*3.1.1.1 Single-phase results of PAO Base fluid*

**Figure 8** illustrates variations of average Nusselt number with Reynolds for the pure PAO base fluid and compares the results with the empirical heat transfer correlations tabulated in **Table 8**. While Stephan correlation was used to predict

heat transfer characteristics within the laminar regime [73], the Gnielinski correlation was exploited for fully developed turbulent flow regime [73] and modified Gnielinski correlation [74] was used for comparison purposes within the transitional flow regime. As shown by the measured data and empirical correlations in **Figure 8**, the Nusselt number consistently increases with the Reynolds number, however it starts to increase at a greater rate for Reynolds greater than 2300 (critical Reynolds), which indicates a transition from laminar to turbulent flow regime. As represented in this figure, the experimental heat transfer results show good agreements with the empirical correlations in both laminar and transitional flow regimes. **Figure 9** shows variations of friction factor with Reynolds number for the pure PAO base fluid and compares the results with the empirical friction factor correlations listed in **Table 9**. The Hagen-Poiseuille correlation was used to predict flow characteristics within the fully developed laminar flow regime inside a circular minichannel [73] whereas the Shah correlation was used to include the entrance effect within the hydrodynamically developing region [73]. As represented in

*Empirical heat transfer correlations used for comparison purposes with their validity ranges.*

**Correlation Conditions Validity Range**

Laminar flow in a circular pipe *Re* <2300

<sup>3000</sup><sup>&</sup>lt; *Re* <sup>&</sup>lt;<sup>5</sup> � <sup>10</sup><sup>4</sup>

2300< *Re* <4500

Constant wall heat flux, fully developed turbulent and transitional flow

Constant wall heat flux, transitional flow in a straight circular pipe

**Figure 8.**

**Table 8.**

**177**

*Stephan correlation* [73] *Nu* <sup>¼</sup> <sup>4</sup>*:*<sup>364</sup> <sup>þ</sup> <sup>0</sup>*:*<sup>086</sup> *RePr<sup>D</sup>* ð Þ*<sup>L</sup>*

*Gnielinski correlation* [73] *Nu* <sup>¼</sup> ð Þ *<sup>f</sup> <sup>=</sup>*<sup>8</sup> ð Þ *Re* �<sup>1000</sup> *Pr* 1þ12*:*7 ffiffi *f* 8 p *Pr*<sup>2</sup> <sup>3</sup>�1 � �

where *<sup>f</sup>* <sup>¼</sup> <sup>1</sup>

*Nu* <sup>¼</sup> ð Þ *<sup>f</sup> <sup>=</sup>*<sup>8</sup> ð Þ *Re* �<sup>1000</sup> *Pr* 1þ12*:*7 ffiffi *f* 8 p *Pr*<sup>2</sup> <sup>3</sup>�1 � � where *<sup>f</sup>* <sup>¼</sup> <sup>3</sup>*:*<sup>03</sup> � <sup>10</sup>�<sup>12</sup><sup>∙</sup> *Re* <sup>3</sup> � <sup>3</sup>*:*<sup>67</sup> � <sup>10</sup>�<sup>8</sup><sup>∙</sup> *Re* <sup>2</sup> <sup>þ</sup> <sup>1</sup>*:*<sup>46</sup> � <sup>10</sup>�<sup>4</sup> <sup>∙</sup> *Re* � <sup>0</sup>*:*<sup>151</sup>

*Average Nusselt number versus Reynolds number for pure PAO fluid.*

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

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

1*:*33

0*:*83

<sup>1</sup>þ0*:*1Pr *Re <sup>D</sup>* ð Þ*<sup>L</sup>*

ð Þ <sup>1</sup>*:*82 log ð Þ� *Re* <sup>1</sup>*:*<sup>64</sup> <sup>2</sup>

*Modified Gnielinski correlation* [74]

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

#### **Figure 8.**

uncertainty in F is the sum of the square of the uncertainties due to each indepen-

uncertainty due to variable*a*. The uncertainties of the heat transfer performance

**Figure 7** illustrates the minichannel heat exchanger of 12 circular channels designed and fabricated conventionally to conduct the experimental tests. The material and geometry of the minichannel test section was shown earlier in **Table 5**,

