Radiative heat transfer to flow of a combustible mixture in a vertical pipe

The flow of a combustible gas in a vertical cylinder, in the presence of radiative heat transfer, affords the closest model to biomass moving bed gasifier operating at temperatures between 750 and 1500 K. This problem forms the subject matter of the paper under the simplistic assumption of a binary reaction A → B. Under the general differential approximation for radiation, the temperature is perturbed about the wall temperature, and the nonlinear differential equations are subsequently integrated in a closed form. Consequences of the effect of the Arrhenius activation energy are discussed quantitatively.     

Radiative heat transfer to flow of a combustible mixture in a vertical channel

The paper studies the flow of a combustible mixture in a vertical channel in the presence of radiative heat transfer as a model for biomass moving bed gasifiers operating in the temperature range 750–1500 K. The simplistic binary reaction A → B is assumed, and both the optically thick (high density gas) and the optically thin (low density gas) situations are considered for the radiative heat transfer. Analytical and numerical solutions are obtained and discussed quantitatively.     

Modeling of hot gas flow through a feed stream within a horizontal pipe

High heat penetration into a feed stream within a horizontal pipe is described mathematically with a gas flow and heat transfer model. Influences of varied factors on the gas flow and heat transfer in porous media are examined for different conditions. The temperature of the packed‐bed particles and the gas velocity distribution curves are obtained for the feeding service at interruption and at regular charge operating conditions. The numerical results show that the thermal effect to the packed‐bed particles by the seepage flow fluid is high only in the position near the gas entrance. The thermal penetration depth tends to increase with the seepage flow velocity and decrease with the feed rate. The operating conditions and the porosity of solid bed have importance effects on the gas velocity and temperature field in the thermal penetration zone. The model results are found to compare favorably with the experimental data available in the literature. © 2003 Wiley Periodicals, Inc. Heat Trans Asian Res, 32(6): 553–565, 2003; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.10109     

Internal flow structure description of slug flow-pattern in a horizontal pipe

Utility of the hot-film anemometry technique in describing the internal flow structure of a horizontal slug flow-pattern is discussed within the scope of intermittent nature of slug flow. It is shown that a single probe can be used for identifying the gas and liquid phases and for differentiating the large elongated bubble group from the small bubbles present in the liquid slug. Analyzing the nature of voltage signals, a signal processing scheme is developed for measurements of time-averaged void fractions of small and large bubbles as well as for the measurements of local mean axial velocity and turbulent intensity in the liquid phase. Some results of local measurements of time-averaged void fractions of small and large bubble groups, axial mean velocity and turbulent intensity are presented at relatively low and high gas and liquid flows for a horizontal slug flow-pattern in a 50.3 mm i.d. pipe.     

Prediction of pressure drop in multiphase horizontal pipe flow

Empirical correlations were tested against reliable two phase pipe flow data for the prediction of pressure drop. Correlations are recommended for the prediction with stratified and annular type flows. When these correlations were adapted to three phase gas–water–oil pipe flow in general they predicted for intermittent slug type flows. Momentum balance models could not be successfully adapted to the prediction of pipe three phase pressure drop.     

生物质成型燃料技术及应用前景

分析了现国内生物质电厂集中存在的燃料问题,而生物质成型燃料能够解决秸秆运输、储存、防火等问题,具有广阔的发展前景。对比介绍了生物质成型技术,分析生物质成型燃料的燃料特性。结果表明,生物质成型燃料可以改善燃烧特性,燃烬时间长,有利于提高生物质灰熔点。     

Two instability criteria for the stratified flow in horizontal pipe at cocurrent flow conditions

This paper presents a theoretical approach of two instability criteria from the stratified to nonstratified flow in horizontal pipes at cocurrent flow conditions. The new theoretical instability criteria for the stratified and nonstratified flow transition in horizontal pipe have been developed by hyperbolic equations in two-phase flow. These criteria, onset of slugging criterion and critical flow condition criterion, at cocurrent flow condition correspond to imaginary and zero characteristics which occur when the hyperbolicity of a stratified two-phase flow are broken, respectively. Through comparison between results predicted by the present theory and the Kukita et al.'s experimental data, it is shown that they are in good agreement with experimental data.     

