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Fisher, Timothy S.

Publications and source records attributed to Fisher, Timothy S..

7 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2024

Local Thermochemical Mechanisms in Direct Solar Graphite Synthesis from Methane

Methane pyrolysis is known to produce hydrogen and solid carbon in a variety of thermal processes. However, the generated carbon product typically belongs to a low-value amorphous type. Here, we elucidate the thermochemical mechanisms of a reaction that produces high-quality graphite via direct solar methane pyrolysis on a porous substrate. By comparing graphite deposition rates and local reaction zone temperatures of exposed and shadowed regions from the same experiment, we clarify the effects of thermolysis and photolysis in this emission-free process that both decarbonizes a fuel and produces a critical material for the sustainable energy transition.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2023

Thermohydraulic experiments on a supercritical carbon dioxide–air microtube heat exchanger

Heat exchangers are critical components in supercritical CO 2 Brayton cycles and provide necessary heat input, recovery, and dissipation. Tubular heat exchangers with unconventionally small tube sizes (tube diameters less than 5 mm) are promising components for supercritical CO 2 cycles and potentially provide excellent structural stability with wide scope of application. This paper provides essential design and fabrication guidelines for a compact shell-and-tube heat exchanger with microtubes (with an inner diameter equal to 1.75 mm). A heat exchanger test rig is used to evaluate the thermohydraulic performance of this heat exchanger with supercritical CO 2 and air as working fluids. Thermohydraulic data are reported for more than forty sets of experiments with varying Reynolds numbers for shell and tube flows. Critical performance metrics are calculated from the data and compared with predictions from a previously described numerical model. The average deviations between the experimental and model results fall within 10% for all critical metrics. This excellent agreement validates the numerical model for supercritical CO 2 heat exchanger optimization and scale-up.

42 ENGINEERING↗
DOE OSTI2022

Technoeconomic optimization of superalloy supercritical CO 2 microtube shell-and-tube-heat exchangers

Heat exchangers are critical components of aerospace systems and improve efficiency of operation by providing waste heat recovery. Supercritical CO has emerged as a promising candidate working fluid due to its potential in increasing the power density of heat exchangers. In this paper, a generalized costing model is developed to estimate the capital costs incurred to manufacture microtube shell-and-tube heat exchangers. This model is utilized in conjunction with an accurate and efficient 2D numerical shell-and-tube heat exchanger performance prediction model to conduct optimization studies with two key objectives – minimization of cost and maximization of heat exchanger power density – on supercritical CO microtube heat exchangers utilizing superalloy Haynes 282 as the solid material. A methodology is then demonstrated to optimize these heat exchangers using Particle Swarm Optimization for aerospace applications and highly compact and cost-effective optimal designs with power density around 20 and cost per conductance less than 5 $ ∙ K/W are obtained.

42 ENGINEERING↗
DOE OSTI2022

Modeling of Supercritical CO2 Shell-and-Tube Heat Exchangers Under Extreme Conditions. Part I: Correlation Development

Abstract High-temperature supercritical CO2 Brayton cycles are promising possibilities for future stationary power generation and hybrid electric propulsion applications. Heat exchangers are critical components in supercritical CO2 thermal cycles and require accurate correlations and comprehensive performance modeling under extreme temperatures and pressures. In this paper (Part I), new Colburn and friction factor correlations are developed to quantify shell-side heat transfer and friction characteristics of flow within heat exchangers in the shell-and-tube configuration. Using experimental and computational fluid dynamics (CFD) data sets from existing literature, multivariate regression analysis is conducted to achieve correlations that capture the effect of multiple critical geometric parameters. These correlations offer superior accuracy and versatility as compared to previous studies and predict the thermohydraulic performance of about 90% of the existing experimental and CFD data within ±15%. Supplementary thermohydraulic performance data are acquired from CFD simulations with supercritical CO2 as working fluid to validate the developed correlations and demonstrate its capability to be applied to supercrtical CO2 heat exchangers.

Engineering↗
DOE OSTI2022

Modeling of Supercritical CO2 Shell-and-Tube Heat Exchangers Under Extreme Conditions: Part II: Heat Exchanger Model

Abstract Heat exchangers play a critical role in supercritical CO2 Brayton cycles by providing necessary waste heat recovery. Supercritical CO2 thermal cycles potentially achieve higher energy density and thermal efficiency operating at elevated temperatures and pressures. Accurate and computationally efficient estimation of heat exchanger performance metrics at these conditions is important for the design and optimization of sCO2 systems and thermal cycles. In this paper (Part II), a computationally efficient and accurate numerical model is developed to predict the performance of shell-and-tube heat exchangers (STHXs). Highly accurate correlations reported in Part I of this study are utilized to improve the accuracy of performance predictions, and the concept of volume averaging is used to abstract the geometry and reduce computation time. The numerical model is validated by comparison with computational fluid dynamics (CFD) simulations and provides high accuracy and significantly lower computation time compared to existing numerical models. A preliminary optimization study is conducted and the advantage of using supercritical CO2 as a working fluid for energy systems is demonstrated.

Engineering↗