**Fluid behaviors**

error being less than 7.2%.

264 Heat Exchangers – Basics Design Applications

It is observed that with a rising the mass flow rate of the cold side, the outlet temperatures decrease, as shown in Fig. 8a; however, for the same flow rate condition, both the heat transfer rates of the hot side and cold side increase. As the mass flow rate of the cold side increases, the heat transfer rate for the cold side increases at a slightly higher rate than that for the hot side. It is also observed that the actual effectiveness for the microchannel heat exchanger increases with a rising the mass flow rate of the cold side, as shown in Fig. 8b. The results obtained from the effectiveness (NTU method) are lower than those obtained from the actual effectiveness, as shown in Fig. 8.b. Hence, a conclusion can be drawn for the heat exchanger under study: at constant inlet temperature and mass flow rate of the hot side, it is more effective to use the heat exchanger with high mass flow rate of cold side. However, leakage of liquid out of the microchannel heat exchanger can occur when the mass flow rate of the cold side increases above 0.854 g/s, as a result of the excessive

> 0.50 0.55 0.60 0.65 0.70 0.75 0.80 0.85 0.90 0.95

c) Overall heat transfer coefficientd) Heat flux

Fig. 8. Comparison between numerical and experimental results at constant inlet

**Heat flux, W/cm2**

**Effectiveness**

0.1600 0.2000 0.2400 0.2800 0.3200 0.3600 0.4000 **Mass flow rate of cold side, g/s**

0.1600 0.2000 0.2400 0.2800 0.3200 0.3600 0.4000 **Mass flow rate of cold side, g/s**

Num. results Exp. results

Num. results of actual eff. Exp. results of actual eff. Num. results of eff. Exp. results of eff.

pressure exerted on the system under study.

Num. results for hot side Exp. results for hot side Num. results for cold side Exp. results for cold side

0.1600 0.2000 0.2400 0.2800 0.3200 0.3600 0.4000 **Mass flow rate of cold side, g/s**

0.1600 0.2000 0.2400 0.2800 0.3200 0.3600 0.4000 **Mass flow rate of cold side, g/s**

temperature and mass flow rate for the hot side.

Num. results Exp. results

a) Outlet temperature b) Effectiveness

30

0.000

0.200

0.400

0.600

**Overall heat transfer coefficient k,**

**W/(cm2K)**

0.800

1.000

34

38

**Outlet temperature, o**

**C**

42

46

50

The boundary conditions of the two outlets of the hot side and the cold side are at the atmospheric pressure. Fig. 9 shows the velocity field along channels of the microchannel heat exchanger. The streamlines of water pass from the microchannels to the manifold. At the edge between channels and manifold, the streamlines appear to be curved in shape. The velocity field at the outlet of the manifold is parabolic which is consistent with that predicted by the laminar flow theory for fluid in a channel.

rates of the cold side are shown in Figs. 8a-8d, respectively, with the maximum percentage

Fig. 9. The velocity field along channels of the microchannel heat exchanger.

Fig. 10 shows the pressure distribution in channels of the hot side at the mass flow rate of 0.2321 g/s and the inlet temperature of 45 ºC. The pressure decreases gradually from the first channel to the last one, with the first channel being the nearest one to the entrance of the inlet of the manifold [25, 26].

Single-Phase Heat Transfer and Fluid Flow Phenomena of Microchannel Heat Exchangers 267

Fig. 12 shows a comparison between numerical and experimental results for the pressure drops and the mass flow rates at various inlet water temperatures. As shown in the figure, the pressure drop decreases as the inlet water temperature increases. The maximum difference between the two results obtained is 131 Pa, with a maximum percentage error of

> Exp. results of water at 25 ºC Num. results of water at 25 ºC Exp. results of water at 52 ºC Num. results of water at 52 ºC

0.1500 0.3500 0.5500 0.7500 0.9500 **Mass flow rate, g/s**

Fig. 12. Comparison between numerical and experimental results for the pressure drop and

Under various conditions for all cases studied up to now, the maximum percentage errors between the results obtained from numerical simulations and those from experimental data

