Theoretical and experimental study of thermal conductivity and operational limits of a sodium axial heat pipe relative to the gravity vector orientation

This paper presents an assessment of the influence of gravitational force vector orientation on the performance of a sodium axial heat pipe. The effect of axial heat pipe orientation on its limiting operational factors, such as the sonic limit, viscous limit, boiling limit, and capillary limit, is demonstrated. Comprehensive numerical simulation was carried out based on a detailed two-dimensional thermal-hydraulic model, which takes into account the spatial distribution of parameters along the pipe length, such as temperature, pressure, and velocity fields. This approach also enables the determination of the distribution of the maximum allowable heat input to the evaporator zone of the axial heat pipe. It was established that the limiting factor is the sonic limit, which amounts to 450 W for an axial heat pipe with a length of 350 mm, wick thickness of 0.5 mm, vapor channel diameter of 7 mm, and casing diameter of 10 mm. Investigation of the dependence on the inclination angle showed a moderate increase in thermal resistance and a decrease in the average temperature within the axial heat pipe circuit. Thus, this work has localized a "weak link" in the design—the vapor channel, whose performance is constrained by the sonic limit. The obtained results determine the feasibility of optimizing the geometry of the vapor path to increase the overall power of the axial heat pipe.
EDN: DRHATK, https://elibrary/drhatk

References

[1] Ferskhter A.L., Solovyev S.L. High-temperature heat pipes with alkali metals. Promyshlennaya teplotekhnika, 1990, vol. 12, no. 4, pp. 45–52. (In Russ).

[2] Lee S., Kang S., Kim Y. et al. Thermal design framework of heat pipe heat exchanger for efficient waste heat recovery. Energy, 2025, vol. 318, art. 134731, doi: https://doi.org/10.1016/j.energy.2025.134731

[3] Faghri A. Advances and challenges in micro/miniature heat pipes. Annu. Rev. Heat Transfer, 2003, vol. 12, pp. 1–25, doi: https://doi.org/10.1615/AnnualRevHeatTransfer.v12.30

[4] Yun M., Hsu W.T., Shim D.I. et al. Design and fabrication of heat pipes using additive manufacturing for thermal management. Appl. Therm. Eng., 2024, vol. 236, art. 121561, doi: https://doi.org/10.1016/j.applthermaleng.2023.121561

[5] Reay D., Kew P., McGlen R. Heat pipes. Butterworth-Heinemann, 2013. 288 p.

[6] Zhang Z., Chai X., Wang C. et al. Numerical investigation on startup characteristics of high temperature heat pipe for nuclear reactor. Nucl. Eng. Des., 2021, vol. 378, art. 111180, doi: https://doi.org/10.1016/j.nucengdes.2021.111180

[7] Vershinin S.V., Maydanik Y.F. Hysteresis phenomena in loop heat pipes. Appl. Therm. Eng., 2007, vol. 27, no. 5–6, pp. 962–968, doi: https://doi.org/10.1016/j.applthermaleng.2006.08.016

[8] Pastukhov V.G., Maydanik Y.F. Low-noise cooling system for pc on the base of loop heat pipes. Appl. Therm. Eng., 2007, vol. 27, no. 5–6, pp. 894–901, doi: https://doi.org/10.1016/j.applthermaleng.2006.09.003

[9] Maydanik Yu.F. Loop heat pipes. Appl. Therm. Eng., 2005, vol. 25, no. 5–6, pp. 635–657, doi: https://doi.org/10.1016/j.applthermaleng.2004.07.010

[10] Maydanik Y.F., Vershinin S.V. Development and tests of ammonia miniature loop heat pipes with cylindrical evaporators. Appl. Therm. Eng., 2009, vol. 29, no. 11–12, pp. 2297–2301, doi: https://doi.org/10.1016/j.applthermaleng.2008.11.016

[11] Pastukhov V.G., Maydanik Y.F. Active coolers based on copper-water LHPs for desktop PC. Appl. Therm. Eng., 2009, vol. 29, no. 14–15, pp. 3140–3143, doi: https://doi.org/10.1016/j.applthermaleng.2009.04.016

[12] Jang J.H., Faghri A., Chang W.S. Analysis of the one-dimensional transient compressible vapor flow in heat pipes. Int. J. Heat Mass Transf., 1991, vol. 34, no. 8, pp. 2029–2037, doi: https://doi.org/10.1016/0017-9310(91)90214-Y

[13] Borshchev N.O., Antonov V.A. Thermal-hydraulic model of a two-phase circuit with a compensation cavity. Teplovye protsessy v tekhnike [Thermal Processes in Engineering], 2022, vol. 14, no. 4, pp. 167–177, doi: https://doi.org/10.34759/tpt-2022-14-4-167-177 (in Russ.).

[14] Wang C., Wu Q., Fang Z. et al. Developments and prospects in temperature control technique of loop heat pipe for spacecraft. Front. Heat Mass Transf., 2025, vol. 23, no. 4, pp. 1261–1280, doi: https://doi.org/10.32604/fhmt.2025.066473

[15] Pan R., Zhang K., Wang W. et al. Heat transfer characteristics analysis of heat pipe based on COMSOL. Ann. Nucl. Energy, 2024, vol. 204, art. 110549, doi: https://doi.org/10.1016/j.anucene.2024.110549