Heat Transfer on a Flat Surface Under a Region of Turbulent Separation

1994 ◽  
Vol 116 (1) ◽  
pp. 57-62 ◽  
Author(s):  
R. B. Rivir ◽  
J. P. Johnston ◽  
J. K. Eaton
Keyword(s):  
Heat Transfer ◽  
Pressure Gradient ◽  
Heat Transfer Rate ◽  
Transfer Rate ◽  
Separation Bubble ◽  
Adverse Pressure Gradient ◽  
Constant Heat Flux ◽  
Opposite Wall ◽  
Turbulent Separation ◽  
Full Separation

Fluid dynamics and heat transfer measurements were performed for a separation bubble formed on a smooth, flat, constant-heat-flux plate. The separation was induced by an adverse pressure gradient created by deflection of the opposite wall of the wind tunnel. The heat transfer rate was found to decline monotonically approaching the separation point and reach a broad minimum approximately 60 percent below zero-pressure-gradient levels. The heat transfer rate increased rapidly approaching reattachment with a peak occurring slightly downstream of the mean reattachment point. The opposite wall shape was varied to reduce the applied adverse pressure gradient. The heat transfer results were similar as long as the pressure gradient was sufficient to cause full separation of the boundary layer.

Heat Transfer on a Flat Surface Under a Region of Turbulent Separation

1992 ◽  
Author(s):  
R. B. Rivir ◽  
J. P. Johnston ◽  
J. K. Eaton
Keyword(s):  
Heat Transfer ◽  
Pressure Gradient ◽  
Heat Transfer Rate ◽  
Transfer Rate ◽  
Separation Bubble ◽  
Adverse Pressure Gradient ◽  
Constant Heat Flux ◽  
Opposite Wall ◽  
Turbulent Separation ◽  
Full Separation

Fluid dynamics and heat transfer measurements were performed for a separation bubble formed on a smooth, flat, constant-heat-flux plate. The separation was induced by an adverse pressure gradient created by deflection of the opposite wall of the wind tunnel. The heat transfer rate was found to decline monotonically approaching the separation point and reach a broad minimum approximately 60% below zero-pressure-gradient levels. The heat transfer rate increased rapidly approaching reattachment with a peak occuring slightly downstream of the mean reattachment point. The opposite wall shape was varied to reduce the applied adverse pressure gradient. The heat transfer results were similar as long as the pressure gradient was sufficient to cause full separation of the boundary layer.

scholarly journals Enhancing Heat Transfer in Tube Heat Exchanger by Inserting Discrete Twisting Tapes with Different Positions

2019 ◽  
Vol 25 (8) ◽  
pp. 39-51
Author(s):  
Nassr Fadhil Hussein ◽  
Abdulrahman Shakir Mahmood
Keyword(s):  
Heat Transfer ◽  
Heat Exchanger ◽  
Heat Transfer Rate ◽  
Transfer Rate ◽  
Constant Heat Flux ◽  
Vertical Position ◽  
Maximum Heat ◽  
Tube Heat Exchanger ◽  
Twisted Tapes ◽  
Maximum Heat Transfer

Enhancement of heat transfer in the tube heat exchanger is studied experimentally by using discrete twisted tapes. Three different positions were selected for inserting turbulators along tube section (horizontal position by α= 00, inclined position by α= 45 0 and vertical position by α= 900). The space between turbulators was fixed by distributing 5 pieces of these turbulators with pitch ratio    PR = (0.44). Also, the factor of constant heat flux was applied as a boundary condition around the tube test section for all experiments of this investigation, while the flow rates were selected as a variable factor (Reynolds number values vary from 5000 to 15000). The results show that using discrete twisted tapes enhances the heat transfer rate by about 60.7-103.7 % compared with plane tube case. Also, inserting turbulators with inclined position offers maximum heat transfer rate by 103.7%.  

