Christopher P. Cadou

Effect of structural conduction and heat loss on combustion in micro-channels

This paper presents a simple analytical model for the effects of heat exchange within the structure of a micro-channel combustor, and heat loss from the structure to the environment. This is accomplished by extending reasoning similar to that employed by Mallard and Le Chatelier in their thermal theory for flame propagation. The model is used to identify some of the basic parameters that must be considered when designing an efficient micro-combustor and its predictions are compared with the results of a numerical simulation of stoichiometric premixed combustion of a hydrogen–air mixture stabilized between two parallel plates. The simulation incorporates a one-dimensional continuity/energy equation solver with full chemistry coupled with a model for thermal exchange in the structure. The results show that heat exchange through the structure of the micro-combustor can lead to a broadening of the reaction zone. Heat loss to the environment decreases the broadening effect and eventually results in flame quenching. This behaviour, which arises from the thermal coupling between the gas and the structure, influences the maximum achievable power density of microscale combustors.

Preliminary development of a hydrocarbon-fueled catalytic micro-combustor

Abstract This paper reports development of a hydrocarbon-fueled micro-combustion system for a micro-scale gas turbine engine for power generation and micro-propulsion applications. A three-wafer catalytic combustor was fabricated and tested. Efficiencies in excess of 40% were achieved for ethylene–air and propane–air combustion. A fabrication process for a six-wafer catalytic combustor was developed and this device was successfully constructed.

Investigation of the Dynamic Characteristics of a Piezohydraulic Actuator

A piezohydraulic actuator is a hybrid device consisting of a hydraulic pump driven by piezo stacks, that is coupled to a conventional hydraulic cylinder via a set of fast-acting valves. Because the performance of the actuator is strongly related to the pumping frequency, a good understanding of the dynamics of the system is essential for designing a high-efficiency actuator. This article describes the development of a frequency domain model to quantify the dynamics of a piezohydraulic hybrid actuator. The analysis treats the hydraulic circuit as a series of fluid transmission lines, each represented by a transfer matrix that determines the relationship between the pressure and velocity at the inlet and at the outlet. The model includes the effects of fluid compressibility, inertia and viscosity. An experimental procedure to measure the frequency response of the device is described, and is used to validate the analysis. The effect of tubing length and fluid viscosity on the dynamic characteristics of the system is investigated. Longer tubing lengths result in lower resonant frequencies of the system, while increasing fluid viscosity results in a decrease in the magnitude of the resonant peak.

Development of a Dynamometer for Measuring Small Internal-Combustion Engine Performance

Small hobby engines with masses less than 1 kg are attractive for use in low-cost unmanned air vehicles, because they are mass-produced and inexpensive. However, very little information about their performance is available in the scientific literature. This paper describes the development of a dynamometer system suitable for measuring the power output and efficiency of these small engines and presents detailed performance measurements for a particular engine with a mass of 150 gm that could be suitable for powering a low-cost unmanned air vehicle. When the mixture setting is adjusted according to the manufacturers instructions, the peak power of this engine is 112 W at 9450 rpm with a brake specific fuel consumption of 3.0 kg/k Wh. The performance can be improved to 159 W at 12,000 rpm and brake specific fuel consumption of approximately 2.1 kg/kWh by controlling the mixture.

Performance Modeling of a Piezo-Hydraulic Actuator

A simple quasi-static model has been developed as an engineering tool for improving the performance of a piezo-hydraulic actuators fluid system. The models predictions compare reasonably well with experimental data at low frequencies (<150 Hz) as trends in the dependence of actuation speed on driving frequency and applied load are captured within 30%. The model indicates that there is an optimum load and driving frequency that corresponds to maximum power output but that the operating conditions for optimum power output and efficiency are different. The efficiency of the fluid system decreases with increasing frequency at a rate that depends on the load. Viscous losses through the valves and tubing are negligible compared with the inertial losses associated with accelerating and decelerating the load. This acceleration and deceleration process occurs twice per piezo cycle because of the configuration of the fluid system used to rectify the oscillatory motion of the piezo stack. Accordingly, the inertia of the load dominates the behavior of the device at high frequencies. The performance of the fluid system is most sensitive to the stiffness of the fluid in the pumping chamber, which should be maximized for maximum power output.

