The present thesis investigated experimentally and numerically the combustion of LPG, DME as well as kerosene within the PIB under various equivalence ratios and thermal load input conditions. The heat recirculation mechanism instigated by the conduction and radiation feedback in the PM allowed the burner to operate in excess enthalpy combustion condition. In the light of this, an extensive study was performed to assess the applicability of the PIB for the fuel-efficient cooking stoves that produced minimal level of CO and soot emissions. The following summarises the major finding of this research:
The first part of this thesis aimed at developing a numerical solver to predict the combustion of air-fuel mixture inside a single-layer PIB considering detailed chemical kinetics. In particular, the performance of LPG-fueled PIB was evaluated based on comparisons to CH4 flame inside the PIB as well as to the combustion of LPG in FF mode. LPG combustion in the PIB was found to have a higher operating range than that of CH4. The LFL of LPG combustion in the PIB was found to be less than that of CH4
combustion in the PIB, and FF combustion of LPG. Radiant heat flux and the total heat flux were more with LPG combustion in the PIB. For the same equivalence ratio, CO emission of the PIB with LPG was much lower than with its combustion in the FF mode.
However, it was higher than that for CH4 combustion in the PIB.
After the detailed parametric study of the single-layer PIB, the second part of this thesis was devoted to the analyses of combustion characteristics of LPG in a two-layer PIB
118 Conclusions and future scope
using both computational and experimental techniques. Thermal performance of the burner was investigated for various geometric and operating parameters such as equivalence ratio, thermal load, and thickness of the PM. Five different burners (CB1- CB5) with different thicknesses of Al2O3 and SiC sections were considered. CO emissions of all the burners operating at 0.4 or below were found to be lower than 5.0 ppm and were below the emission standard prescribed by WHO. The preheating temperature was increased by 60.7 K using a thicker SiC PM integrated PIB. The LFL was extended to 0.31 with CB3 and CB4 burners. The LFL of 0.37 was found for the CB1 burner. CO emissions of CB3 and CB4 burners were found to be less than that of CB1 and CB2 burners for all thermal loads. With the use of CB3 burner instead of CB1 burner, an average reduction of 67.7% and 69.6% was achieved in CO emission for
0.6and 0.7, respectively. In all cases, CO emissions of CB3 burner and the CB4 burner were nearly identical. To meet the accepted indoor air quality norms, the LPG stove had to be operated below 0.8. Thickness of the Al2O3 section had insignificant effect on CO emissions, preheating temperature and LFL. For 0.4, LPG stove with CB1 burner was found to be most effective considering high thermal efficiency and CO emission standard limits permitted by WHO. However, for 0.4,stove with CB3 burner was found to be more appropriate. The sensitivity of CO emission to the burner and s was also studied. Decreasing by 50% and increasing s by 10 times of the base value, the CO emissions from the CB3 burner was found to follow the WHO guideline.
One of the objectives of this thesis was to study the effect of DME addition on LPG-air flame inside the PIB. The reaction pathway and flux analyses were performed by using an extended USC-Zhao kinetic mechanism to gain insights into the combustion chemistry of various LPG-DME-air flames within the PIB. The analyses revealed that for all values of
, combustion of fuel-air mixture inside PIB with higher DME level was found to have a higher filtration velocity than that of pure LPG combustion. Sensitivity analyses of filtration velocity demonstrated that and displayed negative sensitivity, whereas and s exhibited positive sensitivity. However, the filtration velocity was found to be insensitive to the density and specific heat capacity of the PIB material. For a fixed input operating condition, with the increase of , the peak radical pool concentration
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increased and the flame location moved to the upstream region of the intersection of the PIB. In addition, the maximum measured and predicted gas temperatures observed in the burner were higher than their respective adiabatic FF temperatures and excess enthalpy combustion was achieved in the PIB. Furthermore, the reaction zone thicknesses of DME flames within PIB were found to be thinner than that of LPG flames.
In the work presented here, the mole fraction profiles, heat release rate and the production rate of dominant reactions responsible for the generation of major chemical species inside the PIB were also investigated for various LPG-DME blends. The results showed that fuel-lean and fuel-rich flames with higher DME fractions generated highest CH3 and CH2O radicals within the PIB. For the combustion of LPG inside the PIB, the peak concentrations of soot precursors such as (C2H2, C3H3, C6H6, C6H5, C5H5 and C5H6) were found to be more than their corresponding FF values, however the peak values of these soot precursors and CH3CHO mole fractions decreased monotonically when DME was added to the LPG-air mixture in the PIB. With 0.5, 19.0% reduction in C2H2
formation and 21.0% reduction in C3H3 formation was achieved as compared to pure LPG combustion. In case of 1.0,an order of magnitude decrease in C2H2 and C3H3 was observed than that of LPG flame. The reactions involving CH3, HCO and CH2O were found to be dominant contributors to the overall heat release rate for DME combustion inside the PIB. The flux analyses revealed that for the LPG flame, the C3H3 formation pathways through aC3H4 and pC3H4 species were found to be major contributors in the aromatic ring formation and, with the addition of the DME, these pathways completely vanished.
