Thermal Transport to Sessile Droplets on Superhydrophobic Surfaces With Rib and Cavity Features

2013 ◽  
Author(s):  
R. Hays ◽  
J. Crockett ◽  
D. Maynes ◽  
B. W. Webb
Keyword(s):  
Nusselt Number ◽  
Substrate Temperature ◽  
Thermal Transport ◽  
Saturation Temperature ◽  
Infrared Camera ◽  
Transfer Rates ◽  
Video Cameras ◽  
Lower Cavity ◽  
Temperature Heat ◽  
Water Drops

This paper reports on measurements of thermal transport to solitary sessile water drops placed on heated superhydrophobic substrates at constant temperature. Data was obtained by heating the surfaces to specified constant temperatures and gently placing a single water droplet of nominally 3 mm diameter on the surface. The droplet was allowed to evaporate completely while two video cameras and one infrared camera imaged it during the evaporation process. The images were post-processed to yield transient geometric and thermal information, including droplet volume, projected droplet-substrate contact area, and droplet temperature. The total evaporation time and Nusselt and Grashof numbers were determined from the measurements. For all scenarios, the substrate temperature was maintained below the saturation temperature of water and was varied from 60 to 100 °C. Three different rib-patterned superhydrophobic substrates were investigated of 0.5, 0.8, and 0.95 cavity fraction, respectively. The rib features ranged in width from 2 to 30 μm and in height from 15 to 20 μm, while the cavities between the ribs ranged in width from 30 to 38 μm. Results were also obtained for a smooth hydrophobic substrate for comparison purposes. Droplet evaporation times increase with substrate cavity fraction and decrease with increasing substrate temperature. Heat transfer rates decrease with increasing substrate cavity fraction and increase with substrate temperature. The Nusselt number generally increases with the Grashof number raised to the 1/4 power, and Nusselt number is larger for lower cavity fraction substrates.

scholarly journals Natural convection of Bingham fluids in rectangular cross-sectional cylindrical annuli with differentially heated vertical walls

2019 ◽  
Vol 29 (1) ◽  
pp. 43-77 ◽  
Author(s):  
Sahin Yigit ◽  
Nilanjan Chakraborty
Keyword(s):  
Boundary Condition ◽  
Nusselt Number ◽  
Natural Convection ◽  
Yield Stress ◽  
Thermal Transport ◽  
Second Order ◽  
Bingham Fluids ◽  
Cross Sectional ◽  
Content Type ◽  
The Mean

PurposeThis paper aims to numerically analyse natural convection of yield stress fluids in rectangular cross-sectional cylindrical annular enclosures. The laminar steady-state simulations have been conducted for a range of different values of normalised internal radius (ri/L1/8 to 16, whereLis the difference between outer and inner radii); aspect ratio (AR=H/Lfrom 1/8 to 8 whereHis the enclosure height); and nominal Rayleigh number (Rafrom 103to 106) for a single representative value of Prandtl number (Pris 500).Design/methodology/approachThe Bingham model has been used to mimic the yield stress fluid motion, and numerical simulations have been conducted for both constant wall temperature (CWT) and constant wall heat flux (CWHF) boundary conditions for the vertical side walls. The conservation equations of mass, momentum and energy have been solved in a coupled manner using the finite volume method where a second-order central differencing scheme is used for the diffusive terms and a second-order up-wind scheme is used for the convective terms. The well-known semi-implicit method for pressure-linked equations algorithm is used for the coupling of the pressure and velocity.FindingsIt is found that the mean Nusselt number based on the inner peripheryNu¯iincreases (decreases) with an increase inRa(Bn) due to augmented buoyancy (viscous) forces irrespective of the boundary condition. The ratio of convective to diffusive thermal transport increases with increasingri/Lfor both Newtonian (i.e.Bn= 0) and Bingham fluids regardless of the boundary condition. Moreover, the mean Nusselt numberNu¯inormalised by the corresponding Nusselt number due to pure conductive transport (i.e.Nu¯i/(Nu¯i)cond) shows a non-monotonic trend with increasingARin the CWT configuration for a given set of values ofRa,Pr,Lifor both Newtonian (i.e.Bn= 0) and Bingham fluids, whereasNu¯i/(Nu¯i)condincreases monotonically with increasingARin the CWHF configuration. The influences of convective thermal transport strengthen while thermal diffusive transport weakens with increasingAR, and these competing effects are responsible for the non-monotonicNu¯i/(Nu¯i)condvariation withARin the CWT configuration.Originality/valueDetailed scaling analysis is utilised to explain the observed influences ofRa,BN,ri/LandAR, which along with the simulation data has been used to propose correlations forNu¯i.

