A Phase-Field Solidification Model of Almost Pure ITS-90 Fixed Points

Large, Matthew J.; Pearce, J. V.

doi:10.1007/s10765-014-1685-2

Cited by 3 publications

(5 citation statements)

References 34 publications

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“…Related to this, Large and Pearce [29] presented detailed simulations of solidification based on the phase-field method, which indicated the formation of macroscopically (i.e. thicknesses of a few mm) sized solid 'fingers' or 'cells' during solidification in the volume surrounding the bottom of the re-entrant tube, again on account of the geometry in this region.…”

Section: Geometric Effectsmentioning

confidence: 93%

“…The glass thermal conductivity, heat capacity, density and emissivity is 1.3 W m −1 K −1 , 792 J kg −1 K −1 , 2200 kg m −3 and 0.9 respectively. The boundary condition around the outer surface of the reentrant tube, representing the heat transfer between the fixedpoint cell and the re-entrant tube, is a convective heat flux q 0 = h • (T ext − T) where h = 10 W m −2 K −1 ; this value was found to provide the best characterisation of the actual SPRTfixed point thermal interaction, at least at the freezing point of tin [29]. The ambient temperature T ext is param eterised using an empirical function 5 to represent the high degree of uniformity within the 'virtual' fixed point cell at T fp = 692.677 K along about the first 200 mm and the gradual drop to the external ambient temperature T amb = 293.15 K further up the tube.…”

Section: Finite Element Analysismentioning

confidence: 99%

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Immersion effects in zinc ITS-90 fixed-point cells

Pearce¹,

Gray²,

Veltcheva³

et al. 2019

Meas. Sci. Technol.

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The freezing point of zinc (419.527 °C) is an important defining fixed point of the International Temperature Scale of 1990 (ITS-90). To realize it, a crucible of high purity graphite containing an ingot of high purity zinc is placed in a glass container containing an inert gas, with a central re-entrant tube, also glass, to allow insertion of a standard platinum resistance thermometer (SPRT). A key problem with these cells is difficulty in obtaining good immersion of SPRTs; it is commonly thought that the glass construction causes ‘light piping’ whereby the glass could facilitate heat transfer away from the region close to the SPRT sensing element by radiation, both directly and via reflections. In this paper it is shown that at the zinc freezing temperature light piping via reflections between the surfaces of the glass components is not an important heat transfer mechanism. It is also shown that light piping via transmission within the glass components is insignificant because of attenuation at wavelengths where the magnitude of the Planck function is important. In the case of the zinc fixed point, direct line-of-sight radiative heat transfer and thermal effects within the ingot appear to be the most important contributors to spurious immersion effects. It is shown that where the immersion profile is poor, a short platinum coil wrapped around the SPRT in the vicinity of the sensing element to improve radial conduction and to block vertical radiative heat transfer substantially improves the performance.

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Section: Geometric Effectsmentioning

confidence: 93%

Section: Finite Element Analysismentioning

confidence: 99%

Immersion effects in zinc ITS-90 fixed-point cells

Pearce¹,

Gray²,

Veltcheva³

et al. 2019

Meas. Sci. Technol.

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“…In the case of Sn, three very different furnace profiles with outer mantle initiation (see Figure 1) were performed. The objective was to compare the results with the ones obtained in previous studies [4] developed with a different model and software. It must be noticed that the furnace profiles in this study have been lineally extended to the cell full length because in the case of [4] only the part corresponding to the container of the fixed point element was simulated.…”

Section: Influence Of the Furnace Temperature Profile In The Freezingmentioning

confidence: 99%

“…The objective was to compare the results with the ones obtained in previous studies [4] developed with a different model and software. It must be noticed that the furnace profiles in this study have been lineally extended to the cell full length because in the case of [4] only the part corresponding to the container of the fixed point element was simulated. The furnace was initially set 1.3 K above Sn freezing point (505.078 K) and then cooled down to 1.3 K below the freezing point and from this offset (503.778 K) the furnace temperature profiles were added.…”

Section: Influence Of the Furnace Temperature Profile In The Freezingmentioning

confidence: 99%

The use of computational fluid dynamics to study furnace effects in ITS-90 fixed points realizations

et al. 2016

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Together with the impurities, the thermal fluxes are one of the major sources of uncertainty during the realization of the International Temperature Scale of 1990 (ITS-90) defining fixed points. The use of computational fluid dynamics (CFD) is a valuable tool to develop models that describe the time evolution of the phase transformation (essentially the evolution of the solid-liquid interface) as a function of given theoretical assumptions and given parameters (furnace thermal gradients, freezing initiation, ambient temperature and insulation). The models can be validated by observing the impact of the selected parameters on the observed corresponding melting curves and used to achieve a full understanding of these thermal effects and their impact on uncertainty. This paper proposes an ITS-90 metallic fixed points CFD model together with some results about the influence of the furnace thermal gradients and the freezing initiation techniques.

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“…Some authors have hypothesised that a cause may be related to the conditions within the cell, due to, for example, 'bridges' of solid in the ingot due to sub-optimal initiation of the freeze and/or thermal conditions [9][10][11][12] or the appearance of macroscopic convex solid formations [13]. Local variations in temperature distribution arising from the immersion test itself were also investigated [14].…”

Section: Introductionmentioning

confidence: 99%

Understanding immersion in zinc fixed-point cells using factorial design of experiments

Silva¹,

Pearce²

2020

Metrologia

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The objective of this study was to investigate the ‘lift-off effect’ in zinc open cells, where an anomalously large change in temperature is observed when the standard platinum resistance thermometer (SPRT) is lifted off the bottom of the re-entrant tube during an immersion profile measurement. The goal was to determine the most important influences and hence what measures could be taken to mitigate it. The influences investigated were the filling gas, re-entrant tube material, re-entrant tube exterior surface condition, and the SPRT design. For each of these influence variables, three options were chosen. Since testing all combinations of different configurations arising would be extremely time consuming, the authors employed the optimal ‘design of experiments’ method using orthogonal arrays described by the Taguchi method, which is commonly used in manufacturing to optimise processes. Nine test configurations were identified, giving the necessary information on the interrelation between all influence variables. According to these results, the ideal configuration that yields the optimum immersion profile is a borosilicate tube, with full sandblasting and using nitrogen as filling gas for the cell. In addition, one particular SPRT design was found to be the least sensitive to the lift-off effect.

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A Phase-Field Solidification Model of Almost Pure ITS-90 Fixed Points

Cited by 3 publications

References 34 publications

Immersion effects in zinc ITS-90 fixed-point cells

Immersion effects in zinc ITS-90 fixed-point cells

The use of computational fluid dynamics to study furnace effects in ITS-90 fixed points realizations

Understanding immersion in zinc fixed-point cells using factorial design of experiments

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