Large Particle 3D Concrete Printing—A Green and Viable Solution

The Large Particle 3D Concrete Printing (LP3DCP) process presented in this paper is based on the particle bed 3D printing method; here, the integration of significantly larger particles (up to 36 mm) for selective binding using the shotcrete technique is presented. In the LP3DCP process, the integration of large particles, i.e., naturally coarse, crushed or recycled aggregates, reduces the cement volume fraction by more than 50% compared to structures conventionally printed with mortar. Hence, with LP3DCP, the global warming potential, the acidification potential and the total non-renewable primary energy of 3D printed structures can be reduced by approximately 30%. Additionally, the increased proportion of aggregates enables higher compressive strengths than without the coarse aggregates, ranging up to 65 MPa. This article presents fundamental material investigations on particle packing and matrix penetration as well as compressive strength tests and geometry studies. The results of this systematic investigation are presented, and the best set is applied to produce a large-scale demonstrator of one cubic meter of size and complex geometry. Moreover, the demonstrator features reinforcement and subtractive surface processing strategies. Further improvements of the LP3DCP technology as well as construction applications and architectural design potentials are discussed thereafter.

Preliminary Design of a Fuel Element With Divergent Hot Gas Channel In Particle Bed Reactor for Nuclear Thermal Propulsion

The nuclear thermal propulsion (NTP) system can shorten the travel time in deep space exploration and reduce the initial mass of the launch vehicle due to its superior characteristics including high specific impulse and large thrust. Particle bed reactor (PBR) is one of the most appropriate reactor concepts to equip the NTP systems. To make the best use of PBR, the thermal-hydraulic design of the fuel element should be carefully considered and a flow-power matching technology should be developed. In this paper, a novel design employing a divergent hot gas channel is proposed to achieve a uniform flow distribution with lower maximum temperature and pressure drop. Through the analysis of the 1D modified momentum equation in the inlet plenum and hot gas channel, the model of pressure drop is established. Then, the differential equation of the ideal cross-section of the hot gas channel is derived. At last, the flow and heat transfer process in the fuel element with divergent hot gas channel is simulated by using computational fluid dynamics (CFD) code, and the reduction of pressure drop and temperature verifies the theoretical model. This study shows that the proposed design of the divergent hot gas channel can provide a new idea for thermal-hydraulic optimization of the PBR fuel element.

Numerical Study on Discharging Characteristics of Entangled Cluster of Particles in Particle Bed

To better understand the flow features of the particle cluster in a particle bed, discharging of the particle entangled cluster is simulated by the discrete element method (DEM). The particle entangled cluster is composed of eight particles connected by rigid bonds, and the simulated entangled cluster models are divided into two types: axisymmetric u-particles and distorted z-particles. The simulation starts with the closed discharge outlet, and the bonded clusters with different IDs are randomly added from the entrance section. The particles fall freely and accumulate freely in the particle bed. The discharge hole opens after all the particles are stationary for a period. Then, the particles are discharged from the particle bed under gravity. The discharging process has time-dependent bulk-movement behavior. There is not much mixing between layers on the boundary. The vertical end not only makes the packing loose but also intensifies the interaction between particles due to entanglement. Consequently, the discharge features of particle entangled clusters of different included angles were quantified. The results show that the particle discharging speeds depend on the entanglement angle (α of u-particles and η of z-particles) and discharging outlet diameter. A large included angle may play the role of retarding or inhibiting the discharging flow rate. Therefore, the entanglement of particle components also always plays the key role of retarding the discharge.