Experimental and Numerical Investigation of Air Suction in Domestic Gas-Burning Heaters to Increase Efficiency

Transcription

1 Internatonal Journal of Materals, Mechancs and Manufacturng, Vol. 1, No. 2, May 2013 Expermental and Numercal Investgaton of Ar Sucton n Domestc Gas-Burnng Heaters to Increase Effcency Peyman Zahed, S. Mohammad Javad, Kanoosh Yousef, and Afshn Pakdel Abstract In ths paper, the performance of domestc gas burner heatng unt s enhanced by optmzng secondary ar of combuston that t s controlled by control of area entrance vent of secondary ar and the heater second furnace vent. In order to determne the approprate extent of the area of ar sucton vent, second furnace vent was closed by expermental methods and a valve wth openng and closng capabltes was appled to the under part of the frst furnace of the heater and the effect of closng ths valve on ncreasng the heater effcency was studed. The heatng unt s frst modeled as a three dmensonal physcal doman for flow of gases, and conservaton equatons of mass, momentum, energy, speces and radaton are dscretzed over the meshng system of fnte volume method provded n the doman. Expermental set-up to measure and valdate the numercal results s equally establshed. Results show that by closng 84% of secondary ar sucton vent and complete closure of the second furnace vent, mass flow rate of ar sucton becomes kg/s. In ths condton, a 9% ncrease n the mean heat transfer rate from furnace surface of the heater wll occur. Index Terms Ar sucton vent, furnace, gas-burnng heater. I. INTRODUCTION Consderng the mportance of optmzng fuel consumpton and crtcal proporton of ndoor warmng n energy consumpton, optmzng the structure of ndoor gas-burnng equpment to ncrease the effcency s very mportant. At the same tme, the extent of usng ndoor gas-burnng heaters has multpled the mportance of the studes of optmzng ths equpment. Complex geometrcal structure, smultanety of dfferent phenomenon such as combuston, dfferent models of heat transfer (free convecton, forced convecton and radaton) and also the effect of buoyancy energy n provdng combuston ar cause weakness n emprcal method of try and error to mprove the effcency and hghlght the mportance of computer smulaton. Because of more complete understandng of flud, heat and combuston processes, usng smulaton allow us to control these processes toward effcency and heat transfer mprovement. Consderng the appled methods of better combuston wth less pollutant and better effcency of gas-burnng heaters, the amount of ar whch enters the heater and also the amount of ar whch enters as secondary ar has a great effect on heater effcency and the amount of pollutant. The extent of excessve ar sutable for optmum performance of a heater Manuscrpt receved October 20, 2012; revsed February 2, Peyman Zahed, S. Mohammad Javad, Kanoosh Yousef, and Afshn Pakdel are wth Mechancal Engneerng Department, Islamc Azad Unversty, Mashhad Branch, Mashhad, Iran (e-mal: pemanzahed@gmal.com; mohammad.javad@gmal.com; kanoosh_py@yahoo.com; afshn.pakdel@gmal.com ). s calculated and also the percent of prmary and secondary ar of ths sutable ar for mnmum polluton and maxmum effcency s calculated [1]. The effect of parameters such as envronment condton, heght and dameter of chmney, thermal barrers of furnace and radaton heat transfer rato and fan nstallaton of the heater on the effcency mprovement have been studed [2]. and meanwhle, the rato of fuel to utlzed ar n gas-burn heater has been measured and accordng to the results the extent of ar whch flows through a heater set, n normal condton, s almost fourfold of the extent of the requred ar n stochometry mxture whch ths s sgnfcantly effectve n decreasng the effcency of regular gas-burn heaters [3]. The extent of passng flow through an alght heater depends on ts desgn parameters ncludng the area of ar entrance nto t, so optmzng the heater to sucks adequate ar for stochometry combuston s mportant. II. HEATER STRUCTURE ORIENTATION To optmze the process, frst studyng a sample heater was done. In Fg. 1 the overall scheme of the heater and ts parts are shown. As t can be seen, the heater ncludes burner, two furnace and chmney whch those two furnaces are connected by two cylndrcal tubes wth 100 mm dameter.input fuel s njected through a nozzle nto the burner whch sucks n some ar along wth tself whch play the role of prmary ar for combuston. When pre-mxed gas flow enters the frst furnace, t gntes. Flame s created and provdes ts requred remaned ar by the sucked ar of frst furnace (due to suckng nature of chmney). Combuston products are transmtted to the second furnace by two cylndrcal tubes. To allow the ncrease of the resdence tme and complete dffuson of consumpton products n second furnace, some barrers are put n the way of consumpton products nto the second furnace