Development and scale effects of small Swiss-roll combustors

Proceedings

Proceedings of the Combustion Institute 31 (2007) 3243–3250

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of the

CombustionInstitute

Development and scale effects of smallSwiss-roll combustors

Nam Il Kim a, Satoshi Aizumi b, Takeshi Yokomori b, Soichiro Kato c,Toshiro Fujimori c, Kaoru Maruta b,*

a School of Mechanical Engineering, Chung-Ang University, 221, Heukseok, Dongjak, Seoul 156-756, Republic of Koreab Institute of Fluid Science, Tohoku University, 2-1-1 Katahira, Aoba, Sendai 980-8577, Japan

c Ishikawajima-Harima Heavy Industry Co. Ltd., 1 Shin-nakahara, Isogo, Yokohama 235-8501, Japan

Abstract

The configuration of Swiss-roll was adopted as a promising structure for the development of sub-mil-limetre scale combustors. In this study, the scaling effects are investigated both experimentally and analyt-ically. Based on our first proto-type (64 mm in diameter), scaled-down combustors (45 mm) with differentdesign parameters were fabricated; diameter of the combustor, top plate thickness, channel size and com-bustor material. A simple one-dimensional analytical model was constructed to interpret the combustioncharacteristics of the Swiss-roll combustors. Two energy equations for combustor body and mixture, alongwith species equations employing two-step global reaction model were solved. In spite of the simplicity ofthe present model, important characteristics of the combustors such as upper and lower flammability lim-its, effect of cap thickness on flammability limits were predicted well. Based on the analytical results, severalsmaller coin-size combustors (diameter 26 and 20 mm) were additionally developed and their characteris-tics were investigated. This study shows that the effects of design parameters are important in the practicaldevelopment of smaller combustors.� 2006 The Combustion Institute. Published by Elsevier Inc. All rights reserved.

Keywords: Scale effect; Swiss-roll combustor; Micro combustor; Flame stability; Flammability limits

1. Introduction

Higher energy densities of fossil fuels motivat-ed the development of small scale combustorswhich can be applied for portable energy supplysystems [1,2]. However, increased heat loss dueto large surface-to-volume ratio of small combus-tors hinders the flame stabilization thus resultingin flame quenching and narrower flammabilitylimits. To design a small combustor, some specific

1540-7489/$ - see front matter � 2006 The Combustion Institdoi:10.1016/j.proci.2006.08.077

* Corresponding author. Fax: +81 22 217 5311.E-mail address: [email protected] (K. Maruta).

techniques such as high pressure, high tempera-ture, and catalytic reaction can be applied. How-ever, determining the flammability limit andselecting the best configuration for these combus-tors using heat-regeneration are other essentialissues. Based on these requirements, a Swiss-rollcombustor was introduced as one of the promis-ing configuration [3–5], which consists of a pairof spiral channels and a combustion space at thecenter.

Even though many studies have been conduct-ed on Swiss-roll combustors, successful operationswere achieved only for combustors with theirdimensions larger than the critical size (quenching

ute. Published by Elsevier Inc. All rights reserved.

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3244 N.I. Kim et al. / Proceedings of the Combustion Institute 31 (2007) 3243–3250

distance) or when catalytic materials were used[3,6]. Our previous study [7] reported the evidenceof flame stabilization in small Swiss-roll combus-tors without catalyst. The channel width was lessthan the ordinary quenching distance at standardstate for these experiments.

The present work is the continuance of the pre-vious work to determine the small size limit of theSwiss-roll combustor. A detailed study on thescale effect of Swiss-roll combustor is required tounderstand the flame behaviour and stabilizationcharacteristics with the decrease in the combustordimensions. This is due to the fact that interactionbetween gas reaction and combustor body plays asignificant role at such small scales. Therefore, it isimportant to examine the effects of a number ofconfiguration parameters and the developmentof a simple analytical model is necessary for amore efficient development process.

