Anna University, Chennai
HEAT AND MASS TRANSFER
DEP ARTMENT OF MECHANICAL ENGINEERING
HEAT AND MASS TRANSFER
TWO MARK QUES TIO NS WITH ANS WERS
ACEDAMIC YEAR 2013-20 14
P REPAR ED BY
MRS.R. SIVA RANJANI.DST, BTECH .,
QUE STION AND ANSWERS
1 . Define He at transfe r?
Heat transfer can b e de fined as the tran smission o f en ergy from one re gion to anothe r regio n to te mpe ra ture diffe re nce. `
2 . What are the mode s of he at tran sfer?
Conductio n Convection Rad iatio n.
3 . What is conduction ?
Heat condu ctio n is a mechanism o f he at tran sfer from a region of high temp erature to a regio n of low temperature with in a medium (S olid, liquid or Gases) or d ifferen t medium in dire ct phy sic al c on ta ct. In conduc tion , ene rgy excha nge take s p la ce b y the kin ema tics motio n or d irec t impa ct o f mo le cu le s .Pure conduc tion is found on ly in solid s.
4 . State Fourier’s la w of c onduc tio n.
The rate of h ea t co nduction is prop ortional to the area measured n ormal to the direc tion of hea t flo w and to th e tempe rature grad ient in that dire ction.
5 . Define T herma l conduc tivity
Thermal conductivity is define d as the ab ility of a sub stance to con duct hea t.
6 . List down th e three typ es of bounda ry conditio ns
P rescribe d tempe rature P rescribe d heat flux Boundary cond ition s.
7 . Define con vec tio n.
Convection is a process of hea t tra nsfer tha t will oc cu r be tween solid su rfac e and a flu id medium when they are at diffe rent tempe ra tu res. Convection is po ssible on ly in th e pre sence of fluid med ium.
8 . Define Ra diation
T h e h ea t tra n sfe r fro m o n e b o dy to a no th e r w ith o u t a ny tra n sm ittin g medium is known a s rad ia tio n .It is a n ele ctromagnetic wave p henomenon.
9 . State Newton’s law o f cooling o r conv ec tion la w.
Heat tran sfer by c onvec tio n is give n by Newton ’s law of coo ling Q = hA(Ts-T )
Whe re ;-A- Are a exposed to he at tran sfer in m2 h - H ea t transfe r coeffic ie nt o f the surfa ce in KT s-Tempera tu re o f the surface in K- Tempe rature of the flu id in K
1 0.Define ove ra l he at tran sfer co-e fficien t.
The ove rall he at tran sfer b y combined mode s is usually expresse d in terms of an overall con ductan ce o r overall hea t transfer co-effic ient’ Heat tra nsfe r, Q
1 1.Define fins o r extended surfaces.
It is p ossib le to in crea se the heat tran sfer rate by inc reasing the su rface of h ea t tra nsfe r. The surfaces u sed fo r in crea sing h ea t transfe r are c alled exte nded surfa ce s some times kn own a s fin s.
1 2.State the app lic atio ns o f fin s.
1. Coo ling o f e le ctronic components.2. Coolin g of motor cyc le engines.3. Cooling o f sma l ca pa city comp ressors4. Coo ling o f transformers.
1 3.Define fin effic ie nc y.
The effic iency of a fin is d efin ed a s the ratio of ac tual heat tran sferred to the ma ximum po ssible to heat tran sferred by th e fin .
1 4.Define F in e ffectiv eness.
F in effectiveness is the ratio of heat tran sfer with fin to that w ithout fin
F in e f fe c t iv e n e s s = Q w it h f i n /Q w i t h o u t f in .
1 5.What is mea nt by stead y sta te hea t c onduc tio n?
If th e temperature of a bo dy does not va ry with time , it is sa id to b e in a ste ady state and tha t type of con ductio n is known a s stead y sta te h ea t c onduc tio n.
1 6.What is mea nt by transient he at co ndu ction o r un stea dy state conduc tion ?
If th e te mpe rature of a bod y va rie s with time, it is sa id to be in a transie nt state a nd tha t ty pe of con ductio n is k nown a s transie nt hea t conduction or unsteady state cond uc tion .
