Transient Start-up Research Articles

A methodology to study the possible occurrence of chugging in liquid rocket engines during transient start-up

An investigation on the low frequency combustion instabilities due to the interaction of combustion chamber and feed line dynamics in a liquid rocket engine is carried out implementing a specific module in the system analysis software EcosimPro. The properties of the selected double time lag model are identified according to the two classical assumptions of constant and variable time lag. Module capabilities are evaluated on a literature experimental set up consisting of a combustion chamber decoupled from the upstream feed lines. The computed stability map results to be in good agreement with both experimental data and analytical models. Moreover, the first characteristic frequency of the engine is correctly predicted, giving confidence on the use of the module for the analysis of chugging instabilities. As an example of application, a study is carried out on the influence of the feed lines on the system stability, correctly capturing that the lines extend the stable regime of the combustion chamber and that the propellant domes play a key role in coupling the dynamics of combustion chamber and feed lines. A further example is presented to discuss on the role of pressure growth rate and of the combustion chamber properties on the possible occurrence of chug instability during engine start-up and on the conditions that lead to its damping or growth.

Tuning the control penalty factor of a minimum variance adaptive controller

This paper presents a startup and tuning procedure for an adaptive control system based on a minimum variance controller, proposing a successive dual setting of the control penalty factor (ρ) which weights the influence of the control variance in a cost criterion function. The main idea consists of a differentiated approach of the transient startup regime (including the following steps: open-loop initialization of the parameter estimates, initial tuning of the controller parameters, closing the loop, synchronization between the controller and the plant, retuning of the control penalty factor), respectively of the stable steady-state regime reached by the control system, by tuning the controller using two distinct values of factor ρ. Firstly, in the control system startup phase, characterized by strong transient regimes which could destabilize the system, a high value of ρ factor is chosen. Such a value of ρ ensures a strong penalization of the control and, at the same time, smoothes the transient responses, leading the control system to a stable steady-state regime. However, a too high value of ρ factor is not proper for an efficient rejection of disturbances which perturb the plant during this stable state. Based on these considerations, a retuning of the controller is required through a significant decrease of the ρ factor, thus increasing the dynamic of control, which becomes able to provide an efficient rejection of the disturbances acting on the plant. The new proposed startup and tuning procedure was tested and validated for the case of an excitation control system of an induction generator connected to a power system. The novelty of the paper consists of this dual setting of the control penalty factor (ρ).

Optimisation of stand-alone hybrid energy systems supplemented by combustion-based prime movers

A comparative analysis is undertaken between a baseline PV/Batt system, meeting a dynamic load profile, and systems hybridised with supplementary combustion-based prime movers such as Internal Combustion Engines (ICEs) or Micro Gas Turbines (MGTs) of 30–65kW rating. This study sheds light for the first time on a number of research questions not addressed in earlier studies. The main contributions of the work are namely to: (i) analyse the effects of the start-up threshold and the type of supplementary prime mover on the Cost of Energy (COE, $/kWh), lifetime CO2 emissions, and (unrecovered) waste heat for a specified reliability (Loss of Power Supply Probability-LPSP); (ii) investigate the effects of including the transient start-up periods of prime movers on systems sizing; and (iii) look into the effects of using two smaller sized (tandem) supplementary prime movers versus a single larger one on the operational characteristics. The research also analyses (iv) the effects of the methods used (e.g. temporal resolution of simulations, Genetic Algorithm (GA) population size) on the COE, lifetime CO2 emissions, and (unrecovered) waste heat.The results of this study indicate that PV/Batt and PV/Batt/ICE systems have comparable COEs but are preferable to PV/Batt/MGT. The minimum starting thresholds of supplementary devices (ICE or MGT) have significant effects on renewable energy penetration, genset running hours, waste heat generation, and Life Cycle Emission (LCE, kg CO2-eq/yr), but insignificant effects on the COE. The results also show that the transient start-up of supplementary devices has a negligible influence on overall system sizing. The COE resulting from the use of larger capacity prime movers (60kW ICE or 65kW MGT) is comparable to deploying two smaller capacity prime movers (30kW) but results in higher renewable energy penetration, an improved duty factor with lower LCEs. Additionally, the COE increases slightly (2∼5%) when the models run at 15min temporal resolution compared to 60min.

