Mechanical Engineering

This study presents the simulation, experimentation, and design considerations of a Poly(vinylidene fluoride co-trifluoroethylene)/ Polyethylene Terephthalate (PVDF-TrFe / PET), laser-cut, flexible piezoelectric energy harvester. It is possible to obtain energy from the environment around autonomous sensor systems, which can then be used to power various equipment. This article investigates the actuation means of ambient vibration, which is a good candidate for using piezoelectric energy harvester (PEH) devices. The output voltage characteristics were analyzed in a wind test apparatus. Finite element modeling (FEM) was done for von Mises stress and modal analysis. Resonance frequency sweeps, quality factors, and damping ratios of the circular plate were given numerically. For a PVDF-TrFe piezoelectric layer thickness of 18 µm and 1.5 mm radius, a damping ratio of 0.117 and a quality factor of 4.284 was calculated. Vmax was calculated as 984 mV from the wind setup experiments and compared with the FEM outputs.

This study presents the simulation, experimentation, and design considerations of a Poly(vinylidene fluoride co-trifluoroethylene)/ Polyethylene Terephthalate (PVDF-TrFe / PET), laser-cut, flexible piezoelectric energy harvester. It is possible to obtain energy from the environment around autonomous sensor systems, which can then be used to power various equipment. This article investigates the actuation means of ambient vibration, which is a good candidate for using piezoelectric energy harvester (PEH) devices. The output voltage characteristics were analyzed in a wind test apparatus. Finite element modeling (FEM) was done for von Mises stress and modal analysis. Resonance frequency sweeps, quality factors, and damping ratios of the circular plate were given numerically. For a PVDF-TrFe piezoelectric layer thickness of 18 µm and 1.5 mm radius, a damping ratio of 0.117 and a quality factor of 4.284 was calculated. Vmax was calculated as 984 mV from the wind setup and compared with the FEM outputs.

Developments in hybrid propellants over the last decade make the hybrid motor a viable candidate and possibly an enabling technology for a Mars Ascent Vehicle (MAV) as part of a Mars Sample Return (MSR) campaign. Fast regression rate fuels such as paraffin allow for single port hybrid designs that overcome the disadvantages associated with classical hybrid fuels. Additionally, paraffin has a both a weak and low glass transition temperature, making it an ideal candidate for a Mars application. Nytrox, a high performance oxidizer made of a mixture of nitrous oxide and oxygen, can be chosen to match the temperatures on Mars. The hybrid MAV can survive the harsh Martian environment with minimal or no thermal conditioning and it can meet the design challenges posed by coordinating with the other aspects of MSR.

The fuel regression rate expressions reported in the hybrid literature often depend explicitly on the physical dimensions of the system such as the fuel port diameter. Typically, when these dimensional formulas are applied to systems with significantly different scales, they produce grossly inaccurate results. This paper addresses the development of scalable space-time averaged regression rate formulas for hybrid rockets. The derivation process hinges on the assumption that the local instantaneous regression rate is a function of the local mass flux and the axial port distance in the power law format as predicted by the classical theory developed by Marxman. In this study, we have developed physics-based nondimensional formulas for the space-time averaged regression rate and used these expressions to develop a scalable regression rate law for a selected propellant combination, paraffin-based SP-1a/ GOX, from limited motor data. Initially, space and time averaging are treated separately, which were later combined to develop a technique that allows for the coupling between the spatial and time variations to predict the port diameter and mass flow rate profiles as functions of time. Finally, a comprehensive technique to estimate the systematic and random errors on the regression rate and mass flux data is also outlined.

