The Numerical Simulation And Characteristics Study Of Reciprocal Porous Media Combustion With Heat Recirculation

With increasing of the energy consumption and environmental protection, it is significantly important to make the utilization of fuel gases effectively and cleanly, especially for the low heat valve fuel gases. With the support of the National High-tech Research and Development Program (863, 2006AA05Z223) and the Natural Science Foundation of Zhejiang Province (Y506071), based on the principle of porous media combustion, a systematic research on a novel reciprocal porous media combustion system with heat recirculation (RPMCSHR) was investigated by the methods of experiment and numerical simulation for the combustion of low heat value fuel gases effectively and cleanly.Firstly, the characteristic of pressure dynamic, flow resistance in the RPMCSHR was studied experimentally. The effects of reciprocating half-period, superficial velocities, secondary air ratio and porous medium structure on the pressure dynamic distribution were investigated in detail. The results showed that the flow resistance and dynamic pressure varied periodically with rectangle waves when the system operated steadily and periodically. A minimum reciprocating half-period is needed to help the system operation steadily, which is more influenced by superficial velocities than porous media diameter. The results also found the ratio of secondary air and superficial velocity have strong influence on the flow resistance, pressure dynamic, while the half-period and pore size of porous media have relative small effects. The pressure dynamic and flow resistance increase rapidly with increase of the secondary air ratio and superficial velocities. Application of a gradually-varied porous media structure is good to decrease the pressure drop, and pressure dynamic of the system. Empirical correlation coefficients were given based on Ergun equation and test data for different combination of porous media.In addition, the influence of primary air, secondary air ratio, and porous media structure on the producing characteristics of the simulating high temperature air was investigated in detail. Results showed the simulating flow of high temperature air can be obtained when the secondary air ratio is no less than 1. The increase in primary air and secondary air ratio will conduce to the producing of simulating high temperature air, and increase the absolute amount of simulating high temperature air. However, the split ratio is little influenced by primary air, and decreases with increasing of the secondary air ratio. It was also found the gradually-varied porous media in the combustor is the better structure on increasing the split ratio than the other structure combinations.Secondly, the temperature profiles and emission characteristics of the system at one period and between periods were firstly experimentally investigated. The results show the temperature of any position in the system and emissions varied periodically while the system operated steadily and periodically, which indicates the dynamic process of switching flow combustion. Two temperature peak zones exist in the system, and the temperature distribution in the system has an "M" shape. The nearer to the combustion flame, the larger temperature dynamic is. For the low heat valve gases combustion, the effects of half periods, secondary air ration, and equivalent ratio on the temperature profiles, NO and CO emissions were also experimentally tested and discussed. Two temperature peaks come forth easily at small half period. The temperature in combustion zone firstly increases, and then decreases with increase of half periods and secondary air ratio, and increase with increase equivalent ratio. The flame is gradually propagated to the upstream of the combustor with the increase of secondary air ratio and equivalent ratio. Along with highly perfect combustion efficiency, Low emission levels were obtained and well compared with the experimental results of Hoffmann and Fabinao. The CO emission is mostly below 100ppm, and NO emission is below 20ppm. This sufficiently proves the feasibility of low heat valve gases combustion in the system.Thirdly, based on the former research, a dynamic "two temperature" model was founded according to the symmetrical structure and flowing periodically of the RPMCSHR. The ignition location has small influence on the temperature, flame location, and maximum temperature when the combustion come to steady. The simulation results were well agreement with the corresponding experimental results. According to the experimental range of RPMCSHR, the effect of half-period, dimensionless heat valve, and Reynolds on the temperature distribution and "excess enthalpy" were further investigated in detail. The simulation conclusions were also compared by the corresponding experimental results, and a good agreement was obtained.The theoretical flammable limit is also obtained by the model with a equivalence ratio ofφ=0.125 (fuel heat value of 300KJ/Nm3 ) The relative value of excess enthalpy was mostly influenced by fuel gas heat value with a relation of hyperbolic curve. The combustion efficiency of simulation well agrees with the experimental results. The effective of the model was conformed, which will further provide a good reference to the optimization and improvement of the system.Forthly, directed against the conventional reciprocal porous media combustion (RSCP), the unsteady "two temperature model" was also founded to investigate the influence of porosity layer distributions and gradually varied distributions on the combustion characteristics. The ignition stabilization of the process in (RSCP) was firstly analyzed by the method of finite volume methods with dimensionless form, in addition to the dynamic characteristics between periods, and compared with the experimental results of the dynamic characteristic in RPMCSHR. The principle of producing the "excess enthalpy combustion" was explained by analyzing the temperature between the "solid and gases". The results showed that when the operating parametric varies, the temperature distribution in the combustor varies from the reversal "V" to "M" under the condition of heat loss in combustion area, or varies from the reversal "V" to trapezium without the heat loss. When the temperature becomes the reverse "V", the system is at the limitation of combustion. Application of a gradually-varied porous media structure is good to further extend the lean combustion limit. It can be obtained withφ=0.125 under the condition of heat loss, orφ= 0.02 without heat loss. In the end, the effects of parameters on the relative excess enthalpy, combustion efficiency, and combustion flame distance were also analyzed. The relative value of excess enthalpy is gradually decreasing with increase of the fuel gas heat value- a major factor, and the high combustion efficiency is obtained.It is obviously that the combustion of low heat value gas in the novel RPMCSHR is completely feasible with a high efficiency and low emission. Under the existing experimental condition, the steady combustion can be extended to an equivalence ratio of 0.2, corresponding to the heat value of 620KJ/Nm~3. the theoretical lean flammable limit with an equivalence ratio of 0.125 is also obtained by the numerical simulation. The influence of porosity layers and gradually varied distribution on combustion characteristics is better than the uniform distribution, and the lean flammable limit is further extended. The paper results will provide a good reference on further study of the porous media combustion with a reciprocal flow.