4.1 The Concepts of Force and Mass - UNIOS...Title 4.1 The Concepts of Force and Mass Author Michael...
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Prijenos topline
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konvekcija
kondukcija
radijacija
termik
kiler
luminoznost Najčešći kvar perilice rublja
Zašto “pregori” grijač perilice?
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13.1 Strujanje
STRUJANJE (konvekcija)
Strujanje je proces u kojem se toplina s jednog mjesta na drugoprenosi gibanjem dijelova fluida (zbog razlike gustoća toplijihi hladnijih dijelova).
konvekcijske struje
konvekcija = prijenos topline strujanjem čestica fluida
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Konceptualni primjer 1 Radijatorske cijevi i hladnjaci
Radijatorske cijevi s vrućom vodom postavljene su na zid blizu poda.Rashladna jedinica hladnjaka postavljena je pri vrhu hladnjaka.Svaki od tih položaja odabran je tako da najviše pogoduje razvojukonvekcijskih struja. Objasnite.
13.1 Strujanje
konvekcijskestruje rashladna
jedinica
radijatorska cijev
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Piloti jedrilica koriste termike za uspinjanje.
13.1 Strujanje
termik = stup uzlaznog strujanja zraka
toplije tlo
hladnije tlo
hladnije tlo
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Prisilno strujanje
13.1 Strujanje
cilindri
pumpa
ventilator
hladna tekućina
vrućatekućina
kiler = hladnjak automobila (njem. Kühler, engl. chiller)
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13.2 Vođenje
VOĐENJE (kondukcija)
Vođenje je proces u kojem se toplina s jednog mjesta na drugoprenosi kroz materijal, ali bez gibanja (makroskopskih)dijelova materijala.
Jedan od mehanizama vođenja temelji se na titranju atoma ili molekula.Čestice u toplijem dijelu titraju intenzivnije nego česticeu hladnijem dijelu.
Objašnjeno međusobnim sudarima: molekule s većim energijama predajudio svoje energije susjednim molekulama s manje energije.
Materijale koji dobro vode toplinu nazivamo toplinskim vodičima,a one koji slabo vode toplinu toplinskim izolatorima.
kondukcija = prijenos topline kroz medij bez transporta mase
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Iznos topline Q koji se vođenjem prenese kroz šipkuovisi o nekoliko faktora:
1. trajanju vođenja2. razlici temperatura krajeva šipke3. poprečnom presjeku šipke4. duljini šipke
13.2 Vođenje
hladnijetijelo
toplije tijelo
toplinski tok
poprečni presjek A
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VOĐENJE TOPLINE KROZ MATERIJAL
Toplina Q prenesena vođenjem za vrijeme t kroz šipku duljineL poprečnog presjeka A je
Jedinica SI toplinske vodljivosti: J/(s·m·Co)
toplinska vodljivost
13.2 Vođenje
hladnijetijelo
toplije tijelo
toplinski tok
poprečni presjek A
Q =k AΔT t
L
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13.2 Vođenje
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Šupljikavi materijali običnosu izvrsni toplinski izolatori.
13.2 Vođenje
zarobljeni zračni mjehurići
sićušnekonvekcijskestruje
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Primjer 4 Slojevita toplinska izolacija
Jedan je zid kuće pokriven šperpločom.Toplinske vodljivosti zida i šperploče su,redom, 0,030 and 0,080 J/(s·m·Co), apovršina zida je 35 m2.
Kolika se toplina izgubi kroz taj zidu jednome satu?
13.2 Vođenje
šperploča (0,019 m)
zid (0,076 m)
unutrašnjost(25,0 oC)
vanjski dio(4,0 oC)
toplina
temperatura međusloja T
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Najprije treba naći temperaturu međusloja.
