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It is conventionally believed that there are no self-sustained thermoacoustic oscillations in the absence of acoustic modes in combustors. However, such oscillations (also known as intrinsic thermoacoustic instability) are recently found to occur in a premixed combustor with a mean flow present but no acoustic eigenmodes involved. Practical combustors are associated with entropy waves, pressure jump and mean flow, which are ignored in previous studies without justification. In this work, an entropy-involved energy measure is defined and used to study the stability behaviors of intrinsic thermoacoustic modes. The concepts and methods are exemplified with the classical time-delay n–τ unsteady heat release model. The intrinsic thermoacoustic eigenmodes are found to be related to not only a flame transfer/describing function but also the acoustic impedance at the flame, which is boundary-dependent. It is shown that the predicted frequency ωfr of the intrinsic modes and the critical gain nc depend on the ratio T¯2/T¯1 between the after- and before-combustion temperatures and the inlet mean flow Mach number M¯1. Comparison is then made between the present results and those available in literature. Good agreement is obtained for ωfr. Furthermore, the predicted stability of intrinsic modes based on calculated nc is found to agree well with direct numerical simulations (DNS). It is also interesting to show that as T¯2/T¯1→1, the critical gain as predicted from the previous models is nc→+∞, which means that all intrinsic eigenmodes are stable. However, the present works shows that nc→1.0. Further illustration is then performed by conducting case studies of measured flame transfer and describing functions in premixed combustors. The present work opens up an alternative but more applicable way to study intrinsic thermoacoustic oscillations via the entropy-involved energy measure.

It is conventionally believed that there are no self-sustained thermoacoustic oscillations in the absence of acoustic modes in combustors. However, such oscillations (also known as intrinsic thermoacoustic instability) are recently found to occur in a premixed combustor with a mean flow present but no acoustic eigenmodes involved. Practical combustors are associated with entropy waves, pressure jump and mean flow, which are ignored in previous studies without justification. In this work, an entropy-involved energy measure is defined and used to study the stability behaviors of intrinsic thermoacoustic modes. The concepts and methods are exemplified with the classical time-delay n–τ unsteady heat release model. The intrinsic thermoacoustic eigenmodes are found to be related to not only a flame transfer/describing function but also the acoustic impedance at the flame, which is boundary-dependent. It is shown that the predicted frequency ωfr of the intrinsic modes and the critical gain nc depend on the ratio T¯2/T¯1 between the after- and before-combustion temperatures and the inlet mean flow Mach number M¯1. Comparison is then made between the present results and those available in literature. Good agreement is obtained for ωfr. Furthermore, the predicted stability of intrinsic modes based on calculated nc is found to agree well with direct numerical simulations (DNS). It is also interesting to show that as T¯2/T¯1→1, the critical gain as predicted from the previous models is nc→+∞, which means that all intrinsic eigenmodes are stable. However, the present works shows that nc→1.0. Further illustration is then performed by conducting case studies of measured flame transfer and describing functions in premixed combustors. The present work opens up an alternative but more applicable way to study intrinsic thermoacoustic oscillations via the entropy-involved energy measure.

Abstract This study presents a detailed kinetic investigation into ultra-rich oxidation of H 2 S-CH 4 under high temperature (900–1250 °C) and ambient pressure. Effects of temperature, initial H 2 S/CH 4 ratio and equivalence ratio (Φ) on reactants conversions and products distributions were experimentally studied in a tubular flow reactor and kinetically analyzed by CHEMKIN software. A detailed kinetic mechanism involving 85 species and 515 reactions has been developed and validated using reference data for H 2 S-CH 4 decomposition and results from extended experimental conditions involving the O 2 addition. For H 2 S-CH 4 system, conversion of H 2 S increased steady with the rising temperature while reactivity of CH 4 was weak at temperature below 1000 °C. At temperature higher than 1000 °C, conversion of CH 4 increased rapidly and devoted further formation of H 2 and CS 2 mainly via reacting with H 2 S decomposition products. The H 2 production efficiency was negatively associated with initial H 2 S fraction as H 2 S decomposition was dominant H 2 source within 1150 °C. The stoichiometric ratio for H 2 S/CH 4 merely showed its advantage in H 2 production at higher temperature under which CH 4 reached its equilibrium conversion swiftly. Introduction of little amount of O 2 (Φ = 6 or higher) accelerated the whole reaction process and triggered H 2 S partial oxidation and H 2 formation at lower temperature. CH 4 explicitly showed inferior position in oxidation competition with H 2 S and maintained poor conversion at temperature below 950 °C. The results of rate of production (ROP) analysis at condition without O 2 showed that CH 4 reactivity showed dependence on free S radical via S + CH 4 = SH + CH 3 , and the formed CH 3 was mainly converted via reacting with SH and H radicals. CH 3 could be concurrently reverted to CH 4 via reactions with H 2 S and H 2 . O 2 activated the whole system by forming chain branching radicals O and OH. These radicals promoted H 2 S and CH 4 conversions to form richer S, H and CH 3 radicals. SH + CS = CS 2 + H was important for CS 2 formation and with presence of O 2 , CS 2 was likely to be consumed via oxidation reactions. Finally reaction pathways for H 2 S, CH 4 conversion and H 2 , CS 2 formation were presented.

