Onset of Natural Convection from a Suddenly Heated Horizontal Cylinder

1980 ◽  
Vol 102 (4) ◽  
pp. 636-639 ◽  
Author(s):  
J. R. Parsons ◽  
J. C. Mulligan

A study of the onset of transient natural convection from a suddenly heated, horizontal cylinder of finite diameter is presented. The termination of the initial conductive and “locally” conuectiue heat transfer regime which precedes the onset of global natural convection is treated as a thermal stability phenomenon. An analysis is presented wherein the effects of finite cylinder diameter, cylinder heat capacity, and cylinder thermal conductivity are included in calculations of the convective delay time. A simple experimental apparatus is described and data presented. The thermal stability analysis is confirmed experimentally and data is presented which indicates localized natural convection prior to global motion.

Author(s):  
Titan C. Paul ◽  
A. K. M. M. Morshed ◽  
Elise B. Fox ◽  
Ann E. Visser ◽  
Nicholas J. Bridges ◽  
...  

A systematic natural convection heat transfer experiment has been carried out of nanoparticle enhanced ionic liquids (NEILs) in rectangular enclosures (lengthxwidthxheight, 50×50×50mm and 50×50×75mm) heated from below condition. In the present experiment NEIL was made of N-butyl-N-methylpyrrolidinium bis{(trifluoromethyl)sulfonyl} imide, ([C4mpyrr][NTf2]) ionic liquid with 0.5% (weight%) Al2O3 nanoparticles. In addition to characterize the natural convection behavior of NEIL, thermophysical properties such as thermal conductivity, heat capacity, and viscosity were also measured. The result shows that the thermal conductivity of NEIL enhanced ∼3% from the base ionic liquid (IL), heat capacity enhanced ∼12% over the measured temperature range. The natural convection experimental result shows consistent for two different enclosures based on the degrading natural convection heat transfer rate over the measured Rayleigh number range. Possible reasons of the degradation of natural convection heat transfer may be the relative change of the thermophysical properties of NEIL compare to the base ionic liquid.


Author(s):  
Titan C. Paul ◽  
A. K. M. M. Morshed ◽  
Elise B. Fox ◽  
Ann Visser ◽  
Nicholas Bridges ◽  
...  

This paper presents an experimental study of natural convection heat transfer for an Ionic Liquid. The experiments were performed for 1-butyl-2, 3-dimethylimidazolium bis(trifluoromethylsulfonyl)imide, ([C4mmim][NTf2]) at a Rayleigh number range of 1.13×107 to 7.7×107. In addition to determining the convective heat transfer coefficients, this study also included experimental determination of thermophysical properties of [C4mmim][NTf2] such as, density, viscosity, heat capacity, and thermal conductivity. The results show that the density of [C4mmim][NTf2] varies from 1.437–1.396 g/cm3 within the temperature range of 10–50°C, the thermal conductivity varies from 0.125–0.12 W/m.K between a temperature of 10 to 70°C, the heat capacity varies from 1.015 J/g.K–1.760 J/g.K within temperature range of 25–340°C and the viscosity varies from 243cP–18cP within temperature range 10–75°C. The results for density, thermal conductivity, heat capacity, and viscosity were in close agreement with the values in the literature. Measured dimensionless Nusselt number was observed to be higher for the ionic liquid than that of DI water. This is expected as Nusselt number is the ratio of heat transfer by convection to conduction and the ionic liquid has lower thermal conductivity (approximately 20% of DI water) than DI water.


1995 ◽  
Vol 299 ◽  
pp. 241-265 ◽  
Author(s):  
Sang W. Joo

An evolution equation is derived that describes the nonlinear development of the interface between two viscoelastic fluids flowing, under the action of imposed pressure gradient and gravity, in a vertical channel. The channel walls are kept at different temperatures, resulting in heat transfer across the layers. The equation, based on the lubrication approximation, models the effects of stratifications in density, viscosity, elasticity, shear thinning, and thermal conductivity. It also describes the capillary and thermocapillary effects, as well as the sensitivity of viscosities to temperature. Linear-stability analysis is performed based on the evolution equation to understand the competing effects of viscous, elastic, and Marangoni instabilities. Particular attention is paid to the active control of the interfacial instabilities through the thermocapillarity.


Author(s):  
Shijo Thomas ◽  
C. B. Sobhan ◽  
Jaime Taha-Tijerina ◽  
T. N. Narayanan ◽  
P. M. Ajayan

Nanofluids are suspensions or colloids produced by dispersing nanoparticles in base fluids like water, oil or organic fluids, so as to improve their thermo-physical properties. Investigations reported in recent times have shown that the addition of nanoparticles significantly influence the thermophysical properties, such as the thermal conductivity, viscosity, specific heat and density of base fluids. The convective heat transfer coefficient also has shown anomalous variations, compared to those encountered in the base fluids. By careful selection of the parameters such as the concentration and the particle size, it has been possible to produce nanofluids with various properties engineered depending on the requirement. A mineral oil–boron nitride nanofluid system, where an increased thermal conductivity and a reduced electrical conductivity has been observed, is investigated in the present work to evaluate its heat transfer performance under natural convection. The modified mineral oil is produced by chemically dispersing boron nitride nanoparticles utilizing a one step method to obtain a stable suspension. The mineral oil based nanofluid is investigated under transient free convection heat transfer, by observing the temperature-time response of a lumped parameter system. The experimental study is used to estimate the time-dependent convective heat transfer coefficient. Comparisons are made with the base fluid, so that the enhancement in the heat transfer coefficient under natural convection situation can be estimated.


Author(s):  
Aditya Kuchibhotla ◽  
Debjyoti Banerjee

Stable homogeneous colloidal suspensions of nanoparticles in a liquid solvents are termed as nanofluids. In this review the results for the forced convection heat transfer of nanofluids are gleaned from the literature reports. This study attempts to evaluate the experimental data in the literature for the efficacy of employing nanofluids as heat transfer fluids (HTF) and for Thermal Energy Storage (TES). The efficacy of nanofluids for improving the performance of compact heat exchangers were also explored. In addition to thermal conductivity and specific heat capacity the rheological behavior of nanofluids also play a significant role for various applications. The material properties of nanofluids are highly sensitive to small variations in synthesis protocols. Hence the scope of this review encompassed various sub-topics including: synthesis protocols for nanofluids, materials characterization, thermo-physical properties (thermal conductivity, viscosity, specific heat capacity), pressure drop and heat transfer coefficients under forced convection conditions. The measured values of heat transfer coefficient of the nanofluids varies with testing configuration i.e. flow regime, boundary condition and geometry. Furthermore, a review of the reported results on the effects of particle concentration, size, temperature is presented in this study. A brief discussion on the pros and cons of various models in the literature is also performed — especially pertaining to the reports on the anomalous enhancement in heat transfer coefficient of nanofluids. Furthermore, the experimental data in the literature indicate that the enhancement observed in heat transfer coefficient is incongruous compared to the level of thermal conductivity enhancement obtained in these studies. Plausible explanations for this incongruous behavior is explored in this review. A brief discussion on the applicability of conventional single phase convection correlations based on Newtonian rheological models for predicting the heat transfer characteristics of the nanofluids is also explored in this review (especially considering that nanofluids often display non-Newtonian rheology). Validity of various correlations reported in the literature that were developed from experiments, is also explored in this review. These comparisons were performed as a function of various parameters, such as, for the same mass flow rate, Reynolds number, mass averaged velocity and pumping power.


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