Heat Transfer Measurement on a Turbulent Spot Using the Energy Balance Method

Abstract

A turbulent spot is a small turbulent patch, surrounded by laminar flow at a boundary layer transition. It is an important mechanism, clearly showing the characteristics of highly turbulent flow and heat transfer inside the body. However, measurement in heat transfer from a flat plate to a fluid via a turbulent spot remains difficult due to the unsteady and unstable nature of the site. This paper detailed a temperature measurement technique using a coating of thermochromic liquid crystal. An analytical solution, derived from the developed energy balance method and an image processing technique were combined to visualize the thermal footprint of a turbulent spot. The experiment was performed over an isothermal flat plate in a low turbulence water tunnel where the turbulent intensity was lower than 1%. Reynolds number was initially kept at 75,000, as a reference rate for laminar flow. Transitional flow was bypassed by a small disturbance injection. The results were presented qualitatively and quantitatively using color, temperature, the heat transfer coefficient and heat flux as spots were propagated downstream. The locations of the leading and trailing edges were also clearly visualized. Turbulent spot information (spot celerities and the half spreading angle) were in agreement with those obtained by the other researchers, confirming the reliability of this technique. Thus, this energy balance method can be applied effectively to obtain insight into the bypass boundary layer transition over a flat plate.