The operation of the Fluidyne heat engine at low differential temperatures

A dynamic model for the efficiency optimization of an oscillatory low grade heat engine

“…The heat transfer coefficient associated with single phase convection is much lower (typically by orders of magnitude) than that associated with phase change, and hence we neglect all modes of heat transfer other than evaporation and condensation. Evaporation occurs when working fluid in the liquid phase comes into contact with the surface of the hot heat exchanger (10), whereas condensation occurs when working fluid in the vapor phase comes into contact with the cold heat exchanger (11). The latter is the dominant heat transfer process at the instant portrayed in Fig.…”

“…The latter is the dominant heat transfer process at the instant portrayed in Fig. 1, when the vapor-liquid interface (7) is at the height of the cold heat exchanger (11).…”

“…These desirable oscillations are driven by and give rise to heat and fluid flows, which involve the evaporation (boiling) and condensation of the working fluid. Hence, the NIFTE can also be considered a two-phase realization of a class of devices known as "thermofluidic oscillators", which includes thermoacoustic engines [4][5][6][7], dry free-liquidpiston (Fluidyne) engines [8][9][10][11], free-piston Stirling engines [12][13][14], pulsejets and pulse-tubes [15][16][17][18][19]. In common with many thermofluidic oscillators, the NIFTE is particularly well suited to the conversion of low grade heat.…”

Optimizing the Non-Inertive-Feedback Thermofluidic Engine for the Conversion of Low-Grade Heat to Pumping Work

“…S th = ρ g,0 s fg U th (13) where ρ g,0 is the time-averaged density of the working fluid in the vapor phase and s fg is the entropy change due to vaporization. Further, assuming that the thermal processes are governed by phase change with small perturbations about a certain timemean saturation temperature T 0 (and pressure P 0 ), the coupling equation between temperature and pressure is:…”

“…The NIFTE produces work in a way similar to a Fluidyne engine, which is a Stirling engine featuring liquid pistons [13,14]. A Stirling engine, be it with liquid or solid pistons, operates by cyclically compressing and expanding the working fluid (gas) while heat is exchanged (added and removed) at different temperatures.…”