T.L. Chan

Validation of a two-dimensional pollutant dispersion model in an isolated street canyon

Abstract A two-dimensional numerical model based on Reynolds-averaged Navier–Stokes equations coupled with a series of standard, Renormalization Group (RNG) and realizable k–e turbulence models was developed to simulate the fluid-flow development and pollutant dispersion within an isolated street canyon using the FLUENT code. In the present study, the validation of the numerical model was evaluated using an extensive experimental database obtained from the atmospheric boundary layer wind tunnel at the Meteorological Institute of Hamburg University, Germany (J. Wind Eng. Ind. Aerodyn. 62 (1996) 37). Among the studied turbulence models, the RNG k–e turbulence model was found to be the most optimum turbulence model coupled with the two-dimensional street canyon model developed in the present study. Both the calculated and measured dimensionless pollutant concentrations have been shown to be less dependent on the variation of wind speed and source strength conditions for the studied street canyon aspect ratio of the B/H=1 case. However, the street canyon configuration has significant influence on the pollutant dispersion. The wider street and lower height of the buildings are favorable to pollutant dilution within the street canyon. The fluid-flow development has demonstrated that the rotative vortex or vortices generated within the urban street canyon can transport the pollutants from a line source to the wall surfaces of the buildings.

A New Moment Method for Solving the Coagulation Equation for Particles in Brownian Motion

A new numerical approach for solving coagulation equation, TEMOM model, is first presented. In this model, the closure of the moment equations is approached using the Taylor-series expansion technique. Through constructing a system of three first-order ordinary differential equations, the most important indexes for describing aerosol dynamics, including particle number density, particle mass and geometric standard deviation, are easily obtained. This approach has no prior requirement for particle size spectrum, and the limitation existing in the log-normal distribution theory automatically disappears. This new approach is tested by comparing it with known accurate solutions both in the free molecular and the continuum regime. The results show that this new approach can be used to solve the particle general dynamic equation undergoing Brownian coagulation with sufficient accuracy, while less computational cost is needed.

Characterisation of diesel exhaust particle number and size distributions using mini-dilution tunnel and ejector-diluter measurement techniques

Abstract This paper presents the characteristics of diesel exhaust particle number and size distributions. These were measured for different engine load conditions from 10% to 100% of full engine load at a maximum torque of constant speed, using mini-dilution tunnel and sampling (MDTS) and ejector–diluter and sampling (EDS) systems. The exhaust particles sampled were analysed using a scanning mobility particle sizer (SMPS). In general, the particle number concentration increased with increasing engine load using both measurement systems. Comparing the particle number and volume concentrations, the MDTS system measures a lower level in the nanoparticle range, D p nm , but a higher level in the accumulation mode size range, 50 p nm , than the EDS system. The measurements also showed that the MDTS system shifted the particle count median diameter (CMD) to larger particle diameter and the particle number size and volume distribution for all engine load conditions. It is mainly because the mini-dilution tunnel leads to the particle transformations of nucleation and condensation taking place simultaneously when the exhaust particle emissions are cooled and diluted. However, the effect of coagulation on the total number particle concentration was shown to be negligible. On the other hand, the EDS measurement system can minimise the particle transformations taking place on the exhaust particle number and size distributions during the heated dilution process. Hence, the EDS measurement system can provide more reliable diesel exhaust particle number and size distributions than MDTS measurement system.

Heat transfer characteristics of a slot jet impinging on a semi-circular convex surface

Abstract Surface heat transfer characteristics of a heated slot jet impinging on a semi-circular convex surface have been investigated by using the transient heating liquid crystal technique. Free jet velocity, turbulence and temperature characteristics have been determined by using a combination of an X-wire and a cold wire anemometry. The parametric effects of jet Reynolds number (ReW) ranging from 5600 to 13,200 and the dimensionless slot nozzle-to-impingement surface distance (Y/W) ranges from 2 to 10 on the local circumferential heat transfer have been studied. Local circumferential Nusselt number (NuS) decreases with increasing the dimensionless circumferential distance (S/W) from its maximum value at the stagnation point up to S/W=3.1. The transition in the wall jet from laminar to turbulent flow was completed by about 3.3⩽S/W⩽4.2 which coincided with a secondary peak in heat transfer. Correlations of local and average Nusselt numbers with ReW, Y/W and S/W have been established for the stagnation point and the circumferential distribution. The rate of decay of average circumferential Nusselt numbers around the semi-circular convex surface is much faster than that which occurs laterally along the flat surface. As Y/W increases, the effect of surface curvature becomes apparent and the difference between the flat surface correlation and the convex surface becomes more pronounced.

