Flame stabilization and emission of small Swiss-roll combustors as heaters

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Combustion and Flame 141 (2005) 229–240www.elsevier.com/locate/combustflam

Flame stabilization and emission of small Swiss-rollcombustors as heaters

Nam Il Kim a,∗, Souichiro Katob, Takuya Kataokaa, Takeshi Yokomoria,Shigenao Maruyamaa, Toshiro Fujimorib, Kaoru Marutaa

a Institute of Fluid Science, Tohoku University, Katahira 2-1-1, Aoba-ku, Sendai 980-8577, Japanb Ishikawajima-Harima Heavy Industry Co. Ltd., 1 Shin-nakahara, Isogo, Yokohama 235-8501, Japan

Received 13 July 2004; received in revised form 20 December 2004; accepted 10 January 2005

Available online 9 February 2005

Abstract

The characteristics of small Swiss-roll combustors were investigated experimentally in detail. Three tSwiss-roll combustors of different designs and two cases of heat transfer conditions were studied. Thof design parameters on the performance of these combustors were examined. Each combustor conscombustion region at the center (called the combustion room) and double spiral-shaped channels, thewhich were smaller than the minimum quenching distance of a propane premixed flame at a normal statecould be stabilized successfully for a wide range of equivalence ratios and mean velocities by using the recheat from the burned gas, and blow-off was not observed. Temperature distributions of the combustors,of gas temperature, and the concentrations of the exhaust gas from the combustors were also investigatemperatures of the combustors were found to be governed by both the radiant heat loss from the combuthe total chemical energy liberated by the combustors. Efficiencies of the combustors as heaters were eAs a combustor became smaller, its thermal efficiency as a heater increased and its NOx emission decreased, whithe emission of CO increased. By adding a catalytic reactor at the exhaust port, it was found that the emCO could be eliminated. This study provides new experimental results for a small Swiss-roll combustorrepresents an essential step toward the development of a microcombustor. 2005 The Combustion Institute. Published by Elsevier Inc. All rights reserved.

Keywords:Swiss-roll combustor; Flammable limits; Heat recirculation; Emission; Small-scale combustor

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1. Introduction

A heater that employs combustion has much bter energy efficiency than an electric heater, beca

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

(N.I. Kim).

0010-2180/$ – see front matter 2005 The Combustion Institutdoi:10.1016/j.combustflame.2005.01.006

it uses the energy in the fuel to heat objects direcrather than using electric power. Nevertheless, elecheaters have been used widely due to their flexiity in design and control. When a specific distributiof the temperature is required, particularly in mid-small-scale heating, use of a combustion heater isconvenient. If an easily controllable, small combution heater were to be developed, it could be appin many energy-consuming areas instead of elec

e. Published by Elsevier Inc. All rights reserved.

http://www.elsevier.com/locate/combustflame

mailto:[email protected]

230 N.I. Kim et al. / Combustion and Flame 141 (2005) 229–240

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heaters. For example, by constructing a matrix ofmerous small combustors (like a grid system in nmerical simulation) combustion heaters could be ueven for the specific requirements of temperaturetribution.

Many studies of small combustors using fossil eergy recently have been conducted in the attempdevelop smaller and higher density power sourfor small-scale devices. They have been sumrized by Vican et al.[1] and Fernandez-Pello[2].Analytical studies of flame stabilization in a hearecirculating combustor of the countercurrent-tycombustors have been conducted[3,4]. These stud-ies were motivated by the fact that energy densityhydrocarbon fuel is much greater than that of comercial batteries (from 12- to more than 50-foldvolume[1]). To date, however, the small combusas a power generator has remained at a basic pof development; one of the obstacles is the difficuof flame stabilization due to increased heat loss blarge surface-to-volume ratio. A combustion heahowever, can use this heat loss positively to hup objects. Therefore, development of a small-sccombustor as a heater is the first step of the clenging development of a small-scale combustor apower generator.

Generally it is known that the flame quenchidistance near the flammability limit is proportionto the flame thickness[5]. Thus, creating a thin (orapidly reacting) flame and reducing the heat lhave been major goals in the development of a smscale combustor. Use of high pressure, high temature, catalytic reactions, and certain fuels can pvide a stable flame within a small combustor. Evthough all these approaches can be successfullyplied in most combustors, there is an essentialimportant question: What are the limitations of a cobustor which uses only the heat recirculation fromburned gas?

Jones et al.[6] described various combustowhich use recirculated heat from burned gas. Weberg [7] introduced a countercurrent heat converwhich uses a large amount of recirculated heat. Lloand Weinberg[8,9] investigated a Swiss-roll-type reactor, which consists of a space for combustion atcenter and double spiral channels for heat recirction from the burned gas to the unburned mixture. Tspace at the center will be called a combustion roin this paper, since the flame is located within thspace in most cases, especially near the flammlimits. Compared with a simple linear countercurreheat converter, the Swiss-roll structure has a mlarger surface area for heat exchange since each cnel of unburned mixture or burned gas passes betwa pair of the channels of burned gas or unburned mture, respectively. However, in previous experimen

-

studies of Swiss-roll combustors[8–10], the sizes ofthe channels were larger than the quenching distaat a normal state (300 K, 1 atm).

