Master's Theses

Date of Award

2013

Document Type

Thesis

Department

Mechanical Engineering

Keywords

Sensible Heat, Packed Bed, Free Convenction

Abstract

The present study seeks to develop a robust system-scale heat transfer model for a sensible heat storage system. A novel mechanism to simulate axial dispersion and heat loss from the boundary of the sensible heat system to the environment is introduced. The most challenging topics to develop a robust heat transfer model are related to heat loss term, selecting the more reliable method to calculate heat transfer coefficient between fluid and solid, and investigating the effect of neglecting temperature dependency of materials properties. As the storage heat system has a vertical storage vessel, the dominant heat transfer mechanism from the boundaries of the vessel is free heat convection. In other words, there is no heat loss from the walls except the heat loss happening between walls of the vessel and the ambient by free convection. This model shows heat transfer coefficient is a function of time because of the changing hot zone length. The hot zone length is increased over time and this causes increase of Grashof number and finally Nusselt number or heat convection coefficient to the ambient. Material property changes are another matter considered in the model. The temperature of the system is varied considerably (absolute temperature is designed to change by roughly a factor of two) over a storage cycle and this affects the material properties of the solid beads and fluid flow. Results show that considering this point has a very significant effect when temperatures are (as expected in application) far above the ambient.

 
 

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