**4.3 Physical properties of Field's alloy nanoparticle slurry in HFE7100**

For the silane monolayer modified Field's alloy nanoparticles, the optical transmittance of nanoparticle suspensions in HFE7100 fluid was monitored by a portable spectrometer, and the optical transmittance kept nearly the same over 1 month, which indicates the nanoparticle suspension is highly stable for more than a month after the 1H, 1H, 2H, 2H-perfluorooctyltriethoxysilane modification. The physical properties of nano-PCM slurry in HFE7100 with 10, 23, and 30% particle mass fraction, where the corresponding particle volume fractions of slurry are 2.3, 6.4, and 8.0%, and its components are presented in **Table** 1. The bulk viscosity of Field's alloy nanoparticle slurry is measured by using a calibrated Cannon-Fenske viscometer. Other parameters, such as density, thermal conductivity, and latent heat of Field's alloy nano-PCM slurry, are calculated according to the reference data and mixture equations [41]. The specific heat of nanoparticles for solid/liquid phases is derived from the superposition calculations involving HFE7100 and Field's alloy (solid/liquid).

*Synthesis, Properties, and Characterization of Field's Alloy Nanoparticles and Its Slurry DOI: http://dx.doi.org/10.5772/intechopen.84224*


*a http://www.Matweb.com [42].*

*b 3M Data Book, HFE7100 for Heat Transfer, 2002 [43].*

*c Bulk physical properties of slurries are calculated from those of solid PCM particles.*

#### **Table 1.**

*Nanoemulsions - Properties, Fabrications and Applications*

thermal contact resistance at the interface.

coefficient was then obtained by the following equation [40]:

calculations involving HFE7100 and Field's alloy (solid/liquid).

h = *<sup>Q</sup>*⁄

inlet (non-melting conditions).

The coolant within the reservoir was maintained at a constant temperature using a combination of an immersion heater and proper thermal insulation to ensure the test was under 1 atmosphere pressure. An acrylic cylinder with aluminum lids, which was 300 mm tall and 200 mm in diameter, was used as the test chamber. The test chamber contained approximately 500 ml of working slurry (pure HFE7100/ nano-PCM mixture). Heat was applied to the heater surface through a copper plate using a resistive heater controlled by a HP 6030A DC power supply system. The heater was built by soldering a 10×10×2-mm-copper block onto a matching size, 1-mm-thick resistive heater. **Figure 6C** showed the heater details. The resistive heater was made of a 5-ohm-thick film resistor with BeO substrate made by Barry Industries Inc. Soldering the thick film resistor to the copper plate minimizes the

The temperature of slurry entering the test chamber was maintained at 56°C, 5°C below the liquid saturation temperature. Due to the large supercooling of nano-PCM particles (solidification temperature at 17.1°C), to ensure the PCM in solid phase, the slurry was chilled to 5°C by ice water. The nozzle inlet temperature (56°C) was controlled by an auxiliary heater powered by an AC regulator. The test heater surface was fastened at the bottom of the container using epoxy resin. The test vessel was insulated with a 25-mm-thick fiber glass blanket. A Bakelite layer (with a thermal conductivity less than 1 W/mK) underneath the heater was found to be a sufficient insulator. The uniformity of the Joule heating over the resistor surface was within 5%. The heat fluxes were also controlled at steady state for each set of testing. The jet impingement nozzle (a TG 0.7 full cone nozzle with the insert removed) was procured from Spraying Systems Co., and it had a passage with circular cross section, 0.76 mm in diameter. The coolant was allowed to enter the plenum of the nozzle where it flowed through a converging section as it existed to the chamber. Once the steady-state flow and temperature conditions were attained, the mean temperature of the heater and inlet and outlet temperatures of the slurry were calculated by the arithmetic mean of temperature readings. The heat transfer

where *Tw* is the average of the two surface temperatures extrapolated from the two embedded thermocouple readings. The pressure drop across the nozzle was measured at different mass fractions of slurry at a temperature of 20°C at the nozzle

