Transient response of laminar premixed flame to a radially diverging/converging flow

Laminar flamelets are often used to model premixed turbulent combustion. The libraries of rates of conversion from chemical to thermal enthalpies used for flamelets are typically based on counter-flow, stained laminar planar flames under steady conditions. The current research seeks further understanding of the effect of stretch on premixed flames by considering laminar flame dynamics in a cylindrically-symmetric outward radial flow geometry (i.e., inwardly propagating flame). This numerical model was designed to study the flame response when the flow and scalar fields align (i.e., no tangential strain on the flame) while the flame either expands (positive stretch) or contracts (negative stretch, which is a case that has been seldom explored) radially. The transient response of a laminar premixed flame has been investigated by applying a sinusoidal variation of mass flow rate at the inlet boundary with different frequencies to compare key characteristics of a steady unstretched flame to the dynamics of an unsteady stretched flame. An energy index (EI), which is the integration of the source term in the energy equation over all control volumes in the computational domain, was selected for the comparison. The transient response of laminar premixed flames, when subjected to positive and negative stretch, results in amplitude decrease and phase shift increase with increasing frequency. Other characteristics, such as the deviation of the EI at the mean mass flow rate between when the flame is expanding and contracting, are nonmonotonic with frequency. Also, the response of fuel lean flames is more sensitive to the frequency of the periodic stretching compared to a stoichiometric flame. An analysis to seek universality of transient flame responses across lean methane-air flames of different equivalence ratios (i.e., 1.0 to 0.7) using Damköhler Numbers (i.e., the ratio of a flow to chemical time scales) had limited success.

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Transient response of laminar premixed flame to a radially diverging/converging flow

10.32920/14636262.v1 ◽

2021 ◽

Author(s):

Meysam Sahafzadeh ◽

Larry W. Kostiuk ◽

Seth B. Dworkin

Keyword(s):

Mass Flow Rate ◽

Transient Response ◽

Mass Flow ◽

Flow Rate ◽

Laminar Flame ◽

Premixed Flames ◽

Scalar Fields ◽

Premixed Flame ◽

Computational Domain ◽

Tangential Strain

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Numerical Study on the Main Coolant Pump of Sodium-Cooled Fast Reactor

Volume 1A: Symposia, Part 2 ◽

10.1115/ajkfluids2015-02403 ◽

2015 ◽

Author(s):

Sungho Ko ◽

Yeon-tae Kim

Keyword(s):

Mass Flow Rate ◽

Mass Flow ◽

Flow Rate ◽

Fast Reactor ◽

Numerical Study ◽

Pressure Rise ◽

Head Loss ◽

Computational Domain ◽

Coolant Pump ◽

Pump Head

A numerical study was conducted to predict the performance curve of a downscaled model of the main coolant pump for a sodium-cooled fast reactor and to reduce the head loss by the optimization of the diffuser blade. The ANSYS CFX program was utilized to obtain flow characteristics inside the pump as well as the overall pressure rise across the pump operating on- and off-design points. Computational domain was divided into several blocks to achieve high grid quality effectively and 7.5 million nodes were used totally to resolve small leakage flows as well as the flow inside the rotating impeller. The corresponding experiment was conducted to validate CFD computed results. The comparison between the CFD and experimental data shows excellent agreement in terms of mass flow rate and head rise on and near design operating points. The DOE (design of experiments) and RSM (response surface method)[1] were utilized to reduce the head loss by the diffuser blade in the pump. The diffuser blade was defined as four geometric parameters for DOE. The analysis of 25 cases was made to solve the output parameters for all design points which are defined by the DOE. RSM was fitting the output parameter as a function of the input parameters using regression analysis techniques. The optimized model increased the total pump head on the design point and the low mass flow rate point, but total pump head on 130% of operating mass flow rate was reduced than the initial model.

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The response of the solid fuel ramjet combustor to the inflow excitations

Proceedings of the Institution of Mechanical Engineers Part G Journal of Aerospace Engineering ◽

10.1177/09544100211053405 ◽

2021 ◽

pp. 095441002110534

Author(s):

AmirMahdi Tahsini ◽

Seyed Saeid Nabavi

Keyword(s):

