TfNH
TOANKHUfcH
TANBUI THEO
MO HINHGAUSS CO K^ D^N VAN
T 6 C ROI COA HATBUI
ThS. Nguyen Huy Tie'n
Vipn Khoa hgc va Ky thuat Mdi tmdng Trudng Dal hge XSy dung
Tdm tat: Hidn nay md hinh Gauss dugc sU dung kha pho biin di tinh toSn du bSo d nhiim trong cdng tSe dSnh giS va du bSo d nhidm mdi trudng khdng khi.
Vdi md hinh Gauss, khi tinb toSn ed nhan mpt sd gia thiit gin dung, trong dd cd gia thiit IS chit d nhidm khdng ling dgng trong quS trinh khuich tan. Thuc ti, trgng lugng ciia bui Idn hgn rat nhiiu so vdi chat khi, do vSy sd xiy ra hidn tugng ling dgng bui trong quS trinh khuich tSn. BSi bao gidi thidu phuong phSp tinh lan toi ehit d nhiem cd ki din su ling dgng eiia bui trong quS trinh lan truyin khdng khi, ding thdi tinh toSn so sanh su khSc bidt vi phSn bd ndng dp bui kbi khing ki de'n va ed ki din vpn td'c rai cua bui trong quS trinh khuich tSn trong khdng khi.
Summary: Currently, Gausses model is used commonly to calculate and predict pollution in assessment of air environment Gausses model, when calculating is based on some assumptions, in which the pollutants are not deposited during their dispersion. Actually, gravity of particulate matter is so larger than gases.
Hence, they are setting down while transferring into atmosphere. This paper gives out air pollution dispersion method, which mentions to depositing of particulate matters during transfer Simultaneously, calculation and comparison the difference of particulate matter ambient concentrations between two options:
mention or not mention to deposition of the dusts.
I . O A T V A N D E
o i du bdo d nhiem mdi trUdng do cdc ngudn diem cao gdy ra dd'i vdi mdi trudng khdng khi xung quanh, trong cdng tac ddnh gid tde ddng mdi trudng ngUdi ta thudng sd dyng cac md hinh d i tinh todn. Mdt trong nhOmg md hinh dupe sd dung kha phd bien hidn nay la md hinh Gauss.
Theo md hinh Gauss, ndng dp C cua chd't d nhiem tai toa dp x, y, z bat ky trong khdng gian dugc xde djnh nhu sau:
^(x V z) = -^r-^ ( e x p - ^ ) ^ exp ixy.z; 2r7'ua,Oz 2oJ I
- ( H - Z ) ^ 2a + exp
-(H + Z)2
2a mg/m^ (1)
Trong dP: C,,,.,, - ndng dp chit d nhiem tai diim cd tog dp x, y, z, mg/m^;
y - khoang edeh tir diim tinh todn trdn mdt ngang d i n tmc vet khdi, m;
z - chiiu cao diim tinh todn, m;
TAP CH( KHOA HOC C0NGNGH$XAY DUNG sd3/2008 63
M - tai lupng P nhiim eda ngudn thai, mg/s;
u - tdc dp gio trung binh, m/s;
Oy, Oj - he sd khuich tan cda chat o nhiem trong khi quyen theo phUOng y v^
theo phuong z, m;
H - chiiu eao hieu qua cua dng khdi, m.
Md hinh nay thudng dupe dp dyng de tinh toan khuech tdn chd't d nhiim Id khi hoac bui trong mdi trudng khdng khi. Khi tinh eho byi, ngudi ta da ngim hieu ring day Id byi kich thudc hat r i t be, d dang Id Idng vTnh vien trong khdng khi vd ed dp khuich tan gidng nhu chit khi.
Thue t i hat byi khdng gidng nhu ede chat khi, byi ed khdi lupng ridng Idn hon rat nhiiu (lii hang trdm tdi hang nghin lin) so vdi ede chit khi, do vdy bui co van tdc roi tuong ddi Idn, trong qua trinh lan tmyin, van tde ndy se anh hudng tdi su phdn bd ndng dp eda byi trong khdng khi.