For this section, single-phase heat transfer and fluid flow phenomena obtained from experimental data and numerical simulations for the microchannel heat exchanger T1 were investigated. Two cases of flow arrangements will be discussed for the heat exchanger under investigation: (1) the counter-flow arrangement and (2) the parallel-flow arrangement. The dimensions of this microchannel heat exchanger are shown in Fig 3 and its geometric parameters are listed in Table 2. The conditions of the numerical simulation

For the experiments carried out in this section, the inlet temperature and the mass flow rate of the cold side were fixed at 22.5 ºC and 0.2043 g/s, respectively. For the hot side, the mass flow rate was fixed at 0.2321 g/s and the inlet temperatures were varying from 45 to 70 ºC (Dang et al. [26]). The thermal boundary conditions of the top and bottom walls of the heat exchanger are assumed to be constant heat flux. The temperature profile of the microchannel heat exchanger is shown in Fig. 13 for the inlet temperature of 45 ºC at the hot

7.8%.

0

the flow rate at various inlet water temperatures.

**4.2 Effects of flow arrangements** 

are found to be less than 9% and are in good agreement.

and experimental data are indicated in more detail in [3, 22-25].

1000

2000

**Pressure drop, Pa**

3000

4000

5000

For microchannel heat exchanger used in this study, at an inlet temperature of 25 ºC, the pressure drop increases from 889 to 4,411 Pa, with the mass flow rate rising from 0.1812 to 0.8540 g/s. In addition, the pressure drop decreases as the inlet temperature increases, since as the inlet temperature increases, the dynamic viscosity decreases. Because that the Poiseuille number (*Po = f Re*) depends only on the geometry of the microchannel, the pressure drop decreases with a rising inlet temperature of water. This conclusion is in agreement with [6]. Fig. 11 shows the pressure drop obtained experimentally as a function of the inlet water temperature for various mass flow rates. At a mass flow rate of 0.4972 g/s, the pressure drop decreases from 2,437 to 1,776 Pa, with the inlet temperature rising from 25 ºC to 52 ºC.

Fig. 10. Pressure distribution of the hot side of the heat exchanger.

Fig. 11. Pressure drop as a function of the inlet water temperature for various mass flow rates.

Fig. 12 shows a comparison between numerical and experimental results for the pressure drops and the mass flow rates at various inlet water temperatures. As shown in the figure, the pressure drop decreases as the inlet water temperature increases. The maximum difference between the two results obtained is 131 Pa, with a maximum percentage error of 7.8%.

Fig. 12. Comparison between numerical and experimental results for the pressure drop and the flow rate at various inlet water temperatures.

Under various conditions for all cases studied up to now, the maximum percentage errors between the results obtained from numerical simulations and those from experimental data are found to be less than 9% and are in good agreement.

#### **4.2 Effects of flow arrangements**

266 Heat Exchangers – Basics Design Applications

For microchannel heat exchanger used in this study, at an inlet temperature of 25 ºC, the pressure drop increases from 889 to 4,411 Pa, with the mass flow rate rising from 0.1812 to 0.8540 g/s. In addition, the pressure drop decreases as the inlet temperature increases, since as the inlet temperature increases, the dynamic viscosity decreases. Because that the Poiseuille number (*Po = f Re*) depends only on the geometry of the microchannel, the pressure drop decreases with a rising inlet temperature of water. This conclusion is in agreement with [6]. Fig. 11 shows the pressure drop obtained experimentally as a function of the inlet water temperature for various mass flow rates. At a mass flow rate of 0.4972 g/s, the pressure drop decreases from 2,437 to 1,776 Pa, with the inlet temperature rising from 25

Fig. 10. Pressure distribution of the hot side of the heat exchanger.

20 25 30 35 40 45 50 55 **Water temperature, <sup>o</sup>**

Fig. 11. Pressure drop as a function of the inlet water temperature for various mass flow rates.

**C**

0.1812 g/s 0.3555 g/s 0.4972 g/s 0.6718 g/s 0.8540 g/s

0

1000

2000

**Pressure drop, Pa**

3000

4000

5000

ºC to 52 ºC.

