OPTIMIZATION AND NEURAL MODELLING OF PULSE COMBUSTORS FOR DRYING APPLICATIONS

ABSTRACT

Results of investigations of a valved pulse combustor to choose optimal geometry, which covered measurements of the flow rates of air and fuel, pressure oscillations, including pressure amplitude and frequency and flue gas composition are presented in the paper. Experimental studies compsiring the operation of the pulse combustor coupled with a drying chamber and working separately are described. It was found that coupling of the pulse combustor with a drying chamber had no significant effect on the pulse combustion process. Smoother runs of pressure oscillations in the combustion chamber, lower noise level and slightly higher NO_x emission were observed. The velocity flow field inside the drying chamber was measured by LDA technique. Results confirmed a complex character of pulsating flow in the chamber. A large experimental data set obtained from measurements enabled developing a neural model of pulse combustion process. Artificial neural networks were trained to predict amplitudes and frequencies of pressure oscillations, temperatures in the combustion chamber and emission of toxic substances. An excellent mapping performance of the developed neural models was obtained. Due to complex character of the pulse combustion process, the application of artificial neural networks seems to be the best way to predict inlet parameters of a drying agent produced by the pulse combustor     

An analysis of a pulse combustion drying system

The paper presents a theoretical and experimental analysis of a pulse combustion spray drying system. Measurements of the velocity flow field inside the drying chamber and extensive tests on drying and water evaporation were carried out for various feed rates and operating parameters of the pulse combustor. Each test included the analysis of temperature distribution in the dryer, evaporation level and sprayed material structure. LDA and PDA techniques were employed to determine the character of pulsating flow in the chamber, amount of water evaporated and to perform a profound analysis of spray structure. Experimental results show an intensive and efficient drying process. An attempt was made to perform theoretical predictions of velocity and temperature distribution in the drying chamber. The CFD technique was used to calculate time-dependent flow in the chamber. Results show vanishing velocity, pressure and temperature oscillations along the length of the drying chamber. Temperature oscillations decline faster than oscillations of pressure and velocity. Satisfactory agreement between calculations and experimental results was found in certain regions of the drying chamber. Discrepancies might be caused by simplification of the system geometry and flow pattern which were assumed to perform calculations in reasonable time.     

A Parametric Study of Spray Drying of a Solution in a Pulsating High-Temperature Turbulent Flow

Spray drying of a concentrated common salt (NaCl) solution carried out in the intense oscillating high-temperature turbulent flow field generated in the tailpipe of a pulse combustor was simulated. Simulation of such transport process problems is especially crucial since the environmental conditions are too hostile for detailed and reliable measurements. The momentum, heat, and mass transfer processes between the gas and droplet phases during drying were simulated using a computational fluid dynamic solver. The simulated profiles of flow field, temperature, and humidity of gaseous phase, and particle trajectories in a drying chamber are presented and discussed. The effects of gas temperature, pulse frequency and amplitude, and gas mass flow rate on the transient flow patterns, droplet trajectories, and overall dryer performance were investigated. Different turbulence models were also tested. Simulation results show that the flow field and droplet drying conditions vary widely during a single pulsating period. Very short drying times and very high drying rate characterize pulse combustion spray drying. Thus, pulse combustion drying can be applied to drying of fine droplets of highly heat-sensitive materials although the jet temperature initially is extremely high.     

A Parametric Study of Spray Drying of a Solution in a Pulsating High-Temperature Turbulent Flow

Spray drying of a concentrated common salt (NaCl) solution carried out in the intense oscillating high-temperature turbulent flow field generated in the tailpipe of a pulse combustor was simulated. Simulation of such transport process problems is especially crucial since the environmental conditions are too hostile for detailed and reliable measurements. The momentum, heat, and mass transfer processes between the gas and droplet phases during drying were simulated using a computational fluid dynamic solver. The simulated profiles of flow field, temperature, and humidity of gaseous phase, and particle trajectories in a drying chamber are presented and discussed. The effects of gas temperature, pulse frequency and amplitude, and gas mass flow rate on the transient flow patterns, droplet trajectories, and overall dryer performance were investigated. Different turbulence models were also tested. Simulation results show that the flow field and droplet drying conditions vary widely during a single pulsating period. Very short drying times and very high drying rate characterize pulse combustion spray drying. Thus, pulse combustion drying can be applied to drying of fine droplets of highly heat-sensitive materials although the jet temperature initially is extremely high.     

