This thesis documents the results of high pressure testing of hole-pattern annular gas seals conducted at the Texas A&M University's Turbomachinery Laboratory. The testing conditions were aimed at determining the test seals sensitivity to pressure ratio, inlet fluid preswirl, rotor speed, and rotor to seal clearance. The rotordynamic coefficients showed only small changes resulting from the different pressure ratios tested. Only the damping terms at the lower frequencies showed some influence. One other notable result from the testing of different pressure ratios is that the seals were tested in a choked flow condition, and there was not a significant change in the seal behavior when the seals transitioned to the choked condition. The inlet fluid preswirl only had a notable effect on the cross-coupled stiffness in the larger clearance tests. These results lead to the conclusion that a swirl brake could have some rotordynamic value, but only if the seals have sufficiently large clearance. Conversely this also means that if hole-pattern seals are being implemented with a small clearance, then a swirl brake would not be an effective way to improve the rotordynamic stability of the system. The only significant effect that the rotor speeds had on the rotordynamic coefficients were that the cross-coupled coefficients increased as the rotor speed increased. This is the expected result because as the rotor speed increases there is a greater shear force on the gas as it passes through the seal resulting in more fluid circumferential velocity, which results in stronger cross-coupled coefficients. The changes in clearance resulted in drastic changes in the magnitude of the coefficients. The smaller clearance yielded much higher coefficients than the larger clearance. All of the rotordynamic coefficients were predicted well by ISOTSEAL. The code was found to do a good job predicting the seal leakage as well. This gives more credence to the coefficients and leakage that ISOTSEAL predicts.

This thesis documents the results of high pressure testing of hole-pattern annular gas seals conducted at the Texas A&M University's Turbomachinery Laboratory. The testing conditions were aimed at determining the test seals sensitivity to pressure ratio, inlet fluid preswirl, rotor speed, and rotor to seal clearance. The rotordynamic coefficients showed only small changes resulting from the different pressure ratios tested. Only the damping terms at the lower frequencies showed some influence. One other notable result from the testing of different pressure ratios is that the seals were tested in a choked flow condition, and there was not a significant change in the seal behavior when the seals transitioned to the choked condition. The inlet fluid preswirl only had a notable effect on the cross-coupled stiffness in the larger clearance tests. These results lead to the conclusion that a swirl brake could have some rotordynamic value, but only if the seals have sufficiently large clearance. Conversely this also means that if hole-pattern seals are being implemented with a small clearance, then a swirl brake would not be an effective way to improve the rotordynamic stability of the system. The only significant effect that the rotor speeds had on the rotordynamic coefficients were that the cross-coupled coefficients increased as the rotor speed increased. This is the expected result because as the rotor speed increases there is a greater shear force on the gas as it passes through the seal resulting in more fluid circumferential velocity, which results in stronger cross-coupled coefficients. The changes in clearance resulted in drastic changes in the magnitude of the coefficients. The smaller clearance yielded much higher coefficients than the larger clearance. All of the rotordynamic coefficients were predicted well by ISOTSEAL. The code was found to do a good job predicting the seal leakage as well. This gives more credence to the coefficients and leakage that ISOTSEAL predicts.

