Heat Transfer in the Forced Laminar Wall Jet

The mean and fluctuating characteristics of a plane, unsteady, laminar, wall jet were investigated experimentally for a constant wall-temperature boundary condition. Temperature and streamwise velocity profiles, including the downstream development of the thermal and hydrodynamic boundary layer thicknesses, were obtained through simultaneous hot and cold wire measurements in air. Even at relatively low temperature differences, heating or cooling of a floor surface sufficiently altered the mean velocity profile in the inner, near-wall region to produce significant effects on the jet stability. Selective forcing of the flow at the most amplified frequencies produced profound effects on the temperature and velocity fields and hence the time-averaged heat transfer and shear stress. Large amplitude excitation of the flow ( up to 2 percent 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 65 percent, with an increase in the maximum wall heat flux as high as 45 percent. The skin friction and wall heat flux were much less susceptible to low-frequency excitation.