Hybrid rocket propulsion is a tipping point technology in the sense that a small, short term investment could have game changing consequences in developing green, safe, affordable and high performance systems needed in future space missions. In order to demonstrate the advantages of hybrids most effectively, the effort should be concentrated on improving the Technology Readiness Level (TRL) of the technology for a carefully selected class of missions. Arguably upper stage motors used in small launch vehicles constitute a perfect platform for this purpose due their relatively small scale and high performance requirements. The advanced hybrid rockets that are being developed by SPG are believed to have the capability to deliver high performances desirable for upper stages, while retaining the cost, environmental and simplicity advantages of the classical hybrids. In order to demonstrate the performance capabilities of advanced hybrid rockets, a design study has been conducted to replace the Orion 38 solid rocket motor with a LOX/paraffin-based system. The LOX hybrids delivering the same level of total impulse as Orion 38 system are determined to be 15-18% lighter. It has been shown that switching to higher performance hybrid upper stages could lead to payload increases up to 40% for a typical launch vehicle. The additional cost, environmental, safety, stop/restart/throttling advantages are expected to make the hybrids desirable alternatives to the existing upper stage systems.

The Peregrine Sounding Rocket Program is a joint basic research program of NASA Ames Research Center, NASA Wallops, Stanford University and the Space Propulsion Group, Inc. (SPG). The goal is to determine the applicability of liquifying hybrid technology to a small launch system. The approach is to design, build, test and fly a stable, efficient liquefying fuel hybrid rocket vehicle to an altitude of 100 km. The program was kicked off in October of 2006 and has seen considerable progress in the subsequent 18 months. Two virtually identical vehicles will be constructed and flown out of the NASA Sounding Rocket Facility at Wallops Island. This paper presents the current status of the project as of June 2008. For background on the project, the reader is referred to last year's paper. 1

This paper addresses the safety issues associated with the oxidizer nitrous oxide (N 2 O) with emphasis on propulsion systems. Even though N 2 O is a widely used energetic material, the number of decomposition related accidents are quite limited due to its abnormally slow decomposition kinetics. However hazards do exist especially in propulsion systems where large quantities of N 2 O are stored at room temperature in thin walled vessels. Moreover the closely coupled combustion chamber is a significant source for ignition which does not naturally exist in other applications. A detailed kinetics model for the N 2 O decomposition process is presented. It is shown that a simplified single step first order kinetics model accurately captures the decomposition process at pressures larger than 40 atm. With use of the kinetics data, it has been shown that, at the same pressure and temperature, the N 2 O decomposition rate is six orders of magnitude slower than the decomposition of hydrogen peroxide (H 2 O 2), making it a much safer propellant. Models for homogenous and local thermal ignition are also presented. It is shown that the estimated minimum ignition energy for pure N 2 O is approximately 450 mJ which is three orders of magnitude larger than the ignition energy for a stoichiometric CH 4 /air mixture. Small concentrations of diluents (i.e. N 2 , O 2 or He) further increase the ignition energy making the mixture extremely difficult to ignite at dilution levels higher than 30%. The results of a model developed to predict the pressure rise in a closed vessel subject to decomposition is presented to demonstrate the significant hazard that exists in the N 2 O tank. The model predicts a 20 fold increase in pressure over a time period of many seconds for tanks that are in the range of 1-3 meters in length. Finally, a list of safety related recommendations unique to N 2 O operations have been included. The general conclusion is that despite its potential decomposition hazard, if handled properly, N 2 O is one of the safest oxidizers being used in rocket propulsion systems. I) Nomenclature A: Surface area of the vessel r A : Reaction rate coefficient i C : Non-dimensional molar concentration of the ith species pf C : Specific heat of the products at constant pressure p C : Average specific heat at constant pressure across the flame v ĉ : Average molar heat capacity of the gas mixture

Hybrid rocket fuel average regression rate is one of the most important values to accurately determine in the hybrid rocket design process and for rocket performance prediction. Yet there is no comprehensive theory that can be used to reliably predict this quantity. Additionally, regression rate data is difficult to measure. Measured data often contains a high degree of scatter, suffers from scale effects and is generally a closely held secret by those performing the experiments and therefore is unavailable for many propellant combinations. This paper presents a regression rate model that has been developed based on the results of several previous studies. The model is applicable to vaporizing fuels in a cylindrical grain configuration that do not form significant char or melt layers. It accounts for the presence of a pre-combustion chamber upstream of the fuel grain and also for variable gas properties (to a limited degree). The model is compared with existing published regression rate data. The results of the comparison are reasonable given the level of approximation in the model but additional work is required before models of this type will supplant regression rate measurements for rocket design purposes.

