Gregory J. Michna

Experimental Investigation of Single-Phase Microjet Array Heat Transfer

The heat transfer performance of two microjet arrays was investigated using degassed deionized water and air. The inline jet arrays had diameters of 54 μm and 112 μm, a spacing of 250 μm, a standoff of 200 μm (S/d=2.2 and 4.6, H/d=1.8 and 3.7), and jet-to-heater area ratios from 0.036 to 0.16. Average heat transfer coefficients with deionized water were obtained for 150≤Re d ≤3300 and ranged from 80,000 W/m 2 K to 414,000 W/m 2 K. A heat flux of 1110 W/cm 2 was attained with 23°C inlet water and an average surface temperature of 50°C. The Reynolds number range for the same arrays with air was 300≤Re d ≤4900 with average heat transfer coefficients of 2500 W/m 2 K to 15,000 W/m 2 K. The effect of the Mach number on the area-averaged Nusselt number was found to be negligible. The data were compared with available correlations for submerged jet array heat transfer.

Flow Boiling of R134a in Circular Microtubes—Part I: Study of Heat Transfer Characteristics

An experimental study of two-phase heat transfer coefficients was carried out using R134a in uniformly heated horizontal circular microtubes with diameters from 0.50 mm to 1.60 mm over a range of mass fluxes, heat fluxes, saturation pressures, and vapor qualities. Heat transfer coefficients increased with increasing heat flux and saturation pressure but were independent of mass flux. The effects of vapor quality on heat transfer coefficients were less pronounced and varied depending on the quality. The data were compared with seven flow boiling correlations. None of the correlations predicted the experimental data very well, although they generally predicted the correct trends within limits of experimental error. A correlation was developed, which predicted the heat transfer coefficients with a mean average error of 29%. 80% of the data points were within the ±30% error limit.

Single-Phase Microscale Jet Stagnation Point Heat Transfer

An investigation of the pressure drop and impingement zone heat transfer coefficient trends of a single-phase microscale impinging jet was undertaken. Microelectromechanical system (MEMS) processes were used to fabricate a device with a 67-μm orifice. The water jet impinged on an 80-μm square heater on a normal surface 200 μm from the orifice. Because of the extremely small heater area, the conjugate convection-conduction heat transfer process provided an unexpected path for heat losses. A numerical simulation was used to estimate the heat losses, which were quite large. Pressure loss coefficients were much higher in the range Re d,o <500 than those predicted by available models for short orifice tubes; this behavior was likely due to the presence of the wall onto which the jet impinged. At higher Reynolds numbers, much better agreement was observed. Area-averaged heat transfer coefficients up to 80,000 W/m 2 K were attained in the range 70 <Re d <1900. This corresponds to a 400 W/cm 2 heat flux at a 50°C temperature difference. However, this impingement zone heat transfer coefficient is nearly an order-of-magnitude less than that predicted by correlations developed from macroscale jet data, and the dependence on the Reynolds number is much weaker than expected. Further investigation of microjet heat transfer is needed to explain the deviation from expected behavior.

An Experimental Study of the Friction Factor and Mass Transfer Performance of an Offset-Strip Fin Array at Very High Reynolds Numbers

Thermal-hydraulic performance data for offset-strip fin arrays are readily available in the range Re<10,000. However, in emerging applications in automotive and aerospace systems, where fan power is not a constraint and compactness is important, it may be desirable to operate offset-strip fin heat exchangers at very high Reynolds numbers. In this paper, friction factor and mass transfer performance of an offset-strip fin array at Reynolds numbers between 10,000 and 120,000 are characterized. A scale-model, eight-column fin array is used in pressure drop and naphthalene sublimation experiments, and the data are compared to predictions of performance given by available analytical models and extrapolations of the best available correlations. The friction factor data follow the correlation-predicted trend of decreasing monotonically as the Reynolds number is increased to 20,000. However, at higher Reynolds numbers, the friction factor increases as the Reynolds number increases and local maxima are observed in the data. Over the range investigated, the modified Colburn j factor decreases monotonically as the Reynolds number increases. For Reynolds numbers in the range 10,000< Re < 120,000, well beyond that covered by state-of-the-art correlations, both the friction factor and Colburn j factor are roughly twice that predicted by extrapolating the best available correlations. The higher-than-predicted Colburn j factor at very high Reynolds numbers is encouraging for the use of offset-strip fin heat exchangers in emerging applications where compactness is of high importance.

