Electronics Protection - July/August 2013 - (Page 14)

Feature Harnessing Laboratory Wind Tunnels Bahman Tavassoli, PhD Advanced Thermal Solutions, Inc. Man-made wind tunnels have been used for more than a century. Today’s models have many different shapes and sizes, from just 30 mm long to large enough to contain a passenger jet. They are commonly employed in the aerospace, automotive and defense industries. The world’s largest wind tunnel is at NASA’s Ames Research Center in California. Here, engineers tested a 50-ft diameter parachute design that would help bring the Curiosity rover safely to the Martian surface in August 2012. Wind tunnels also have an important role in electronics thermal management studies. For these lab applications, the most common types are desk top and floor models used in laboratories. Large or small, all wind tunnels are basically alike. Their 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 Figure 1. Servo Controlled Wind Tunnel tunnel and through the and Control Console from Analysis Tech [1] test section at a controlled rate, usually driven by a fan. Wind tunnels used for heat studies in electronics are found in many research labs and universities around the world. While performance varies among models and their operators, most laboratory wind tunnels share similar design components. The most common characteristics of lab wind tunnels are a blade assembly, power supply, test chamber, control unit and a data acquisition system that interfaces with a PC. To achieve uniform and good quality flow in the test section, research quality wind tunnels include a settling chamber and contraction systems to smooth the airflow. A good quality wind tunnel should have a flow uniformity of 0.5 to 2.0 percent, turbulence intensity of 0.5 to 2.0 percent and temperature uniformity of 0.1°C 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 air flow through the test section. The test chamber is typically inside a clear-walled enclosure allowing clear observation of the test in progress. Many laboratory wind tunnels will fit on a bench top, while others, with larger test areas, are floor models. Omega’s advanced wind tunnel is designed to give a highly uniform flow rate over a 6 inch (152 mm) test section. A powerful 12 amp motor with variable speed from 0 to 10,000 Figure 2. Laboratory Grade Benchtop RPM is adjustable to give a Wind Tunnel from Omega [2]. particular flow rate using a motor control unit. The uniform flow rate is determined by monitoring a highly repeatable differential pressure sensor, which has been calibrated to each individual wind tunnel as a system. Each wind tunnel is supplied with two restrictive plates for achieving optimum low flow rates. The established differential pressure measurements versus flow rates are listed from 25 to 9,000 FPM. Calibration sheets are included, which makes calibrating different flow sensors simple. The differential pressure measurements used to establish known flow rates will be affected by barometric pressure and temperature conditions during testing. Depending on the application, humidity may also be a factor. To control these issues, Omega offers a wind tunnel package with an environmental monitoring system that measures barometric pressure, room temperature, humidity and differential pressure. By monitoring room conditions, standard differential pressures can be converted to actual differential pressure readings to ensure accurate flow rates. Closing the Loop 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 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. The second type of wind tunnel is the closed loop version, whose 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 100°C (212°F). Thermal Studies Because the thermal resistance of air-cooled electronic devices depends strongly on air flow velocity, accurate measurement and control of flow speed is a must for accurate test results. With a subject device set in the test enclosure, thermal resistance measurements can be performed over a range of air flow speeds. A console displays the air flow 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. 14 Figure 3. Closed Loop Wind Tunnels Like the CLWT-115 from ATS, Provide More Control of Air Temperatures within the Test Section [3]. July/August 2013 www.ElectronicsProtectionMagazine.com http://www.ElectronicsProtectionMagazine.com

Table of Contents for the Digital Edition of Electronics Protection - July/August 2013

Electronics Protection - July/August 2013
Newer Technology Releases Next-Generation Power2U AC/USB In Wall Charging Solution
Subzero Engineering Introduces PolarXpress DCiM SaaS Monitoring Solution
Specifiers of Enclosures for Components in Outdoor Applications: Be Aware of Material Selection Issues
Formable Phase Change Materials as Latent Heat Sinks for Portable Electronic Devices
How to Protect Electronic Circuits from Power Surges
Diamond Heat-Spreaders: Growth Methods and Applications
Standards-Based Design & Electronic Packaging Solutions
Reducing Room-Level Bypass Airflow Creates Opportunities to Improve Cooling Capacity and Operating Costs
Harnessing Laboratory Wind Tunnels
New DVR Enclosure with Prepackaged Thermoelectric Cooler
New Silver Conductive Aqueous Based Sodium Silicate System for EMI/RFI Shielding
Citizen and MechaTronix LED Cooling Cooperate On New Generation CitiLED COB’s
Eaton Expands 5P UPS Product Line to Provide Efficient Protection for Any IT Environment
Flex-Block System Puts Free-Standing Enclosures on New Footing
Gore Introduces Acoustic Vent for Protecting Electronic Devices
Industry News
Calendar of Events

Electronics Protection - July/August 2013