Lloyd and Weinberg [3] and Weinberg [4]introduced an analytical model of Swiss-roll com-bustors and presented the results of their pioneer-ing experimental achievements. However, theanalytical model was based on a schematic ther-mal efficiency considering the heat regenerationfrom burned gas to unburned mixture, in whichneither a combustion model nor a configurationof the combustor was considered. An analyticalstudy on the flame in a small channel [8] notedthat the heat conduction through the tube wallsextends the flammability limits. Ju and Choi [9]studied flame instability in a counter current chan-nel. Ronney [10] proposed a simple analyticalmodel of a small combustor with a well-stirred-reaction model. Another analytical model forSwiss-roll combustor was proposed by Chen andBuckmaster [11] for the specific experimentalresults of Ahn et al. [6]. Unfortunately, these

Table 1Specifics of fabricated Swiss-roll combustors

Case Do (mm) H (mm) Dc (mm) C

d

W 64 16 12.7 6S 64 16 3.5 6D 64 27 3.5 15RD 45 26 3.5 15RS 46 20.5 2.5 10CS1 26 6 3.3 3CS2 26 12 3.3 4CS3 26 12 3.3 4CC 26 8 3.3 4cS 20 8 3.3 4

W, wide combustion space; S, shallow combustor; D, deep com(70%); CS1�CS3, coin-size combustor of stainless steel; CC,bustor of stainless.Do, diameter of the combustor; H, height of the combustor; Dc

w, width of channel; Nt, turning number of inlet channel; tw,plates).

models are not applicable to the general configu-rations of Swiss-roll combustors mainly due tothe differences in the configuration and mecha-nism of the flame stabilization.

In this study, several Swiss-roll combustors ofthe reduced scales were fabricated to comparetheir characteristics with the previous first proto-type Swiss-roll combustor (64 mm in diameter)[7] and to examine the essentials of the scale effect.A general analytical model was constructed toanalyse the effects of various design parameters.Four coin-sized combustors (20–26 mm diameter)were fabricated based on the analytical modelresults and their flame stabilization characteristicswere examined.

2. Experimental results

Ten combustors were fabricated and their spe-cifics are shown in Table 1. Among them, threecases of W-, S-, and D-combustors are identicalto those in previous study [7]. They have the samediameters of 64 mm and channel width of 2 mm,which is slightly smaller than the quenching dis-tance (�2.2 mm) of propane/air mixture at a stan-dard state (298 K, 1 atm). Distinctive features ofthese combustors are as follows; the W-combustorhas a wide combustion space at the center; S-com-bustor has a shallow channel; and D-combustorhas a deep channel.

The photographs of CC- and cS-combustorsand cross-sectional structures of Swiss-roll com-bustors are shown in Fig. 1. Thermocouples wereimplanted on the surface of the bottom plates ofthe combustors, and the mean temperature ofthe combustor was evaluated by averaging theirtemperature. Mean velocity Vm was defined as

hannel tw (mm) tcap (mm)

w Nt

2 3 1.5 52 4 1.5 52 4 1.5 62 2 1.5 5

.5 1.4 4 1.05 51 2.5 0.75 1.51 2.5 0.75 40.8 3.5 0.75 41 2.5 0.75 1(3)0.8 2.5 0.75 2

bustor; RD, reduced diameter (70%); RS, reduced scalecoin-size combustor of copper; cS, small coin-size com-

, diameter of the combustion space; d, depth of channel;wall thickness; tcap, thickness of the cap (top or bottom

0.5 1.0 1.5 2.0 2.50

1

2

3

4

5

6

7

]s/m[ yticolev nae

M

Equivalence ratio

W S D RS RD

Fig. 2. Flame stabilization conditions of variouscombustors.

Fig. 1. Coin-size combustors; (a) CC-combustor(26 mm in diameter), (b) cS-combustor (20 mm indiameter) and (c) cross sectional structure of Swiss-rollcombustor.

N.I. Kim et al. / Proceedings of the Combustion Institute 31 (2007) 3243–3250 3245

the flow rate divided by the cross-sectional area ofthe channel of width w and depth d. All the com-bustor surfaces except upper surface were insulat-ed as shown in Fig. 1c. The premixed mixture wassupplied with two mass flow controllers. Moredetails on the experimental method are presentedin Ref. 7.

