Analysis of heat transfer from single wires close to walls

Two-dimensional numerical investigations of the forced heat convection from a microcylinder in laminar cross-flow, both in free stream and in near-wall flow, were carried out aiming at a better understanding of the physics behind the wall effects on hot-wire near-wall measurements. In the physical model, an infinitely thin plate with the same properties as the fluid (air) was used as an
artificial wall. The conjugate heat transfer between the flow regions on both sides of the plate was taken into account. The effect of the conjugate thermal conditions (temperature distribution and diffusive heat flux) at the interface of the two flow regions on the heat transfer from the wire was investigated by varying the flow conditions on the side opposite to the wire location. Careful energy balance analysis was performed for both the free-stream case and the near-wall case. This enabled the authors to verify their own understanding of the physical mechanism responsible for the wall effect on hot-wire measurements and to examine other mechanisms proposed in the literature. The numerical results showed that the heat diffusion from the wire is significantly enhanced in the case of small wire-to-wall distances (Y+ < 3). This is mainly caused by modifications of the thermal boundary condition (diffusive effect) at the fluid-wall interface. In contrast, the flow distortion (enhanced convection) was shown not to be the most important influencing factor for the heat transfer of a hot wire. Although the present model study was performed for a laminar
flow, the results obtained are applicable to hot-wire measurements in turbulent flows, as stated in the literature.