**1. Introduction**

248 Heat Exchangers – Basics Design Applications

[23] Poling, B.E., Prausnnitz, J.M. et.al, M. (2000). *The Properties of Liquids & Gases*, 5th

[24] Todd, B. & Young, J.B., J. (2002). Thermodynamic and transport properties of gases for

[25] Mueller, F.; Jabbari, F.; Gaynor, R. & Jacob, B., J. (2007). Novel solid oxide fuel cell

[26] Aguiar, P.; Chadwick, D. & Kershenbaum, L., J. (2004). Effect of methane slippage on an

[27] Steen, M. & Ranzani, L., J. (2000). Potential of SiC as a heat exchanger material in

[28] Yasar, I., J. (2004). Finite element model for thermal analysis of ceramic heat exchanger

[29] Akiyoshi, M.; Takagi, I.; Yano, T.; Akasaka, N. & Tachi, Y., J. (2006). Thermal

combined cycle plant, *Ceramics International*, 26(2000) 849-854.

use in solid oxide fuel cell modeling, *Journal of Power Sources* 110 (2002) pp.186-200.

system controller for rapid load following, *Journal of Power Sources* 172 (2007)

indirect internal reforming solid oxide fuel cell, *Chemical Engineering Science*

tube under axial non-uniform convective heat transfer coefficient, *Materials and* 

conductivity of ceramics during irradiation, *Fusion Engineering and Design* 81 (2006)

Edition, McGraw-Hill, New York, 2000.

pp.308-323.

321-325.

59(2004) pp.87-97.

*Design* 25(2004) 479-482.

In recent years, microfabrication technologies have been utilized in the fields of process engineering using microchannel devices as heat exchangers. The microchannel heat transfer means is of importance to the areas of small and confined spaces, high heat flux devices for cooling electronic components, or other cooling applications in thermal and chemical engineering. Increasing the heat transfer rate and decreasing characteristic dimension of a heat exchanger are key design requirements, and a micro heat exchanger satisfies these needs.

A review of micro heat exchanger related issues such as flow behaviors, fabrication methods, and practical applications was done by Bowman and Maynes [1]. The review firstly introduced the experimental and numerical investigations of channel flow. Subsequently, Friction and heat transfer measurements of gas flow and liquid flow were discussed in the paper. The paper indicated that the transition Reynolds number is a function of surface roughness and channel geometry. Moreover, in the paper, the heat exchanger designs – including their comparison and optimization – were also reviewed. Furthermore, several fabrication methods including micromachining, chemical etching, laser machining, electroplating, and lamination, were discussed.

Review of the experimental results concerning single-phase convective heat transfer in microchannels was presented by Morini [2], with additional review of the results obtained for friction factor, laminar-to-turbulent transition, and Nusselt number in channels having hydraulic diameters less than 1 mm. Dang [3] and Dang et al. [4] presented the fluid flow

Single-Phase Heat Transfer and Fluid Flow Phenomena of Microchannel Heat Exchangers 251

Fig. 1. Schematic of the integrated preferential oxidation-heat exchanger [3,4,9,10].

Ceramic microstructure devices in the forms of counter-flow and cross-flow microchannel heat exchangers were manufactured by Alm et al. [13] and used in thermal and chemical process engineering. The peak experimental heat transfer coefficient for the cross-flow heat exchanger was observed to reach 22 kW/(m2K). Hallmark et al. [14] presented an experimental investigation on the heat transfer response of plastic microcapillary lms (MCF). Thermal power removed by the MCF heat exchanger was shown to be a function of input electrical power for an increasing flow rate of the MCF. Jiang et al. [15] investigated fluid flow and forced convection heat transfer in microchannel heat exchanger (MCHE). The transition from laminar to turbulent ow in the microchannel heat exchanger was observed to occur at Re 600. A new method of fabrication of heat transfer surfaces with micro-structured prole was presented by Schulz et al. [16]. By the ion track etching combined with metal electro-plating method, arrays of copper whiskers with high aspect ratio were produced on surfaces of heat transfer tubes. At the same temperature, the structured tube had higher heat flux or heat transfer coefficient value than that of the smooth one. Lee et al. [17] studied a polymer type micro-heat exchanger applicable to 272 BGA multi-chip module (MCM) and selected polydimethylsiloxane (PDMS) as the package material. The design was evaluated numerically using the Fluent CFD simulation tool. Results obtained from both the experiment and the simulation for each fabricated heat exchanger were compared; difference of temperature distribution in chip was less than 2 C. Surface temperature of the chip was found to be a function of pressure drop, the temperature decreased with a rising of the pressure drop for all the conditions being tested. Wei [18] fabricated a stacked microchannel heat sink using microfabrication techniques. Experiments were conducted to study the thermal performance of stacked microchannel structure, and overall thermal resistance was determined to be less than 0.1 K/W for both counter-flow and parallel-flow configurations. In the study, the numerical simulations were

