Free Convective Heat Transfer in a Liquid-Filled Vertical Annulus

1985 ◽  
Vol 107 (3) ◽  
pp. 596-602 ◽  
Author(s):  
V. Prasad ◽  
F. A. Kulacki
Keyword(s):  
Heat Transfer ◽  
Nusselt Number ◽  
Laminar Flow ◽  
Prandtl Number ◽  
Convective Heat Transfer ◽  
Flow Regime ◽  
Convective Heat ◽  
Radius Ratio ◽  
Number Range ◽  
Vertical Annulus

An experimental study of convective heat transfer in liquid-filled vertical annulus of radius ratio κ = 5.338 has been conducted for the height-to-gap width ratio A = 0.5, 1, and 1.5. By using water, heptane, and ethylene glycol as the test fluids, a Rayleigh number range of 8 × 106 < Ra < 3 × 1010, and a Prandtl number range of 4 < Pr < 196 have been covered. Curvature effects on the temperature field are significant and result in a lower effective sink temperature for the boundary layer on the isothermally heated inner wall. The Nusselt number Nu thus increases with radius ratio κ. However, the slope of ln (Nu) versus ln (κ) curve is not a constant, and decreases with an increase in κ. The effect of Prandtl number is weak. In the laminar flow regime, the Nusselt number is weakly dependent on the aspect ratio when Nu and Ra are considered in terms of the annulus height L. The start of laminar flow regime is delayed with an increase in radius ratio. For A = 0.5, κ = 5.338, the critical Grashof number is GrL = 7 × 104, which decreases with an increase in A. Turbulence is initiated when the local Grashof number Grx ≃ 4 × 109.

Flow and Convective Heat Transfer Characteristics of Nanofluids With Various Shapes of Alumina Nanoparticles

2009 ◽  
Author(s):  
Kyo Sik Hwang ◽  
Hyo Jun Ha ◽  
Seung Hyun Lee ◽  
Hyun Jin Kim ◽  
Seok Pil Jang ◽  
...  
Keyword(s):  
Heat Transfer ◽  
Laminar Flow ◽  
Convective Heat Transfer ◽  
Flow Regime ◽  
Convective Heat ◽  
Circular Tube ◽  
Al2o3 Nanoparticles ◽  
Heat Transfer Characteristics ◽  
Laminar Flow Regime ◽  
Transfer Characteristics

This paper is to investigate flow and convective heat transfer characteristics of nanofluids with various shapes of Al2O3 nanoparticles flowing through a uniformly heated circular tube under fully developed laminar flow regime. For the purpose, Al2O3 nanofluids of 0.3 Vol.% with sphere, rod, platelet, blade and brick shapes are manufactured by a two-step method. Zeta potential as well as TEM image is experimentally obtained to examine suspension and dispersion characteristics of Al2O3 nanofluids with various shapes. To investigate flow characteristics, the pressure drop of Al2O3 nanofluids with various shapes are measured. In order to investigate convective heat transfer characteristics, the effective thermal conductivities of Al2O3 nanofluids with various shapes, the temperature distribution at the tube surface and the mean temperature of nanofluids at the inlet are measured, respectively. Based on the experimental results, the convective heat transfer coefficient of Al2O3 nanofluids with various shapes is compared with that of pure water and the thermal conductivity of Al2O3 nanofluids with various shapes. Thus, the effect of nanoparticles shape on the flow and convective heat transfer characteristics flowing through a uniformly heated circular tube under fully developed laminar flow regime is experimentally investigated.

