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.