and heat transfer characteristics for rectangular-shaped microchannel heat exchangers, both numerically and experimentally. Effects of flow arrangement on the performance index (expressed as the ratio of the heat transfer rate to the pressure drop) of a microchannel heat exchanger were evaluated. In addition, influences of configurations on the performance index of microchannel heat exchangers were presented.

Brandner et al. [5] described microstructure heat exchangers and their applications in laboratory and industry. In their paper, several micro heat exchangers were introduced, including polymer microchannel heat exchanger with aluminum separation foil, electrically powered lab-scale microchannel evaporator, ceramic counter-flow microstructure heat exchanger, etc. An analysis of effectiveness and pressure drop in micro cross-flow heat exchanger was presented by Kang and Tseng [6]. For each effectiveness, the heat transfer rate and pressure drop as a function of average temperature were obtained. The results indicated that pressure drop was reduced with a rising average temperature. Using silicon or copper as the materials for the microchannel heat exchangers, the difference in heat transfer rates between these two types of heat exchangers was found to be minimal. This was due to the fact that the substrate thicknesses between the hot and the cold channels were very thin; as a result, their thermal conductivity resistances were very small. Henning et al. [7] made three devices – the "standard" channel device with straight layout and a hydraulic diameter of 153 m, the "short" channel with straight layout and a hydraulic diameter of 149 m, and the "wavy" channel with wavy layout and a hydraulic diameter of 125 m. Their experimental results indicated that the standard channel device was the best choice for heating at moderate and high flow rates.

The crossflow microstructure heat exchanger made of stainless steel W316L was studied by Brandner [8]. It was observed that heat transfer in a microstructure heat exchanger was enhanced by using staggered microcolumn array heat exchangers which were designed to operate in the transition or turbulent flow regime. Results obtained from experiments and from modeling of an integrated preferential oxidation-heat exchanger (ProxHeatex) microdevice were presented by Delsman et al. [9]. The ProxHeatex consisted of two heat exchangers and a cooled reactor, as shown in Fig. 1. Those researchers also improved a new version of a ProxHeatex from an earlier prototype [10]. Heat recovery efficiency of the ProxHeatex device was found to be a function of the reformatted gas flow rate. The overall heat recovery of the device varies between 73% and 95%, with the higher values corresponding to higher ow rates and higher oxygen excess.

Shen and Gau [11] presented a paper dealing with design and fabrication of sensors and heaters for the study of micro-jet impingement cooling. The local Nusselt number distribution along the wall, Nux, was found to be a function of Z/B ratio, where Z is nozzleto-wall distance and B is the slot width. A heat exchanger for power electronic components was studied by Gillot et al. [12]. The prototype was composed of four elementary modules. Each module was composed of two IGBT (Insulated Gate Bipolar Transistor) chips directly brazed onto a piece of copper where rectangular channels were machined. The thermal resistance of the chips was calculated using 3D finite element simulation tool (FLUX 3D). Numerical and experimental values of the metal temperature at five testing locations were in good agreement, with the maximum percentage error being 1.7%. The pressure drop observed to be increased with a rising flow rate. The heat spread effect was observed to be a function of the heat transfer coefficient.

and heat transfer characteristics for rectangular-shaped microchannel heat exchangers, both numerically and experimentally. Effects of flow arrangement on the performance index (expressed as the ratio of the heat transfer rate to the pressure drop) of a microchannel heat exchanger were evaluated. In addition, influences of configurations on the performance

