Numerical Analysis on Nanoparticles-laden Gas Film Thrust Bearing

Transcription

1 CHINESE JOURNAL OF MECHANICAL ENGINEERING Vol. 6, No. 4, DOI: /CJME , available online at Nueical Analysis on Nanopaticles-laden Gas Fil Thust Beaing YANG Zhiu 1, DIAO Dongfeng 1,, *, and YANG Lei 1 1 Key Laboatoy of Education Ministy fo Moden Design and Roto-Beaing Syste, Xi an Jiaotong Univesity, Xi an , China Institute of Nanosuface Science and Engineeing (INSE), Shenzhen Univesity, Shenzhen , China Received Octobe 7, 01; evised Januay 8, 013; accepted Mach, 013 Abstact: Nanopaticles can be taken as additives and added into vaious fluids to ipove thei lubicating pefoances. At pesent, eseaches in this aea ae ainly concentated on the ipoveent effects of nanopaticles on the lubicating pefoances of liquid such as oil and wate. Nanopaticles will also affect gas lubication, but few elated studies have been epoted. Nanopaticles-laden gas fil (NLGF) is foed when adding nanopaticles into gas beaing. Then, the lubicating pefoances of gas beaing including pessue distibution and load-caying capacity will change. The vaiations of pessue distibution and load-caying capacity in nanopaticles-laden gas fil thust beaing ae investigated by nueical ethod. Taking account of the copessibility of gas and the inteactions between gas and nanopaticles, a coputational fluid dynaics odel based on Navie-Stokes equations is applied to siulate the NLGF flow. The effects of inlet nanopaticles volue faction and oifice adius on fil pessue distibution and load-caying capacity of the NLGF ae calculated. The nueical calculation esults show that both of the fil land pessue and the axiu fil pessue both incease when the nanopaticles ae added into gas beaing, and the fil pessues incease with the ising of the inlet nanopaticles volue faction. The nanopaticles have an enhanceent effect on load-caying capacity of the studied beaing, and the enhanceent effect becoes geate as the fil thickness decease. Theefoe, nanopaticles can effectively ipove the lubicating pefoance of gas beaing. The poposed eseach povides a theoetical basis fo the design of new-type nanopaticles-laden gas fil beaings. Key wods: load-caying capacity, pessue distibution, nanopaticles-laden gas fil, coputational fluid dynaics 1 Intoduction Nanopaticles could be used as eithe lubicant [1] o lubicant additives []. As additives, nanopaticles was added in vaious fluids to ipove thei lubicating pefoances [3 4]. Thee wee any studies on adding nanopaticles in lubicating oil [5 9]. RAPOPORT, et al [5], found that addition of even a sall aount of nanopaticles into oil could incease its load-caying capacity. LIU, et al [6], found that the oil exhibited excellent load-caying capacity when the aveage size of the additive paticles was less than 30 n. HSIN, et al [7] and LIN, et al [8], both confied that nanopaticles of diffeent ateial ipoved the load-caying capacity of lubicating oil. LEE, et al [9], analyzed the echanis of load-caying capacity enhanceent of nanopaticles on lubicating oil. The above-entioned liteatues ae all about enhanceent effects of nanopaticles on load-caying capacity of lubicating oil. When paticles wee added in gas, the pessue * Coesponding autho. E-ail: dfdiao@ail.xjtu.edu.cn This poject is suppoted by National Natual Science Foundation of China (Gant No ) Chinese Mechanical Engineeing Society and Spinge-Velag Belin Heidelbeg 013 distibution of the ixtue flow changed. Gas pessue in an oifice o nozzle outlet was significantly affected by paticles concentation in a gas-paticles ixtue flow [10 16]. YUU, et al [10], epoted that the pesence of paticles educed the pessue dop by as uch as 30% at the outlet. CHU, et al [11], found the pessue dop deceased with the inceasing of paticles concentation in a cyclone sepaato. WANG, et al [1 13] and LEE, et al [14], studied the aeodynaic focusing of nanopaticles, and povided nueical and expeiental ethods to investigate the gas-nanopaticles ixtue flowing though a nozzle. MA, et al [15] and KIM, et al [16], studied pessue dop in gas-paticles ixtue flow using nueical and theoetical appoach, espectively, and thei odel successfully pedicted the echanis of the pessue vaiation. Howeve, thee ae few studies discussing the effects of paticles in gas-nanopaticles ixtue which was applied as lubicant. In this study, in ode to study the effects of inlet nanopaticles volue faction and oifice adius on fil pessue distibution, a nanopaticles-laden gas fil (NLGF) foed by adding silica (SiO ) nanopaticles into ai beaing is siulated by coputational fluid dynaics (CFD) ethod. Then, the fil pessue distibution and load-caying capacity in NLGF thust beaing ae

