半圆形微通道内纳米流体流动与传热特性

为了研究流体流经半圆形微通道的传热与流动特性,对去离子水、Cu-水纳米流体及Al-水纳米流体在21个当量直径为612μm的平行半圆形微通道热沉(微型散热片)中的流动与对流换热特性进行了实验研究。研究发现:与截面为矩形的常规形状相比,半圆形微通道也具有很好的换热效果,与去离子水相比,添加Al和Cu纳米颗粒的纳米流体压降损失增大。当纳米流体的质量浓度为0.5%时,在微通道换热器中的纳米流体效应由于粘度过大等原因发生了恶化,并且这种恶化在高流速下也出现了。根据实验数据得到了半圆形微通道内低浓度纳米流体的层流对流换热以及摩擦阻力系数关联式,对热性能系数的分布曲线进行了综合分析,研究结果对于集成高效芯片散热系统设计具有重要意义。     

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。     

基于格子Boltzmann方法的微通道传热特性数值研究

应用格子Boltzmann方法研究单相流体流过矩形和三角形微通道结构的对流换热特性。数值模拟结果展示出流体在不同微通道中的速度场和温度场。从温度场结果可发现,流体流经微通道时会在热壁面附近形成热边界层,并且热边界层的厚度随雷诺数Re的增大而变薄。另外,通过出口温度和努塞尔数Nu等物理量定量研究两种微通道结构的换热特性。数值结果表明,矩形微通道结构的换热性能优于三角形微通道结构。在场协同原理基础上,进一步分析造成两种微通道换热性能不同的原因。     

矩形微通道内流体流动特性的数值研究

对矩形微通道实体模型进行简化处理,并建立微通道内流体流动的数学模型.设定矩形微通道水力直径Dh=120～480 μm,入口雷诺数Re=ll.9～3 817.1,以20℃蒸馏水为流动工质,借助FLUENT分别对不同水力直径的三组矩形微通道内流体流动特性展开数值模拟研究,并将数值模拟结果与理论预测值及其他学者的研究结论进行对比.结果表明:随着微通道水力直径的减小,摩擦阻力系数、速度梯度和压强梯度都呈现增大趋势;在微尺度下,矩形微通道内临界Re提前,而且水力直径越小,临界Re值越小.     

圆管内对流换热过程中潜热型功能流体流动阻力特性的实验研究

实验研究了由正十四烷和尿素甲醛树脂制成的相变微胶囊和水混合制成的潜热型功能流体在流过恒热流圆管进行对流换热时的流动阻力特性,获得了压降随流速的变化关系、摩擦阻力系数和表观黏度随R e的变化关系。并在同样条件下用单相水进行了对比实验。相变微胶囊的加入导致流体流动阻力较单相流体有显著增大。管路中扰动件导致单相流体的流动阻力特性在低R e条件下呈湍流特征;功能流体则呈不同规律,扰动仅导致流动阻力进一步增大,而流动阻力特性仍呈层流特征。     

泡沫铝通道内瞬态流动的平均换热系数

针对泡沫铝金属填充矩形通道内的对流换热开展了瞬态实验研究,分析了泡沫铝孔径(孔隙率)、流体流量(流速)等关键参数的影响。为了有效地处理实验数据,重新定义并推导了平均换热系数的计算公式,得到了泡沫铝通道内流动的平均换热系数,并引入了基于渗透率的雷诺数和达西数,确定了相关换热、流动准则数关系。实验研究表明,流速的增大有利于对流换热的强化:而平均换热系数对泡沫金属孔径较敏感;对于低孔隙率泡沫金属,渗透率成为影响换热强度的主要因素,相同或接近的孔隙率下,孔径越大,渗透率和达西数越大,越有利于换热,且压损减小。     

内插多孔介质微通道热沉的数值模拟

以水为流动介质,在微通道内添加堆叠金属丝网多孔介质,采用局部非热平衡假设和双能量方程模型,分析了内插不同目数金属丝网的微通道在层流状况下的传热与阻力特性,并采用数值计算方法对微通道热沉进行了数值模拟。结果表明,在微通道内插入多孔介质能显著提高热沉的对流换热系数、降低加热面平均温度,但阻力增加较大,且当插入的金属丝网目数为100目时,微通道热沉的对流换热系数较大,与填充其他目数金属丝网相比阻力增加较小。     

