Evaluation of Wettability of Solid Surface with Oil/ Refrigerant Mixture

Transcription

1 Purdue Unversty Purdue e-pubs Internatonal Compressor Engneerng Conference School of Mechancal Engneerng 16 Evaluaton of Wettablty of Sold Surface wth Ol/ Refrgerant Mxture Mtsuhro Fukuta Shzuoka Unversty, Japan, Junk Sumyama Graduate school of engneerng, Shzuoka Unversty, Masaak Motozawa Shzuoka Unversty, Japan, Akfum Hyodo Graduate school of engneerng, Shzuoka Unversty, Tadash Yanagsawa Shzuoka Unversty, Japan, Follow ths and addtonal works at: Fukuta, Mtsuhro; Sumyama, Junk; Motozawa, Masaak; Hyodo, Akfum; and Yanagsawa, Tadash, "Evaluaton of Wettablty of Sold Surface wth Ol/Refrgerant Mxture" (16). Internatonal Compressor Engneerng Conference. Paper Ths document has been made avalable through Purdue e-pubs, a servce of the Purdue Unversty Lbrares. Please contact epubs@purdue.edu for addtonal nformaton. Complete proceedngs may be acqured n prnt and on CD-ROM drectly from the Ray W. Herrck Laboratores at Herrck/Events/orderlt.html

2 54, Page 1 Evaluaton of Wettablty of Sold Surface wth Ol/Refrgerant Mxture Mtsuhro FUKUTA 1 *, Junk SUMIYAMA 2, Masaak MOTOZAWA 1 Akfum HYODO 2, Tadash YANAGISAWA 1 1 Shzuoka Unversty, Department of Mechancal Engneerng, Hamamatsu, Shzuoka, Japan , fukuta.mtsuhro@shzuoka.ac.jp 2 Shzuoka Unversty, Graduate school of Engneerng, Hamamatsu, Shzuoka, Japan * Correspondng Author ABSTRACT In order to reduce an ol amount dscharged wth refrgerant from a refrgerant compressor, ol separaton from the refrgerant n a compressor shell or an ol separator has been nvestgated by many researchers. The ol separaton n the compressor shell or the separator strongly depends on droplet sze of ol mst generated n the compressor and condton of ol flm formed on an nner surface of the compressor shell or the separator. The ol mst dameter and generaton mechansm of the ol mst are affected by surface tenson of the ol, whle the condton of the ol flm on the nner surface s nfluenced by wettablty of the surface wth the ol. In addton, dssoluton of the refrgerant nto the ol changes the surface tenson and the wettablty of the surface. In ths study, the surface tenson of the ol/refrgerant mxture and the wettablty of the sold surface wth the ol/refrgerant mxture were examned as a frst step of the study on the generaton mechansm and the droplet behavor of the ol mst n the compressor. The surface tenson of PAG ol/co 2 mxture was measured by a pendant drop method and the wettablty of a PTFE sold surface wth the mxture was evaluated by measurng a contact angle. It s found that the surface tenson of the PAG/CO 2 mxture decreases steeply wth ncreasng the refrgerant concentraton n the ol. The droplet of the mxture keeps ts shape wth equlbrum condton on the PTFE plate. The contact angle on the PTFE surface decreases wth the refrgerant concentraton n the ol due to reducton of the surface tenson. 1. INTRODUCTION A part of lubrcaton ol stored n an ol sump of a refrgeraton compressor becomes fne ol mst n a compressor shell by motons of sldng parts and a dscharge valve, an entranment nduced by gas flow and rotaton of a motor rotor. Although some of the ol mst s separated from refrgerant n the compressor shell, a certan amount of the ol mst s dscharged wth refrgerant nto a refrgeraton cycle. The ol crculated wth refrgerant n the cycle degrades the heat transfer effcency of heat exchangers, ncreases pressure drop n lnes and the heat exchangers and sometmes reduces relablty of the compressor when an ol return from the cycle to the compressor s nsuffcent. It s therefore mportant to reduce the ol dscharge from the compressor to the cycle. There are many researches on the separaton of the ol mst n the compressor shell or an ol separator n a dscharge lne. The ol separaton n the compressor shell or the separator strongly depends on droplet sze of the ol mst generated n the compressor and condton of ol flm formed on an nner surface of the compressor shell or the separator. The ol mst dameter and generaton mechansm of the ol mst are affected by surface tenson of the ol, whle the condton of the ol flm on the nner surface s nfluenced by wettablty of the surface wth the ol. In addton, dssoluton of the refrgerant nto the ol changes the surface tenson and the wettablty of the surface. 23 rd Internatonal Compressor Engneerng Conference at Purdue, July 11-14, 16

