**Abstract**

The present study experimentally and numerically investigates the flow and heat transfer characteristics of a novel nanostructured heat transfer fluid, namely, ethanol/polyalphaolefin nanoemulsion, inside a conventionally manufactured minichannel of circular cross section and a microchannel heat exchanger of rectangular cross section manufactured additively using the Direct Metal Laser Sintering (DMLS) process. The experiments were conducted for single-phase flow of pure polyalphaolefin (PAO) and ethanol/PAO nanoemulsion fluids with two ethanol concentrations of 4 wt% and 8 wt% as well as for two-phase flow boiling of nanoemulsion fluids to study the effect of ethanol nanodroplets on the convective flow and heat transfer characteristics. Furthermore, the effects of flow regime of the working fluids on the heat transfer performance for both the minichannel and microchannel heat exchangers were examined within the laminar and transitional flow regimes. It was found that the ethanol/PAO nanoemulsion fluids can improve convective heat transfer compared to that of the pure PAO base fluid under both single- and two-phase flow regimes. While the concentration of nanoemulsion fluids did not reflect a remarkable distinction in single-phase heat transfer performance within the laminar regime, a significant heat transfer enhancement was observed using the nanoemulsion fluids upon entering the transitional flow regime. The heat transfer enhancement 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 twophase flow boiling, heat transfer coefficients of ethanol/PAO nanoemulsion fluids were further enhanced when the ethanol nanodroplets underwent phase change. A comparative study on the flow and heat transfer characteristics was also implemented between the traditionally fabricated minichannel and additively manufactured microchannel of similar dimensions using the same working fluid of pure PAO and the same operating conditions. The results revealed that although the DMLS fabricated microchannel posed a higher pressure loss, a substantial heat

transfer enhancement was achieved as compared to the minichannel heat exchanger tested under the same conditions. The non-post processed surface of the DMLS manufactured microchannel is likely to be the main contributor to the augmented heat transfer performance. Further studies are required to fully appreciate the possible mechanisms behind this phenomenon as well as the convective heat transfer properties of nanoemulsion fluids.

By shifting to the smaller channel dimensions, some of the conventional princi-

i. some changes realized in the fundamental principles; as an illustration, the

microchannels, or a deviation arisen from an enhanced effect of some

ii. uncertainties originating in those factors extracted empirically from experiments on the large-size channels, for instance pressure loss

iii. uncertainties originating from microscale measurements, either in

In the heat transfer applications, the reasons which drive such a shift towards

ii. Improved dissipation of heat flux in the microelectronic circuits and devices,

iii. Development of micro-scale devices which require the equally small

The use of smaller channels provides a better performance in heat transfer, albeit accompanied mostly with an increase in the pressure drop. An optimal balance between these parameters results in the various channel dimensions for different applications. Take as an illustration, in automobile industry, the dimensions of flow passages in evaporators and radiators have reached to nearly 1 mm as a result of the balance between the cleanliness standards, heat transfer, and pumping power. Similarly, the high heat fluxes generated by microelectronic devices as well as the geometric and dimensional constraints imposed by the micro-scale devices and microelectromechanical systems (MEMS) require a drastic reduction in the dimensions of flow channels designed for their cooling systems. Also, the mirrors used for high-power laser devices employ the cooling systems having extremely small footprint. The continuous advances in the fields of genetic and biomedical engineering are contingent upon the precise transport control and thermal control of fluid flow in the micro-scale passages. Hence, a solid understanding of heat transfer process and fluid flow in such micro-scale systems is crucial to the design

The hydraulic diameter can serve as an indicator for taking into account a channel's dimensions and then classifying the flow channels. The reduction in channel dimensions has different impacts on various processes. Although the derivation of particular criteria based on different process parameters seems to be fascinating, a simple dimensional-based classification is typically employed in the literature due to the abundance of process parameters arising in the transition from

coefficients of fluid flow at the tube entrance and exit,

reevaluation for validation or possible revisions. The following three main reasons can be mentioned to address the difference in the fluid flow modeling between

continuum hypothesis may not be valid for gas flows in mini/

ples of fluid flow, mass transport, and energy transport need to undergo

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

conventional and mini/microchannels [3]:

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

forces (e.g., electrokinetic),

geometry or operating conditions.

i. Substantial enhancement of heat transfer,

smaller flow passages are as follows [2]:

cooling systems.

and operation.

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**1.1 Classification of flow channels**

**Keywords:** ethanol/polyalphaolefin nanoemulsion, minichannel, additively manufactured microchannel, single-phase flow, two-phase flow boiling, heat transfer enhancement