Using the PAO base fluid, the flow and heat transfer characteristics of the minichannel test section were carefully evaluated to verify the integrity of the experimental facility and test procedures. The experiments on the PAO base fluid were first performed, and the test results were used as baseline data to compare with those of Ethanol/PAO nanoemulsion fluids of 4% and 8% wt% ethanol. A range of heat fluxes addressed earlier in **Table 3** was selected as heat inputs to simulate single-phase and two-phase flow heat transfer conditions. All the experiments performed in the present study were repeated five times, and the relative errors of test data were found to be less than 5%. The SANS measurements were performed on the samples only prior to initiating the experiments. However, previous studies on a similar water/PAO system have demonstrated that there are no remarkable structure changes of the nanodroplets before and after pool boiling tests

**Figure 8** illustrates variations of average Nusselt number with Reynolds for the

pure PAO base fluid and compares the results with the empirical heat transfer correlations tabulated in **Table 8**. While Stephan correlation was used to predict

*Traditionally manufactured minichannel heat exchanger developed for the present comparative study.*

*<sup>∂</sup><sup>c</sup> <sup>δ</sup><sup>c</sup>* � �<sup>2</sup> <sup>þ</sup> …

where *δa* stands for the

*<sup>∂</sup><sup>b</sup> <sup>δ</sup><sup>b</sup>* � �<sup>2</sup> <sup>þ</sup> *<sup>∂</sup><sup>F</sup>*

**3. Conventionally manufactured minichannel heat exchanger**

coupled with the operating conditions engaged to run the experiments.

h i0*:*<sup>5</sup>

*<sup>∂</sup><sup>a</sup> <sup>δ</sup><sup>a</sup>* � �<sup>2</sup> <sup>þ</sup> *<sup>∂</sup><sup>F</sup>*

*Heat Transfer - Design, Experimentation and Applications*

parameters are calculated and represented in **Table 7**.

dent variable, *<sup>δ</sup><sup>F</sup>* <sup>¼</sup> *<sup>∂</sup><sup>F</sup>*

inside an enclosed system [63].

*3.1.1.1 Single-phase results of PAO Base fluid*

**3.1 Results and discussions**

*3.1.1 Experimental results*

**Figure 7.**

**176**

*Average Nusselt number versus Reynolds number for pure PAO fluid.*


#### **Table 8.**

*Empirical heat transfer correlations used for comparison purposes with their validity ranges.*

heat transfer characteristics within the laminar regime [73], the Gnielinski correlation was exploited for fully developed turbulent flow regime [73] and modified Gnielinski correlation [74] was used for comparison purposes within the transitional flow regime. As shown by the measured data and empirical correlations in **Figure 8**, the Nusselt number consistently increases with the Reynolds number, however it starts to increase at a greater rate for Reynolds greater than 2300 (critical Reynolds), which indicates a transition from laminar to turbulent flow regime. As represented in this figure, the experimental heat transfer results show good agreements with the empirical correlations in both laminar and transitional flow regimes.

**Figure 9** shows variations of friction factor with Reynolds number for the pure PAO base fluid and compares the results with the empirical friction factor correlations listed in **Table 9**. The Hagen-Poiseuille correlation was used to predict flow characteristics within the fully developed laminar flow regime inside a circular minichannel [73] whereas the Shah correlation was used to include the entrance effect within the hydrodynamically developing region [73]. As represented in

### **Figure 9.**

*Friction factor versus Reynolds number for pure PAO fluid.*


transitional and the early stage of turbulent flow regimes. The results show a 24% increase in average Nusselt number for 8 wt% nanoemulsion fluid and a 11% increase in average Nusselt for 4 wt% nanoemulsion fluid compared to that of pure PAO at the same Reynolds number of 3400. The heat transfer enhancement in the transitional regime can be attributed to the enhanced interaction and interfacial thermal transport between ethanol nanodroplets and PAO base fluid, so that the increase in density and size of nanodroplets at higher concentrations of ethanol can contribute to a stronger mixing and mass exchange effects within the transitional

*Nusselt number versus Reynolds for pure PAO and nanoemulsion fluids.*

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

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

**Figure 11** compares experimental heat transfer data for 8 wt% ethanol/PAO nanoemulsion with conventional heat transfer correlations suggested in the literature for internal flow. As shown in this figure, the experimental results agree very well with those predicted by the empirical correlations in both laminar and transitional flow regimes. While the classical Gnielinski correlation overestimates the Nusselt number for the transitional flow, the Stephan and modified Gnielinski correlations provide a better prediction of Nusselt number for the laminar and

and turbulent flow regimes.