1 7.What is Pe riod ic heat flow?
In Pe riod ic hea t flow, the temp erature va rie s on a re gu la r ba sis E xa mple;
1. Cylind er o f a n IC engin e.
2. Su rfac e of ea rth durin g a pe rio d of 24 hours.
1 8.What is non Pe riod ic hea t flow?
In non P eriodic heat flow , the temperature a t an y po in t within th e sy stem varies n on lin early with time. Examp le :
1. Hea ting of an ingo t in furnace .
2. Coo ling o f b ars.
1 9.What is mea nt by Newtonia n heating or cooling proc ess?
The process in which the inte rnal resistan ce is a ssume d as n egligible in compa rison with its surfa ce re sista nce is kno wn a s Newtonian hea tin g or coo lin g proc ess.
2 0.What is mea nt by Lumped he at analysis?
In a Newton ian he ating or coo lin g proce ss the tempera tu re th roughou t the so lid is
considered to be uniform a t a give n time. Such an ana lysis is ca lle d Lumped h ea t capacity analysis.
2 1.What is mea nt by semi-in fin ite solid s?
In sem i-infin ite solids, at any in stan t of time , the re is a lw ays a po in t where the effec t
of hea tin g or cooling at one o f its b ounda rie s is not felt a t all .At this po in t the temp erature rema ins unchan ge d .In semi infin ite solid s, the bio t nu mbe r value s is
2 2.What is mea nt by infinite solid?
A so lid whic h extends itself infin itely in all d irec tio ns o f sp ac e is kn own a s infin ite so lid. In in finite so lid s, th e biot nu mbe r va lue is in be tween0.1 a nd 100. 0.1 < Bi< 1 00
2 3.Define Bio t nu mbe r.
It is d efined a s the ratio of interna l cond uc tiv e re sista nce to the surface conductive re sista nc e. Interna l cond uctive resistan ce Bi= -----------------------------------su rfac e conductive resistan ce .
2 4.What is the signific ance o f Biot number?
Biot number is u se d to find Lumped he at a na ly sis, Se mi infin ite solid s an d in fin ite solid s If Bi< 0.1 Lumped heat analysis.Bi= 0.1 < Bi< 10 025.
2 5.What are the factors affec ting the therma l co nduc tivity?
1. Mo isture2. Density o f mate rial3 . P ressu re4. Tempe ratu re 5. Structural of material.
2 6.Exp lain th e sign ific ance o f therma l diffu sivity.
The physic al signific ance of thermal diffusivity is that it te lls us h ow fa st heat is prop agated or it d iffuses throu gh a material during change s of temperature with time .
2 7.What are He islers c harts?
In Heisle rs c ha rt, the solution s fo r temp erature distribution s and heat flows in plane wa ls, lo ng cy lin ders and sphere s w ith finite in te rnal a nd surfa ce resistance are prese nted. He islers ch arts are no thing but a ana lytic al so lu tio ns in the form of graph s
QUE STION AND ANSWER
2 8.What is dimen sio nal analysis?
Dimen sional ana lysis is a ma th ema tic al me thod which ma kes use of the stu dy o f the d ime nsions for so lv ing seve ral e ngin ee ring p ro blems. T his method can be applie d to all types o f fluid re sista nce, hea t flow prob le ms in flu id mec han ics and the rmodynamics.
2 9.State Buck in gh am’s the orem. Buck in gham’s theo rem state s as fo llow s:
“If there are n v aria bles in a dimen sionally homogene ou s equa tion and if th ese contain M dimen sion s, then the v ariable s are a rran ged in to (n-m) d imension le ss te rms. Th ese dime nsio na l terms a re called B uck ingh am’s the orem.
3 0.What are a l th e advantages of d imensiona l a na ly sis?
1. It e xp re sses the fu nc tional re latio nship be tween the va ria bles in d im ensiona l terms.
2. It enable s ge tting u p a theoretic al so lu tio n in a simplifie d dimen sional fo rm.
3. The results of one series o f test can be app lie d to a la rge nu mbe r of o ther simila r prob le ms with the he lp of d imen sion al ana lysis.
