The Impact of the Reynolds Number of Airflow on Heat Sink Performance
Key Takeaways

Reynolds number of airflow plays an important role in bringing out the optimal performance of heat sinks.

An increase in the Reynolds number attributes to the reduction of junction temperature in electronic components.

The thermal conductance increases with the Reynolds number and improves the heat transfer rate.
Finned heatsinks and heat exchangers are common electronic thermal management systems
Highly populated circuit boards are often incorporated into complex systems to increase profitability. To improve the performance of such highdensity boards, electronics cooling is employed to transport heat. Electronic thermal management using heat sinks for cooling depend on the heat dissipation, airflow, cost, and size of the design.
Finned heatsinks and heat exchangers are common thermal management systems used for heat transfer from electronic components. The performance of heat sinks is influenced by the material, heat sink geometry, fin spacing, and the Reynolds number of airflow or any coolant fluid flow. In this article, we will discuss the role of the Reynolds number of airflow in heat sink performance.
Electronic Thermal Management Using Heat Sinks
Downsizing package sizes have increased the power dissipation in electronic components. The heat flux produced by increased power dissipation is challenging for electronic thermal management systems. To maintain a safe operating temperature, heat sinks are utilized.
Heat sinks are attached to components to remove heat. The heat transfer mechanisms for thermal flux removal in electronics using heat sinks can be either conduction, convection, radiation, or a combination of these. Heat transfer mechanisms can be natural or forced in heat sinks.
There are different geometries of heat sinks used to enhance electronic component thermal performance. The materials used in heat sink manufacturing are selected such that material properties, such as thermal conductance, speed up the thermal transport from the electronic components.
Airflow or fluid flow is a major factor that influences heat sink performance, especially in forced convective heat transfer. Reynolds number of airflow plays an important role in bringing out the optimal performance of a heat sink.
Reynolds Number and Fluid Flow Type
Generally, Reynolds number is a dimensionless measurement that describes the degree of laminar or turbulent fluid flow. Fluid can be gas or liquid, and the value of the Reynolds number determines whether the flow is laminar or turbulent. When the Reynolds number is between 2300 and 4000, the fluid flow is assumed to be transient. As the Reynolds number increases above 4000, the fluid flow becomes turbulent. Reynolds numbers with a value of less than 2300 indicate a laminar flow of the fluid.
Reynolds number is the ratio of inertial forces to viscous forces and this number categorizes fluid systems. It reveals the effect of fluid viscosity in controlling the flow pattern or velocity of the fluid. Mathematically, the Reynolds number can be given by the following equation:
Note that is the density of the fluid, is the velocity of the fluid, l is the characteristic length of the fluid flow, is the dynamic viscosity of the fluid, and v is the kinematic viscosity of the fluid.
The Relationship Between Reynolds Number of Airflow and Heat Sink Performance
Flow type is an important factor in increasing the cooling efficiency and performance of a heat sink. As an example, let's consider an impingement airflow heat sink with pin array fins. A jet of air impinges through a nozzle onto the heat sink for fast heat removal. In the abovementioned thermal management system, the Reynolds number can be defined as:
Note that u is the velocity of the air impinging the heat sink from the nozzle, D is the diameter of the nozzle, and v is the kinematic viscosity of the fluid. As the flow rate increases, the Reynolds number increases. With the knowledge of the Reynolds number of airflow or operating flow rates, it is possible to optimize heat sink pin spacing, which improves heat sink performance.
The effect of the Reynolds number on cooling efficiency is significant. As the Reynolds number increases, the turbulence generated in the air impingement increases. When the air jet mixes with the atmospheric air, the turbulence intensity increases further. The turbulence generated enhances the heat transfer coefficient. An increase in the Reynolds number attributes to the reduction of junction temperature in electronic components. It is observed that as the Reynolds number increases, the thermal resistance related to the heat sink decreases. In other words, the thermal conductance increases with the Reynolds number, which improves the heat transfer rate. Other applications of the Reynolds number includes microfluidics, aerodynamics, thermal engineering, wind energy engineering, aircraft design, hydraulics, and fluid flow measurements.
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