*4.2.1 Experimental results*

After verifying the integrity of the experimental configuration and test procedures, heat transfer and flow characteristics of the single-phase flow of pure PAO fluid inside the additively manufactured microchannel were experimentally investigated and then compared with those of minichannel heat exchanger introduced earlier. All the experiments conducted in the present study were repeated five times, and the relative errors of test data were found to be less than 5%. Since Poisueille number remains constant with variations of Reynolds in the laminar regime, the experimental data for each set was averaged over the laminar flow regime.

**Figure 18** represents the variations of the measured average Nusselt number with Reynolds for single-phase flow of the pure PAO within the laminar regime (100 < Re < 2000) for both microchannel and minichannel heat exchangers. As shown in this figure, while the Nusselt number gradually increases with Reynolds for both microchannel and minichannel, a significant enhancement of heat transfer is observed using the microchannel compared to the minichannel heat exchanger under the same operating conditions. Furthermore, the experimental results were compared with the Stephon's correlation for internal flow. As illustrated in **Figure 18**, the Stephon's correlation underestimates the Nusselt number inside the microchannel whereas it agrees well with the minichannel. The main reason for the reflected deviation is attributed to the larger surface roughness of the additively manufactured microchannel using the DMLS technique compared to that of the

*Convective Heat Transfer of Ethanol/Polyalphaolefin Nanoemulsion in Mini… DOI: http://dx.doi.org/10.5772/intechopen.96015*

#### **Figure 18.**

To characterize internal surfaces of the part including the roughness of each surfaces and true cross-section area, a computed X-ray tomography (CT Scan) was used. The CT scan takes a series of 2D images of an object and then incorporates them to form a 3D reconstruction of the object using the software algorithms. Through this method, the external and internal surfaces can be determined along with the channel surface roughness. An arithmetic roughness average, *Ra*, was calculated using the height differences between the CT scan data and the designed

After verifying the integrity of the experimental configuration and test procedures, heat transfer and flow characteristics of the single-phase flow of pure PAO fluid inside the additively manufactured microchannel were experimentally investigated and then compared with those of minichannel heat exchanger introduced earlier. All the experiments conducted in the present study were repeated five times, and the relative errors of test data were found to be less than 5%. Since Poisueille number remains constant with variations of Reynolds in the laminar regime, the experimental data for each set was averaged over the laminar flow

**Figure 18** represents the variations of the measured average Nusselt number with Reynolds for single-phase flow of the pure PAO within the laminar regime (100 < Re < 2000) for both microchannel and minichannel heat exchangers. As shown in this figure, while the Nusselt number gradually increases with Reynolds for both microchannel and minichannel, a significant enhancement of heat transfer is observed using the microchannel compared to the minichannel heat exchanger under the same operating conditions. Furthermore, the experimental results were compared with the Stephon's correlation for internal flow. As illustrated in

**Figure 18**, the Stephon's correlation underestimates the Nusselt number inside the microchannel whereas it agrees well with the minichannel. The main reason for the reflected deviation is attributed to the larger surface roughness of the additively manufactured microchannel using the DMLS technique compared to that of the

channel wall surface. The results have shown that the *Ra=Dh*value is 0.23.

*Additively manufactured (AM) microchannel heat exchanger developed for the present study.*

*Heat Transfer - Design, Experimentation and Applications*

**4.2 Results and discussions**

*4.2.1 Experimental results*

regime.

**186**

**Figure 17.**

*Average Nusselt number versus Reynolds for the pure PAO fluid in both minichannel and microchannel heat exchangers.*

traditionally manufactured minichannel heat exchanger. Stimpson et al. [84] and Snyder et al. [85] have investigated the effects of surface roughness and printing orientation on flow structures in DMLS manufactured parts and proved that the existing empirical correlations for heat transfer and friction factor are no longer valid for the additively fabricated surfaces with high roughness values. This is also important to point out that the non-post processed surface of the DMLS manufactured microchannels is likely to be the main contributor to the augmented heat transfer performance. Future study is then required to better appreciate the possible mechanisms behind the phenomenon observed here.

