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The paper describes the effects of heat transfer enhancement and gas-flow characteristics by wing-type-vortex-generators inside a rectangular gas-flow duct of a plate-fin structure exhaust gas recirculation (EGR) cooler used in a cooled-EGR system. The analyses are conducted using computational fluid dynamics (CFD). The numerical modelling is designed as a gas-flow rectangular duct of an EGR cooler using two fluids with high temperature gas and coolant water whose flow directions are opposite. The gas-flow duct used to separate two fluids is assembled with a stainless steel material. The inlet temperature and velocity of gas flowed inside gas-flow duct are 400 °C and 30 m/s, respectively. Coolant water is flowed into two ducts on both a top and a bottom surface of the gas-flow duct, and the inlet temperature and velocity is 80 °C and 0.6 m/s, respectively. Wing-type-vortex-generators are designed to achieve good cooling performance and low pressure drop and positioned at the center of the gas-flow duct with angle of inclination from 30 to 150 degrees at every 15 degrees. The temperature distributions and velocity vectors gained from numerical results were compared, and discussed. As a result, it is found that the vortices guided in the proximity of heat transfer surfaces play an important role in the heat transfer enhancement and low pressure drop. The collapse of the vortices is caused by complicated flow induced in the corner constituted by two surfaces inside gas-flow duct.

Exhaust gas recirculation (EGR) has been recently focused on enhancement of fuel consumption and reduction of nitrogen oxides in small type of internal combustion engines for automobiles.

The numerical simulation was conducted with the heat-fluid-analysis software, STAR-CCM+ of CD-adapco.

In order to obtain the solution, three equations were used. The Navier-Stokes equation is used as RANS (Reynolds Averaged Navier-Stokes Simulation) with turbulent models. Two other equations are equation of continuity and energy equation. As turbulent vortex viscosity models, K-ε model was used since it has advantage of better convergence in calculation and is frequently used for numerical analyses of EGR coolers. Maximum step numbers in steady analyses and total mesh numbers are 450 and about 800,000, respectively. Polyhedral meshes with 10 to 15 planes provided by Star-CCM+ were employed as mesh shapes to highly enhance the numerical accuracy and convergence. Prism layer was applied to reproduce behaviors of boundary layer in the vicinity of the heat transfer plane. The prism layer was divided into 10 layers and its total thickness amounts to 0.1.

In order to clarify characteristics of a single vortex generator, thermal fluid

analysis investigated the flow and heat transfer by changing angles of inclination with respect to a delta wing and a rectangular wing.

The shape of the vortex generator is an isosceles triangle with each side of 0.5 H and height H and a rectangular wing with each side of 0.4 H and 0.8 H. The set-up condition is defined in such a way that X and Y axis are the gas flow direction and the rotating axis, respectively and angles of inclination ψ are angles between X axis and the delta wing. Angles of inclination ψ were parametrically studied by six different angles inclined to the flow direction, with the backward inclination (ψ = 30, 45, 60 degrees) and forward inclination (ψ = 120, 135, 150 degrees) as shown in

Angles of inclination Ψ [degree] | Projected area of vortex generators [%] | |
---|---|---|

Delta Wing | Rectangular Wing | |

30, 150 | 13 | 16 |

45, 135 | 18 | 23 |

60, 120 | 22 | 28 |

velocity vectors described by Equation (1) and the x component of vorticity ω_{x} by Equation (2). Here ω_{x} indicates the rotation of fluid particles on the plane normal to velocity u in the downstream direction.

ω = ∇ × U = r o t U = c u r l U (1)

ω x = ∂ w ∂ Y − ∂ v ∂ Z (2)

ω y = ∂ u ∂ Z − ∂ w ∂ X (3)

ω z = ∂ v ∂ X − ∂ u ∂ Y (4)

where U ( u , v , w ) is velocity vector, w transverse velocity, v spanwise velocity, and ω ( ω x , ω y , ω z ) vorticity. _{x} rotating around the flow direction, which are called longitudinal vortices. The longitudinal vortices are rotating along the surface of the delta wing from the tip angle position down toward the heat transfer plane at the bottom of the delta wing. This is because the flow separation caused at the tip angle of the wing generates downwash owing to the forward inclination of the wing. As a result, temperature distributions show that great change of the temperature occurs extremely close to the heat transfer plane. The vortices cause the downwash to convey the hot gas at the center of the duct down toward the heat transfer plane where convective heat transfer is enhanced.

This section describes flow field and heat characteristics induced by a group of vortex generators, changing the array of vortex generators. As the configuration parameters of the vortex generators, six arrays of the vortex generators were totally evaluated. Four arrays are delta wings or rectangular ones of setting the same inclined angles with ψ = 45˚ or 135˚ respectively. Other two arrays are delta wings or rectangular ones of arranging ψ = 45˚ and 135˚ alternately. The pitch P between the wings are the same distance as P = 1.5 H, where H is the height of gas duct. The total number of generators is 18. These groups of vortex generators were investigated, setting in the center of the gas duct.

downstream direction. This leads to the high enhancement of heat transfer. Regarding pressure drop, there simultaneously exist two vortices generated from an upstream generator and a downstream one. The vortices rotating in the same direction interact each other and at the contact plane they have the opposite velocity vectors, which induce the high pressure drop. As a result, it follows that delta and rectangular wings with ψ = 135˚ have high cooling performance, but pressure drop is also high. Delta and rectangular wings with ψ = 45˚ and 135˚ alternately generate the weakest vorticity distributions. This is because the vortices generated by backward inclined wings and forward inclined wings rotate each other in opposite direction, which is considered to weaken the strength of vorticity and leads to lowest pressure loss.

The paper numerically studied the enhancement of heat transfer and array conditions for a single generator as well as a group of the delta and rectangular wings in one duct. As a result, the following were concluded.

1) Whether vortex generators are inclined backward or forward toward the flow direction makes a significant difference in the flow field. Forward inclined vortex generators induce the twin strong longitudinal vortices behind the generators. The longitudinal vortices induce the mainstream with hot gas toward the heat transfer plane to enhance the heat transfer efficiently. A delta and a rectangular wing show the similar trend.

2) A group of delta and rectangular wings with ψ = 135˚ have high cooling performance, but pressure drop is also high. Delta and rectangular wings with ψ = 45˚ and 135˚ alternately generate the weakest vorticity distributions. This is because the vortices generated by wings with backward inclination and wings with forward inclination rotate each other in opposite direction.

This work has been supported by Japan Grant-in-Aid for Scientific Research (C) under contract No. 17K06174.

The authors declare no conflicts of interest regarding the publication of this paper.

Ogawa, S. and Usui, S. (2018) Heat Transfer Enhancement by Vortex Generators for Compact Heat Exchanger of Automobiles. Open Journal of Fluid Dynamics, 8, 321-330. https://doi.org/10.4236/ojfd.2018.83020