Pyrolysis of biomass in a semi-industrial scale reactor: Study of the fuel-nitrogen oxidation during combustion of volatiles

In this work, an experimental study of the NOx-fuel formation, carried out on a semi-industrial scale reactor during combustion of volatiles of the pyrolysis, is performed. Two different biomasses with different nitrogen contents such as a mixture of organic sludge and wood were tested. Results show that the temperature of pyrolysis does not obviously affect the production of NOx-fuel because of the most active precursors (NH₃ and HCN) are already released at low temperatures (400 °C). In the case of sludge mixture, the combustion conditions play the discriminating role in the production of NOx-fuel: the higher the excess air ratio the larger the production of nitrogen oxides from N-fuel.     

Performance evaluation of producer gas burner for industrial application

The purpose of this study is to evaluate the performance of premixed type industrial burner with producer gas, in terms of emission, axial and radial flame temperature. The developed burner was a concentric tube type where the air supplied through a central tube, which is surrounded by another one. It having 150 kWth capacity and tested on open core downdraft type gasifier. The burner consists of swirl vane for mixing the air and producer gas, mixing tube and bluff body for flame stabilization. Swirl angle and bluff body diameter was kept constant throughout the study. Burner was evaluated with open core downdraft type gasifier. In order to understand the qualitative differences in the temperature profile and emissions during combustion of producer gas, an experiment was conducted on three-flow rate and air-fuel ratio. Study shows low NO_x and CO emission at 125 Nm³ h⁻¹ as compared to that of 75 and 100 Nm³ h⁻¹. Maximum flame temperature (753 °C) was recorded at of 10 cm axial and 10 mm radial distance.     

Interfacial condensation for countercurrent steam–water stratified wavy flow in a horizontal circular pipe

We experimentally investigate the interfacial condensation heat transfer for a steam–water countercurrent stratified flow in a horizontal pipe. In contrast to the previous work of Chu et al. [I.C. Chu, S.O. Yu, M.H. Chun, Interfacial condensation heat transfer for countercurrent steam–water stratified flow in a circular pipe. J. Korean Nucl. Soc. 32 (2) (2000) 142–156] that investigated only the interfacial condensation heat transfer in a stratified smooth flow in a horizontal pipe, this work investigates the steam–water stratified wavy flow. A total of 105 local interfacial condensation heat transfer coefficients for a wavy interface have been obtained. The parametric effects of the flow rates of steam and subcooled water and the degree of subcooling on condensation heat transfer were examined. The empirical Nusselt number correlation was developed on the basis of the bulk flow properties. This correlation agrees with the experimental data within ±32% with a RMS error of 16.2%. Its applicable ranges for a steam–water countercurrent stratified flow in a horizontal pipe are as follows: the ranges of Reynolds numbers are 4000–14 000 for the water and 12 000–23 000 for the steam, and the Jakob numbers for the water are 43.5–180.     

CFD modeling for slurry flow through a horizontal pipe bend at different Prandtl number

The present work shows the slurry flow characteristics of bottom ash particulates having density 2219 kg/m³ at different Prandtl number through horizontal pipe bend. The simulation is carried out by adopting Eulerian two-phase model in conjunction with RNG k-ε turbulence model using available commercial software ANSYS Fluent. The transportation of solid particulates has the settling behaviour in the slurry pipeline and that leads to the sedimentation and blockage of the pipeline resulting more power and pressure drop in the pipeline. Therefore, it is important to know the transport capability of the solid particulates at different Prandtl fluids to minimise the pressure loss. The fluid properties at four Prandtl numbers i.e., 1.34, 2.14, 3.42 and 5.83 are used to carry the bottom ash concentration ranging from 40 to 60% (by weight) at mean flow-velocity ranging from 1 to 5 ms^?1. The obtained computational results for pressure drop are validated with the published data in the literature and found in good agreement. The findings show that the pressure drop rises with escalation in flow velocity and Prandtl number for chosen efflux concentration range. The bottom ash particulates flowing at higher Prandtl fluid experiences less pressure drop through bend cross section in comparison to bottom ash particulates flowing at low Prandtl fluid. Finally, the contours of granular pressure, granular temperature and wall shear stress are predicted and discussed in details through the bend cross section to understand the complex slurry flow for chosen Prandtl numbers.     

