Analysis of Entropy Generation in Diffusion HTAC Processes

Transcription

1 nternational Journal of Applied Phyic and Mathematic, ol. 4, No. 4, July 214 Analyi of Entropy Generation in Diffuion HAC Procee Deodat Makhanlall and Peixue Jiang, Meme ACS Atract hermodynamic irreveriility ignificantly limit fuel energy converion into ueful ork. Hence, the evaluation of thermodynamic irreveriility i an important apect in the development of comution technique for etter fuel utilization. hi evaluation i performed for the HAC (high-temperature air comution) proce through entropy eration analyi. he primary irreverile procee of HAC are determined and the influence of key operating parameter uch a the comution air temperature, and oxy-dilution are tudied. ndex erm Comution, entropy eration, HAC mode, thermodynamic.. NRODUCON HAC (high-temperature air comution) i an emerging comution technique that offer the important enefit of fuel aving. Burning of le fuel mean reduced greenhoue ga emiion. Comution in the HAC mode i alo ditinctive y it trong flame taility and homoeity [1]. n addition, HAC procee have ignificantly increaed radiation heat tranfer rate [1]. Furnace operating in HAC mode may hoever exhiit an important draack: the increae of the comution air temperature promote NO-formation [2]-[4]. Recently, many tudie have een conducted to invetigate NO-formation in HAC procee. hee tudie ho that an oxy-diluted environment i required for uppreing NO-formation [3]-[6]. n addition to NO-related iue, there are other apect that till need e tudied to provide adequate inight into HAC procee. For intance, thermodynamic analye of the HAC mode are till lacking. Hence, thi paper aee the thermodynamic irreveriilitie of HAC. Such irreveriilitie caue entropy eration hich limit fuel energy converion into ueful ork [7]. oth the Arrheniu and Eddy-diipation reaction rate, and take the minimum of thee to rate a the net reaction rate [9]. he model for thermal radiation are: ith 4 4 a, d. 1 a r, r r, n d, n 1 (1) (2), (3) Here, a and are the pectral aorption and pectral cattering coicient, repectively. hey are determined y a eighted um of gray ga model. i the pectral radiative intenity, i the pectral lackody intenity, i the olid angle, i the cattering phae function, i the all emiivity, i a unit poition vecto r i a unit direction vecto and n i an outard unit normal vector.. MAHEMACAL MODELNG A. Computational Methodology he tudy i carried out ith the FLUEN 6.3 CFD code. Fig. 1 ho the prolem etup, hich i 2-D axi-ymmetric. Computational detail are pecified in ale. he HAC proce i modeled uing the to-tep reaction mechanim ith CO intermediate [8]. he RNG k-e turulence model i ued, hile turulence-chemitry interaction i olved uing the finite-rate/eddy diipation model. hi model, compute Manucript received March 3, 214; revied May 12, 214. hi ork i upported financially y inghua Univerity. he author are ith the inghua Univerity, the Department of hermal Engineering, Beijing, 184 PR China ( deodat@mail.tinghua.edu.cn, jiangpx@mail.tinghua.edu.cn). Fig. 1. Prolem chematic. ABLE : COMPUAONAL DEALS Parameter SDC HAC nlet Air emperature (K) nlet oxy volume fraction (%) Fuel-air ratio Aorption coicient (m -1 ) SGG SGG Scattering coicient (m -1 ).1.1 all emiivity all heat tranfer coicient (m -2 K -1 ) 1 1 DO: /JAPM

