G. P. Peterson

Experimental investigation of temperature and volume fraction variations on the effective thermal conductivity of nanoparticle suspensions (nanofluids)

An experimental investigation was conducted to examine the effects of variations in the temperature and volume fraction on the steady-state effective thermal conductivity of two different nanoparticle suspensions. Copper and aluminum oxide, CuO and Al2O3, nanoparticles with area weighted diameters of 29 and 36nm, respectively, were blended with distilled water at 2%, 4%, 6%, and 10% volume fractions and the resulting suspensions were evaluated at temperatures ranging from 27.5to34.7°C. The results indicate that the nanoparticle material, diameter, volume fraction, and bulk temperature, all have a significant impact on the effective thermal conductivity of these suspensions. The 6% volume fraction of CuO nanoparticle/distilled water suspension resulted in an increase in the effective thermal conductivity of 1.52 times that of pure distilled water and the 10% Al2O3 nanoparticle/distilled water suspension increased the effective thermal conductivity by a factor of 1.3, at a temperature of 34°C. A two-factor ...

Convective heat transfer and flow friction for water flow in microchannel structures

Abstract The single-phase forced convective heat transfer and flow characteristics of water in microchannel structures/plates with small rectangular channels having hydraulic diameters of 0.133 – 0.367 mm and distinct geometric configurations were investigated experimentally. The results indicated that the geometric configuration had a significant effect on the single-phase convective heat transfer and flow characteristics. The laminar heat transfer was found to be dependent upon the aspect ratio and the ratio of the hydraulic diameter to the center-to-center distance of the microchannels. The turbulent heat transfer was found to be a further function of a new dimensionless variable, Z, such that Z = 0.5 will be the optimum configuration for turbulent heat transfer regardless of the groove aspect ratio. The friction factor or flow resistance reached a minimum value as Z approaches 0.5. The turbulent flow resistance was usually smaller than that predicted by classical relationships, and the Reynolds number for flow transition to fully developed turbulent flow became much smaller than the ordinary channel flow. Empirical correlations were suggested for calculating both the heat transfer and pressure drop.

Nanostructured Copper Interfaces for Enhanced Boiling

Phase change through boiling is used in a variety of heat-transfer and chemical reaction applications. The state of the art in nucleate boiling has focused on increasing the density of bubble nucleation using porous structures and microchannels with characteristic sizes of tens of micrometers. Traditionally, it is thought that nanoscale surfaces will not improve boiling heat transfer, since the bubble nucleation process is not expected to be enhanced by such small cavities. In the experiments reported here, we observed unexpected enhancements in boiling performance for a nanostructured copper (Cu) surface formed by the deposition of Cu nanorods on a Cu substrate. Moreover, we observed striking differences in the dynamics of bubble nucleation and release from the Cu nanorods, including smaller bubble diameters, higher bubble release frequencies, and an approximately 30-fold increase in the density of active bubble nucleation sites. It appears that the ability of the Cu surface with nanorods to generate stable nucleation of bubbles at low superheated temperatures results from a synergistic coupling effect between the nanoscale gas cavities (or nanobubbles) formed within the nanorod interstices and micrometer-scale defects (voids) that form on the film surface during nanorod deposition. For such a coupled system, the interconnected nanoscale gas cavities stabilize (or feed) bubble nucleation at the microscale defect sites. This is distinct from conventional-scale boiling surfaces, since for the nanostructured surface the bubble nucleation stability is provided by features with orders-of-magnitude smaller scales than the cavity-mouth openings.

FRICTIONAL FLOW CHARACTERISTICS OF WATER FLOWING THROUGH RECTANGULAR MICROCHANNELS

Experiments were conducted to investigate the flow characteristics of water flowing through rectangular microchannels having hydraulic diameters of 0.133-0.367 mm and H/W ratios of 0.333-1. Experimental results indicated that the laminar flow transition occurred at Reynolds numbers of 200-700. This critical Re for the laminar transition was strongly affected by the hydraulic diameter, decreasing with corresponding decreases in the microchannel. In addition, the size of the transition range was diminished and fully developed turbulent flow also occurred at much lower Re. The friction behavior of both the laminar and turbulent flow was found to depart from the classical thermqfluid correlations. lite friction factor, f, was found to be proportional to Re−1.98 rather than Re for the laminar condition, and proportional to Re−1.72i for turbulent flow. The geometric parameters, hydraulic diameter, and H/W were found to be the most important parameters and had a critical effect on the flow. Generally, increasing...

Thermal Control of Electronic Equipment and Devices

Publisher Summary Thermal control of electronic components has one principal objective, to maintain relatively constant component temperature equal to or below the manufacturers maximum specified service temperature, typically between 85 and 100°C. It is noted that even a single component operating 10°C beyond this temperature can reduce the reliability of certain systems by as much as 50%. Therefore, it is important for the new thermal control schemes to be capable of eliminating hot spots within the electronic devices, removing heat from these devices and dissipating this heat to the surrounding environment. Several strategies have developed over the years for controlling and removing the heat generated in multichip modules, which include advanced air-cooling schemes, direct cooling, and miniature thermosyphons or free-falling liquid films. The chapter summarizes analytical, numerical, and experimental work in literature, in order to facilitate the improvement of existing schemes and provide a basis for the development of new ones. The chapter focuses on investigations performed over the past decade and includes information on the thermal control of semiconductor devices, modules, and total systems.