First, two combustors were examined to distin-guish the effect of scale difference; the RD-com-bustor has a reduced diameter of �45 mm,about 70% of the D-combustor while the cross-sectional size of the channel was the same to thatof D-combustor. The RS-combustor has areduced scale of about 70% of the D-combustorin all scales. Flammable conditions of these twoscale-down combustors are compared with previ-ous results of W-, S-, and D-combustors inFig. 2. The regime above each line correspondsto the self-sustainable flammable regimes in thesteady state.

Results show that the flammable regime of theRD-combustor is almost the same to that of theD-combustor, while that of the RS-combustor iswider than those of the S- and W-combustors inspite of the much smaller width of the RS-com-bustor channel. Flammable regimes become largerin the order of S-, RS-, and D-combustors (orRD-combustor). This order is similar to that ofthe perimeter-length of the channel. This resultimplies that the flammable conditions are signifi-

cantly affected by convective heat transferbetween the combustor walls and mixture.

Figure 2 also shows that the flammability limitof the S-combustor is wider than that of the W-combustor. It is contrary to the usual expectationthat a sudden flow divergence coupled with vol-ume change enhances the flame stabilization. Inthe previous study, shorter channel length of theW-combustor compared to that of the S-combus-tor was suggested as the reason of narrower flam-mable regime. However, present results of largerflammable regime of RD- and RS-combustorsshow that the channel length is not a dominantfactor in these cases.

At this stage, the difference between a Swiss-rollcombustor of an ideal two-dimensional configura-tion and our practical Swiss-roll combustors isnotable. The former has an infinitely deep channel(d fi1) or a perfectly insulated top and bottomplates (caps of the combustor) in the depth-direc-tion, while the latter has a welded metal capthrough which a significant amount of heat istransferred. From the viewpoint of the small com-bustor development, however, the ideal two-di-mensional Swiss-roll combustor is not optimum.If the top and bottom plates are adiabatic and ifthe heat transfer occurs only through the wallbetween the channels, flame will propagate furtherupstream because heat regeneration is larger thanheat loss, and finally will attach to the inlet port.Then significant amount of heat may leak by theexhaust gas or by the heat loss through the out-side-surface of the combustor thus dropping theheat regeneration efficiency drastically. Finally,the advantage of employing the Swiss-roll configu-ration will be lost under such operating conditions.

There is a need to reappraise the role of thecombustor caps. According to the experimentalresult, the steady state temperature distribution

10 100

1000

40040

CS2 CS3 CC

800

600

400

]K[ erutarep

met naeM

Input energy [W]

W S D RS RD

Fig. 3. Mean temperature with the input energy forvarious combustors.

Fig. 4. Description of the analytical model.


of the Swiss-roll combustors showed a similartrend. Therefore, the mean temperatures of thecombustors can be a representative parameterand it is compared with the input energy Qin,which was evaluated from the flow rate and thelow heating value of propane (46,357 kJ/kg) asshown in Fig. 3. Experimental results of eight com-bustors are plotted and most of the data are dis-tributed close to the two lines in log-log scale.Although these results pretend that there was aclear deviation between two lines, it is mainlydue to the significant differences in the scale ofthe combustors and cross-sectional area of thechannel. Location of the lines depends on the dif-ference in the thermal efficiency, heat transfer coef-ficient and size of the combustor. Small coin-sizecombustors exhibited relatively lower temperatureat lower input energy. One notable fact is that theirslopes are close to a quarter and this implies thatheat loss from the combustor is dominated bythe radiative heat loss. Detailed discussion wasdriven in the previous study by Kim et al. [7]. Fur-ther discussion on the coin-size combustors (CS2,CS3, and CC) will be made later.

