Doctoral Dissertations

Keywords and Phrases

Axial Dispersion Model; Mixing Process


"Proper analyses of axial dispersion and mixing of the coolant gas flow and heat transport phenomena in the dynamic core of nuclear pebble-bed reactors pose extreme challenges to the safe design and efficient operation of these packed pebble-bed reactors.

The main objectives of the present work are advancing the knowledge of the coolant gas dispersion and extent of mixing and the convective heat transfer coefficients in the studied packed pebble-beds. The study also provides the needed benchmark data for modeling and simulation validation. Hence, a separate effect pilot-plant scale and cold-flow experimental setup was designed, developed and used to carry out for the first time such experimental investigations. Advanced gaseous tracer technique was developed and utilized to measure in a cold-flow randomly packed pebble-bed unit the residence time distribution (RTD) of gas. A novel, sophisticated fast-response and non-invasive heat transfer probe of spherical type was developed and utilized to measure in a cold-flow packed pebble-bed unit the solid-gas convective heat transfer coefficients. The non-ideal flow of the gas phase in pebble bed was described using one-dimensional axial dispersion model (ADM), tanks-in-series (T-I-S) model and central moments analyses (CMA) method. Some of the findings of this study are:

  • The flow pattern of the gas phase does not much deviate from the idealized plug-flow condition which depends on the gas flow rate and bed structure of the pebble-bed.
  • The non-uniformity of gas flow in the studied packed pebble bed can be described adequately by the axial dispersion model (ADM) at different Reynolds numbers covers laminar and turbulent flow conditions. This has been further confirmed by the results of tanks in series (T-I-S) model and the central moment analyses (CMA).
  • The obtained results indicate that pebbles size and hence the bed structure strongly affects axial dispersion and mixing of the flowing coolant gas while the effect of bed height is negligible in packed pebble-bed. At high range of gas velocities, the change in heat transfer coefficients with respect to the gas velocity reduces as compared to these at low and medium range of gas velocities.
  • The increase of coolant gas flow velocity causes an increase in the heat transfer coefficient and the effect of 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 results and findings clearly indicate that one value as overall heat transfer coefficient cannot represent the local heat transfer coefficients within the bed and hence correlations to predict radial and axial profiles of heat transfer coefficient are needed"--Abstract, page iii.


Al-Dahhan, Muthanna H.

Committee Member(s)

Liapis, Athanasios I.
Neogi, P. (Partho), 1951-
Smith, Joseph D.
Mueller, Gary Edward, 1954-


Chemical and Biochemical Engineering

Degree Name

Ph. D. in Chemical Engineering


Nuclear Energy Research Initiative (U.S.)
United States. Department of Education. Graduate Assistance in Areas of National Need


Missouri University of Science and Technology

Publication Date

Fall 2013


xvii, 236 pages

Note about bibliography

Includes bibliographical references (pages 224-235).


© 2013 Rahman Abdulmohsin, All rights reserved.

Document Type

Dissertation - Open Access

File Type




Subject Headings

Pebble bed reactors
Gas dynamics
Heat -- Transmission
Heat exchangers -- Fluid dynamics

Thesis Number

T 10375

Electronic OCLC #