**Figure 19** shows the variations of friction factor with a range of Reynolds numbers lying in the laminar regime for both the minichannel and microchannel heat exchangers. As clearly represented in this figure, the friction factor decreases with the increase of Reynolds number for both the test sections experimented in the present study. It can also be observed that the average friction factor of the PAO fluid inside the AM microchannel remains noticeably higher compared to that of the minichannel, and the entrance effect is more pronounced at lower Reynolds

#### **Figure 19.**

*Friction factor versus Reynolds number for the pure PAO fluid in both minichannel and microchannel heat exchangers.*

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 DMLS manufactured microchannel as discussed earlier.

at a certain Reynolds number of 500 using the 8 wt% nanoemulsion fluid compared

This is important to note that while the simulation results in **Figure 20** show a Nusselt value of 22 for the fully developed flow of pure PAO, the experimental results introduced in **Figure 18** show a Nusselt value of almost 20 at the same Reynolds of 500, which indicates a relative deviation of approximately 10% between the simulation and experimental results. This deviation is mainly due to

As vividly seen from the simulation results in **Figure 20**, there is a declining trend of local Nusselt number along the early locations of the microchannel. The significantly higher Nusselt number at the early locations and its subsequent sharp drop is due to the thermal entrance region at the inlet of the microchannel test section where the internal liquid single-phase flows are still neither thermally nor hydrodynamically fully developed. As a result of having a thermally developing flow in the entrance region, the thermal boundary layer is extremely thin which causes larger values of Nusselt and HTC compared to those of locations outside the

Using the Eq. (13), the hydrodynamic entry length for the laminar flows of PAO and nanoemulsion in the microchannel was found to be 17.5 mm. As illustrated in **Figure 20**, the local Nusselt numbers for each of the working fluids take relatively constant values after the entry length of 17.5 mm where the boundary layer develops fully across the cross section of the microchannel and appears to be independent of the channel length. Since the working fluids are PAO and nanoemulsion with Prandtl numbers greater than 1 (Pr > 1), the hydrodynamic

In this study, the flow and heat transfer characteristics of a novel nanostructured

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

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

to that of the pure PAO inside the microchannel heat exchanger.

*DOI: http://dx.doi.org/10.5772/intechopen.96015*

*Convective Heat Transfer of Ethanol/Polyalphaolefin Nanoemulsion in Mini…*

the development of model which fails to study the effect of the DMLS manufactured surfaces on the flow and heat transfer characteristics.

entrance region where both of the working fluids are fully developed.

boundary layer develops more quickly than the thermal boundary layer.

heat transfer fluid (i.e., ethanol/polyalphaolefin nanoemulsion) inside a

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

**5. Conclusions**

laminar flow regime.

**189**

#### *4.2.2 Simulation results*

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 flux boundary conditions imposed on the top and bottom surfaces of the 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 reasonable computational time.

**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%

#### **Figure 20.**

*Variations of local Nusselt number along the microchannel at Re = 500 for pure PAO and 8% ethanol/PAO nanoemulsion fluids with the entrance effects.*

#### *Convective Heat Transfer of Ethanol/Polyalphaolefin Nanoemulsion in Mini… DOI: http://dx.doi.org/10.5772/intechopen.96015*

at a certain Reynolds number of 500 using the 8 wt% nanoemulsion fluid compared to that of the pure PAO inside the microchannel heat exchanger.

This is important to note that while the simulation results in **Figure 20** show a Nusselt value of 22 for the fully developed flow of pure PAO, the experimental results introduced in **Figure 18** show a Nusselt value of almost 20 at the same Reynolds of 500, which indicates a relative deviation of approximately 10% between the simulation and experimental results. This deviation is mainly due to the development of model which fails to study the effect of the DMLS manufactured surfaces on the flow and heat transfer characteristics.

As vividly seen from the simulation results in **Figure 20**, there is a declining trend of local Nusselt number along the early locations of the microchannel. The significantly higher Nusselt number at the early locations and its subsequent sharp drop is due to the thermal entrance region at the inlet of the microchannel test section where the internal liquid single-phase flows are still neither thermally nor hydrodynamically fully developed. As a result of having a thermally developing flow in the entrance region, the thermal boundary layer is extremely thin which causes larger values of Nusselt and HTC compared to those of locations outside the entrance region where both of the working fluids are fully developed.

Using the Eq. (13), the hydrodynamic entry length for the laminar flows of PAO and nanoemulsion in the microchannel was found to be 17.5 mm. As illustrated in **Figure 20**, the local Nusselt numbers for each of the working fluids take relatively constant values after the entry length of 17.5 mm where the boundary layer develops fully across the cross section of the microchannel and appears to be independent of the channel length. Since the working fluids are PAO and nanoemulsion with Prandtl numbers greater than 1 (Pr > 1), the hydrodynamic boundary layer develops more quickly than the thermal boundary layer.