Method for CFD facilitated pressure rise calculation due to deflagration in heat recovery steam generator

For combined cycle operations, especially during startup and shutdown, safety concerns have always been the top priority. Residual fuels can escape to and accumulate in the downstream heat recovery steam generator (HRSG) if appropriate purge is not taken, which may cause deflagration, an explosion at subsonic condition, during startup when heat source exists. Residual fuel deflagration will lead to pressure rise and therefore, structural and or tube damages depending on the level of pressure increase. Therefore, a careful study of the residual fuel distribution in HRSG and the pressure rise due to possible deflagration is wanted.Historically, thermodynamics relationships and chemical equilibrium calculations are used to estimate the pressure rise. These approaches either assume that all fuel energy is converted into heat, or make assumptions on the amount of fuel in chemical equilibrium that contributes to the pressure rise. These assumptions tend to overestimate the pressure rise. With the development of computational fluid dynamics (CFD) simulation, a more accurate prediction of the fuel concentration in HRSG during transient startup and/or shutdown process is possible. The local fuel concentrations can now be calculated and therefore, how much fuel that are within the explosion limits and contribute to the pressure rise can be detected readily.This article presents the transient CFD modeling results of residual fuel concentration in a typical HRSG configuration during a gas turbine/combined cycle startup failure process. The article then refines the adiabatic mixing model with the predicted local fuel concentrations to provide a better estimation of the pressure rise due to deflagration. © 2017 American Institute of Chemical Engineers Process Saf Prog 36: 408–413, 2017

A numerical study on the aerodynamic performance and the self-starting characteristics of a Darrieus wind turbine considering its moment of inertia

Self-starting characteristics is a challenging issue in the field of Darrieus-type vertical axis wind turbines. Traditionally, the numerical simulations were performed at several constant rotational velocities for the turbine and the generated torques were reported for each rotational velocity, neglecting the effects of the turbine inertia on the transient start-up motion of the turbine. In the current study, a numerical method is proposed to study the self-starting characteristics of a Darrieus wind turbine, considering the turbine moment of inertia. The simulation starts from the initial stationary state and continues until the final steady-periodic condition. At each time step, the instantaneous rotational velocity of the turbine is computed based on the Newton’s second law, according to the instantaneous aerodynamic and mechanical forces acting on the turbine. Results indicate that as the rotor inertia increases, it takes a longer time for the turbine to reach its final velocity, in a manner that, the turbine might even fail to reach the final condition and the rotation halts. Results also show that as the rotor inertia decreases, the oscillations amplitude of the turbine rotational velocity increases. This can enable the turbine to pass higher resistant torques than those computed based on the traditional method.

Modeling startup and shutdown transient of the microlinear piezo drive via ANSYS

The article describes the construction-design of the micro linear piezo drive intended for a peripheral cord tensioner in the reflecting surface shape regulator system for large-sized transformable spacecraft antenna reflectors. The research target –the development method of modeling startup and shutdown transient of the micro linear piezo drive. This method is based on application software package ANSYS. The method embraces a detailed description of the calculation stages to determine the operating characteristics of the designed piezo drive. Based on the numerical solutions, the time characteristics of the designed piezo drive are determined.

Optimal Design of a Thermoelectric Cooling/Heating System for Car Seat Climate Control (CSCC)

In the present work, the optimum design of thermoelectric car seat climate control (CSCC) is studied analytically in an attempt to achieve high system efficiency. Optimal design of a thermoelectric device (element length, cross-section area and number of thermocouples) is carried out using our newly developed optimization method based on the ideal thermoelectric equations and dimensional analysis to improve the performance of the thermoelectric device in terms of the heating/cooling power and the coefficient of performance (COP). Then, a new innovative system design is introduced which also includes the optimum input current for the initial (transient) startup warming and cooling before the car heating ventilation and air conditioner (HVAC) is active in the cabin. The air-to-air heat exchanger's configuration was taken into account to investigate the optimal design of the CSCC.

Modeling of flashing-induced flow instabilities for a natural circulation driven novel modular reactor