**Figure 10.**

**Figure 11.**

**179**

transitional flow regimes, respectively.

*Nusselt number versus Reynolds for 8 wt% ethanol/PAO nanoemulsion.*

#### **Table 9.**

*Empirical friction factor correlations used for comparison purposes with their validity ranges.*

**Figure 9**, the measured values of friction factor for the pure PAO flowing inside the minichannel agree well with the empirical correlations during laminar flow regime. The friction factor decreases when Reynolds number increases up to around 2000. Afterwards, the friction factor starts to increase sharply for Reynolds greater than 2000, which indicates transition from laminar to turbulent flow regime.

#### *3.1.1.2 Single-phase results of ethanol/PAO nanoemulsions*

After confirming the integrity of the test loop using the experimental results of pure PAO fluid, heat transfer and flow characteristics of Ethanol/PAO nanoemulsion fluids with 4 and 8 wt% ethanol were experimentally investigated by following a similar test procedure.

**Figure 10** represents variations of average Nusselt for the base fluid and nanoemulsion fluids against a range of Reynolds lying in the laminar and transitional flow regimes. As shown in this figure, Nusselt increases with Reynolds for all the working fluids tested in the present study within either laminar regime (Re < 2300) or transitional regime (Re > 2300). **Figure 10** also illustrates that increase in ethanol concentration of nanoemulsion fluids does not reflect a remarkable distinction in heat transfer performance within the laminar regime. However, when ethanol concentration of nanoemulsions increases from pure PAO (0%) to 8 wt% nanoemulsion, Nusselt number exhibits significant enhancements within the

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

**Figure 10.** *Nusselt number versus Reynolds for pure PAO and nanoemulsion fluids.*

transitional and the early stage of turbulent flow regimes. The results show a 24% increase in average Nusselt number for 8 wt% nanoemulsion fluid and a 11% increase in average Nusselt for 4 wt% nanoemulsion fluid compared to that of pure PAO at the same Reynolds number of 3400. The heat transfer enhancement in the transitional regime can be attributed to the enhanced interaction and interfacial thermal transport between ethanol nanodroplets and PAO base fluid, so that the increase in density and size of nanodroplets at higher concentrations of ethanol can contribute to a stronger mixing and mass exchange effects within the transitional and turbulent flow regimes.

**Figure 11** compares experimental heat transfer data for 8 wt% ethanol/PAO nanoemulsion with conventional heat transfer correlations suggested in the literature for internal flow. As shown in this figure, the experimental results agree very well with those predicted by the empirical correlations in both laminar and transitional flow regimes. While the classical Gnielinski correlation overestimates the Nusselt number for the transitional flow, the Stephan and modified Gnielinski correlations provide a better prediction of Nusselt number for the laminar and transitional flow regimes, respectively.

**Figure 11.** *Nusselt number versus Reynolds for 8 wt% ethanol/PAO nanoemulsion.*

**Figure 9**, the measured values of friction factor for the pure PAO flowing inside the minichannel agree well with the empirical correlations during laminar flow regime. The friction factor decreases when Reynolds number increases up to around 2000. Afterwards, the friction factor starts to increase sharply for Reynolds greater than

**Correlation Conditions Validity Range**

Laminar flow in a circular channel

Laminar flow inside circular channel with consideration of entrance length

*Re* <2300

*Re* <2300

After confirming the integrity of the test loop using the experimental results of

nanoemulsion fluids with 4 and 8 wt% ethanol were experimentally investigated by

**Figure 10** represents variations of average Nusselt for the base fluid and nanoemulsion fluids against a range of Reynolds lying in the laminar and transitional flow regimes. As shown in this figure, Nusselt increases with Reynolds for all the working fluids tested in the present study within either laminar regime (Re < 2300) or transitional regime (Re > 2300). **Figure 10** also illustrates that increase in ethanol concentration of nanoemulsion fluids does not reflect a remarkable distinction in heat transfer performance within the laminar regime. However, when ethanol concentration of nanoemulsions increases from pure PAO (0%) to 8 wt% nanoemulsion, Nusselt number exhibits significant enhancements within the

2000, which indicates transition from laminar to turbulent flow regime.