Recently, even though very small Swiss-roll gemetries have been successfully fabricated, stabiliza flame has been difficult and only flame stabiliztion using a catalytic reaction or a hydrogen mixtuhas been successful[1,2]. However, it is notable thacombustion phenomena are strongly affected bywall close to the flame when the size of the combtor becomes smaller. This implies that the geomeof the combustor becomes more significant withdecrease in size and that generalization of comtion characteristics becomes more difficult in a smcombustor. Therefore parametric studies are requespecially in a small Swiss-roll combustor, whimay be the best geometry of a very small combtor.

In this study combustion characteristics of a smSwiss-roll combustor that is used as a heater winvestigated. “Small” implies that the characterislength scale is smaller than the minimum value ofstandard quenching distance. Standard in this smeans reactants that are not preheated but are at 2and 1 atm. The stabilization of self-sustaining flais difficult without using recirculated heat. Thretypes of small Swiss-roll combustors, having dferent depths, and different sizes of the combusroom at the center, were examined. The effectsthermal environments were also studied using twoditional kinds of heat transfer environments. Flastabilization conditions, temperature characteristand concentration were investigated. Efficienciesthe combustion heaters were estimated and the pbility of developing a smaller combustor was exained.

2. Combustors and experimental method

A Swiss-roll combustor consists of a combustiroom at the center and a pair of long channelsheat recirculation from the burned gas to theburned mixture.Fig. 1a shows a Swiss-roll combustin which a flame was successfully stabilized. Thtypes of Swiss-roll combustors were precisely fabcated using the electro discharge machining (EDtechnique to the specifications shown inTable 1.The material of the combustor was stainless s(SS304). The width of the channel was 2 mm, whis smaller than the quenching distance of a propflame (2.1 mm[11]) at room temperature and presure. Wall thickness between the burned gas andunburned mixture was 1.5 mm. Those three typecombustors are denoted as S-, W-, and D-combusin Table 1. The S-combustor had asmall combus-

N.I. Kim et al. / Combustion and Flame 141 (2005) 229–240 231

m)

gas and

Table 1Experimental conditions: three kinds of combustors and heat transfer environments

Type Case Cap Insulation Combustion room (Dc, mm) Channel depth (mm) Channel turn Height (m

S Si Quartz All sides Narrow (3.5) 6 4 16Sq Quartz Except one side Narrow (3.5) 6 4 16S S/S Except one side Narrow (3.5) 6 4 16

W W S/S Except one side Wide (12.7) 6 3 16

D D S/S Except one side Narrow (3.5) 15 4 27

Note. S/S: Stainless steel (SS304), outer diameter of the combustor: 64 mm. Thickness of the wall between burnedunburned mixture: 1.5 mm. Channel width: 2 mm,D0 = 2 mm,Di = 1 mm (at the inlet to the combustion room).

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Fig. 1. Flame and the Swiss-roll combustors: (a) photogrof a stable flame in Swiss-roll combustor, (b) geometrycombustion room, (c) arrangement of thermocouples anniter.

tion room andshallowchannels (6 mm in depth), thD-combustor had asmallcombustion room anddeepchannels (15 mm in depth), and the W-combushad awide combustion room andshallow channels(6 mm). The diameter of the combustion room of tW-combustor was 12.7 mm, while the diametersthe other combustors were the same (3.5 mm). Ually the bottom and side surfaces of the combuswere insulated well with refractory blankets as shoin Fig. 1c. This insulation can simplify the geometof heat transfer to avoid the complex effect by covective flow near the combustor.

Two additional cases (called Sq and Si) wereamined using the S-combustor. For the Sq case, twas aquartzplate cap instead of a stainless plate cfor visualization as shown inFig. 1a. For the Si caseall surfaces of the combustors wereinsulated. There-fore, the cases of S, Si, and Sq have different therenvironments, while their cross-sectional geometwere the same.

The equivalence ratio and mean velocity were ctrolled with two mass-flow controllers. The mean v

locity was defined as the mass-flow rate dividedproduct of the density of the reactants and the crosectional area of the channel. The density was evated at the normal state (300 K, 1 atm), and the crsectional area was the product of the depth ofchannel as shown inTable 1and the width of 2 mmPure propane (99.5%) and air were used as the rtants. If the width of the channel is sufficiently largthan the quenching distance of a flame, flashbackcurs when the mean velocity is sufficiently low. Thflame position can be controlled easily by adjustthe mean velocity. This characteristic can be usestabilize the flame at the center at the initial statea small combustor, however, a flame that is ignitedthe exhaust port of the combustor cannot propagto the center (the combustion room) by adjustmenthe flow rate. This is because the scale of the chnel of the combustor is smaller than the quenchdistance. In this study, therefore, preheated gasallowed to flow till the temperature of the combutor reached a sufficiently high point for ignition. Thflame was ignited by an electric spark at the centethe combustor provided by a 50 Hz, 7 kV dischasource, as shown inFig. 1c.