Emission reduction from diesel engine using fumigation methanol and diesel oxidation catalyst

This study is aimed to investigate the combined application of fumigation methanol and a diesel oxidation catalyst for reducing emissions of an in-use diesel engine. Experiments were performed on a 4-cylinder naturally-aspirated direct-injection diesel engine operating at a constant speed of 1800 rev/min for five engine loads. The experimental results show that at low engine loads, the brake thermal efficiency decreases with increase in fumigation methanol; but at high loads, it slightly increases with increase in fumigation methanol. The fumigation method results in a significant increase in hydrocarbon (HC), carbon monoxide (CO), and nitrogen dioxide (NO(2)) emissions, but decrease in nitrogen oxides (NO(x)), smoke opacity and the particulate mass concentration. For the submicron particles, the total number of particles decreases. In all cases, there is little change in geometrical mean diameter of the particles. After catalytic conversion, the HC, CO, NO(2), particulate mass and particulate number concentrations were significantly reduced at medium to high engine loads; while the geometrical mean diameter of the particles becomes larger. Thus, the combined use of fumigation methanol and diesel oxidation catalyst leads to a reduction of HC, CO, NO(x), particulate mass and particulate number concentrations at medium to high engine loads.

Calibrating for viewing angle effect during heat transfer measurements on a curved surface

Abstract Liquid crystal thermography (LCT) has been widely used for the determination of surface heat transfer distribution. However, this technique is sensitive to illumination and viewing angle and therefore limited to surfaces with only slight curvature. A liquid crystal calibration technique using true-colour image processing system has now been developed to alleviate the effect of viewing angle on oblique/curved surfaces. Application of the calibration and transient liquid crystal thermographic techniques and uncertainty analysis to a heated air slot jet impinging on a semi-cylindrical convex surface has been demonstrated. It is shown that the local heat transfer coefficient may be overestimated by up to 39.1% at a viewing angle of 69° from the normal under test conditions. However, the overall uncertainty in heat transfer coefficient can be significantly reduced from the maximum value of 36.3% to within 11.1% by using the implemented viewing calibration technique.

NUMERICAL SIMULATION OF LAMINAR FORCED CONVECTION IN AN AIR-COOLED HORIZONTAL PRINTED CIRCUIT BOARD ASSEMBLY

A numerical solution of the steady-state forced convection for air flowing through a horizontally oriented simulated printed circuit board (PCB) assembly under laminar flow condition has been developed. The considered assembly consists of a channel formed by two parallel plates. The upper plate is thermally insulated, whereas the bottom plate is attached with uniformly spaced identical electrically heated square ribs perpendicular to the mean air flow. The bottom plate is used to simulate the PCB, and the ribs with heat generation are used to simulate the electronic components. A second-order upwind scheme is adopted in the calculation and a very fine mesh density is arranged near the obstacle and the channel surface to achieve higher calculation accuracy. Four Nusselt numbers (Nu) are of particular interest in this analysis: local distribution along the ribs surfaces, mean value for individual surfaces of the rib, overall obstacle mean value, and overall PCB mean value between the central lines of two o...A numerical solution of the steady-state forced convection for air flowing through a horizontally oriented simulated printed circuit board (PCB) assembly under laminar flow condition has been developed. The considered assembly consists of a channel formed by two parallel plates. The upper plate is thermally insulated, whereas the bottom plate is attached with uniformly spaced identical electrically heated square ribs perpendicular to the mean air flow. The bottom plate is used to simulate the PCB, and the ribs with heat generation are used to simulate the electronic components. A second-order upwind scheme is adopted in the calculation and a very fine mesh density is arranged near the obstacle and the channel surface to achieve higher calculation accuracy. Four Nusselt numbers (Nu) are of particular interest in this analysis: local distribution along the ribs surfaces, mean value for individual surfaces of the rib, overall obstacle mean value, and overall PCB mean value between the central lines of two obstacles. The effect of the obstacle size and the separation between two obstacles is discussed systematically.

Experimental investigation on regulated and unregulated emissions of a diesel/methanol compound combustion engine with and without diesel oxidation catalyst.

