Fletcher Miller

Characterizing fingering flamelets using the logistic model

We apply the logistic equation to a class of flame spread that occurs in near-extinction, weakly convective environments such as microgravity or vertically confined spaces. The flame under these conditions breaks into numerous ‘flamelets’ which form a Turing-type reaction–diffusion fingering pattern as they spread across the fuel. Flamelets are steady, based on flame spread measurements, and reach a critical state near extinction where a spread rate plateau reflects a critical heat flux for ignition. Our analysis of experiments performed in a buoyancy-reducing, vertically confined flow tunnel reveals the presence of statistical order in the seemingly random patterns. Flamelets as a group form a dynamic population that interacts competitively for the limited available oxygen. Flamelets bifurcate and extinguish individually, but as a whole, the group maintains a stable size. Flamelets show an exponentially decaying lifetime and a uniform pattern of dispersion. We utilize the continuous logistic model with a time lag to describe the flamelet population growth and fluctuation around a stable population characterized by the carrying capacity based on environmental limitations. We discuss how the physics of the system is expressed through the model parameters.

Microgravity Flame Spread in Exploration Atmospheres: Pressure, Oxygen, and Velocity Effects on Opposed and Concurrent Flame Spread

Microgravity tests of flammability and flame spread were performed in a low-speed flow tunnel to simulate spacecraft ventilation flows. Three thin fuels were tested for flammability (Ultem 1000 (General Electric Company), 10 mil film, Nomex (Dupont) HT90-40, and Mylar G (Dupont) and one fuel for flame spread testing (Kimwipes (Kimberly-Clark Worldwide, Inc.). The 1g Upward Limiting Oxygen Index (ULOI) and 1g Maximum Oxygen Concentration (MOC) are found to be greater than those in 0g, by up to 4% oxygen mole fraction, meaning that the fuels burned in 0g at lower oxygen concentrations than they did using the NASA Standard 6001 Test 1 protocol. Flame spread tests with Kimwipes were used to develop correlations that capture the effects of flow velocity, oxygen concentration, and pressure on flame spread rate. These correlations were used to determine that over virtually the entire range of spacecraft atmospheres and flow conditions, the opposed spread is faster, especially for normoxic atmospheres. The correlations were also compared with 1g MOC for various materials as a function of pressure and oxygen. The lines of constant opposed flow agreed best with the 1g MOC trends, which indicates that Test 1 limits are essentially dictated by the critical heat flux for ignition. Further evaluation of these and other materials is continuing to better understand the 0g flammability of materials and its effect on the oxygen margin of safety.

Three-Dimensional Fluid Dynamics and Radiative Heat Transfer Modeling of a Small Particle Solar Receiver

We present an investigation of the effects of the solar irradiation and mass flow conditions on the behavior of a Small Particle Solar Receiver employing our new, three-dimensional coupled fluid flow and radiative heat transfer model. This research expands on previous work conducted by our group and utilizes improved software with a set of new features that allows performing more flexible simulations and obtaining more accurate results. For the first time, it is possible not only to accurately predict the overall efficiency and the wall temperature distribution of the solar receiver, but also to determine the effect on the receiver of the window, the outlet tube, real solar irradiation from a heliostat field, non-cylindrical geometries and 3-D effects. This way, a much better understanding of the receiver’s capabilities is obtained. While the previous models were useful to observe simple trends, this new software is flexible and accurate enough to eventually act as a design and optimization tool for the actual receiver.The solution procedure relies on the coupling of the CFD package ANSYS Fluent to our in-house Monte Carlo Ray Trace (MCRT) software. On the one hand, ANSYS Fluent is utilized as the mass-, momentum- and energy-equation solver and requires the divergence of the radiative heat flux, which constitutes a source term of the energy equation. On the other hand, the MCRT software calculates the radiation heat transfer in the solar receiver and needs the temperature field to do so. By virtue of the coupled nature of the problem, both codes should provide feed-back to each other and iterate until convergence. The coupling between ANSYS Fluent and our in-house MCRT code is done via User-Defined Functions.After developing the mathematical model, setting up and validating the software, and optimizing the coupled solution procedure, the receiver has been simulated under fifteen different solar irradiation and mass flow rate cross combinations. Among other results, the behavior of the receiver at different times of the day and the optimum mass flow rate as a function of the solar thermal input are presented. On an average day, the thermal efficiency of the receiver is found to be over 89% and the outlet temperature over 1250 K at all times from 7:30 AM to 4:00 PM (Albuquerque, NM) by properly adapting the mass flow rate. The origin of the losses and how to improve the efficiency of the Small Particle Solar Receiver are discussed as well.Copyright