Temperatures of the combustor were measuwith seven thermocouples installed at locations shoin Figs. 1b and 1c. One thermocouple was placedthe center of the cross-sectional area of the cha5 mm upstream from the combustion room in ordto measure the temperature of the unburned mixat the inlet of the combustion room, and another thmocouple was located at the exhaust port.

All experimental results were obtained after cobustors reached a steady state, when the variatioall measured temperatures became less than±0.2 K/

min. Since the time required for realizing steady stis proportional to the ratio of the total heat capacof the combustor to the heat release from a flaa longer time is required as a combustor becomsmaller; the time also depends on the types of inlation used. Major species in the exhaust gas wmeasured with gas analyzers (HORIBA PG-240 aFIA-510).


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Fig. 2. Flame stabilization conditions for various combustand different heat transfer conditions.

3. Results and discussion

3.1. Flame stabilization and combustor temperatu

A premixed flame is stabilized when the flamspeed is balanced with the flow velocity. The flamspeed is affected by pressure, temperature, and ealence ratio. In this experiment, the pressure dthrough the Swiss-roll combustor was negligible. Ttemperature of the unburned mixture is affectedmany parameters, including the flow rate and equalence ratio. Thus, the flammable limits of varioSwiss-roll combustors were investigated for varioequivalence ratios and mean velocities. Resultsshown inFig. 2. Mean velocity was defined as thflow rate at the inlet of the Swiss-roll combustdivided by the cross-sectional area of the chanA steady flame existed in the Swiss-roll combusat the regime above each line inFig. 2.

Depending on the ignition method, flame coube ignited under the conditions outside of the flamable regime. However, it was only a transient pnomenon and the flame was eventually extinguisbefore the combustor reached a steady state. Ineral, a premixed flame in a channel can be stabiliwhen the mean velocity is similar to the laminar buing velocity. Thus, mean velocity for stabilized flamhas a maximum value near the stoichiometry athe case with laminar burning velocity. The flameblown off when the velocity is greater than the criticvelocity [12]. However, all flame stabilization conditions in this study had opposite trends, as shoin Fig. 2. Close to stoichiometry, mean velocitiesthe flammable limits had minimum velocities. Minmum velocities of each flammable regime of theperimental cases decreased in the order of W, SqSi, and D. Minimum mean velocities of W-, S-, anD-combustors were 3, 1.7, and 0.42 m/s, respectively.Under most of the flammable conditions, mean velities were larger than the maximum laminar burn

velocity of the propane flame, about 0.44 m/s[13]. Inaddition to this, blow-off by large flow velocity wanot observed within our experimental conditions.the case of the S-combustor, flame was stable eif the mean velocity was faster than 10 m/s, morethan 20 times the laminar burning velocity at the nmal state, and the flame was visible in most caOccasionally, flame can be stabilized at the fuel ccentration, which is smaller than the lean flammaity limits of the propane flame at the normal sta(∼0.5 in equivalence ratio). Many researchers hreported such a lean combustion, so-called supeabatic flame, using large-scale Swiss-roll combus[6] or using porous materials[14,15].

In most cases, a flame is stabilized at the ceof the combustion room as shown inFig. 1a. Thus,the volume of the combustion room is important frothe viewpoints of flame quenching and hydrodynamflame stabilization. Compared with the combustrooms of the D-combustor or of the S-combustthe combustion room of the W-combustor haswider cross-sectional area and a smaller surfacevolume ratio (as shown inTable 1). These characteristics should have suppressed the occurrencflame quenching. In addition to this, the wide cobustion room enhances the flow divergence or flrecirculation near the inlet to the combustion rooand the mean velocity within the combustion roobecomes smaller. This should have enhanced hydynamic flame stabilization as well. Nevertheless,flammable regime of the W-combustor was narrowthan the other cases. The most conspicuous charaistic, which can hinder flame stabilization, is the fathat the W-combustor has shorter channels or a fenumber of turns of the channel than the other cobustors. This reduces the temperature of the unbumixture at the inlet of the combustion room (Tc) andmakes flame stabilization difficult because the mconsumption rate of a propane flame is approximaproportional to the mixture temperature[16]. Giventhese facts, it is known that heat transfer and temature of the combustor are important for obtaininstable flame in a small combustor.

Although the cross-sectional geometry of tD-combustor and that of the S-combustor weresame, the depth of the channel of the D-combuwas 2.5 times larger than those of the other combtors. As a result, the D-combustor had a much smaheat loss compared with the heat generation. Thisdue to the fact that most of the heat loss occurthrough only the upper side of the combustor, whthe total heat generation in the combustor was appimately proportional to the mass-flow rate (or to tdepth of the channel multiplied by the mean veloity). Thus, the flammable regime of the D-combuswas much larger than those of the others. This la


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deviation of mean velocities also implies that hbalance is the most important factor for obtainingstable flame.