The use of methanol in combination with diesel fuel is an effective measure to reduce particulate matter (PM) and nitrogen oxides (NOx) emissions from in-use diesel vehicles. In this study, a diesel/methanol compound combustion (DMCC) scheme was proposed and a 4-cylinder naturally-aspirated direct-injection diesel engine modified to operate on the proposed combustion scheme. The effect of DMCC and diesel oxidation catalyst (DOC) on the regulated emissions of total hydrocarbons (THC), carbon monoxide (CO), NOx and PM was investigated based on the Japanese 13 Mode test cycle. Certain unregulated emissions, including methane, ethyne, ethene, 1,3-butadiene, BTX (benzene, toluene, xylene), unburned methanol and formaldehyde were also evaluated based on the same test cycle. In addition, the soluble organic fraction (SOF) in the particulate and the particulate number concentration and size distribution were investigated at certain selected modes of operation. The results show that the DMCC scheme can effectively reduce NOx, particulate mass and number concentrations, ethyne, ethene and 1,3-butadiene emissions but significantly increase the emissions of THC, CO, NO(2), BTX, unburned methanol, formaldehyde, and the proportion of SOF in the particles. After the DOC, the emission of THC, CO, NO(2), as well as the unregulated gaseous emissions, can be significantly reduced when the exhaust gas temperature is sufficiently high while the particulate mass concentration is further reduced due to oxidation of the SOF.

Flow and Forced-Convection Characteristics of Turbulent Flow Through Parallel Plates with Periodic Transverse Ribs

ABSTRACT Two general turbulence models, the standard k–ϵ model and the Reynolds stress model (RSM), were used to predict the forced convection of a fully developed turbulent flow through an assembly of two horizontally oriented parallel plates in the Reynolds number range 22,000 < Re D < 94,000. The upper smooth plate was thermally insulated, whereas the bottom plate, attached with rectangular-cross-sectional ribs perpendicular to the mean air flow, was provided with a uniform heat flux. The ribs were uniformly spaced with the pitch-to-height ratio of p/e = 4, a height-to-hydraulic-diameter ratio of e/D = 0.25, and a width-to-height ratio of w/e = 2. The numerical approaches were based on the finite-volume technique. A second-order upwind scheme was applied in the calculation and a very fine mesh density was arranged in the regions near the wall boundaries. The SIMPLE algorithm was adopted to handle the pressure–velocity coupling in the calculation. Local Nusselt number distribution along the heated bottom ribbed surface was investigated, which was validated against corresponding experimental results conducted by Lorenz et al. [1]. It was found that in the simulation of the turbulent forced convection in this two-dimensional channel with a ribbed surface, the standard k–ϵ model had superiority over the Reynolds stress model. An anticlockwise vortex was found in the downstream region of a rib by using either of the two models; however, the length and relative strength of the vortex predicted by these two models were significantly different. Recirculating flow pattern was formed in the cavity between two adjacent ribs, while no reattachment of the mainstream flow was observed at the present pitch-to-height ratio of p/e = 4.

Monte Carlo simulation of nitrogen oxides dispersion from a vehicular exhaust plume and its sensitivity studies

Abstract The pollutant dispersion behavior from the vehicular exhaust plume has a direct impact on human health, particularly to the drivers, bicyclists, motorcyclists, pedestrians, people working nearby and vehicle passengers. A two-dimensional pollutant dispersion numerical model was developed based on the joint-scalar probability density function (PDF) approach coupled with a k – e turbulence model to simulate the initial dispersion process of nitrogen oxides, temperature and flow velocity distributions from a vehicular exhaust plume. A Monte Carlo algorithm was used to solve the PDF transport equations in order to obtain the dispersion distribution of nitrogen oxides concentration. The model was then validated by a series of sensitivity experimental studies in order to assess the effects of vehicular exhaust tailpipe velocities, wind speeds and chemistry on the initial dispersion of NO and NO 2 mass concentrations from the vehicular exhaust plume. The results show that the mass concentrations of nitrogen oxides decrease along the centerline of the vehicular exhaust plume in the downstream distance. The dispersion process can be enhanced when the vehicular exhaust tailpipe velocity is much larger than the wind speed. The oxidation reaction of NO plays an important role when the wind speed is large and the vehicular exhaust exit velocity is small, which leads to chemical reduction of NO, and the formation and accumulation of NO 2 in the exhaust plume. It is also found that the effect of vehicular exhaust-induced turbulence in the vicinity of the exhaust tailpipe exit is more dominant than the effect of wind turbulence, while the wind turbulence gradually shows a significant role for the dispersion of nitrogen oxides along with the development of exhaust plume. The range of dispersion of nitrogen oxides in the radial direction is increased along with the development of vehicular exhaust plume.