Coke deposition and heat transfer deterioration in curved cooling channels have drawn increasing attention. In this work, the influence of asymmetric truncated micro-ribs on flow and heat transfer in a curved cooling channel is numerically investigated. Three-dimensional numerical model is established for the reacting flow of aviation kerosene, RP-3 in both the smooth and ribbed cooling channels. The results show that compared with the non-ribbed and the top heated wall-ribbed channels, the bottom heated wall-ribbed channel renders lower temperature and less coke deposition on the top wall with more uniform temperature field in the channel. Heat transfer deterioration at the top heated wall is significantly improved in the bottom heated wall-ribbed channel. As rib number of the bottom heated wall-ribbed channel increases from 0 to 18, the velocity of the secondary flow in curved channel increases by 68.7 %, heat transfer coefficient of the top wall rises by 50.6 % while coke deposition on top wall is reduced by 34.9 %, respectively. With the height of ribs in the bottom heated wall-ribbed channel increasing from 0 mm to 0.4 mm, temperature and coke deposition on the top heated wall decreases by up to 300 K and 59.26 %, respectively. Meanwhile, heat transfer coefficient at the top wall increases by as much as 69.23 %. The interaction between vortex structures and the micro-ribs enhances heat near the heating wall surface. Additionally, coke formation is inhibited due to enhanced secondary flow in the ribbed channel. However, pressure drop of the cooling channel rises rapidly with the increase of height and number of ribs. Considering both the overall thermal performance and flow resistance of the cooling channel, the bottom heated wall-ribbed curved channel with rib number of 9 and rib height of 0.2 mm is recommended in the present study.

Dynamic coke deposition has significant influence on flow and heat transfer characteristics of regenerative cooling using hydrocarbon fuel. In the present work, the effect of coke deposition on flow and heat transfer performance of hydrocarbon fuel, n-Decane is numerically investigated under different heating modes including the constant heat flux (CHF), the periodic heat flux (PHF) and staged heat flux (SHF). The results show that as the heating mode varies from CHF to SHF, locally high temperature zones are formed. The flow field structure is varied by dynamic heating condition and coke deposition. The reaction rate of n-Decane also varies periodically with heat flux, resulting in an irregular "S" shaped density distribution. The two main coke precursors i.e., propylene and aromatics are highly sensitive to temperature due to secondary reactions. The thickness of coke deposition increases significantly with time regardless of the heating modes, which indicates that coke deposition process is irreversible. Compared with CHF heating mode, the thickness of coke deposition increases by nearly 25% for the SHF heating mode. Coking mechanism undergoes transition from catalytic coke to lateral coke with increasing temperature. For the CHF heating mode, the heat transfer coefficient increases from 1200 W/(m2 & sdot;K) to 4400 W/(m2 & sdot;K) along the flow direction. The PHF and SHF heating modes render higher Nusselt number and heat transfer coefficient than the CHF heating mode as the periodically varying wall heat flux results in locally higher heat flux and smaller temperature difference between the fluid and solid zones. In addition, the thermal conductivity and flow velocity in the regenerative cooling channel are also enhanced for the PHF and SHF heating modes. However, coke deposition leads to serious heat transfer deterioration for PHF and SHF with a decrease of up to 13% in heat transfer coefficient adjacent to the outlet section of the cooling channel. The new heat transfer correlation considering chemical reaction and coke deposition effects of supercritical hydrocarbon fuel is established for different wall heating conditions.

The micro structure of porous media has significant impact on transpiration cooling performance. In this study, the flow distribution and transpiration cooling performance are investigated in heterogeneous porous media. A local thermal non-equilibrium model coupling with cracking reaction model of the coolant is developed. Compared with the non-reacting flow, the cooling efficiency increases significantly for the reacting flow while the flow distribution becomes more non-uniform. With the porosity increasing from 0.2 to 0.5, the average cooling efficiency decreases by 7.5% while the standard deviation of cooling efficiency increases by 119.3%. It is found that porosity has the most significant impact on transpiration cooling performance for low to medium blowing ratios. For blowing ratio at 0.3%, the average cooling efficiency increases by 10.3% while the standard deviation increases by 249.8% with porosity increasing from 0.2 to 0.5. In order to further improve the transpiration cooling performance, 6 sets of heterogeneous porous structures with spatially varying porosity functions are analyzed. Different from the homogeneous porous media, the heterogeneous porous media render better flow distribution and cooling performance. Compared with the homogeneous porous media, the average cooling efficiency remains above 60% for all the heterogeneous porous media while the standard deviation of the cooling efficiency is reduced from 13.1% to 1.7% for the optimized porous structure. The results indicate the spatially periodic porous structure is promising in rendering better overall transpiration cooling performance. This work provides novel insight into flow distribution and transpiration cooling mechanism in heterogeneous porous media using reacting coolant.

