The Characteristics f Wall Cnfluent Jets fr Ventilated Enclsures 9 th Internatinal Cnference n Air Distributin in Rs niversity f Cibra PORTGAL Y.J. Ch **, Hazi Awbi** & Taghi Kariipanah* *) Fresh AB, SWEDEN **) niversity f Reading, K 30/08/2004 1
Intrductin The ethd f supplying cnditined air t a r r a zne in a building has a direct ipactin n: - The theral cfrt in the zne - The air quality in the zne - The energy cnsuptin f the HVAC syste - The facility anageent schedules 30/08/2004 2
Definitin In ventilatin, a cnfluent jet is fred, when circular jets issuing fr different apertures in the sae plane flw in parallel directins, and at a certain distance dwnstrea they calesce and ve as a single jet. Tw type f cnfiguratins: (a) Ceiling level (b) flr level (blwing t the wall) 30/08/2004 3
Spacing: Cnfluent Jets The distance fr the utlets at which the jets erge depends n the distance between and the area f the utlets. (a) (b) (c) Velcity prfiles fr cnfluent circular jets. (a) Clsely-spaced (b) widely-spaced Awbi (2003) S/ A = 5.6, Re=5.8 10 S/ A = 2.8, Re=8 10 4 (c) Velcity prfiles fr five cnfluent circular jets S / A 2.86, Re=4.9 10 =. 4 30/08/2004 4 4
Definitin: Cnfluent Jets /Cn Awbi (2003) (a) (b) (a) Velcity decay fr three cnfluent jets (circular & rectangular penings). (b) Velcity decay fr five cnfluent jets (circular & rectangular penings). 30/08/2004 5
MECHANICAL VENTILATION Mixing Air Distributin A ixing ventilatin syste ais t dilute the indr pllutants by ixing fresh air with r air. sually, an air jet is supplied at high h level t achieve the ixing and prvide acceptable velcities at lw level. l. Ventilatin effectiveness < 100% (<50% in SI syste). Displaceent Ventilatin Here, air is usually supplied directly int the ccupied zne at lw velcity and heat and indr pllutants are displaced upwards due e t the plues rising abve heat surces in the r. The ventilatin effectiveness is usually >>100% (>> 50% in SI syste). Wall DV units are nly suitable fr sall cling lads (<40W/ 2 ) and fr higher lads chilled ceiling panels r chilled beas are used d t suppleent the cling. 30/08/2004 6
MECHANICAL VENTILATION /Cnt.. Cnfluent Jets Ventilatin Here a jet f air is supplied dwnward n t the flr with quite a high entu (i.e. resebling ixing ventilatin) but the velcity decays very rapidly away fr the pint f ipact n the flr. These ethd is suitable fr cling and heating. The ventilatin Effectiveness is usually >>100% (>> 50% in SI syste). Cnfluent jets units are suitable fr high cling lads (>40W/ 2 p t 67W/ 2 was tested in Reading cliate chaber). 30/08/2004 7
Cnfluent jets - Benefits It is knwn that fr exaple wall-unted displaceent ventilatin systes are re efficient than ixing ventilatin systes but have their wn liitatins, e.g. nt suitable fr heating. The cnfluent jet systes have the ptential f lw energy air distributin and at the sae tie d nt have the liitatins f the displaceent systes. The reasn is cntrlled balance between theral frces and entu frces. 30/08/2004 8
Experients The experients were carried ut at a fixed air flw rate and fixed teperature difference between the supply and r air in the cling de. The ai f the velcity easureents was t establish a basis fr diensining physical paraeters fr the use in the design f cnfluent jets ventilatin systes. 30/08/2004 9
OBJECTIVES The verall bjective f this prject is t evaluate the abve syste fr HVAC applicatins and t iprve existing designs using results fr labratry tests. 30/08/2004 10
