Abstract: The cold flow gasification technology of dry pulverized coal flow bed is widely used. The gasifier in this process is prone to inaccurate liquid level measurement in the quench chamber, resulting in poor washing effect, ash carryover of syn-gas, and wear of downstream equipment and pipelines. Previous studies on the downcomer and riser structure of the quench chamber mostly focused on the adjustment of process operating parameters after on-site blockage, but few studies were conducted on the flow field simulation and the characteristics of bubble nucleation, aggregation and breakage of this structure. Therefore, in view of the problems such as large liquid level fluctuation and difficult liquid level control of the quench chamber of an industrial gasifier, the structure of the quench chamber was planned to be transformed and 3D CFD simulation was carried out. Based on the Euler+multi fluid VOF model and PBM group balance model, the washing situation of crude syn-gas in the water bath at the bottom of the quench chamber through the downcomer at 100% load was discussed under three different diameters combined structures of downcomer and riser, and The distributions of velocity, gas holdup, turbulent kinetic energy, bubble diameter were compared under three different structures to confirm the best bubble breaking and washing effect. The results show that the gas-liquid two-phase flow patterns under all three structures belong to ideal annular gap flow, and the gas-liquid flows stably in parallel in the annular gaps of the downcomer and riser. The structure with 100 mm diameter expansion of the downcomer has the largest impinging depth, greater turbulence energy in the annular gap, and more complete gas-liquid two-phase contact. And the average diameter of the bubbles measured on each key path of the flow field is smaller, and the diameter distribution is more uniform along the path, indicating that the structure has less liquid level disturbance, less water carried by the syn-gas, and better washing effect. The study can provide theoretical reference for the future transformation direction of similar structures.