abstract: Production from a high pressure gas well at a high production-rate encounters the risk of operating near the choking condition for a compressible flow in porous media. The unbounded gas pressure gradient near the point of choking, which is located near the wellbore, generates an effective tensile stress on the porous rock frame. This tensile stress almost always exceeds the tensile strength of the rock and it causes a tensile failure of the rock, leading to wellbore instability. In a porous rock, not all pores are choked at the same flow rate, and when just one pore is choked, the flow through the entire porous medium should be considered choked as the gas pressure gradient at the point of choking becomes singular. This thesis investigates the choking condition for compressible gas flow in a single microscopic pore. Quasi-one-dimensional analysis and axisymmetric numerical simulations of compressible gas flow in a pore scale varicose tube with a number of bumps are carried out, and the local Mach number and pressure along the tube are computed for the flow near choking condition. The effects of tube length, inlet-to-outlet pressure ratio, the number of bumps and the amplitude of the bumps on the choking condition are obtained. These critical values provide guidance for avoiding the choking condition in practice. / Dissertation/Thesis / M.S. Mechanical Engineering 2013

This thesis is concerned with experimental and computational studies on pulse detonation engine (PDE) that has been envisioned as a new concept engine. These engines use the high pressure generated by detonation wave for propulsion. The cycle efficiency of PDE is either higher in comparison to conventional jet engines or at least has similar high performance with much greater simplicity in terms of components. The first part of the work consists of an experimental study of the performance of PDE under choked flame and partial fill conditions. Detonations used in classical PDEs create conditions of Mach numbers of 4-6 and choked flames create conditions in which flame achieves Mach numbers near-half of detonation wave. While classical concepts on PDE's utilize deflagration-to-detonation transition and are more intensively studied, the working of PDE under choked regime has received inadequate attention in the literature and much remains to be explored. Most of the earlier studies claim transition to detonation as success in the working of the PDE and non-transition as failure. After exploring both these regimes, the current work brings out that impulse obtained from the wave traveling near the choked flame velocity conditions is comparable to detonation regime. This is consistent with the understanding from the literature that CJ detonation may not be the optimum condition for maximum specific impulse. The present study examines the details of working of PDE close to the choked regime for different experimental conditions, in comparison with other aspects of PDEs. The study also examines transmission of fast flames from small diameter pipe into larger ducts. This approach in the smaller pipe for flame acceleration also leading to decrease in the time and length of transition process. The second part of the study aims at elucidating the features of deflagration-to-detonation transition with direct numerical simulation (DNS) accounting for and the choice of full chemistry and DNS is based on two features: (a) the induction time estimation at the conditions of varying high pressure and temperature behind the shock can only be obtained through the use of full chemistry, and (b) the complex effects of fine scale of turbulence that have sometimes been argued to influence the acceleration phase in the DDT cannot be captured otherwise. Turbulence in the early stages causes flame wrinkling and helps flame acceleration process. The study of flame propagation showed that the wrinkling of flame has major effect on the final transition phase as flame accelerates through the channel. Further, flame becomes corrugated prior to transition. This feature was investigated using non-uniform initial conditions. Under these conditions the pressure waves emanating from corrugated flame interact with the shock moving ahead and transition occurs in between the flame and the forward propagating shock wave. The primary contributions of this thesis are: (a) Elucidating the phenomenology of choked flames, demonstrating that under partial fill conditions, the specific impulse can be superior to detonations and hence, allowing for the possibility of choked flames as a more appropriate choice for propulsive purposes instead of full detonations, (b) The use of smaller tube to enhance the flame acceleration and transition to detonation. The comparison with earlier experiments clearly shows the enhancements achieved using this method, and (c) The importance of the interaction between pressure waves emanating from the flame front with the shock wave which leads to formation of hot spots finally transitioning to detonation wave.

