Instantaneous Measurement of Heat Transfer From an Oscillating Wire in Free Convection

1970 ◽  
Vol 92 (3) ◽  
pp. 439-445 ◽  
Author(s):  
B. H. Thrasher ◽  
W. J. Schaetzle
Keyword(s):  
Heat Transfer ◽  
Reynolds Number ◽  
Free Convection ◽  
Forced Convection ◽  
Internal Resistance ◽  
Appreciable Amount ◽  
Significant Difference ◽  
Experimental Apparatus ◽  
Free And Forced Convection ◽  
The Wire

The instantaneous heat transfer properties are measured as a function of time for an oscillating wire (20 to 40 Hz) in still air. The wire is oscillated by thermal contractions and expansions which match the natural frequency based on wire mass and tension. The temperature variation results from the internal resistance heating of an alternating current. The wire temperature and velocity are measured as a function of time by photocells. This eliminates any instrumentation interference with the heat transfer. The results are plotted as a function of instantaneous and average Reynolds’ number. The oscillatory heat transfer data are divided into two regimes of free and forced convection by the critical Reynolds number. Oscillatory heat transfer rates are smaller for forced convection and greater for free convection than those for steady state conditions recommended by McAdams [2] for the respective regimes. No significant difference is found in the heat transfer for oscillations in the vertical and horizontal planes. Due to the time variation of the variables an appreciable amount of emphasis is placed on the experimental apparatus and the recording of data. The recorded data is basically corrected by assuming first order linear systems.

scholarly journals Effect of Reynolds and Prandtl Numbers on Mixed Convection in an Obstructed Vented Cavity

2011 ◽  
Vol 3 (2) ◽  
pp. 271-281
Author(s):  
M. M. Rahman ◽  
M. M. Billah ◽  
M. A. Alim
Keyword(s):  
Heat Transfer ◽  
Finite Element Method ◽  
Reynolds Number ◽  
Finite Element ◽  
Free Convection ◽  
Prandtl Number ◽  
Mixed Convection ◽  
Forced Convection ◽  
Thermal Field ◽  
Heat Transfer Characteristics

A numerical investigation is conducted to analyze the steady flow and thermal fields as well as heat transfer characteristics in a vented square cavity with a built-in heat conducting horizontal solid circular obstruction. Hydrodynamic behavior, thermal characteristics and heat transfer results are obtained by solving the couple of Navier-Stokes and energy equations by using a weighted residuals Finite element method. The computation was made for different Reynolds number, Prandtl number ranging from 50 to 200 and from 0.71 to 7.1 at the three different convective regimes. Three different regimes are observed with increasing Ri: forced convection (with negligible free convection), mixed convection (comparable free and forced convection) and free convection dominated region (with higher free convection). The results are presented to show the effects of the Reynolds number, Prandtl number on flow pattern, thermal field and heat transfer characteristics at the three convective regimes. It is found that the flow and thermal field strongly depend on the Reynolds number, Prandtl number as well as Richardson number. As the Reynolds number and Prandtl number increase, the heat transfer rate increases but average fluid temperature in the cavity and temperature at the cylinder center decrease at the three convective regimes.Keywords: Mixed convection; Finite element method; Obstructed vented cavity; Prandtl number.© 2011 JSR Publications. ISSN: 2070-0237 (Print); 2070-0245 (Online). All rights reserved.doi:10.3329/jsr.v3i2.4344                J. Sci. Res. 3 (2), 271-281 (2011)

scholarly journals EXPERIMENTAL STUDY OF HEAT TRANSFER OF A SINGLE-ROW BUNDLE OF FINNED TUBES IN MIXED CONVECTION OF AIR

2017 ◽  
Vol 60 (4) ◽  
pp. 352-366 ◽  
Author(s):  
A. B. Sukhotskii ◽  
G. S. Sidorik
Keyword(s):  
Heat Transfer ◽  
Experimental Study ◽  
Reynolds Number ◽  
Free Convection ◽  
Mixed Convection ◽  
Forced Convection ◽  
Reynolds Numbers ◽  
Convection Flow ◽  
Single Bundle ◽  
Single Row

