The thermal ground plane is an advanced planar heat pipe designed for cooling microelectronics in high gravitational fields. A thermal resistance model is developed to predict the thermal performance of the thermal ground plane, including the effects of the presence of noncondensable gases. Viscous laminar flow pressure losses are predicted to determine the maximum heat load when the capillary limit is reached. This paper shows that the axial effective thermal conductivity of the thermal ground plane decreases when the substrate and/or wick are thicker and/or with the presence of noncondensable gases. Moreover, it was demonstrated that the thermal-fluid model may be used to optimize the performance of the thermal ground plane by estimating the limits of wick thickness and vapor space thickness for a recognized internal volume of the thermal ground plane. The wick porosity plays a significant role in maximum heat transport capability. A large adverse gravitational field strongly decreases the maximum heat transport capability of the thermal ground plane. Axial effective thermal conductivity is mostly unaffected by the gravitational field. The maximum length of the thermal ground plane prior to reaching the capillary limit is inversely proportional to input power.

Thermal ground planes (TGPs) are flat, thin (external thickness of 2 mm) heat pipes which utilize two-phase cooling. The goal is to utilize TGPs as thermal spreaders in a variety of microelectronic cooling applications. TGPs are novel high-performance, integrated systems able to operate at a high power density with a reduced weight and temperature gradient. In addition to being able to dissipate large amounts of heat, they have very high effective axial thermal conductivities and (because of nano-porous wicks) can operate in high adverse gravitational fields. A three-dimensional (3D) finite element model is used to predict the thermal performance of the TGP. The 3D thermal model predicts the temperature field in the TGP, the effective axial thermal conductivity, and the evaporation and the condensation rates. A key feature of this model is that it relies on empirical interfacial heat transfer coefficient data to very accurately model the interfacial energy balance at the vapor-liquid saturated wick interface. Wick samples for a TGP are tested in an experimental setup to measure the interfacial heat transfer coefficient. Then the experimental heat transfer coefficient data are used for the interfacial energy balance. Another key feature of this model is that it demonstrates that for the Jakob numbers of interest, the thermal and flow fields can be decoupled except at the vapor-liquid saturated wick interface. This model can be used to predict the performance of a TGP for different geometries and implementation structures. This paper will describe the model and how it incorporates empirical interfacial heat transfer coefficient data. It will then show theoretical predictions for the thermal performance of TGP’s, and compare with experimental results.

In recent years, electronics have significantly reduced in size at maintained or increased functionality. This trend has led to an increased demand for more capable thermal management solutions at smaller scales. However, miniaturization of conventional fan and heat sink cooling systems introduces significant size, weight and efficiency challenges. In this study the flow performance of a novel thin form-factor cooling solution, the advanced dual piezoelectric cooling jet (DCJ), is evaluated. A DCJ is a micro-fluidic device that disturbs the boundary layer over a hot component and hence increases heat transfer. The design of an equivalent fan-curve experiment is described in detail. A first ever fan curve for a bimorph DCJ device is presented. This is coupled to a thermal performance analysis using an experiment simulating thin profile consumer electronics.

Heat pipes are commonly used in electronics cooling applications to spread heat from a concentrated heat source to a larger heat sink. Heat pipes work on the principles of two-phase heat transfer by evaporation and condensation of a working fluid. The amount of heat that can be transported is limited by the capillary and hydrostatic forces in the wicking structure of the device. Thermal ground planes are two-dimensional high conductivity heat pipes that can serve as thermal ground to which heat can be rejected by a multitude of heat sources. As hydrostatic forces are dependent on gravity, it is commonly known that heat pipe and thermal ground plane performance is orientation dependent. The effect of variation of gravity force on performance is discussed and the development of a miniaturized thermal ground plane for high g operation is described. In addition, experimental results are presented from zero to −10g acceleration. The study shows and discusses that minimal orientation or g-force dependence can be achieved if pore dimensions in the wicking structure can be designed at micro/nano-scale dimensions.

