Roger R. Riehl

Modeling of a Loop Heat Pipe for Ground and Space Conditions

This paper presents a mathematical model of a LHP, where its results were compared with experimental test results and the LHP behavior was predicted for Space conditions. In the LHP condenser, the condensate film thickness in the tube is determined by the solution of the conjugate equations of energy, momentum and mass balance in the control volume. Evaporator and compensation chamber are both described by a few transient nodes with generalized key parameters, whose values were adjusted by test results. Experimental LHP consists of cylindrical stainless steel evaporator with an integral compensation chamber, where the primary wick is of micropore UHMW polyethylene and the condenser is an aluminum plate, with acetone as the working fluid. The tests conditions have been reproduced in the mathematical model and its parameters were adjusted using the measured temperatures. The model was applied to predict the LHP behavior for Space conditions, corresponding to the circular low Earth equatorial orbit.

Characteristics of an Open Loop Pulsating Heat Pipe

This paper presents an experimental investigation of an open loop pulsating heat pipe (OLPHP), where several issues related to its performance were evaluated. Tests were conducted with different working fluids for the OLPHP operating at both vertical and horizontal orientations. The experimental results show that the system presented better performance when operating at horizontal orientation, as lower evaporation section temperatures were achieved. Regarding the working fluids used, the system showed better performance when acetone was used on vertical orientation and methanol on horizontal orientation.

Performance Improvement in Loop Heat Pipe Using Primary Wick with Circumferential Grooves

This paper presents an investigation of LHPs that operate with acetone as working fluid, for maximum operational heat loads of 80 W, where the results of tests performed in laboratory conditions are presented. Two identical LHPs were tested, where one presents the capillary evaporator primary wick with axial grooves, while the other presents the design of circumferential grooves and both evaporators present microgrooves in their internal diameter. For the same active length, there has been a gain of 20% on the contact area when using circumferential grooves on the primary wick, which resulted in better thermal performance. When the LHP operates at its maximum operational heat load, the heat source temperature for the evaporator with the primary wick with circumferential grooves is kept at levels 50% lower then when using the one with axial grooves. Besides the fact the an increase on the contact area has been achieved, other factors related to the design of the capillary evaporator have also resulted in the overall improvement. This represents a possibility of using the new designed LHPs operating at higher heat loads while keeping the source temperature at lower levels.

Modeling of a Loop Heat Pipe with Evaporator of Circumferential Vapor Grooves in Primary Wick

A new configuration of internal geometry of LHP evaporator is investigated. Cylindrical evaporator has circumferential vapor escape channels instead of axial channels, commonly used in evaporators; only one axial groove is presented to provide vapor collection and direction to the vapor line. These circumferential grooves are combined with circumferential micro-threads on evaporator case internal surface. Mathematical model of LHP has been developed with emphasis on simulation of heat-mass transfer in evaporator. The model was verified by the test results obtained for the LHP with axial-groove wick evaporator. The model was then modified and applied to the new evaporator wick geometry. Comparative studies reveal the main reason for considerable improvement of the performance for new configuration: significant reduction of the two-phase layer between wall and wick. Two mechanisms are found to be able to contribute in such thickness reduction. There is effect of liquid trapping in the microthreads, and like-heat-pipe effect along circumferential grooves.

Comparing the Behavior of a Loop Heat Pipe With Different Elevations of the Capillary Evaporator

During the life tests required for the development of a loop heat pipe (LHP), the device must operate at different capillary evaporator elevations in order to compare its thermal behavior, specially related to the operation temperature. As an effort in developing LHPs for future space missions, a LHP was designed and tested for a system operating with acetone as the working fluid. Tests were performed for the LHP at horizontal position and with the capillary evaporator above and below the condenser. The experimental results show the operationability of the LHP tested at all situations, where the system presented reliable operation at power levels as low as 1 W. Tests with the capillary evaporator above and below the condenser also show the reliability of the LHP operation, showing higher operation temperatures with the evaporator above and lower operation temperatures with the evaporator below. The test results present the robust design of the LHP and its capability as a thermal control device for future space missions.