2 nternational Journal of Applied Phyic and Mathematic, ol. 4, No. 4, July 214 B. Entropy Generation Entropy eration i a pot-proceing quantity. Once the governing equation are olved, the local volumetric rate of entropy eration i computed a folloing [1]: S S f S ch S m S c S ae r. (4) L exp exp r, 2kc 4 hc k 1 exp hc k hc k 1 ln exp hc k 1 ln (12) here S S, f (5) H M i, (6) ch Ji, i i S, m (7) k S 2 c (8) 2 he total entropy eration rate i computed y integrating (5)-(9) over volume and (11) over urface area ith (13) ae S G, tot SG, f SG, ch SG, m SG, c SG, r SG, r S G, f S f d (14) S G, ch S, chd (15) S, S, d (16) G m m ith S ae, r a a, ddd r dd r (9) S S S G, c S ae G r, S, S G r A c ae, r, r d d da (17) (18) (19) hc k ln 2hc (1) are the local volumetric entropy eration rate aociated ith vicou diipation, chemical reaction, ma tranfe heat conduction and convection, and radiation tranfer in the medium, repectively., k, and J i, are the ective vicoity, ective thermal conductivity, and ective ma diffuion flux, repectively. i the vicou diipation function, i reaction rate, H i enthalpy, M i mean molecular eight, i chemical potential, h i Planck contant, k i Boltzmann contant, and c i peed of light. Entropy eration due to all radiation tranfer procee need alo e conidered hen computing the overall entropy eration rate [11]. he local radiation entropy eration rate per unit urface area i defined a:. RESULS AND DSCUSSON olumetric entropy eration rate of the HAC mode are hon in Fig. 2. Comparion i made ith the tandard diffuion comution (SDC) mode. olumetric entropy eration rate due to vicou diipation, the chemical reaction, ma tranfe and heat conduction and convection are much loer in the HAC diffuion flame. hee reduced volumetric entropy eration rate are due to the trong homoeity of the HAC mode [1]. Hoeve radiation entropy eration rate are increaed in the HAC proce. r, S (11) r, n d d, r L 4 r here 276

3 nternational Journal of Applied Phyic and Mathematic, ol. 4, No. 4, July 214 Fig. 2. Ditriution of local volumetric entropy eration rate under SDC and HAC mode. he highet volumetric rate of entropy eration, in oth the high-temperature air and the conventional diffuion comution mode, i aociated ith heat conduction and convection. By accounting for over 85% of the total entropy eration, heat conduction and convection alo form the primary irreverile procee in the tandard diffuion flame (Fig. 3). Radiation thermodynamic i of minor importance in 277

4 nternational Journal of Applied Phyic and Mathematic, ol. 4, No. 4, July 214 the conventional comution proce. Hoeve the ect of thermal radiation cannot e neglected in the HAC mode. hermal radiation account for more than 8% of the overall entropy eration in the HAC proce (Fig. 3). hi hift from heat conduction and convection to thermal radiation a the main irreverile proce i due largely to the pre-heated air temperature. t i ell knon that radiative heat tranfe and it aociated thermodynamic ect, ecome more ignificant at elevated temperature [1]. Oxy-dilution i one of the primary feature of the HAC mode ecaue of it ect on NO-formation [1]. he ect of oxy-dilution on entropy eration in the HAC mode i depicted in Fig. 5. he overall radiation entropy eration rate increae, principally due to all radiation procee, hile entropy eration aociated ith heat conduction and convection reduce a the oxy volume fraction in air ecome loer. he overall thermodynamic ect of diluted air in the HAC mode involve a mall increae of the overall entropy eration rate. Fig. 3. mpact of variou procee on entropy eration in HAC mode. Fig. 4 depict the relationhip eteen air temperature and entropy eration rate. ith increaing air temperature, the overall radiation entropy eration rate increae trongly, hile overall entropy eration rate due to heat conduction and convection reduce. he increae of radiation entropy eration rate i primarily due to all radiation tranfer procee. Radiation entropy eration in the medium ecome relatively le important ith increaing air temperature. Fig. 4 alo ho that in HAC procee the overall entropy eration rate firt reduce and then increae ith air temperature. Overall entropy eration rate i minimum at 55K. Fig. 5. Effect of oxy volume fraction of inlet comution air and all emiivity on entropy eration in HAC mode (air temperature i 1K). For the HAC mode, thermal radiation tranfer procee at olid all i the main mechanim of entropy eration. he all emiivity i an important parameter that control all radiation heat tranfer. all emiivity alo affect radiation entropy eration at olid all. Fig. 5 ho that the overall radiation entropy eration rate reduce ith increaing all emiivity. Overall entropy eration in the HAC proce alo reduce ith increaing all emiivity. hu, high all emiivity value hould e elected in order to reduce thermodynamic irreveriilitie in the HAC mode. Fig. 4. Effect of comution air temperature on entropy eration in HAC mode (oxy volume fraction 15%, all emiivity.7).. CONCLUSON Compared to tandard diffuion comution, entropy eration aociated ith vicou diipation, ma tranfe chemical reaction and heat conduction and convection ha ignificantly reduced rate in diffuion HAC procee. Radiation entropy eration rate are hoever trongly increaed in HAC procee. hermal radiation i the main caue of entropy eration in HAC flame. Radiation entropy eration rate increae trongly ith the preheated 278