Coupled Fluid Flow and Radiation Modeling of a Cylindrical Small Particle Solar Receiver

This research expands on previous work by coupling the in-house Monte Carlo Ray Trace (MCRT) radiation model with the more sophisticated fluid dynamics modeling capabilities of ANSYS FLUENT. This allows for the inclusion of more realistic inlet and outlet geometries in the receiver, as well as a turbulence model and much finer grid sizing. Taken together, these features give a more complete picture of the heat transfer, mixing, and temperature profiles within the receiver than previous models. This flow solution is coupled to the MCRT code, by using the in-house MCRT radiation solver to provide the source term of the energy equation. The temperature data output from FLUENT is then fed back into the FORTRAN MCRT code, via a User Defined Function written in C#, and the two models iterate until convergence. The solar input has been modified from the previous model to provide a Gaussian fit to a calculated flux distribution, which is more realistic than a uniform flux. Initial results for a 5 MW solar input agree with the trend identified in Ruther’s work regarding the influence of particle mass loading on heating in the receiver. The maximum outlet temperature reached is 1430K, which is on target for driving a Brayton cycle gas turbine. Cylinder wall temperatures are consistently below those of the gas boundary layer, and significantly below the maximum gas temperature in the receiver cavity.Copyright

Thermodynamic Cycles for a Small Particle Heat Exchange Receiver Used in Concentrating Solar Power Plants

Gas-cooled solar receivers for concentrating solar power plants are capable of providing high temperature, pressurized gas for electrical power generation via a Brayton cycle. This can be accomplished by expanding hot, pressurized gas directly through a turbine, or through using a heat exchanger to indirectly heat pressurized air. Gas-cooled receivers can be divided into two basic technologies. In tube based solar receivers, thermal energy is transferred to air through convection with the heated tube wall. This limits receiver efficiency since the tube wall needs to be substantially hotter than the gas inside due to the relatively poor gas heat transfer coefficient. In volumetric receivers, solar energy is absorbed within a volume, rather than on a surface. The absorption volume can be filled with ceramic foam, wires, or particles to act as the absorbing medium. In a small particle heat exchange receiver, for example, sub-micron sized particles absorb solar radiation, and transfer this energy as heat to a surrounding fluid. This effectively eliminates any thermal resistance, allowing for higher receiver efficiencies. However, mechanical considerations limit the size of volumetric, pressurized gas-cooled receivers. In order to solve this problem, several thermodynamic cycles have been investigated, each of which is motivated by key physical considerations in volumetric receivers. The cyclic efficiencies are determined by a new MATLAB code based on previous Brayton cycle modeling conducted by Sandia National Laboratories. The modeling accounts for pressure drops and temperature losses in various components, and parameters such as the turbine inlet temperature and pressure ratio are easily modified to run parametric cases. The performance of a gas-cooled solar receiver is largely a function of its ability to provide process gas at a consistent temperature or pressure, regardless of variations in solar flux, which can vary due to cloud transients or apparent sun motion throughout the day. Consistent output can be ensured by combusting fuel within the cycle, effectively making a solar/fossil fuel hybrid system. Several schemes for hybridization with natural gas are considered here, including externally fired concepts and combined receiver/combustor units. Because the efficiency of hybridized cycles is a function of the solar thermal input, the part load behavior of the recuperated cycle is examined in depth. Finally, a brief report of economic costs inherent to solar powered gas turbine engines is given. Possibilities for the future of solar power gas turbine power plants are discussed, with key issues regarding thermal storage techniques.Copyright

Dome Window Geometry for a Large Scale Solar Receiver

Windows are being evaluated for use in some high temperature solar receivers to reduce radiative and convective losses. The design process of a 1.7 meter diameter quartz dome window is explained to arrive at a window geometry able to maintain acceptable stresses when exposed to pressure differentials. The dome must be able to withstand the operational differential pressures of 0.5 MPa where the efficiency of the solar receiver/power cycle is maximized, and maximum temperatures upwards to 800°C may be observed. Brittle materials like glass need the tensile stresses to be reduced or eliminated to maximize the reliability of the dome window. However, glass does not possess a consistent characteristic strength and it is dependent on the flaw size. The dome mount is critical to maintaining an environmental seal, but careful attention must be taken in the glass-metal interface to minimize tensile bending stresses that can cause a catastrophic or rapid failure.A method to characterize the strength of the quartz dome is discussed and aides in determining the maximum design stresses allowable during operation of the solar receiver. To determine an accurate model of the dome stress, a statistical analysis based on strength data has been carried out using the Weibull failure probability method. Finite Element Analysis (FEA) analysis of the dome is discussed, and a design trade study of the dome window geometry (ranging from a shallow angle to a full hemisphere) is presented. The combination of these perspectives will give insight on the process to design a glass dome window for this challenging environment and to predict the reliability.Copyright