3. Analytical model

Variation of the gas phase temperature Tg wasevaluated coupled with the temperature of thecombustor body Tc, and species equations weresolved by employing a two-step reaction model.For the simplification of the model configuration,it was assumed that the two channels of unburnedand burned sides are located at the same orbit of aspiral having a pitch distance a = (w + tw)/p andthe initial position of the spiral wasxc ” (Dc + 2tw + w)/2 as shown in Fig. 4a. Thelength of the channel along the spiral can be pre-

sented by integrating the Eqs. (2a) and (2b). Whena finite volume shown in Fig. 4b is used, the heatbalance for the combustor and mixture can bewritten as shown in Eqs. (3) and (4), respectively.

rðhÞ ¼ ahþ xc ð0 6 h 6 2pN tÞ: ð1Þ

dx ¼ dr ðwhen r < xc or 2paN t þ xc < rÞ; ð2aÞ

dx ¼ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi1þ ðr=aÞ2

qdr ðwhen xc 6 r 6 2paN þ xcÞ:

ð2bÞ

kcd

drAr

dT c

dr

� �� d

drAnq00rad

� �

� hod

drAnðT c � T1Þ½ � þ hi

d

drAp T g � T c

� � ¼ 0

ð3Þ

_mcpgdT g

dx

� �þ kg

d

dxAg

dT g

dx

� �

þ hid

dxAp T g � T c

� � þ Ag

Xi

_xiMih0f ;i ¼ 0

ð4Þ

_mdY i

dxþ kg

cp

d

dxAg

dY i

dx

� �þ _xiMiAg ¼ 0 ð5Þ

where _m is the mass flow rate through the channel.Specific heat cp and thermal conductivity of mix-ture kg were assumed as 1321 J/kg K and

0 100 200 300

0

5

10

]s/m[ yticolev

wolF

hi [J/m2K]

upper-limit lower-limit experimental range

Fig. 5. Effect of inner heat transfer coefficient on theflame stabilization conditions at the stoichiometry con-dition (RS-combustor).


6.2 · 10�2 W/mK at the 1000 K, respectively.Radiative heat loss was noted asq00rad ¼ erðT 4

c � T 41Þ, where r is Stefan–Boltzmann

constant. Convective heat transfer coefficient onthe outer surface is defined as ho ¼ NuLkg=L,where L = Do/4, NuL ¼ 0:54Ra1=4

L , andRaL ¼ PrL3ðT c � T1Þðgb=m2Þ. Another convectiveheat transfer coefficient on the inner surface ofthe channel hi ¼ Nuwh

kg=wh was estimated fromthe hydraulic width defined as wh = 2wd/(w + d).It is known that the Nuw asymptoticallyapproaches to 4 in a sufficiently small channel[12]. Surface area for the convective heat transferbetween mixture and the combustor wasAp = 2wddx.

In the case of the RS-combustor, for example,if the mean temperature of the combustor is about900 K, following values, L � 11.5 mm,RaL � 3935, NuL � 4:3, ho � 12 J/m2 K,wh � 2.47 mm, and hi � 75 J/m2 K were used.Since these convective heat transfer coefficientsare enlarged by flow disturbance, it was assumedas ho � 15 J/m2 K. Concerned to hi, however,greater deviation may happen due to the signifi-cant temperature variation, complex flow pattern,and configuration of channel, as well as radiativeheat transfer. This effect will be investigated in theanalytical results.

Two-step global reaction model by Westbrooket al. [13] written in Eqs. (6) and (7) was employedbecause it is the simplest one which can describethe incomplete combustion process in a smallcombustor.

