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Investigations of Flow Patterns in Ventilated Rooms Using Particle Image Velocimetry: Applications in a Scaled Room with Rapidly Varying Inflow and over a Wall-Mounted Radiator
KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Building Service and Energy Systems.
2015 (English)Doctoral thesis, comprehensive summary (Other academic) [Artistic work]
Abstract [en]

This thesis introduces and describes a new experimental setup for examining the effects of pulsating inflow to a ventilated enclosure. The study aimed to test the hypothesis that a pulsating inflow has potential to improve ventilation quality by reducing the stagnation zones through enhanced mixing. The experimental setup, which was a small-scale, two-dimensional (2D), water-filled room model, was successfully designed and manufactured to be able to capture two-dimensional velocity vectors of the entire field using Particle Image Velocimetry (PIV). Using in-house software, it was possible to conclude that for an increase in pulsation frequency or alternatively in the flow rate, the stagnation zones were reduced in size, the distribution of vortices became more homogeneous over the considered domain, and the number of vortices in all scales had increased. Considering the occupied region, the stagnation zones were moved away in a favorable direction from a mixing point of view. In addition, statistical analysis unveiled that in the far-field occupied region of the room model, stronger eddies were developed that we could expect to give rise to improved mixing. As a fundamental experimental study performed in a 2D, small-scale room model with water as operating fluid, we can logically conclude that the positive effect of enhanced mixing through increasing the flow rate could equally be accomplished through applying a pulsating inflow.

In addition, this thesis introduces and describes an experimental setup for study of air flow over a wall-mounted radiator in a mockup of a real room, which has been successfully designed and manufactured. In this experimental study, the airflow over an electric radiator without forced convection, a common room-heating technique, was measured and visualized using the 2D PIV technique. Surface blackening due to particle deposition calls for monitoring in detail the local climate over a heating radiator. One mechanism causing particle deposition is turbophoresis, which occurs when the flow is turbulent. Because turbulence plays a role in particle deposition, it is important to identify where the laminar flow over radiator becomes turbulent. The results from several visualization techniques and PIV measurements indicated that for a room with typical radiator heating, the flow over the radiator became agitated after a dimensionless length, 5.0–6.25, based on the radiator thickness.

Surface properties are among the influencing factors in particle deposition; therefore, the geometrical properties of different finishing techniques were investigated experimentally using a structured light 3D scanner that revealed differences in roughness among different surface finishing techniques. To investigate the resistance to airflow along the surface and the turbulence generated by the surfaces, we recorded the boundary layer flow over the surfaces in a special flow rig, which revealed that the types of surface finishing methods differed very little in their resistance and therefore their influence on the deposition velocity is probably small. 

Abstract [sv]

Det övergripande syftet med den första studien i avhandlingen var att undersöka hypotesen att ett pulserande inflöde till ett ventilerade utrymme har en potential till att förbättra ventilationens kvalitet genom att minska stagnationszoner och därigenom öka omblandningen. För genomförande av studien byggdes en experimentuppställning i form av en tvådimensionell (2D) småskalig modell av ett ventilerat rum. Strömningsmediet i modellen var vatten. Det tvådimensionella hastighetsfältet registrerades över hela modellen med hjälp av Particle Image Velocimetry (PIV). Vid ett stationärt tillflöde bildas ett stagnationsområde i centrum av rumsmodellen. Vid ett pulserade inflöde genererades sekundära virvlar. Med en egen utvecklad programvara var det möjligt att kvantifiera statistiken hos virvlarna. Det pulserade inflödet gjorde att inom området där det vid stationärt tillflöde fanns en stagnationszon ökade antalet virvlar i alla storlekar och fördelningen av virvlar blev mera homogen än tidigare. Detta kan förväntas ge upphov till förbättrad omblandning. Baserat på en grundläggande experimentell studie utförd i en småskalig tvådimensionell rumsmodell med vatten som strömningsmedium kan vi logiskt dra slutsatsen att ett pulserande tilluftsflöde har en potential att förbättra omblandningen. 

