Wind Tunnels Find Use in Thermal Studies
Knowing about this equipment can help harness the power of laboratory wind tunnels.
Modern wind tunnels have many different shapes and sizes — from just over 1" long to large enough to contain a passenger jet. Used for more than a century, man-made wind tunnels often are employed in the aerospace, automotive and defense industries. In fact, the world’s largest wind tunnel is located at NASA’s Ames Research Center in California. It is so large that in August 2012, engineers were able to test a 50' dia. parachute design that would help bring the Curiosity rover safely to the surface of Mars.
Yet beyond their other-worldly pursuits, large or small, all wind tunnels are basically alike. The main compartment contains a central test section, where objects with attached sensors are positioned, or where air-velocity sensors requiring calibration are lined up. Air streams through the tunnel and through the test section at a controlled rate, usually driven by a fan.
Wind tunnels play an important role in thermal management studies for electronics. For these laboratory applications, the most common types of wind tunnels are desktop and floor models. While performance varies among models and their operators, most laboratory wind tunnels share similar design components.
The most common characteristics of laboratory wind tunnels are:
• Blade assembly.
• Power supply.
• Test chamber.
• Control unit.
• A data acquisition system that interfaces with a PC.
To achieve uniform and high quality flow in the test section, research-quality wind tunnels include a settling chamber and contraction systems to smooth the airflow. A high quality wind tunnel can be expected to offer:
• Flow uniformity of 0.5 to 2.0 percent.
• Turbulence intensity of 0.5 to 2.0 percent.
• Temperature uniformity of 0.18 to 0.90°F (0.1 to 0.5°C) at the inlet of the test section.
In basic operation, air is drawn through an entry site into the test section by a variable-speed fan. A properly designed tunnel will ensure laminar airflow through the test section. Typically, the test chamber is inside a transparent-wall enclosure, allowing clear observation of the test in progress. Many laboratory wind tunnels for electronics thermal management studies will fit on a benchtop; others with larger test areas are floor models.
From a functional standpoint, there are two basic kinds of wind tunnels: open and closed loop. The open type draws its air from the ambient environment and exits it back to the ambient. This kind of wind tunnel does not provide practical temperature control. The air follows the ambient temperature when there is no heating element at the intake.
In the second type of wind tunnel — the closed-loop design — the internal air circulates in a loop. This separates it from outside ambient air. The temperature in a closed-loop wind tunnel can be controlled using a combination of heaters and heat exchangers. Air temperatures can be achieved from sub-ambient to more than 212°F (100°C).
Thermal Studies for Electronics
Because the thermal resistance of air-cooled electronic devices depends largely on airflow velocity, accurate measurement and control of flow speed are musts for accurate test results. With a subject device set in the test enclosure, thermal resistance measurements can be performed over a range of airflow speeds. A console displays the airflow speed in feet per minute. Air speed can be controlled manually or programmed into a PC-based thermal analyzer. The airflow speed can be indexed to the next value in a test regimen after equilibrium is reached in a current test.
In 2012, NASA engineers at the Glenn Research Center developed a downsized supersonic wind tunnel with a 5.91 by 5.91 by 27.95" (15 by 15 by 71 cm) test section.1 This tunnel typically is used for instrument development and calibration of supersonic flows, or for fundamental studies of supersonic flow phenomena. Air enters a plenum-contraction section and travels into a nozzle assembly with fixed-geometry nozzle sections that can rotate 90° for each of Mach number changes. The Mach number ranges from 1.3 to 3.0. The tunnel offers 160 ports that allow ±15 pressure differential measurements. The supersonic wind tunnel has been used for simulation studies of aerodynamic shocks and laser-based shock sensing for in-flight and ground electronics.2
Users choosing a wind tunnel for a laboratory must factor in cost and space constraints. Larger wind tunnels require more space to have all the conditioning elements in place. A desktop wind tunnel will sacrifice features such as higher-end flow quality, but most models are acceptable for practical engineering purposes.
1. NASA. https://Rt.grc.nasa.gov.
2. NASA/TM 2012-217439, Wind Tunnel Testing of a One-Dimensional Laser Beam Scanning and Laser Sheet Approach to Shock Sensing, March 2012.
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