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A Ventilation Strategy Based on Confluent Jets: An Experimental and Numerical Study
University of Gävle, Faculty of Engineering and Sustainable Development, Department of Building, Energy and Environmental Engineering. Linköpings universitet, Energisystem.
2015 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

This study presents air distribution systems that are based on confluent jets; this system can be of interest for the establishment of indoor environments, to fulfill the goals of indoor climate and energy-efficient usage. The main objective of this study is to provide deeper understanding of the flow field development of a supply device that is designed based on wall confluent jets and to investigate the ventilation performance by experimental and numerical methods. In this study, the supply device can be described as an array of round jets on a flat surface attached to a side wall. Multiple round jets that issue from supply device apertures are combined at a certain distance downstream from the device and behave as a united jet or so-called confluent jets. Multiple round jets that are generated from the supply device move downward and are attached to the wall at the primary region, due to the Coanda effect, and then they become wall confluent jets until the floor wall is reached. A wall jet in a secondary region is formed along the floor after the stagnation region.

The characteristics of the flow field and the ventilation performance of conventional wall confluent jets and modified wall confluent jets supply devices are investigated experimentally in an office test room. The study of the modified wall confluent jets is intended to improve the efficiency of the conventional one while maintaining acceptable thermal comfort in an office environment. The results show that the modified wall confluent jets supply device can provide acceptable thermal comfort for the occupant with lower airflow rate compared to the conventional wall confluent jets supply device.

Numerical predictions using three turbulence models (renormalization group (RNG k– ε), realizable (Re k– ε), and shear stress transport (SST k– ω) are evaluated by measurement results. The computational box and nozzle plate models are used to model the inlet boundary conditions of the nozzle device. In the isothermal study, the wall confluent jets in the primary region and the wall jet in the secondary region, when predicted by the three turbulence models, are in good agreement with the measurements. The non-isothermal validation studies show that the SST k– ω model is slightly better at predicting the wall confluent jets than the other two models. The SST k– ω model is used to investigate the effects of the nozzle diameter, number of nozzles, nozzle array configuration, and inlet discharge height on the ventilation performance of the proposed wall confluent jets supply device. The nozzle diameter and number of nozzles play important roles in determining the airflow pattern, temperature field, and draught distribution. Increased temperature stratification and less draught distribution are achieved by increasing the nozzle diameter and number of nozzles. The supply device with smaller nozzle diameters and fewer nozzles yields rather uniform temperature distribution due to the dominant effect of mixing. The flow behavior is nearly independent of the inlet discharge height for the studied range.

The proposed wall confluent jets supply device is compared with a mixing supply device, impinging supply device and displacement supply device. The results show that the proposed wall confluent jets supply device has the combined behavior of both mixing and stratification principles. The proposed wall confluent jets supply device provides better overall ventilation performance than the mixing and displacement supply devices used in this study.

This study covers also another application of confluent jets that is based on impinging technology. The supply device under consideration has an array of round jets on a curve. Multiple jets issue from the supply device aperture, in which the supply device is positioned vertically and the jets are directed against a target wall. The flow behavior and ventilation performance of the impinging confluent jets supply device is studied experimentally in an industrial premise. The results show that the impinging confluent jets supply device maintains acceptable thermal comfort in the occupied zone by creating well-distributed airflow during cold and hot seasons.

Place, publisher, year, edition, pages
Linköping: Linköping University Electronic Press, 2015. , 78 p.
Series
Linköping Studies in Science and Technology. Dissertations, ISSN 0345-7524 ; 1671
Keyword [en]
Multiple interacting jets, round jets, confluent jets, wall jet, ventilation strategy, air distribution system, air supply device, ventilation performance, thermal comfort, energy-saving potential, measurement, numerical predictions, RANS turbulence models, renormalization group (RNG k– ε), realizable (Re k– ε), and shear stress transport (SST k– ω)
National Category
Energy Engineering
Identifiers
URN: urn:nbn:se:hig:diva-19369DOI: 10.3384/diss-diva-117442ISBN: 978-91-7519-063-1 (print)OAI: oai:DiVA.org:hig-19369DiVA: diva2:814059
Public defence
2015-05-29, ACAS, Hus A, Campus Valla, 10:15 (English)
Opponent
Supervisors
Available from: 2015-05-26 Created: 2015-05-26 Last updated: 2015-05-27Bibliographically approved
List of papers
1. Experimental investigation of a new supply diffuser in an office room
Open this publication in new window or tab >>Experimental investigation of a new supply diffuser in an office room
2009 (English)Conference paper (Refereed)
National Category
Energy Engineering
Identifiers
urn:nbn:se:hig:diva-10759 (URN)
Conference
Proceedings of 11th International Conference on Air Distribution in Rooms - ROOMVENT 2009, Busan, Korea, May 24 -27, 2009
Available from: 2011-10-19 Created: 2011-10-19 Last updated: 2017-01-03Bibliographically approved
2. Experimental investigation of a ventilation system based on wall confluent jets
Open this publication in new window or tab >>Experimental investigation of a ventilation system based on wall confluent jets
2014 (English)In: Building and Environment, ISSN 0360-1323, E-ISSN 1873-684X, Vol. 80, 18-31 p.Article in journal (Refereed) Published
Abstract [en]

