The two-phase flow in vertical pipe can be classified according to the geometry of the interfaces. The α(r) profiles are plotted together for different values of ¯U ws for each value of β, as shown in Figure 8(a) and (b). c) The α(r) distribution of Vigneaux et al.
Introduction
The potential of this technique has been experimentally demonstrated by investigations of fluid flow and heat transfer using different fluids, such as nanofluids based on distilled water, aluminum oxide (Al2O3), and iron oxide (Fe3O4). To demonstrate the potential of this technique, fluid flow and heat transfer studies were performed using different fluids such as distilled water, aluminum oxide (Al 2 O 3 ), and iron oxide (Fe 3 O 4 ) nanofluids at concentrations of 1 and 2.5%.
Materials and methods
- Nanoparticles preparation
- Fabrication of the heat sink microchannel
- Experimental procedures
- Heat transfer calculations
The temperature of the plate was set at 60°C, while the flow rate of the liquids was controlled by a syringe pump (Harvard) connected to the inlet of the heat sink. The flow of the Fe 3 O 4 nanofluid was analyzed by optical microscopy at a flow rate of 10 μL/min. The setup and calibration procedures of the camera were performed as in Teodori et al.
To evaluate the influence of the properties of nanofluids in the microfluidic device of the refrigerator, tests were carried out using distilled water, Fe 3 O 4 in a concentration of 1 and 2.5% and Al 2 O 3 in the same concentrations. The properties of nanofluids were obtained considering the fundamental equations described in previous studies [39, 40]. Nanofluid density and heat capacity were calculated using a weighted average of the individual properties of both the NPs and the base fluid.
Consequently, a mathematical approach that better enabled heat transfer estimation was described by Ma et al.
Results and discussion
Influence of the nanofluid properties
The heat transfer rate, Q, represents the amount of heat energy that the fluid takes as it flows through the channels and is given by Eq. This parameter was obtained for each mass flow rate, m ̇ , after the temperature at the inlet, T in , and at the outlet, T out , was measured. Convective heat transfer coefficient for distilled water, alumina, and iron oxide nanofluids as a function of flow rate.
In fact, a decrease in the convective heat transfer coefficient was observed when the NP concentration increased to 2.5%. Temperature difference between inlet and outlet for nanofluids tested in the proposed PDMS cooler microfluidic device. The main possible reasons for the first deterioration of heat transfer include both aggregation and sedimentation of NPs.
Currently, our group is engaged in both experimental and numerical work to identify the exact causes for such phenomena.
Optical and thermal imaging analyses
This verified roughness was caused by the ABS master mold produced by the FDM 3D printer. To improve the surface roughness, the ABS master molds should undergo an acetone vapor treatment before performing the PDMS casting procedure. Another interesting advantage of this PDMS microfluidic device is the ability to visualize both flow and thermal performance of the system using a thermographic camera, as shown in Figure 12.
Note that the temperatures obtained came from the surface of the heat sink and not directly from the working fluid flowing in the microchannels. Therefore, the thickness of the top walls should be reduced in future experiments to obtain temperatures more closely related to the working fluids flowing through the microchannels. A very interesting observation was the ability to detect bubbles likely to exist in microfluidic devices.
In figure 13 it is possible to visualize an air bubble inside the microchannel and the thermal performance of the heat sink.
Limitations and future directions
In fact, PDMS is transparent to visible radiation, but partially opaque to infrared (IR) radiation. Also, the heat sink walls were rough because of the ABS master mold used in the manufacturing technique, and the thickness should be reduced. The stability of the nanofluids has yet to be optimized since the deposition of NPs on the heat sinks was detected.
Conclusion
These results were only possible due to the optical transparency of the PDMS heat sink. Therefore, by using this device, it was possible to visualize several flow phenomena of the nanofluids, such as the formation, growth, and degradation of NPs clusters. From these latter observations, it was possible to conclude that one of the main reasons for the formation of the clusters was the high roughness of the PDMS surface channels caused by the surface roughness of the ABS master mold produced by the FDM 3D printer.
Overall, the simplicity, low cost, and unique properties of the proposed PDMS heat sink microfluidic device may prove to be a viable alternative tool to investigate nanofluid flow and heat transfer phenomena that are not feasible by the current traditional systems. The authors also acknowledge FCT for partially funding the research within the framework of the project UTAP-EXPL/CTE financiado no amibito do Projeto 5665-Parcerias Internacionais de Ciência e Tecnologia, UT Austin Programme. Moita also acknowledges the FCT for its contract in connection with the recruitment program FCT Investigator (IF and exploratory project associated with it.