In this paper, a theoretical investigation of the transient regression rates that occur in hybrid rockets is presented. We specifically concentrate on the thermal lags in the solid fuel grain, since they are believed to be rate limiting step in determining the dynamic response of this combustion system. A model is developed that approximately represents the physical and chemical processes occurring in the solid and at the surface. Two distinct cases, one with constant wall temperature and the other with an Arrhenius type of dependence of the regression rate to the surface temperature, are defined. Numerical and analytical solution techniques are developed and used to understand the behavior of the 4 regression rate during several transient cases. The analytical solution is simpler, but requires a constant heat flux and an assumed temperature contour. The numerical solution is quite general and treats both the case of a variable surface temperature and a time dependent heat flux. Consequently it is suitable for all forms of transients. The perturbation treatment also * Graduate student, Student member AIAA

Developments in hybrid propellants over the last decade make the hybrid motor a viable candidate and possibly an enabling technology for a Mars Ascent Vehicle (MAV) as part of a Mars Sample Return (MSR) campaign. Fast regression rate fuels such as paraffin allow for single port hybrid designs that overcome the disadvantages associated with classical hybrid fuels. Additionally, paraffin has a both a weak and low glass transition temperature, making it an ideal candidate for a Mars application. Nytrox, a high performance oxidizer made of a mixture of nitrous oxide and oxygen, can be chosen to match the temperatures on Mars. The hybrid MAV can survive the harsh Martian environment with minimal or no thermal conditioning and it can meet the design challenges posed by coordinating with the other aspects of MSR.

I would like to remember all the people who have helped me during the research and the writing of this thesis, with suggestions, criticisms and observations, or only being by my side. Foremost, I would like to express my sincere gratitude to my advisor Prof. Daniele Pavarin and to my co-advisor Prof. Arif Karabeyoğlu, for their excellent guidance and because without them none of this would have been possible. I would also like to thank Francesco Barato and Marta Lazzarin, for their continuous support and patience during all the time of my research. I would like to thank my mother, who always does everything possible to help me and to give me invaluable advices, and during the writing of this thesis even more. I would like to thank my father, who is a precious reference, and I know that he is always there for me. I would also like to thank my brothers, because in their own way, with a bit of good fraternal competition, they have spurred me to get to this point. I would also like to thank my friends, the old and the new ones, who helped me relaxing and having fun during my thesis period. Finally, I would like to thank Carlotta, that is always by my side and has constantly encouraged me to do my best, without stopping to believe in me. v

Introduced as a renewable energy source in many studies, ammonia (NH3) is known as a potential fuel to be combusted in power engines. With its high hydrogen density and already existing infrastructure, it is believed to be an excellent green fuel that can be used in energy generation and transportation systems. Combustion of ammonia has certain challenges (associated with its low flame speed and fuel bond NOx emissions) that need to be addressed before its widespread use in practical systems. A comprehensive numerical study [1] is accomplished focusing on the major combustion characteristics of ammonia and ammonia-hydrogen flames in a wide range of conditions. In the first section of the study we investigate the combustion characteristics of ammonia-air and ammonia-hydrogenair mixtures at elevated pressure and lean conditions which are encountered in gas turbine combustors. Laminar premixed freely propagating flame and homogenous reactor models are used to calculate the combustion properties. The improvement by hydrogen addition to the fuel mixture in the key combustion characteristics such as laminar flame speed and ignition delay time is noticeable. Based on ammonia decomposition sensitivity analysis, it is found that the OH radicals have a leading role in controlling the fuel mole conversion and the laminar flame speed. The complex chemistry of NOx formation is highly dependent on the fuel mixture composition which controls the contribution of fuel NOx and thermal NOx levels. This could lead to a trade-off situation in choosing the optimum fuel composition for engine applications. The results of sensitivity study of total NO formation with respect to the equivalence ratio reveal the possibility of localized rich combustion as an effective way to reduce the NO concentration down to levels that are the same order as the modern gas turbine engines (Figure 1). In the second part of the study, by considering a wide range of conditions in terms of pressure, fuel mixture, and equivalence ratio we develop two reduced mechanisms [2] based on the Konnov mechanism [3]. The reduced mechanisms are capable of predicting total NOx emission level and laminar flame speed in an acceptable accuracy under wide range of conditions (Figure 2). Evaluating performance of the reduced mechanisms with respect to the full mechanism and experimental data shows that the mechanisms are able to predict the combustion properties with almost the same accuracy as the full Konnov mechanism and with nearly five times less CPU time expense (Figure 3). As another important section of the study, by using our reduced mechanisms we have performed CFD simulations to investigate the flame characteristics in a basic axis-symmetric burner configuration. As the most critical parameter in practical systems, NOx formation is carefully taken into account. The effects of mixing and bypass air dilution level on the exhaust emission are investigated. The results are to be compared with experimental data which will be obtained in near future.

