Investigation of Heat Transfer Mechanisms in Dilute Particle Clouds for Thermochemical Energy Storage Applications
Restricted (Penn State Only)
- Author:
- Umer, Muhammad
- Graduate Program:
- Mechanical Engineering
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- December 10, 2024
- Committee Members:
- Linxiao Zhu, Major Field Member
Jacqueline O'Connor, Major Field Member
Brian Fronk, Chair & Dissertation Advisor
Saya Lee, Outside Unit & Field Member
Robert Kunz, Professor in Charge/Director of Graduate Studies - Keywords:
- Particle heat exchangers
Thermochemical energy storage
Particle cloud heating
Heat transfer in particles under sub-atmospheric pressure
Heat transfer coefficient
Wall convection
Free-falling particle system
Convection and radiation in dilute particle flow.
Sub-atmospheric pressure
Dilute particle flow - Abstract:
- The potential of reactive particles for thermochemical energy storage and solar fuel production is highly dependent on the efficiency of energy transport to and from these particles. A critical component in concentrated solar power (CSP) systems is the heat exchanger/reactor (HX), which facilitates energy recovery from metal oxide (MOX) particles to the supercritical carbon dioxide (sCO2) loop, used as the power fluid. Unlike inert particle heat exchangers, reactive systems operate with particle flows in the presence of an oxidizer (i.e., air) at significantly lower feed rates per kilojoule of energy recovered, resulting in a dilute configuration (solid volume fraction, fv < 0.01). A common reactor design involves an active heating or cooling of particles, which are simultaneously reduced or oxidized as they descend under gravity, thus enabling rapid heat exchange between the particles and the heat exchanger surfaces. The movement of particles within an enclosure, combined with the quiescent or uprising hot air adjacent to the wall, presents a fascinating yet highly complex problem. When the system operates at sub-atmospheric pressure, the problem becomes even more intricate, making it nearly impossible to derive an analytical expression for convective heat transfer. Currently, there is considerable uncertainty in characterizing the total thermal duty in such dilute particle flow systems. Consequently, the effective coupling of particles with heat transfer fluid across the solid boundary poses a major challenge to achieve high HX effectiveness. A thorough understanding of the heat transfer mechanism between the solid-gas mixture and the wall is essential for the cost-effective design of heat exchangers in these systems. This study aims to investigate the heat transfer phenomena within solid-gas mixture in various configurations. Prior to experimentations, a one-dimensional hydrodynamics model of free-falling particles was coupled with a heat transfer model to assess particle entrainment in counter flow solid-gas system, focusing on the relative contributions of conduction, convection, and radiation to heat transfer in a sCO2 fluid. The results indicated that convection dominates heat transfer at lower temperatures, while radiation becomes increasingly significant at higher temperatures, contributing more than half to the overall heat transfer coefficient. To isolate the effects of radiation from convection, an independent Monte Carlo Ray Tracing study was conducted to predict the optical properties of the particle cloud, under varying solid concentration, surface emissivity and heat transfer geometry. The findings demonstrated a strong dependence of radiation absorption and transmission on solid volume fraction, and channel geometry. A test facility was developed to quantify the individual and overall heat transfer coefficients in gravity-driven particle flow systems using a tubular furnace capable of operating at temperature of up to 900oC. The results indicated that the overall heat transfer coefficient is highly dependent on particle feed rate, surface temperature, and pressure. The presence of moving particles adjacent to the heating surface in quiescent gas resulted in the wall convective coefficient 4-6 times higher than what is typically expected from natural convection in a single-phase gas. In subsequent experiments, the effect of pressure ranging from atmospheric down to vacuum conditions of 0.2 Pa was investigated. The results showed a significant drop in heat transfer performance between 1000 Pa to 10 Pa, primarily attributed to the diminishing effect of combined particle and wall convection. Additionally, the findings suggested that convection may be nearly eliminated at pressure below 10 Pa, where radiation becomes the dominant mode of heat transfer. Finally, a series of experiments were performed in a countercurrent solid-gas mixture flow, varying key parameters including particle loading ratio, particle dimeter, gas flow rates and furnace wall temperature. The overall heat transfer coefficient exhibited a nearly linear increasing trend with the rising Reynolds number of the counterflow gas indiscriminative of particle size. The effect of increasing particle feed rate on heat transfer was more pronounced for smaller particles due to their higher particle-to-wall surface area ratio. The experimental data presented here are applicable to characterizing heat transfer in the design of dilute particle systems (fv < 1%) including heat exchangers, indirect solar receivers, small-scale combustors, and furnaces used for the reduction or oxidation of metal oxide particles.