Selected Past Research Activities

The research interests of Dr. Peterson are in the thermal/fluids area of mechanical engineering. Broad areas of interest of both past and current work include combustion diagnostics, measurement and modeling of heat transfer processes, development of techniques for modeling microscale heat transfer, meso- and micro-scale energy conversion devices, and cooling of electronic components. Several specific research project now completed are listed below.

1.) Determination of Thermal Properties of Thin Films

This project concerns the development of a unique thermal property measurement technique that can simultaneously determine thermal conductivity, specific heat, and thermal diffusivity of a thin film sample in the direction perpendicular to the plane of the sample. A fully developed technique is currently being applied to thin polymeric samples (polyimide films ~ 25 microns thick) and very thin (1 micron) dielectric films on silicon substrates. A continuation of this work extended the film thickness range for SiO2 on silicon to sub-micron ranges The ultimate goal of the work was to develop a versatile measurement technique that can be applied to a wide range of films and film thicknesses.  The technique currently stands as a method with applicability to dielectric films only.
 

2.) Development of a Mesoscale Cryocooler

Mesoscale cryocoolers have a footprint about the size of a glass microscope slide. Current coolers able to reach cryogenic conditions are rather complex, expensive, and often require large compressors. Stirling machines have been proven as a reliable cooler for many applications and further miniaturization can be achieved. Past work by Dr. Peterson's group demonstrated the developed for a mesoscale cryocooler components using fabrication techniques at OSU as part of the MECS initiative. Micro-channel arrrays for regenerators and heat exchangers, small expansion engines, and miniature fluid channels, valves and nozzles were fabricated for use in the cooler. The device was based on concepts being developed by Prof. Peterson to overcome some the the traditional problems associated with Stirling machines. These concepts would allow a simple cryocooler to be fabricated to meet cooling needs over a large useful temperature range. Furthermore, a device based on these new techniques would be of use in small-scale cooling applications, e.g. sensors and instruments operating at cryogenic temperatures.
 

3.) Miniaturizing Thermomechanical Devices for Energy Conversion Applications

Shrinking a heat pump or thermal engine to a characteristic size of 1 cm brings some interesting challenges to the design of energy conversion devices. An array of 1 cm diameter devices might form a flat panel having an area of perhaps 0.25 to 1.0 square meter. Such a device would have technologically useful heat pumping or energy production capability and would be a robust design having "graceful" failure modes. Furthermore, decentralization could result, thus allowing the device to be located where the function (i.e. heat pumping, cooling, or power production) is required. Past theoretical work in this area has shown that heat transfer plays a critical role in determining overall thermodynamic performance of small-scale systems.  In fact, heat transfer must be included in the analysis as a finite rate process in order to understand the impact of scaling thermomechanical devices to smaller dimensions.  Dr. Peterson's research group has accomplished several studies in this area of scaling small-scale energy conversion devices, especially ones that depend on temperature differences for their operation.  This past work is supporting ongoing efforts to develop miniature heat pumps, coolers, and heat engines.
 

4.) Micro Combustion in Regenerative Devices

A source of energy is needed to drive meso- and micro-devices. At the macro-scale, combustion is an often used source of heat to drive processes such as chemical processing and power production. The Micro Combustion project is a research study on the feasibility of creating micro sources of high temperature heat with energy release rates in the sub-watt range. The concept used is this completed project was to employ a microscale counter-flow heat exchanger to feed reactants to a micro combustor. The heat exchanger was designed to isolate the combustor from ambient temperature conditions to minimize heat loss. Also, catalytic processes at the hot end were used to promote combustion with the reactants being hydrogen and hydrocarbons mixed with air.  The project concluded with the reporting in the literature of the smallest self-sustaining combustor of this type with a heat release of approximately 0.25 Watts in a volume of 0.050 mm3 using hydrogen and air.  Propane and air was also used for a small-scale combustor with sub-Watt heat release rates.

Because of the inherent operating principles of thermomechanical devices, a relatively large temperature difference must be established and maintained across a small separation.  This presents new thermal design challenges especially when microscale engineered structures are involved.  One area currently being explored is the effect of heat transfer on the thermodynamic performance of heat engines and cryocoolers as the length scale of the device is reduced.  Cycles involving steady state temperature differences are of greatest interest and include the Brayton cycle, Rankine cycle (vapor compression), Stirling and Ericsson cycle.  The latter two employ a temperature difference separated by an intervening section of material that functions as a regenerator.  This configuration is a good model to explore the effects of heat transfer on performance since closed form solutions can be developed for both the thermal efficiency of the overall cycle and the heat transfer through the regenerator.  A Stirling cycle analysis has been carried out in order to determine the lower practical size limits on this class of thermomechanical device. It included a finite rate process (heat transfer) coupled with thermodynamic considerations and has resulted in the first closed form solution that we are aware of for the first law efficiency of a heat engine (with parasitic heat transfer effects). The closed form solution is,

This expression has the engine compression ratio, r, the temperature ratio, t, the specific heat ratio, g, and a term that incorporates conduction heat loss effects , l (referred to as the conduction parameter). Also in the expression is the regenerator effectiveness, e. This is a closed form solution and represents a unique application of finite-rate analysis to engine performance.  This expression was used in subsequent work to examine the scaling of regenerative heat engines in,

R.B. Peterson, “Size Limits for Regenerative Heat Engines,” Microscale Thermophysical Engineering, 2, 121-131 (1998).

This is another expression that arises out of our work on modeling small-scale energy systems.  For a combustor supplying heat to an ideal engine, the expression gives the thermal efficiency of the overall system when operating at the maximum power point.  The combustor has a heat exchanger of specified effectiveness, and also has an internal conductive heat loss term.  These effects are taken into account through epsilon and lambda.  Furthermore, the combustor burns a mixture with an adiabatic flame temperature Taf where the surroundings are at T0 .  This type of analysis is useful for determining the scaling characteristics of small-scale energy systems since efficiency is now described with quantities that can be tied to physical parameters such as a length.  With a classic thermodynamic expression typified by Carnot's efficiency equation, no quantities are present to perform a scaling analysis. The above expression is derived and further analysis is given in a forthcoming paper:

R.B. Peterson, “A Scaling Study of a Combined Micro Combustor and Heat Engine System,” to be published in Journal of Power and Energy, (2005).