Controlling instability waves on vertical natural convection using a buoyant impinging jet

Abstract

Natural convective heat transfer is typically enhanced by installing fin arrays and other devices that increase the surface area; however, large surfaces present various problems in their implementation. In contrast, the impact of a jet on a natural convection boundary layer has been reported to produce periodically alternating T- and L-flows, whose disturbance may yield flow resonance with the periodic flows present in the transition length of the natural convection boundary layer. The control of instability waves, including flow resonance, is expected to have applications in enhancing convective heat transfer over large surfaces. This study focuses on the jet temperature (between ambient temperature 296 K and wall temperature 316 K), and jet velocity (Reynolds number 95 ≤ Re ≤ 150). Based on frequency analysis, the instability waves downstream of the heated wall are classified into three categories: resonance, convective instability, and turbulence. The analysis reveals that when resonance and turbulence take place, the heat transfer is enhanced compared to natural convection, however, when convective instability takes place, heat transfer depends on the jet temperature. Furthermore, we show that an appropriate choice of jet temperature and velocity can control the instability waves over the natural convection boundary layer.