This thesis addresses the experimental investigation of the cavitation phenomenon and its main consequences on the normal operation of a safety relief valve (SRV). More particularly, limitation of the mass flux discharged and alteration of the hydraulic fluid forces behavior is of main interest for the proper design and sizing of such devices. In nuclear or thermal engineering systems, the use of SRVs is mandatory since it represents the ultimate protection device before an accident occurs, caused by a sudden pressurization of the system. A careful design and sizing of the SRV is therefore essential. The complete understanding of the physics taking place in the flow through the valve is required to guaranty and optimize the security of the protected process.<p><p>In order to investigate the above effects of cavitation in a SRV, two different orifice sized valves (API 2J3 type and a transparent model based on an API 1 1/2G3 type) are tested in two different experimental facilities expressly built for this purpose. Instead of using a spring, the design of both valves allows the adjustment of the disc at any desired lift. Hence the static behavior of the valves is investigated. Both facilities, operating at different magnitude scales, allow the study of single phase and cavitating flow conditions required to properly determine the most important hydraulic characteristics, and access on any potential scaling effect between both sized SRVs. Experimental techniques used for the determination of the hydraulic characteristics include temperature, flow rate, fluid forces and pressure measurements both upstream and downstream the test sections. <p><p>Results show a similar influence of cavitation on the flow characteristics of both valves, minimizing any potential scaling effect. The liquid pressure recovery factor FL, which is normally used to identify a choked flow condition in a control valve, is experimentally determined for the first time in a SRV. The existence of a local minimum located at small openings of the lift indicates a change on the flow characteristics of both valves, which is related to the location of the minimum cross section of the flow that does not remain constant for every lift position. An extended experimental campaign is performed to analyse the effect of the blowdown ring adjustment located around the nozzle of the API 2J3 valve. Results confirm that the position of the ring has an important contribution for the hydraulic forces acting on the valve disc. <p><p>In the second part of the research, precise optical diagnostic techniques are successfully applied in the transparent valve to locally characterize the flow topology in a SRV experiencing cavitation. These results are innovative and enrich the experimental database available in the literature for the characterization and understanding of the flow physics in such devices. In a first configuration, high speed visualization is applied to observe qualitatively the flow pattern and the inception of liquid vaporization. Particle tracking results suggest that vapor bubbles are formed in the core of vortices detached from the shear layers attached to the valve. These rotational structures promote lower pressure regions allowing the liquid to vaporize. In the second configuration, particle image velocimetry is applied to extract the velocity field in both single phase and cavitating flow conditions. Results of PIV confirm the existence of a submerged jet just downstream the minimum section. This jet is characterized by two non-symmetric shear layers at its sides. Under cavitation conditions, PIV results confirm that vapor bubbles are formed preferentially inside the jet shear layers. The phenomenon of mass flux limitation caused by cavitation is reproduced at small openings of the valve and interaction with the flow topology is highlighted. It is observed that limitation of the flow occurs when the vena contracta is shifted towards the minimum geometrical section of the flow. Finally, instabilities of the flow downstream the critical section are investigated in the frequency domain by means of time resolved data. Results suggest that vortex shedding mechanism is dominated by a constant Strouhal number which is slightly affected by the valve opening. <p><p>In the last part of the research, the methodology used in water is extended and applied to cryogenic liquids. Two different geometries are investigated experimentally and numerically using water and liquid nitrogen as working fluids. Results suggest that both the flow coefficient (determined at single flow conditions), and the liquid recovery factor (used to identify choked flows), are independent on the fluid properties and therefore, an hydraulic similarity relation can be proposed.<p><p>This research project was carried out at the von Karman Institute for Fluid Dynamics (VKI), in Belgium, in close collaboration and with the funding of Centre Technique des Industries Mécaniques (CETIM) in France. / Doctorat en Sciences de l'ingénieur / info:eu-repo/semantics/nonPublished

The work deals with the design of a system of orifices for high pressure difference. The task of this work is to design a device for controlled discharge of steam-gas mixture from a volume compensator with an overpressure of 12,27 MPa to a tank with an overpressure of 0,02 MPa at a constant mass flow of 40 kg/h. The first part of the thesis contains the theory and also the basic principles of calculations. In the next part of the work, the theoretical properties of flow, such as the speed of sound in wet steam, are determined. This knowledge then serves the main goal of the work, namely to design a system of orifices to release steam-gas mixture from the volume compensator.