2. SU L A N G OQNG C O A BUI KHI LAN TRUYEN TRONG KHONG KHi
Trong qua trnh khuich tan trong khdng khi, su ling dpng eda eae hat byi phy thuoc nhiiu vdo kieh thude cda hat byi. Vdi cdc hat byi cd kieh thudc 5 > 20 (jm ed van tde roi Idn, se nhanh chdng ling dpng xudng mdt dit theo ehiiu gid thdi. Oe ke de'n sy ling dpng trong qua trinh lan truyen eda byi, Bosanquet vd cdng sy [1] da nghidn edu ly thuyet phuong phdp xdc djnh lupng ling dpng trung binh cda byi, trong dd ed tinh d i n van tde rdi eda byi trong diiu kidn thdi tiit ed gid.
Cudng dd ling dpng trung binh eda hat byi trdn gde cung 45° d eudi hUdng gid dupe xdc djnh nhusau:
G - ^ 4 ^ ^ f ^ f ^ e-^. g/m^s (2)
HY U - ^ ' ^ pu [px!
Trong dd: M^ - tai lupng phat thai eua byi, g/s;
a - ty Id thdi gian cd gid n i m trong gde cung 45° v i cudi hudng gid;
p - he sd khuich tan ddng cda Bosanquet, cd the nhdn p = 0,05;
V, - van td'c rOi tu do tdi han cda hat byi, m/s;
r(1+ v,ypu) - hdm sd gamma r cda dai lupng 1+ v/pu;
H - ehiiu eao hieu qua cua dng khdi, m;
X - khoang each td ngudn thai tdi diim tinh toan, m Cudng dp ling dpng tren tryc ngudn dupc xde djnh nhu sau:
0 - = - ^ ^ ^ ^ ^ { ^ f e-^ g/m^.s (3)
HY 1 + ^ pu LPXJ
Qua edng thdc (2) va (3) cho tha'y vdi cudng dp ling cua byi phy thude vao van tdc roi (v,) tde Id phy thudc vdo dudng kinh (5) va khdi lupng don vi (p) cua hat byi. Byi ed 8, p cdng Idn thi eudng dp ling cang cao. Do ed van td'c roi eda hat byi, se lam giam dp ndng eda ludng khdi (byi), tde la se lam giam ehiiu cao hidu qua (H) cua ngudn thai vd se cd qua trinh "tach bui" ra khdi ludng khdi, dd ehinh Id lupng bui ling dpng.
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3. TiNH TOAN KHUECH TAN BUI THEO MO HJNH GAUSS CO KE DEN VAN TOC ROI CUA HAT BUI
Theo md hinh Gauss khi khdng cd sU phan xa cua mdt dd't ddi vdi ludng khdi, mdt dd't h i p thy ehit o nhiem, ndng dp chat d nhiim xdc dinh nhU sau:
M
'(x y.z) - ( e x p - i y ) e x p
^ y " Z
- ( H - Z ) ^
2o mg/m^ (4)
o i dp dyng md hinh Gauss tinh toan khuech tan byi cd ke d i n van tdc roi eda hat bui ta phai hidu chinh:
- Oai lupng H trong md hinh Gauss se dupc hidu chinh bing cdch trd di doan dUdng md hat byi roi dupe trong khoang thdi gian T. Ooan dudng roi cda hat byi se la V,.T md T = X/U (x- khoang each theo true gid tinh td ngudn, u - van tde gid).
- Vd tai lupng M Id tai lupng cua cd hat bui dang xem xet, dupc ky hieu Id M,,.
- Bui ling dpng se bi h i p thy hodn todn bdi mdt dat.
Khi do cong thdc (4) se trd thdnh:
C(x y z) =:; ^—(exp-i5-)exp ("y.z) 2rTua„o, 2o„
("-¥)•
2o mg/m-' (5)
o i tinh toan dupe theo cdng thdc trdn thi phai biit dp phan c i p cda byi, tdc Id ty Id theo khdi lupng cac loai cd hat eda byi phdt thai vd van tde roi cda hat byi.
Phdn c i p cda hgt bui theo kich thude tuy thudc vdo loai byi phdt thai.Theo tdi lidu cda Nga thi thudng ngUdi ta phdn chia byi theo 6 c i p cd hat: < 5; 5^10; 10-^20; 20+40; 40-=-60 vd >
60 nm. Khi tinh todn mdi cd hat phai lay mdt trj sd dudng kinh trung binh de tinh toan. Vdi byi xi mdng, phdn e i p cac cd hat theo 6 cd'p ndu trdn theo khdi lupng nhu sau (ty Id theo sd thdp phdn): 0,37; 0,18; 0,08; 0,06; 0,12; 0,19 vd khdi lupng don vj eda byi xi mdng Id p6= 2840 kg/m^
Tai lupng theo cd hat dupc xdc djnh nhu sau:
Mb = ms X M
Trong dd: M^ - tai IUpng byi theo c3 hat kich thudc 5, mg/s;
ms- ty Id phdn e i p cd hgt bui theo khdi lupng;
M - tai lupng tdng cdng cda byi, mg/s.
