Enhanced Cooling via Boiling in Porous Layers: The Effect of Vapor Channels

1999 ◽  
Vol 121 (1) ◽  
pp. 205-210 ◽  
Author(s):  
A. K. Stubos ◽  
J.-M. Buchlin
Keyword(s):  
Heat Transfer ◽  
Porous Layer ◽  
Heated Surface ◽  
Heat Removal ◽  
Vapor Film ◽  
Enhanced Heat Transfer ◽  
Porous Layers

Enhanced heat transfer via boiling in a porous layer covering the heated surface is considered analytically. The effect of vapor channels traversing the porous layer on the power for which a vapor film on the heated surface occurs, is examined by comparing the heat removal capability of the system with and without channels. A significant increase of the attained coolability level is obtained theoretically leading to the possibility of new design configurations for the cooling of intensely heated surfaces.

Numerical Investigation of a Liquid Droplet Impinging on a Heated Surface

2009 ◽  
Author(s):  
Andres Diaz ◽  
Alfonso Ortega ◽  
Ryan Anderson
Keyword(s):  
Heat Transfer ◽  
Numerical Investigation ◽  
Fundamental Problem ◽  
Water Drop ◽  
Liquid Droplet ◽  
Heated Surface ◽  
Heat Removal ◽  
Droplet Spreading ◽  
Droplet Shape ◽  
Spreading Dynamics

Previous studies, most of them experimental, reveal that the cooling effectiveness of a water drop impinging on a heated surface depends on the wall temperature, droplet shape and velocity. All previous studies focus on the behavior of a droplet falling in a quiescent environment, such as still air. Evidence in the literature also shows that gas assisted droplet sprays, in which a gas phase propels the droplets, are more efficient in heat removal than sprays consisting of droplets alone. It is conjectured that this is due to an increase in the maximum droplet spreading diameter upon impact, a thinner film, and consequently an increase in the overall heat transfer coefficient. Recent experiments in the author’s group [1, 2] show that the carrier gas jet strongly influences droplet spreading dynamics by imposing normal and shear forces on the liquid surface. The heat transfer is greatly augmented in the process, compared to a free falling droplet. To date, there has been no fundamental investigation of the physics of gas assisted spray cooling. To begin to understand the complicated process, this paper reports on a fundamental problem of a single liquid droplet that impinges on a heated surface. This paper contributes a numerical investigation of the problem using the volume of fluid (VOF) technique to capture droplet spreading dynamics and heat transfer in a single drop event. The fluid mechanics is investigated and compared to the experimental data. The greatest uncertainty in the simulation is in the specification of the contact angle of the advancing or receding liquid front, and in capturing the onset of the three-dimensional fingering phenomena.

Reduction of Gaseous Emission From Turbine Combustor

2005 ◽  
Author(s):  
R. S. Amano ◽  
J. Xie ◽  
Shyam Singh ◽  
R. E. Peck
Keyword(s):  
Heat Transfer ◽  
Porous Layer ◽  
Operating Conditions ◽  
Ceramic Foam ◽  
Spray Combustion ◽  
Hydrocarbon Emissions ◽  
Flame Zone ◽  
Enhanced Heat Transfer ◽  
Firing Rates ◽  
Unburned Hydrocarbon

A study of spray combustion with porous inserts was performed using an on-axis fuel used in a concentric Jet-A. Combustion performance was evaluated by measuring exhaust emissions and gaseous temperatures for different operating conditions with and without ceramic foam inserts. The results indicated that the enhanced heat transfer in the flame zone could reduce nitrogen oxides and unburned hydrocarbon emissions. Placing a second porous layer downstream could yield further reductions in both emissions. The results for different firing rates and equivalence ratios revealed the residence time in the porous layer is an important factor in controlling the combustor performance.

