Heat Transfer Enhancement Caused by Sliding Bubbles

2003 ◽  
Vol 125 (3) ◽  
pp. 503-509 ◽  
Author(s):  
Baris B. Bayazit ◽  
D. Keith Hollingsworth ◽  
Larry C. Witte
Keyword(s):  
Heat Transfer ◽  
Liquid Crystal ◽  
Heated Surface ◽  
Local Heat Transfer ◽  
Thin Foil ◽  
Local Heat ◽  
Superheated Liquid ◽  
Enhancement Of Heat Transfer ◽  
Inclined Plates ◽  
Inclined Surface

Measurements that illustrate the enhancement of heat transfer caused by a bubble sliding under an inclined surface are reported. The data were obtained on an electrically heated thin-foil surface that was exposed on its lower side to FC-87 and displayed the output of a liquid crystal coating on the upper (dry) side. A sequence of digital images was obtained from two cameras: one that recorded the response of the liquid crystal and one that recorded images of the bubble as it moved along the heated surface. With this information, the thermal imprint of the bubble was correlated to its motion and position. A bubble generator that produced FC-87 bubbles of repeatable and controllable size was also developed for this study. The results show that both the microlayer under a sliding bubble and the wake behind the bubble contribute substantially to the local heat transfer rate from the surface. The dynamic behavior of the bubbles corresponded well with studies of the motion of adiabatic bubbles under inclined plates, even though the bubbles in the present study grew rapidly because of heat transfer from the wall and the surrounding superheated liquid. Three regimes of bubble motion were observed: spherical, ellipsoidal and bubble-cap. The regimes depend upon bubble size and velocity. A model of the heat transfer within the microlayer was used to infer the microlayer thickness. Preliminary results indicate a microlayer thickness of 40–50 μm for bubbles in FC-87 and a plate inclination of 12 deg.

An Experimental Study of High Rayleigh Number Mixed Convection in a Rectangular Enclosure With Restricted Inlet and Outlet Openings

1987 ◽  
Vol 109 (2) ◽  
pp. 446-453 ◽  
Author(s):  
L. Neiswanger ◽  
G. A. Johnson ◽  
V. P. Carey
Keyword(s):  
Heat Transfer ◽  
Heat Transfer Coefficient ◽  
Rayleigh Number ◽  
Mixed Convection ◽  
Transfer Coefficient ◽  
Heated Surface ◽  
Local Heat Transfer ◽  
Local Heat ◽  
Test Fluid ◽  
Rectangular Enclosure

Measured local heat transfer data and the results of flow visualization studies are reported for cross-flow mixed convection in a rectangular enclosure with restricted inlet and outlet openings at high Rayleigh number. In this study, experiments using water as the test fluid were conducted in a small-scale test section with uniformly heated vertical side walls and an adiabatic top and bottom. As the flow rate through the enclosure increased, the enhancement of heat transfer, above that for natural convection alone, also increased. The variation of the local heat transfer coefficient over the heated surface was found to be strongly affected by the recirculation of portions of the forced flow within the enclosure. Mean heat transfer coefficients are also presented which were calculated by averaging the measured local values over the heated surface. A correlation for the mean heat transfer coefficient is also proposed which agrees very well with the experimentally determined values. A method of predicting the flow regime in this geometry for specified heating and flow conditions is also discussed.

A Technique for Processing Transient Heat Transfer, Liquid Crystal Experiments in the Presence of Lateral Conduction

2004 ◽  
Vol 126 (2) ◽  
pp. 247-258 ◽  
Author(s):  
John P. C. W. Ling ◽  
Peter T. Ireland ◽  
Lynne Turner
Keyword(s):  
Heat Transfer ◽  
Liquid Crystal ◽  
Film Cooling ◽  
Cooling System ◽  
Local Heat Transfer ◽  
Local Heat ◽  
Three Dimensions ◽  
Transient Heat Transfer ◽  
Adiabatic Wall ◽  
Full Intensity