SIMULATION OF SPRAY DRYING OF A SOLUTION ATOMIZED IN A PULSATING FLOW

Spray drying of NaCl solution was carried out under an intense oscillating flow field generated by a pulse combustor. A pulse combustion spray drying system was constructed. An optical analyzer was used to measure the particle diameter distribution of droplets atomized by a pulsating flow. The momentum, heat and mass transfer in both gaseous and particulate phases during spray drying inside the drying chamber were simulated using the computational fluid dynamics method. The simulated profiles of flow field, temperature and humidity of the gaseous phase, as well as the particulate phase, in the drying chamber were presented. The simulation showed changes of the flow field and particle trajectories in the drying chamber during one pulsating period. A large-scale vortex was observed in the upper part of the drying chamber because of the unstable state of flow field and particle trajectories. Short drying time and large evaporation rate are characteristics of pulsating spray drying. The influence of gas stream pulsation frequency on the drying process is also analyzed.     

Special features of the combustion process in a channel with a supersonic velocity at the entrance

Results of combustion studies in channels are presented. These researches deal with anomalous phenomena in steady turbulent flows: unsteady heat transfer, two-layer flow, oscillations of a gas column of different intensity and frequency. Under certain conditions, these phenomena lead to substantial redistributions of enthalphy and stagnation pressure at the exit of the combustion chamber, which significantly affects the integral characteristics of the combustor. Translated fromFizika Goreniya i Vzryva, Vol. 35, No. 4, pp. 24–32, July–August 1999.     

波瓣喷嘴燃烧室燃烧特性的数值模拟

建立了波瓣式燃油多点喷射燃烧室模型,考察了波瓣诱发涡系对燃烧室燃烧特性的影响。采用文献的多点喷射燃烧室实验的空载、30%载荷、巡航与起飞4种工况,对波瓣喷嘴燃烧室内的流场涡系结构、燃烧多物量场及燃烧特性进行了数值模拟。结果表明,不同油气质量比下随空气质量流量增加,每个工况下的流向涡、正交涡等无量纲涡量逐渐增大,出口温场品质逐渐提高,NOx排放逐渐降低,燃烧效率和出口温度场改善。波瓣喷嘴燃烧室实验台的水流模型实验结果验证了模型计算结果的正确性。     

Influence of Atomizing Parameters on Droplet Properties in a Pulse Combustion Spray Dryer

A pulse combustor employed in a spray-drying system offers a new approach for liquid atomization that yields high-quality powders at low cost. Using a pulse combustion atomizer, there is no need for any form of nozzle dispersion and its atomization mechanism differs from those of conventional atomizers, such as rotary atomizers and pressure and pneumatic nozzles. In this work, based on the analysis of atomization mechanism, experiments of unsteady pulsating atomization were carried out in an experimental system of a Helmholtz-type pulse combustor. An optical analyzer was used for measuring the mean diameter of atomized droplet and droplet distribution. The effects of liquid feed rate, air flow oscillatory frequency, and liquid viscosity on atomized droplet size and size distribution were investigated and analyzed. The results indicate that the uniform droplet size distribution can be obtained under the conditions of a low feed rate, high-frequency pulsating flow, and moderate viscosity. The range of the droplets' Sauter mean diameter (SMD) is between 50 and 80 µm. The pulsating air flow from the pulse combustor can be used to atomize liquid or slurry without a nozzle and the atomizing quality can meet the requirements of spray drying.     

Influence of Atomizing Parameters on Droplet Properties in a Pulse Combustion Spray Dryer

A pulse combustor employed in a spray-drying system offers a new approach for liquid atomization that yields high-quality powders at low cost. Using a pulse combustion atomizer, there is no need for any form of nozzle dispersion and its atomization mechanism differs from those of conventional atomizers, such as rotary atomizers and pressure and pneumatic nozzles. In this work, based on the analysis of atomization mechanism, experiments of unsteady pulsating atomization were carried out in an experimental system of a Helmholtz-type pulse combustor. An optical analyzer was used for measuring the mean diameter of atomized droplet and droplet distribution. The effects of liquid feed rate, air flow oscillatory frequency, and liquid viscosity on atomized droplet size and size distribution were investigated and analyzed. The results indicate that the uniform droplet size distribution can be obtained under the conditions of a low feed rate, high-frequency pulsating flow, and moderate viscosity. The range of the droplets' Sauter mean diameter (SMD) is between 50 and 80 µm. The pulsating air flow from the pulse combustor can be used to atomize liquid or slurry without a nozzle and the atomizing quality can meet the requirements of spray drying.     

