Showing papers by "Avram Bar-Cohen published in 2014"

Pulsed Thermoelectric Cooling for Improved Suppression of a Germanium Hotspot

Abstract: As hotspots become an increasingly important factor in the design of electronic devices, it has become essential to develop novel near-junction cooling methods. Steady-state thermoelectric cooling has previously been considered for the removal of localized hotspots on various substrates. In this paper, the transient behavior of a germanium thermoelectric self-cooler, in which the chip substrate is used as a leg of the thermoelectric circuit, is described. A 3-D thermoelectric numerical model was created in the commercial FEA package ANSYS and is used to explore the effects of various initial conditions, current pulse durations, current pulse magnitudes, pulse shapes, and die thicknesses. The results suggest that pulsed transient thermoelectric cooling has the potential to improve hotspot temperature reduction by approximately 30% relative to what is achievable in steady state. In addition, it was found that larger currents generally cause more rapid thermoelectric cooling, but also result in large overshoot temperatures and that the applied current profile has a strong effect on the transient behavior of the cooler.

Thermal performance and reliability characterization of bonded interface materials (BIMs)

Abstract: Thermal interface materials (TIMs) are an important enabler for low thermal resistance and reliable electronics packaging for a wide array of applications. There is a trend towards bonded interface materials (BIMs) because of their potential for low thermal resistance (<;1 mm 2 -K/W). However, due to coefficient of thermal expansion mismatches between various layers of a package, thermomechanical stresses are induced in BIMs and the package can be prone to failures and integrity risks. Deteriorated interfaces can result in high thermal resistance in the package and degradation and/or failure of the electronics. The Defense Advanced Research Projects Agency's (DARPA) Thermal Management Technologies (TMT) Program has addressed this challenge, supporting the development of mechanically compliant, low resistivity nano-thermal interface (NTI) materials. Prior development of these materials resulted in samples that met DARPA's initial thermal performance and synthesis metrics. In this present work, we describe the testing procedure and report the results of thermal performance and reliability characterization of an initial sample set of three different NTI-BIMs tested at the National Renewable Energy Laboratory.

Energy Efficient Two-Phase Microcooler Design for a Concentrated Photovoltaic Triple Junction Cell

Abstract: The potential application of an R134a-cooled two-phase microcooler for thermal management of a triple junction solar cell (CPV), under concentration of 2000 suns, is presented. An analytical model for the triple-junction solar cell temperature based on prediction of two-phase flow boiling in microchannel coolers is developed and exercised with empirical correlations from the open literature for the heat transfer coefficient, pressure drop, and critical heat flux. The thermofluid analysis is augmented by detailed energy modeling relating the solar energy harvest to the “parasitic” work expended to provide the requisite cooling, including pumping power and the energy consumed in the formation and fabrication of the microcooler itself. Three fin thicknesses, between 100 μm and 500 μm, a variable number of fins, between 0 and 9, and 5 channel heights between 0.25 mm and 3 mm, are examined for a R134a flow rate of 0.85 g/s to determine the energy efficient microcooler design for a 10 mm × 10 mm triple junction CPV cell.

High vapor quality two phase heat transfer in staggered and inline micro pin fin arrays

Abstract: We investigated two phase heat transfer in staggered and inline micro pin fin arrays, using deionized water and HFE-7200 as working fluids with exit qualities up to 90%,. The inline and staggered arrays have equivalent pin fin width of 153μm and height of 305μm on a heated base area of 0.96cm width by 2.88cm length. Mass fluxes ranged from 200 kg/m²s to 600 kg/m²s for HFE-7200 and 400 kg/m²s to 1300 kg/m²s for water. Base heat fluxes ranged from 27 W/cm² to 118 W/cm² for water, and 1 W/cm² to 36 W/cm² for HFE-7200. Average heat transfer coefficient data were obtained and successfully correlated to 3% MAE for water and 10-13% MAE for HFE-7200 in these pin fin arrays.

Non-Intrusive Optical Validation of Two-Phase Flow Regimes in a Small Diameter Tube

Abstract: A non-intrusive optical method for two-phase flow pattern identification was developed to validate flow regime maps for two-phase adiabatic flow in a small diameter tube. Empirical measurements of film thickness have been shown to provide objective identification of the dominant two-phase flow regimes, representing a significant improvement over the traditional use of exclusively visual and verbal descriptions. Use of this technique has shown the Taitel-Dukler, Ullmann-Brauner, and Wojtan et al. phenomenological flow regime mapping methodologies to be applicable, with varying accuracy, to small diameter two-phase flow.Copyright © 2014 by ASME

Numerical modeling and simulation of laser diode diamond microcoolers

Abstract: High heat flux management schemes in laser diodes require appropriate cooling applications Micro channel coolers are now widely used in high power laser diode industry with the highest total thermal resistance reported as low as 003 cm 2 -K/W with pressure drops as low as 10~50 psi Since, the geometries, flow rates as well as high heat fluxes of current SOA LD micro-coolers differ, it is necessary to understand their thermal performance relative to conductive, convective and caloric thermal resistance To do this comparison an equivalent effective micro-cooler thermal model is developed and then iterated with scaled input parameters from SOA LD micro-coolers The objective is to identify the dominant thermal resistance - that plays the major role in decreasing the total thermal resistance This paper will then predict a micro cooler that will perhaps be able to reduce total thermal resistance lower than 001 K-cm 2 /W with minimal pressure drop for next generation high heat flux applications The current study will be restricted to only single phase liquid cooling

A Uniform Temperature Heat Sink for Cooling of High Concentrator Photo-Voltaic Systems

Abstract: Here we investigate experimentally application of single-phase water micro-pin-fin cooler for ‎thermal management of a triple-junction solar cell under concentration of 500 suns. The ‎micro-pin fin heat sink was fabricated of copper using the electric discharge machining‎ (EDM). Silver over nickel ‎plating was applied to the heat sink to allow soldering of the resistor to the heat sink. The heat ‎sink of 1×1 cm 2 has pin-fins of 0.35×0.35 mm, 1 mm height, and pitch of 0.45 mm. The ‎dimensions were taken from typical micro-scale devices described in the literature. To ‎minimize the pressure drop the pin-fins were designed in staggered arrangement, and pinfin ‎corner was normal to the flow direction. Experiments were performed in the range of heat flux ‎q=5.2-24.7 W/cm 2 and mass flux G=10.7-39.1 kg/m 2 s. A thermal high speed imaging ‎radiometer was utilized to study the temperature field on the electrical heater. We compared ‎temperature non uniformity (on the heated surface) under conditions of convective heat transfer ‎of water in micro-channel without pin-fins. With pin-fin cooling, the temperature distribution ‎on the whole heater, as recorded by the IR camera, showed that the standard deviation from ‎the average temperature was 0.3-1.9 K and the maximum difference of the wall temperature ‎between different points did not exceed 3-5 K. By cooling at the same conditions using ‎rectangular micro-channel without pinfins the difference between wall temperature at the ‎inlet and the outlet was about 17-28 K, depending on heat flux and mass flow rate. This study, ‎to the best of our knowledge, is the first presentation of the method that makes use of a uniform ‎temperature microchannel single-phase heat sink‎.