Brandner et al. [5] described microstructure heat exchangers and their applications in laboratory and industry. In their paper, several micro heat exchangers were introduced, including polymer microchannel heat exchanger with aluminum separation foil, electrically powered lab-scale microchannel evaporator, ceramic counter-flow microstructure heat exchanger, etc. An analysis of effectiveness and pressure drop in micro cross-flow heat exchanger was presented by Kang and Tseng [6]. For each effectiveness, the heat transfer rate and pressure drop as a function of average temperature were obtained. The results indicated that pressure drop was reduced with a rising average temperature. Using silicon or copper as the materials for the microchannel heat exchangers, the difference in heat transfer rates between these two types of heat exchangers was found to be minimal. This was due to the fact that the substrate thicknesses between the hot and the cold channels were very thin; as a result, their thermal conductivity resistances were very small. Henning et al. [7] made three devices – the "standard" channel device with straight layout and a hydraulic diameter of 153 m, the "short" channel with straight layout and a hydraulic diameter of 149 m, and the "wavy" channel with wavy layout and a hydraulic diameter of 125 m. Their experimental results indicated that the standard channel device was the best

The crossflow microstructure heat exchanger made of stainless steel W316L was studied by Brandner [8]. It was observed that heat transfer in a microstructure heat exchanger was enhanced by using staggered microcolumn array heat exchangers which were designed to operate in the transition or turbulent flow regime. Results obtained from experiments and from modeling of an integrated preferential oxidation-heat exchanger (ProxHeatex) microdevice were presented by Delsman et al. [9]. The ProxHeatex consisted of two heat exchangers and a cooled reactor, as shown in Fig. 1. Those researchers also improved a new version of a ProxHeatex from an earlier prototype [10]. Heat recovery efficiency of the ProxHeatex device was found to be a function of the reformatted gas flow rate. The overall heat recovery of the device varies between 73% and 95%, with the higher values

Shen and Gau [11] presented a paper dealing with design and fabrication of sensors and heaters for the study of micro-jet impingement cooling. The local Nusselt number distribution along the wall, Nux, was found to be a function of Z/B ratio, where Z is nozzleto-wall distance and B is the slot width. A heat exchanger for power electronic components was studied by Gillot et al. [12]. The prototype was composed of four elementary modules. Each module was composed of two IGBT (Insulated Gate Bipolar Transistor) chips directly brazed onto a piece of copper where rectangular channels were machined. The thermal resistance of the chips was calculated using 3D finite element simulation tool (FLUX 3D). Numerical and experimental values of the metal temperature at five testing locations were in good agreement, with the maximum percentage error being 1.7%. The pressure drop observed to be increased with a rising flow rate. The heat spread effect was observed to be a

index of microchannel heat exchangers were presented.

choice for heating at moderate and high flow rates.

corresponding to higher ow rates and higher oxygen excess.

function of the heat transfer coefficient.

Fig. 1. Schematic of the integrated preferential oxidation-heat exchanger [3,4,9,10].

Ceramic microstructure devices in the forms of counter-flow and cross-flow microchannel heat exchangers were manufactured by Alm et al. [13] and used in thermal and chemical process engineering. The peak experimental heat transfer coefficient for the cross-flow heat exchanger was observed to reach 22 kW/(m2K). Hallmark et al. [14] presented an experimental investigation on the heat transfer response of plastic microcapillary lms (MCF). Thermal power removed by the MCF heat exchanger was shown to be a function of input electrical power for an increasing flow rate of the MCF. Jiang et al. [15] investigated fluid flow and forced convection heat transfer in microchannel heat exchanger (MCHE). The transition from laminar to turbulent ow in the microchannel heat exchanger was observed to occur at Re 600. A new method of fabrication of heat transfer surfaces with micro-structured prole was presented by Schulz et al. [16]. By the ion track etching combined with metal electro-plating method, arrays of copper whiskers with high aspect ratio were produced on surfaces of heat transfer tubes. At the same temperature, the structured tube had higher heat flux or heat transfer coefficient value than that of the smooth one. Lee et al. [17] studied a polymer type micro-heat exchanger applicable to 272 BGA multi-chip module (MCM) and selected polydimethylsiloxane (PDMS) as the package material. The design was evaluated numerically using the Fluent CFD simulation tool. Results obtained from both the experiment and the simulation for each fabricated heat exchanger were compared; difference of temperature distribution in chip was less than 2 C. Surface temperature of the chip was found to be a function of pressure drop, the temperature decreased with a rising of the pressure drop for all the conditions being tested.

