Experimental and numerical investigation of unsteady behaviour in the near-field of pure thermal planar plumes

A near-field, unsteady buoyant plume exhibits puffing behaviour, characterised by the periodic formation of large-scale vortical structures (puffs). It has been observed in numerical simulations of pure thermal plumes with a finite area source that the periodic formation of puffs is associated with an instability of the lapping flow that develops over the heated region away from the plume centreline, producing bulge-like structures. Experiments using a shadowgraph technique and two-dimensional, two-component particle image velocimetry are conducted to validate our numerical results for the near-field, unsteady behaviours of a pure thermal planar plume, with water as the working fluid. The formations of bulges in the lapping flow and associated puffs are also observed in transient flow fields obtained in the experiments. Further, quantitative comparison of mean velocity field and oscillation frequencies obtained in the experiments and numerical simulation shows reasonable agreement. This experimental study confirms the important effect of the lapping flow instability on the near-field unsteady behaviour, which has been highlighted in numerical simulations.

► The near-field unsteady behaviour of a pure thermal plume was experimentally studied.
► A low cost, in-house PIV system was used.
► Consistent unsteady behaviour was observed in experiments and numerical simulations.
► Statistics also showed good agreement in experiments and numerical simulations.