Experiment 5

ERT 245- Heat and Mass Transfer Transfer in Biological Systems

Laboratory Module

EXPERIMENT 5 EVALUATE HEAT TRANSFER COEFFICIENT OF FREE & FORCE CONCVECTION HEAT EXCHANGER 1.0OBJECTIVES 11 11 To demo demons nstr trat ate e t!e t!e rela relati tion ons! s!i" i" bet# bet#ee een n "o#e "o#err

in"u in"utt and and surf surfac ace e

tem"erature in free and force con$ection 12 To demonstrate demonstrate t!e use of e%tended surface to im"ro$e !eat transfer transfer from t!e surface

2.0INTRODUCTION Heat transfer by simultaneous conduction and con$ection& #!et!er free or force& forms forms t!e t!e basis basis of most most indu industr stria iall !eat !eat e%c! e%c!an anger gers s and and relate related d e'ui" e'ui"me ment nt T!e T!e measurem measurement ent and "redictio "rediction n of !eat !eat transfer transfer coeffi coefficien cients ts for suc! circumstance circumstances s is ac!ie$ed in t!e (ree and (orce )on$ection Heat E%c!anger *""aratus by studying t!e tem"erat tem"erature ure "rofiles "rofiles and !eat flu% in an air duct duct #it! #it! associat associated ed flat and e%tend e%tended ed transfer transfer surfaces surfaces T!e $ertical $ertical duct is so constru constructed cted t!at t!e air tem"erat tem"erature ure and $elocity can be readily measured& and a $ariety of +"lug-in, modules of !eated solid surfaces of no#n dimensions can be "resented to t!e air stream for detailed study * fan situated situated at t!e to" of t!e duct "ro$id "ro$ides es t!e air strea stream m for for force forced d con$e con$ecti ction on e%"erime e%"eriments nts .sing .sing t!e instrume instrumentat ntation ion "ro$ide "ro$ided& d& free and forced forced con$ect con$ecti$e i$e !eat !eat transfer coefficients may be determined for/ 1 * flat surface 2 *n array of cylinders 0"inned !eat sin  *n array of fins 0finned !eat sin

ERT 245- Heat and Mass Transfer Transfer in Biological Systems

Laboratory Module

3.0THEORY 3.1 Fr C!"#$%!"  * !eated surface dissi"ates dissi"ates !eat "rimarily t!roug! a "rocess called con$ection con$ection Heat is also dissi"ated by conduction and radiation& !o#e$er t!ese effects are not considered in t!is e%"eriment *ir in contact #it! t!e !ot surface is !eated by t!e surface and rises due to a reduction in density T!e !eated air is re"laced by cooler  air #!ic! is in turn !eated by t!e surface and rises T!is "rocess is called free con$ection T!e !otter t!e tem"erature of t!e surface& t!e greater t!e con$ecti$e currents and more !eat 0"o#er #ill be dissi"ated 3f more "o#er is su""lied to a surface& t!e tem"erature of t!e surface must rise to dissi"ate t!is "o#er

3.2 F!r$' C!"#$%!" 3n free con$ection t!e !eat transfer rate from t!e surface is limited by t!e small mo$ements of air generated by t!is !eat More !eat is transferred if t!e air $elocity is increased o$er t!e !eated surface T!is "rocess of assisting t!e mo$ement of air o$er  t!e !eated surface is called forced con$ection T!erefore a !eated surface e%"eriencing e%"eriencing force con$ection #ill !a$e a lo#er surface tem"erature t!an t!at of t!e same surface in free con$ection& for t!e same "o#er in"ut

3.3 E(%"'' S)r*+$ Heat transfer from an obect can be im"ro$ed by increasing t!e surface area in contact #it! t!e air 3n "ractice it may be difficult to increase t!e sie of t!e body to suit 3n t!ese circumstances t!e surface area in contact #it! t!e air may be increased by adding fins or "ins normal to t!e surface T!ese features are called e%tended surfaces * ty"ical e%am"le is t!e use of fins on t!e cylinder and !ead of an air cooled "etrol engine T!e effect of e%tended surfaces can be demonstrated by com"aring finned and "inned surfaces #it! a flat "late under t!e same conditions of "o#er in"ut and airflo#