An analytical study based on frequency domain analysis is presented on the flashing-induced flow instability in a natural circulation test facility, which was designed to investigate the flow instability for a BWR-type novel modular reactor (NMR). To address the flashing phenomena at low pressure conditions, such as initial startup transients or accidents, the liquid enthalpy change in the P-T diagram due to reduced hydrostatic head in the riser or chimney was treated as an axially uniform heat flux. Based on the drift flux model, the system transfer function was obtained through small perturbations about the steady state in the frequency domain. The D-partition method was used to determine the neutral stability boundary in the dimensionless stability plane, which was constituted of the subcooling number and phase change number. From the frequency domain analysis, the flashing stability boundary and the density wave oscillations boundary could be predicted. Some parametric studies had been performed on the system pressure and the inlet flow resistance coefficients in the stability analysis. The results showed that the flashing stability boundary was more sensitive to the system pressure than the density wave oscillations. In addition, the theoretical stability boundaries were benchmarked against the experimental stability boundaries from quasi-steady state tests. Although the general stability boundary agreed well with the experiments, certain discrepancies still existed due to the assumptions of thermal equilibrium in current study. In the future, the thermal non-equilibrium conditions including subcooled boiling will be taken into account in the flashing induced stability analysis.

Real-time aerosol measurements in pilot scale coal fired post-combustion CO2 capture

Coal fired plants account for nearly 40% of the world's electric power generation and are a large source of CO2 emissions. Post-combustion CO2 capture using amine based solvents is a relatively mature technology. Recent pilot plants test data indicate that amine and its degradation produce emissions in the form of aerosols, which must be controlled for commercial success of this technology. In this study, the aerosols from CO2 depleted flue gas at the National Carbon Capture Center (NCCC) Pilot Solvent Test Unit (PSTU) and Slipstream Solvent Test Unit (SSTU) were measured using an isokinetic probe and a Dekati Electric Low Pressure Impactor (ELPI+TM). Multiple tests were conducted to quantify the effects of different process changes. PSTU measurements demonstrated sample sensitivity to transient intercooler start-up conditions, dilution gas temperatures and absorber beds in operation. During this test, the typical concentration (expressed in number of particles per volume) ranged from E+06 to E+07cm−3 and the peak occurred near 0.12µm with an observed increase in sub 0.01µm aerosol concentration during transient intercooler start-up conditions. The measurements also demonstrated sample sensitivity to dilution gas temperatures by showing that aerosol size and concentration peaks shifted at higher diluter operating temperatures, especially above 100°C. Several tests were also conducted at the absorber inlet and wash tower outlet of the SSTU with varying dilution ratios and temperatures. Comparison of SSTU and PSTU results are presented in this paper. The stage cuts for different impactor stages of different ELPI+ instruments are calibrated which showed minimum deviation in the range of 0.02–0.70% from original calibration. Generally, the concentrations were higher at the absorber inlet compared to the wash tower outlet. Also, a higher concentration was observed for the smallest size fraction at the wash tower outlet compared to the absorber inlet. Similar to the PSTU, the concentration measured at the SSTU was in the range of E+06cm−3. These results are promising and will enable the development of process control strategies to mitigate solvent losses and reduce operational and maintenance expenses.

Analytical solutions of channel and duct flows due to general pressure gradients

Pursued herein are the closed-form solutions of the Navier–Stokes equations for both planar channel and circular duct flows, influenced by either periodic or aperiodic pressure gradients, of which the amplitudes are sufficiently low to yield laminar incompressible flows. The analyses conducted for the unsteady flows parallel to the walls lead to the analytical solutions that encompass not only the long-time oscillations by periodic pressure gradients, but also the transient start-up flows commencing from zero velocity due to arbitrary aperiodic pressure gradients. With the standard methods employed, the present solutions generalizing the classic ones are written in the forms rendering the explicit dependence on the pressure gradient, and are numerically validated by the existing solutions of simple sinusoidal oscillations and a flow involving an aperiodic impulsive pressure gradient. By virtue of their functional forms, the present solutions can be applied with any pressure gradients, even if the gradients are not in closed forms.

Adapting Steady-State Solar Power Models to Include Transients

Quite a few computer programs have been developed to model power plant performance. These software codes are geared toward modeling steady-state operations, which are usually sufficient for conventional power plants. Solar thermal power plants undergo prolonged transient start-up and shut-down operations due to the periodic nature of solar radiation. Moreover, the large size of the solar field brings about large residence time that must be considered to accurately lag power generation. A novel scheme has been developed to fine-tune steady-state solar power generation models to accurately take account of the impact of those transient operations. The suggested new scheme is implemented by adjusting solar radiation data input to the model and has been shown to clearly improve modeling accuracy by moving modeled results closer to matching real operating data.

