The Development of Active Flow Control Devices from Shape Memory Alloys for the Convective Enhancement of Heated Surfaces

A study of the convective heat transfer enhancement of heated surfaces through the use of active delta
wing vortex generators is reported in this paper. The surface-mounted vortex generators (VGs) change
their shape to intrude further into the flow at high temperatures to enhance heat transfer, while maintaining
a low profile at low temperatures to minimise flow pressure losses. The VGs are made from shape
memory alloys and manufactured in a selective laser melting process. Experiments have been carried out
in a rectangular duct supplied with laminar-transition air flow. In the test section, a single, and a pair of
active delta wing VGs were placed near the leading edge of a heated plate and tested separately for their
heat transfer enhancement effects using infrared thermography. The pressure difference across the test
section was also measured to determine the pressure drop penalty associated with the obstruction
caused by the vortex generators in their active positions. Promising shape memory response was
obtained from the active VG samples when their surface temperatures were varied from 20 �C to
65 �C. The vortex generators responded by increasing their angles of attack from 10� to 38� and as the
designs were two-way trained, they regained their initial position and shape at a lower temperature.
At their activated positions, maximum heat transfer improvements of up to 90% and 80% were achieved
by the single and double wings respectively along the downstream direction. The flow pressure losses
across the test section, when the wings were activated, increased between 7% and 63% of the losses at
their de-activated positions, for the single and double VG respectively