Numerical and Experimental Study of the Effect of Secondary Surfaces Fixed Over a Rectangular Vortex Generator

2019 ◽  
Vol 11 (6) ◽  
Author(s):  
Uddip Kashyap ◽  
Koushik Das ◽  
Biplab Kumar Debnath
Keyword(s):  
Heat Transfer ◽  
Heat Transfer Rate ◽  
Transfer Rate ◽  
Heated Surface ◽  
Linear Regression Analysis ◽  
Vortex Generator ◽  
Base Plate ◽  
Constant Heat Flux ◽  
Heated Plate ◽  
Longitudinal Vortices

In order to cool a heated surface surrounded by fluid flow, vortex generator plays a significant role. The presence of a vortex generator in the flow creates both latitudinal and longitudinal vortices. The vortices energize the boundary layer over the heated surface and excel convective mode of heat transfer. Therefore, the strength of these vortices is directly proportional to the heat transferal rate. The present study considers a vortex generator attached to a heated base plate. The system is studied numerically and experimentally. The existing rectangular vortex generator is modified computationally with a goal to escalate the overall heat transferal rate. The role of secondary surfaces fixed over the primary surface of the rectangular vortex generator is discussed. Water flows over the surface of the base plate at a Reynolds number of 350. And the plate has a constant heat flux of 1 kW/m2. The results show that the secondary surfaces fixed parallel to the heated plate over the vortex generator significantly augment the heat transfer rate to about 13.4%. However, it enhances the drag by 5.7%. A linear regression analysis predicts the suitable placement of the secondary surface with an enhancement of heat transfer rate of about 7.6%, with a decrease in the drag by about 0.7%. In order to validate the obtained results, the best configuration is fabricated and tested experimentally. The experimental outcomes are found to complement the numerical results. In this experiment, the modification yields 25% enhancement in heat transfer rate.

Effects of Nanoparticle Migration on Water/Alumina Nanofluid Flow Inside a Horizontal Annulus with a Moving Core

2014 ◽  
Vol 31 (3) ◽  
pp. 291-305 ◽  
Author(s):  
A. Malvandi ◽  
D. D. Ganji
Keyword(s):  
Heat Transfer ◽  
Heat Flux ◽  
Boundary Conditions ◽  
Heat Transfer Rate ◽  
Transfer Rate ◽  
Outer Wall ◽  
Constant Heat Flux ◽  
Constant Heat ◽  
Thermal Boundary Conditions ◽  
Alumina Nanofluid

AbstractThe present study is a theoretical investigation of the laminar flow and convective heat transfer of water/alumina nanofluid inside a horizontal annulus with a streamwise moving inner cylinder. A modified, two-component, four-equation, nonhomogeneous equilibrium model is employed for the alumina/water nanofluid, which fully accounts for the effect of the nanoparticle volume fraction distribution. To determine the effects of thermal boundary conditions on the migration of the nanoparticles, two cases are considered: constant heat flux at the outer wall with an adiabatic inner wall (Case A) and constant heat flux at the inner wall with an adiabatic outer wall (Case B). The numerical results indicate that the thermal boundary conditions at the pipe walls significantly affect the nanoparticle distribution, particularly in cases where the ratio of Brownian motion to thermophoretic diffusivities is small. Moreover, increasing the velocity of the moving inner cylinder reduces the heat transfer rate for Case A. Conversely, in Case B, the movement of the inner cylinder enhances the heat transfer rate, and anomalous heat transfer enhancement occurs when the thermophoretic force is dominant (in larger nanoparticles).