Extensive Development of the Loop Heat Pipe Technology

The development of LHPs for space applications have shown the possibility of using them in several ground applications, such as water heating systems, electronics cooling, etc. Some issues related to the use of LHPs rely on their design and development depending on the required use. In this case, the most indicated configuration for their reliable operation will depend on the correct selection of working fluid, materials and configuration regarding the maximum heat management requirement. This paper presents what has been developed in this institute regarding the LHP technology, which includes devices for electronics cooling operating at its classical design (one evaporator and condenser), reversible, ramified and miniature LHPs. The results obtained have shown the great potential in using LHPs as passive thermal control devices. Continuous development have shown that the LHPs can promote a reliable control of the heat source temperature within the required designed limits, even when considering the issue of miniaturizing these devices.

Thermal Control of Surveillance Systems Using Pulsating Heat Pipes

Important applications can be found on surveillance systems using high performance thermal control devices, especially passive ones using heat pipe technology. With the growing need for heat dissipation presented by this type of system, usually hybrid solutions are designed. This is usually applied when the heat source is located far from the heat sink and the use of liquid cooling or any other active thermal control system is not possible. Since most of the surveillance systems being designed today require thermal control devices that operate under adverse orientation, some restrictions apply. Therefore, the technology that is currently used and disseminated for aerospace can find many other applications in surveillance systems for defense purposes. With severe restrictions regarding available space for integration of common thermal control devices, the design and application of pulsating heat pipes (PHPs) becomes the most indicated solution for the present investigation, together with the use of heat pipes for proper heat rejection. Based on this fact, this investigation is focused on presenting the thermal control management of electronic components of a surveillance system being done by PHPs configured as open loops with conventional heat pipes. Despite the relatively high temperature difference observed between the heat source and the sink (up to 25 o C), the open loop PHP was able to transport the rejected heat (up to 40 W) from the electronic components to a remote heat dissipation area, while keeping their temperatures within the required range established by the project (below 80 o C) and relatively high thermal conductances (up to 1.6 W/ o C), as demonstrated by experimental results. The heat pipe has demonstrated the capability of spreading the heat evenly, positively affecting the operation of the PHP. The operation of this combined solution has shown to be stable and reliable.

Design Optimization of Loop Heat Pipes with Cylindrical Evaporator and Integral Reservoir for Space Application

Design optimization of a LHP system for a space application is considered. The system is composed of the LHP itself, an interface with the heat source (saddle) and a radiator. The criterion is minimal system mass while meeting the operational requirements. The optimization is performed with simultaneous consideration of hot and cold conditions with respect to imposed heat loads to the evaporator and external heat fluxes over the radiator panel. The design parameters of the system optimized are the active length of the evaporator, internal and external diameters of the primary wick, volume and size of the reservoir, thickness and width of the saddle, diameters and tube thickness of the transport lines and condenser, length of the condenser, dimensions of the radiator panel and the amount of the LHP working fluid charged. The LHP mass and optimal design parameters are obtained for three working fluids: ammonia, propylene and acetone; a comparative study of the optimal mass characteristics is performed. Fixe...

Evaluating Loop Heat Pipes Performances Regarding Their Geometric Characteristics

This paper presents the performance tests of small-scale loop heat pipes (LHPs) that have being developed in order to perform the heat dissipation using acetone as working fluid. Two LHPs have being designed and built to accomplish the thermal management on power cycles of up to 80 W, where each LHP has a different set of compensation chamber/capillary evaporator, being one detached from the evaporator while the other is an integral part of it. Laboratory life tests have shown reliable operation with negligible non-condensable gas influence along time. The basic difference between tests with both LHPs is that the one with an integral compensation chamber showed higher operation temperatures due to the smaller thermal resistances between the evaporator and the compensation chamber. The life tests results show the potential in using acetone as an alternative working fluid in LHPs with reduced active lengths.

Loop Heat Pipe: Design and Performance During Operation

Loop heat pipes (LHPs) have been extensively investigated and considered for the thermal control of satellites and other space equipments, but some geometric limitations, as well as the use of hazardous working fluids must be considered. Focusing on such concerns, a LHP was designed and built to accomplish certain requirements towards its future application in space missions. The designing procedure had to consider some limitations, such as a reduced scale capillary evaporator and the use of an alternative working fluid. Thus, an experimental LHP was built and tested for acetone as the working fluid to manage up to 70 W of heat transfer rate. The experimental results showed a good thermal management performance of the proposed LHP for the imposed limitations to its design. The proposed LHP presented to be a reliable thermal management device for applying in future space missions, especially when considering the use of a less hazardous working fluid.