Diffuser is one key component of the gas turbine combustor following the compressor. Its primary function is to slow down the air flow delivered by the compressor in order to promote efficient combustion and avoid large total pressure losses. The impacts of pre-diffuser wall angle and dump gap ratio on the performance of dump diffusers have been discussed in the previous work, but few work has been undertaken on designing diffusers with greater pre-diffuser wall angle. It was observed that apparent flow separation occurred on pre-diffuser wall when pre-diffuser wall angle amplified to certain degree. The pre-diffuser exit flow was distorted, indicating that the uniform exit conditions typically assumed in the diffuser design were violated. Skew distribution of the pre-diffuser outlet flow can result in strong transverse mixing for liquid, the total pressure loss of pre-diffuser increases significantly. In this paper, vortex generators were introduced to delay pre-diffuser separation. PIV system was used to obtain the velocity filed of dump diffuser in the non-reacting condition. Comparisons between experimental results and numerical simulations using different turbulence models show that Reynolds Stress Model (RSM) can better predict flow field of diffuser Calculations result indicated that vortex generators delayed pre-diffuser separation observably and the total pressure drop was reduced to five percent of previous value