More in­focen­ter top­ics: Meas­ure­ment Tech­no­logy | Sig­nal pro­cessing| Visu­al­iz­a­tion

What is the dif­fer­ence between act­ive in­stabil­ity con­trol and pass­ive meth­ods?

Ex­cept for in­fre­quent spe­cial-pur­pose ap­plic­a­tions (such as so-called pulse com­bus­tors), com­bus­tion in­stabil­it­ies are un­desir­able ef­fects det­ri­ment­ally in­flu­en­cing life, per­form­ance and en­vir­on­ment­al char­ac­ter­ist­ics of any com­bus­tion sys­tem. Ac­cord­ingly, solu­tions are act­ively sought that allow the avoid­ance of this kind of in­stabil­ity or - if os­cil­lat­ory prob­lems are already being en­countered - to sup­press them or at least re­duce them to ac­cept­able levels. Meas­ures taken in this re­gard can be sub­divided in prin­ciple into two dis­tinct groups: pass­ive and act­ive ones.


Passive meth­ods

In gen­er­al, this term refers to sys­tems devoid of "in­tel­li­gence", i.e. of closed-loop con­trol; this means that they do not in­flu­ence or con­trol os­cil­la­tions arising with­in a com­bus­tion sys­tem by spe­cific­ally in­ter­pret­ing the course of cer­tain sys­tem para­met­ers. To sim­pli­fy even more, pass­ive meth­ods may be said to be char­ac­ter­ized by re­quir­ing no ex­tern­al en­ergy sup­ply.

Below, some ex­amples for pass­ive meth­ods as well as high­lights on modes of ac­tion, be­ne­fits and dis­ad­vant­ages:

  • Damp­ing ele­ments and si­len­cers: acous­tic en­ergy is dis­sip­ated as heat. Will not pre­vent os­cil­la­tions but cap­able of re­du­cing sound pres­sure amp­litudes oc­cur­ring. Low-price stand­ard com­pon­ents but fre­quently bulky; en­tail ad­di­tion­al pres­sure losses.
  • Helm­holtz and lambda/4 res­on­at­ors: dis­turb the res­on­ant sound field and thus the evol­u­tion of com­bus­tion in­stabil­it­ies. Ef­fect­ive­ness lim­ited to cer­tain fre­quen­cies; low fre­quen­cies need large di­men­sions.
  • Baffles: dis­turb the propaga­tion of sound waves and thus any res­on­ant sound field that might form. Un­com­plic­ated con­struc­tion but sub­ject to burn­ing off; fre­quently det­ri­ment­al to com­bus­tion it­self; ad­di­tion­al pres­sure losses.
  • "De­tun­ing" - geo­met­ric­al modi­fic­a­tions used to modi­fy ei­gen­fre­quen­cies with­in a com­bus­tion sys­tem. No sig­ni­fic­ant draw­backs for com­bus­tion and ef­fi­ciency but often ex­pens­ive and com­plic­ated to build. Moreover, ef­fect­ive­ness fre­quently lim­ited to spe­cif­ic fre­quen­cies; thus, there is some danger that os­cil­la­tions will merely shift to some other fre­quency.

Above all due to these draw­backs and the lim­ited ef­fect­ive­ness of pass­ive meth­ods, act­ive meth­ods con­sti­tute an in­ter­est­ing al­tern­at­ive spe­cific­ally for sys­tems op­er­at­ing under highly vari­able con­di­tions.


Act­ive meth­ods

With "Act­ive In­stabil­ity Con­trol (AIC)", i.e. act­ive sup­pres­sion of self-ex­cited com­bus­tion in­stabil­it­ies, sys­tem para­met­ers are meas­ured and fed into a con­trol sys­tem that - de­pend­ing upon the input sig­nal - uses an ac­tu­at­or to in­flu­ence the cor­res­pond­ing sys­tem so that no os­cil­la­tions are gen­er­ated. A simple ex­ample: a pres­sure sensor de­tects pres­sure fluc­tu­ations arising with­in a com­bus­tion sys­tem for a closed-loop con­trol sys­tem to de­term­ine a sig­nal act­ing upon a loud­speak­er. As a func­tion of sig­nals re­ceived, the loud­speak­er gen­er­ates an anti-sound field to be su­per­im­posed upon the ini­tial sound field in the sys­tem so as to can­cel both fields out.

