M.R. Ravi

On the high-Rayleigh-number structure of steady laminar natural-convection flow in a square enclosure

Natural-convection flow in an enclosure with adiabatic horizontal walls and isothermal vertical walls maintained at a fixed temperature difference has been investigated. At high values of the natural-convection parameter, the Rayleigh number, a recirculating pocket appears near the corners downstream of the vertical walls, and the flow separates and reattaches at the horizontal walls in the vicinity of this recirculation. There is also a considerable thickening of the horizontal layer. In some previous studies by different authors, this corner flow was considered to be caused by an internal hydraulic jump, and the jump theory was used to predict bifurcation of the steady flow into periodic flow. The present work examines the corner phenomenon closely to decide if it is indeed caused by a hydraulic jump. The results of the analysis reveal the oversimplification of the problem made in the previous studies: there is no connection of the corner phenomenon with a hydraulic jump. The separation of flow at the ceiling is not a feature of hydraulic jumps, and the essential energy loss associated with hydraulic jumps is not observed in the corner flow. It is shown that the corner structure is caused by thermal effects. Owing to the temperature undershoots in vertical boundary layer, which are known to be caused by the stable thermal stratification of the core, relatively cold fluid reaches the upper corner. This cold fluid detaches from the ceiling like a plume at high Rayleigh numbers, and causes the separation and recirculation.

Development of a semi-empirical model for pyrolysis of an annular sawdust bed

Abstract A model for simulating pyrolysis of sawdust in a packed bed of annular shape has been developed. Heat transfer is assumed to be purely due to conduction, and chemical reaction of pyrolysis has been approximated to a pseudo-first order reaction. The reaction rate constant has been obtained as a function of temperature for the species of sawdust used in the present work using thermogravimetric analysis, and its variation with heating rate has been accounted for. Thermal conductivity of the packed bed of sawdust has been estimated experimentally by temperature measurement in the sawdust bed. The sawdust bed was pyrolysed using an electric heater in contact with the inner surface of the annulus as the source of energy. Using the history of temperature profiles measured on the heater wall at various points of time, a simulation of the experiment has been carried out. The predicted mass loss history compares reasonably well with the measured one. A sensitivity analysis using the model suggests that the predictions may be improved by considering the flow of volatiles in the void space of char bed, and by accounting for secondary pyrolysis due to residence of volatiles in the bed.

Use of CFD simulation as a design tool for biomass stoves

Design of biomass stoves has relied mostly on empirical information and trial-and-error experiments backed up by simple thermodynamic and heat transfer calculations. Quite frequently, the details of fluid flow inside the biomass stove, which is caused by buoyancy due to the high temperature prevailing in the combustion region, have been ignored and correlations of heat transfer based on forced flow have been used in the heat transfer analysis of stoves. This paper presents an approach in which detailed CFD simulations of the flow, heat transfer, pyrolysis and combustion in th e configuration of a simple sawdust stove are used to evolve simple algebraic equations that describe individual phenomena. Such equations are needed in the field for performance analysis and prediction, and could also be used for the optimization of stove geometry for performance. The paper describes the development of the building-blocks of the detailed simulation model and its use in the derivation of simple model equations relating design and performance parameters.

Aerosol and carbon monoxide emissions from low-temperature combustion in a sawdust packed-bed stove

