A Theoretical Study of Film Condensation in Horizontal Microfin Tubes

2001 ◽  
Vol 124 (1) ◽  
pp. 94-101 ◽  
Author(s):  
Hiroshi Honda ◽  
Huasheng Wang ◽  
Shigeru Nozu
Keyword(s):  
Heat Transfer ◽  
Heat Transfer Coefficient ◽  
Transfer Coefficient ◽  
Flow Model ◽  
Density Ratio ◽  
Stratified Flow ◽  
Film Condensation ◽  
Vapor Flow ◽  
Liquid Density ◽  
Microfin Tubes

A stratified flow model of film condensation in helically grooved, horizontal microfin tubes has been developed. The height of stratified condensate was estimated by extending the Taitel and Dukler model for a smooth tube to a microfin tube. For the upper part of the tube exposed to the vapor flow, laminar film condensation due to the combined effects of gravity and surface tension forces was assumed. For the lower part of the tube exposed to the stratified condensate flow, the heat transfer coefficient was estimated by an empirical equation for the internally finned tubes developed by Carnavos. The theoretical predictions of the circumferential average heat transfer coefficient by the present model and previously proposed annular flow model were compared with available experimental data for five tubes and five refrigerants. It was shown that the stratified flow model was applicable to wide ranges of mass velocity and quality as long as the vapor to liquid density ratio was larger than 0.05. Comparison was also made with the predictions of previously proposed empirical equations.

Heat Transfer Coefficient in Rapid Solidification of a Liquid Layer on a Substrate

2000 ◽  
Vol 122 (4) ◽  
pp. 792-800 ◽  
Author(s):  
P. S. Wei ◽  
F. B. Yeh
Keyword(s):  
Heat Transfer ◽  
Heat Transfer Coefficient ◽  
Transfer Coefficient ◽  
Biot Number ◽  
Equilibrium Phase ◽  
Density Ratio ◽  
Solid Substrate ◽  
Time Dependent ◽  
Bottom Surface ◽  
Melting Front

The heat transfer coefficient at the bottom surface of a splat rapidly solidified on a cold substrate is self-consistently and quantitatively investigated. Provided that the boundary condition at the bottom surface of the splat is specified by introducing the obtained heat transfer coefficient, solutions of the splat can be conveniently obtained without solving the substrate. In this work, the solidification front in the splat is governed by nonequilibrium kinetics while the melting front in the substrate undergoes equilibrium phase change. By solving one-dimensional unsteady heat conduction equations and accounting for distinct properties between phases and splat and substrate, the results show that the time-dependent heat transfer coefficient or Biot number can be divided into five regimes: liquid splat-solid substrate, liquid splat-liquid substrate, nucleation of splat, solid splat-solid substrate, and solid splat-liquid substrate. The Biot number at the bottom surface of the splat during liquid splat cooling increases and nucleation time decreases with increasing contact Biot number, density ratio, and solid conductivity of the substrate, and decreasing specific heat ratio. Decreases in melting temperature and liquid conductivity of the substrate and increase in latent heat ratio further decrease the Biot number at the bottom surface of the splat after the substrate becomes molten. Time-dependent Biot number at the bottom surface of the splat is obtained from a scale analysis. [S0022-1481(00)01004-5]

One Dimensional Model for Rotating Channels in the Turbine System: Part 2 — Formulation and Validation for Film Condensation

2013 ◽  
Author(s):  
Murali Krishnan R. ◽  
Zain Dweik ◽  
Deoras Prabhudharwadkar
Keyword(s):  
Heat Transfer ◽  
Heat Transfer Coefficient ◽  
Transfer Coefficient ◽  
Steam Turbine ◽  
Compressible Flow ◽  
Saturation Temperature ◽  
Film Condensation ◽  
Bulk Temperature ◽  
Dimensional Model ◽  
One Dimensional

This paper provides an extension of the previously described [1] formulation of a one-dimensional model for steady, compressible flow inside a channel, to the steam turbine application. The major challenge faced in the network simulation of the steam turbine secondary system is the prediction of the condensation that occurs during the engine start-up on the cold parts that are below the saturation temperature. Neglecting condensation effects may result in large errors in the engine temperatures since they are calculated based on the boundary conditions (heat transfer coefficient and bulk temperature) which depend on the solution of the network analysis. This paper provides a detailed formulation of a one-dimensional model for steady, compressible flow inside a channel which is based on the solution of two equations for a coupled system of mass, momentum and energy equations with wall condensation. The model also accounts for channel area variation, inclination with respect to the engine axis, rotation, wall friction and external heating. The formulation was first validated against existing 1D correlation for an idealized case. The wall condensation is modeled using the best-suited film condensation models for pressure and heat transfer coefficient available in the literature and has been validated against the experimental data with satisfactory predictions.

