Huseyin Bostanci

Saturation boiling of HFE-7100 from a copper surface, simulating a microelectronic chip

Abstract Experiments are performed, which investigated the effect of inclination angle, θ , on saturation pool boiling of HFE-7100 dielectric liquid from a smooth, 10×10 mm copper surface, simulating a microelectronic chip. For θ ⩽90° and surface superheats, Δ T sat >20 K, nucleate boiling heat flux decreases with increased θ , but increases with θ for Δ T sat T sat >13 K, nucleate boiling heat flux decreases with increased inclination, but at lower surface superheats the trend is inconclusive. The developed nucleate boiling correlation is within ±10% of the data and the developed correlations for critical heat flux (CHF) and the surface superheat at CHF are within ±3% and ±8% of the data, respectively. Results show that CHF decreases slowly from 24.45 W/cm 2 at 0° to 21 W/cm 2 at 90°, then decreases fast with increased θ to 4.30 W/cm 2 at 180°. The surface superheat at CHF also decreases with θ , from 31.7 K at 0° to 19.9 K at 180°. Still photographs are recorded of pool boiling at different heat fluxes and θ =0°, 30°, 60°, 90, 120°, 150° and 180°. The measured average departure bubble diameter from the photographs taken at the lowest nucleate boiling heat flux of ∼0.5 W/cm 2 and θ =0° is 0.55±0.07 mm and the calculated departure frequency is ∼100 Hz.

Spray Cooling With Ammonia on Microstructured Surfaces: Performance Enhancement and Hysteresis Effect

Experiments were performed to investigate spray cooling on microstructured surfaces. Surface modification techniques were utilized to obtain microscale indentations and protrusions on the heater surfaces. A smooth surface was also tested to have baseline data for comparison. Tests were conducted in a closed loop system with ammonia using RTIs vapor atomized spray nozzles. Thick film resistors, simulating heat source, were mounted onto 1 ×2 cm 2 heaters, and heat fluxes up to 500 W/cm 2 (well below critical heat flux limit) were removed. Two nozzles each spraying 1 cm 2 of the heater area used 96 ml/cm 2 min (9.7 gal/in. 2 h) liquid and 13.8 ml/cm 2 s (11.3 ft 3 /in. 2 h) vapor flow rate with only 48 kPa (7 psi) pressure drop. Comparison of cooling curves in the form of surface superheat (ΔT sat =T surf ―T sat ) versus heat flux in the heating-up and cooling-down modes (for increasing and decreasing heat flux conditions) demonstrated substantial performance enhancement for both microstructured surfaces over smooth surface. At 500 W/cm 2 , the increases in the heat transfer coefficient for microstructured surfaces with protrusions and indentations were 112% and 49% over smooth surface, respectively. Moreover, results showed that smooth surface gives nearly identical cooling curves in the heating-up and cooling-down modes, while microstructured surfaces experience a hysteresis phenomenon depending on the surface roughness level and yields lower surface superheat in the cooling-down mode, compared with the heating-up mode, at a given heat flux. Microstructured surface with protrusions was further tested using two approaches to gain better understanding on hysteresis. Data indicated that microstructured surface helps retain the established three-phase contact lines, the regions where solid, liquid, and vapor phases meet, resulting in consistent cooling curve and hysteresis effect at varying heat flux conditions (as low as 25 W/cm 2 for the present work). Data also confirmed a direct connection between hysteresis and thermal history of the heater.

Spray cooling of power electronics using high temperature coolant and enhanced surface

A spray cooling system was developed and tested for thermal management of power inverter modules utilized in automotive applications. System featured an array of 1×2 pressure atomized nozzles that used 90 °C antifreeze coolant with 0.15 L/min-cm2 liquid flow rate and 145 kPa pressure drop. Two cm2 simulated device, having an enhanced spray surface with micro scale structures, reached up to 400 W/cm2 heat flux with only 14 °C surface superheat. These experimental results demonstrated the capability of greatly reducing the overall thermal resistance of the inverter modules that are commonly cooled with single phase convective systems. Performance of the presented system proved the spray cooling of power electronics as an attractive option that enables high power densities while maintaining satisfactory and uniform device temperatures. In addition, due to the use of high temperature coolant at low flow rates, spray cooling offers compact and efficient system design.

