Computational Investigation of Convective Heat Transfer on Ice-Roughened Aerodynamic Surfaces

摘要

Ice accretion on aerodynamic surfaces of aircraft is a fundamental safety hazard. Accreted ice has an abundance of geometric scales, from the size of the gross features such as horns and scallops down to the minute details of individual frozen droplets. The smallest geometric scales take the form of surface roughness, which has a firstorder effect on the ice growth. This presents many challenges to analysis methods, especially for Computational Fluid Dynamics (CFD). Directly resolving roughness with CFD can increase the size of the computational mesh by orders of magnitude. Equivalent Sand-Grain Roughness (SGR) models are the only commonly used way to account for surface roughness in CFD; however, these methods are fundamentally unable to resolve the physics relevant to convective heat transfer.;The Discrete Element Roughness Method (DERM) is proposed and evaluated as an engineering solution to the problem of convective heat transfer on rough surfaces. DERM shows potential to improve heat transfer predictions beyond the capability of SGR models while only slightly increasing the computation time. As part of the present work, DERM is derived for and implemented in a general-purpose CFD solver and explored as a way of modeling the sub-resolved roughness scales. In order to establish DERM for ice roughness, a fundamental understanding of the relevant flow physics and heat transfer is developed.;High-resolution Computerized Tomography (CT) scans of ice-roughened airfoils are used to generate Computer Aided Drafting (CAD) models. The CAD models are analyzed to develop a deeper understanding of the geometric character of the ice roughness. Reynolds-Averaged Navier Stokes (RANS) simulations of the flow over an iced airfoil as well as a Large Eddy Simulation (LES) are used to develop understanding of how the roughness interacts with the boundary layer near the leading edge of an airfoil. The details of the turbulent flow field are extracted from the LES and compared with the RANS model predictions.;These models are also used to develop an understanding of the DERM modeling assumptions. One such assumption is neglect of the "dispersive stresses" and "dispersive energy fluxes"; these are quantified from the LES of an ice-roughened airfoil. The results show that the dispersive stresses are not negligible; in fact, they can be quite large compared with the Reynolds stresses. This result indicates that DERM may be improved for use with ice-roughened airfoils by development of computational models for the dispersive terms.;Finally, DERM-based CFD solutions are coupled with LEWICE to predict ice-growth on airfoils. The DERM-LEWICE predictions are compared with ice shapes generated in experiments and indicate that DERM has potential to improve ice-shape predictions in the glaze-icing regime.