Heat Transfer Characteristics of Single Wrap-around Loop Heat Pipe for Different Working Fluids

A wrap-around loop heat pipe (WLHP) heat exchanger is mounted upstream and downstream of an HVAC cooling coil to lower the coil’s sensible load and reduce reheat energy use. Prior studies have focused on exchang-level performance and economics, while single-channel wrap-around loop heat pipes have been evaluated mostly with R-134a, whose high global-warming potential (GWP = 1430) limits long-term suitability. Here we build a single WLHP with a 15° incline and compare four working fluids water, ethanol, acetone, and R-1233zd(E) under identical geometry and boundary conditions. The condenser water was held at 10 °C with a flow of 0.5 LPM, and the applied heat flux spanned 2.5–30 kW/m² for all fluids except R-1233zd(E), which was tested to 20 kW/m². The adiabatic-section temperature matched the saturation temperature inferred from pressure within 1.3 °C, with a mean difference of 0.2 °C, indicating stable saturated operation. Thermal resistance overlapped across fluids up to 7.5 kW/m², after which water became clearly superior. Water reached Rth = 0.097 K/W at 10 kW/m² and the study minimum of 0.054 K/W at 30 kW/m², while ethanol, acetone, and R-1233zd(E) tended to level near 0.10 K/W at mid to high heat flux. The evaporator heat-transfer coefficient for water rose sharply with heat flux, reaching 3.04, 3.47, and 3.66 kW/m²⋅K at 15, 20, and 30 kW/m². R-1233zd(E) increased monotonically to 2.55 kW/m²⋅K at 20 kW/m², whereas ethanol and acetone showed mid-range dips before recovering. Only water exhibited a sustained increase in condenser performance, attaining 3.05 kW/m²⋅K at 30 kW/m², while the other fluids remained within 1.2–1.6 kW/m²⋅K. These results quantify fluid-dependent behavior in a controlled WLHP and show that fluid choice materially affects performance.