**1. Introduction**

Fluid flow within the channels can be viewed as the heart for plenty of natural and industrial systems. Heat and mass transfer are carried out along the walls of channels existing in biological systems, for instance blood vessels, kidney, lungs, and brain, as well as in many of industrial systems, for example heat exchangers, air separation plants, water desalination systems, and nuclear power reactors [1]. **Figure 1** illustrates a range of channel dimensions applied for different systems. While the smallest channel dimensions are observed in the biological systems undergoing mass transport, the larger dimensions are employed for the transportation of fluids. From a technological point of view, a steady transition from the larger channel dimensions, order of magnitudes of 10 to 25 mm, to the smaller channel dimeters, order of magnitudes of tens to hundreds of μm, can be seen in the recent years.

Generally, the energy transport process takes place along the channel wall, while the bulk flow occurs through the channel's cross-sectional area. The transport rate varies with the surface area, being in a linear proportion to the channel diameter (D), while the flow rate shows a direct proportion to the cross-sectional area (D2 ). Hence, the ratio of channel surface area to the volume is proportional to 1/D. Obviously, a reduction in the channel diameter leads to the increase in the ratio of surface area to volume.

**Figure 1.**

*Channel diameter ranges applied for different applications [2].*

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

By shifting to the smaller channel dimensions, some of the conventional principles of fluid flow, mass transport, and energy transport need to undergo reevaluation for validation or possible revisions. The following three main reasons can be mentioned to address the difference in the fluid flow modeling between conventional and mini/microchannels [3]:


In the heat transfer applications, the reasons which drive such a shift towards smaller flow passages are as follows [2]:


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 and operation.

#### **1.1 Classification of flow channels**

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

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

Fluid flow within the channels can be viewed as the heart for plenty of natural and industrial systems. Heat and mass transfer are carried out along the walls of channels existing in biological systems, for instance blood vessels, kidney, lungs, and brain, as well as in many of industrial systems, for example heat exchangers, air separation plants, water desalination systems, and nuclear power reactors [1]. **Figure 1** illustrates a range of channel dimensions applied for different systems. While the smallest channel dimensions are observed in the biological systems undergoing mass transport, the larger dimensions are employed for the transportation of fluids. From a technological point of view, a steady transition from the larger channel dimensions, order of magnitudes of 10 to 25 mm, to the smaller channel dimeters, order of magnitudes of tens to hundreds of μm, can be seen in the recent years.

Generally, the energy transport process takes place along the channel wall, while the bulk flow occurs through the channel's cross-sectional area. The transport rate varies with the surface area, being in a linear proportion to the channel diameter (D), while the flow rate shows a direct proportion to the cross-sectional area (D2

Hence, the ratio of channel surface area to the volume is proportional to 1/D. Obviously, a reduction in the channel diameter leads to the increase in the ratio of

).

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

transfer properties of nanoemulsion fluids.

*Heat Transfer - Design, Experimentation and Applications*

transfer enhancement

surface area to volume.

**Figure 1.**

**166**

*Channel diameter ranges applied for different applications [2].*

**1. Introduction**

conventional to micro dimensions. The channel classification suggested in [4] has categorized the range of 1–100 μm as the microchannels, 100 μm to 1 mm as the mesochannels, 1–6 mm as the compact passages, and the range above 6 mm as the conventional channels.

Kandlikar et al. [3] improved their channel classification reported earlier in [5], and then presented a more general classification according to the minimum channel dimension, shown in **Table 1**. In this table, D indicates the channel diameter. However, in case the channel is non-circular, the smallest channel dimension is recommended to be taken for D; for instance, in a rectangular channel the smaller side is considered for D. This channel classification may be used for either of singlephase or two-phase flow applications.

For the case of phase-change heat transfer in particular, the channels with various scales are classified according to the Bond number proposed by Cheng et al. [6] for expressing the transition from macroscale heat transfer to microscale heat transfer. Bond number takes into consideration the impacts of pressure, temperature, and some thermophysical properties of a fluid and is given as follows:

$$Bo = \left(\frac{D\_h}{l\_C}\right)^2\tag{1}$$

In the present study, we follow and meet both the selection criteria stated above for channel classification (i.e., **Tables 1** and **2**) to ensure proper differentiation in performance between minichannels and microchannels as well as proper collection of the literature associated with heat transfer of mini and/or microchannels.