For the silane monolayer modified Field's alloy nanoparticles, the optical transmit-

tance of nanoparticle suspensions in HFE7100 fluid was monitored by a portable spectrometer, and the optical transmittance kept nearly the same over 1 month, which indicates the nanoparticle suspension is highly stable for more than a month after the 1H, 1H, 2H, 2H-perfluorooctyltriethoxysilane modification. The physical properties of nano-PCM slurry in HFE7100 with 10, 23, and 30% particle mass fraction, where the corresponding particle volume fractions of slurry are 2.3, 6.4, and 8.0%, and its components are presented in **Table** 1. The bulk viscosity of Field's alloy nanoparticle slurry is measured by using a calibrated Cannon-Fenske viscometer. Other parameters, such as density, thermal conductivity, and latent heat of Field's alloy nano-PCM slurry, are calculated according to the reference data and mixture equations [41]. The specific heat of nanoparticles for solid/liquid phases is derived from the superposition

**4.3 Physical properties of Field's alloy nanoparticle slurry in HFE7100**

*<sup>A</sup>*(*Tw* <sup>−</sup> *Tf*) (1)

**114**

*Physical properties of Field's alloy nano-PCM slurry and its components.*

**Figure 7.** *Effect of temperature on apparent viscosity of pure HFE7100 and nano-PCM slurry.*

A relation exists between the temperature of HFE7100 with Field's alloy nanoparticles slurry and its apparent viscosity. **Figure 7** shows the measured experimental data that exhibits the effect of temperature on apparent viscosity of pure HFE7100 and slurry with 30% (wt%) particle fraction. The results show that the ratio of the slurry viscosities keeps almost the same at 1.4 over the tested temperature range.

#### **4.4 Pressure drop**

An important parameter was pressure drop between the inlet and the outlet of microchannel heat exchanger. In ideal case, the pressure drop should be as small

**Figure 8.**

*Jet impingement pressure drop data with different Field's nanoparticle concentrations (nozzle: TG0.76, H = 20 mm); the inlet temperature is controlled at 20°C.*

as possible to minimize pumping power. When the velocity of the jet was varied between 4.2 and 15.5 m/s, the pressure drop across the nozzle was measured at flow rates of 1.95–7.15 cc/s. As shown in **Figure 8**, when the nozzle inlet temperature of the liquid temperature was controlled at 20°C, the jet impingement pressure drop results were very close even with different particle mass fractions. It can be assigned to the comparable increases in Field's alloy nanoparticle slurry viscosity and density with increased nanoparticle mass fraction (see **Table** 1). The pressure drop was related to the nozzle Reynolds number as △P ∝ *Red* −0.25 [44]. Since the nozzle Reynolds number did not change too much with increased particle mass fraction, the pressure drop should not vary significantly with particle mass concentration.

### **4.5 Heat transfer performance of Field's alloy nanoparticle slurry**

Heat transfer performance of the nanoparticle slurries was evaluated by measuring their convective heat transfer coefficients in jet impingement configuration. When no solid-liquid and liquid-vapor phase change occurs in the slurry, the jet impingement heat transfer coefficient can be predicted by the Martin correlation [45]. **Figure 9** showed the heat transfer coefficient of jet impingement test for pure HFE7100 when the inlet nozzle temperature was kept at 20°C with a flow rate from 2.05 to 6.95 cc/s. When the flow rate was from 2.05 to 6.95 cc/s, the heat transfer coefficient was increased from 4.6 to 9.4 103 W/m2 K. The difference between the experimental data and the Martin correlation was within 10% of the Martin correlation.

In order to compare the heat transfer performance between HFE7100 and the slurries with different particle mass fractions, the flow rate constant was set at 7.15 cc/s, and the temperature of the liquids at the nozzle inlet was fixed at 56°C. The heater surface temperature was varied between 56 and 75°C by controlling the heater power input. The comparison for the overall performance of pure HFE7100 (0%) and slurries with 10, 23, and 30% particle mass fraction, using heat flux as a measure, was provided in **Figure 10**. It can be seen that even before the melting temperature of

**117**

**Figure 10.**

*from 62 to 66°C.*

**Figure 9.**

*is 20°C.*

*Synthesis, Properties, and Characterization of Field's Alloy Nanoparticles and Its Slurry*

Field's alloy and the boiling point of HFE7100 were reached, the slurries had a higher heat flux at surface temperature between 56 and 60°C. This can be attributed, in part to their higher thermal conductivity. At the heater surface of 62°C, the results clearly