In the third part of this study, ultra-lean DME flame was used to evaluate the performance of the PIB. From the stand point of higher rad and lower CO emission, the PIB showed better performance at the ultra-fuel-lean condition, and DME-air mixture with 0.4 and Qth = 1.4 kW was found to be the most desirable operating condition for the burner.
Further analyses highlighted the advantage of the pure DME flame over LPG flame inside the PIB. When DME was used instead of LPG in the PIB integrated stove, the maximum allowable equivalence ratio could be extended from 0.4 to 0.5,and the thermal load from Qth = 4.0 kW to Qth = 5.0 kW, to meet the permitted CO emission limit prescribed by WHO. It was found that with the use of DME instead of LPG, an
120 Conclusions and future scope
average reduction of 47.1% and 27% was achieved in the CO emission for 0.5and
0.6,respectively. Moreover, for all cases, the radiant efficiency of the PIB with DME flame was found to be higher than the LPG flame.
Another important novelty introduced in the present thesis is to address the soot formation and particle growth process inside the SiC-based PIB. In order to accomplish this purpose, a comprehensive soot kinetic mechanism based upon DSM has been incorporated into the core numerical model, taking into account the radiative heat losses from the gas species and soot particles. It was revealed that the PIB suppressed the soot formation and delayed the manifestation of the soot particulates as compared to the FF case. Moreover, along the PIB axis prominent decline in particle density profile and slower growth of soot particle size were observed. It was shown that the coagulation and agglomeration process of small particles to form large soot aggregates were also depressed by the less prolonged presence of free radicals in the post-flame region of the PIB. When operated under the same input conditions,the computed FV and DSP valuesfor the PIB ranged from 5×10-10 - 3.4×10-8 and 2 nm - 22 nm, respectively, while in case of the FF the same were in the range of 10-7 - 10-6 and 4 nm - 51 nm, respectively. For the PIB the maximum ND was found to be 8.12×1011 particles/cm3, while for the FF combustion the same was 1.13×1011 particles/cm3. The results showed that the soot particles and the aggregates were evolved at a higher height above the PIB, and they achieved their peak values at the further downstream side of the burner as compared to the FF case.
Sensitivity analyses of various soot-governing parameters revealed that slight changes in gas- and solid-phase temperature profiles due to the perturbation of different thermal and optical properties of the burner resulted in significant variations in sensitivities of FV, DSP
and XC H16 10,while ND,
6 6
XC H and
10 8
XC H were found to be mildly sensitive towards the properties of the SiC matrix. It was further demonstrated that a burner with highs,and low, , dp and values should be preferred for the further reduction of PAHs and soot production from the PIB. Eventually, once the numerical model had been developed to interpret the soot evolution phenomenon for PMC, this model was used to demonstrate the soot suppression effect within the kerosene flame inside the PIB.
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8.1. Future Scope
The numerical model developed as part of this thesis provides valuable insight in improving the thermal performance of gaseous- and liquid-fuelled PIBs, as well as it permits constructive theoretical guidance for the experimental analysis. Based on the current research, more studies could be conducted in the future for the further improvement of the PIB. The following are the recommendation for the future study:
In this study, ceramic foam (SiC) and alumina beds have been used as burner materials for the two-layer PIB. To enhance the durability of the burner, a higher resistant material to thermal cycles needs to be explored.
The present work predicts the suppression effect of soot formation with the use of PMC technique. More research is recommended to confirm this prediction through experimental investigations.
In the mathematical analysis, the modeling of evaporation mechanism was neglected assuming complete vaporization of the liquid fuel before its entry to the PIB. In this regard, to obtain a close approximation for the combustion of liquid fuels inside the PIB, a complete numerical model including the evaporation process need to be developed.
Though, the current thesis has mainly focused on the in-depth study of PIB with particular emphasis on lower and medium-scale cooking stoves, the applicability of the PMC can be extended to other relevant domestic as well as industrial purposes.