A stochastic micromixing model based on the turbulent diffusion length scale

A new micromixing model to close probability density function (pdf) models is proposed. The model is based on the proposition that each computational timestep, stochastic particles move within the scalar space (on average) by a distance equal to the turbulent diffusion length scale. At each timestep, the model evaluates the distance in scalar space between all particles. During the timestep, a discrete pdf is computed for the distance between unmixed particles and the cumulative integral for the mean calculated. A filter is applied to retain the lower portion of the distance domain so that the cumulative integral is equal to the average diffusion length required to decrease the scalar variance. A sample (a pair of particles) is chosen from this filtered part of the domain and the particles mixed using Modified Curl's model. The complete interparticle-distance pdf is re-evaluated for each pair to ensure that there is sufficient capacity to mix to meet the variance decay requirements. Preliminary tests show that this model obeys several fundamental properties required of micromixing models, including conservation, correct decay of variance and relaxation to Gaussian pdf

Application of hybrid binomial Langevin-multiple mapping conditioning method to reacting jet flow

The hybrid binomial Langevin-MMC (Multiple Mapping Conditioning) method combines the advantages of the binomial Langevin and MMC models in a consistent manner to overcome difficulties in each. The binomial Langevin method provides joint velocity-scalar statistics, but the treatment of scalars is complex since specification of the bounds is not trivial. The MMC method is capable of dealing with the mixing of any number of scalars, but it can be difficult to specify coefficients involving averages of the scalars and the introduced reference space. The difficulties are overcome by using the velocity statistics from the binomial Langevin model to obtain the reference variable for MMC and, subsequently, the mixing of MMC scalars is performed in a manner that minimises the difference between the mixture fractions for each submodel. The current work expands past studies of NO conversion in a mixing layer to include a study of the Sandia D Flame in preparation for the application to more complex combustion phenomena. Results compare favourably with experimental data and other models

Utilising tablet PCs in tutorials to aid external students

[Abstract]: A tablet PC (a computer with a special screen so a stylus can write directly into an active program) was used in a tutorial setting to record pen-strokes and audio subsequently edited as a video. Because the subject primarily involved mathematical manipulation, ordinary text-based presentations might not be as beneficial. The intention was to provide a fuller tutorial experience for external students compared to the usual static page of worked solutions, which does not allow the student to discover the deeper thought processes behind the flow of the solution. A broader range of senses can be engaged, deepening the learning process and assisting those students who tend to learn aurally. Students were surveyed regarding their impressions of how useful the videos were in conjunction with the static page of worked solutions. Also studied was the inclass learning experience due to the use of the computer instead of the traditional board

The study of air fuel ratio for open furnace MILD combustion of biogas on bluff-body burner

Economical fuel cost is very critical in the heating industry. Lean combustion with high air fuel ratio (AFR) is normally practiced by the industry. Low air fuel ratio or rich combustion will result in unburned hydrocarbons (UHC). UHC is a waste and pollution to the environment. This paper discussed on the modelling of air fuel ratio for the moderate and intense low oxygen dilution (MILD) combustion of biogas on bluff-body burner. Biogas is a low calorific value (LCV) gas which was formulated by using 50% methane, 20% hydrogen and 30% carbon dioxide. AFR is the ratio of air and fuel injected to the combustion chamber. Nozzle outlet size for air and fuel plays important role to determine AFR. In this study, the air and fuel nozzle size ratio used is 23:1. The AFR will be evaluated based on the UHC produced by the combustion. Stoichiometric AFR occurred when zero UHC and zero excess oxygen flow through the EGR pipe. The result shows that when AFR is 4.0, zero UHC was detected in the EGR. UHC in EGR will be waste and create unwanted combustion at the wrong location

Peer review of teamwork for encouraging equal commitment to the group effort

An important graduate attribute is the ability to work in teams, so many university courses incorporate this as part of the learning experience. However, it is inevitable that in some teams there will be members who do not contribute as much to the overall effort as others, leading to frustration in those members who carry the majority of the burden. When there are students enrolled in distance-education mode, this can be exacerbated because many of the teams cannot meet face-to-face, so it can be difficult to exert sufficient influence to force problematic individuals to amend their behaviour. In an effort to mitigate against this problem, self and peer assessment was used for both team assignments in a Problem-Based Learning (PBL) course and the results of the peer assessment were used to scale the team mark for the corresponding assignment to obtain individual grades. After submitting their final assignment, a survey instrument was used to investigate the success of this process. The students overwhelmingly supported the idea of distributing marks based on the value of the individual’s contribution because in many teams it had the desired effect of motivating underperforming members to involve themselves more in the second assignment. There was some dissatisfaction about the process used to distribute marks, which the authors will attempt to address by providing better scaffolding in subsequent uses of the software tool. Regardless of these difficulties, we found that a transparent mechanism for distributing team marks to individual grades is beneficial for encouraging equal commitment to the team effort by all team members

Mixture-fraction based hybrid binomial-langevin–MMC modelling applied to auto-ignition in vitiated flow

A new hybrid binomial Langevin–MMC (Multiple Mapping Conditioning) modelling approach is proposed. The mixture fraction derived from the binomial Langevin model is used to specify the reference variable for MMC. The modified Curl’s model is used to close the stochastic MMC mixing term. The new model is applied to a jet burner with a vitiated co-flow (the 'Cabra burner') with methane as the fuel. The first- and second-order statistics show good agreement with experimental data

Comparative study of spray characteristics of butanol, acetonebutanol-ethanol, butanol-acetone/diesel blends

Butanol is widely investigated as a renewable biofuel additive in Compression-Ignition (CI) engines due to its ability to improve diesel fuel properties and reduce emission levels. Because Acetone-butanol-ethanol (ABE) and butanol-acetone (BA) are intermediate mixtures in bio-butanol production, they present cost benefits compared to butanol production by reducing energy consumption and the number of recovery processes. This paper evaluates and compares the effect of using butanol (B), BA and ABE additives with diesel (D) on macroscopic spray characteristics. Spray tests were carried out in a constant volume vessel (CVV) under different injection conditions. A high-speed camera was used to record spray images. Macroscopic spray characteristics including spray penetration, spray cone angle and spray volume were measured. The experimental results showed that spray penetration (S) was increased as a result of addition of all alcohols to diesel fuel as well as of increased injection pressure; spray cone angle () was slightly widened while it was slightly narrowed as a consequence of increase fuel injection. The spray volume of the alcohol-diesel blends showed a higher value compared to that of neat diesel due to high spray penetration length. Spray penetration and spray volume of BA-diesel blend were higher compared to ABE-diesel and Bdiesel blends

Design and development of mild combustion burner

This paper discussed the design and development of the Moderate and Intense Low oxygen Dilution (MILD) combustion burner using Computational Fluid Dynamics (CFD) simulations. The CFD commercial package was used to simulate preliminary designs for the burner before the final design was sent to workshop for the fabrication. The burner is required to be a non-premixed and open burner. To capture and use the exhaust gas, the burner was enclosed within a large circular shaped wall with an opening at the top. An external EGR pipe was used to transport the exhaust gas which was mixed with the fresh oxidant. To control the EGR and exhaust flow, butterfly valves were installed at the top opening as a damper to close the exhaust gas flow at the certain ratio for EGR and exhaust out to atmosphere. High temperature fused silica glass windows were installed to view and capture images of the flame and analyse the flame propagation. The burner simulation shows that MILD combustion was achieved for the oxygen mole fraction between 3-13%. The final design of the burner was fabricated and ready for the experimental validation