Modeling the dynamic behavior of turbine runner blades during transients using indirect measurements

Turbine start-up transients are induced by the wicket gates opening sequence and generate high amplitude stress cycles. These stress cycles have a detrimental effect leading to faster crack growth in the runner blades. Using a series of direct measurements taken on a prototype runner in order to find the optimal start-up parameters exposes both the runner and the instrumentation to a series of successive damaging transient events during the optimization process. To solve this, finding sensors strongly correlated to strain gauges and whose signals can be easily obtained to identify a model to predict the strain, instead of directly measuring it, would reduce the risk, cost and downtime associated with a measurement campaign. This paper shows that turbine shaft torsion measurements is highly correlated to the strain at a runner blade hotspot, and we demonstrate that the ARMAX model can be used to represent the dynamic system in order to minimize the strain on blades.

Coal particle and bed physical transformational behaviour after thermochemical conversion in a fixed bed

This paper examines the physical transformational behaviour that coal particles and the bed structure undergo in a fixed bed reactor. The transient overfeed configuration is used with feed gas entering from the bottom of the 1.2m reactor with an inner diameter of 104mm. The fuel bed loading, of 3.3kg on average, consisted of non-devolatilized seam two coal of 4, 6 and 8mm particles from the Highveld region in South Africa and was inertinite-rich. Transient exit gas composition and temperature profile inside the fixed bed of particles, is presented along with a full chemical and physical characterisation of the feed coal and the partially converted bed contents consisting of coal, char and ash particles. The bed dissection method implemented was designed to limit mechanical fragmentation by minimizing particle handling and bed structural disturbance during sampling. Particle size distribution and the particle size distribution width showed a significant variation in the oxidation zone and the ash bed which was not previously quantified on industrial scale experiments. The bulk, true as well as the apparent (measured by hydrostatically weighing in mercury) densities are reported for the different reaction zones of pyrolysis, reduction, oxidation and the ash bed, and showed significant variation in each zone. The particles and the fixed bed structural change were established by the measured particle porosity and the fixed bed voidage. The extensive role of particle agglomeration in the lower sections of the fixed bed was demonstrated in the bed voidage data and is evidently tracked as the reaction front moves up through the fixed bed of particles. The mode of operation simulated the transient start-up procedure and subsequent particle and bed transformational behaviour during industrial fixed bed combustion and gasification.

Optimization of a Rocket Nozzle Side Load Reduction Device

During transient start-up and shutdown of rocket engines under sea-level conditions, the flow inside the supersonic part of the nozzle separates from the wall. The separated flow is asymmetrically distributed around the circumference and hence induces side loads. A cold flow subscale test campaign was carried out at DLR to study a device that reduces the side loads of a thrust-optimized parabolic nozzle under transient conditions. A detailed parametrical study was performed. The experimental data demonstrated and quantified the side load reduction potential of the device.

Coal particle chemical transformational behaviour after thermochemical conversion in a fixed bed

This paper examines the chemical transformational behaviour that coal particles undergo during conversion in deep packed beds, typically found in industrial combustion and gasification operations. Experiments were performed in a transient overfeed packed bed laboratory scale reactor fed with 4, 6 and 8mm coal particles. The reactor was operated at atmospheric pressure and is 1.2m in length and has an inner diameter of 104mm. The coal loading was 3.3kg and resulted in a bed height of 520mm. A typical inertinite-rich South African seam four coal from the Highveld region was used as fuel in the reactor. The transient temperature profile in the descending bed as well as the exit gas composition is presented here. A post experiment dissection of the bed contents along with full chemical characterisation was undertaken. Care was taken to ensure that the dissection method, specifically designed for minimal disturbance to the fixed bed structure, could provide an accurate representation of the characteristic reaction zones. The residual volatile matter and the overlap in the reduction and pyrolysis zones were insensitive to particle size variation and mainly determined by the maximum temperature in each zone. Where it was found useful, these reaction zone profiles are compared to industrial and pilot operations. The reaction front velocity and heating rates that particles experience in the different reaction zones was obtained and showed significant variation during the transient start-up stage, but are remarkably comparable once the stable reaction front is formed. This current mode of operation represents fixed bed combustion and gasification operations particularly during the transient start-up stages.