Measurement of Local Heat Transfer Coefficient and Film Cooling Effectiveness Through Discrete Holes

2000 ◽  
Author(s):  
R. F. Martinez-Botas ◽  
C. H. N. Yuen
Keyword(s):  
Heat Transfer ◽  
Heat Transfer Coefficient ◽  
Liquid Crystal ◽  
Transfer Coefficient ◽  
Film Cooling ◽  
Density Ratio ◽  
Local Heat Transfer ◽  
Double Row ◽  
Cooling Effectiveness ◽  
Film Cooling Effectiveness

An efficient steady-state wide band liquid crystal technique is used to study the film cooling performance of a variety of geometries in a flat plate: a single row of holes, a double row of holes (both in-line and staggered), and a single cooling hole. This method allows temperature information to be captured in one image, without the difficulty involved in a transient experiment. The streamwise inclinations tested are 30°, 60°, and 90°. The freestream is maintained at 13m/s, and at ambient temperature. The range of blowing ratios varied from 0.33 to 2.0. Both heat transfer coefficient and adiabatic cooling effectiveness are measured for all the cases. Air is used to produce a density ratio near unity. From the range of blowing ratios tested, the most effective film cooling is achieved at a value close to 0.5, for near unity density ratio. It has been revealed that film cooling effectiveness is improved when the jet remains attached to the surface, however, this is generally coupled with an augmentation in heat transfer owing to the disturbance the attached jet causes to the boundary layer. The 30° inclined holes show to be the most effective. Results demonstrate the full coverage capability of liquid crystal thermography.

The Effect of Upstream Thermal Boundary Condition on Convective Heat Transfer Coefficient on a Film-Cooled Flat Plate

2013 ◽  
Author(s):  
James E. Mayhew ◽  
D. Andrew Sowders ◽  
Benjamin B. Fuller
Keyword(s):  
Heat Transfer ◽  
Heat Transfer Coefficient ◽  
Transfer Coefficient ◽  
Flat Plate ◽  
Convective Heat Transfer ◽  
Convective Heat ◽  
Enhancement Factor ◽  
Convective Heat Transfer Coefficient ◽  
Density Ratio ◽  
Surface Heating

The convection heat transfer coefficient on a film-cooled flat plate with and without upstream surface heating is investigated using liquid crystal thermography. The experiments were conducted with a turbulent boundary layer and low freestream turbulence at mass flux ratios of 0.5, 1.0, and 1.5 and density ratio of unity, using cylindrical holes at a 30° injection angle. Results show that upstream surface heating produces a lower convective heat transfer coefficient as expected, and the spanwise-averaged heat transfer enhancement factor is increased by up to 5% over approximately 60% of the film-cooled region. As blowing ratio increased, this area of increased enhancement factor moved further downstream of the holes.

scholarly journals Effects on Heat Transfer Coefficient and Adiabatic Effectiveness in Combined Backside and Film Cooling With Short-Hole Geometry

2019 ◽  
Author(s):  
Renzo La Rosa ◽  
Jaideep Pandit ◽  
Wing Ng ◽  
Brett Barker
Keyword(s):  
Heat Transfer ◽  
Reynolds Number ◽  
Heat Transfer Coefficient ◽  
Transfer Coefficient ◽  
Film Cooling ◽  
Density Ratio ◽  
Transfer Model ◽  
Cooling Effectiveness ◽  
Film Cooling Effectiveness ◽  
Adiabatic Effectiveness

Abstract Heat transfer experiments were done on a flat plate to study the effect of internal counter-flow backside cooling on adiabatic film cooling effectiveness and heat transfer coefficient. In addition, the effects of density ratio (DR), blowing ratio (BR), diagonal length over diameter (L/D) ratio, and Reynolds number were studied using this new configuration. The results are compared to a conventional plenum fed case. Data were collected up to X/D = 23 where X = 0 at the holes, an S/D = 1.65 and L/D = 1 and 2. Testing was done at low L/D ratios since short holes are normally found in double wall cooling applications in turbine components. A DR of 2 was used in order to simulate engine-like conditions and this was compared to a DR of 0.92 since relevant research is done at similar low DR. The BR range of 0.5 to 1.5 was chosen to simulate turbine conditions as well. In addition, previous research shows that peak effectiveness is found within this range. Infrared (IR) thermography was used to capture temperature contours on the surface of interest and the images were calibrated using a thermocouple and data analyzed through MATLAB software. A heated secondary fluid was used as ‘coolant’ in the present study. A steady state heat transfer model was used to perform the data reduction procedure. Results show that backside cooling configuration has a higher adiabatic film cooling effectiveness when compared to plenum fed configurations at the same conditions. In addition, the trend for effectiveness with varying BR is reversed when compared with traditional plenum fed cases. Yarn flow visualization tests show that flow exiting the holes in the backside cooling configuration is significantly different when compared to flow exiting the plenum fed holes. We hypothesize that backside cooling configuration has flow exiting the holes in various directions, including laterally, and behaving similar to slot film cooling, explaining the differences in trends. Increasing DR at constant BR shows an increase in adiabatic effectiveness and HTC in both backside cooling and plenum fed configurations due to the decreased momentum of the coolant, making film attachment to the surface more probable. The effects of L/D ratio in this study were negligible since both ratios used were small. This shows that the coolant flow is still underdeveloped at both L/D ratios. The study also showed that increasing turbulence through increasing Reynolds number decreased adiabatic effectiveness.

