The present study describes the Computational Fluid Dynamics (CFD) modeling of two phase flow boiling for the prediction of Critical Heat Flux (CHF) and its location in a tube, an annulus and a 4×4 rod bundle geometry. A number of different closure models employed for the calculation of the interface momentum exchange are assessed based on a comparison of wall temperature predictions against available experimental data. From detailed and systematic validations, a mean relative error of 15% and 7% for tube and annulus cases respectively was noticed. These simulations identify the CHF location in an annulus and in a rod bundle. The wall temperature variations are predicted for the concentric and eccentric annulus as well as a 4×4 rod bundle geometry. Eccentricity was found to promote early occurrence of CHF in the narrow gap and delay in the wide gap regions. For the 4×4 rod bundle considered, the corner rod facing the pressure tube was found to have an early shoot up in the wall temperature. © 2019

B. V.S.S.S. Prasad

Department of Mechanical Engineering

Harish Pothukuchi

B. S.V. Patnaik

Computational fluid dynamics

Computational geometry

Two phase flow

Computational fluid dynamics modeling

Critical heat flux(CHF)

ECCENTRIC ANNULI

MEAN RELATIVE ERROR

MOMENTUM EXCHANGE

Pressure tubes

TWO-PHASE FLOW BOILING

Wall temperatures

Heat flux

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IIT Madras

Applied Thermal Engineering

The subcooled flow boiling phenomenon usually occurs in a number of industrial applications, such as Pressurized Heavy Water Reactors (PHWR) and heat exchangers in nuclear power plants. The pressure tube deformation, presence of spacer pads, etc., may result in the eccentricity (e) of the rod bundle which causes coolant flow maldistribution and hence influence the heat transfer characteristics. In the present study, the entire rod bundle is modeled as a single inner rod and Eulerian Eulerian Multiphase Flow (EEMF) and Wall Heat Flux Partition (WHFP) model framework is used to simulate the effect of eccentricity (e). This model is validated against the available experimental data in the literature and found to be in reasonably good agreement. This framework is also compared and contrasted against the subchannel analysis predictions to highlight the importance of detailed CFD simulations. Furthermore, in the present study, the thermal hydraulics of narrow and wide gap regions are numerically investigated in an eccentric annulus. Due to eccentricity, early phase change, higher wall temperature and vapour volume fraction were noticed in the narrow gap region, compared to the concentric case (e=0.0), which is undesirable in the PHWR type reactors. © 2017 Elsevier Ltd

Numerical investigation of subcooled flow boiling in an annulus under the influence of eccentricity

An accurate estimation of dryout power and its location (zcr) is central to the safety of nuclear reactors. In the present study, a sub-channel analysis code is developed by extending the standard single phase DIANA algorithm to two phase flow conditions. The mass, momentum and energy conservation equations are solved, using a mixture model, which is validated against available experimental data. Numerical simulations are performed to determine the dryout location for a circular 19 rod bundle, in conjunction with a film thickness model. In critical sub-channels, a sudden jump in wall temperature was noticed at the dryout location. The effect of eccentricity(e) on the dryout location was investigated in the range of 0.0⩽e⩽0.7, under different operating conditions. It was observed that, eccentricity causes flow maldistribution in different sub-channels, and in turn affects the dryout location. For low inlet mass fluxes, sub-channels which never experienced dryout (for e=0.0) were found to experience dryout (for e&lt;0.0) The effect of blockage (b) was also systematically studied for 0.0⩽b⩽0.3. In flow regimes with higher vapor quality, the blockage leads to a disturbance in the continuous liquid film, resulting in an early occurrence of dryout. Two types of axial power distribution (APD) viz uniform and sinusoidal heat flux imposition were numerically simulated. The latter was found to delay the occurrence of dryout, compared to the former. © 2016 Elsevier B.V.

