An experimental study of the transfer function of a ducted, laminar premixed flame

This paper presents an experimental study of the transfer function of a ducted, laminar, premixed flame. This transfer function is defined as the ratio of the fluctuations in flame heat release to the flow velocity modulations at the base of the flame. A conical, laminar, propane/air premixed flame stabilised at the rim of the burner and confined in a glass tube is considered. The flame is excited by incident acoustic waves generated bya loud speaker over a range of forcing frequencies. The fluctuations of heat release due to corrugation of the flame by the acoustic wave is measured using a photo-multiplier tube (PMT)while the flow velocity fluctuation is determined by considering the loud speaker diaphragm motion and assuming conservation of classical acoustic energy within the burner. In keeping with other studies, the results clearly show qualitatively the low-pass filter nature of the flame. The decay of the amplitude of theflame transfer function by increase of forcing frequency is further supported by images of the excited flame.</p

Gas-phase transport and entropy generation during transient combustion of single biomass particle in varying oxygen and nitrogen atmospheres

Transient combustion of a single biomass particle in preheated oxygen and nitrogen atmospheres with varying concentration of oxygen is investigated numerically. The simulations are rigorously validated against the existing experimental data. The unsteady temperature and species concentration fields are calculated in the course of transient burning process and the subsequent diffusion of the combustion products into the surrounding gases. These numerical results are further post processed to reveal the temporal rates of unsteady entropy generation by chemical and transport mechanisms in the gaseous phase of the reactive system. The spatio-temporal evolutions of the temperature, major chemical species including CO, CO2, O2, H2 and H2O, and also the local entropy generations are presented. It is shown that the homogenous combustion of the products of devolatilisation process dominates the temperature and chemical species fields at low concentrations of oxygen. Yet, by oxygen enriching of the atmosphere the post-ignition heterogeneous reactions become increasingly more influential. Analysis of the total entropy generation shows that the chemical entropy is the most significant source of irreversibility and is generated chiefly by the ignition of volatiles. However, thermal entropy continues to be produced well after termination of the particle life time through diffusion of the hot gases. It also indicates that increasing the molar concentration of oxygen above 21% results in considerable increase in the chemical and thermal entropy generation. Nonetheless, further oxygen enrichment has only modest effects upon the thermodynamic irreversibilities of the syste

Nonlinear dynamics of thermoacoustic instability using a kinematic, premixed flame model

This paper considers a simple, nonlinear model of a ductedlaminar flame. Ducted flames are susceptible to thermoacoustic instability, in which perturbations in the flame heat release drive acoustic modes of the duct that, in turn, drive the flame perturbations. Both the forced response of the flame and the selfexcited response of system are studied numerically. The overall system demonstrates limit cycles in the heat release, duct velocityand static pressure. The effect of varying the duct geometry is examined, and the form of the system’s steady state behaviour is found to be strongly dependent on the system’s configuration. This final result infers that the use of a simple saturation element to model the flame non-linearity is inappropriate.</p

Effects of nanofluid and radiative heat transfer on the double-diffusive forced convection in microreactors

Understanding transport phenomena in microreactors remains challenging owing to the peculiar transfer features of microstructure devices and their interactions with chemistry. This paper, therefore, theoretically investigates heat and mass transfer in microreactors consisting of porous microchannels with thick walls, typical of real microreactors. To analyse the porous section of the microchannel the local thermal non-equilibrium model of thermal transport in porous media is employed. A first order, catalytic chemical reaction is implemented on the internal walls of the microchannel to establish the mass transfer boundary conditions. The effects of thermal radiation and nanofluid flow within the microreactor are then included within the governing equations. Further, the species concentration fields are coupled with that of the nanofluid temperature through considering the Soret effect. A semi-analytical methodology is used to tackle the resultant mathematical model with two different thermal boundary conditions. Temperature and species concentration fields as well as Nusselt number for the hot wall are reported versus various parameters such as porosity, radiation parameter and volumetric concentration of nanoparticles. The results show that radiative heat transfer imparts noticeable effects upon the temperature fields and consequently Nusselt number of the system. Importantly, it is observed that the radiation effects can lead to the development of a bifurcation in the nanofluid and porous solid phases and significantly influence the concentration field. This highlights the importance of including thermal radiation in thermo-chemical simulations of microreactors

Experimental investigation of the linear and non-linear dynamics of a ducted, laminar premixed flame

This work investigates experimentally the linear and non-linear dynamics of a ducted, conical, laminar premixed flame under forced acoustic excitation. Upstream travelling acoustic waves hit the flame at varying amplitudes. The flame transfer function is measured. In particular, for velocity disturbance amplitudes close to the mean flow velocity the phase of the transfer function shows significant difference to that at weak excitation. Instantaneous images of the flame under excitation suggest that the non-linearity is associated with the movement of the flame base at high levelsof excitation, which is assumed fixed in the kinematic linear theory. Further work by the group will attempt to explain how non-linearity comes into play at higher forcing amplitudes.</p

Investigation of thermochemical process of coal particle packed bed reactions for the development of UCG