In prac­tice, AIC un­for­tu­nately is a little more com­plic­ated than that. Vari­ous as­pects have to be taken into ac­count, and the mech­an­ism trig­ger­ing self-ex­cited com­bus­tion in­stabil­it­ies has to be care­fully re­searched be­fore a de­cision can be made re­gard­ing a suit­able AIC strategy. Sensors, ac­tu­at­ors and con­trol sys­tem ar­chi­tec­ture de­pend to a large ex­tent on the com­bus­tion sys­tem con­cerned (power, fuel, geo­metry), on fre­quen­cies arising and on sound fields gen­er­ated in the pro­cess. However, any extra ex­pense en­tailed by AIC will be jus­ti­fied by the be­ne­fits linked with it:

  • Hardly any need to modi­fy com­bus­tion sys­tems, small foot­print, re­l­at­ively un­com­plic­ated ret­ro­fit­ting.
  • No det­ri­ment­al ef­fects on heat re­lease and com­bus­tion con­trol, no ad­di­tion­al losses.
  • In cer­tain cases, pol­lut­ant emis­sions and in­com­plete burnout can even be re­duced.
  • True closed-loop con­trol re­acts flex­ibly to ac­tu­al sys­tem be­ha­vi­or and its changes. No in­ter­ven­tion ex­cept if and while ne­ces­sary.
  • Upon sys­tem sta­bil­iz­a­tion by AIC, ac­tu­at­ing sig­nals are re­duced, thus de­creas­ing power con­sump­tion by the ac­tu­at­or and in­creas­ing life of the ac­tu­at­or.

What are the causes of acous­tic feed­back?

Acous­tic feed­back is char­ac­ter­ized by ei­gen­fre­quen­cies and ei­gen­modes. With most self-ex­cited com­bus­tion in­stabil­it­ies, feed­back is provided by acous­tics. To un­der­stand the causes lead­ing to self-ex­cited com­bus­tion in­stabil­it­ies, it is thus in­dis­pens­able to first con­sider in de­tail the acous­tics of a com­bus­tion sys­tem. In this con­text, po­ten­tially res­on­ant acous­tic fre­quen­cies (ei­gen­fre­quen­cies) and the as­so­ci­ated ei­gen­modes of sound pres­sure and particle ve­lo­city are of major im­port­ance. Ei­gen­modes are defined as the stand­ing waves of sound pres­sure and particle ve­lo­city pro­duced by res­on­ance.

The fol­low­ing il­lus­tra­tion demon­strates, for a simple tube closed at one end and open at the other, the ei­gen­modes cor­res­pond­ing to the first, second and third har­mon­ic along the lon­git­ud­in­al axis of the tube. The bound­ary con­di­tions are ideal­ized and as­sumed to be acous­tic­ally closed for the left end and acous­tic­ally open for the right one.

In con­trast to this simple ex­ample, ei­gen­modes in a real com­bus­tion sys­tem look sig­ni­fic­antly dif­fer­ent owing to changes in cross-sec­tion, to the in­flu­ence ex­er­ted by flow con­di­tions, to tem­per­at­ure fluc­tu­ations with­in the com­bus­tion zone and to the com­plex bound­ary con­di­tions at the in­lets and out­lets of the sys­tem.

Com­bus­tion dy­nam­ics: rum­bling, hum­ming, screech­ing or pulsa­tion.

Pre­mix flames are favored in mod­ern com­bus­tion sys­tems to re­duce emis­sions, be­cause they have low NOx emis­sions due to their low flame tem­per­at­ures. Un­for­tu­nately, with this type of com­bus­tion, and in con­junc­tion with com­bus­tion-cham­ber acous­tics, so-called self-ex­cited com­bus­tion dy­nam­ics emerge very quickly. These are often also re­ferred to as com­bus­tion in­stabil­it­ies, ther­moacous­tic in­stabil­it­ies, or, in ac­cord­ance with the aud­ible fre­quen­cies, also known as rum­bling, hum­ming, screech­ing or pulsa­tion.