3 54, Page 2 Weber number s nversely proportonal to the surface tenson and s one of the domnant parameters of lqud breakup (Majtha et al., 08; Sakura et al., 12). Khosharay and Varamnan (14) measured the surface tenson of (CH 4+H 2O), (C 2H 6+H 2O), (CO 2+H 2O) and (C 3H 8+H 2O) bnary system by a pendant drop method wth changng pressure and temperature. The surface tenson of the ol/refrgerant mxture was measured by Seeton and Hrnjak (06) by a maxmum bubble pressure method. Jensen and Jackman (1984) measured the surface tenson of the mxture by a rng method and proposed a correlaton between the surface tenson and the refrgerant concentraton. Fukuta (16) also measured the surface tenson of the mxture by the maxmum bubble pressure method under hgh temperature pressure condton. The condton of the ol flm formed on the sold surface affects the mst generaton due to the entranment by the flow. It s nfluenced by the wettablty of the surface wth the ol and the wettablty s evaluated by a contact angle of ol droplet on the surface. It s preferable that the contact angle of the ol on the nner wall of the compressor shell s small to avod the entranment of the ol mst by the gas flow. Vadgama and Harrs (07) measured a quas-statc advancng contact angle of refrgerant R134a on copper and alumnum surfaces by a drect optcal observaton technque where the lqud menscus at the surface of a vertcal plate was captured. It s shown that the contact angle vares between 8.3 and 5.6 for the alumnum and between 5.1 and 6.5 for the copper when the temperature rses from 0 C to 80 C. Ln et al. (16) nvestgated the wettng behavor of refrgerant-ol mxture on copper surface by the capllary rse method. They found that the protuberant lqud flm occurs n front of menscus durng evaporaton of the refrgerant-ol mxture, and the presence of ol sgnfcantly enhances the surface wettablty of refrgerant. In ths study, the surface tenson of the ol/refrgerant mxture and the wettablty of the sold surface wth the ol/refrgerant mxture are examned as a frst step of the study on the generaton mechansm and the droplet behavor of the ol mst n the compressor. The surface tenson of the ol/refrgerant mxture s measured by the pendant drop method that can be applcable to a pressurzed condton lke a refrgerant atmosphere. The contact angle of the mxture on the sold surface s measured by a Sessle Drop Method n parallel wth the surface tenson measurement. Polyalkylene glycol (PAG) ol and CO 2 are used as refrgeraton ol and refrgerant respectvely. As the test surface, polytetrafluoroethylene (PTFE) plate s used n the measurement of the contact angle of the mxture on the sold surface. 2. EXPERIMENT 2.1 Expermental Setup Fgure 1 shows a schematc of an expermental setup. A man vessel s desgned to wthstand pressure up to MPa. Two sght glasses are nstalled at opposte sdes of the man vessel. One s used to lght up an nsde of the vessel, and the other s used to take pctures of a droplet on a test plate. The ol/refrgerant mxture,.e. PAG/CO 2 mxture n ths study, s prepared n a sub vessel pror to the experment and fed to the man vessel by a gear pump. The PAG/CO 2 mxture flows through a capllary tube and the droplet drops from the tp of capllary tube on the test plate. Snce volume of the droplet n a Sessle Droplet Method s less than 4 μl, the capllary tube of 0.5 mm n outer dameter s used. The tp of the capllary tube s set at 5 mm above the test plate. The test plate s fxed on a rotary stage and the rotary stage s set on a thrust bearng. The rotary stage has two neodymum magnets on a sde of the stage. Ths confguraton allows us to rotate the test plate by operatng other magnets from outsde of the man vessel so that the measurement can be repeated at the same condton wth changng the droppng poston of the droplet by the rotaton of the test plate to confrm reproducblty. Polytetrafluoroethylene (PTFE) plate s used n ths study as the test plate. Refrgerant concentraton s measured by a samplng method. In the experment, the measurements of the contact angle, the surface tenson and the refrgerant concentraton of the mxture are carred out wth changng the pressure and the temperature of the PAG/CO 2 mxture. Purty of CO 2 used n ths experment s 99.95%. Vscosty grade of PAG used n ths study s 0 mm 2 /s at C. 2.2 Measurement of Contact Angle The contact angle of the mxture s measured by the Sessle Drop Method for the wettablty evaluaton. Fgure 2 shows a pcture of the droplet on the test plate taken by a CCD camera. The contact angle of the droplet s an angle between a horzontal lne and a tangental lne of the droplet at a contact pont of an edge of the droplet. The tangental lne s calculated by a curve fttng method. Frstly, detecton of droplet boundary s conducted 23 rd Internatonal Compressor Engneerng Conference at Purdue, July 11-14, 16