A variety of industries and military sectors have faced the challenge of finding effective and efficient thermal management solutions as the electronic systems used can output heat flux as high as 100 W/cm<sup>2</sup> [7–11]. While many advanced works have been performed to develop high performance heat exchangers with varieties of shape, size and tube surface augmentation, the bottleneck of improvement has fall into how to develop efficient heat transfer fluids with significantly improved thermal properties over those currently available. To date, several heat transfer fluid candidates have been reported, which include, but not limited to, nanofluids [12–34], dilute emulsion [35, 36], and emulsion [37–41]: Nanofluid has been intensively studied since it was proposed in 1995 by Choi [42]. It is consisted of a mixture of solid nanoparticles and base fluid, and it has been reported to be potentially useful in applications such as nuclear power system, solar collector, and compact high power density electronics system. Emulsion and dilute emulsion fluid are essentially similar systems made of a mixture of two immiscible liquids, while the "dilute emulsion" has 5 vol% or less dispersed component. Using emulsion to enhance heat transfer can be dated back to 1959 by Moore [43], and it has attracted interests of researchers [35–45]. One of the most detailed descriptions of how emulsions boil is the work of Bulanov and Gasanov [38–41, 44, 45], in which they proposed chain-reaction boiling of the droplets as an explanation for the observed superheated droplets and bubble dynamics on the heat surface. In addition, Rosele [46] et al. carried out an experimental study of boiling heat transfer from a horizontal heated wire, including visual observations in which the heat transfer could be

**1.2 Ethanol/polyalphaolefin nanoemulsion: a novel heat transfer fluid**

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

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

enhanced in dilute emulsions compared to that of water as a base fluid.

by self-assembly inside another immiscible fluid, as part of a broad class of

light. A comparison of nanoemulsion with emulsion (dilute emulsion) is

**Property Nanoemulsion Emulsion** Appearance Transparent Turbid Interfacial tension Ultra-low (< <1 mN/m) Low Droplet size <50 nm >500 nm

Phase stability Thermodynamically stable Thermodynamically unstable Preparation Self-assembly Need of external shear Viscosity Newtonian Non-Newtonian

represented in **Table 3** [47, 48].

*Comparison of Nanoemulsion and emulsion (dilute emulsion).*

**Table 3.**

**169**

Recently, the authors have proposed a new type of heat transfer fluid called "nanoemulsion" [47]. Nanoemulsion is a suspension of liquid nanodroplets formed

multiphase colloidal dispersions [48]. The nanoemulsion eliminates the presence of solid particles, which usually cause abrasion and erosion issues even with extremely fine particles such as nanoparticles [49–53], and instead, uses liquid nanostructures [54–63]. The droplets typically have a length scale less or equal to 50 nm, which makes the nanoemulsion fluid thermodynamically stable and transparent to natural

where *Dh* stands for hydraulic diameter, and *lC* accounts for capillary length expressed as:

$$l\_C = \sqrt{\frac{\sigma}{\mathcal{g} \left(\rho\_1 - \rho\_V\right)}}\tag{2}$$

For water at 373 K, the capillary length (*lC*) of water is practically 2.72 mm. Based on Cheng's et al. classification, the channels with the range of a hydraulic diameter (*Dh*) between 600 μm and 4,720 μm can be considered as minichannels for the applications using water as the base liquid. **Table 2** shows the channel classification based on Bond Number (Bo) for water at 373 K.


#### **Table 1.**

*Channel classification based on dimensions.*


**Table 2.**

*Channel classification based on bond number for water at 373 K.*

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

In the present study, we follow and meet both the selection criteria stated above for channel classification (i.e., **Tables 1** and **2**) to ensure proper differentiation in performance between minichannels and microchannels as well as proper collection of the literature associated with heat transfer of mini and/or microchannels.
