氧化石墨烯对脉动热管传热性能的影响

实验选用外径为4mm、内径为2mm的铜质脉动热管研究了氧化石墨烯对以去离子水和体积分数为50%的乙醇溶液为工质的脉动热管传热性能的影响。实验分别采用加有少量氧化石墨烯的去离子水溶液(简称氧化石墨烯水溶液)和体积分数为50%的乙醇溶液(简称氧化石墨烯乙醇溶液),氧化石墨烯质量分数均为0.03%。实验发现：氧化石墨烯对以去离子水为工质的脉动热管传热性能具有强化作用,对以体积分数为50%的乙醇溶液为工质的脉动热管传热性能的影响较差,但都和脉动热管的加热功率密切相关。对于以去离子水为工质的脉动热管,在加热功率低于20W时,氧化石墨烯对脉动热管的强化作用较弱;当加热功率在30～60W之间时,氧化石墨烯对脉动热管的强化作用较强,在3.71～11.33%之间,且强化作用随加热功率的增大呈逐渐增强趋势;但随着功率继续增大,氧化石墨烯的强化作用逐渐减弱,当加热功率达到80W后,热管传热性能减弱,原因可能是氧化石墨烯颗粒出现了沉降现象。     

脉动热管传热性能的实验研究

对脉动热管的运行进行了可视化实验,分析了影响脉动热管运行性能的因素,并且在不同的充液率、传热量和不凝性气体含量的条件下,对脉动热管的运行进行了测试,实验结果表明:脉动热管的整体热阻受充液率、传热量、不凝性气体的影响,存在最佳的充液率,在一定的条件下脉动热管的运行达到最佳。     

单回路脉动热管传热特性研究

试验研究了单回路紫铜-水脉动热管在3种充液率下的传热性能,理论分析了不同加热功率和充液率下工质的干度、流速、显热和潜热及其份额的变化特性。结果显示:较小传热功率时,减小充液率或增大加热功率会提高热管的传热性能;而较高传热功率时,充液率和加热功率对热管的传热性能影响较小。增加传热功率或减小充液率,会提高管内工质的流速及流量,提高热管的潜热传热量及潜热传热份额;显热量随加热功率和充液率的增加而增大。     

基于BP和RBF神经网络的乙烷脉动热管对比研究

为研究BP和RBF神经网络对脉动热管热阻的预测及改善脉动热管性能,将加热功率、倾角及工作温区作为输入参数,热阻作为输出参数,建立BP和RBF神经网络模型。利用大量实验数据对BP及RBF神经网络进行训练并预测,将预测值与实验值比较,以验证BP和RBF神经网络预测性能。结果表明:BP和RBF神经网络均能较好地预测热阻;采用RBF神经网络,训练数据及测试数据线性回归决定系数R~2分别为0.999 44和0.969 76,预测结果相对误差分别为0.89%和2.97%,均方误差分别为1.43×10~(-7)和3.13×10~(-6);与BP神经网络相比,线性回归决定系数R~2更接近1,相对误差和均方误差更小,能更精确地预测热阻。     

环路型脉动热管的工质流动和传热特性实验研究

建立了部分可视化的环路型铜-乙醇脉动热管试验台,研究了充液率、倾斜角度、环路数目等因素对脉动热管传热性能的影响。结果表明:不能形成脉动效应时工质的流型是间歇振动,形成脉动效应时工质的流型是弹状流或环状流;最佳倾角为70°~90,°最佳充液率在50%左右;热阻随着环路数目的增加而减小。     

脉动热管水平及小倾角条件下的传热性能

为了研究脉动热管放置方式对其传热性能的影响,以超纯水作为工质,对水平及倾角为30°放置的脉动热管的传热性能进行研究,用壁面温度振荡性能和传热热阻来描述其传热能力。在不同的放置条件下,着重分析不同加热功率和充液率(35%,50%,70%)对其传热性能的影响。研究表明：水平放置时,充液率为35%和50%时脉动热管不能启动,充液率70%时可以启动运行;脉动热管在运行时存在临界热量输入值,倾角为30°时,临界值为60 W,但水平放置条件下临界值为90 W;水平放置下的脉动热管传热热阻在不同加热功率下,显著高于倾角为30°的情况;倾角为30°,充液率为35%时的脉动热管适合在低加热功率范围运行,此时传热热阻要低于充液率为50%的情况,但传热范围很窄,传热极限低;30°倾角时,与充液率35%和50%相比,高充液率70%的脉动热管整体传热性能最优。     