13.2 Vođenje
šperploča (0,019 m)
zid (0,076 m)
unutrašnjost(25,0 oC)
vanjski dio(4,0 oC)
toplina
temperatura međusloja TQ = Qzid = Qšperploča
Q = Q1 = Q2
k1 A (t 1−2) tL1
=k2 A( t−t 2) t
L2
T = 5,8o C
0,030J /(s m o C) A (25o C−T ) t0,076m
=0,080J /(s mo C)A (T −4 o C)t
0,019m
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13.2 Vođenje
šperploča (0,019 m)
zid (0,076 m)
unutrašnjost(25,0 oC)
vanjski dio(4,0 oC)
toplina
temperatura međusloja T
Q =0,030J /(s m o C)35 m2
(25o C−5,8o C)60⋅60s0,076 m
= 9,5⋅105 J
izo-staklo
termo-fasada
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Konceptualni primjer 5 Zaleđeni hladnjak
Toplina se u hladnjaku odvodi hladnomrashladnom tekućinom koja cirkulira krozcjevasti prostor u metalnoj ploči. Bi liploča trebala biti aluminijska ili čelična?Hoće li sustav bolje ili lošije raditi kadje prekriven slojem leda?
13.2 Vođenje
toplina toplina
ledmetal
rashladnatekućina
metalna ploča
cjevasti prostor kroz kojiteče rashladna tekućina
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Najčešći kvar perilice rublja
Zašto “pregori” grijač perilice?
Najčešći kvar perilice rublja
Zašto “pregori” grijač perilice?
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13.3 Zračenje
ZRAČENJE (radijacija)
Zračenje je proces u kojem se toplina(openito, energija) s jednog mjesta nadrugo prenosi elektromagnetskim valovima.
Materijal koji je dobar apsorber ujednoje i dobar emiter.
Materijal koji potpuno apsorbira zračenjenazivamo savršenim crnim tijelom.
radijacija = prijenos topline kroz prostor elektromagnetskim valovima
blok pokriven čađom posrebren blok
temperaturaraste brzo
temperaturaraste sporo
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STEFAN-BOLTZMANNOV ZAKON ZRAČENJAEnergija Q koju tijelo - temperature T, površine A i stupnja emisije e -emitira za vrijeme t dana je izrazom
Stupanj emisije e je bezdimjenzijski broj između nula i jedan.To je omjer energije koju tijelo emitira i energije koju bi tijelo emitiralo da je savršeni emiter.
Stefan-Boltzmannova konstanta
13.3 Zračenje
σ = 5,67⋅10−8 J s−1 m−2 K−4
Q = e σT 4 A t
luminoznost = snaga zračenja svemirskoga izvora
L =Qt
= e σT 4 A
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Primjer 6 Zvijezda superdiv
Superdivovska zvijezda Betelgeuse ima površinsku temperaturuoko 2900 K, a snaga kojom zrači je oko 4 · 1030 W.
Odredite polumjer te zvijezde uz pretpostavku da je kugla i savršeni emiter.
13.3 Zračenje
Q = eσT 4 A t
4 r2π
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13.3 Zračenje
P = σT 4⋅4 r2
π
P =Qt
e = 1
r2=
P
4 πσT 4 r =1
T 2 √ P4 πσ
r =1
(2900 K)2 √ 4⋅1030 W
4 π⋅5,67⋅10−8 J s−1m−2 K−4= 3⋅1011 m
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13.4 Primjene
staklo
posrebrene stijenke
vruća ilihladnatekućina
brtva
brtva
čep
TERMOSICA
Prijenos topline (vođenjem, strujanjemi zračenjem) sveden je na minimum.
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Halogensko kuhalo emitira elektromagnetskuenergiju koja prolazi kroz keramički poklopaci apsorbira se izravno u dnu vrča.
13.4 Primjene
halogenskalampa
keramičkipoklopac
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PITANJA ZA PONAVLJANJE
1. Strujanje (konvekcija)
2. Vođenje (kondukcija)
3. Vođenje topline kroz materijal
4. Toplinski izolator
5. Zračenje (radijacija)
6. Stefan-Boltzmannov zakon
7. Savršeno crno tijelo
8. Zaleđeni hladnjak i iglu
9. Termosica
10. Izo-staklo i termo-fasada
PITANJA ZA PONAVLJANJE