The flow behavior of isothermal and non-isothermal wall confluent jets (WCJ) ventilation system was investigated experimentally in a mock-up office environment. Two flow regions were identified: first, a primary region is developed below the supply device, with axis along the inlet wall, and a secondary wall-jet forms along the floor. The velocity and temperature fields were recorded by traversing a hot wire and thermistor anemometers for both primary and secondary regions. The results show self-similarity characteristic of the velocity and temperature profiles for both isothermal and non-isothermal WCJ. Maximum velocity decay and its spreading rate are linear and independent of the inlet airflow rate. Minimum temperature difference and its spread also show linear decay. The spreading rates for maximum velocity are similar in both the primary and secondary regions normal to the inlet wall and floor, respectively, which is consistent with previous studies on wall jets. The variation of draught rating is evaluated via mean velocity, turbulence intensity and temperature. Although the WCJ has slow velocity decay, which enables it to cover almost the entire floor of the test room, the draught is acceptable in the occupied zone according to ISO 7730. PMV (predicted mean vote) and PPD (predicted percentage dissatisfied) are presented for the occupied zone of the room. 

Keyword
Draught rate, Measurement, Supply device, Thermal comfort indices, Ventilation system, Wall confluent jets, Floors, Inlet flow, Isotherms, Measurements, Thermal comfort, Velocity, Ventilation, Experimental investigations, Minimum temperature differences, Temperature profiles, Thermal comfort index, Turbulence intensity, Velocity and temperature fields, Ventilation systems, Air, airflow, experimental study, jet flow, measurement method, temperature profile, turbulence, velocity profile
National Category
Building Technologies
Identifiers
urn:nbn:se:hig:diva-18465 (URN)10.1016/j.buildenv.2014.05.011 (DOI)000340323300003 ()2-s2.0-84902119008 (ScopusID)
Available from: 2014-12-10 Created: 2014-12-10 Last updated: 2017-01-03Bibliographically approved
3. Numerical study of a ventilation system based on wall confluent jets
Open this publication in new window or tab >>Numerical study of a ventilation system based on wall confluent jets
2014 (English)In: HVAC & R RESEARCH, ISSN 1078-9669, E-ISSN 1938-5587, Vol. 20, no 8, 846-861 p.Article in journal (Refereed) Published
Abstract [en]

This study presents numerical investigation of an air supply device based on wall confluent jets in a ventilated room. Confluent jets can be described as multiple round jets issuing from supply device apertures. The jets converge, merge, and combine at a certain distance downstream from the supply device and behave as a united jet, or so-called confluent jet. The numerical predictions of the velocity flow field of isothermal confluent jets with three Reynolds-averaged Navier-Stokes turbulence models (renormalization group k-epsilon, realizable k-epsilon, and shear stress transport k-omega) are reported in the present study. The results of the numerical predictions are verified with detailed experimental measurements by a hot wire anemometer and constant temperature anemometers for two airflow rates. The box method is used to provide the inlet boundary conditions. The study of the airflow distribution shows that a primary wall jet (wall confluent jet) exists close to the supply device along the wetted wall, and a secondary wall jet is created after the stagnation region along the floor. It is presented that the flow field of the primary and secondary wall jet predicted by turbulence models is in good agreement with the experimental data. The current study is also compared with the literature in terms of velocity decay and the spreading rate of the primary and secondary wall jet, the results of which are consistent with each other. Velocity decay and the spreading rate of the secondary wall jet in vertical and lateral directions were studied for different inlet airflow rates and inlet discharge heights. The comparative results demonstrate that the flow behavior is nearly independent of the inlet flow rate. Inlet discharge height is found to have impact close to the inlet, where the velocity decays faster when the jet discharges at higher level. The decay tendency is similar as the jet enters into the room for all discharge heights.