Blade Tip Heat Transfer and Aerodynamics in a Large Scale Turbine Cascade With Moving Endwall

2006 ◽  
Author(s):  
P. Palafox ◽  
M. L. G. Oldfield ◽  
P. T. Ireland ◽  
T. V. Jones ◽  
J. E. LaGraff
Keyword(s):  
Heat Transfer ◽  
Nusselt Number ◽  
Large Scale ◽  
Infrared Camera ◽  
Suction Side ◽  
Constant Heat Flux ◽  
Tip Clearance ◽  
Airfoil Surface ◽  
Nusselt Number Distribution ◽  
Blade Tip

High resolution Nusselt number (Nu) distributions were measured on the blade tip surface of a large, 1.0 meter-chord, low-speed cascade representative of a high-pressure turbine. Data was obtained at a Reynolds number of 4.0 × 105 based on exit velocity and blade axial chord. Tip clearance levels ranged from 0.56% to 1.68% design span or equally from 1% to 3% of blade chord. An infrared camera, looking through the hollow blade, made detailed temperature measurements on a constant heat flux tip surface. The relative motion between the endwall and the blade tip was simulated by a moving belt. The moving belt endwall significantly to shifts the region of high Nusselt number distribution and reduces the overall averaged Nusselt number on the tip surface by up to 13.3%. The addition of a suction side squealer tip significantly reduced local tip heat transfer and resulted in a 32% reduction in averaged Nusselt number. Analysis of pressure measurements on the blade airfoil surface and tip surface along with PIV velocity flow fields in the gap give an understanding of the heat transfer mechanism.

On the Teaching of Condensation Heat Transfer

2004 ◽  
Author(s):  
A. O¨zer Arnas ◽  
Daisie D. Boettner ◽  
Michael J. Benson ◽  
Bret P. Van Poppel
Keyword(s):  
Heat Transfer ◽  
Nusselt Number ◽  
Convective Heat Transfer Coefficient ◽  
Saturation Temperature ◽  
Condensation Heat Transfer ◽  
Condensate Film ◽  
Vapor Interface ◽  
Simplifying Assumptions ◽  
Thermo Physical Properties ◽  
Vapor Temperature

The topic of condensation heat transfer is usually included in a chapter on Boiling and Condensation in most Heat Transfer textbooks. The assumptions made are those of laminar liquid film with constant thermo-physical properties, uniform vapor temperature equal to the saturation temperature of the vapor, negligible shear at the liquid-vapor interface, and negligible momentum and energy transfer by advection in the condensate film. The results presented are normally for the film thickness, the local convective heat transfer coefficient, and the Nusselt number. However, no means are presented to the student to determine if all of these simplifying assumptions are actually satisfied for a given problem. This investigation clarifies these points to improve teaching of the material and understanding by the student at the undergraduate and graduate level.

Three-Dimensional Visualization and Measurement of Temperature Field Using Liquid Crystal Scanning Thermography

2001 ◽  
Vol 123 (5) ◽  
pp. 1006-1014 ◽  
Author(s):  
P. M. Lutjen ◽  
D. Mishra ◽  
V. Prasad
Keyword(s):  
Nusselt Number ◽  
Liquid Crystal ◽  
Three Dimensional ◽  
Temperature Fields ◽  
Working Fluid ◽  
Transfer Rates ◽  
Numerical Predictions ◽  
Fluid Systems ◽  
Empirical Relations ◽  
Rayleigh Numbers

A Liquid Crystal Scanning Thermography (LCST) technique is developed and implemented to visualize and measure three-dimensional temperature fields. Results are reported for natural convection in a differentially heated vertical cavity. Experiments are conducted under steady state conditions at Rayleigh numbers of about 104 with glycerin as the working fluid. The scanning arrangement is described in detail together with the calibration scheme, and image processing routines that enable the processing of the thermographs and extraction of quantitative temperature information from them, including local and global heat transfer rates. The resultant temperature fields and Nusselt number values are computed and validated against both standard empirical relations and numerical predictions. To date, LCT has not been used to determine Nusselt number for enclosed fluid systems. The ability of LCST to perform this task makes this novel experimental technique a useful tool.

Analysis of Laminar Slip-Flow Thermal Transport in Microchannels With Transverse Rib and Cavity Structured Superhydrophobic Walls at Constant Heat Flux

2011 ◽  
Author(s):  
D. Maynes ◽  
B. W. Webb ◽  
V. Soloviev
Keyword(s):  
Heat Flux ◽  
Nusselt Number ◽  
Laminar Flow ◽  
Thermal Transport ◽  
Peclet Number ◽  
Average Nusselt Number ◽  
Slip Flow ◽  
Constant Heat Flux ◽  
Constant Heat ◽  
Péclet Number

This paper presents an analytical investigation of the thermally developing and periodically fully-developed flow in a parallel-plate channel comprised of superhydrophobic walls. The superhydrophobic walls considered in this paper exhibit alternating micro-ribs and cavities positioned perpendicular to the flow direction and the transport scenario analyzed is that of constant wall heat flux through the rib surfaces with negligible thermal transport through the vapor cavity interface. Axial conduction is neglected in the analysis and the problem is one of Graetz flow with apparent slip-flow and periodicity of constant heating. Closed form solutions for the local Nusselt number and wall temperature are presented and are in the form of infinite series expansions. Previously it has been shown that significant reductions in the overall frictional pressure drop can be expected relative to the classical smooth channel laminar flow. The present results reveal that the overall thermal transport is markedly influenced by the relative cavity region (cavity fraction), the relative rib/cavity module width, and the flow Peclet number. The following conclusions can be made regarding thermal transport for a constant heat flux channel exhibiting the superhydrophobic surfaces considered: 1) Increases in the cavity fraction lead to decreases in the average Nusselt number; 2) Increasing the relative rib/cavity module length yields a decrease in the average Nusselt number; and 3) as the Peclet number increases the average Nusselt number increases. For all parameters explored, the limiting upper bound on the fully-developed average Nusselt number corresponds to the limiting case scenario of classical laminar flow through a smooth-walled channel with constant heat flux.