CHARACTERISTICS OF EVAPORATION HEAT TRANSFER ON HEATED SURFACE WITH THIN POWDER POROUS LAYER

2013 ◽  
Vol 4 (3) ◽  
pp. 241-250
Author(s):  
Shigeki Hirasawa ◽  
Y. Takeuchi ◽  
Tsuyoshi Kawanami ◽  
Katsuaki Shirai ◽  
M. Nakatsuka ◽  
...  
Keyword(s):  
Heat Transfer ◽  
Porous Layer ◽  
Heated Surface ◽  
Evaporation Heat ◽  
Evaporation Heat Transfer

Unsteady CFD Analysis of Turbulent Flow and Heat Transfer in a Gas Turbine Blade Trailing Edge Subjected to Rotation

2012 ◽  
Author(s):  
Domenico Borello ◽  
Giovanni Delibra ◽  
Cosimo Bianchini ◽  
Antonio Andreini
Keyword(s):  
Heat Transfer ◽  
Turbulent Flow ◽  
Gas Turbine ◽  
Turbine Blade ◽  
Heated Surface ◽  
Heat Removal ◽  
Gas Turbine Blade ◽  
Trailing Edge ◽  
Flow And Heat Transfer ◽  
The Impact

Internal cooling of gas turbine blade represents a challenging task involving several different phenomena as, among others, highly three-dimensional unsteady fluid flow, efficient heat transfer and structural design. This paper focuses on the analysis of the turbulent flow and heat transfer inside a typical wedge–shaped trailing edge cooling duct of a gas turbine blade. In the configuration under scrutiny the coolant flows inside the duct in radial direction and it leaves the blade through the trailing edge after a 90 deg turning. At first an analysis of the flow and thermal fields in stationary conditions was carried out. Then the effects of rotational motion were investigated for a rotation number of 0.275. The rotation axis here considered is normal to the inflow and outflow bulk velocity, representing schematically a highly loaded blade configuration. The work aimed to i) analyse the dynamic of the vortical structures under the influence of strong body forces and the constraints induced by the internal geometry and ii) to study the impact of such motions on the mechanisms of heat removal. The final aim was to verify the design of the equipment and to detect the possible presence of regions subjected to high thermal loads. The analysis is carried out using the well assessed open source code OpenFOAM written in C++ and widely validated by several scientists and researchers around the world. The unsteadiness of the flow inside the trailing edge required to adopt models that accurately reconstructed the flow field. As the computational costs associated to LES (especially in the near wall regions) largely exceed the available resources, we chose for the simulation the SAS model of Menter, that was validated in a series of benchmark and industrially relevant test cases and allowed to reconstruct a part of the turbulence spectra through a scale-adaptive mechanism. Assessment of the obtained results with steady-state k-ω SST computations and available experimental results was carried out. The present analysis demonstrated that a strong unsteadiness develops inside the trailing edge and that the rotation generated strong secondary motions that enhanced the dynamic of heat removal, leading to a less severe temperature distribution on the heated surface w.r.t the non rotating case.

scholarly journals Bubble Enhanced Heat Transfer from a Vertical Heated Surface

2008 ◽  
Vol 15 (2) ◽  
pp. 159-169 ◽  
Author(s):  
Brian Donnelly ◽  
TADHG S. O'Donovan ◽  
Darina B. Murray
Keyword(s):  
Heat Transfer ◽  
Heated Surface ◽  
Enhanced Heat Transfer

Imaging Measurements in Nano-Particle Enhanced Spray Cooling

2010 ◽  
Author(s):  
J. Torres ◽  
A. Perdones ◽  
A. Garcia ◽  
F. J. Diez
Keyword(s):  
Heat Transfer ◽  
Heat Flux ◽  
High Speed ◽  
Heated Surface ◽  
Heat Removal ◽  
Thermal Control ◽  
Spray Cooling ◽  
Nano Particles ◽  
Copper Block ◽  
Alumina Oxide

