Spacer Grid Induced Heat Transfer Enhancement In A Rod Bundle Under Reflood Conditions

Open Access
Author:
Riley, Michael Patrick
Graduate Program:
Mechanical Engineering
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
December 09, 2014
Committee Members:
  • Fan Bill B Cheung, Dissertation Advisor
  • Fan Bill B Cheung, Committee Chair
  • Timothy Miller, Committee Chair
  • Dr Stephen Bajorek, Committee Member
  • Savas Yavuzkurt, Committee Member
  • Cengiz Camci, Committee Member
Keywords:
  • Reflood
  • Dispersed Flow Film Boiling
  • Spacer Grid
  • Heat Transfer Enhancement
  • Two-Phase Flow
  • Loss of Coolant Accident
Abstract:
A new correlation has been developed in this work that accurately models and predicts the effects of both the upstream and downstream spacer grids on single-phase heat transfer. When the steam flow encounters a spacer grid, it must restructure in order to get around the spacer grid straps. In addition, the spacer grids cause a sudden contraction and expansion of the flow. This causes the turbulence in the flow to increase at the spacer grid that subsequently decays downstream. Since the flow is a continuum, there cannot be any discontinuities. In other words, the spacer grid effect also occurs upstream of the spacer grid. The single-phase upstream-downstream spacer grid heat transfer enhancement model developed in the present study accurately predicts both the downstream and upstream effects of a spacer grid on the heat transfer enhancement. Similar spacer grid effects occur in two-phase flow, under the DFFB regime with the addition of a second-stage effect occurring downstream of a wet spacer grid. The second-stage enhancement occurs approximately ten diameters downstream of the trailing edge of the spacer grid, as observed experimentally. Due to the high relative velocity between the vapor and the liquid film on the spacer grid, liquid ligaments are being sheared off of the spacer grid and subsequently break up into liquid droplets. The flow must restructure itself in order to get around the droplets, similar to flow restructuring around spacer grid straps. In this work, a theoretical model is developed that describes the underlying physics behind the second-stage heat transfer enhancement observed downstream of a wet spacer grid. The large break loss of coolant accident (LOCA) is a hypothetical design basis scenario that must be addressed as part of nuclear power plant licensing. This particular accident scenario is analyzed in order to ensure that the emergency core cooling system (ECCS) is sufficient to prevent fuel rod cladding temperatures from exceeding the regulatory limit of 1478 K (2200 degrees F). The core operating power is generally dependent on the large break LOCA analysis. During a LOCA, the fuel rods become uncovered and the temperature could increase rapidly. Without sufficient cooling, the fuel rods and other materials in the reactor, collectively known as the corium, could melt and relocate to the bottom of the reactor vessel. If this were to happen, the corium could potentially breach the vessel, resulting in a catastrophic nuclear accident. In order to effectively terminate the progression of the accident in the event of a LOCA, the rod bundle is flooded with coolant to ensure that the temperature of the fuel rods and other materials does not rise above acceptable limits. This calls for an accurate prediction of the thermal hydraulics during reflood transients such that nuclear reactors can be safely designed and operated. At the beginning of reflood, the rod bundle is cooled by single-phase steam convection. As more coolant is injected into the rod bundle, cooling becomes dominated by film boiling. Depending on the rate of coolant injection, three film boiling regimes may exist: dispersed flow film boiling (DFFB), inverted annular film boiling (IAFB), and inverted slug film boiling (ISFB). DFFB typically occurs under low flooding rates and is characterized by the flow of a continuous vapor with dispersed liquid droplets. IAFB typically occurs under high flooding rates and is characterized by the flow of a continuous liquid core surrounded by the flow of a continuous vapor near the wall. ISFB typically occurs under high flooding rate conditions as well, but occurs downstream of IAFB when the continuous liquid core has broken up, as a result of hydrodynamic instability, into large liquid slugs. Fuel assemblies consist of an array of fuel rods and a thimble tube that contain control rods that can shut down the reactor when inserted. The rod bundles are structurally maintained by several spacer grids, along with the top and bottom nozzles of the assemblies. The spacer grids have been found to have significant effects on flow in a rod bundle during a LOCA. The present work focuses on single-phase steam cooling and the effects that spacer grids have on the thermal hydraulics of single-phase flow and heat transfer. It is important to understand and accurately model single-phase flow and heat transfer to provide a basis for understanding and modeling two-phase heat transfer. The models developed in this work can be implemented into safety analysis codes to more accurately predict the thermal hydraulics behavior of reflood transients. The models can also be used as a basis for the understanding and development of two-phase heat transfer models.