Measurements of Two-Dimensional Distribution of Refrigerant Concentration in EHL Film Using Micro...

Measurements of two-dimensional distribution of refrigerant concentration

in EHL film using micro FT-IR and effect of variation of concentration on

oil film thickness

Shinji Tanakaa, Tsunamitsu Nakaharab and Keiji Kyogokub

aDepartment of Mechanical Sciences and Engineering, Tokyo Institute of Technology, Tokyo 152-8552, JapanbDepartment of Mechanical and Aerospace Engineering, Tokyo Institute of Technology, Tokyo 152-8552, Japan

Received 24 December 2001; accepted 2 May 2002

The dissolution of refrigerant into a lubricant causes a decrease in viscosity of the oil and it gives a large effect on the lubrication

of sliding parts in a refrigerant compressor. This paper describes an application of micro FT-IR to measure the two-dimensional

concentration distribution of refrigerant held in solution in the EHL film surrounded by the refrigerant gas and discusses the

refrigerant concentration variation in the vicinity of the Hertzian contact area. In order to measure the concentration distribution, an

apparatus which can observe the EHL film in a point contact in the refrigerant atmosphere was developed. The refrigerant con-

centration was measured using micro FT-IR through a CaF2 window from outside of the apparatus with polyol ester as a base oil in

an atmosphere pressurized with HFC-134a refrigerant gas. The results indicate that the concentration of HFC-134a refrigerant

reduces in the inlet boost region of EHL contact and the Hertzian contact area but in the side region of Hertzian contact area it is

greater than that in the bulk fluid. In addition, the effect of the variation on the oil film thickness is discussed.

KEY WORDS: EHL; HFC-134a refrigerant; polyol ester; concentration variation; micro FT-IR

1. Introduction

Chlorofluorocarbons (CFCs) and hydrochloro-fluorocarbons (HCFCs) containing chlorine, widely usedas refrigerants in refrigerators and air conditioners, werereplaced by hydrofluorocarbons (HFCs), without chlor-ine, in order to protect the earth’s ozone layer.

However, this refrigerant change causes a tribologicalproblem, lowering the life and reliability of a refrigerantcompressor due to the loss of the boundary lubricationeffect of chlorine. The oil in a refrigerant compressorcontains the refrigerant in solution. The refrigerantdissolution lowers the viscosity and viscosity-pressurecoefficient of lubricant, so that the oil film thicknessbetween sliding surfaces decreases. Additionally, slidingparts with high contact pressures such as a vane tip of arotary compressor operate under mixed lubricationconditions, with partial elastohydrodynamic lubrication(EHL) which is dependent on the rheological char-acteristics of the oil. In the case of the refrigerant atmo-sphere, the lubrication characteristics are significantlyaffected by the refrigerant concentration in the lubri-cating oil film. Therefore, it is important to know therefrigerant concentration in the EHL film.

In recent years, Fourier transform infrared (FT-IR)spectroscopy has been applied to the investigation oflubricating oil films. Using the FT-IR, the alignmentand the concentration of additives in lubricating oil films[1–3] and the concentration of thickeners in greaselubricating films [2] have been measured.

Hoshi et al. [4] have used micro FT-IR with an EHLpoint contact to study the behavior of additives in theEHL film and have shown that the concentration of theadditive in the Hertzian contact area decreases com-pared to that of the bulk fluid. The authors [5] havemeasured the concentration of refrigerant in the EHLfilm in the refrigerant atmosphere and found the phe-nomenon that the concentration of refrigerant decreasesin the inlet region and the Hertzian contact area.Moreover, Ono et al. [6,7] have measured the con-centration of blend oil in the EHL film and have foundalso a decrease in concentration of constituents in theinlet region of EHL contact. These phenomena of con-centration change are very interesting and important forthe lubrication mechanism.

This paper describes an application of micro FT-IRto measure the two-dimensional concentration dis-tribution of refrigerant in the EHL film in the refrigerantatmosphere and discusses the refrigerant concentrationchange in the Hertzian contact area by comparison withthe measured results of other papers.

