Valve Lift Measurement By Optical Techniques in Compressors

Transcription

1 Purdue Uiversity Purdue e-pubs Iteratioal Compressor Egieerig Coerece School o Mechaical Egieerig 22 Valve Lit Measuremet By Optical Techiques i Compressors G. Buliga Electrolux Compressors N. Paoe Uiversità degli Studi di Acoa G. M. Revel Uiversità degli Studi di Acoa E. P. Tomasii Uiversità degli Studi di Acoa Follow this ad additioal works at: Buliga, G.; Paoe, N.; Revel, G. M.; ad Tomasii, E. P., " Valve Lit Measuremet By Optical Techiques i Compressors " (22). Iteratioal Compressor Egieerig Coerece. Paper This documet has bee made available through Purdue e-pubs, a service o the Purdue Uiversity Libraries. Please cotact epubs@purdue.edu or additioal iormatio. Complete proceedigs may be acquired i prit ad o CD-ROM directly rom the Ray W. Herrick Laboratories at Herrick/Evets/orderlit.html

2 C13-5 VALVE LIFT MEASUREMENT BY OPTICAL TECHNIQUES IN COMPRESSORS Gialuca Buliga, Electrolux Compressors M.E., Seior N&V Researcher via Cosorziale 13, 3317 PORDENONE, ITALY; Tel.: ; Fax: Nicola Paoe, Gia Marco Revel (*), Erico Primo Tomasii Dipartimeto di Meccaica, Uiversità degli Studi di Acoa via Brecce Biache, 6131 ANCONA, ITALY, Tel.: , Fax: *Author or Correspodece ABSTRACT The paper addresses the developmet o a o-ivasive measuremet method or valve motio i hermetic compressors or domestic rerigerators; the mai aim is vibro-acoustic comort improvemet. The methodology developed is based o a sigle poit laser Doppler vibrometer. These tests required a special compressor case equipped with trasparet widows or optical access to the valves. Mai metrological problems are related to optical access: the paper discusses the eects o optical sigals o a variety o disturbig iputs ad evirometal eects, which aect sigal quality ad measuremet ucertaity. Eects o beam icidece agle, valve surace roughess, depth o ield ad oil used or lubricatio are aalyzed ad miimized. NOMENCLATURE : reractio idex T: temperature ψ,ϑ: icidece agles λ: laser wavelegth a : Doppler requecy shit v: valve velocity LDV: Laser Doppler Vibrometer DOF: Depth O Field INTRODUCTION Compressor valve desig has a eect o the ollowig compressor perormaces: 1. Thermodyamic eiciecy. Both static ad dyamic mechaical properties o a reed valve aect the lit uctio uder give luid-dyamic orces, hece the gas low at opeig phase ad ially cylider illig ad COP. 2. Reliability. No part iside a sealed shell is accessible; thereore ay ailure causes the whole compressor replacemet. Valves are strogly stressed to atigue ad must be desiged to last or the whole compressor lie. Reed valves lie is related to the umber o atigue cycles doe or each thermodyamic cycle. The umber o atigue cycles per thermodyamic cycle is give rom the opeig ad closig motio o the valve ad the other superimposed vibratios. 3. Noise level. Vibratios arise rom valve iteractio with valve-seat, lit-stop, rerigerat ad oil-layer are trasmitted via solid structure or gas ad cotribute to oise geeratio. Numerical modelig is used to evaluate the correlatio betwee valve desig ad compressor perormaces. I order to validate ad tue a model o the valve motio ad valve vibratio uder give luid-dyamic orces, valve motio at workig coditios should be measured. At preset o straightorward measuremet techique has bee proposed or compressor valve lit motio. The ideal method should be o-ivasive, reliable ad lexible. Mai problems are: Valves are small ad light parts: o cotact trasducers ca be attached to them without chagig their behavior; Valves are ot directly accessible; The space available or trasducers - without modiyig head or compressio chamber volumes ad thus operatig coditios - is small; The eviromet is harsh, due to temperature ad oil droplets movig with the low; The duratio o valve opeig ad closig is a ractio o a cycle, i.e. a ractio o about 2 ms: too much a short time or may trasducers, due to their requecy rage;

