Experimental investigation of falling liquid film flow of palm-based Bio-Oil using image processing techniques

A falling liquid film evaporator can provide high heat transfer rates for evaporating palm-based bio-oil in biofuel production through fluid catalytic cracking. Although liquid-film hydrodynamics strongly affects heat transfer and evaporation, the hydrodynamic characteristics of palm-based bio-oil falling liquid film flow remain insufficiently investigated. The optical opacity of palm-based bio-oil prevents direct use of standard optical diagnostics, requiring an alternative measurement approach. This study experimentally investigates palm-based bio-oil flow on the outer surface of a vertical tube under various Reynolds numbers. Flow visualization was performed using a standard DSLR camera, while film thickness was measured using image stacking and average-intensity projection methods. Results demonstrate that increasing Reynolds number accelerates the transition from smooth-to-wavy flow and from two-dimensional to three-dimensional wavy regimes, shifting transition points upstream along the tube through secondary instability mechanisms. Comparisons with empirical correlations show that the Nusselt model significantly overestimates film thickness by 78.6%. Tube-based correlations, such as Takahama and Kato, provide closer approximations but still underestimate the experimental results by approximately 38%. These discrepancies indicate that geometric or fluid-property similarity alone is insufficient for accurate prediction, highlighting the need for dedicated empirical correlations for palm-based bio-oil falling films on vertical tubular geometries.