Design Considerations for the Effects of Liquid Compressibility in Microchannel Flow Boiling

2006 ◽  
Author(s):  
David W. Fogg ◽  
Ken E. Goodson
Keyword(s):  
Single Channel ◽  
Flow Boiling ◽  
Vapor Bubbles ◽  
Confined Geometries ◽  
Local Pressure ◽  
Eulerian Model ◽  
Forced Convective Flow ◽  
Pressure Perturbations ◽  
Vapor Formation ◽  
Chip Geometry

Forced convective flow boiling in microchannels is characterized by the nucleation and rapid growth of vapor bubbles in confined geometries. Confined boiling flows are highly transient yielding periods of rapid vapor formation followed by a refilling of the channel with liquid. This behavior in a single microchannel with constant flow rates can only be correlated with the growth of single bubbles in the channel. Using a one-dimensional Lagrangrian-Eulerian model, Fogg and Goodson [1] showed that reflections of these pressure waves create local pressure depressions that may trigger nucleation at temperatures not predicted by incompressible analysis. This study extends the work of Fogg and Goodson [1] by examining the influence of channel and chip geometry on the propagation of pressure perturbations within microchannels. A set of equations are proposed to estimate the amplitude of the initial pulse and its evolution through various geometries such as converging/diverging channels and sudden expansions/contractions. Simulations of two single channel experimental structures show that the flow delivery condition plays a minimal role in the reflection and propagation of pressure perturbations and that channel design may impact the nucleation characteristics of microchannels.

Considerations about mass and energy flow in discontinuous evaporating crystallizers

2016 ◽  
pp. 514-516
Author(s):  
Martin Bruhns
Keyword(s):  
Energy Flow ◽  
Static Pressure ◽  
Bubble Formation ◽  
Boiling Temperature ◽  
Vapor Bubbles ◽  
Local Pressure ◽  
Heating Steam ◽  
Flow Changes ◽  
Local Superheating ◽  
Local Supersaturation

The massecuite circulates in a loop within the evaporating crystallizing vessel. The massecuite flows upwards through the heating tubes. In the room above the calandria the massecuite flow changes its direction to radial inwards and then to vertical downwards. An impeller in the central tube forces the circulation. Below the calandria the main direction of flow is radially outwards until threads of the massecuite stream enter the heating tubes in upwards direction. Within the tubes heat is transferred to the massecuite. At low temperature differences between heating steam and massecuite and higher levels of the massecuite in the crystallizer vapor bubbles are not found in the tubes. Vapor bubbles can be formed at a massecuite level in the crystallizer where the temperature of the massecuite is higher than the local boiling temperature of water, which depends on the local pressure (including the static pressure of the massecuite at this point) and the boiling point elevation of the mother liquor. The surface tension of the liquid is a resistance against the bubble formation, which has to be overcome by the local superheating i.e. the part of the enthalpy of the massecuite exceeding the local boiling temperature. The formation and the flow of the bubbles change the density of the massecuite/bubbles mixture and has an influence on the massecuite flow. The formation of a vapour bubble is connected with a local drop of the massecuite temperature which changes the local supersaturation. Today the heat transfer into the magma is quite well known but the process of bubble formation is quite unknown. Some basic considerations about the formation of bubbles and its influence on local supersaturation based on calculation of heat and mass balances and models of bubble formation are be given and discussed. Experiments for basic investigations are proposed.

Numerical Study of the Effect of Inlet Constriction on Bubble Growth During Flow Boiling in Microchannels

2005 ◽  
Author(s):  
Abhijit Mukherjee ◽  
Satish G. Kandlikar
Keyword(s):  
Stokes Equations ◽  
Numerical Study ◽  
Flow Boiling ◽  
Navier Stokes ◽  
Channel Cross Section ◽  
Vapor Bubbles ◽  
Reversed Flow ◽  
Cross Sectional ◽  
Parallel Microchannels ◽  
Energy Equations

Flow boiling through microchannels is characterized by nucleation of vapor bubbles on the channel walls and their rapid growth as they fill the entire channel cross-section. In parallel microchannels connected through a common header, formation of vapor bubbles often results in flow maldistribution that leads to reversed flow in certain channels. The reversed flow is detrimental to the heat transfer and leads to early CHF condition. One way of eliminating the reversed flow is to incorporate flow restrictions at the channel inlet. In the present numerical study, a nucleating vapor bubble placed near the restricted end of a microchannel is numerically simulated. 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 results show that with no restriction the bubble moves towards the nearest channel outlet, whereas in the presence of a restriction, the bubble moves towards the distant but unrestricted end. It is proposed that channels with increasing cross-sectional area may be used to promote unidirectional growth of the vapor plugs and prevent reversed flow.

scholarly journals Upscaled phase-field models for interfacial dynamics in strongly heterogeneous domains

2012 ◽  
Vol 468 (2147) ◽  
pp. 3705-3724 ◽  
Author(s):  
Markus Schmuck ◽  
Marc Pradas ◽  
Grigorios A. Pavliotis ◽  
Serafim Kalliadasis
Keyword(s):  
Porous Media ◽  
Free Energy ◽  
Phase Field ◽  
Single Channel ◽  
Confined Geometries ◽  
Science And Engineering ◽  
Hilliard Equation ◽  
Phase Field Models ◽  
Cahn Hilliard Equation ◽  
Double Well Potential

