Design of a microbreather for two-phase microchannel devices

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2008.This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.Includes bibliographical references (p. 50-52).Multiphase flows in microchannels are encountered in a variety of microfluidic applications. Two-phase microchannel heat sinks leverage the latent heat of vaporization to offer an efficient method of dissipating large heat fluxes in a compact device. In microscale methanol-based fuel cells, the chemical reactions produce a two-phase flow of methanol solution and carbon dioxide gas. Differences in the underlying physics between microscale and macroscale systems, however, provide a new set of challenges for multiphase microscale devices. In thermal management devices, large pressure fluctuations caused by the rapid expansion of vapor are prevalent in the flow channels. In fuel cells, the gaseous carbon dioxide blocks reaction sites. In both of these cases, dry-out is a problem that limits device performance. We propose a design for a microscale breather that uses surface chemistry and microstructures to separate gas from a liquid flow to improve two-phase microchannel performance. To better understand the physics and governing parameters of the proposed breather, we have designed and fabricated test devices that allow cross-sectional visualization of the breathing events. We have conducted various experiments to examine the effects of device channel hydraulic diameters ranging from 72 [mu]m to 340 [mu]m and liquid inlet flow rates ranging from 0.5 cm/s to 4 cm/s on the maximum gas removal rate. We demonstrated a maximum breather removal rate of 48.1 [mu]l/min through breather ports with a hydraulic diameter of 4.6 [mu]m connected to a microchannel with a hydraulic diameter of 72 [mu]m, and a liquid inlet flow velocity of 0.5 cm/s. A model was developed that accurately predicts the exponential dependence of the maximum gas removal rate on a non-dimensional ratio of the pressure across the breather ports compared to the pressure drop in the main channel caused by the venting bubble.(cont.) These results serve as design guidelines to aid in the development of more efficient and sophisticated breathing devices. The successful implementation of a microchannel with an efficient breather will allow for new technologies with higher heat removal capacities or chemical reaction rates that can be effectively used by industry.by Brentan R. Alexander.S.M

Coal fired power generation scheme with near-zero carbon dioxide emissions

Thesis (S.B.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2007.This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.Includes bibliographical references (leaves 91-93).Humans are releasing record amounts of carbon dioxide into the atmosphere through the combustion of fossil fuels in power generation plants. With mounting evidence that this carbon dioxide is a leading cause of global warming and with energy demand exploding, it is time to seek out realistic power production methods that do not pollute the environment with CO2 waste. The relative abundance and low cost of fossil fuels remains attractive and clean coal technologies are examined as a viable solution. This paper helps identify the many options currently available, including post-combustion capture, pre-combustion capture, and a number of oxy-fuel combustion schemes. One cycle design in particular, the Graz cycle, holds some promise as a future power generation cycle. A model of the Graz cycle developed in this paper predicts a cycle efficiency value of 56.72%, a value that does not account for efficiency losses in the liquefaction and sequestration of carbon dioxide, or the efficiency penalty associated with the gasification of coal. This high efficiency number, coupled with the low technological barriers of this cycle compared to similar schemes, is used as a justification for investigating this cycle further.(cont.) A sensitivity analysis is performed in order to identify key system parameters. Using this information, a computational optimization algorithm based on a simulated annealing scheme is devised and used to alter the parameters until an overall efficiency of 60.11% is achieved. Another optimization scheme which accounts for hardware limitations and plant capital costs is also discussed. This optimization yields a total efficiency of 58.76% while limiting the system high pressure to 110 bar. With such high efficiency values for this cycle, it is suggested that further study with more advanced models be conducted to better assess the viability of the Graz cycle as a clean technology.by Brentan R. Alexander.S.B