Augmentation of heat transfer in a laminar wall jet by selective forcing

Abstract

In this investigation an attempt was made to understand the dominant mechanisms of heat and momentum transport in an externally excited wall jet. The mean and fluctuating characteristics of this flow were experimentally evaluated for a constant wall temperature boundary condition. Temperature and streamwise velocity profiles were obtained through simultaneous hot and cold wire measurements in air. Selective forcing of the flow at the most amplified frequencies produced profound effects on the temperature and velocity fields and hence the time-averaged wall heat transfer and shear stress. Large amplitude excitation of the flow (up to 2% of the velocity measured at the jet exit plane) at a high frequency resulted in a reduction in the maximum skin friction by as much as 60% with an increase in the maximum wall heat flux as high as 30%. The skin friction and wall heat flux were much less susceptible to low frequency excitation. These profound effects on the skin friction and heat transfer present a breakdown in the Reynolds analogy. The Reynolds analogy factor increased significantly relative to the unforced case by as much as 200% for high frequency forcing at a large excitation level. Thus, the ability to predict the heat transfer in a wall jet from a known hydrodynamic solution is restricted in the presence of large amplitude disturbances. The amplitude and phase distributions for the fluctuating streamwise velocity and temperature demonstrate, that for large excitation levels, a sub-harmonic wave experiences substantial growth in the measurement domain. Significant distortion of the sub-harmonic component of the fluctuating temperature provides evidence that these large scale structures are responsible for the significant widening of the boundary layer and the transport of energy and momentum away from the surface. This motion may also explain the increase in the temperature gradient near the surface since the unsteady upward coherent transport is increased compared to diffusive transport in this region. Temperature and velocity profiles were also acquired at different spanwise locations. Consistent with previous flow visualization studies, it was found that the transition process (including the coherent transport of the sub-harmonic wave) is two-dimensional in nature.