Abstract This study presents a detailed kinetic investigation into ultra-rich oxidation of H 2 S-CH 4 under high temperature (900–1250 °C) and ambient pressure. Effects of temperature, initial H 2 S/CH 4 ratio and equivalence ratio (Φ) on reactants conversions and products distributions were experimentally studied in a tubular flow reactor and kinetically analyzed by CHEMKIN software. A detailed kinetic mechanism involving 85 species and 515 reactions has been developed and validated using reference data for H 2 S-CH 4 decomposition and results from extended experimental conditions involving the O 2 addition. For H 2 S-CH 4 system, conversion of H 2 S increased steady with the rising temperature while reactivity of CH 4 was weak at temperature below 1000 °C. At temperature higher than 1000 °C, conversion of CH 4 increased rapidly and devoted further formation of H 2 and CS 2 mainly via reacting with H 2 S decomposition products. The H 2 production efficiency was negatively associated with initial H 2 S fraction as H 2 S decomposition was dominant H 2 source within 1150 °C. The stoichiometric ratio for H 2 S/CH 4 merely showed its advantage in H 2 production at higher temperature under which CH 4 reached its equilibrium conversion swiftly. Introduction of little amount of O 2 (Φ = 6 or higher) accelerated the whole reaction process and triggered H 2 S partial oxidation and H 2 formation at lower temperature. CH 4 explicitly showed inferior position in oxidation competition with H 2 S and maintained poor conversion at temperature below 950 °C. The results of rate of production (ROP) analysis at condition without O 2 showed that CH 4 reactivity showed dependence on free S radical via S + CH 4 = SH + CH 3 , and the formed CH 3 was mainly converted via reacting with SH and H radicals. CH 3 could be concurrently reverted to CH 4 via reactions with H 2 S and H 2 . O 2 activated the whole system by forming chain branching radicals O and OH. These radicals promoted H 2 S and CH 4 conversions to form richer S, H and CH 3 radicals. SH + CS = CS 2 + H was important for CS 2 formation and with presence of O 2 , CS 2 was likely to be consumed via oxidation reactions. Finally reaction pathways for H 2 S, CH 4 conversion and H 2 , CS 2 formation were presented.

An electric and thermal model of the hybrid device consisting of a dye-sensitized solar cell (DSSC) and a thermoelectric generator (TEG) is studied for exploiting the solar full spectrum. Analytical expressions for the power outputs and efficiencies of the DSSC, TEG and hybrid device are derived. Temperature-dependent coefficients of the DSSC are introduced and their values being consistent with the experimental data are determined. The effects of the operating electric current, working temperature and temperature-dependent coefficient in the DSSC on the performance of the hybrid device are discussed in detail. The optimum performance characteristics of the hybrid derive at different temperature conditions and the DSSC at the reference temperature condition are compared. The parametric design criteria of the optimal coupling are given. These results obtained here may provide some guidance for the optimum design of practical hybrid devices.

An electric and thermal model of the hybrid device consisting of a dye-sensitized solar cell (DSSC) and a thermoelectric generator (TEG) is studied for exploiting the solar full spectrum. Analytical expressions for the power outputs and efficiencies of the DSSC, TEG and hybrid device are derived. Temperature-dependent coefficients of the DSSC are introduced and their values being consistent with the experimental data are determined. The effects of the operating electric current, working temperature and temperature-dependent coefficient in the DSSC on the performance of the hybrid device are discussed in detail. The optimum performance characteristics of the hybrid derive at different temperature conditions and the DSSC at the reference temperature condition are compared. The parametric design criteria of the optimal coupling are given. These results obtained here may provide some guidance for the optimum design of practical hybrid devices.

Abstract A high heat storage capacity form-stable composite phase change material (CPCM) with enhanced flame retardancy that integrated modified glass fibers with form-stable PCM was proposed. The modified glass fibers were wrapped by a composite flame retardant coating. The thermal and flame retardant properties of the CPCM were measured and compared to other CPCM samples. The results of vertical burning test indicated that the glass fibers improved the mechanical properties of the CPCM and prevented it from fracturing during the burning process. The modified glass fibers could further improve the flame retardancy of CPCM, and V-0 burning rating was achieved while the content of paraffin was maintained at 70 wt%, which means the proportion of flame retardants could be reduced. TGA results showed that the modified glass fibers could enhance the thermal stability and retard the degradation process of the CPCM, and the char residue was increased to 15.3%. Thermal cycling results indicated that the CPCM has good thermal reliability. The results of cone calorimeter test indicated that the peak heat release rate (PHRR) of flame retardant form-stable CPCM dropped by 58.8%, and the combustion rate could be greatly slowed down due to the protection of carbon layers formed by modified glass fibers. In addition, the thermal conductivity of CPCMs were greatly enhanced and the CPCM has good thermal reliability.