Florin Bode

Optimization of a Lobed Perforated Panel Diffuser - A Numerical Study of Orifice Arrangement

Abstract Heating Ventilating and Air Conditioning (HVAC) systems are primarily designed for ensuring good indoor air quality and thermal comfort. However, building energy requirements tend to put demand on reducing air change rates. Passive control of jet flows in order to enhance mixing and entrainment may be a solution to this problem. Our purpose is to develop new air diffusers, in order to ameliorate the users’ thermal comfort and air quality. When the diffuser is a lobed perforated panel (Meslem et al, 2010), the optimization of jet entrainment consists of optimizing the spacing between neighboring orifices and their relative arrangement on the panel. The objective is to conceive a perforated panel diffuser with high entrainment and high perforation rate. In a recent study (Meslem et al, 2011) the flow field of a turbulent twin cross-shaped jet was investigated experimentally and numerically using different turbulence models. In comparison to Particle Image Velocimetry (PIV) measurements, it was shown that among the investigated turbulence models, the SST k-ω model is capable of reproducing reasonably well jet interaction, global expansion and ambient air entrainment when the flow is numerically resolved through the lobed diffuser. Based on the previous experimental validation of the SST k-ω turbulence model, in this work, numerical simulations of parallel cross-shaped jets in different flow configurations are analysed using this model. This study deals with the effect of orifice arrangement and spacing on jet entrainment. The parametric study put into evidence an optimal configuration which will be transferred to the scale of a real air diffuser.

Influence of the choice of the inlet turbulence intensity on the performance of numerically simulated moderate Reynolds jet flows - Part 1 - the near exit region of the jet

A real problem when trying to develop a numerical model reproducing the flow through an orifice is the choice of a correct value for the turbulence intensity at the inlet of the numerical domain in order to obtain at the exit plane of the jet the same values of the turbulence intensity as in the experimental evaluation. There are few indications in the literature concerning this issue, and the imposed boundary conditions are usually taken into consideration by usage without any physical fundament. In this article we tried to check the influence of the variation of the inlet turbulence intensity on the jet flow behavior. This article is focusing only on the near exit region of the jet. Five values of the inlet turbulence intensity Tu were imposed at the inlet of the computational domain, from 1.5% to 30%. One of these values, Tu= 2% was the one measured with a hot wire anemometer at the jet exit plane, and another one Tu= 8.8% was issued from the recommendation of Jaramillo (1). The choice of the mesh-grid and of the turbulence model which was the SST k-ω model were previously established (2). We found that in the initial region of the jet flow, the mean streamwise velocity profiles and the volumetric flow rate do not seem to be sensitive at all at the variation of the inlet turbulence intensity. On the opposite, for the vorticity and the turbulent kinetic energy (TKE) distributions we found a difference between the maximum values as high as 30%. The closest values to the experimental case were found for the lowest value of Tu, on the same order of magnitude as the measurement at the exit plane of the jet flow. Mean streamwise velocity is not affected by these differences of the TKE distributions. Contrary, the transverse field is modified as it was displayed by the vorticity distributions. This observation allows us to predict a possible modification of the entire mean flow field in the far region of the jet flow.