entrance vent. After passng the abovementoned barrers, the flow changes ts drecton towards the chmney entrance vent. Also n ths part to prevent quck exhaust of consumpton products nto chmney a barrer s nstalled n entrance. Man task of the second furnace s decreasng the chmney sucton mpact and mnmzng the negatve effect of chmney reversed ar flow on torch flame stablty and ts combuston and also decreasng the exhaust speed of the gases derved from combuston and sendng ncreased heat transfer of hot gases nto the envronment. Bottom part of ths furnace s completely opened to release the reversed penetrated flow from the chmney. The sucton created n the chmney depends on factors such as: chmney heght, chmney area, fume temperature and gases produced from combuston. So by changng each of these above-mentoned DOI: /IJMMM.2013.V

2 Internatonal Journal of Materals, Mechancs and Manufacturng, Vol. 1, No. 2, May 2013 condtons, the possbltes of reversed sucton ncrease. III. METHOD OF EXPERIMENT To examne the performance of consdered heater and the credblty of the numercal soluton, densty parameters of combuston products and the temperature of the nner parts of the chmney and also temperature dstrbuton on the heater and furnace external body were measured. Combuston products densty and chmney temperature have been measured by usng Testo 327 machne. Also surface temperature of the heater and temperature of nsde the furnace are measured by usng Testo925 thermometer wth contact probe. Steps of the exprmentng are n ths way that frst the pressure of nput gas of the heater s regulated at 178 mm of water column by usng a barometer and then heater s turned on, now we should wat tll the heater reach the stable mode whch wll usually occur after 15 mnutes of the start. By stablty we mean the tme n whch the temperature of exhaust combuston products of the heater chmney becomes almost stable, now combuston products are sampled by usng gas analyzer. Consdered experments are measured for a chmney wth the length of 165 cm. effcency mprovement, the ncrease of exhaust CO s also evdent. Ths experment s measured n three modes of low, average and hgh flame whch almost n every three modes results are the same and show ncreasng pattern of effcency tll the extent of 75% of ar entrance area closure and after that because of the lack of sucked ar, cause effcency drop. But about the extent of exhaust CO tll 40% of ar entrance vent closure, the extent of CO has steady pattern and t can be seen that by more closng the sucton vent, more ncreasng occur n CO pattern whch accordng to OSHA (Occupatonal Safety & Health Admnstraton), ths extent of CO should be no more than 250 ppm. IV. NUMERICAL SOLUTION METHOD By usng SoldWorks software, geometrcal modelng was executed and created model n SoldWorks software s llustrated n Fg. 3. To exactly study effectve parameters of effcency and heater temperature dstrbuton, Fluent6.3 computatonal software has been used. The flow nsde the consdered combuston chamber s of turbulent flow type wth densty change of chemcal materals derved from combuston. Domnatng equaton on ths phenomenon, mass conservaton equatons, movement, turbulence equatons, transmsson of speces and energy are consdered by the assumpton of steady wth respect of tme. To model the expresson resulted from turbulence RNG k-ε method and for modelng combuston flow and calculatng transmsson of speces Eddy-Dsspaton method has been used [4]. Fg. 1. Overall scheme of heater and ts parts By completely closng the second furnace vent, we begn to gradually close the secondary ar entrance vent by the transtve valve whch s ndcated n Fg. 2. By gradual closure of ar entrance vent n every step and after heater stablty durng consdered tme wth appled sucton condton of new ar, we measure the extent of CO and combuston effcency and combuston products temperature n the chmney. Fg. 3. Created model n SoldWorks software Equatons of transmsson of speces (N-1 equatons whch N s the number of speces) can be expressed as: u m 2 J 2 R 2 (1), x x where J D 2 2,, m m Sc x And dffuson factor of speces s calculated as follows: (2) Fg. 2. Transtve valve of furnace sucton wth openng and closng capabltes Expermental results as t can be seen from Fg. 9 and Fg. 10, show combuston effcency mprovement whch leads to overall effcency mprovement of the heater that along wth D (1 x ) / X / D (3) , m j, j j j j And dffuson factor of speces s calculated as follows: P x x x j uu j j j (4) 144

