Passives

(Re)designing resistors with computational fluid dynamics

5th June 2020
Alex Lynn

Here, Damiano Natali, research and development engineer at power resistor manufacturer Cressall, explains how Computational Fluid Dynamics (CFD) improves product designs that have stayed much the same for decades.

The Mini Cooper’s iconic design dates back to the 1950s and has elegantly withstood the test of time. If you asked Mini Cooper engineers, they would tell you that there are several generations of Minis, each different from the rest.

However, if you asked the average person on the street, they would probably say there’s only one Mini. This goes to show that often product redesigns can go unnoticed, despite significant improvement.

CFD combines the principles of fluid mechanics with numerical algorithms to analyse the behaviour of a fluid, such as air or water, and the forces it exerts.

Like structural and electromagnetic packages, CFD is one of the Computer Aided Engineering (CAE) technologies that has flourished in the last decade. CAE has progressively become a tool of overwhelming importance given its flexibility in allowing manufacturers to improve product performance quicker and with fewer resources.

By providing a reliable and accurate digital simulation of how a product will perform in a given situation, the manufacturer can identify potential drawbacks and improvements early in the design process.

Moreover, CAE tools such as CFD allow engineers to perform simulations on existing products without having to design, set up and maintain a highly complex testing rig.

Enclosures

Apart from the primary function of providing electrical insulation, post insulators play a fundamental part in the structure of an electrical enclosure. But the brittle nature of the porcelain ceramic from which they are made can cause design issues. Post insulators need to withstand the impact of the journey from factory to site, and the strong winds they will be facing throughout their lifetime, especially for offshore or near coast projects such as wind farms.

By using CFD, Cressall’s design engineers can assess the wind force that an enclosure can be subjected to, ensuring that the product is fit for purpose and for the environment it will operate in.

Natural air convection

Power resistors reach high temperatures, which makes their cooling mechanism a critical part of the design process. The principle cooling method is defined by the exact application of the resistor and location in which it is going be installed. The three methods used are natural or forced air convection or forced water cooling.

For all types, CFD allows engineers to understand the performance of the cooling system and make adjustments to refine a design. Furthermore, by experimenting with different set-ups, design engineers can identify the best solution and maximise the power rating that can be assigned to the resistor — all using digital tools.

Water-cooled resistors

The EV2 water cooled resistor has a very high-power density, which enables Cressall to offer a compact solution where water cooling is a preferred option. An accurate design aimed at streamlining and reducing all possible pressure drops is therefore crucial to enable customers to save energy in the long run.

A particular example of how CFD enhances resistor design springs to mind. Cressall was asked to reduce the pressure drop in one of the systems where an EV2 water-cooled resistor was installed. In particular, the customer was limited on the amount of pressure available from the pumping system in such a way that the minimum flow rate needed to cool down the EV2 modules could not be reached. In this case, Cressall was able to use CFD to accurately replicate the bottle-neck in the system and quickly identify the necessary improvements.

Cressall designed a new assembly to fit these customer requirements. The design went directly into production following a simple proving test, without time-consuming physical testing. The revised product was delivered in-line with the customers maintenance schedule.

The applications of CFD are almost endless, but the bottom line is that the software enables designers to pin-point specific performance areas of new products and improvement established designs with confidence. Just like the Mini, the proof of good design is its ability to withstand the test of time.

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