矩形和圆形槽道脉动热管传热性能的实验研究

为了探讨不同截面形状的脉动热管在复杂工况下的传热性能,本文设计了矩形和圆形两种截面形状的脉动热管,采用对比实验的方法探究不同截面形状的脉动热管在水平、竖直工况下,不同功率对其传热性能的影响。实验结果表明:水平工况下矩形脉动热管表面温度和最大温差均低于圆形管;随着功率负荷增加,两种脉动热管表面温度及最大温差均增大,在高功率区,圆形管最大温差明显高于矩形管;竖直工况相比于水平工况,脉动热管表面温度及最大温差均降低。因此,矩形脉动热管相比于圆形脉动热管更适合在电子芯片散热领域中应用。     

下冷式脉动热管太阳能集热器传热性能实验研究

实验研究和分析了一种新型下冷式脉动热管太阳能集热器的传热特性。与传统太阳能集热器相比,下冷式脉动热管太阳能集热器水箱位于底部,真空玻璃管不盛水并嵌于水箱上,脉动热管作为传热元件,一部分置于真空玻璃管内,另一部分封于水箱中。实验过程中通过调整脉动热管冷热段间的比例,研究不同情况下集热器启动、运行特性及传热性能。结果表明,在集热器倾角为70°、脉动热管充液率为55%的情况下,脉动热管的当量导热系数随冷热段比例的减小而先升后降;当冷热段比例为47%时,脉动热管的启动温度最低、传热性能最优。     

纳米流体在不同类型热管中的传热研究进展

纳米流体作为一种新型的高导热材料引起了持续关注,本文针对不同的纳米流体材料在振荡热管、重力热管、平板热管中的传热效果,概述了近年来纳米流体的制备、热特性以及在热管中应用的传热研究,总结了目前纳米流体应用于热管中存在的颗粒滞纳性、不均匀分散等问题并进行了分析。大量研究表明,使用金属纳米颗粒、合适的纳米流体配比等可有效强化热管传热,在体积分数为0.25%~1.50%或质量分数为0.01%~3.00%范围下,传热效率存在峰值。此外,对若干最新的纳米流体热管相变传热模型开展了传热模型新实验工况的适用性分析,发现Mohseni模型在新条件下的平均计算偏差约为±13.8%,并基于该模型提出更适用于新工况的模型,其平均误差为±3.4%。提出寻找颗粒聚合纳米流体的二次非接触反聚合手段是未来纳米流体在热管中可靠应用的重点研究方向。     

倾斜角及充液率对脉动热管运行性能的影响

在内径分别为1 mm和2 mm的细铜管弯曲而成的40弯头脉动热管试验装置上,采用R123、水和酒精为工作介质,研究了倾斜角及充液率对脉动热管传热性能的影响.结果表明:在相同的热负荷下,当倾斜角从+90°逐渐转到0°,最后转到-90°时,平均蒸发温度有不同程度的提高;在较低负荷下,倾斜角对运行性能的影响比较明显;对于多弯头数管状脉动热管,重力作用对其传热性能的影响仍然存在,但影响程度随着热负荷的增加及管内径的减小而减弱;最佳充液率范围与管内径、加热模式、热负荷及工质种类等因素有关,在工程应用中,最佳充液率可近似取管内总体积的55%左右.     

A new empirical correlating equation for calculating effective viscosity of nanofluids

In this paper, an empirical correlation for the nanofluid viscosity is proposed. The new equation for the nanofluid effective dynamic viscosity, normalized by the dynamic viscosity of the base liquid, is derived from a wide selection of experimental data available in the literature. This correlation presented the viscosity of the nanofluid as a function of the base fluid viscosity, nanoparticle volume fraction, nanoparticle diameter, nanoparticle temperature, and mass density of the base fluid. The new correlation was evaluated against 898 experimental data for the viscosities of nanofluid collected from the literature. The experimental data included different working nanofluids, such as alumina, Iron, and silica, where the diameter of nanoparticles was ranging between 10 and 350 nm, suspended in water, propylene glycol, and kerosene. The predicted results were then compared with many other published experimental results for different nanofluids and very good concordance between these results was observed. In general, this correlation has higher accuracy and precision.     