C3H8 þ 3:5 O2 þ 3:76N2ð Þ! 3COþ 4H2Oþ 13:16N2 ð6aÞ

COþ 0:5O2 ! CO2 ð6bÞ

_xC3H8¼ �1:0� 1012

� expð�15098=T Þ C3H8½ �0:1 O2½ �1:65 ð7aÞ

_xCO ¼ �1014:6 expð�20131=T Þ½CO�½O2�0:25 H2O½ �0:5

þ 5:0� 108 expð�20131=T Þ CO2½ �ð7bÞ

Though this reaction model is simple, it canpresent qualitative characteristics of the smallcombustors. The cross-sectional area of a streamtube varies in the combustion space at the centerand it can contribute to the hydrodynamic flamestabilization. To describe this variation, the widthof the channel was modelled based on the experi-mental geometry. Even though the flow pattern inthe combustion space cannot be defined clearly,two boundary conditions are available. One ismatching the size of the channel at the inlet ofthe combustion space (x = xc) as expressed inEq. (8a). Another is that the integrated area of

the width along the channel cannot be larger thanthe cross sectional area of the combustion space asexpressed in Eq. (8b). Thus, the width of the chan-nel is given in Eq. (9), in which the maximumwidth of the stream tube at the center wasassumed as the diameter of the combustion spacew0 = Dc.

wðxÞ ¼ wch; ðwhen x < �xc or xc < xÞ; ð8aÞ

wð0Þ ¼ w0; w xcð Þ ¼ wb; 2

Z xc

0

wðxÞdx ¼ px2c ;

ð8bÞ

wðxÞ ¼ �3pRþ 3wb þ 3w0ð Þ 2x=Doð Þ2

þ 3pR� 2wb � 4w0ð Þ 2x=Doð Þ þ w0: ð9Þ

4. Analytical results

Figure 5 shows the effect of the convective heattransfer coefficient in the channel hi on the flamestabilization for RS-combustor. It was found thatthere are upper- and lower-limits of the mixturevelocity for flame stabilization. However, theexperimental results of the RS-combustor onlyshowed the lower-limit and the correspondingvelocity was about 1 m/s at stoichiometric condi-tion as shown in Fig. 2. Although the upper limitof mixture velocity in the experiment was not plot-ted in Fig. 2, it was limited to 3.75 m/s because themean temperature exceeds 1100 K, which is closeto the material limit. Such velocity limits of exper-imental conditions were drawn with two horizon-tal lines in Fig. 5. By adjusting the lower limits ofthe experimental condition to the analytical pre-

0 10 20

900

1000

1100

flame location

ed

c

b

a

T C]

K[

r [mm]

k [J/mK] tcap [mm]a : 1.60 10.0b : 16.0 5.0c : 16.0 10.0d : 16.0 40.0e : 160.0 10.0

Fig. 7. Effect of the thermal conductivity and thethickness of the cap of combustors on the flamestabilization and flame position. (Vm = 2 m/s).


diction, a reference value of the convective heattransfer coefficient was decided (hi = 125) andwas used in the following results of the RS-com-bustor. Accordingly it was shown that the analyt-ical upper-limit is larger than the experimentallimit. Additionally, it is also noted that thereexists a critical hi below which lower limits doesnot exist (or significantly decrease) while theupper limits exist for all conditions. If hi decreasesto zero, it becomes adiabatic condition. Thus aflame will not be quenched but upper limit stillexists only if the velocity is larger than the propa-gation velocity of the adiabatic flame.

Variation of the gas temperature along thechannel is shown in Fig. 6. Unburned mixtureenters at 300 K and its temperature increases tothe maximum value at the flame position andthen it decreases. Three cases of the temperatureprofiles corresponding to the conditions close tothe lower-limit (1 m/s), stable combustion (2 m/s), and close to the upper-limit (4 m/s) wereshown. Near the lower-limits, flame is stabilizednear the center or undergoes oscillations. Adetailed discussion on a similar phenomenonwas presented in another study [14] and it is out-side the scope of the present work. Flame movesto the upstream direction with the increase in themixture velocity as observed in experiment [7].The movement of the flame slows down andthe blow-off takes place till it reaches the outermost position. In a similar fashion, flame posi-tion moves upstream as the heat loss from theouter surface decreases.

