Effects of pressure on thermal transport in plutonium oxide powder.
- Patricia
- F. Coyne
- Douglas Kirk
- Jerry
Radial temperature profiles in plutonium oxide (PuO{sub 2}) powder were measured in a cylindrical vessel over a pressure range of 0.055 to 334.4 kPa with two different fill gases, helium and argon. The fine PuO{sub 2} powder provides a very uniform self-heating medium amenable to relatively simple mathematical descriptions. At low pressures (<0.1 kPa), the effective thermal conductivity of the powder bed was approximately the same with either helium or argon since the dominant mechanisms are thermal radiation between particles and solid-solid conduction pathways. At high pressures, the effective thermal conductivity of the powder bed is typically assumed in the literature to be dominated by the gas thermal conductivities. However, from experimental measurements at high pressures, the effective thermal conductivity of the powder bed with argon as a fill gas is approximately three times higher than would be predicted from the gas thermal conductivities. Additionally, a significant pressure dependence was measured at pressures greater than atmospheric where the gas thermal conductivity would typically be assumed to be in the continuum limit and independent of pressure. An analytical model was developed for heat conduction in the fine ceramic powder with conduction pathways in parallel and in series through the gaseous and solid components. Many analytical models in the literature were unsuitable for this system because they make limiting assumptions about the particle dimensions and shape and are developed for packed beds with higher packing fractions. PuO{sub 2} powder has small particle sizes (on the order of 1 to 10 {mu}m), random particle shapes, and high porosity so a more general model was required for this system. The model correctly predicts the temperature profiles of the powder over the wide pressure range for both argon and helium as fill gases. The effective thermal conductivity of the powder bed exhibits a pressure dependence at higher pressures because the pore sizes in the interparticle contact area are relatively small (less than 1 {mu}m) and the Knudsen number remains above the continuum limit at these conditions for both fill gases. Also, the effective thermal conductivity with argon as a fill gas is higher than expected at higher pressures because the solid pathways account for over 80% of the effective powder conductivity. The results obtained from this model help to bring insight to the thermal conductivity of very fine ceramic powders with different fill gases.
- Research Organization:
- Los Alamos National Laboratory (LANL), Los Alamos, NM (United States)
- Sponsoring Organization:
- USDOE
- OSTI ID:
- 977925
- Report Number(s):
- LA-UR-04-7867; TRN: US1003644
- Resource Relation:
- Journal Volume: 49; Journal Issue: 17-18; Conference: Submitted to: AIChE 2004 Annual Meeting, November 2004, Austin, TX
- Country of Publication:
- United States
- Language:
- English
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