1. Measurement of Thermal Gap Conductance Using the Laser Flash Method
Zhuorui Song1, Luke Scoggins1, Chris Martinez1, Heng Ban1, Pavel Medvedev2 1Mechanical and Aerospace Engineering, Utah State University, 2Idaho National Laboratory
Motivation
Classic Theory of Gap Conductance
Laser Flash Method
Previous Experimental Study
Theoretical Study
Sensitivity Analysis
Evaluation of the 1D Assumption
1D model
Analytical solution
GOAL: Experimental data on the gap conductance between HT9 and SS316 and between advanced fuels and advanced cladding in the Accident Tolerant Fuels program.
SPECIFIC OBJECTIVES: Investigate the laser flash method on measurement of thermal gap conductance.
WORKING CONDITIONS: Helium in a gap of 5mm to 50mm at 1-3 atm and 0~500°C, with Kn of Thermal behavior falls in continuum region and transient region.
Laser pulse
IR temperature detector
5~50mm gap
~1mm in thickness
Temperature rise is strongly related to thermal diffusivity of materials in most cases, but thermal gap conductance instead is determined together with heat convection coefficients at front and rear surfaces and absorbed laser heat.
Grooves
Spacer
Species 1&2 and gap
Domain used for COMSOL simulations (Case2)
Radial grooves greatly diminish the impact of shunting heat transfer through spacers.
Heat losses at side walls have insignificant influence.
Better approach to the 1D model with the presence of grooves
L L2 … Li … Ln
0 x x xi xn
Laser flash
Heat loss
… i … n
Free molecular regime
Transient regime (0.01<Kn<100)
Continuum regime (Kn<0.01)
Clad
Fuel
g1 d g2
Gas
g predicted by GAPCON model
Temperature jump distance (g) accounts for imperfect collisions of gas molecules with the solid surface. GAPCON, Kennard, Lloyd models are used estimate temperature jump distance in a generalized format:
Measurements
The temperature rise at the rear surface due to laser flash was solved using Laplace transform,
SS316: 1mm in thickness
Gap: ~50microns
GAPCON
Kennard
Lloyd
Gap distance can be determined from the dependence of thermal gap conductance on gas pressure
Thermal gap conductance between UO2 fuel and Zircaloy-4 cladding were measured by Garnier and Begej in 1979.
Extra error may exist due to the groove on Zr species according to our simulation results. The groove was initially designed to decrease the shunting heat transfer through spacers.
Definition:
Results
Cases
L1,L3
(mm)
k1,k3
(W/m K)
r 1, r 3
(kg/m3)
cp1, cp3
(J/kg K) L2
kgas
r 2
cp2
Qd(t) (kJ/m2)
hf, hr
(W/m2 K)
Case1
0.5
17.14
7758
495.4 10
0.0367
0.736
1052.5 15 50
Case2
a. Independence of gap thermal diffusivity
b. Precise measurement of gap conductance is possible.
Conclusions
Thermal gap conductance can be accurately determined by using the laser flash method, based on the sensitivity analysis and experimental results .
Gap distance can be measured by the dependence of thermal gap conductance on pressure.
Heat storage of the gap gas has insignificant influence on heat transfer process in the measurement.
Heat conduction through spacers may cause large uncertainties of the gap conductance, and the influence could be greatly reduced by the addition of grooves on specimens.