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Distribution of Ventilation Air and Heat by Buoyancy Forces inside Buildings: An Experimental Study
KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering.
2009 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The main task of the ventilation system in a building is to maintain the air quality and (together with the heating or cooling system) the thermal climate at an acceptable level within the building. This means that a sufficient amount of ventilation air at the appropriate temperature and quality must be supplied to satisfy thermal comfort and air quality demands and that this air is distributed to the parts of the building where people reside. Air movements caused by buoyancy forces can determine the distribution of ventilation air within buildings. The purpose of this thesis is to advance the state of knowledge of buoyancydriven air movements within buildings and to determine their importance both for ventilation air distribution and the maintenance of thermal comfort and air quality in buildings. The work is focused on studying thermally-driven air movements through large openings, both horizontal and vertical (i.e. doorways). The properties of a special type of thermally-driven currents, so called gravity currents, have also been explored.

Large vertical openings like doorways are important for air exchange between rooms within a building. Air movements through doorways separating rooms with different air temperatures are often bidirectional and the buoyancy-driven flow rates are often greater than those caused by the mechanical ventilation system alone. Bidirectional flows through doorways can effectively spread contaminants, for example, from a kitchen or a hospital rooms, yet the results of this study indicate that the conversion of a thermally-driven bidirectional flow to a unidirectional flow via an increase of the mechanically forced flow rate requires forced flows that are more than three times greater than the thermally-driven flows. Experiments conducted in this project indicate that the resistance to buoyancy-driven flows in horizontal openings is significantly greater than that in vertical openings. Model tests have shown, however, that this problem may be mitigated if a simple model of a staircase located in the centre of the room (being ventilated) is linked to the horizontal ventilation opening. Gravity currents in rooms occur in connection with so called displacement ventilation as cool gravity currents propagate along the floor that are driven by the density difference of the ventilation air and the ambient, warmer air within the room. As these gravity currents easily pass obstacles and to a certain extent are self-controlling, they can effectively distribute the cool air within rooms in a building.

Likewise, warm gravity currents occur when warmer air introduced in a room rises and spreads along the ceiling plane. One application where warm gravity currents may be used to advantage is when converting buildings from electric heating to district hot water heating thus, avoiding the introduction of an expensive hydronic heating system. This report includes a full-scale laboratory study of the basic properties of thermally-driven warm air gravity currents in a residential building and examines the possibilities of using the resulting air movements for the distribution of ventilation air as well as heat. Results from laboratory tests show that this conversion method may prove effective if certain conditions on the layout of the building are fulfilled.

Place, publisher, year, edition, pages
Stockholm: KTH , 2009. , xi, 56 p.
Keyword [en]
building ventilation, thermal forces, buoyancy, gravity currents, large openings, heating, air quality, forced convection, free convection
National Category
Building Technologies
Identifiers
URN: urn:nbn:se:kth:diva-10615ISBN: 978-91-7415-322-4 (print)OAI: oai:DiVA.org:kth-10615DiVA: diva2:220990
Public defence
2009-06-09, Sal 33:202, Kungsbäcksvägen 47, Högskolan i Gävle, Gävle, 10:15 (English)
Opponent
Supervisors
Note
QC 20100705Available from: 2009-06-03 Created: 2009-06-03 Last updated: 2010-07-20Bibliographically approved
List of papers
1. To what extent can one with mechanical ventilation control the air motions within a building
Open this publication in new window or tab >>To what extent can one with mechanical ventilation control the air motions within a building
1996 (English)In: ROOMVENT'96: 5th International Conference on Air Distribution in Rooms, 1996, 265-272 p.Conference paper, Published paper (Other academic)
Abstract [en]

There are a number of methods available concerning with air distribution in buildings. Within control research, one can find new control algorithms which have not been used in practice yet. These new algorithms open the possibility of developing and implementing of new demand controlled ventilation systems.