Convective Heat Transfer of Al2O3 Nanofluids in Porous Media

2016 ◽  
Vol 139 (3) ◽  
Author(s):  
Navid O. Ghaziani ◽  
Fatemeh Hassanipour
Keyword(s):  
Heat Transfer ◽  
Heat Flux ◽  
Porous Medium ◽  
Porous Media ◽  
Heat Transfer Rate ◽  
Transfer Rate ◽  
Particle Concentration ◽  
Metal Foam ◽  
Volume Fraction ◽  
Constant Heat Flux

In this study, the performance of a heat sink embedded with a porous medium and nanofluids as coolants is analyzed experimentally. The nanofluid is a mixture of de-ionized water and nanoscale Al2O3 particles with three different volumetric concentrations: ζ = 0.41%, 0.58%, and 0.83%. The experimental test section is a rectangular minichannel filled with metal foam, which is electrically heated to provide a constant heat flux. The porous medium is assumed to be homogeneous and the flow regime is laminar. The result of heat transfer enhancement by slurry of Al2O3 nanofluid in porous media is studied under various flow velocities, heat flux, porous media structure, and particle concentration of nanofluid. The effect of particles volume fraction on heat transfer coefficient is also studied. This experimental study discovers and/or confirms the following hypotheses: (1) nanoparticle slurry in conjunction with metal foam has a significant effect on heat transfer rate; (2) there is an optimum permeability for the foam resulting in maximal heat transfer rate; (3) for a fixed particle concentration, smaller particles are more effective in enhancing heat transfer; and (4) increasing particle concentration results in some gains, but this trend weakens after a threshold.

scholarly journals Experimental implementation of thermal enhancement performance of air heat exchanger’s pipes utilizing unconventional turbulator

2021 ◽  
pp. 35-44
Author(s):  
Ali Abdulwahab Ismaeel ◽  
Nassr Fadhil Hussein ◽  
Kadhim H. Suffer ◽  
Zuradzman M Razlan
Keyword(s):  
Heat Transfer ◽  
Heat Transfer Rate ◽  
Thermal Performance ◽  
Heat Exchangers ◽  
Transfer Rate ◽  
Heated Surface ◽  
Constant Heat Flux ◽  
Working Fluid ◽  
Practical Implementation ◽  
Cross Sectional

Heat exchangers are widely used in industry, however, raising their performance are important for the variety of applications. Consequently, efficiency improvement associated with low production cost is considered in this experimental work. The current study aims to enhance the rate of heat transfer in pipe-type heat exchangers experimentally by using a novel nozzle as a turbulator. The cross-sectional shape of the nozzle is hexagonal, and the diameter ratio DR is equal to 0.5. Constant heat flux was maintained in the vicinity of the section of the test tube, while the working fluid was pumped into the open system at six discrete Reynolds number values ranging from 6000 to 19500. To investigate the effect of distance among the pieces, three turbulators with different numbers were assigned and named as (N=4, 5 and 6). The results indicated an increase of 172 %, 194 % and 216 % of the heat transfer rate for cases 4, 5 and 6 respectively comparing to the benchmark tube. On the other hand, the friction factor values increased remarkably due to the inserting of turbulators by about of 722.9 % for N=4, 823.9 % for N=5 and 886.7 % for N=6 compared to a plain tube case. Moreover, it has been established that with the insertion of 6 pieces two enhancements was observed; heat transfer rate and thermal performance, where, thermal performance of all cases exceeds unity (maximum thermal performance of 1.62 has been obtained by inserting 6 pieces of hexagonal nozzles turbulators). A comparison with another types of vortex generators shows the gap between the turbulator and heated surface offers a solution for problems occurred in the pipes of heat exchanger. The study therefore suggests a wider practical implementation of the turbulators

Combined Free and Forced Convection Film Condensation on a Finite-Size Horizontal Wavy Plate

2002 ◽  
Vol 124 (3) ◽  
pp. 573-576 ◽  
Author(s):  
Chi-Chang Wang ◽  
Cha’o-Kuang Chen
Keyword(s):  
Heat Transfer ◽  
Pressure Gradient ◽  
Heat Transfer Rate ◽  
Transfer Rate ◽  
Finite Size ◽  
Wave Number ◽  
Film Condensation ◽  
Alternating Direction ◽  
Free And Forced Convection ◽  
Method Effects

Mixed-convection film condensation with downward flowing vapors onto a finite-size horizontal wavy plate is studied by a simple mathematical model and the spline alternating-direction implicit method. Effects of the wavy geometry, the interfacial vapor shear and the pressure gradient on the local condensate film thickness and the heat transfer characteristics have been studied independently. Results show that the pressure gradient tends to increase the heat transfer rate and to decrease the influence of the wavy amplitude. The appropriate wave number which can enhance the maximum condensation heat transfer rate is found in the neighborhood of lunder all circumstances.

