Rengasamy Ponnappan

Thermal conductivity improvement in carbon nanoparticle doped PAO oil: An experimental study

The present work involves a study on the thermal conductivity of nanoparticle-oil suspensions for three types of nanoparticles, namely, carbon nanotubes (CNTs), exfoliated graphite (EXG), and heat treated nanofibers (HTT) with PAO oil as the base fluid. To accomplish the above task, an experimental analysis is performed using a modern light flash technique (LFA 447) for measuring the thermal conductivity of the three types of nanofluids, for different loading of nanoparticles. The experimental results show a similar trend as observed in literature for nanofluids with a maximum enhancement of approximately 161% obtained for the CNT-PAO oil suspension. The overall percent enhancements for different volume fractions of the nanoparticles are highest for the CNT-based nanofluid, followed by the EXG and the HTT. The findings from this study for the three different types of carbon nanoparticles can have great potential in the field of thermal management.

Experimental Investigation of Oscillating Heat Pipes

Operation requirements of oscillating heat pipes (OHPs) are proposed. Based on the requirements, OHPs with nonflammable fluorocarbon fluids, FC-72 and FC-75, as the working fluid are developed. The OHPs have an inner diameter of 1.75 mm, a total length of 446 mm, and 40 tubing turns. There are two condensers on both outer sides and one evaporator in the middle of the OHPs. Thermal performance tests are conducted at various operating temperatures and heat rates. The working fluid fill ratio is varied. A high-performance OHP with FC-72 has been indicated for the first time. The FC-72 OHP can transport a 2040-W heat rate without dryout. The gravitational acceleration does not have a noticeable influence on the performance of the fluorocarbon OHP. The thermal performances of the fluorocarbon OHPs are compared with the case of an acetone OHP.

Computational Modeling of Spray Cooling: Current Status and Future Challenges

A survey on the current status of computational modeling of spray cooling is reported. From the survey, it is inferred that modeling heat transfer of the spray on thin liquid film on a hot surface will help one to understand and improve the performance of spray cooling. The computer modeling of multiphase flow using the level set method to identify the interface of vapor and liquid is presented. Computed velocity vector and temperature contour diagram are plotted for a thin liquid film of 73.62 μm. The transient Nu on the heated wall is plotted. From that the importance of transient conduction in the liquid film leading to high heat flux in a short time is identified. The mechanics of spray cooling is explained from the modeling study.

LARGE AREA SPRAY COOLING

A multi -nozzle plate with 48 miniature nozzles is designed to generate an array of 4×12 spray cones for the cooling of high power directed energy source components. A closed loop spray cooling test setup with a large cooling area of 19.3 cm 2 is established. The initial thermal performance test is carried out at a spray distance of 10 mm using FC-72 as the working fluid. Spray cooling tests are performed in two orientations of the spray target surface - (a) vertical and (b) horizontal facing downward. In both cases, the spray cone axis remains normal to the cooling surface. Critical heat flux (CHF) is investigated at various spray saturation temperatures and nozzle pressure drops. It is found that the thermal performance of the spray cooling is higher for the horizontal facing downward surface than for the vertical surface. Compared with the previous data for a small cooling surface area of 2.0 cm 2 , the maximum heat transfer coefficient and CHF of the large area (19.3 cm 2 ) spray cooling are lower by around 30% and 34% respectively.

Experimental Investigation of an Air Microjet Array Impingement Cooling Device

A microjet impingement cooling device for high power electronics was constructed from silicon wafers using microelectromechnical systems fabrication techniques. The array of 221, 0.277-mm-diam jets was tested using air as the coolant for jet diameter Reynolds numbers from 4.65 × 10 2 to 1.405 x 10 3 . Heat transfer and pressure drop data were obtained for a range of mass rates extending up to the point of choked flow and also for variable heat fluxes. The results were compared to an existing Nusselt correlation for jet impingement arrays that was found to significantly under-predict the heat transfer. A new correlation is provided that also accounts for variable air properties

