Effects of parallel magnet bars and partially filled porous media on magneto-thermo-hydro-dynamic characteristics of pipe ferroconvection

https://doi.org/10.1016/j.ijheatmasstransfer.2019.03.085Get rights and content

Highlights

  • This study intended to address heat transfer enhancement for forced ferroconvection.

  • Influence of magnetic field would be significant specially in low Re number.

  • Synergy of porous media and magnetic field lead to heat transfer enhancement up to 2.2-fold.

  • With given geometry, the effect of porous media is much more than magnetic field.

  • Heat transfer enhancement results in decreasing insulation heat loss.

  • Transverse uniform magnetic field causes 3D vortices and confer laminar flow a mixing feature.

Abstract

In this experimental-numerical study, effects of transverse magnetic field of parallel magnet bars perpendicular to the fluid flow direction and partially filled porous media are investigated. Moreover, constant heat flux is applied on the outer surface of a circular straight horizontal pipe and concentric porous media is embedded inside it. Magnet bars are located right at the entrance of pipe and a thick layer of insulation is wrapped the pipe to prevent heat loss. In addition, a 3D numerical simulation is performed using CFD techniques to capture the influence of these two factors on the thermohydrodynamic behavior of ferrofluid flow. Hybrid scheme of finite volume method and SIMPLE algorithm are employed to discretize and solve governing equations as well as TDMA for solving the linear algebraic matrix equation. Mineral oil is used as a base fluid and ferrofluid with three different volume fractions 0.5, 1 and 2% are prepared for the experiment. Finally prepared experimental setup configuration in this investigation leads to this conclusion that in comparison with magnetic field, although porous media induce more unfavorable pressure loss, it will bring more heat transfer enhancement as well.

Introduction

Currently, due to many state-of-the-art practical applications such as geothermal energy extraction, thermal energy storage, petroleum excavation, though porous shale and ferrofluid flows through filtering instruments, interdisciplinary investigations consist of mechanics of non-conducting magnetic fluid exposure to a thermal evolution as well as magnetic field, emerge hot and essential. For high-tech companies, practical concerns of this subject and its plentiful applications in astrophysical and geophysical phenomena were enough to invest a huge amount of money in order to globally be a leading and decisive one in a close future.

From the beginning of ferrofluid emergence, as a unique branch of nanofluids, by NASA in 1963 [1], till now, due to its special thermophysical properties and rheological controllability characteristics, ferrofluid has attracted the researchers’ attention such that many scientific works have been published to explain the ferrofluid governing equations [2], [3], [4], [5] and its application in rotary machines as a seal [6], in a magnetic damper as an absorber [7], in drug delivery as a carrier [8] and in journal bearing as a lubricant [9] as well as its special usage in convection heat transfer, which is so-called ferroconvection, [10], [11], [12], [13], [14], [15].

Recently, the curiosity of testing the synergy of the active approach of a magnetic field and conventional passive approach of embedded porous media results in the hybrid application of ferroconvection and partially filled porous media becoming the attractive subject [16], [17], [18], [19], [20], as the efforts of researchers toward the aim of maximizing heat transfer. Porous media is used in a partially filled condition so that exploiting its benefits to reduce the thermal boundary layer and enhance heat transfer coefficient (HTC) and simultaneously avoid from unfavorable pressure drop as much as possible. Although, this situation depends on flow geometry, Reynolds number and applied external forces but for the simple case of axisymmetric circular pipe flow under no external force partially filled with concentric porous media, Teamah et al. [21], [22] showed that there is a critical radius at which Nusselt number would be maximum.

Some features make the topic of this paper valuable. First, the considered geometry is a circular pipe which is the common carrier of fluid and energy in more than 99% of industries. Second, a magnet bar does not require extra energy to produce magnetic field like electrical magnets. Third, porous media, as a passive method of heat transfer enhancement, is still on the way of progress to be constructed in new alloy, geometry, and applications [26], [27]. These facts accompanied by exploiting special distribution of the magnetic field may represent one of the best solutions for high heat flux problem. There are few or no trace of publications in literature in which exploiting this arrangement of these entire features. Results of this study while suggesting a new way for heat transfer enhancement, simultaneously, may lead to a deeper understanding of the interdisciplinary field of magneto-thermohydrodynamics. An extensive experimental data and numerical simulation address steady-state laminar forced convection heat transfer of Newtonian ferrofluid under magnetic field through the horizontal straight circular pipe subjected to constant heat flux and partially filled with concentric metal foam porous media.

Section snippets

Governing equation

In this part governing equations for the simulation of forced convection heat transfer in the test section is presented. The test section is the part of the circular pipe in which copper foam as a porous media (ε=0.75,rp=5mm,Lp=L=101cm) is embedded concentric to the pipe and constant heat flux is applied by the electrical wire heater uniformly wrapped over the pipe external surface as well, Fig. 1. In Fig. 1, cylindrical porous media is displayed at the center of the pipe (with pink color)

Experimental setup

In the experimental setup, Fig. 4, some instrumentation devices have been used which their specifications are presented in Table 2. Inspiring experimental work recently performed by Shafii and Keshavarz [31] in this area. Ferrofluid is pumped into the test section where electrical heater element uniformly wrapped the pipe and thick insulation cover the whole. Data from Inlet and outlet temperature as well as wall temperature have been acquired by the thermocouples in order to calculate the HTC.

Result and discussion

Although the code authenticity has been validated already in [10], [17], [19], before presenting the numerical results, a new comparison was made with the numerical work of Shakiba and Vahedi [29]. Their numerical investigation dedicated to the ferrofluid (water and 4 vol% Fe3O4) flow in a counter-current horizontal double pipe heat exchanger, which was exposed to a non-uniform transverse magnetic field with different intensities. As it is presented in Fig. 5, the ratio of averaged Nu number

Conclusion

This study was intended to address heat transfer enhancement by investigation of steady-state laminar forced convection heat transfer of Newtonian incompressible ferrofluid flow through horizontal straight circular pipe partially filled with concentric porous media under the transverse magnetic field of two parallel magnet bars located at the entrance of test section.

From experimental-numerical results, it is concluded that the influence of the magnetic field would be significant specially in

Conflict of interest

The authors declared that there is no conflict of interest.

References (32)

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