Reheat effect on the improvement in efficiency of the turbine driven by pulse detonation

Due to the strong unsteadiness of pulse detonation,large flow losses are generated when the detonation wave interacts with the turbine blades,resulting in low turbine efficiency.Considering that the flow losses are dissipated into the gas as heat energy,some of them can be recycled during the expansion process in subsequent stages by the reheat effect,which should be helpful to improve the detonationdriven turbine efficiency.Taking this into account,this paper developed a numerical model of the detonation chamber coupled with a two-stage axial turbine,and a stoichiometric hydrogen-air mixture was used.The improvement in turbine efficiency attributable to the reheat effect was calculated by comparing the average efficiency of the stages with the efficiency of the two-stage turbine.The research indicated that the first stage was critical in suppressing the flow unsteadiness caused by pulse detonation,which stabilized the intake condition of the second stage and consequently allowed much of the flow losses from the first stage to be recycled,so that the efficiency of the two-stage turbine was improved.At a 95%confidence level,the efficiency improvement was stable at 4.5%—5.3%,demonstrating that the reheat effect is significant in improving the efficiency of the detonation-driven turbine.

Structural evolution of non dendritic AlSi7Mg alloy during reheating

The structural evolution of non dendritic AlSi7Mg alloy during reheating in resistance furnace was studied. The alloy ingots were produced by electromagnetic stirring during solidification. It was found that, the Si phase in eutectic dissolves in a way of diffusion toward α phase, its appearance changes from flake to dot like, and tends to be fine and spheroidal with increasing reheating temperature. The thinner the flake, the lower the temperature for the occurrence of this process, and the higher the transforming speed. The eutectic melts partially when Si phase dissolves to some extent, and the morphology and size of primary α phase begin to change. The dendrite and rosette α phases tends to sphericize. The size of the former becomes larger, while the size of the latter reduces to be 1/2～1/4 of the original size. The spheroidal primary α phase has a tendency of grain growth.

REHEATING TEMPERATURE CONTRAST AND MICROSTRUCTURES OF 7075 Al ALLOY CAST BY LIQUIDUS SEMI-CONTINUOUS CASTING

In this study, reheating of liquidus semi-continuous cast billets of 7075 Al alloy was carried out in a resistance furnace, and the temperature contrast of the outer and the center of the reheated billets was investigated, then the reheating microstructures were investigated. Results show that: the difference of temperature between the outer and center is small and the difference of their microstructures are also small. During reheating at 576℃ the spheroidization of grains is significant after 5min and no rosettes are visible after 20min by optical microscopy. Similar observations were madeon materials reheated at 596℃, but the ripening process is faster. The grains growup to 30-60μm, fine enough for thixoforming.

Kinetics of bainite-to-austenite transformation during continuous reheating in low carbon microalloyed steel

A dilatometer was used to study the kinetics of bainite-to-austenite transformation in low carbon microalloyed steel with the initial microstructure of bainite during the continuous reheating process. The bainite-to-austenite trans- formation was observed to take place in two steps at low heating rate. The first step is the dissolution of bainite, and the second one is the remaining bainite-to-austenite transformation controlled by a dissolution process. The calculation result of the kinetics of austenite formation shows that the two steps occur by diffusion at low heating rate. However, at high heating rate the bainite-to-austenite transformation occurs in a single step, and the process is mainly dominated by shear. The growth rate of austenite reaches the maximum at about 835℃ at different heating rates and the growth rate of austenite as a function of temperature increases with the increase in heating rate.

Advanced control of walking-beam reheating furnace

Rehearing furnace is an important device with complex dynamic characteristicsin steel plants. The temperature tracing control of reheating furnace has great importance both tothe quality of slabs and energy saving. A model-based control strategy, multivariable constrainedcontrol (MCC) for the reheating furnace control is used. With this control method, the furnace istreated as a six-input-six-output general model with loops coupled in nature. Compared with thetraditional control, the proposed control strategy gets better temperature tracing accuracy andexhibits some energy saving feature. The simulation results show that the performance of the furnaceis greatly improved.

Cellular automata modelling of austenite grain coarsening during reheating——I. Normal grain coarsening

A two-dimensional cellular automaton (CA) model has been developed for thedescription of the normal grain coarsening process. The probabilistic CA method incorporatingMoore's definition of the neighbourhood is used to simulate the normal grain coarsening process witha new transition rule. The model simulates the grain coarsening process in as much detail that ispossible, from the point of initial nucleation to subsequent coarsening with computational times.The unique result is that the grain coarsening speed can be controlled by the specific method, thisresult is vital to model the grain coarsening quantitatively. In order to make this model valid,experimental work has been done to provide solid evidence for this model. Comparison of themodelling result and the experimental result has been carried out.