2. Experimental procedures

Figure 1 shows the schematic view of the experi-mental apparatus for oil-film observation in the refrig-erant atmosphere. This apparatus is a ball-on-disk typeof friction tester set in a pressure vessel. The refrigerant

Tribology Letters, Vol. 14, No. 1, January 2003 (# 2003 ) 9

1023-8883/03/0100-0009/0 # 2003 Plenum Publishing Corporation

concentration in the oil film between the steel ball andthe disk was measured using micro FT-IR through aninfrared transmission disk made of CaF2, which playsboth roles of a contact surface and a pressure-proofwindow. The apparatus is mounted on an X-Y-Z micropositioning table underneath the micro FT-IR, and themicroscope is focused through the vertical movement ofthe table. The applicable pressure range of the vessel isfrom 0.7 Pa(abs.) to 1.1 MPa(abs.), and the pressure ofthe refrigerant gas in the vessel is adjusted with a pres-sure regulator. The temperature of sample oil in thevessel is controlled with a silicone rubber heater, whichis attached to the outside of the vessel.

The steel ball is rotated using an AC servomotorthrough a magnetic coupling. By the rotation of the steelball, the sample oil is supplied to the contact surfacebetween the steel ball and the disk. A scraper made ofPTFE is installed in an oil reservoir in order to removethe oil which adheres on the surface of the steel ballbecause it was supposed that the lubricating oil filmformed on the steel ball is not replaced with new oil inthe oil reservoir.

The infrared transmission disk is attached to the diskholder which is sealed by the same size of bellows sealsand loaded against the steel ball. Since the bellows sealsare the same size, the pressure in the vessel doesn’t sig-nificantly affect the load. The load was measured with aload cell outside of the vessel. The mechanical propertiesof the steel ball and the infrared transmission disk areshown in table 1.

The measurements were carried out under lubricationwith refrigerating machine oil containing dissolvedrefrigerant, as found inside a refrigerant compressor.The combination of refrigerant and refrigerating

machine oil is HFC-134a/polyol ester (POE). Theproperties of oil and the standard experimental con-ditions in this study are shown in tables 2 and 3respectively.

Figure 1. Oil film observation apparatus in refrigerant atmosphere.

Table 1

Mechanical properties of steel ball and infrared transmission disk.

Steel ball Disk

Material JIS SUJ-2 CaF2

Size, mm �50.8 �40� 10

Surface roughness, �m Ra 0.005 Ra 0.005

Young’s modulus, GPa 207.8 75.8

Poisson’s ratio 0.30 0.26

Table 2

Properties of oil.

Kinematic viscosity, �10�6 m2/s

—————————————— Density @15 �C

40 �C 100 �C kg/m3

Polyol ester 56.0 6.7 980

Table 3

Standard experimental conditions.

Refrigerant HFC-134a

Refrigerant pressure 0.4MPa (abs.)

Refrigerant concentration in bulk fluid 11.0 mass%

Oil Polyol ester (non additive)

Oil temperature 40 �C

Load (Maximum Hertzian pressure) 5N (174MPa)

Hertzian radius 116�m

Sliding velocity 4.0m/s

10 S. Tanaka et al./Measurements of two-dimensional distribution of refrigerant concentration in EHL film

Figure 2 shows the infrared absorption spectra ofHFC-134a/POE and the transmittance of the infraredtransmission materials. HFC-134a and POE showstrong absorption peaks of C�F (1286 cm�1) and C��O(1740 cm�1) stretching vibrations respectively. In orderto measure these absorption peaks, an infrared trans-mission disk made of CaF2 was used for the measure-ment. The peak height ratio of the peaks at 1286 cm�1

and 1740 cm�1 and the peak area of the C��O stretchingvibr-ation were used as measures of HFC-134a refrig-erant concentration and the oil film thickness betweenthe steel ball and the disk, respectively.