Measurement and Analysis of Laminar Heat Transfer Coefficients in Micro and Mini-Scale Ducts and Channels

2014 ◽  
Author(s):  
Y. S. Muzychka ◽  
M. Ghobadi
Keyword(s):  
Heat Transfer ◽  
Nusselt Number ◽  
Reynolds Numbers ◽  
Transfer Coefficients ◽  
Constant Wall Temperature ◽  
Fully Developed Flow ◽  
Transfer Rates ◽  
Heat Transfer Coefficients ◽  
Advantages And Disadvantages ◽  
Nusselt Numbers

Heat transfer in micro and mini-scale ducts and channels is considered. In particular, issues of thermal performance are considered in systems with constant wall temperature at low to moderate Reynolds numbers or small dimensional scales which lead to conditions characteristic of thermally fully developed flows or within the transition region leading to thermally fully developed flows. An analysis of two approaches to representing experimental data is given. One using the traditional Nusselt number and another using the dimensionless mean wall flux. Both approaches offer a number of advantages and disadvantages. In particular, it is shown that while good data can be obtained which agree with predicted heat transfer rates, the same data can be problematic if one desires a Nusselt number. Other issues such as boundary conditions pertaining to measuring thermally developing and fully developed flow Nusselt numbers are also discussed in detail.

Influence of multiple slips and chemical reaction on radiative MHD Williamson nanofluid flow in porous medium

2019 ◽  
Vol 15 (3) ◽  
pp. 630-658 ◽  
Author(s):  
Nilankush Acharya ◽  
Kalidas Das ◽  
Prabir Kumar Kundu
Keyword(s):  
Porous Medium ◽  
Nusselt Number ◽  
Chemical Reaction ◽  
Homotopy Perturbation Method ◽  
Velocity Slip ◽  
Nanofluid Flow ◽  
Content Type ◽  
Transfer Rates ◽  
Order Chemical Reaction ◽  
Multiple Slips

Purpose The purpose of this paper is to focus on the influence of multiple slips on MHD Williamson nanofluid flow embedded in porous medium towards a linearly stretching sheet that has been investigated numerically. The whole analysis has been carried out considering the presence of nth-order chemical reaction between base fluid and nanoparticles. Design/methodology/approach A similarity transformation technique has been adopted to convert non-linear governing partial differential equations into ordinary ones and then they are solved by using both the RK-4 method and Laplace transform homotopy perturbation method. The consequences of multiple slip parameters on dimensionless velocity, temperature and concentration and heat and mass transfer rates have been demonstrated using tabular and graphical outline. Findings The investigation explores that the Nusselt number reduces for escalating behaviour of velocity slip and thermal slip parameter. Fluid’s temperature rises in the presence of generative reaction parameter. Originality/value A fine conformity of the current results has been achieved after comparing with previous literature studies. Considering destructive chemical reaction, reduced Nusselt number is found to decrease, but reverse consequence has been noticed in the case of generative chemical reaction. Mass transport diminishes when the order of chemical reaction amplifies for both destructive and generative reactions.

scholarly journals The cold Leidenfrost regime

2019 ◽  
Vol 5 (6) ◽  
pp. eaaw0304 ◽  
Author(s):  
Philippe Bourrianne ◽  
Cunjing Lv ◽  
David Quéré
Keyword(s):  
Substrate Temperature ◽  
Solid Surface ◽  
Surface Texture ◽  
Room Temperature ◽  
Vapor Condensation ◽  
Water Repellent ◽  
Heating Water ◽  
Temperature Heat ◽  
Water Adhesion ◽  
Leidenfrost Temperature

Superhydrophobicity (observed at room temperature) and Leidenfrost phenomenon (observed on very hot solids) are classical examples of nonwetting surfaces. It was found that combining the two effects by heating water-repellent materials leads to a marked yet unexplained decrease of the Leidenfrost temperature of water. We discuss here how heat enhances superhydrophobicity by favoring a “cold Leidenfrost regime” where water adhesion becomes nonmeasurable even at moderate substrate temperature. Heat is found to induce contradictory effects (sticking due to vapor condensation, and lift due to the spreading of vapor patches), which is eventually shown to be controllable by the solid surface texture. The transition to the levitating Leidenfrost regime is observed to be continuous as a function of temperature, contrasting with the transition on common solids.

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