Determination of the phase state of phase change materials surrounding finned tube heat exchangers

Kurzfassung

Latent heat thermal energy storage (LHTES) systems compensate for the low thermal conductivity of the phase change material (PCM) by means of extended fin structures. The numerical models used to optimise these geometries are validated against the experimentally determined liquid fraction and heat transfer. However, established methods for determining the liquid fraction provide either low spatial resolution or no temperature information.

This thesis characterises the infrared behaviour of an eutectic NaNO3-KNO3 mixture melting at 222°C and quantifies the uncertainty of an infrared-thermography-based determination of the local phase state and of the overall liquid fraction. A representative cross-section of a shell-and-tube LHTES was observed and realised as a hexagonal enclosure containing either a fin structure or a bare tube.

Across the phase change, the equivalent emissivity increases from 0.660 to 0.684. This change is sufficiently small for a single radiance threshold to separate the two phase states, for which a common value of 204.4Wm-2sr-1 was determined. The resulting mean propagated uncertainty of the liquid fraction amounts to ± 7.0% for the fin structure and ± 2.0% for the bare tube, of which 83% to 87% originates from the boundary of the complex fin structure.

The method yields the liquid fraction and the stored heat from a single measurement. Its accuracy is governed by the perimeter of the evaluation mask and by the requirement of an approximately two-dimensional phase front, a condition that was not fulfilled during solidification under the investigated boundary conditions.