Benchtop windtunnels: Calibration, thermal analysis

Benchtop wind tunnels have practical value in engineering, product design, and academia, as well as in research and development. Benchtop devices follow the general principles of wind tunnel design, such as controlled air flow, low turbulence, flow accuracy, and repeatability, explains Omega Engineering Inc.

07/01/2009


Benchtop thermal evaluation wind tunnel. Image courtesy of Omega Engineering.

Benchtop wind tunnels have practical value in engineering, product design, and academia, as well as in research and development. Benchtop devices follow the general principles of wind tunnel design, such as controlled air flow, low turbulence, flow accuracy, and repeatability, explains Omega Engineering Inc.

Benchtop wind tunnels often are open-loop systems. Air is drawn from and is expelled into the room. A test section or air chamber can be the smallest part of the device. It usually has either a round or square cross-section where measurements are made. Fans, blowers, honeycombs, vanes, and filters produce and control air flow, reducing air turbulence and producing a laminar flow. An area of varying duct cross sections shape the flow as it moves into the test chamber. Applications include anemometer air flow calibration. Anemometers measure air velocity, especially for HVAC system installation and maintenance, air balancing, flow measurements, troubleshooting, and to monitor air velocity in spray booths, fume hoods, clean rooms and laminar flow workstations. They’re also used in industrial processes, weather and environmental studies, and other research.

A wind tunnel designed for calibrating anemometers needs no special knowledge to operate. The test chamber is usually configured to accept specific models, either vane type or hot-wire anemometers, and the air flow is pre-calibrated against a National Institute of Standards and Technology (NIST) standard. Mount the unit under test, select the air flow, and read the output on the test unit. An instrument panel can control the unit, and a multiposition switch can select the air flow rate.

A laboratory grade windtunnel can be used for calibration or air flow studies (aerodynamic studies of models for product design and development, R&D projects, and university experiments). The test chamber can accommodate custom mounting fixtures and instrumentation and includes capability to measure temperature, humidity, and barometric pressure. These wind tunnels usually have a wider range of flow rates than simpler calibration wind tunnels: 25-9,000 fpm (feet per minute) versus 500-3,000 fpm. Also, flow rates are continuously variable and not preset, unlike a calibration wind tunnel. To achieve the lowest flow rates, specially designed restrictive plates reduce air flow and maintain high uniformity and low turbulence.

Electrical components’ heat flow

A third design of benchtop windtunnels can help provide thermal evaluation of electrical and electronic components: active devices like circuit boards and powered components, and passive devices like heat sinks and heat exchangers. In these units, the air chamber is the largest part. Fans on the left (see photo) can be controlled individually and draw air through the right, where a honeycomb filter suppresses turbulence and sets up a uniform flow.

The unit under test is suspended in the air chamber on a universal mounting fixture that can accept a wide range of test objects. Openings in the test chamber allow for instrumentation, such as temperature sensors and anemometers. Air flow is continuously variable so different test conditions can be established, and temperature profile measurements can be made. It operates via a small control box or a PC interface.

For more information, contact Omega Engineering, www.omega.com .

Edited by Mark T. Hoske, Control Engineering editor in chief, MHoske@cfemedia.com .





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