S.K. Som

Numerical prediction of air core diameter, coefficient of discharge and spray cone angle of a swirl spray pressure nozzle

Abstract Theoretical predictions of air core diameter, coefficient of discharge and spray cone angle of a swirl spray pressure nozzle have been made from numerical computations of flow within the nozzle. The diameter of central air core that stabilizes inside the nozzle has been predicted from a hydrodynamic situation that yields the minimum resistance to the liquid flow in the nozzle at given operating conditions. The coefficient of discharge and spray cone angle have been evaluated from the distributions of the different velocity components of liquid flow at the nozzle orifice. It has been observed that the coefficient of discharge Cd decreases, while the air core diameter da and spray cone angle ψ increase with the increase in nozzle flow in its lower range. However, all these parameters Cd, da and ψ finally become independent of nozzle flow. Predicted values of da, Cd and ψ for different geometrical dimensions of the nozzle have been compared with the empirical data available in the literature.

Combustion and emission characteristics in a gas turbine combustor at different pressure and swirl conditions

Abstract The practical challenge in research in the field of gas turbine combustion mainly centres around a clean emission, a low liner wall temperature, a uniform exit temperature distribution for turbomachinery applications along with a fuel economy of the combustion process. An attempt to meet up the challenge has been made in the present paper in developing a computational model based on stochastic separated flow analysis of a typical diffusion controlled spray combustion of liquid fuel in a gas turbine combustor to study the influences of combustor pressure and inlet swirl on the combustion and emission characteristics within the combustor. A standard k–ϵ model with wall function treatment for near wall region has been adopted for the solution of conservation equations in gas phase. The initial spray parameters are specified by a suitable PDF size distribution and a given spray cone angle. A radiation model for the gas phase, based on first order moment method, has been adopted in consideration of the gas phase as a grey absorbing–emitting medium. Formation of thermal NOx, as a post combustion reaction process, is determined from Zeldovich mechanism. It has been recognized that an increase in swirl number reduces the NOx emission level and improves the pattern factor at all combustor pressures. However, though at lower pressure an increase in swirl number reduces combustion efficiency, the trend is exactly the reverse at higher pressure. With increase in pressure, pattern factor improves while the combustion efficiency falls and the level of NOx emission increases.

Design and Optimization of Single-Phase Liquid Cooled Microchannel Heat Sink

Semiconductor devices for demanding automotive applications generate a large amount of heat (>100 Wcm2). These high power devices can be cooled off very effectively by liquid coolant flowing through the microchannel heat sink. Microchannel heat sinks are very attractive because of their compactness, light weight, and large surface-to-volume ratio. Higher surface-to-volume ratio results in enhanced cooling performance. In this paper, a systematic robust analytical method is presented for design and optimization of single-phase liquid cooled microchannel heat sink. Effects of various design parameters such as eccentricity and footprint of heat source or device, thickness of the heat sink base, channel aspect ratio, number of microchannels or fins, coolant flow rate, and thermal conductivity of heat sink material on heat sink thermal resistances and pressure drop are delineated. Finally, analytical results are compared with experimental data and good agreement is obtained. The analytical method helps to reduce the design cycle time and time-to-market significantly.

Entropy balance and exergy analysis of the process of droplet combustion

An entropy balance and subsequent exergy analysis of the process of combustion of a liquid fuel droplet in a quiescent gaseous surrounding at high temperature has been performed in order to determine the second-law efficiency of the process. Velocity and species concentration fields for the gas phase and the temperature field both for the gas and for the droplet phases have been evaluated from the numerical solution of the equations of conservation of mass, momentum and heat, accordingly. The rate of generation of entropy due to transport processes and chemical reaction in the gas phase has been determined from the generalized entropy transport equation. A theoretical model for exergy analysis of the process of droplet combustion has been developed in order to predict the second-law efficiency in terms of the pertinent controlling parameters, namely, the ratio of free stream to initial droplet temperatures and the initial Damkohler number. It has been observed that, in a typical diffusion-controlled droplet combustion process, in which the rate of chemical reaction is much faster than the rates of diffusion of heat, mass and momentum, the irreversibility rate has, in contrast, a lower value due to chemical reaction than that due to diffusion processes taken together. A low value of the initial Damkohler number (as close as possible to its limiting value for initiation of ignition) and a high value of free stream temperature should be preferred for the process of droplet combustion from the viewpoint of energy economy in relation to thermodynamic utilization of available energy.