Optical Analysis of a Window for Solar Receivers Using the Monte Carlo Ray Trace Method

Concentrated solar power (CSP) systems use heliostats to concentrate solar radiation in order to produce heat, which drives a turbine to generate electricity. We, the Combustion and Solar Energy Laboratory at San Diego State University, are developing a new type of receiver for power tower CSP plants based on volumetric absorption by a gas-particle suspension. The radiation enters the pressurized receiver through a window, which must sustain the thermal loads from the concentrated solar flux and infrared reradiation from inside the receiver. The window is curved in a dome shape to withstand the pressure within the receiver and help minimize the stresses caused by thermal loading. It is highly important to estimate how much radiation goes through the window into the receiver and the spatial and directional distribution of the radiation. These factors play an important role in the efficiency of the receiver as well as window survivability.Concentrated solar flux was calculated with a computer code called MIRVAL from Sandia National Laboratory which uses the Monte Carlo Ray Trace (MCRT) method. The computer code is capable of taking the day of the year and time of day into account, which causes a variation in the flux. Knowing the concentrated solar flux, it is possible to calculate the solar radiation through the window and the thermal loading on the window from the short wavelength solar radiation. The MIRVAL code as originally written did not account for spectral variations, but we have added that capability.Optical properties of the window such as the transmissivity, absorptivity, and reflectivity need to be known in order to trace the rays at the window. A separate computer code was developed to calculate the optical properties depending on the incident angle and the wavelength of the incident radiation by using data for the absorptive index and index of refraction for the window (quartz) from other studies and vendor information. This method accounts for regions where the window is partially transparent and internal absorption can occur.A third code was developed using the MCRT method and coupled with both codes mentioned above to calculate the thermal load on the window and the solar radiation that enters the receiver. Thermal load was calculated from energy absorbed at various points throughout the window. In our study, window shapes from flat to concave hemispherical, as well as a novel concave ellipsoidal window are considered, including the effect of day of the year and time of the day.Copyright

Developing the Small Particle Heat Exchange Receiver for a Prototype Test

The concept of absorbing concentrated solar radiation volumetrically, rather than on a surface, is being researched by several groups with differing designs for high temperature solar receivers. The Small Particle Heat Exchange Receiver (SPHER), one such design, is a gas-cooled central receiver capable of producing pressurized air in excess of 1100 C designed to be directly integrated into a Brayton-cycle power block to generate electricity from solar thermal power. The unique heat transfer fluid used in the SPHER is a low-density suspension of carbon nano-particles (diameter ∼ 200 nm) to absorb highly concentrated solar radiation directly in a gas stream, rather than on a fixed absorber like a tube or ceramic foam. The nano-particles are created on-demand by pyrolyzing a small flow of natural gas in an inert carrier gas just upstream of the receiver, and the particle stream is mixed with air prior to injection into the receiver. The receiver features a window (or multiple windows, depending on scale) on one end to allow concentrated sunlight into the receiver where it is absorbed by the gas-particle suspension prior to reaching the receiver walls. As they pass through the receiver the carbon nano-particles oxidize to CO2 resulting a clear gas stream ready to enter a downstream combustor or directly into the turbine. The amount of natural gas consumed or CO2 produced is miniscule (1–2%) compared to what would be produced if the natural gas were burned directly to power a gas turbine.The idea of a SPHER, first proposed many years ago, has been tested on a kW scale by two different groups. In the new work, the engineering for a multi-MW SPHER is reported. An in-house Monte Carlo model of the radiation heat transfer in the gas-particle mixture has been developed and is coupled to FLUENT to perform the fluid dynamic calculations in the receiver. Particle properties (size distribution and complex index of refraction) are obtained experimentally from angular scattering and extinction measurements of natural gas pyrolysis in a lab-scale generator, and these are corroborated using image analysis of Scanning Electron Microscope (SEM) pictures of particles captured on a filter. A numerical model of the particle generator has been created to allow for scale-up for a large receiver. We have also designed a new window for the receiver that will allow pressurized operation up to 10 bar with a 2 m diameter window. Recent progress on overcoming the engineering challenges in developing this receiver for a prototype test is reported.Copyright