Figure 3 shows the relation between the oil filmthickness and the ratio of peak heights which corre-sponds to HFC-134a refrigerant concentration. In thecase of constant refrigerant concentration (11.0 mass%)in the bulk fluid, the refrigerant concentration changesgreatly at an oil film thickness below 0.5�m because ofan increase in the noise during the measurement usingthe micro FT-IR. Results obtained in a position where

the film thickness is 0.5�m or less are indicated by adotted line.

Static calibration curves of the refrigerant con-centration and the film thickness of HFC-134a/POE areshown in figure 4. The static calibration of the refrig-erant concentration was carried out at constant oiltemperature (40 �C) and oil film thickness (0.95�m) bychanging the refrigerant pressure. Before the measure-ment for the static calibration of the refrigerant con-centration, the steel ball was rotated in order to dissolvethe refrigerant into the oil sufficiently. The ball wasmade to rest during the measurement. The calibrationdata of the refrigerant concentration were transformedto an experimental formula. The film thickness wascalibrated with the ball profile and the peak area of thespectra taken at different positions from the contactcenter under constant refrigerant concentration (11.0mass%). In addition, the refrigerant concentrationdidn’t greatly affect the calibration data of the oil filmthickness. The calibration data of the oil film thicknesswere interpolated by a spline function.

Figure 5 shows the measured positions of the infraredabsorption spectra. The center of the Hertzian contactwas determined by the observed interference fringes atthe start of the experiment, and a micrometer measure-ment was used to measure the relative position. Toobtain the two-dimensional concentration distributionof refrigerant in the surrounding region of the Hertziancontact area, the spectra were taken at 50�m intervalsfrom the inlet to the exit of the EHL contact zone in the

Figure 2. Infrared absorption spectra of HFC-134a / POE.

Figure 3. Relation between film thickness and ratio of peak height.

Figure 4. Calibration curves of HFC-134a / POE.

Figure 5. Measured positions of infrared absorption spectra.

S. Tanaka et al./Measurements of two-dimensional distribution of refrigerant concentration in EHL film 11

sliding direction and 100�m intervals from the center ofthe steel ball to a distance of 300�m in the shaftdirection. The sampling area of spectra was25� 25 �m2, and spectra were collected by co-adding128 scans at a resolution of 4 cm�1.

3. Experimental results and discussion

Figure 6 presents the measured results of oil filmthickness under the standard experimental conditions.The measured results were compared with the calculatedones obtained from the familiar central film thicknessformula by Chittenden et al. [8].

hcRx

¼ 4:31�0u

ERx

� �0:68

ð�EÞ0:49w

ER2x

� ��0:073

� 1� e�1:23ðRy=RxÞ2=3

n oð1Þ

where

E ¼ equivalent Young’s modulus (Pa)hc ¼ central film thickness (m)Rx ¼ equivalent curvature radius in sliding direction

(m)Ry ¼ equivalent curvature radius in shaft direction

(m)u ¼ entrainment velocity (m/s) (= sliding velocity/2)w ¼ load (N)� ¼ viscosity-pressure coefficient (Pa�1)�0 ¼ viscosity ðPa � sÞ

In order to obtain the viscosity-pressure coefficient �of the oil in which the refrigerant dissolved, So andKlaus’s experimental formula [9], which is widely used

for mineral oils, is expanded for the lubricant dissolvingrefrigerant:

� ¼ 1:030þ 3:509 ðlog �0Þ3:0627

þ 2:412� 10�4 m5:19030 ðlog �0Þ

1:5976

� 3:387 ðlog �0Þ3:0975�0:11620 ð2Þ

where

m0 ¼ viscosity-temperature property from theASTM-Walther equation [10]

�0 ¼ kinematic viscosity of oil, m2/s�0 ¼ density of oil, kg/m3

The viscosity and the density of oil in which therefrigerant dissolved and the dissolved concentration ofthe refrigerant were obtained from data measured by anoil company. Figure 6 illustrates that the measuredresults are roughly the same as the calculated onesexcept for a cavitation area.

