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Optimering av dysa avseende induktion hos fri luftstråle - CFD-beräkning i Femlab 3
2004 (Swedish)Independent thesis Advanced level (degree of Master (One Year))Student thesis
Abstract [en]

Stifab Farex AB is a company that manufactures and markets products and solutions for ventilation and indoor climate systems. One of the company’s most important product groups is induction units. An induction unit uses supply air that is forced through small nozzles causing air jets to develop. These jets entrain indoor air from the room. The unit is constructed in a way that the indoor air that the jets entrain goes through a heating or cooling coil. The mixture of supply air and conditioned indoor air is delivered to the room to produce the desired climate. The rate of entrainment inside the unit is of decisive importance for the efficiency of the unit. The aim of this essay is to study the shape of the supply air nozzles with the view of finding the most important parameters that determine the rate of entrainment and the shape that generates the greatest possible rate of entrainment. Furthermore, the physics of entrainment are studied. This study is performed within the Department of Engineering, Physics and Mathematics at Karlstad University in Sweden. CFD simulation (Computional Fluid Dynamics) is used to study the influence of the nozzle shape on the rate of entrainment. The computer software used for the simulation is Femlab 3. A basic model that resembles an authentic nozzle, manufactured by Stifab Farex, is created. For validation of the basic model, LDV measurements (Laser Doppler Velocimetry) on a jet created with the authentic nozzle are performed in the laboratory. In addition to this, pressure measurements with a pitot tube in the centre of the jet are performed in the laboratory to investigate if the physics of entrainment can be explained with a difference in static pressure between the jet and the ambient air. Data from the laboratory measurements are prepared in the computer software Matlab, producing suitable charts and tables. With a starting point in the basic model, all modelling of new shapes are made with computer simulation, i.e. new models where only the shape of the nozzle differ from the basic model are created and solved with Femlab 3. The results from the computer simulation are analysed and compared with other studies concerning air jets and their entrainment. The pressure measurements with a pitot tube in the centre of the jet show that the physics of entrainment can not be explained with a difference in static pressure between the jet and the ambient air. Instead, the entrainment depends only upon the viscous forces between the jet and the ambient air, producing eddies that mix the ambient air with the jet. The simulation model does not agree well enough with the laboratory measurement to allow quantitative conclusions to be drawn. However, trends of certain variations in the nozzle shape should be possible to study qualitatively. The nozzle height and its degree of divergence at the outlet have the largest impact on the entrainment. A low height gives a larger entrainment than a high height and a convergent outlet gives a larger entrainment than a divergent outlet. The degree of divergence at the outlet also affects the flow through the nozzle. A convergent outlet gives a larger flow through the nozzle than a divergent outlet. In addition, also the radius of the curvature of the nozzle affects the flow through the nozzle. A small radius gives a smaller flow through the nozzle than a large radius. Even though a large flow trough the nozzle is favourable to the entrainment, the parameters of the nozzle that affects the flow and the entrainment oppose each other. The nozzle shape that best combines these parameters is the one currently in use in Stifab Farex’s products.

Place, publisher, year, edition, pages
2004. , 58 p.
Identifiers
URN: urn:nbn:se:kau:diva-54429Local ID: EMI-7bOAI: oai:DiVA.org:kau-54429DiVA: diva2:1103150
Subject / course
Environmental and Energy Systems
Available from: 2017-05-30 Created: 2017-05-30

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