Experimental and Numerical Analysis of Diffuse Ceiling Ventilation: Performance of diffuse ceiling ventilation system
Authors
Taradajko, Piotr Pawel ; Chodor, Anna Diana
Term
4. term
Publication year
2013
Submitted on
2013-03-14
Pages
90
Abstract
Denne afhandling undersøger, hvordan et system for diffus loftventilation (DCV) fungerer under realistiske forhold. DCV forsyner rummet med luft gennem et stort loftareal med lav hastighed for at skabe en jævn fordeling. Den eksperimentelle del bygger på fuldskala-målinger af DCV under forskellige varme- og luftstrømsforhold. Et hovedfokus er, hvordan placering og fordeling af varmebelastninger i rummet påvirker luftstrømsmønstre. Vi tester scenarier med jævnt fordelte varmekilder, punktkilder placeret forskellige steder, varmekilder kun på én side af rummet samt forskellige lodrette placeringer. Ved jævnt fordelte varmebelastninger kan der opstå ustabile luftstrømme; vi undersøger årsagen til denne ustabilitet. Vi ser også på betydningen af størrelsen af loftets forsyningsareal. Alle resultater omsættes til et designdiagram, som gør det muligt at finde maksimalt acceptable værdier af q og ΔT (temperaturforskel), så trækrisikoen holdes inden for et acceptabelt niveau. Derudover måles tryktab over et reelt loft, og vi vurderer, hvordan loftets konstruktion påvirker trykfaldet. Den numeriske del omfatter CFD-modellering i Star CCM+. En af de eksperimentelle cases danner grundlag for modellens geometri og randbetingelser, og modellen valideres ved at sammenligne simuleringer med målinger. Til sidst ændres modelgeometrien ved at reducere rumhøjden for at undersøge, hvordan rumhøjden påvirker opdriftskræfterne (luftens bevægelse pga. temperaturforskelle), som er de primære drivkræfter for luftbevægelser i et DCV-system.
This thesis examines the performance of a diffuse ceiling ventilation (DCV) system under realistic conditions. DCV supplies air gently through a large ceiling area to promote even distribution. The experimental part uses full-scale measurements across a range of thermal and airflow conditions. A main focus is how the location and distribution of heat loads in the room affect airflow patterns. We test scenarios with evenly distributed heat sources, point sources placed in different locations, sources only on one side of the room, and different vertical positions. When heat loads are evenly distributed, unstable airflow patterns can occur; we investigate the cause of this instability. We also assess how important the size/extent of the ceiling supply area is. All results are consolidated into a design chart that identifies maximum acceptable values of q and ΔT (temperature difference) to keep the risk of uncomfortable drafts within an acceptable range. In addition, we measure the pressure loss across a real ceiling and examine how the ceiling construction influences this pressure drop. The second part is computational fluid dynamics (CFD) modeling using Star CCM+. One of the experimental cases provides the basis for the model geometry and boundary conditions, and the model is validated by comparing simulations with measurements. Finally, the room height in the model is reduced to study how room height affects buoyancy forces (air movement due to temperature differences), which are the main drivers of airflow in a DCV system.
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