For this section, single-phase heat transfer and fluid flow phenomena obtained from experimental data and numerical simulations for the microchannel heat exchanger T1 were investigated. Two cases of flow arrangements will be discussed for the heat exchanger under investigation: (1) the counter-flow arrangement and (2) the parallel-flow arrangement. The dimensions of this microchannel heat exchanger are shown in Fig 3 and its geometric parameters are listed in Table 2. The conditions of the numerical simulation and experimental data are indicated in more detail in [3, 22-25].

For the experiments carried out in this section, the inlet temperature and the mass flow rate of the cold side were fixed at 22.5 ºC and 0.2043 g/s, respectively. For the hot side, the mass flow rate was fixed at 0.2321 g/s and the inlet temperatures were varying from 45 to 70 ºC (Dang et al. [26]). The thermal boundary conditions of the top and bottom walls of the heat exchanger are assumed to be constant heat flux. The temperature profile of the microchannel heat exchanger is shown in Fig. 13 for the inlet temperature of 45 ºC at the hot

Single-Phase Heat Transfer and Fluid Flow Phenomena of Microchannel Heat Exchangers 269

a) Cross-section for counter-flow b) Cross-section for parallel-flow

c) Counter-flow for 2D d) Parallel-flow for 2D

The temperature profiles of the microchannel heat exchanger are shown in Fig. 15 for the inlet temperature of 45 ºC at the hot side at the cross-section through three points: (-0.01, 0.02, -0.02), (-0.01, 0.02, 0.00), and (-0.01, 0, 0.00). Fig. 15a and Fig. 15b show the temperature profiles of the cross-section in the microchannel heat exchanger for the cases with counterflow and parallel-flow at the conditions specified above. At this cross-section, it is observed that the temperature profiles varying along the distance measured from the substrate for counter-flow arrangement are more evenly distributed (as shown in Fig. 15c) than those for parallel-flow arrangement with the hot temperature region skewed to the right-side of the

As a result, the range of temperature gradient obtained from counter-flow arrangement is smaller than that obtained from parallel-flow one. For the counter-flow one, the temperature gradients obtained numerically ranging from 50.3 to 529.1 K/m, as shown in Fig. 16a. However, the range is from 16.6 to 574 K/m for parallel-flow one, as shown in Fig. 16b. The

Fig. 15. Temperature profiles of a cross-section of the microchannel heat exchanger.

microchannel, as shown in Fig. 15d.

side. Fig. 13a and Fig. 13b show the temperature profiles for the cases with counter-flow and parallel-flow at the conditions specified above.

The 3D temperature profiles of the microchannel heat exchanger were shown in more detail in [24-26] also. Profiles of the temperature gradients shown in Fig. 14 indicate the temperature gradients from heat exchanger's cold region towards its hot region, with Fig. 14a being the counter-flow and Fig. 14b being the parallel-flow. Distribution of the temperature gradients varies along the channel length of the heat exchanger with counterflow and parallel-flow configurations. In the middle of the heat exchanger with counterflow arrangement, the temperature gradients are in fishbone shapes. However, the temperature gradients are in the perpendicular direction towards the substrate of the heat exchanger with parallel-flow arrangement.

Fig. 13. Temperature profiles of the microchannel heat exchanger.

Fig. 14. The profiles of temperature gradients of the microchannel heat exchanger.

side. Fig. 13a and Fig. 13b show the temperature profiles for the cases with counter-flow and

The 3D temperature profiles of the microchannel heat exchanger were shown in more detail in [24-26] also. Profiles of the temperature gradients shown in Fig. 14 indicate the temperature gradients from heat exchanger's cold region towards its hot region, with Fig. 14a being the counter-flow and Fig. 14b being the parallel-flow. Distribution of the temperature gradients varies along the channel length of the heat exchanger with counterflow and parallel-flow configurations. In the middle of the heat exchanger with counterflow arrangement, the temperature gradients are in fishbone shapes. However, the temperature gradients are in the perpendicular direction towards the substrate of the heat

a) Counter-flow b) Parallel-flow

a) Counter-flow b) Parallel-flow

Fig. 14. The profiles of temperature gradients of the microchannel heat exchanger.