THERMAL STABILITY OF NON-EQUILIBRIUS MICROSTRUCTURE IN MICROALLOYED STEEL DURING REHEATING

Cooled in water after isothermal relaxation of deformed austenite for different times, an Nb-bearing microalloyed steel always exhibits synthetic microstructures, in which bainitic ferrite ,dominates. Dislocation configurations and distributions of strain induced precipitates inside bainitic ferrite of samples relaxed for different times were distinct. When compared with the austenite model steel, which maintained fcc structure even at room temperature, the strain induced precipitates were not found in the sample without relaxation whereas these were distributed outside dislocations in sample relaxed for 1000s. Most of the strain induced precipitates distribute along dislocations and pin dislocations in sample relaxed for appropriate time. After bainitic transformation, the dislocations formed in deformed austenite remain to be pinned by the precipitates. When these samples were reheated to and held at 650 or 700℃, the non-equilibrloas microstructures tended to evolve into equilibrioas ones. The sample relaxed for 60s displayed the highest thermo-stability, whereas microstructure evolution was the quickest in the sample relaxed for 1000s even though it was the softest prior to reheating. Dislocations inside laths got rid of pinning of precipitates, and their polygonization became the precursor to the evolution of microstructures during reheated and held, followed by gradual disappearance of lath boundaries caused by dislocation climbing. Finally, recrystallization occurred and polygonal ferrite appeared. By hardness measurement, it was found that softening is not a single process occurring during reheated, in which hardness fluctuates with time. There were two peaks in the hardness-time curve of each sample having undergone relaxation, while single peak occured in the curve of the sample not being relaxed. These results indicated that the thermo-stability of microstructures was determined by their history of formation to a considerable degree.

Software sensor for slab reheating furnace

It has long been thought that a reheating furnace, with its inherent measurement difficulties and complex dynamics, posed almost insurmountable problems to engineers in steel plants. A novel software sensor is proposed to make more effective use of those measurements that are already available, which has great importance both to slab quality and energy saving. The proposed method is based on the mixtures of Gaussian processes (GP) with the expectation maximization (EM) algorithm employed for parameter esti- mation of the mixture of models. The mixture model can alleviate the computational complexity of GP and also accords with the changes of operating condition in practical processes. It is demonstrated by on-line estimation of the furnace gas temperature in 1580 reheating furnace in Baosteel Corporation (Group).

Numerical simulation of fluid flow in a reheating furnace with multi-swirling-burners

A general numerical simulating program for three-dimensional (3-D) andtime-dependent fluid flow for a reheating furnace with multi-swirling-burners has been developedbased upon an arbitrary Lagrangian-Eulerian scheme (ALE) with the finite volume method. Theparameters of fluid flow in a reheating furnace with multi-swirling-burners was calculated and the3-D velocity distributions were obtained. The design of the burners was optimized for forming betterswirling flow. The simulation shows that the fluid flow in the reheating furnace with the optimizedburners is reasonable.

Mathematical Model of Dynamic Operation and Optimum Control of Billet Reheating Furnace

This paper provides a mathematical model for the billet reheating process in furnace.A new optimum method is brought up that the objective function is the integral value of enthalpy increasing process of a billet.Different delays are simulated and calculated,some proper delay strategies are ob- tained.The on-line computer control model is de- veloped.The real production conditions simulated, the temperature deviation of drop out billet from the target temperature is kept within±15℃.

CELLULAR AUTOMATA MODELLING OF GRAIN COARSENING DURING REHEATING AND VALIDATION WITH THE EXPERIMENTAL RESULTS

A novel 2D cellular automata (CA) model has been developed for description of normal grain coarsening and abnormal grain coarsening process. The program reflects the grain coarsening quite well even through the average grain size becomes very large. Follow results have been obtained: (a) The model reflect the normal grain growth kinetics gradually increase with probability and grain growth speed can be controlled. Based on this result, temperature can be coupled in the model. (b) Abnormal grain growth is modelled successfully. (c) Methodology has been put forward to find the relationship between the experiment results and modelling results. The experimental work on the grain coarsening has been carried out. Graphical output matched the realistic microstructure in every detail. Because many physical parameters can be taken into account in the CA programme, this CA model could not only qualitatively demonstrate the grain growth process, but also quantitatively predict and analyse the grain coarsening process.

Induction reheating of A356.2 aluminum alloy and thixocasting as automobile component

The development work for producing an automobile component by thixocasting using A356.2 alloy was introduced.As the first step,the alloy was electromagnetically stirred and solidified to produce a billet with non-dendritic microstructure.The microstructure depended on several process parameters such as stirring intensity,stirring frequency,cooling rate,and melt initial superheat.Through a series of computational studies and controlled experiments,a set of process parameters were identified to produce the best microstructures.Reheating of a billet with non-dendritic microstructure to a semisolid temperature was the next step for thixo-casting of the components.The reheating process was characterized for various reheating cycles using a vertical-type reheating machine.The induction heating cycle was optimized to obtain a near-uniform temperature distribution in radial as well as axial direction of the billet,and the heating was continued until the liquid fraction reached about 50%.These parameters were determined with the help of a computational fluid dynamics(CFD) model of die filling and solidification of the semisolid alloy.The heated billets were subsequently thixo-cast into automobile components using a real-time controlled die casting machine.The results show that the castings are near net shape,free from porosity,good surface finish and have superior mechanical properties compared to those produced by conventional die casting processes using the same alloy.

