Abstract
Development of commercial nuclear fusion power plants is a major goal for mankind in the 21st century. The principle behind current technology is magnetic confinement of hydrogen isotopes in a high energy plasma state. The hydrogen fuses to form helium, releasing large amounts of energy. One of many remaining technical challenges is the management of high temperature waste helium plasma. The plasma temperature must be decreased prior to extraction of the helium and a possible solution is to cascade heat-resistant solids through the waste streams. In a conceptual cascade system design which is being developed by the UKAEA, the optimal solids are coarse and dense (d = 0.0035 m and ρ = 4500 kg m-3) and flow rates of at least 500 kg s-1 are anticipated. The hot solids can be re-used after passing them through a helium fluidized-bed heat exchanger at high pressure, and the preferred technology for returning cooled solids to the cascade is dense-phase pneumatic conveying as this mode of conveying causes the least damage to solids and pipework. The high pressure in the fluidized bed allows direct feeding of the solids to the conveying lines, however the factors involved in the design of this kind of conveyor are not well understood. For example, reliable prediction of key operating parameters, e.g. pressure gradients and flow rates, is not yet possible for vertical dense-phase pneumatic conveying of solids such as those to be used in the cascade. A pilot-scale rig has been designed and built for the study of vertical pneumatic conveying of coarse, dense solids started directly from the side of a fluidized bed. No system like this has been reported in the open literature. A rotary-lobe blower is used for recirculation of fluidizing air through the solids feed pressure vessel and pressure is maintained using a compressed air utility line. Solids pass into the conveying line via an opening in the side of the pressure vessel and flow into a 3 or 4 m vertical section via a long 90° bend. Conveying lines of diameter 71.4 mm or 46.4 mm were fabricated from clear PVC to allow video footage of the conveying to be taken. An overhead collection vessel continuously weighs the solids in order to determine their flow rate, whilst gas flow rate is calculated from the compressed air make-up flow rate. Air pressures were measured at key points throughout the system using pressure transducers. For the 71.4 mm conveyor, conveying in the dense-phase plug-flow regime was possible for solids flow rates of up to 4-5 kg s-1. Transition to turbulent flow occurred at less than 2 kg s-1 for the 41.6 mm conveyor. For the same solids flux the solids-gas ratio was greater for the larger conveyor diameter. Solids were conveyed in square-nosed plugs with variable length and velocity and solids down-flow between plugs occurred gradually or through sudden disintegration of the rear of the plug. According to Konrad & To-tah (1989), falling solids generate frontal stresses upon impact with rising solids plugs, resulting in wall friction and a decrease in conveying efficiency. Earlier steady-state pressure drop prediction models for the plug-flow regime (e.g. Konrad & Totah (1989), Singh (1978) and Leung & Towler (1973)) are shown to be unsatisfactory and a new relationship for predicting outlet pressure as a function of inlet pressure, solids flow rate, gas flow rate, material properties and conveying line dimensions is proposed. The solids flow rate from the fluidized bed into the conveying line is found to be linearly related to the gas flow rate when conveying in the plug flow regime.