Merit Number vs. Thermal Resistance: Rethinking Working Fluid Selection in Loop Heat Pipes

Merit Numbers are still widely used to screen working fluids for Loop Heat Pipes (LHPs) because they condense multiple thermophysical properties into a single metric. However, their relevance to actual LHP performance is insufficiently quantified. In this work, a previously validated 1-D transient Lumped Parameter Model (LPM) is used to generate a database of 150 simulations spanning three initial temperatures (20, 40, and 60 °C), five heat loads (50–300 W), and a range of conventional fluids and low-GWP refrigerants. Fluid rankings derived from literature Merit Numbers are compared with rankings based on thermal resistance predicted by the LPM. Systematic discrepancies are observed, demonstrating that Merit Numbers cannot reliably predict thermal performance. A new Design Number, incorporating geometrical parameters and heat flux through dimensionless groups, is proposed and predicts thermal resistance with a mean absolute percentage error of 22% relative to LPM results. While this index retains acceptable accuracy under out-of-range conditions and in an independent methanol campaign, ranking inconsistencies still persist. This confirms that the limitation is fundamental to the use of a single figure of merit as a surrogate for LHP performance as even enhanced merit-based indices fail to reproduce performance-based rankings consistently. Consequently, while Merit Numbers can offer some initial insight, they are not sufficient as standalone criteria for working fluid selection and should be applied only in a preliminary context. Reliable prediction of thermal performance requires LPMs and, where feasible, CFD.