Final Boiling Assignment (1)
Short Description
Boiling assignment...
Description
BOILING ASSIGNMENT BEng Mechanical Engineering (Minor: Energy Systems) Thermal Engineering II (MECH !"#$ ) Coor%inator:& Mr' Mr' A' hoo%ar hoo%arth th *resente% +y:& •
BA**OO *alla,i+aye ("-####)
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BEEG.N /ar,esh ("#"#00)
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1A.2EE Bi+i Haseenah ("#"3!-4)
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5AMBH.N6.N 5oni Noor,eer ("#"7#8!)
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5.NGEN 5amalingm Brian ("#""#78)
1. What is is th the di difference be between evaporation and boiling? Evaporation occurs at the liquid–vapor interface Boiling occurs at the when the vapor pressure solid–liquid interface is less than the saturation when a liquid is brought pressure of into contact with a the liquid surface maintained at a at a given temperature sufficie su fficiently ntly above the saturation temperature. temperature of the liquid. Cengel & Ghajar !"1#
1. What is is th the di difference be between evaporation and boiling? Evaporation occurs at the liquid–vapor interface Boiling occurs at the when the vapor pressure solid–liquid interface is less than the saturation when a liquid is brought pressure of into contact with a the liquid surface maintained at a at a given temperature sufficie su fficiently ntly above the saturation temperature. temperature of the liquid. Cengel & Ghajar !"1#
$he main differences of evaporation and boiling are%
'vaporation ta(es place at all temperatures while boiling occurs at a particular temperature.
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'vaporation ta(es place at the surface whereas the entire liquid boils.
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'vaporation can occur using the internal energy of the system while boiling requires an e)ternal source of heat.
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'vaporation produces cooling but boiling does not.
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'vaporation is a slow process while boiling is a rapid process.
!. What is is th the di difference be between pool boiling and flow boiling? boiling? Flow Boiling
Pool Boiling *oiling is called pool boiling in the absence of bul( fluid flow.
+ny motion of the fluid is due to natural convection currents and the motion of the bubbles
under the influence of buoyancy
*oiling is called flow boiling
in the presence of bul( fluid flow. ,n flow boiling the fluid is
forced to move in a heated pipe or over surface by e)ternal means such as a pump.
a
-. What is the difference between subcooled and saturated boiling?
Subcooled Boiling
*oiling is subcooled if temperature of
main body of fluid is below the saturation temp $sat /i.e. bul( of liquid is subcooled0. ,t occurs at early stages of boiling.
*ubbles formation and disappearance near
hot surface. *ubbles disappear as they transfer heat to
surrounding subcooled liquid. *oiling is confined to locality of hot
Cengel & Ghajar !"1#
surface so also called local boiling. *ubbles serve as energy movers and
-. What is the difference between subcooled and saturated boiling? Saturated Boiling
*oiling is saturated if temperature of main body of fluid is equal to the saturation temperature $sat /i.e. bul( of liquid is saturated0.
,t occurs when entire liquid body reaches saturation temperature .
Cengel & Ghajar !"1#
*ubbles rise to the top.
. 2raw the boiling curve and identify the different boiling regimes. +lso e)plain the characteristics of each regime.
3atural boiling regime% the fluid motion is governed by natural convection and heat transfer from heating surface to fluid is by natural convection.
3ucleate boiling regime% bubbles form at various preferential sites on the heating surface and rise to the top.
$ransition boiling regime% part of the surface is covered by a vapor film.
4ilm boiling regime% the heater surface is completely covered by a continuous stable vapour film and heat transfer is by combined convection and radiation.
#. 5ow does film boiling differ from nucleate boiling?
,n film boiling regime the heater surface is completely covered by a continuous stable vapour film and heat transfer is by combined convection and radiation.
,n nucleate boiling regime the heater surface is covered by the liquid and heat transfer is through direct transfer from the surface to the liquid in motion at the surface.
$he boiling heat flu) in the stable film boiling regime can be higher or lower than that in nucleate boiling regime as can be seen in the boiling curve.
6. 2raw the boiling curve and identify the burnout point on the curve. ')plain how burnout is caused. Why is the burnout avoided in the design of boilers?
$he burnout point is the point C.
*urnout is caused by%
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$he heater surface being blan(eted by a continuous layer of vapour film at increased heat flu)es.
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9ise in temperature in heater surface temperature in order to maintain the same heat transfer rate across a lowconducting vapour film.
+ny attempt to increase the heat flu) beyond the ma)imum heat flu) will cause the operation point on the boiling curve to suddenly jump from point C to point '.
,n most cases the surface temperature that corresponds to point ' is beyond the melting point of heater materials.
$his causes burnout to occur.
$his should be avoided in the design of boilers to prevent disastrous e)plosions from happening.
:. 2iscuss some methods of enhancing pool boiling heat transfer permanently.
