Numerical investigation of heat transfer in parallel channels with water at supercritical pressure

Abstract

Thermal phenomena such as heat transfer enhancement, heat transfer deterioration, and flow instability observed at supercritical pressures as a result of fluid property variations have the potential to affect the safety of design and operation of Supercritical Water-cooled Reactor SCWR, and also challenge the capabilities of both heat transfer correlations and Computational Fluid Dynamics CFD physical models. These phenomena observed at supercritical pressures need to be thoroughly investigated.An experimental study was carried out by Xi to investigate flow instability in parallel channels at supercritical pressures under different mass flow rates, pressures, and axial power shapes. Experimental data on flow instability at inlet of the heated channels were obtained but no heat transfer data along the axial length was obtained. This numerical study used 3D numerical tool STAR-CCM+ to investigate heat transfer at supercritical pressures along the axial lengths of the parallel channels with water ahead of experimental data. Homogeneous axial power shape HAPS was adopted and the heating powers adopted in this work were below the experimental threshold heating powers obtained for HAPS by Xi. The results show that the Fluid Centre-line Temperature FCLT increased linearly below and above the PCT region, but flattened at the PCT region for all the system parameters considered. The inlet temperature, heating power, pressure, gravity and mass flow rate have effects on WT (wall temperature) values in the NHT (normal heat transfer), EHT (enhanced heat transfer), DHT (deteriorated heat transfer) and recovery from DHT regions. While variation of all other system parameters in the EHT and PCT regions showed no significant difference in the WT and FCLT values respectively, the WT and FCLT values respectively increased with pressure in these regions. For most of the system parameters considered, the FCLT and WT values obtained in the two channels were nearly the same. The numerical study was not quantitatively compared with experimental data along the axial lengths of the parallel channels, but it was observed that the numerical tool STAR-CCM+ adopted was able to capture the trends for NHT, EHT, DHT and recovery from DHT regions. The heating powers used for the various simulations were below the experimentally observed threshold heating powers, but heat transfer deterioration HTD was observed, confirming the previous finding that HTD could occur before the occurrence of unstable behavior at supercritical pressures. For purposes of comparing the results of numerical simulations with experimental data, the heat transfer data on temperature oscillations obtained at the outlet of the heated channels and instability boundary results obtained at the inlet of the heated channels were compared. The numerical results obtained quite well agree with the experimental data. This work calls for provision of experimental data on heat transfer in parallel channels at supercritical pressures for validation of similar numerical studies.

Assessment of heat transfer correlations in the sub-channels of proposed rod bundle geometry for supercritical water reactor

“…From the assessment results of the turbulence model results obtained, it can be inferred that the most suitable turbulence models capturing the coolant temperature obtained by Waata in this analysis were found to be SST κ-ω (Menter's) and standard (Wilcox) κ - ω as displayed in Figures 2, 3, 4, and 5. Nevertheless, SST κ-ω (Menter's) turbulence model was selected due to its widely used and recommendations are given by several researchers (Ampomah-Amoako and Ambrosini, 2013; Podila and Rao, 2014; Shitsi et al., 2017) who have carried out analysis on heat transfer behaviour at supercritical conditions related to SCWRs. Hence, the SST κ-ω (Menter's) turbulence model with a low y + wall treatment was used to carry out the numerical simulations in this study.…”

“…The Physical Models adopted in this study includes Continuity equation, Momentum equations and Energy equation. The detailed treatments or descriptions of these Physical Models are given in the literatures, Podila and Rao (2014), CD-ADAPCO, 2015; Xi et al., 2014a, Xi et al., 2014b, and Shitsi et al. (2017).…”

“…The treatments or description equations for turbulent kinetic energy κ, turbulent dissipation rate ε , and Specific dissipation rate ω, in the various turbulence models are detailed in the literatures, CD-ADAPCO, 2015; Xi et al. (2014b), and Shitsi et al. (2017).…”

“…Most of the heat transfer correlations in the last category have been developed in recent years based on the “mechanisms of heat transfer at supercritical pressures”. With a view to increasing an in-depth knowledge on the mechanisms of heat transfer enhancement and deterioration phenomena, some researchers have stated that buoyancy and thermal acceleration play dominant and significant roles in the prediction of wall temperatures (Cheng et al., 2009; Chen and Fang, 2014; Shitsi et al., 2017). The HTC in the supercritical water must be addressed prior to, and during the formulation and development of new empirical correlations.…”