Wei [18] fabricated a stacked microchannel heat sink using microfabrication techniques. Experiments were conducted to study the thermal performance of stacked microchannel structure, and overall thermal resistance was determined to be less than 0.1 K/W for both counter-flow and parallel-flow configurations. In the study, the numerical simulations were

Single-Phase Heat Transfer and Fluid Flow Phenomena of Microchannel Heat Exchangers 253

Hasan [19] Silicon Water Re = 50 = f (Kr) 7.8 kPa For rectangular

5.8×10-6 m3/s

0.1158 g/s

0.3625 g/s

0.401g/s

(Note: Rect- Rectangular, Q-heat transfer rate, q-heat flux, R-thermal resistance, h-heat transfer coefficient, Kv-volumetric heat transfer coefficient, *Nu*-average Nusselt number, SLM-standard liter per minute, m-mass flow rate, V-volume flow rate, PDMS- polydimethylsiloxane, Z- nozzle-to-wall distance, L- distance from nozzle to the breakdown point of the jet, HE-heat exchanger), Kr-thermal

Table 1. Summary of the microchannel heat exchangers with single phase flow [3,4].

also done by using Fluent CFD package. Hasan et al. [19] evaluated the effect of the size and shape of channels for a counter-flow microchannel heat exchanger by using Fluent CFD numerical simulation. The effect of various channels showed that the circular shape achieved the best overall performance, with the second being the square channels. Results obtained from the numerical analyses and the experimental data of [18, 19] were in good agreement with the maximum error being 5.1% and the maximum difference in wall temperature being 1.7 K. Ameel et al. [20] presented an overview of the miniaturization technologies and their applications to energy systems. Based on the MEMS (microelectromechanical systems) technologies, the processes (including silicon-based micromachining, deep X-ray lithography, and the micro mechanical machining) were discussed in the context of applications to fluid flow, heat transfer, and energy

A study on the simulations of a trapezoidal shaped micro heat exchanger was presented by Dang et al. [21]. Using the geometric dimensions and the flow condition associated with this micro heat exchanger, a heat flux of 13.6 W/cm2 was determined by the numerical method. The effects of flow arrangement on the heat transfer behaviors of a microchannel heat exchanger were presented by Dang et al. [22-25]. For all cases done in these studies, the heat flux obtained from the counter-flow arrangement was observed to be always higher than that obtained from the parallel-flow: the value obtained from the counter-flow was evaluated to be 1.1 to 1.2 times of that obtained from the parallel-flow. The authors also presented an experimental study of the effects of gravity on the behaviors of heat transfer and pressure drop of a microchannel heat exchanger. The results showed that for microchannel heat exchangers, the influence of gravity on the pressure drop and heat

**fluid Flow rate Heat transfer Pressure** 

R: 0.24×10-4 – 0.12 ×10-4 Cm2/W

q: 12 – 13.6 W/cm2

q: 6.5 – 8.2 W/cm2

q: 14.3 – 17.8 W/cm2 

 : 13.9-21.7 W/kPa

**drop Comments** 

channels

channel

With the mass

With the mass flow rate of the hot side of 0.1667 g/s

flow rate of the hot side of 0.2321 g/s

p = f(V) for several

None

500-1400 Pa

**References Material/** 

Dang [21- 31]

systems.

**Shape** 

Silicon/ Trapezoidal

ect

ect

Aluminum/R

Aluminum/R

conductivity ratio, and - performance index)

transfer behaviors was negligibly small [26, 27].

