Heat transfer characteristics of double layer gas injection pipe for underground coal gasification

The forced oxidation ignition technology is to heat the air or oxygen enriched gas to a certain temperature on the ground and then transport it to the underground coal seam through the inner pipe of the double-layer gas injection pipe, so as to carry out forced oxidation of the coal seam until the coal seam is on fire.In order to obtain the process parameters of forced oxidation ignition technology for underground coal gasification, the heat transfer characteristics of double-layer gas injection pipe device are experimentally studied, the fluid temperature parameters of two different media under different gas injection conditions are obtained, and the heat transfer process of air and oxygen media in double-layer gas injection pipe is analyzed, The local heat loss and heat transfer coefficient of the corresponding double-layer injection pipe under different conditions are calculated.Based on the Fourier heat transfer theory, the calculation model of the fluid temperature in the fully developed stable flow in the circular pipe under the two-dimensional cylindrical system is established, and the fluid temperature parameters of the 1 000 m double-layer injection pipe are obtained.The results show that the temperature rise rate of oxygen is obviously faster than that of air, and the temperature when reaching the steady state is also higher; With the increase of fluid velocity, the fluid temperature at the outlet of gas injection pipe increases; The local heat transfer coefficient gradually decreases and tends to a constant value in the transition flow region, in which the air is about 42.229 W/(m²·℃),and the oxygen is about 41.384 W/(m²·℃); The local heat loss decreases gradually with the extension of distance, in which the stable heat loss value of hot air is 3.28 J/s and that of hot oxygen is 2.68 J/s; The relative error between the experimental temperature distribution value and the theoretical temperature calculation model is less than 2%.Based on the established temperature calculation model, when the distance between the double-layer gas injection pipe reaches 1 000 m, the air outlet temperature is about 104.52 ℃ and the oxygen outlet temperature is about 114.36 ℃.The above results can provide theoretical guidance for the engineering application of forced oxidation ignition technology.