Heat Transfer From Interrupted Plates

1983 ◽  
Vol 105 (1) ◽  
pp. 172-177 ◽  
Author(s):  
R. L. Zelenka ◽  
R. I. Loehrke
Keyword(s):  
Heat Transfer ◽  
Reynolds Number ◽  
Heat Transfer Rate ◽  
Transfer Rate ◽  
Separation Bubble ◽  
Flow Direction ◽  
Leading Edge ◽  
Transitional Flow ◽  
Thin Plates ◽  
Plate Spacing

The forced convection heat transfer from two plates aligned with the flow direction in a wind tunnel was measured. The effects of leading edge bluntness, plate spacing distance, and Reynolds number on the leading and trailing plate average heat transfer rate were studied. The low Reynolds number, steady laminar and transitional flow regimes investigated are typical for compact heat exchangers. The measured heat transfer rate from the leading plate agrees well with laminar theory for thin plates when the leading edge is rounded. The heat transfer rate from the leading plate with a blunt nose ranges from slightly below theoretical at a Reynolds number which gives a long, steady separation bubble to well above theoretical under conditions of laminar separation and turbulent reattachment. The heat transfer rate from the second plate is influenced by the leading edge configuration of the first plate only at small plate spacing distances and high Reynolds number. At large spacings the mixing provided by the unsteady wake of the first plate dominates that due to the turbulence formed by leading edge separation on the first plate. The leading edge configuration of the second plate is important only at large values of plate spacing. The heat transfer rate from the second plate is generally higher than that predicted by theory for laminar, steady flow over thin plates and may be higher than that on the leading plate.

Heat Transfer Through a Compressible Laminar Boundary Layer

1962 ◽  
Vol 13 (3) ◽  
pp. 255-270 ◽  
Author(s):  
N. Curle
Keyword(s):  
Heat Transfer ◽  
Boundary Layer ◽  
Pressure Gradient ◽  
Heat Transfer Rate ◽  
Laminar Boundary Layer ◽  
Wall Temperature ◽  
Transfer Rate ◽  
Boundary Layer Separation ◽  
Accurate Solution ◽  
Simple Method

SummaryA simple method is presented for calculating the heat transfer rate through a compressible laminar boundary layer. The temperature of the wall is assumed to be uniform, the viscosity being proportional to the absolute temperature with a ratio which may vary in an appropriate manner with position along the wall; the Prandtl number is arbitrary but greater than about 0·5.The method assumes, following Lighthill, that heat transfer rates are determined mainly by the form of the velocity field in regions close to the wall. After applying the Ulingworth-Stewartson transformation the velocity is expanded in powers of the distance Y normal to the wall, three terms being retained. The first term, proportional to the skin friction, is zero near to a position of boundary layer separation. The second term is proportional to the pressure gradient and also to the wall temperature. Accordingly it can become close to zero when the wall is sufficiently cooled. The third term becomes important only when the first two are simultaneously close to zero and is proportional to the heat transfer rate. Since the three terms cannot, in fact, ever be all zero at the same position they form a uniformly valid non-trivial approximation to the velocity close to the wall.With this velocity profile, and following similarity arguments first given by Liepmann, it is possible to reduce the integrated thermal energy equation to a first-order ordinary differential equation for the heat transfer rate, which is easily solved.A comparison with an accurate solution of Poots, for a particular pressure gradient and wall temperature, yields agreement to within 1 per cent at one third of the distance from leading edge to separation and 3 per cent at twice this distance, with the error rising to 16 per cent at separation where the heat transfer rate is, of course, very low. The modesty of this error is very encouraging.

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