Thermodynamic Irreversibilities and Second Law Analysis in a Spray Combustion Process

A theoretical model of exergy analysis, based on availability transfer and flow availability, in the process of spray combustion has been developed to evaluate the total thermodynamic irreversibility and second law efficiency of the process at various operating conditions. The velocity, temperature and concentration fields in the combustor, required for the evaluation of the availabilities and irreversibilities, have been computed numerically from a two phase separated flow model of the spray along with a suitable reaction kinetics for the gas phase reaction. The thermodynamic irreversibilities associated with the gas phase processes in the combustor have been obtained from the entropy transport equation, while that due to the interphase transport processes have been obtained as a difference of gas phase irreversibilities from the total irreversibility. The thermodynamic irreversibilities associated with different processes and a comparative picture of the variation of second law efficiency at various ope...

Energy and exergy balance in a gas turbine combustor

Abstract An energy and exergy balance in the process of spray combustion in a model tubular gas turbine combustor has been made to determine the combustion efficiency and second law efficiency of the process at different operating conditions. The velocity, temperature and species concentration fields in the combustor have been evaluated numerically from a two-phase separated flow model of the spray along with suitable reaction kinetics for the gas phase reaction. A theoretical model of exergy analysis, based on availability transfer and flow availability, has been developed to predict the second law efficiency of the combustion process in the gas turbine combustor. A comparative picture of the functional relationships of combustion efficiency and second law efficiency of the process with the operating parameters of the combustor have been made to throw light on the trade-off between the effectiveness of combustion and the lost work due to thermodynamic irreversibility.

Evaporation of multicomponent liquid fuel droplets

A theoretical investigation on evaporation of a two-component liquid fuel droplet in high-temperature quiescent gaseous surroundings has been made from the numerical solution of conservation equations of heat, mass and momentum transports in the carrier and droplet phases. Liquid fuel droplets containing (i) components of widely varying volatilities, namely, n-hexane and n-hexadecane and (ii) components of closely spaced volatilities, namely, n-hexane and benzene, have been considered for the studies. The evaporation characteristics, namely mass depletion, droplet temperature and droplet composition histories with time, have been evaluated in terms of the pertinent input parameters, namely the initial composition of the drop constituents and the free stream temperature. The present studies have been made on the basis of both (i) the interdiffusion (finite diffusion in the droplet phase) model, and (ii) the rapid mixing (infinite diffusion in the droplet phase) model. The results from both the models have been compared to ascertain the accuracy of the rapid mixing model.

Thermodynamics of flame impingement heat transfer

A theoretical model for entropy generation and utilization of work potential (exergy) in flame impingement (both premixed and diffusion) heat transfer has been developed in this article, to offer physical insights on the optimal operational regimes, depicting high values of the surface heat flux with minimal exergy destruction, within the practical constraints. The irreversibility components due to different physical processes have been evaluated from a general entropy transport equation. The velocity, temperature, and species concentration fields required for the solution of entropy transport equation have been determined from the numerical computation of flow-field in the flame. Global two-step chemical kinetics has been considered with methane (CH4) and air as fuel and oxidizer, respectively. The results have been predicted in terms of average nondimensional heat flux, expressed as Nusselt number at the target plate, the irreversibility components, and second law efficiency, as functions of the pertine...

Effects of spray characteristics on combustion performance of a liquid fuel spray in a gas turbine combustor

Spray characteristics like mean drop diameter and spray cone angle play an important role in the process of combustion within a gas turbine combustor. In order to study their effects on wall and exit temperature distributions and combustion efficiency in the combustor, a numerical model of a typical diffusion controlled spray combustion in a can-type gas turbine combustion chamber has been made. A simple k–ϵ model with wall function treatment for near-wall region has been adopted for the solution of conservation equations in carrier phase. The initial spray parameters are specified by a suitable PDF for size distribution and a given spray cone angle. A radiation model for the gas phase, based on modified first order moment method, and in consideration of the gas phase as a grey absorbing–emitting medium, has been adopted in the analysis. It has been recognized that an increase in mean drop diameter improves the pattern factor. However, the combustion efficiency attains its maximum at an optimum value of the mean diameter. Higher spray cone angle increases the combustion efficiency and improves the pattern factor, but at the same time, increases the wall temperature. Copyright

Migration of a surfactant-laden droplet in non-isothermal Poiseuille flow

The motion of a surfactant-laden viscous droplet in the presence of non-isothermal Poiseuille flow is studied analytically and numerically. Specifically, the focus of the present study is on the role of interfacial Marangoni stress generated due to imposed temperature gradient and non-uniform distribution of bulk-insoluble surfactants towards dictating the velocity and direction of motion of the droplet when the background flow is Poiseuille. Assuming the thermal convection and fluid inertia to be negligible, we obtain the explicit expression for steady velocity of a non-deformable spherical droplet when the droplet is located at the centerline of the imposed unbounded Poiseuille flow and encountering a linearly varying temperature field. Under these assumptions, the interfacial transport of surfactants is governed by the surface Peclet number which represents the relative strength of the advective transport of surfactant molecules over the diffusive transport. We obtain analytical solution for small and ...