Confined bubble growth during flow boiling in a mini/micro-channel of rectangular cross-section Part I: Experiments and 1-D modelling

In this paper, a simultaneous visualization and measurement study have been carried out to investigate bubble nucleation frequency of water in micro-channel at various heat fluxes and mass fluxes. A single micro-channel with an identical rectangular cross-section having a hydraulic of 137 μm and a heating length of 30 mm was used in this experiment. It is shown that the frequency of bubble nucleation increased drastically with the increase of heat flux and was also strongly dependent on the mass flux. A dimensionless frequency of bubble nucleation was correlated in terms of the Boiling number. The predictions of bubble nucleation frequency in the microchannel are found in good agreement with experimental data with a MAE of 10.4%.

Download Full-text

Numerical Analysis of Vapor Bubble Growth and Wall Heat Transfer During Flow Boiling of Water in a Microchannel

Heat Transfer, Part B ◽

10.1115/imece2005-82365 ◽

2005 ◽

Author(s):

Abhijit Mukherjee ◽

Satish G. Kandlikar

Keyword(s):

Heat Transfer ◽

Cross Section ◽

Vapor Bubble ◽

Bubble Growth ◽

Stokes Equations ◽

Flow Boiling ◽

Navier Stokes ◽

Channel Cross Section ◽

Wall Heat Transfer ◽

Wall Superheat

The present study is performed to analyze the wall heat transfer mechanisms during growth of a vapor bubble inside a microchannel. The microchannel is of 200 μm square cross section and a vapor bubble begins to grow at one of the walls, with liquid coming in through the channel inlet. The complete Navier-Stokes equations along with continuity and energy equations are solved using the SIMPLER method. The liquid vapor interface is captured using the level set technique. The bubble grows rapidly due to heat transfer from the walls and soon turns into a plug filling the entire channel cross section. The average wall heat transfer at the channel walls is studied for different values of wall superheat and incoming liquid mass flux. The results show that the wall heat transfer increases with wall superheat but is almost unaffected by the liquid flow rate. The bubble growth is found to be the primary mechanism of increasing wall heat transfer as it pushes the liquid against the walls thereby influencing the thermal boundary layer development.

Download Full-text

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

ASME 2009 7th International Conference on Nanochannels, Microchannels and Minichannels ◽

10.1115/icnmm2009-82119 ◽

2009 ◽

Cited By ~ 3

Author(s):

S. Gedupudi ◽

Y. Q. Zu ◽

T. G. Karayiannis ◽

D. B. R. Kenning ◽

Y. Y. Yan

Keyword(s):

Aspect Ratio ◽

Cross Section ◽

High Aspect Ratio ◽

Transient Flow ◽

Flow Boiling ◽

Rectangular Cross Section ◽

Computing Time ◽

Pressure Fluctuations ◽

Design Tool ◽

D Model

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.

Download Full-text

Local Heat Transfer Analysis in a Single Microchannel with Boiling DI-Water and Correlations with Impedance Local Sensors

Energies ◽

10.3390/en13236473 ◽

2020 ◽

Vol 13 (23) ◽

pp. 6473

Author(s):

Mohammadmahdi Talebi ◽

Sahba Sadir ◽

Manfred Kraut ◽

Roland Dittmeyer ◽

Peter Woias

Keyword(s):

Heat Transfer ◽

Heat Transfer Coefficient ◽

Transfer Coefficient ◽

Cross Section ◽

Flow Boiling ◽

Rectangular Cross Section ◽

Local Heat Transfer ◽

Local Heat ◽

Experimental Conditions ◽

Impedance Measurements

Determination of local heat transfer coefficient at the interface of channel wall and fluid was the main goal of this experimental study in microchannel flow boiling domain. Flow boiling heat transfer to DI-water in a single microchannel with a rectangular cross section was experimentally investigated. The rectangular cross section dimensions of the experimented microchannel were 1050 μm × 500 μm and 1500 μm × 500 μm. Experiments under conditions of boiling were performed in a test setup, which allows the optical and local impedance measurements of the fluids by mass fluxes of 22.1 kg·m−2·s−1 to 118.8 kg·m−2·s−1 and heat fluxes in the range of 14.7 kW·m−2 to 116.54 kW·m−2. The effect of the mass flux, heat flux, and flow pattern on flow boiling local heat transfer coefficient and pressure drop were investigated. Experimental data compared to existing correlations indicated no single correlation of good predictive value. This was concluded to be the case due to the instability of flow conditions on one hand and the variation of the flow regimes over the experimental conditions on the other hand. The results from the local impedance measurements in correlation to the optical measurements shows the flow regime variation at the experimental conditions. From these measurements, useful parameters for use in models on boiling like the 3-zone model were shown. It was shown that the sensing method can shed a precise light on unknown features locally in slug flow such as residence time of each phases, bubble frequency, and duty cycle.

Download Full-text

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

ASME 2009 7th International Conference on Nanochannels, Microchannels and Minichannels ◽

10.1115/icnmm2009-82118 ◽

2009 ◽

Author(s):

Y. Q. Zu ◽

S. Gedupudi ◽

Y. Y. Yan ◽

T. G. Karayiannis ◽

D. B. R. Kenning

Keyword(s):

Heat Transfer ◽

Numerical Simulation ◽

Fluid Mechanics ◽

Cross Section ◽

Flow Boiling ◽

Rectangular Cross Section ◽

Pressure Fluctuations ◽

High Heat ◽

Inlet Flow ◽

Bubble Liquid

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/m2K for incompressible and compressible inlet flow conditions. The inlet pressure was about 1.12 bar and the mass flux was 747.5 kg/m2s 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.

Download Full-text

NONLINEAR THIN-WALLED BEAMS WITH A RECTANGULAR CROSS-SECTION — PART I

Mathematical Models and Methods in Applied Sciences ◽

10.1142/s0218202511500163 ◽

2012 ◽

Vol 22 (03) ◽

pp. 1150016 ◽

Cited By ~ 14

Author(s):

LORENZO FREDDI ◽

MARIA GIOVANNA MORA ◽

ROBERTO PARONI

Keyword(s):

Cross Section ◽

Elastic Energy ◽

Rectangular Cross Section ◽

Three Dimensional ◽

Scaling Factor ◽

One Dimensional ◽

Thin Walled ◽

The Cross ◽

Limit Models ◽

Cross Section Part

Our aim is to rigorously derive a hierarchy of one-dimensional models for thin-walled beams with rectangular cross-section, starting from three-dimensional nonlinear elasticity. The different limit models are distinguished by the different scaling of the elastic energy and of the ratio between the sides of the cross-section. In this paper we report the first part of our results. More precisely, denoting by h and δh the length of the sides of the cross-section, with δh ≪ h, and by [Formula: see text] the scaling factor of the bulk elastic energy, we analyze the cases in which δh/εh → 0 (subcritical) and δh/εh → 1 (critical).

Download Full-text