Numerical simulations of two phase (gas and liquid) flow are being pursued using Computational Fluid Dynamic (CFD) modeling. The simulations predict the breakup and coalescence of gas bubbles as they are sparged into the bottom of bioreactors and collide with each other and the spinning impellers. The results of our CFD model of gas holdup and oxygen transfer in a bioreactor are being compared to actual experimental data from our laboratory. The goal of this research is to use the CFD model to determine the optimum bioreactor configuration (i.e. sparger location etc…) for given operating conditions (RPM, type cell line..).