Hybrid solar-Brayton systems utilize both solar thermal energy and supplementary renewable fuels to provide controllable and dispatchable power output, which renders them a promising way to meet the growing energy demand and reduce the carbon footprints. However, existing testing facilities for key components in such hybrid systems often fail to accomplish the testing requirements, hence impeding the improvement of the renewable energy share and the overall efficiency. A novel testing facility is urgently needed in order to thoroughly stimulate and analyze the component characteristics. This research work focuses on the development of a gas turbine simulator as an innovative testing facility for hot, pressurized components in hybrid solar-Brayton systems. The dual-flow choked nozzle based flow control has been proposed, explained, and analyzed in comparison to the single-flow layout. The basic idea of gas turbine simulator has been experimentally implemented and validated on a prototype, verifying its functionality. By incorporating a PLC-based control system, an automated gas turbine simulator has been designed and modified based on the prototype. Its performance with regard to stabilizing boundaries and tracking trajectories has been evaluated by experiments. Based on the experimental results, the gas turbine simulator prototype has proven its ability to establish controllable boundary conditions and migrate operating points for the impinging receiver. Through manual adjustments, excellent quasi-steady state performance has been obtained, with the precision for pressure control reaching ±0.005 bar at ambient temperature and ±0.015 bar at high temperature of 797.1-931.5 °C. The manual operation time has been identified at 23.1 s for establishing the receiver boundaries, and at 70 s for changing operating points. With the help of the proposed control strategy, the automated gas turbine simulator has eliminated the need for manual adjustments, and demonstrated the ability to maintain the safe and convergent operation for the receiver. The performance in boundary condition stabilization has been satisfactory, with enhanced steady-state accuracy comparing to the prototype by virtue of the PID controller. The transient-state fluctuations in pressure control have been effectively restrained within an acceptable region with deviations of ±0.018 bar to ±0.076 bar from the desired 2.400 bar operating pressure. The capability of tracking linear and nonlinear trajectories has also been testified, with the precision level between ±0.023 bar and ±0.037 bar. Finally, in view of the good stability, high precision, and rapid response manifested in the experimental studies, the gas turbine simulator has validated its ability to imitate the steady and transient characteristics of gas turbines on the boundaries of the test section. It also grants the possibilities to conduct control variable studies and wide-range transition studies. The gas turbine simulator is a suitable testing facility for the key components in hybrid solar-Brayton systems. / Hybrid solenergi-Brayton-system använder både solvärmeenergi och kompletterande förnybara bränslen för att ge kontrollerbar och sändbar effekt, vilket gör dem till ett lovande sätt att möta den växande energiefterfrågan och minska koldioxidavtrycken. Men befintliga testanläggningar för nyckelkomponenter i sådana hybridsystem misslyckas ofta med att uppfylla testkraven, vilket hindrar förbättringen av andelen förnybar energi och den totala effektiviteten. En ny testanläggning behövs omgående för att grundligt stimulera och analysera komponentens egenskaper. Detta forskningsarbete fokuserar på utvecklingen av en gasturbinsimulator som en innovativ testanläggning för varma, trycksatta komponenter i hybridsolar-Brayton-system. Den dubbelströms strypta munstycksbaserade flödeskontrollen har föreslagits, förklarats och analyserats i jämförelse med enkelflödeslayouten. Den grundläggande idén med gasturbinsimulator har experimentellt implementerats och validerats på en prototyp, vilket verifierar dess funktionalitet. Genom att införliva ett PLC-baserat styrsystem har en automatiserad gasturbinsimulator designats och modifierats utifrån prototypen. Dess prestanda med avseende på stabilisering av gränser och spårning av banor har utvärderats genom experiment. Baserat på de experimentella resultaten har prototypen av gasturbinsimulatorn bevisat sin förmåga att upprätta kontrollerbara gränsförhållanden och migrera arbetspunkter för den träffande mottagaren. Genom manuella justeringar har man erhållit utmärkt prestanda i nästan konstant tillstånd, med precisionen för tryckkontroll som når ±0,005 bar vid omgivningstemperatur och ±0,015 bar vid hög temperatur på 797,1-931,5 °C. Den manuella drifttiden har identifierats till 23,1 s för att fastställa mottagargränserna och till 70 s för att byta arbetspunkter. Med hjälp av den föreslagna styrstrategin har den automatiserade gasturbinsimulatorn eliminerat behovet av manuella justeringar och visat förmågan att upprätthålla en säker och konvergent drift för mottagaren. Prestandan vid gränstillståndsstabilisering har varit tillfredsställande, med förbättrad steady-state noggrannhet jämfört med prototypen tack vare PID-regulatorn. De transienta tillståndsfluktuationerna i tryckregleringen har effektivt begränsats inom ett acceptabelt område med avvikelser på ±0,018 bar till ±0,076 bar från det önskade 2,400 bar arbetstrycket. Förmågan att spåra linjära och olinjära banor har också vittnats, med precisionsnivån mellan ±0,023 bar och ±0,037 bar. Slutligen, med tanke på den goda stabiliteten, höga precisionen och snabba responsen som manifesteras i de experimentella studierna, har gasturbinsimulatorn validerat sin förmåga att imitera de stabila och transienta egenskaperna hos gasturbiner på gränserna för testsektionen. Det ger också möjlighet att genomföra kontrollvariabelstudier och omfattande övergångsstudier. Gasturbinsimulatorn är en lämplig testanläggning för nyckelkomponenterna i hybridsolar-Brayton-system.

