“KEEP COOL” – using CFD simulation to improve thermal management of devices

Dario Denzler, Mechanical Engineer
Reading time: 5 min

Insight in Brief

In order to obtain a robust, safe, durable and high-performance device, thermal management is a key aspect in the development of any electronic product. This article deals with the following topics:

  • Importance of thermal management in product design
  • Application of computational fluid dynamics (CFD) in concept phases
  • Verification of numerical simulations with experimental measurements
  • Potential and limitations of using numerical simulations in the early product development stage

Introduction

”Keep cool“ is not only important in the hot summer months or during crucial project phases. Did you know that the life expectancy of electronic components (i.e. semiconductors) is highly dependent on operating temperature?

With an increase of ten degrees, life expectancy is halved (logarithmic relationship) and the overall performance decreases drastically. Moreover, overheating can lead to fire, resulting in catastrophic failure modes and needs to be considered in order to comply with regulatory standards. Therefore, thermal management is a key aspect in the development of any electronic product in order to obtain a robust, safe, durable and high-performance device.

At IMT, state-of-the-art simulation tools are used to address such requirements at a very early stage of the development phase to achieve an optimal product layout and design. However, not only are colorful marketing pictures produced…we also challenge the computational models and verify them with experimental measurements.

Problem

Within the project concept phase, IMT was commissioned to investigate possible options to increase the cooling efficiency of a product. The need for this optimization arose from direct market feedback, as long-term observations showed that the devices should be more robust in terms of thermal management.

Restrictions were imposed by the connector layout for proper use and the resulting internal wiring. Another important consideration was the maintenance requirements. Every component of the device should be easily accessible and exchangeable.

Due to the tight schedule, multiple iterations on prototypes were not an option.

Method

Computational fluid dynamics (CFD) is a branch of fluid mechanics that uses numerical analysis and data structures to analyze and solve problems that involve fluid flows.[1] Using this method, product designs can be investigated in terms of their aerodynamic behavior including thermal effects, without the need for physical prototypes. Thus, design changes and their impacts can be studied in a very early development phase.

Numerical Model

After some research on enclosure layout, a basic concept for the device was worked out taking into account the restrictions imposed (i.e. PCB layouts, cable length, etc.). A numerical model to perform CFD simulations was setup in the Solidworks® flow simulation software to obtain a high-performance prototype at the first attempt. This is intended to save time and costs and the only way to comply with the schedule laid down. For efficient iterations, the concept of the enclosure was abstracted in a block model approach in order to be able to analyze the device on the system level (Figure 1).

ID

Name

1

Outer case
2

Inner case

3

Device-specific unit (A)
4

Device-specific unit (B)

5

Mainboard

6

Power supply
7

Power board

8

Display board

9

Battery
10

Fan

Figure 1: Block model of concept device

The idea is to increase cooling efficiency for critical components by separating and guiding the airflow at the inlet of the main unit into a primary (blue) and secondary (orange) stream (Figure 2). This allows better tuning of the spatial distribution of the mass flow. Additionally, fresh inlet air can be directly guided to critical areas with high power dissipation to benefit from a higher temperature gradient.

The concept of the enclosure design splits the device into two sections by a “false bottom”, where the fan is located. The mass flow distribution inside the upper part can be adjusted by changing the position and size of the opening at the inlet and outlet of the main device respectively.

Components with higher heat loss such as the power board and power supply are encapsulated and located in the upper part. This not only allows more control over heat accumulation, but also supports electrical insulation to comply with standards such as EN 60601-1, EN 61010-1, IEC62368-1 etc. Additionally, the component orientation and outlet position are optimized to take full advantage of natural convection leading to a well-formed flow over the mainboard.

Figure 2: Intended airflow separation / guidance

Pre-Studies

The fan was implemented on the basis of a mathematical description to reduce the required calculation time. Therefore, the fan curve, provided by the manufacturer, was implemented by means of a lookup table. The implementation and mesh dependency was investigated in a preliminary study (Figure 3).

Figure 3: Mathematical model of fan

In order to obtain meaningful results from the simulation, a Mesh Sensitivity Study (MSS) was performed to determine the required spatial discretization of the numerical model. Particularly for thermal loads, such studies are essential in order to achieve the required resolution for the boundary layer where the heat transfer takes place (Figure 4).

