Tadej Semenic

Thermophysical Properties of Biporous Heat Pipe Evaporators

Thirty biporous slugs with 3 different cluster diameters and 5 different particle diameters (15 combinations with 2 repetitions) and 12 monoporous slugs with 6 different particle diameters were sintered from spherical copper powder, and thermophysical properties were measured. The neck size ratio for all the particles was approximately 0.4. The porosity of monoporous samples was found to be independent of particle diameter and was equal to 0.28, and the porosity of biporous samples was found to be independent of cluster and particle diameters, and was equal to 0.64. The liquid permeability and maximum capillary pressure of small pores were found to be a linear function of the particle diameter. Similarly, vapor permeability was found to be a linear function of the cluster diameter The thermal conductivity of monoporous samples was measured to be 142 ±3 WImK at 42±2°C, and it was independent of particle diameter The thermal conductivity of biporous samples was found to be a function of cluster to particle diameter ratio.

Loop Thermosyphon Design for Cooling of Large Area, High Heat Flux Sources

Two-phase flow loop technologies capable of acquiring high heat fluxes (>1kW/cm 2 ) from large area heat sources (10cm 2 ) are being considered for the next generation naval thermal requirements. A loop thermosyphon device (~1 meter tall) was fabricated and tested that included several copper porous wick structures in cylindrical evaporators. The first two were standard annular monoporous and biporous wick designs. The third wick consists of an annular evaporator wick and an integral secondary slab wick for improved liquid transport. In this configuration a circular array of cylindrical vapor vents are formed integral to the primary and secondary transport wick composite. Critical heat fluxes using these wick structures were measured between 240W/cm 2 and 465W/cm 2 over a 10cm 2 area with water as the working fluid at 70°C saturation temperature. A thermosyphon model capable of predicting flow rate at various operating conditions based on a separated flow model is presented.

Thermophysical Properties of Monoporous Sintered Copper

Thermophysical properties of monodispersed-sintered copper are measured. An apparatus to measure effective thermal conductivity of dry and wet samples is built. It is calibrated using bulk samples with known thermal conductivity. Permeability is measured based on flow resistance though the porous samples. Velocity at different pressure drops is measured and the permeability calculated using Darcy’s law. The experiment is performed using water and silicone oil as working liquids. The error of the measurement is less then five percent. Capillary pressure for all samples is measured based on amount of liquid that is held by the porous sample. The Young-Laplace relationship is used to relate capillary pressure to effective pore radius. Porosity of the samples is calculated by measuring the dimension of samples and weighing the amount of liquid in fully saturated samples. Thermal conductivity and capillary pressure are found to decrease as powder diameter increases; permeability and porosity increases with powder diameter.Copyright

Use of Liquid Film Evaporation in Biporous Media to Achieve High Heat Flux Over Large Areas

Boiling characteristics of three biporous and one monoporous sintered wick are tested. The monoporous wick has the same wick thickness as a comparable biporous wick. Diameters of the clusters of the comparable biporous wick are equal to the powder diameter of the monoporous wick. A second biporous wick has the same configuration as the first, but is sintered in a thicker layer. The third biporous wick that is tested has smaller cluster sizes then the first two. All three biporous wicks have clusters sintered from powder with the same size distribution. The results demonstrate the advantages of a biporous capillary structure. All biporous wicks reached higher critical heat flux (CHF) then the monoporous wick. Experiments show that larger clusters are better than smaller. Comparing two different wick thicknesses, we can see that even though there is a dryout region inside the thick wick, it is still able to continuously remove heat at constant superheat. No sudden changes in superheat are seen. This process of heat removal is not possible with the thin wick. The working fluid in all runs is methanol. 4-mm thick wick with powder diameter ranging from 53 to 63 microns and cluster diameter ranging from 500 to 707microns is able to remove 377W/cm2 at temperature difference 110°C. A partial pressure inside the test chamber at this heat flux is 0.68atm and the interface temperature 167°C.Copyright