Efficacy of Microscopic Interconnected Channels and Surfactants on Enhancing Pool Boiling Heat Transfer

2007 ◽  
Author(s):  
Rene Reyes Mazzoco
Keyword(s):  
Heat Transfer ◽  
Heat Transfer Coefficient ◽  
Transfer Coefficient ◽  
Boiling Heat Transfer ◽  
Bubble Diameter ◽  
Pool Boiling ◽  
Liquid Mixtures ◽  
Vapor Flow ◽  
Pool Boiling Heat Transfer ◽  
The Heat Transfer Coefficient

Nucleate pool boiling heat transfer increases with certain liquid mixtures and some coatings over the heater’s surface. The effects of these modifications are best measured by the relative values of the convective heat transfer coefficient that quantify the ability for transferring heat. The mechanisms that increase pool boiling heat transfer are reflected in the formation of smaller bubbles that escape away from the heater’s surface at a higher velocity, than those formed under not enhanced conditions. The bubble diameter depends on a chemical effect from the liquid composition acting at the bubble’s interface, and on the physical effect of the porous coverings to break the bubbles and to allow the resulting vapor flow. The reduction in bubble diameter in liquid mixtures comes from the action of intermolecular forces at the liquid-vapor interface similar to those associated to surfactants. Several studies have concentrated on increasing the heat transfer coefficient in pool using surfactants in concentrations close to the critical micelle concentration (cmc) of the surfactant in the liquid. The surfactants achieve the highest reduction of bubble diameter by accommodating the lowest surface of their molecules at the interface. However, the mixture of 16% ethanol in water also showed an increase in the convective heat transfer coefficient by producing the lowest size of bubbles from any other ethanol-water mixture. Surface tension and sessile drop contact angle for this mixture have a behavior similar to the cmc; therefore, the mixture effect on boiling is explained through the presence of ethanol-hydrated-states accommodated at the interface. Other liquid mixtures, containing propylene glycol, ethylene glycol, ethanol and water, with cmc behavior had been found through surface tension and sessile contact angle measurements, and showed that they increased the heat transfer coefficient. The mechanical effect that increases the heat transfer coefficient with porous coverings has been explained as the breaking of emerging bubbles at the heater’s surface and the proper handling of the resulting vapor flow away from the covering. Experiments with a mesh located at a distance half the bubble diameter, at a specific power supplied, released the bubbles from the heater before finishing its formation increasing their departure frequency. An array of layers of the same mesh produced and additional increment in the heat transfer coefficient if the array is accommodated to favor the gas flow out of the heater’s region.

Film Cooling Jet Injection Effect in Heat Transfer Coefficient Augmentation for the Pressure Side Cooling of Turbine Vane

2014 ◽  
Author(s):  
Hossein Nadali Najafabadi ◽  
Matts Karlsson ◽  
Mats Kinell ◽  
Esa Utriainen
Keyword(s):  
Heat Transfer ◽  
Heat Transfer Coefficient ◽  
Transfer Coefficient ◽  
Film Cooling ◽  
Density Ratio ◽  
Leading Edge ◽  
Suction Side ◽  
Pressure Side ◽  
Time Resolved ◽  
Turbine Vane

Improving film cooling performance of turbine vanes and blades is often achieved through application of multiple arrays of cooling holes on the suction side, the showerhead region and the pressure side. This study investigates the pressure side cooling under the influence of single and multiple rows of cooling in the presence of a showerhead from a heat transfer coefficient augmentation perspective. Experiments are conducted on a prototype turbine vane working at engine representative conditions. Transient IR thermography is used to measure time-resolved surface temperature and the semi-infinite method is utilized to calculate the heat transfer coefficient on a low conductive material. Investigations are performed for cylindrical and fan-shaped holes covering blowing ratio 0.6 and 1.8 at density ratio of about unity. The freestream turbulence is approximately 5% close to the leading edge. The resulting heat transfer coefficient enhancement, the ratio of HTC with to that without film cooling, from different case scenarios have been compared to showerhead cooling only. Findings of the study highlight the importance of showerhead cooling to be used with additional row of cooling on the pressure side in order to reduce heat transfer coefficient enhancement. In addition, it is shown that extra rows of cooling will not significantly influence heat transfer augmentation, regardless of the cooling hole shape.

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