Van tde rdi cda hat bui dUdc xde djnh theo djnh ludt Stokes [4] nhu sau:
.. p»g5'
18M m/s (6)
Trong dd: Pb - khdi lupng ddn vj eua bui, kg/m^
g - gia tde trpng trudng, m/s^;
5 - dudng kinh hat byi, m;
\i - hd sd nhdt ddng lue, Pa.s
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4. ANH HUdNG CUA VAN TOC ROI TCJl SU PHAN BO NONG DO BUI KHI TINH T O A N THEO MO HJNH GAUSS
Oe thay sU khdc biet gida vide xae djnh phdn bd ndng dp bui khi khdng ke d i n van tde roi theo edng thdc (1) vd cd ke d i n van td'c roi theo cdng thdc (4) theo mo hinh Gauss. Xet vi du tinh todn sau day:
V i d u l :
Tinh todn ndng dp byi do dng khdi nha may xi mdng gdy ra tren mdt dd't, dpc theo tryc gid di qua ehan dng khdi, xuoi theo chiiu gid each chdn d'ng khdi 1 khoang Id 2km. Khi biit chieu eao hidu qua dng khdi: H = 120 m; tai lupng byi M = 1298.10' mg/s; van tde gid Id 2 m/s; he sd khuich tan CT, = 300m; CT^ = 350m; dp nhdt dpng lUc eda khdng khi n =1,85.10"^ Pa.s; bui xi mdng phan eip cdc ed hat theo khdi lupng nhu sau (ty Id theo sd thdp phan): m<5= 0,37; ms.n=
0,18; m,o 20=0,08; m2o^o=0,06; m4o.6o=0,12; m,6o=0,19 va p= 2840 kg/m^ Oiem tinh toan nSm trdn mat d i t (z = 0), tai true ludng (y = 0).
Sarrg 1. Kit qua tinh toSn Cd hat trung binh
Jim
< 5 5-10 10-20 20-40 40-60
>60
M, mg/s 480.260 233.640 103.840 77.880 155.760 246.620
H m
120
xSe djnh ndng dp bui cua vl du 1 u
m/s
2
CT,
m
300
m
350
V,
m/s 0,021 0,054 0,188 0,753 2,092 3,012 Ning dp ting cpng eiia eSc ed hat khi ki din van tde rai eiia hat bui
Ndng dp khi khdng ki din van tde rai eda hat bui
C mg/m' 0,364 0,177 0,079 0,059 0,119 0,189 0,987 1,968 Qua kit qua tinh toan vi du cho thiy, ed sy sai khae rat Idn v i phan bd ndng dp byi tong edng gida vide khong k i d i n va cd k i d i n van tdc roi cda byi trong qua trinh lan truyen.
