Ryan Anderson

Study of Convection Heat Transfer in a Very High Temperature Reactor Flow Channel: Numerical and Experimental Results

Abstract Very high temperature reactors (VHTRs) with helium-cooled prismatic cores are one type of Generation IV gas-cooled reactors proposed for implementation in next-generation nuclear power plants. To contribute to the VHTR development, a high-temperature/high-pressure test facility has been constructed and used to investigate the convection heat transfer of gaseous coolants. This test facility consisted of a single flow channel with a diameter of 16.8 mm in a graphite column with a length of 2.7 m (9 ft) equipped with four 2.3-kW heaters. Convection heat transfer experiments were conducted with air, nitrogen, and helium for inlet Reynolds number (Re) values ranging from 500 to 70000. Extensive three-dimensional numerical modeling was also performed using a commercial finite element package, COMSOL Multiphysics. The numerical results agreed with the convection heat transfer data, with maximum error percentages under 15%. Based on this agreement, important information was extracted from the numerical model regarding the axial and radial velocity and temperature profiles as well as the axial variations in gas properties. This work examines deteriorated turbulent heat transfer and flow laminarization for a wide range of Re, including laminar, transition, and turbulent flows.

An Analysis of Two-Phase Flow Pressure Drop in Operating Proton Exchange Membrane Fuel Cell Channels With the Lockhart-Martinelli Approach

Proton exchange membrane fuel cells (PEMFCs) are considered one of the most promising alternatives for the automotive industry owing to their high energy efficiency, zero emission at the vehicle use stage, and low temperature operation. Water as a byproduct plays a complex role in fuel cell operation. In particular, the inevitable occurrence of liquid water leads to gas-liquid two-phase flows in various components of PEMFCs including flow channels of which diameters range from micrometers to millimeters. In conventional minichannels and microchannels, the Lockhart-Martinelli (LM) approach has been employed to predict the two-phase pressure drop of gas-liquid systems. This approach has previously been updated by our group to more accurately reflect the introduction of liquid water into the flow channels of a PEMFC i.e. from a porous media perpendicular to the gas flow. Importantly, the LM method normalizes the data independent of the flow field design and operating conditions like temperature, pressure, and relative humidity. This paper analyzes the increasing amount of experimental data on two-phase flow pressure drops/two-phase flow multipliers in the literature with these approaches. The focus is the cathode side (therefore an air/water system), and data is collected from multiple research groups using active fuel cells (electrochemically produced water). The traditional LM approach greatly under-predicts the two-phase pressure drop at low current densities. However, the analysis is applied over a range of current densities, and it better predicts results at higher current densities (>600 mA cm−2). Literature correlations for the Chisholm parameter C, a flow regime dependent parameter in the LM equation, have been proposed for non-active (external water injection) fuel cells but do not match the results from operating fuel cells. C is shown here to vary with current density, flow stoichiometry (gas velocity), gas diffusion layer, and slightly with relative humidity.Copyright

Characterization of gas-liquid two-phase flow in a proton exchange membrane fuel cell

This thesis explores two-phase flow phenomena relevant to water management in PEM fuel cells. Particularly, pressure drop hysteresis is explored in depth, which occurs when the gas and liquid flow rates are increased and decreased along a set path but exhibit different pressure drops. The hysteresis effect is explored here experimentally in three studies: non-operating cold model to study hydrodynamics, non-operating hot model at fuel cell operating conditions to study increasingly relevant hydrodynamics, and an operating study to explore pressure drop hysteresis in an active cell. This is the first time pressure drop hysteresis has been studied in a PEM fuel cell. A specially designed visualization fuel cell, allowing for observation into the cathode flow field channels, is utilized to further understand these results. The pressure drop hysteresis occurs because liquid water accumulates in the cathode flow channels during the descending approach. The cathode air stoichiometry and temperature play a major role, as lower stoichiometries and lower temperatures lead to more water accumulation in the channels, which increases the hysteresis problem. The gas diffusion layer is not a main parameter affecting pressure drop hysteresis. Additionally, several other variables are studied through the three experimental setups to understand the hysteresis behavior. This thesis then examines anode water removal (AWR) as a diagnostic tool to determine maximum fuel cell performance in the absence of mass transfer limitations on the cathode. By exacerbating cathode flooding and using a variety of cathode GDLs, large voltage increases occur through the AWR process when the cathode GDL is under flooding conditions. Larger voltage gains occur during the AWR process with the use of GDLs without an MPL when the cathode gas stream is fully humidified. Both studies, pressure drop hysteresis and AWR, improve overall fuel cell performance by better understanding water management in PEM fuel cells. Understanding the pressure drop hysteresis is important to limit the parasitic power losses associated with higher pressure drops, and AWR is a novel tool researchers can use to evaluate new GDLs in terms of their ability to prevent voltage losses due to flooding.

