**4. Additively manufactured microchannel heat exchanger**

#### **4.1 Additive manufacturing of microchannel prototype**

While micro- and minichannels show promising potential and have been incorporated in a wide variety of unique, compact, and efficient cooling applications, manufacturing of micro- and minichannels made of high temperature alloys is still a challenging task. Recently, additive manufacturing (AM) technology has shown its promising application in manufacturing [75–79]. The use of additive manufacturing, or widely known as 3D printing technique, has revolutionized the traditional manufacturing process, which now eliminates the traditional constrictions in geometry design and manufacturability. AM based manufacturing technique has opened the design potential of traditionally impossible geometries and structures.

The metal based additive manufacturing techniques, including: Selective Laser Melting (SLM), Selective Laser Sintering (SLS), and Electron Beam Melting (EBM), are the most widely studied. Of the aforementioned techniques, Direct Metal Laser Sintering (DMLS) method, as a subset of the SLS process, is particularly appropriate for building production-grade parts due to its ability to use high temperature alloys in the process [77, 80–83].

With DMLS, thin layers of atomized fine metal powder are evenly distributed using a coating mechanism onto a substrate plate, usually metal, that is fastened to an indexing table that moves in the vertical (Z) axis. This takes place inside a chamber containing a tightly controlled atmosphere of inert gas, either argon or nitrogen at oxygen levels below 500 parts per million. Once each layer has been distributed, each 2D slice of the part geometry is fused by selectively melting the powder. This is accomplished with a high-power laser beam, usually an ytterbium fiber laser with hundreds of watts. The laser beam is directed in the X and Y directions with two high frequency scanning mirrors. The laser energy is intense enough to permit full melting (welding) of the particles to form solid metal. The process is repeated layer after layer until the part is complete.

One of the unique characteristics of DMLS manufacturing process is that the surfaces of manufactured parts are intrinsically rough due to the nature of laser sintering process [80]. While the outward-facing surfaces can be smoothed via machining, internal surfaces cannot be post-processed easily. Stimpson et al. examined the effects of surface roughness on flow structures in DMLS parts containing microchannels [84]. In another study, Snyder et al. [85] analyzed the effect of building direction on flow structures in similar DMLS parts, paying particular attention to the different roughness features generated by different printing orientations. The authors of both studies [84, 85] reported relative roughness values ranging between 20% and 38% of the channel hydraulic diameters and proved that correlations for friction factor and heat transfer are no longer valid for surfaces with such high roughness values. Thus, it is imperative to characterize the internal surface of DMLS manufactured parts.

Many investigations have also been conducted to study flow behavior in microchannels of different cross-sections such as rectangular, circular, trapezoidal, triangular, and elliptical [86–88]. Rectangular and trapezoidal cross-sections have been extensively studied for a wide range of applications, mainly due to practical considerations such as fabrication techniques, cost, and ease of manufacturing.

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

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 works.

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 performance is beyond the scope of this study.

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 developed for the present study.

**Figure 16.** *EOSINT M280 machine used to develop the microchannel prototype.*


#### **Table 11.**

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

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*

While micro- and minichannels show promising potential and have been incorporated in a wide variety of unique, compact, and efficient cooling applications, manufacturing of micro- and minichannels made of high temperature alloys is still a challenging task. Recently, additive manufacturing (AM) technology has shown its promising application in manufacturing [75–79]. The use of additive manufacturing, or widely known as 3D printing technique, has revolutionized the traditional manufacturing process, which now eliminates the traditional constrictions in geometry design and manufacturability. AM based manufacturing technique has opened the design potential of traditionally impossible geometries and structures. The metal based additive manufacturing techniques, including: Selective Laser Melting (SLM), Selective Laser Sintering (SLS), and Electron Beam Melting (EBM), are the most widely studied. Of the aforementioned techniques, Direct Metal Laser Sintering (DMLS) method, as a subset of the SLS process, is particularly appropriate for building production-grade parts due to its ability to use high temperature alloys

With DMLS, thin layers of atomized fine metal powder are evenly distributed using a coating mechanism onto a substrate plate, usually metal, that is fastened to an indexing table that moves in the vertical (Z) axis. This takes place inside a chamber containing a tightly controlled atmosphere of inert gas, either argon or nitrogen at oxygen levels below 500 parts per million. Once each layer has been distributed, each 2D slice of the part geometry is fused by selectively melting the powder. This is accomplished with a high-power laser beam, usually an ytterbium fiber laser with hundreds of watts. The laser beam is directed in the X and Y directions with two high frequency scanning mirrors. The laser energy is intense enough to permit full melting (welding) of the particles to form solid metal. The

One of the unique characteristics of DMLS manufacturing process is that the surfaces of manufactured parts are intrinsically rough due to the nature of laser sintering process [80]. While the outward-facing surfaces can be smoothed via machining, internal surfaces cannot be post-processed easily. Stimpson et al. examined the effects of surface roughness on flow structures in DMLS parts containing microchannels [84]. In another study, Snyder et al. [85] analyzed the effect of building direction on flow structures in similar DMLS parts, paying particular attention to the different roughness features generated by different printing orientations. The authors of both studies [84, 85] reported relative roughness values ranging between 20% and 38% of the channel hydraulic diameters and proved that correlations for friction factor and heat transfer are no longer valid for surfaces with such high roughness values. Thus, it is imperative to characterize the internal

Many investigations have also been conducted to study flow behavior in microchannels of different cross-sections such as rectangular, circular, trapezoidal, triangular, and elliptical [86–88]. Rectangular and trapezoidal cross-sections have been extensively studied for a wide range of applications, mainly due to practical considerations such as fabrication techniques, cost, and ease of manufacturing.

**4. Additively manufactured microchannel heat exchanger**

**4.1 Additive manufacturing of microchannel prototype**

*Heat Transfer - Design, Experimentation and Applications*

process is repeated layer after layer until the part is complete.

in the process [77, 80–83].

surface of DMLS manufactured parts.

**184**

.

**Figure 17.** *Additively manufactured (AM) microchannel heat exchanger developed for the present study.*

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

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

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

manufactured microchannels is likely to be the main contributor to the augmented heat transfer performance. Future study is then required to better appreciate the

**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

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

important to point out that the non-post processed surface of the DMLS

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

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

possible mechanisms behind the phenomenon observed here.

**Figure 18.**

*exchangers.*

**Figure 19.**

*exchangers.*

**187**
