A comparative numerical study is conducted to evaluate the effect of inlet velocity on the gas-liquid-solid phase change, the separation of phases and the coke formation. The numerical procedure is constructed within the Eulerian framework in which the liquid phase is treated as a continuous phase while gas and solid are both considered as dispersed phases. The simplified reaction net of crude oil is used in order to predict the thermal cracking of the crude oil. The temperature distribution, flow field, liquid-gas phase separation, and coke formation are predicted and discussed for different inlet velocities. The information predicted by the CFD model can be utilized in the optimal design of industrial fired furnaces.