National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:hig:diva-18208 (URN)10.1080/10789669.2014.957111 (DOI)000344407100002 ()2-s2.0-84910660769 (ScopusID)
Available from: 2014-12-03 Created: 2014-11-28 Last updated: 2017-01-03Bibliographically approved
4. Investigation of design parameters for an air supply device based on wall confluent jets
Open this publication in new window or tab >>Investigation of design parameters for an air supply device based on wall confluent jets
2015 (English)Article in journal (Other academic) Submitted
Keyword
Wall confluent jets supply device, Numerical predictions, Parametric studies, Ventilation performance, Ventilation efficiency, Thermal comfort
National Category
Energy Engineering
Identifiers
urn:nbn:se:hig:diva-19372 (URN)
Available from: 2015-05-26 Created: 2015-05-26 Last updated: 2017-01-03Bibliographically approved
5. Numerical investigation of ventilation performance of different air supply devices in an office environment
Open this publication in new window or tab >>Numerical investigation of ventilation performance of different air supply devices in an office environment
2015 (English)In: Building and Environment, ISSN 0360-1323, E-ISSN 1873-684X, Vol. 90, 37-50 p.Article in journal (Refereed) Published
Abstract [en]

The aim of this study was to compare ventilation performance of four different air supply devices in an office environment with respect to thermal comfort, ventilation efficiency and energy-saving potential, by performing numerical simulations. The devices have the acronyms: Mixing supply device (MSD), Wall confluent jets supply device (WCJSD), Impinging jet supply device (IJSD) and Displacement supply device (DSD). Comparisons were made under identical set-up conditions, as well as at the same occupied zone temperature of about 24.2°C achieved by adding different heat loads and using different air-flow rates. Energy-saving potential was addressed based on the air-flow rate and the related fan power required for obtaining a similar occupied zone temperature for each device. Results showed that the WCJSD and IJSD could provide an acceptable thermal environment while removing excess heat more efficiently than the MSD, as it combined the positive effects of both mixing and stratification principles. This benefit also meant that this devices required less fan power than the MSD for obtaining equivalent occupant zone temperature. The DSD showed a superior performance on heat removal, air exchange efficiency and energy saving to all other devices, but it had difficulties in providing acceptable vertical temperature gradient between the ankle and neck levels for a standing person. 

Keyword
Air supply devices, Energy-saving potential, Thermal comfort, Ventilation performance, Air, Energy efficiency, Flow rate, Mixing, Ventilation, Air exchange efficiencies, Air supply, Energy saving potential, Numerical investigations, Thermal environment, Ventilation efficiency, Vertical temperature gradients, Energy conservation, building, computer simulation, indoor air, numerical method, performance assessment
National Category
Mechanical Engineering Building Technologies
Identifiers
urn:nbn:se:hig:diva-19292 (URN)10.1016/j.buildenv.2015.03.021 (DOI)000356189000004 ()2-s2.0-84926464708 (ScopusID)
Funder
Swedish Research Council Formas, 242-2008-835Knowledge Foundation, 2007/0289
Available from: 2015-05-05 Created: 2015-05-05 Last updated: 2017-01-03Bibliographically approved
6. A Newly Designed Supply Diffuser for Industrial Premises
Open this publication in new window or tab >>A Newly Designed Supply Diffuser for Industrial Premises
2010 (English)In: The International Journal of Ventilation, ISSN 1473-3315, E-ISSN 2044-4044, Vol. 9, no 1, 59-67 p.Article in journal (Refereed) Published
Abstract [en]

The results of this investigation revealed the airflow distribution from a new design of supply diffuser under non-isothermal conditions. To illustrate the indoor climate parameters in the occupied zone, for both the heating and cooling seasons, an experimental investigation was carried out in industrial premises. The indoor climate was explored at ankle, waist and neck levels for a standing person at different positions, to determine the variation of the thermal comfort indexes and draught rating (DR) with position in the facility. The observed PPD and DR values indicate acceptable levels of thermal comfort in the facility for both summer and winter cases. The conclusion can be drawn that well-distributed airflow saves energy by removing the need for an additional heating and cooling systems during cold and hot weather seasons.

Keyword
industrial ventilation, experimental investigation, indoor climate, supply diffuser
National Category
Energy Engineering
Identifiers
urn:nbn:se:hig:diva-10272 (URN)10.1080/14733315.2010.11683867 (DOI)000279036000006 ()2-s2.0-77953582933 (ScopusID)
Conference
9th International Conference on Industrial Ventilation Location: ETH, OCT 18-21, 2009 , Zürich, Switzerland
Available from: 2011-09-21 Created: 2011-09-21 Last updated: 2017-01-03Bibliographically approved

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