Numerical Simulation On Post-dryout Dispersed Flow Film Boiling In Helical Coiled Tubes

Helical Coil once-through Steam Generator(OTSG)has been widely used in integral small modular reactors(SMR).The heat transfer and flow characteristic of post-dryout flow in OTSG helically coiled tube play an essential part in the heat exchange performance between the primary and secondary side of OTSG.A numerical thermal-hydraulic model is developed on the basis of the mechanism of dispersed flow film boiling(DFFB)which is also called mist flow.In this model,Eulerian model is selected as the conservation equations for DFFB and RNG k-? model is used to account for the turbulence phenomenon.In order to confine the conservation equations,the mass,momentum and energy transfers between the vapor and liquid are highly investigated and programmed as UDF code which are the closure source terms.Also,the model is validated by the M.A.STYRIKOVICH&V.S.POLONSKY's experimental data.In this essay,the integrated numerical model for post-dryout mist flow in helical coils is set up in ANSYS Fluent,one of CFD software.Closure source terms are programmed in the form of user defined functions(UDF)and added into the conservation model.By exploiting such a model,the flow and heat transfer characteristic of the mist flow in the inner layer,middle layer and the outer layer coil of the OTSG are calculated and analyzed respectively.Before the calculation,mesh division and mesh independency analysis are carried out.Results show the flow and heat transfer distributions in the tube's cross section are not uniform.In addition,the second flow in the cross section are observed,whose mechanism is clarified.On the basis of preceding numerical model,the influences of both thermal parameters and geometry parameters on post-dryout DFFB in helical tube are analyzed.Result shows:(1)Increasing the helical diameter,increasing the inner diameter of the coiled tube,decreasing the mass velocity,increasing the inlet dryness and increasing the secondary wall temperature can reduce the inhomogeneity of the heat transfer coefficient distribution on the wall of the helical coil;(2)decreasing the helical diameter and inner diameter of coils,increasing inlet dryness and increasing secondary side wall temperature can shorten the distance from mist flow to dry saturated steam in the channel;(3)The pressure drop loss can be reduced by increasing the helical diameter of the helical tube and increasing the inner diameter of the coil tube,decreasing the mass flow rate,increasing the inlet dryness and decreasing the secondary side wall temperature.