New techniques for processing transient liquid crystal heat transfer experiment have been developed. The methods are able to measure detailed local heat transfer coefficient and adiabatic wall temperature in a three temperature system from a single transient test using the full intensity history recorded. Transient liquid crystal processing methods invariably assume that lateral conduction is negligible and so the heat conduction process can be considered one-dimensional into the substrate. However, in regions with high temperature variation such as immediately downstream of a film-cooling hole, it is found that lateral conduction can become significant. For this reason, a procedure which allows for conduction in three dimensions was developed by the authors. The paper is the first report of a means of correcting data from the transient heat transfer liquid crystal experiments for the effects of significant lateral conduction. The technique was applied to a film cooling system as an example and a detailed uncertainty analysis performed.

Three-Dimensional Transient Heat Conduction Equation Solution for Accurate Quantification of Heat Transfer Coefficient in Transient Liquid Crystal Experiments

2019 ◽  
Author(s):  
Shoaib Ahmed ◽  
Prashant Singh ◽  
Srinath V. Ekkad
Keyword(s):  
Heat Transfer ◽  
Heat Transfer Coefficient ◽  
Liquid Crystal ◽  
Transfer Coefficient ◽  
Three Dimensional ◽  
Jet Impingement ◽  
Local Heat Transfer ◽  
Local Heat ◽  
Transfer Coefficients ◽  
Conduction Model

Abstract Liquid crystal thermography and infrared thermography techniques are typically employed to measure detailed surface temperatures, where local heat transfer coefficient (HTC) values are calculated by employing suitable conduction models. One such practice, which is very popular and easy to use, is the transient liquid crystal thermography using one-dimensional semi-infinite conduction model. In these experiments, a test surface with low thermal conductivity and low thermal diffusivity (e.g. acrylic) is used where a step-change in coolant air temperature is induced and surface temperature response is recorded. An error minimization routine is then employed to guess heat transfer coefficients of each pixel, where wall temperature evolution is known through an analytical expression. The assumption that heat flow in the solid is essentially in one-dimension, often leads to errors in HTC determination and this error depends on true HTC, wall temperature evolution and HTC gradient. A representative case of array jet impingement under maximum crossflow condition has been considered here. This heat transfer enhancement concept is widely used in gas turbine leading edge and electronics cooling. Jet impingement is a popular cooling technique which results in high convective heat rates and has steep gradients in heat transfer coefficient distribution. In this paper, we have presented a procedure for solution of three-dimensional transient conduction equation using alternating direction implicit method and an error minimization routine to find accurate heat transfer coefficients at relatively lower computational cost. The HTC results obtained using 1D semi-infinite conduction model and 3D conduction model were compared and it was found that the heat transfer coefficient obtained using the 3D model was consistently higher than the conventional 1D model by 3–16%. Significant deviations, as high as 8–20% in local heat transfer at the stagnation points of the jets were observed between h1D and h3D.

Internal Heat Transfer Enhancement by Two Perforated Baffles in a Rectangular Channel

1998 ◽  
Author(s):  
Prashanta Dutta ◽  
Sandip Dutta ◽  
Jamil A. Khan
Keyword(s):  
Heat Transfer ◽  
Heated Surface ◽  
Rectangular Channel ◽  
Internal Heat ◽  
Internal Flow ◽  
Jet Impingement ◽  
Local Heat Transfer ◽  
Boundary Layer Separation ◽  
Local Heat ◽  
Augmentation Techniques

The effect of two in-line inclined baffles on the local heat transfer distributions and the associated frictional losses for a turbulent flow with uniform heating from the top surface of a rectangular channel is presented for different Reynolds numbers. A combination of two baffles of same overall size is used in this experiment. The upstream baffle remains attached to the top heated surface and the position, orientation, and geometry of the other is varied. These inclined perforated baffles combine the three major heat transfer augmentation techniques, i.e., jet impingement, internal flow swirls, and boundary layer separation. The results indicate that placement of two inclined baffles augment the overall heat transfer coefficient significantly along with the local heat transfer distribution. The pattern of local Nusselt number ratio strongly depends on the position, orientation, and geometry of the second plate. Like single inclined baffles and rib mounted channels, two baffles offer more pressure drop at higher flow Reynolds number.