3 Internatonal Journal of Materals, Mechancs and Manufacturng, Vol. 1, No. 2, May 2013 where tensor stress s u u j 2 ul j j x j x 3 x l Conservaton of energy equaton n ths process wth chemcal reacton (combuston) s x x T u ( E P ) h J u S eff 2 2 j j h j j eff x 2 where S h s the source term aroused from the heat released from the chemcal reacton, also 2 u (5) (6) P E h (7) 2 where h s calculated from the deal gas defnton. Effectve heat conducton factor, whch s heat conducton factor of flud and turbulence effects on t, wth usng the RNG k-ε method calculated as below: C (8) eff p eff For calculatng the turbulence effects on the propertes of flow and calculatng the effectve heat conducton factor and effectve vscosty two assstance equatons (k-ε) has been utlzed. Also t s the turbulent vscosty determned from the followng equaton [5]: C s constant factor. t C 2 k (9) The default values of constant factors for k-ε model are: oxygen percentage exsts n the ar, whch s 22% n normal condtons. Ths equaton n bound of 0 1 s the governng equaton for the complete combuston wth excess ar. Mass of entrance fuel ( mf ) s calculated n terms of fuel heat capacty (LCV), molecular mass ( M f ) and gas-burner heat capacty ( Q ). m Q. M / LCV (13) f f whch LCV s calculated n terms of enthalpy of combuston products ( H p ) and enthalpy of reactants ( H R ). In turbulent flows wth chemcal reactons Aranus rate of reacton (for lamnar flows) or Eddy-Dsspaton rate of reacton (for turbulent flows) or both calculated accordng to the defnton of the problem for usng n the source term of the transmsson of the speces equaton. In ths study rate of reacton s used from Eddy-Dsspaton model on the bass of Magnesen & Hertager, 1976 [7]. V. GRID GENRRATION AND BOUNDARY CONDITIONS One of the most mportant parts n numercal solvng s producng proper geometry of the under studed system whch has the least errors n meshng, by consderng geometrc complexty and numerous ntervene parts, a trangular grd has been used for meshng. In the best created grd by Gambt 2.4 software, furnace s dvded nto meshes whch n Fg. 4 and Fg. 5 you can see soluton envronment meshng and also defned boundary condton for solvng the ssue. Consderng ntense changes of the varables of nsde the furnace and partcularly near the burner entrance valve, meshes have become smaller than other parts. C C 1.92, 1.3, 1.0, 1.44, C In ths study combuston of methane-ar assumed wth two stage combuston mechansm as mentoned below [6]: CH +1.5O k CO+2H O Step (10) CO+0.5O CO Step2 (11) 2 2 On the bass of ths mechansm, the products of methane oxdzaton are carbon monoxde and water vapor. In the next stage carbon doxde formed from carbon monoxde oxdzaton. Because of complete oxdzaton of methane n dlute complexes, n combuston wth excess ar the equaton of combuston s expressed as: CH 4 O2 N CO 2H O 2 O N (12) In ths equaton s the rato of the amount of stochometrc ar to the amount of actual ar and s the Fg. 4. Soluton envronment meshng n Gambt software Fg. 5. Appled boundary condtons 145

4 Internatonal Journal of Materals, Mechancs and Manufacturng, Vol. 1, No. 2, May 2013 VI. VALIDATION OF NUMERICAL SOLUTION To vald and assess the accuracy of numercal soluton, temperature dstrbuton on external surfaces of the heater and at some ponts of the furnace has also been determned by testng. Temperature of furnace upper surfaces and two connectng tubes between furnaces are measured and compared wth numercal results. In Fg. 6 temperature dstrbuton of numercal soluton on the external body s ndcated. In ths fgure computatonal temperature dstrbuton s as a constant dstrbuton and measured values n numbers are wrtten on the fgure. In Fg. 7. temperature dstrbuton of nsde the furnace n a plane rght at the center of the furnace s llustrated. Comparson of the results show that obtaned temperature change procedure from smulaton corresponds very well wth expermental data. Also n regard to temperature extent, maxmum fault s about 8%. By complete closure of second furnace vent, as t can be seen from Fg. 11 and Fg. 12, when 84% of secondary ar entrance vent s closed and the extent of sucked ar and needed ar for complete and stochometrc combuston are approachng each other, we wll have 9% ncrease n mean heat transfer of furnace bodes and then by more closng of ar sucton vent because of the lack of nput ar and creaton of ncomplete combuston, heat transfer rate has been decreased. Fg. 8. Examnng the mpact of the area of second furnace vent on mass rate of sucked ar As t s shown n Fg. 12, the temperature of combuston products and heat transfer rate relatve to the area of ar sucton vent have been ncreased and n the condton of 84% closure ar sucton vent, combuston products have the hghest temperature and then temperature of combuston products n chmney decrease because of ncomplete combuston and lack of needed ar, and as the same reason mass fracton of CO n numercal method after 78% closng of ar sucton area ncrease as t s seen n Fg. 10. Fg. 6. Temperature dstrbuton of heater furnace body Fg. 9. Investgaton of extent the area of ar nput to combuston effcency n three modes of low, average and hgh flame Fg. 7. Temperature dstrbuton of nsde the furnace VII. NUMERICAL SOLUTION RESULTS By examnng nput ar rate from bottom part of the furnace, as t s seen n Fg. 8, t can be concluded that by closng the vent of the second furnace nput ar rate from secondary ar sucton vent ncrease and by closng completely of second furnace cent mass flow rate of ar sucton ncrease by 71% because of chmney sucton effect. Fg. 10. CO densty dagram n chmney resulted from combuston relatve to the closure of ar sucton vent 146

