# Experimental Study on Microchannel with Addition of Microinserts Aiming Heat Transfer Performance Improvement

^{1}

^{2}

^{*}

## Abstract

**:**

## 1. Introduction

## 2. Experiment

#### 2.1. Experimental Set-Up

#### 2.2. Scaling Parameters and Uncertainty

^{3}, $U$ is the fluid velocity in the unit of m/s,$\mu $ is the dynamic viscosity of the fluid in the unit of Pa-s, and ${D}_{h}$ is the hydraulic diameter at the microchannel inlet in the unit of mm. The hydraulic diameter is defined in terms of width $W$ and the height $H$, both in the unit of mm, as follows:

^{2}. $\rho $ is fluid density expressed in kg/m

^{3}, measured at the arithmetic mean of temperature at the inlet and outlet of the microchannel. Microchannel length is denoted by $L$ in the unit of mm.

^{2}, at which fluid and microchannel are in contact, and is defined as:

## 3. Results and Discussion

#### 3.1. Pressure Drop Characteristics

#### 3.2. Heat Transfer Characteristics

#### 3.3. Overall Performance Evaluation

_{s}) and heat transfer performance enhancement factor (Nu/Nu

_{s}) against the Reynolds number, where f

_{s}and Nu

_{s}are the friction factor and Nusselt number, respectively, for the plain microchannel. It is clear that as the Reynolds number increases, the values of f/f

_{s}are found to increase. However, rate of increment of f/f

_{s}is low. At an equal Reynolds number, values of f/f

_{s}for a microchannel with microinserts are higher than that without microinserts. The f/f

_{s}of a microchannel with microinserts are found to be in the range of 0.003–0.014 and 0.014–0.044 for channel sizes 1 mm and 2 mm, respectively, for the whole Reynolds number. From Figure 7, it is evident that at the same Reynolds number, f/f

_{s}for the microchannel with the microinsert is larger as compared to plain microchannel. This result suggests that flow resistance for the channel without microinserts gets more influenced by the decrement of dynamic viscosity due to the increase in fluid temperature than that for the channel without microinserts.

_{s}first increase and then continuously decrease with an increasing Reynolds number for the 1 mm channel. An increasing Reynolds number value of Nu/Nu

_{s}continuously decreases for the 2 mm channel. It is found that the variation trends of Nu/Nu

_{s}with the Reynolds number are different for a 1 mm size. For 1 mm, Nu/Nu

_{s}values increase quickly at first, then decrease sharply, and later decrease slowly with an increase in the Reynolds number. At last, at a higher Reynolds number, it is observed to be an increment in Nu/Nu

_{s}. For the 2 mm channel, the decrement is sharp and then decreases slowly, and at last the value nearly approaches a limiting value. This suggests that for the 2 mm channel, for a higher Reynolds number, enhancement of performance is not quite sensitive to increments in the Reynolds number. This means an increment in flow rate will not enhance heat transfer performance further.

_{s}values for 2 mm channels with or without microinserts are almost the same and there is little variation in the values of Nu/Nu

_{s}for a 1 mm channel with or without microinserts. This concludes that thermophysical properties that depend on temperature have influence on the heat transfer performance for lower size microchannels. Nu/Nu

_{s}are found to be in the range of 0.081–0.177 and 0.098–0.321 for respective 1 mm and 2 mm channel sizes with microinserts.

_{s}are seen to be steadily declining with rising Reynolds numbers, with a sharp decline at lower Reynolds numbers and a steady decline at higher Reynolds numbers. Smaller channel sizes respond similarly to big channel sizes throughout a moderate range of Reynolds numbers. Nu/Nu

_{s}, however, first quickly increase at low Reynolds numbers before continuing to slowly decrease. It is discovered that the increase in Nu/Nus is substantially steeper at low Reynolds numbers, as the channel size continues to shrink. Heat transport is influenced by the inclusion of microinserts being simple for all channel sizes. Additionally, as the channel size gets smaller, microinserts become more important because they speed up heat transfer. This tendency is seen in smaller size channels where temperature-dependent thermo-physical features have a greater impact on heat transfer performance. The insertion of inserts has a significant impact on heat transfer enhancement. It can be concluded that the features that are temperature-dependent are crucial for making such observations.

