Abstract: The flow of dual radial ventilation system without fan for a hydrogenerator was simulated with parallel computing CFD code. The multiple implicit rotating frame (MRF) method and porous media model were used, averaged 3D NavierStokes (NS) equations and RNG kε equations were solved. The flow characteristic of different parts was analyzed. In order to get accurate heat transfer coefficients of the wall for thermal stress analysis with finite element method, the heights of the fist meshes near the wall were controlled strictly and the values of y+ were controlled between 30 and 100. Because the flow flux to cool the end of stator bars is small, the style of cover of pole ( static or rotating) and the leakage gaps between poles and stators will impact the cooling of stator bars. The rotating cover with leakage gaps is the best style for the cooling of stator bars. The flow from leakage gaps between poles and stators forms “air wall” with rotating velocity, which generates a recirculating zone between internal stator bars and induces the air from cooler into the zone, the heat transfer coefficient of internal stator bars is higher than the one of external stator bars. The heat transfer coefficient of internal stator bars is reduced after removing the leakage gaps. Due to rotational effects of the poles, the main ventilation loss is generated at the entrance of stator ventilation slots, the windward and leeward zones are formed around stator bars and recirculating zones are easy to be formed in leeward zones. The heat transfer coefficient in windward zones is higher than the one in leeward zones. The research is helpful for the optimization of the ventilation system design.