Unsteady heat transfer and flow structure of a row of laminar impingement jets, including vortex development

Considered is a cylindrical channel with a single row of 10 aligned impinging jets, with exit flow in the axial direction at one end of the channel. For the present predictions, each jet is laminar with a Reynolds number of 200. An unsteady FLUENT solver is employed for predictions of flow characteristics within and nearby the 10 impingement jets. Instantaneous, local surface Nusselt numbers oscillate as time progresses, such that the local Nusselt number signature from one individual vortex diminishes in intensity, as the opposite vortex in the same pair then produces an increase in the local, instantaneous Nusselt number signature. Spectrum analysis of different flow quantities shows frequencies associated with laminar jet and vortex oscillations, and evidence more orderly flow for Re = 200, without the chaos and broad-band mixing associated with the turbulent flow when Re = 15,000. Laminar flow spectra also evidence increased flow unsteadiness as cross-flows accumulate within the impingement channel with streamwise development as Z/D increases. In some cases, this increased unsteadiness manifests itself through the formation of multiple spectral peaks, in place of single peak spectra. Similar with turbulent jet arrangements, the unsteady, local static pressure gradient variations along interfaces between laminar jets and cross flow are also a key flow feature, which is connected to the initiation and development of the Kelvin-Helmholtz instability induced vortices. (C) 2015 Elsevier Ltd. All rights reserved.