Numerical prediction of dryout in a 19 rod bundle under the effect of eccentricity and blockage
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<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.
Numerical simulation of flow through an eccentric annulus with heat transfer
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.
Thermal hydraulics of rod bundles: The effect of eccentricity
The effect of eccentricity on the fluid flow and heat transfer through a 19-rod bundle is numerically carried out. When the whole bundle shifts downwards with respect to the outer (pressure) tube, flow redistribution happens. This in turn is responsible for changes in mass flux, pressure and differential flow development in various subchannels. The heat flux imposed on the surface of the fuel rods and the mass flux through the subchannels determines the coolant outlet temperatures. The simulations are performed for a coolant flow Reynolds number of 4 × 105. For an eccentricity value of 0.7, the mass flux in the bottom most subchannel (l) was found to decrease by 10%, while the surface temperature of the fuel rod in the vicinity of this subchannel increased by 250% at the outlet section. Parameters of engineering interest including skin friction coefficient, Nusselt number, etc., have been systematically explored to study the effect of eccentricity on the rod bundle. © 2013 Elsevier B.V.
A mechanistic model for embryo size prediction at boiling incipience: ‘Work of formation’ based approach
The initial size of the embryo, which is formed at the inception of boiling, plays a vital role in the accurate prediction of component scale wall boiling phenomenon. Embryo size predictions are typically calculated using the classical theory of nucleation. However, in recent times, the predictive capability of this theory was found to have limitations. Hence, there is need for a more fundamental and mechanistic model to overcome some of the drawbacks. In this paper, we propose a ‘work of formation’ based model for the embryo formation. This model is mechanistic and includes a Van der Waals based real gas treatment for the vapour. It also incorporates Lewins surface tension model that is a function of the boiling-nucleus size. The present model also accounts for the boiling occurrence in the presence of undissolved nanobubbles on the surface. The embryo formation model has been extensively tested for both low and high pressures, horizontal and vertical test section orientation, and for different surfaces and fluids. The energy required for the embryo formation was found to be higher, when the initial gas bubble is intact compared to when the gas bubble diffuses into the embryo. Some of the contradictory claims on the suitability of classical theory of nucleation to nanosurfaces have been tested. From the present embryo formation model, the physics of nucleation such as, the effect of pressure fluctuations and energy dissipation mechanisms involved in the formation is explained.
Heat transfer from eccentric annuli of small gap ratio
A Computational Fluid Dynamics (CFD) study has been reported on the eccentric annuli with a wide range of radius ratios (α=0.5, 0.65, 0.8 and 0.95) and dimensionless eccentricity values (0.0, 0.3, 0.5 and 0.7) representing small gap ratios. All the geometric cases are investigated either by imposing a constant heat flux or peripherally varying heat flux on the inner wall. The narrow zone in the annular channel has been observed to have encountered drastic variations in hydrodynamic (velocity and Darcy friction factor) and thermal (temperature and Nusselt number) characteristics. The velocity in the narrow gap increases by 17% for values of α ranging from 0.5 to 0.65, and decreases thereafter. For a typical eccentricity value of 0.7, the mass flux in the narrow gap zone decreases by 75% and Darcy friction factor by 23%. The maximum temperature at the inner cylinder surface is found to increase by 188% at this eccentricity for constant heat flux case and 140% for varying heat flux. Copyright © 2012 by ASME.
Sub-channel analysis of rod bundle thermal hydraulics: Effect of eccentricity and blockage
A comprehensive study of rod bundle thermal-hydraulics in a nuclear reactor, enables us with safe and reliable margins for its operation. Although computational fluid dynamics (CFD) is an ideal choice, the number of degrees of freedom and the detail increases exponentially with increase in bundle size. Performing a large number of simulations, over the entire range of operating conditions and other parametric combinations makes CFD prohibitively expensive. Hence, it is preferable to pursue simple and effective reduced order models. To this end, a wide range of sub-channel analysis codes have been devised in the literature. One such model is the popular deformation and intermixing analysis in nuclear assemblies (DIANA) algorithm. In the present study, this algorithm is numerically implemented and validated for different rod bundle configurations. The effect of eccentricity and blockage and their combined effect on the thermal hydraulics of a 19 rod bundle is systematically analyzed. The thermal-hydraulics simulations are performed in the eccentricity (e) range of 0.0 ≤ e ≤ 0.7. It was observed that, eccentricity leads to flow maldistribution in some sub-channels, which in turn effects the coolant temperature distribution at the outlet. Using the present approach, a number of what-if type scenarios such as, blockage, bundle deformation and their combinations are also numerically investigated. It was noticed that, the presence of blockage in a sub-channel results in a reduction in the local coolant mass flux and hence the local clad temperature shoots up. In the narrowest sub-channel, for a typical case of eccentricity e = 0.5 and blockage ratio (b) = 0.4, maximum decrease in mass flux was observed to be 85% and increase in the coolant and clad temperatures was found to be 150% and 304% respectively. To emphasize the utility of the present study, the simulations are extended for a 217 sub-assembly with multiple blockages.
CFD investigation and assessment of wall heat flux partitioning model for the prediction of high pressure subcooled flow boiling
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.