The technique and results of experimental study of heat transfer of a single bundle consisting of bimetallic tubes with helically knurled edges, in natural and mixed convection of air are presented. Mixed convection, i.e. a heat transfer, when the contribution of free and forced convection is comparable, was created with the help of the exhaust shaft mounted above the heat exchanger bundle and forced air movement was created by the difference in density of the air in the shaft and the environment. The experimental dependence of the heat transfer of finned single row of bundles in the selected ranges of Grashof and Reynolds numbers has been determined. It is demonstrated that heat transfer in the mixed convection is 2.5−3 times higher than in free one and the growth rate of heat transfer with increasing Reynolds number is more than in the forced convection. Different forms of representation of results of experiments were analyzed and it was determined that the Nusselt number has a single power dependence on the Reynolds number at any height of the exhaust shafts. A linear dependence of the Reynolds number on the square root of the Grashof number was determined as well as the proportionality factors for different shaft heights. It is noted that the characteristics of the motion of air particles in the bundle in free convection is identical to the motion of particles in forced convection at small Reynolds numbers, i.e. a free convection flow smoothly flows into a forced convection one without the typical failures or surges if additional driving forces arise.

Effect of Vibration on Heat Transfer From Spheres

1969 ◽  
Vol 91 (3) ◽  
pp. 337-343 ◽  
Author(s):  
C. B. Baxi ◽  
A. Ramachandran
Keyword(s):  
Heat Transfer ◽  
Reynolds Number ◽  
Free Convection ◽  
Flow Velocity ◽  
Forced Convection ◽  
Vertical Plane ◽  
Convection Heat Transfer ◽  
Transfer Coefficients ◽  
Sinusoidal Vibration ◽  
Vibrational Velocity

The effect of vibration on free and forced convection heat transfer from spheres was investigated. Test spheres made of copper were subjected to sinusoidal vibration in the vertical plane, this being perpendicular to the direction of airstream in the case of forced convection studies. In free convection studies the amplitude of vibration was varied from 4 mm to 25.5 mm and the frequency of vibration from 150 cpm to 930 cpm. It was found that the effect of vibration on Nusselt number was negligible for values of vibrational Reynolds number less than 200. For values of vibrational Reynolds number greater than 200, the vibration increased the heat transfer coefficient considerably and values of heat transfer coefficients as high as seven times the free convection values without vibration were obtained. The following correlations were obtained for heat transfer from spheres to air: freeconvectionwithoutvibration,NNu=2+0.401(NGr)0.25for4×103<NGr<6×104 and free convection with vibration: hvho=0.83(NRe)v0.5(a/D)0.1(NGr)0.251.28 In the case of forced convection studies with vibration, the amplitude of vibrations varied between 4 mm and 12.4 mm, and the frequency of vibration from 200 cpm to 1600 cpm. The flow velocity was varied from 24.5 ft/sec to 84 ft/sec. The results in the absence of vibration could be represented by: NNu = 0.304 (NRe)0.56 or NNu = 2 + 0.222 (NRe)0.587 in the range 6 × 103 < NRe < 3.3 × 104. Nusselt numbers were not found to be affected by the imposition of vibrational velocity even as high as 19.6 percent of the flow velocity.

An Experimental Investigation of Heat Transfer and Buoyancy Induced Transition from Laminar Forced Convection to Turbulent Free Convection over a Horizontal Isothermally Heated Plate

1978 ◽  
Vol 100 (3) ◽  
pp. 429-434 ◽  
Author(s):  
H. Imura ◽  
R. R. Gilpin ◽  
K. C. Cheng
Keyword(s):  
Heat Transfer ◽  
Free Convection ◽  
Forced Convection ◽  
Leading Edge ◽  
Finite Amplitude ◽  
Heated Plate ◽  
Transfer Rates ◽  
Longitudinal Vortices ◽  
Turbulent Flow Regime ◽  
Laminar Forced Convection

The flow over a horizontal isothermally heated plate at Reynolds numbers below that at which hydrodynamic instabilities exist, is characterized by a region of laminar forced convection near the leading edge, followed by the onset of longitudinal vortices and their growth to a finite amplitude and finally a transition to a turbulent flow regime. Results are presented for the temperature profiles, the thermal boundary layer thickness, and the local Nusselt number. They are used to identify the various flow regimes. It was found that the transition from laminar forced convection to turbulent convection was characterized by the parameter Grx/Rex1.5 falling in the range 100 to 300. For values of this parameter greater than 300 the heat transfer rates were independent of Reynolds number and typical of those for turbulent free convection from a horizontal surface.