Organization of a pulsed mode of combustion in scramjets

A new method is proposed to organize the working process in the combustion chamber of a scramjet. The flow velocity in the combustor is maintained close to the velocity of sound. In a constant-area channel, this situation is achieved by organizing combustion in a pulsed wave structure of the pseudo-shock type whose position is determined by parameters of the external thermal-gas-dynamic pulsed-periodic action on the flow. In the channel part with a variable cross section, the mean Mach number close to unity is maintained by choosing an appropriate degree of combustor expansion and appropriate places of fuel injection. The pulsed mode assists in improvement of fuel-air mixing and in reduction of the combustion zone length. The main advantage of this method is the high efficiency of the process determined by the minimum loss of the total pressure and the maximum increase in temperature. Experimental results are given to confirm the possibility of realization of the pulsed combustion mode.     

纸张脉动燃烧尾气冲击流干燥过程的实验研究

脉动燃烧因其传热效率高、结构紧凑、环境污染小等优点而广泛应用于生产生活中,冲击干燥同样也能形成强烈的热质交换,本文将上述两种干燥技术结合应用。为了探讨脉动燃烧尾气冲击流干燥的性能,研制了小型Helmh01tz脉动燃烧器,并对脉动燃烧尾气冲击流干燥技术应用于纸张的干燥过程进行了实验研究。分析了气流辐射半径和燃料流量两个参数对干燥过程的影响,可以看出随着气流辐射半径的增加,干燥速度降低。此外还对不同干燥条件下的纸张干燥时间进行了估计,为实际生产提供参考。     

Development of a catalytically assisted combustor for a gas turbine

A catalytically assisted low NO_x combustor has been developed which has the advantage of catalyst durability. This combustor is composed of a burner section and a premixed combustion section behind the burner section. The burner system consists of six catalytic combustor segments and six premixing nozzles, which are arranged alternately and in parallel. Fuel flow rate for the catalysts and the premixing nozzles are controlled independently. The catalytic combustion temperature is maintained under 1000°C, additional premixed gas is injected from the premixing nozzles into the catalytic combustion gas, and lean premixed combustion at 1300°C is carried out in the premixed combustion section. This system was designed to avoid catalytic deactivation at high temperature and thermal or mechanical shock fracture of the honeycomb monolith. In order to maintain the catalyst temperature under 1000°C, the combustion characteristics of catalysts at high pressure were investigated using a bench scale reactor and an improved catalyst was selected for the combustor test. A combustor for a 20 MW class multi-can type gas turbine was designed and tested under high pressure conditions using LNG fuel. Measurements of NO_x, CO and unburned hydrocarbon were made and other measurements were made to evaluate combustor performance under various combustion temperatures and pressures. As a result of the tests, it was proved that NO_x emission was lower than 10 ppm converted at 16% O₂, combustion efficiency was almost 100% at 1300°C of combustor outlet temperature and 13.5 ata of combustor inlet pressure.     

Experimental measurements of the NOx and CO concentrations operating in oscillatory and non-oscillatory burning conditions

The effects of combustion driven acoustic oscillations in carbon monoxide and nitrogen oxides emission rates of a combustor operated with liquefied petroleum gas (LPG) were investigated. Because the fuel does not contain nitrogen, tests were also conducted with ammonia injected in the fuel, in order to study the formation of fuel NO_x. The main conclusions were: (a) the pulsating combustion process is more efficient than the non-pulsating one and (b) the pulsating combustion process generates higher rates of NO_x, with and without ammonia injection, as shown by CO and NO concentrations as function of the O₂ concentration. An increase in the LPG flow rate, keeping constant the air to fuel ratio, increased the acoustic pressure amplitude and the frequency of oscillation. The injection of ammonia had no influence on either pressure amplitude or frequency.     

