Abstract Laminar-to-turbulent flow transition in microchannels can be useful to enhance mixing and heat transfer in microsystems. Typically, the small characteristic dimensions of these devices hinder in attaining higher Reynolds numbers to limit the total pressure drop. This is true especially in the presence of a liquid as a working medium. On the contrary, due to lower density, Reynolds number larger than 2000 can be easily reached for gas microflows with an acceptable pressure drop. Since microchannels are used as elementary building blocks of micro heat exchangers and micro heat-sinks, it is essential to predict under which conditions, the laminar-to-turbulent flow transition inside such geometries can be expected. In this paper, experimental validation of a two equations transitional turbulence model, capable of predicting the laminar-to-turbulent flow transition for internal flows as proposed by Abraham etal. (2008), is presented for the first time for microchannels. This is done by employing microchannels in which Nitrogen gas is used as a working fluid. Two different cross-sections namely circular and rectangular are utilized for numerical and experimental investigations. The inlet mass flow rate of the gas is varied to cover all the flow regimes from laminar to fully turbulent flow. Pressure loss experiments are performed for both cross-sectional geometries and friction factor results from experiments and numerical simulations are compared. From the analysis of the friction factor as a function of the Reynolds number, the critical value of the Reynolds number linked to the laminar-to-turbulent transition has been determined. The experimental and numerical critical Reynolds number for all the tested microchannels showed a maximum deviation of less than 12%. These results demonstrate that the transitional turbulence model proposed by Abraham etal. (2008) for internal flows can be extended to microchannels and proficiently employed for the design of micro heat exchangers in presence of gas flows.

中文翻译：

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可压缩微流的两方程 RANS 过渡湍流模型的实验验证

摘要 微通道中层流到湍流的转变可用于增强微系统中的混合和传热。通常，这些装置的小特征尺寸阻碍获得更高的雷诺数以限制总压降。这在存在液体作为工作介质的情况下尤其如此。相反，由于密度较低，对于具有可接受压降的气体微流，很容易达到大于 2000 的雷诺数。由于微通道被用作微型热交换器和微型散热器的基本构建块，因此必须预测在何种条件下，可以预期此类几何结构内的层流到湍流的转变。在本文中，实验验证了一个两方程过渡湍流模型，能够预测亚伯拉罕等人提出的内部流动的层流到湍流的转变。(2008)，首次提出微通道。这是通过采用其中氮气用作工作流体的微通道来完成的。两种不同的横截面，即圆形和矩形用于数值和实验研究。气体的入口质量流速变化以涵盖从层流到完全湍流的所有流态。对横截面几何形状和摩擦系数结果进行了压力损失实验，并比较了数值模拟。通过对作为雷诺数函数的摩擦因数进行分析，确定了与层流到湍流转变相关的雷诺数的临界值。所有测试微通道的实验和数值临界雷诺数显示出小于 12% 的最大偏差。这些结果证明了亚伯拉罕等人提出的过渡湍流模型。(2008) 的内部流动可以扩展到微通道，并熟练地用于设计存在气流的微型换热器。