Wednesday July 23, 2014

Silent Air Cooling is a New Approach to Thermal Management

by Jennifer Hand, Contributing Editor

A multi-disciplinary team at Tessera Technologies has leveraged simulation to develop a new concept for cooling the thin and light tablets used for entertainment, clarification, and communication.

Thin and light tablets have become an essential tool for entertainment, clarification, and communication. Serious data handling, however, calls for the power of a bigger device and when tablet users, accustomed to silent running, turn on a notebook, or an even thinner Ultrabook, the first thing they notice is…noise.

For the best performance from electronic devices, it’s critical to manage heat. For portable computers, up to now, thermal management has been handled with small mechanical fans. The trend towards thin Ultrabooks, though, means that a typical fan unit is now squeezed into a height of less than 10 mm. With a couple of millimeters allowance top and bottom for air gaps and casing, the actual fan blade measures only a few millimeters and that is pushing the limit of effectiveness.

Although some of the latest fans can operate in cavities of less than 5 mm, performance is diminished because smaller blades move less air with each revolution and thus must turn faster, creating even more noise. The truth is that engineers have nearly exhausted the potential for improving fan technology, just as the consumer tolerance for noise has fallen even further.

As a University of Washington student in 2001, Nels Jewell-Larsen recognized the need for research in this area and began a development program focused on finding a silent replacement for rotary fans. For the past five years he has been Senior Program Manager heading up a specialist development effort within Tessera Technologies Inc. Recently, Tessera Technologies has launched a new product based on Jewell-Larsen’s research called Tessera Silent Air Cooling.

Silent Air Cooling is not a fan, since it does not use a rotating blade to move air. It uses an electric field and charged air to create airflow, a totally different concept. The technology relies on an electric field that charges and pushes nitrogen molecules in ambient air; these collide with other molecules in the air, transferring momentum and producing a continuous stream of laminar airflow.

Multiphysics plot of a two stage Silent Air Cooling air mover

Here is a Multiphysics plot of a two stage Silent Air Cooling air mover. Space charge density as a surface map (red color indicates higher space charge density) with space charge flux lines shown by white lines. Air velocity displayed as arrows, with arrow length linearly scaled to velocity.

Operation involves applying a voltage between two electrodes, which generates a very high electric field near one of them that creates positively charged nitrogen ions. The ions created are pushed towards the second electrode, generating a constant pressure source (see photo below). The electric field does not change with time, so there are no pressure waves, and therefore, virtually no sound. In addition to being noise-free, Silent Air Cooling (SAC) fits in a thin cavity of less than 4 mm, since it doesn’t need rotating fan blades and doesn’t need an air plenum above or below it.

silent air cooling

Silent Air Cooling uses an electric field and charged air to create airflow. An electric field charges and pushes nitrogen molecules in ambient air, which collide with other molecules in the air, transferring momentum and producing a continuous stream of laminar airflow.

“Simulation has been at the core of our product development because the team needed to take in account electrostatics, charge generation and transport, fluid dynamics and heat transfer. We are in a niche field and there was no dedicated simulation software for this when we started, so we looked at several offers,” commented Jewell-Larsen. “We even considered a custom-made software at one point. We then began using COMSOL Multiphysics because of its in-built flexibility.”

Gustavo Joseph, Head Thermal Engineer, noted, “Many software packages can easily simulate the movement of fluid or electrostatic forces independently. What is difficult to simulate is the generation of ions, their transport in an electric field and the force generated on air molecules resulting in the needed cooling flow. COMSOL Multiphysics allowed us to build all our own equations and couple them into the already available fluid dynamics and electrostatics capabilities.”

silent air cooling pencil

Silent Air Cooling’s size compared to a pencil.

Joseph and Jewell-Larsen used the program to design the core engine of the new technology, termed ‘the blower.’ “As the main objective was to maximize pressure and airflow for cooling, we simulated different geometry and materials to optimize these parameters. After we designed the key aspects of the blower in Multiphysics we ported that over to a CAD software to design the rest of the system.”

The end result is a reliable, compact unit that operates at less than 15 dBA, which is below the average threshold for hearing. Additional features include a self-cleaning system and, as there are no bearings, the system is easy to maintain. The target market includes suppliers of portable computer devices.

Concluded Jewell-Larsen, “We have demonstrated that the technology works in Ultrabooks and our objective is to have this technology in as many devices as possible, with designers free to build form factors that are thinner and thinner. This technology is going to have a positive impact for users.”

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