*Empirical friction factor correlations used for comparison purposes with their validity ranges.*

pure PAO fluid, heat transfer and flow characteristics of Ethanol/PAO

*3.1.1.2 Single-phase results of ethanol/PAO nanoemulsions*

*Friction factor versus Reynolds number for pure PAO fluid.*

*Heat Transfer - Design, Experimentation and Applications*

*ξ* p <sup>1</sup>þ2*:*12�10�<sup>4</sup> *ξ*2

*Hagen-Poiseuille correlation* [73]

*<sup>ξ</sup>* <sup>p</sup> <sup>þ</sup> <sup>16</sup>þ0*:*3125*ξ*�3*:*<sup>44</sup>ffi

� �

following a similar test procedure.

**Figure 9.**

*f*∙ *Re* ¼ 64

*<sup>f</sup>*<sup>∙</sup> *Re* <sup>≈</sup> <sup>4</sup> <sup>3</sup>*:*<sup>44</sup>ffiffi

where <sup>ξ</sup><sup>¼</sup> *<sup>x</sup> D* � �*= Re*

**Table 9.**

**178**

*Shah correlation* [73]

**Figure 12** represents variations of friction factor with Reynolds number for 8 wt % ethanol/PAO nanoemulsion, and compares the experimental results with those predicted by the empirical correlations, in which a very similar trend can be observed with that of the single-phase PAO base fluid as previously shown in **Figure 9**. As illustrated in **Figure 12**, the friction factor of nanoemulsion within the laminar regime decreases consistently with Reynolds number up to the Reynolds of 2000, indicating a slightly earlier entrance into the transitional flow regime compared to that of the pure PAO. Upon entering the transitional regime, friction factor is found to increase and then starts to flat out at Reynolds number of around 3000. As clearly seen in this figure, the experimental data measured for friction factor of nanoemulsion showed good agreements with those predicted by the Hagen-Poiseuille and Shah correlations.

#### *3.1.1.3 Two-phase results of ethanol/PAO nanoemulsions*

One of the reasons to replace conventional heat transfer fluids with nanoemulsion is to achieve significant heat transfer enhancements when the phase changeable nanodroplets undergo nucleation. Previous studies have demonstrated a significantly improved heat transfer coefficient and critical heat flux using nanoemulsion with phase changeable nanodroplets undergoing the nucleate boiling [57, 61, 62]. In the present study, the ethanol nanodroplets formed inside the nanoemulsion are expected to function as phase change nuclei at elevated temperatures during the two-phase flow boiling experiments. The maximum flow rate needs to be limited to less than 4.46 m/s (or Re = 1136) to maintain a wall temperature high enough to trigger flow boiling. Accordingly, all the flow boiling data collected and shown in the present study reflect heat transfer behavior within the laminar flow regime.

and the flow lies in the state of two-phase flow boiling. Another interesting observation was the delay in nucleation boiling temperature or the onset of nucleate boiling (ONB). As illustrated in **Figure 13**, the nucleation did not start until the average surface temperature of the minichannels reached a temperature around 140°C while the boiling temperature for ethanol is 78°C. Similar findings of delayed ONB were previously reported for pool boiling experiments of sub-cooled ethanol/ PAO nanoemulsion fluids [57]. The delayed ONB can be attributed to the inefficient thermal transport between each surfactant molecule and its surrounding PAO fluid, in which the PAO molecules are not packed closely near the hydrophilic head-group of the surfactant molecule and could not provide efficient thermal pathway in

*Evolution of the average wall temperature with time for pure PAO and nanoemulsions.*

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

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

**Figure 14** shows the variations of average heat transfer coefficient with Reynolds number for flow boiling of nanoemulsion fluids, and compares them with those of single-phase flow of pure PAO and nanoemulsions. In addition to the fact that the Ethanol/PAO nanoemulsion fluids exhibit slightly higher heat transfer coefficients (HTCs) compared to those of pure PAO in single-phase flow due to the minor improvement in thermophysical properties, the HTCs were found to be significantly enhanced when the nanoemulsion fluids underwent two-phase flow boiling. It is also apparent that the phase change of the ethanol nanodroplets is the main contributor to the heat transfer improvement. **Figure 14** also reveals that the ethanol concentration of nanoemulsion has a positive impact on the overall heat transfer coefficients in both single-phase flow and two-phase flow boiling. As shown in this figure, an average HTC enhancement of 50 70% was achieved with Ethanol/PAO

The prototype of the minichannel 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

between the micelles and base fluid [59, 60].

nanoemulsion compared to that of the PAO base fluid.

