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.

B. V.S.S.S. Prasad

Department of Mechanical Engineering

B. S.V. Patnaik

Department of Applied Mechanics

DIFFERENTIAL FLOWS

EFFECT OF ECCENTRICITIES

ENGINEERING INTERESTS

FLOW REDISTRIBUTION

Fluid flow and heat transfers

SKIN FRICTION COEFFICIENT

Surface temperatures

Thermal hydraulics

Atmospheric temperature

Coolants

Heat flux

Nusselt number

Reynolds number

Two phase flow

IIT Madras is a public technical and research university located in Chennai, Tamil Nadu. Founded in 1959, it is recognised as an Institute of National Importance.

IIT Madras has been ranked as the top engineering institute in India for four years in a row by the National Institutional Ranking Framework of the MHRD

It currently offers undergraduate, postgraduate and research degrees across 16 disciplines in Engineering, Sciences, Humanities and Management. About 596 faculty belonging to science and engineering departments and centres of the Institute are engaged in teaching, research and industrial consultancy.

IIT Madras

Nuclear Engineering and Design

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. © 2016 Elsevier B.V. All rights reserved.

Sub-channel analysis of rod bundle thermal hydraulics: 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&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.

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

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

Convective heat transfer for an annulus, having its inner cylinder wrapped with a helical wire, has been numerically studied. Constant heat flux boundary condition is applied at the inner wall of the annulus whereas the outer wall is insulated. The computer code with k-omega SST turbulence model has been validated with the results of bare annulus. Several numerical results such as variations in velocity, pressure, vorticity, turbulent kinetic energy, temperature, friction factor and Nusselt number are presented for various annulus diameter ratios (D / d = 1.5, 1.8 and 2.0), pitch ratios (P / d = 9.09, 18.18 and 30.30), wire diameter ratios (Dw / d = 0.15, 0.25 and 0.30) and Reynolds number (Re = 20000 to 180000). The zones of possible hot-spots in the cusp region close to the wrap-wire are discerned. © 2008 Elsevier Masson SAS. All rights reserved.

International Journal of Thermal Sciences

Flow and heat transfer characteristics in an annulus wrapped with a helical wire

A comparative CFD investigation of helical wire-wrapped 7, 19 and 37 fuel pin bundles and its extendibility to 217 pin bundle

Simulations of turbulence, heat transfer and mixing across narrow gaps between rod-bundle subchannels

International Journal of Heat and Mass Transfer

A dominant geometrical parameter affecting the turbulent mixing rate in rod bundles

Fluent Aphasia

Non-fluent and fluent aphasic speakers. What are the differences?

Thermal hydraulics of rod bundles: The effect of eccentricity

Journal	Nuclear Engineering and Design
ISSN	00295493
Open Access	No