Figure 7 shows the variation of the tempera-ture profile of the combustor depending on theconduction parameters. In the analytical model,the thickness of the cap was assumed as 2 · tcap,since heat transfer occurs through both top and

0 10 200

1500

3000

Flow-out

Flow-in

4.96 m/s3 m/s 4 m/s

1m/s

2m/s

blow-off

Tg

]K[

r [mm]

Tmax Flow-in Flow-out

Oscillation amplitude

Fig. 6. Variation of the gas temperature depending onthe mean velocity and flame blow-off condition (RS-combustor, hi = 125 J/m2 K).

bottom caps. The conductive heat transfer is pro-portional to thermal conductivity and combustorcaps thickness. Since the velocity condition of2 m/s is not close to the flammable limit, flameis located upstream. Though this might be sus-pected as discrepancy with the present thermalmodel which essentially assumes symmetry, thismodel can be justified because overall thermalcharacteristics of the Swiss-roll combustor canbe described with the averaged temperature ofthe combustor body. The temperature profilesbecame steeper with the decrease in kctcap, andthat flames location shifted to the upstream. Con-clusively, the thickness of the cap was an impor-tant design parameter which affects the

1 10 1000

1

2

3

4

5

6

upper limit

lower limit

]s/m[ ytic olev nae

M

tcap [mm]

Fig. 8. The effect of cap thickness on flame stabilizationfor coin-size combustor.

0.8 1.0 1.2 1.4

1

5

3

0.7

0.3

0.5

]s/m[ yticolev nae

M

Equivalence ratio

CC CS2 CS3 cS

Fig. 9. Experimental results of flame stabilization ofcoin-size combustors.


combustion characteristics, thus the effect of tcap

on the flammable limits was examined further.Figure 8 shows the variation of the flame sta-

bilization conditions depending on the thicknessof the top and bottom caps, tcap. It is noted thatthe lower limit significantly broadens when thetcap is sufficiently small. In the case of thin com-bustor, conductive heat transfer is suppressed thusthe local temperature of the combustor shouldincrease. If the thickness becomes infinitely small,heat transfer in radial direction decreases. Thusthe temperature of the plate follows the gas tem-perature ultimately. This means that the deviationof the temperature between the flame and capdecreases and heat loss from the flame to the com-bustor also decreases. Since the area of the hightemperature is limited close to the flame, the over-all heat loss from the combustor by radiation maydecrease in spite of its higher local temperature.Eventually the flame quenching will be sup-pressed. For the case of a thick combustor cap,significant amount of heat is transferred to neigh-bouring plates and then is transferred to the out-side by radiation. Thus the range of theflammable conditions decreased with the thicknessof the combustor cap.

5. Development of smaller combustors

Based on the analytical approaches, four kindsof coin-size combustors (26 mm diameter) such asCS1, CS2, CS3 and CC (diameter 26 mm) weredesigned as already shown in Table 1. Accordingto the analytical model, the CS1-combustor shouldhave a wider flammable regime because of its thincap. However, stabilizing a flame within the CS1-combustor was difficult in practice. One possiblereason is as follows. In a combustor with a thincap, temperature gradient in the cap near the flamebecomes steep as shown in Fig. 7 thus the flameresponding to a velocity disturbance suffers greatertemperature deviation with the combustor, whichcan easily lead to the extinction. Another possiblereason is that the temperature distribution of thethin cap is easily affected by the configuration ofthe channel. Thus more complex thermal interac-tion may occur depending on the configurationof the combustor. Further study is required to con-sider the effects of these specific phenomena, whichis beyond the scope of this study.

On the other hand, the experimental results ofthe CS2 and CS3-combustors showed successfulflame stabilization as shown in Fig. 9. More thanfive times of experiments were conducted for eachcases. This proves the combustor with narrowerchannel-width of 0.8 mm (CS3) was also stable.It is noted that the coin-size combustors exhibitedthe upper flammability limits which was notobserved by larger combustors experimentally.The minimum velocity for flame stabilization in

the CS2-combustor was about 0.5 m/s, whichwas smaller than that of the RS-combustor(Fig. 2) in spite of its much smaller scale. The cop-per combustor (CC) also showed successful flamestabilization, while the flammable regime waslocated at higher velocity range. This trend canbe explained by the result of Fig. 8. Consideringthe conductivity of copper (390 J/mK), which is25 times larger than that of stainless steel, the low-er limits are expected to exist at higher mixturevelocities. Based on these analytical and experi-mental results, much smaller Swiss-roll combustor(the cS-combustor) having 20 mm diameter wasfabricated as shown in Fig. 1 and it also showedsuccessful flame stabilization as shown in Fig. 9.Thus we expect that developing smaller combus-tors is still possible.