Figure 7 shows an enlarged view of the oil filmthickness in the Hertzian contact area along the centerline in the sliding direction. The measured film thicknessin the Hertzian contact area is larger than the filmthickness that is calculated by assuming that the refrig-erant concentration in the Hertzian contact area is equalto that in the bulk fluid (11.0 mass%). The refrigerantconcentration in the Hertzian contact area was identifiedby comparing the measured film thickness with thecalculated value. The effective refrigerant concentrationin the EHL film was estimated at 6.2 mass% as shown infigure 7.

The two-dimensional concentration distribution ofHFC-134a refrigerant under the standard experimentalconditions is shown in figure 8. This figure illustratesthat the HFC-134a refrigerant concentration in theHertzian contact area and the inlet zone in the EHLcontact are less than that in the bulk fluid (11.0 mass%).The average refrigerant concentration in the Hertzian

Figure 6. Oil film thickness under standard conditions. Figure 7. Enlarged view of oil film thickness in Hertzian contact area.


contact area along the center line in the sliding directionis 6.6 mass%, and this value is in good agreement withthe estimated one. In addition, the refrigerant con-centration in the side region of the Hertzian contact area(sliding direction: �116 � þ116 �m, shaft direction:116 � 300 �m) is greater than that in the bulk. There-fore, these results mean that the refrigerant solute isfiltered in the high-pressure region in the vicinity of theHertzian contact area, and the rejected solute is dis-charged into the side region of the Hertzian contactarea.

A similar phenomenon has been reported in otherstudies. Hoshi et al. [4] have measured the concentrationvariation of oiliness agents in the EHL film as shown infigure 9. The vertical and the horizontal axes indicate theconcentration of the oiliness agent and the measuredposition respectively. The Hertzian radius is 78.4�m.The measured oils are polyalphaolefin (PAO), poly-butene (PB) and mineral oil (P-400), which all contain10 wt% stearic acid butyl ester. The additive con-centrations of all oils decrease in the Hertzian contactarea. They have assumed that the factor of this con-centration decrease is the solubility of additive in thebase oil. However, the cause is not clear.

Ono and Yamamoto [7] have measured the con-centration of the blend oil in a point contact, as shown infigure 10. The Hertzian radius was 70�m. The blend oilconsists of PAO400 (395 cSt @40 �C) and PAO5 (5.1 cSt@40 �C). PAO100/0 and PAO50/50 mean that the weightratio of PAO400 and PAO5 was 100:0 and 50:50respectively. As shown in figure 10, the blend ratio of theblend oil, PAO50/50, decreases in the inlet zone of theEHL contact. This figure suggests that molecules withlow viscosity are easier to get into the Hertzian contactarea in a point contact than those with high viscosity.However, this result is opposite to the result in the case ofthe combination of HFC-134a refrigerant gas and POE.

According to Henry’s law, the solubility of a gas in aliquid increases in proportion to the pressure of gas.However, the phenomenon in this study is different fromthe Henry’s law situation in the following two respects.One is that the Hertzian pressure distribution is dis-similar in character from the atmosphere pressure of thegas, and the other is that the refrigerant dissolved in theHertzian contact area cannot directly exchange with therefrigerant in the surrounding region.

Figure 11 shows the effect of load between the steelball and the CaF2 disk on the concentration distributionof HFC-134a refrigerant. The results were measured on

Figure 8. Two-dimensional concentration distribution of HFC-134a

under standard conditions.

Figure 9. Concentration-change of additive in the EHL film [4].

Figure 10. Concentration of blend oil in point contact [7].

Figure 11. Effect of load.


the center line of the steel ball. In this figure, the posi-tions where the refrigerant concentration hasn’t beenmeasured properly because of the thin film thickness(less than 0.5�m) are illustrated in a dotted line.Although the maximum Hertzian pressure increasesfrom 174MPa (Hertzian radius: 116�m) to 220MPa(Hertzian radius: 146�m) upon increasing the load, theconcentration distribution of HFC-134a refrigerant inthe EHL film is not significantly affected by the load.