Fig. 13. Temperature profiles of the microchannel heat exchanger.

parallel-flow at the conditions specified above.

exchanger with parallel-flow arrangement.

Fig. 15. Temperature profiles of a cross-section of the microchannel heat exchanger.

The temperature profiles of the microchannel heat exchanger are shown in Fig. 15 for the inlet temperature of 45 ºC at the hot side at the cross-section through three points: (-0.01, 0.02, -0.02), (-0.01, 0.02, 0.00), and (-0.01, 0, 0.00). Fig. 15a and Fig. 15b show the temperature profiles of the cross-section in the microchannel heat exchanger for the cases with counterflow and parallel-flow at the conditions specified above. At this cross-section, it is observed that the temperature profiles varying along the distance measured from the substrate for counter-flow arrangement are more evenly distributed (as shown in Fig. 15c) than those for parallel-flow arrangement with the hot temperature region skewed to the right-side of the microchannel, as shown in Fig. 15d.

As a result, the range of temperature gradient obtained from counter-flow arrangement is smaller than that obtained from parallel-flow one. For the counter-flow one, the temperature gradients obtained numerically ranging from 50.3 to 529.1 K/m, as shown in Fig. 16a. However, the range is from 16.6 to 574 K/m for parallel-flow one, as shown in Fig. 16b. The

Single-Phase Heat Transfer and Fluid Flow Phenomena of Microchannel Heat Exchangers 271

0

0

5

10

15

**Performance index, W/kPa**

20

25

40 45 50 55 60 65 70 75 **Inlet temperature of hot side, <sup>0</sup>**

40 45 50 55 60 65 70 75 **Inlet temperature of hot side, <sup>0</sup>**

Counter Parallel

Counter Parallel

**C**

**C**

5

10

**Heat transfer rate,** 

**W**

15

20

25

30

percentage error is 5.3%.

**Pressure drop, Pa**

40 45 50 55 60 65 70 75 **Inlet temperature of hot side, <sup>o</sup>**

40 45 50 55 60 65 70 75 **Inlet temperature of hot side, <sup>0</sup>**

Hot side-counter Hot side-parallel Cold side-counter Cold side-parallel

**C**

a) Outlet temperature b) Heat transfer rate

**C**

c) Pressure drop d) Performance index

Fig. 17. Comparison of the experimental results between the counter-flow and parallel-flow.

When the inlet temperature of the hot side is increased, the heat transfer rate *Q* of the heat exchanger increases also. As a result, the heat transfer result obtained from the effectiveness (NTU method) increases with rising inlet temperature at the hot side, as shown in Fig. 18a. The figure shows a comparison between numerical and experimental results of the effectiveness (NTU method) for the microchannel heat exchanger with counter-flow arrangement. The maximum difference of the effectiveness is 0.009; it occurs at low inlet temperature of the hot side, and the maximum percentage error is 1.6%. Fig. 18b shows the comparison of the performance indexes between the numerical and experimental results for the case with counter-flow arrangement. Since the performance index obtained from the simulation is in the vicinity of that obtained from the experiment, the results obtained from the simulation are judged to be in good agreement with those obtained from the experiments. The maximum difference of the performance index is 0.413 W/kPa; it occurs at low inlet temperature of the hot side for the counter-flow arrangement, and the maximum

Hot side-Counter Hot side-Parallel Cold side-Counter Cold side-Parallel

34

**O utlet tem** 

38

42

**peratures, o**

**C**

46

50

profiles of the temperature gradients in 3D and in these three planes (x-y, y-z, and z-x planes) for the whole subdomains of the heat exchanger were shown in more detail in [24- 26].

Fig. 16. Profiles of 2D temperature gradients of the microchannel heat exchanger.

Under the condition stated above, the inlet temperature and the mass flow rate of the cold side were fixed at 22.5 ºC and 0.2043 g/s, respectively. For the hot side, the mass flow rate was fixed at 0.2321 g/s and the inlet temperatures were varying from 45 to 70 ºC [22-25]. A relationship of the experimental results between the counter-flow and the parallel-flow cases is shown in Fig. 17.

