Gas-Solid Flow in a Fluidized-Particle Tubular Solar Receiver: Off-Sun Experimental Flow Regimes Characterization

The fluidized particle-in-tube solar receiver concept is promoted as an attractive solution for heating particles at high temperature in the context of the next generation of solar power tower. Similar to most existing central solar receivers, the irradiated part of the system, the absorber, is composed of tubes in which circulate the fluidized particles. In this concept, the bottom tip of the tubes is immersed in a fluidized bed generated in a vessel named the dispenser. A secondary air injection, called aeration, is added at the bottom of the tube to stabilize the flow. Contrary to risers, the particle mass flow rate is controlled by a combination of the overpressure in the dispenser and the aeration air velocity in the tube. This is an originality of the system that justifies a specific study of the fluidization regimes in a wide range of operating parameters. Moreover, due to the high value of the aspect ratio, the particle flow structure varies along the tube. Experiments were conducted with Geldart Group A particles at ambient temperature with a 0.045 m internal diameter and 3 m long tube. Various temporal pressure signal processing methods, applied in the case of classical risers, are applied. Over a short acquisition time, a cross-reference of the results is necessary to identify and characterize the fluidization regimes. Bubbling, slugging, turbulent and fast fluidization regimes are encountered and the two operation modes, without and with particle circulation, are compared.

Flowing Particle Fluidized Bath Design and Heat Transfer

Any proposed particle to working fluid heat exchanger as part of a CSP Particle Heating Receiver system is challenging. A principal challenge is achieving adequate heat exchange (HX) from the high temperature particles to the working fluid such as sCO2 or air flowing in tubes or other passages. To reduce the required HX area, a high particle side heat transfer coefficient is needed, and counterflow is always the best overall arrangement. Consequently, a promising approach is implementing an open channel flow of fluidized particles actually flowing in a general counterflow with respect to the working fluid, which is contained in tubes or passages immersed in the channel. This arrangement provides (1) excellent particle side heat transfer, (2) convenient particle re-circulation, and (3) almost ideal counterflow with the working fluid. To advance the understanding and support the design and applications of such exchangers, this investigation has been conducted to study the possibility of local effects of the particle flow path on the fluidized heat transfer. To this end, a series of smaller fluidized bed heat exchangers were built utilizing an axially flowing open channel for the moving bed of fluidized particles. These designs featured a serpentine flow path representative the full scale HX design proposed by others. The proposed serpentine flow design is based on an existing particle cooling system; however, questions were raised about this design that had not yet been conclusively answered and promoted this investigation. The test bath supporting this investigation contains one bend around which the particulate flows prior to exiting the heat exchanger. The intent of this larger scale apparatus is to observe the variables affecting the stability or uniformity of the particle flow and provide insight into potential problems with the operational unit. The test rig consists of two stacked sections. The lower container is the fluidizing air plenum, which provides a uniformly distributed airflow through the bottom plane of the upper container. The interface comprises a structural perforated plate, stacked layers of filter paper to balance the pressure drop, and a fine stainless steel wire mesh to ensure that the particulate remains in the upper container. This upper container represents the particulate flow area. Clear conductive PETG polymer walls were used for the fluidized bath to reduce electrostatic buildup while still providing a transparent material through which the flow can be observed. The current design uses an air conveyor to recirculate the particulate from one end of the test bath back to the other closing the particle loop. The tests described investigate the effectiveness of fluidization in specific regions of the serpentine path. Measurements have been taken in these regions to determine the local heat transfer coefficient. This is accomplished by inserting a cartridge heater with a known power input and heated area, instrumented with a fine bead surface thermocouple to measure the heater surface temperature. In addition, two probes are immersed in the fluidized bed surrounding the cartridge heater to measure the free stream temperature in the bed. The air input for fluidization and air conveyor lift are also measured and recorded as test parameters along with approximate bed height in each region. In addition to the quantitative measurements of the flow, the test unit is used to observe the effect of fluidization, bed height, and outlet locations on the axial mass flow rate of the particulate. These results will be presented in the proposed paper. Going forward, this setup will allow for testing of various mass flow control schemes for the system. Currently this design, with the instrumented heater and free stream temperature probes, allows measurement of the local heat transfer properties anywhere in the particle flow path. The present tests provide a localized map of heat transfer coefficients in the fluidized bath design and a description of the flow behavior which will be reported and presented to support future open channel particle to sCO2 heat exchanger designs.

Numerical Simulations of Wave-induced Soil Erosion in Silty Sand Seabeds

Silty sand is a kind of typical marine sediment that is widely distributed in the offshore areas of East China. It has been found that under continuous actions of wave pressure, a mass of fine particles will gradually rise up to the surface of silty sand seabeds, i.e., the phenomenon called wave-induced soil erosion. This is thought to be due to the seepage flow caused by the pore-pressure accumulation within the seabed. In this paper, a kind of three-phase soil model (soil skeleton, pore fluid, and fluidized soil particles) is established to simulate the process of wave-induced soil erosion. In the simulations, the analytical solution for wave-induced pore-pressure accumulation was used, and Darcy flow law, mass conservation, and generation equations were coupled. Then, the time characteristics of wave-induced soil erosion in the seabed were studied, especially for the effects of wave height, wave period, and critical concentration of fluidized particles. It can be concluded that the most significant soil erosion under wave actions appears at the shallow seabed. With the increases of wave height and critical concentration of fluidized particles, the soil erosion rate and erosion degree increase obviously, and there exists a particular wave period that will lead to the most severe and the fastest rate of soil erosion in the seabed.

Influence of Thermal Conditions on Particle Properties in Fluidized Bed Layering Granulation

Fluidized bed layering granulation is frequently used to formulate particles of high quality. From previous studies, it is well known that the dynamic behavior of the process, as well as the product properties depend on operating parameters. The process is characterized by heat and mass transfer between fluidized particles and the surrounding fluidization medium. To investigate the mutual influence between particle phase and fluidization medium, a dynamic model is introduced. The model comprises two parts: a population balance model to describe the evolution of the particle sizes and a system of ordinary differential equations to account for thermal conditions. For the first time, the dynamic model considers the bidirectional coupling of particles and fluidization medium in fluidized bed layering granulation. By means of simulations, it is shown that the derived model is capable of reproducing the experimental findings.