Numerical analysis of flow and heat conditions during braking of a disc brake

V magistrskem delu smo preučevali tokovne in toplotne razmere pri zaviranju v zavornem disku in njegovi okolici. Pri zaviranju avtomobilski zavorni sistem pretvarja kinetično energijo celotnega avtomobila v toploto, katere večji del se kopiči v diskih, z njihove površine pa se širi v okolico s pomočjo konvekcije in sevanja. Razmere pri zaviranju na posebnem testu proizvajalca diskov smo simulirali s pomočjo računalniške dinamike tekočin. Uporabili smo programski paket Ansys CFX. Vse začetne predpostavke in poenostavitve smo skozi izdelavo magistrskega dela ovrednotili in skušali zmanjšati njihov vpliv na končne rezultate. Pri raziskavi nas je zanimalo tokovno polje, predvsem potrebna velikost računskega območja in gostota numerične mreže, ustrezen turbulentni model ter robni pogoji in vpliv časovne odvisnosti modela. Izkazalo se je, da je najprimernejše čim večje računsko območje, gostota mreže mora biti največja tik ob disku, ustrezen turbulentni model je SST (transport strižnih napetosti), robni pogoj odprtina, glede na opazovano geometrijo pa izbira časovno odvisnega modela ni smotrna. Glede temperaturnega polja smo se spraševali o ustreznosti uporabe časovno odvisnega oziroma neodvisnega tokovnega polja, o vključevanju sevanja v model, o temperaturni odvisnosti materialnih lastnosti in o primernem načinu predpisovanja izvora toplote ter vrtilne frekvence diska. Ugotovili smo, da je tudi pri temperaturnem polju za prve ocene dovolj natančen preračun s stacionarnim tokovnim poljem, v model pa je potrebno vključiti tako sevanje kot tudi temperaturno odvisne materialne lastnosti diska in zraka. Izvor toplote mora biti predpisan časovno odvisno, zmanjševanje vrtilne frekvence diska pa vpliva predvsem na temperaturno polje v zraku.In the Master’s thesis we have studied flow and heat conditions during braking in a disc brake and its surroundings. During braking the kinetic energy is converted into heat which is accumulated mostly in the discs. The accumulated heat is then transported from the discs’ surfaces to the environment with convection and radiation. The conditions during braking at a special test of a manufacturer were simulated using Computational Fluid Dynamics. We have used software package Ansys CFX. All of the initial assumptions and simplifications were evaluated during the research. We tried to minimize their influence over the final results. In the research we were interested in the flow field, especially in the required computation area, mesh density, the appropriate turbulence model and boundary conditions as well as in the influence of the time dependency of our model. It turned out that it is the most suitable if we use larger computation area, the mesh density has to be greatest near the disc, the appropriate turbulence model is SST (Shear Stress Transport) and the appropriate boundary condition is opening. The use of transient model is not reasonable considering the observed geometry. As for the temperature field we have studied the applicability of transient or stationary flow field, the radiation calculation in the model, the temperature-dependence of material properties and the most suitable way of applying the heat source and disc’s rotational frequency. We have found that for initial evaluation the calculation using the stationary flow field is sufficient. Radiation and time-dependent material properties need to be incorporated into the model. Heat source should be time-dependent. The reduction of rotational frequency influences especially the air temperature field