*3.1.2 Simulation results*

**181**

**Figure 13.**

**Figure 13** represents the variations of average transient wall temperature data for all the tested working fluids (4 and 8 wt% ethanol/PAO nanoemulsions and pure PAO) with time, which overlapped well with each other within single-phase flow regime. However, the wall temperatures of nanoemulsions started to deviate from the single-phase trend line, followed by a sudden drop in the wall temperature which indicates an increase in heat transfer coefficients due to the flow boiling. Using the sight flow indicator located next to the outlet of the minichannel test section, it was observed that there were bubbles coming out of the minichannel heat exchanger, which confirms that the ethanol nanodroplets underwent nucleation

**Figure 12.** *Friction factor versus Reynolds number (Nanoemulsion fluid).*

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

**Figure 13.** *Evolution of the average wall temperature with time for pure PAO and nanoemulsions.*

and the flow lies in the state of two-phase flow boiling. Another interesting observation was the delay in nucleation boiling temperature or the onset of nucleate boiling (ONB). As illustrated in **Figure 13**, the nucleation did not start until the average surface temperature of the minichannels reached a temperature around 140°C while the boiling temperature for ethanol is 78°C. Similar findings of delayed ONB were previously reported for pool boiling experiments of sub-cooled ethanol/ PAO nanoemulsion fluids [57]. The delayed ONB can be attributed to the inefficient thermal transport between each surfactant molecule and its surrounding PAO fluid, in which the PAO molecules are not packed closely near the hydrophilic head-group of the surfactant molecule and could not provide efficient thermal pathway in between the micelles and base fluid [59, 60].

**Figure 14** shows the variations of average heat transfer coefficient with Reynolds number for flow boiling of nanoemulsion fluids, and compares them with those of single-phase flow of pure PAO and nanoemulsions. In addition to the fact that the Ethanol/PAO nanoemulsion fluids exhibit slightly higher heat transfer coefficients (HTCs) compared to those of pure PAO in single-phase flow due to the minor improvement in thermophysical properties, the HTCs were found to be significantly enhanced when the nanoemulsion fluids underwent two-phase flow boiling. It is also apparent that the phase change of the ethanol nanodroplets is the main contributor to the heat transfer improvement. **Figure 14** also reveals that the ethanol concentration of nanoemulsion has a positive impact on the overall heat transfer coefficients in both single-phase flow and two-phase flow boiling. As shown in this figure, an average HTC enhancement of 50 70% was achieved with Ethanol/PAO nanoemulsion compared to that of the PAO base fluid.

### *3.1.2 Simulation results*

The prototype of the minichannel 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

**Figure 12** represents variations of friction factor with Reynolds number for 8 wt % ethanol/PAO nanoemulsion, and compares the experimental results with those predicted by the empirical correlations, in which a very similar trend can be observed with that of the single-phase PAO base fluid as previously shown in **Figure 9**. As illustrated in **Figure 12**, the friction factor of nanoemulsion within the laminar regime decreases consistently with Reynolds number up to the Reynolds of 2000, indicating a slightly earlier entrance into the transitional flow regime compared to that of the pure PAO. Upon entering the transitional regime, friction factor is found to increase and then starts to flat out at Reynolds number of around 3000. As clearly seen in this figure, the experimental data measured for friction factor of nanoemulsion showed good agreements with those predicted by the Hagen-

Poiseuille and Shah correlations.

laminar flow regime.

**Figure 12.**

**180**

*Friction factor versus Reynolds number (Nanoemulsion fluid).*

*3.1.1.3 Two-phase results of ethanol/PAO nanoemulsions*

*Heat Transfer - Design, Experimentation and Applications*

One of the reasons to replace conventional heat transfer fluids with

significantly improved heat transfer coefficient and critical heat flux using

nanoemulsion is to achieve significant heat transfer enhancements when the phase changeable nanodroplets undergo nucleation. Previous studies have demonstrated a

nanoemulsion with phase changeable nanodroplets undergoing the nucleate boiling [57, 61, 62]. In the present study, the ethanol nanodroplets formed inside the nanoemulsion are expected to function as phase change nuclei at elevated temperatures during the two-phase flow boiling experiments. The maximum flow rate needs to be limited to less than 4.46 m/s (or Re = 1136) to maintain a wall temperature high enough to trigger flow boiling. Accordingly, all the flow boiling data collected and shown in the present study reflect heat transfer behavior within the