6. Conclusions

The scale effects of small Swiss-roll combustorswere investigated both experimentally and analyt-ically. Flames could be stabilized in scale-downSwiss-roll combustors of different channel size.Flammable limits and temperature distributionsof the combustor bodies were examined. Basedon the experimental results, a simple one-dimen-sional analytical model was developed. Two ener-gy equations of combustor and mixture weresolved together with species equations employingthe two-step global reaction model. Flammabilitylimits and the characteristics of various parame-ters were investigated with the model. This modelclearly interprets the present experimental results.Based on the model, four kinds of coin-size com-bustors (of diameter 26 or 20 mm) with differentsizes and materials were fabricated and successfulflame stabilizations were achieved. Both of the


experimental and analytical results imply thatdeveloping smaller Swiss-roll combustors usingthe heat regeneration is still possible.

Acknowledgment

This work was performed as the R&D projectentitled ‘‘Micro-combustors as Heaters’’ which issupported by the New Energy and IndustrialTechnology Development Organization (NEDO),Japan.

References

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[2] A.C. Fernandez-Pello, Proc. Combust. Inst. 29(2002) 883–899.

[3] S.A. Lloyd, F.J. Weinberg, Nature 251 (1974) 47–49.

[4] F.J. Weinberg, Advanced combustion methods, Aca-demic Press, London, New York, 1986.

[5] L. Sitzki, K. Borer, E. Schuster, P.D. Ronney, in:The Third Asia-Pacific Conference on Combustion,Seoul, 2001, pp. 473–476.

[6] J. Ahn, C. Eastwood, L. Sitzki, P.D. Ronney, Proc.Combust. Inst. 30 (2005) 2463–2472.

[7] N.I. Kim, S. Kato, T. Fujimori, et al., Combust.Flame 141 (2005) 229–240.

[8] V.V. Zamashchikov, S.S. Minaev, Combust. Explo-sion Shock Waves 37 (1) (2001) 21–26.

[9] Y. Ju, C.W. Choi, Combust. Flame 133 (2003) 483–493.

[10] P.D. Ronney, Combust. Flame 135 (2003) 421–439.[11] M. Chen, J. Buckmaster, Combust. Theory Model. 8

(2004) 701–720.[12] E.P. Incropera, D.P. De Witt, Fundamentals of

Heat and Mass Transfer, fifth ed., Wiley, NewYork, 2002.

[13] C.K. Westbrook, F.L. Dryer, Prog. Energy Com-bust. Sci. 10 (1984) 1–57.

[14] K. Maruta, T. Kataoka, N.I. Kim, S. Minaev, R.Fursenko, Proc. Combust. Inst. 30 (2005) 2429–2436.

Comment

Felix Weinberg, Imperial College London, UK. Inconsidering the overall dimensions of these devices,should we not have to include some kind of micro com-pressor? As you make the channels narrower, maintain-ing the velocity by going to faster-burning mixtures, itseems unlikely that pressure losses can be neglected.

Reply. The pressure loss level of the 45 mm diametercombustor reaches some 100 mm H2O at high velocityoperation and it is much lower in the coin size combus-

tor due to lower upper limit velocities. Thus, practicalsystem should have low-pressure level pumping deviceas you pointed out. Note that no significant change incombustion characteristics was found for such pressurelevel. This is the reason why we denoted that pressureloss is negligible. Besides this, pressure loss is not an is-sue for replacement of industrial electric heaters, whichis our primary target. Combinations of small ejectorand pressurized liquid hydrocarbon fuel might be onecandidate for portable power system applications.

Development and scale effects of small Swiss-roll combustors

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