Choosing a heat pipe working fluid involves more than matching the operating temperature range or ensuring material compatibility. The heat pipe fluid figure of merit (FOM) is a critical metric that combines multiple fluid properties to predict performance and optimize the system’s efficiency. This article explains the FOM in a bit more detail while touching on several other criteria used for selecting the best heat pipe working fluid. For brevity’s sake only water, methanol, and ammonia are discussed.
Heat Pipe Fluid ‘Figure of Merit’
The FOM equation includes the following temperature-dependent variables:
- Latent heat of vaporization (hfg): in J/kg, joules per kilogram. Higher latent heat enables efficient phase change, allowing the fluid to transfer more heat with minimal mass flow.
- Surface tension (σ): in N/m, newton meters. Surface tension drives capillary action in the heat pipe’s wick, enabling fluid return from the condenser to the evaporator.
- Liquid density (ρ) in kg/m³. Fluid density affects the mass flow rate and pressure drop, both of which influence heat transfer efficiency.
- Liquid viscosity (μ): in Pa·s, pascal-second. Lower viscosity reduces flow resistance, enhancing fluid circulation within the heat pipe.
Figure 1: Heat Pipe Working Fluid Figure of Merit
Figure 1 plots the FOM across a range of operating temperatures for the most common heat pipe working fluids. As a starting point, engineers should choose the working fluid with the highest FOM across the specified operating temperature range of the device. However, other factors need to be considered.
Other Heat Pipe Working Fluid Selection Criteria
Beyond the variables accounted for in the fluid figure of merit at different operating temperatures, several other factors should be considered before choosing the most appropriate heat pipe working fluid.
Figure 2: Factors other than FOM That Affect Heat Pipe Working Fluid Choice
Choosing the right working fluid and enclosure material is essential for effective heat pipe design. By evaluating factors such as the fluid figure of merit, operating temperature range of the liquid, toxicity, flammability, compatibility, and cost, engineers can optimize performance for specific applications.