In this study, a packed bed reactor was developed to investigate the gasification process of coal particles. The effects of coal particle size and heater temperature of reactor were examined to identify the thermochemical processes through the packed bed. Three different coal samples with varying size, named as A, B, and C, are used, and the experimental results show that the packed bed with smaller coal size has higher temperature, reaching 624 °C, 582 °C, and 569 °C for coal A, B, and C, respectively. In the case of CO formation, the smaller particle size has greater products in the unit of mole fraction over the area of generation. However, the variation in the porosity of the packed bed due to different coal particle sizes affects the reactions through the oxygen access. Consequently, the CO formation is least from the coal packed bed formed by the smallest particle size A. A second test with the temperature variations shows that the higher heater temperature promotes the chemical reactions, resulting in the increased gas products. The findings indicate the important role of coal seam porosity in underground coal gasification application, as well as temperature to promote the syngas productions

Modelling of waste heat recovery of a biomass combustion plant through ground source heat pumps- development of an efficient numerical framework

Development of a reliable and convenient dynamic modelling approach for ground source heat pumps remains as an important unresolved issue. As a remedy, in this work a novel, computationally-efficient modelling framework is developed and rigorously validated. This is based upon an implicit computational modelling approach of the ground together with an empirical modelling of heat and fluid flow inside U-tube ground heat exchangers and waste heat calculations. The coupled governing equations are solved simultaneously and the influences of parameters on the performance of the whole system are evaluated. The outcomes of the developed framework are, first, favorably compared against two different existing cases in the literature. Subsequently, the underground storage and recovery process of the waste heat through flue gases generated by a biomass combustion plant are modelled numerically. This reveals the history of temperature distributions in the ground under different configurations of the system. The results show that for a biomass combustion plant generating flue gases at 485.9 K as waste heat with the mass flow rate of 0.773 kg/s, the extracted heat from the ground is increase by 7.6%, 14.4% and 23.7% per unit length of the borehole corresponding to 40 °C, 50 °C and 60 °C storage temperatures. It is further shown that the proposed storage system can recover a significant fraction of the thermal energy otherwise wasted to the atmosphere. Hence, it practically offers a sizable reduction in greenhouse gas emissions

Dynamic response of a ducted laminar premixed flame, part I: low amplitude forcing

This paper presents an experimental study of the dynamics of a ducted, conical, laminar premixed flame subjected to low amplitude, acoustic excitation. The heat release response of the flame to velocity disturbances is investigated through measurement of the so called ‘flame transfer function’ for a wide range of forcing frequencies. The results agree well with those predicted by existing linear kinematic theories. The level of agreement, particularly in phase, for some of the experimental conditions appeared to be higher than any other previously reported validation of these theories. In keepingwith others, high speed photography of the flame clearly shows that the flame response includes waves on the flame front which are formed at the base and then convect along the flame.</p

Numerical modelling of unsteady transport and entropy generation in oxy-combustion of single coal particles with varying flow velocities and oxygen concentrations

Unsteady generation of entropy and transfer of heat and chemical species in the transient oxy-combustion of a single coal particle are investigated numerically. The burning process takes place in oxygen and nitrogen atmospheres with varying chemical compositions and under either quiescent or active flows. The combustion simulations are validated against the existing experimental data on a single coal particle burning in a drop-tube reactor. The spatio-temporal evolutions of the gas-phase temperature and major gaseous species concentration fields as well as that of entropy generation are investigated for the two types of gas flow. It is shown that the rates of production and transport of chemical species reach their maximum level during the homogenous combustion of volatiles and decay subsequently. Yet, the transient transfer of heat of combustion continues for a relatively long time after the termination of particle life time. This results in the generation of a large amount of thermal entropy at post-combustion stage. The analyses further indicate that the entropy generated by the chemical reactions is the most significant source of unsteady irreversibilities. Most importantly, it is demonstrated that a slight oxygenation of the atmosphere leads to major increases in the total chemical entropy generation and, thus it significantly intensifies the global irreversibilities of the process. However, upon exceeding a certain mole fraction of oxygen in the atmosphere, further addition of oxygen only causes minor increases in entropy generation. This trend is observed consistently in both quiescent and active flow cases

Utilization of H2O and CO2 in coal particle gasification with an impact of temperature and particle size

Aiming at improving the quality of syngas, the thermochemical behavior of syngas formation during a single coal particle gasification process is investigated based on a validated numerical model. Initially, simulations of coal gasification with steam (H2O) are conducted in a reactor, and the results show that the steam gasification generally favors the production of H2 and CH4. However, the switch of agent to CO2 into the gasifier influences the gasification products having H2, CO, and CH4. This then prompts the investigation of a mixture of H2O and CO2 agent in the reactor’s environment, and the results show a promising indicator in producing an overall better syngas quality. Moreover, the influence of the coal particle size and gasification temperature on the syngas production is studied. The results identify that the concentration of syngas products is higher when using smaller coal particles as the behavior of heterogeneous reactions of CO formation is affected by the particle size. Finally, high temperature promotes the chemical reactions of the gasification process, resulting in the improved production of syngas