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Energy Balance of a High- Pressure Sodium Arc Tube

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Energy Balance of a High- Pressure Sodium Arc Tube Energy balance of a high- pressure sodium arc tube H idezoh AKUTSU* and Naoki SAITO** This paper describes in detail the dependence of the energy batance of a high･pressure sodium arc tube on discharge parameters. The total radiation and the thermal conduction loss of the arc tube were estimated by calculating the thermal dissipation loss from the measured wall tem- perature of the tube envelope. The results thus estimated showed a rea- sonable consistency with experimental data on the absolute visible radiant power and the published results by previous authors as well. in the low sodium vapor pressure region, the total radiation Pr (W/cm) is influenced sharply by various discharge parameters, while in the high sodium vapor pressure region (above about 70 to 100 Torr), it is deter- mined mainly by the power input Pin (W/cm) through the linear equation Pr;~~0.80 (Pin-6.7) for a sapphire arc tube. It has been concluded that the high efficacy of a high-pressure sodium lamp stems from the low thermal conduotion loss of its arc tube, as well as its high luminous efficacy of visible radiation. properties of the arc tube. Furthermore, a gen- 1. Introduction eral guidance fcr improving the luminous ef ficacy Knowledge about the energy balance in an arc of a high-pressure sodium lamp is shown. tube of a lamp will offer a foothold not only for understanding the processes occurring in the arc tube also fcr improving its luminous efficacy. The 2. Experimental procedures energy balance of a high-pressure sodium arc tube Fig. I illustrates the construction of arc tubes has been studied by several authors ;i)~) theoreti- employed in our experiments. The tube envelope cally, through calculating the radiant power and was made of sapphire, with inner diameter 8.0 mm the thermal conduction loss on the basis of the and wall thickness 0.8 mm. Electrodes were sealed temperature distribution in the arc tube, and ex- with a gap of around 80 mm between the tips. The perimentally, through zneasurement of the absolute sodium amalgam having 66% sodium mole rate radiant power from the arc tube. The obtained was introduced in the arc tube at a fixed weight results have not shown the whole aspect of de- of 20 mg. Xenon or neon-argon Penning mixture pendence of the energy balance on various dis- was also put in the arc tube as a starting gas at charge parameters, partly because of relatively 2 different pressure levels, 4 and 30 Torr. The limited ranges of the discharge parameters chosen thermal protective layer of tantalum foil was ap- in those studies. In this paper another method for analyzing the energy balance is discussed, in whieh the radiant -80 9 l~ power and the thermal conduction loss over a wide o range of each diseharge parameter are estimated through calculating the thermal dissipation loss 3 2 984 56 7 from the measured wall temperature of the tube J SQpphire Tube 2. envelope. The estimated results are examined in Alumino End CGP comparison with experimental data on the radiant s- Nb - Tube 4. W - Coil E ectrode 5. Sodium ArnolgGm 6, Xe-Gas or Ne-0.5eleAr Mixture * Research Laboratory 7. Thern~GI ProtecNve LO yer ** Light Division 8, Coldest Spot 9. pt - pt, 13alcRh Thermocoup!e Matsushita Electronics Corporation, Takatsuki, Osaka, Ja pan Fig. 1 Colestructio,e of the are tube. J. Light & Vis. Env. Vo!. 3 No. 2 1979 ll plied at both ends of the arc tube to adjust the I 300 o Na -ernolgem 20mg ( Na 66mole Qlo) cold-spot temperature, viz. the temperature of the coldest spot which exists inside either one of the ~ ~~~~t~)~~/ Pewer Input Pifl (W/cm) ends of the arc tube. It should be noted that, in 200 our experiments, the sodium and mercury vapor o 5~(~e~ ¥:~',~ ~~ pressures inside the arc tube were determined only ~ by the cold-spot temperature, as the sodium amal- ~t~~~1~ ¥~Q~¥ tS~ 'b:0~1" gam composition was kept the same. When iumi- i OO nous output and spectral distribution were meas- ¥~~~ ured the arc tube was mounted in an evacuated ~ E ¥ ~"'e~" outer-bulb. ~ Arc tubes were operated horizontally on 60 Hz l OOO ¥.,. ,..~ ac supply with choke ballast in series. Each tube ~ Xe 50 Tor~ was measured for its wall temperature, its lumi- 'e-o" Xe 4 Torr nous output and spectral distribution against two 900 variables ; power input and cold-spot temperature. 650 700 These 2 variables were independently adjusted by Cold600 - Spo? Temperature 750 ('C ) 800 changing the dimensions of the thermal protective layer. For measuring the wall temperature the arc Fig. 2 Wall teemperature as a fut~ction of tube was set up in an evacuated demountable ap- eold-spot tev~rperature at the ce?