Strong pres­sure fluc­tu­ations at one or more fre­quen­cies are the un­der­ly­ing cause of such vi­bra­tions. Pres­sure fluc­tu­ations can achieve such high amp­litudes that the com­bus­tion sys­tem it­self or com­pon­ents con­nec­ted up­stream and down­stream are des­troyed. These vi­bra­tions can be ob­served in a range of situ­ations from house­hold heat­ing sys­tems to large-scale fir­ings and from sta­tion­ary gas tur­bines to rock­et en­gines.

The IfTA PreCurs­or is a meth­od for early de­tec­tion of com­bus­tion dy­nam­ics- even be­fore high amp­litudes occur which can lead to dam­age.

How does ex­per­i­ment­al modal ana­lys­is work?

Owing to wide fluc­tu­ations of all sound field para­met­ers with­in sys­tems sub­ject to self-ex­cited com­bus­tion in­stabil­it­ies, merely de­term­in­ing ei­gen­modes by meas­ur­ing local sound-pres­sure amp­litudes and phases does not lead to use­ful res­ults

There­fore, we de­veloped a spe­cial ana­lyt­ic­al meth­od based on cor­rel­a­tion meas­ur­ing tech­niques. In sim­pli­fied terms, a mi­cro­phone is placed at a ref­er­ence point while an­oth­er one is used to de­term­ine sound-pres­sure amp­litude and phase re­la­tion­ships in the fre­quency do­main. Com­ple­men­ted by av­er­aging in the fre­quency do­main, this meth­od yields the ne­ces­sary in­sights.

How does the nu­mer­ic­al modal ana­lys­is work?

For com­put­ing pur­poses, any geo­metry to be ana­lyzed is rendered dis­crete by means of cyl­indric­al and con­ic­al ele­ments that per­mit the mod­el­ing of ac­tu­al cross-sec­tions to any de­gree of pre­ci­sion re­quired. This dis­cret­iz­ing pro­ced­ure is backed by com­pre­hens­ive ex­per­i­ment­al know-how; after all, the qual­ity of any res­ults ob­tained is de­term­ined to a large ex­tent by ap­pro­pri­ate par­ti­tion­ing of the do­main to be ana­lyzed. Dis­cret­ized geo­metry, to­geth­er with fur­ther para­met­ers - tem­per­at­ure, flow ve­lo­city and acous­tic bound­ary con­di­tions - con­sti­tute the basic data for a spe­cial nu­mer­ic­al code. Acous­tic bound­ary con­di­tions, in­clud­ing fre­quency-de­pend­ent ones, can be im­posed at will. In the event that com­plic­ated bound­ary con­di­tions can­not be suf­fi­ciently form­al­ized by the­or­et­ic­al ap­proaches, we offer ex­per­i­ment­al stud­ies per­formed ac­cord­ing to the n mi­cro­phone meth­od. The pro­gram code used was writ­ten at IfTA; new fea­tures are being added con­tinu­ously.

This ver­sion makes it pos­sible to eas­ily vary the length or cross-sec­tion of any given tube ele­ment to tar­get re­search on spe­cif­ic shifts in mode or fre­quency.

What defines the Rayleigh cri­terion?

The Rayleigh cri­terion is an im­port­ant sta­bil­ity cri­terion for self-ex­cited com­bus­tion os­cil­la­tions

If any res­on­ant fre­quency is to be ex­cited with­in a com­bus­tion cham­ber, a source of sound is re­quired that con­stantly feeds en­ergy into the sys­tem at the cor­rect os­cil­lat­ory fre­quency. With any com­bus­tion sys­tem, en­ergy is sup­plied by heat re­lease rates os­cil­lat­ing around their mean value.

