Rehman Danish, Morini Gian Luca, Hong Chungpyo
Microfluidics Laboratory, Department of Industrial Engineering (DIN), University of Bologna, Via Zamboni, 33, 40126 Bologna BO, Italy.
Microscale Heat Transfer Laboratory, Department of Mechanical Engineering, Kagoshima University, Kagoshima Prefecture 890-8580, Japan.
Micromachines (Basel). 2019 Feb 28;10(2):171. doi: 10.3390/mi10030171.
In this paper, a combined numerical and experimental approach for the estimation of the average friction factor along adiabatic microchannels with compressible gas flows is presented. Pressure-drop experiments are performed for a rectangular microchannel with a hydraulic diameter of 295 μ m by varying Reynolds number up to 17,000. In parallel, the calculation of friction factor has been repeated numerically and results are compared with the experimental work. The validated numerical model was also used to gain an insight of flow physics by varying the aspect ratio and hydraulic diameter of rectangular microchannels with respect to the channel tested experimentally. This was done with an aim of verifying the role of minor loss coefficients for the estimation of the average friction factor. To have laminar, transitional, and turbulent regimes captured, numerical analysis has been performed by varying Reynolds number from 200 to 20,000. Comparison of numerically and experimentally calculated gas flow characteristics has shown that adiabatic wall treatment (Fanno flow) results in better agreement of average friction factor values with conventional theory than the isothermal treatment of gas along the microchannel. The use of a constant value for minor loss coefficients available in the literature is not recommended for microflows as they change from one assembly to the other and their accurate estimation for compressible flows requires a coupling of numerical analysis with experimental data reduction. Results presented in this work demonstrate how an adiabatic wall treatment along the length of the channel coupled with the assumption of an isentropic flow from manifold to microchannel inlet results in a self-sustained experimental data reduction method for the accurate estimation of friction factor values even in presence of significant compressibility effects. Results also demonstrate that both the assumption of perfect expansion and consequently wrong estimation of average temperature between inlet and outlet of a microchannel can be responsible for an apparent increase in experimental average friction factor in choked flow regime.
本文提出了一种结合数值和实验的方法，用于估算沿绝热微通道的可压缩气流的平均摩擦系数。对水力直径为295μm的矩形微通道进行了压降实验，雷诺数变化范围高达17000。同时，对摩擦系数进行了数值计算，并将结果与实验工作进行了比较。经验证的数值模型还用于通过改变矩形微通道的纵横比和水力直径（相对于实验测试的通道）来深入了解流动物理。这样做的目的是验证次要损失系数在估算平均摩擦系数中的作用。为了捕捉层流、过渡流和湍流状态，通过将雷诺数从200变化到20000进行了数值分析。数值计算和实验计算的气流特性比较表明，与沿微通道对气体进行等温处理相比，绝热壁处理（范诺流）能使平均摩擦系数值与传统理论更好地吻合。对于微流动，不建议使用文献中可用的次要损失系数的恒定值，因为它们在不同组件之间会发生变化，并且对于可压缩流的准确估算需要将数值分析与实验数据处理相结合。本文给出的结果表明，沿通道长度进行绝热壁处理，再加上从歧管到微通道入口的等熵流假设，会产生一种自维持的实验数据处理方法，即使在存在显著压缩效应的情况下，也能准确估算摩擦系数值。结果还表明，微通道进出口之间完美膨胀的假设以及由此导致的平均温度错误估算，可能是导致阻塞流状态下实验平均摩擦系数明显增加的原因。
