# Heat Transfer and Pressure Drop Characteristics in Straight Microchannel of Printed Circuit Heat Exchangers

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## Abstract

**:**

## 1. Introduction

## 2. Experimental Setup and Data

#### 2.1. Microchannel PCHE

#### 2.2. Experimental Setup

#### 2.3. Experimental Conditions and Results Analysis

_{c}is the free flow area, A

_{s}is the total heat transfer area and L

_{f}is the length of the flow stream in a channel. On the hot side, A

_{c}is 31.7 mm

^{2}and A

_{s}is 26,037 mm

^{2}. On the cold side, A

_{c}is 42.2 mm

^{2}and A

_{s}is 34,716 mm

^{2}. L

_{f}is 137 mm and D

_{h}is 0.6685 mm on both sides.

_{m}denotes the gap between the hot and cold side channels—which is 0.4 mm—the thermal conductivity of the heat transfer plate is 16.2 W/m·K, and the average heat transfer area respectively. The hot-side heat transfer coefficient, h

_{h}and the cold-side heat transfer coefficient, h

_{c}were obtained by using the modified Wilson plot method [31]. The measurement error was calculated using Equation (9):

_{m}is the average density across the flow path and G

_{p}denotes the mass flux at the inlet port. Note that the effect of hydrostatic pressure is neglected. The pressure drop was the measured sum of the microchannel, the inlet ports, and the outlet ports [32]. Experimental uncertainty was calculated by using ASME PEC 19.1 [33] and NIST Technical Note 1297 [34]. The total uncertainty consists of bias error and precision error as shown in Equation (11). When propagating errors, Equation (12) gives the uncertainty of the calculated parameters based upon the measured variables:

## 3. Experimental Results and Discussion

#### 3.1. Heat Transfer Characteristics

#### 3.2. Pressure Drop Characteristics

_{N}, which is the result value of the pressure drop according to Reynolds number. N indicates the number of lamination layers of the hot side. The total pressure drop was divided by the number of lamination layers. The friction factor correlation is represented by the function of the Reynolds number, and is as follows:

## 4. Conclusions

- The average heat transfer rate of the counterflow PCHE is about 6.8, and the UA of the heat transfer performance is excellent to the extent of approximately 10%–15%.
- As the Reynolds number of the hot and cold sides increases and the inlet temperature increases, the average heat transfer rate also increases. This increase was the general performance characteristic of the heat exchanger according to the increase of the flow rate.
- As the Reynolds number of the hot and cold sides increases, the pressure drop increases. If the inlet temperature of the hot side is constant, the pressure drop according to the change of Reynolds number of the cold side shows equal results.
- The heat transfer performance is not affected by the change in the inlet temperature of the hot side, but if the inlet temperature is high at the time of the pressure drop, which shows a slight pressure drop.
- The heat transfer coefficient correlations of the hot and cold sides using the modified Wilson plot method are proposed. The Reynolds number range of these correlations is 100–850.
- The friction factor f
_{N}was calculated using the pressure drop results. The application scope is the same as above. It is expected that the experimental results obtained in this study will be usable as the basis for future performance experimental data.

## Acknowledgments

## Author Contributions

## Conflicts of Interest

## Nomenclature

A_{c} | Minimum free flow area (mm) ^{2} |

A_{s} | Total effective heat transfer area (mm) ^{2} |

B | Bias error |

C_{p} | Specific heat (J/kg·K) |

D_{h} | Hydraulic diameter (mm) |

f | Friction factor |

G | Core mass velocity (kg/m ^{2}·s) |

G_{p} | Fluid mass velocity in the port (kg/m ^{2}·s) |

H | Thickness of metal sheet (mm) |

j | Colburn j-factor |

L | Length of metal sheet (mm) |

Nu | Nusselt number |

Pr | Prandtl number |

Re | Reynolds number |

UA | Heat transfer performance (W/K) |

h | Heat transfer coefficient (W/m ^{2}·K) |

k | Thermal conductivity (W/m·K) |

N | Stacked number of metal sheet |

ΔP | Pressure drop (kPa) |

Q | Heat transfer rate (W) |

ΔT_{LMTD} | Log mean temperature difference (K) |

W | Width of metal sheet (mm) |

Greek Symbols | |
---|---|

ρ | Fluid density (kg/m ^{3}) |

µ | Dynamic viscosity (N·s/m ^{2}) |

Π | Uncertainty |

Subscripts | |
---|---|

c | Cold |

i | Inlet |

o | Outlet |

h | Hot |

m | Mean |

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**Figure 1.**Flow cross-section of a printed circuit heat exchanger (PCHE) fabricated using diffusion bonding [1].