3 Measuremets must be accurate over the whole valve lit, typically rom to some mm s, with the capability o resolvig small bouces o the order o 5 µm; I literature some works ca be oud or measuremet o poppet valve kiematics, by eddy curret proximity sesors. The disadvatages are: o-liearity, decreasig sesitivity with distace, calibratio curve ilueced by temperature, possibility to measure oly the base o the valve (to locate the trasducer away rom the lux), low spatial resolutio. I [2] iber-optic displacemet sesors were used to measure valve motio i a VIP (Valve i Pisto) compressor. The istallatio o the iber budle eeds small seat holes o the cylider head. Video cameras - with the aid o stroboscopic light - were also recetly proposed i [3] ad tested successully. I this case two edoscopes were istalled iside the compressor. The mai limitig actors are the complex experimetal set-up, the itroductio o additioal dead volumes caused by the edoscope seats, the eed o steady-state operatig coditios, the low samplig requecy ad the eed o large memory usage ad itesive image processig. The advatage o usig a optical istrumet such as a Laser Doppler Vibrometer (LDV) is extesively documeted i [1]. Based o the available experiece reported i [1], a approach based o a sigle-poit LDV has bee developed i this work. A LDV is a istrumet which ca measure idepedetly oe compoet o velocity ad displacemet o a movig solid surace by meas o the Doppler eect o a scattered light wave [4,5]. Velocity ad displacemet are measured alog the optical axis o the system with o cotact. A major characteristic o these sesors is that they do ot require calibratio ad their output does ot strogly deped o target material ad o surace coditios. Their use or measurig valve kiematics is well justiied by several reasos like dyamic rage ad badwidth. Resolutio ad accuracy are superior to ay other available sesor ad allow the use o oly oe istrumet to measure the maximum valve lit ad bouces. Measuremets are take keepig the laser head away rom the object: the oly eed is optical access to the poit. TEST BENCH The ZEM GQY 75 AA model chose or tests is a part o the high eiciecy compressor amily or R134a rerigerat. It is a hermetic sigle-cylider, alterative compressor or low back pressure household rerigerators drive by a iductio motor at a omial speed o 2915 rpm. Bore x stroke measures are x mm. The compressor housig was modiied by isertig quartz widows, allowig a optical access or the laser beam to the valves surace. Two widows are eeded to provide optical access i a hermetic compressor: the irst oe lets the beam eter the exteral case o the compressor, while the secod oe is eeded to cross the compressor head. Such widows do ot alter sigiicatly the geometry o the machie; thereore ivasio is kept to a miimum. This was veriied measurig compressor thermodyamic perormaces ad comparig them to those o a stadard compressor. DISTURBING INPUTS DUE TO OPTICAL ACCESS The mai problems cocerig optical access are due to: 1. Light scatterig ad relectios by optical widows; 2. Light scatterig by lubricatio oil. The irst questio is related to the act that each optical iterace scatters light back to the laser vibrometer: the vibrometer will the measure the iteraces motio ad ot the valve motio, as the sigal itesity scattered by the valve is too low i compared to the lat widows oe. Two strategies must the be adopted: the scatterig eiciecy o valve surace must be maximized ad a proper optical aligmet must be obtaied. Figure 2 highlights the dieret optical characteristics o smooth versus rough suraces, whe hit by a collimated laser beam. Whe surace roughess is icreased, a trasitio rom specular relectio to diused scatter is observed [9]. The two lat optical widows have two relectig suraces each, which relect about 4% o icomig power i the visible rage. Also the valve s surace, beig a thi metal oil, shows a relective character. However, it ca be treated to icrease its surace roughess, thus chagig its behavior ito a diusive surace. This does ot modiy its uctioalities, but allows the solutio o the irst over metioed problem: i act, as the valve scatters the light i a solid agle, the measuremet ca be perormed without a perect ortogoality betwee surace ad beam. This immediately produces two advatages: a. The laser beam ca be put at a agle so that light relected by the lat widows does ot reach ay more the vibrometer head;

