An investigation of wall effects on hot-wire measurements using a bent sublayer probe

Abstract

The effects of aluminium and Perspex walls on near-wall hot-wire measurements were investigated in a fully developed channel flow of 250 mm width, 1.6 mm (laminar) or 16.6 mm (turbulent) height using specially made bent sublayer probes of 5 μm, 1.27 μm and 0.63 μm diameter that had minimum aerodynamic interference. Previous experiments by Wills indicated that the heat loss from the wire expressed in terms of Nusselt number (Nu) versus the Reynolds number (Rew based on velocity at the wire location and wire diameter) for various fixed distances from the wall does not converge to the wall-remote behaviour at large Rew or Y+ (the distance from the wall normalized by the viscous length scale). Other works show conflicting trends of the effects of wall thermal conductivity, wire diameters and overheat ratio on the operation of the hot wire in the vicinity of the wall. The present effort represents a systematic approach to evaluate each of these parameters on the hot-wire performance near the wall in terms of Nu and measured dimensionless velocity U+ (taken wrt wall shear velocity) for comparison to previous works and to resolve some of the apparent controversies. Experiments were also conducted in both fully developed laminar and turbulent channel flows to determine the near-wall correction curves in order to resolve the contradictory suggestions by Wills, Demin and Kuraev, and Zemskaya et al regarding laminar and turbulent near-wall corrections. The present investigations found that contrary to the previous work of Wills, the behaviour of Nu versus Rew for a hot wire at given fixed distances from the wall converges to the wall-remote condition for large Rew or K+. This finding was consistent for the three different wire diameters used and two walls of different thermal conductivities. The discrepancy with Wills' results is attributed to aerodynamic interference and blockage effects caused by the near-wall hot-wire prongs in a narrow channel flow. The critical distances from the wall, Yc +, beyond which the wall influences on hot-wire measurements can be considered insignificant, were found to decrease with decreasing wire diameter and decreasing wall thermal conductivity. They were found to be approximately 5.0, 3.7, 3.0 for the aluminium wall and 3.0, 2.7, 2.2 for the Perspex wall for respective wire diameters of 5 μm, 1.27 μm, 0.63 μm. It was also found that the overheat ratio plays an insignificant role provided that the length to diameter ratio of the wire is sufficiently larger than 200 to minimize three-dimensionaj effects. The resultant mean U+ distribution wrt Y+ obtained in fully developed channel flow was found to resemble the counterpart obtained in laminar flow. This finding concurs with the experimental results of Zemskaya et al performed in boundary layer flows, but contradicts the conclusion by Wills who recommended half the laminar correction for turbulent flow. It is suspected that the experimental results of Wills are affected by different degrees of prong interference and blockage effects for the laminar and turbulent channel flows which may account for the discrepancy.