A high-pressure laser ignition and combustion system with adjustable oxidizer gas atmosphere is established to investigate the ignition and combustion characteristics of boron-magnesium (BM) composite powders. An ignition and combustion model of BM powders is established and validated in the present study. The results show that increasing water content, O2 content and Mg content all result in shorter ignition delay time of BM powders, among which the effect of water content is the most obvious. However, ignition delay time increases as pressure increases. The combustion time decreases with increasing Mg content and ambient pressure but increases with water content. With the increase of O2 content, combustion time of BM powders first increases and then decreases, which means a critical O2 content exists above which combustion time decreases. The results show that there exists a trade-off between ignition and combustion performance of BM composite powders.

Conjugate heat and mass transfer at heterogeneous burning surface has significant effect on combustion characteristics of AP/HTPB/Al composite propellant. In the present study, the conjugate heat and mass transfer characteristics of AP/HTPB/Al composite propellant are studied experimentally and numerically based on formulated combustion model and experimental facilities. With the increase of pressure and Al content, flame temperature increases significantly and leads to stronger heat transfer to the burning surface. With the content of Al varying from 18 wt% to 9 wt%, the total heat transfer coefficient decreases for all the sampled positions of the burning surface with a maximum decrease of 12% at the AP/HTPB interface. It is found that heat convection accounts for more than 97% of the total heat transfer coefficient. However, with pressure increasing from 2 MPa to 10 MPa, the total heat transfer coefficient and the respective contribution of heat convection and radiation vary slightly. Owing to the heterogeneous sandwich-like structure of the propellant, the temperature distribution at the burning surface of propellant is highly non-uniform. Therefore, heat transfer characteristics vary significantly at different positions of the burning surface. The heterogeneity of the burning surface is exacerbated by the non-uniform distribution of heat transfer coefficients at different positions of the burning surface. The burning rate of the propellant varies from 0.5 mm/s to 9.5 mm/s for different positions of the burning surface at 10 MPa. The results of micro-scale flame structure show that the mass transfer of fine Al particles in the combustion of AP/HTPB/Al composite propellant can be basically divided into four stages, i.e., migration, melting, agglomeration and oxidation/combustion. Coral reef-like aggregate structure is formed by some of the fine Al particles before oxidation and combustion. Increasing pressure is conducive to avoiding formation of large Al agglomerates and enhancing mass transfer at the heterogeneous burning surface.

The effect of mixing and heat transfer on regression rate of TAGN-based fuel of a segregated AP/TAGN solid motor is studied in the present study. A regression rate model of TAGN-based fuel under coupled combustion is developed and validated based on detailed reaction mechanism. The two-dimensional mixing characteristics in boundary layer and the effect on regression rate of TAGN-based fuel are analyzed in detail. The simulation results show that the increase of initial temperature of oxidizer gas leads to better mixing in the axial direction and higher temperature of burning surface. However, the mass concentration of O2 decreases greatly due to the increasing consumption by primary combustion reactions of AP-derived gaseous species. Therefore, the regression rate of burning surface decreases slightly. With inflow Mach number increasing from 0.32 to 0.75, mixing in the boundary layer is greatly enhanced. Therefore, both the temperature and regression rate increase with inflow Mach number. As pressure increases from 4 MPa to 10 MPa, the primary reactions of AP-derived gas species are also enhanced. Therefore, the mass concentration of O2 decreases rapidly, which leads to oxygen-lean combustion in the combustion chamber. Therefore, the temperature of burning surface decreases while the regression hardly varies despite of the better mixing near the burning surface at high pressure. The results show that the coupled combustion reactions between inflow oxidizer gas and fuel gas mainly occur in a narrow belt-like region of the boundary layer. The results show that the regression rate of solid fuel in the segregated solid motor is controlled by the competition between primary combustion and coupled combustion.

Coking and heat transfer deterioration of supercritical aviation kerosene pose grave challenges to regenerative cooling of scramjet engines. In the present study, reaction and heat transfer characteristics of supercritical China aviation kerosene No. 3, RP-3 undergoing simultaneous pyrolysis and steam reforming reactions in a corrugated channel are numerically studied. Compared with reacting flow of RP-3 in the smooth channel, temperature and coke deposition are much lower in majority part of the corrugated channel. The maximum temperature difference between the smooth and corrugated channels is up to 300K. The maximum heat transfer coefficient in the corrugated channel is almost three times of that in the smooth channel under the same condition. However, low-velocity and high-temperature zones are formed near the corrugated micro-structures and cause coke accumulation and heat transfer deterioration along the flow direction of the corrugated channel. With the increase of wall heat flux, both temperature and velocity increase significantly in the corrugated channel. The conversion of RP-3 and formation of coke are also enhanced with increasing wall heat flux. Moreover, the total heat transfer coefficient first increases and then decreases with wall heat flux. The maximum total heat transfer coefficient increases from 4000W/m2 center dot K to 18,000W/m2 center dot K with wall heat flux increasing from 0.3MW/m2 to 1.8MW/m2. Periodic wavy structures are found at the interface between the corrugated micro-structure and the bulk flow, which is pronounced when wall heat flux is 1.3MW/m2 or above. With the increase of flow time, local turbulent kinetic energy decreases due to the interaction between the corrugated micro-structures and the wavy structures. The interaction between the corrugated structures and complex reaction and heat transfer leads to intersection of the distributions of temperature and coke in the smooth and the corrugated channels. An empirical heat transfer correlation formula is obtained as Nu=0.1Re0.65Pr1.94 for RP-3 reacting flow in the corrugated channel. The study provides better insight into interaction mechanism between chemically reacting flow and micro corrugated structures.