*HFE7100 jet impingement (56–68°C) heat flux at the flow rate of 7.15 cc/s; (note: the inlet Tin was controlled at 56°C, nozzle: TG0.76, H = 20 mm). Slurry with 30% particle mass fraction improved average heat transfer coefficient by 70% when compared to pure HFE7100 for jet impingement at the temperature range* 

*Experimental and Martin correlation of heat transfer coefficients of pure HFE7100 when the inlet temperature* 

*DOI: http://dx.doi.org/10.5772/intechopen.84224*

*Synthesis, Properties, and Characterization of Field's Alloy Nanoparticles and Its Slurry DOI: http://dx.doi.org/10.5772/intechopen.84224*

**Figure 9.**

*Nanoemulsions - Properties, Fabrications and Applications*

as possible to minimize pumping power. When the velocity of the jet was varied between 4.2 and 15.5 m/s, the pressure drop across the nozzle was measured at flow rates of 1.95–7.15 cc/s. As shown in **Figure 8**, when the nozzle inlet temperature of the liquid temperature was controlled at 20°C, the jet impingement pressure drop results were very close even with different particle mass fractions. It can be assigned to the comparable increases in Field's alloy nanoparticle slurry viscosity and density with increased nanoparticle mass fraction (see **Table** 1). The pressure drop

*Jet impingement pressure drop data with different Field's nanoparticle concentrations (nozzle: TG0.76,* 

Reynolds number did not change too much with increased particle mass fraction, the pressure drop should not vary significantly with particle mass concentration.

Heat transfer performance of the nanoparticle slurries was evaluated by measuring their convective heat transfer coefficients in jet impingement configuration. When no solid-liquid and liquid-vapor phase change occurs in the slurry, the jet impingement heat transfer coefficient can be predicted by the Martin correlation [45]. **Figure 9** showed the heat transfer coefficient of jet impingement test for pure HFE7100 when the inlet nozzle temperature was kept at 20°C with a flow rate from 2.05 to 6.95 cc/s. When the flow rate was from 2.05 to 6.95 cc/s, the heat transfer coef-

mental data and the Martin correlation was within 10% of the Martin correlation. In order to compare the heat transfer performance between HFE7100 and the slurries with different particle mass fractions, the flow rate constant was set at 7.15 cc/s, and the temperature of the liquids at the nozzle inlet was fixed at 56°C. The heater surface temperature was varied between 56 and 75°C by controlling the heater power input. The comparison for the overall performance of pure HFE7100 (0%) and slurries with 10, 23, and 30% particle mass fraction, using heat flux as a measure, was provided in **Figure 10**. It can be seen that even before the melting temperature of

**4.5 Heat transfer performance of Field's alloy nanoparticle slurry**

−0.25 [44]. Since the nozzle

K. The difference between the experi-

was related to the nozzle Reynolds number as △P ∝ *Red*

*H = 20 mm); the inlet temperature is controlled at 20°C.*

ficient was increased from 4.6 to 9.4 103 W/m2

**116**

**Figure 8.**

*Experimental and Martin correlation of heat transfer coefficients of pure HFE7100 when the inlet temperature is 20°C.*

#### **Figure 10.**

*HFE7100 jet impingement (56–68°C) heat flux at the flow rate of 7.15 cc/s; (note: the inlet Tin was controlled at 56°C, nozzle: TG0.76, H = 20 mm). Slurry with 30% particle mass fraction improved average heat transfer coefficient by 70% when compared to pure HFE7100 for jet impingement at the temperature range from 62 to 66°C.*

Field's alloy and the boiling point of HFE7100 were reached, the slurries had a higher heat flux at surface temperature between 56 and 60°C. This can be attributed, in part to their higher thermal conductivity. At the heater surface of 62°C, the results clearly

**Figure 11.**

*Heater surface temperature at full range (56–75°C) at the flow rate of 7.15 cc/s; (note: the inlet Tin was controlled at 56°C, nozzle: TG0.76, H = 20 mm), black arrow was the critical heat flux.*

indicated that the slurry with 30% particle mass fraction provides a much higher heat flux than pure HFE7100. The heat flux removal increased from 15 to 30 W/cm2 , a 100% improvement. At 64°C, the results showed that the slurry with 30% particle mass fraction increases heat flux from 22 to 38 W/cm2 , a 73% improvement. The results also show that at 66°C, the slurry with 30% particle mass fraction had a higher heat flux, 30 W/cm2 vs. 44 W/cm2 , a 47% improvement. On the average, the slurry with 30% particle mass fraction provided a heat transfer enhancement of 70% when compared to pure HFE7100.

