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3 Experiments in Pebble Bed Heat Transfer
3.3.4.3 Using Improved Inverse Method for T2–T5 Without Wall
Effects
With the same discussion of Sect. (3.3.3.1), this section will show the results with the
same boundary and middle sensor choice of Sect. (3.3.3.1). Figure (3.18a, b) show
the effective thermal diffusivities of two vacuum tests and two helium tests with the
spline-piecewise cubic-polynomials retrieved by the improved inverse method. Figures (3.18c) and (3.18d) show the effective thermal conductivities converted from the
diffusivities by Eqs. (3.41) and (3.42). Compared with the results of Sect. (3.3.3.1),
the standard deviation of diffusivities of different azimuthal sets is smaller with this
improved inverse method, which implies the final experimental errors will also be
lower. The more flexible function type and adjustable positions make the inverse
method closer to the actual situation at each azimuthal set.
Moreover, in Fig. (3.18b), it is interesting that the distinct dips are identified
by spline-piecewise cubic-polynomials of the effective thermal diffusivities in all
helium tests at the temperature of 400
◦ C, which reveals the dynamic heat transfer
performance of a pebble bed descends slightly within low-temperatures at constant
Fig. 3.18 Effective thermal diffusivities (a, b) and Effective thermal conductivities (c, d) of C1–C5
in two vacuum (a, c) and two helium (b, d) tests with the improved method (T2–T5);