• space application
• hybrid heat pipe
• numerical results comparison
• heat transfer devices
• flow boiling
• two phase

With an aim to reach farther in to space, research is going on to develop advanced propulsion systems and miniaturisation of electronics. This leads to the need to dissipate excess heat from the systems and high heat fluxes. Two phase devices fit in to the solution set due to their capability to transfer heat up to 10 m distance, and with an accuracy up to 0.1 K. They can be used over wide ranges of temperatures. Pulsating heat pipe is one such device which can cater to the needs of deep space mission’s thermal control requirements. It is light, reliable, passive, cheap and has satisfactory thermal performance. A hybrid heat pipe, also known as, loop thermosyphon has been proposed for space applications. To better understand the physical phenomenon of the loop thermosyphon, experimental and analytical study is required. Hence, a numerical model has been studied and is validated against the experimental results.
For this purpose, two models have been studied. First model considered, is based on conservation of momentum equation. Mass flow rate is calculated for different working fluids at various fill ratios. The mass flow rate is as a function of input heat power. The second model is based on the consideration that the two-phase flow is a homogeneous mixture which is in thermal equilibrium. The flow model is solved with a hyperbolic solver based on Godunov method. The model considers subcooled liquid and overheated vapor as well as phase transition. To carry out the numerical analysis, the equations are discretised in a one-dimensional space using finite volume approach. MATLAB code has been developed to give the initial steady state condition of the flow.
The first model is based on lumped capacitance model and it accurately depicts the changing trends in the mass flow rate with the variation of power. It is validated against the experimental results for the working fluid ethanol and water for different fill ratios. The second model is thermodynamic equilibrium model which can reproduce satisfactorily the steady state response of a classical loop and variation of temperature, density, vapor mass fraction with the change in gravity. As a future work, both the models can be merged to create a new model which can estimate the variation of operating parameters and can be used as an initial point for better understanding of the dynamics of loop thermosyphon in space.