Effect of Condenser Temperature on Pulsating Heat Pipe Performance

Pulsating heat pipes (PHPs) excel at transferring heat efficiently over a small area. As electrical components continue to miniaturize, heat flux is increasingly important in thermal management. In this work, the effect of condenser temperature in a PHP was explored. Significantly reduced thermal resistance in the PHP was achievable by increasing the condenser temperature. It was found that for a given heater input, the evaporator temperatures with a 30°C condenser temperature were lower than with a 20°C condenser temperature. It is hypothesized that this is the result of more favorable fluid properties (viscosity and vapor pressure) at those temperatures. It is therefore possible, in some cases, to reduce the temperature of the device being cooled by placing the condenser in a warmer environment.Copyright

Exploration of Experimental Techniques to Determine the Condensation Heat Flux in Microchannels and Minichannels

This study seeks to analyze and explore experimental methods to study condensation heat transfer in micro- and mini-channel. Following, an experimental setup was built and initial results are presented. Several experimental techniques were reviewed, while two, thermoelectric coolers and a copper-heat-flux-sensor were analyzed in detail for condensation heat flux. It was concluded that thermoelectric coolers were not suitable as heat flux sensors for single-phase validation, but the copper-heat-flux-sensor was appropriate to measure heat transfer coefficients at the mini-scale. Condensation heat transfer coefficients were obtained experimentally in seven parallel square mini-channels of diameter 1mm. Existing condensation correlations were applied to these data; micro- and mini-scale correlations captured the appropriate trends, but the macro-scale Shah (1979) correlation fit the data best.Copyright

Energy Efficiency of Refrigeration Systems for High-Heat-Flux Microelectronics

Increasingly, military and civilian applications of electronics require extremely high-heat fluxes on the order of 1000 W/cm2. Thermal management solutions for these severe operating conditions are subject to a number of constraints, including energy consumption, controllability, and the volume or size of the package. Calculations indicate that the only possible approach to meeting this heat flux condition, while maintaining the chip temperature below 65°C, is to utilize refrigeration. Here, we report an initial thermodynamic optimization of the refrigeration system design. In order to hold the outlet quality of the fluid leaving the evaporator to less than approximately 20%, in order to avoid reaching critical heat flux, the refrigeration system design is dramatically different from typical configurations for household applications. In short, a simple vapor-compression cycle will require excessive energy consumption, largely because of the additional heat required to return the refrigerant to its vapor state before the compressor inlet. A better design is determined to be a “two-loop” cycle, in which the vapor-compression loop is coupled thermally to a pumped loop that directly cools the high-heat-flux chip.

Geometric Evaporator Enhancements of an Oscillating Heat Pipe

The oscillating heat pipe (OHP) is a passive two-phase cooling device that is capable of transferring large amounts of thermal energy. Previous research conducted on OHPs indicates that it is a viable option for developing high-heat flux cooling requirements, particularly in the field of electronics cooling. OHPs consist of evaporator, adiabatic, and condenser sections connected by multiple interconnected meandering channels. A two-phase working fluid, in this study acetone, fills the channels and acts as the heat transfer medium. The focus of this study is to further the development of OHPs to improve performance and operation by conducting a comparison between two different evaporator geometries. The first was a traditional straight channel geometry. The second consisted of circular pins centered in the channels with circular cavities surrounding the pins to allow fluid flow. The results of this study showed that the traditional straight channel configuration preformed best. The lowest fill ratio, 35%, performed best for all cases. The lowest thermal resistance observed was 0.11 K/W for the straight channels, and 0.16 K/W for the enhanced channels. The enhanced channels likely did not improve the performance because of an increase in pressure drop through the evaporator section.© 2015 ASME

Properties of Brassica Carinata and Camelina Sativa Meals and Fast Pyrolysis Derived Bio-Oils

Fast pyrolysis is one method of creating bio-oil from various sources of biomass. In this research, fast pyrolysis of Brassica carinata and Camelina sativa meals were performed using a fluidized bed reactor. Chemical and physical properties of each oil sample were analyzed to determine the initial characteristics of the samples produced. Karl Fischer method was used to determine the water content and a total acid test was used to determine the total number of strong acids in each oil sample. A bomb calorimeter was used to determine the energy content of each bio-oil sample. Calorimetry and particle sizing were also done on the meals, on “dried” samples and “as received” samples. Particle size distributions of ground and unground samples of the feedstocks were determined. The results from this study can be used to assess the possibilities of using Brassica carinata and Camelina sativa meals as viable biomass sources for producing bio-oil. This could add value to these meals by turning a by-product of the oil extraction process into a resource for production of bio-oil.Copyright

Experimental Investigation of Single-Phase Microjet Array Impingement Cooling

The heat transfer performance of two microjet arrays using degassed deionized water was investigated. The in-line jet arrays had a spacing of 250 μm, a standoff of 200 μm, and diameters of 54 and 112 μm. Average heat transfer coefficients were obtained for 150 < Red < 3300 and ranged from 80,000 to 414,000 W/m2 -K. A heat flux of 1,110 W/cm2 was attained with 23 °C water and a surface temperature of 50 °C.Copyright