Wei [18] Silicon/Rect Water V: 1.4×10-6 –

**Working** 

Water m: 0.0579–

Water m: 0.1859-

Water m: 0.2043-


Polymer Water m: 10-400 kg/h q: 0.2-1.1 W/cm2 0-0.4 MPa

m: 0.0643-0.07

kg/s

kg/s m: 0.1803- 0.0027 kg/s

Water m: 0-400 kg/h

Methanol, water

with 40% glycol

Plastic Water V: 30 or 60 or

Jiang [15] Copper/ Rect Water m: 0.0093-0.34

Alm [13] Ceramic Water m: 10-140 kg/h h: 7-22 kW/m2K

Schulz [16] Copper/ tube Water V: 4 L/m q: 1,000-17,000

Shen [11] Silicon/ Rect Air None 0.4

m: 0-300 kg/h

120 mL/min

Lee [17] PDMS/ Rect Water None Q: 0-14 W 0-10 kPa The top chip's

kg/s

m: 0.1746- 0.0026 kg/s

m: 0.0663-0.724

**fluid Flow rate Heat transfer Pressure** 

Q: 2,690-2,925 W

Q: 500-7,300 W

Q: 2,780-3,030 W

Q: 500-7,500 W

Q: 0-8,500 W

Q: 0-12,000 W

0.045Re 

*<sup>Z</sup> Nu*

0.052Re

*<sup>Z</sup> Nu*

(For crossflow

h: 125-230 W/m2K

Kv:11-38.5 MW/m3K

W/m2

V: 130-280 ml/s R: 100-110 K/kW 50-200

HE)

 

V: 2.5-6 SLM Heat recovery efficiency of the ProHeatex: 73-

*L* 0.5

*L*

Metal Water m: 64 kg/h Q: 3,000 W None Electrical power

95%

**drop Comments** 

Given the same effectiveness

Given the same temperature

Given the same effectiveness Given the same temperature

up to 3 kW with effectiveness ~ 0.95

For hydraulic diameter of 70

For staggered microcolumns

For the case before jet breakdown For the case after jet break down

m

0.16-0.28 MPa (Hot side) 0.22-0.44 MPa (Cold side)

10-400 kPa (Hot side) 10-900 kPa (Cold side)

0-6.2x105 Pa None

None

None

kPa

20-450 kPa (For counterflo w HE )

None

3.3-90 kPa

None Overheat from 4-16 oC

> temperature was 125 oC

**References Material/** 

Kang [6] Silicon/Rect

Brandner [5]

Henning [7]

Brandner [8]

Delsman [9,10]

Hallmark [14]

**Shape** 

Copper/Rect

Stainless steel/ Rect

Stainless steel/ Rect

Gillot [12] Copper/ Rect Water

**Working** 

Water

Water


(Note: Rect- Rectangular, Q-heat transfer rate, q-heat flux, R-thermal resistance, h-heat transfer coefficient, Kv-volumetric heat transfer coefficient, *Nu*-average Nusselt number, SLM-standard liter per minute, m-mass flow rate, V-volume flow rate, PDMS- polydimethylsiloxane, Z- nozzle-to-wall distance, L- distance from nozzle to the breakdown point of the jet, HE-heat exchanger), Kr-thermal conductivity ratio, and - performance index)

Table 1. Summary of the microchannel heat exchangers with single phase flow [3,4].

also done by using Fluent CFD package. Hasan et al. [19] evaluated the effect of the size and shape of channels for a counter-flow microchannel heat exchanger by using Fluent CFD numerical simulation. The effect of various channels showed that the circular shape achieved the best overall performance, with the second being the square channels. Results obtained from the numerical analyses and the experimental data of [18, 19] were in good agreement with the maximum error being 5.1% and the maximum difference in wall temperature being 1.7 K. Ameel et al. [20] presented an overview of the miniaturization technologies and their applications to energy systems. Based on the MEMS (microelectromechanical systems) technologies, the processes (including silicon-based micromachining, deep X-ray lithography, and the micro mechanical machining) were discussed in the context of applications to fluid flow, heat transfer, and energy systems.

A study on the simulations of a trapezoidal shaped micro heat exchanger was presented by Dang et al. [21]. Using the geometric dimensions and the flow condition associated with this micro heat exchanger, a heat flux of 13.6 W/cm2 was determined by the numerical method. The effects of flow arrangement on the heat transfer behaviors of a microchannel heat exchanger were presented by Dang et al. [22-25]. For all cases done in these studies, the heat flux obtained from the counter-flow arrangement was observed to be always higher than that obtained from the parallel-flow: the value obtained from the counter-flow was evaluated to be 1.1 to 1.2 times of that obtained from the parallel-flow. The authors also presented an experimental study of the effects of gravity on the behaviors of heat transfer and pressure drop of a microchannel heat exchanger. The results showed that for microchannel heat exchangers, the influence of gravity on the pressure drop and heat transfer behaviors was negligibly small [26, 27].

