Detonation initiation by shock focusing at elevated pressure conditions in a pulse detonation combustor

This work contains experimental investigations on the correlation of the detonation initiation process via a shock-focusing device with various initial pressures and mass flow rates. A pulse detonation combustor is operated with stoichiometric hydrogen--air--oxygen mixtures in single cycle operation. A rotationally symmetric shock-focusing geometry evokes the onset of a detonation by the focusing of the reflected leading shock wave, while a blockage plate at the rear end of the test rig is applied to induce an elevated initial pressure. The results show that the reactivity has a major influence on the success rate of detonation initiation. However, measurements with different blockage plates suggest that the mass flow rate has to be considered as well when predicting the success rate. Three main statements can be drawn from the results. (1) An increase in the mean flow velocity induces higher velocity fluctuations which result in a stronger leading shock ahead of the accelerating deflagration front. (2) An increase in the initial static pressure reduces the critical shock strength that must be exceeded to ensure successful detonation initiation by shock focusing. (3) Since the initial pressure is directly linked to the mass flow rate, these contrary trends can cancel each other out, which could be observed for 40% vol. of oxygen in the oxidizer. High-speed images were taken, which confirm that the detonation is initiated in the center of the converging--diverging nozzle due to focusing of the leading shock.

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Dynamic Response Formulation for Pneumatically Driven Liquid in a Pressure Vessel

ASME 1991 Computers in Engineering Conference: Volume 2 — Finite Elements/Computational Geometry; Computers in Education; Robotics and Controls ◽

10.1115/cie1991-0095 ◽

1991 ◽

Author(s):

Richard B. Loucks

Keyword(s):

Mass Flow Rate ◽

Aluminum Powder ◽

Mass Flow ◽

Flow Rate ◽

Pressure Vessel ◽

Ideal Gas ◽

Liquid Oxygen ◽

Mean Flow ◽

Containment Vessel ◽

Thermal Output

Abstract The Thermal Radiation Simulator (TRS) at the U.S. Army Ballistic Research Laboratory uses aluminum powder reacting with liquid oxygen to create a large jet like flame. The flame acts as a large thermally radiant wall, exposing targets to a nuclear weapon equivalent. The aluminum powder is driven pneumatically to the combustion chamber from a pressurized containment vessel. Unfortunately the thermal output of the flame oscillates with large amplitude relative to the mean yield. The fluctuating mass flow rate of aluminum powder from the aluminum powder containment vessel seemed the cause of the unstable output. A computer model of the aluminum vessel was constructed to determine the pressure dynamics in the pressure vessel. The aluminum powder was assumed to behave as a Newtonian liquid. The pneumatic fluid was assumed to be an ideal gas. The model concentrated inside the vessel and at the exit. The result was to determine the mass flow rate of aluminum from the exit given the inlet gas pressures. The model did reveal the source of mass flow fluctuations not to be caused directly by the existing pneumatic set-up. The variation was shown to be perturbated by forces outside the pressure vessel. Once the outside influence was eliminated, the model showed a clean mean flow rate of aluminum powder. The results were applied to the TRS and the thermal output was stabilized.

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Lean Operation of a Pulse Detonation Combustor by Fuel Stratification

Volume 4B: Combustion, Fuels, and Emissions ◽

10.1115/gt2020-16169 ◽

2020 ◽

Author(s):

Fabian E. Habicht ◽

Fatma C. Yücel ◽

Niclas Hanraths ◽

Neda Djordjevic ◽

Christian Oliver Paschereit

Keyword(s):

Gas Turbines ◽

Equivalence Ratio ◽

Pressure Ratio ◽

Initial Pressure ◽

Detonation Initiation ◽

Nox Emissions ◽

Shock Focusing ◽

Pulse Detonation ◽

Lean Combustion ◽

Fuel Stratification

Abstract Pressure gain combustion is a promising concept to substantially increase the thermal efficiency of gas turbines. One possible implementation that has been frequently investigated are pulse detonation combustors (PDCs), as they permit stable and reliable operation. At the same time, the need for part-load operation and low NOx emissions requires combustion concepts in the lean regime. However, realizing lean combustion is still very challenging in PDCs since the deflagration to detonation transition (DDT) is very sensitive to the reactant composition. The present work investigates an approach to realize lean combustion in PDC by applying fuel stratification experimentally. The scope is to find the necessary increase of fuel concentration inside the pre-detonation chamber to provide reliable DDT with respect to the overall equivalence ratio. Emission measurements in the exhaust of the PDC allow for a quantification of the NOx emissions as a function of the injected fuel profile. A valveless PDC test rig is used, which contains a shock focusing geometry for detonation initiation and is ignited by a spark plug close to the upstream end wall. The subsequent expansion of the burned gas and interaction of the flame front with turbulence leads to the formation of a leading shock inside the pre-detonation chamber, which is then focused inside a converging-diverging geometry. The successful initiation of a detonation wave by shock focusing is very sensitive to the pressure ratio across the leading shock, which can be influenced by initial pressure, reactant composition and flow velocity. Results reveal that fuel stratification allows for reliable detonation initiation at a global equivalence ratio of ϕglob = 0.65, whereas repeatable successful operation with non-stratified fuel injection is limited to ϕglob ≥ 0.85.