Low-temperature combustion in biomass-burning stoves used for cooking results in poor thermal efficiency and high emissions. A sawdust packed-bed stove has been shown to give more stable combustion at higher temperatures than woodstoves. The study examines pollutant emissions from this stove and their dependence on stove dimensions, specifically the vertical port radius and the stove-pot spacing. Emission rates of particulate matter (PM)—along with size resolution—and of carbon monoxide (CO) were measured during steady-state combustion. The stove power increased with increased spacing and vertical port radius. However, the air-flow rate, combustion temperature, and air-fuel ratio showed complex variations with stove dimensions from the described coupling among the pyrolysis, combustion, induced air flow, and mixing. Emission rates of PM (0.21–0.36 gh−1 and CO (3–8 gh−1 and were a factor of ten lower than those previously measured from woodstoves. Emission rates of CO decreased, while PM increased, with increasing combustion temperature. Aerosol size distributions were unimodal with mass median aerodynamic diameters (MMAD) of 0.24–0.40 𝛍 a factor of two smaller than from woodstoves. Cool combustion at 534–625°C gave lower PM emission rates but particles of larger MMAD, while hot combustion at 625–741°C gave higher PM emission rates with smaller particle MMAD. The OC/EC ratio obtained for cool combustion was higher (1.20) than that for hot combustion (0.96). Greater elemental carbon formation was seen at the higher temperatures. PM and CO emission rates followed opposite trends with combustion temperature and stove configuration, resulting in no single configuration at which both CO and PM emissions were minimized. However, its superior thermal efficiency and significantly lower emissions than wood stoves should motivate further study of this device to optimize thermal and emissions performance.

Effect of Hydrogen Content and Dilution on Laminar Burning Velocity and Stability Characteristics of Producer Gas-Air Mixtures

Producer gas is one of the promising alternative fuels with typical constituents of H2, CO, CH4, N2, and CO2. The laminar burning velocity of producer gas was computed for a wide range of operating conditions. Flame stability due to preferential diffusional effects was also investigated. Computations were carried out for spherical outwardly propagating flames and planar flames. Different reaction mechanisms were assessed for the prediction of laminar burning velocities of CH4, H2, H2-CO, and CO-CH4 and results showed that the Warnatz reaction mechanism with C1 chemistry was the smallest among the tested mechanisms with reasonably accurate predictions for all fuels at 1 bar, 300 K. To study the effect of variation in the producer gas composition, each of the fuel constituents in ternary CH4-H2-CO mixtures was varied between 0 to 48%, while keeping diluents fixed at 10% CO2 and 42% N2 by volume. Peak burning velocity shifted from to 1.1 as the combined volumetric percentage of hydrogen and CO varied from 48% to 0%. Unstable flames due to preferential diffusion effects were observed for lean mixtures of fuel with high hydrogen content. The present results indicate that H2 has a strong influence on the combustion of producer gas.

Prediction of film cooling effectiveness over a flat plate from film heating studies

ABSTRACT Film cooling is widely used to protect surfaces exposed to gases at a high temperature in gas turbine engines. Film heating is the reverse of film cooling, where hot secondary fluid is injected onto the walls to protect against a relatively cold mainstream. In the literature, the latter has often been used as an experimental analogue of the former, since mainstream flow rates are substantially higher, and it is relatively simpler to heat the smaller stream of secondary fluid for experiments. In this paper, the results obtained from a numerical study of film cooling and film heating over a flat plate through single-slot injection are presented. Since the objective of the work is to evaluate the suitability of film heating as a proxy for film cooling, it was decided to keep computational simple, using two-dimensional simulations. The effect of a density ratio of injectant-to-mainstream in the range of 0.2–5 is studied numerically to cover film heating and film cooling. Numerical simulations were carried out for three blowing ratios, M = 1, 2, and 3 at a fixed mainstream Reynolds number of 1.5 × 105 for three injection angles, 30°, 45°, and 60°. Numerical simulations were also carried out for a wide range of momentum flux ratio for film heating and film cooling at an injection angle of 30°. The results show that film heating and film cooling are not equivalent, especially when the density ratio deviates from unity substantially. Based on numerical study, it appears possible to predict film cooling effectiveness from film heating effectiveness for a wide range of density ratios, even though the effectiveness values obtained in regard to film cooling and film heating differ significantly.

A Numerical Study on the 2D Film Cooling of a Flat Surface

In this article, the results obtained from a detailed numerical investigation of 2D film cooling over a flat plate through single-slot injection are presented. The effects of mainstream Reynolds number, blowing ratio, density ratio, and injection angle on the effectiveness of film cooling were investigated in the present work. Numerical simulations were carried over a wide range of density ratio ranging from 1.1 to 5 at two mainstream Reynolds numbers (8 × 104 and 1.5 × 105), three blowing ratios (ranging from 1 to 3), and six injection angles (ranging from 15° to 90°). The results show that at lower injection angles of 15°–45°, maximum film-cooling effectiveness occurs at a particular value of velocity ratio which is found to be independent of mainstream Reynolds number, blowing ratio, and density ratio. Based on a combined effect analysis of blowing ratio, density ratio, and injection angle, a relation was obtained for velocity ratio that gives an optimum film-cooling effectiveness.