;ool boiling can be enhanced permanently by% ,ncreasing the number of nucleation sites on the
heater surface. $his can be achieved by coating the surface with a very thin layer of a very porous material or by mechanically machining cavities on the surface. $his will help in the continuous formation of vapour. ,ntroducing microchannels at the bottom of
surface to increase capillary flow. $hus increasing
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,ntroducing microchannels at the bottom of surface to increase capillary flow. $hus increasing the heat transfer and critical heat flu). ose% to atmos>heric >ressre' Negligi+le losses 9rom heater to srron%ings' The satration tem>eratre o9 ,ncropera
Analysis:
Accor%ing to the +oiling cr,e (gre"!'#= Incro>era)= ncleate +oiling occrs' 1rom Ta+le "!'"DIncro>era= 1or a mechanically >olishe% steel >an= the ,ale o9 is !'!"- an% corres>on%ing ,ale o9 n is "'!
$he 9ohsenow relation which gives nucleate boiling heat flu) per unit area is given by% > @nowing that
> A $herefore
b. The rate of evaporation
oration
Fgs
1". Water is boiled at sea level in a coffee ma&er e'uipped with a #(cm long .) cm diameter immersion(type electric heating element made of mechanically polished stainless steel. The coffee ma&er initially contains 1* of water at 1+o". ,nce boiling starts% it is observed that half of the water in the coffee ma&er evaporates in #$ min. -etermine i/ power rating of electric heating element% ii/surface temperature of heating element% iii/how long it will ta&e to raise the temperature of 1* of cold water from 1+ o" to the boiling temperature.
1".
=chematic diagram
Assumptions made:
=teady state operating conditions e)ist.
+t sea level pressureB 1 atm saturation temperature of boiling water $satB 1""oC.
3egligible heat losses from coffee ma(er to surroundings.
*oiling regime is nucleate boiling.
Given information
2B diameter of heating element B ".cm
Blength of heating element B!"cm
Properties of water 4rom $able +6 /,ncropera and 2ewitt0 When $satB 1""oC Dl B Df B !:7 ) 1" 6 3sEm!
hfg B !!#: (FE(g
C pl B C pf B .!1: (FE(g@
;r l B ;r f B 1.:6
σ = #8.7 ) 1"- 3Em
vf B 1.")1"- m-E(g
l B 1Evf B 7#:.8# (gEm -
vg B 1.6:7 m -E(g
vB1Evg B ".#7#6 (gEm-
4rom $able 1".1 / ,ncropera and 2ewitt0
4or mechanically polished stainless steel surfacewater combination% Csf B "."1-! n B 1."
Analysis: i0 l B 7#:.8# (gEm- which is equivalent to ".7#:8# (gE. $herefore ".# corresponds to a mass of ".:87- (g 5eat required /H0 to evaporate half of the water in !# min is% HB q t B m C p I$ B m hfg tB time ta(en to evaporate the water in seconds mB mass of water evaporated C p B specific heat capacity at average temperature I$ B temperature change of water
ii0 =urface area of heating element +s B 2 B "."""8 J m! 5eat flu) qKK B q E +s B !86 :!7.6 WEm!
$o determine the surface temperature $s 9ohsenowKs equation for heat flu) due to nucleate boiling is used.
$herefore $s B 11-.6 o C iii0 +verage temperature of water B /1""L180E! B#7 oC ,nterpolating from $able +6 to find C p at #7 oC we get C p B.187 (FE(g@ . =ince H B q t B m C p I$ t B /m C p I$0 Eq B #6.1- s B :.6 min
11. 0 $(cm long% #(cm diameter brass heating element is to be used to boil water at 1# ". 2f the surface temperature of the heating element is not to e3ceed 1#$ "% determine the highest rate of steam production in &g4h. o
o
=chematic diagram
Assumptions made:
=teady state operating conditions e)ist.
3egligible heat losses from boiler to surroundings.
Given information
2B diameter of heating element B ! cm
Blength of heating element B6# cm
$s B surface temperature B 1!# oC
$sat B saturation temperature B 1!" oC B -7-.1# @
Properties of water ,nterpolating from $able +6 /,ncropera and 2ewitt0 When $satB -7-.1#@ D B Df B !-".: ) 1"6 3sEm!
l
hfg B !!"!.8: (FE(g
C B C pf B .! (FE(g@
pl
;r l B ;r f B 1.!7
σ = #.7:) 1"- 3Em
v B 1."6"8)1"- m-E(g
f
B 1Evf B 7!.68 (gEm
l
vg B ".7"16 m-E(g
vB1Evg B 1.1"7 (gEm-
4rom $able 1".1 / ,ncropera and 2ewitt0
4or brass surfacewater combination% Csf B ".""6" nB1."
Analysis: I$e B $s M $sat B1!# 1!" B#C
5eat transfer rate q B qKK +s =9 K#'-" F6Fg& l K 9 K ("8" @ "! &4) N&sm*rl K *r9 K "'"# K (#8'3 @ "! &) N&m h9g K -""- F6Fg
4rom $able 1".1 /,ncropera0% For mechanically polished stainless steel: •
Csf B "."1-!
a0 $he 9ohsenow relation is used to obtain the ma)imum heat flu) in nucleate boiling%
b0 $he rate of evaporation is given by%
c0 4rom $able 1". in 5eat & Qass $ransfer Cengel we can obtain the coefficient C for a horiSontal cylindrical heating element. T ".1!
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