Modeling of supersonic reacting flow fields

A detailed understanding of the scramjet combustor flow field is critical to the achievement of a successful design. Even though the combustor flow field is quite complex, it can be realistically viewed as a collection of spatially developing and reacting supersonic mixing layers that are initially discrete, but that ultimately merge into larger more complex zones. These mixing layers begin downstream of a set of fuel injectors that introduce gaseous hydrogen in both a parallel and transverse direction into a supersonic air stream entering from the engine inlet. The behavior of the initial portion of the combustor flow, in the mixing layers near the fuel injectors, appears to be most critical, since this is where the mechanism for efficient high speed mixing must be established to achieve the required degree of combustion downstream. Because of the structure of the flow field in this initial portion of the combustor, a single supersonic, spatially developing and reacting mixing layer serves as an excellent physical model for the overall flow field. Even though this reacting mixing layer flow is geometrically simple, it can still be made to retain all of the fluid mechanical and chemical complexities present in the actual combustor flow field

The direct simulation of high-speed mixing-layers without and with chemical heat release

A direct numerical simulation of high speed reacting and non-reacting flows for H2-air systems is presented. The calculations are made for a convective Mach number of 0.38 with hyperbolic tangent initial profile and finite rate chemical reactions. A higher-order numerical method is used in time accurate mode to time advance the solution to a statistical steady state. About 600 time slices of all the variables are then stored for statistical analysis. It is shown that most of the problems of high-speed combustion with air are characterized by relatively weak heat release. The present study shows that: (1) the convective speed is reduced by heat release by about 10 percent at this convective Mach number M(sub c) = 0.38; (2) the variation of the mean and rms fluctuation of temperature can be explained on the basis of temperature fluctuation between the flame temperature and the ambient; (3) the growth rate with heat release is reduced by 7 percent; and (4) the entrainment is reduced by 25 percent with heat release. These differences are small in comparison with incompressible flow dynamics, and are argued to be due to the reduced importance of heat release in comparison with the large enthalpy gradients resulting from the large-scale vortex dynamics. It is finally suggested that the problems of reduced mixing in high-speed flows are not severely complicated by heat release

Experimental studies on combustible fuel block strategy for cooking

This paper presents experimental results of a study on the feasibility of making highly densified fuel block from agro residues that could be used for applications such as domestic cooking and barbecuing. A strategy had been adopted to determine the best suitable raw materials which meet both the criteria of performance and economy. In this regard several experiments were conducted with various raw materials in different proportions and it was found that fuel block composed of 40% biomass, 40% charcoal powder, 15% binder and 5% oxidizer fulfills the requirement of performance as well as economy. The unique geometry of this kind of fuel block (cylindrical one with a number of holes extending from top to bottom unlike traditional biomass briquette with single or no holes) helps in smokeless operation with reasonably steady thermal output. The geometry of the fuel block is so designed that it operates in partially premixed mode of combustion thus leading to better combustion and thereby lower emission. A typical fuel block for cooking weighing about 700-800g provides a thermal output of 1.5 kWth, with a burn time of 1.5 hours. Water boiling tests have indicated a thermal efficiency in the range of 55-58%

Variable property analysis-is there anything to it?

This paper discusses a few situations in combusting flows and attempts to demonstrate that including variable thermodynamic and transport properties in the analysis does more than simply improve the accuracy of predictions. The qualitative behaviour of the result itself is altered. Three examples are considered-single droplet combustion, forced convective turbulent boundary layer combustion, and free convective combustion. The flame to droplet radius ratio is very well predicted by variable property theory and the improvement is a direct consequence of the property variation. In the case of turbulent boundary layer combustion of solid/liquid fuels it is shown that the fuel exerts a significant influence on regression rate as is found in the experiments. The constant property theory, however, shows relative independence of regression rate with regard to the nature of the fuel. The prediction of regression rate is improved substantially in the case of free convective combustion

Flow studies in non circular tubes with wall injection

An analysis of inviscid incompressible flow in a tube of sinusoidally perturbed circular cross section with wall injection has been made. The velocity and pressure fields have been obtained. Measurements of axial velocity profiles and pressure distribution have been made in a simulated star shaped tube with wall injection. The static pressure at the star recess is found to be more than that at the star point, this feature being in conformity with the analytical result. Flow visualisation by photography of injected smoke seems to show simple diffusion rather than strong vortices in the recess

Biomass derived producer gas as a reciprocating engine fuel-an experimental analysis

