Search

Search Results

• 1. [Article] On the optimization of performance in fractal-like branching microchannel heat transfer devices

  Fractal-like branching microchannel flow networks have been found to improve wall temperature uniformity and decrease both pressure drop and flow power compared to arrays of straight microchannels. The ...

  Citation × Citation Citation Abstract Copy Title: On the optimization of performance in fractal-like branching microchannel heat transfer devices Author: Heymann, Douglas Benjamin Fractal-like branching microchannel flow networks have been found to improve wall temperature uniformity and decrease both pressure drop and flow power compared to arrays of straight microchannels. The present study seeks to maximize the benefits of fractal-like branching channels by means of a gradient-based optimization algorithm. The algorithm identifies the geometric parameters that yield the highest ratio of benefit to cost; the benefit being the heat transfer with cost being flow power. The stream-wise pressure and wall temperature distributions were determined numerically using one-dimensional models validated using experimental diagnostics and computational flow analyses. Pressure distributions were used to assess flow power, and wall temperature was used as an optimization constraint. Several geometric constraints were imposed during optimization to ensure sufficient bonding area for fabrication, to maximize convective surface area and to prevent channel overlap. Three fractal-like devices were studied and optimized: a single-phase heat sink, a two-phase heat sink and a single-phase oil-to-water heat exchanger. The flow rate of the three devices was constrained using a maximum wall temperature constraint for the single-phase heat sink, the critical heat flux for the two-phase heat sink, and a fixed cold side mass flow rate and temperature rise in the case of the heat exchanger. The optimized solutions were found to depend highly on both the geometric and flow constraints imposed. As the terminal channel width was reduced, flow power was reduced in both single-phase and two-phase heat sinks. However, the flow power was increased in the heat exchanger due to the constrained volumetric flow rate. In addition, results show that if a maximum wall temperature constraint is employed, single-phase flow outperforms two-phase flow in terms of reduction in flow power. However when constrained by a critical heat flux, two-phase networks outperform single-phase networks by up to 150% Title: On the optimization of performance in fractal-like branching microchannel heat transfer devices Author: Heymann, Douglas Benjamin Fractal-like branching microchannel flow networks have been found to improve wall temperature uniformity and decrease both pressure drop and flow power compared to arrays of straight microchannels. The present study seeks to maximize the benefits of fractal-like branching channels by means of a gradient-based optimization algorithm. The algorithm identifies the geometric parameters that yield the highest ratio of benefit to cost; the benefit being the heat transfer with cost being flow power. The stream-wise pressure and wall temperature distributions were determined numerically using one-dimensional models validated using experimental diagnostics and computational flow analyses. Pressure distributions were used to assess flow power, and wall temperature was used as an optimization constraint. Several geometric constraints were imposed during optimization to ensure sufficient bonding area for fabrication, to maximize convective surface area and to prevent channel overlap. Three fractal-like devices were studied and optimized: a single-phase heat sink, a two-phase heat sink and a single-phase oil-to-water heat exchanger. The flow rate of the three devices was constrained using a maximum wall temperature constraint for the single-phase heat sink, the critical heat flux for the two-phase heat sink, and a fixed cold side mass flow rate and temperature rise in the case of the heat exchanger. The optimized solutions were found to depend highly on both the geometric and flow constraints imposed. As the terminal channel width was reduced, flow power was reduced in both single-phase and two-phase heat sinks. However, the flow power was increased in the heat exchanger due to the constrained volumetric flow rate. In addition, results show that if a maximum wall temperature constraint is employed, single-phase flow outperforms two-phase flow in terms of reduction in flow power. However when constrained by a critical heat flux, two-phase networks outperform single-phase networks by up to 150% Close

• 2. [Article] A microchannel-based thermal management system for hydrogen storage adsorbent beds

  Hydrogen has been shown to be a promising replacement for fossil fuels for use in light duty vehicles because it is a clean, renewable and plentiful resource with a high gravimetric energy density. However, ...

  Citation × Citation Citation Abstract Copy Title: A microchannel-based thermal management system for hydrogen storage adsorbent beds Author: Steigleder, Leif J. Hydrogen has been shown to be a promising replacement for fossil fuels for use in light duty vehicles because it is a clean, renewable and plentiful resource with a high gravimetric energy density. However, in order to obtain an acceptable volumetric energy density, densification of the hydrogen is required. Adsorptive materials have been shown in the literature to increase volumetric and gravimetric storage densities. A major issue with adsorptive storage is that the adsorption process generates heat and optimal storage conditions are at temperatures below 100 K at pressures up to 50 atm. There is a need to develop heat exchanging architecture that enables adsorbents to be a viable method for hydrogen storage by managing the thermal environment of the storage tank. Based on previous modeling efforts to determine an acceptable bed module height for removal of heat via microchannel cooling plates, a thermal management system has been designed and tested capable of removing the heat of adsorption within adsorbent-filled hydrogen storage tanks. The system uses liquid nitrogen cooling to maintain tank temperatures of below 80 K at 50 atm. System studies show that the microchannel architecture offers a high cooling capacity with about a 6% displacement volume. Simulations and experiments have been conducted to evaluate the design for the cooling capacity, pressure drop, and flow distribution between and across the cooling plates, stress due to the pressurized environment, and thermal stress. Cost models have been developed to demonstrate that the system can be manufactured for a reasonable cost at high production volumes. Experimental results show that the modular system offers an acceptable cooling capacity and pressure drop with good flow distribution while adequately managing thermal stresses during operation. Title: A microchannel-based thermal management system for hydrogen storage adsorbent beds Author: Steigleder, Leif J. Hydrogen has been shown to be a promising replacement for fossil fuels for use in light duty vehicles because it is a clean, renewable and plentiful resource with a high gravimetric energy density. However, in order to obtain an acceptable volumetric energy density, densification of the hydrogen is required. Adsorptive materials have been shown in the literature to increase volumetric and gravimetric storage densities. A major issue with adsorptive storage is that the adsorption process generates heat and optimal storage conditions are at temperatures below 100 K at pressures up to 50 atm. There is a need to develop heat exchanging architecture that enables adsorbents to be a viable method for hydrogen storage by managing the thermal environment of the storage tank. Based on previous modeling efforts to determine an acceptable bed module height for removal of heat via microchannel cooling plates, a thermal management system has been designed and tested capable of removing the heat of adsorption within adsorbent-filled hydrogen storage tanks. The system uses liquid nitrogen cooling to maintain tank temperatures of below 80 K at 50 atm. System studies show that the microchannel architecture offers a high cooling capacity with about a 6% displacement volume. Simulations and experiments have been conducted to evaluate the design for the cooling capacity, pressure drop, and flow distribution between and across the cooling plates, stress due to the pressurized environment, and thermal stress. Cost models have been developed to demonstrate that the system can be manufactured for a reasonable cost at high production volumes. Experimental results show that the modular system offers an acceptable cooling capacity and pressure drop with good flow distribution while adequately managing thermal stresses during operation. Close

