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The isotropic turbulent flow field is constructed by a four-fan hedging electrothermal experimental device, and the turbulence intensity and ambient temperature can be adjusted by changing the fan speed and thermal power. Based on this experimental setup, the effect of turbulence intensity on the heat transfer characteristics of millimeter-sized particles was investigated, and a particle heat transfer correlation equation considering the turbulent Reynolds number correction was proposed, according to which the heat transfer enhancement process of coal particles under the turbulent flow field was further investigated, revealing the enhancement effect of turbulent mixing on the heating rate and ignition of millimeter-sized fuel particles.
Heat transfer and ignition characteristics of millimeter-sized particles in a turbulent flow field
Units: 1. Key Laboratory of Thermal Science and Power Engineering, Ministry of Education, Tsinghua University; 2. School of Safety Engineering, China University of Mining and Technology; 3. School of Energy and Power Engineering, Shanghai University of Technology
The ignition of solid fuels in hot gas streams is an important process in their combustion, in which the ignition of fuel particles is influenced by various factors, including coal type, particle size, heating method, etc. In addition, the airflow environment (natural or forced convection) in which the particles are located also has an important influence on the ignition process. KHATAMI et al. investigated the ignition mechanism of pulverized coal in different coal classes under natural and forced convection in a drop tube furnace and found that forced convection slowed down the heating rate and increased the ignition delay time of coal particles compared to natural convection. Although opposite test results were obtained on different test benches, the results further indicate that the process of heating up the fuel particles in the hot air stream to the ignition point occupies a large part of the whole combustion history of the particles, and the accurate prediction of the particle heating and temperature rise process is of great significance for the study of the ignition characteristics and ignition stability of fuel particles and the optimization of combustion conditions.
With the introduction of the dual carbon target, the use of biomass coupled with coal for power generation has become a major factor in the large-scale reduction of CO2The most feasible measure, due to the difficulty of biomass crushing, the current biomass pellet particle size is generally millimeter, and its combustion process is quite different from the micron particles. The heating and combustion process of the fuel in fluidized bed coal-fired boilers is also the heating and ignition of millimeter-sized coal particles in a strongly turbulent dense-phase bed.
Previous studies have generally investigated the combustion processes of millimeter-sized large particle fuels through experiments and numerical simulations with experimental equipment such as thermogravimetric analyzers, drop-tube furnaces, planar flame methods, and single-particle reactors, which can only provide warming and ignition environments at low Reynolds numbers, i.e., focusing on the combustion characteristics of large-diameter fuel particles in laminar flow conditions or low pulsation flow fields. With the emphasis on efficient and environmentally friendly combustion technology, low-oxygen dilution combustion technology with high-speed jet characteristics is gaining attention and is considered as a new generation of efficient and clean combustion technology. In this technology, the interior of the actual furnace is an environment with strong turbulence and flame stretching rate in which fuel particles such as coal and biomass are heated and burned. The strongly turbulent pulsating flow field characteristics have a significant impact on the heat transfer, ignition and combustion of solid fuel particles. In gas fuel combustion studies, it has been found that turbulent pulsations in the flow field cause the flame surface to become folded, thereby increasing the flame area and, within certain limits, the turbulent combustion rate. In the combustion of solid fuels, turbulent pulsations enhance the gas-phase heat and mass transfer at the particle boundary by breaking the thermal boundary layer of solid particles.
When considering the particle size effect, large size particles have a stronger response to large scale energy-containing vortex clusters in turbulent vortices due to their longer relaxation times, and have better dispersion and tendency to follow due to the fact that the relaxation times of small micron-sized particles are close to the Kolmogorov times in turbulent vortices. Therefore, the effect of turbulence on the heat transfer of large particle fuels is more significant, and the effect of turbulence on the ignition process of large particle fuels needs to be studied. However, relevant experimental studies are still very limited. Typical test methods in turbulence-enhanced gas and liquid heat and mass transfer tests are the jet flame method, the hedge flame method, and the hedge fan method . The jet flame method increases the pulsation velocity by increasing the incoming flow velocity, which means that a larger pulsation velocity requires a higher time-averaged velocity and it is difficult to distinguish the effects of forced convection from turbulent pulsation. The hedonic flame method investigates the effect of turbulent pulsation on gas transfer by studying it on a hysteresis surface where the intermediate mean velocity is close to zero, but again requires a higher incoming velocity to achieve the desired pulsation velocity. The hedonic fan method is designed to form a uniform isotropic turbulent region with mean velocity close to 0 in the center of space by hedonic flow of fans, and the effect of turbulent pulsation on liquid fuel droplet evaporation, gas flame propagation, etc. is studied in this region. 8 fans are arranged symmetrically by BIROUK et al. to construct a controlled turbulent flow field by placing 8 fans at 8 corners of a cube, and isotropic turbulence measurements are carried out to verify this. And carried out a systematic study of the effects of turbulence intensity, droplet particle size, temperature and pressure on droplet evaporation, etc. The advantage of this method is that the pulsation velocity magnitude can be directly regulated by the fan speed, and the operation is simple and efficient.