Mesh 1

Mesh 2

Global mesh [-] 3 Global mesh [-] 5
Local mesh [-] 0 Local mesh [-] 3
Cell count [n] 11,244 Cell count [n] 72,294

Mesh 3

Mesh 4

Global mesh [-] 6 Global mesh [-] 7
Local mesh [-] 5 Local mesh [-] 6
Cell count [n] 288,664 Cell count [n] 671,192
Figure 4: Mesh sensitivity study (MSS) on convection

The correct definition of the thermal loads turned out to be the crux of the model. Generally, the heat loss can be approximated by the power dissipation.

For distinct hot spots such as integrated circuits (IC), the calculation is straightforward as long as the required data is available. However, for extended heat sources, such as the power board in this case, another solution had to be found. Using a reverse engineering approach, the block model of the PCB was calibrated. Using thermal imaging, the steady-state temperature during operation in a free convection setup was measured (Figure 5).

PB power block

PB charger block

Figure 5: Thermal imaging of the power board

The resulting temperature could be used in a corresponding CFD simulation as the boundary condition to determine heat flux, which could then be used in the block model of the device (Figure 6).

Figure 6: CFD model of the powerboard

With these pre-studies, the physical phenomena involved were investigated and their implementation in the simulation verified. Now the block model of the device could be set up.

 

Results

Simulation

First, the initial layout was analyzed in order to understand if the intended principal of airflow separation works and to identify problematic locations.

Front

Left

Back

Figure 7: Airflow for the initial design

According to the simulation, the general idea has proven successful (Figure 7). However, detailed investigation revealed a problematic vortex that needed to be resolved. Additionally, a support plate had to be included to increase structural stability. Within hours, the design change was simulated and the effect could be assessed. The integrated support plate leads to an improvement of the vortex and overall heat discharge (Figure 8).

Initial design

Optimized design

Figure 8: Critical vortices leading to heat accumulation

Compared to the “free convection” measurements, the ventilation of the enclosure was shown to work efficiently with both designs and lead to a significant reduction of the temperature of electrical components at the different probe locations (Figure 9).

Figure 9: Comparison of overall performance

Satisfied with the results predicted by simulation, the prototype could be released.

Experimental

At IMT, we not only put trust in our tools, we question them and our work, always striving for evidence rather than belief. Once the prototype was assembled, it was fitted with thermocouples at probe locations from the numerical model and put through a stress test (Figure 10).

MB SSD

MB CFB

UI Stepper Drive

PB Charger block DB Valens

DV Outlet

Figure 10: Prototype fitted with thermocouples

The simulation results were compared with the experimental measurements showing a discrepancy of around 15 – 25% (Figure 11). This sounds like a lot, but given the high level of abstraction, the results met the expectations.

Figure 11: Verification of simulation

During development, it was noticed that the concept device would allow the implementation of a larger fan, which could further optimize thermal management. Without further ado, the prototype was adapted to examine the effect of a larger fan. Since the fan control is designed to operate the fan in low and high RPM mode, this parameter was investigated as well (Figure 12).

Figure 12: Concept device with small and large fan

Most important was the comparison with the current device, which was used as benchmark. Comparing the internal sensors, which are equivalent in both devices, a tremendous temperature reduction could be achieved for all sensor positions (Figure 13).

Interestingly, the concept device with large fan achieves a performance equivalent to the configuration with small fan operating in high RPM mode. This is essential for customers regarding noise emission, as the large fan option was shown to be even more quiet, while providing a margin for extreme conditions.

Figure 13: Comparison between concept device and benchmark

[1] https://en.wikipedia.org/wiki/Computational_fluid_dynamics

Summary

Simulations allow product designs to be investigated in a very early development phase and reduce the need for prototyping. Additionally, physical effects can be visualized, which could not be detected or only with great effort using experimental methods.

This leads to very high efficiency in the development of new products or identification of weaknesses in existing designs, while reducing valuable resources such as time and money. Moreover, it facilitates communication between different departments or stakeholders to better understand requirements. This simplifies decision-making in crucial questions…true to the motto: “Keep cool”!

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