Proceedings of the 23rd CANCAM

HEAT TRANSFER ENHANCEMENT IN VENTILATED BRAKE DISC USING AIRFOIL VANES

A. Nejat1, E. Mirzakhalili2, M. Aslani3, R. Najian Asl4 Department of Mechanical Engineering

University of Tehran Tehran, Iran

ABSTRACT

In this research, the curved vane brake rotors are studied via a detailed numerical simulation. To increase the air pumping efficiency a novel design is used by means of the airfoils. In order to enhance the ventilating capacity, the airfoil geometry, the arrangement and its installation angle are studied to find a maximum cooling performance. A steady state scenario for braking is investigated using CFD and the corresponding heat dissipation rate is computed for different velocities. Some specific recommendations are made to achieve better thermal efficiency for the brake system.

Keywords: Brake disc, Airfoil vanes, CFD.

INTRODUCTION

The brake discs utilize rubbing or friction surfaces to decelerate the ongoing car. In this action, the generated heat due to energy transform is conducted to the components of the operating (brake) system. What is called the brake fade is the tragic result of this cyclic heating. To wipe out the braking fade thus maximizing vehicle safety many solutions have been proposed.

Fig. 1: Ventilated Brake Disc using curved vanes

1- Amir Nejat, PhD (email: nejat@ut.ac.ir) 2- E.Mirzakhalili, (email: e.mirzakhalili@ut.ac.ir) 3- M.Aslani, (email: m.aslani@ut.ac.ir) 4- R.Najian Asl, (email: r.najian@ut.ac.ir )

One of the most important solutions is to use vented

discs consisting of two rubbing surfaces separated by radial vanes. These ventilated discs act like an air pump, circulating air from center (through the passages) to the outside of the rotor (Fig. 1).

Many profiles are designed for the mean layer of the co-rotating disc in order to improve the aerodynamic cooling properties of the disc thus increasing the heat transfer coefficient (HTC).

Research studies in the literature are mainly focused on finding a true model for this complex thermal-fluid system. R. Limpert [1] compared the solid and the ventilated rotor thermal performance. Also the temperature distribution of the solid rotor has been investigated by Limpert using Duhamel’s theorem by the assumption of a constant heat flux during constant-speed downhill braking [2]. Sisson [3] used Duhamel’s theorem to integrate an analytical slab solution with a nodal solution to come up with a general formula for the straight vanes. L. Wallis et.al [4] investigated different vane geometries including pillar posts and round radial vanes. F. Talati and S. Jalalifar [6&7] obtained the governing heat equations for the disc-pad system in form of transient heat equations in which heat generation is dependent on time and space.

In this research a novel design for the geometry of the vanes is presented. A NACA type airfoil is employed as the well known profile to accelerate the air flow. Being familiar with the physics of the flow over these profiles, a superior shape and arrangement for vanes are suggested. The new design is numerically tested for brake disc cooling and its promising performance is compared with the classic straight vane design in some vehicle speeds. The main drive in our research is to increase the speed of the air passed by vanes and this has been a general guide considered in almost all articles in the field. Also another heat transfer improving factor, i.e. keeping the flow attached to the vanes, is explored in detail through our CFD simulations.

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MODELING THE BRAKE DISC A. Theory

In extreme braking operations such as steep descents or

repeated high speed brake applications, the sufficient heat dissipation becomes crucial to ensure reliable continued braking. The heat is dissipated by radiation, conduction and convection. According to Limpert [1] the amount of heat dissipation through radiation under normal braking conditions is less than 5% of the total heat dissipated form the rotor. Therefore as many other studies in the literature, here, the radiation is neglected. Although conduction plays an important role in terms of heat transfer in the brake disc, it has adverse results in the nearby components as mentioned before. The third and the most important way to dissipate heat from the discs is convection. Convection may happen in two ways, sides of the rotor and the vanes of a ventilated disc. The dissipation from the vanes can be expressed by the following formula:

E convection=A ventilation ×h× (T-Tout) (1)

Ventilation area (Aventilation) is confined to the limited space available between two co-rotating discs. This might change slightly, depending on the type of the ventilation and the vanes geometry, but the overall effect is very slight and can be ignored when discs of the same diameters are compared. Therefore the most reasonable way to accomplish our goal, i.e. increasing the dissipation rate, is to increase the convective heat transfer coefficient.

B. Verification

Different strategies maybe available depending on the driving situations, but as we want to assess the capacity of the brake discs in extreme braking operations, a steady state model of a moving car with applied brakes in a monotonous fashion is considered. In this case the disc reaches the maximum temperature which can be obtained from a simple heat transfer computation. In this computation the disc brake is assumed to be isolated so that the effect of cooling is ignored. In this way we can find a maximum steady temperature field for the disc. The obtained temperature field depends on different parameters like inner and outer radius of the disc, angular velocity, friction coefficient and applied pressure on the disc. To model our brake disc the temperature of the vanes of the disc is assumed to be 600°C. This obtained temperature (the detail of its computation is ignored here for brevity) well matches the result of studies like [6] or the analytical and experimental results [1, 2 &3].

A sensible and true modeling is the one validated with a former experimental work. To verify our model, we did a comparison with two important and well-known empirical correlations of Limpert [1] and Sisson [3] and a numerical approach by L. Wallis et.al [4] (Fig. 2). These results are obtained for straight vane geometry. The same results will

also be used for a base comparison when new geometries are introduced.

Table1 shows the detail conditions of our CFD simulation. The unstructured (triangle) meshing is also used for all cases in this study as it can successfully and easily cover the complex geometry curvatures. The reason of creating the same mesh for all of the geometries is the fact that, in this way, they are all introduced to almost equal error when the geometry is discretized. Therefore, there is a reasonable consistency in our result comparison.