We derive a new, effective macroscopic Cahn–Hilliard equation whose homogeneous free energy is represented by fourth-order polynomials, which form the frequently applied double-well potential. This upscaling is done for perforated/strongly heterogeneous domains. To the best knowledge of the authors, this seems to be the first attempt of upscaling the Cahn–Hilliard equation in such domains. The new homogenized equation should have a broad range of applicability owing to the well-known versatility of phase-field models. The additionally introduced feature of systematically and reliably accounting for confined geometries by homogenization allows for new modelling and numerical perspectives in both science and engineering. Our results are applied to wetting dynamics in porous media and to a single channel with strongly heterogeneous walls.

scholarly journals Differential responsiveness of glabrous and non-glabrous skin to local transmural pressure elevations; the impact of 5 weeks of iterative local pressure loading

2021 ◽  
Author(s):  
Michail E. Keramidas ◽  
Roger Kölegård ◽  
Patrik Sundblad ◽  
Håkan Sköldefors ◽  
Ola Eiken
Keyword(s):  
Transmural Pressure ◽  
Glabrous Skin ◽  
Local Pressure ◽  
Wide Range ◽  
Forearm Skin ◽  
Differential Responsiveness ◽  
Pressure Resistance ◽  
The Impact ◽  
Pressure Perturbations

We examined the in vivo pressure-flow relationship in human cutaneous vessels during acute and repeated elevations of local transmural pressure. In 10 healthy men, red blood cell flux was monitored simultaneously on the non-glabrous skin of the forearm and the glabrous skin of a finger during a vascular pressure provocation, wherein the blood vessels of an arm were exposed to a wide range of stepwise increasing distending pressures. Forearm skin blood flux was relatively stable at slight and moderate elevations of distending pressure, whereas it increased ~3-4-fold at the highest levels (P = 0.004). Finger blood flux on the contrary, dropped promptly and consistently throughout the provocation (P < 0.001). Eight of the subjects repeated the provocation trial after a 5-week pressure-training regimen, during which the vasculature in one arm was exposed intermittently (40 min, 3 times･week-1) to increased transmural pressure (from +65 mmHg week-1 to +105 mmHg week-5). The training regimen diminished the pressure-induced increase in forearm blood flux by ~34% (P = 0.02), whereas it inhibited the reduction in finger blood flux (P < 0.001) in response to slight and moderate distending pressure elevations. The present findings demonstrate that, during local pressure perturbations, the cutaneous autoregulatory function is accentuated in glabrous compared to in the non-glabrous skin regions. Prolonged intermittent regional exposures to augmented intravascular pressure blunt the responsiveness of the glabrous skin, but enhance arteriolar pressure resistance in the non-glabrous skin.

An investigation of horizontal and vertical flow boiling in a single channel with a confinement number beyond the threshold of micro-scale flow

2021 ◽  
Vol 33 (11) ◽  
pp. 113302
Author(s):  
Sira Saisorn ◽  
Adirek Suriyawong ◽  
Pochai Srithumkhant ◽  
Pakorn Wongpromma ◽  
Somchai Wongwises
Keyword(s):  
Single Channel ◽  
Flow Boiling ◽  
Vertical Flow ◽  
Micro Scale

Experimental Evaluation of Pressure Drop Elements and Fabricated Nucleation Sites for Stabilizing Flow Boiling in Minichannels and Microchannels

2005 ◽  
Author(s):  
Satish G. Kandlikar ◽  
Daniel A. Willistein ◽  
John Borrelli
Keyword(s):  
Pressure Drop ◽  
Experimental Evaluation ◽  
Flow Boiling ◽  
Superheated Liquid ◽  
Working Fluid ◽  
Vapor Bubbles ◽  
Liquid Environment ◽  
Nucleation Sites ◽  
Boiling Process ◽  
Flow Configurations

The flow boiling process suffers from severe instabilities induced due to nucleation of vapor bubbles in a minichannel or a microchannel in a superheated liquid environment. In an effort to improve the flow boiling stability, several modifications are introduced and experiments are performed on 1054 × 197 μm microchannels with water as the working fluid. The cavity sizes and local liquid and wall conditions required at the onset of nucleation are analyzed. The effects of an inlet pressure restrictor and fabricated nucleation sites are evaluated as a means of stabilizing the flow boiling process and avoiding the backflow phenomena. The results are compared with the unrestricted flow configurations in smooth channels.

Closed-Loop Flow Boiling Module With a Plated Heat Sink

2007 ◽  
Author(s):  
Hitoshi Sakamoto ◽  
Kazuyuki Mikubo
Keyword(s):  
Heat Transfer ◽  
Flow Boiling ◽  
Working Fluid ◽  
Cold Plate ◽  
Vapor Bubbles ◽  
Cross Sectional ◽  
Heat Rejection ◽  
Attractive Option ◽  
Cooling Module ◽  
Water Holding

A compact flow boiling module was developed for cooling a 100-W class package of about one-inch square in size. The cold plate, where heat is transferred from the package was made with a porous plating inside to augment boiling heat transfer. Heat transfer increased by a maximum of 50 percent when an organic refrigerant HFE-7100 was used, while the conditions for heat rejection to the ambient were kept unchanged. The heat rejection was achieved with an 80-mm fan with a matching corrugated fin radiator, whose effectiveness limits the overall size of the cooling module. The microscopic structure in the cold plate negatively influenced boiling of water, holding large patches of vapor bubbles on the surface. When the convective effect was increased by decreasing the cross sectional area of the channel by 10 times, heat transfer was further augmented approximately by 2 folds, making the use of the organic refrigerant an attractive option as the working fluid.

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