• 3. [Article] Enhancement and modeling of cryogenic solid-state hydrogen storage systems with a novel microchannel thermal management device

  Compact storage systems demonstrating increased hydrogen storage capacity are the key to advancing the growth of the hydrogen economy throughout the world and reducing the global dependence on fossil fuels ...

  Citation × Citation Citation Abstract Copy Title: Enhancement and modeling of cryogenic solid-state hydrogen storage systems with a novel microchannel thermal management device Author: Loeb, Christopher S. Compact storage systems demonstrating increased hydrogen storage capacity are the key to advancing the growth of the hydrogen economy throughout the world and reducing the global dependence on fossil fuels for transportation. The use of highly porous adsorbent materials for reversible hydrogen storage is a promising pathway towards achieving storage densities capable of reaching driving ranges comparable to gasoline automobiles without high-pressure compressed gas storage systems (>200 bars H₂). The focus of this effort is the enhancement of solid-state hydrogen adsorption systems through the development and inclusion of a microchannel thermal management device for efficient heat removal and uniform gas distribution in compressed beds of activated carbon and metal organic framework. The stainless steel microchannel device utilizes 250 μm walls to remove heat from the contacting adsorbent bed with liquid nitrogen coolant flow through 250 μm channel. Cryogenic (<140 K) hydrogen adsorption experiments were completed with and without liquid nitrogen cooling at pressures between 50 and 55 bars. The experiments demonstrate that the use of liquid nitrogen coolant effectively lowers the adsorbent bed temperature by removing the majority of the heat generated during the adsorption process, resulting in an 11% increase in stored H₂ gas with MOF-5 and a 3% increase with activated carbon compared to their respective non-cooled counterparts. The liquid cooled MOF-5 adsorbent bed increases average hydrogen storage throughout the experimental vessel by 14% compared to a cryo-compressed gas system at similar conditions (121 K and 55 bars). A functional development tool representing the experimental adsorption system was developed in COMSOL Multiphysics and utilized derived conservation equations and the modified Dubinin-Astakhov equation to characterization the gas flow, energy transport, and physical adsorption of supercritical hydrogen on MOF-5 adsorbent. Simulated adsorption experiments, modeled with development tool, were validated against experimental data; the predicted temperature responses at embedded thermocouple locations show a strong correlation to experimental data. The development tool may assist in the design and development of future storage systems utilizing adsorbent materials with known physical, thermal, and adsorption parameters. Title: Enhancement and modeling of cryogenic solid-state hydrogen storage systems with a novel microchannel thermal management device Author: Loeb, Christopher S. Compact storage systems demonstrating increased hydrogen storage capacity are the key to advancing the growth of the hydrogen economy throughout the world and reducing the global dependence on fossil fuels for transportation. The use of highly porous adsorbent materials for reversible hydrogen storage is a promising pathway towards achieving storage densities capable of reaching driving ranges comparable to gasoline automobiles without high-pressure compressed gas storage systems (>200 bars H₂). The focus of this effort is the enhancement of solid-state hydrogen adsorption systems through the development and inclusion of a microchannel thermal management device for efficient heat removal and uniform gas distribution in compressed beds of activated carbon and metal organic framework. The stainless steel microchannel device utilizes 250 μm walls to remove heat from the contacting adsorbent bed with liquid nitrogen coolant flow through 250 μm channel. Cryogenic (<140 K) hydrogen adsorption experiments were completed with and without liquid nitrogen cooling at pressures between 50 and 55 bars. The experiments demonstrate that the use of liquid nitrogen coolant effectively lowers the adsorbent bed temperature by removing the majority of the heat generated during the adsorption process, resulting in an 11% increase in stored H₂ gas with MOF-5 and a 3% increase with activated carbon compared to their respective non-cooled counterparts. The liquid cooled MOF-5 adsorbent bed increases average hydrogen storage throughout the experimental vessel by 14% compared to a cryo-compressed gas system at similar conditions (121 K and 55 bars). A functional development tool representing the experimental adsorption system was developed in COMSOL Multiphysics and utilized derived conservation equations and the modified Dubinin-Astakhov equation to characterization the gas flow, energy transport, and physical adsorption of supercritical hydrogen on MOF-5 adsorbent. Simulated adsorption experiments, modeled with development tool, were validated against experimental data; the predicted temperature responses at embedded thermocouple locations show a strong correlation to experimental data. The development tool may assist in the design and development of future storage systems utilizing adsorbent materials with known physical, thermal, and adsorption parameters. Close