Convection velocity of temperature fluctuations in a natural convection boundary layer

Abstract

Measurements of instantaneous temperature are carried out in a natural convection boundary layer (NCBL) developing over a vertical hot plate using a combination of an infra-red sensor, three cold-wire probes and a Resistance Temperature device (RTD). The aim of this study is to establish experimental procedures for high spatial and temporal resolution temperature measurements, and use that information to measure the convective speed of thermal structures in the flow. The evolution of NCBL along the hot plate is compared against existing empirical models. Statistics of mean and fluctuating temperature are in excellent agreement with the previously reported experimental data for a turbulent NCBL. The boundary layer thickness is observed to grow as y0.45, where y is the vertical distance along the heated plate. Separate experiments are conducted by varying the heat input to investigate its effect on convection velocity (Uc) of thermal structures in the flow. It is found that Uc remained nominally constant in the region 0 ≤ x ≤ δT/2 (δT is the thickness of thermal boundary layer) and decreased in a log-linear fashion in the outer region of NCBL. The results of Uc are found to be consistent with the shadowgraph measurements carried out in the same test rig, and the normalised convection velocity, Uc*=UcδTν is found to depend on Rayleigh number as Uc*∝Ra0.82, where ν is the kinematic viscosity of air. Importantly, these experimental observations are in agreement with the three layer model put forth by Wells and Worster (2008).