Beeinflussung der Naturumlaufverdampfung durch sekundäre Alkylsulfonate (SAS) (Volume 54) (English shop)

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This research work investigates the influence of secondary alkane sulfonates (SAS) on the operating behaviour of a single-tube natural circulation evaporator at miniplant scale. By varying the mass-based surfactant fraction ξ, the operating pressure p, as well as the apparent liquid level h* and the driving temperature difference ΔTglo, the effects of the surfactant on the heat transfer coefficient and the circulation velocity are examined. The addition of SAS results in an increase in the circulation velocity of up to 136 % compared with the circulation velocity of pure water, which is attributed to the reduction of the two-phase pressure drop caused by the presence of the surfactant. Contrary to expectations, the increase in circulation velocity does not lead to an increase in the integral heat transfer. Through a differentiated consideration of the heat transfer in the preheating and evaporation zones, it could be shown that the influence of the driving temperature difference on the heat transfer coefficient is more pronounced than the influence of the fluid dynamics, quantified by the Reynolds number. This result permits the conclusion that heat transport in the preheating zone is dominated by the mechanism of subcooled nucleate boiling. Building on this, it is assumed that the increase in circulation velocity caused by the surfactant at the same driving temperature difference leads to a suppression of subcooled nucleate boiling, whereby the heat transfer coefficient in the preheating zone is reduced.

The influence of the surfactant on the two-phase heat transfer in the evaporation zone is small. When comparing the heat transfer in the evaporation zone, however, it must be taken into account that both the heat transfer coefficients and the overall heat transfer coefficients are subject to high uncertainties depending on the operating conditions. The greater the product-side heat transfer, the greater the uncertainty of the coefficients. This dependency in particular reduces the reliability of the experimentally determined heat transfer coefficients in the evaporation zone. In the lower operating range of the evaporator at ΔTglo ≤ 30 K and h* ≤ 90 %, increased temporal fluctuations of the circulation flow were observed, which suggest the occurrence of the geysering flow instability. The addition of SAS partially reduces the flow instabilities at the highest surfactant fraction. Since geysering is based on the cyclic formation of slug bubbles, it is presumed that the surfactant stabilises the flow due to its coalescence-inhibiting effect.

In order to quantify the influence of the surfactant, or rather of the reduced surface tension, on the operating behaviour of the natural circulation evaporator, two dimensionless model equations were developed to describe the Nusselt number and the Reynolds number on the basis of the Jakob number and the apparent liquid level. The influence of the surfactant is taken into account by means of a newly developed dimensionless number as well as by normalising the surface tension of the substance system to the surface tension of pure water. Both correlation equations reproduce the experimental data with a mean deviation of 16 %. With regard to the description of the Reynolds number, an extended database is required in order to verify the suitability of the power-law approach for representing the surfactant influence. As next steps, an extension of the variation range of the surfactant fraction and of the driving temperature difference is therefore proposed.