International Journal of Nuclear Energy Science and Engineering Volume 3 Issue 4, December 2013 doi: 10.14355/ijnese.2013.0304.02
www.ijnese.org
Study on Influence of Rewetting on Conduction Heat Transfer for AHWR Fuel Bundle Re-flooding Phenomena Mithilesh Kumar1, D. Mukhopadhay1, A. K. Ghosh2, Ravi Kumar3 Reactor Safety Division1, Bhabha Atomic Research Centre, Mumbai, India Raja Ramanna Fellow2, Bhabha Atomic Research Centre, Mumbai, India Indian Institute of Technology, Roorkee3, India mithiles@barc.gov.in
1
Abstract Numerical study on re-flooding behavior of AHWR fuel bundle has been carried out to assess influence of circumferential and axial rewetting on the conduction heat transfer. As the AHWR fuel bundle quenching under accident condition is designed primarily with radial jets at several axial locations, however, bottom re-flooding still remain as an another option. A 3-D (r, θ, z) transient conduction fuel pin model has been developed to carry out the study with a Finite Difference Method (FDM) technique with Alternating Direction Implicit (ADI) scheme. Both the analyses are carried out with same fluid temperature and heat transfer coefficients as boundary conditions. It has been found from the analyses that for radial jet, the circumferential conduction is significant and overall the fuel temperature falls in the quench plane with the initiation of quenching event. Axial conduction is found to be very poor for bottom reflooding case and the fuel temperature only falls when the quench front reaches the location. Sensitivity studies with respect to the direction of solving by ADI are carried out to assess directional influence on the solution. The study shows that direction (r- θ-z) of solution is insensitive to the solution for axial and circumferential conduction solutions. Keywords Re-flooding; Rewetting; Heat Conduction; Numerical Method; Sensitivity Study
Introduction Rewetting of hot surface is a process in which a liquid wets a hot surface by displacing its own vapour that otherwise prevents the contact between the solid and liquid phases. This has generated immense interest in studying rewetting through both theoretical simulation by Yamanouchi, Coney M.W.E. and experimental studied was carried by Yamanouchi, Duffey R.B. Falling film rewetting for several vertical geometries such as plates (Coney M.W.E., Tien C.L.),
rods [Blair J.M.] and tubes [Satapathy A.K.] have been modeled by a number of researchers. In general, in all models, a moving rewetting front that divides the solid into two distinct regions is considered. Most of the models also consider a constant rewetting velocity that reduces the problem into a quasi-static one. Initial efforts were made to formulate one-dimensional conduction models [Yamanouchi] that are reasonably successful in correlating rewetting phenomena at low Peclet number. Tien and Yao presented the asymptotic solutions of a two-dimensional conduction model which clearly demonstrates the different physical pictures for the cases of high and low coolant flow rates. A variety of techniques have been used for solving two-dimensional conduction models for falling film rewetting. Some of the important studies are elaborated. Because of mathematical difficulty, most two-dimensional analyses are either approximate or numerical ones. Duffey and Porthouse first considered for solving the rewetting problem by separation of variables. They retained only the first term in the series solution. However, Coney M.W.E reported that using a small number of terms in a series yields inaccurate results due to slow convergence of the series. An approximate solution to the same model for a cylindrical rod was presented by Blair J.M. Tien and Yao first applied the Wiener–Hopf technique to a twodimensional rewetting problem of a rectangular slab, while an exact solution to the same problem was presented by Castiglia et al., employing the method of separation of variables. Numerical solutions of conduction controlled rewetting were provided by Satapathy et al., Thompson, and Raj and Date by using the finite difference technique. Heat Balance Integral Method (HBIM) is one of many semi-analytical methods used to solve conduction problems (Eckert and Goodman). This is analogous to classical integral
85