INTRODUCING AN IMPROVED NOVEL GEOMETRY

In this section, a new design is presented for ventilating vanes which improves the overall convection heat transfer characteristics significantly. A very new design of the vanes was presented by S. J. Ruiz and R. Beach [5]. This design makes use of the curved vanes similar to airfoil shapes in its leading edge and combines the properties of a nozzle and diffuser. The study of these geometries led us to the two following cases. In each of these cases we tried to maximize the cooling efficiency of the brake disc.

Fig. 2: Comparison of the results of straight vanes disc

Table 1- Flow Condition properties

Meshing

Parameters

Mesh Type Unstructured Triangles

Number of Cells 56000-95000

Element Size (m2) 2.75×10-4 ~2.627×10-3

Solution Properties

Spatial Discretization

Second Order Upwind

Turbulence Realizable k-ε

Fluid

Properties

Reynolds no.*

2.46×104~1.23×105

Prandtl no. 0.685

* Diameter of the disc is used for Reynolds number

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A. Case1: Single Airfoil

According to the previous discussion, we are motivated to use airfoils for the vane profile of the brake disc. The leading edge curvature of the airfoil rapidly accelerates the flow with minimum pressure drag (as the airfoil is designed for such purpose) making the airfoil a very good candidate to perform the cooling task. The airfoils arrangement creates a suitable passage for the airflow over the brake rotor. In the first case, simple NACA 0009, due to its superior lift to drag ratio among other NACA airfoil series, is selected as a candidate to explore its performance in the brake disc cooling. Fig. 3 shows the geometry of the vanes and in Fig. 4 the contours of the velocity magnitudes are compared with straight vanes.

 

Fig. 3: Geometry of the vanes in Case1

 Fig. 4: Contours of Velocity Magnitude, a) Straight, b) Case1

The Fig. 4 clearly explains the effect of the flow

acceleration of the new. The nozzle shape at the entrance of the vane passage speeds up the air to a maximum magnitude but then the air decelerates because it enters the diffusing area. The tendency of the airflow to stick more to the airfoil surface than to separate from the back vane region improves the HTC of this geometry as well. Here the HTC is increased over 6 to 15% in comparison with the straight case for different radial velocities.

B. Case 2: Double Airfoils

Based on the model obtained above, one main flaw in the flow field is obvious and that is the existence of a dead part of the vane which happens to be in backward or in shadow of the neighbor’s vane. To cure this defect partially, a secondary NACA-0009 with smaller relative chord is placed close to the end of the brake disc between the passages of the main airfoils (Fig. 5) in such a way to boost the momentum of the airflow again and limits the separation, i.e. the dead region, over the rear part of the brake disc. The result of this new design is demonstrated in Fig. 6 in which the velocity contours of both cases are shown. The effect of added airfoil is obvious due to the considerable reduction of the dead region.

 

Fig. 5: Geometry of the vanes Case2

Fig. 6: Contours of Velocity Magnitude, a) Case1, b) Case2

To reveal the best quality of this design, in Fig. 7, the

contours of the temperature are shown. Since the air flow is accelerated, it has assisted cooling down the vanes especially in areas where the single airfoil design failed to cool down appropriately. Also the regions with high temperature are eliminated in the new design in comparison with straight vanes. To find the proper setting for the secondary airfoils, some test cases including the position and its deviation angle

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from radial line were performed. Results showed that for ±5 degrees of deviation from the radial line the overall change in HTC number were lower than 0.1% when compared to the original neutral positioning.

In all of these design steps, we followed an approach to improve the HTC of the ventilated brake disc by the vanes modification and step by step an improvement in HTC was observed. Table 2 summarizes the test cases and the results that obtained in this research.

Fig. 7: Contours of Temperature distribution for Case1 & 2

CONCLUSION

The flow fields of the various air passage topologies were

modeled via CFD simulation. Some detailed analysis was performed by studying the velocity and temperature distribution around vanes. The result showed that increasing the flow momentum and limiting the flow separation region especially close to leading edge of the vanes are the key factors in overall HTC improvement. Improving the air pumping efficiency noticeably using a novel design was introduced by means of two airfoils as primary and secondary vanes. The overall HTC was enhanced between 18 to 30 percent for different angular velocities using the new design. As a future work we intend to optimize the location and the shape of the secondary vane to achieve the optimal HTC for a double vane brake disc.

REFERENCES

[1] Limpert, R., 1975, Cooling analysis of disc brake rotors, SAE paper number: 751014.

[2] Limpert, R., 1999, Brake design and safety, Second

edition, SAE International, Warrendale, PA. [3] Sisson, A.E., 1978, Thermal analysis of vented

brake rotors, SAE paper number: 780352. [4] Wallis, L., Leonardi, E., Milton, B., and Joseph, P.,

2002, Air flow and heat transfer in ventilated disc brake,

rotors with diamond and tear-drop pillars, Numerical heat transfer, part a 41: 643-655.

[5] Ruiz, S.J., and Beach R., 2004, VENTILATED BRAKE

ROTOR, Patent No.: US 6,796,405 B2. [6] Talati, F. and Jalalifar, S., 2008, Investigation of

heat transfer phenomena in a ventilated disc brake rotor with straight radial rounded vanes. Journal of applied sciences, 20:3583-3592.

[7] Talati, F., and Jalalifar, S., 2009, Analysis of heat conduction in a disc brake system, heat mass transfer, 45:1047–1059.

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