For the counter-flow case, the outlet temperature at the cold side is higher than that obtained at the hot side (see Fig. 17a). However, for the parallel-flow case, the outlet temperature at the cold side is lower than that obtained at the hot side. As a result, the heat transfer rate obtained from the counter-flow arrangement is higher than that obtained from the parallel-flow arrangement of the microchannel heat exchanger, as shown in Fig. 17b. It is noted that to compute the heat transfer rates for an adiabatic heat exchanger, these rates were based on those of the cold side. Fig. 17c shows the comparison of pressure drops of both cases for the counter- and parallel-flow arrangements. It is observed that the pressure drop obtained from the hot side is higher than that obtained from the cold side; this is consistent with the fact that the mass flow rate of the hot side is also higher than that of the cold side. It is also observed that the pressure drop obtained from the counter-flow arrangement is the same as that obtained from parallel-flow one. As a result, the performance index obtained from the counter-flow arrangement is higher than that obtained from the parallel-flow one: the value obtained from the counter-flow is 1.192 to 1.2 times of that obtained from the parallel-flow, as shown in Fig. 17d.

profiles of the temperature gradients in 3D and in these three planes (x-y, y-z, and z-x planes) for the whole subdomains of the heat exchanger were shown in more detail in [24-

a) Counter-flow b) Parallel-flow

that obtained from the parallel-flow, as shown in Fig. 17d.

Fig. 16. Profiles of 2D temperature gradients of the microchannel heat exchanger.

Under the condition stated above, the inlet temperature and the mass flow rate of the cold side were fixed at 22.5 ºC and 0.2043 g/s, respectively. For the hot side, the mass flow rate was fixed at 0.2321 g/s and the inlet temperatures were varying from 45 to 70 ºC [22-25]. A relationship of the experimental results between the counter-flow and the parallel-flow

For the counter-flow case, the outlet temperature at the cold side is higher than that obtained at the hot side (see Fig. 17a). However, for the parallel-flow case, the outlet temperature at the cold side is lower than that obtained at the hot side. As a result, the heat transfer rate obtained from the counter-flow arrangement is higher than that obtained from the parallel-flow arrangement of the microchannel heat exchanger, as shown in Fig. 17b. It is noted that to compute the heat transfer rates for an adiabatic heat exchanger, these rates were based on those of the cold side. Fig. 17c shows the comparison of pressure drops of both cases for the counter- and parallel-flow arrangements. It is observed that the pressure drop obtained from the hot side is higher than that obtained from the cold side; this is consistent with the fact that the mass flow rate of the hot side is also higher than that of the cold side. It is also observed that the pressure drop obtained from the counter-flow arrangement is the same as that obtained from parallel-flow one. As a result, the performance index obtained from the counter-flow arrangement is higher than that obtained from the parallel-flow one: the value obtained from the counter-flow is 1.192 to 1.2 times of

26].

cases is shown in Fig. 17.

Fig. 17. Comparison of the experimental results between the counter-flow and parallel-flow.

When the inlet temperature of the hot side is increased, the heat transfer rate *Q* of the heat exchanger increases also. As a result, the heat transfer result obtained from the effectiveness (NTU method) increases with rising inlet temperature at the hot side, as shown in Fig. 18a. The figure shows a comparison between numerical and experimental results of the effectiveness (NTU method) for the microchannel heat exchanger with counter-flow arrangement. The maximum difference of the effectiveness is 0.009; it occurs at low inlet temperature of the hot side, and the maximum percentage error is 1.6%. Fig. 18b shows the comparison of the performance indexes between the numerical and experimental results for the case with counter-flow arrangement. Since the performance index obtained from the simulation is in the vicinity of that obtained from the experiment, the results obtained from the simulation are judged to be in good agreement with those obtained from the experiments. The maximum difference of the performance index is 0.413 W/kPa; it occurs at low inlet temperature of the hot side for the counter-flow arrangement, and the maximum percentage error is 5.3%.