Hence, environmental effects of the heat transshould be understood. Two kinds of additional eperimental environments were imposed in the casthe S-combustor (Si and Sq) to estimate the effeof heat loss and to observe the flame behaviorrectly. Flammable regimes greatly varied dependon the insulating conditions. The minimum valuethe mean velocities for flammable conditions of thecase was 50% smaller than that of the Sq case. Instudy, flame could be stabilized only inside of eiththe combustion room or the channel upstream frthe combustion room. Within the downstream chnel after the combustion room, the temperature gdient in flow direction was negative, while the flampropagation was positively sensitive to the tempeture. Thus, a small disturbance of flame positionthe downstream direction reduces the mixture teperature and the propagation speed decreases asThis results in the flame moving farther downstreand it becomes definitely unstable within the dowstream channel.

Since all experimental conditions of the Si caand Sq case were the same except the insulatingdition, comparison of these two cases directly depthe effect of heat loss from the surface of the combtor to the ambient air. It can be said that as the hloss increases, the flammable regime becomesrower. One notable result is that the flammable regof the S case is wider than that of the Sq case. Thisplies that heat loss of the Sq case is larger than ththe S case even though quartz glass has much smheat conductivity than a stainless-steel plate. Thisbe discussed again after the investigation of tempture characteristics and the heat-transfer mechanof the combustors.

As the thermal characteristics of the combustare important for heaters, temperatures of the cbustors were measured. In some cases, thermoco(T1 ∼ T5) were located at the upper surface fortimation of heat transfer from that surface. In thecases, however, the upper surface became ruggeto many thermocouples and their supporting strtures. Such a complex surface geometry affectsheat transfer mechanism and thus results are not cparable with those for the Sq case in which a quaplate is used. Mean temperatures of the upper surand the bottom surface of the D-combustor are copared inFig. 3a. Here, the mean temperature is dfined as an average of the temperatures of the comtor (Tm = ∑5

i=1 Ti/5). As the depth of the combustobecomes larger, the temperature difference betwthe top and bottom surfaces increases. However,temperature difference was less than 40 K, even in

l.

Fig. 3. Surface temperature distributions of D-combus(a) temperature difference between upper surface andtom surface, (b) temperature distributions in radial direct(measured at the locations marked inFig. 1).

D-combustor of the largest depth, and was negligin most cases due to the large thermal conductivitthe stainless body of the combustor. Thus, temptures at the bottom of combustors were comparethis study.

One of the merits of a Swiss-roll combustor dto its structure is small deviation of temperaturethe surface. Each channel of unburned mixtureburned mixture is located side by side. Temperatuof the wall between burned and unburned mixturesclose to the average temperatures of those chanIn addition to this, the width of the channel (2 mmand the wall thickness (1.5 mm) between channwere smaller than the thickness of the surface p(∼6 mm) in this experiment. Thus, a large differenof the temperature between the burned gas and thburned mixture on the inner surfaces of the channdecreases due to the heat conduction in the radiarection through the cap plate of the combustor.Fig. 3bshows that the temperature deviation in the radialrection increases with the mean velocity. Here,distance between thermocouples was 12.5 mm. Ifthickness of the cap is sufficiently large, then the htransfer in the radial direction will be enhanced athe temperature deviation on the outer surface wilreduced to a negligible level.


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Fig. 4. Mean velocity and measured temperatures of D-cbustor (measured at the locations marked inFig. 1).

Fig. 4 shows temperatures of the combustor bo(T1 ∼ T5), temperature of the unburned mixture at tcenter (Tc), the exhaust gas temperature (Tex), andthe mean temperature (Tm) when the equivalence ratio is 0.9 for the D-combustor. As the mean velocincreased, all measured temperatures increased.temperature of the unburned mixture at the inletthe combustion room (Tc) was similar to the surfactemperature at the center (T3) provided that the meavelocity was less than 2.8 m/s in the D-combustorWhen the mean velocity was greater than 2.8 m/s, theflame front propagated upstream of the thermocouTc, not downstream. This forward propagation of tflame was also observed during the experiment ofSq case using a quartz plate as a cap.

