Purpose
This paper aims to numerically investigate the heat transfer and entropy generation characteristics of water-based hybrid nanofluid in natural convection flow inside a concentric horizontal annulus.
Design/methodology/approach
The hybrid nanofluid is prepared by suspending tetramethylammonium hydroxide-coated Fe3O4 (magnetite) nanoparticles and gum arabic (GA)-coated carbon nanotubes (CNTs) in water. The effects of nanoparticle volume concentration and Rayleigh number on the streamlines, isotherms, average Nusselt number and the thermal, frictional and total entropy generation rates are investigated comprehensively.
Findings
Results show the advantageous effect of hybrid nanofluid on the average Nusselt number. Furthermore, the study of entropy generation shows the increment of both frictional and thermal entropy generation rates by increasing Fe3O4 and CNT concentrations at various Rayleigh numbers. Increasing Rayleigh number from 103 to 105, at Fe3O4 concentration of 0.9 per cent and CNT concentration of 1.35 per cent, increases the average Nusselt number, thermal entropy generation rate and frictional entropy generation rate by 224.95, 224.65 and 155.25 per cent, respectively. Moreover, increasing the Fe3O4 concentration from 0.5 to 0.9 per cent, at Rayleigh number of 105 and CNT concentration of 1.35 per cent, intensifies the average Nusselt number, thermal entropy generation rate and frictional entropy generation rate by 18.36, 22.78 and 72.7 per cent, respectively.
Originality/value
To the best knowledge of the authors, there are not any archival publications considering the detailed behaviour of the natural convective heat transfer and entropy generation of hybrid nanofluid in a concentric annulus.
This paper reports the two-dimensional (2D) transient numerical simulation of a phase change material (PCM) based finned heat sink to investigate the heat transfer performance for passive cooling of electronic devices. The finned heat sinks of 2 mm and 3 mm fin thickness are employed with a constant fin volume fraction of 9%, acting as thermal conductivity enhancer (TCE). The n-eicosane is employed as a PCM inside the heat sink to store the heat generated from the electronic device applied at the heat sink base. Transient numerical simulations are performed using finite-volume-method and conjugate heat transfer and melting/solidification phenomenon are investigated by applying various power levels. The numerical results show that the employed PCM with low temperature keeps the heat sink base temperature in lower limits and uniform melting is observed inside the finned heat sink.With the increase of heating power level, the PCM melting time is decreased for fin thickness heat sinks. By increasing the power level from 4 to 6W, for the case of 3 mm fin thickness, the melting time increases by 6.63%, 3.59% and 1.90% by 3 mm fin thickness heat sink, compared to the 2 mm fin thickness heat sink. The developed equations of liquid fraction and modified Nusselt number are obtained as function of modified Fourier number, Stefan number, and Rayleigh number which provide guidelines for generalizing the performance of PCM based finned heat sinks.
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