Niedbalski, Nicholas Paul (2017-08). Experimental and Theoretical Investigation of a Multiphase Reaction in a Compact Heat Exchanger-Reactor (HEX Reactor). Doctoral Dissertation. Thesis uri icon

abstract

  • Heat transfer enhancement research has long been concerned with keeping pace to the rapidly increasing cooling demands of high performance electronics. In addition to requiring the removal of higher heat fluxes as device miniaturization continues, the operating temperature limits remain essentially the same. The rapid removal of large quantities of low quality heat presents a formidable challenge to thermal systems engineers. In such applications, single-phase forced convection thermal management (TM) schemes are no longer adequate. High energy density thermophysical phase change materials, such as the boiling of water, have proven capable of handling high heat fluxes, but suffer the drawback of requiring temperatures that are outside the acceptable range for high-power electronics cooling. Recently, the use of high energy density endothermic chemical reactions as an alternative to thermophysical phase change materials has shown promise. One particularly attractive reaction for thermal management purposes is the endothermic decomposition of ammonium carbamate (AC), due to both its high energy density and comparatively low reaction temperatures that are amenable to electronics cooling. In this study, we propose to facilitate the development of thermal management systems based on AC (or similar reactions) by a combined experimental and theoretical approach. The objective is to elucidate the combined effect of heat transfer, mass transfer, momentum transfer, and chemical kinetics on the thermal management capabilities of a heat exchanger-chemical reactor (HEX reactor) utilizing an AC-heat transfer fluid slurry. A model to describe the reaction kinetics in the presence of a liquid iii solvent, which is presently lacking the literature, is critical to the design and understanding of thermochemical reaction-based TM systems. Further, this model must be rooted in a sound theoretical and empirical basis. Currently, there are no published experimental chemical kinetics data for the decomposition of AC in a heat transfer fluid. A systematic investigation of the reaction kinetics within the range of temperatures typical for electronics cooling was conducted to obtain real-time calorimetric and species concentration data. Fundamental insights gained from the experimental chemical kinetics study were used to develop a general reaction model framework for AC decomposition in the presence of a solvent. This model serves as the source term in the thermal energy conservation equation, which in turn is required to model and predict HEX reactor performance. The reaction model parameters were estimated from the experimental results using numerical optimization and validated at temperature between 55?C and 70?C, concentrations between 25 g/L an 50 g/L, and particle sizes between 800?m and 100?m. A 1-dimensional, multi-phase HEX reactor model was developed, incorporating the parameterized reaction model and validated against data from the literature. In this report, we demonstrate the experimental findings and the subsequent model development, comparison to experimental trends, validation, and comparison to published data for HEX reactor thermal performance with AC decomposition. Critical factors of design interest are identified and explored to improve fundamental understanding of the complex thermal, hydrodynamic, and chemical phenomena governing AC HEX reactor performance.
  • Heat transfer enhancement research has long been concerned with keeping pace to the rapidly increasing cooling demands of high performance electronics. In addition to requiring the removal of higher heat fluxes as device miniaturization continues, the operating temperature limits remain essentially the same. The rapid removal of large quantities of low quality heat presents a formidable challenge to thermal systems engineers. In such applications, single-phase forced convection thermal management (TM) schemes are no longer adequate. High energy density thermophysical phase change materials, such as the boiling of water, have proven capable of handling high heat fluxes, but suffer the drawback of requiring temperatures that are outside the acceptable range for high-power electronics cooling. Recently, the use of high energy density endothermic chemical reactions as an alternative to thermophysical phase change materials has shown promise. One particularly attractive reaction for thermal management purposes is the endothermic decomposition of ammonium carbamate (AC), due to both its high energy density and comparatively low reaction temperatures that are amenable to electronics cooling.

    In this study, we propose to facilitate the development of thermal management systems based on AC (or similar reactions) by a combined experimental and theoretical approach. The objective is to elucidate the combined effect of heat transfer, mass transfer, momentum transfer, and chemical kinetics on the thermal management capabilities of a heat exchanger-chemical reactor (HEX reactor) utilizing an AC-heat transfer fluid slurry. A model to describe the reaction kinetics in the presence of a liquid iii solvent, which is presently lacking the literature, is critical to the design and understanding of thermochemical reaction-based TM systems. Further, this model must be rooted in a sound theoretical and empirical basis. Currently, there are no published experimental chemical kinetics data for the decomposition of AC in a heat transfer fluid.

    A systematic investigation of the reaction kinetics within the range of temperatures typical for electronics cooling was conducted to obtain real-time calorimetric and species concentration data. Fundamental insights gained from the experimental chemical kinetics study were used to develop a general reaction model framework for AC decomposition in the presence of a solvent. This model serves as the source term in the thermal energy conservation equation, which in turn is required to model and predict HEX reactor performance. The reaction model parameters were estimated from the experimental results using numerical optimization and validated at temperature between 55?C and 70?C, concentrations between 25 g/L an 50 g/L, and particle sizes between 800?m and 100?m. A 1-dimensional, multi-phase HEX reactor model was developed, incorporating the parameterized reaction model and validated against data from the literature.

    In this report, we demonstrate the experimental findings and the subsequent model development, comparison to experimental trends, validation, and comparison to published data for HEX reactor thermal performance with AC decomposition. Critical factors of design interest are identified and explored to improve fundamental understanding of the complex thermal, hydrodynamic, and chemical phenomena governing AC HEX reactor performance.

publication date

  • August 2017