The labyrinth seal is a non-contact annular type sealing device used to reduce the internal leakage of the working fluid which is caused by the pressure difference between each stage in a turbomachine. Reducing the leakage mass flow rate of the working fluid through the labyrinth seal is desirable because it improves the efficiency of the turbomachine. The carry-over coefficient, based on the divergence angle of the jet, changed with flow parameters with fixed seal geometry while earlier models expressed the carry-over coefficient solely as a function of seal geometry. For both compressible and incompressible flows, the Reynolds number based on clearance was the only flow parameter which could influence the carry-over coefficient. In the case of incompressible flow based on the simulations for various seal geometries and operating conditions, for a given Reynolds number, the carry-over coefficient strongly depended on radial clearance to tooth width ratio. Moreover, in general, the lower the Reynolds number, the larger is the divergence angle of the jet and this results in a smaller carry-over coefficient at lower Reynolds numbers. However, during transition from laminar to turbulent, the carry-over coefficient reduced initially and once the Reynolds number attained a critical value, the carry-over coefficient increased again. In the case of compressible flow, the carry-over coefficient had been slightly increased if radial clearance to tooth width ratio and radial clearance to tooth pitch ratio were increased. Further, the carry-over coefficient did not considerably change if only radial clearance to tooth width ratio was decreased. The discharge coefficient for compressible and incompressible flows depended only on the Reynolds number based on clearance. The discharge coefficient of the tooth in a single cavity labyrinth seal was equivalent to that in a multiple tooth labyrinth seal indicating that flow downstream had negligible effect on the discharge coefficient. In particular, for compressible fluid under certain flow and seal geometric conditions, the discharge coefficient did not increase with an increase in the Reynolds number. It was correlated to the pressure ratio, Pr. Moreover, it was also related to the fact that the flow of the fluid through the constriction became compressible and the flow eventually became choked. At low pressure ratios (less than 0.7), Saikishan’s incompressible model deviated from CFD simulation results. Hence, the effects of compressibility became significant and both the carry-over coefficient compressibility factor and the discharge coefficient compressibility factor needed to be considered and included into the leakage model. The carry-over coefficient compressibility factor, phi, had two linear relationships with positive and negative slopes regarding the pressure ratios. This result was not associated with the seal geometry because the seal geometry ratios for each instance were located within the nearly same ranges. Further, the phi-Pr relationship was independent of the number of teeth regardless of single and multiple cavity labyrinth seals. The discharge coefficient compressibility factor, psi, was a linear relationship with pressure ratios across the tooth as Saikishan predicted. However, in certain flow and seal geometric conditions, Saikishan’s model needed to be modified for the deviation appearing when the pressure ratios were decreased. Hence, a modified psi-Pr relationship including Saikishan’s model was presented in order to compensate for the deviation between the simulations and his model.