Experimental & numerical study of different orifice configurations for helical coil Once Through Steam Generator

Abstract

The main aim of the present study is to aid the selection of suitable orifice configuration for helical coil once-through steam generators being designed for SMRs. The selection of orifice is based on the pressure drop occurring in the orifice over the desired flow range, the minimum length required for the orifice, repeatability of the results, and ease of fabrication.In the present study, three different tube inlet orifice configurations are evaluated for a helical coiled Once Through Steam Generator. Orifice length is derived for all three configurations based on the orifice pressure drop required for avoiding flow instabilities. Orifice pressure drop required to avoid flow instabilities is estimated as factor k times the sum of two phase pressure drop and super-heating zone pressure drop of the tube side flow, where minimum value of k is taken as 2 (Han et al., 2019; Kang et al., 2007). Various power levels (10%–100%) are considered to obtain the governing length of the orifice. Pressure drop across the orifice over the flow range 10%–100% is estimated theoretically using various correlations available in the literature (Idelchik). Numerical simulations are carried out for all three configurations of the orifice. Flow through the orifice falls in the turbulent regime and hence, three different turbulent models viz. k-ε model, RNG k-ε model and Realizable k-ε model are compared for their capability to predict pressure drop across the orifice. Theoretical and Numerical results are verified with experimental pressure drop measurements over the flow range corresponding to a power range of 10%–100%. The effect of manufacturing tolerances on pressure drop is studied experimentally for all three configurations. Five nos. of each orifice configuration were manufactured and tested experimentally for pressure drop over 10%–100% flow range to see the effect of manufacturing tolerances.It is concluded from the study that irrespective of the orifice configuration, the minimum length required for the orifice is governed by the lowest power operation i.e, the lowest flow operation. Also, a comparison of various turbulence models viz. k-ε model, RNG k-ε model and Realizable k-ε model, reveals that, RNG model is best suited for orifice configurations that utilize sudden expansion and contraction for creating pressure drop. Whereas, all three turbulence models compared in the present study gives similar results for orifice configurations in which pressure drop occurs mainly due to wall friction and gradual but smooth change in flow direction. Orifice configuration-3 which utilizes a helical flow path is finally selected based on the selection criteria stated above.