4 54, Page 3 by usng an mage processng software and x-y coordnates (x, y) of ponts on the boundary are obtaned. Then, an approxmaton curve of the droplet boundary s calculated by the curve fttng wth an assumpton that the shape of the droplet boundary s perfect crcle when the volume of the droplet s small. Equaton (1) s used to obtan the approxmaton curve of the boundary by a least square method wth dentfyng a coordnate of the center (a, b), and radus r of the crcle from the dscrete ponts on the boundary. 2 2a x 2b = x y a2 + b2 r 2 x x y x y y y ( ( x3 + x y2 x2 y + y3 2 2 x + y ( ) ) ) (1) The broken lne n Fgure 2 s the approxmaton curve obtaned by Equaton (1). The tangental lne at the contact pont s calculated from the approxmaton curve and the coordnate at the contact pont. The contact angle θ s obtaned from the angle between the horzontal lne and the tangental lne of the approxmaton curve at the contact pont. γs, γl, and γsl shown n Fgure 2 expresses a balance of surface tenson and nterfacal tenson, and are explaned n the followng secton. Fgure 1 Expermental setup of wettablty evaluaton Fgure 2 Measurement of contact angle 23rd Internatonal Compressor Engneerng Conference at Purdue, July 11-14, 16

5 54, Page Measurement of Surface Tenson Snce the surface tenson of lqud s closely related to the contact angle of the droplet on the sold surface, the surface tenson of the ol/refrgerant mxture s also measured by the pendant drop method n the same expermental setup shown n Fgure 1. Durng drppng the droplet from the capllary tube, the surface tenson of the droplet s calculated by the shape of the droplet when the dameter of the droplet becomes the maxmum. When the maxmum dameter of the droplet s de, and the dameter at the pont where the heght from the bottom of the droplet s equal to de s ds (see upper left n Fgure 3), the surface tenson of the droplet s calculated from Equaton (2). γ = J g ρ (d e )2 (2) where, γ s surface tenson, g s the gravtatonal acceleraton, ρ s densty of the ol/refrgerant mxture. J s a correlaton factor and calculated by the followng equaton (Fordham, 1948). J = x x x (x = d s / de ) (3) 3. EXPERIMENTAL RESULTS AND DISCUSSIONS 3.1 Contact Angle of Pure PAG on PTFE Plate The contact angle of pure PAG on the PTFE test plate s measured at frst. An observaton results of the droplet shape are shown n Fgure 3 when the droplet s drpped from the capllary tube and colldes aganst the plate. The temperature condton s kept to 60 C durng the experment. The heght from the plate to the capllary tube s set to 5 mm. Tme s zero when the droplet colldes wth the plate. The droplet shape changes from sphere to hem-sphere on the plate at an nstant of the collson. Then, t slghtly spreads and becomes equlbrum condton after 5 seconds from the collson. The contact angle at each moment s obtaned from the mages and tme hstory of the contact angle of the pure PAG on the PTFE plate s shown n Fgure 4. It s found that the contact angle decreases at the nstant of the collson wth the plate and becomes stable after 2 second after the collson. In the followng experment, the contact angle s measured by the mage taken at seconds after the collson. In addton, the surface tenson of the droplet can be measured by the pendant drop method as prevously explaned at the secton 2.3. Fgure 3 PAG Droplet drpped from capllary tube on PTFE plate 23rd Internatonal Compressor Engneerng Conference at Purdue, July 11-14, 16