Thermophysical Properties of Biporous Sintered Copper

Thermophysical properties of bidispersed (biporous)-sintered copper are measured. An apparatus to measure effective thermal conductivity of dry samples is built. It is calibrated using bulk samples with known thermal conductivity. Permeability is measured based on flow resistance though the porous samples. Velocity at different pressure drops is measured and the permeability calculated using Darcy’s law. The experiment is performed using silicone oil as working liquid. The error of the method is less than three percent. Capillary pressure for all samples is measured based on amount of liquid that is held by the porous sample. The Young-Laplace relationship is used to relate capillary pressure to effective pore radius. Porosity of the samples is measured using density method. According to the measurement results, effective thermal conductivity of biporous samples is much lower than for comparable monoporous samples. Permeability and porosity of biporous samples are much higher than the monoporous samples. Capillary pressure of the biporous samples is very close to the one measured for the monoporous samples.Copyright

Biporous Sintered Copper for Closed Loop Heat Pipe Evaporator

Parameters that determine a critical heat flux (CHF) inside a biporous evaporator (wick) for a closed loop heat pipe have been studied. In a present work, a biporous wick structure was sintered from copper powder 53–63μm diameter into clusters 500–710μm diameter; the clusters were then sintered into 20mm long and 3mm wide wicks with different wick thickness on copper bases with three different lengths (5mm, 7.5mm and 10mm). Total of six wicks were made and tested. Copper base(mm) to wick thickness(mm) ratios of the wicks tested are: 5/5, 7.5/5, 10/5, 5/3, 7.5/3 and 10/1.5. Narrow (3mm) wicks with different copper base lengths allowed sidewise observation of the boiling inside the wick at different heat loads. Best-performed 10/1.5 wick, second best was 5/3 and then following 7.5/3, 5/5, 7.5/5, 10/5. Tests were run at atmospheric pressure and absolute ethanol as working fluid.Copyright

Heat Removal and Thermophysical Properties of Biporous Evaporators

This paper presents a study of different biporous evaporators as potential candidates for cooling high-power electronic devices. A biporous evaporator has two characteristic pore size distributions, which due to an enhancement in capillarity and vapor permeability greatly increase the critical heat flux (CHF) of the evaporator. In this work, eleven biporous evaporators are made by sintering together clusters of copper particles. Particles with diameters of 58, 76, 83, and 98μm are sintered into clusters with diameters of 302, 605, and 855μm. Clusters are subsequently sintered into evaporators with constant thickness-to-cluster diameter ratios of 3.3 and 0.32cm2 evaporator area. Finally, they are tested with degassed distilled water at 0.07bar. The highest CHF, 471W/cm2 at 149°C wall temperature and 104°C superheat, is measured for the 855/58 evaporator. A comparison of heat fluxes removed at a constant wall temperature of 125°C for all eleven evaporators shows that the highest heat flux of 388W/cm2 is removed with the 302/83 evaporator. A statistical regression analysis on heat fluxes at 125°C wall temperatures for all evaporators tested yields a correlation that relates the heat fluxes to cluster and particle diameters and is further used to predict a region of particle and cluster diameters with the highest heat flux.Copyright

High temperature semiconductor diode laser pumps for high energy laser applications

Existing thermal management technologies for diode laser pumps place a significant load on the size, weight and power consumption of High Power Solid State and Fiber Laser systems, thus making current laser systems very large, heavy, and inefficient in many important practical applications. To mitigate this thermal management burden, it is desirable for diode pumps to operate efficiently at high heat sink temperatures. In this work, we have developed a scalable cooling architecture, based on jet-impingement technology with industrial coolant, for efficient cooling of diode laser bars. We have demonstrated 60% electrical-to-optical efficiency from a 9xx nm two-bar laser stack operating with propylene-glycolwater coolant, at 50 °C coolant temperature. To our knowledge, this is the highest efficiency achieved from a diode stack using 50 °C industrial fluid coolant. The output power is greater than 100 W per bar. Stacks with additional laser bars are currently in development, as this cooler architecture is scalable to a 1 kW system. This work will enable compact and robust fiber-coupled diode pump modules for high energy laser applications.

Oxygen Fluorescence as a Means for Temperature and Pressure Measurement

The application of interest in this work is the measurement of air temperature and pressure using broadband laser-induced fluorescence (LIF). Because of the cost, bulkiness, and operating complications of ArF lasers, a xenon flashlamp is used instead of LIF. The oxygen is excited by photons in the 185–200nm range. Large wavelength windows (100nm and 65nm) are used to reduce the fluctuations in the total measured fluorescence. A correlation between temperature and fluorescence ratio is developed with an error in fluorescence ratio of approximately ±1.3%, resulting in an error in temperature of ±1.35° C for temperatures from 25–50° C.Copyright