Experimental study on the effect of pulsating inflow to an enclosure for improved mixing

• Published in: The International journal of heat and fluid flow 2013-12, Vol.44, p.108-119
• Main Authors: Fallenius, Bengt E.G., Sattari, Amir, Fransson, Jens H.M., Sandberg, Mats
• Format: Article
• Language: English
• Subjects: Applied sciences; Building technical equipments; Buildings; Buildings. Public works; Computational fluid dynamics; Environmental engineering; Exact sciences and technology; Flow rate; Fluid flow; Inflow; Kelvin-Helmholtz instability; Pulsating flow; Pulsation; Roll-up of vortices; Room ventilation; Stagnation; Ventilation; Ventilation. Air conditioning; Vortex statistics; Vortices; Wall-jet instability
• ISSN: 0142-727X; 1879-2278; 1879-2278
• DOI: 10.1016/j.ijheatfluidflow.2013.05.004

•Pulsating inflow is compared to an increased flow rate from a mixing point of view.•PIV measurements are performed on a 2-D room model with water as medium.•An in-house vortex detection program is used to extract statistics about vortical structures.•The number of vortices is quadrupled when the inflow is pulsated.•Number and size of stagnation zones are reduced as the inflow is pulsated. Optimal control of inlet jet flows is of broad interest for enhanced mixing in ventilated rooms. The general approach in mechanical ventilation is forced convection by means of a constant flow rate supply. However, this type of ventilation may cause several problems such as draught and appearance of stagnation zones, which reduces the ventilation efficiency. A potential way to improve the ventilation quality is to apply a pulsating inflow, which has been hypothesised to reduce the stagnation zones due to enhanced mixing. The present study aims at testing this hypothesis, experimentally, in a small-scale two-dimensional water model using Particle Image Velocimetry with an in-house vortex detection program. We are able to show that for an increase in pulsation frequency or alternatively in the flow rate the stagnation zones are reduced in size and the distribution of vortices becomes more homogeneous over the considered domain. The number of vortices (all scales) increases by a factor of four and the swirl-strength by about 50% simply by turning on the inflow pulsation. Furthermore, the vortices are well balanced in terms of their rotational direction, which is validated by the symmetric Probability Density Functions of vortex circulation (Γ) around Γ=0. There are two dominating vortex length scales in the flow, namely 0.6 and 0.8 inlet diameters and the spectrum of vortex diameters become broader by turning on the inflow pulsation. We conclude that the positive effect for enhanced mixing by increasing the flow rate can equally be accomplished by applying a pulsating inflow.