6 54, Page 5 Fgure 5 shows the contact angle of PAG on the PTFE plate measured wth changng the temperature as a parameter. The contact angle s measured 5 tmes at each condton, and an average value s plotted aganst the temperature. The contact angle decreases wth ncreasng the temperature. Under the equlbrum condton, Young s formula expressed by Equaton (4) s establshed as shown n Fgure 2. γ = γ cos θ + γ (4) S L SL where, γ S s sold surface tenson, γ L s surface tenson of lqud phase, γ SL s sold-lqud nterfacal tenson, and θ s the contact angle. Accordng to Equaton (4), the contact angle depends on the lqud surface tenson f the soldlqud nterfacal tenson are assumed to be constant aganst the temperature snce the sold surface tenson hardly changes n ths temperature range. The surface tenson of PAG measured by the pendant drop method s shown n Fgure 6 aganst the temperature. It s shown that the surface tenson of PAG decreases wth ncreasng the temperature, and t causes the reducton of the contact angle shown n Fgure 5. Relatonshp between the surface tenson and the contact angle s shown n Fgure 7. It s found that the contact angle ncreases wth the surface tenson. Duple s equaton shown by the followng equaton expresses a work of C 70 Contact angle [ ] 60 PAG PTFE plate Tme [s] Fgure 4 Change of contact angle from collson Contact angle [ ] 60 PAG PTFE plate Temperature [ C] Fgure 5 Contact angle of PAG on PTFE versus temperature 23 rd Internatonal Compressor Engneerng Conference at Purdue, July 11-14, 16

7 54, Page 6 adheson, whch s the work per unt area requred to separate the drop from the sold surface. W = γ + γ γ (5) A s L SL The work of adheson s an ndex of the wettablty and t s ndcated that the wettablty becomes better wth large work of adheson. Wth consderng that the droplet of the PAG mantans ts shape on the PTFE plate wth equlbrum condton, Young-Duple equaton s derved from Equatons (4) and (5) as follows. W γ ( 1+ cosθ ) (6) A = L The work of adheson under the equlbrum condton s a functon of the surface tenson of lqud and the contact angle as expressed by Equaton (6). The work of adheson s obtaned by the surface tenson and the contact angle, and found to be 45.5 mj/m 2 [mn/m] when the surface tenson s 29 mn/m. As a reference value, the work of adheson of PTFE flm wth glycerol s shown to be 45.9 mn/m by Glman (14). The contact angle calculated by Equaton (6) wth an assumpton of constant work of adheson of 45.5 mn/m s shown n Fgure 7 by sold lne. Although the calculated contact angle shows the smlar tendency as the expermental one, gradent of the calculated contact angle s larger than expermental one. It may be caused by the fact that the work of adheson vares wth the Surface tenson [mn/m] PAG Temperature [ C] Fgure 6 Surface tenson of PAG versus temperature Contact angle [ ] PAG PTFE plate W A = 45.5 mj/m 2 Expermental value Calculated by Eq. (6) Surface tenson [mn/m] Fgure 7 Influence of surface tenson on contact angle (PAG on PTFE) 23 rd Internatonal Compressor Engneerng Conference at Purdue, July 11-14, 16

8 temperature. It s also supposed that contamnaton on the PTFE surface affects the wettng phenomena. 54, Page Contact Angle of PAG/CO2 Mxture on PTFE Plate The nfluence of refrgerant dssolved nto the ol on the contact angle s nvestgated. Fgure 8 shows the relatonshp between the mass fracton of CO 2 dssolved n the PAG and the surface tenson of the mxture measured by the pendant drop method at C. The surface tenson of the PAG/CO 2 mxture decreases steeply wth ncreasng the CO 2 concentraton n the mxture. The contact angle of the PAG/CO 2 mxture s shown n Fgure 9 aganst the CO 2 concentraton n the mxture. It s shown that the contact angle of the PAG/CO 2 mxture on the PTFE plate decreases wth ncreasng the CO 2 concentraton n the mxture due to the reducton of the surface tenson of the mxture. Therefore, t s supposed that the wettablty of the sold surface wth the PAG/CO 2 mxture havng hgh CO 2 concentraton becomes hgh. The contact angle s re-arranged aganst the surface tenson as shown n Fgure. When the surface tenson of the mxture becomes smaller than 22.5 mn/m, the contact angle calculated by Equaton (6) wth the work of adheson of 45.5 mj/m 2 becomes zero, whch means that the wettng becomes a perfect one n whch the droplet spreads and does not keep the droplet shape. In the experment, however, the droplet of the mxture wth the surface tenson of mn/m keeps the equlbrum contact angle. Ths dfference may be caused by the fact that the work of adheson s not constant. Contamnaton on the PTFE surface s another factor that affects PAG/CO 2 mxture C Surface tenson [mn/m] CO 2 concentraton [wt%] Fgure 8 Surface tenson of PAG/CO 2 mxture versus refrgerant concentraton PAG/CO 2 mxture PTFE plate C Contact angle [ ] CO 2 concentraton [wt%] Fgure 9 Influence of refrgerant concentraton on contact angle of PAG/CO 2 mxture on PTFE 23 rd Internatonal Compressor Engneerng Conference at Purdue, July 11-14, 16

