Model Small Gas Turbine Jet Engines

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Model

THIRD EDITION

BY THOMAS KAMPS

THE MODELLER' S WORLD s e::e::- s R..

,

THIRD EDITION

BY THOMAS KAMPS

© Auflage 1995 by Verlag fiir Technik und I landwt:rk Postfach 22-·•. 76492 Baden-Bade n English Language © 1995 Traplel Publications Limitnl Translated from the original German by Keith T homas Technical suppon by Tom Wilkinson

© 2005 Traplet Publications Ltd

All rights reserved. All trademarks an fittc:d standard for full-size gas turbincs. Modem jet engines with a radial turbine . In the:- courst' o f time the radial turhave extremely complt:x compressors consisting of up to bine has been supt:rst:c.Jed almost t::n tire ly hr rhe axial 17 stages and even morc. The rt:s uh is an increaSt: in prc:stype . Evc:-n by the: 50s tht: radial turbine only survived sure of up to 30 times. occ:1sionallr in lo w-power shaft power engines. Howcver. The: r.tdial compressor is muc h simpkr in constmcfor modcl jct engines this typ e of turhinc could still bt: of interest. t ion and therefore muc h mo r c s u itable for model c:ngines. The air flows into thc wheel i11 the axfal dircction and is then flung outward b)' centrifugal force. On it:. The question of efficiency own this device is know n as a centrifugal compressor. We will n ow c onsider the p rocc:sses inside:- the:- gas t urOnce again a single stage con sists of a rotor and a stato r, bine somewhat m o re closc:ly. If wt: adopt tht: prot:t:!>s although the pressure:: inc rt·aSt: per stage is much higher described here , the: engint: can 111)' function if the turthan with an axial compressor stage. As a result gas turbine produces sufficient powe r to d rivt: the compressor. I ' nfortunatcly turbines a nd compressors are not zt·robines wi th radial comprt·ssors can often managc with loss machines. In each stage friction and turbulence only one stagc. absorb pan o f th e e n ergy an d w aste it as h t:at. To mini· Additional advantages of the radial comprcssor arc its robui.t nature and ih inhe rent rdiabilit)'. The disad vanm ise friction losses there musr he a gap betwee n the frontal a rea of th t· machine. Gas turbines tage" is the ｬ ｡ ｾ･＠ rotor bhtdes and the ho usinµ to avoid any danger of foulwith a radial compressor art' the refore always somew hat ing. This c lear.m et: tht:n allows a proportion o f th e:- f?.aS hulkT simply to slip past the rotor. 'fhe second conunuo us flow mach ine in thc gas turTo counter this problem and still kct:p tht: e ngine: runbine: ii. the actual turbine:. This can he visualised as a comn ing it is essential to kt:ep the tcmperature of thc gas prc:ssor " in reversc". The tu rbin e con vert s pressure and thcrcfort: its power capacity - high cnough to comenergy into rhc:- !>haft p ower which is require d to drivt: pt:nsate for th e losses. However. the permissible: gas temthe comprt:ssor. Sinct: the h ot gases contain much m ort: pc:raturc is not infinite ly high . ·ntl' maximum tt:mpt:r.tturc is limitt:d by the strenµth o f the materials used in tht: encrgy than the compressor absorbs, the:- system is selfsustaining. If the llnal t and its only task is to heat air by m e:ms of the combu stion of fuc::I. there are:: consider.tbk problc::ms involvt:tl in optimising th e:: doubk jets wit h ont: o pening for the idle into the primary zont' under high pressure. In de\·t:loping range and additional injc::ctor cross-section al area for full this technology Schreckling confronted many and va rious throttk. p roblems. but h is experimental work certainly producc:d Nt:vt'rtheless, direct fuel injection app,-ars to Ix: ft:asia workable system. bk for small engine::s. Because of the high tempt·ratures to which th e c.:oil of In small profession ally-made:: gas turbines a simple tubing is subjected, it is not possible to solder injec to r hut very effective solu tion has heen adopted: fuel is jets to the tubing, which means that the e ntire injec tion injected into the combustion chamber through the h o lｰ ｲｯ｣･ｾｳ＠ must take place: by means of accurately cut low rotor shaft. The fud is pumped t hrough the comholes a.lone. The length and arrangement ot tht' vaporipressor undt:r low prt:ssure. then into the e n gint:'s ser an: crucial, and musl be "just so". lf the coil is too revolving shaft. At the a ppro priate p o int it passes in a s ho rt, or located in th e cold area, too much fuel lt:aves finc:ly atomised for m int o the combustion chambt'r the vaporiser in liquid form, with poor c.:ombustio n and through small openings, whert:by the spinning s h aft a wake o f tire str eaming behind the t:ngine the net works as a ct:ntrifugal pump. l11e adv:mtagt: of th is technology is it s simp licity. The atomiser com: is exactly circular, which promotc::s even tempera ture distri bution. Even at low rot ational speeds th e process results in fine atomisation of the fuel. The crucial drawback of s haft inject ion is the complex air p ath through the enwne. The: combustion chamber must be immediately adjacent to the ｾｨ｡ｦｴ Ｌ＠ and this a rrangemen t closes off the air supply to the inside of the combustion chamber. It also makes it ｩｭｰｯｾｳ｢ｬ･＠ to use a shaft tunnel t o Fuel iP{jection by m eans of booked tubes. strengthen the engine . l11e Turbomeca Marbore exploits this injection technology. a nd in this case air flows into the internal space o f the engin e through hollow guidt: blades. This is an intt:rt:sting solution, hut raih e::r complex for our purposes. In the model sp here fuel vaporisation systems are gene rall y used. ln principle: these systems are:- simple h eat exchan ger.> whic h feed part of the heat of combust ion to th e fuel. However, these ｾＱ ﾷ ｳｴ｣Ｚ ｭｳ＠ are not as efficient as the term · vaporisor·