板式换热器单边流动与对角流动数值模拟

基于传热学控制方程,采用数值计算方法,对板式换热器单边流动和对角流动时的流动与换热特性进行分析。在分析过程中保持换热器的结构参数不变,只改变进出口的流动方式,结果发现:在相同的流速下,单边流动的总对流换热系数要高于对角流动,而总压降单边流动要低于对角流动,在流速u=0.6 m/s工况下,努谢尔数单边流动比对角流动高出10.87%,压降对角流流动比单边流动高出5.13%。随着进口流速的增大,单边流动与对角流动的冷热流体进出口温差均减小,而且减小的趋势对角流动要大于单边流动,摩擦因子f和传热因子j逐渐减小。单边流动的流动和传热特性要优于对角流动。     

垂直矩形窄通道流动沸腾换热特性实验研究

流动沸腾换热是典型的两相流问题。窄通道与常规通道相比较,其流动沸腾换热系数有较大提高,换热机理也更加复杂。针对截面为250 mm×5 mm的竖直矩形窄缝通道,在低压、入口温度过冷、不同质量流速及加热功率密度的条件下,对水流动沸腾换热特性进行实验研究。通过实验分析可知:入口温度27～60℃、质量流速2.22～3.49 kg/(m2.s)及加热功率密度0～12 kW/m2对饱和沸腾起始点和过冷段长度有重要影响;高的空泡份额和通道结构的限制使汽液两相流动不稳定而影响换热系数,换热系数随着功率的增大而减小,流体进入完全对流沸腾阶段;由于实验段通道顶部结构的限制,干度的增加不会出现干涸点,换热不会得到恶化,换热系数随着功率的增大基本不变。     

非圆形微通道热沉的流动换热特性数值模拟

建立了非圆形硅微通道内单相流动和换热过程的三维模型,并分别对三角形、矩形和梯形微通道中流动换热进行了数值模拟.研究发现,截面平均努塞尔数在通道入口处数值最大,然后沿流体流动方向急剧减小,直至流动充分发展时趋于恒定.固体和流体温度沿流动方向近似线性升高.换热面壁温仅沿流动方向升高,在垂直于流动方向,温度则基本保持均衡;雷诺数对微通道的流动与换热特性存在着较大的影响,雷诺数越大,其对应的努塞尔数也越大.对3种微通道的热经济性分析比较发现,三角形通道的热有效性最高.     

Fluid flow and heat transfer characteristics in rectangular microchannels

Experiments were conducted to investigate flow and heat transfer characteristics of water in rectangular microchannels. All tests were performed with deionized water. The flow rate, the pressures, and temperatures at the inlet and outlet were measured. The friction factor, heat flux, and Nusselt number were obtained. The friction factor in the microchannel is lower than the conventional value. That is only 20% to 30% of the convectional value. The critical Reynolds number below which the flow remains laminar in the microchannel is also lower than the conventional value. The Nusselt number in the microchannel is quite different from the conventional value. The Nusselt number for the microchannel is lower than the conventional value when the flow rate is small. As the flow rate through the microchannel is increased, the Nusselt number significantly increases and exceeds the value of Nusselt number for the fully developed flow in the conventional channel. The micro‐scale effect was exhibited. The Nusselt number is also affected by the heat flux. The Nusselt number remains the constant value when the flow rate is small. The Nusselt number increases with the increase in the heat flux when the flow rate is large. © 2008 Wiley Periodicals, Inc. Heat Trans Asian Res, 37(4): 197–207, 2008; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20206     

Forced convective heat transfer of nanofluids in microchannels

Convective heat transfer coefficient and friction factor of nanofluids in rectangular microchannels were measured. An integrated microsystem consisting of a single microchannel on one side, and two localized heaters and five polysilicon temperature sensors along the channel on the other side were fabricated. Aluminum dioxide (Al₂O₃) with diameter of 170 nm nanofluids with various particle volume fractions were used in experiments to investigate the effect of the volume fraction of the nanoparticles to the convective heat transfer and fluid flow in microchannels. The convective heat transfer coefficient of the Al₂O₃ nanofluid in laminar flow regime was measured to be increased up to 32% compared to the distilled water at a volume fraction of 1.8 volume percent without major friction loss. The Nusselt number measured increases with increasing the Reynolds number in laminar flow regime. The measured Nusselt number which turned out to be less than 0.5 was successfully correlated with Reynolds number and Prandtl number based on the thermal conductivity of nanofluids.     

