Secondary steam flow characteristics and heat transfer effects in a horizontal tube falling film evaporator based on CFD-distributed parameter model

Abstract

This study establishes a three-dimensional Distributed Parameter(DP) model for a horizontal tube falling film evaporator(HTFFE) in a Low-Temperature Multi-effect Desalination (LT-MED) system. An innovative computational method integrating the DP model and Computational Fluid Dynamics methods is proposed, which fully couples the flow and heat transfer processes inside, outside, and between the tubes. This method effectively resolves the simulation problems in previous studies where secondary steam flow process were either uncalculable or oversimplified. Using an actual two-pass HTFFE as a case study, the model reveals distributions of inter-tube pressure, secondary steam velocity, and other thermal parameters. The concept of a “flow center” for secondary steam was defined, and it was found that over 70 % of the secondary steam exits through the side boundary of the tube bundle. The study clarifies the mechanism by which secondary steam influences evaporator performance: an increase in inter-tube pressure elevates the seawater temperature, which thereby reduces the effective heat transfer temperature difference(ΔT_eff), leading to thermal parameter inhomogeneity along the tube bundle. The impact of secondary steam flow on ΔT_eff was quantified under varying apparent heat transfer temperature differences(ΔT_ap) and total tube column numbers. It demonstrates a reduction in ΔT_eff of up to 17.32 %, with a local loss reaching 56.04 % when combined with seawater boiling point elevation under certain conditions. Furthermore, local and overall relative change rate indicators of thermal parameters were defined. These indicators reflect that the location of the local ΔT_eff loss occurs in the region near the heating steam outlet for each pass, while the secondary steam flow enhances the overall heat transfer coefficient but reduces both the average ΔT_eff and the total water production.