In addition, the application of nanoparticles in some of the green technologies such as MQL and MQCL using vegetable oils not only provides superior cooling and lubricating properties and minimizes the use of cutting fluids, but also creates new machining solutions, especially for difficult-to-cut materials.
Hard machining under MQL condition using nanofluid
The improvement of cutting performance
Through experimental results, the cutting force components Fx, Fy and Fz when changing the cutting speed and based fluid are shown in Figures 2-4. Under NFMQL with emulsion-based oil, the cutting forces Fx, Fy and Fz decrease with increasing cutting speed from 18 to 30 m/min. It can be clearly seen that the cutting forces decrease due to the better lubrication performance of soybean oil compared to emulsion liquid.
When increasing the cutting speed from 18 to 30 m/min, MQL with emulsion-based fluid did not provide sufficient lubrication effects, so the cutting temperature rose rapidly and damaged the end mills. Therefore, soybean oil-based nanofluid has excellent lubrication effects to reduce the friction coefficient in the cutting area due to easier oil formation. In addition, the Al2O3 nanoparticles with almost spherical morphology suspended in the oil mist as "rollers" play an important role in improving the cooling and lubrication effects.
Furthermore, the cutting speed is also increased from 18 to 30 m/min using MQL emulsion-based nanofluid, which reveals better cooling and lubricating effects compared to pure fluids.
The important parameter of MQL nanofluid
It can be clearly observed from Figure 10 that, for better surface roughness, the low value of nanoparticle concentration around 0.5 wt% is more preferable than the larger ones (1.0 and 1.5 wt%). In contrast, the higher concentration (about 1.0 and 1.5 wt%) contributes to the significant reduction of cutting forces and cutting temperature compared to the lower one (0.5 wt%). Of those, the wear rate is greatly reduced by increasing the concentration of Al2O3 nanoparticles to 1.0-1.5 wt%, so the tool life is extended (Figures 12-15).
Therefore, the concentration of nanoparticles must be chosen not only to ensure good tool life, but also to maintain high surface quality. From Figure 15, it clearly shows that during the first 40 minutes, the surface roughness values are higher when the high concentration of nanoparticles (1.0-1.5 wt%) is used. It is the new observation obtained from the validation experiments, which is carried out until the life of the tool is over to see the actual phenomena after obtaining the ANOVA and RSM results.
Interestingly, the tool life in the case of soybean-based nanofluid 1.5 wt% is equal to emulsion-based nanofluid 0.5 wt%.
Hard machining under MQCL condition
Using a real cooling method assisted with MQL technique to form MQCL state is a new approach. The deep study on hard grinding of SKD 11 steel (52–60 HRC) in terms of surface quality under MQCL condition was done, and the results were compared with dry and MQL conditions. From Figure 16, hard grinding under the MQCL method brought out better surface roughness than that under dry and MQL conditions.
The main reason is that the MQCL technique provides sufficient cooling and lubrication effects, especially the cooling effect, which helps to reduce the cutting temperature and tool wear. White layer and burn marks were significantly reduced in MQL and MQCL conditions compared to dry cutting due to improved cooling and lubrication. Burn marks under MQCL conditions are smaller than those of the MQL method due to better cooling efficiency (Figures 19(a), 20(a)).
In addition, compared to dry and MQL conditions, the compression of the worked surface observed from the surface profile decreases much (Figure 18(b), 19(b), 20(b)).
MQCL hard machining using nanofluids
The protective film decreases and disappears when the concentration increases to 0.8%, which causes a negative effect on the surface quality [37].
Conclusion
Effect of Nanoscale Textures on the Cutting Performance of WC/Co-Base Ti55Al45N Coated Tools in Dry Cutting. A study of the effect of palm oil as an MQL lubricant on high speed drilling of titanium alloys. Performance of Al2O3 nanofluids in minimum quantity lubrication in hard grinding of 60Si2Mn steel using cemented carbide tools.
An experimental study of the effect of nanoparticle concentration on the lubricating properties of nanofluids for MQL grinding of Ni-based alloys. Effect of addition of hBN nanoparticles to nanofluid-MQL on tool wear patterns, tool life, roughness and temperature in turning Ni-based Inconel 625. Effect of alumina nanofluid concentration on minimum lubrication hard machining for sustainable manufacturing.
A study of droplet sizes, their distribution and heat transfer for minimum quantity cooling lubrication (MQCL).