5. SO
S A N H P H A N B 6NONG DO BUI THEO MO HINH GAUSS KHI KHONG KE DEN vA CO Ki DEN VAN TOC ROI
Oe ed the so sanh eho nhieu trudng hpp khae nhau ein phai lap trinh tinh toan di ve ban dd d nhiem byi cd ke de'n van tde roi eda bui tren dudng lan truyin vd so sanh vdi trudng hpp khdng k i d i n van tdc roi eda hat byi. Odng thdi ve dupc biiu dd ndng dp byi trdn mat da't dpc theo tryc gid, cho tdng cd hat ridng biet vd eda tdng edng t i t ea eae ed hat byi ed trong ngudn thai. Dya trdn cdng thdc (1) vd (5) nhdm tac gia tiin hanh lap trinh tinh toan so sanh su khac nhau ve phan bd ehit d nhiem gida hai trudng hpp de cap d trdn. Xet eae vi dy sau:
Vi du 2:
Ngudn thai la d'ng khdi Id nung xi mdng: chiiu d'ng khdi: h = 120m; dudng kinh D = 4,5m;
tai lupng byi M = 598.10' mg/s; luu lUpng khi thai 135m'/s; nhiet dp khi thai 150°C. Oieu kien khi quyen: van tde gid 2m/s: nhiet dp khdng khi xung quanh 28°C; cd'p dn djnh khi quyen Id B; Kit qua tinh toan phan od ndng dp byi, theo cdc cd hat va ndng dp byi tdng cdng nhu sau:
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Hinh 1: PhSn bd ndng ndng dp bui dgc theo true gid di qua chSn dng khdi Vi du 3:
o i tha'y rd sy phdn bd ndng dp byi tren mat d i t dpc theo tryc gid eho cac trudng hpp byi ddng nhit nhung ed hgt khae nhau khi ed ke d i n van td'c roi. Cdc cd hat bui xem xet nhu sau:
Trudng hpp 1: toan bd 100% Id cd byi d = 5 nm Trudng hpp 2: todn bp 100% Id cd byi d = 25 nm Trudng hpp 3: todn bp 100% Id ed byi d = 45 nm Trudng hpp 4: todn bd 100% la cd byi d = 65 nm
Chay chuong trinh vdi sd lieu ngudn gia djnh (ed ghi tren bieu dd), ta se thiy dng vdi cd hat byi cdng Idn thi eae diim eye dai eda biiu dd edng cd trj sd Idn vd cdng nim g i n chdn ngudn thai. So sdnh vdi k i t qua tinh theo Gauss thdng thudng ta thiy ed sy khac biet dang ke.
Hinh 2. PhSn bd ndng dp bui ddng nhit theo tUng ed hat (KH = 3; v=2,5 m/s; tN = 30 "C) Qua cae vi dy trdn cho thdy, van tde roi v, cda hat byi anh hudng rit Idn tdi su phdn bd ndng dd trong khdng khi. Do anh hudng eua van tde roi v„ cae hat byi cd kieh thudc ldn ed xu hudng ling dpng trdn b i mdt dit, trdn dudng Ian tmyin g i n chdn ngudn thai, eae hat byi cd kich thude nhd cd xu hudng phdt tdn ra xa ngudn thai xudi theo chieu gid. Do vdy ndng dp bui tdng cdng cac ed hat se thip hon trong trudng hpp khdng ke de'n van tde roi trdn dudng lan truyin.
TAP CHf KHOA HOC C O N G N G H ^ XAY DUNG SO 3/2008 67
6. KET LUAN
- Khi tinh todn khuich tan d nhiim byi trong khdng khi, khdc vdi cdc chit khi, cdc hat bui cd kieh thude vd trpng lupng Idn se ehju anh hudng cda van tdc roi v„ byi se ling dpng xud'ng mdt dit.
- Khi tinh toa.i khuich tan byi cd k i d i n van tde roi v, vd khdng ke de'n van td'c rdi v„ thi phan bd ndng dd byi trong trudng hpp cd ke de'n van tdc roi v, thd'p hon so vdi trudng hdp khdng k i din van tdc roi v,. Oiiu nay hodn todn phu hpp vi ed su ling dpng cua hat byi trdn dUdng lan truyin.
Tidu ehuan Viet Nam TCVN 5937-2005 cd quy djnh ndng dp cho phep cda 2 loai bui:
bui tdng edng (TSP) vd bui ed kich thUde < 10 nm (PMIO), do vdy sd dyng phuong phap tinh toan khuich tdn cd ke van td'c roi v,ndu trdn se du bdo dupe ndng dp bui PMIO trong khdng khi xung quanh ma theo md hinh Gauss thdng thudng khdng xac djnh dupe.
Tai lieu tham khao
1. Magil P. L. Holden F. R., Aekley C. - Air Pollution Handbook. McGraw^-Hill, Inc., New York, 1956.
2. Kari B. Schnelle, Jr, Partha R. Dey - Atmospheric Dispersion Modelling Compliance Guide.
McGraw-Hill, NewYork, 2000.
3. Tran Nggc Chan - O nhiem khong khi vd xd ly khi thai. (Tap 1): O nhiem khdng khi vd tinh todn khuich tdn chat d nhiim. Nxb Khoa hpc vd Ky thuat. Hd Npi, 2000.
4. Trin Nggc Chin - 6 nhidm khdng khi vd xd ly khi thai. (TSp 2): Co hpc v i bui vd phuong phdp xd ly byi. Nxb Khoa hpe va Ky thudt. Ha Ndi, 2001.
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