Characterization of Flashing Phenomena with Cryogenic Fluid Under Vacuum Conditions

Flashing occurs when liquid undergoes a sudden depressurization and triggers a violent atomization and vaporization. This phenomenon may happen at the transient startup of the upper-stage rocket engine. In this paper, experimental investigation of cryogenic fluid injection into a low-pressure environment is performed using liquid nitrogen instead of the real cryogenic propellants (liquid oxygen and liquid methane). With the help of a high-speed shadowgraph technique and with the temperature measurement, the characteristics of flashing are explored, including its evolution process, the spray angle, the spray thermal behaviors along the injection centerline, and the spray phase solidification. The results show that bubble nucleation is a reasonable controlling factor for the flashing evolution process. The nondimensional energy barrier is conditionally indicative of the flashing transition process, and it works well under low injection temperature conditions. The drastic temperature drop due to flashing phenomena causes liquid jet solidification, which may pose a challenge for the engine startup process. An analysis of the spray lateral propagation indicates that superheat exerts a substantial effect on the spray angle enhancement; however, this enhanced effect is found to decrease with an increase of liquid superheat.

Benchmarking the Performance of Boost-Derived Converters Under Start-Up and Load Transients

The control of boost-derived converters presents a special challenge due to its non-minimum phase behavior, forcing a slow transient response when traditional linear techniques are employed. In order to improve the dynamic behavior, more complex controllers have been proposed (e.g., adaptive, nonlinear, boundary, etc.), but the improvements introduced by each one of them cannot be objectively assessed in the absence of a theoretical performance limit standard reference. This work introduces a benchmarking tool based on the evaluation of the converters’ performance under start-up, load current, and input voltage step-up/step-down transients. Six performance indices are defined using the theoretical performance limits as reference. In this way, the converters’ dynamic performance can be quantitatively assessed in six different aspects, enabling a thorough benchmarking procedure. The ideal parameters are found by exploring the performance limits of boost converters, providing valuable insight into the behavior of the topology. These reference parameters are found in a normalized domain and characterized by closed-form expressions, providing simple, intuitive, and general results valid for any combination of LC parameters and voltage levels. The characterization of the dynamic performance limits is validated by experimental results. The proposed benchmarking procedure is illustrated by evaluating the dynamic performance of two different dc–dc boost converters.

Flow and Acoustics of Clustered Rockets During Startup

The plume produced by a cluster of two large-area-ratio thrust-optimized parabolic contour nozzles is visualized over a range of nozzle pressure ratios by way of retroreflective shadowgraphy. Both nozzles exhibit free-shock separated flow, restricted-shock separated flow, and an end-effects regime before flowing full. Transient (startup) operations of the nozzles are studied, with the primary focus being the pulsations that form during the end-effects regime. This occurs at a pressure ratio of 37 for these nozzles and is associated with elevated sound levels in the immediate vicinity of the nozzles and vehicle. The shadowgraphy images reveal the formation of turbulent large-scale structures, on the order of the nozzle diameter, during the end-effects regime. These large-scale structures are driven by the intermittent opening of the last trapped annular separation bubble to the ambient and grow rapidly within the first two nozzle diameters.

Shear rheology of polylactide (PLA)–cellulose nanocrystal (CNC) nanocomposites

A simple method for the preparation of polymer-cellulose nanocrystal (CNC) nanocomposite is shown to yield good dispersion of CNCs within a polylactide (PLA) matrix, which consequently resulted in the lowest rheological percolation threshold reported so far for polymer-CNC systems. The rheological behavior of the nanocomposites was determined in dynamic, transient, and steady-shear flow fields in the molten state. The complex viscosity and storage modulus of the nanocomposites increased markedly with CNC content, particularly at low frequencies; the samples were highly shear thinning and exhibited a transition from liquid- to solid-like behavior as the CNC concentration increased. Larger values for steady-state viscosity, yield stress, shear stress, and first normal stress difference were reported for the more concentrated nanocomposites. Also, pronounced overshoots in the transient start-up viscosity of the nanocomposites were observed. These results could be ascribed to the formation of an interconnected CNC network within the PLA matrix.

PALS - An Auspicious New Gas Turbine Seal

This article highlights various aspects of a new gas turbine shaft seal called the pressure activated leaf seal (PALS). The paper in detail discusses the architecture and working of a gas turbine seal. PALS is designed to use changing pressure drop forces across the seal to eliminate rub. The seal elements stay clear of the rotor seal surface during start-up and shutdown transients, and subsequently close to a small, non-contacting, steady-state running clearance. During start-up or shutdown, when the axial pressure difference across PALS is small, the leaves are in a relaxed open position, providing a general clearance gap for possible rotor seal surface eccentricities. At operating speeds, the resulting axial pressure difference causes the leaf element to elastically deflect and close, reaching the design clearance when they contact the support member. The test results show that the PALS concept provides for a potentially viable, robust, low leakage seal for gas turbine applications.