Combustion

Experimental investigations are carried out to establish the thermal and emission characteristics of a Can combustor. Temperature and emission levels at the combustor exit are measured for different swirler vane angles and air fuel ratios (AFR). Swirler vane angle is varied from 150 to 600 in steps of 150. AFR is varied in the range of 41 to 51. Experimental analysis is carried out using methane as fuel. Measured temperature variation at combustor outlet indicates that the hot product of combustor flows near the liner wall. Gradient of temperature near the wall decreases as the swirler vane angle (and corresponding swirl number) is increased. The peak temperature reduces at higher value of AFR. Emission level of carbon monoxide (CO) decreases with increase in AFR and swirler vane orientation.  A higher level of NOX emission is observed for AFR of 45. This is due to change in shape and strength of the recirculation region in the primary zone of the combustor.
Large Eddy Simulation of a Co-axial Combustor is carried out Accuracy of Large Eddy simulation (LES) depends on the ability of the Sub-Grid Scale (SGS) models to predict the turbulent viscosity. The sensitivity of LES results for different SGS models is established for a coaxial annular combustor. The radial, axial and tangential velocity distribution predicted by four sub-grid scale models is compared with the experimental results of Sommerfeld and Qiu. It is observed that the flow physics is captured more accurately by WALE model as compared to the other SGS models. The predictions of WALE model are closer to the experimental results for all stations in the flow domain.
A tubular (CAN) combustor comprises of cylindrical liner mounted concentrically inside a cylindrical casing.  The mixing of fuel and oxidizer takes place in the primary region. Primary jet and swirl air creates strong re-circulating region in the primary zone. This recirculation controls the shape, size and stability of the flame. This way of generating swirl flow by passing air through the swirl vanes located in upstream of primary zone is called backward swirl. The present research is towards development of an upward swirl CAN type combustor. In this arrangement, primary air is introduced parallel to the combustor axis. Re-circulating region is formed due to change in flow direction of primary air in the primary region. During alteration of direction, air will mix with the fuel and the combustion takes place. This annular entry will confine flame to a region nearer the axis and reduce the number of scattered pockets with steep temperature gradient. The entrainment of flame near the axis will also leave liner at relatively low temperature. This in turn will reduce the wall cooling requirement. Upward swirl also helps in reducing the emission level of NOX from the gas turbine combustor by reducing the resident time of hot gas.
A new concept of reverse fuel injection is introduced to enhance the fuel-burning rate in upward swirl can-type gas turbine combustor. The existing combustor has conical shaped injector where fuel jets are arranged on 90 cone. In reverse fuel injection, the injectant exits the fuelling device in reverse axial direction towards the wall of hemispherical dome. The dome wall deaccelerates the fuel jet first and then fuel flows in all directions. The injector length is chosen as design variable for analysis of reverse fuel injection flow field. Combustion experiments are carried out for existing conical injector and different reverse fuel injectors. Large Eddy Simulations (LES) are carried out to simulate non-premixed turbulent combustion flow field and it is combined with Discrete Ordinates (DO) method for the radiation modelling. The turbulence-chemistry interaction is modelled using presumed shape Probability Density Function (PDF) approach. Results obtained show that reverse fuel injection provides rapid near field mixing and better fuel penetration. Improved mixing results into release of large amount of heat energy during combustion, which increases combustion efficiency and reduces CO emission level without adversely affecting the NOx emission level. The obtained results show that injector length influences mixing efficiency, residence time and emission level. There exists an optimal penetration height for efficient mixing and total pressure recovery efficiency and it is achieved when injector length is kept at 5mm in the present study. Thermal imaging of outer wall of combustor demonstrates that reverse fuel injection does not adversely affect the wall temperature.
A Relative assessment of thermal and emission characteristics of conventional and reverse air flow Can type gas turbine combustor is carried out . Large Eddy Simulations (LES) are carried out with Wall Adaptive Local Eddy (WALE) viscosity model for sub grid scale stresses. Turbulence-chemistry interaction is modelled using presumed shape Probability Density Function (PDF) approach. Discrete Ordinates (DO) model is used for radiative heat transfer. Experimental measurements are carried out of temperature and NOx emission level at the exit of combustors. Flow pattern and flame shape obtained from numerical analysis for conventional and reverse air flow combustor are correlated to combustor liner wall heating, exit temperature characteristics and NOx emission. The liner wall region of conventional combustor is dominated by hot combustion gases whereas the location of hot flame in reverse air flow combustor is in the vicinity of centreline. The exit temperature measurement shows that the wall side temperature in reverse air flow combustor is relatively low compared to conventional combustor. Reverse air flow arrangement in combustor eliminates the hot spots of high temperature gradient from primary region of combustor which results into significant reduction in NOx emission level in reverse air flow combustor as compared to conventional combustor. Thermal imaging of outer wall of combustors demonstrates that the radiative energy transfers from liner wall to outer wall is small in reverse air flow combustor compared to conventional combustor. This depicts that the wall-cooling requirement decreases in reverse air flow combustor.
This research aims to assess the potential of hydrogen in the form of a supplementary fuel to accelerate combustion chemistry and reduce CO emissions of methane fuelled upward swirl gas turbine combustor. Effects of hydrogen enrichment on flame characteristics and chemical kinetics are analysed using Large Eddy Simulations (LES). Flame visualization is performed and measurements of temperature and emissions at the exit of combustor are carried out . For the same energy input, flames are relatively broader and shorter at higher hydrogen concentrations. Augmentation of hydrogen is advantageous in terms of flame velocity, temperature, rate of chemical reactions and CO emissions. Higher flame temperature favours NOx emissions at higher hydrogen content. At a constant volumetric fuel flow, reduction in carbon-generated species is attributed to hydrocarbon substitution and chemical kinetic effects are less. Hydrogen addition increases flame temperature, decreases flame dimensions and reduces CO emissions with marginal increase in NOx emissions.