Fully-developed thermal transport in combined electroosmotic and pressure driven flow of power-law fluids in microchannels

Thermally fully-developed heat transfer has been analyzed for combined electroosmotic and pressure driven flow of power-law fluid in a microchannel. Analytical expressions for transport parameters are presented in terms of the flow behavior index, the length scale ratio (ratio of Debye length to half channel height), dimensionless pressure gradient, and Joule heating parameter (ratio of Joule heating to surface heat flux). Closed form solutions are obtained for some specific values of the flow behavior index, while numerical solutions are presented for general cases. The results show that the temperature variation across the channel increases with increasing the pressure gradient. To reduce the length scale ratio is found to decrease the temperature variation, particularly for shear-thinning fluids. To increase the Joule heating parameter is to enlarge the temperature variation in the channel, especially for shear-thickening fluids. The Nusselt number can be increased by decreasing the length scale ratio due to the electroosmotic effect. Also, the Nusselt number increases with decreasing the values of flow behavior index and dimensionless pressure gradient.     

Numerical investigation of nonuniform transverse magnetic field effects on the flow and heat transfer of magnetic nanofluid in a sintered porous channel

In this paper, fluid flow and convective heat transfer of a ferrofluid (water and 4 vol% Fe₃O₄) in sintered Aluminum porous channel, which is subjected to a nonuniform transverse magnetic field have been studied. The numerical simulations supposed an ordinary cubic and staggered arrangement organized by uniformly sized particles with a small contact area for the porous media and constant heat flux at the surface of the microchannel. A wire, in which the electric current passes creates a nonuniform magnetic field, which is perpendicular to the flow direction. To do this simulation, the control volume technique and the two‐phase mixture model have been employed. The results show that the obtained local heat transfer coefficient on the channel surface increased with increasing mass flow rate and decreased slightly along the axial direction. Moreover, exerting the above‐mentioned magnetic field increases the Nusselt number that enhances the heat transfer rate while it has no effect on the pressure drop along the channel.     

Laminar convective heat transfer in a microchannel with internal longitudinal fins

A numerical study was conducted to investigate the fluid flow and heat transfer characteristics of a square microchannel with four longitudinal internal fins. Three-dimensional numerical simulations were performed on the microchannel with variable fin height ratio in the presence of a hydrodynamically developed, thermally developing laminar flow. Constant heat flux boundary conditions were assumed on the external walls of the square microchannel. Results of the average local Nusselt number distribution along the channel length were obtained as a function of the fin height ratio. The analysis was carried out for different fin heights and flow parameters. Interesting observations that provide more physical insight on this passive enhancement technique, and the existence of an optimum fin height are brought out in the present study.     

An experimental study of convective heat transfer in silicon microchannels with different surface conditions

An experimental investigation has been performed on the laminar convective heat transfer and pressure drop of water in 13 different trapezoidal silicon microchannels. It is found that the values of Nusselt number and apparent friction constant depend greatly on different geometric parameters. The laminar Nusselt number and apparent friction constant increase with the increase of surface roughness and surface hydrophilic property. These increases become more obvious at larger Reynolds numbers. The experimental results also show that the Nusselt number increases almost linearly with the Reynolds number at low Reynolds numbers (Re<100), but increases slowly at a Reynolds number greater than 100. Based on 168 experimental data points, dimensionless correlations for the Nusselt number and the apparent friction constant are obtained for the flow of water in trapezoidal microchannels having different geometric parameters, surface roughnesses and surface hydrophilic properties. Finally, an evaluation of heat flux per pumping power and per temperature difference is given for the microchannels used in this experiment.     