I en fortsatt studie i avhandlingen visuliserades och mättes hastighetsfältet och därefter beräknades statistiska värden av exempelvis medelhastighet, standardavvikelse och skjuvspänning hos hastighetsfluktuationerna i luftströmmen över en väggmonterad radiator med 2D-PIV-teknik.  Bakgrunden till studien är att en bidragande orsak till partikelavsättning på väggytor är turbofores som uppträder vid en turbulent luftström. Studien genomfördes genom uppbyggnad av en fullskalig rumsmodell. Eftersom turbulens spelar en roll vid partikelavsättning genom turbofores är det viktigt att identifiera var det laminära flödet över radiatorn blir turbulent. Resultaten baserat på visualisering och PIV-mätningar indikerade att, för ett rum med denna typ av radiatoruppvärmning, blev flödet över radiatorn turbulent efter en dimensionslös längd lika med 5,0‒6,25 gånger radiatorns tjocklek.

Ytors egenskaper är viktiga vid partikelavsättning. Därför har de geometriska egenskaperna hos några olika metoder för ytbehandling undersökts experimentellt med hjälp av en scanner för strukturerat 3D-ljus. Resultaten visar på skillnader i ytråhet hos de olika ytbehandlingsmetoderna. För att undersöka motståndet mot luftströmning längs ytan och den turbulens som genereras av ytorna registrerade vi gränsskiktsflödet över ytorna i en speciell luftströmningsrigg. Detta påvisade att motståndet hos de olika typerna av ytbehandlingsmetoder skilde sig mycket litet åt och därför är troligt vid deras påverkan på depositionshastigheten mycket liten. 

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2015. , vi, 38 p.
Series
TRITA-IES, 2015:02
Keyword [en]
Particle Image Velocimetry (PIV), experimental study, structured light 3D scanning system, ventilation, varying flow rate, room model, wall-mounted radiator, air, water, flow
Keyword [sv]
Particle Image Velocimetry (PIV), experimentell studie, scanningsystem för strukturerat 3D-ljus, ventilation, varierande tilluftsflöde, rumsmodell, väggmonterad radiator
National Category
Other Engineering and Technologies not elsewhere specified Civil Engineering Mechanical Engineering
Research subject
Engineering Mechanics; Civil and Architectural Engineering; Energy Technology
Identifiers
URN: urn:nbn:se:kth:diva-167804ISBN: 978-91-7595-600-8 (print)OAI: oai:DiVA.org:kth-167804DiVA: diva2:813475
Public defence
2015-06-10, Hall F3, Lindstedtsvägen 26, KTH, Stockholm, 15:40 (English)
Opponent
Supervisors
Note

QC 20150525

Available from: 2015-05-25 Created: 2015-05-22 Last updated: 2015-05-25Bibliographically approved
List of papers
1. PIV Visualisation study in a two-dimensional room model with rapid time varying ventilation flow rates
Open this publication in new window or tab >>PIV Visualisation study in a two-dimensional room model with rapid time varying ventilation flow rates
2011 (English)In: ROOMVENT, The 12th International Conference on Air Distribution in Rooms / [ed] Vojislav Novakovic, Sten Olaf Hanssen, Hans Martin Mathisen, 2011Conference paper, Published paper (Refereed)
Abstract [en]

Optimal control of inlet jet flows is of wide applicative interest in order to enhance mixing and reduce stagnation in a ventilated room. The general approach in mechanical ventilation is to use a constant flow rate forced convection system providing the ventilation air. This type of ventilation may cause several problems such as draught, stagnation at certain occupied locations, and subsequently low ventilation efficiencies. An alternative to increase the ventilation quality that has been of interest in this study is to introduce flow variations, which is considered as a potential to reduce stagnation and increase efficiency of the ventilation. The study was conducted as a model experiment in a small-scale, two-dimensional (2-D) room model with dimensions 30200.9 cm3 with water as operating fluid. The size of the model made it possible to investigate the 2-D velocity vector field within the entire room using Particle Image Velocimetry (PIV) method and further consequent dynamical and statistical analyses have been done from the resulted PIV vector fields. The comparison between cases of constant flow rate and flow variations have been conducted for the cases of two set of base flow rates and for each one, the cases of constant flow rate and flow variations with frequencies of 0.3, 0.4 and 0.5 Hz, is considered. In this investigation we show that the calm region, with a large stagnation zone, without pulsating inflow condition becomes more active in the sense that the stagnation points are moved and that the small-scale structures are grown for increasing pulsation frequency.

