Daniel Kintea

Numerical investigation of ice particle accretion on heated surfaces with application to aircraft engines

Under certain conditions a porous ice/water layer builds up on hot surfaces in aircraft engines or on heated probes and eventually leads to their malfunction. In this study a numerical algorithm for the computation of heat and mass fluxes within and over the system boundaries of such a porous ice/water layer is developed. The code predicts the amount of liquid in the porous layer, which potentially can help to model ice shedding. This solver accounts for phase transitions between the solid, liquid and gaseous state as well as for the heat fluxes in the substrate on which the ice accretes and in the porous ice/water layer. A verification of the code is carried out using test cases for which analytical or experimental data are available. These cases encompass all the physical mechanisms which eventually lead to ice accretion and shedding. Numerically predicted results show excellent agreement with available data in all the test cases. Existing accretion experiments are well reproduced by means of the code and yield insight into the governing physical processes. Finally, an analysis of parameters of high influence on the accretion reveals limits for icing.

Shape evolution of a melting nonspherical particle.

In this study melting of irregular ice crystals was observed in an acoustic levitator. The evolution of the particle shape is captured using a high-speed video system. Several typical phenomena have been discovered: change of the particle shape, appearance of a capillary flow of the melted liquid on the particle surface leading to liquid collection at the particle midsection (where the interface curvature is smallest), and appearance of sharp cusps at the particle tips. No such phenomena can be observed during melting of spherical particles. An approximate theoretical model is developed which accounts for the main physical phenomena associated with melting of an irregular particle. The agreement between the theoretical predictions for the melting time, for the evolution of the particle shape, and the corresponding experimental data is rather good.

On the influence of surface tension during the impact of particles on a liquid-gaseous interface

A numerical study of the water entry of non-rotating and rotating rigid spheres under varying impact angles and Weber numbers is presented. The numerical algorithm uses a finite-volume discretization and the interface between the liquid and the gaseous phase is described by means of a volume-of-fluid method. An appropriate mesh translation allows the boundary condition at the surface of the moving and rotating particle to be accounted for. The simulation results are validated with experiments and found to be in very good agreement both qualitatively (evolution of cavity shape) and quantitatively (motion of particle with respect to time). An investigation of the influence of particle rotation on its water entry behavior is carried out as well as an analysis of the effect of wettability upon cavity formation. Notably, wettability of the sphere plays a role during the penetration of a free liquid surface, even at higher Weber numbers. During impact of small particles at low Weber numbers, the influence of capillary forces rises and the force emerging at the three phase contact line becomes predominant. This force is taken into account and its influence on the impact behavior is presented. It is shown that the interface penetration behavior, either water entry or escaping from water, mostly depends on the Weber number, the solid to liquid density ratio, and the particle’s wettability, while the impact angle has nearly no influence.

Inception of ice accretion by ice crystal impact

In this experimental and theoretical study the ice accretion phenomena on a heated cylinder in Braunschweig Icing Wind Tunnel are investigated. The ice crystal size, velocity, the liquid-to-total mass ratio are accurately controlled. The evolution of the cylinder temperature, the time required for the inception of the ice accretion, and the ice accretion rate are measured for various operating conditions. The surface temperature of the solid target is determined by balancing the heating power in the wall and the cooling effect of the stream of ice particles. We have discovered that the inception of the ice crystal accretion is determined by the instant when the surface temperature of the heated target reduces to the freezing temperature. This result will help to model the phenomena of ice crystal accretion.