We employ a validated first-order lumped dynamic model of the Up-THERM converter, a two-phase unsteady
heat-engine that belongs to a class of innovative devices known as thermofluidic oscillators, which contain
fewer moving parts than conventional engines and represent an attractive alternative for remote or off-grid
power generation as well as waste-heat recovery. We investigate the performance the Up-THERM with
respect to working-fluid selection for its prospective applications. An examination of relevant working-fluid
thermodynamic properties reveals that the saturation pressure and vapour-phase density of the fluid play important
roles in determining the performance of the Up-THERM – the device delivers a higher power output
at high saturation pressures and has higher exergy efficiencies at low vapour-phase densities. Furthermore,
working fluids with low critical temperatures, high critical pressures and exhibiting high values of reduced
pressures and temperatures result in designs with high power outputs. For a nominal Up-THERM design
corresponding to a target application with a heat-source temperature of 360 ◦C, water is compared with
forty-five other pure working fluids. When maximizing the power output, R113 is identified as the optimal
fluid, followed by i-hexane. Fluids such as siloxanes and heavier hydrocarbons are found to maximize the
exergy and thermal efficiencies. The ability of the Up-THERM to convert heat over a range of heat-source
temperatures is also investigated, and it is found that the device can deliver in excess of 10 kW when utilizing
thermal energy at temperatures above 200 ◦C. Of all the working fluids considered here, ammonia, R245ca,
R32, propene and butane feature prominently as optimal and versatile fluids delivering high power over a
wide range of heat-source temperatures.