Mass Flow Rate ◽

Mass Flow ◽

Flow Rate ◽

Solid Fuel ◽

Ambient Pressure ◽

Excitation Frequency ◽

Engine Performance ◽

Computational Domain ◽

Rocket Motors ◽

Fuel Mass

The response of the solid fuel ramjet to the imposed excitations of the ambient pressure is investigated using full part computation of the system including the intake, combustion chamber, and exhaust nozzle. The finite volume solver of the turbulent reacting compressible flow is used to simulate the flow field, where two grid blocks are considered for discretizing the computational domain. Both impulsive and oscillatory excitations are imposed to predict the response of the solid fuel mass flow rate. The results demonstrate that strong fuel flow overshoot occurs in the case of sudden impulsive excitation which is omitted for gradual impulsive excitations. In addition, the oscillatory excitations eventually lead to regular oscillatory response with frequencies similar to the imposed excitations and decrease the average fuel mass flow rate independent of the excitation frequency. But the amplitude of the response depends on the excitation frequency and amplification occurs in some frequencies. This behavior is not related to the combustion instabilities and is similar to the L-star instability in the solid rocket motors. In the design and analysis of the solid fuel ramjets, the coupling of the flight dynamics and the engine performance must be considered, and this study is the first step of such complete methodology to have more accurate predictions.

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Review and Analysis of Cross Flow Heat Exchanger Transient Modeling for Flow Rate and Temperature Variations

Journal of Thermal Science and Engineering Applications ◽

10.1115/1.4031222 ◽

2015 ◽

Vol 7 (4) ◽

Cited By ~ 3

Author(s):

Tianyi Gao ◽

James Geer ◽

Bahgat Sammakia

Keyword(s):

Heat Exchanger ◽

Mass Flow Rate ◽

Transient Response ◽

Mass Flow ◽

Flow Rate ◽

Heat Exchangers ◽

Cross Flow ◽

Inlet Temperature ◽

Flow Rate Change ◽

Flow Heat

Heat exchangers are important facilities that are widely used in heating, ventilating, and air conditioning (HVAC) systems. For example, heat exchangers are the primary units used in the design of the heat transfer loops of cooling systems for data centers. The performance of a heat exchanger strongly influences the thermal performance of the entire cooling system. The prediction of transient phenomenon of heat exchangers is of increasing interest in many application areas. In this work, a dynamic thermal model for a cross flow heat exchanger is solved numerically in order to predict the transient response under step changes in the fluid mass flow rate and the fluid inlet temperature. Transient responses of both the primary and secondary fluid outlet temperatures are characterized under different scenarios, including fluid mass flow rate change and a combination of changes in the fluid inlet temperature and the mass flow rate. In the ε-NTU (number of transfer units) method, the minimum capacity, denoted by Cmin, is the smaller of Ch and Cc. Due to a mass flow rate change, Cmin may vary from one fluid to another fluid. The numerical procedure and transient response regarding the case of varying Cmin are investigated in detail in this study. A review and comparison of several journal articles related to the similar topic are performed. Several sets of data available in the literatures which are in error are studied and analyzed in detail.

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Prediction of the Transient Thermodynamic Response of a Closed-Cycle Regenerative Gas Turbine

Journal of Engineering for Gas Turbines and Power ◽

10.1115/1.1806449 ◽

2005 ◽

Vol 127 (1) ◽

pp. 57-64 ◽

Cited By ~ 1

Author(s):

T. Korakianitis ◽

J. I. Hochstein ◽

D. Zou

Keyword(s):

Heat Exchanger ◽

Gas Turbine ◽

Mass Flow Rate ◽

Transient Response ◽

Mass Flow ◽

Flow Rate ◽

Transient Flow ◽

Closed Cycle ◽

Flow Heat ◽

Component Models

Instantaneous-response and transient-flow component models for the prediction of the transient response of gas turbine cycles are presented. The component models are based on applications of the principles of conservation of mass, energy, and momentum. The models are coupled to simulate the system transient thermodynamic behavior, and used to predict the transient response of a closed-cycle regenerative Brayton cycle. Various system transients are simulated using: the instantaneous-response turbomachinery models coupled with transient-flow heat-exchanger models; and transient-flow turbomachinery models coupled with transient-flow heat-exchanger models. The component sizes are comparable to those for a solar-powered Space Station (radial turbomachinery), but the models can easily be expanded to other applications with axial turbomachinery. An iterative scheme based on the principle of conservation of working-fluid mass in the system is used to compute the mass-flow rate at the solar-receiver inlet during the transients. In the process the mass-flow rate of every component at every time step is also computed. Representative results of different system models are compared and discussed.

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Numerical Investigation on Flow Characteristics of Low Pressure Exhaust Hood Under Off-Design Conditions for Steam Turbines

Volume 5B: Oil and Gas Applications; Steam Turbines ◽

10.1115/gt2013-95257 ◽

2013 ◽

Cited By ~ 1

Author(s):

Shuai Shao ◽

Qinghua Deng ◽

Heshuang Shi ◽

Zhenping Feng ◽

Kai Cheng ◽

...

