Heat Transfer and Applied Thermodynamics Laboratory

University of Colorado at Colorado Springs

James Stevens

Areas: transient and steady-state conduction, convection, thermodynamics

Applications:  thermal history, thermal response, electronic cooling, temperature profiles, thermal design, heat flow rate determination, renewable energy, HVAC, air engines, novel heat engines, and more

Representative Projects

liquid piston stirling engines
A liquid piston Stirling engine (sometimes termed a "fluidyne" engine) consists of a working fluid (usually air) that is repeatedly shifted between a hot space and a cold space by the motion of a liquid oscillating between the two vertical columns of a u-tube. An additional liquid column serves to maintain the proper phasing for operation as a power producing engine.  An extremely wide variety of geometric configurations are possible and many have been demonstrated in laboratory devices. Most share the attributes of great simplicity and robustness in operation. 

Liquid piston engines possess several significant advantages over other heat engine implementations. Several advantages are related to the simplicity of the device. The working fluid is isolated by the liquid pistons so sliding seals and tight machining tolerances are not required. This eliminates all sealing problems which comprise some of the most difficult challenges faced by most Stirling machine implementations. It also leads to very low capital costs relative to typical Stirling and other power producing heat engines. With proper design, functional, though inefficient, models can be built from a few pieces of pipe and tubing. Other advantages are associated with the fact that these devices operate as external combustion engines. Since the heat source is independent of the engine operation, there are few restrictions on the nature of an appropriate primary energy source. Combustion from low heating value biomass, industrial waste heat, concentrated solar energy, geothermal energy, as well as most traditional fuels could all be used as primary energy sources for liquid piston engines.  The three main technical obstacles to widespread use of these engines are relatively low power density, relatively low efficiency, and lack of a convenient method of generating electricity from the oscillating motion of the liquid or from the working fluid pressure variations.
Ongoing research at UCCS addresses these obstacles.   These three obstacles are fundamentally mutually dependent on understanding the thermodynamic cycle, the engine dynamics, and the engine losses consisting mainly of flow and thermal losses. The investigative approach to developing a quantitative understanding of the thermodynamic cycle including mechanical dynamics and losses is based on mutually supportive programs of development of a detailed numerical model and comprehensive laboratory testing of partial and fully functioning fluidyne engines.

Past, current, and future projects include: oscillating flow loss quantification, solar powered demonstration, quantification of the phasing/work output relationship,  dynamometer development and characterization, exploration of pressurized operation, use of liquid metals,  vacuum operation, applications for unattended pumping andwater desalination 

ground source heat engine

At most locations outside the tropics, the diurnal temperature changes in the air near ground level are  large compared with the temperature changes even a very short distance below the surface of the ground.  In concept, these daily temperature differences can be harnessed by a heat engine to produce power.  In practice, the temperature differences are of the order of one  to ten Kelvins, in surroundings close to 300 K, so that the thermal efficiency of such heat engines is relatively small.  This means that a large amount of heat must be moved through the device for each unit of power produced.  
Ground-source energy harvesting systems use one or more thermoelectric modules to generate electric power from the temperature difference between the ground and the air.  A very tiny amount of electricity can be produced very reliably and continuously for powering unattended sensors or transmitters.  Heat transfer considerations figure prominently in the power generation system design. 

finite bath quench

Heat treatment is widely used to improve hardness, toughness, and wear characteristics of metals across a wide variety of material applications.  Appropriate heat treatment requires close control of the metal temperature and the time that it is held at a specific temperature.  Quenching, which involves placing a hot metal into a cooler fluid bath in order to quickly bring it to the desired temperature, normally involves a large tank of quench fluid which is held at the desired temperature. Usually, the heat capacity of the bath is much larger (essentially, infinite in size) than that of the metal parts so that the bath remains at approximately constant temperature. It would be possible to bring the parts to the desired temperature faster, while saving energy and requiring less quench fluid by using a smaller bath that increases in temperature as the parts cool down.  This research explores the uncertainty (controllability) of the quench process in a finite bath.

deep subsurface feature identification

The goal of this research is to develop a set of methodologies to identify features to depths of one or more meters below the surface of the ground relying only on remotely sensed surface temperatures.  The unprecedented depth sensitivity will be achieved by utilizing temperature changes over a long time-scale—typically on the order of a year.  This time-scale has the complementary advantage of allowing the anomaly identification process to use surfac
e temperature information only, i.e. detailed information needed to estimate an imposed heat flux such as local wind speed, local cloud cover, and local solar insolation will not be required.

universal thermal shock

A sudden change in surrounding fluid temperature for a body at an initial uniform temperature causes internal stresses termed thermal shock.  For one-dimensional cases of rectangular, cylindrical, and spherical coordinates, single curves can describe the maximum temperature differences of a quenched body across all three coordinate systems and across four orders of magnitude of Biot number.  It is postulated that these results would be applicable to any case in which the heat transfer could be characterized as approximately one dimensional.  While potentially useful for estimating transient heat transfer effects in any application, these results are directly applicable to estimating maximum thermal tension stresses due to quenching.