~tral paratus, with a Pt-Pt, 13%Rh thermocouple (di- poi,et of the (~rc tubes filled with ameter 200 ,~) cemented at the measuring spot of xenon. the tube surface. The wall temperature T~,* at the spot confronting the coldest spot inside the arc I 300 tube was measured with the assumption that it oNe-emeleem 20mg (NQ66melee/*) represented the nearest value of the cold-spot tem- O perature, as shown in Fig. 1. All the measured ~S¥ values were reproducible within a maximum :200 - o ¥ ¥~:~~ ~~' _~~__e--j~~~~ spread of :t:5'C. ~~ h ¥ 1~q.¥ ~~c~_~~ /~~~.~ 3. Results oa b¥ ¥~e¥.eL. 11,~~~ The experimental data in this paper were ana- ¥e ¥¥ ¥ lyzed mainly as a function of the cold-spot tem- ~ ~~_ l; 8 oo ~:s Power ~~p:Jt Pin (W'er~~ perature T,~... Based on experimental results of ~ ':4~ Ozaki5) on the relationship between the sodium vapor pressure and the self-reversal width of ~ 4~- Ne-o.5S<,Ar 30Torr -o*e- Ne-0.50/.Ar 4Torr broadened sodium D-lines, the cold-spot temper- 90 o ature T~,, of 650 to 750'C in our experiments seems to correspond with sodium vapor pressure of 20 to 600 6 so 700 7 50 800 80 Torr. coid -Spot Tempe~a tu~e ('C ) Fig. 3 Wall te,?rper(~ture as a function of cold-spot temperature at the central 3.1 Luminous efficacy and wall tem- poi,et of the arc tubes filled with perature ,eeo,e-argo,~ mixture. Fig. 2 shows the wall temperature T~(O) at the central point of the arc tubes filled with xenon, as difi:erence between the arc tubes filled with xenon a function of the cold-spot temperature T~,.. The and neon-argon was such that the wall temperature power input per unit length Pi~ which will be de- T~(O) went higher with the increase of neon-argon fined later on was chosen as a parameter. Within filling pressure and, even in the region of high the region below the cold-spot temperature T~,, of cold-spot temperature, it still continued dropping 700'C, the wall temperature T,.(O) at a fixed power in different modes depending on neon-argon filling input dropped rather sharply with the rising of the pressure as the cold-spot temperature was raised. cold-spot temperature, and took a slightly lower Then these tubes were fabricated to make lamps. locus to the increase of xenon filling pressure from The overall luminous efficacy eyL and the luminous 4 to 30 Torr. Then, as the cold-spot temperature efficacy per watt of visible radiation K of each lamp was raised higher still, the wall ternperature T~ (O) were obtained from the measurements of its lumi- approached a given level for each power input, nous output and spectral distribution, respectively. irrespective of the xenon filling pressure. Fig. 3 Its visible radiant efficiency ~.~ was calculated by : shows the wall temperature T~ (O) of the arc tubes filled with neon-argon mixture, which intrinsically ~･~= vi/K ･ ･ ･ ･ ･ ･ ･ ･ ･ ･ ･ ･ ･ (1) gives a higher wall temperature than xenon. The Figs. 4 and 5 show the obtained radiant proper- 12 J. Illum. Engng. inst. Jpn. 舛o oNo－omotgom20mg〔封G66mole％l o至thew呂ll七e瓢perature．掛hisindic我tes捻att盗e oXe50Torr wall temperature is useful as a 搬eas縫re for ana－ シ 蝸、κ＼ 1yzing t負e energy b＆1ance in the arc t厭be． E 130 考＼ E 3、2 ↑o看a箋rad韮a重韮on and漁erm…蝸 J ω 120 co轟d“cl韮on望OSS η之‡マrv・K Poweゴ1鵬ut PinlWlc鮪 →←r〆一；～58．5 T難e eiectyical power iBp就P伽supPlied per unit 一悼：～46．5 lengt難 of the arc t駁be まs 最rst traゑsferred to the o l lG 瞬一1～34．5 駐rc plasma並the＆xiai core portion of the tube、 O，36 Almost all o妻the tr＆nsfeyred powerまs co強sidered タ ＾420 旦 一K ／／F一 to be delivered to the tube w段ll，except in the vicinity of t虹e electrodes，by various processes such ⊂400 0，34 ＆s radi＆tioR，the℃m＆1 con（luction and di登usion o£ ≧ 石 ！ ch＆rged or excited pεtどt重cles． T｝1e energy b＆lance ① α：380 ㌦ 0．32 equεしtion on 七簸e tube envelope for unit lengtL of ＼ t｝注e翫rc t摂be t五us becomes： む 3560－ 暑＼ ・o．30 ω ｝ P搬＝Pr＋P麟｝側……一………・一…………（2｝ in which PT a豊d p翫，加（ienote t蓋e tot31r＆di鼠tioB per 028 E 340 u麟lengthandt鼓ether撚aHossd重ssまpatedfro加 650 700 ア50 施et騒beenvelopeperunitlength，respectively，and Pt1、，船isgivenby： Cold－S轟ot TefnperGtロre ｛。C l P傭，御＝Pc季切十P7，盟 Fig．4 Rα識α？鴬斜¢0ηθゲ撹θSαSα∫％冗0一 一3・え（κ）d2釜婆）＋πφ・σ・ε（」じ）・姻4…（3｝ 玩0π0／ooJ4－Sρ0言 むθ？πpε？ヤα重％ゲε 0∫ 古んε αゲ0 む％bθ ガ泥θd ω茗亡1乞 where P。，，．is the thermal cond縫ction loss to the end の6？30？30 direction through the tubing w呂11（sapphire）per unit｝ength，P．，，，is塩e thermal radねtion loss from oNo－omαgom20mg｛NG66mo1豊％｝ the tube e簸velope pey駿niも1ength，x is t蓋e distance 120 o鯉6一〇・5％Ar 30Torr x＿ ン x1x’xイ £rom the central poiηt to施e end direction of the ε uO ／ tube，Tω（x）is the wall temper我撮re我t distaRce」‘， 8 重s t難e cross－sectional 翫rea of the t級bing wailン iOO ノ ノ 入（x） and ε（x） are the thermal conductまvity εm（i E… 田 the emissivity， yespectively， of the t罎be envelope go J at （iis七＆nce κンσ is Ste賃an－Boltzmann consta盤t and φo is t溢e o級ter di我狐eもer of the tube． 80 o 艸：～33，0 丁簸etot段1r呂di駐tioゑper臓nitlengt簸P．c3nbe obt我童ne（i fro磁Eq雛εしtion（2） by giving both values， 440 o，35 》 pi札我ndP漁，”丁烈ev＆lueP｛帆三nW／c瓢wasesもimated
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