(216l) Effect of Surfactant Drag-Reduction Agent On Fluid Flow and Heat Transfer for DI Water Flowing in a Mock Aviation Coolant Loop

Cooling systems in military aircraft are insufficient to handle the additional thermal loads of advanced avionics and weaponry. As such, drop-in replacements are being sought. Surfactant Drag Reducing Agents (SDRA) have been proposed as one possible solution in that for equal pressure drop, fluids can circulate faster without undergoing permanent shear degradation.

However, these surfactants form rod-like micelles that - it is thought – assemble in larger structures that force the flow to become laminar even at high Reynolds numbers.  Hence, significant drag reduction is obtained by the addition of SDRAs to DI water.  However, this approach has two pitfalls.  The first one is the effect of abrupt changes in geometry on pressure drop.  When the surfactant solution passes though regions with abrupt change in geometry (for example, sharp elbows, valves, or pumps), the induced laminar flow is disrupted, causing a temporary increase in pressure drop until the induced laminar flow is recovered.  The second one is the minimization of radial motion that reduces film heat transfer coefficient. 

To avoid the first problem, changes in geometry have been minimized so that the savings in pressure drop with SDRAs is large enough to increase volumetric flow of the liquid solution.  For the second problem, temporary disruption of the micellar structures before the surfactant solution enters the heat exchanger promotes turbulence, enhancing the heat transfer coefficient.  It is proposed that at the system level, the net reduction in pressure drop in the system will be sufficient to increase the flow through the heat exchanger enough to enhance the heat transfer coefficient.

In this study, the thermal performance of water is compared to that of a water/SDRA solution as they flow through both laminar and turbulent style heat exchangers.  The comparison is carried out at the same total pressure drop in the coolant loop.  The effect of surfactants on heat transfer coefficient for each of the coolant and on each of the two types of heat exchangers is discussed.