Numerical investigations on the heat transfer enhancement in a wavy channel using nanofluid

In this paper, laminar copper–water nanofluid flow and heat transfer in a two-dimensional wavy channel is numerically investigated. The Reynolds number and nanoparticle volume fraction considered are in the ranges of 100–800 and 0–5% respectively. Numerical solutions are obtained by solving the governing equation of stream function, vorticity transport and energy in curvilinear coordinates using the finite difference method. The effects of nanoparticle volume fraction, the wavy channel amplitude and wavelength and the Reynolds number on the local skin-friction coefficient, local and average Nusselt number and the heat transfer enhancement are presented and discussed. Results show that the friction coefficient and Nusselt number increase as the amplitude of wavy channel increases. As the nanoparticle volume fraction increases, the Nusselt number is found to be significantly increased, accompanied by only a slight increase in the friction coefficient. In addition, it was found that the enhancement in heat transfer mainly depends on the nanoparticle volume fraction, amplitude of the wavy wall and Reynolds number rather than the wavelength.     

Averaging approach for microchannel heat sinks subject to the uniform wall temperature condition

The present paper is devoted to modeling methods for thermal analysis of microchannel heat sinks. The averaging approach presented in earlier works for the case of constant surface heat flux is extended to the problems subject to the uniform wall temperature condition. The solutions for velocity and temperature distributions are obtained by solving one-dimensional averaged governing equations without resorting to a two-dimensional direct numerical simulation. General solutions for both high-aspect-ratio and low-aspect-ratio microchannel heat sinks are presented. Asymptotic solutions in high-aspect-ratio and low-aspect-ratio limits are also given in explicit form. The solutions presented in the paper are validated by comparing them with the results of direct numerical simulation. The friction factors, Nusselt numbers and thermal resistances for microchannel heat sinks with a uniform base temperature are obtained from the presented solutions. The effects of the aspect ratio and the porosity on the friction factor and the Nusselt number are presented. Finally, characteristics of the thermal resistance of the microchannel heat sink are discussed.     

Numerical study of flow and heat transfer characteristics of alumina-water nanofluids in a microchannel using the lattice Boltzmann method

In the present study, mathematical modeling is performed to simulate force d convection flow of Al₂O₃/water nanofluids in a microchannel using the lattice Boltzmann method (LBM). Simulations are conducted at low Reynolds numbers (Re ≦ 16). Results indicate that the average Nusselt number increases with the increase of Reynolds number and particle volume concentration. The fluid temperature distribution is more uniform with the use of nanofluid than that of pure water. Furthermore, great deviations of computed Nusselt numbers using different models associated with the physical properties of a nanofluid are revealed. The results of LBM agree well with the classical CFD method for predictions of flow and heat transfer in a single channel and a microchannel heat sink concerning the conjugate heat transfer problem, and consequently LBM is robust and promising for practical applications.     

Entropy generation minimization of a MHD (magnetohydrodynamic) flow in a microchannel

The dissipative processes that arise in a microchannel flow subjected to electromagnetic interactions, as occurs in a MHD (magnetohydrodynamic) micropump, are analyzed. The entropy generation rate is used as a tool for the assessment of the intrinsic irreversibilities present in the microchannel owing to viscous friction, heat flow and electric conduction. The flow in a parallel plate microchannel produced by a Lorentz force created by a transverse magnetic field and an injected electric current is considered assuming a thermally fully developed flow and conducting walls of finite thickness. The conjugate heat transfer problem in the fluid and solid walls is solved analytically using thermal boundary conditions of the third kind at the outer surfaces of the walls and continuity of temperature and heat flux across the fluid-wall interfaces. Velocity, temperature and current density fields in the fluid and walls are used to calculate the global entropy generation rate. Conditions under which this quantity is minimized are determined for specific values of the geometrical and physical parameters of the system. The Nusselt number is also calculated and explored for different conditions. Results can be used to determine optimized conditions that lead to a minimum dissipation consistent with the physical constraints demanded by the microdevice.