Thermal control is a major constraint in spacecraft development as increased demand on electronics performance requires large heat dissipation from smaller surfaces which has led to increased challenges for thermal control. Spray cooling has a great amount of application in industrial processes as a heat removal method. It is thought to be the future in thermal management systems in space because of its capability for ‘close’ and accurate control of heat removal. Spray cooling is based on phase change heat transfer generating high heat transfer rates for low superheats. This last term is used to describe the difference in temperature between the heated surface and the cooling fluid. When the temperature of the surface to be cooled rises above the saturation temperature of the fluid splashed to the surface, a phase change occurs at the solid liquid interface during the boiling regime. However, the most interesting phase (regime) is the nucleating boiling where the critical heat flux, CHF, is reached. The CHF is then achieved due to the vapor generation is such as great that the liquid cannot still be in contact with the surface. Thus the heat is transferred through the vapor if there is not enough cold fluid. The thermal conductivity of vapor is lower and so the efficient of the cooling process. This turns out in a decrease on heat flux. Nowadays it is being taken more into account nanofluids as a technique capable of enhancing heat transfer. Nanofluids, a mix of nano-size particles in a base fluid, have been found to have a very high thermal conductivity as compared to the base fluid. In You et al., 2003; Kim et al., 2004a; Moreno et al., 2005 water was used with various Al2O3 particle concentration in a flat plate nucleate pool boiling system. They came across with no change in the heat transfer coefficient but a dramatic enhancement in CHF. They also found that high concentrations can degrade nucleate boiling. The aim of this project is study the effects of spray cooling with suspended nano-particles as an enhanced method for heat transfer removal. The working fluid was water with different concentrations of alumina-oxide particles added. The alumina oxide particles were supplied by Nanophase Technologies (Nano Tek® Alumina Oxide AL-01000-003-025) which had a mean diameter of 60 nm. Three different concentrations were used and the following: .5 g/L, 1 g/L, 2 g/L. Since clumping of particles can affect the heat transfer properties of the droplets, the solution was placed on inside an ultrasonic bath and left there for at least 24 hrs and immediately used in the experiments. Two nozzles were used in this experiment to study a wide range of sauter diameter of droplets. The experiment was carried out using three experimental techniques which looked into different characteristics of spray cooling. In the first mode, the fluid was sprayed onto a copper block heater surface while it was imaged with a high speed camera and synchronized with a high speed Nd-YAG laser. 9 thermocouples were positioned inside the copper block heater, as seen on Figure 1, to measure critical heat flux, while a camera was used to record different impact properties and the influence of nano-particles. Some of these properties were pool buildup size, spread, and duration of pool. For the second imaging technique, the spray on the heated surface was also considered to be an impinging jet, so to visualize the flow of this jet and how the heated surface affected it, PIV (Particle Image Velocimetry) was used in the study. A third imaging technique was used to study the droplet behavior when in contact with a heated surface. A transparent glass heater made of aluminum silicate glass and coated with an ITO (indium tin oxide) film was used as the heater. The size of the drops had an average diameter of 2.38 mm. When compared to the copper block study, this method allows images to be taken from directly below the clear glass heater. Furthermore, these images allow for a clear edge detection of drops as they spread on the surface and what characteristics they develop when the droplets have different concentrations of nanoparticles, as seen on Figure 2. The experiment used a pulsed laser to provide the background illumination. This project is a continuing research project.

Microjet Impingement Cooling With Phase Change

2004 ◽  
Author(s):  
Evelyn N. Wang ◽  
Juan G. Santiago ◽  
Kenneth E. Goodson ◽  
Thomas W. Kenny
Keyword(s):  
Heat Transfer ◽  
Cooling System ◽  
Heated Surface ◽  
Heat Removal ◽  
Transfer Coefficients ◽  
Two Phase ◽  
Impingement Cooling ◽  
Heat Transfer Coefficients ◽  
High Heat Transfer ◽  
Wall Superheat

The large heat generation rates in contemporary microprocessors require new thermal management solutions. Two-phase microjet impingement cooling promises high heat transfer coefficients and effective cooling of hotspots. We have fabricated integrated microjet structures with heaters and temperature sensors to study local heat transfer at the impingement surface of a confined microjet. Circular jets with diameters less than 100 μm are machined in glass. Preliminary temperature measurements (for Rej = 100–500) suggest that heat transfer coefficients of 1000 W/m2C close to the jet stagnation zone can be achieved. As the flowrate of the jet is increased, a tradeoff in heat removal capability and wall superheat is observed. To aid in understanding the mechanism for wall superheat during boiling at the heated surface, the devices allow for optical access through the top of the device. However, the formation of vapor from the top reservoir makes visualization difficult. This study aids in the design of microjet heat sinks used for integration into a closed-loop cooling system.

scholarly journals SIMULATION OF WATER DROPLET IMPINGEMENT ON A HEATED SURFACE- SINGLE AND DOUBLE DROPLETS

2021 ◽  
Vol 25 (4) ◽  
pp. 95-113
Author(s):  
Ruaa B. Namaa ◽  
◽  
Adnan A. Rasool ◽  
Keyword(s):  
Heat Transfer ◽  
Fluid Flow ◽  
Water Drop ◽  
Heated Surface ◽  
Heat Removal ◽  
Heat Transfer Fluid ◽  
Heated Plate ◽  
Single Drop ◽  
Transfer Coefficients ◽  
Numerical Work