However, any such thermal power os­cil­la­tion must not only have the cor­rect fre­quency to trig­ger self-ex­cited com­bus­tion in­stabil­it­ies but also the cor­rect phase angle suit­able for re­in­for­cing the sound pres­sure fluc­tu­ations arising with­in the sys­tem. In 1878, Lord Rayleigh first de­term­ined the cri­terion that now bears his name. Ac­cord­ing to it, com­bus­tion in­stabil­it­ies are ex­cited if the os­cil­la­tion of sound-pres­sure and a flame's heat re­lease rate are in-phase, or damped down, if they are anti-phase. Put­nam and Den­nis took up this idea and provided a form­al math­em­at­ic­al rep­res­ent­a­tion of Rayleigh's cri­terion, the so-called "Rayleigh in­teg­ral":

This in­teg­ral means that the product of thermal power Q(t) and sound pres­sure os­cil­la­tion p(t), in­teg­rated over the peri­od of os­cil­la­tion T (=1/f [s]), must be pos­it­ive for any com­bus­tion os­cil­la­tion to be ex­cited. Whenev­er it is neg­at­ive, os­cil­la­tions are damped.

Ap­ply­ing this equa­tion to any har­mon­ic os­cil­la­tion, the con­di­tion is met if the ab­so­lute value for the phase dif­fer­ence between the os­cil­la­tions of heat re­lease and of sound pres­sure is less than or equal to 90°.

The re­search tools now avail­able from us for the one and two-di­men­sion­al Rayleigh in­dexes con­sti­tute a lo­gic­ally con­sist­ent ap­plic­a­tion of this cri­terion. The Rayleigh index is a local ver­sion of the glob­ally ex­pressed in­equal­ity above al­low­ing to identi­fy areas sub­ject to ex­cit­a­tion and damp­ing with­in any com­bus­tion zone. With this in­form­a­tion, the in­ter­ac­tions between thermal and acous­tic os­cil­la­tions can be in­flu­enced de­lib­er­ately.

The above il­lus­tra­tion shows the one and two-di­men­sion­al Rayleigh in­dexes com­puted from ex­per­i­ment­al data ob­tained for a li­quid fueled com­bus­tor. The one-di­men­sion­al Rayleigh index is plot­ted versus burn­er length (bot­tom), the two-di­men­sion­al one as an in­tens­ity plot to­geth­er with the burn­er geo­metry (top). It is ob­vi­ous that flame sec­tions sub­ject to ex­cit­a­tion and damp­ing al­tern­ate along the com­bus­tion cham­ber and that ex­cit­ing do­mains pre­dom­in­ate, a fact which leads to the ob­served com­bus­tion in­stabil­it­ies.

How do self-ex­cited or self-in­duced com­bus­tion os­cil­la­tions de­vel­op?

Given cer­tain op­er­at­ing con­di­tions, in­dus­tri­al-type com­bus­tion and propul­sion sys­tems may be sub­ject to self-ex­cited com­bus­tion in­stabil­it­ies, some­times des­ig­nated flame pulsa­tions or os­cil­la­tions, which are char­ac­ter­ized by pres­sure os­cil­la­tions arising at dis­crete fre­quen­cies. With low-power sys­tems, such as do­mest­ic or aux­il­i­ary vehicle heat­ing sys­tems, com­bus­tion in­stabil­it­ies nor­mally only pro­duce high levels of noise.

With high­er-power sys­tems, such as re­gen­er­at­ing air heat­ing units, pro­cess gas heat­ers, gas tur­bines or rock­et en­gines, the pres­sure amp­litudes reached are some­times so high that the al­tern­at­ing loads en­tailed by them may res­ult in mech­an­ic­al sys­tem fail­ure of the com­bus­tion cham­bers as well as the up­stream and down­stream com­pon­ents. For in­stance, pres­sure amp­litudes meas­ured for pro­cess gas heat­ers reached 0.5 bar, and even 5 bars for a rock­et com­bus­tion cham­ber.

In ad­di­tion to pres­sure fluc­tu­ations, self-ex­cited com­bus­tion in­stabil­it­ies in­vari­ably in­duce, via sound particle ve­lo­cit­ies, equi­val­ent fluc­tu­ations of flow ve­lo­cit­ies that lead, in their turn, to sub­stan­tially in­creased amounts of heat being trans­ferred to com­bus­tion cham­ber walls. Thus, thermal loads will in­crease, in ad­di­tion to mech­an­ic­al ones, so that there are def­in­ite thermal haz­ards that might even des­troy the com­bus­tion cham­ber. When amp­litudes reach sub­stan­tial val­ues, burn­er flames can often no longer be sta­bil­ized so that they may flash back or be ex­tin­guished.

