Towards laminar flow control via cooling: exploring system feasibility and potential from a preliminary aircraft design perspective

Friction drag reduction through laminar flow control is a promising approach for decreasing fuel consumption, thereby contributing to more environmentally sustainable aviation. In this paper, different system layouts for laminar flow control via cooling are compared. A simplified two-dimensional model of a flat plate sandwich structure element is developed mathematically to investigate the dependence of the transition location on different sandwich structure geometries and materials, specifically aluminum and CFRP composites. The simplifications include neglecting in-plane heat conduction in the outer skin and reducing the geometry to a $$10\,\text {m}$$ long flat plate. Given these simplifications, the absolute uncertainty is high; however, relative trends can still be captured, which is especially important for the application throughout the preliminary aircraft design phase. As the paper focuses on the application for civil passenger aircraft, the study is conducted at a Mach number of $$\textit{Ma}=0.7$$ at an altitude of $$11,000\,\text {m}$$ . Moreover, two synthetic coolants (R21 and R22) as well as two natural coolants ( $$\textrm{N}_2$$ and $$\textrm{He}$$ ) are qualitatively compared and evaluated regarding their applicability for the described use case. Since R22 and $$\textrm{He}$$ are considered to be most suitable, the effects of different coolant flow velocities are addressed, reaching from $$0.5\, \frac{\textrm{m}}{\textrm{s}}$$ to $$2.5\, \frac{\textrm{m}}{\textrm{s}}$$ for R22 and from $$10\, \frac{\textrm{m}}{\textrm{s}}$$ to $$30\, \frac{\textrm{m}}{\textrm{s}}$$ for $$\textrm{He}$$ , respectively. The geometry of the coolant channel within the sandwich structure is assumed to be rectangular with a channel height of $$h=0.1\, \text {m}$$ and a width of $$b=0.2\, \text {m}$$ for R22 and $$h=0.005\, \text {m}$$ by $$b=0.02\, \text {m}$$ for $$\textrm{He}$$ , respectively. While the investigation is based on several simplifying assumptions, it could be shown that the transition location and thus the overall friction drag is very sensitive to the used coolant, the coolant temperature and the coolant velocity. Depending on the chosen setting, the entire flat plate segment could be laminarized. Putting the results in the context of the preliminary aircraft design, an overall drag reduction of −11.8% was estimated.