Effect of flow choking on experimental average friction factor of gas microflows

Pressure drop experiments are performed for a rectangular channel having a hydraulic diameter of 295μm (w=360μm, h=250μm) up to Re 16000. A validated numerical model is used to gain insight of flow physics inside employed microchannel test assembly. Comparison of numerical and experimentally calculated flow properties considering two different data reduction methodologies show that adiabatic treatment of gas results in a better agreement of average friction factor values with conventional theory in turbulent regime. Minor loss coefficients available in literature are not valid for microflows as they change from one assembly to other. This necessitates an estimation of minor loss coefficients as a priori which can be established using a validated numerical model of the experimental test rig. However, such a treatment of minor loss coefficients adds an additional step of establishing a well posed numerical model before each experiment and hence is not convenient at all from experimentalist point of view. An adiabatic treatment of the gas along the length of the channel coupled with isentropic flow assumption from manifold to microchannel inlet results in a self-sustained experimental data reduction and therefore should be followed in consequent gas flow studies. Furthermore, assumption of perfect expansion and wrong estimation of average gas temperature between inlet and outlet results in an apparent increase of experimental friction factor in highly turbulent choked regime. Literature has been divided into two main approaches for establishing experimental average frictional characteristics in micro channels (MCs). When a total pressure drop and inlet temperature are available, a classical methodology is to invoke minor loss coefficients and subtract pressure losses associated to inlet/outlet manifold. Resulting pressure difference is then utilized along with measured temperature at manifold inlet to calculate average isothermal fanning friction factor. Such a treatment is quite realistic when an incompressible liquid working fluid is utilized but has been applied to compressible flows as well in the past [1]. In reality, a gas microflow does not stay isothermal and shows a strong temperature decrease close to outlet for adiabatic walls. For an adiabatic flow, temperature estimation at MC outlet can be done using a quadratic equation proposed by [2]. Data reduction methodology where minor losses are utilized along with the temperature estimation at outlet, is referred to as M1 in the subsequent text. An alternative methodology (M2), originally proposed by [2] is to estimate MC inlet flow properties by assuming isentropic flow between inlet manifold and MC inlet. This automatically caters for a reduction in MC inlet pressure and hence inlet coefficient is not required. Main aim of current study is to investigate underlying differences and their effects on experimental average friction factor between above stated methodologies in the presence of flow choking. An establishment of a unique methodology for future compressible gas experimentalists is also intended.