It was noted that the heat transfer enhancement was heat-flux dependent. It begun to decrease as the heat flux increased because of the increasingly higher temperature difference between the wall and the slurry. High heat flux can shift the slurry temperature out of the melting range (>62°C). **Figure 11** showed the heat removal results at a flow rate of 7.15 cc/s over a wider surface temperature range, between 56 and 75°C. It was interesting to note that, at surface temperature higher than 73°C, the critical heat flux decreased as the particle fraction increased from 10 to 30%. This could be explained by the increase in viscosity and latent heat of melting depletion. These combined effects could reduce the overall heat removal capability of nano-PCM slurry. At a surface temperature of 75°C and flow rate of 7.15 cc/s, the heat fluxes of the slurry with 30% particle mass fraction and that of pure HFE7100 were 59 and 69 W/cm2 , respectively. **Figure 12** showed the heat transfer coefficients of slurries with several particle mass fractions vs. the heater surface temperature at a flow rate of 7.15 cc/s. The figure showed the heat transfer coefficients of nano-PCM slurries peak at 60–63°C.

The reported heat transfer results in **Figures 10**–**12** were the average values from three consecutive repeated tests. Depending on the temperature of the heater, a heat loss of 4.5% of the electrical power input was estimated by calibration. The heat flux at the surface of the copper plate was obtained from the measured electrical power. Data in **Figures 10** and **11** were the heat transfer coefficients from five consecutive tests after accounting for the heat loss. All experiments had random

**119**

RMSE = √

**Figure 12.**

*H = 20 mm).*

\_\_ \_\_1 *<sup>n</sup>*(∑<sup>1</sup> *n* ( *<sup>h</sup>* <sup>−</sup> *hexp* \_\_\_\_\_\_

degree of accuracy.

**5. Conclusions**

*Synthesis, Properties, and Characterization of Field's Alloy Nanoparticles and Its Slurry*

errors which occurred inevitably during measurements. These errors may be analyzed with the calculation of the root-mean-square of errors (RMSE) defined as

*Heat transfer coefficients of slurry with 30% particle mass fraction vs. heater surface temperature of full range (56–75°C) at the flow rate of 7.15 cc/s (note: the inlet Tin was controlled at 56°C, nozzle: TG0.76,* 

for pure HFE7100, 10% slurry, 23% slurry, and 30% slurry are 0.013, 0.036, 0.04, and 0.049, respectively. During these measurements the maximum root-meansquare of errors (RMSE) was found to be less than 5% representing a reasonable

Our data from the slurries with nano-PCMs demonstrated very consistent thermal performance. The 30% nano-PCM slurry attains 97% of its initial heat transfer performance after 5000 thermal cycles. This implied that encapsulation of the nano-PCM particles with a shell made of materials such as silica, normally used to prevent coalescence of molten nanoparticles, was not needed for the slurry featuring HFE7100 and Field's alloy nano-PCM. The main reason was thought to be the existence of oxide shells around the nanoparticles. Oxidation of Field's alloy was unavoidable during the synthesis process and can provide a thin protective shell for the core material for a long time. Furthermore, two other possible mechanisms might be helping the bare nano-PCMs. First, the added perfluorooctyltriethoxysilane surfactant helped resist coalescence of molten Field's alloy nanoparticles and ensured the stability of colloidal suspension. Second, bare Field's alloy nanoparticles had residual charges which generate repulsive electrical force to prevent the agglomeration of molten nanoparticles.

In this chapter, a facile one-step method was developed for the production of Field's alloy nanoparticles or slurry using nanoemulsification technique. The composition, size, morphology, and thermal properties of as-prepared nanoparticles were characterized by XRF, TEM, and DSC, respectively. The slurry with modified Field's alloy nanoparticle dispersed in PAO or HFE7100 exhibits good thermal

*<sup>h</sup>* )) where n was the size of the sample (n = 5). The RMSE values

*DOI: http://dx.doi.org/10.5772/intechopen.84224*

*Synthesis, Properties, and Characterization of Field's Alloy Nanoparticles and Its Slurry DOI: http://dx.doi.org/10.5772/intechopen.84224*