In a building the internal air motions are due both to differences in temperature and due to pressure differences induced by the ventilation system. Therefore, one fundamental question is to what extent one can with fan powered ventilation control the air motions within a building.

The aims of this paper is to report on development of methods to study the air motions in a multi room residence apartment using various combinations of exhaust and supply air management. The experimental work includes measurements of air flow rates in door openings in both directions and use of various tracer gas methods to determine the supply air flow to each room, and identifying flow paths.

In an accompanying paper (Björsell 1996) is reported on the results from a simulation of the performance of different control algorithms.

Keyword
Flows in doorways, temperature control, tracer gas technique, velocity measurements, demand controlled ventilation
Identifiers
urn:nbn:se:kth:diva-13953 (URN)4-924557-01-3 (ISBN)
Note
QC 20100705Available from: 2010-07-05 Created: 2010-07-05 Last updated: 2010-07-20Bibliographically approved
2. Measurements and Control of Air Movements within a Building
Open this publication in new window or tab >>Measurements and Control of Air Movements within a Building
1997 (English)In: 18th AIVC Conference: Ventilation and Cooling, 1997, 427-436 p.Conference paper, Published paper (Other academic)
Abstract [en]

There are a number of methods available concerning with distribution of air in buildings. Within control research, one can find new control algorithms which have not yet been used in practice. These new algorithms open the possibility of developing and implementing of new demand controlled ventilation systems.

In a building the internal air motions are due both to differences in temperature and pressure differences caused by the ventilation system. Therefore, one fundamental question is to what extent it s possible to control the air motions within a building using fan powered ventilation in combination with temperature control.

The aims of this paper is to report on measurements done to examine the influence of temperature differences between rooms on the air exchange through open doors in a building and to explore the use of modern control technique to minimise the temperature difference.

The result of the measurements shows that even very small (0.1-0.2°C) temperature differences between rooms cause bi-directional air flows in the doorways of a magnitude that exceed the flow rates caused by the mechanical ventilation system. Therefore it is necessary to control the temperatures in the rooms to make it possible for the ventilation system to distribute the air to those parts of the building where it is needed.

National Category
Building Technologies
Identifiers
urn:nbn:se:kth:diva-13955 (URN)
Note
QC 20100705Available from: 2010-07-05 Created: 2010-07-05 Last updated: 2010-07-20Bibliographically approved
3. Transition from Bi-directional to Unidirectional Flow in a Doorway
Open this publication in new window or tab >>Transition from Bi-directional to Unidirectional Flow in a Doorway
1998 (English)In: ROOMVENT '98 : proceedings: 6th International Conference on Air Distribution in Rooms, 1998, 539-546 p.Conference paper, Published paper (Refereed)
Abstract [en]

The air flow in a doorway is governed by density difference caused by temperature difference and pressure difference caused by mechanical ventilation. Tests have been carried out in a unique indoor test house where the room to room to temperature difference could be controlled very accurately with a new control system. In addition to these tests some tests were carried out in a scale model with water as the operating fluid. Two main criteria of unidirectional flow in a doorway have been explored:

 

1a.     The recorded mean velocity is unidirectional

1b.     The neutral height is equal to the height of the door

2.       Unidirectional flow in the sense that

 

there is no transfer of contaminant from one room to another. To explore condition one the velocity profile in the doorway have been recorded by transversing the door opening. Condition two has been explored by using tracer gas technique.

