Thomas M. Kiehne

Use of Fractal Geometry to Model Turbulent Combustion in SI Engines

Abstract Use of fractal geometry to model the effects of turbulence on flame propagation in an engine is explored using a quasidimensional, 4-stroke, homogeneous charge, SI engine code. This application of fractal geometry requires a new interpretation of the effect of turbulence on the combustion process in an engine. Specifically, flame wrinkling, rather than entrainment, is assumed to be the dominant effect of turbulence on the combustion process. Various simplifications are made in the formulation of the engine model to allow this fractal technique to be investigated as expeditiously as possible. Model predictions are compared to experimental data from an engine with an axisymmetric pancake-shaped combustion chamber. The sensitivity of the model predictions to the fractal dimension, to the effects of flame stretch, and to the ratio of maximum-to-minimum flame wrinkling scales is investigated. It is shown that the predicted initial rate of pressure rise is a strong function of the fractal dimension but...

The significance of intermediate hydrocarbons during wall quench of propane flames

A one-dimensional model is used to study end-on wall quench using a detailed chemical kinetics mechanism for propane. Previous models using detailed chemical kinetics mechanisms for methane, methanol, and acetylene revealed that intermediate hydrocarbons exist at much lower levels than unreacted fuel molecules during quench, and thus led to the general conclusion that one-step global chemistry (which accounts only for the rate of disappearance of the fuel) is adequate for studying hydrocarbon evolution during wall quench. However, these fuels are extremely simple, low molecular weight molecules with very limited paths available for forming intermediate hydrocarbons. In the present study, wall quench is studied for propane-air mixtures at equivalence ratios of 0.9, 1.0, and 1.1; pressures of 1, 10, and 40 atmospheres; and wall temperatures of 400 and 500 K. It is shown that intermediate hydrocarbons exist at higher levels and at greater distances from the wall during quench than unreacted fuel. Furthermore, the intermediate hydrocarbons are oxidized less rapidly and persist at significant levels much longer after quench. The persistence of the intermediate hydrocarbons is aggravated by lower wall temperatures, lower pressures, and equivalence ratios both greater than and less than stoichiometric. At lower pressures, the rate of oxidation of the intermediate hydrocarbons is slowed to an even greater extent than is the fuel oxidation rate. The conclusion that intermediate hydrocarbons contribute significantly to wall quench hydrocarbon evolution indicates that one-step global chemistry is inadequate for studying turbulent wall quench, bulk quench, and crevice volume quench of higher hydrocarbon fuels and thus casts doubt on the use of the results of previous theoretical wall quench studies to obtain general conclusions regarding wall quench hydrocarbon evolution in practical engines using practical fuels.

Dynamic simulation of ship-system thermal load management

Anticipating highly dynamic and reconfigurable future ships, the US Navy has sought to develop modeling and simulation capabilities for transient, electrical-mechanical-thermal, shipboard interactions at the system level. In support of this work, an object-oriented Dynamic Thermal Modeling and Simulation (DTMS) framework written in C++ has been in use for several years. As reported in this paper, DTMS has recently been augmented to model two-phase flow and heat transfer for simulation of a shipboard vapor-compression chiller and its attendant loads. A controls methodology has been implemented in the heat exchanger models to monitor their relevant states, chilled water enthalpy, and refrigerant liquid level. These heat exchangers have been integrated with a heavily-customizable, centrifugal compressor model focused on required power input rather than the detailed dynamics of fluid compression. The heat exchangers and centrifugal compressor, along with a model of a thermostatic expansion valve, have been used to assemble a simulation of a 200-ton marine chiller predicated on baseline parameters for the Navys current destroyer. This chiller has been connected with thermal loads of varying magnitude to demonstrate controller response during full-load and part-load operation. The final simulation reported here consists of 22 thermal loads ranging from 8 to 256 kW with chilled water supplied by two chillers. Results are compared with both steady-state-predicted values and previous dynamic simulations using commercial software.

Thermal aspects of a shipboard integrated electric power system

Development and validation of a dynamic model focused on thermal aspects of the integrated propulsion system (IPS) on a notional all-electric ship (AES) is presented. This model serves as a baseline for investigation of thermal management technologies and architectures focused on identifying tradeoffs, improving system-level efficiency, and increasing the performance of an AES. The IPS model includes component models for gas turbine engines, synchronous generators, motor converters, propulsion motors, fixed-pitch propellers, and a ship hull. These component models are integrated into a system-level simulation and dynamic results for a ship-maneuver called a “crash-back” are presented. The resulting thermal-mechanical dynamics are discussed in depth.

All-Electric Ship Energy System Design Using Classifier-Guided Sampling

The addition of power-intensive electrical systems on the U.S. Navys next-generation all-electric ships (AES) creates significant new challenges in the area of total-ship energy management. Power intensive assets are likely to compete for available generation capacity, and thermal loads are expected to greatly exceed current heat removal capacity. To address this challenge, a total-ship zonal distribution model that includes electric power, chilled water (CW), and refrigerated air (RA) systems is developed. Classifier-guided sampling (CGS), a population-based optimization algorithm for solving problems with discrete variables and discontinuous responses, is used to identify high-performance configurations with respect to fuel consumption. This modeling approach supports early-stage design decisions and performance analyses of notional systems in response to changing operating modes and damage scenarios. A set of configurations that enhance survivability is identified. Results of a comparison study demonstrate that CGS improves the rate of convergence toward superior solutions, on average, when compared to genetic algorithms (GAs).

Dynamic assessment of thermal management strategies aboard naval surface ships

Quantifying the close relationship between shipboard thermal, mechanical, and electrical sub-systems is of fundamental importance to understanding the nature of a large, integrated system like the all-electric ship (AES). Our research efforts in support of development of an AES are focused on physics-based, dynamic models of components and subsystems that approximate, at the system-level, the notional architecture of an AES. This research has resulted in the development of a general purpose thermal management tool coded in C++ and known as the Dynamic Thermal Modeling and Simulation (DTMS) framework. The DTMS simulation environment provides the ability to model thermal systems and subsystems relevant to the AES. The summary presented here describes the modeling approach used in DTMS and provides several examples of its use in large, complex, system-level, · dynamic simulations. The examples emphasized here include a: dynamic model for variable and constant air volume heating, ventilation, and air conditioning that permits investigation of alternatives to conventional constant air flow systems and the power savings associated with variable air flow systems. · Co-simulation that dynamically links a thermally dependent electrical power distribution network, and its consequent transient heat loads, with a thermal resistive heat flow network that connects these loads to the thermal management network at the system-level.

Thermo-Kinetic Representation and Transient Simulation of a Molten Carbonate Fuel Cell

There are a growing number of models in the literature dealing with the transient behavior of fuel cells. However, few, if any, employ fundamental kinetic theory to model the fuel reformation process while simultaneously simulating fuel cell behavior from a transient, system-level perspective. Thus a comprehensive, transient fuel cell model has been developed that includes all the relevant thermodynamics, chemistry, and electrical characteristics of actual fuel cell operation. The model tracks the transient temperature response of a fuel cell stack, chemical specie concentrations of exhaust gases, efficiency of the fuel reformation equipment, and electrical output characteristics. Model results provide a concise, parametric evaluation of the influence of operating conditions and user-controlled parameters on fuel cell performance. The model is validated against transient Molten Carbonate fuel cell (MCFC) data from a subscale stack.Copyright