Application of CFD in the Design and Analysis of a Piezoelectric Hydraulic Pump

The ability of smart materials to deliver large block forces in a small package while operating at high frequencies makes them extremely attractive for converting electrical power into mechanical power. This has led to the development of hybrid actuators consisting of co-located smart material actuated pumps and hydraulic cylinders. The overall success of the hybrid concept hinges on the effectiveness of the coupling between the smart material and the fluid. This study presents the results of two and three-dimensional (3D) simulations of fluid flow in a prototype hybrid actuator being developed for aerospace applications. The steady simulations show that losses in the device result primarily from three-dimensional effects like radial acceleration of the fluid in the pumping chamber, and the formation of vortex ring structures that block the flow. The effects of varying design parameters like pumping chamber height, discharge tube location, and discharge tube chamfer are explored and are found to have significant impacts on performance. Analytical expressions for the scaling of pressure losses with driving frequency are presented.

Heat Transfer in Mini∕Microchannels With Combustion: A Simple Analysis for Application in Nonintrusive IR Diagnostics

An analytical solution for the temperature distribution in 2D laminar reacting flow between closely spaced parallel plates is derived as part of a larger effort to develop a nonintrusive technique for measuring gas temperature distributions in millimeter and submillimeter scale channel flows. The results show that the exact solution, a Fourier series, which is a function of the Peclet number, is approximated by second and fourth order polynomial fits to an R value of almost unity for both fits. The slopes of the temperature near the wall (heat fluxes) are captured to within 20% of the exact solution using a second order polynomial and to within 2% of the exact solution using a fourth order polynomial. The fits are used in a nonintrusive Fourier transform infrared spectroscopy technique and enable one to infer the temperature distribution along an absorbing gas column from the measured absorption spectrum. The technique is demonstrated in a silicon-walled microcombustor.

Scaling of Miniature Piston Engine Performance Part 2: Energy Losses

Previous work based on measurements of the overall performance of small, (<10  cc displacement) two-stroke piston engines has shown that power and efficiency decrease more rapidly with size in miniature (<10  cc displacement) than in larger-scale engines. The present work seeks explanations for these trends by using energy balance analyses to identify and quantify the principal loss mechanisms in miniature engines. Energy losses in order of decreasing importance are found to be: incomplete combustion, heat transfer, sensible enthalpy in the exhaust, and friction. This ordering is almost opposite to that observed in conventional-scale engines. The results show that the scale-dependence of thermal and mechanical efficiency drive the drop in overall efficiency but that the biggest problem is incomplete combustion that consumes 50–60% of the fuel energy. Power output decreases somewhat less rapidly than efficiency because delivery ratio increases with decreasing scale in the set of engines investigated here. ...

Quantifying Unmanned Undersea Vehicle Range Improvement Enabled by Aluminum–Water Power System

Aluminum is an attractive energy storage material for underwater propulsion because of its high density and strongly exothermic reaction with seawater. However, the degree to which an aluminum–seawater power system could outperform other systems has remained unknown because of uncertainties about volume and energy costs associated with the balance of plant. This work addresses this problem by developing a thermodynamic model for a complete Rankine-cycle propulsion system based on the aluminum–seawater reaction and combining this with a scaling methodology for inferring the system’s effective energy density. The results show that replacing battery-based power systems with aluminum combustion based ones could increase range/endurance by factors of four to ten over competing technologies. Overall system efficiency is maximized by adjusting the water mass flow to fuel mass flow ratio so as to control the temperature and quantity of steam. Although increasing the amount of combustion byproduct, hydrogen, impro...

Two-dimensional analytical model of heat transfer for flames in channels

A two-dimensional model for heat transfer in reacting channel flow with a constant wall temperature is developed along with an analytical solution that relates the temperature field in the channel to the flow Peclet number. The solution is derived from first principles by modeling the flame as a volumetric heat source and by applying jump conditions across the flame for plug and Hagen-Poiseuille velocity profiles and is validated via comparison with more detailed computational fluid dynamics solutions. The analytical solution provides a computationally efficient tool for exploring the effects of varying channel height and gas velocity on the temperature distribution in a channel in which a flame is stabilized. The results show that the Peclet number is the principal parameter controlling the temperature distribution in the channel. It is also found that although the Nusselt number is independent of the Peclet number (or velocity) in the postflame region, it can change by nearly 3 ord ers o f magnitude in the preflame region over the range of Peclet numbers (or velocities) expected in microcombustors. This has important implications for quasi-onedimensional numerical modeling of micro/mesoscale combustion, in which it is usual to select a single Nusselt value from the heat transfer literature.Acorrelation to facilitate incorporation of the streamwise Nusselt number variation is provided.