No abstract

The NASA-standard thermohydraulic analyzer, SINDA/ FLUINT (Ref 1), has been used to model various aspects of loop heat pipe * (LHP) operation for more than 12 years. Indeed, this code has many features that were specifically designed for just such specialized tasks, and is unique in this respect. Furthermore, SINDA is commonly used at the vehicle (integration) level, has a large user base both inside and outside the aerospace industry, has several graphical user interfaces, preprocessors, postprocessors, has strong links to CAD and structural tools, and has built-in optimization, data correlation, parametric analysis, reliability estimation, and robust design tools.

In recent years, loop heat pipe (LHP) technology has transitioned from a developmental technology to one that is flight ready. The LHP is considered to be more robust than capillary pumped loops (CPL) because the LHP does not require any preconditioning of the system prior to application of the heat load, nor does its performance become unstable in the presence of two-phase fluid in the core of the evaporator. However, both devices have a lower limit on input power: below a certain power, the system may not start properly. The LHP becomes especially susceptible to these low power start-ups following diode operation, intentional shutdown , or very cold conditions. These limits are affected by the presence of adverse tilt, mass on the evaporator, and noncondensible gas in the working fluid. Based on analytical modeling correlated to start-up test data, this paper will describe how the minimum power required to start the loop is increased due to the presence of mass, noncondensible gas, and adverse tilt. The end-product is a methodology for predicting a "safe start" design envelope for a given system and loop design.

Automated design space exploration was implemented and demonstrated in the form of the multidisciplinary optimization of the design of a space-based telescope. Off-the-shelf software representing the industry standards for thermal, structural, and optical analysis were employed. The integrated thermal/structural/optical models were collected and tasked with finding an optimum design using yet another off-the-shelf program. Using this integrated tool, the minimum mass thermal/structural design was found that directly satisfied optical performance requirements without relying on derived requirements such as isothermality and mechanical stability. Overdesign was therefore avoided, and engineering productivity was greatly improved. This ambitious project was intended to be a pathfinder for integrated design activities. Therefore, difficulties and lessons learned are presented, along with recommendations for future investigations. * To better enable comparisons with the original design case, buckling was again neglected.

This paper describes the need for dynamic (transient) simulation of automotive air conditioning systems, the reasons why such simulations are challenging, and the applicability of a general purpose off-the-shelf thermohydraulic analyzer to answer such challenges. An overview of modeling methods for the basic components are presented, along with relevant approximations and their effect on speed and accuracy of the results. THE MOTIVATION: THE NEED FOR DYNAMIC MODELING Major Department of Energy (DoE) objectives include developing innovative transportation technologies and systems that decrease vehicle fuel consumption and emissions across the nation, thereby reducing the nation's reliance on foreign oil consumption. Recent changes to the Federal Test Procedure have added SC03 and US06 drive cycles to form the Supplemental Federal Test Procedure (STFP), with corresponding requirements for evaluating vehicle emissions during additional driving conditions. In particular, the SC03 drive cycle is specifically intended to evaluate vehicle emissions while the air conditioning (A/C) system is operating in typical high-temperature, high solar load conditions. The US06 drive cycle is intended to evaluate vehicle emissions during more high speed, high acceleration conditions. The addition of the SC03 drive cycle creates a significant need for better understanding the impact of dynamic conditions (i.e., vehicle external environments, passenger compartment environments, etc.) on the vehicle A/C systems and their dynamic response to these conditions. Since vehicle A/C systems represent the major auxiliary load on the engine of light-duty passenger vehicles, sport-utility vehicles (SUV), and heavy-duty vehicles, the A/C system performance has a dramatic effect on fuel consumption and exhaust emissions. Recent studies (Ref 1) have shown that, during the SC03 drive cycle, the average impact of the A/C system over a range of light-duty vehicles was to increase 1) fuel consumption by 28%, 2) carbon monoxide emissions by 71%, 3) nitrogen oxide emissions by 81%, and 4) non-methane hydrocarbons by 30%.