2 676 YANG Zhiu, et al: Nueical Analysis on Nanopaticles-laden Gas Fil Thust Beaing calculated. Finally, the vaiation tendency of load-caying capacity with nanopaticle volue faction and fil thickness is deived. Nueical Analysis Method.1 Coputational fluid dynaics odel Fig. 1 shows the geoetical configuation of the aeostatic NLGF thust beaing with a single inlet pot. The adius of the cicula pad is 5. An oifice esticto with adius of is located at the cente of the pad. The nanopaticles-laden gas is consisted of ai and unifoly dispesed silica nanopaticles (30 n in diaete). The ixtue flows though oifice, then entes the beaing cleaance and flows out fo the beaing edge at last. Because the flow field in the beaing is axisyetic, the calculation can be siplified fo thee-diensional (3D) to two-diensional (D), and the coss-section of the flow field (the shaded aea in Fig. 1) is used fo the D nueical siulation. v, v, v, ρ ρv, x ρv, t x p v, v, x µ x x 1 v, v, v, µ µ F, whee ρ is the density of the ixtue; v,x, v, ae the velocities of the ixtue in axial diection and adial diection, espectively; p is the pessue of the ixtue; µ is the viscosity of the ixtue; F x, F ae the dag foces of the ixtue in axial diection and adial diection, espectively. The dag foce of the ixtue in axial and adial diection can be expessed as follows: 1 Fx αρ 1 1 ( vd1, xvd1, x) ( vd1, vd1, x) x 1 αρ ( vd, xvd, x) ( vd, vd, x), x () (3) 1 F αρ 1 1 ( vd1, xvd1, ) ( vd1, vd1, ) x 1 αρ ( vd, xvd, ) ( vd, vd, ), x (4) Fig. 1. Basic beaing odel fo nueical analysis CFD ethod was widely used in the nueical analysis of gas fil lubication [17 18]. The Mixtue Model was the coon odel to siulate the unifo two coponent flow [19]. In this odel, the nanopaticles-laden gas ixtue is teated as one fluid. The ai and silica nanopaticles ae consideed as two coponents of the fluid, and the su of thei volue factions is one. The copessibility of gas and the inteactions between gas and nanopaticles ae both consideed in the flow. Theefoe, the Navie-Stokes equations (include continuity equation and oentu equation), the volue faction equation [0], the gas state equation and the enegy consevation equation [1] ae necessay fo the analysis of the odel. In ode to cobine the inteactions between gas and nanopaticles in the odel, two ites (F x, F ) ae added in the oentu equations. Then, the oentu equations fo the ixtue in axial and adial diection can be expessed as follows: v, x v, x v, x p ρ ρv, x ρv, t x x v, x µ v, x v, µ ρ g Fx, x x (1) whee α 1, α ae the volue factions of ai and silica nanopaticles; ρ 1, ρ ae the densities of ai and silica nanopaticles; vd1, x, v d,x ae the dift velocity of ai and silica nanopaticles in axial diection; vd1,, v d, ae the dift velocity of ai and silica nanopaticles in adial diection, espectively. Based on the above entioned equations, the fil pessue distibution of the NLGF is obtained. The load-caying capacity of NLGF is the integal of fil pessue distibution, and it can be expessed as o π ( )d, 0 W p (5) whee o is the pad adius and p() is the fil pessue distibution along fil cente in adial diection.. Calculation gids and bounday conditions To solve the Navie-Stokes equations of the ixtue, bounday conditions need to be defined and the fluid needs to be discetized to finite eleent gids. Fig. shows the gids used in NLGF lubication analysis. The flow field is divided into 500 eleents in the axial diection and in the adial diection. The eleent size is in the ange fo to In ode to ipove the accuacy of the calculation, the distance between gid points becoes salle as the gid position is close to the walls, as shown in the enlaged view of Fig..