Experimental investigation of local heat transfer measurement having inclined discrete ribs of solar air heater duct using LCT technique

2021 ◽  
pp. 1-15
Author(s):  
Yogendra Rathor ◽  
K.R. Aharwal
Keyword(s):  
Heat Transfer ◽  
Liquid Crystal ◽  
Local Heat Transfer ◽  
Local Heat ◽  
Uniform Heat Flux ◽  
Solar Air Heater ◽  
Distribution Profile ◽  
Air Heater ◽  
Staggered Arrangement ◽  
Transfer Measurement

Abstract The steady-state experiment using liquid crystal thermography has been conducted for the analysis of Nusselt number distribution over the absorber surface of solar air heater duct having a gap with the staggered arrangement in inclined rib geometry to recuperate its thermal performance. The heat transfer experiment is performed with uniform heat flux and thermo-chromic liquid crystal is utilized to show the temperature distribution profile over the ribbed surfaces of the rectangular duct of aspect ratio of 5. The colored pattern image of TLCs was acquired using a 3CCD camera and exported to a TIFF file format using frame grabbing SOFTWARE SAPERALT which was further processed to get HSI values. The flow parameters considered in this present investigation are Re, d/W, and g/e varied from 4000-12500,0.15-0.45 and 1-4 respectively. Experiments has been performed with fixed P/e, r/e, p′/P, a and e/Dh of 10, 2, 0.6, 60° and 0.0303 respectively. The influence of relative gap position and relative gap width on flow pattern has been analyzed. The maximum augmentation in Nu and f over the smooth duct was obtained as 4.01 and 4.28 times respectively at the optimum value of d/W = 0.35 and g/e = 2 under similar flow conditions. The maximum value of THPP obtained at d/W & g/e of 0.35 & 2 respectively and Reynolds number of 12445.

The Influence of Including a Partially Smooth Section in the 2nd Leg of an Internally Ribbed Two Pass Cooling Channel

2006 ◽  
Author(s):  
Detlef Pape ◽  
Sean Jenkins ◽  
Jens von Wolfersdorf ◽  
Bernhard Weigand ◽  
Martin Schnieder
Keyword(s):  
Heat Transfer ◽  
Liquid Crystal ◽  
Three Dimensional ◽  
Local Heat Transfer ◽  
Local Heat ◽  
Internal Cooling ◽  
Fluid Temperature ◽  
Turbine Engine ◽  
Pressure Losses ◽  
Uniform Manner

Internal cooling schemes for blades in a gas turbine engine often are subject to compromises between increased pressure losses in return for greater levels of heat transfer required to maintain durability levels in the engine’s harsh environment. Rib configurations have been the subject of much study in past years, however these configurations are normally presumed to be used in “full-coverage” mode, meaning that the ribs are placed in the channel in a continuous and uniform manner. This study investigates the interaction between the bend effects downstream of a 180° bend, which cause higher local heat transfer, and the effect of ribs. Some of the ribs directly downstream of the 180° bend in the 2nd leg of a two pass high aspect ratio (4:1) channel were removed and the effect on heat transfer was assessed. Experimental results showed that the heat transfer level recovered quickly once ribs were encountered. As expected, some decrease in heat transfer was observed in the region where ribs were removed; however total pressure losses in the channel were also much lower. Results include detailed two-dimensional heat transfer distributions determined by the transient liquid crystal method as well as an analysis of the balance between pressure recovery and local heat transfer levels. Generally, for the accuracy of the transient liquid crystal technique in complex three-dimensional flows, strongly varying fluid temperatures present in and downstream of the bend region must be taken into account. For this study, time and position dependent fluid temperature distributions were measured to account for these effects, making it possible to obtain high quality heat transfer results in those regions.

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