Velocity Measurement of Rarefied Gas Flow using Hot Wire Anemometry (HWA)

Abstract

This work presents the experimental study of velocity measured using Hot Wire Anemometry (HWA) for isothermal subsonic rarefied nitrogen flow for a straight tube. HWA is widely used to measure velocity in the continuum condition. However the velocity measurement using HWA in the subsonic rarefied gas flow condition is a challenging task as the density and velocity of flow are very less. To achieve this objective correlation used for the velocity determination for continuum flow needs to be modified including the temperature jump and slip effect over the hot wire. In the present work, HWA is calibrated for the continuum flow with the help of Pitot static tube. A density correction factor is applied to measure the velocity using HWA in subsonic rarefied gas flow condition. Measurements of the velocity are carried out by inserting HWA in axial direction for larger size tube and in the radial directions for small size tube. Continuum to rarefied flow conditions are developed by using tubes of different diameters and lengths and applying additional resistance. The additional resistance is generated using the smaller dimensional tube between the test section and outlet reservoir. Using these concepts, measurement of the velocity using HWA is possible up to 0.0067 Knudsen number. The corresponding velocity found is 22.24 m/s. Conduction losses and radiation losses are calculated by incorporating conductivity and resistivity variation along the length wire. By considering the properties variation, convective heat transfer is improved by 5-6 % than previous study. Two correlations between Nusselt, Reyolds and Knudsen number are developed to find the velocity for the moderately rarefied gas flow.