Keyword
PIV, room model, ventilation, varying flow rate
National Category
Other Engineering and Technologies not elsewhere specified
Identifiers
urn:nbn:se:kth:diva-87074 (URN)9788251928120 (ISBN)
Conference
ROOMVENT, The 12th International Conference on Air Distribution in Rooms, June 19-22, Trondheim
Note
QC 20120411Available from: 2012-02-14 Created: 2012-02-14 Last updated: 2015-05-25Bibliographically approved
2. PIV Study of Ventilation Quality in Certain Occupied Regions of a Two-Dimensional Room Model with Rapidly Varying Flow Rates
Open this publication in new window or tab >>PIV Study of Ventilation Quality in Certain Occupied Regions of a Two-Dimensional Room Model with Rapidly Varying Flow Rates
2013 (English)In: The International Journal of Ventilation, ISSN 1473-3315, E-ISSN 2044-4044, Vol. 12, no 2, 187-194 p.Article in journal (Refereed) Published
Abstract [en]

The use of supply jet flows is the most common type of air distribution for general ventilation. Usually the supply flow rate is constant or slowly varying (VAV-systems) to cope with a varying load. A novel air distribution method, with the potential to reduce stagnation and to increase the ventilation efficiency, is to introduce rapid flow variations (pulsations). This paper reports on a fundamental study of this type of air distribution. The purpose of the study was to explore the effect of flow variations on stagnant zones and the levels of the turbulent kinetic energy and the relative turbulence intensity. A small scale room model is used that consists of an enclosure with a ventilation supply at the bottom and an extract at the top of the opposite wall. Water was used as an operating fluid and the model had a design which mainly generated a two-dimensional flow. The size of the model made it possible to investigate the two-dimensional velocity vector field using the Particle Image Velocimetry (PIV) method in regions corresponding to occupied regions. Further post processing was conducted from the resulting vector fields. The comparison between cases of constant inflow and pulsated inflow (flow variations with frequency of 0.5 Hz) was conducted for three domains: two belonging to the far-field occupied zone and one belonging to the near-field, downstream of the supply wall jet.            

Place, publisher, year, edition, pages
Veetech Ltd, 2013
Keyword
PIV, ventilation, varying flow rate, flow pulsation, room model
National Category
Other Engineering and Technologies not elsewhere specified
Research subject
Civil and Architectural Engineering; Energy Technology; Engineering Mechanics
Identifiers
urn:nbn:se:kth:diva-149299 (URN)
Note

QC 20140826

Available from: 2014-08-19 Created: 2014-08-19 Last updated: 2017-12-05Bibliographically approved
3. Experimental study on the effect of pulsating inflow to an enclosure for improved mixing
Open this publication in new window or tab >>Experimental study on the effect of pulsating inflow to an enclosure for improved mixing
2013 (English)In: International Journal of Heat and Fluid Flow, ISSN 0142-727X, E-ISSN 1879-2278, Vol. 44, 108-119 p.Article in journal (Refereed) Published
Abstract [en]

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.

Keyword
Pulsating flow, Room ventilation, Vortex statistics, Wall-jet instability, Kelvin-Helmholtz instability, Roll-up of vortices
National Category
Engineering and Technology
Identifiers
urn:nbn:se:kth:diva-139869 (URN)10.1016/j.ijheatfluidflow.2013.05.004 (DOI)000329594600009 ()2-s2.0-84888432075 (Scopus ID)
Funder
Swedish Research Council
Note

QC 20140122

Available from: 2014-01-22 Created: 2014-01-15 Last updated: 2017-12-06Bibliographically approved
4. Plaster finishes in historical buildings – Measurements of surface structure, roughness parameters and air flow characteristics
Open this publication in new window or tab >>Plaster finishes in historical buildings – Measurements of surface structure, roughness parameters and air flow characteristics
2013 (English)In: Plaster finishes in historical buildings: Measurements of surface structure, roughness parameters and air flow characteristics / [ed] A. Troi, E. Lucchi, 2013, 69-75 p.Conference paper, Published paper (Refereed)
Abstract [en]

