Showing papers by "Sateesh Gedupudi published in 2009"

1-D Modelling and 3-D Simulation of Confined Bubble Formation and Pressure Fluctuations During Flow Boiling in a Microchannel With a Rectangular Cross-Section of High Aspect Ratio

Abstract: A simple 1-D model with low requirements for computing time is required to investigate parametric influences on the potentially adverse effects of pressure fluctuations driven by confined vapour bubble growth in microchannel evaporative cooling systems operating at high heat fluxes. A model is developed in this paper for the particular conditions of a channel of rectangular cross-section with high aspect ratio with a constant inlet flow rate (zero upstream compressibility). (The model will later be extended to the conditions of finite upstream compressibility that lead to transient flow reversal). Some parametric trends predicted by the model are presented. The simplifying assumptions in the model are examined in the light of a 3-D simulation by a commercial CFD code, described in an accompanying paper by the same authors. The predictions of pressure changes are in reasonable agreement. It is suggested that the 1-D model will be a useful design tool.Copyright © 2009 by ASME

Modelling pressure fluctuations during flow boiling in microchannels with inlet compressibility and resistance

Abstract: Confined bubble growth during flow boiling at low pressures in microchannels generates pressure fluctuations that may cause transient flow reversals that disturb the flow distribution in heat sinks formed of parallel channels joined by plena. A simple model is developed for the effects of upstream compressibility and flow resistance at the channel inlet on the magnitude of the pressure transient during the growth of one bubble in a single channel. Preliminary results are presented.

Numerical Simulation and Experimental Observations of Confined Bubble Growth During Flow Boiling in a Microchannel With Rectangular Cross-Section of High Aspect Ratio

Abstract: Bubble nucleation and growth to confinement during flow boiling in microchannels lead to high heat transfer coefficients. They may also create pressure fluctuations that change the superheat driving evaporation and cause flow reversals that promote transient dry-out and uneven distribution of flow between parallel channels. The work described in this paper is part of a programme to develop models for these processes that will aid the design of evaporative cooling systems for devices operating at high heat fluxes. Video observations of water boiling in a single copper channel of rectangular cross-section, 0.38 × 1.6 mm and a heated length 40 mm, were performed. The top side of the channel was a glass window. Results are presented for a heat flux, averaged over the area of the three metal sides, of 210 and 173 W/m2 K for incompressible and compressible inlet flow conditions. The inlet pressure was about 1.12 bar and the mass flux was 747.5 kg/m2 s for both conditions examined. The results demonstrated the strong influence of compressibility on the mode of bubble detachment and growth and therefore on flow patterns, pressure fluctuations and heat transfer rates. The fluid mechanics of boiling in this size channel were also successfully investigated by 3-D numerical simulation for bubbles growing at a defined rate with a fixed inlet flow rate using the 3-D CFD code FLUENT 6 (no upstream compressibility). The study examined the fluid mechanics of bubble motion with heat transfer, but the mass transfer across the bubble-liquid interface was not simulated in the present work. A small vapour bubble was injected at the wall to ensure the bubble generation is under a quasi nucleation condition. Its growth was driven by an internal source of vapour, at a rate derived by analysis of the experimental measurements of growth. The simulation reproduced well the observed motion and shape of the bubble. The simulation was then extended to model bubbles generated and growing randomly in a 2-D channel.Copyright © 2009 by ASME

1-D modelling of pressure fluctuations due to confined bubble growth during flow boiling in a microchannel

Abstract: A simple 1-D model is presented to study the effects of the wall heat flux, location of the nucleation site, channel dimensions and fluid properties on the pressure fluctuations caused by confined bubble growth in rectangular microchannels, for zero upstream compressibility. A simple isothermal compressibility model is then used to study the effects of the initial volume of non-condensable gas trapped in the upstream plenum, initial inlet velocity and nucleation site location on local pressure fluctuations and transient flow reversal. Both acceleration pressure drop and viscous pressure drop are considered.