Basic­ally, any self-ex­cited com­bus­tion in­stabil­ity is due to sev­er­al quant­it­ies in­ter­act­ing phys­ic­ally and lead­ing, given ap­pro­pri­ate pre­con­di­tions, to res­on­ance. In order to allow any such os­cil­la­tion to arise in a self-ex­cited man­ner, sound-pres­sure and heat-re­lease os­cil­la­tions have to re­in­force each other. For this to hap­pen, there must be a feed­back mech­an­ism in­du­cing heat re­lease fluc­tu­ations strong enough to re­in­force sound pres­sure os­cil­la­tions. In most cases, feed­back is caused by the acous­tics of the com­bus­tion sys­tem con­cerned. However, other mech­an­isms have like­wise been ob­served.

For in­stance, struc­tur­al vi­bra­tions in rock­et drives may mod­u­late fuel flows sup­ply­ing the com­bus­tion cham­ber and thus pro­duce os­cil­la­tions of the heat re­lease rate of the flame. The vi­bra­tions caused by these peri­od­ic changes of the heat re­lease rate in turn re­in­force the struc­tur­al os­cil­la­tions, thus clos­ing the loop.

BrummenSelf-ex­cited com­bus­tion dy­nam­ics are also often re­ferred to as com­bus­tion in­stabil­it­ies, ther­moacous­tic in­stabil­it­ies, or, in ac­cord­ance with the aud­ible fre­quen­cies, also known as rum­bling, hum­ming, screech­ing or pulsa­tion.

What defines thermal heat re­lease rates of flames?

Un­steady heat re­lease rates of flames, i.e. fluc­tu­ations of the en­ergy in­tro­duced into the flow of air by com­bus­tion per unit of time, con­sti­tute an im­port­ant para­met­er for self-ex­cited com­bus­tion in­stabil­it­ies. Fre­quently, mod­u­lated flows of air-fuel mix­tures into the com­bus­tion zone cause this type of fluc­tu­at­ing heat re­lease. Quan­ti­fy­ing these fluc­tu­ations con­sti­tutes an im­port­ant pre­con­di­tion for ana­lyz­ing any un­steady com­bus­tion and its clas­si­fic­a­tion as self-ex­cited or ex­tern­ally ex­cited.

The com­bus­tion of hy­dro­car­bons in oxy­gen or air is not a single bulk re­ac­tion but pro­ceeds over a se­quence of nu­mer­ous (some­times sev­er­al hun­dred) in­ter­me­di­ate steps in­volving a cor­res­pond­ing num­ber of chem­ic­al spe­cies. Many of those in­ter­me­di­ate re­ac­tions pro­ceed ex­tremely rap­idly, res­ult­ing in cor­res­pond­ingly short-lived spe­cies, such as OH rad­ic­als whose life­time is less than 600 ns. OH rad­ic­als emit ra­di­ation at 306.4 nm that can be meas­ured using suit­able probes; this para­met­er provides a suf­fi­ciently pre­cise fig­ure for the burn­ing rate and thus for the heat re­lease rate.

In ad­di­tion to spa­tially in­teg­rated meas­ure­ments of the ra­di­ation emit­ted by OH rad­ic­als (total heat re­leased), IfTA of­fers 1D and 2D meas­ure­ments for cer­tain points or seg­ments of a flame front. We use spe­cial photo mul­ti­pli­ers for in­teg­ral meas­ure­ment pur­poses. 1D and 2D meas­ure­ments are per­formed by means of spe­cial fiber op­tics. The meas­ured sig­nals are ana­lyzed in the fre­quency do­main in order to de­term­ine the amp­litude and phase re­la­tion­ship between fluc­tu­at­ing heat re­lease rates and os­cil­la­tions of the sound field para­met­ers meas­ured at the same time. This type of data may be used not only for dir­ect ana­lyt­ic­al pur­poses but also to com­pute the Rayleigh-Index for ex­ample.