Soiling of surfaces in historical buildings by deposition of particles is a common problem.  Minimizing soiling is an important goal for conservation of structures and objects. The surfaces give rise to an interference with the air motions along the surfaces. Properties of surfaces may therefore influence the particle deposition. It is well known that with increasing roughness of the surfaces the particle deposition rate increases. The properties of surfaces in historical buildings are not well documented.  We have investigated samples of surfaces finished by wood float finish, steel float finish and brushed finish. As a reference we have used an MDF board. The geometrical properties of the surfaces have been documented by using the stripe projection method. The resistance to airflow along the surface and the turbulence generated by the surfaces have been investigated by recording the boundary layer flow over the surfaces in a special flow rig. The work reported is part of a project where the process of soiling is studied both in laboratory and in field studies. The air velocity adjacent to the surfaces will be recorded with both PIV (Particle Image Velocimetry) and hot-wire technique. The temperature gradient close to the walls will be recorded with cold-wire technique.  

Keyword
Soiling, Plasters, Particle deposition, Surface structure, Surface roughness
National Category
Other Engineering and Technologies
Research subject
Engineering Mechanics; Energy Technology
Identifiers
urn:nbn:se:kth:diva-167803 (URN)978-88-88307-26-8 (ISBN)
Conference
The 3rd European Workshop on Cultural Heritage Preservation (EWCHP) Bozen/Bolzano, Italy
Note

QC 20150525

Available from: 2015-05-22 Created: 2015-05-22 Last updated: 2015-05-25Bibliographically approved
5. PARTICLE IMAGE VELOCIMETRY (PIV) VISUALIZATION OF AIR FLOW OVER A WALL-MOUNTED RADIATOR
Open this publication in new window or tab >>PARTICLE IMAGE VELOCIMETRY (PIV) VISUALIZATION OF AIR FLOW OVER A WALL-MOUNTED RADIATOR
2014 (English)Conference paper, Published paper (Refereed)
Abstract [en]

A common room heating technique is to use a wall-mounted radiator without forced convection. The cold surrounding air passes adjacent to the warm surfaces of the radiator, gets heated, and the buoyancy difference gives this heated air a momentum to rise along the wall surface (as plume) and finally circulate and get mixed into the whole room. The properties of heated plumes are important for assessing the risk of soiling of the wall surfaces through particle deposition driven by thermophoresis and turbophoresis. It is important to identify where there is a transition from laminar to turbulent flow. With the objective to characterize the plume of heated air flow in the vicinity of wall surface, the airflow over the radiator is visualized and measured using the two-dimensional Particle Image Velocimetry (2D PIV) technique. The PIV technique yields two-dimensional vector fields of the flow. The resulted vector maps are size and peak validated and post processed using in house developed software to provide the average streamlines. In the near wall PIV measurements there are practical problems; generating a homogeneous global seeding that makes it possible to study both the plume and the surrounding entrainment region, and optical problems due to strong laser reflection from the wall surface which limits the investigation area. These issues are dealt with in the present study. In addition to visualization with PIV, visualization with a CMOS video camera was also conducted.

Keyword
radiator, PIV, thermophoresis, turbophoresis, visualization
National Category
Other Engineering and Technologies not elsewhere specified
Identifiers
urn:nbn:se:kth:diva-148834 (URN)
Conference
ROOMVENT, The 13th International Conference on Air Distribution in Rooms, October 19-22, São Paulo, Brazil
Note

QC 20140828

Available from: 2014-08-13 Created: 2014-08-13 Last updated: 2015-05-25Bibliographically approved
6. Particle Image Velocimetry Visualization and measurement of Air Flow over a Wall-Mounter Radiator
Open this publication in new window or tab >>Particle Image Velocimetry Visualization and measurement of Air Flow over a Wall-Mounter Radiator
(English)Manuscript (preprint) (Other academic)
National Category
Other Engineering and Technologies not elsewhere specified
Identifiers
urn:nbn:se:kth:diva-167992 (URN)
Note

QS 2015

Available from: 2015-05-25 Created: 2015-05-25 Last updated: 2015-05-25Bibliographically approved

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