Numerical investigation of film cooling on a 2D corrugated surface

ABSTRACT A detailed numerical study on the film cooling of a corrugated surface through a single slot has been presented in this paper. The effects of the blowing ratio, density ratio (DR), and injection angle on the film cooling of the corrugated surface are discussed. Numerical simulations are carried out over a wide range of DRs ranging from 0.2 to 5.0 at a fixed mainstream Reynolds number of 1.5 × 105, three blowing ratios of 1, 2, and 3, and five injection angles ranging from 30° to 90°. Results show that the velocity profile on a corrugated surface is strongly influenced by the injection of the secondary fluid. It is observed that the film cooling effectiveness of the corrugated surface increases monotonically with an increase in the blowing ratio. The density ratio and injection angle also have a strong influence on the film cooling.

Thermodynamics and Kinetics of Gasification

This chapter deals with the basic thermodynamics and chemical kinetics pertaining to the various physicochemical phenomena that are collectively termed as the phenomenon of gasification. Although the phenomena associated with the gasification of various feedstocks differ from each other in detail, the underlying thermodynamics is more or less common and is attempted to be captured here. The technology of gasification also has a wide variety, and this results in different phenomena having varying grades of importance in each. Thermodynamics of a phenomenon is described in terms of conservation equation for mass and the first law, often discussed under the headings of stoichiometry and energetics of a phenomenon, and in terms of the second law, which determines the equilibrium state at the end of the phenomenon, and thus defines the product compositions in gasification when the reactor is maintained at a given pressure and temperature. The variety of phenomena involved in gasification, namely drying of feedstock, its pyrolysis, homogeneous and heterogeneous reactions which form part of the gasification in the form of oxidation and reduction reactions, proceed at different rates in a given system, and also vary widely between different types of gasification systems. Hence, it is important to study the kinetics of these phenomena, in addition to the study of thermodynamic equilibrium states pertaining to these phenomena. Owing to the fact that each of these phenomena is extremely complex, in mathematical modelling of these phenomena, often apparent mechanism and their thermodynamics and kinetics are studied. This leads to a variety of models and thermodynamic and kinetic data in the literature, often in apparent conflict with each other. This chapter also attempts to identify some of these conflicts through the experience of the authors in modelling gasification phenomena.

Performance and Emissions of Natural Gas and Hydrogen/Natural Gas Blended Fuels in Spark Ignition Engine

The basic intent of the present work is to evaluate the potential of using alternative gaseous fuels like compressed natural gas (CNG) and H2 /CNG as a spark ignition (SI) engine (lean burn engines) fuel. Computer modeling of internal combustion engine is useful in understanding the complex processes that occur in the combustion chamber. This research deals with quasi-dimensional, two-zone thermodynamic simulation of four-stroke SI engine fueled with CNG and H2 /CNG. The fraction of hydrogen in the H2 /CNG blend, for simulation was varied from 0–60% by volume. The developed computer model has been used for the prediction of the combustion and emission characteristics of H2 /CNG blended fuel in SI engines, which includes the power, thermal efficiency, cylinder pressure-crank angle history, exhaust emissions (NOx and CO), fuel consumption, combustion duration, ignition delay, etc. Predicted results indicate that the presence of hydrogen in H2 /CNG blend can improve combustion duration as it has a higher flame speed. There are increases in oxides of nitrogen emissions, but decrease in carbon monoxide and hydrocarbon emissions, when comparing H2 /CNG blended fuel to neat CNG. The validity of the model has been carried out by comparing the computed results with experimental data obtained under same engine setup and operating conditions. The results obtained from the theoretical model when compared with those from experimental ones show a good agreement. Also, the effects of the many operating parameters such as equivalence ratio, engine speed, and spark timing have been studied.Copyright