The combustion of fuels in strong turbulence is widely present in real industrial installations, and the study of the role of turbulent pulsations on fuel heat transfer and ignition is important for an accurate understanding of the combustion process. At present, processes such as evaporation and ignition of gaseous fuels and liquid droplets under turbulent pulsation conditions have been studied in great detail, while most experimental methods for the study of solid particle heating and ignition, such as drop tube furnaces, single particle furnaces and flat flame burners, have been performed under laminar flow conditions.
The main methods studied under turbulent flow conditions include high-speed jets, one-dimensional furnaces and cyclonic burners, but generally suffer from poor optical visibility and difficulty in tuning the turbulence intensity. To this end, a near-uniform isotropic turbulent field with adjustable turbulent intensity was established by building a four-fan hedging experimental setup capable of operating at high temperatures, and the role of turbulent pulsations on the warming and ignition of millimeter-scale single particles was investigated.
The transient velocity distribution of the flow field and the temperature rise curve of the particles were measured at different fan speeds and ambient temperatures to obtain the heat transfer characteristics of the particles at different ambient temperatures and turbulence intensities. Based on the heat transfer test results of copper spheres with a particle size of 4.4 mm, a heat transfer model of particles considering the effect of turbulent pulsation is proposed, and the model is verified by using the temperature rise test data of copper spheres with a particle size of 2.0 mm.
The results show that the pulsation velocity of the flow field in the measurement area of the established test bench has isotropic characteristics and is much larger than the time-averaged velocity, and the pulsation velocity magnitude increases linearly with the fan speed. The increase of pulsation velocity makes the coal particles ignite earlier and the heating rate of copper ball particles accelerate, which indicates that the enhanced effect of turbulence on particle heat transfer is not negligible.
The generalized Ranz-Marshall formulation can significantly underestimate the heating history of particles under strongly turbulent conditions, which in turn results in large calculated particle ignition delay times. By introducing an additional turbulence effect term into the Ranz-Marshall formulation and fitting the coefficients therein to experimental results in a strongly turbulent field, the enhanced effect of turbulence on heat transfer to large particles can be accurately characterized.
Schematic of uniform isotropic turbulence test rig
Average velocity and pulsation velocity distribution of the flow field at different speeds
Cloud plot of uniformity index and isotropic rate distribution of flow field (2000r/min)
Probability density function of instantaneous velocity at different speeds
Heat-up results of 4.4mm copper balls at different temperatures and fan speeds
Predicted copper sphere heating results when gas pulsation is not considered
NuThe calculated values are compared with the inverse values based on the experimental results
Predicted copper sphere heating results when gas pulsation is considered
Photos of coal particles at different speeds and moments
Yuxin Wu is an associate professor and PhD supervisor in the Department of Energy and Power Engineering at Tsinghua University. His main research areas are clean combustion technologies for fossil fuels, high-fidelity numerical methods for turbulent multiphase reaction flows, and low-carbon energy-saving applications. In recent years, he has published more than 150 papers, including more than 70 SCI papers and 100 EI papers with more than 1200 citations, one ESI highly cited paper, one ESI hot paper and one Journal of Chemical Engineering highly cited paper. He has granted more than 20 patents and published one monograph. Since his appointment, he has taken charge of more than 30 scientific and technical projects, including the key projects of the National Natural Science Foundation of China (NSFC), youth projects, two special projects (special topics), the National Key Research and Development Program (sub-projects), the National Science and Technology Support Program (sub-projects), the sub-projects of the National 863 Project, and the cooperation projects with domestic and foreign enterprises and institutions. Among them, high brine reuse oilfield steam injection boiler technology, high precision water-cooled heat flow meter and other technologies have been applied and promoted by enterprises. The scientific and technological achievements have won 2 provincial and ministerial first-class awards, 1 second-class award, 5 industry first-class awards, 1 gold medal at the National Invention Exhibition and 1 gold medal at the Geneva International Invention Exhibition.
Wu Yuxin, Guo Huina, Feng Lele, et al. Heat transfer and ignition characteristics of millimeter-sized particles in turbulent flow field [J]. Journal of Coal,2022,47(1):489-498.
Disclaimer: The above content is reproduced from the Journal of Coal, and the content posted does not represent the position of this platform.
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