Boiling water reactor (BWR) is the main contributor of nuclear energy in Sweden. However, the understanding of boiling phenomenon in flow channels still needs to be advanced so that safer and more efficient BWRs could be designed. Entrainment is a crucial and fundamental process in flow boiling since it is closely related to the heat and mass transfer between water and vapor. A lot of correlations have been proposed based on experimental data which is usually one-dimensional and based on average parameters. However, 3D localized models are needed to predict entrainment more accurately. With the development of hardware and numerical methods, much more insights of entrainment might be obtained by running Computational Fluid Dynamics (CFD) simulations. This can help us develop the desired 3D localized model. As the first step, this project will focus on investigating the onset of entrainment using CFD method. The results will be used to improve the boiling prediction in BWR. In addition, it will also be beneficial to other entrainment related applications in chemical engineering and oil industry.