Maintaining the temperature in the motor cycle engines is essential for better performance. Hence, techniques to enhance the heat transfer rate must be employed in the most efficient way. One such popular method is the addition of the fins to the engine cylinder. Addition of fins provides larger surface area leading to larger heat transfer rates. This can be further improved by keeping more number of fins to the same body. However, adding a large number of fins would reduce the fin pitch (distance between the adjacent fins). Lower pitch resists the flow in the region between the fins lowering the improvement in heat transfer coefficient beyond a certain point. Therefore, the current project addresses this issue to report the best fin pitch.
Computational Fluid Dynamics (CFD) is an engineering tool that can be used to solve various complex fluid flow problems using the numerical approach. This provides
The primary aim of this project is to find the optimal fin pitch.
The following are the objectives of the project:
Although it is obvious that vehicles do not travel at the same speed all the time, it has been assumed that the fin system encounters a wind with uniform velocity. However, to take into account of the varying speed, two common speeds (60 and 40 km/hr) have been employed to obtain the heat transfer predictions and the subsequent analysis is done with the same.
Further to save the computational cost, only one fin section is taken applying the symmetric condition on the above and below to obtain back the entire finned system. Further the single fin has been halved from the top view and the front view horizontally (see Fig 2. and 3.) as the flow conditions and subsequent results would be identical. Hence all the simulations have been performed on a quarter of the fin.
The heat production in an internal combustion engine does not take place in a continuous manner. Hence the inner wall temperatures are subject to fluctuations for the entire cycle. But, to make the computations simple, this effect has been neglected and a constant wall temperature has been assigned.
The body under investigation has two parts viz. the main cylindrical body and the circular fins. The main cylindrical body has an inner diameter of 62mm and an outer diameter of 78mm and a height of 120mm. The circular fins begin from the outer wall of the cylinder with an initial thickness of 2mm. The fins taper down to a single edge of diameter of 148mm. The profile is tapered into in a convex arc. (see Fig 1.)
To make sure that the results obtained do not depend on the grid size, the Grid Independence Study is performed on various sizes starting from a coarse mesh to finer meshes. The Heat Transfer Coefficient is used as the parameter to observe the variations. It is found that for about 5.5 lakh elements the results showed the least deviation. Hence, for all the forth coming simulations the same mesh settings is used.
The current numerical investigation has been carried out in the commercial CFD software ANSYS Fluent (student version). The grid independence studies showed for about 5.5 lakh elements, the variation in the average Heat Transfer Coefficient and the Average Heat Flux through the surface remained almost constant.
The speeds at which the study is considered naturally falls into the turbulent flow conditions. Hence, appropriate turbulence model needs to be considered. In this case, boundary layer separation occurs on both the surfaces. Hence, the region near the wall must be carefully discretized to resolve the boundary layer and also its separation. The most suitable model is the k-ε model with enhanced wall treatments.
The fluid flow domain contains an inlet to which the wind speed is specified along with the ambient temperature. At the top, bottom and the symmetry line symmetric boundary condition is applied as they are periodic. On the free end of the fluid domain, zero shear is specified. In the solid domain, the inner wall is specified with constant wall temperature. All interfaces of the solid with fluid have no slip condition.
Fin pitch have been varied as the following for the reason as they correspond to the shown number of fins that can be uniformly put on the cylindrical body. The maximum number of fins possible are 20 considering the thickness of the fins.
| No of Fins. | Fin Pitch (mm) |
| 6 | 18 |
| 10 | 11.45 |
| 14 | 8.4 |
| 16 | 7.41 |
For each of the fin pitch used at the two specified speeds, the Heat Transfer Coefficient and the Total Heat Loss are found. It is observed that the Heat Transfer Coefficient increases until a point and then decreases as the fin pitch decreases. However, the Total Heat Loss behaves similarly except that it starts decreasing at a later point. (See Fig 5. and 6.)
From the graph it is clear that after the pitch corresponding to 14 fins for the specified system, the Total Heat Loss starts to reduce. This is true regardless of the operation speed. Hence to have the maximum benefit from the cooling fins, it is strongly recommended to use 14 finned system only.
The peculiar behaviour of the first increasing then decreasing Heat Transfer Coefficient can be attributed to the fact that in the larger pitch the heat transfer increases as the area allowing it increases. While on further reducing the pitch, there is increased resistance to the wind flowing between the fins reducing the effective speed and hence the Heat Transfer Coefficient. It can also be attributed to the fact that on lowering the pitch, the thermal boundary layers also overlap to some extent.
Although the Heat Transfer Coefficient reaches its peak, the Total Heat Loss reaches only later. This can also be understood by comparing the way the area available for heat transfer behaves in relation with the heat transfer coefficient. Despite the reduction in Heat Transfer Coefficient it is the total area which is the dominating factor enhancing the Total Heat Loss. However, further reduction in Heat Transfer Coefficient, overpowers the effect of area and hence dominates to lower the Total Heat Loss. Therefore the Heat Transfer Coefficient alone is not a determining factor for the optimal fin pitch.
The project employs CFD as a tool to optimise the design parameter of fin pitch to obtain the best performance. It has demonstrated that for the specified engine dimensions, 14 equally spaced fins is the best choice and hence is strongly recommended.
Air-Cooling Effects of Fins on a Motorcycle Engine, (Yoshida et al.,) JSME International Journal, Series B Vol 49, No. 3, 2006.
Report prepared on May 7, 2024, 5:12 p.m. by:
Report reviewed and approved by Nikesh Shetty [Piston] on May 10, 2024, 7:27 a.m..