低热值燃料高效低污染的催化燃烧

以两段法等制作的普通煤气及化工、冶金企业中含可燃组分的废气等,都是低热值燃料。它们的高效燃烧和废热回收,需借助高活性催化剂支持的CST燃烧法以提高热效、降低空气污染,因而颇具实用价值。研制活性组分和优质载体是关键因素,要求前者有好的活化氧的能力,后者则比表面大且稳定。该技术用于热交换器,亦是节能领域一新举。     

STAGED COMBUSTOR CONTROL USING ARTIFICIALNEURAL NETWORK-BASED PROCESS MODELS

Process controllers using trained, feed-forward, multi-layer-perceptron (FMLP) neural networks as complex process models have been successfully demonstrated for the active, on-line control of selected species emitted from a two-stage combustion reactor. In the first case, as compared to a proportional-integral-derivative controller, faster control of exhaust oxygen content with nearly no offset was achieved using a proportional controller with a variable bias value as determined by an FMLP. In the second case, effective and rapid control of exhaust nitrogen oxide, after a separate feed stream disturbance and a set point change, was achieved using a controller comprised of two clusters of FMLP neural networks. The first cluster identified the process disturbance and adjusted the manipulated variable. The second cluster served as a Smith time-delay compensator. All the FMLP networks used were trained off-line using steady-state data obtained from both experiments and from direct combustor simulations based on detailed chemical reactions.     

Numerical and experimental study of oscillatory processes in small-size combustion heaters of air

Results of a comprehensive numerical and experimental study of oscillatory processes in combustors of small-size combustion heaters of air are reported. Methods for prediction and experimental determination of regular features of changes in spectral characteristics of pressure oscillations in the combustor in the nominal operation mode are presented. The data obtained in the study can be used for the development of various combustion heaters, including those for testing combustors of air-breathing engines for advanced flying vehicles.     

HYBRID NEURAL MODELLING OF FLUIDISED BED DRYING PROCESS

The aim of this study was to investigate the applicability of hybrid neural models in modelling of drying process. A study aimed at extending a neural network mapping was also carried out. In this approach dimensionless numbers (Re, Ar, H/d) were used as inputs to predict the heat transfer coefficient in a fluidised bed drying process. To produce a data set necessary to train the networks, trials of drying different materials in a fluidised bed were carried out. On the basis of this network, a hybrid model describing the process of drying in a fluidised bed dryer was built. Results obtained were compared not only with available experimental data but also with results obtained using other types of models: a pseudo-dynamic neural model and a classical mathematical model. The analysis of results leads to a conclusion that hybrid models constitute a solid alternative method of process modelling.     

HYBRID NEURAL MODELLING OF FLUIDISED BED DRYING PROCESS

The aim of this study was to investigate the applicability of hybrid neural models in modelling of drying process. A study aimed at extending a neural network mapping was also carried out. In this approach dimensionless numbers (Re, Ar, H/d) were used as inputs to predict the heat transfer coefficient in a fluidised bed drying process. To produce a data set necessary to train the networks, trials of drying different materials in a fluidised bed were carried out. On the basis of this network, a hybrid model describing the process of drying in a fluidised bed dryer was built. Results obtained were compared not only with available experimental data but also with results obtained using other types of models: a pseudo-dynamic neural model and a classical mathematical model. The analysis of results leads to a conclusion that hybrid models constitute a solid alternative method of process modelling.     

去耦室压力变化对脉动燃烧器尾管传热的影响

去耦室是脉动燃烧器的重要部件,除降低燃烧噪声外,其另一作用是保证尾管出口声学边界条件,维持整个燃烧器的运行性能。在一台无阀自激脉动燃烧器尾部建立了去耦室压力控制系统,通过调节引风机前的阀门开度来改变去耦室内部压力即尾管出口压力大小（调节范围-10～10 kPa）,实验研究了去耦室压力变化对脉动燃烧器尾管传热的影响。结果表明：当去耦室压力高于或低于大气压力时,尾管中的传热系数均能提高,而去耦室压力为负值时,尾管传热系数相对较高;燃烧室压力幅值的大小和速度比的大小均能反映传热系数的高低。