Corrections for the hydrodynamic instability based critical heat flux models in pool boiling – effects of viscosity and heating surface size

This paper presents corrections for existing hydrodynamic instability based Critical Heat Flux (CHF) models in pool boiling by taking into account the effect of the viscosity, geometry and size of the liquid-vapour interface. Based on the existing literature, the Kelvin – Helmholtz theory, used by the most commonly adopted CHF models, can lead to noticeable errors when predicting the instability conditions. The errors are mainly due to the inaccuracy of the inviscid flow assumptions and the oversimplification of the interface geometry. In addition, the literature suggests the most unstable condition predicted by the Viscous Correction for Viscous Potential Flow (VCVPF) theory for the cylindrical interfaces best match the observed air column breakup conditions in water. In this paper, the most unstable instability conditions predicted by the VCVPF theory are used to correct the existing CHF models. The comparison between the existing and corrected CHF models suggests that the corrected models always predict a higher CHF value. In addition, the corrected Zuber model predicts similar CHF value to the Lienhard and Dhir model. The comparison with experimental data suggests that the correction to the Zuber model can increase its prediction accuracy in most cases, but not necessary for the Lienhard and Dhir model. When compared to experimental CHF data for boiling cryogens at different pressures, the corrected CHF models are consistently more accurate than the original CHF models

Correcting mass measurement of diesel particulate filters at non-ambient temperatures

Diesel particulate filters (DPFs) are becoming a widespread method for reducing the particulate matter (PM) emissions from both on-highway and off-highway automotive diesel engines. Mass measurements of DPFs are commonly used to determine rapidly both the amount of PM trapped by the filter and the amount regenerated (removed) by regeneration systems. To avoid issues with adsorption of atmospheric water the filters are often weighed at elevated temperatures. It is shown in this work that at elevated temperatures the filters weigh less than at lower temperatures as a direct result of the buoyant hot air within the filter substrate. This study shows that consideration of the buoyancy forces allows for correction of the mass measurement for the errors relating to the non-ambient temperature of the filter, allowing mass measurements at elevated temperatures while avoiding adsorption of atmospheric water on to the filter substrate and, therefore, improving the accuracy of mass-measurement-based studies of filtration and regeneration performance of DPFs. It is demonstrated that a filter with approximately 85 per cent overall porosity weighed at 150 °C in ambient temperatures will have an error of about 0.3 g/l (typically about 10 per cent of the trapped PM mass) in the mass measurement when not correcting for the temperature. By way of an example, this is shown to have potentially an important effect on the calculated trapped PM

Analysis and optimization of gel-cast ceramic foam diesel particulate filter performance

Gel-cast ceramic foams potentially offer a more robust configurable alternative filtration medium to monolithic wall flow filters (WFFs) for the reduction in particulate matter (PM) emissions from diesel internal combustion engines. The fundamental back pressure and filtration efficiency characteristics of gel-cast ceramic foam diesel particulate filters (DPFs) have been investigated. Methodology is developed for the first time that allows the calculation of the effect of local PM loading on the pressure drop characteristics from experimental data without problems caused by the non-uniform PM loading in the filter that can be applied to all depth bed filtration media. The back pressure and filtration efficiency relationships were used to develop graphical design spaces to aid development of application-specific DPFs. Effects of PM distribution on the pressure drop of the filter are presented. Filters with a non-even distribution of PM were found to have lower pressure drops than filters with an evenly distributed PM for the same average specific PM loadings. The predictions showed that gel-cast ceramic foams can achieve comparable back pressure, filtration volume, and PM holding capacity with WFFs with a lower filtration efficiency of about 80 per cent. The model demonstrated that greater than 90 per cent filtration efficiency can be achieved with filter volumes of about 0.6 times the volume of a WFF with a lower PM holding capacity

Measuring pore diameter distribution of gelcast ceramic foams from two-dimensional cross sections

Increasing applications for gelcast ceramic foams is making the effective, accurate and cost effective measurement of pore diameter and distribution of significant value to a wide range of research fields. Current methods either do not directly measure pore diameter or they require high equipment and time costs. Measuring pore diameter directly from sample cross sections is both rapid and cost effective but, due to the random nature of the pore location during sectioning of the sample, it under predicts the pore diameter. The proposed method identified that the mean measured pore diameter was 79% (2 s.f.) of the actual pore diameter. Numerical methods for correcting the pore distribution as well as the average pore diameter are presented

A diesel particulate filter regeneration model with a multi-step chemical reaction scheme