Such flame propagation upstream can beplained as being due to two mechanisms. One ispreviously noted fact that the mass consumptionof a deflagrating propane flame is proportional totemperature of the unburned mixture in the casepropane flame[16]. Due to the heat released from tflame, temperature at the combustion room behthe flame front is much higher than the temperatof the heated mixture. Much of the heat from tburned gas is transferred to the combustor in the cbustion room since it has the maximum temperatdifference and the longest residence time. This hgeneration is proportional to the mean velocity ofunburned mixture under the assumption that mosthe fuel burns well. Therefore, a larger flow ratecreases the temperature of the unburned mixtureresults in a shift of the flame in the upstream directi

The other mechanism is the initiation of the raction when the local temperature of the combusreaches an ignition temperature, which can be csified as a chemical explosion. The temperaturethe unburned mixture (Tc) at the critical mean velocity for sudden flame propagation was about 1100and the mean temperature at the critical mean ve

Fig. 5. Mean velocity and mean temperatures: (a) expmental results of the small-scale combustors; (b) estimconvective and radiant heat losses.

ity was identified asTm,i , about 1000 K in the casof the D-combustor. These temperatures are mhigher than the spontaneous ignition temperaturthe propane/air mixture of about 777 K[17]. Whilethe spontaneous ignition temperature was evalufor a static premixed gas in a sufficiently large chaber, combustion in a narrow channel (or in a tubestrongly affected by the distribution of flow or temperature. For example, a premixed flame near theundergoes a positive flame stretch, generated bygradients of local burning velocity and the flow vlocity [18], and this gradient increases as the chanbecomes narrower or as the velocity increases. Ethough it is not clear how much it affects the crical temperatureTm,i , it can increase the temperaturequired for flame generation. The determinationchemical branches on the channel wall can be anocause as well.

In our experiment, the temperatures of the cobustor body (T1 ∼ T5) had a similar trend and thdeviations from the mean temperatures were smTherefore, the mean temperature of the combuwas considered to be a representative parametecomparison of various experimental conditions. Tdependency of the mean temperature on the mea


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locity was investigated at stoichiometry for the ficases of this study. The results are shown inFig. 5a.As the mean velocity increased, the mean tempture also increased for all conditions. From the viepoint of a combustion heater, the heat transferred fthe combustor to the surrounding area is availableheating an object. However, from the viewpoint of tflame in the combustor, this heat corresponds toheat lossfrom the combustor. Since the flammablimits of the combustors are being discussed hereterm “heat loss” will be used. Heat loss from the cobustor occurs through three paths. The first is the hloss from the insulated surfaces to the insulating mterial. The second is convective heat loss fromopen surface. The last is radiant heat loss throughopen (or upper) surface.

First, although most surfaces of the combustwere insulated by refractory blankets, some amoof heat loss was inevitable. This heat loss could betimated by comparing the results of the Sq caseSi case inFig. 2. The results of the Si case indicathe heat loss through all insulated surfaces. Bysulating the upper surface, the mean velocity atflammable limits of the Si case can be reducedless than 40% of that of the Sq case. For simpletimation of the magnitude of heat loss at insulasurfaces, it was assumed that the temperature ocombustor is uniform and that the magnitude ofheat loss is proportional to the mean velocity ofmixture (or input chemical energy). Thus, the raof the heat loss between the cases of Si and Sqbe presented ashi(Ai + A0)/(hiAi + h0A0) ∼ 0.4,whereAi is the area of the insulated surface andA0is the area of noninsulated surface of the Sq cHeat-transfer coefficients of the insulated surfacethe open surface are denoted ashi and h0, respec-tively. Since the surface ratio of the S-combustor wabout 2(Ai/A0 ∼ 2), the ratio of heat transfer coefficient became 0.18(hi/h0 ∼ 0.18). Even thoughthis estimation used slightly excessive assumptioat present, it can be said that heat loss through thsulated surface is very small.

Second, the magnitude of the convective heatat the surface of the Swiss-roll combustor maydetermined by a natural convection, since the abient air is not moving. Thus the amount of heloss depends on the Rayleigh number[19], which isdefined asRaL = gβ(Tm − T0)L3/να, whereg isgravity acceleration,T0 is the room temperature,νis kinematic viscosity, andα is thermal diffusivity.Properties were evaluated at the average temper(film temperature) of the surface and the room teperature((Tm + T0)/2). A coefficient of volumetricthermal expansion is inversely proportional to theerage temperature(β = 2/(Tm + T0)) under the assumption of ideal gas.L is characteristic length de

fined as the ratio of the surface area to the perim(L ∼ 16 mm) of the noninsulated surface. In this eperiment, the Rayleigh number was in the range104 < RaL < 2×104 for the mean temperature of thcombustor, 600 K< Tm < 1000 K. Thus, the averagNusselt number can be evaluated from the followequation[19]:

(1)NuL = 0.54Ra1/4L

for 104 � RaL � 107.

Thus, the average convective heat loss can bemated as follows:

(2)Qconv= A0kNuL(Tm − T0)/L,

wherek is the thermal conductivity of the combustoThis convective heat loss was between 15 and 4for the experimental temperature range as showFig. 5b.

Finally, the magnitude of the radiant heat loss frthe surface to the ambient can be presented roughfollows under the assumption of uniform temperatof the open surface:

(3)Qrad= A0εσ(T 4

m − T 4∞),

whereε is emissivity, andσ is the Stefan–Boltzmanconstant. Radiant heat losses are plotted inFig. 5bas well. Both convective and radiant heat losses wcomparable for the mean temperature of the surof the combustor. When the emission coefficient wclose to 1.0, radiant heat loss was much greaterconvective heat loss under all conditions correspoing to the experimental range.