3 1.What are a l th e limitations of d imensional a na lysis?
1. The comp lete information is no t p ro vided by d ime nsiona l a na ly sis .It on ly in dica te s th at there is so me relationship between the p arame ters.
2. No information is g iv en abo ut the in ternal mechanism o f ph ysical phenomenon .
3. Dimen sional ana lysis doe s not giv e any clu e re gard ing the selectio n of variab le s.
3 2.Define Re ynolds numb er (Re)
It is de fin ed a s the ratio of in ertia force to v isco us force. Inertia force R e= --------------- Viscous force.
3 3.Define P ra ndtl number (P r)
It is the ratio of th e momentum d iffusiv ity to the the rma l d iffusiv ity. P r=Mo men tum diffusivity / the rma l d iffusiv ity
3 4.Define Nu sselts Nu mbe r (Nu).
It is d efined a s th e ra tio of the heat flow by convec tio n p roce ss unde r an u nit temp erature grad ient to the hea t flo w ra te b y conduction unde r an unit temp erature gra dient through a sta tio nary thic kne ss (L) of meter.
3 5.Define Grasho f number (Gr).
It is de fined as the ratio o f p roduc t o f ine rtia forc e and b uoy ancy force to the squa re o f visc ou s force
3 6.Define S ta nton number (St)
It is the ratio of Mu ssels numbe r to the p rodu ct o f Reyno lds numbe r and P ran dtl numbe r.
3 7.What is mea nt by Newtonia n and non- Newton ia n flu ids?
The flu id s whic h obey the N ewton ’s la w of v isco sity are calle d Newtonian s flu ids a nd those wh ich do not obey a re ca lled non Newton ia n flu ids.
3 8.What is mea nt by lamin ar flow and turbulent flow?
Laminar flow :
Laminar flow is so metimes called streams line flow .In this type o f flo w , the flu id m ove s in la ye r and each fluid pa rticle fo llo ws a smooth c ontinuous pa th . The flu id p artic als in each layer remain in an orderly seque nce with out mixing with ea ch o ther.
Tu rb ulen t flo w:
In add ition to the lamina r type of flow, a d istin ct irre gu la r flo w is fre quen tly ob se rved in n ature. This type of flow is ca lled tu rb ulen t flo w .T he pa th o f a ny ind iv id ua l pa rticle is z igza g an d irregular.
3 9.What is hydro dynamic s bo unda ry layer?
In hy drodynamics b oun da ry layer, velo city o f the fluid is less th an 9 9% of free stre am ve locity.
4 0.What is therma l boun dary layer?
In the rmal b ou nda ry la ye r, te mpe ra tu re o f the fluid is le ss tha n 99 % o f free stre am temp erature.
4 1.Define con vec tio n.
Convection is a process o f he at tran sfer that will o ccur betwe en a solid su rfac e and a flu id mediu m w hen th ey a re at d ifferen t temperatures.
4 2.What is mea nt by free or na tu ra l c on vectio n?
If the flu id mo tio n is p ro du ced due to c han ge in density resultin g from temp erature
gra dients, the mode of hea t transfe r is said to be free o r na tural c onvection.
4 3.What is fo rc ed convection?
If th e flu id m otio n is artific ially crea ted by means o f an exte rnal force lik e a blow er o f fan ,that type of h ea t tra nsfe r is k nown as forc ed convec tio n.
4 4.What is th e fo rm of equation u sed to calcu late transfe r fo r flo w through cylindrica l pipe s?
Nu = 0 .023 (R e) 0 .8 (P r) n n = 0.4 for hea tin g of fluids=0.3 for co olin g of flu ids
4 5.What are the d imensional p arameters used in forced c onv ec tio n?
1. Re yno lds n umbe r (Re )
2. Nusselt Nu mbe r (Nu)
3. Prandtl n umber (P r)
4 6.Define bounda ry la ye r thickness.
The th ickness of the boundary layer has been defin ed a s the d istanc e from the su rface at w hich the loc al veloc ity o r te mpe rature reache s 99% o f th e exte rnal velocity o r te mpe ra ture.
4 7.In dica te th e concep t of sign ifica nce of bou ndary laye r.