**Figure 13** represents the variations of average transient wall temperature data for all the tested working fluids (4 and 8 wt% ethanol/PAO nanoemulsions and pure PAO) with time, which overlapped well with each other within single-phase flow regime. However, the wall temperatures of nanoemulsions started to deviate from the single-phase trend line, followed by a sudden drop in the wall temperature which indicates an increase in heat transfer coefficients due to the flow boiling. Using the sight flow indicator located next to the outlet of the minichannel test section, it was observed that there were bubbles coming out of the minichannel heat exchanger, which confirms that the ethanol nanodroplets underwent nucleation

Ethanol/PAO nanoemulsion, respectively, within the fully developed transitional region. This reveals a heat transfer enhancement of around 13% at a certain Reynolds number of 2500 using the 8 wt% nanoemulsion fluid compared to that of the

*Variations of local Nusselt number along the minichannel at Re = 2500 for pure PAO and 8% ethanol/PAO*

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

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

nanoemulsion, respectively, the experimental results of pure PAO and 8% nanoemulsion introduced earlier in **Figures 8** and **11** show the Nusselt values of around 21 and 24 at the same Reynolds of 2500, respectively. This indicates a relative deviation of approximately 9% between the model and experimental

region where both of the working fluids are fully developed.

This is important to point out that while the simulation results in **Figure 15** show the Nusselt values of 23 and 26 for the fully developed flows of pure PAO and 8%

As also observed from the simulation results in **Figure 15**, there is a declining trend of Nusselt number along the early locations of the minichannel. 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 minichannel 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

The hydrodynamic (*xfd*,*hyd*) and thermal (*xfd*,*th*) entry lengths for an internal

The hydrodynamic entry length for the transitional flows of PAO and 8% nanoemulsion in the minichannel was found to be 25 mm. As demonstrated in **Figure 15**, the local Nusselt numbers for each of the working fluids take relatively constant values after the entry length of 25 mm where the boundary layer develops fully across the cross section of the microchannel and appears to be independent of

*<sup>D</sup>* <sup>≈</sup>0*:*<sup>05</sup> *Re <sup>D</sup>* (13)

*<sup>D</sup>* <sup>≈</sup>0*:*<sup>05</sup> *Re DPr* (14)

flow in a circular channel can be calculated as follows, respectively [73]:

*xfd*,*hyd*

*xfd*,*th*

pure PAO inside the minichannel heat exchanger.

*nanoemulsion fluids with the entrance effects*

results.

**183**

**Figure 15.**

flux boundary conditions imposed on the top and bottom surfaces of the minichannel heat exchanger, and thermophysical properties of the working fluids are set to remain constant for the values addressed earlier in **Table 4**.

Three types of meshes were developed in the present study to examine accuracy of simulation results as well as to confirm mesh independency of the results, including finer, fine, and normal. The size settings for each mesh are shown in **Table 10**. The finer mesh size was eventually chosen as it allows to conduct sufficiently accurate analysis while still maintaining a reasonable computational time.

**Figure 15** represents the variations of local Nusselt number at a certain Reynolds number of 2500 (i.e., the beginning of transitional regime) along the minichannel heat exchanger with the same geometry and dimensions used previously for the experimental investigations as summarized in **Table 5**. As illustrated in this figure, the local Nusselt number decreases along the minichannel at early positions and then reaches relatively constant values of 23 and 26 for the pure PAO and 8 wt%


#### **Table 10.**

*Mesh types developed to conduct numerical analysis.*

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

#### **Figure 15.**

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

of simulation results as well as to confirm mesh independency of the results, including finer, fine, and normal. The size settings for each mesh are shown in **Table 10**. The finer mesh size was eventually chosen as it allows to conduct sufficiently accurate analysis while still maintaining a reasonable computational time. **Figure 15** represents the variations of local Nusselt number at a certain Reynolds number of 2500 (i.e., the beginning of transitional regime) along the minichannel heat exchanger with the same geometry and dimensions used previously for the experimental investigations as summarized in **Table 5**. As illustrated in this figure, the local Nusselt number decreases along the minichannel at early positions and then reaches relatively constant values of 23 and 26 for the pure PAO and 8 wt%

**Description Finer Fine Normal** Calibrate for Fluid dynamics Fluid dynamics Fluid dynamics Maximum element size 0.148 0.212 0.4 Minimum element size 0.016 0.04 0.12 Curvature factor 0.4 0.5 0.7 Resolution of narrow regions 0.95 0.8 0.7 Maximum element growth rate 1.08 1.13 1.4 Number of elements 684,571 200,829 86,383 Mesh shape Triangle Triangle Triangle Computational time 14 min 25 sec 3 min 19 sec 1 min 22 sec

are set to remain constant for the values addressed earlier in **Table 4**.