Experimental investigation on frequency pulsation effects on a single pass plate heat exchanger performance

This paper investigates combined heat transfer improvement methods. These methods include introducing pulsating flow, adding nanofluids, and manipulating the flow's characteristics in a corrugated plate heat exchanger. Tests are carried out with multi-walled carbon nanotube (MWCNT), graphene nanoplate (GNP), and a mixture of GNP and MWCNT meeting the requirement of 0.01% nanofluids volume fraction and exposed to pulsation. Results demonstrated that the use of pulsating frequencies from 0 to 30 Hz of GNP-water, MIX nanofluids–water, and MWCNT–water nanofluids with a constant concentration of 0.01 wt% leads to a significant improvement in heat transfer. Using pure water at frequency f = 0 Hz as a benchmark, the Nusselt number of the mixture nanofluid increases by 15.2%, 27.5%, 40.4%, and 52.8% with the increase of frequency pulsation from 0 to 30 Hz with a slight effect on the pressure-drop at this low used constant nanofluid concentration = 0.01%. The highest Nusselt number value for GNP-water nanofluid improved by an amount of 58.3% at the highest frequency compared with pure water at f = 0 Hz.     

Effect of thermophoresis and Brownian motion on the melting heat transfer of a Jeffrey fluid near a stagnation point towards a stretching surface: Buongiorno's model

This analysis intends to address the coupled effect of phase change heat transfer, thermal radiation, and viscous heating on the MHD flow of an incompressible chemically reactive nanofluid in the vicinity of the stagnation point toward the stretching surface, taking a Jeffrey fluid as the base fluid. Convergent analytical solutions for the nonlinear boundary layer equations are obtained by the successive application of scaling variables and the highly efficacious homotopy analysis method. Error analysis is implemented to endorse the convergence of the solutions. Through parametric examination, influence of various physical parameters occurring in analysis of the profiles of velocity, temperature, and nanoparticle concentration, coefficient of surface drag, rates of mass and heat transfer is explored pictorially. The Deborah number and the melting parameter are found to enhance velocity, and the associated momentum boundary layers are thicker, whereas the magnetic field depreciates the flow rate. Temperature is observed to enhance with the thermophoresis parameter, Prandtl number and Eckert number, whereas a reduction is seen with the thermal radiation parameter and Brownian motion parameter. Nanoparticle concentration is depleted by the chemical reaction parameter, the thermophoresis parameter, and the Lewis number.     

开式脉动热管传热极限的试验研究

在由内径分别为1 mm和2 mm的细铜管弯曲而成的2组40弯头开式回路脉动热管试验装置上,采用R123为工作介质,定性分析了充液率及加热方式对传热极限的影响,并将试验值与Katpradit传热极限关联式的计算值进行了比较,结果表明:随着充液率的提高,传热极限先增大而后减小,存在一个最佳充液率(约50%);在3种加热方式中,垂直底部加热有助于脉动热管取得较大的传热极限值,而垂直项部加热则对应较小的传热极限值.通过对Katpradit传热极限关联式进行适当修正,得到了新的试验关联式.     

纳米流体对U型深埋管换热特性影响的研究

结合西安市某个埋深为2505 m的U型深埋管换热系统,在原位实验验证的基础上建立三维全尺寸数值计算模型,进而模拟分析埋管内热流载体种类对强化埋管换热的效果.埋管内热流载体除常规使用的水外,还选用Al2O3/水、CuO/水和SiO2/水3种纳米流体.通过改变这3种纳米流体的纳米颗粒体积浓度分别为0.1％、0.3％、0.5...     

Effect of thermal radiation on magnetohydrodynamic three‐dimensional motion of a nanofluid past a shrinking surface under the influence of a heat source

An analytical technique known as the homotopy analysis method is used to acquire solutions for magnetohydrodynamic 3‐D motion of a viscous nanofluid over a saturated porous medium with a heat source and thermal radiation. The governing nonlinear partial differential equations are changed to ordinary differential equations employing appropriate transformations. Validation of the present result is done with the help of error analysis for flow and temperature. The influences of pertinent parameters on momentum, energy, and Nusselt number are studied and discussed. The major findings are: the velocity of the nanofluid is affected by the nanoparticle volume fraction and the thickness of the thermal boundary layer becomes thinner and thinner subject to sink, whereas the effect is revered in case of the source.