Study on the mechanisms of coal thermal properties and microstructural differences affecting heat and mass transfer characteristics during liquid nitrogen flow boiling

Significant differences exist in the thermal properties and microstructures of coals; however, the influence of these characteristics on heat and mass transfer, as well as on the cost of coal fracturing using liquid nitrogen(LN 2 ), remains unclear. Furthermore, theoretical models linking coal thermal properties and porosity parameters to heat transfer in LN 2 are currently lacking. In view of the above problems, this study conducted transient flow boiling experiments between LN 2 and six coal samples, recording coal temperature, the net mass loss of LN 2 , and heat flux variations. Differences in thermal properties among the coal samples, including thermal conductivity, specific heat capacity, and thermal diffusivity, were compared. The evolution of these thermal parameters with temperature was analyzed to elucidate the mechanisms by which coal thermal properties and porosity influence the flow boiling heat and mass transfer of LN 2 . The results show that the thermophysical properties and pore structure characteristics of coal samples are associated with notable changes in the flow boiling heat and mass transfer characteristics of LN 2 . Thermal conductivity is associated with the internal heat conduction capability of the coal matrix, while pore structure influences heat transfer by altering heat transfer pathways and local thermal resistance. In addition, specific heat capacity and thermal diffusivity govern the cooling response characteristics of coal samples. Compared with bituminous coal, anthracite exhibits a faster cooling rate due to its higher thermal conductivity and thermal diffusivity as well as lower specific heat capacity and porosity, resulting in an approximately 40% reduction in the Leidenfrost stage duration. A segmented heat flux empirical correlation was developed that comprehensively accounts for the thermal conductivity of coal samples, pore structure parameters, LN 2 superheat, and time effects. The model yields a mean absolute percentage error (MAPE) below 9.02%. Moreover, the peak value of the boiling heat transfer efficiency factor and its occurrence time are jointly influenced by the thermal properties and pore structure of coal samples. The highest peak value reaches 79.80 °C/g, which is 121.7% higher than the lowest value of 35.99 °C/g. The coal sample corresponding to this peak value features high porosity, low thermal conductivity. The findings provide a quantitative basis for calculating and evaluating heat flux in LN 2 boiling heat transfer for different coal samples.