In the boundary laye r c oncep t the flow fie ld o ve r a body is d iv ided in to two regio ns: A th in reg ion ne ar the b ody called the b ounda ry la yer whe re the v eloc ity and the temp erature grad ients are large . The re gion ou tside the boun da ry lay er where the ve locity and the te mpera tu re gra dien ts a re very ne arly equal to th eir fre e stre am va lues.
4 8.Define d isplac ement th ickn ess.
The disp lacement thickne ss is the d istan ce , measured pe rpen dicu lar to the boun da ry , by which the free stream is d isp lac ed on accoun t of fo rmatio n of bou ndary laye r.
4 9.Define mo men tu m th ic kness.
The momentum thickness is de fin ed as the dista nce through wh ich the total lo ss o f
momentum per seco nd be equal to if it were passing a stationa ry plate.
5 0.Define ene rgy th ickness.
The energy thic kne ss can be defined as the distance, measured perpend ic ular to the o f the solid b od y ,by which the bounda ry should be displac ed to compensate fo r the reduction in k in etic ene rgy of the flo win g flu id on ac count o f boundary la ye r formation
QUE STION AND ANSWERS
5 1.Define boilin g.
The ch an ge o f phase fro m liqu id to v apour state is known as boiling
5 2.What is mea nt by c onden sa tio n?
The ch an ge o f phase fro m vapou r to liq uid state is known as con densation.
5 3.Give the app lic atio n of boiling a nd conden sa tio n.
Bo iling a nd con den sa tio n process fin ds wid e application a s mentio ned be lo w.
1. Thermal and nuc lea r power poin t
2. Re frigeration systems.
3. Process of he atin g and cooling.
4. Air cond ition in g sy stem .
5 4.What is mea nt poo l boilin g?
If hea t is ad ded to a liquid fro m a submerge d so lid surfa ce , the bo iling p rocess is referred to as pool bo iling. In th is c ase the liquid a bove the hot surface is e ssen tia lly sta gn ant and it motion ne ar th e su rfac e is due to free c onvectio n and mixing ind uced by bub ble growth and detachm ent.
5 5.What are the mode s of co ndensation ?
The re are two modes of c ondensatio n
1. Film wise co ndensation.
2. Drop wise conde nsation
5 6.What is Film wise cond ensation?
The liq uid co nde nsate we ts th e so lid surfa ce , spread ou t and fo rm s a continuo us film over th e en tire surface is k nown as film wise co ndensation .
5 7.What is Drop wise condensation?
The vapo ur conden se s into sma ll liq uid drop le ts of various size s whic h fa ll do wn the surfa ce in a rando m fa shion.
5 8.What is heat exchange?
The hea t e xchan ge is d efin ed a s equipment which tran sfers the hea t fro m a h ot fluid to a co ld fluid.
5 9.What are the types of hea t e xchan ge r?
The type s o f heat excha nger are as fo llow s.
1. Direc t con tact heat e xcha ngers.
2. In dire ct con tac t heat exchangers
3. Surface he at e xc hangers
4. Parallel flow hea t exchan ge r
6 0.What is mea nt by in direc t contact hea t e xchan ge rs?
In this typ e of heat exc hangers, the tran sfer o f heat between two flu id s cou ld be
ca rried out by transmission throu gh a wa ll wh ic h sepa ra tes the two fluids
6 1.What is mea nt by Re ge ne ra to rs?
In th is type o f heat exchangers, ho t an d co ld fluids flow a lterna tely th rough the same . Examp le s: IC engin es, Ga s turbine .
6 2.What is mea nt by Recu pe ra to rs or surface heat exchangers?
Th is is th e most common type of heat e xchangers in which the h ot and cold flu id do no t come into dire ct contac t with each othe r bu t are se pa ra ted by a tube wall o r a surfa ce E xamples: Au to mob ile rad ia tors, Air pre he aters, e con omiz ers
6 3.What is mea nt by p aralle l flow heat exch angers?
In th is ty pe o f h ea t exchan ge rs, h ot and c old flu ids move in the same dire ction.
6 4.What is mea nt by Coun te r flo w he at e xc hangers?