9 54, Page PAG/CO 2 mxture PTFE plate C Contact angle [ ] 60 W A = 45.5 mj/m 2 Expermental value Calculated by Eq. (6) Surface tenson [mn/m] Fgure Contact angle of PAG/CO 2 mxture on PTFE versus surface tenson the wettng phenomena. Snce metal materals have much larger sold surface tenson or work of adheson than those of the PTFE, the wettng on the metal surface wth the ol/refrgerant mxture wll tend to be the perfect one. 4. CONCLUSIONS In ths study, the wettablty of sold surface wth ol/refrgerant mxture s evaluated expermentaly. The followng conclusons are obtaned 1. The expermental apparatus developed n ths study s applcable to measure the contact angle and the surface tenson of ol/refrgerant mxture under hgh pressure and temperature condton. 2. The droplet of pure PAG on the PTFE plate becomes equlbrum condton after 2 second from the collson wth the plate. The contact angle of the pure PAG on the PTFE plate decreases wth ncreasng temperature due to the reducton of the surface tenson of the PAG. 3. The droplet of PAG/CO 2 mxture keeps the droplet shape wth the equlbrum condton on the PTFE plate, and the contact angle decreases wth ncreasng the CO 2 concentraton n the mxture snce the surface tenson of the mxture steeply decreases wth ncreasng the CO 2 concentraton. 4. The wettng of the ol/refrgerant mxture on the metal surface s supposed to be the perfect one due to the much hgher sold surface tenson of the metal than that of the PTFE. REFERENCES Fordham, S. (1948). On the calculaton of surface tenson from measurements of pendant drops, Proceedngs of Royal socety of London, Volume194, A. Fukuta, M., Sumyama, J., Motozawa, M., Yanagsawa, T. (16). Surface tenson measurement of ol/refrgerant mxture by maxmum bubble pressure method. Int. J. Refrg., submtted. Glman, A., Pskarev, M., Yablokov, M., Kechek yan, A., Kuznetsov, A. (14). Adhesve propertes of PTFE modfed by DC dscharge. J. Physcs: Conference Seres, 516, rd Internatonal Compressor Engneerng Conference at Purdue, July 11-14, 16

10 54, Page 9 Jensen, M.K., Jackman, D.L. (1984). Predcton of nucleate pool bolng heat transfer coeffcents of refrgerant-ol mxtures. J. Heat Transfer, 6, Khosharay, S., Varamnan, F. (14). Expermental and modelng nvestgaton on surface tenson and surface propertes of (CH 4+H 2O), (C 2H 6+H 2O), (CO 2+H 2O) and (C 3H 8+H 2O) from K to K and pressures up to 60 bar. Int. J. Refrg., 47, Ln, L., Peng, H., Dng, G. (16). Influence of ol concentraton on wettng behavor durng evaporaton of refrgerant-ol mxture on copper surface, Int. J. Refrg., 61, Majtha, A.K., Hall, S., Bowen P.J. (08). Droplet breakup quantfcaton and processes n constant and pulsed arflows. Proceedngs of the 22 nd European Conference on Lqud Atomzaton and Spray System, ILASS Sakura, Y., Kobayash, K., Fujkawa, T., Sanada, T., Watanabe, M. (12). Observaton of deformaton of sngle droplet mpact on sold surface (Effects of lqud propertes, surface roughness and surroundng gas pressure). Japanese Journal of Multphase flow, Vol.26 No.2, (n Japanese) Seeton, C.J., Hrnjak, P. (06). Thermophyscal propertes of CO 2-lubrcant mxtures and ther affect on 2-phase flow n small channels (less than 1 mm). Proceedngs of the Internatonal Refrgeraton and Ar Condtonng Conference at Purdue, R170. Vadgama, B., Harrs, K. D. (07). Measurements of the contact angle between R134a and both alumnum and copper surfaces. Expermental Thermal and Flud Scence, 31, ACKNOWLEDGEMENT The densty measurement of the ol/refrgerant mxture was done wth help by Mr. Tatsuhro Suzuk, workng at DENSO corp., when he was a student at graduate school of engneerng, Shzuoka Unversty, and the authors would lke to apprecate for hs effort. 23 rd Internatonal Compressor Engneerng Conference at Purdue, July 11-14, 16