The objective of this investigation is to evaluate the ability of analytical and computational models to describe the momentum and heat transfer between the gas and particles in a falling-bed heat exchanger. Experimental data are presented for a test falling-bed heat exchanger. Measured temperatures, pressures, and overall heat transfer rates are compared to predicted values from analytical and computational models, and the capabilities and deficiencies of these modeling methods are discussed. In addition, the effect of the addition of a particle distributor on the performance of the falling-bed heat exchanger is measured.
In the falling-bed heat exchanger, solid particles fall through a vertical column against a counterflowing gas stream flowing upward with a velocity less than the terminal velocity of the particle. Heat is exchanged between the falling particles and rising gas. This arrangement has been proposed for heat recovery and regeneration in power plants and other process applications.
The ability to model and predict the heat transfer rate between the gas and particles is critical to the design of the falling-bed heat exchanger. The heat transfer between the gas and solid particles in these devices has typically been modeled by assuming steady-state and ideal, uniform, one-dimensional flow of the continuous fluid and the particle or droplets. This model, termed the uniform mixing model in this study, has been used in many instances to estimate the effective heat transfer coefficient and Nusselt number of the falling droplets and particles as a function of effective Reynolds number from experimental data.
The addition of a particle distributor has been shown to increase the heat transfer effectiveness of the falling-bed heat exchanger in experiments. It has been determined that the uniform mixing model generally does not provide an accurate representation of the falling-bed heat exchanger, as it cannot account for gas and particle maldistributions such as those created by a particular particle distributor design. Computational fluid dynamics, which can permit the modeling of these spatial maldistributions, has been used to model the falling-bed heat exchanger. The predictions of the overall heat transfer rate from computational fluid dynamics are in better agreement with the measured values. However, discrepancies between the predicted and measured pressures and local temperatures indicate that the modeling of the turbulent mixing of momentum and energy is inadequate.