The Influence Of Burner Material Properties On The .
Proceedingsof theCombustionInstituteProceedings of the Combustion Institute 30 (2005) 1741–1748www.elsevier.com/locate/prociThe influence of burner material propertieson the acoustical transfer function of radiantsurface burnersK.R.A.M. Schreel*, E.L. van den Tillaart, L.P.H. de GoeyDepartment of Mechanical Engineering, Section Combustion Technology,Eindhoven University of Technology, P.O. Box 513,5600 MB Eindhoven, The NetherlandsAbstractModern central heating systems use low NOx premixed burners with a large modulation range. This canlead to noise problems that cannot be solved via trial and error, but need accurate modeling. An acousticalanalysis as part of the design phase can reduce the time-to-market considerably, but the acoustical responseof the flame is an unknown and complex key factor. A large class of burners currently used in boiler equipment are formed by radiant surface burners. These burners are generally made of a porous material onwhich a (nearly) flat flame can be stabilized. This will lead to heat transfer from the flame to the burner,and consequently a cooler flame and reduced NOx emissions. However, the heat loss towards the burnerhas a significant influence on the behavior of the flame when the flow velocity is acoustically disturbed. Inthat case at certain frequencies a feedback loop can occur, and the velocity fluctuations are amplified. Inthis paper, a detailed model for the heat transfer between the gas and the burner is used to numericallystudy the influence of several different material properties on the acoustical transfer coefficient. The modelis tested with measurements of the transfer function for four different burner deck types with known material properties. It is found that an accurate description of the heat transfer in the porous material is essential for a good modeling of the acoustical transfer function.Ó 2004 The Combustion Institute. Published by Elsevier Inc. All rights reserved.Keywords: Laminar flames; Acoustics; Transfer function; Heat transfer ceramic burners1. IntroductionAcoustical problems are almost inherent toany closed system with a flowing medium. Oftenthese problems can be avoided by using simplerules-of-thumb or eliminated by trial-and-errormethods. In the case of small (condensing) domestic central heating systems and boilers, two mod*Corresponding author. Fax: 31 40 2433445.E-mail address: k.r.a.m.schreel@tue.nl (K.R.A.M.Schreel).ern developments have given rise to a severeincrease in acoustical problems. First, to reduceNOx emissions, the burner is operated undermuch more fuel lean conditions than before,implicating a somewhat worse acoustical behavior. The most difficult development, however, isthe trend to use very large modulation ranges.Where acoustical problems often can be solvedrelatively easily for one setting, a well-behavingsystem for a range of operating conditions is muchmore difficult to develop. It is clear that this cannot be solved via trial and error anymore, and an1540-7489/ - see front matter Ó 2004 The Combustion Institute. Published by Elsevier Inc. All rights reserved.doi:10.1016/j.proci.2004.08.167
1742K.R.A.M. Schreel et al. / Proceedings of the Combustion Institute 30 (2005) 1741–1748accurate acoustical analysis as part of the designphase of the boiler becomes necessary.For long wavelengths relative to the typicaldiameters found in small boilers, the system canacoustically be analyzed via a 1D network analysis, in which every acoustical element is describedby a transfer matrix. The key element with themost complex acoustical properties is the flame/burner combination, which strongly depends onthe flame dynamics due to acoustical forcing.The burners used in small boilers can be classifiedaccording to their operating principle. A largeclass of burners currently used in boiler equipmentare formed by surface burners. These burners aremade of some porous material (can be either ceramic or metal), on which a (nearly) flat flame is stabilized for gas velocities lower than the adiabaticburning velocity. In this case, the flame will loseheat to the burner, which leads to a cooler flame,and consequently reduced NOx emissions. However, the heat loss towards the burner has a significant influence on the dynamics of the flame whenthe flow velocity is acoustically disturbed. In thatcase at certain frequencies a feedback loop can occur, and the velocity fluctuations are amplified.The acoustical transfer of surface burners hasrecently been studied theoretically by the groupof McIntosh [1,2], and numerically, theoretically,and experimentally by the group of De Goey [3–5]. For a broader overview, the reader is referredto the paper by Schreel et al. [5]. All modeling attempts have shown a reasonable to good correspondence with the experimental results, withthe worst results for the case of radiant surfaceburners. Their behavior can partly be explainedby their elevated surface temperature, but significant differences with experiments remain. Allmodels used so far assume a perfect heat transferbetween the flame and the flame holder, but this isnot necessarily true.Heat transfer in porous media has been a topicof wide interest. It plays an essential