Modeling of a High-Temperature Latent Heat Thermal Storage Module for Brayton Cycle Applications

Concentrating solar power (CSP) plants with thermal energy storage offer several advantages to plants without storage. Thermal energy storage (TES) allows CSP plants to produce power for longer periods of time each day, making them produce energy more like traditional, fossil fuel power plants. TES also gives the ability to time shift production of energy to times of peak demand, allowing the plant to sell the energy when prices are highest. A CSP plant with storage can increase turbine performance and reach higher levels of efficiency by load leveling production and can remain productive through cloud transients.Power tower CSP plants are capable of producing extremely high temperatures, as they have the ability to oversize their solar field and achieve a greater concentration ratio. Studies have been conducted on variable working fluids, leading to higher working temperatures. This theoretically allows power towers to use more efficient, higher temperature cycles including the recuperated air Brayton cycle, although none currently exist on a commercial scale. This research focuses on developing a model of a high temperature TES system for use with an air Brayton cycle for a power tower CSP plant.In this research we model one module of a latent heat TES system designed to meet the thermal needs of a recuperated Brayton engine of 4.6 MWe capacity for six hours. A metal alloy, aluminum-silicide (AlSi), is considered as the phase change medium. The storage tank is approximately 161 m3, or a cylinder with a 5 m diameter that is 8 m tall filled with AlSi with several air pipes throughout the volume. We model the volume around a single pipe in a 2-D cylindrical coordinate system, for a module size of 0.2 m in diameter and 8 m long. The advantages of using AlSi alloys is that they have variable melting temperatures depending on the relative concentration of the two metals, from 577 C for the eutectic composition of 12.6% silicon to 1411 C for 100% silicon. This attribute is taken advantage of by the TES model as the composition of the AlSi alloy will vary axially. This will allow a cascaded type storage system within one tank and with one material. The use of FLUENT to model this problem is first validated by several analytical solutions including Neumann’s exact solution for a one-dimensional Cartesian geometry and the Quasi-Steady Approximation in a 1-D cylindrical geometry. The model developed will establish charge/discharge times for the storage system, round-trip efficiency of the system, ability of the system to meet the demand of the Brayton cycle, and the validity of using off-eutectic metal alloys in a cascade as a latent heat TES medium.Copyright

A study of the effectiveness of a narrow channel apparatus in simulating microgravity flame spread over thin fuels

NASA’s current flammability testing method for non-metallic solids is NASA-STD-(I)-6001A Test 1. Materials that allow for an upward flame propagation of six inches or more fail the flammability test. The flames in the Earth-based Test 1 are dominated by the upward-flowing buoyant gases, and this is not representative of actual flame behavior in microgravity, where there are no buoyant effects on flames. Scientists at NASA have shown that by spatially confining a horizontally spreading flame in the vertical direction, buoyant forces can be minimized in an Earth-based flame spread test. Results from flammability tests conducted in San Diego State University’s Narrow Channel Apparatus (SDSU NCA) closely match results from NASA’s Narrow Channel Apparatus at 1 atmosphere of pressure and 21% oxygen. The advantage of the SDSU NCA is that it not only minimizes buoyant effects, but it also allows flammability tests to be performed at normoxic equivalent atmospheres that more closely match future spacecraft cabin atmospheres. Normoxic conditions are achieved in the SDSU NCA by varying the total pressure, opposed flow oxidizer velocity, and oxygen concentration in the test channel. In our previous research we measured the flame spread rate across Whatman 44 ashless filter paper at total pressures from .27 to 1.0 atm and oxygen mole fractions from 0.77 to 0.21 respectively, along the normoxic curve for a gap height of 5 mm above and below the thin fuel. Significant differences in the flame spread rate were found, with much higher spread rates at the low pressure/high oxygen concentrations. This paper extends that work by comparing flame spread results from flammability tests conducted on Kimwipes ® in the SDSU NCA and in true microgravity in NASA’s Zero-Gravity Research Facility at several points along the normoxic curve. Also, a scaling analysis is conducted to study the effect of pressure on certain flame length scales.