3 CHINESE JOURNAL OF MECHANICAL ENGINEERING 677 of 1 and beaing adius o of 30. It can be seen that the nueical esults and the expeiental esults have the sae vaiation tendency, and it coincides with Yoshioto s nueical esults [18]. Theefoe, the CFD odel can be used in NLGF flow field analysis. Table 1. Calculation paaetes Fig.. Scheatic view of gid fo calculation of flow field Bounday conditions ae defined as follows. Solid walls (edge AB, CD, and DE): The walls ae consideed to be ipeeable and adiabatic. No-slip condition is set up at the walls, and the flow velocities in all diections at the walls ae zeo. Syetic axis (edge AF): At the cental axis of the beaing, the density, the velocities and the enegy ae syetic. The noal velocity coponent and the noal gadients fo all vaiables at the axis ae assued zeo. Pessue inlet (edge EF): In the pesent siulation, the calculations ae initiated fo edge EF. The inlet pessue is 0.3 MPa and its diection is pependicula to the inlet edge EF. The tepeatue at the inlet is 300 K. Pessue outlet (edge BC): The outlet pessue is 0.1 MPa, which equals to abient pessue, and its diection is pependicula to the outlet edge BC. The tepeatue at the outlet is also 300 K. A coecial code (FLUENT 6.3) was eployed to establish the CFD odel, as well as to calculate the fil pessue distibution and load-caying capacity of the NLGF. Fistly, the Mixtue Model in FLUENT 6.3 was chosen to define the NLGF flow. SiO nanopaticles wee defined as the paticle phase. The diaete, density and volue faction of the nanopaticles wee set. Secondly, enegy equation was utilized, and ideal gas was defined as the gas phase. Thidly, bounday conditions wee set. Fouthly, the solution schee was defined. The contol volue-based technique was adopted to solve the flow govening equations. The heat tansfe tes wee centally diffeenced, and the pessue tes wee upwind diffeenced. Finally, iteations of the solution loop ended when the conveged solution was obtained. All of the calculation paaetes, including ateial popeties of nanopaticles, stuctual paaetes of beaing and bounday conditions, ae suaized in Table 1. Mateial popety Stuctual paaete Bounday paaete Calculation paaete Nanopaticles density ρ (kg 3 ) Nanopaticles viscosity μ (kg s 1 1 ) Inlet nanopaticles volue faction α% Value , 10, 0, 30, 40, 50, 60 Fil thickness hμ 0, 30, 40, 50, 60 Oifice adius i 0.1, 0., 0.3, 0.4, 0.5 Pad adius o 5 Oifice inlet pessue p impa 0.3 Abient pessue p ompa 0.1 Tepeatue TK 300 Fig. 3. Validation of nueical odel The pessue distibution along the fil cente (h) in adial diection was concened in the study. Fig. 4 shows the effect of the inlet nanopaticles volue faction on fil pessue distibution unde the conditions of h0 μ, p i 0.3 MPa, and i Nueical Results Analysis and Discussion In ode to validate the CFD odel, the nueical esults of the CFD odel with clean gas wee copaed with the foe expeiental esults []. The noalized pessue of nueical esults and expeiental esults ae shown in Fig. 3, in which the conditions wee fil thickness h of 90 μ, oifice inlet pessue p i of 0.4 MPa, oifice adius i Fig. 4. Effect of the inlet nanopaticles volue faction on fil pessue distibution The noalized pessue fist showed a sudden dop nea

4 678 YANG Zhiu, et al: Nueical Analysis on Nanopaticles-laden Gas Fil Thust Beaing oifice, then inceased to axiu pessue and finally deceased slowly with the inceasing of noalized adius along fil cente. The iniu pessue nea oifice, naed the fil land pessue, affected the woking stability of the NLGF; and the axiu pessue inside the fil affected the load-caying capacity of the NLGF. Theefoe, the two pessues wee concened in the esults analysis. It was clea that the fil land pessue and axiu fil pessue had the sae vaiation tend, and both of the inceased when the nanopaticles wee added. In addition, as the inlet nanopaticles volue faction (α ) vaied fo 0 to 60%, the inceent of fil land pessue educed and it was alost close to zeo when the faction was 60%. Fig. 5 shows the fil pessue distibutions with diffeent oifice adius when α is 0%, h is 0 μ, and p i is 0.3 MPa. It was found that the fil land pessue inceased as the oifice adius vaied fo 0.1 to 0.5. Fig. 5. Effect of the oifice adius on fil pessue distibution The load-caying