Annals of the Assembly for International Heat Transfer Conference 13

ISBN 1-56700-225-0 / CD 1-56700-226-9

Year 2006

T. N. Ngonda
University of the Witwatersrand, South Africa

T. J. Sheer
University of the Witwatersrand, South Africa

This paper proposes a general model for frost growth on a flat plate during the bulk growth and densification period for both subsaturated and supersaturated supply air. Frost growth is considered as a two-faceted problem comprising a heat and mass transfer aspect and a fluid flow aspect. The model treats frost as a porous medium with variable porosity, density and thermal conductivity. The model improves on the earlier diffusion-based models, particularly in the way that diffusion within the frost layer is handled. The temperature dependency of water vapour diffusion inside the frost layer is not neglected. The model adopts a modified effective diffusion coefficient that contains a factor that accounts for varying pore blockage. The pore blockage factor must be correlated from experimental data. Unlike the heat and mass transfer aspects of the frost formation, the fluid flow aspects for the two conditions of subsaturated and supersaturated air are different. For subsaturated conditions, the fluid flow is basically single-phase. However for supersaturated conditions, the flow is multi-phase. Flow of the supersaturated air is modelled as fully developed particle-laden turbulent flow comprising saturated air as the carrier gas and ice crystals as the dispersed particles. The carrier gas flow is modelled based on compressible gas flow principles using momentum, mass continuity and energy equations. The Langrangian approach is then used to track the motion of the ice crystals in the flow field. Thus there are two types of mass transfer on the frost-air interface, mass accretion due to entrapment of ice particles and mass transfer from the diffusion-based processes. This approach is unique to the model. The model will be solved numerically using computational fluid dynamics software. On the air side the heat transfer, the mass transfer and the mass accretion from the particle-laden flow are computed at the frost-air interface. The energy and mass conservation equations are then solved for the frost layer using the boundary conditions for the frost-air interface and the cold surface. Comparison of model results with limited previously-published data is promising; new experimental data are being generated in order to validate the model fully.

MST-04 pages DOI: 10.1615/IHTC13.p10.40 Download article

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