Single-Phase Heat Transfer and Fluid Flow Phenomena of Microchannel Heat Exchangers 255

P

T

Heat exchanger

P

Balance

T

Exhaust air valve

Pre-heater

Pump

Buffer tank

Pump

Fig. 2. Schematic of the test loop for the heat exchangers [3, 22-31].

glass wool are listed in Table 3 [32].

Water tank

**No. Type** 

In a microchannel heat exchanger, all channels are connected by manifolds for the inlet and outlet of hot water and for those of cold water, respectively. The manifolds of the heat exchangers are of the same cross-sections: having a rectangular shape with a width of 3 mm and a depth of 300 m. Figs. 3a and 3b show the dimensions of the S-types and I-type, respectively, with three S-types and one I-type being designed and manufactured and their dimensions listed in Table 2. Fig. 4 shows the photos of the microchannel heat exchangers with S-type and I-type manifolds. These test sections were manufactured by precision micromachining [20]. Each inlet hole or outlet hole of the heat exchangers has a crosssectional area of 9 mm2. The four sides of the heat exchanger were thermally insulated by the glass wool with a thickness of 5 mm. To seal the microchannels, two layers of PMMA (polymethyl methacrylate) were bonded on the top and bottom sides of the substrate by UV (ultraviolet) light process, as shown in Fig. 4. The physical properties of the PMMA and the

Buffer tank

Heater

**Dimensions of the substrate** 

L WT Wc Dc

**Dimensions of the** 

Balance

**channel (m)** 

**(mm)** 

Table 2. Geometric parameters of the microchannel heat exchangers [3, 25].

T1 S- Type (Microchannel) 46 26.5 1.2 500 300 T2 S- Type (Microchannel) 46 26.5 2 500 300 T3 S- Type (Microchannel) 46 26.5 1.2 500 180 T4 I- Type (Microchannel) 54 26.5 2 500 300

Dang and Teng [28, 29] studied the effects of the configuration (such as substrate thickness, cross-sectional area, and inlet/outlet location) on the behaviors of heat transfer and fluid flow of the microchannel heat exchangers. It was found that the actual heat transfer rate was observed to vary insignificantly with the substrate thickness in the range from 1.2 to 2 mm. Moreover, a comparison of the pressure drop and heat transfer behaviors between the microchannel and minichannel heat exchangers was done by Dang et al. [30]. Furthermore, numerical simulations of the microchannel heat exchangers using solver with the capability of dealing with steady-state and time-dependent conditions were carried out [31]. Numerical studies of the behaviors of the microchannel heat exchangers with 3D singlephase fluid flow and heat transfer in [22-26, 28-31] were done by using the COMSOL Multiphysics software, version 3.5. The algorithm of this software was based on the finite element method. The results obtained from the simulation were in good agreement with those obtained from the experiments, with the maximum percentage error being less than 9%.

To summarize, Table 1 listed the heat transfer and fluid flow behaviors for single phase microchannel heat exchangers [3, 4]. The heat exchangers were manufactured by different materials with a variety of shapes. Water was the most frequently used working fluid. The heat transfer coefficient and pressure drop were observed to be functions of the mass flow rate. The staggered microcolumn array and the micro-structured surface were found to enhance heat transfer rate in the micro heat exchangers. Because that the substrate thickness (between the hot and the cold channels) of micro heat exchangers was very thin, so the differences between the heat transfer rates obtained from these heat exchangers were negligibly small for several materials used in the studies.

From the above literatures, it is important to better understand the behaviors of heat transfer and pressure drop of the fluid through the microchannel heat exchangers in order to improve their design and optimize their performance. For the present study, single phase heat transfer and uid ow phenomena obtained from experiments and numerical simulations for rectangular-shaped microchannel heat exchangers were investigated. In the following sections, five heat exchangers with different geometrical congurations will be discussed.