Keyword(s):

Mass Flow Rate ◽

Mass Flow ◽

Flow Rate ◽

Static Pressure ◽

Computational Domain ◽

Exhaust Hood ◽

Flow Conditions ◽

Back Flow ◽

Three Stages ◽

Last Stage

In this paper, numerical investigations on the aerodynamic characteristics of the last three stages and the exhaust hood for a large power steam turbine were conducted under a series of mass flow conditions (100% ∼ 10% of the design condition) using the commercial CFD software ANSYS-CFX. The single passages of the last three stages and the whole exhaust hood are combined together as the computational domain. The main objective of this present work is to analyze the aerodynamic performance and the flow behavior of the exhaust hood. The variations of the static pressure recovery coefficient and the total pressure loss coefficient while the mass flow rate decreasing were analyzed. The static pressure distributions along the diffuser surface under different flow conditions were illustrated. The development of the vortex near the outlet of the diffuser was demonstrated through the velocity vector distribution at the meridional plane of the exhaust hood. The windage conditions were analyzed under 20% and 10% mass flow rate of the design condition. In addition, the back flow phenomenon was observed when the mass flow rate was below 50% of the design condition, and it starts from the hub region of the last stage rotor and grows up along the radial direction. The back flow also induces a sharp turning on the span-wise distribution of the angle θ (defined in Fig. 9) at the outlet of the last stage rotor. The three-dimensional streamlines inside the exhaust hood under different mass flow conditions were also compared.

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Response of Laminar Premixed Flame to Mass Flow Rate and Pressure Fluctuations

Heat Transfer, Volume 2 ◽

10.1115/imece2006-16250 ◽

2006 ◽

Author(s):

Baidurja Ray ◽

Ishita Chakraborty ◽

Achintya Mukhopadhyay ◽

Subhashis Ray ◽

Swarnendu Sen

Keyword(s):

Mass Flow Rate ◽

Surface Area ◽

Mass Flow ◽

Flow Rate ◽

Closed Form Solution ◽

Form Solution ◽

Premixed Flame ◽

Flame Surface ◽

Kinematic Equation ◽

Flame Response

In this work, we develop an analytical model to describe laminar premixed flame response to an oscillating flow and use this model to predict the relationship between the heat release rate and the instantaneous flow field. Fully developed pulsating flow through a channel is considered. The flow is driven by pressure gradients. To facilitate direct comparison with experiments, the transient velocity profile is obtained in terms of mass flow rate fluctuations. The flame is anchored at the channel wall. The flame is assumed to be a thin surface, separating the reactants and the products. Flame displacement speed is assumed to be constant. The flame displacement is described by a single-valued function of the transverse coordinate. The flame dynamics is represented by a kinematic equation describing the displacement of the surface. The assumption of constant flame speed and fully developed flow allows closed-form solution of the flame response. The temporal variation of the mass flow rate and the flame surface area are compared to determine the gain and phase difference of the flame transfer function, relating the fluctuations in flame surface area to fluctuations in the mass flow rate.

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Prediction of the Transient Thermodynamic Response of a Closed-Cycle Regenerative Gas Turbine

Volume 2: Combustion and Fuels; Oil and Gas Applications; Cycle Innovations; Heat Transfer; Electric Power; Industrial and Cogeneration; Ceramics; Structures and Dynamics; Controls, Diagnostics and Instrumentation; IGTI Scholar Award ◽

10.1115/93-gt-136 ◽

1993 ◽

Cited By ~ 1

Author(s):

T. Korakianitis ◽

J. I. Hochstein ◽

D. Zou

Keyword(s):

Heat Exchanger ◽

Gas Turbine ◽

Mass Flow Rate ◽

Transient Response ◽

Mass Flow ◽

Flow Rate ◽

Transient Flow ◽

Closed Cycle ◽

Flow Heat ◽

Component Models

This paper presents instantaneous-response and transient-flow component models for the prediction of the transient response of gas turbine cycles. The component models are based on applications of the principles of conservation of mass, energy, and momentum. The models are coupled to simulate the system transient thermodynamic behavior, and used to predict the transient response of a closed-cycle regenerative Brayton cycle. Various system transients are simulated using: the instantaneous-response turbomachinery models coupled with transient-flow heat-exchanger models; and transient-flow turbomachinery models coupled with transient-flow heat-exchanger models. The component sizes are comparable to those under consideration for the solar-powered Space Station (radial turbomachinery), but the models can easily be expanded to other applications with axial turbomachinery. An iterative scheme based on the principle of conservation of working-fluid mass in the system is used to compute the mass-flow rate at the solar-receiver inlet during the transients. In the process the mass-flow rate of every component at every time step is also computed. Representative results of different system models are compared and discussed.

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