Papers presented to the 11th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, South Africa, 20-23 July 2015.
This paper considers an energy-conversion heat-engine concept termed ‘Up-THERM’. This machine is capable of converting low- to medium-grade heat to useful positivedisplacement work through the periodic evaporation and condensation of a working fluid in an enclosed space. These
alternating phase-change processes drive sustained oscillations of thermodynamic properties (pressure, temperature, volume) as the
working fluid undergoes an unsteady thermodynamic heatengine cycle. The resulting oscillatory flow of the working fluid is converted into a unidirectional flow in a hydraulic load arrangement where power can be extracted from the machine.
The engine is described with lumped dynamic models
constructed using electrical analogies founded on previously developed thermoacoustic and thermofluidic principles, which are extended here to include a description of the phase-change
heat-transfer processes. For some sub-components of the engine,
such as the gas spring, valves and the temperature profile in the
heat exchangers, deviations from the linear theory are nonnegligible.
These are modelled using non-linear descriptions. In particular, the results of linear and non-linear descriptions of the
gas spring are compared using three important performance
indicators — efficiency, power output and frequency.
The non-linear description of the gas spring results in morerealistic predictions of the oscillation frequency compared to
direct measurements on an experimental prototype of a similar engine. Owing to its mode of operation and lack of moving parts, the Up-THERM engine does offer a much simpler and more
cost-efficient solution than alternative engines for heat recovery and solar applications. The results from this work suggest that
this technology can be a competitive alternative in terms of cost
per unit power in low-power, small-scale applications, especially in remote, off-grid settings, for example in developing countries
where minimising upfront costs is crucial.