Particle-based heat transfer fluids for concentrated solar power (CSP) tower applications offer a unique advantage over traditional fluids, as they have the potential to reach very high operating temperatures. The present work studies the heat transfer behavior of dense granular flows through cylindrical tubes as a potential system configuration for CSP tower receivers. Experimental studies were conducted using a bench-scale apparatus to examine the heat transfer to such a flow configuration, as well as examine the effect of system parameters on the heat transfer to the flow. The experimental results corroborate the observations of other researchers; namely, that the discrete nature of the flow limits the heat transfer from a tube wall to the flow due to an increased thermal resistance in the layer of particles adjacent to the heated wall. A two-layer model was developed to describe this heat transfer phenomenon. The model assumed the flow was composed of two layers: a bulk layer characterized by the bulk effective properties of the flow, and a thin layer adjacent to the heated wall (with thickness of a particle radius) characterized by an effective thermal conductivity (ETC). A correlation to approximate the ETC of the wall-adjacent layer was developed, taking into consideration how the ETC may vary with flow rate and the system configuration. The packing fraction of the wall-adjacent layer and the number of particles in contact with the wall were found to control the heat transfer from the heated wall. Discrete Element Method (DEM) simulations indicated that both these parameters decrease with increasing flow rate, which manifests in a decrease in heat transfer with increasing flow rate. Data from DEM simulations for six system configurations was used to develop empirical correlations to predict these parameters for different flow rates and system configurations.
Incorporating the empirical correlations into the ETC correlation developed for the wall-adjacent layer allowed the heat transfer to the flow to be predicted using the two-layer model. The model showed good agreement with the experimental results taken for different tube diameters, particle diameters, and flow rates (within the experimental uncertainty). The experimental results and model suggest that, at low operating temperatures (<200°C), the heat transfer to a flow increases with increasing tube diameter and/or decreasing particle diameter due to the relative size of the wall-adjacent layer within the flow.
As a final step, the heat transfer to dense flows at operating temperatures more characteristic of CSP applications was studied. The effect of temperature on the heat transfer to the flow was examined for a single flow rate. The results demonstrated an increase in the heat transfer with increasing temperature due to enhanced thermal properties at higher temperatures. The influence of radiation was also examined by developing a simple radiation model for the wall-adjacent layer. For the small particle diameters tested in the present study (<320μm), radiation did not contribute significantly to the overall heat transfer. The model suggests, however, that the radiation contribution will increase with increasing particle diameter due to the increased void size. The results from all aspects of the current work provide an understanding of the parameters controlling the heat transfer to a dense granular flow and pave the way for designing future dense flow systems.
|School:||North Carolina State University|
|School Location:||United States -- North Carolina|
|Source:||DAI-B 80/01(E), Dissertation Abstracts International|
|Keywords:||Concentrated solar power, Dense granular flows, Heat transfer, High operating temperatures, Thermal resistance, Wall-adjacent layer|
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