Effects of Free Convection and Axial Conduction on Forced-Convection Heat Transfer Inside a Vertical Channel at Low Peclet Numbers

1984 ◽  
Vol 106 (2) ◽  
pp. 297-303 ◽  
Author(s):  
L. C. Chow ◽  
S. R. Husain ◽  
A. Campo
Keyword(s):  
Heat Transfer ◽  
Free Convection ◽  
Forced Convection ◽  
Convection Heat Transfer ◽  
Vertical Channel ◽  
Reynolds Numbers ◽  
Convection Heat ◽  
Constant Property ◽  
Axial Conduction ◽  
Forced Convection Heat Transfer

A numerical investigation was conducted to study the simultaneous effects of free convection and axial conduction on forced-convection heat transfer inside a vertical channel at low Peclet numbers. Insulated entry and exit lengths were provided in order to assess the effect of upstream and downstream energy penetration due to axial conduction. The fluid enters the channel with a parabolic velocity and uniform temperature profiles. A constant-property (except for the buoyancy term), steady-state case was assumed for the analysis. Results were categorized into two main groups, the first being the case where the channel walls were hotter than the entering fluid (heating), and the second being the reverse of the first (cooling). For each group, heat transfer between the fluid and the walls were given as functions of the Grashof, Peclet, and Reynolds numbers.

Thermal Performance of Sierpinski Carpet Fractal Fins in a Forced Convection Environment

2016 ◽  
Author(s):  
David Calamas ◽  
Daniel Dannelley ◽  
Gyunay Keten
Keyword(s):  
Heat Transfer ◽  
Reynolds Number ◽  
Forced Convection ◽  
Thermal Performance ◽  
Reynolds Numbers ◽  
Unit Mass ◽  
Thermal Testing ◽  
Sierpinski Carpet ◽  
Fractal Pattern ◽  
Sierpiński Carpet

When certain fractal geometries are used in the design of fins or heat sinks the surface area available for heat transfer can be increased while system mass can be simultaneously decreased. The Sierpinski carpet fractal pattern, when utilized in the design of an extended surface, can provide more effective heat dissipation while simultaneously reducing mass. In order to assess the thermal performance of fractal fins for application in the thermal management of electronic devices an experimental investigation was performed. The first four fractal iterations of the Sierpinski carpet pattern, used in the design of extended surfaces, were examined in a forced convection environment. The thermal performance of the Sierpinski carpet fractal fins was quantified by the following performance metrics: efficiency, effectiveness, and effectiveness per unit mass. The fractal fins were experimentally examined in a thermal testing tunnel for a range of Reynolds numbers. As the Reynolds number increased, the fin efficiency, effectiveness and effectiveness per unit mass were found to decrease. However, as the Reynolds number increased the Nusselt number was found to similarly increase due to higher average heat transfer coefficients. The fourth iteration of the fractal pattern resulted in a 6.73% and 70.97% increase in fin effectiveness and fin effectiveness per unit mass when compared with the zeroth iteration for a Reynolds number of 6.5E3. However, the fourth iteration of the fractal pattern resulted in a 1.93% decrease in fin effectiveness and 57.09% increase in fin effectiveness per unit mass when compared with the zeroth iteration for a Reynolds number of 1.3E4. The contribution of thermal radiation to the rate of heat transfer was as high as 62.90% and 33.69% for Reynolds numbers of 6.5E3 and 1.3E4 respectively.

Forced Convection Laminar Pulsating Flow in a 90-deg Bifurcation

2020 ◽  
Vol 13 (3) ◽  
Author(s):  
Fatih Selimefendigil ◽  
Hakan F. Öztop ◽  
Jay M. Khodadadi
Keyword(s):  
Heat Transfer ◽  
Reynolds Number ◽  
Forced Convection ◽  
Pulsating Flow ◽  
Working Fluid ◽  
Constant Wall Temperature ◽  
Numerical Work ◽  
Average Value ◽  
Number Variation ◽  
Volume Method

Abstract Numerical investigation of laminar forced convection of pulsating flow in a 90-deg bifurcation was performed with the finite volume method. The inlet velocity varies sinusoidally with time while constant wall temperature is utilized. The working fluid is air with constant properties and the numerical work is conducted for a range of the Reynolds numbers (100–2000), dividing flowrates (0.3–0.7) and Strouhal numbers (0.1–10). It is observed that the amplitudes of oscillating heat transfer are damped as the value of the Strouhal number increases. The average value of Nu number rises for higher Reynolds number and the dividing flowrate for the downstream wall of the y-channel branch. As the value of the dividing flowrate increases from 0.3 to 0.7, heat transfer is less effective in the vicinity of the branch at the Reynolds number of 500. The effects of the Reynolds number on the average Nu number variation is more pronounced for the y-branch wall for different values of dividing flowrates. Resonant type behavior of average Nu number is obtained for the y-branch channel for diving flowrates of 0.3 and 0.5.

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