The effect of different inlet geometries on laminar flow combined convection heat transfer inside a horizontal circular pipe

The effect of different inlet geometries on laminar air flow combined convection heat transfer inside a horizontal circular pipe has been experimentally investigated for Reynolds number range of 400–1600, and the Grashof number range from 3.12 × 10⁵ to 1.72 × 10⁶. The experimental setup consists of an aluminum circular pipe as a heated section with 30 mm inside diameter and 900 mm heated length (L/D = 30) with different inlet geometries. A wall boundary heating condition of a uniform heat flux was imposed. The inlet configurations used in this paper are calming sections having the same inside diameter as the heated pipe but with variable lengths of L_calm. = 600 mm (L/D = 20), L_calm. = 1200 mm (L/D = 40), L_calm. = 1800 mm (L/D = 60), L_calm. = 2400 mm (L/D = 80), sharp-edged and bell-mouth. It was found that the surface temperature values for calming section length corresponding to (L/D = 80) were higher than other inlet geometries due to the lower mass flow rate and higher flow resistance. It was also observed that the Nusselt number values for bell-mouth inlet geometry were higher than other inlet geometries due to the differences in the average temperatures and densities of the air. The average heat transfer results were correlated with an empirical correlation in terms of dependent parameters of Grashof, Prandtl and Reynolds numbers. The proposed correlation was compared with available literature and it shows reasonable agreement.     

Combustion,cofiring and emissions characteristics of torrefied biomass in a drop tube reactor

The study investigates cofiring characteristics of torrefied biomass fuels at 50% thermal shares with coals and 100% combustion cases. Experiments were carried out in a 20 kW, electrically heated, drop-tube reactor. Fuels used include a range of torrefied biomass fuels, non-thermally treated white wood pellets, a high volatile bituminous coal and a lignite coal. The reactor was maintained at 1200 °C while the overall stoichiometric ratio was kept constant at 1.15 for all combustion cases. Measurements were performed to evaluate combustion reactivity, emissions and burn-out.Torrefied biomass fuels in comparison to non-thermally treated wood contain a lower amount of volatiles. For the tests performed at a similar particle size distribution, the reduced volatile content did not impact combustion reactivity significantly. Delay in combustion was only observed for test fuel with a lower amount of fine particles. The particle size distribution of the pulverised grinds therefore impacts combustion reactivity more.Sulphur and nitrogen contents of woody biomass fuels are low. Blending woody biomass with coal lowers the emissions of SO₂ mainly as a result of dilution. NO_X emissions have a more complex dependency on the nitrogen content. Factors such as volatile content of the fuels, fuel type, furnace and burner configurations also impact the final NO_X emissions. In comparison to unstaged combustion, the nitrogen conversion to NO_X declined from 34% to 9% for air-staged co-combustion of torrefied biomass and hard coal. For the air-staged mono-combustion cases, nitrogen conversion to NO_X declined from between 42% and 48% to about 10%–14%.     

The effect of inlet subcooling on two‐phase flow instabilities in a horizontal pipe system with augmented surfaces

This research has been conducted to investigate the effect of inlet subcooling on two‐phase flow instabilities in a horizontal pipe system with augmented surfaces. Five different inlet temperatures are used to study the effect of inlet subcooling for five different heat transfer surface configurations. All experiments are carried out at constant heat input, system pressure and exit restriction. The effect of inlet subcooling on the steady‐state characteristics and two‐phase flow instabilities are studied for each configuration. The bound aries for the appearance of pressure‐drop‐type and density‐wave‐type instabilities are found and the effect of the inlet subcooling on these oscillations is studied for each configuration. Copyright © 2002 John Wiley & Sons, Ltd.     

Modelling of two phase solid-liquid flow in horizontal pipe using computational fluid dynamics technique

In this paper three-dimensional fluid flow characteristics of slurry pipeline has been studied using commercial Computational Fluid Dynamics (CFD) code FLUENT at different velocities and solid concentrations. An Euler-Lagrange multiphase approach was used to simulate the flow of bottom ash slurry in a 50 mm diameter straight pipeline. Simulations were carried out with bottom ash at velocity range of 1–3 ms^–1. The solid concentration of bottom ash particles was varied from 40 to 60% (by weight). The simulations were carried out using SST k-ω turbulence model and the results obtained were compared with experimental results. It was found that the SST k-ω turbulence model shows good agreement with experimental data. Numerical simulation results show that the pressure drop in pipeline increases non-linearly with increase in velocity and solid concentration.     