Diesel particulate filters (DPFs) are considered necessary in order to meet future global diesel engine emissions legislation. Various regeneration methods have been developed to clean DPFs by periodic oxidation of trapped particulate matter (soot). To achieve this goal, it is important to understand the fundamentals of the regeneration process. Previous soot oxidation regeneration models relied on tunable chemical kinetic parameters to achieve agreement between model and experimental results. In the work reported in this paper, a multistep chemical reaction scheme is incorporated in a model to study the thermal regeneration process. The regeneration model does not require tunable parameters and its results compare well with experimental findings. The effects on regeneration of various gas species are also studied, in addition to O2 and N2, such as CO and H2O that are present in the exhaust gas. The model is also used to demonstrate the effects of quenching the regeneration process and its impact on partial filter regeneration

The effects of soot properties on the regeneration behaviour of wall-flow diesel particulate filters

In recent years, significant effort has been put into studying the regeneration process of diesel particulate filters (DPFs) either through experiments or modelling. However, less attention is paid to understanding the important influence of soot properties on the regeneration process. In this paper, for the first time, five fundamental soot properties, namely activation energy, frequency factor of the reaction, soot bulk density, porosity and mean soot particulate diameter, are investigated. Sensitivity analyses are carried out for each of these parameters based on a one-dimensional generalized DPF regeneration model. It is found that activation energy is the most important factor in the regeneration process, followed by frequency factor, bulk density, porosity and mean particulate size. In addition, the results also indicate that the concentration of exhaust gas oxygen has a significant influence on the role played by each parameter. This clearly shows the importance of gas diffusion in the regeneration process

Effects of intake-port throttling on combustion behaviour in diesel low-temperature combustion

This article describes the effects of intake-port throttling on diesel low-temperature combustion at a low and medium load condition. These conditions were known for their characteristically high hydrocarbon emissions predominantly from over-mixed and under-mixed mixture zones, respectively. The investigation was carried out to supplement current findings in the literature with valuable information on the formation of high hydrocarbon emissions with increasing swirl levels generated by intake-port throttling. This was achieved through the use of cycle-resolved high hydrocarbon measurements in addition to cycle averaged emissions and in-cylinder pressure-derived metrics. While there was negligible overall effect at the moderately dilute low-load conditions, increasing swirl has been shown to be beneficial to premixing efficacy under highly dilute conditions with extended ignition delay. This potential advantage was found to be nullified by the swirl-induced confinement of fuel and combustion products to the central region of the cylinder leading to poor late cycle burn rates and increased smoke emissions. High hydrocarbon emissions from the squish and head quench regions were reduced by an increase in swirl ratio

An investigation of string cavitation in a true-scale fuel injector flow geometry at high pressure

String cavitation has been studied in an optical automotive size fuel injector with true-scale flow geometry at injection pressures of up to 2050 bar. The multihole nozzle geometry studied allowed observation of the hole-to-hole vortex interaction and, in particular, that of a bridging vortex in the sac region between the holes. A dependency on Reynolds number was observed in the formation of the visible, vapor filled vortex cores. Above a threshold Reynolds number, their formation and appearance during a 2 ms injection event was repeatable and independent of upstream pressure and cavitation number

A review of split-cycle engines

This article reviews split-cycle internal combustion engine designs. The review includes historical work, assessment of prototypes and discussion of the most recent designs. There has been an abundance of split-cycle engine designs proposed since the first in 1872. Despite this, very few prototypes exist, and no split-cycle engines are reported to be in series production. The few split-cycle prototypes that have been developed have faced practical challenges contributing to limited performance. These challenges include air flow restrictions into the expansion cylinder, late combustion, thermal management issues, and mechanical challenges with the crossover valve actuation mechanism. The main promoted advantage of split-cycle engines is the increased thermal efficiency compared to conventional internal combustion engines. However, an efficiency improvement has not thus far been demonstrated in published test data. The thermodynamic studies reviewed suggest that split-cycle engines should be more efficient than conventional four-stroke engines. Reasons why increased thermal efficiency is not realised in practice could be due to practical compromises, or due to inherent architectural split-cycle engine design limitations. It was found that the number of split-cycle engine patents has increased significantly over recent years, suggesting an increased commercial interest in the concept since the possibility of increased efficiency becomes more desirable and might outweigh the drawbacks of practical challenges

CFD modelling of cavitation phenomenon in piston ring / cylinder liner conjunction

CFD modelling of cavitation phenomenon in piston ring / cylinder liner conjunctio