scholarly journals Asymmetrical Thermal Boundary Condition Influence on the Flow Structure and Heat Transfer Performance of Paramagnetic Fluid-Forced Convection in the Strong Magnetic Field

Fluids ◽  
2020 ◽  
Vol 5 (4) ◽  
pp. 246
Author(s):  
Lukasz Pleskacz ◽  
Elzbieta Fornalik-Wajs ◽  
Sebastian Gurgul
Keyword(s):  
Magnetic Field ◽  
Heat Transfer ◽  
Boundary Condition ◽  
Heat Transfer Enhancement ◽  
Strong Magnetic Field ◽  
Flow Structure ◽  
Thermal Boundary Condition ◽  
Solid Particles ◽  
Transfer Enhancement ◽  
Dimensionless Numbers

Continuous interest in space journeys opens the research fields, which might be useful in non-terrestrial conditions. Due to the lack of the gravitational force, there will be a need to force the flow for mixing or heat transfer. Strong magnetic field offers the conditions, which can help to obtain the flow. In light of this origin, presented paper discusses the dually modified Graetz-Brinkman problem. The modifications were related to the presence of the magnetic field influencing the flow and asymmetrical thermal boundary condition. Dimensionless numerical analysis was performed, and two dimensionless numbers (magnetic Grashof number and magnetic Richardson number) were defined for paramagnetic fluid flow. The results revealed the heat transfer enhancement due to the strong magnetic field influence accompanied by possible but not essential flow structure modifications. On the other hand, the flow structure changes can be utilized to prevent the solid particles’ sedimentation. The explanation of the heat transfer enhancement including energy budget and vorticity distribution was presented.

Numerical Investigation of Flow Structure and Heat Transfer Produced by a Single Highly Confined Bubble in a Pressure-Driven Channel Flow

2016 ◽  
Author(s):  
John R. Willard ◽  
D. Keith Hollingsworth
Keyword(s):  
Heat Transfer ◽  
Boundary Condition ◽  
Heat Transfer Enhancement ◽  
Numerical Investigation ◽  
Flow Structure ◽  
Recent Experiment ◽  
Single Phase ◽  
Heated Wall ◽  
Transfer Enhancement ◽  
Cold Fluid

Confined bubbly flows in millimeter-scale channels produce significant heat transfer enhancement when compared to single-phase flows. This enhancement has been demonstrated in experimental studies, and some of these studies conclude that the enhancement persists even in the absence of active nucleation sites and bubble growth. This observation leads to the hypothesis that the enhancement is driven by a convective phenomenon in the liquid phase around the bubble instead of sourcing from microlayer evaporation or active nucleation. Presented here is a numerical investigation of flow structure and heat transfer due to a single bubble moving through a millimeter-scale channel in the absence of phase change. The simulation includes thermal boundary conditions designed to match those of a recent experiment. The channel is horizontal with a uniform-heat-generation upper boundary condition and an adiabatic lower boundary condition. The Lagrangian framework allows the simulation of a channel of arbitrary length using this smaller computational domain. The fluid phases are modeled using the Volume-of-Fluid method with full geometric reconstruction of the liquid/gas interface. The liquid around the bubble moves as a low-Reynolds-number unsteady laminar flow. In a square region from the trailing edge of the contact line to one nominal bubble diameter behind the bubble, the area-averaged Nusselt number is, at its greatest, 4.7 times the value produced by a single-phase flow. Bubble shape and speed compare well to observations from the recent experiment. The heat transfer enhancement can be attributed to flow structures created by bubble motion. Multiple regions have been observed and are differentiated by their respective vortex characteristics. The primary region exists directly behind the bubble and exhibits the highest enhancement in heat transfer. It contains channel-spanning vortices that move cold fluid along the centerline and edge of the vortices from near the far wall of the channel to the heated wall. The cold fluid delivered by this motion tends to thin the thermal gradient region near the wall and directly behind the bubble and results in the highest local heat transfer coefficients. This vortex drives a bulk exchange of fluid across the channel and elongates the area of heat transfer enhancement to several bubble diameters. The secondary region is a set of vortices that exist to the side and slightly behind the bubble. These vortices rotate at a shallow angle to the primary flow direction and are weaker than those in the other regions.

Numerical Investigation of Heat Transfer Enhancement on a Small Scale Vortex Generator

2009 ◽  
Author(s):  
Jiansheng Wang ◽  
Zhiqin Yang
Keyword(s):  
Heat Transfer ◽  
Heat Transfer Enhancement ◽  
Flow Structure ◽  
Rectangular Channel ◽  
Vortex Generator ◽  
Local Heat Transfer ◽  
Local Heat ◽  
Vortex Generators ◽  
Small Scale ◽  
Transfer Enhancement

The heat transfer characteristic and flow structure of fluid in the rectangular channel with different height vortex generators in small scale are investigated with numerical simulation. Meantime, the properties of heat transfer and flow of fluid in the rectangular channel are compared with the channel which located small scale vortex generator. The variation law of local heat transfer and flow structure in channel is obtained. The mechanism of heat transfer enhancement of small scale vortex generators is discussed in detail. It is found that the influence of vortex generator on heat transfer is not in proportion to the size of vortex generator. What is more, turbulent flow structure near the wall, which influences the temperature distribution near the wall, induces the variety of local heat transfer. The fluid movement towards to the wall causes the heat transfer enhanced. On the contrary, the fluid movement away from the wall decreases the local heat transfer.

Optimization and Development for Fin-and-Tube Heat Exchangers With Vortex Generators

2010 ◽  
Author(s):  
Ya-Ling He ◽  
Pan Chu ◽  
Wen-Quan Tao
Keyword(s):  
Heat Transfer ◽  
Heat Transfer Enhancement ◽  
Flow Structure ◽  
Heat Exchangers ◽  
Pressure Loss ◽  
Numerical Study ◽  
Moderate Pressure ◽  
Computational Domain ◽  
Transfer Enhancement ◽  
Local Flow

In this paper, heat transfer enhancement and pressure loss penalty for fin-and-tube heat exchangers with rectangular winglet pairs (RWPs) were numerically investigated in a relatively low Reynolds number flow. The purpose of this study was to explore the fundamental mechanism between the local flow structure and the heat transfer augmentation. The RWPs were placed with a special orientation for the purpose of enhancement of heat transfer. The numerical study involved three-dimensional flow and conjugate heat transfer in the computational domain, which was set up to model the entire flow channel in the air flow direction. The effects of attack-angle of RWPs, row-number of RWPs and placement of RWPs on the heat transfer characteristics and flow structure were examined in detail. It was observed that the longitudinal vortices caused by RWPs and the impingement of RWPs-directed flow on the downstream tube were important reasons of heat transfer enhancement for fin-and-tube heat exchangers with RWPs. It was interesting to find that the pressure loss penalty of the fin-and-tube heat exchangers with RWPs could be reduced by altering the placement of the same number of RWPs from inline array to staggered array and simultaneously maintain the heat transfer enhancement level. The results showed that the rectangular winglet pairs (RWPs) can significantly improve the heat transfer performance of the fin-and-tube heat exchangers with a moderate pressure loss penalty.

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