capacity was deived fo fil pessue distibution (see Eq. (5)). With the ising of fil pessues, the load-caying capacity inceased. The incease atio of load-caying capacity was defined as the atio of NLGF load-caying capacity inceent (W-W 0 ) to load-caying capacity of gas fil without nanopaticles (W 0 ). Fig. 6 shows the enhanceent effects of inlet nanopaticles volue faction on incease atio of load-caying capacity in vaious fil thickness unde the conditions of p i 0.3 MPa and i 0.4. Fo Fig. 6, it was found that the load-caying capacity inceased with the inceasing of inlet nanopaticles volue faction. This was consistent with the esults of fil pessue distibution in Fig. 4. Moeove, the incease atio of load-caying capacity was geate in thin fils than in thick fils. In this eseach, we found that the pessue and load-caying capacity inceased with the adding of nanopaticles. It was well known that the paticles could unifoly dispese in ai when the size was less than 10-7 (100 n) [3]. When they wee added in a fluid, the viscosity of the fluid inceased [1]. Thus, the pessue inside the fil inceased (pessue was popotional to viscous foce), and then the load-caying capacity (the integal of fil pessue) inceased. In addition, we also found that the load-caying capacity inceased with the inceasing of nanopaticles volue faction. This phenoenon was also caused by the incease of viscosity. When the nanopaticles volue faction inceased, the aveage distances between nanopaticles deceased and the inteactions between the inceased which caused the incease of the viscosity [4]. Due to the copessibility of gas, nanopaticles volue faction inside the fil is diffeent with that at the inlet. The vaiation of nanopaticles volue faction along fil cente was investigated unde the conditions of h0 μ, p i 0.3 MPa, and i 0.4. And the esults ae shown in Fig. 7. It can be seen that the nanopaticles volue faction gadually decease inside the fil. Howeve, the volue factions of nanopaticles inside the fil show the sae tend of inceasing as that of the inlet volue faction. By this esult, we can conclude that the inceasing of inlet volue faction incease the nanopaticles volue faction inside the fil, which contibute to highe load-caying capacity. So it is easonable to use inlet volue faction to epesent the state of the whole fil, and nanopaticles volue faction changing inside the fil will not affect the conclusions. Fig. 7. The vaiation of nanopaticles volue faction along fil cente (h) Fig. 6. Effects of nanopaticles on incease atio of load-caying capacity (LCC) 4 Conclusions (1) A CFD odel of gas-nanopaticles two coponents

5 CHINESE JOURNAL OF MECHANICAL ENGINEERING 679 copessible flow is poposed to descibe the flow field of nanopaticles-laden gas fil thust beaing. () By adding nanopaticles into the gas fil, the fil land pessue and the axiu fil pessue ae both inceased. The fil pessues incease with the ising of the inlet nanopaticles volue faction. (3) The nanopaticles have an enhanceent effect on load-caying capacity of NLGF. The load-caying capacity inceases with the inceasing of inlet nanopaticles volue faction and the incease atio of load-caying capacity is geate in thin fils than in thick fils. Refeences [1] TENNE R. Inoganic nanotubes and fulleene-like nanopaticles[j]. Natue Nanotechnology, 006, 1(11): [] LIU W M. Application of nanopaticles in lubicants[j]. Tibology, 003, 3(4): (in Chinese) [3] ZHANG Y D, YAN J S, SUN L, et al. Fiction educing anti-wea and self-epaiing popeties of nano-cu additive in lubicating oil[j]. 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Tel: ; E-ail: yangzhiu00@16.co DIAO Dongfeng, bon in 1961, is cuently a pofesso, a PhD candidate supeviso and the diecto of Key Laboatoy of Education Ministy fo Moden Design and Roto-Beaing Syste, Xi an Jiaotong Univesity, China. His ain eseach inteests include nano-scale and ico-scale paticles envionent tibology, nano-tibology and suface sciences. Tel: ; E-ail: dfdiao@ail.xjtu.edu.cn YANG Lei, bon in 1987, is cuently a PhD candidate at Key Laboatoy of Education Ministy fo Moden Design and Roto-Beaing Syste, Xi an Jiaotong Univesity, China, in 010. His eseach inteests include nano-scale wea and lubication, nano-suface engineeing. Tel: ; E-ail: yolai1015@stu.xjtu.edu.cn