In th is typ e of hea t exchan gers, ho t an d cold flu ids move in p arallel b ut o pposite d irec tio n
6 5.What is mea nt by c ro ss flow heat exchan gers?
In th is ty pe o f h ea t exchan ge rs, h ot and c old flu ids move a t right angles to e ach othe r.
6 6.What is mea nt by S hell and tube hea t exch an ge rs?
In th is ty pe of hea t e xchan ge rs, one of th e fluids move s through a bund le o f tube s
enclosed by a she ll. T he oth er fluid is fo rc ed throu gh the she ll an d it mov es o ve r the ou tside surface of the tu be s.
6 7.What is mea nt by Compact heat e xcha ngers?
There are man y specia l pu rp ose hea t e xch an ge rs c alled co mpact hea t exchan gers. They a re gene rally e mployed wh en c onvec tio n hea t transfe r co-effic ie nt a ssoc ia ted with one of the fluid s is much sma lle r tha n that associated with the other flu id .
6 8.What is mea nt by LMT D?
We know tha t the temperature diffe renc e be tween the ho t an d co ld fluids in the h ea t exch an ge rs varies from po in t to po in t. In addition various modes o f heat exchan ger are inv olved. Therefore ba sed on c oncep t of app ropriate mea n tempe ra tu re d ifference , also ca lle d lo ga rithmic mean temperatu re d ifference
6 9.What is mea nt Fou lin g fa ctor?
We know, the surfa ce s of heat e xcha nge rs do n ot remain clean after it has be en in use for some time .The su rface beco me fou led with scalin g or deposits .T he effec t of th ese depo sits affecting the value o f overall hea t tra nsfe r co –e fficien t. This e ffec t is taken ca re of by intro du cing a n additio na l the rma l resistan ce called fou ling re sista nce .
7 0.What is mea nt by Effec tive ne ss?
The hea t e xchan ge r effec tivene ss is d efined a s the ratio of actua l heattra nsfe r to the ma ximum possib le heat transfer. £=actual heat transfer/ma ximu m po ssible h ea t tra nsfe r
QUE STION AND UNIT ANSWERS
7 1.Define Ra diation?
The h ea t transfe r from on e body to anoth er withou t any transm ittin g mediu m is known a s radia tion .It is a n elec tromagnetic wave phenomenon.
7 2.Define emissiv e po wer?
The emissive p owe r is de fin ed a s th e total amount of rad iatio n emitte d by a body pe r un it time and unit a rea .It is expressed in W/m2.
7 3.Define mo nochromatic emissive powe r.
The en ergy emitte d by the surfac e at a given length pe r un it time a rea in all dire ction is known as mono chro matic emissive powe r.
7 4.What is mea nt by a bsorptiv e?
Ab so rp tive is define d as the ra tio b etween rad ia tion absorbed a nd inc iden t rad ia tio n
7 5.What is mea nt by reflec tivity?
Re flec tiv ity is d efin ed a s the ratio of radiation reflec te d to the incid ent radia tion
7 6.What is mea nt by transmissivity ?
T ran smissivity is de fine d as th e ratio of rad ia tio n transmitted to the inc iden t rad iatio n
7 7.What is blac k body?
Blac k b ody is a n id eal su rfac e having the fo llo wing prop erties.1. A black bo dy ab so rb s a ll in cide nt rad ia tio n, rega rdless o f wave length an dd irec tio n.2. For a prescribed tempe ra tu re and wave le ngth, n o su rface ca n em it more energy than black body.
7 8.State Wien ’s d ispla cemen t law.
T h e W ie n ’s d isp la ce men t law give s the relation ship b etween temperature and wave len gth co rrespondin g to the ma ximum spe ctra l e missiv e power o f the bla ck body a t tha t temperature.Tma x= C3C3= 2.9 * 1 0-3 Tma x= 2.9 * 10-3mk
7 9.State the S te fan –Boltzma nn law.
The emissive p owe r of a black body is proportion al to the fou rth power of a bsolute temp erature.Eb= Emissive p owe r W/m2
–Boltz mann constant = 5.67 * 10-8W/m2 K4
8 0.Define Emissiv ity.
