Origins of radiometric forces on a circular vane with a temperature gradient

Abstract

Radiometric force on a 0.12 m circular vane is studied experimentally and numerically over a wide range of pressures that cover the flow regimes from near free molecular to near continuum. In the experiment, the vane is resistively heated to about 419 K on one side and 394 K on the other side, and immersed in a rarefied argon gas. The radiometric force is then measured on a nano-Newton thrust stand in a 3 m vacuum chamber and compared with the present numerical predictions and analytical predictions proposed by various authors. The computational modelling is conducted with a kinetic approach based on the solution of ellipsoidal statistical Bhatnagar–Gross–Krook (ES-BGK) equation. Numerical modelling showed the importance of regions with elevated pressure observed near the edges of the vane for the radiometric force production. A simple empirical expression is proposed for the radiometric force as a function of pressure that is found to be in good agreement with the experimental data. The shear force on the lateral side of the vane was found to decrease the total radiometric force.

Experimental measurements and numerical simulations of the Knudsen force on a non-uniformly heated beam

“…11(a and b), the ESBGK simulations demonstrate a net force is generated in the direction opposing the thermal gradient for all cases. The pressure profile is non-uniform over the width, a behavior observed in similar studies on radiometric phenomena [10,3]. This behavior stems from the collisional nature of the flow.…”

“…In the center of the heating element, the number flux of molecules colliding with the surface from all directions is essentially determined by the temperature of the device. Near the edge, the number flux is dictated by both the thermalized molecules above the heating element as well as the cooler molecules approaching from the bulk, leading to an increase in overall number flux in a region near the edge, and ultimately, a net increase in pressure [10]. As the Knudsen number approaches the free molecule limit, the pressure distribution is expected to become uniform due to the corresponding uniform temperature of incident species.…”

“…Interestingly, the shearing stress on the lateral walls of the heating element is upwards for both heating conditions. Based on the findings of previous work [10,13], the shear stress would be expected to act against the influence of normal pressure between the hot and cold surfaces. While this behavior is indeed seen for the positive gradient configuration, the shear stress acts to reinforce the net normal pressure for the negative gradient, an effect resulting from the close proximity of the substrate relative to other boundaries such as vacuum chamber walls.…”

“…The mechanisms governing force production show strong dependence on the nature of thermal gradients within the system, resulting in a complex interplay of thermal creeping effects [10,11]. Expanding on previous experimental [8] and numerical studies [9], the aim of this work is to demonstrate the potential for bidirectional actuation and Knudsen force enhancement through manipulation of thermal gradients and ambient pressure within the system.…”

Effect of vane thickness on radiometric force

“…Note that a full three-dimensional model for the chosen geometrical set-up is not possible, since it would require a prohibitive amount of computer time (up to a year on a modern thousand-processor computer). Using a smaller domain or a coarse three-dimensional grid would unpredictably skew the results, as shown by Selden et al (2009b). Therefore, the axially symmetric/two-dimensional capability of SMOKE has been used in this work.…”