**Figure 2.**Photos of the metal-plates with straight middle sections. (

**A**) Hot-side plate; (

**B**) Cold-side plate.

**Figure 3.**The stack layer and the flow pattern in the microchannel printed circuit heat exchanger (PCHE). (

**A**) PCHE#1 (3 hot/4 cold); (

**B**) PCHE#2 (5 hot/6 cold); (

**C**) Flow configuration.

**Figure 4.**The final shape of the microchannel printed circuit heat exchanger (PCHE). (

**A**) The final shape of the PCHE; (

**B**) Detail design drawing sheet.

**Figure 5.**Cross-sectional view of a microchannel printed circuit heat exchanger (PCHE) fabricated through the diffusion-bonding method.

**Figure 6.**Schematic diagram and photograph of the experimental setup. (

**A**) Photograph of the experimental setup; (

**B**) Flow diagram of the experimental setup.

**Figure 8.**Average heat transfer rate and the heat performance (UA) with the same Reynolds number on hot and cold sides. (

**A**) Average heat transfer rate vs. Reynolds number; (

**B**) UA vs. Reynolds number.

**Figure 9.**Influence of flow configuration (countercurrent vs. parallel). (

**A**) Average heat transfer rate vs. Reynolds number; (

**B**) Heat performance (UA) vs. Reynolds number.

**Figure 10.**Influence of stacked lamination (PCHE#1 vs. PCHE#2). (

**A**) Average heat transfer rate vs. Reynolds number; (

**B**) Heat performance (UA) vs. Reynolds number.

**Figure 11.**Typical modified Wilson plot results for the calibration of the cold-side heat transfer coefficient.

**Figure 12.**Comparision of suggested correlations and experimental data for hot-side heat transfer coefficients. (

**A**) Heat transfer coefficient; (

**B**) Nusselt number.

**Figure 13.**Pressure drop vs. Reynolds number in all experiments. (

**A**) Difference in inlet temperature; (

**B**) Difference in number of lamination layers.

**Figure 14.**Comparison of the friction factor correlation and experimental data for the microchannel printed circuit heat exchanger (PCHE).

Metal-plate material | SUS304L | |
---|---|---|

Dimensions of PCHE (W × L × H), mm | 141 × 40 × 16 | |

Dimensions of plates (W × L × H), mm | 141 × 40 × 1 | |

Dimensions of end plates (W × L × H), mm | 141 | |

Number of plates | Hot side | 3, 5 |

Cold side | 4, 6 | |

Number of channels per plate | 22 | |

Channel width | 800 μm | |

Land (solid) width | 600 μm | |

Channel height | 600 μm |

Parameters | Uncertainty (%) |
---|---|

Temperature, T | 0.6 |

Pressure drop, ΔP | 0.92 |

Flow rate of hot side, ${\dot{m}}_{h}$ | 1.19 |

Flow rate of cold side, ${\dot{m}}_{c}$ | 0.94 |

Averaged heat transfer rate, Q_{m} | 1.19 |

Reynolds number of hot side | 3.13 |

Reynolds number of cold side | 3.29 |

Heat transfer coefficient of hot side | 7.36 |

Heat transfer coefficient of cold side | 7.31 |

Friction factor, f | 5.8 |

© 2015 by the authors; licensee MDPI, Basel, Switzerland This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution license (http://creativecommons.org/licenses/by/4.0/).

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**MDPI and ACS Style**

Seo, J.-W.; Kim, Y.-H.; Kim, D.; Choi, Y.-D.; Lee, K.-J.
Heat Transfer and Pressure Drop Characteristics in Straight Microchannel of Printed Circuit Heat Exchangers. *Entropy* **2015**, *17*, 3438-3457.
https://doi.org/10.3390/e17053438

**AMA Style**

Seo J-W, Kim Y-H, Kim D, Choi Y-D, Lee K-J.
Heat Transfer and Pressure Drop Characteristics in Straight Microchannel of Printed Circuit Heat Exchangers. *Entropy*. 2015; 17(5):3438-3457.
https://doi.org/10.3390/e17053438

**Chicago/Turabian Style**

Seo, Jang-Won, Yoon-Ho Kim, Dongseon Kim, Young-Don Choi, and Kyu-Jung Lee.
2015. "Heat Transfer and Pressure Drop Characteristics in Straight Microchannel of Printed Circuit Heat Exchangers" *Entropy* 17, no. 5: 3438-3457.
https://doi.org/10.3390/e17053438