role in thestability and flash-back properties of ceramicfoam burners, both for surface stabilized flames[6], as for flames stabilized inside ceramic foam[7,8]. Its role in the acoustical transfer function,however, has been neglected so far.In this paper, a study of the influence of the heattransfer between the gas and the burner material onthe acoustical transfer function is presented. Several different material properties are identified,which play a role in the heat transfer, and their relative importance for the transfer function is numerically studied. The numerical simulations are alsocompared to experimental results for four differentburner materials. In the next section, a short introduction to the theoretical background of 1D acoustical transfer is given, and the above-mentionedparameters are introduced. The third section is devoted to the description of the experimental setup,followed by a section describing the numericalmodeling. In the results section, both the numericaland experimental results are presented and discussed. The last section contains the conclusions.2. Acoustical transferIn a flowing medium with an acoustical fieldsuperposed, the velocity can be written asu¼ u þ u0 ;ð1Þin which u is the average velocity and u0 the acoustical fluctuations. For wavelengths much longerthan a typical duct diameter, the system can beconsidered to be acoustically one-dimensional,and a so-called acoustical network analysis [9]can be applied, in which the system is split intoelements. The acoustical velocities and pressuresat both sides of an element can then be relatedvia a transfer matrix.For a burner stabilized flat flame, it can beshown that the transfer matrix has (in the lowMach number approximation) the following form: 0 0 pbpu1 0¼u0b0 T 22 u0uwhich means that the complete transfer matrix isdetermined by T22, the coupling between thevelocity fluctuations upstream of the burner andflame ðu0u Þ, and downstream of the flame ðu0b Þ.As an intuitive model for the interaction between the flame and the acoustical waves, a kinematic description can be applied [4] in which theheat transfer between the burner and the flameis considered to be perfect. This means that thegas temperature and the temperature of the porous solid are always the same. A variation in thestand-off distance of the flame (induced by theacoustical velocity fluctuations) then results in adisplacement of the complete flame-structure, except for the temperature. This leads to a feedbackloop [3,5], through which the flame motion isamplified or damped, depending on the frequency.Assuming a Lewis number equal to 1, the equation for the flame motion can be analytically [4]solved for this system. The transfer function foundshows a resonance like behavior. For low frequencies, the magnitude of T22 is determined by the thermal expansion of the flame, then increasing up to aresonance frequency, after which the magnitudefalls down to 1 for even higher frequencies. The resonance frequency is mainly a function of the standoff distance of the flame, and thus of the surfacetemperature of the burner. The model shows a reasonable agreement with measurements for bothcooled and radiant burners [5].For arbitrary Lewis numbers, an analyticalexpression for the transfer function has beenfound by McIntosh and Rylands [2], based on amatched asymptotic analysis of the full equations.This model has been compared to both measurements and the analytical model by Rook and
K.R.A.M. Schreel et al. / Proceedings of the Combustion Institute 30 (2005) 1741–1748co-workers [5], and shows a reasonable agreementwith the measurements, but the elevated burnersurface temperatures of radiant burners are notaccommodated in the model.When measuring the transfer function for several burner materials, it was found that theassumption of perfect heat transfer is not adequate for radiant burners. With this assumption,the only quantity in which the burner propertiesare reflected is the surface temperature. The largevariation in acoustical behavior, especially forhigher frequencies, can, however, not be explainedby a variation in surface temperature alone. Thus,a more realistic heat transfer model needs to beused. The full model can be found in the paperby Lammers and de Goey [6]. Here, only thetwo coupled enthalpy equations are presented thatare valid inside the porous material. The onedescribing the gas is given by oT goT g ooT g/qg cp;g/kgþ /qg ucp;g oxotoxoxNXhi qi¼ aSðT s T g Þ /ð2Þi¼1and the one for the porous material is given by oT s ooT sð1 /Þss ks ð1 /Þqs cp;soxotoxoqrad;ð3Þ¼ aSðT s T g Þ þoxin which the subscripts ÔsÕ and ÔgÕ indicate whethera quantity relates to the solid material or the gas,respectively. The properties of the solid materialare reflected in the volumetric heat transfer coefficient (aS), the porosity (/), the heat capacity(qscp, s), and the product of tortuosity (ss) and heatconductivity (ssks). It has been confirmed that tortuosity plays no significant role [10] in the gas, andtherefore this is neglected. The term describingradiative heat transfer (qrad) includes both heattransfer inside the porous material and radiativeheat loss at the surface. The latter is describedas rT 4, in which the emissivity ( ) is the fifthand last