Evaluating convective heat transfer performance in packed beds arising in various engineering problems is a complex issue due to the different parameters involved in the media, such as the bed materials, heat transfer mechanisms, packing structure and heat source. Apart from a number of assumptions made in various correlations developed for determining heat transfer performance in packed beds, literature survey also reveals that more experimental researches were conducted with fluid flowing in and out of the medium than heated fluid confined in an enclosed medium. Noted also is that most of the experimental research found in literature were conducted under forced convection compared with the investigation in the present study conducted under natural convection. In a quest to investigate the particle-to-fluid heat transfer characteristics expected in the proposed new fuel design, a basic unit cell (BUC) model is being developed for the theoretical analysis and applied to determine the heat transfer coefficient, h, of the medium. The model adopted a concept in which a single unit of the packed bed was analyzed and taken as representative of the entire bed; it related the convective heat transfer effect of the flowing fluid with the conduction and radiative effect at the finite contact spot between adjacent unit cell particles. As a result, the model could account for the thermophysical properties of sphere particles and the heated gas, the interstitial gas effect, gas temperature, contact interface between particles, particle size and particle temperature distribution in the investigated medium. Although the heat transfer phenomenon experienced in the experimental set-up was a reverse case of the proposed fuel design, the study with the achievement in the validation with the Gunn correlation aided in developing the appropriate theoretical relations required for evaluating the heat transfer characteristics in the proposed nuclear fuel design.
Keywords Packed beds, heat transfer coefficient, basic unit cell model, coated particle nuclear fuel