The potential use of torrefied Nigerian biomass for combustion applications

Many countries are seeking to expand their use of solid biomass for electricity and heat generation. Nigeria, too, is exploring its own potential energy crops and indigenous residues. The use of this biomass for energy production is, however, limited by factors such as high moisture content, low bulk and low energy density. This study examines the torrefaction and combustion properties of four readily available Nigerian woody biomass, Gmelina arborea, Terminalia superba, Nauclea diderrichii, Lophira alata and a residue, palm kernel expeller (PKE). They are considered for their suitability for use in large scale power stations, especially as pulverized fuels.The Fuels were torrefied at 270 and 290 °C for either 30 or 60 min, and assessed for pyrolysis and combustion characteristics in comparison to their untreated counterparts. Energy densities of the woods improved from 19.2 to 21.2 MJ/kg for the raw fuels to 21.5–24.6 MJ/kg for the torrefied fuels. The milling behaviour of the torrefied fuels improved upon torrefaction, especially for Nauclea; however, torrefaction had very little effect on the grindability of PKE. The apparent first order kinetics for pyrolysis were determined by thermogravimetric analysis (TGA). After torrefaction, the fuels become less reactive; Nauclea and Gmelina were the most reactive fuels, whilst PKE was the least reactive. The combustion behavior of selected fuels was visually examined in a methane air flame. This showed that torrefaction resulted in shorter ignition delay, shorter duration of volatile combustion and longer duration of char burn out.     

Forest biomass waste combustion in a pilot-scale bubbling fluidised bed combustor

Combustion experiments of forest biomass waste in a pilot-scale bubbling fluidised bed combustor were performed under the following conditions: i) bed temperature in the range 750-800 °C, ii) excess air in the range 10-100%, and iii) air staging (80% primary air and 20% secondary air). Longitudinal pressure, temperature and gas composition profiles along the reactor were obtained.The combustion progress along the reactor, here defined as the biomass carbon conversion to CO₂, was calculated based on the measured CO₂ concentration at several locations. It was found that 75-80% of the biomass carbon was converted to CO₂ in the region located below the freeboard first centimetres, that is, the region that includes the bed and the splash zone.Based on the CO₂ and NO concentrations in the exit flue gas, it was found that the overall biomass carbon conversion to CO₂ was in the range 97.2-99.3%, indicating high combustion efficiency, whereas the biomass nitrogen conversion to NO was lower than 8%.Concerning the Portuguese regulation about gaseous emissions from industrial biomass combustion, namely, the accomplishment of CO, NO and volatile organic compounds (VOC) (expressed as carbon) emission limits, the set of adequate operating conditions includes bed temperatures in the range 750°C-800 °C, excess air levels in the range 20%-60%, and air staging with secondary air accounting for 20% of total combustion air.     

Simulation of combustion process of a single biomass pellet based on heterogeneous-dimension discretization

Abstract

In this paper, one-dimensional intra-pellet solid model combined with multi-dimensional extra-pellet environment model were used to simulate the combustion processes of a biomass pellet in a furnace. The combustion processes of the solid phase were calculated using a self-written MATLAB code, and the processes of the extra-pellet fluid environment were calculated using the commercial CFD code. The two codes were coupled together by exchanging the data of source terms and boundary conditions in real-time. Experiments on combustion of a single corn-stover pellet in a tube furnace were implemented to validate the calculation method and acceptable agreements were obtained, and the main errors are within 10%. The heterogeneous-dimension discretization method made the updating and debugging of the physical and chemical mechanism of the solid phase very easy. Compared with constant environment conditions, the average error of coupling calculation method is decreased by about 5%. The calculation method can be extended to deal with many other problems.     

Experimental study for the stratified to slug flow regime transition mechanism of gas-oil two-phase flow in horizontal pipe

Theoretical relations that predict the transition from a stratified pattern to a slug pattern, including a onedimensional wave model that contains less empiricism than the commonly used Taitel-Dukler model, and the ideal model for stratified flow for the gas-liquid flow in horizontal pipes are presented. Superficial velocities of each phase, as the onset of slugging occurs, were predicted, and theoretical analysis was conducted on the stratified to slug flow regime transition. The friction, existing between the fluid and pipe wall, and on the interface of two phases, was especially taken into account. A theoretical model was applied to an experiment about air-oil two-phase flow in a 50 mm horizontal pipe. The effect of pipe diameter on the transition was also studied. The results show that this approach gives a reasonable prediction over the whole range of flow rates, and better agreement has been achieved between predicted and measured critical parameters. __________ Translated from Journal of Shanghai Jiaotong University, 2006, 40(10): 1782–1785, 1789 [译自: 上海交通大学学报]