Eric A. Silk

Investigation of Enhanced Surface Spray Cooling

Experiments were conducted to study the effects of enhanced surfaces on heat transfer during spray cooling. The surface enhancements consisted of cubic pin fins, pyramids, and straight fins (uniform cross sectional straight fins) machined on the top surface of copper heater blocks. Each had a cross-sectional area of 2.0 square cm. Measurements were also obtained on a heater block with a flat surface for baseline comparison purposes. A 2x2 nozzle array was used with PF-5060 as the working fluid. Thermal performance data was obtained under nominally degassed (chamber pressure of 41.4 kPa ) and gassy conditions (chamber with N2 gas at 101 kPa). The results show that the straight fins had the largest enhancement in heat flow. Critical heat flux (CHF) for this surface showed an increase of 55% in comparison to the flat surface for the nominally degassed condition. The cubic pin finned and pyramid surfaces provided slightly more than half the heat flux enhancement (30% - 40% greater than the flat surface) of the straight fins. The gassy case showed that the straight fins again provided the largest enhancement (48%) in CHF relative to the flat surface. This was followed by the cubic pin fins, and pyramids which had increases of 31% and 18% respectively. No significant effect was observed in the surface temperature at which CHF occurs for either portion of the study.

Spray Cooling Heat Flux Performance Using POCO HTC Foam

Previous studies have shown that spray cooling heat flux enhancement may be attained using enhanced surfaces. Most enhanced surface spray cooling studies have been performed using either extended or embedded surface structures. The present study investigates the effect of POCO HTC foam on spray cooling heat flux. The copper blocks used in the heat flux performance study had a cross-sectional area of 2.0 cm 2 . The POCO HTC foam pieces were attached to the copper blocks using two different bonding techniques. These were S-Bond® soldering and high thermal conductivity epoxy as the thermal interface material. Measurements were also obtained on a heater block with a flat surface for purposes of baseline comparison. A 2 x 2 nozzle array was used with PF-5060 as the working fluid. Thermal performance data was obtained under nominally degassed conditions (chamber pressure of 41.4 kPa). Results show that the highest heat flux attained was 133 W/cm 2 using the graphite POCO HTC foam with a nozzle-to-foam distance of 17 mm.

Impact of Cubic Pin Finned Surface Structure Geometry Upon Spray Cooling Heat Transfer

Experiments were conducted to study the effects of enhanced surface structures on heat flux using spray cooling. The surface enhancements consisted of cubic pin fins machined on the top surface of copper heater blocks. The structure height, pitch, and width were parametrically varied. Each copper block had a projected cross-sectional area of 2.0 cm2 . Measurements were also obtained on a heater block with a flat surface for baseline comparison purposes. A 2×2 nozzle array was used with PF-5060 as the working fluid. Thermal performance data was obtained under nominally degassed (chamber pressure of 41.4 kPa) and gassy conditions (chamber with N2 gas at 101 kPa) with a bulk fluid temperature of 20.5°C. Results for both the degassed and gassy cases show that structure width and separation distance have a dominant effect upon the heat transfer for the size ranges used. Cubic pin fin height had little impact upon heat flux. The highest critical heat flux (CHF) attained for any of the surfaces was 121 W/cm2 , giving an enhancement of 51% relative to the flat surface case under nominally degassed conditions. The highest CHF in the gassy case was 149 W/cm2 , giving an enhancement of 38% relative to the flat surface case.Copyright

Enhanced Surface Spary Cooling With Embedded and Compound Extended Surface Structures

Experiments were conducted to study the effects of enhanced surface structures on heat flux using spray cooling. The surface enhancements consisted of embedded structures (dimples, pores, and tunnels) and compound extended surface enhancements (straight fins, cubic pin fins and dimples) machined on and within the top surface of copper heater blocks. Each copper block had a projected cross-sectional area of 2.0 cm2. Measurements were also obtained on a heater block with a flat surface for baseline comparison purposes. A 2times2 nozzle array was used with PF-5060 as the working fluid. Thermal performance data was obtained under nominally degassed (chamber pressure of 41.4 kPa) and gassy conditions (chamber with N2 gas at 101 kPa) with a bulk fluid temperature of 20.5 degC. Results for both the nominally degassed and gassy cases show that the highest critical heat flux (CHF) was attained using straight fins and porous tunnels. For the nominally degassed case, both had a CHF of ap142 W/cm2 while for the gassy case their CHF values increased to 175 W/cm2 . This gave an enhancement relative to the respective flat surface degassed/gassy cases of ap 77% and 62% respectively

Spray Cooling Development Effort for Microgravity Environments

Spray cooling is a high heat flux removal technique considered for systems such as advanced lasers and high power density electronics. Several experiments have been conducted using spray cooling in recent years and various designs of spray cooling devices are continually emerging. At this time, one of NASA’s missions is to enhance future space science capabilities through the application of power lasers and electronics. However, the usage of systems having high heat fluxes can only be achieved with the corresponding development of high power thermal control systems. For the reliable performance of these high‐heat‐flux systems, proper thermal management is imperative. The study presented reviews the fabrication of a spray cooling system aimed at addressing issues pertinent to space applications. These issues include heat flux capability, orientation, and volumetric packaging. Computer modeling of spray cooling under microgravity conditions as well as comparison to the analogous 1‐g condition was also perfo...