5 nternational Journal of Applied Phyic and Mathematic, ol. 4, No. 4, July 214 air temperature. hi increae i due to all radiation procee. he all radiation entropy eration can e reduced y increaing all emiivity. Overall entropy eration rate in HAC can alo e minimized y electing an appropriate air temperature. he oxy volume fraction in the comution air ha only a mall affect on the overall entropy eration rate in HAC procee. REFERENCES [1] H. uji, A. K. Gupta,. Haegaa, M. Katuki, K. Kihimoto, and M. Morita, High emperature Air Comution: From Energy Conervation to Pollution Reduction, Boca Raton: CRC, 23, ch. 2, pp [2] K.. Lee and D. H. Choi, Numerical tudy on high-temperature diluted air comution for the turulent jet flame in croflo uing an unteady flamelet model, nternational Journal of Heat and Ma ranfe vol. 52, no , pp , Dec. 29. [3] M. Nihimura,. Suzuki, R. Nakanahi, and R. Kitamura, Lo-NOx comution under high preheated air temperature condition in an indutrial furnace, Energy Conver. Mgmt., vol. 38, no. 1-13, pp , March [4] S. Lille,. Blaiak, and M. Jeartoki, Experimental tudy of the fuel jet comution in high temperature and lo oxy content exhaut gae, Energy, vol. 3, no. 2-4, pp , March 25. [5] G. Choi and M. Katuki, Advanced lo NOx comution uing highly preheated ai Energy Conver. Mgmt., vol. 42, no. 5, pp , March 21. [6] R. ee S. Orino, N. Lallemant, and A. erlaan, Comution of natural ga ith high-temperature air and large quantitie of flue ga, in Proc. Comut. nt., vol. 28, no. 1, pp , 2. [7] S. K. Som and A. Datta, hermodynamic irreveriilitie and exergy alance in comution procee, Prog. Energy Comut. Sci., vol. 34, pp , June 28. [8] C. K. etrook and F. L. Drye Simplified reaction mechanim for oxidation of hydrocaron fuel in flame, Comut. Sci. echnol., vol. 27, no. 1-2, pp , July [9] FLUEN 6.3 Uer Guide, Leanon, 26, ch. 15, pp. 15(1)-15(95). [1] D. Makhanlall, J. L. Munda, and P. Jiang, Radiation energy devaluation in diffuion comuting flo of natural ga, Energy, vol. 61, no. 1-2, pp , July 213. [11] L. H. Liu and S. X. Chu, erification of numerical imulation method for entropy eration of radiation heat tranfer in emitranparent medium, Energy, vol. 61, no. 1-2, pp , July 213. Deodat Makhanlall i a enior memer of ACS. He otained hi PhD in engineering thermophyic from Harin ntitute of echnology (H), Harin, PR China, in June 21. He i a reearch aociate at inghua Univerity, Dept. of hermal Engineering, Beijing, PR China. Hi area of reearch i thermodynamic analyi of fluid flo and heat tranfer procee. Jiang Peixue otained hi PhD from Moco Poer Engineering ntitute, Moco, Ruia in Feruary He i an executive director of General Scientific R&D Office at inghua Univerity and alo the director of ntitute of Engineering hermophyic. He ha carried out extenive reearch on nano-cale thermal tranport, tranpiration cooling, and thermal treatment of ludge. 279