Applied Mechanics and Materials Vols. 592-594

Abstract: Natural convection is studied numerically using finite element based computational procedure. The enclosure used for flow and heat transfer analysis has been bounded by adiabatic top wall, constant temperature cold vertical walls and a horizontal bottom wall. The grid independent study has been made with different grids to yield consistent values. Different grid sizes 30x30, 40x40, 50x50 uniform meshes have been studied. Study shows the convergence of average Nusselt number for a grid size of 41x41. Hence a grid size of 40x40 is used in all computations. Nusselt numbers are computed for different Rayleigh’s numbers (Ra) and aspect ratios of 1,2 and 3. Results are presented in the form of streamlines, isotherm plots and average Nusselt number. The average Nusselt numbers increase with Rayleigh number and for a given Ra, increase in Nu is obtained with increase in aspect ratio for bottom wall.

Abstract: This paper is all about a new type of ignition system for igniting the air-fuel mixture within combustion chamber of a gas turbine engine. In this system there will a separate ignition inside the primary combustion chamber which will be outside the main combustion chamber and responsible for igniting main source of air/fuel mixture inside the combustion chamber. This system is designed to overcome several problems of present ignition system of gas turbine engine and also thermal analysis of this new system has been shown in this paper.

Abstract: In the present work, the heat transfer characteristics of supercritical pressure water are numerically investigated in an upward flow vertical smooth tube. The numerical simulations are carried out by using Ansys-Fluent solver. The objective of the present work is to investigate the effect of heat flux and mass flux on heat transfer characteristics in supercritical water. In order to perform numerical simulation, experimental data of Mokry et al. [2] is considered. Various simulations were carried out for the inlet parameters of temperature 350°C, pressure 240bar; heat flux values ranging from 190 to 884kW/m² and mass flux values ranging from 498 to 1499kg/m²s. Based on the available parameters of heat flux and mass flux, they are segregated as groups with heat flux to mass flux ratios of 0.39 and 0.67. According to computational data, the heat transfer enhancement and heat transfer deterioration phenomenon of supercritical water were analyzed and based on the comparison with experimental data; their occurrence and mechanism were addressed.

Abstract: The Proton Exchange Membrane (PEM) Fuel Cell performance not only depends on the operating parameters like temperature, pressure, the stoichiometric ratio of reactants, relative humidity and back pressure on anode and cathode flow channels, but it also depends on design parameters like channel width to rib width, channel depth and number of passes on the flow channel. In this paper numerical analysis were carried out with six different cross-sections of the channel, namely square, triangle, parallelogram 14^o, parallelogram 26^o, trapezium and inverted trapezium of 1.25 cm² active area with a constant cross sectional area of 0.01 cm² of single pass PEM fuel cell. The model was created and simulated under various pressures and temperature with a constant mass flow rate by using fluent CFD and the influence of the single pass flow channel on the performance of PEM fuel cell has been investigated.

Abstract: Methods for creating thrusters with very low thrust using micronozzles have been actively developed recently. The propellant flow in such micronozzles are pressure driven and are characterized by low Reynolds number. Hence, the flow is always in laminar regime with high viscous losses. Proper design by effectively studying the flow behavior of propellant inside micronozzle is highly essential to minimize the losses. The geometry of the micronozzle is a key factor that affects the performance of the thruster. In this paper numerical examinations of the flow of superheated steam inside a 3D pyramidal micronozzle by solving Navier stoke’s equation with no slip boundary condition and equation of energy conservation. The computational model is validated with available experimental data in the literature. The computations are performed for different mass flow rates and inlet vapour temperatures increased until the exit temperature reaches the saturation temperature of the vapour. The study provides the insight into analysis of flows in the complicated microdevices.

Abstract: Fins are extended surfaces provided to enhance the heat transfer rate of a system. Several attempts have been made in the past to augment the heat transfer rate by using fins of various geometries. In the present study an array of rectangular fins with closed top, standing on a vertical base is analysed under natural convection conditions using commercial CFD code ANSYS FLUENT©. The numerical model is validated with the available experimental results for fins with open top under natural convection conditions. The plate fin heat sink is analysed for a constant heat duty of 60 W. The height, thickness and length of the fins are taken to be constant throughout the analysis. A detailed study is carried out to examine the dependency of the base plate temperature on the thickness of the closed top and on the number of fins. It is concluded based on the analysis that heat fins with closed top are found to have a decreased base plate temperature compared to the conventional rectangular fins.

Abstract: As fossil fuels are becoming less reliable and more costly, the Proton Exchange Membrane Fuel Cell (PEMFC) is emerging as the primary candidate to replace the stationary and transport applications. In this study numerical simulation on PEMFC is done by commercially available Computational Fluid Dynamics (CFD) software. A three-dimensional, model of a single PEM Fuel cell with serpentine flow field design has been used for the study. The numerical model is 3-D steady, incompressible, single phase and isothermal includes the governing of mass, momentum, energy, and species along with electrochemical equations. All of these equations are simultaneously solved in order to get current flux density and H₂, O₂ and H₂O fractions along the flow field design.

Abstract: Initiation of chemical reactions liberating thermal energy in non hypergolic propellants require ignition systems which are reliable as well as robust in nature. This necessitates the igniter and its subsystems be on par or exceed every quality criteria required for the main stage. The ignition systems which provide pilot action and flame holding in supersonic environment adds new challenges in its structural and thermal stability. The paper aims to conduct a numerical analysis of the Augmented Spark Torch igniter required for an air breathing rocket (SCRAMJET) engine with the emphasis on the optimization of the mixing length ratio required to achieve a uniform mixture ratio distribution in the zone of the igniter combustion chamber. Nomenclature

Abstract: Currently, the fixed-displacement oil pump supplies the oil for lubrication. Most of the time, these oil pump consumes more power and deliver significantly higher oil pressure than the required. This leads to the excess power consumption of the engine to run the pump; in turn lower fuel efficiency and higher CO₂ emissions. In this study, the concept of variable delivery oil pump is taken and critical design parameters that affect the oil flow rate are identified and studies are carried out to arrive at the correlation. From the results it can be observed both positive and negative correlation exists between the different critical parameters and the oil flow rate.

Abstract: Of all the renewable sources of energy available, solar thermal energy is the most abundant one and is available in both direct as well as indirect forms. In order to increase the thermal performance of solar collectors, the multipurpose solar collectors were investigated experimentally by the storage tank of the conventional solar water collector is modified as riser tubes and header. It is fitted in the bottom of the solar air heater as an absorber in the normal air heater. The thermal performance of thermosyphon flat plate solar water heater was investigated on both summer and winter seasons. The maximum daily average of 72.05%, 0.0316 kg/sm² and 0.873 m/s were recorded for the relative humidity, mass flow rate and wind speed at summer season respectively. Corresponding figure at for winter was 19.5 % 0.0295 kg/sm² and 0.722 m/s respectively. A minimum daily average of 11.23% and 0.384 m/s for the relative humidity and wind speed respectively. These results show that the mass flow rate obtained was a function of solar radiation and relative humidity. The less humid the ambient air becomes, the higher the heat gained by the system hence higher water flow rate. The maximum value of relative humidity obtained is due to low solar radiation on winter season. The heat removal factor (F_R) and collector efficiency factor (F_o) were found to be higher for Multipurpose solar water heater as compared to conventional solar water heaters for summer as well as winter season. These factors for Multipurpose solar water heater were more during summer and winter.