Nuclear Engineering and Design

Numerical prediction of dryout in a 19 rod bundle under the effect of eccentricity and blockage

The ‘wall heat flux partitioning’ (WHFP) model in conjunction with an Eulerian–Eulerian Multiphase Flow (EEMF) method is found to be an apt tool, to simulate the physics of subcooled flow boiling phenomena. However, the empiricism of the constituent model limits its applicability to predominantly low pressure operating conditions. Since pressurized heavy water reactors (PHWRs) are mostly operated at higher pressures, their reliable usage becomes highly questionable. To this end, we extensively assess and validate the WHFP model in the coupled EEMF–WHFP framework to simulate subcooled flow boiling conditions at high pressures. Based on the simulations, an improved mechanistic correlation combination for use in the WHFP model is recommended in place of the standard combination that is popularly used. This would enable reliable application to nuclear systems at higher pressures. To start with, a suitable high-pressure flow boiling experiment was identified and the coupled EEMF–WHFP method typically used in commercial solvers was assessed. Following this, several new non-standard correlation combinations were examined, in order to understand the dynamics and parametric sensitivities of the WHFP model in the coupled EEMF–WHFP framework. The entire focus is on finding out, the structure of the formulation with a sound physical basis and best possible prediction capability. To this end, a comprehensive literature review of the bubble parameters such as, diameter (D), number density (N) and frequency (f) based correlations, was performed. Popular models were identified, and systematically categorized based on the physical mechanisms behind their modeling. Based on an extensive performance evaluation, it was found that the coupled WHFP model converges and generally predicts physically relevant values across all of the modeling combinations and this robustness is found to be primarily due to the N–Tsup interdependence. It is also shown that the models, which are formulated in terms of the active cavity radius and the bubble growth modulus span very well across various pressures. Based on this study, we recommend a new non-standard correlation combination with a good physical basis that enables excellent predictions for the model, over a wide range of pressures. © 2016 Elsevier Ltd

International Journal of Heat and Mass Transfer

CFD investigation and assessment of wall heat flux partitioning model for the prediction of high pressure subcooled flow boiling

Purpose: The purpose of this paper is to investigate the effect of narrow gap on the fluid flow and heat transfer through an eccentric annular region is numerically. Flow through an eccentric annular geometry is a model problem of practical interest. Design/methodology/approach: The approach involves standard finite volume-based SIMPLE scheme. The numerical simulations cover the practically relevant Reynolds number range of 104-106. Findings: In the narrow gap region, temperature shoot up was observed due to flow maldistribution with an attendant reduction in the heat removal from the wall surfaces. CFD analysis is presented with the aid of, streamlines, isotherms, axial velocity contours, etc. The engineering parameters of interest such as, Nusselt number, wall shear stress, etc., is presented to study the effect of eccentricity and radius ratio. Research limitations/implications: The present investigation is a simplified model for the rod bundle heat transfer studies. However, the detailed study of sectorial mass flux distribution is a useful precursor to the thermal hydraulics of rod bundles. Practical implications: For nuclear reactor fuel rods, the effect of eccentricity is going to be detrimental and might lead to the condition of critical heat flux. A thorough sub-channel analysis is very useful. Social implications: Nuclear safety standards require answers to a wide a range of what-if type hypothetical scenarios to enable preparedness. This study is a highly simplified model and a first step in that direction. Originality/value: The narrow gap region has been systematically investigated for the first time. A detailed sectorial analysis reveals that, flow maldistribution and the attendant temperature shoot up in the narrow gap region is detrimental to the safe operation. © Emerald Group Publishing Limited.

Fulltext

International Journal of Numerical Methods for Heat and Fluid Flow

Numerical simulation of flow through an eccentric annulus with heat transfer

Modeling of subcooled boiling by extending the RPI wall boiling model to ultra-high pressure conditions

CFD modeling of critical heat flux in flow boiling: Validation and assessment of closure models

Journal	Data powered by TypesetApplied Thermal Engineering
Publisher	Data powered by TypesetElsevier Ltd
ISSN	13594311
Open Access	No