Elsevier

Nuclear Engineering and Design

Volume 284, 1 April 2015, Pages 143-152
Nuclear Engineering and Design

Characteristics of convective heat transport in a packed pebble-bed reactor

https://doi.org/10.1016/j.nucengdes.2014.11.041Get rights and content

Highlights

  • A fast-response heat transfer probe has been developed and used in this work.

  • Heat transport has been quantified in terms of local heat transfer coefficients.

  • The method of the electrically heated single sphere in packing has been applied.

  • The heat transfer coefficient increases from the center to the wall of packed bed.

  • This work advancing the knowledge of heat transport in the studied packed bed.

Abstract

Obtaining more precise results and a better understanding of the heat transport mechanism in the dynamic core of packed pebble-bed reactors is needed because this mechanism poses extreme challenges to the reliable design and efficient operation of these reactors. This mechanism can be quantified in terms of a solid-to-gas convective heat transfer coefficient. Therefore, in this work, the local convective heat transfer coefficients and their radial profiles were measured experimentally in a separate effect pilot-plant scale and cold-flow experimental setup of 0.3 m in diameter, using a sophisticated noninvasive heat transfer probe of spherical type. The effect of gas velocity on the heat transfer coefficient was investigated over a wide range of Reynolds numbers of practical importance. The experimental investigations of this work include various radial locations along the height of the bed. It was found that an increase in coolant gas flow velocity causes an increase in the heat transfer coefficient and that effect of the gas flow rate varies from laminar to turbulent flow regimes at all radial positions of the studied packed pebble-bed reactor. The results show that the local heat transfer coefficient increases from the bed center to the wall due to the change in the bed structure, and hence, in the flow pattern of the coolant gas. The findings clearly indicate that one value of an overall heat transfer coefficient cannot represent the local heat transfer coefficients within the bed; therefore, correlations are needed to predict the radial and axial profiles of the heat transfer coefficient.

Introduction

A pebble bed-type of very-high temperature gas-cooled reactor (VHTR) is one of the most probable solutions (Goodjohn, 1991) and the most promising concept (Koster et al., 2003) of the six classes of 4th generation (Gen IV) advanced technologies. In general, the pebble-bed reactor (PBR) is a pyrolytic graphite-moderated and helium gas-cooled nuclear reactor that achieves a requisite high outlet temperature while retaining the passive safety and proliferation resistance requirements of Gen IV designs (Gougar et al., 2003).

In the core of a pebble-bed nuclear reactor, the local fuel temperatures depend not only on the local power generation but on the point heat removal rate (Abdulmohsin and Al-Dahhan, 2012). Hence, for the safe design and efficient operation of packed pebble-bed reactors it is crucial to have detailed information and a proper understanding of the transport of the heat generated during nuclear fission from slowly moving hot fuel pebbles to the flowing coolant gas. All three modes of heat transport (i.e., conduction, convection, and radiation) are important to modeling and predicting the pebble-bed core temperature distribution. During nominal operation of the reactor (relatively high Reynolds numbers), the heat transfer mechanism is governed by forced convection (Fenech, 1981). This heat convection can be quantified and characterized in terms of a convective heat transfer coefficient or non-dimensional Nusselt number. At low Reynolds numbers (in the case of an accident), the effects of free convection, thermal radiation, heat conduction, and heat dispersion are on the same order of magnitude as the contribution of the forced convection (Fenech, 1981). However, little information related to pebble-bed heat transfer is available in the open literature, so this mechanism has not yet been fully understood (Stainsby et al., 2010b). Furthermore, the quantification of the heat transfer coefficient between the heated pebbles and the flowing coolant gas using models or correlations to predict the temperature distributions for design, scale-up, and operation is still lacking.

Section snippets

Literature review

In the open literature, the heat transfer data were obtained by direct measurements (in which the component particles are heated separately) and indirect means (by involving the transient heating of fluid or mass transfer experiments). On the other hand, the measurement techniques applied for packed pebble-bed heat transfer are as follows: (1) the electrically heated single sphere buried in the unheated packing (Achenbach, 1982, Achenbach, 1995, Schroder et al., 2006, Rimkevicius et al., 2006,

Separate effects experimental setup

To simplify the experimental work, the pebble bed was made of fixed bed particles for the purpose of this study. The cold-flow unit of the pebble bed, that was developed as separate effects experimental setup to conduct proper heat transfer coefficient measurements, consists of a Plexiglas column of 0.3 m diameter and variable height of 0.3–0.92 m. The schematic diagram of the separate effect cold-flow experimental setup along with the heat transfer technique and its components is shown in Fig. 1

Effects of gas flow on convective heat transfer coefficients

The effect of coolant gas velocities on the convective heat transfer coefficients was investigated at different axial positions along the bed height with an aspect ratio (D/dp) of 6, as shown in Fig. 4. For all three axial levels (Z/D = 0.5, 1.5, and 2.5), the convective heat transfer coefficients increased gradually with an increase in the gas velocity. It was found that the effect of superficial gas velocity on heat transfer coefficients varies from laminar to turbulent flow regimes for all

Comparison of the heat transfer results with available empirical correlations

As mentioned earlier, numerous studies have been conducted around heat transfer in a packed bed of spheres of small particles (catalysts). However, for the convective heat transfer coefficients, a number of correlations have been reported in the literature in packed pebble beds, which are given by experimental and semi-experimental correlations. The literature shows a great scattering in the heat transfer coefficient predictions of the reported correlations, especially when it comes to fluids

Conclusions

The following conclusions are drawn from the present investigations of the convective heat transfer coefficient in a packed pebble-bed nuclear reactor, with pebbles of 5 cm in diameter using a sophisticated measurement technique:

  • 1.

    A noninvasive sophisticated fast-response heat transfer probe of a spherical type was developed and used in this work.

  • 2.

    The local heat transfer coefficients were measured using such a sophisticated heat transfer probe of spherical type, and the heat transfer experiments

Acknowledgement

The authors acknowledge the financial support provided by the U.S. Department of Energy-Nuclear Energy Research Initiative (DOE-NERI) project (NERI-08-043) for the 4th generation nuclear energy, which made this work possible.

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