In this experiment, stainless-steel surfaces whighly oxidized, and thus the emissivities were higthan 0.8, which may have increased with the tempature[19]. Therefore, radiant heat transfer is the mmechanism of heat loss from the combustor witthe experimental temperature range. Since the mtemperature of the combustor is much higher thanroom temperature, radiant heat loss is roughly pportional to fourth power of the mean temperatu(Qr ∝ T 4

m). If this radiant heat loss from the combutor is proportional to the mean velocity or is weakdependent on the mean temperature, the slope inlogarithmically scaled graph will be close to 4. Thestimation is applicable for explanation of why tslopes of experimental results inFig. 5a are close to 4In the experiment, the increasing rate of the mtemperature was logarithmically proportional to tmean velocity, and the slopes,n, of all cases were between 3.31 and 3.94. The similarity betweenFig. 5aandFig. 5b implies that the heat loss directly depenon the mean flow rate as assumed in the previous pgraph regarding the heat loss through the insulasurface, and thus the domination of radiant heatis again confirmed. Therefore, it is reasonable tha


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Fig. 6. Variation of mean temperatures with equivalence ratio and mean velocity for: (a) D-combustor, (b) S-com(c) W-combustor.
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cases have a similar slope regardless of the theenvironment, even if it is insulated or not.

When the combustor becomes smaller, even thoconvective heat transfer becomes larger compawith the radiant heat loss for the same temperata small combustor of the same geometry requirehigher temperature for flame stabilization. Therefothe radiant heat transfer will dominate the temperaof the combustor only if the temperature and the emsivity of the combustor are sufficiently high. Frothe viewpoint of application of a heater, this simpcorrelation between the flow rate and the combutemperature implies that the temperature controthe combustor heater is readily available.

Now it can be explained why the heat loss of tSq case was larger than that of the S case despitsmaller conductivity of the quartz plate. From the pvious discussion, it is clear that radiant heat transdominates heat loss from the combustor. Thus,possible influential factor is that much greater radiheat loss occurs through the quartz plate. Accordto Wien’s displacement law[19], the wavelength omaximum spectral emissive power is inversely pportional to the absolute temperature, namelyλmax=2897.8/K µm. Since the temperature of the inner sface of the combustion room is much higher thanmean temperature of the combustor, the maximspectral wavelength is shorter than 3 µm (λmax �3 µm). For this wavelength, the spectral transmisity of the fused quartz plate with a thickness of 6 mis larger than 0.9[19]. Therefore radiant heat tranfer can occur directly between the inner surfacethe combustion room and the ambient air, and mgreater heat loss is generated due to the higher ltemperature of the combustion room.

Fig. 7. Measured mean temperatures as a function of eqlence ratio near the flammable limits of various combust

The mean temperatures of the three types of cbustors having different geometries are shownFigs. 6a–6cwith the equivalence ratio against thmean velocity.Figs. 6a–6ccorrespond to the D-, Sand W-combustors, respectively. As the concention is close to stoichiometry, the mean temperatbecomes highest, and it decreases near the flammlimit. These trends can be explained by the heat geration within the combustor, which is proportionalthe mean velocity and becomes maximum near schiometry. Since most of the heat loss is governedthe radiant mechanism, these results will have slarities in terms of input energy, which is proportionto the mean velocity and the cross-sectional arethe channel.

The minimum mean velocity of the W-combustfor flame stabilization was more than 6 times largthan that of the D-combustor, as shown inFig. 2.


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From the viewpoint of the temperature of the cobustor, however, mean temperatures at the flammlimits of the three types of combustors (S-, DW-combustors) showed the small deviations shoin Fig. 7. For the case of the D-combustor, the metemperature for the flammable limit was smaller ththose of other combustors, and the D-combustorfered a smaller heat loss than the S-combustor. FoW-combustor, mean temperature for the flammalimit was higher than those of the other combustoIn this case, the length of the channel was shothan in the other cases, and the size of the combusroom was larger. Heat transfer from burned gasthe unburned mixture does not occur directly withthe combustion room; thus, if the temperatures atflame front are the same, the mean temperaturecomes higher as the combustion room becomes la

Therefore the temperature at the flame front isportant for flame stabilization. This temperature istermined by the total heat generation within the cobustor because the heat loss mechanism is the sfor all experimental cases: radiant heat transfer frthe upper surface dominates heat loss. Considethe heat release per volume, a much larger flowis required when the equivalence ratio is farther frstoichiometry. Additionally, the ignition energy of thpremixed gas also increases as with distance ofequivalence ratio from stoichiometry[12]. Therefore,a higher temperature of the unburned mixture anhigher temperature of the combustor are required

From these results it is expected that a flamebe stabilized even in a much smaller Swiss-roll cobustor only if the wall temperature of the combusis sufficiently high. The temperature of the combuscan be controlled by controlling the equivalence raor the mean velocity. These two results are good nfor the development of a small combustor as a heaHowever, exhaust gas characteristics also are imtant for application of a small Swiss-roll combustora heater.