Spray Cooling Modeling: Droplet Sub-Cooling Effect on Heat Transfer

Spray cooling has become increasingly popular as a thermal management solution for high‐heat flux (>100 W/cm2) applications such as laser diodes and radars. Research has shown that using sub‐cooled liquid can increase the heat flux from the hot surface. The objective of this study was to use a multi‐phase numerical model to simulate the effect of a sub‐cooled droplet impacting a growing vapor bubble in a thin (<100 μm) liquid film. The two‐phase model captured the liquid‐vapor interface using the level set method. The effects of surface tension, viscosity, gravity and phase change were accounted for by using a modification to the incompressible Navier‐Stokes equations, which were solved using the finite difference method. The computed liquid‐vapor interface and temperature distributions were visualized for better understanding of the heat removal process. To understand the heat transfer mechanisms of sub‐cooled droplet impact on a growing vapor bubble, various initial droplet temperatures were modeled (fr...

Investigation of Pore Size Effect On Spray Cooling Heat Transfer With Porous Tunnels

Previous studies have shown that spray cooling heat flux enhancement may be attained using enhanced surfaces (i.e., embedded surfaces). However, most enhanced surface spray cooling studies have been limited to extended surface structures. This study investigates the effect of porous tunnels (and pore size) on spray cooling heat flux. The pores were machined into the top of each heater block leading into the sub‐surface tunnels. Pore diameters varied between 0.25 mm and 1.0 mm. Pore density was held constant for each of the enhanced surfaces tested. Each copper block had a projected cross‐sectional area of 2.0 cm2. Measurements were also obtained on a heater block with a flat surface for purposes of baseline comparison. A 2×2 nozzle array was used with PF‐5060 as the working fluid. Thermal performance data was obtained under nominally degassed (chamber pressure of 41.4 kPa) conditions with a bulk fluid temperature of 20.5 °C. Results show that the highest critical heat flux (CHF) attained was 141 W/cm2 using pores 1.0 mm in diameter. This gave an enhancement of 75% relative to the flat surface case.Previous studies have shown that spray cooling heat flux enhancement may be attained using enhanced surfaces (i.e., embedded surfaces). However, most enhanced surface spray cooling studies have been limited to extended surface structures. This study investigates the effect of porous tunnels (and pore size) on spray cooling heat flux. The pores were machined into the top of each heater block leading into the sub‐surface tunnels. Pore diameters varied between 0.25 mm and 1.0 mm. Pore density was held constant for each of the enhanced surfaces tested. Each copper block had a projected cross‐sectional area of 2.0 cm2. Measurements were also obtained on a heater block with a flat surface for purposes of baseline comparison. A 2×2 nozzle array was used with PF‐5060 as the working fluid. Thermal performance data was obtained under nominally degassed (chamber pressure of 41.4 kPa) conditions with a bulk fluid temperature of 20.5 °C. Results show that the highest critical heat flux (CHF) attained was 141 W/cm2 usi...

Energy Conservation Based Spray Cooling CHF Correlation for Flat Surface Small Area Heaters

A forced convection analysis that emphasizes heat transfer phenomena at the heat exchange surface during spray cooling was used to develop a CHF correlation by recasting the multiphase energy balance equation in the form of the Nusselt number. Although multiphase effects are accounted for, the correlation primarily applies to flow scenarios that are single phase dominated. Test cases from previously published and unpublished studies by various researchers were included. Results for comparison of predicted vs. measured CHF are presented for several different working fluids. Predictions for the modeled cases show an average mean error of ±18% with 88% of the data predicted within a range of ±30%. The correlation applies to both gassy and degassed conditions for perfluorocarbons, water, and methanol.

Thermal Modeling of Spray Cooling: Gravitational Effect on Droplet and Bubble Dynamics

As the need for thermal management of high-power density electronic systems on space-based platforms (i.e., laser diode arrays, multichip modules, etc.) grows, interest in spray cooling as a thermal management solution is also increasing. The present study investigates numerically the effects of microgravity and macrogravity on spray cooling heat transfer as well as the effect of droplet impact on vapor bubble growth and development in a liquid film at the heater surface. A two-dimensional, multiphase flow computer model has been developed that includes the effects of surface tension, viscosity, phase change, and gravity. The liquid-vapor interface is modeled using the level set method. Initially, vapor bubble growth is simulated as pool boiling in the films macroregion (1 to 10 mm normal to the heated wall) for purposes of model verification. Then, bubble merger in a thin film is simulated under microgravity and macrogravity conditions. Finally, droplet impingement is included in the thin-film model, and gravitational effects on the transport properties are discussed. For the thin-film bubble merger and droplet impingement simulation studies, the liquid film adjacent to the heated wall has been approximated as 70 μm thick. Wall heat transfer during droplet impingement was computed in terms of the nondimensional Nusselt number for gravitational constants ranging from 0.0001 to 2g. Computed Nusselt number versus time is presented and explained using spatial velocity vector diagrams for each simulation case. All of the computational studies were performed using FC-72 as the simulated fluid.

Risk management in space systems design and technology development

Abstract Over the past two decades, risk management and risk analysis have emerged throughout the business community in the USA as prominent planning and development strategies used to mitigate risk of failure and to ensure a high return on investment for business endeavours (financial and otherwise). They are generic tools that can be applied to any business regardless of the sector (i.e. government, university, and private) and have been used by the Federal Government in the form of institutional practices aimed at maximizing the probability of success in business activities. One US Federal agency that incorporates risk management and analysis techniques into business and/or engineering activities is the National Aeronautics and Space Administration. The present work is a discussion on mission, spacecraft and instrument design (as well as technology development), and the role of risk management, analysis, and mitigation as fundamental tools in the design process.