ERT 245- Heat and Mass Transfer Transfer in Biological Systems

Laboratory Module

,.0 MATERIALS MATERIALS AND E-UIPMENTS

,.1 DESCRIPTION DESCRIPT ION OF APPARATUS APPARATUS

(igure 1/ (ree and (orce )on$ection Heat E%c!anger *""aratus

1 (an  RT6 sensors 5 7anel

2 *ir flo# column 4 7ortable *nemometer 8 E%c!angeable Heat Transfer System

ERT 29:- Heat and Mass Transfer

Laboratory Module

5.0 5.0 PROC PROCED EDUR URES ES

5.1

E(r/"% 1 T! '/!"%r+% % r+%!" 4%" !r

")% +"'

)r*+$ %/r+%)r " *r $!"#$%!". 511 511

7lace 7lace t!e t!e flat flat finn finned ed !ea !eatt e%c!a e%c!ang nger  er  in to t!e duct

512 512

Recor Record d t!e t!e ambie ambient nt air air tem"e tem"erat rature ure 0t *&

51 51

Set t!e !eater !eater "o#er "o#er control control to 29 ;atts ;atts *llo# *llo# sufficie sufficient nt time to ac!ie$e ac!ie$e stead steady y state conditions before noting t!e !eated "late tem"erature 0t H

514 514

Re"eat Re"eat t!e t!e "roce "rocedure dure at 49& 49& 89 89 and and <9 <9 ;atts ;atts

5.2  E(r/"% 2  T! '/!"%r+% % r+%!" 4%" !r ")% +"' )r*+$ %/r+%)r " *!r$ $!"#$%!" 6finned 6 finned heat exchanger) 5.2.1

Set t!e !eater "o#er control to 59 ;atts *llo# sufficient time to ac!ie$e steady state conditions before noting t!e !eated "late tem"erature 0t H

5.2.2

Set t!e fan s"eed control to gi$e a reading of 95m=s on t!e t!ermal anemometer Record t!e !eated "late tem"erature 0t H

5.2.3

Re"eat t!is "rocedure at 19m=s and 15m=s

5.3 E(r/"% 3 T! '/!"%r+% % ) !* (%"'' )r*+$ %! /r!# +% %r+"*r *r!/ % )r*+$. 51 51

7lace 7lace t!e tube tube bundle bundle "late "late !eat !eat e%c! e%c!ang anger er into into t!e duct duct

52

Set t!e !eater "o#er control control to >5 ;atts ;atts *llo# t!e tem"erature tem"erature to rise to <9?)& <9?)& and t!en adust t!e !eater "o#er control to 15 ;atts until a steady reading is obtained Record !eated "late tem"erature 0t H

5 5

Set Set t!e fan fan s"ee s"eed d contr control ol to gi$e gi$e 1m=s 1m=s

54 54

Re"ea Re"eatt t!is t!is "roced "rocedure ure at at 29 and and 25m 25m=s =s

-4-

ERT 29:- Heat and Mass Transfer

Laboratory Module

7.0 RESULTS AND CALCULATION 81 (or e%"erim e%"eriment ent 1

Input Power  Watts

Plate Temp (t H) C

Ambient Temp (t A) 8C

tH – tA C

20 40 60 80

82 (or e%"eri e%"erimen mentt 2

Air Velocity m/s

Plate Temp (t H) C

Ambient Temp (t A) 8C

tH – tA C

0 0.5 1.0 1.5

8 (or e%"eri e%"erimen mentt 

Velocity m/s

Plate Temp (t H) C

Ambient Temp (t A) 8C

tH - tA C

0 1 2 2.5

84 (or e%"eriment e%"eriment 1& based on t!e data recorded& "lot t!e gra"! of "o#er against against tem"erature 0t H @ t * 85 (or e%"erimen e%"erimentt 2 and & based based on data recorded recorded&& "lot t!e gra"! of air $elocity $elocity against against tem"erature 0t H @ t *

7.0 DISCUSS DISCUSSION ION

>1 ;!y is it necessary to set t!e fan s"eed to ma%imum before s#itc! A(( t!e a""aratus >2 Ho# do t!e t!e e%tended e%tended surface surface for t!e "late !eat !eat e%c!anger e%c!anger #ill im"ro$e im"ro$e t!e !eat !eat transfer transfer "rocess > 6ifferen 6ifferentiat tiate e bet#een bet#een free free and forced forced con$ec con$ection tion >4 6iscuss t!e relations!i relations!i" " bet#een bet#een air air $elocity $elocity and and surface tem"erature tem"erature >5 6iscuss t!e relations!i" relations!i" bet#een bet#een "o#ers "o#ers dissi"ated dissi"ated and and surface surface tem"erature tem"erature

9.0 CONCLUSION <1 Based on t!e e%"erimental "rocedure done and t!e results taen dra# some conclusions to t!is e%"eriment -5-