The idea of the hybrid rocket is to store the oxidizer as a liquid and the fuel as a solid, producing a design that minimizes the chance of a chemical explosion. While the hybrid enjoys many safety and environmental advantages over conventional systems, large hybrids have not been commercially viable. The reason is that traditional systems use polymeric fuels that evaporate too slowly, making it difficult to produce the high thrust needed for most applications. Research at Stanford University and Space Propulsion Group (SPG) has led to the development of paraffin-based fuels that burn at regression rates 3–4 times that of polymeric fuels. Under the action of the oxidizer flow, the new fuels form a thin, hydrodynamically unstable liquid layer on the melting surface of the fuel. Entrainment of droplets from the liquid–gas interface can substantially increase the rate of fuel mass transfer, leading to a much higher surface regression rate than can be achieved with a conventional fuel. ...

Due to its high hydrogen density and extensive experience base, ammonia (NH 3 ) has been gaining special attention as a potential green energy carrier. This study focuses on premixed ammoniaehydrogeneair flames under standard temperature and pressure conditions using an inert silicon-carbide (SiC) porous block as a practical and effective medium for flame stabilization. Combustion experiments conducted using a lab scale burner resulted in stable combustion and high combustion efficiencies at very high ammonia concentration levels over a wide range of equivalence ratios. Noticeable power output densities have also been achieved. Preliminary results of NO x emission measurements indicate NO x concentrations as low as 35 ppm under rich conditions. The remarkable capability of this specific burner to operate efficiently and cleanly at high ammonia concentration levels, which can easily be achieved by partial cracking of NH 3 , is believed to be a key accomplishment in the development of ammonia fired power generation systems.

District heating networks are commonly addressed in the literature as one of the most effective solutions for decreasing the greenhouse gas emissions from the building sector. These systems require high investments which are returned through the heat sales. Due to the changed climate conditions and building renovation policies, heat demand in the future could decrease, prolonging the investment return period. The main scope of this paper is to assess the feasibility of using the heat demand -outdoor temperature function for heat demand forecast. The district of Alvalade, located in Lisbon (Portugal), was used as a case study. The district is consisted of 665 buildings that vary in both construction period and typology. Three weather scenarios (low, medium, high) and three district renovation scenarios were developed (shallow, intermediate, deep). To estimate the error, obtained heat demand values were compared with results from a dynamic heat demand model, previously developed and validated by the authors. The results showed that when only weather change is considered, the margin of error could be acceptable for some applications (the error in annual demand was lower than 20% for all weather scenarios considered). However, after introducing renovation scenarios, the error value increased up to 59.5% (depending on the weather and renovation scenarios combination considered). The value of slope coefficient increased on average within the range of 3.8% up to 8% per decade, that corresponds to the decrease in the number of heating hours of 22-139h during the heating season (depending on the combination of weather and renovation scenarios considered). On the other hand, function intercept increased for 7.8-12.7% per decade (depending on the coupled scenarios). The values suggested could be used to modify the function parameters for the scenarios considered, and improve the accuracy of heat demand estimations.