Investigation of the influence of coflow and partial air premixing on liquid petroleum gas (LPG) flames in a lab-scale co-flow burner is presented. Primary air is supplied along with LPG in the inner core, and secondary air is supplied through the annulus region of the burner. Digital images are analyzed to study the flame shape, color, height, radius, and qualitative laminar flame speed. Concentrations of product gases and emission species are measured using a digital gas analyzer. Results indicate that in a dual air stream configuration, the partial premixing is optimum at % primary air value of around 45%.
Laminar nonpremixed and premixed flames have applications in several residential, commercial and industrial devices such as gas ranges, ovens, heating appliances, and burners. Both premixed and non-premixed laminar flame studies have been carried out employing a wide range of fuels and with even fuel mixtures. Both types of flames have their own advantages as well as disadvantages. For example, premixed flame produces less pollutant species, especially, least CO and soot. On the other hand, it has stability concerns, and it can produce more NO in higher flame temperature cases. Non-premixed flame is quite stable over a wide range of operating parameters. However, it produces soot, nitrogen dioxide, and carbon monoxide and can result in less combustion efficiency in few cases. Partial premixing process would inherit the advantages of both premixed and diffusion flames. Therefore, partial premixing of reactants is gaining importance and is able to meet the rigorous emission standards enforced by different nations around the world. Based on the fuel, there exists a percentage of partial premixing, which can produce lesser emissions and also have higher stability. Hydrocarbon fuel flames have been studied exhaustively due to its usage in several practical devices. Liquefied petroleum gas (LPG) is used as the most feasible fuel in domestic sectors as well as in industrial applications. In India, the majority of the household burners utilize LPG as their fuel. LPG is nontoxic, noncorrosive, and free of tetra-ethyl lead, and it has higher octane rating. These features have resulted in its usage in transport sector also. When compared with natural gas and other fuels, LPG has higher calorific value. Because of its broad flammability limits, it has found its place in lean burn conditions. However, when compared with gasoline and other fuel oils, the equivalent LPG consumption is higher due to its less energy density. As a result, it is less economical and also can result in the formation of higher emissions.
Several studies are reported on LPG-air premixed and diffusion flames. However, the information available on partially premixed LPG-air flames is limited. A study on the flammability limits of LPG-air mixture was done experimentally by Mishra and Rehman [
The schematic of the experimental setup is shown in Figure
Schematic of the experimental setup.
In this study, the LPG flow rate is kept constant at 22.5 g/h. For the given flow rate of secondary air, the primary air flow rate is varied from 0% to 86% of the stoichiometric air flow rate. The effect of secondary air on the flame characteristics is also studied by changing its flow rate between 100%, 150%, and 200% of the stoichiometric air. Air supply has been taken from a compressor storage tank. Controlling of the upstream pressure in the air line is done by a pressure regulator. An inverted funnel arrangement is kept above the burner, without disturbing the flame to collect the hot gases from the flame. The hot gas is subsequently analyzed using Kane KM9106 Quintox gas analyzer for the concentrations of CO, CO2, and NO. The gas analyzer is having an autocalibration setup during every startup. The maximum values of the species concentration that can be measured are 10%, 20%, and 5000 ppm, for CO, CO2, and NO, respectively. The resolution is 0.01% and 1 ppm, and the accuracy being ±5%. Once the flame is stabilized, the gas analyzer is allowed to reach steady state for around 60 to 90 seconds, and the species concentrations are noted down. The above procedure is repeated at least 5 times for all the cases. The variation has been observed to be within ±1.5%. High definition instantaneous digital photographs are taken at each set of readings with high-resolution digital camera. These photographs are used to visualize and analyze the flame shape and sizes. Soot inception points, length of blue region in the flame, flame diameter variation through the length of the flame, and cone angle are investigated with the help of Image J software.
The digital images of flames for different primary and secondary air flow rates are presented in Figure
Flame images: (a) 0% PA, (b) 20% PA, (c) 40% PA, (d) 50% PA, (e) 60% PA, (f) 70% PA, (g) 80% PA, (h) 83% PA, and (i) 86% PA. Left, middle, and right photos in each indicates 100% SA, 150% SA, and 200% SA, respectively.
0% PA
20% PA
40% PA
50% PA
60% PA
70% PA
80% PA
83% PA
86% PA
Variation of soot inception point measured from the burner tip.
When the primary air flow rate is increased to 50% and further to 60% of the stoichiometric value (Figures
The length of the blue region in the flame is found to increase with increase in % PA, until the inner premixed zone is formed around 70% PA. After that, it decreases due to increased availability of oxidizer in the primary mixture. This trend is shown in Figure
Variation of the length of blue region.
For the cases of 83% PA and 86% PA, where the inner premixed flame cone is observed, the laminar flame speed has been approximately calculated using the half-cone angle extracted from the digital image, for the purpose of relative comparison. An increase in the flame speed is observed with an increase in % PA, and a decrease in the flame speed is obtained with an increase in the secondary air flow rate (Figure
Variation of the flame speed.
The concentrations of species such as CO, CO2, and NO from the burner are measured using a portable gas analyzer. The gas analyzer also provides the oxygen concentration in volumetric %, which has been excluded, and the concentration of other species has been recalculated to avoid dilution effects from varying primary and secondary air flow rates. Temperature of the flue gas at the probe tip is recorded to be around 307 K. The variations of individual species are plotted against % PA. Variations of emissions with secondary air flow rates are also compared in the same plot. The concentration of CO (in ppm) displays a decreasing trend with increasing % PA (Figure
Variation of flue gas species: (a) CO (ppm), (b) CO2 (%), and (c) NO (ppm).
The characteristic of LPG-air partially premixed flames is analyzed in this paper. A co-flow burner with premixed reactants in the central core and air in the annular region is employed. At zero and low percentages of primary air, the point of soot inception from the burner tip increases with the increase in the secondary air flow rate. For a given primary air flow rate, a marginal increment in the length of blue region in the flame is observed with the increase in the secondary air. At higher values of % PA, addition of secondary air tends to decrease the flame speed. With increasing air premixing in the core flow, the CO concentration decreases initially and almost remains constant for % PA equal to 40% and above. Increase of the secondary air also results in steeper reduction of CO concentration at lower primary air flows. The concentration of carbon dioxide increases with the increase in the primary and secondary air flow rate indicating oxidation of CO. The variation of NO suggests that a minimum NO is attained around % PA values between 40% and 50%. These indicate that in a dual air stream configuration such as in this study, that partial premixing is optimum around % PA value of around 45% as it gives lower emissions and higher flame stability for LPG-air flames.