the heat transfer rate of water from annulus than through the inner tube flowing nanofluids was approximately 19% greater than for the case which through the inner and outer tubes flow water. The results also show that the convective heat transfer coefficients of the nanofluids and water increased with increasing of the mass flow rate and with the Dean number. The results have been validated by comparison of simulations with the data computed by empirical equations.     

Experimental study of Fe2O3/water and Fe2O3/ethylene glycol nanofluid heat transfer enhancement in a shell and tube heat exchanger

Heat transfer characteristics of Fe₂O₃/water and Fe₂O₃/EG nanofluids were measured in a shell and tube heat exchanger under laminar to turbulent flow condition. In the shell and tube heat exchanger, water and ethylene glycol-based Fe₂O₃ nanofluids with 0.02%, 0.04%, 0.06% and 0.08% volume fractions were used as working fluids for different flow rates of nanofluids. The effects of Reynold's number, volume concentration of suspended nanoparticles and different base fluids on the heat transfer characteristics were investigated. Based on the results, adding nanoparticles to the base fluid causes a significant enhancement of the heat transfer characteristics and thermal conductivity. This enhancement was investigated with regard to various factors; concentration of nanoparticles, types of base fluids, sonication time and temperature of fluids. In this paper, the effect of Fe₂O₃ nanoparticles on the thermal conductivity of base fluids like ethylene glycol and water was studied. The thermal conductivity measurement was made for different concentrations and temperatures. As the concentration of the nanoparticles increased, there was a significant enhancement in thermal conductivity and overall heat transfer due to more interaction between particles. It was also observed that there was an improvement in the thermal conductivity of the base fluid as the temperature increased. The measurements also showed that the pressure drop of nanofluid was higher than that of the base fluid in a turbulent flow regime. However, there was no significant increase in pressure drop at laminar flow.