Novel groove-shaped screen-wick miniature heat pipe

Miniature heat pipes (MHP) are passive heat transport devices mainly considered in electronics packaging for high heat flux acquisition and transport. Their applications could include thermal management of a variety of electronic devices such as computer processors and laser diodes. A novel screen-wick design suitable for MHP is described. The new design promises improved performance and ease of fabrication and is recommended in place of the familiar forms of rectangular groove design MHP. Design and fabrication details, along with steady-stale horizontal orientation performance test results of a proof-of-concept rectangular copper-water heat pipe are presented. Heat flux, temperature difference, and heat transfer coefficient data are compared with the literature data of a comparable flat MHP with a machined groove wick. The performance of the new design matches the comparable groove design. The highest applied evaporator heat flux at the heater surface was 115 W/cm 2 at an operating temperature of 90°C and evaporator-to-adiabatic temperature difference of 37°C. Results on the heat transfer coefficients are also presented.

Heat Dissipation with Pitch-Based Carbon Foams and Phase-Change Materials

The transient response of pitch-based carbon foam with an infiltrated phase-change material to a step temperature input is described. Pitch-based carbon foams exhibit high thermal conductivities and porosities up to 90%. This allows the possibility of infiltrating the foam with a relatively large volume of phase-change material. Phase-change thermal energy storage devices offer potential weight savings by allowing thermal control systems to use a smaller heat sink by absorbing the thermal energy quickly and storing it in the phase-change material. The thermal energy can then be released to a heat sink and the process repeated. Tests are conducted on samples of infiltrated foam. The results show the transient temperature response of the foam samples before, during, and after phase change. A theoretical prediction of the transient response is developed and compared with the experimental results.

All‐Solid‐State Electrochromic Variable Emittance Coatings for Thermal Management in Space

The results presented in this article demonstrate the feasibility of “all‐solid‐state” inorganic electrochromic (EC) variable emittance coatings (VEC) for thermal management in space. VECs were built on glass substrates, flexible polyimide (Kapton™) films, and on high resistance silicon wafers. The best VECs were found to modulate mid‐infrared emittance from 0.15 to 0.46 on a Kapton film and from 0.24 to 0.48 on a silicon wafer that performs the dual role as both infrared window and substrate. The results of thermal estimations for a two‐dimensional plate showed that EC systems with emittance modulation ratio ehigh/elow = 3 can be practically used to advantage in providing enhanced thermal control for lightweight structures in space. In addition, EC variable solar reflectance coatings (VSRCs) were built on polyethylene terephthalate (PET) film. The average reflection of the VSRC in a visual spectral range was 63% in the bleached condition and 20% in the colored condition. Thermal analysis shows that a com...

Electrochromic Devices for Satellite Thermal Control

This paper gives information regarding a new solid‐state thin film electrochromic device (ECD) designed for IR emissivity modulation. The variable emissive properties make the new ECD an ideal spacecraft antenna thermal management system. Specific attention is focused on a new electrochromic system (an Eclipse Energy Systems, Inc. proprietary technology) designed for the infrared (IR) spectral region. Reflectance modulation of the new IR‐ECD system for the 4.2 to 27 micron region is about 60%. In non‐absorbing mode, the IR‐ECD system exhibits a 90% flat IR reflectance spectrum (0.1 emissivity). Upon the application of a 1.6V D.C. voltage (top electrode negative), the device is switched to absorbing mode and the emissivity is increased to up to 0.98 value. The IR‐ECD system switches back to its maximum reflectance condition upon the application of a 0.6V reverse voltage. The IR‐ECD can be used as a smart thermal modulator for the thermal control of satellites and spacecraft by controlling the amount of ene...

Actively pumped two-phase loop for spray cooling

A new closed two-phase loop that combines with a large area spray cooling unit for the cooling of high heat-flux power sources is developed. The fluid circulation is sustained by a magnetic gear pump operating with an ejector unit. The motive flow of the ejector shares the pumping liquid flow with the multinozzle spray. With the assistance of the ejector, the maximum spray pressure drop across the nozzle can be enhanced by at least 0.56 bar at critical heat fluxes (CHF). This increases CHF of the spray cooling by up to 16%. More importantly, the use of the ejector prevents the uncondensed vapor from entering the magnetic gear pump and stabilizes the circulation of the two-phase flow. During the experiment, a multinozzle assembly with 48 miniature nozzles is employed. The target spray cooling area is 19.3 cm 2 . FC-72 and water are used as the working fluid. The present design concept can be applied to cooling systems operating in the aerospace environment.