3.2. Characteristics of exhaust gas

Exhaust gas concentrations of major species wmeasured with gas analyzers and were comparethe basis of dry conditions. To estimate the effectsvarying the combustor size, exhaust gases fromD-combustor and the S-combustor were compare

CO2 exhaust concentration is shown inFig. 8. Forfuel lean conditions, emission of CO2 was similarto the results of both an irreversible one-step retion model for five species and the chemical eqlibrium model for dozens of major species[20]. Forfuel-rich conditions, measured concentrations wslightly larger than the results of the chemical eqlibrium model, and showed a large deviation from t

Fig. 8. Measured CO2 emissions as a function of equivalenratio at various mean velocities for different combustors.

Fig. 9. Measured NOx emission of the D- and S-combustoas a function of equivalence ratio and mean velocity.

results of the ideal one-step model. This is becathe burned gas from a fuel-rich flame contains a laamount of H2 and CO that were not consideredthe ideal one-step model. Generally, however, cobustors are operated under fuel-lean conditions. Teven the one-step model is useful for estimation ofCO2 emission from small Swiss-roll combustors ffuel-lean conditions.

NOx emissions from the D-combustor and tS-combustor are shown inFig. 9 for various equiv-alence ratios. Both combustors had the same crsectional geometry. In the case of the D-combusNOx emission was less than about 100 ppm atequivalence ratio 0.9, and less than 60 ppm atequivalence ratio 0.7. Compared with the NOx emis-sion from uncontrolled combustion (between 10and 4000 ppm[11]), experimental results from evethe D-combustor were relatively small. It is knowthat the reactions associated with NOx generation re-quire flame temperatures higher than 1700 K[17].However, a narrow channel restricts the excestemperature increase in the combustion room e


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Fig. 10. Effect of varying the mean velocity on the emissioof NOx and total hydrocarbons.

though the premixed gas is heated. Therefore the lof NOx emission in a small Swiss-roll combustormuch smaller than in a larger unconfined flame.

Since the temperature of the combustor is strongoverned by the mean velocity as shown inFig. 5,the variation of the NOx emission with the meanvelocity was investigated in more detail.Fig. 10shows results for both NOx and total hydrocarbon(THC). The concentration of NOx was convertedto a value corresponding to the case of 5% excsive oxygen. As the mean velocity increases, Nxemission asymptotically approaches constant vathat depend on the equivalence ratios. In the cof the D-combustor at the equivalence ratio of 0a small peak in NOx emission was observed at thmean velocity of 2.8 m/s. This velocity correspondto the critical velocity above which the flame fropropagates upstream, such as the increment ofTc inFig. 4. For higher velocities the NOx emission didnot increase. Compared with a large-scale combtor, NOx emission was relatively small and becammuch smaller as the combustor became smaller oequivalence ratio was decreased.

Total hydrocarbon (THC) at the exit was negligble as shown inFig. 10. Minimum temperature of thecombustion room in a Swiss-roll combustor is muhigher than that of the ambient air due to heatcirculation. Even though some amount of unburnmixture passes through the dead space close towall, it can react afterward since the temperaturethe combustor or the combustion room is sufficienhigh to cause pyrolysis of propane. In addition, sinthe smaller combustor will requires a slightly hightemperature to operate, THC emission is not anificant obstacle for development of a smaller cobustor. However, it is important to prevent leakageunburned mixture from the upstream channel todownstream channel.

Fig. 11. Measured CO emission of the D- and S-combusas a function of equivalence ratio and mean velocity.

Fig. 12. Effect of the catalyst temperature on the conversof CO.

Fig. 11 shows the concentration of CO with thequivalence ratio against various mean velocitiestwo types of combustors, the S-combustor andD-combustor. Taken as a whole, the S-combustorhigher CO emission and the CO decreases as the mvelocity increases. Compared with the D-combusa small increment of CO was measured nearflammability limit of the S-combustor. Smaller meavelocity causes a lower temperature, as shownFig. 5, which hinders an ideal reaction. So intermdiate chemical products, such as CO, remain andexhausted. Therefore, as the size of the combustocomes smaller, CO emission becomes an imporissue for practical application.

As one of the countermeasures against CO emsion, the catalytic reaction was examined. Exhaustor the unburned mixture was allowed to flow throua small channel filled with fine ceramic balls coatwith a catalyst (Pt, 3 g/L). The correlation betweethe temperature at the inlet to the catalytic reactorconverting rates of CO are shown inFig. 12. Whenthe temperature was higher than 400 K, most of


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CO was eliminated. These results imply that a calytic reaction can be easily applicable to eliminCO. From an additional experiment using a healean premixed gas, it was also found that this catalreaction can convert even THC when the temperais higher than 550 K. This result is helpful in prventing a possible risk caused by the leakage ofunburned mixture. In all of the experimental resuwithin the flammable limits, the temperature of texhaust gas was usually higher than 550 K. Thefore, a catalyst can be installed at a suitable posiwithin or outside the small combustor to improve aplicability.