Series
Meddelande. Installationsteknik, 45
Keyword
Air flow pattern, Air velocity, Convection flows, Full-scale experiments, Tracer gas
National Category
Building Technologies
Identifiers
urn:nbn:se:kth:diva-13957 (URN)
Note
QC 20100705Available from: 2010-07-05 Created: 2010-07-05 Last updated: 2010-07-20Bibliographically approved
4. Spread of Gravity Currents within a Multi-Room Building
Open this publication in new window or tab >>Spread of Gravity Currents within a Multi-Room Building
2000 (English)In: The 7th International Conference on Air Distribution in Rooms: Ventilation for Health and Sustainable Environment, 2000Conference paper, Published paper (Refereed)
Identifiers
urn:nbn:se:kth:diva-13958 (URN)0 080 43017 1 (ISBN)
Note
QC 20100705Available from: 2010-07-05 Created: 2010-07-05 Last updated: 2010-07-20Bibliographically approved
5. A Note on Air Movements through Horizontal Openings in Buildings
Open this publication in new window or tab >>A Note on Air Movements through Horizontal Openings in Buildings
2002 (English)In: The 8th International Conference on Air Distribution in Rooms: Inividual Controlled Environment, 2002Conference paper, Published paper (Refereed)
National Category
Building Technologies
Identifiers
urn:nbn:se:kth:diva-13959 (URN)87-989117-0-8 (ISBN)
Note
QC 20100705Available from: 2010-07-05 Created: 2010-07-05 Last updated: 2010-07-20Bibliographically approved
6. Air Movements through Horizontal Openings in Buildings – A Model Study
Open this publication in new window or tab >>Air Movements through Horizontal Openings in Buildings – A Model Study
2004 (English)In: The International Journal of Ventilation, ISSN 1473-3315, E-ISSN 2044-4044, Vol. 3, no 1, 1-10 p.Article in journal (Refereed) Published
Abstract [en]

A building contains a number of large openings like doors and staircases. When the temperature of the spaces connected by these openings differs, the difference in density will cause air movements through them. Horizontal air movements through vertical openings in buildings like doors and windows are well investigated while studies of air movements through horizontal openings like stairwells are less frequent and therefore this work is focusing on this case.

The paper reports on an experimental study of the possibility of using buoyancy forces to distribute air and heat through horizontal openings. The experiments have been carried out in a scale model with water as the operating fluid.

The result of the study shows that the flow rate through a horizontal opening is roughly half of the flow rate through a vertical opening for the same conditions, probably caused by the more complex flow pattern in the horizontal opening. A staircase below the horizontal opening will guide the flow somewhat and will cause a small increase of the fluid exchange through the opening.

Keyword
air movement, staircase, horizontal openings, salt bath method
National Category
Building Technologies
Identifiers
urn:nbn:se:kth:diva-13960 (URN)
Note
QC 20100705Available from: 2010-07-05 Created: 2010-07-05 Last updated: 2017-12-12Bibliographically approved
7. Conversion of electric heating in buildings An unconventional alternative
Open this publication in new window or tab >>Conversion of electric heating in buildings An unconventional alternative
2008 (English)In: Energy and Buildings, ISSN 0378-7788, E-ISSN 1872-6178, Vol. 40, no 12, 2188-2195 p.Article in journal (Refereed) Published
Abstract [en]

To decrease the electric energy used for heating buildings it has become desirable to convert direct electrical heating to other heat sources. This paper reports on a study of the possibility of using an unconventional method for conversion to avoid installing an expensive hydronic system. The conversion method combines the ventilation and heating systems and uses air instead of water for distribution of heat within the building, taking advantage of thermal forces and the special properties of gravity currents. Full-scale tests have been carried out in a test apartment inside a laboratory hall where the conditions could be controlled. Temperatures and efficiency of ventilation have been measured to ensure that the demands with respect to thermal climate and air exchange were fulfilled. The results show that it is possible to use the method for heating and ventilation when converting the heating system, but further work has to be done to develop a detailed solution that works in practice. (C) 2008 Elsevier B.V. All rights reserved.

Keyword
Thermal forces, Large openings, Gravity currents, Electrical heating, Conversion, Heat transfer, District heating
National Category
Building Technologies
Identifiers
urn:nbn:se:kth:diva-13961 (URN)10.1016/j.enbuild.2008.06.012 (DOI)000260276200010 ()
Note
QC 20100705Available from: 2010-07-05 Created: 2010-07-05 Last updated: 2017-12-12Bibliographically approved

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