VII Brazilian Conference on Rheology

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GrMaTS – Grupo de Pesquisa em Materiais, Tribologia e Superfícies, Universidade Tecnológica Federal do Paraná,. Av. 7 de Setembro, 3165 ZIP CODE ...
VII Brazilian Conference on Rheology – BCR 2015 Curitiba, PR, Brazil, July 05-08

Maintenance of Hydroerosive Grinding Process Efficiency based on the Abrasive Fluid Viscosity Pâmela Portela Moreira ([email protected]) Marina Izabelle Grabarski ([email protected]) Giuseppe Pintaude ([email protected]) GrMaTS – Grupo de Pesquisa em Materiais, Tribologia e Superfícies, Universidade Tecnológica Federal do Paraná, Av. 7 de Setembro, 3165 ZIP CODE 80230-901, Curitiba-PR, Brazil

1 INTRODUCTION The injection system for diesel engines has been investigated aims to improve the quality of air-fuel mixture, which has a direct effect on the pollutant emissions. One of the main components of injection system is the injection nozzle, a high-precision element and responsible to spray the fuel into the chamber engine, such that the better the spray, the better the engine performance (Payri et al, 2005). Diesel injection nozzles have from five to ten injection holes with diameters between 100 and 200 μm and aspect ratios in the range 10:1 to12:1 (Diver et al, 2007). As the geometry of injection holes after Electrical Discharge Machining is not enough adequate, an additional step is required in order to reduce the surface roughness and to increase the rounding radius at the inlet holes, reducing the cavitation and increasing the discharge coefficient (Payri et al, 2005). The removal of material during the hidroerosive grinding is achieved through the flowing of an abrasive fluid inside the confined areas, resulting in rounding at the fuel-entry side of the hole. Figure 1 illustrates in a schematic way the change in hole geometry, after the hidroerosive grinding. In this Figure, Q is the volumetric flow rate before the process, QHE is the volumetric flow rate after the process and rHE is the rounding radius.

(a) (b) Figure 1. Geometry of an abrupt contraction: (a) before hidroerosive grinding and; (b) after hidroerosive grinding. The abrasive fluid is composed by high hardness particles suspended in a mineral oil. The viscosity of abrasive fluid plays an important role on the efficiency of process, once it is related directly to the coupling of particles with the fluid (Coseglio et al., 2015). The process efficiency was quantified by the amount of nozzles produced in a batch, Ni, the volumetric flow rate before the process, Q1(j), for each piece j and the volumetric flow rate reached after the processing Q2(j). 𝑁𝑖

𝐸̅ℎ(𝑖)

𝑄2(𝑗) − 𝑄1(𝑗) 1 = ∑( ) ∙ 100 𝑁𝑖 𝑄1(𝑗)

(1)

𝑗=1

In a previous investigation (Coseglio et al, 2015), the production of Valve Covered Orifice (VCO)type injection nozzles was monitored for a total of 150 hours of hydroerosive grinding. No abrasive particles were added to the abrasive fluid during this time and all the operating conditions were kept

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VII Brazilian Conference on Rheology – BCR 2015 Curitiba, PR, Brazil, July 05-08

