**5. Conclusions**

numbers. Similar to the heat transfer measurements, the friction factor measurements were compared and validated with classical empirical correlations proposed in the literature. As illustrated in **Figure 19**, the friction factor of the PAO fluid flowing inside the minichannel agrees well with the classical Hagen-Poiseuille correlation within the laminar regime. Considering the relatively large dimensions of the microchannel (*H* ¼ 780*μm* � *W* ¼ 640*μm*Þ, Baharmi model [89] underestimates the frication factor, and it is likely due to the rough internal surface of the

The prototype of the microchannel heat exchanger was designed using CREO software and then the model was imported to COMSOL-Multiphysics to conduct numerical heat transfer analysis for the same geometry and operating conditions summarized in **Table 5**. The following assumptions were adopted to conduct the simulations: no slip boundary condition, normal inflow velocity, uniform wall heat

microchannel heat exchanger, and thermophysical properties of the working fluids are set to remain constant for the values listed earlier in **Table 4**. To confirm mesh independency of the simulation results and compare their accuracies, three types of meshes were developed, including finer, fine, and normal. The finer mesh size was ultimately chosen as it provides sufficiently accurate analysis while still sustaining a

**Figure 20** represents the variations of local Nusselt number at a certain Reynolds number of 500 along the microchannel heat exchanger with the same geometry and dimensions used previously for the experimental investigations as listed in **Table 5**. As shown in this figure, the local Nusselt number decreases along the microchannel at early positions and then reaches relatively constant values of 22 and 29 for the pure PAO and 8 wt% Ethanol/PAO nanoemulsion, respectively, within the fully developed laminar region. This reveals a heat transfer enhancement of around 32%

*Variations of local Nusselt number along the microchannel at Re = 500 for pure PAO and 8% ethanol/PAO*

flux boundary conditions imposed on the top and bottom surfaces of the

DMLS manufactured microchannel as discussed earlier.

*Heat Transfer - Design, Experimentation and Applications*

*4.2.2 Simulation results*

reasonable computational time.

**Figure 20.**

**188**

*nanoemulsion fluids with the entrance effects.*

In this study, the flow and heat transfer characteristics of a novel nanostructured heat transfer fluid (i.e., ethanol/polyalphaolefin nanoemulsion) inside a minichannel of circular cross section and a microchannel of rectangular cross section were investigated experimentally and numerically. The experiments were performed for single-phase flow of pure PAO and ethanol/PAO nanoemulsion fluids of 4 wt% and 8 wt% concentrations within the laminar and transitional regimes as well as for two-phase flow boiling of nanoemulsion fluids within the laminar flow regime.

It was revealed that the nanoemulsion fluids thermally outperformed the pure PAO base fluid in single-phase flow of transitional regime, however, it does not reflect an appreciable improvement in single-phase heat transfer performance within the laminar flow regime. The significant heat transfer enhancement achieved at higher concentrations of nanoemulsion within the transitional regime is mainly attributed to the enhanced interaction and interfacial thermal transport between ethanol nanodroplets and PAO base fluid. For two-phase flow boiling, heat transfer coefficients of ethanol/PAO nanoemulsion fluids were further enhanced once the ethanol nanodroplets underwent phase change. A comparative study was also conducted on the flow and heat transfer characteristics of pure PAO between the traditionally manufactured minichannel and additively manufactured microchannel under the same operating conditions. Despite the higher friction factor and pressure loss, significant heat transfer enhancements were achieved using the additively manufactured microchannel compared to the traditionally fabricated minichannel heat exchanger under the same operating conditions, so that

the gained heat transfer enhancement through the microchannel is more prominent than its drawback of increased pressure loss. The non-post processed surface of the DMLS manufactured microchannel is believed to be the main contributor to the augmented heat transfer and pressure drop. Further studies are needed to fully understand the possible mechanisms behind it, and also to gain a deeper insight into the phase-change heat transfer characteristics of nanoemulsion fluids.

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