The present numerical work is concerned with the single drop and double drops impingement on a heated surface. Fluid flow and heat transfer coefficients were modeled using a volume of fluid (VOF) code. The stainless –steel thin plate surface is uniformly heated to reach a constant temperature at (50C°), this was done by using relatively thicker plate underneath the heated plate. The thick plate is made of high conductivity aluminum alloy 2mm thickness. Relatively a lower temperature water drop is used for cooling to ensure that drop temperature remains below the boiling point of water. The drop –plate initial impingement distances were varied in the range (10-60) cm which represent an impact velocity in the range (1.4-3.4) m/s. The single drop fluid flow simulation results are compared with that in the literature ,while the heat transfer fluid flow results are represented as instantuous heat transfer coefficient variation as alternative to values of heat flux on the surface. Double drops impingement results are then presented and its features are compared to the single drop. Results show that the flow characteristics for the double drops are similar to the single drop at small distances with smaller coverage areas during impingement with lower heat removal rates. As distances increase rebound and splash occurs leading to bigger coverage areas during impingement with relatively smaller heat coefficients compared to the single drop one. The present results shows the same behavior for drop deformation when compared with M.pasandideh-Fard et al. [1] numerical results with an agreement of 90 % and 95 % in calculations the spread factor and impact velocities respectively. The calculated average heat coefficients show acceptable values with that given in litreture

Void Fractions in Subcooled Flow Boiling

1969 ◽  
Vol 91 (4) ◽  
pp. 471-476 ◽  
Author(s):  
P. S. Larsen ◽  
L. S. Tong
Keyword(s):  
Heat Transfer ◽  
Flow Boiling ◽  
Heated Surface ◽  
Liquid Core ◽  
Heat Removal ◽  
Two Phase ◽  
Subcooled Flow Boiling ◽  
Void Fractions ◽  
Subcooled Flow ◽  
The Difference

A semianalytic model is presented for the prediction of void fractions in subcooled flow boiling at elevated pressures. The model is based on the formation and growth of a bubble boundary layer adjacent to the heated surface at a rate determined by the difference between the imposed surface heat transfer and the heat removal capability of the subcooled liquid core of the flow. The latter heat transfer rate is determined by the analogy between heat and momentum transfer in the liquid employing empirical friction-factor data for low-quality two-phase flow. The analysis is compared to experimental results.

scholarly journals Enhancement of Mist Flow Cooling by Using V-Shaped Broken Ribs

Energies ◽  
2019 ◽  
Vol 12 (19) ◽  
pp. 3785
Author(s):  
Kuan-Tzu Huang ◽  
Yao-Hsien Liu
Keyword(s):  
Heat Transfer ◽  
Removal Rate ◽  
Heated Surface ◽  
Heat Removal ◽  
High Heat ◽  
Water Mist ◽  
Droplet Impingement ◽  
Square Channel ◽  
Air Volume ◽  
Mist Flow

Substantial heat transfer enhancement can be achieved by cooling with air/water mist flow because of droplet impingement and liquid film/fragment evaporation on the heated surface, which leads to a high heat-removal rate. An experimental investigation was conducted in a square channel with continuous and broken V-shaped ribs. To generate a mist flow, micro droplets were introduced into the gas stream. The rib angle of attack was 45°, and the rib spacing-to-height ratios were 10 and 20. The air Reynolds number ranged from 7900 to 24,000, and the water-to-air volume flow ratio was less than 0.1%. The net heat inputs ranged from 1.1–3.1 W/cm2 and 3.4–9.4 W/cm2 for the air and mist flow cases, respectively. Because the deposited liquid fragments produced uneven temperature distribution on the heated surface, steady-state infrared thermography was used to visualize the heat transfer distribution. Two to seven times higher heat transfer was attained for the broken ribs when using the mist flow than when using air flow. This increase was mainly attributed to the broken structure, which facilitated liquid transport and enhanced liquid coverage. In addition, the broken ribs produced a smaller friction factor than continuous ribs. The broken structures were beneficial for higher thermal performance in the mist flow.

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