5 Internatonal Journal of Materals, Mechancs and Manufacturng, Vol. 1, No. 2, May 2013 VIII. CONCLUSION By consderng the extent of usng ndoor gas-burn heaters and the mportance of optmzng fuel consumpton n them, ths paper has presented expermental and computer smulaton of a regular heater. Frst to ensure the accuracy of modelng, heater wth current geometry modeled and analyzed and results of numercal smulaton have been compared wth results of experment. Wth the purpose of ncreasng the effcency, we began to close the second furnace vent and also the valve of secondary ar sucton n the bottom part of the furnace by expermental method where more closures of sucton valve ncreases the combuston effcency. Fg. 11. Examnng the mpact of the area of ar entrance vent on mass flow of sucked ar and heat transfer rate of furnace body Fg. 12. Examnng proporton of external surface of sucton valve to the temperature of combuston products n chmney and temperature transfer rate from furnace surface In numercal smulaton, as t s ndcated by dagrams, t can be seen that heat transfer rate ncreases by the closure of ar sucton vent where by 84% closure of sucton valve mass rate of enterng ar equals the heater prmary mode durng producton but s accompaned by 9% ncrease n mean heat transfer rate of the heater furnace surface. Because of the lmtatons n complete closure of the second furnace due to safe exhaust of reverted gas from chmney, recommendaton for further works s to choose a mechansm n the second furnace that the vent can be open whenever the exhausted gases revert from chmney to the furnace and come out from ths open vent to prevent reversed gas flows effect on torch flame stablty. Ths condton leads to ncrease n the heat transfer rate of heater furnace and accordngly heater effcency mprovement. m IX. NOMENCLATURE Absorpton coeffcent Mass u P E H T Densty Vscosty Velocty Pressure Internal energy Enthalpy Temperature Oxygen percent Rato of stochometrc ar to actual ar j Tenson tensor Stephan-Boltzman constant s Propagaton constant S Path length s Radaton drecton vector j Kronecker delta r Locaton vector I Radaton ntensty Q Heat capacty of the gas-burner Phase functon Body surface angle REFERENCES [1] M. Bdabad, M. Seddgh, and S. Yousef, Optmzng gas-burn heater, n Proc. Second conference of Iran combuston, [2] A. Kanfar, H. Moen, M. Javad, and A. RashdTorogh, Numercal smulaton of combuston n an ndoor gas-burn heater and examnng effectve parameters of that, n Proc. Ffteenth annual conference of mechanc engneerng, Iran, [3] M. Rahm and V. AbdAghdam, Estmatng the proporton of fuel to used ar n a gas-burn heater set, n Proc. Second conference of Iran combuston, Islamc Azad Unversty of Mashhad, Iran, [4] D. C Howarth, A PDF method for turbulent mxng and combuston on three-dmensonal unstructured deformng meshes, Int. J. Engne Res, 2000, vol. 1, pp [5] M. Muradoglu, S. B Pope, and D. A. Caughey, The hybrd method for the PDF equaton of turbulent reactng flows: consstency condtons and correcton algorthms, J. Comput. Phys, 2001, vol. 172, pp [6] F. L. Dryer and I. Glassman, Hgh temperature oxdaton of CO and CH 4, n Proc. Fourteenth Sym. Int. on Combuston, 1973, pp [7] B. F. Magnussen and B. H. Hjertager, On mathematcal models of turbulent combuston wth specal emphass on soot formaton and combuston, n Proc. 16th Int. Symposum on Combuston, Phladelpha, Peyman Zahed was born n 1987, Iran. He receved B.Sc. degree n mechancal engneerng the feld of Auto-Mechanc from Khayyam unversty of Mashhad, Mashhad, Iran n 2008 wth hgh dstncton. Currently he s M.Sc. student (fnal semester) n Azad Unversty of Mashhad and studed mechancal engneerng the feld of energy conversaton wth great dstncton. Hs research nterests nclude Fuel and Combuston, Heat transfer, Multphase Flows Smulaton, Boundary Layer Control, Boundary Layer Theory, Flud Mechancs, Turbulent Flows and Modelng. Some of the most mportant paper ttles that presented n ICMEAT 2012 conference whch held n Isfahan, Iran are A Revew of Multphase Flows Smulaton Methods, Investgaton for ncrease or decrease the lft and drag coeffcent on the arfol wth sucton and blowng, Numercal nvestgaton on the flame speed of CH4/Ar dluted wth CO2 and vapor, Incompressble flud flow on a crcular cylnder wth heat transfer by fnte element method and Incompressble flud flow on four ellptc cylnder by fnte element method. 147