The effect of sliding velocity on the refrigerant con-centration in a lubrication film is shown in figure 12. Theresults were measured at the position of �200�m on thecenter line of the steel ball. The measured refrigerantconcentration is equal to that in the bulk fluid at lowsliding velocity. As the sliding velocity increases, therefrigerant concentration in the EHL contact decreases.Although the phase inversion has been observed in O/Wtype emulsion lubrication [11], it wasn’t observed in thisstudy. The phase inversion is related to the size of drop-lets that are dispersed into the continuous phase. It isconsidered that the phase inversion doesn’t occur in thecase of the combination of HFC-134a refrigerant andPOE because HFC-134a may dissolve in POE, namelydisperse on a molecular scale.

The effect of oil temperature on the concentrationdistribution of the refrigerant on the center line of thesteel ball is shown in figure 13. The refrigerant con-centration in the bulk fluid is 11.0mass% at all tem-perature. As the oil viscosity decreases from0.0175 Pa � s to 0.0094 Pa � s with an increase in the oiltemperature from 40 to 60 �C, the variation in the con-centration distribution of refrigerant is expanded.

Figure 14 shows the effect of refrigerant concentra-tion in the bulk fluid on the concentration distributionof refrigerant. When the refrigerant concentration in thebulk fluid is low, the refrigerant concentration in theinlet boost region of the EHL contact is almost equal tothat in the bulk fluid. The refrigerant concentration in

the inlet boost region of the EHL contact decreases withan increase in the refrigerant concentration in the bulkfluid.

4. Conclusions

The two-dimensional distributions of refrigerantconcentration in a lubricating film have been measuredin a pressurized refrigerant gas atmosphere using microFT-IR, which can observe the EHL film in a pointcontact. The results show that the concentration ofHFC-134a refrigerant under standard experimentalconditions decreases in the inlet boost region of theEHL contact and the Hertzian contact area, but in theside region of the Hertzian contact area it is greater thanthat in the bulk fluid. The decrease in the refrigerantconcentration in the inlet region of Hertzian contactincreases the EHL film thickness. The concentrationdistribution of HFC-134a refrigerant is affected by thesliding velocity, the oil temperature and the refrigerantFigure 12. Effect of sliding velocity.

Figure 13. Effect of oil temperature.

Figure 14. Effect of refrigerant concentration in bulk fluid.


concentration in the bulk fluid, but not significantlyaffected by the load.

References

[1] R.W.M. Wardle, R.C. Coy, P.M. Cann and H.A. Spikes, Lubr.

Sci. 3 (1990) 45.

[2] P.M. Cann and H.A. Spikes, Tribol. Trans. 34 (1991) 248.

[3] J.L. Lauer and Y-J. Ahn, STLE Trans. 31 (1988) 120.

[4] Y. Hoshi, N. Shimotomai, M. Sato and S. Mori, J. Jap. Soc.

Tribol. 44 (1999) 736. (in Japanese)

[5] S. Tanaka, K. Kyogoku and T. Nakahara, in: Proc. Int. Tribol.

Conference, Nagasaki, 2000 (JST, Tokyo, 2001) p. 1395.

[6] B. Ono and Y. Yamamoto, in: Proc. JAST Tribol. Conference,

Tokyo, 2001 (JST, Tokyo, 2001) p. 289. (in Japanese)

[7] B. Ono and Y. Yamamoto, in: Proc. JAST Tribol. Conference,

Utsunomiya, 2001 (JST, Tokyo, 2001) p. 309. (in Japanese)

[8] R.J. Chittenden, D. Dowson, J.F. Dunn and C.M. Taylor, in:

Proc. Roy. Soc., Vol. A397, London, 1985 (Royal Society of

London, London, 1985) p. 245.

[9] B.Y.C. So and E.E. Klaus, ASLE Trans. 23 (1980) 409.

[10] ASTM Standards : D341.

[11] T. Nakahara, T. Makino and K. Kyogoku, Trans. ASME J.

Tribol., 110 (1988) 348.


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