Ilinca Nastase

Vortex Dynamics and Entrainment Mechanisms in Low Reynolds Orifice Jets

Classical planar 2D-PIV measurements and time-resolved visualizations enriched by low-level processing are used for the reconstruction of the Kelvin-Helmholtz vortex passing in the near field of a circular and a 6-lobed orifice jet flow. In the circular jet, the entrainment is produced in the braid region, being interrupted in the presence of the Kelvin-Helmholtz ring. The latter compresses the streamwise vortices and alters their self-induction role. Conversely, the 6-lobed orifice geometry allows the cutting of the Kelvin-Helmholtz structures into discontinuous ring segments. Consequently, into these discontinuity regions streamwise large scale structures are developing. These streamwise structures are permanent thus controlling and enhancing the jet entrainment which is not altered by the Kelvin-Helmholtz structures passing.

Analysis of jet entrainment mechanism in the transitional regime by time-resolved PIV

The entrainment mechanism in the near field of daisy-shaped and circular orifice jets have been investigated in the transitional regime using time-resolved 2D PIV measurements. The objective is to improve the knowledge from one previous investigation at initial Reynolds number of 800, based on the construction of a pseudo-time resolved PIV fields using the combination of non time-resolved PIV measurements and time-resolved visualizations (Nastase and Meslem J Vis 11(4):309–318, 2008). As expected in the previous work, the entrainment in the circular jet is correlated to the periodic Kelvin–Helmholtz (K–H) ring passing and the entrainment is produced in the braid region where the streamwise structures develop. In the daisy jet, we found that the entrainment rate is not correlated to the periodic K–H vortex passing. The observed small variation of the entrainment rate amplitude in the daisy jet could be related to the K–H dynamics. However, at the studied low Reynolds number the contribution of the K–H vortex on the daisy jet entrainment seems negligible comparing with the streamwise structures role. Furthermore, the real-time resolved measurements allow an indepth analysis of the role played by the K–H ring in the entrainment of circular jet. It is shown that the entrainment is not only produced in the braid region but is also present in the upstream part of the K–H ring. In the downstream part of the ring, the entrainment is dramatically reduced. This new observation opens a question which still has to be answered with time-resolved 3D PIV measurements. The question is “Whether the depression formed due to the ring passing or the streamwise structures rolled-up on the ring is responsible for entrainment at the upstream part of the ring?”Graphical Abstract

Improved inhaled air quality at reduced ventilation rate by control of airflow interaction at the breathing zone with lobed jets

Inhaled air quality at a reduced supply of clean air was studied by controlling the airflow interaction at the breathing zone of a person using lobed jets as part of personalized ventilation (PV). Experiments were performed in a full-scale test room at 23°C (73.4°F) with a breathing thermal manikin seated at a workstation, with realistic free-convection flow around the body and a normal breathing cycle. The air in the room was mixed with tracer gas R134a. Clean air was supplied isothermally from three nozzles with circular, four-leafed clover, and six-edged star openings of 0.025 m (0.08 ft) equivalent diameter. The nozzles were positioned frontally at the face within the boundary layer and centered to the mouth. The enhancement of inhaled air quality by changing the initial velocity (0.2–0.6 m/s, 0.66–1.97 fps) and the distance from the mouth (0.02–0.06 m, 0.07–0.20 ft) was studied. The control over the interaction between the inserted jets and the free convection flow was efficient. Over 80% clean PV air was measured in inhalation. The worst performing nozzle was the four-leafed clover: its best performance yielded 23% clean air inhalation, at the shortest distance and the highest velocity. The other lobed nozzle, the six-edged star, performed similarly to the circular nozzle.

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.

Intelligent control of HVAC systems. Part II: perceptron performance analysis

This is the second part of a paper on intelligent type control of Heating, Ventilating, and Air-Conditioning (HVAC) systems. The whole study proposes a unified approach in the design of intelligent control for such systems, to ensure high energy efficiency and air quality improving. In the first part of the study it is considered as benchmark system a single thermal space HVAC system, for which it is assigned a mathematical model of the controlled system and a mathematical model (algorithm) of intelligent control synthesis. The conception of the intelligent control is of switching type, between a simple neural network, a perceptron, which aims to decrease (optimize) a cost index, and a fuzzy logic component, having supervisory antisaturating role for neuro-control. Based on numerical simulations, this Part II focuses on the analysis of system operation in the presence only of the neural control component. Working of the entire neuro-fuzzy system will be reported in a third part of the study.

An Adaptive Thermal Comfort Model for the Romanian Climate

Human thermal comfort (HTC) embraces two major approaches, Fanger or classical theory and an adaptive one. Adaptive HTC equations make up parts of the worldwide recognized thermal comfort standards. The balance between thermal comfort and energy saving is held by the adaptive approach of thermal comfort. The use of adaptive HTC equations in the evaluation of existing buildings and in the design of new buildings has led to an important decrease in energy consumption and a minimization of building maintenance costs.

First adaptive thermal comfort equation for naturally ventilated buildings in Bucharest, Romania

ABSTRACT National and international adaptive thermal comfort (ATC) standards gain now wider acceptance. These standards propose adaptive comfort equations derived from specific databases. New ATC equations are developed worldwide, under different latitudes and climates. A field survey has been performed in 2013–2014 in six naturally ventilated buildings located in Bucharest (Romania, Köppen type D - temperate continental climate). Comfort parameters were measured and comfort questionnaires were distributed. Using the theory of ASHRAE 55 Standard an ATC equation specific to Romanian climate has been proposed for the first time. Also, the 80% and 90% acceptability bands were determined in this situation. The new ATC equation has been compared with other ATC equations found in literature for similar climates. New thermal comfort investigations have been performed one year later after the first survey, new comfort investigations were performed. The ATC equation has been checked and good agreement has been found.

Preliminary Results Concerning the Thermal Comfort in a Romanian Passive House

Energy saving is a recognized priority worldwide. The goal for the construction industry is to conceive and operate buildings with low energy consumption. However, sometimes energy saving can cause discomfort to the occupants. A field survey was done in a naturally ventilated passive office building during summer. Comfort parameters were measured and occupants were distributed comfort questionnaires consisting of questions related to their thermal sensation and preference.

Thermal Comfort Analyses in Naturally Ventilated Buildings

Abstract Global current requirement is to increase thermal comfort in residential and non residential buildings. A field survey was accomplished in a naturally ventilated university classroom in Bucharest, Romania, in winter and spring. Comfort parameters were measured and comfort questionnaires were distributed to the students. Questions were related to thermal sensation of the occupants. This paper compares the experimental results with the occupant’s response. It analyzes the variation of Predicted Mean Vote (PMV) and Predicted Percent of Dissatisfied (PPD) with temperature. It is made a comparison between PMV and thermal sensation vote. The results show PMV values different from Thermal Sensation Vote (TSV) values which means there is a poor approximation of indoor comfort. In conclusion the comfort parameters should be reviewed and should be proposed other evaluation methods. Possible explanations are discussed in relation with thermal regime of the buildings.

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.