4 b. The light scattered rom the valve always reaches the vibrometer, eve whe valve rotates durig lit. Figure 3 shows the test bech developed to assess the eectiveess o such a approach. A compressor valve is put o a optical table ad aliged to the laser vibrometer; the two optical widows are iserted i rot o the valve to reproduce the optical access coiguratio. Each compoet ca be tilted at a agle. The valve was moved by a stiger put o a electro-dyamic shaker, excited with a step sigal. The velocity measured by the vibrometer was acquired. With the aid o the above described test bech the optimal agles were oud. I such coditios the valve motio is measured correctly, as i the widows were ot preset. Oil particles i the gas ad oil ilms o the widows the secod problem ca compromise the optical access to the valve ad the icreased light scatterig rom the widows will agai compromise the sigal to oise ratio. A way to overcome this problem was to create a protectio scree or the laser beam path iside the compressor shell. Such a scree cosists o two coaxial little copper cyliders, mouted as show i Figure 4 ad does ot modiy the compressor luid-dyamics. It oly prevets oil droplets to iteract with the laser light ad to orm a ilm o the widows. Fially aother questio to poit out cocers depth o ield (DOF). All optical systems have a rage aroud the ocal distace i which objects are still i ocus. DOF depeds o the ocal legth ad the diaphragm aperture D. I our case the DOF has to be miimized i order to isolate the valve respect to the ear widows. This ca be obtaied by icreasig the diaphragm aperture or decreasig the ocal legth, avoidig the receptio o scattered light which icreases oise. DOF should i ay case be larger tha valve lit. EFFECTS OF THE WORKING FLUID ON LDV SENSITIVITY The compressor workig luid has a dieret reractio idex ( Ñ F respect to air ( ~ F: it aects LDV sesitivity, so that velocity data eed to be corrected. The model show i Figure 4 is cosidered. Sell s law describes the optical path o the LDV beam: a siθ a= siψ (1) Ñ should be evaluated at the workig coditios i the compressor housig, because: T [ ( T ) ] T ( T ) = 1+ (2) 1 where T is the gas temperature ad T is a reerece temperature. LDV sesitivity depeds o Ñ [1]. I act, laser wavelegth i the luid is: λ = λ a (3). a LDV sesitivity i air is: D 2 k' = = (4), v λa beig a the measured Doppler shit ad v the target velocity. Iside a dieret luid it becomes: D 2 2 a a k " = = = = k' (5). v λ λa Thereore, i a luid dieret tha air, LDV measures with a dieret sesitivity k. Usig sesitivity k all the same, it will be observed that: v measured = k' D = k" D = vreal (6). a a Thereore measured velocity has to be corrected; the ial correctio is i eq.7, takig ito accout also the agle o icidece ψ: v( t) measured a vreal ( t) = (7). cosψ SIGNAL ACQUISITION AND PROCESSING The aalog displacemet sigal is digitalized by a 8 bit AD coverter at a samplig requecy always larger tha 1 khz. Figure 5 ad Figure 6 show some resultig measuremets.