Influencing Factors of Reheating Shrinkage Rate of Glass Substrate on LTPS Process

Different heating treatments with the variation of heating rates,holding temperatures and holding time were used to simulate the LTPS procedure.The experimental results show that the reheating shrinkage rates of glass substrates are rarely changed with increasing the heating rate,but strongly enhanced by raising the holding temperature and time,which shows that the reheating shrinkage of glass is closely related to heat treatment and structural relaxation.The production process of glass is critical to the reheating shrinkage of glass.

Improvement of Temperature Uniformity of Instant Rice Inside Plastic Rectangular Container Under Microwave Reheating

To improve the temperature uniformity of instant rice in a plastic rectangular container, during microwave reheating, the changes of temperature distribution were analyzed by using experiment and simulation method. A three-dimensional finite element model was established to describe microwave reheating of instant rice to predict the temperature. The results showed that the highest temperature occurred at the corners and bottom layer. The cold spots were located in the sample interior center. The simulation results in the model matched relatively with the experimental results. A method of intermittent microwave reheating was proposed to improve the temperature uniformity of convenient rice, and the optimal combination was the time of microwave reheating was 180 s, and the intermittent ratio was 1 : 3.

Electro-pulse modification and reheating process for production of thixotropic microstructure in AlSi_7Mg alloy

Electro-pulse modification(EPM) was used to change the dendritic structure of AlSi7Mg alloy to globular one.The effects of the modified temperature,electro-pulse frequency and time on the solidified structure were examined.The results show that these parameters play an important role in the solidified microstructures.That is to say,under the same modified temperature,the solidified microstructure will be improved greatly with the increase of electro-pulse frequency and time,but when they exceed to limit values,the solidified microstructure will become worse,resulting from the decrease of the ratio of nucleation.The experimental results indicate that the suitable modified temperature of AlSi7Mg alloy is 720 °C,and the appropriate electro-pulse frequency and time are 5 Hz and 40 s,respectively.Then the microstructures produced by suitable EPM process were reheated at temperatures between liquidus and solidus,the primary-Al grains ripen further and become more spherical,which is favorable to the semi-solid forming of AlSi7Mg alloy.

RESEARCH ON THE DYNAMIC MATHEMATICAL MODEL OF REHEATING FURNACE

This paper presented a dynamical mathematical model for reheating furnace based on energy balance, which consists of three submodels. With the inputting parameters, adopting the finite difference technique, not only the combustion gas temperature but also the temperature distribution of slabs in the furnace can be predicated. The dynamical mathematical model is the base for the further control and it also can be treated as a simulator of a reheating furnace, optimal and advanced controlling strategies can be applied based on the dynamical model.

Development of integrated reheating,rolling and cooling models for stelmor production line

Production characteristics of high speed rod are fast speed,complex section and internal microstructure evolution,which bring great problems to analyze and solve the quality disputes.Therefore,a serial process models have been developed based on the characteristics of high speed rod:due to rod stock to be square section,a two dimension model has been developed to predict the evolution of internal temperature and grain coarsening.As for the rod rolling process,an automatic section design system has been written, which can show section filling condition and process parameters such as temperature,pressure and recrystallization etc per pass.As for rod cooling stage,an integrated model which coupled phase transformation,temperature and final properties was developed and applied in real production with good results.

Present development of rolling reheating furnace technology in Shougang

By analyzing recent development of reheating furnace in Shougang,the direction of rolling reheating furnace is large type,high efficiency,lower pollution and automation etc.Many advanced technologies are integrated in new rolling reheating furnace such as working beam,regenerative combustion,evaporation cooling and hot loading and hot feeding etc.It shows that reheating furnace technology is upgraded.During the process of product structure adjustment in Shougang,because product heating quality is improved,energy-saving of reheating furnace should pay attention to aspects such as energy-saving of heating variety steel,equality of heating steel,heating system,oxidation loss and so on.This will offer reference for energy-saving of domestic and overseas reheating furnace technology.

Analysis of Efflorescence of Castables in Walking Beam Reheating Furnace

When the castable refractory linings are installed without drying out sufficiently and exposed to high humidity for a long time,white fibrous or powdery substance appears on the surface,which is called“efflorescence”.The main reason is the cement of castable refractory linings contacts with C02 and moisture in the atmosphere to form calcium carbonate(CaC03).In general,efflorescence takes place more readily in lightweight castable refractories.In the walking beam reheating furnace construction project,there are many white powdery substances in the walking beams,because the construction procedure causes poor ventilation in the surrounding area,and the internal moisture of castable refractory linings cannot be evaporated effectively.In future,the project programming should consider the environmental humidity and surrounding ventilation.