Single-Phase Heat Transfer and Fluid Flow Phenomena of Microchannel Heat Exchangers 273

hot side of the device having the inlet temperature of 70 ºC and mass flow rate of 0.2321 g/s and for water from the cold side having the inlet temperature of 22.5 ºC and mass flow rate

Fig. 19b shows the comparison of the performance indexes between the counter- and parallel-flow arrangements. The performance index obtained from the counter-flow arrangement is higher than that obtained from the parallel-flow one: the value obtained from the counter-flow is 1.13 to 1.17 times of that obtained from the parallel-flow. The performance index of 21.69 W/kPa was achieved for water from the hot side of the device having the inlet temperature of 70 ºC and the mass flow rate of 0.2321 g/s and for water from the cold side having the inlet temperature of 22.5 ºC and the mass flow rate of 0.401

In order to study the effects of geometrical configurations on the performance of the heat exchangers, all experimental conditions for the four microchannel heat exchangers were kept the same, more detail in [28,29]. Throughout the section, two cases of testing were discussed: the first one for increasing the inlet temperature of the hot side and the second for increasing the mass flow rate of the cold side. Further details of these cases are as follows:

1. Case No. 1 is for the case of increasing the inlet temperature of the hot side: the inlet temperature and the mass flow rate of the cold side were fixed at 22.5 C and 0.2135 g/s, respectively; at the hot side, the mass flow rates was fixed at 0.2308 g/s and the

2. Case No. 2 is for the case of increasing the mass flow rate of the cold side: the inlet temperature and the mass flow rate of the hot side were fixed at 70 ºC and 0.2308 g/s, respectively; at the cold side, the inlet temperature was fixed at 22.5 ºC and the mass

Variable parameters Fixed parameters

mh = 0.2308 g/s mc = 0.2135 g/s Tc,i = 22.5 C

mh = 0.2308 g/s Th,i = 70 C Tc,i = 22.5 C

of 0.401 g/s.

g/s.

**4.3 Effects of geometrical configurations** 

inlet temperature were varying from 45 to 70 C.

flow rates were varying from 0.2135 to 0.401 g/s.

The flow parameters for these two cases are summarized in Table 5.

1 Th,i = 45 70 C

<sup>2</sup>mc= 0.2135 0.401 g/s

Table 5. Flow parameters for the cases under study.

**Case Flow conditions** 

Fig. 18. Comparison between numerical and experimental results.

Fig. 19. Comparison of the experimental results with a rising mass flow rate of the cold side.

Under another experimental condition, for the experiments done in this study, the inlet temperature and the mass flow rate of the hot side were fixed at 70 ºC and 0.2321 g/s, respectively. For the cold side, the inlet temperature was fixed at 22.5 ºC and the mass flow rates were varying from 0.2043 to 0.401 g/s. The outlet temperatures are a function of the mass flow rate at the cold side, as shown in more detail in [22-25]. Contrary to the case of varying inlet temperature of the hot side, the outlet temperatures decrease as the mass flow rate of the cold side increases. For the counter-flow case, the outlet temperature of the cold side is higher than or equal to that obtained at the hot side. However, for the parallel-flow case, the outlet temperature at the cold side is lower than that obtained at the hot side. As a result, for the microchannel heat exchanger, the heat flux obtained from the counter-flow arrangement is higher than that obtained from the parallel-flow arrangement, as shown in Fig. 19a. The heat flux of 17.81×104 W/m2 (or 17.81 W/cm2) was achieved for water from the hot side of the device having the inlet temperature of 70 ºC and mass flow rate of 0.2321 g/s and for water from the cold side having the inlet temperature of 22.5 ºC and mass flow rate of 0.401 g/s.

Fig. 19b shows the comparison of the performance indexes between the counter- and parallel-flow arrangements. The performance index obtained from the counter-flow arrangement is higher than that obtained from the parallel-flow one: the value obtained from the counter-flow is 1.13 to 1.17 times of that obtained from the parallel-flow. The performance index of 21.69 W/kPa was achieved for water from the hot side of the device having the inlet temperature of 70 ºC and the mass flow rate of 0.2321 g/s and for water from the cold side having the inlet temperature of 22.5 ºC and the mass flow rate of 0.401 g/s.