This paper uncovers some of the misconceptions associated with the usage of producer gas, a lower calorific gas as a reciprocating engine fuel. This paper particularly addresses the use of producer gas in reciprocating engines at high compression ratio (17 : 1), which hitherto had been restricted to lower compression ratio (up to 12 : 1). This restriction in compression ratio has been mainly attributed to the auto-ignition tendency of the fuel, which appears to be simply a matter of presumption rather than fact. The current work clearly indicates the breakdown of this compression ratio barrier and it is shown that the engine runs smoothly at compression ratio of 17 : 1 without any tendency of auto-ignition. Experiments have been conducted on multi-cylinder spark ignition engine modified from a production diesel engine at varying compression ratios from 11.5 : 1 to 17 : 1 by retaining the combustion chamber design. As expected, working at a higher compression ratio turned out to be more efficient and also yielded higher brake power. A maximum brake power of 17.5 kWe was obtained at an overall efficiency of 21% at the highest compression ratio. The maximum de-rating of power in gas mode was 16% as compared to the normal diesel mode of operation at comparable compression ratio, whereas, the overall efficiency declined by 32.5%. A careful analysis of energy balance revealed excess energy loss to the coolant due to the existing combustion chamber design. Addressing the combustion chamber design for producer gas fuel should form a part of future work in improving the overall efficiency

Prediction of flame liftoff height of diffusion/partially premixed jet flames and modeling of mild combustion burners

In this article, a new flame extinction model based on the k/ε turbulence time scale concept is proposed to predict the flame liftoff heights over a wide range of coflow temperature and O2 mass fraction of the coflow. The flame is assumed to be quenched, when the fluid time scale is less than the chemical time scale (Da < 1). The chemical time scale is derived as a function of temperature, oxidizer mass fraction, fuel dilution, velocity of the jet and fuel type. The present extinction model has been tested for a variety of conditions: (a) ambient coflow conditions (1 atm and 300 K) for propane, methane and hydrogen jet flames, (b) highly preheated coflow, and (c) high temperature and low oxidizer concentration coflow. Predicted flame liftoff heights of jet diffusion and partially premixed flames are in excellent agreement with the experimental data for all the simulated conditions and fuels. It is observed that flame stabilization occurs at a point near the stoichiometric mixture fraction surface, where the local flow velocity is equal to the local flame propagation speed. The present method is used to determine the chemical time scale for the conditions existing in the mild/flameless combustion burners investigated by the authors earlier. This model has successfully predicted the initial premixing of the fuel with combustion products before the combustion reaction initiates. It has been inferred from these numerical simulations that fuel injection is followed by intense premixing with hot combustion products in the primary zone and combustion reaction follows further downstream. Reaction rate contours suggest that reaction takes place over a large volume and the magnitude of the combustion reaction is lower compared to the conventional combustion mode. The appearance of attached flames in the mild combustion burners at low thermal inputs is also predicted, which is due to lower average jet velocity and larger residence times in the near injection zone

Simulation of fluid flow in a high compression ratio reciprocating internal combustion engine

This paper discusses the detailed three-dimensional modelling of a reciprocating engine geometry comprising a flat cylinder head and a bowl-in-piston combustion chamber, simulating the motoring or non-firing conditions. The turbulence is modelled using a standard K-ε model and the results are compared against experimental results from the literature. Computed velocity profiles at time steps close to top centre (TC) are presented. The effect of squish and reverse squish becomes significant in a high compression ratio reciprocating engine. This enhanced fluid movement during a reverse squish regime could have an effect on burn rate, particularly in a spark ignition engine fuelled with biomass-derived producer gas, which has optimum ignition timing close to TC

Zero-dimensional modelling of a producer gas-based reciprocating engine

A zero-dimensional modelling study has been conducted using wrinkled flame theory for flame propagation to understand the in-cylinder pressure behaviour with time in a reciprocating internal combustion engine. These are compared with experiments conducted on the engine operated on biomass derived from producer gas and air mixture. The required inputs on the laminar burning velocity and turbulence parameters are obtained from separate studies. The data related to laminar burning velocity for producer gas and air mixture at thermodynamic conditions typical of unburned mixture in an engine cylinder were obtained from one-dimensional flame calculations. The turbulence parameters were obtained by conducting a three-dimensional computational fluid dynamics study on a bowl-in-piston geometry to simulate motored or non-firing conditions. The above mentioned data were used in the zero-dimensional model to make pressure-time (p-θ) computation over the complete engine cycle, for a range of test cases at varying compression ratio (CR) and ignition timing. The computational results matched reasonably well with experimental p-θ curves at advanced ignition timing at all CRs. The error in computed indicated power (IP) at advanced ignition setting (18°-27°CA) is around 3-4 per cent for CR = 17.0 and 11.5, and between 6 and 9 per cent for CR = 13.5. However, at less advanced ignition setting, the error in computed IP is larger and this is attributed to enhanced fluid dynamic effect due to reverse squish effect. And, whenever major part of the combustion occurred during this period, the deviation in the computed result appeared to be larger. This model has also been used to predict output of a commercially available producer gas engine of 60 kW. The optimum ignition timing on this particular engine was experimentally found to be 22°-24° before top centre. The zero-dimensional model has been used in a predictive mode and results compared with brake power under wide throttle open condition