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A Critical Review on Thermo-Hydraulic Performance of Wire Screen Matrix Packed Solar Air Heaters

Distributed Generation & Alternative Energy Journal ◽

10.13052/dgaej2156-3306.3521 ◽

2021 ◽

Author(s):

Prashant Verma ◽

Abhishek Saxena ◽

L. Varshney

Keyword(s):

Heat Transfer ◽

Mass Flow Rate ◽

Mass Flow ◽

Flow Rate ◽

Hydraulic Performance ◽

Major Influence ◽

Pumping Power ◽

Bed Porosity ◽

Wire Screen ◽

The Impact

Solar air heaters (SAHs) have an important role in applications such as space heating and industrial drying worldwide. The packing of SAH bed not only increases the heat transfer area but also increases the pumping power losses thereby limiting the thermo-hydraulic performance. In the present study, efforts have been made for a critical assessment of the literature dealing with the impact of collector bed and operating parameters over thermal and thermo-hydraulic performance for different configurations of wire screen matrix packed SAH. The porosity of bed and mass flow rate of the air have a major influence on the thermo-hydraulic performance of wire screen matrix packed SAH. It is found that the enhancement in the volumetric heat transfer coefficient due to a decrease in bed porosity is obtained at the expense of increase in pumping power which ultimately affects the thermo-hydraulic performance of wire screen matrix packed SAH. In general it is observed that porosity is an important parameter that affects the thermo-hydraulic performance. It is seen that matrix having porosity 0.937 yields thermo-hydraulic performance of 68% at mass flow rate 0.023 kg/s where as for the same mass flow rate porosity of 0.887 results thermo-hydraulic performance of only 42%.

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Lean Operation of a Pulse Detonation Combustor by Fuel Stratification

Journal of Engineering for Gas Turbines and Power ◽

10.1115/1.4048775 ◽

2020 ◽

Author(s):

Fabian Habicht ◽

Fatma C. Yücel ◽

Niclas Hanraths ◽

Neda Djordjevic ◽

Christian Oliver Paschereit

Keyword(s):

Gas Turbines ◽

Equivalence Ratio ◽

Pressure Ratio ◽

Initial Pressure ◽

Detonation Initiation ◽

Nox Emissions ◽

Shock Focusing ◽

Successful Operation ◽

Pulse Detonation ◽

Fuel Stratification

Abstract Pressure gain combustion is a promising concept to substantially increase the thermal efficiency of gas turbines. One possible implementation are pulse detonation combustors (PDCs), as they permit stable and reliable operation. Besides, the need for part-load operation and low NOx emissions requires combustion concepts in the lean regime. The present work investigates an approach to realize lean combustion in a PDC by applying fuel stratification experimentally. The necessary increase of fuel concentration inside the pre-detonation chamber to provide reliable DDT with respect to the overall equivalence ratio is identified. Emission measurements in the exhaust allow for a quantification of the NOx emissions as a function of the injected fuel profile. A valveless PDC test rig is used, which contains a shock-focusing geometry for detonation initiation and is ignited by a spark plug close to the upstream end wall. The subsequent expansion of the burned gas and interaction of the flame front with turbulence leads to the formation of a leading shock inside the pre-detonation chamber, which is then focused inside a converging-diverging geometry. The successful initiation of a detonation wave by shock focusing is very sensitive to the pressure ratio across the leading shock, which can be influenced by initial pressure, reactant composition and flow velocity. Results reveal that fuel stratification allows for reliable detonation initiation at a global equivalence ratio of 0.65, whereas repeatable successful operation with non-stratified fuel injection is limited to a global equivalence ratio greater than 0.85.

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Changes in turbulent dissipation in a channel flow with oscillating walls

Journal of Fluid Mechanics ◽

10.1017/jfm.2012.97 ◽

2012 ◽

Vol 700 ◽

pp. 77-104 ◽

Cited By ~ 38

Author(s):

Pierre Ricco ◽

Claudio Ottonelli ◽

Yosuke Hasegawa ◽

Maurizio Quadrio

Keyword(s):

Drag Reduction ◽

Mass Flow Rate ◽

Channel Flow ◽

Mass Flow ◽

Flow Rate ◽

Plane Channel ◽

Quasi Equilibrium ◽

Mean Flow ◽

Dominant Term ◽

Turbulent Dissipation

AbstractHarmonic oscillations of the walls of a turbulent plane channel flow are studied by direct numerical simulations to improve our understanding of the physical mechanism for skin-friction drag reduction. The simulations are carried out at constant pressure gradient in order to define an unambiguous inner scaling: in this case, drag reduction manifests itself as an increase of mass flow rate. Energy and enstrophy balances, carried out to emphasize the role of the oscillating spanwise shear layer, show that the viscous dissipation of the mean flow and of the turbulent fluctuations increase with the mass flow rate, and the relative importance of the latter decreases. We then focus on the turbulent enstrophy: through an analysis of the temporal evolution from the beginning of the wall motion, the dominant, oscillation-related term in the turbulent enstrophy is shown to cause the turbulent dissipation to be enhanced in absolute terms, before the slow drift towards the new quasi-equilibrium condition. This mechanism is found to be responsible for the increase in mass flow rate. We finally show that the time-average volume integral of the dominant term is linearly related to the drag reduction.

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