Prcedure Test r (2.78 x 2.78 x2.3 height) The ai f the velcity easureents was t establish a basis fr diensining physical paraeters fr the use in the design f cnfluent jets ventilatin systes. During the tests, a flw rate f 25 l/s, supply teperature f 18 C C and an average r teperature f 22 C C were used. 30/08/2004 11
The jet cnfiguratin Fig.1. Scheatic f wall cnfluent jets 30/08/2004 12
Measuring pints S = 2h (a) Hrizntal easureent (b) Vertical easureent Fig. 2. The easureent cnfiguratin f cnfluent jets 30/08/2004 13
Fig. 3. Velcity prfiles fr cnfluent circular jets (a) Near wall ( x/h = 1.6) (b) Centerline ( x/h = 2.9) = 0.16 r h r h = 2.9 = 1.00 = 0.60 r h = 5.1 = 0.78 = 0.71 r h = 5.9 = 0.68 = 0.61 x h e = 7.2 = 0.47 = 0.44 r 11 h = = 0.41 = 0.37 r 13.1 h = = 0.38 = 0.33 r 15.5 h = = 0.36 = 0.27 r 17.6 h = = 0.35 = 0.18 r 18.1 h = = 0.25 30/08/2004 14
Fig.4. Three regins f wall cnfluent jets ( x/h = 1.6) ( x/h = 2.9) - Regin I - - II - - Regin III - - Regin I - - II - - Regin III - The flw within these jets ay be classified int three regins: Regin I : Free Jet Regin Regin II: Canda Effect Regin Regin III: Wall Jet Regin. 30/08/2004 15 r/h
Fig. 5. Nn - diensinal velcity prfiles f wall cnfluent jets r/h : u erf 1/7 x/ = 1.48 ηx [1 (0.68 ηx)] η x = x/ x0.5 The epirical equatin fr the nn- diensinal velcity prfile f a plane wall jet is expressed as abve and there x is the distance fr the wall and x0.5 is the distance where the jet velcity is a half f axiu velcity. 30/08/2004 16
Fig. 6. Spreading rate f the wall cnfluent jet 35 30 Exp. x 0.5 x/h x0.5/h 25 20 15 10 5 ( x / h) = 0.0976( r/ h) + 5.6871 0.5 0 0 50 100 150 200 250 300 The spreading rate f the wall cnfluent jets is 0.0976 which is siilar t that f the radial wall jet, i.e. given in Launder and Rdi (1983): 0.5 r/h r/h dx / dr = 0.09 ± 0.005 30/08/2004 17
Cparisn f wall cnfluent jets with ther jets Fr a plane free jet : Fr a plane wall jet : Fr free cnfluent jets: / = 2.47/ l r / h c / = 3.50/ l c 1.5 / = 2.887 c where l = / l Fr wall cnfluent jets: 0.79 / = 2.96( lc) Fr r / h >4 c r h 30/08/2004 18
Fig. 7. The variatin in hrizntal velcity prfiles at different distances fr supply r/ h x/ 1.0 0.9 0.8 0.7 6.6 0.6 0.5 0.4 5.9 15.5 0.3 5.1 0.2 4.5 2.9 7.8 11.7 0.1 0.0 1.0 2.0 3.0 4.0 5.0 6.0 x h Distance fr the wall, / Distance fr wall, r/ h 2.9 4.5 5.1 5.9 6.6 7.8 11.7 15.5 30/08/2004 19
Experiental set-up & CFD delling: Cnfluent Jets 30/08/2004 20
Experiental set-up & CFD delling: Cnfluent Jets /Cn Cn Mannequin - Cputer bx & heated plate 30/08/2004 21
Velcities, CFD 30/08/2004 22
Teperatures, CFD 30/08/2004 23
Measureents in BMG classr, GävleG Cliate chaber A 2,3 8,40 4,5,6 B 2,3 1 a b 7,20 30/08/2004 24
Measureents in BMG classr, GävleG Air Exchange Efficiency [%] Displaceent 43 Cnfluent 58 0 10 20 30 40 50 60 70 30/08/2004 25
Measureents in BMG classr, GävleG Cparisn between Displaceent syste and Cnfluent jets syste in classr with 4 supply terinals (ne in each crner with 50 l/s) 1,4 Deplacerande syste Cnfluent jets syste Lcal Air Exchange >1 Gd Air Exchange Lcal Air Exchange Index 1,2 1 0,8 0,6 0,4 0,2 0 2 3 4 Pint n., see Figure (Classr) 5 6 Cnfluent jets syste Deplacerande syste 30/08/2004 26
The flw field f the wall cnfluent jet is classified int three regins: Free jet regin, Canda effect regin, Wall jet regin. The wall cnfluent jets have a better self preservatin characteristics than ther types f air jets. Overall, thrugh cparisn with the thrw cnstant f the ther jets, the slw diffusin f the wall cnfluent jet is due t the lwer velcity decay. In ther wrds, the jet entu f the wall cnfluent jets can be re cnserved than ther jets. Wall cnfluent jets can successfully be used fr bth heating and cling a cbined effect f ixing and displaceent systes. 30/08/2004 27