A Numerical Study of the Effect of Triangular Waves on Natural Convective Heat Transfer From an Upward Facing Heated Horizontal Isothermal Surface

2016 ◽  
Author(s):  
Patrick H. Oosthuizen
Keyword(s):  
Heat Transfer ◽  
Nusselt Number ◽  
Rayleigh Number ◽  
Prandtl Number ◽  
Surface Waves ◽  
Convective Heat Transfer ◽  
Convective Heat ◽  
Numerical Study ◽  
Heated Surface ◽  
Isothermal Surface

A numerical study of natural convective heat transfer from an upward facing, heated horizontal isothermal surface imbedded in a large flat adiabatic surface has been undertaken. On the heated surface is a series of triangular shaped waves. Laminar, transitional, and turbulent flow conditions have been considered. The flow has been assumed to be two-dimensional and steady. The fluid properties have been assumed constant except for the density change with temperature giving rise to the buoyancy forces. This was with treated using the Boussinesq approach. The numerical solution has been obtained using the commercial CFD solver ANSYS FLUENT©. The k-epsilon turbulence model with full account being taken of buoyancy force effects has been employed. The heat transfer rate from the heated surface expressed in terms of a Nusselt number is dependent on the Rayleigh number, the number of waves, the height of the waves relative to the width of the heated surface, and the Prandtl number. This study obtained results for a Prandtl number of 0.74 which is effectively the value for air. An investigation of the effect of the Rayleigh number, the dimensionless height of the surface waves, and the number of surface waves on the Nusselt number has been undertaken.

Forced Convective Heat Transfer of Nanofluids in Microchannels

2006 ◽  
Author(s):  
Jung-Yeul Jung ◽  
Hoo-Suk Oh ◽  
Ho-Young Kwak
Keyword(s):  
Heat Transfer ◽  
Heat Transfer Coefficient ◽  
Laminar Flow ◽  
Transfer Coefficient ◽  
Convective Heat Transfer ◽  
Flow Regime ◽  
Convective Heat ◽  
Volume Fraction ◽  
Convective Heat Transfer Coefficient ◽  
Laminar Flow Regime

Convective heat transfer coefficient and friction factor of a nanofluid in rectangular microchannel were measured. An integrated microsystem consisting of a single microchannel on one side and two localized heaters and five polysilicon temperature sensors along the channel on the other side were fabricated. Aluminum dioxide (Al2O3) nanofluids with various particle volume fractions were used in experiment to investigate the effect of the volume fraction of the nanoparticles to the convective heat transfer and fluid flow in microchannels. The convective heat transfer coefficient of the Al2O3 nanofluid in laminar flow regime was measured to be increased up to 15% compared to the distilled water at a volume fraction of 1.8 volume percent without major friction loss. The Nusselt number measured increases with increasing the Reynolds number in laminar flow regime. A new type of convective heat transfer correlation was proposed to correlate experimental data of heat transfer coefficient for nanofluids in microchannels.

A Numerical Study of the Simultaneous Natural Convective Heat Transfer From the Upper and Lower Surfaces of a Thin Isothermal Horizontal Circular Plate

2016 ◽  
Author(s):  
Patrick H. Oosthuizen ◽  
Abdulrahim Kalendar
Keyword(s):  
Heat Transfer ◽  
Nusselt Number ◽  
Rayleigh Number ◽  
Prandtl Number ◽  
Convective Heat Transfer ◽  
Heat Transfer Rate ◽  
Convective Heat ◽  
Transfer Rate ◽  
The Mean ◽  
Adiabatic Section

Natural convective heat transfer from the top and bottom surfaces of a thin circular isothermal horizontal plate which, in general, has a centrally placed adiabatic section has been numerically investigated. The temperature of the plate surfaces is higher than the temperature of the surrounding fluid. The range of conditions considered is such that laminar, transitional, and turbulent flow occurs over the plate. The heat transfer from the upper and lower surfaces of the plate as well as the mean heat transfer rate from the entire surface of the plate have been considered. The flow has been assumed to be axisymmetric and steady. The k-epsilon turbulence model with account being taken of buoyancy force effects has been used and the solution has been obtained using the commercial CFD solver ANSYS FLUENT©. The heat transfer rate from the heated plate has been expressed in terms of a Nusselt number based on the outside plate diameter and the difference between the plate temperature and the fluid temperature far from the plate. The mean Nusselt number is dependent on the Rayleigh number, the ratio of the diameter of the inner adiabatic section to the outer plate diameter, and the Prandtl number. Results have only been obtained for a Prandtl number of 0.74, i.e., effectively the value for air. The variations of the mean Nusselt number averaged over both the upper and lower surfaces and of the mean Nusselt numbers for the upper surface and for the lower surface with Rayleigh number for various adiabatic section diameter ratios have been studied. The use of a reference length scale to allow the correlation of these mean Nusselt number-Rayleigh number variations has been investigated.