constant. The hydroerosive grinding unit that was monitored produces a wide variety of injection nozzles in different-sized batches according to the internal demand. A significant decrease was observed in the viscosity of abrasive fluid: after approximately 100 hours of production, the viscosity dropped 50%. At the same time, there was a 28% reduction in the volume concentration of solids in the abrasive fluid after 100 hours of use. This reduction was primarily caused by the loss of abrasive particles that deviate from the carrier fluid trajectory and become trapped in the nozzle near the injection hole. These particles are eliminated from the circuit during flushing operation, resulting in a higher volume percentage of fine particles in the abrasive fluid circulating in the hydroerosive grinding station. The interaction between abrasive particles and carried fluid can be evaluated by means of the equilibrium moment (λ), a non-dimensional parameter that means as a balance between the viscous and inertial forces acting on the particles. Table 1 presents possible values for this parameter and the resulting coupling between particles and carried fluid. A low coupling can imply in a loss of particles during the processing, meaning in a reducing of efficiency (Humphrey, 1990). Table 1. Possible values for equilibrium moment of a particle within a fluid. λ1 Viscous forces predominance: Equilibrium between the Inertial forces predominance: the particles tend to follow the flow viscous and inertial forces is particles do not tend to follow the lines easily. verified: the prediction of flow lines. particle behavior is difficult. High coupling between the Moderate coupling. Low coupling between the particle particle and the carried fluid. and the carried fluid. Coseglio et al (2015) attributed the loss in the process efficiency to the decrease in the oil viscosity along the hidroerosive grinding process. In this way, a methodology could be planned to correct this effect along the production cycle. This investigations aims to evaluate the effect of the adjustment of abrasive fluid on the process efficiency, taken into account the systematic addition of a mineral oil with high viscosity. Beside this, the causes that gave rise to the decrease in the viscosity were also evaluated. 2 EXPERIMENTAL PROCEDURES AND RESULTS The methodology employed in the current investigation is based on the fact that a periodic adjustment is required to keep the viscosity of abrasive fluid along the production. The dynamic viscosity of mineral oil used during the processing is 58.2 mPa.s at 25 ºC and another mineral oil, with the same physicochemical characteristics was chosen to make the viscosity correction, with a higher viscosity, 161.2 mPa.s at 25 ºC. The additions of high-viscosity mineral oil were performed at each 40 hours of production. Always in the previous day before the addition, a sample of abrasive fluid was taken and filtered, to measure its viscosity. Knowing this value, the volume of high-viscosity mineral oil can be calculated using Eq. (2), based on the Arrhenius model (Gao and Li, 2012).

𝑉adc =

(𝑉𝑓 ∗ 𝑙𝑜𝑔𝜇𝑓 − 𝑉𝑖 ∗ 𝑙𝑜𝑔𝜇𝑖 ) − ((𝑉𝑓 − 𝑉𝑖 ) ∗ 𝑙𝑜𝑔𝜇𝐴 ) (𝑙𝑜𝑔𝜇𝐵 − 𝑙𝑜𝑔𝜇𝐴 )

where: Vadc: volume of oil to be added; Vi: volume of tank before the adjustment; Vf: final volume of tank after adjustment; μf: aimed viscosity; μi: viscosity of abrasive fluid before the adjustment; μA: nominal viscosity of the process oil; and μB: nominal viscosity of the oil to be added.

2

(2)

VII Brazilian Conference on Rheology – BCR 2015 Curitiba, PR, Brazil, July 05-08

The viscosity was measured using rotating cylinder viscometer Brookfield, equipped with a sensor Searle. The measurements were performed at 25°C. The variation of dynamic viscosity can be observed in Figure 2. In this Figure the previous results (Coseglio et al, 2015) are also presented, when any kind of interference was made during the processing. One can observe a significant difference between the current investigation and that performed by Coseglio et al (2015) with respect to the maintenance of the dynamic viscosity of abrasive fluid, along the production cycles.

Dynamic viscosity (mPa.s)

60 50 40 30 20 10

Coseglio, 2015

Current study

Viscosity adjustments

0 0

0,2 0.2

0,4 0.4

0,6 0,8 0.6 0.8 Time (Normalized)

1

11,2 .2

Figure 2. Variation of oil viscosity as a function of the production time. Another kind of result that confirms the necessity to adjustment of viscosity is the size distribution of particles. The size distribution of particles was determined in a laser granulometer (Microtrac S3500). The particles were suspended in isopropyl alcohol and kept in an ultrasound bath up to the measurements to avoid any agglomeration. Again, the obtained values of statistical parameters d10, d50 and d90 (grain sizes at which a specified percentage of the grains are coarser for 10, 50 and 90% in volume, respectively) are compared with those obtained by Coseglio et al (2015). This comparison can be observed in Table 2, for two different times of production. In the current investigation, no significant difference along time is observed, a very different behavior when any adjustment was performed in the previous study, once a significant decrease in d10 was noted.