3.3. Thermal efficiency

Finally, the thermal efficiencies of small Swisroll combustors as heaters were estimated usingexperimental results. As a heater, all heat transferheat loss) from the combustor can be used to hecertain object. Therefore, for maximum efficiencya heater, it was assumed that all energy except theergy losses due to incomplete combustion and theexhaust gas could be used as heating energy. Theefficiency can be presented as

(4)η = (Qin − Lf − Lex)/Qin,

whereQin is the total input energy by premixed gaLf is energy loss due to incomplete combustion, aLex is the energy loss due to hot exhaust gas.

Total heat generation by combustion is the saregardless of the heat recirculation. These energyputs, combustion loss, and exhaust loss can besented relative to room temperature and can bespectively written as

Qin = m

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(5)Lex = m

Tex∫

T0

cp(T ) dT ,

wherem is mass-flow rate,T0 is room temperatureTex is the exhaust gas temperature, andcp is the spe-cific heat of the gas. The averaged values evaluat the mean temperatures between the adiabatic fland the exhaust gasT1 = (Tad + Tex)/2 or betweenthe adiabatic flame and the roomT2 = (Tad+ T0)/2were used. The adiabatic flame temperature(Tad) andthe specific heat of the burned gas(cp) were cal-culated under chemical equilibrium conditions[20]since most of the fuel burns and the CO2 compo-sitions of the exhaust gas were approximately tof chemical equilibrium, especially at the fuel-leside as shown inFig. 8. The fraction of fuel loss,β,

Fig. 13. Thermal efficiencies of the combustors.

was assumed to beβ ≈ YCO/YCO2. This fraction wasevaluated based on experimental results and wasthan 1% for the D-combustor and less than 2% forS-combustor. Therefore, the efficiency of the Swiroll combustor as a heater can be simplified as

(6)η ≈ cp(T1)

cp(T2)

Tad− Tex

Tad− T0− β.

The results are plotted inFig. 13, and the maxi-mum efficiency was up to 85% for the D-combusand decreased with the mean velocity. Higher veity reduces the residence time of the gas flow witthe combustor, and this restricts the heat recirction between burned gas and unburned mixture.S-combustor had higher efficiency than that ofD-combustor at the same mean velocity. The surfato-volume ratio of the S-combustor was much larthan that of the D-combustor. Hence, the efficienas a heater becomes greater for the same mean vity. On the other hand, since a smaller combustorquires a higher temperature and higher mean velofor flame stabilization, the maximum efficiency of tsmaller combustor decreases. Thus, a greater turnumber of heat-converting channels will be requifor a smaller combustor, another issue in the facation of a smaller combustor. Nevertheless, therefficiencies were still much higher compared with tenergy efficiency of electric power generation (ab40%). Through this study, it was found that develoment of a smaller combustor is possible, which wbe helpful in reducing energy consumption.

4. Concluding remarks

Combustion characteristics of Swiss-roll combtors were experimentally investigated. Three tyof Swiss-roll combustors having shallow chann(S-combustor), deep channels (D-combustor),


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wide combustion room (W-combustor) were fabcated, the width of the channel being smaller ththe quenching distance at normal state. And twadditional conditions of different thermal enviroments were compared.

Self-sustaining flames were successfully estlished in these small Swiss-roll combustors. Asheat loss increased, the flammable regime becnarrower and higher mean velocity was requirHowever, an upper limit of mean velocity (or blowoff limit) was not observed in this study, and even tmean velocity was 20 times greater than the lamburning velocity. Mean velocities at flammable limits were quite different depending on the geometand shapes of the combustors, while the mean tperatures of the combustors at the flammable limwere similar and depended on the equivalence raResults of temperature measurement imply thatmean temperatures and the flammable limits ofcombustors were governed by both the radiant hloss from the combustors and the total chemicalergy introduced into the combustors.

Characteristics of the exhaust gas of small Swroll combustors and their thermal efficiencies asheater were examined. CO2 emission was similar tothat estimated for chemical equilibrium. The levelNOx emission was relatively low, compared to largscale flames, and the NOx further decreased as thcombustor became smaller. Emission of total hydcarbon was negligible. Only the emission of COcreased as the combustor became smaller. Thisemission will be a major issue for the practical aplication of a small combustor. However, a catalyreaction was shown to be an effective countermeafor CO emission. Additionally, the thermal efficiencof the combustor as a heater was much higher thantypical efficiency of electric power generation. Tnew experimental results of this study represent astep in the long-term development of a small-scpower generators.

Acknowledgment

This work was performed as the R&D project etitled “Micro-combustors as Heaters” which is su

ported by the New Energy and Industrial TechnoloDevelopment Organization (NEDO), Japan.

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Flame stabilization and emission of small Swiss-roll combustors as heaters

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