5 Valve lit traces show a ast valve opeig, ollowed by damped oscillatios aroud maximum valve lit, ad the the valve closes. This procedure has bee applied successully both to the itake valve ad to the discharge valve (see Figure 8); measuremets have bee take i dieret workig coditios o the compressor (see Figure 7); moreover it was possible to measure valve motio durig trasiet (see Figure 1). Compressor was also equipped with covetioal seismic accelerometers (see Figure 9). I order to assure realistic coditios, gas temperature at suctio ad oil temperature were cotrolled ad rerigerat pressure at oticeable positios was moitored. CONCLUSION A measuremet method or compressor valve lit based o a sigle-poit laser Doppler techique has bee developed. The paper has discussed the eects o optical disturbaces o the LDV sigal; sigal to oise ratio i operatig coditios has bee maximized by proper aligmet o the laser head ad by protectig the laser beam path rom oil. I such coditio the valve motio is measured correctly, as i widows ad oil droplets were ot preset. The measuremet method provides iormatio without relevat modiicatios to compressor s geometry ad to its characteristic volumes; thereore it is ar less ivasive tha ay other measuremet method proposed up to ow. Measuremets have bee take i various operatig coditios icludig trasiet. ACKNOWLEDGMENTS The authors wish to thak Ig. C. Gradozzi ad Ig. F.Sbarbati or their cotributio to the experimetal measuremets. REFERENCES [1] N.Paoe, C.Satolii, E.P.Tomasii, "Applicatio o a laser Doppler vibrometer to evaluate egie poppet valve kiematics", 12 It.Modal Aalysis Co., Hawaii, geaio [2] B.G.Shiva Prasad ad D. Woollatt, Valve dyamic measuremets i a vip compressor, Proc. o 2 Iteratio Compressor Egieerig Coerece at Purdue, July 25-28, 2, West Laayette, Idiaa, USA. [3] A. Ludu, A. Betto, G. Reger, Edoscope video o compressor valve motio ad pressure measuremet assist simulatios or desig improvemets, Proc. o 2 Iteratio Compressor Egieerig Coerece at Purdue, July 25-28, 2, West Laayette, Idiaa, USA. [4] L.E. Drai, The Laser Doppler Techique, Ed. Wiley, New York, 198. [5] P. Castellii, G.M. Revel, E.P. Tomasii, Laser Doppler Vibrometry: a Review o Advaces ad Applicatios, The Shock ad Vibratio Digest, Vol. 3, No. 6, November 1998, pp [6] C. Gradozzi Laser Doppler measuremet techiques or the experimetal studies o valve lit i hermetic compressor, degree thesis, Acoa 2. [7] R. Marsili, L. Pizzoi ad G. Rossi Ucertaity aalysis measuremets o tools iside luids by laser Doppler techiques, XXX [8] N. Paoe ad G. M. Revel Problems i vibratio measuremet by laser techiques through combustig lows, 2 d It. Co. Vibratio measuremets by Laser Techiques: Advaces ad Applicatios, pp , SPIE vol. 2868, Acoa [9] P.Beckma, A.Spizzichio, The scatterig o electromagetic waves rom rough suraces, ed. Pergamo Press, Oxord, [1] R.Marsili, L.Pizzoi, G.L.Rossi, Ucertaity aalysis o vibratio measuremets o tools iside luids by laser Doppler techiques, Proc. 3 rd It. Co. Vibratio measuremets by Laser Techiques: Advaces ad Applicatios, pp.92-1, SPIE vol. 3411, Acoa, 1998.

6 Figure 1 Experimetal setup. Figure 2 Scatterig eiciecy o suraces: a) relective surace, b) real rough suraces. Figure 3 The optical test bech. Figure 4 Fial coiguratio with tilted optical axis ad laser beam protectio rom oil spray.

7 LDV θ a WINDOW ψ VALVE Figure 4 Optical path through a widow ad 2 dieret luids. 1 x acq. N 1 acq. N 2 acq. N agle re. TDC [deg] Figure 5 Three measuremets doe at dieret times, with same operatig coditios, o suctio valve. 2 x acq. N 1 acq. N agle re. TDC [deg] Figure 6 Two measuremets doe at dieret times, with same operatig coditios, o discharge valve.

8 1 x /55 C 23.3/55 C 1/55 C agle re. TDC [deg] Figure 7 Suctio valve displacemet at dieret evaporatig/codesig temperatures. 2 x Suctio valve Discharge valve agle re. TDC [deg] Figure 8 Suctio ad discharge valve displacemet superimposed o the same graph.

9 15 1 crakcase acceleratio [mm/sec2] agle re. TDC [deg] Figure 9 Acceleratio simultaeously measured o the compressor crakcase by a covetioal piezoelectric trasducer. 1 x Suctio valve durig start agle re. TDC [deg] Figure 1 Suctio valve motio durig start.