Beyond single-pipe paradigm: conjugate heat transfer modeling reveals developed heat transfer correlation for concentric-pipe in artificial ground freezing

Abstract

The artificial ground freezing (AGF) method is an effective reinforcement technique in water-rich and weak strata. In the numerical simulation of AGF temperature fields, scholars typically assume convection heat transfer or fixed temperature boundaries at the surface of the freezing pipe. This simplification reduces modeling complexity by eliminating the need to simulate brine flow, thereby enhancing computational efficiency. For single-pipe, single-phase forced convection heat transfer, several well-established heat transfer correlations exist. However, in AGF applications, freezing pipes are often composed of two concentric pipes. Consequently, it is crucial to propose heat transfer correlations applicable within these concentric pipes. In this study, based on single-pipe heat transfer correlations, a convective heat transfer model is established to validate the single-pipe conjugate heat transfer model. Subsequently, a conjugate heat transfer model for concentric pipes is also developed. Results indicate that the single-pipe heat transfer correlation is not suitable for concentric pipes, with 7 points exhibiting temperature differences exceeding 0.6 °C, among which the maximum reached 0.86 °C. Based on classical heat transfer correlations, a new correlation for the annular space is proposed, demonstrating excellent agreement with most temperature differences controlled within 0.2 °C. Further comparison with the model tests shows that the results are also in good agreement under different flow rates, freezing pipe sizes, and freezing durations. This study provides significant reference value for more accurate investigations of the AGF process using numerical simulation methods.