Free Convective Heat Transfer Across Inclined Air Layers

1976 ◽  
Vol 98 (2) ◽  
pp. 189-193 ◽  
Author(s):  
K. G. T. Hollands ◽  
T. E. Unny ◽  
G. D. Raithby ◽  
L. Konicek
Keyword(s):  
Heat Transfer ◽  
Nusselt Number ◽  
Convective Heat Transfer ◽  
Convective Heat ◽  
Horizontal Layer ◽  
Experimental Measurements ◽  
Number Range ◽  
Transfer Rates ◽  
Air Layers ◽  
Free Convective Heat Transfer

This paper presents new experimental measurements on free convective heat transfer rates through inclined air layers of high aspect ratio, heated from below. The Rayleigh number range covered is from subcritical to 105; the range of the angle of inclination, φ measured from the horizontal is: 0 < φ < ∼70 deg. Although it was anticipated that the results might be identical to the results for the horizontal layer if one replaced Ra by Ra cos φ, significant departures from this behavior were observed, particularly in the range 1708 < Ra cos φ < 104, 30 deg ≤ φ < 60 deg. A recommended relationship giving the Nusselt number as a function of Ra cos φ and φ is reported. This relationship fits all data closely.

Convective Heat Transfer of Ethanol/Polyalphaolefin Nanoemulsion Inside Circular Minichannel Heat Exchanger

2017 ◽  
Author(s):  
Fana Zewede ◽  
Henok Argaw ◽  
Thanh Tran ◽  
Jiajun Xu
Keyword(s):  
Heat Transfer ◽  
Heat Exchanger ◽  
Laminar Flow ◽  
Convective Heat Transfer ◽  
Flow Regime ◽  
Convective Heat ◽  
Heat Transfer Fluid ◽  
Phase Flow ◽  
Two Phase ◽  
Current Test

This work experimentally studied the convective heat transfer characteristics of a novel nanostructured heat transfer fluid: “Ethanol/Polyalphaolefin(PAO) nanoemulsion fluids” flowing through a heat exchanger made of twelve circular minichannels. Ethanol/PAO nanoemulsion fluid is a thermodynamically stable system formed by dispersing ethanol into a mixture of PAO and surfactants, in which the ethanol added inside forms self-assembled nanodroplets of tens of nanometers in diameter. These ethanol nanodroplets can serve as pre-seed boiling nuclei at elevated temperature. The Reynolds number was varied between 140 and 1200 to maintain the entire range of flow regime remained at laminar flow for both single- and two-phase convective heat transfer experiments. Pure PAO was also tested under same conditions and used as baseline data for comparison. It is found that: for single phase flow, there is no significant increase in Nusselt number of Ethanol/PAO nanoemulsion compared to that of PAO fluid in laminar flow regime. However, when the nucleation of ethanol nanodroplets inside the nanoemulsion fluid was initiated, it showed a substantial increase in heat transfer coefficient compared to that of PAO fluid: a 75% enhancement can be achieved under current test conditions. While its mechanism is not completely clear yet, it is believed that such an effect is likely related to the latent heat carried by ethanol bubbles, as well as the increased turbulence and mixing generated during the two-phase flow of nanoemulsion which can increase the heat transfer rate.

Sign in / Sign up

Export Citation Format

Share Document