,Hodel.fet Engines

61

Fro111 pan of t/Je comb11slion dmml>e•· witlJ six /Jcx>ke1l t11bes.

result. If the vaporiser is too long the te mperature tends to rise un comforrnhly h ig h , w ith the following result: w he n the th rottle is closed tht: fuel heats up to a point above its thennal srnhility (for .JPl n 1issembk d sound , at w hich p oint t he 11eedle should you should immediate ly Parl 15.4 lie inside of a stick open the t hrottle a little fu rther and switc h the '.f!J Bing pressure w ith prior experie nce of jet engines to b uild a m odel je t should not exceed o.: bar. With other special heat-resise ngine based o n an y rurhocharger rotor. This approach tant materials you should call a halt at a maximum o f 1 bar to pre!'>t:rve the bearings. a:. th b figure corresponds to exploits the fact tha t m ost c(lmpressor w h eels of this t ype usually exh ibit similar geometry, and the refore their around 105,000 rpm at s tandard temperature and pressure. charncteristic values are also similar. O f course, the formulae stated h ere cannot be expected to coincide exactly Once you have completed a few rest nms, starting the w ith the throughputs ant! pressure:. p roduced hy differe ngine becom es pure ly a matter of routine. The imporent w h eds. For this reason I cannot guarantee that the t ant point is to acquire a feelinA fo r when the fan is needed, and w hen the turbine is able:: to run up to speed g;is turbine you make w ill n ecessarilr work. That is w h y I apcc::d for an y c ross-sectio n al area. Tht:sc: formu lae a re p artic ularly use ful in so far llS they allow us to d tec k the compressor's supply valu e . II c lln also he productivt' a nd w o nhwhilt: to t'stah lish th e e ngine 's fue l con sumption. All you ne::e::d to do is set up i1 calihratt'd cylinder as a fut'! 1:111k, the n )' O U c an use a st o p wa td1 t o m t:as ure con s u m ption vt:ry a c curately u ndc:r d ifft:rt:nt Opt'rat inp. cond itions. T o find the ac tual consumption figure for model flying we just h ;1ve to multiply the fue l volumt' by t he c o rrt:spon ding fluid d t:nsity. This g ives a ｾｯ ＼ｬ＠ idt:a of tht: size of fue l tan k you will net'.'d in you r modt:I. Ano the r in teresting v:1lue:: is specilk fut:I con sumptio n, w h ich tells us h ow man y kilogr.tmm es o f fue l are:: c onsumed pt'r h o u r anc.l pt:r Nt:wton of t h rnst. This va lut' w ill \•ary w id t:ly acco rdi n g t o t ht' e ng ine ' s r o t a ti o n al

1n = A xp xC :tpp lit•s at the outlet of the e x haust cone , and the:: gas de nsity can be cak ulatnl from t'ht.' m easure d e::xhaust tt:mp th at a sim p k formul a c a n be deri\'nl from tht:st: t:quations w givt: c ngint: th roughput:

SUMMARY OF ESSENTIAL MEASURED VALUES AND THE FORMUI..AE FOR CAI.CULATING THEM Paran1e 1er

Formulae

Unit

Peripheral spt:ed :

m/s

l'res!>urt: ratio : C.;1!> c.lensirr

p=P 1 T I R

Through p ut:

ｲｩＱ］

Outflow spt:t:d :

kg/nH

ｦ Ｏ ｣ ］ｾ

Ｈａ

c =F / m

］ ｾ Ｈｆ

ｉ＠

ＮＬ ｆ ｸ ｰＩ＠

kg/s

A 'p

m/s

Spt.·citk fud consumption:

kwN.h

Jel power:

Watt

13umi ng e flkien q •:

1]., =( I',.

+ 1i1 xc p x (T 1 -

Pressure: kvt:I (com pressor):

VI= 2 x CP xT 0 xC1r" 0 '" -

Spt:cifk thrust:

F/Eng ine mass

T 11 )) / (ri1.,

xh0u )

I) I u !

Measured paramete rs and co n stants n = Rotational spt:t:d P,; = Excess housing pn:ssurt: P11 = Atmosphe ric p n:sMtrt: A Nozzle: cro!>.vsectional a rt:a F Engine thrust

rpm Pascal (N/ml ) I Pa= 0 .0 I m bar Pascal (N/m!) I Pa = 0 .0 l mbar m ! (See d escription ) N

ril = l'ud Consumptio n T , = Exhaust gas tt:mperat ure T0 = Inlet Tt:mperature R Gas constant for air Cr = Specific heat of air h0 ., = Sp t•ci!k heat of fuel

kg.ls Kdvin Kelvin 287 J/kg/K 1000 ) /kg/K -B 3 MJ/kg (for J et A I la:ro!>t:lle )

= = =

J\lodelJd CllJ!.i11es

speed. as specific consumption is much lower at higher pressure ratios and efflux speeds. Neverth eles.o; it remains true that a model jet engine at full throttle requires two o r three times as much kerosene per Newton of thmst as other engine., of comparable size. It is even possible to relate the quantity of heat which is fed to the air in the combustion chamber to the ｣｡ｬｯ ｲｩｴ ｾ＠ ic value of the fuel used. This calculation gives us the efficiency of th e fuel burning process in the combustion chamber, w he reby the converted calorific power corresponds to the sum of the power from exhaust h eat and jet power. The burning efficiency of the Mii:ro-Turbine rises w ith increasing rotational speed and reaches just over (X)% at full throttk, taking into account measuring inaccuracit:s. Thus about I 0 % of the fuel leaves the engine unused. Industrial miniature gas n1rbines achieve a burning e::fficiency of more than 99.5%, so there is certainly scope for improvement.

Using jet engines in model aircraft Fundamental special features In comparison with propeller engines and powerful impellers (ducted fans) the thrust produced by the model jet e ngine seems to he on the low side. At takeoff the mo A =

mI c Ip = 0 . 1">I40/ 1.22">

=0.00306m' = 30.6cm ' 111 c p

Engine th roughput at full throttle in kg/s Maximu m inkt speed (here 40 m/s) Air d e n si t y (under normal conditio n s l.22"> kg/m 3)

that sp eed more than 60% of th c:: c::nergy h as a lready been converted I into pressure]. W ith a semi-scale o r scale modc::l you a re bound to t he full-size machim:·s intake size. but w ith a sp o rts or ex p erimc::ntal mode l you can incorpo ra te anr ty p e and size o f air opening. TI1e id eal fo rm o f inlet for a mod e l jet would then be w ha1 is known as a ventu ri. w hich consists of a ro unded n ozzle opening followed by an integral difft1ser. This form of inta ke gives good rc::.'>ults in most flight s ituatio n s a11d does not incur a serious p ressure los.s. The airflow speed at the narrowest point can the n be t u ned to corresp ond to the mmld'i. c::xpected maximum airspeed.

Cooling the fuselage Since model jet engines an: no t usually w hat we might' call lightweight. they usually h ave to he installc::d d ose to the mo del's Centre of Gr.ivit)'. As a result it i.'> virtuall)' inevitable that delicate pans o f t he model e n d up close to the h ot exh aust gas flow. Good layouts for jet-engi nell model aircraft therefore include tr pes w ith the cc; a long way back. and especially flrin A ｷｩ ｮ ｾ｣［＠ and Ll11gs.

g rllS!> is al1•etuly r•isible. ( Pboto: A 111ta va11 de

Goor). Controlling the