Thermal Management of Power Inverter Modules at High Fluxes via Two-Phase Spray Cooling

A spray cooling system was developed and tested for thermal management of power inverter modules utilized in automotive applications. The system featured an array of 1×2 pressure atomized nozzles that used 88°C boiling point antifreeze coolant with 0.15-l/min.cm2 liquid flow rate and 145-kPa pressure drop. A 2-cm2 simulated device, having two kinds of enhanced spray surface with microscale structures, reached up to 400-W/cm2 heat flux with as low as 14 °C surface superheat. These experimental results demonstrated the capability of greatly reducing the overall thermal resistance of the inverter modules that are commonly cooled with single-phase convective systems. The long-term reliability of the spray cooling was assessed with 2000 h of testing time. Performance of the presented system proved the spray cooling of power electronics as an attractive option that enables high power densities while maintaining acceptable and uniform device temperatures. In addition, due to the use of high temperature coolant at low flow rates, the spray cooling offers a compact and efficient system design.

Evaluation of Compact and Effective Air-Cooled Carbon Foam Heat Sink

This study investigates a V-shaped corrugated carbon foam heat sink for thermal management of electronics with forced air convection. Experiments were conducted to determine the heat sink performance in terms of heat transfer coefficient and pressure drop. The test section, with overall dimensions of 51 mm L X51 mm W ×19 mm H, enabled up to 166 W of heat dissipation, and 3280 W/m 2 K and 2210 W/m 2 K heat transfer coefficients, based on log mean and air inlet temperatures, respectively, at 7.8 m/s air flow speed, and 1320 Pa pressure loss. Compared to a solid carbon foam, the V-shaped corrugated structure enhances the heat transfer, and at the same time reduces the flow resistance. Physical mechanisms underlying the observed phenomena are briefly explained. With benefits that potentially can reduce overall weight, volume, and cost of the air-cooled electronics, the present V-shaped corrugated carbon foam emerges as an alternative heat sink.

Jet Impingement Heat Transfer Using Air-Laden Nanoparticles With Encapsulated Phase Change Materials

Nanoparticles made of polymer encapsulated phase change materials (PCM) are added in air to enhance the heat transfer performance of air jet impingement flows applied to cooling processes. Encapsulation prevents agglomeration of the PCM (paraffin) nanoparticles when they are in the liquid phase. The sizes of the particles are chosen to be small enough so that they maintain near velocity equilibrium with the air stream. Small solid paraffin particles can absorb a significant amount of energy rapidly from a heat source by changing phase from solid to liquid. Nanoparticle volume fraction is found to play an important role in determining the overall pressure drop and heat transfer of the jet impingement process. Specifically, air jets laden with 2.5% particulate volume fraction were shown to improve the average heat transfer coefficient by 58 times in the air flow speed range of 4.6 to 15.2m/s when compared to that of pure air alone. In addition, the structural integrity of the encapsulating shells was demonstrated to be excellent by the repeated use of the nanoparticles in closed loop testing. [DOI: 10.1115/1.4023563]

Microscale surface modifications for heat transfer enhancement.

In this experimental study, two surface modification techniques were investigated for their effect on heat transfer enhancement. One of the methods employed the particle (grit) blasting to create microscale indentations, while the other used plasma spray coating to create microscale protrusions on Al 6061 (aluminum alloy 6061) samples. The test surfaces were characterized using scanning electron microscopy (SEM) and confocal scanning laser microscopy. Because of the surface modifications, the actual surface area was increased up to 2.8× compared to the projected base area, and the arithmetic mean roughness value (Ra) was determined to vary from 0.3 μm for the reference smooth surface to 19.5 μm for the modified surfaces. Selected samples with modified surfaces along with the reference smooth surface were then evaluated for their heat transfer performance in spray cooling tests. The cooling system had vapor-atomizing nozzles and used anhydrous ammonia as the coolant in order to achieve heat fluxes up to 500 W/cm(2) representing a thermal management setting for high power systems. Experimental results showed that the microscale surface modifications enhanced heat transfer coefficients up to 76% at 500 W/cm(2) compared to the smooth surface and demonstrated the benefits of these practical surface modification techniques to enhance two-phase heat transfer process.

