Design challenges in surface-modified falling film evaporators: Heat transfer gains vs. scaling penalties

Artikel i vetenskaplig tidskrift, 2026

Highly corrugated heat-transfer surfaces are often proposed to intensify heat transfer in falling-film evaporators, but their effect on crystallization fouling remains poorly quantified. We use multiphase DNS to analyse evaporating falling films of an aqueous sodium-carbonate solution, representative of Kraft black liquor. We quantify how surface geometry (corrugation pitch and height) influences the heat-transfer coefficient and near-wall supersaturation, a precursor of crystallization fouling. Compared to a smooth reference surface (Nu = 0.28), corrugated surfaces increase the Nusselt number by 32%–86% (Nu = 0.37–0.52) depending on pitch and height, but this heat-transfer benefit is followed by an increase in near-wall salt supersaturation S. In the entrance region, the domain-averaged near-wall supersaturation for the modified surfaces remains just below the nucleation threshold (S≈0.98–1.00), yet persistent local pockets with S>1 appear immediately downstream of the corrugations, most markedly for the highest-heat-transfer (base) geometry, while further along the tube the smooth surface itself reaches mean near-wall values of S≈1.05–1.10. A heat-flux decomposition shows that the enhanced advective transport responsible for the heat-transfer gain simultaneously transports concentrated solute from the interface towards the wall. We further show that reducing the corrugation height, while lowering the peak heat-transfer gain (Nu = 0.37, +32% relative to smooth), produces a stratified concentration field that shifts the onset of supersaturation away from the wall and towards the bulk, thereby favouring bulk over wall nucleation. These results indicate that maximizing heat transfer and minimizing fouling risk are competing objectives, and that moderate, carefully proportioned surface modifications, rather than the most aggressive corrugation, offer the best compromise for long-term evaporator performance.