Water addition to aviation kerosene draws increasing attention in increasing heat sink and suppressing coke formation for regenerative cooling of scramjet engines. However, the effect of water on flow, reaction and heat transfer, particularly the thermal inhomogeneity in curved cooling channel is rarely reported. In the present study, the effect of water on coking and heat transfer inhomogeneity characteristics of China aviation kerosene, RP-3 studied in a uniformly heated curved mini-channel. A transient three-dimensional numerical simulation model with hybrid pyrolysis and steam reforming reactions of RP-3 is established and validated. The results show that there exists serious mass and thermal stratification in the curved mini-channel due to secondary flow and species diffusion. Specifically, RP-3 and water are diffused to concentrate at the bottom heating wall surface while coke deposition and light gaseous species, e.g., H2, CH4, CO are concentrated at the top heating wall surface. The thermal difference between the top and bottom heating walls is significant. Temperature and coke content of the top heating wall are much higher than those of the bottom heating wall. The gas film and coke deposition formed at the top heating wall surface lead to serious heat transfer deterioration. Therefore, the heat transfer coefficient at the bottom wall of the curved channel is much higher than that of the top wall. Water addition enhances secondary flow and thus exacerbates the coking and heat transfer inhomogeneity in the curved cooling channel. With water content increasing from 0 to 15 wt%, the secondary flow velocity increases by 288.2%, the conversion of RP-3 increases by 114.8% and the average coke content on the heating walls decreases by 40%. Meanwhile, the average heat transfer coefficient increases by 33.9% at the bottom heating wall but decreases by 18.0% at the top heating wall with the increase of water content.

Regenerative cooling technology plays an important role for development of scramjet engine. In order to further improve thermal performance of regenerative cooling system with hydrocarbon fuel, a distributed regenerative cooling system using catalytic steam reforming of supercritical aviation kerosene and distributed supply of water is evaluated in the present study. Thermal performances of distributed catalytic steam reforming system (DCSR) and conventional catalytic steam reforming system (CSR) are compared. The effects of pressure and secondary injection ratio of water on DCSR are numerically analyzed. The results show that DCSR renders higher heat transfer coefficient and larger heat absorption capacity than CSR. Coke content of DCSR decreases by up to 20% while heat transfer coefficient increases by 12.02% compared with CSR. With secondary injection ratio increasing from 30.98% to 71.60%, coke content decreases by up to 37.53% while heat transfer coefficient increases by up to 44.45%. As coke deposition is significantly removed by in situ steam gasification reaction, no heat transfer deterioration is found near the outlet of the cooling mini-channel. Moreover, the cumulative heat absorption capacity of DCSR increases by 24.3% than CSR. This study provides preliminary insight into thermal performance of DCSR for scramjet engines.

The effect of pressure on transient micro-combustion and heat release characteristics of AP/HTPB/Al composite propellant is investigated in the present study. Detailed combustion mechanism with 42 species and 136 re-actions and segmented regression rate model are established and validated for the propellant fuel. The results show that temporal evolution of regression rate is not synchronous with temperature due to the heterogeneous properties of the burning surface. With combustion time increasing from t = 0.6 ms to t = 6.0 ms, the standard deviation of temperature of the burning surface increases by 65.08% while the standard deviation of regression rate decreases by 18.80%. Although raising pressure increases both the temperature and regression rate of the burning surface, the distribution uniformity of temperature and regression rate exhibits distinct response char-acteristics to pressure. With pressure increasing from 2 MPa to 10 MPa, the standard deviation of temperature of burning surface decreases by as much as 36.50% due to enhanced heat transfer to the burning surface. However, the standard deviation of regression rate increases by as much as 66.68% with pressure. Both the gas-phase temperature and solid-phase temperature increase and become more uniform with pressure. With progress of combustion, total heat transfer coefficient decreases slightly due to the reduced combustion heat release rate. Although the contribution of heat radiation increases significantly during the combustion process, heat con-vection is dominant heat transfer. With the increase of pressure, the combustion heat release rate increases significantly due to the enhanced oxidation reactions in diffusion flame. With consumption of the propellant, the combustion heat release rate decreases and squeezes to a smaller region. The experimental results of extin-guished burnt surface show that AP particles become higher than the surface of HTPB binder as pressure in-creases. The non-uniformity of morphology of the burning surface of the AP/HTPB/Al composite propellant increases with pressure.