Cryogenic ceramic 277 watt Yb:YAG thin-disk laser

A ceramic ytterbium:yttrium aluminum garnet (Yb:YAG) thin-disk laser is investigated at 15°C (288 K) and also at 80 K, where it behaves as a four-level laser. We introduce a new two-phase spray cooling method to cool the Yb:YAG. One system relies on R134a refrigerant while the other uses liquid nitrogen (LN 2 ). The use of two systems allows the same disk to be tested at the two temperatures. When the Yb:YAG is cooled from room to cryogenic temperatures, the lasing threshold drops from 155 W to near 10 W, while the slope efficiency increases from 54% to a 63%. A 277 W laser with 520 W of pump is demonstrated. We also model the thermal and structural properties at these two temperatures and estimate the beam quality.

Spray cooling with ammonia on micro-structured surfaces

Experiments were performed to investigate spray cooling enhancement on micro-structured surfaces. Surface modification techniques were utilized to obtain micro-scale indentations and protrusions on the heater surfaces. A smooth surface was also tested to have baseline data for comparison. Tests were conducted in a closed loop system with ammonia using RTFs vapor atomized spray nozzles. Thick film resistors, simulating heat source, were mounted onto 1 cm times 2 cm heaters and heat fluxes up to 500 W/cm2 (well below critical heat flux (CHF) limit) were removed. Two nozzles each spraying 1 cm2 of heater area used 96 ml/cm2-min (9.7 gal/in2-hr) liquid and 13.8 ml/cm2-s (11.3 ft3/in2-hr) vapor flow rate with only 48 kPa (7 psi) pressure drop. Results for micro-structured surfaces with protrusions and indentations offered significant performance enhancement of 115% and 52% increase in heat transfer coefficient over smooth surface respectively.

Preliminary Analysis of an Innovative Rotary Displacer Stirling Engine

Stirling engines are an external combustion heat engine that converts thermal energy into mechanical work that a closed cycle is run by cyclic compression and expansion of a work fluid (commonly air or Helium) in which, the working fluid interacts with a heat source and a heat sink and produces network. The engine is based on the Stirling cycle which is a subset of the Carnot cycle. The Stirling cycle has recently been receiving renewed interest due to some of its key inherent advantages. In particular, the ability to operate with any form of heat source (including external combustion, flue gases, alternative (biomass, solar, geothermal) energy) provides Stirling engines a great flexibility and potential benefits since it is convinced as engines running with external heat sources. However, several aspects of traditional Stirling engine configurations (namely, the Alpha, Beta, and Gamma), specifically complexity of design, high cost, and relatively low power to size and power to volume ratios, limited their widespread applications to date. This study focuses on an innovative Stirling engine configuration that features a rotary displacer (as opposed to common reciprocating displacers), and aims to utilize analytical and numerical analysis to gain insights on its operation parameters. The results are expected to provide useful design guidelines towards optimization. The present study starts with an overview of the Stirling cycle and Stirling engines including both traditional and innovative rotary displacer configurations, and their major advantages and disadvantages. The first approach considers an ideal analytical model and implements the well-known Schmidt analysis assumptions for the rotary displacer Stirling engine to define the effects of major design and operation parameters on the performance. The analytical model resulted in identifying major variables that could affect the engine performance (such as the dead volume spaces, temperature ratios and the leading phase angle). It was shown that the dead volume could have a drastic effect over the engine performance and the optimum phase angle of the engine is 90o. The second approach considers a non-ideal analytical model and aims to identify and account the main sources of energy losses in the cycle to better represent the engine performance. The study showed that the ideal efficiency and the non-ideal efficiency could have 15% difference that could have as an enormous effect on the engine performance.