An Updated Two-Phase Flow Regime Map in Active PEM Fuel Cells Based on a Force Balance Approach

The water balance in proton exchange membrane (PEM) fuel cells still remains a topic of much investigation in order to maintain satisfactory cell performance. One specific water management issue relates to the gas-liquid flows that occur when water enters the reactant flow field channels, which are typically microchannels or minichannels. Due to its unique water introduction, the Lockhart-Martinelli (LM) approach has been revised for its applicability in predicting the two-phase pressure drop in these channels where water emerges from a gas diffusion layer perpendicular to the direction of gas flow. In the revised LM approach, the Chisholm parameter C is found not to vary strongly as a function of key fuel cell operating variables (relative humidity, temperature, materials, gas stoichiometry), whereas it does vary as a function of flow regime and current density. A new flow regime map was proposed based on all pressure drop data collected from active fuel cells, where an accumulating flow regime is presented in addition to single-phase, film/droplet, and slug flow. The proposed accumulating regime is linked to water droplet dynamics, namely, water droplet emergence, growth, and detachment. A force balance approach shows when detachment will occur, which clarifies the bounds of the accumulating regime in terms of superficial gas velocity (gas stoichiometry ratio) and liquid velocity (current density). The balance considers different wetting scenarios in the channels and a range of superficial velocities of importance to PEM fuel cells.Copyright

Forced Convection Heat Transfer of a Phase Change Material (PCM) Nanoemulsion

Phase Change Materials (PCM) are suitable for use in Thermal Energy Storage (TES) systems as they can store and release both sensible heat and latent heat during phase change. This investigation examines the thermophysical properties and heat transfer properties of a beeswax nanoemulsion during forced convection in a circular tube. First, the beeswax nanoemulsion was synthesized using surfactants and water, which possesses a relatively low viscosity to enhance pumpability, as well as a high beeswax percentage by mass for greater latent heat storage capacity. The test section was a circular stainless steel tube, 11.3 mm in diameter and heated uniformly using an Ohmic heating method. To determine the heat transfer coefficient, the inlet and exit nanoemulsion temperatures and tube wall temperatures were measured at several axial locations. The forced convection heat transfer coefficient results were first compared to water in order to verify the setup accuracy as well as the degree of success of the PCM in heat storage ability. The experimental results indicate suitable heat transfer coefficients for a stable beeswax nanoemulsion, making it a potential candidate for charging and discharging thermal energy in thermal storage applications.Copyright

Thermal Energy Storage for the Supercritical CO2 Brayton Cycle

This paper examines the operation of a simple sensible thermal energy storage (TES) unit for use in concentrated solar power (CSP) plant applications using supercritical CO2 (sCO2) as the heat transfer fluid. The heat transfer characteristics of the system are described and it is shown that an advancing heat front, with a very high temperature gradient, is achieved through proper design. Typical charge and discharge times of 6 hours are studied to show how this method can be used in practical applications. It is shown that the TES can be effectively matched to a conceptual CSP plant to allow it to operate at night or during periods of reduced sunlight.Copyright