#### **Figure 12.**

*Nanoemulsions - Properties, Fabrications and Applications*

indicated that the slurry with 30% particle mass fraction provides a much higher heat flux than pure HFE7100. The heat flux removal increased from 15 to 30 W/cm2

*Heater surface temperature at full range (56–75°C) at the flow rate of 7.15 cc/s; (note: the inlet Tin was* 

*controlled at 56°C, nozzle: TG0.76, H = 20 mm), black arrow was the critical heat flux.*

mass fraction increases heat flux from 22 to 38 W/cm2

vs. 44 W/cm2

heat flux, 30 W/cm2

**Figure 11.**

compared to pure HFE7100.

a 100% improvement. At 64°C, the results showed that the slurry with 30% particle

results also show that at 66°C, the slurry with 30% particle mass fraction had a higher

with 30% particle mass fraction provided a heat transfer enhancement of 70% when

respectively. **Figure 12** showed the heat transfer coefficients of slurries with several particle mass fractions vs. the heater surface temperature at a flow rate of 7.15 cc/s. The figure showed the heat transfer coefficients of nano-PCM slurries peak at 60–63°C. The reported heat transfer results in **Figures 10**–**12** were the average values from three consecutive repeated tests. Depending on the temperature of the heater, a heat loss of 4.5% of the electrical power input was estimated by calibration. The heat flux at the surface of the copper plate was obtained from the measured electrical power. Data in **Figures 10** and **11** were the heat transfer coefficients from five consecutive tests after accounting for the heat loss. All experiments had random

It was noted that the heat transfer enhancement was heat-flux dependent. It begun to decrease as the heat flux increased because of the increasingly higher temperature difference between the wall and the slurry. High heat flux can shift the slurry temperature out of the melting range (>62°C). **Figure 11** showed the heat removal results at a flow rate of 7.15 cc/s over a wider surface temperature range, between 56 and 75°C. It was interesting to note that, at surface temperature higher than 73°C, the critical heat flux decreased as the particle fraction increased from 10 to 30%. This could be explained by the increase in viscosity and latent heat of melting depletion. These combined effects could reduce the overall heat removal capability of nano-PCM slurry. At a surface temperature of 75°C and flow rate of 7.15 cc/s, the heat fluxes of the slurry with 30% particle mass fraction and that of pure HFE7100 were 59 and 69 W/cm2

,

,

, a 73% improvement. The

, a 47% improvement. On the average, the slurry

**118**

*Heat transfer coefficients of slurry with 30% particle mass fraction vs. heater surface temperature of full range (56–75°C) at the flow rate of 7.15 cc/s (note: the inlet Tin was controlled at 56°C, nozzle: TG0.76, H = 20 mm).*

errors which occurred inevitably during measurements. These errors may be analyzed with the calculation of the root-mean-square of errors (RMSE) defined as RMSE = √ \_\_ \_\_1 *<sup>n</sup>*(∑<sup>1</sup> *n* ( *<sup>h</sup>* <sup>−</sup> *hexp* \_\_\_\_\_\_ *<sup>h</sup>* )) where n was the size of the sample (n = 5). The RMSE values for pure HFE7100, 10% slurry, 23% slurry, and 30% slurry are 0.013, 0.036, 0.04, and 0.049, respectively. During these measurements the maximum root-meansquare of errors (RMSE) was found to be less than 5% representing a reasonable degree of accuracy.

Our data from the slurries with nano-PCMs demonstrated very consistent thermal performance. The 30% nano-PCM slurry attains 97% of its initial heat transfer performance after 5000 thermal cycles. This implied that encapsulation of the nano-PCM particles with a shell made of materials such as silica, normally used to prevent coalescence of molten nanoparticles, was not needed for the slurry featuring HFE7100 and Field's alloy nano-PCM. The main reason was thought to be the existence of oxide shells around the nanoparticles. Oxidation of Field's alloy was unavoidable during the synthesis process and can provide a thin protective shell for the core material for a long time. Furthermore, two other possible mechanisms might be helping the bare nano-PCMs. First, the added perfluorooctyltriethoxysilane surfactant helped resist coalescence of molten Field's alloy nanoparticles and ensured the stability of colloidal suspension. Second, bare Field's alloy nanoparticles had residual charges which generate repulsive electrical force to prevent the agglomeration of molten nanoparticles.