The effect of particle size on the effective thermal conductivity of Al2O3-water nanofluids

A steady-state method was used to evaluate the effective thermal conductivity of Al2O3∕distilled water nanofluids with nanoparticle diameters of 36 and 47nm. Tests were conducted over a temperature range of 27–37°C for volume fractions ranging from 0.5% to 6.0%. The thermal conductivity enhancement of the two nanofluids demonstrated a nonlinear relationship with respect to temperature, volume fraction, and nanoparticle size, with increases in the volume fraction, temperature, and particle size all resulting in an increase in the measured enhancement. The most significant finding was the effect that variations in particle size had on the effective thermal conductivity of the Al2O3∕distilled water nanofluids. The largest enhancement difference observed occurred at a temperature of approximately 32°C and at a volume fraction of between 2% and 4%. The experimental results exhibited a peak in the enhancement factor in this range of volume fractions for the temperature range evaluated, which implies that an opt...

Experimental Investigations on the Effects of Cerium Oxide Nanoparticle Fuel Additives on Biodiesel

This paper reports the results of experimental investigations on the influence of the addition of cerium oxide in the nanoparticle form on the major physicochemical properties and the performance of biodiesel. The physicochemical properties of the base fuel and the modified fuel formed by dispersing the catalyst nanoparticles by ultrasonic agitation are measured using ASTM standard test methods. The effects of the additive nanoparticles on the individual fuel properties, the engine performance, and emissions are studied, and the dosing level of the additive is optimized. Comparisons of the performance of the fuel with and without the additive are also presented. The flash point and the viscosity of biodiesel were found to increase with the inclusion of the cerium oxide nanoparticles. The emission levels of hydrocarbon and NOx are appreciably reduced with the addition of cerium oxide nanoparticles.

Experimental investigation of heat transfer in flat plates with rectangular microchannels

Abstract An experimental investigation was conducted to determine the heat transfer characteristics and cooling performance of rectangular-shaped microgrooves machined into stainless steel plates. Using methanol as the cooling fluid, grooves with different aspect ratios and a variety of center-to-center spacings were evaluated. The influence of liquid velocity, subcooling, property variations and microchannel geometric configuration on the heat transfer behavior, cooling performance, and heat transfer and liquid flow mode transition were analyzed experimentally. Measurements made to clarify the flow nucleate boiling attributes indicated an increased heat transfer rate and a behavior that was quite different from what typically occurs in larger tubes or channels, due to the relatively large portion of the surface area associated with the thin-film region.

Experimental Investigation of Micro Heat Pipes Fabricated in Silicon Wafers

An experimental investigation was conducted to determine the thermal behavior of arrays of micro heat pipes fabricated in silicon wafers. Two types of micro heat pipe arrays were evaluated, one that utilized machined rectangular channels and the other that used an anisotropic etching process to produce triangular channels. Once fabricated, a clear pyrex cover plate was bonded to the top surface of each wafer using an ultraviolet bonding technique to form the micro heat pipe array. These micro heat pipe arrays were then evacuated and charged with a predetermined amount of methanol. Using an infrared thermal imaging unit, the temperature gradients and maximum localized temperatures were measured and an effective thermal conductivity was computed. The experimental results were compared with those obtained for a plain silicon wafer

Parametric Study of Pool Boiling on Horizontal Highly Conductive Microporous Coated Surfaces

To better understand the mechanisms that govern the behavior of pool boiling on horizontal highly conductive microporous coated surfaces, a series of experimental investigations were designed to systematically examine the effects of the geometric dimensions (i.e., coating thickness, volumetric porosity, and pore size, as well as the surface conditions of the porous coatings) on the pool-boiling performance and characteristics. The study was conducted using saturated distilled water at atmospheric pressure (101 kPa) and porous surfaces fabricated from sintered isotropic copper wire screens. For nucleate boiling on the microporous coated surfaces, two vapor ventilation modes were observed to exist: (i) upward and (ii) mainly from sideways leakage to the unsealed sides and partially from the center of porous surfaces. The ratio of the heater size to the coating thickness, the friction factor of the two-phase flow to single-phase flow inside the porous coatings, as well as the input heat flux all govern the vapor ventilation mode that occurs. In this investigation, the ratio of heater size to coating thickness varies from 3.5 to 38 in order to identify the effect of heater size on the boiling characteristics. The experimental results indicate that the boiling performance and characteristics are also strongly dependent on the volumetric porosity and mesh size, as well as the surface conditions when the heater size is given. Descriptions and discussion of the typical boiling characteristics; the progressive boiling process, from pool nucleate boiling to film boiling; and the boiling performance curves on conductive microporous coated surfaces are all systematically presented.