Diameter dp (μm) d10 d50 d90

Table 2. Size distribution of B4C particles. Samples from current study Coseglio et al (2015) Fresh abrasive Th = 2.78 h Th = 67.67 h Th = 0 h Th = 150 h 2.5 3.6 3.7 2.5 1.7 6.8 6.2 6.3 6.0 6.0 10.7 11.0 10.4 10.7 10.2

A hypothesis to explain the significant drop in the viscosity along the production time is the contamination of mineral oil used in hydroerosive grinding (processing oil) by another one, which is used to check the initial volumetric flow rate (check oil). This oil has a lower dynamic viscosity (15.2 mPa.s) than the oil used to process.

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VII Brazilian Conference on Rheology – BCR 2015 Curitiba, PR, Brazil, July 05-08

In order to verify this possibility, 10 injection nozzles were analyzed after the check step, before the hydroerosive grinding. These pieces were washed using hexane, then they were heated at 200 °C to evaporate and 1.5 g of oil was found. Knowing the amount of processed injection nozzles along the investigated cycles, and knowing that after 40 hours of production a drop of 10 mPa.s in the dynamic viscosity occurred, a simulation was made in a bench. For 100 mL of oil used in the process, 4 mL of check oil would be enough to promote this decrease, equivalent to those 1.5 g found. However, the measured amount of oil was not sufficient to reduce the viscosity in 10 mPa.s, following Table 3. A higher amount of oil was added to reach the expected drop, 13.5 mL. Table 3. Variation of viscosity as a function of check oil addition to the processing oil. Volume of check oil (mL) 0.0 2.5 4.0 7.0 11.0 13.5 Viscosity at 25 °C (mPa.s) 58.25 55.8 54.5 52.5 49.9 48.25 The difference observed in the residual amount of oil after the washing and evaporation and that it would be necessary to cause the specific decrease in the viscosity (13.5 mL following Table 3) could be explained by an unexpected removal of mass due to the evaporation. The used temperature must have been high enough to remove more oil than that is actually should be found after the check step. In this way, this contamination was then confirmed using mass spectroscopy analysis, but these results are not be presented here in detail. 3 CONCLUSIONS The maintenance of hydroerosive grinding efficiency, which is directly affected by the abrasive fluid viscosity, can be achieved using systematic addition of high-viscosity oil along the production time to manufacture diesel injection nozzles. Other important characteristic of abrasive fluid is kept constant with this addition, the distribution size of particles did not change along the processing, proving that the viscosity is a key aspect to guarantee the coupling of particles and mineral oil. The significant drop in the viscosity of mineral oil used to process is caused by the contamination due to the oil used to check the initial volumetric flow rate before the hydroerosive grinding, once it has a low viscosity. 4 ACKNOWLEDGEMENTS The authors would like to express their gratitude to Robert Bosch Ltda for providing funding as part of project ACT 02/2013. G. Pintaude would like to thank CNPq for the financial support provided under Project 306727/2011‐0. 5 REFERENCES Coseglio, M. S. D. R., Moreira, P.P., Procopio, H.L., Pintaude, G. Analysis of the efficiency of hydroerosive grinding without renewal of abrasive particles, submitted to Journal of Manufacturing Science and Engineering, 2015. Diver, C., Atkinson, J., Befrui, B., Helml, H.J., Li, L. 2007, Improving the geometry and quality of a micro-hole fuel injection nozzle by means of hydroerosive grinding. Proceedings of the Institution of Mechanical Engineers, vol. 221 Part B: Journal of Engineering Manufacture, pp. 1-9. Gao, Y., Li, K. 2012, New models for calculating the viscosity of mixed oil, Fuel, vol. 95, pp. 431-437. Humphrey, J. 1990, Fundamentals of fluid motion in erosion by solid particle impact. International Journal of Heat and Fluid Flow, vol. 11, No. 3, pp. 170-195. Payri, R., García, J.M., Salvador, F.J., Gimeno, J., 2005. Using spray momentum flux measurements to understand the influence of diesel nozzle geometry on spray characteristics. Fuel, vol. 84, pp. 551– 561.

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