*Heat Transfer - Design, Experimentation and Applications*

**Figure 14.**

**Table 10.**

**182**

*Mesh types developed to conduct numerical analysis.*

minichannel heat exchanger, and thermophysical properties of the working fluids

*Average heat transfer coefficient versus Reynolds for single-phase and two-phase flow of the working fluids.*

Three types of meshes were developed in the present study to examine accuracy

*Variations of local Nusselt number along the minichannel at Re = 2500 for pure PAO and 8% ethanol/PAO nanoemulsion fluids with the entrance effects*

Ethanol/PAO nanoemulsion, respectively, within the fully developed transitional region. This reveals a heat transfer enhancement of around 13% at a certain Reynolds number of 2500 using the 8 wt% nanoemulsion fluid compared to that of the pure PAO inside the minichannel heat exchanger.

This is important to point out that while the simulation results in **Figure 15** show the Nusselt values of 23 and 26 for the fully developed flows of pure PAO and 8% nanoemulsion, respectively, the experimental results of pure PAO and 8% nanoemulsion introduced earlier in **Figures 8** and **11** show the Nusselt values of around 21 and 24 at the same Reynolds of 2500, respectively. This indicates a relative deviation of approximately 9% between the model and experimental results.

As also observed from the simulation results in **Figure 15**, there is a declining trend of Nusselt number along the early locations of the minichannel. 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 minichannel 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.

The hydrodynamic (*xfd*,*hyd*) and thermal (*xfd*,*th*) entry lengths for an internal flow in a circular channel can be calculated as follows, respectively [73]:

$$\frac{\mathcal{X}\_{fd,hyd}}{D} \approx 0.05 \text{ } Re\_D \tag{13}$$

$$\frac{\mathcal{X}\_{\text{fd},th}}{D} \approx 0.05 \ Re\_D Pr \tag{14}$$

The hydrodynamic entry length for the transitional flows of PAO and 8% nanoemulsion in the minichannel was found to be 25 mm. As demonstrated in **Figure 15**, the local Nusselt numbers for each of the working fluids take relatively constant values after the entry length of 25 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 *xfd*,*th* >*xfd*,*hyd* .

Most researchers have employed rectangular cross-sections for studying friction and pressure drop in microchannels. Since microchannels length is normally long (compared to other dimensions), inlet and exit effects have been neglected in most

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

In the present study, the additively manufactured microchannel heat exchanger was developed using the EOSINT M280 machine at the University of the District of Columbia (UDC). The machine takes advantage of the DMLS technique to 3D print the designed prototype. **Figure 16** represents the EOSINT M280 machine which can be used to seamlessly manufacture complex heat exchanger designs. The process parameters (material scaling, layer thickness, and beam offset) applied to the machine are listed in **Table 11**. Adjusting process parameters and investigating different building directions to understand their impacts on the prototype perfor-

Using the aforementioned fabrication process, a rectangular cross-sectional microchannel heat exchanger was designed and additively manufactured with 316 L Stainless Steel. Each channel is designed to be 640*μm*in width and 760*μm*in height, with a length of 120 *mm*. A total of 30 microchannels were fabricated along the center of the heat exchanger. Flanges were manufactured on each end of the heat exchanger to fit the existing test loop and facilitate the experimental investigations. **Figure 17** represents the additively manufactured microchannel heat exchanger

works.

**Figure 16.**

**Table 11.**

**185**

mance is beyond the scope of this study.

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

developed for the present study.

*EOSINT M280 machine used to develop the microchannel prototype.*

*Settings applied to EOSINT M280 machine to develop the prototype.*

**Parameter Setting** Material 316 L Stainless Steel Process Gas Nitrogen Laser Power 400 W Material Scaling X 0.045% Material Scaling Y 0.16% Beam Offset 0.11 mm Layer Thickness 40 micrometer

Software EOSTATE Magics RP (materialize)
