2024-03-29T15:26:25Zhttps://keep.lib.asu.edu/oai/requestoai:keep.lib.asu.edu:node-1497852021-08-30T18:53:49Zoai_pmh:all149785
https://hdl.handle.net/2286/R.I.9081
http://rightsstatements.org/vocab/InC/1.0/
All Rights Reserved
2011
ix, 54 p. : ill. (some col.)
Masters Thesis
Academic theses
Text
eng
Miner, Mark
Phelan, Patrick E
Herrmann, Marcus
Chen, Kangping
Arizona State University
Partial requirement for: M.S., Arizona State University, 2011
Includes bibliographical references (p. 50-54)
Field of study: Mechanical engineering
Microchannel heat sinks can possess heat transfer characteristics unavailable in conventional heat exchangers; such sinks offer compact solutions to otherwise intractable thermal management problems, notably in small-scale electronics cooling. Flow boiling in microchannels allows a very high heat transfer rate, but is bounded by the critical heat flux (CHF). This thesis presents a theoretical-numerical study of a method to improve the heat rejection capability of a microchannel heat sink via expansion of the channel cross-section along the flow direction. The thermodynamic quality of the refrigerant increases during flow boiling, decreasing the density of the bulk coolant as it flows. This may effect pressure fluctuations in the channels, leading to nonuniform heat transfer and local dryout in regions exceeding CHF. This undesirable phenomenon is counteracted by permitting the cross-section of the microchannel to increase along the direction of flow, allowing more volume for the vapor. Governing equations are derived from a control-volume analysis of a single heated rectangular microchannel; the cross-section is allowed to expand in width and height. The resulting differential equations are solved numerically for a variety of channel expansion profiles and numbers of channels. The refrigerant is R-134a and channel parameters are based on a physical test bed in a related experiment. Significant improvement in CHF is possible with moderate area expansion. Minimal additional manufacturing costs could yield major gains in the utility of microchannel heat sinks. An optimum expansion rate occurred in certain cases, and alterations in the channel width are, in general, more effective at improving CHF than alterations in the channel height. Modest expansion in height enables small width expansions to be very effective.
Mechanical Engineering
critical heat flux
cross-section
divergent channel
flow boiling
Heat Transfer
microchannel
Heat flux--Mathematical models.
Heat flux
Water--Boiling--Mathematical models.
Water
Heat sinks (Electronics)--Mathematical models.
Heat sinks (Electronics)
Heat--Transmission--Mathematical models.
A theoretical analysis of microchannel flow boiling enhancement via cross-sectional expansion