It is de fine d as the ab ility of the surface of a bod y to radiate hea t .It is a lso d efin ed a s the ratio of emissive powe r of anybo dy to the e missiv e po wer of a b lack body o f eq ua l temp erature
8 1.What is mea nt by gray bo dy?
If a bo dy absorbs a definite pe rcen ta ge o f inc id en t radia tion irre sp ec tive of the ir wave len gth, the bo dy is kn own a s gray bo dy. The emissive p owe r of a gray b ody is a lway s less tha n that of the black body.
8 2.State Kircho ff’s law of radiation .
Th is la w states that th e ra tio o f total emissive p owe r to the ab so rp tiv ity is co nstant fo r all surfac es wh ich are in therma l equilib rium with th e su rround in g. It a lso sta tes tha t the e missiv ity o f the body is a lway s equa l to its absorptivity when the body rema ins in the rma l equilibriu m w ith its su rrou ndin gs.
8 3.Define inten sity of ra diation (Ib ).
It is de fined as the ra te o f energy leavin g a spac e in a given d ire ctio n pe r un it so lid an gle p er unit a rea of the e mittin g surfa ce normal to the mean direc tion in space.
8 4.State Lambe rt’s co sin e la w.
It state s that the total emissive po wer Eb from a rad iatin g plane surfac e in a ny direc tion propo rtional to the cosine o f the angle of e missio n
8 5.What is the purpose of ra diation shie ld ?
Radiation shield c onstructed from lo w emissivity (high re fle ctiv e) materials. It is used to redu ce the ne t radia tion tran sfer be tween two surfaces.
8 6.Define Irra diation (G)?
It is de fin ed a s th e total ra diation inc id en t upon a su rface pe r unit time per un it area . It is expresse d in W/m2
8 7.What is ra diosity ( J)
It is used to ind ic ate the tota l radia tion le avin g a su rface pe r unit time pe r un it area. It is expresse d in W/m2.
8 8.What are the a ssumptio ns made to calcu late radia tion e xc hange be tween the su rfa ces?
1. All surfa ce s are con side red to be e ith er b la ck of gray.
2. Ra diation and refle ction p roce ss a re assu med to b e diffu se .
3. The a bsorptivity o f a surface is tak en equa l to the emissiv ity and in depen dent o f temp erature o f the sou rce of the incid ent radiation.
8 9.What is mea nt by shape fac tor and mention its physic al sign ifica nce .
The sh ape facto r is de fined a s
“The fraction o f the rad ioac tive ene rgy tha t is diffu se d from one su rface e le men t a nd strikes th e othe r su rface direc tly with no intervening reflection “ it is rep re sented by Fij.O th er na mes for radia tion shape fa ctor are view factor, an gle fa ctor a nd configura tion fac to r .The shape facto r is u se d in the analysis of rad io ac tive h ea t exch an ge between two surface s
9 0.Disc uss th e rad ia tio n charac te ristics of c arbo n dioxide a nd wa te r v apour.
T h e C O 2 andH2 O both ab so rb and emit ra diation over ce rtain wa ve len gth re gion s ca lle d ab so rp tion ban ds. The radiation in these ga se s is a vo lu me phen omenon. The em issivity of CO2and the emissiv ity of H2O at a pa rticula r temperature inc reases w ith pa rtial pressu re and mean beam length.
9 1.Define Schm id t Nu mbe r?
It is d efined as the ratio of the molec ular diffusivity o f momentum to the mo le cu la r
diffusivity of ma ss
9 2.Define Sche rwoo d number.
It is de fin ed a s the ratio of conce ntra tio n grad ie nt at the bo undary.
9 3.Define Ma ss Concen tra tion .
Mass of a component p er unit volume of the mixture .It is e xp re ssed in k g/m3.
9 4.Define Ma ss Concen tra tion .
Mass of a component p er unit volume of the mixture .It is e xp re ssed in k g/m3
9 5.Define ma ss frac tio n.
The mass fra ctio n is de fine d as the ra tio o f mass concentra tion of species to the to ta l density of th e mixture
9 6.Define mo le frac tio ns.
The mole fra ctio n is de fine d as the ra tio o f mole concentra tion of species to the to ta l molar concentration.