burner material property of interest.Radiative heat loss in the flue gas is neglected asit is considered to be of insignificant importancein this case. Even if it were significant, its effect(a lower flame temperature) would partly be compensated by absorption of the same radiation atthe burner surface. The parameters aS, /, and ssmainly describe the structural properties of theporous material, where the other parameters(qscp,s, ks, and ) are ÔtrueÕ material properties.3. ExperimentalThe experimental setup and method is identicalto the one presented in a previous paper [5].1743A short description will be given here. The readeris referred to the other paper for more details.The setup consists essentially of an approximately 60 cm long tube with an inner diameterof 5 cm, placed vertically to minimize buoyancyeffects. The gas inlet is placed at the bottom ofthe tube, and the flame holder is placed at a distance approximately 7 cm from the exit. Somegrids are fitted right after the entrance to settlethe flow. In the lower part of the tube, a hole ismade in the side, which is coupled with a flexiblehose to a loudspeaker. The part of the tube downstream of the flame holder is water cooled at nominally 50 C to avoid condensation of water. Theacoustical properties of the tube itself have beenstudied, and it was found that a rather weak resonance shows up around 640 Hz. This is differentthan reported previously [5], but this frequency region was not used for that research. In this case,frequencies up to 800 Hz are applied, avoidingthe 600–680 Hz band.The sound wave upstream of the flame holderis measured by using the two microphone method[9,11]. For this, two pressure transducers aremounted in the wall of the tube. Downstream ofthe flame, the velocity is measured phase correlated by means of laser Doppler velocimetry(LDV). Optical access is provided by three smallholes at 4 mm above the flame holder. The LDVequipment consists of a 20 mW HeNe laser usedin forward scatter mode and a counter based signal analyzer (Disa 55L series). In principle, onedoes not measure the transfer matrix element ofthe flame in this way, but the transfer matrix element of the flame combined with the flame holder.Test measurements showed however that thetransfer matrix element of the flame holder itselfis very close to unity for all materials at the frequencies of interest and can be neglected.Four materials are used for the flame holder.All of them are ceramic. Material 1 consists of sintered fibers with a diameter of 25 lm. The fibershave an inner core of some silicate and an outershell of silicon carbide. The fibers are randomlyplaced within a layer. Material 2 is the unsinteredversion of the previous one, with additional holesperforated in the plate. Because the material is notsintered, the porosity is noticeably larger. Thethird material is a perforated ceramic plate. Itconsists of foamed cordierite, but with a closedcell structure. The porosity is thus entirely determined by the perforation pattern, which is hexagonal (pitch 1.95 mm) and has a hole diameter of1 mm. The fourth material is a ceramic foammade of cordierite. It has a reticulated structurewith 24 pores/cm and an average hole size of0.3 mm. All relevant material properties are presented in Table 1. The value of the volumetric heattransfer coefficient is based on the application ofknown Nusselt relations for an approximategeometry of the solid (these are discussed with
1744K.R.A.M. Schreel et al. / Proceedings of the Combustion Institute 30 (2005) 1741–1748Table 1Values for the relevant properties of the burner materialsBurner materialaS (W/cm3 K)/qscp, s (J/cm3 K)ssks(W/cm K) .900.050.050.020.020.850.850.850.85Sintered ceramic fibersUnsintered ceramic fibersPerforated ceramic plateCeramic foamthe presentation of the measurements). The porosity is derived from a comparison between the measured density of the porous material and theliterature value of the density of the solid. Forthe heat capacity and conductivity, literature values are used. The (apparent) emissivity is assumedto be 0.85 for all materials.4. NumericalNumerical computations are performed usingthe software package Chem1D, developed inhouse [12]. With this program, a one-dimensionalflame can be calculated time resolved with complex chemistry. Several add-ons can be used to include, for example, radiation, stagnation flow(with and without heat transfer), surface chemistry, and heat transfer inside the flame holder.The combustion equations [13] for species and energy conservation are solved for the gas phase, together with an energy conservation equation forthe solid (Eqs. (2) and (3)). For the purpose of thiswork, the chemical model as proposed by Smooke[14] is used. This model is limited to lean CH4/airflames, but it can accurately describe the flamedynamics and offer a high computational performance gain over the more complex models likeGRI [15]. The diffusion is modeled by using theEGlib library of Ern and Giovangigli [16], whichincorporates complex transport processes including the Dufour and Soret effect.To allow for 1D modeling, the porous solid ismodeled with a volume-averaged continuum approach. The actual properties of the materialcan be described by the parameters in Eqs. (2)and (3). The pore size and shape are reflected inthe values for aS and /. The radiation term (qrad)in Eq. (3) is in most cases described Rosselandmodel inside the porous material, with an extinction coefficient of 15 cm 1. A discrete transfermodel is used close to (numerical) flash-back [6].At the surface, the radiative heat loss is describedas rT4.When modeling a stationary flame, the temperature distribution inside the ceramic is clearly different for the gas and the solid (see Fig. 1 for anexample of material 1). At the surface, the solidtemperature is significantly lower than the gastemperature due to the radiative heat loss at thesurface. The solid, however, is a much better heatFig. 