A multidisciplinary team, consisting of participants from GE, Qorex LLC, AFL Telecommunications and Sandia National Labs, is engaged in a DoE-sponsored program to develop an optical sensor suite for the measurement of distributed temperature and pressure in geothermal wells. Our effort in the first year has been focused on the development and validation of specific sub-systems including both fiber and sensors to show the reliability of the sensing approach for temperatures up to 374 C and pressures of 220 bars in the presence of hydrogen. This paper discusses the overall program scope and presents some initial results from three key tasks, point pressure sensor development, fiber Bragg grating pressure sensor development, and fiber tensile strength testing at elevated temperatures. FIBER OPTIC SENSING SYSTEM FOR ENHANCED GEOTHERMAL SYSTEMS Temperature and pressure sensing in enhanced geothermal systems is one of the primary needs at present. The harsh environment of such wells leads to difficulty in designing robust sensors. This current project is aimed at developing a fiber optic system that will combine several different types of measurement systems into a single cable. Fiber is currently being evaluated for use at high temperatures (374 C) and high pressures (220 bars). Both its mechanical strength and resistance to hydrogen darkening effects are key concerns. Fiber which is found to be robust under these conditions will be used to operate a MEMS pressure sensor at the bottom of the geothermal well. Another fiber will be used to interrogate Fiber Bragg grating pressure and temperature sensors distributed along the length of the fiber, and especially near the lower end of the well. A graded index multimode fiber will be used as part of a Raman distributed temperature measurement system (DTS). A single mode step index fiber will be used for Brillouin distributed temperature and strain sensing (DTSS), and another fiber for Rayleigh coherent optical time domain reflectometry (COTDR) for distributed strain measurements. All of these fibers will be bundled into a single cable with appropriate strain relief and corrosion resistance with the objective of making these measurement in wells up to 10 km deep for a lifetime of at least six months with a pressure measurement accuracy of better than 1% and a long term drift of less than 1%. The program is funded by DOE with cost sharing for a period of two years for a total cost of $2.6M. The first year of the project is aimed at developing the various subsystems. In the second year, the subsystems will be integrated into a cable and down hole testing will be undertaken. GE Global Research and GE Sensing are primarily involved in designing and fabricating the MEMS and FBG pressure sensors. Qorex is testing various commercially available fibers for their suitability in the harsh geothermal environment, and is evaluating commercially available instruments for Raman DTS, Brillouin DTSS and Rayleigh COTDR measurements. Qorex will also design the fiber cable, which will be fabricated by AFL Telecommunications. Down hole fiber cable testing will be supervised by Sandia National Laboratory. In this report, we discuss research efforts related to the MEMS and FBG sensor development, and the initial fiber testing results related to year 1 project tasks. MEMS POINT PRESSURE SENSOR It is desirable to have a highly accurate pressure sensor at the bottom of the geothermal well. We have selected a MEMS pressure sensor because of its high accuracy and general suitability for the geothermal environment. MEMS devices, of course, have been employed in a wide variety of pressure sensors. For pressure measurements in a geothermal well, we have chosen an approach similar to that described in (Andres, 1986). A laser at one wavelength is modulated at the resonant frequency of the MEMS sensor. The heat generated in the resonator by absorption of the laser beam causes the resonator to vibrate. The vibration in turn varies the cavity formed by the resonator and the end of the fiber. The reflected beam from a second continuous laser is thereby modulated at the frequency of the resonator vibration and detected. A simplified block diagram of our sensor design is shown in Fig. 1. It differs from that described by Andres in that a single fiber is used to deliver both the drive and read laser beams. The cavity is formed from surfaces integral to the sensor and, unlike the previously described sensor, is relatively insensitive to the fiber-to-sensor gap other than for an approximately inverse square drop in signal intensity as the fiber gap is increased, as shown in Fig. 2. A priori, there is no reason for the optimum spot on the resonator for the drive laser to be the same as that for the read laser. Extensive finite element analysis has been carried out to determine a resonator design that satisfies the constraint of a single fiber to simplify the sensor package and fiber cable. With proper alignment of the fiber to the sensor and a resonator that is driven within its linear optical response range, a reasonably sinusoidal readback signal can be obtained as shown in Fig. 3. The results in Fig. 3 were obtained at atmospheric pressure for a MEMS sensor optimized for pressures below 2 bars and driven electrically. Sensors have now been designed for this high pressure/high temperature geothermal application and will be tested and calibrated using a complete optical system including a feedback loop as shown in Fig. 1. Other challenges in the system design that are being addressed include the fiber-to-sensor splice, the fiberto-sensor package connection, and the sensor package design that enables the well pressure to be conveyed detector Feedback electronics Laser driver/ modulator MEMS sensor drive laser Resonant frequency

ac electrokinetic manipulations of particles and fluids are important techniques in the development of lab-on-a-chip technologies. Most of these systems involve planar micro-electrode geometries, generating high strength electric fields. When these fields are applied to a dielectric medium, Joule heating occurs. Understanding electrothermal heating and monitoring the temperature in these environments are critical for temperature-sensitive investigations including biological applications. Additionally, significant changes in fluid temperature when subjected to an electric field will induce electrohydrodynamic flows, potentially disrupting the intended microfluidic profile. This work investigates heat generated from the interaction of ac electric fields and water at various electrical conductivities (from 0.92 mS/m to 390 mS/m). The electrode geometry is an indium tin oxide (ITO) electrode strip 20 μm wide and a grounded, planar ITO substrate separated by a 50 μm spacer with microfluidic features. Laser-induced fluorescence is used to measure the experimental changes in temperature. A normalization procedure that requires a single temperature-sensitive dye, Rhodamine B (RhB), is used to reduce uncertainty. The experimental electrothermal results are compared with theory and computer simulations.

As power densities in electronic devices have increased dramatically over the last decade, advanced thermal management solutions are required. A significant part of the thermal resistance budget is commonly taken up by the heat spreader, which serves to reduce the input heat flux and connect to an increased area for heat removal. Thermal ground planes are devices that address this issue by utilizing two-phase heat transfer achieving higher effective thermal conductivities than conventional solid heat spreaders. This study describes the need for and design of a charging station to accurately dispense the working fluid and a thermal characterization experiment to characterize performance. The design study includes detailed analysis of accuracy and validation of the setup.

Heat pipes have been gaining a lot of popularity in electronics cooling applications due to their ease of operation, reliability, and high effective thermal conductivity. An important component of a heat pipe is the wick structure, which transports the condensate from condenser to evaporator. The design of wick structures is complicated by competing requirements to create high capillary driving forces and maintain high permeability. While generating large pore sizes will help achieve high permeability, it will significantly reduce the wick’s capillary performance. This study presents a novel experimental method to simultaneously measure capillary and permeability characteristics of the wick structures using fluorescent visualization. This technique will be used to study the effects of pore size and gravitational force on the flow-related properties of the wick structures. Initial results are presented on wick samples visually characterized from zero to nine g acceleration on a centrifuge. These results will provide a tool to understand the physics involved in transport through porous structures and help in the design of high performance heat pipes.