OCR Text |
Show CO to diffuse into the venting hole. The CO gas further reacts with 02 in the hole to form CO 2 , This is evident by the blue tongue of light which is radiated by the excited CO 2 gas. The end product of combust jon is a structurally intact briquette of light dehydrated clay and calcium sulfate with almost no residual carbon content. The ideal combustion characterjstic of the small model can easily be scaled into a much larger combustion system with additional automatic control by a computer. Due to the inherent stability of the ablative combustion process, the process may be subjected to automatic control in order to further enhance the overall combustion characteristic. Before considering such a control process, it is necessary to study the detailed combustion process involving the inter reaction and distribution of the reactants, C, 02' CO, and CO2, in the fuel system. The key to the success of flat-bed combustion is due to the regularity of the preformed venting holes. The diameter to length ratio of the holes is very critical for a slow combustion rate. Due to the complexity of the analytical simulation for the problem, only qualitative description of the analysis will be presented in this paper. It has been noted that for ideal combustion of the char the temperature within the fuel block must be kept at a near uniform temperature of 1000 ± 2000 C at all time. Heat is removed from the fuel block by regulating the recirculation rate of hot flue gas through the vent holes of the fuel block. Thus the transfer of heat to the heat exchanger is by both forced convection and radiative heat-transfer. The temperature in the fuel block is regulated by the injection rate of preheated 02 gas or air into the forced recirculating flue gas. Under this condition, the distribution of 02 and CO within the vent hole is depicted in Fig. 2. for both high and low rate of combustion. Under low combustion rate the gas flow-speed in the hole 1s very slow. Consequently, the CO gas formed in the wall of the fuel block will have time to diffuse across the center line of the hole to react with 02 in the tube. Consequently, the 02 gas is nearly depleted near the exit of the vent. Thus for a proper design, the diameter to length ratio of the vent should be sufficiently large to prevent complete depletion of 02 before reaching the exhaust end of the vent. Under high combustion rate, there will always be high concentration of 02 at the exhaust end, because the boundary layer depicting the concentration of 02 is very thin, see Fig. 2. In this case, the boundary layer depicting the concentration of CO is also thin. Hence, the secondary combustion of CO to CO 2 will have to take place outside the venting hole, which gives rise to a long blue tongue of fire. By changing the geometry of the venting hole, ideal combustion characteristics can be covered over a wide range of combustion rates. That is, for a high combustion rate more holes with a smaller diameter to length ratio is needed. The actual design values are to be found by both theoretical and experimental analyses. 4 |