Charles E. Lents

Collaborative Engineering in Integrated Aircraft Power System Design

Integrated aircraft power system design can provide for performance increases, weight reductions, and cost reductions by exploiting synergies and opportunities for functional integration between systems. A method for bringing together discipline expertise into a common environment for defining system specifications and interfaces that exploit these synergies is presented here. The goal is to create the broadest possible use of the trade-space for aircraft applications. An integrated process and tool environment are described. This process has been applied in the areas of airframe-propulsion integration, thermal management and environmental control systems, control system integration and in the evaluation of more electric aircraft architectures.Copyright

Design of Thermoelectric Modules for High Heat Flux Cooling

Thermoelectric (TE) coolers work on the Seebeck effect, where an electrical current is used to drive a heat flux against a temperature gradient. They have applications for active cooling of electronic devices but have low coefficients of performance (COP 10 W/cm2, dT = 15 K). While the active elements (TE material) in a TE cooling module lead to cooling, the nonactive elements, such as the electrical leads and headers, cause joule heating and decrease the coefficient of performance. A conventional module design uses purely horizontal leads and vertical active elements. In this work, we numerically investigate trapezoidal leads with angled active elements as a method to improve cooler performance in terms of lower parasitic resistance, higher packing fraction and higher reliability, for both supperlattice thin-film and bulk TE materials. For source and sink side temperatures of 30 °C and 45 °C, we show that, for a constant packing fraction, defined as the ratio of active element area to the couple base area, trapezoidal leads decrease electrical losses but also increase thermal resistance. We also demonstrate that trapezoidal leads can be used to increase the packing fraction to values greater than one, leading to a two times increase in heat pumping capacity. Structural analysis shows a significant reduction in both tensile and shear stresses in the TE modules with trapezoidal leads. Thus, the present work provides a pathway to engineer more reliable thermoelectric coolers (TECs) and improve their efficiency by >30% at a two times higher heat flux as compared to the state-of-the-art.

Design and Simulation of a Commercial Hybrid Electric Aircraft Thermal Management System

The baseline design of the Thermal Management System (TMS) of a parallel, hybrid electric aircraft engine for a commercial, single aisle aircraft with batteries for energy storage has been completed. The Hybrid Electric Propulsion (HEP) system features a low spool motor to assist the propulsor, its attendant motor drive, propulsion batteries, and supplementary batteries to cover TMS electric loads during electric augmentation on takeoff and climb. The TMS further includes the heat loads sunk to engine oil including bearings, the fan drive system, and the accessory gearbox. The model was executed under hot day conditions (ISA + 15) over the mission sizing points when electric augmentation is active. REHEATS, a proprietary, object-oriented modeling tool created at the United Technologies Research Center, was used to model the TMS and find the solution with minimal fuel consumption. This study establishes a baseline for comparison of energy storage using batteries for future comparison. The results predict that the TMS of a HEP aircraft increases fuel consumption by 3.4% during takeoff, climb, and cruise.

Experimental Characterization of Thin-Film Thermoelectric Based Active Cooling Modules

Improvements in electronic devices have led to increased power densities and need for small scale miniature cooling solutions. To address this issue, Defense Advanced Research Projects Agency (DARPA), Microsystems Technology Office (MTO) created the Active Cooling Modules (ACM) effort to develop technology solutions able to provide small scale (4 cm2) active high heat flux cooling of a 100W device (25 W/cm2) at a 15 °C temperature lift with Coefficient Of Performance (COP) comparable to state of the art thermoelectric coolers. Thin-film thermoelectric cooling devices are well suited to provide high heat flux active cooling. In the present work, an experimental apparatus is developed to characterize the performance of a subscale thin-film thermoelectric cooling modules 1/144th, 1/36th and 1/9th the size of a full scale 4 cm2 device. An in-situ calibration methodology is proposed to characterize the performance of these thermoelectric microcoolers. In this early development work, vacuum conditions are maintained to minimize thermal losses between the thermoelectric module sink and source sides. The small size of the subscale devices and vacuum conditions introduce additional uncertainties into the system and could lead to errors in COP measurement (>0.5). The additional sources of errors as the device dimensions shrink are identified and minimized leading to energy balance in the system.Copyright