Experimental study on frost formation on the surface of ultra-low temperature circular tubes under forced convection conditions

Combined engine precooler systems typically utilize ultra-low temperature refrigerants such as liquid nitrogen or hydrogen as cooling media. This operational configuration inevitably results in frost formation on microtube bundle surfaces. The frost growth characteristics on precooler tube bundles differ markedly from conventional surfaces, exhibiting distinct growth rates, densities, and ice crystal structures under ultra-low temperatures. To systematically examine frost layer formation under ultra-low temperature forced convection conditions, this study conducts experimental investigations on circular tubes, focusing on the effects of humid air parameters: temperature, flow velocity and moisture content on frost growth characteristics. Comprehensive single-tube experiments establish that frost layer thickness exhibits a positive correlation with both flow velocity and moisture content, while demonstrating an inverse relationship with ambient temperature. Data from the experimental section establishes a proportional dependence of humid air pressure drop on accumulated frost thickness. In tube bundles, initial frost growth mirrors single-tube behavior. Intertube frost bridges subsequently form and merge adjacent layers, ultimately creating a continuous coating that envelops the entire bundle. The first tube's trailing edge showed delayed frost growth with airflow disruption versus isolated tubes. The last row's trailing edge maintained more stable frost accumulation with reduced shedding. Moreover, experimental results demonstrate the efficacy of ethanol-based defrosting. These findings provide key insights into precooler tube-bundle frost formation, enabling better prediction models and frost-control strategies.