## 4. Conclusions

- The microinserts in the channel resulted in higher fluid outlet temperatures, causing lower base temperature when compared with the channel without microinserts.
- Microinserts performed in enhanced heat transfer, however, also caused a larger pressure drop. Pressure drops of the channel with microinserts were increased by a factor of 1.01–1.32 and 1.05–2.08, corresponding to 1 mm and 2 mm, respectively. The presence of microinserts resulted in increased flow resistance. It was obvious that temperature-dependent thermo-physical properties influenced the flow resistance.
- The heat transfer coefficients, effectiveness, NTU, and Nu of channels with microinserts were found to be increased, as compared to that of the channel without microinserts. The values of Nu were found to be larger by a factor of 1.01–1.08 in the case of the 1 mm and 1–1.07 for 2 mm channel sizes. It is indicated that the thermal performance of channels with microinserts improved. Microinserts effectively enhanced the heat transfer performance for both channel sizes.
- The performance evaluation criteria were employed to assess the overall performance of different channels. The results obtained by this method concluded that the overall performance of the channel with microinserts is better than that for the channel without microinserts for both channel sizes. It was found that microinserts result in the best overall performance at a lower Reynolds number. At a higher Reynolds number, microinserts improve the overall performance only marginally.

## Author Contributions

## Funding

## Institutional Review Board Statement

## Informed Consent Statement

## Data Availability Statement

## Conflicts of Interest

## Nomenclature

Symbols | Descriptions | Unit |

A_{s} | Contact surface area of the fluid and microchannel | mm^{2} |

c_{p} | Specific heat of water | J/kg-K |

D_{h} | Hydraulic diameter | mm |

f | Friction factor | |

H | Height of the microchannel | mm |

h | Heat transfer coefficient | W/m^{2}-K |

k_{f} | Thermal conductivity of fluid | J/s-m-K |

K_{s} | Solid thermal conductivity | J/s-m-K |

L | Length of the microchannel | mm |

m | Mass | kg |

Nu | Nusselt number | |

p | Pressure | Pa |

Re | Reynolds Number | |

T | Temperature | K |

TPF | Thermal performance factor | |

U | Fluid velocity | m/s |

W | Width of the microchannel | mm |

Δp | Pressure difference | |

ΔT | Temperature difference | |

Greek symbols | ||

ρ | Fluid density | kg/m^{3} |

µ | Dynamic viscosity | Pa-s |

Subscript | ||

f | Fluid | |

s | Solid |

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**Figure 3.**(

**a**) Geometric parameters, (

**b**) centre plate containing microchannel and flow path of hot and cold fluid and (

**c**) photographic view of a microchannel with microinserts (unit: mm).

**Figure 4.**Variation in pressure drop and friction factor against Reynolds number (

**a**) for a 1 mm channel and (

**b**) for a 2 mm channel.

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**MDPI and ACS Style**

Kumar, S.R.; Singh, S.
Experimental Study on Microchannel with Addition of Microinserts Aiming Heat Transfer Performance Improvement. *Water* **2022**, *14*, 3291.
https://doi.org/10.3390/w14203291

**AMA Style**

Kumar SR, Singh S.
Experimental Study on Microchannel with Addition of Microinserts Aiming Heat Transfer Performance Improvement. *Water*. 2022; 14(20):3291.
https://doi.org/10.3390/w14203291

**Chicago/Turabian Style**

Kumar, Shailesh Ranjan, and Satyendra Singh.
2022. "Experimental Study on Microchannel with Addition of Microinserts Aiming Heat Transfer Performance Improvement" *Water* 14, no. 20: 3291.
https://doi.org/10.3390/w14203291