Elsevier

Applied Thermal Engineering

Volume 72, Issue 1, 5 November 2014, Pages 114-119
Applied Thermal Engineering

Natural convection around a pair of hot and cold horizontal microtubes at low Rayleigh numbers

https://doi.org/10.1016/j.applthermaleng.2014.01.013Get rights and content

Abstract

Natural convection around a pair of hot and cold horizontal microtubes in a relative large square enclosure was numerically studied using multi-block Lattice Boltzmann method (LBM). Five typical arrangements of the tube pair were made by placing the hot tube at various positions relative to the cold tube, such as side-by-side with, above or under. The hot and cold horizontal microtubes were put at the middle of a horizontal square enclosure. No slip flow conditions were given for all of the tube and the enclosure walls. Constant temperature conditions were given for the hot and cold tubes, respectively, and the adiabatic condition for the square enclosure. The computational code was validated by performing a benchmark case of a single hot isothermal horizontal microtube in the closed cold square cavity. A simple experimental loop was set up in order to make a comparison with the obtained numerical results. Two stainless microtubes with outer diameter of 1.00 mm were used for heat transfer test with a counter-current arrangement. The inlet water temperatures through the hot and cold microtubes were separately controlled by two water baths. The numerical simulated range of Rayleigh number is Ra < 1500. A correlation of Nusselt number with Rayleigh number was proposed based on the numerical results, which is in general agreement with the obtained experimental data.

Introduction

Natural convection is a common but complex phenomenon of fluid flow driven by a kind of internal body force, such as the buoyancy in the gravity field, and it is still an interesting topic in many scientific areas. Recent reviews on natural convection cover various aspects representing the up-to-date achievement, which are meaningful for many industry applications [1], [2]. This paper reports a numerical modeling of natural convection between two parallelized horizontal microtubes enclosed in a relative large rectangular channel. To our knowledge, there is no previous work that directly related to this configuration. In order to give an evaluation of the numerical simulation in the present paper, a simple experimental loop was set up and made a comparison with the obtained numerical results except that the rectangular channel in the numerical simulation was replaced by a cylinder.

Section snippets

Mathematical model and configuration descriptions

The geometry configuration of the problem is as shown in Fig. 1. The hot and cold microtubes were placed in the middle of a 20d × 20d sized square, where d is the diameter of the microtube, and the distance between the two microtubes or tube spacing is 1.5d. All the solid boundaries were given as no slip boundary. The hot and cold tubes were at constant temperatures, and given by 2 and 1, respectively. The enclosure wall was adiabatic, and its initial temperature was given by the averaged hot

Numerical results and discussions

While the Rayleigh number is given as zero, it is corresponding to the case of heat conduction. The calculated mean Nusselt numbers for both the hot and cold tubes are approximate to 1.02, which is close to unity of the pure heat conduction state in an infinite large space. Fig. 4 shows the temperature and velocity profiles at various Rayleigh numbers while the hot and cold tubes are side by side. It shows that there is only one vortex clockwise surrounding the tube pair at very low Rayleigh

Comparison with the experimental results

An experimental loop was set up in order to make a comparison with the obtained numerical results. The experimental system includes a test section, a cold and a hot water loop, a flow rate control system and a data acquisition system.

In the test section, a plexiglass circular pipe was used as the tube shell, which is 300 mm in length, and the inner diameter and thickness are respectively 20 mm and 2.5 mm. Two stainless steel microtubes are circulated respectively with cold and hot water, both

Conclusions

The natural convection of a parallelized hot and cold tube pair in a rectangular enclosure was numerically studied in a range of Ra < 1400. The results show that the vortex structure of fluid flow is more complicated with increasing Rayleigh number than that of a free convection for a horizontal cylinder. The arrangement of hot tube right over cold tube could have the most complicated vortex structure at relative large Rayleigh number (Ra ≈ 103), and its averaged Nusselt number is the lowest

Acknowledgements

The work was supported by National Program on Key Basic Research Project fund under contract 2011CB707203 and NSF of China under grant No. 40972160.

References (9)

There are more references available in the full text version of this article.

Cited by (24)

  • Natural circulation flow distribution within a multi-branch manifold

    2019, International Journal of Heat and Mass Transfer
View all citing articles on Scopus
View full text