Modeling effects of external convective heat transfer on rotating blades in anti-icing operations

Abstract

Calculating the unsteady convective heat transfer on helicopter blades is a first step in the prediction of ice accretion and the design of ice-protection systems. CFD simulations are considered a high-fidelity model of the complex aerodynamics of rotors as well as the heat transfer on blade surfaces. The literature indicates that coupling methods have emerged to model aerodynamic problems in a fast and efficient way, convenient for a conceptual design. Coupling methods are defined as the coupling of classical aerodynamic methods with viscous CFD databases, with the purpose of increasing the classical model fidelity yet maintaining its relatively computationally inexpensive solution. The main objective of this thesis is to develop numerical tools based on a new methodology of coupling to calculate the external convective heat transfer onto rotating blades while heated during anti-icing. Four specific objectives are set : 1- Build a viscous and heat transfer database for an airfoil; 2- Develop a low-fidelity rotor aerodynamic tool coupled with the CFD database; 3- Develop a medium fidelity rotor aerodynamic tool coupled with the CFD database; and 4 – Compare results from the low and medium fidelity tools to those of experimental work conducted on a heated fixed wing and 2- blade rotor. The novelty of this research originates from the introduction of an added layer of the coupling technique to predict rotor blade heat transfer using the Blade Element Momentum Theory (BEMT) and the Unsteady Vortex Lattice Method (UVLM). The new approach implements the viscous coupling of the two methods from one hand and introduces a link to a new airfoil CFD-determined heat transfer correlation. This way, the convective heat transfer on rotor blades is estimated while benefiting from the viscous extension of the BEMT & UVLM. An airfoil viscous and heat transfer database is built using CFD RANS simulations for a wide range of Re and α as well as two different thermal boundary conditions TBC. A curve fitting method is applied on the obtained average and maximum Frossling Number FrAvg and FrMax. Four correlations are then proposed for the FrAvg and FrMax, two for each TBC. The CFD heat transfer prediction is verified using existing correlations for a flat plate and validated by comparing the predicted airfoil heat transfer to experimental data from the literature. Thrust predictions by the implemented UVLM and BEMT agree within 92% and 80% compared to experimental data for rotors in hover, axial and forward flight. Tip vortex locations by the UVLM are predicted within 90%. The end results present as an estimate of the heat transfer for a typical lightweight helicopter tail rotor for four flight modes. Regarding the fixed wing experiments, comparisons CFD simulations indicate a discrepancy between 5% and 32%, suspected to be due to flow transition effects. For the rotor tests, most points of the experiments agreed with BEMT-RHT and UVLM-RHT predictions between 5% and 12%, although a maximum discrepancy of 80% was found. The rotor tests showed that for a rotor subjected to a freestream of air, turbulence will be present throughout the blade sections. This will cause increases of the heat transfer throughout the airfoil sections that are higher than what’s found on a no-to-low turbulence airfoil flow. The use of the fully turbulent CFD database with the BEMT-RHT and UVLM-RHT was therefore useful in calculating the heat transfer on the rotor blades. Its main limitation however was in the lack of a proper estimation of the transition effect on heat transfer.

Date	23 Feb 2021
Original language	American English
Awarding Institution	École de technologie supérieure
Supervisor	François Morency (Supervisor) & Christophe Volat (Co-supervisor)