Advanced salt hydrate thermochemical heat pipe: Experimental validation and numerical analysis

Abstract

This study investigates the use of salt hydrate in a heat pipe as a thermochemical substance for the enhancement of thermal energy transfer, both numerically and experimentally. The main objective is to assess the impact of chemical (hydration/dehydration) reactions on heat conduction and convection within the heat pipe. For this, relying on its high thermal stability, solubility, energy density, and expected positive boiling characteristics, CaCl₂.6H₂O solution was selected as the working fluid. A lab-scale setup of the concept was fabricated and evaluated in the experimental phase. 2D thermal-fluid simulation of two-phase boiling and condensation phenomena was conducted to investigate thermophysical properties, including thermal conductivity, specific heat capacity, evaporation rate, and vapor pressure. Numerical analysis was used to evaluate the effect of salt addition on heat transfer, evaporation rate, and boiling regime stability. Results show that the CaCl₂ solution decreases thermal resistance by 35.4% at 80 °C and by 48.7% at 100 °C compared to pure water. In temperature range of 80–100 °C, the heat transfer mechanisms conduction and convection are dominant, not the chemical reaction. The increased vaporization enthalpy and boiling stability compensate for the solution's lower vapor pressure and thermal conductivity. The presence of salt is proven to prevent unstable boiling modes such as geyser boiling, thereby enhancing the transient behavior of the heat pipe. An analysis of different CaCl₂.6H2O concentrations shows that increasing salt concentration decreases bubble size and dry-out regions below saturation levels where no precipitation occurs.