1. The temperature, of the gas and solid for astationary calculation for the reference case. The verticaldashed lines represent the boundaries of the porousmaterial.conductor than the gas, and at some position inside the flame holder the gas and solid temperatures are equal. Below that point, the solidtemperature is higher than the gas temperature.5. ResultsFirst, modeling results are presented for a variation of aS, /, qscp,s, ssks, and . The experimental results are presented afterward, and comparedto modeling results based on the best known values of the material properties.5.1. Numerical studyBurner material 1 is chosen as the reference casefor the numerical modeling. From a variation of theparameters, their relative importance can bejudged, and from this the accuracy can be estimatedwith which they need to be known for modeling anarbitrary material. For these calculations, a fuellean methane–air flame is used at a stoichiometricratio of 0.8 and a gas mixture velocity of 17 cm/s.The volumetric heat transfer coefficient is theoretically the parameter that determines whether itcan be assumed that a material exhibits perfectheat transfer. A series of transfer function calculations with aS in the range of 1–80 W/cm3 K is presented in Fig. 2. For large values of aS, aresonance like shape is found, which is dampedfor lower values of aS. The cut-off frequency also
K.R.A.M. Schreel et al. / Proceedings of the Combustion Institute 30 (2005) 1741–17481745Fig. 2. The absolute value and phase of the acoustical transfer coefficient (T22) as a function of frequency forburner material 1 with an aS (in W/cm3 K) value range of 1 (lower curve), 2, 4, and 7 (dashed line); and 10, 40, and 80(upper curve).decreases with lower values for aS. There is onlyinfluence on the low frequency behavior of thetransfer coefficient for very low values of aS.For values of aS larger than 40 W/cm3 K, thetransfer function becomes insensitive to variationsin aS, indicating that the heat transfer becomesideal. It should be noted that the transfer functionis particularly sensitive for variations in the region1–10 W/cm3 K, which corresponds to the range ofvalues encountered in the burner materials underconsideration. Clearly the approximation thatthe heat transfer is ideal is not valid. This parameter should be given special attention when modeling an arbitrary material if no directcomparison with measurements of the transferfunction is available, but is very hard to estimatefrom first principles [7,17].A series of transfer functions with varyingporosity is presented in Fig. 3. Also in this casethe influence on the transfer function is significant, but now the magnitude of the transfer function is affected for the frequency range below thefall-off frequency. The fall-off frequency itself ishardly affected. Although the porosity is important, it is not difficult to measure it accurately enough to be able to model an arbitrary material.The influence of the heat conductivity (ssks) isalmost negligible and is not presented in a figure.We found only significant influence on the transfer function for variations of an order of magnitude. This can be understood from Eq. (3). Theterm in which the heat conductivity is present isproportional to oTs/ox. As can be seen from Fig.1, the slope of the temperature is quite moderate.Normally the heat conductivity is known with anaccuracy better than a factor of 2, which meansthat no special attention is needed.Even less significant is the heat capacity of thesolid. Again from Eqs. (2) and (3), it can be seenthat, since the
mainly describe the structural properties of the porous material, where the other parameters (q sc p,s, k s, and ) are true material properties. 3. Experimental The experimental setup and method is identical to the one presented in a previous paper [5]. A short description will be given her
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rich in fat, oil or alcohol (e.g. rum, cognac, wine). - Unattended cooking on a hob with fat or oil can be dangerous and may result in fire. NEVER try to extinguish a . 3 Semirapid burner 4 Power burner 5 Auxiliary burner 6 Power burner control knob 7 Semirapid burner control knob 8 Semirapid burner control knob 9 Auxiliary burner control knob.
Tube burner 1.59 mm curved-wall pinhole burner Pinhole burner 6.35 mm curved-wall pinhole burner Burner Diameter (mm) Quenching Mass Flow Rate (mg/s) H 2 Quenching Limits Three burner types are shown. For large d the limits converge. Heat losses are greatest for pinholes, lea
