5 Experimental Procedure for Rotating Ventilated Disk

This chapter illustrates experimental procedure and rig development for a ventilated brake disk. Ventilated brake disk is the state of the art technology in automobile braking system. It is well known that the braking capability of brake disk is affected by the rate at which heat is dissipated through forced convection. The rapid increase and decrease of the brake disk temperature could lead to catastrophic failure of the brake disk due to high thermal strains. Heat transfer property enhancement in any ventilated disk design is an important interest to researches. The heat transfer analysis can be conducted experimentally or numerically. Experiment can be carried out on vehicle testing and in laboratory testing procedures. This chapter describes a method for laboratory testing procedure for vehicle disk brake design heat transfer property evaluation.


Introduction
This chapter illustrates experimental procedure and rig development for a ventilated brake disk.Ventilated brake disk is the state of the art technology in automobile braking system.It is well known that the braking capability of brake disk is affected by the rate at which heat is dissipated through forced convection.The rapid increase and decrease of the brake disk temperature could lead to catastrophic failure of the brake disk due to high thermal strains.Heat transfer property enhancement in any ventilated disk design is an important interest to researches.The heat transfer analysis can be conducted experimentally or numerically.Experiment can be carried out on vehicle testing and in laboratory testing procedures.This chapter describes a method for laboratory testing procedure for vehicle disk brake design heat transfer property evaluation.
A rotating test rig is described with unique outflow measurement casing design to restrict outflow with minimum leakage.The casing is important to measure the total air metered through the ventilated disk passage due to inductive air flow.The chapter includes the test procedure and the post processing of the data collected.The devices, such as infrared sensors, thermocouples, flow meter, magnetic speed pick-up and pressure transducers are all connected to Data Acquisition System (DAQ), where the signals are converted to raw data files.The data acquisition module and graphical outputs are shared during the transient recording.The stationary but rotating test rig allows designer to eliminate cross wind effect and other fittings in the car wheel components to focus on the blade geometrical aspect alone.The immobility of the test rig makes it easier to measure the flow pass through and heat transfer from the disk brake.

Brake disk experimental rig
A preliminary experimental rig is initially developed to measure the amount of air metered into the ventilated brake disk and the pressure between the rotating blades.The inlet velocity is measured by taking the velocity integral in the inlet ducting using pitot-static tube.Schematic diagrams of the test rig with the components positions shown in picture below.The initial test rig was worth the effort to minimize lead time in designing the rotating test rig with DAQ system and sophisticated measuring equipment.

Part
No.

Description
Part No.   .There are Pt-type, K-type and infrared type thermocouples involved in the measurements of temperature.The tube around the casing is to measure the outlet static pressure.

Error analysis
Experiments are basically done to verify some statement, by determining objectives, methodology and research are carried out also at first, followed by progress, results, and analysis with discussion and finally conclusions are made.These are the basic steps on how an experiment is being conducted, and there are no limitations for sub-stages to be included in each step.Specifically, analysis can be categorized to few sub-stages, such as measuring and obtaining some data, tabulate it and plotting a graph based on table.Besides that, error analysis is vital part and it's divided into two fraction; random error and systematic error.These analysis are done mainly to shirk wrong analysis as well provide good accuracy during collecting data from the measurement device.

Random error
Random errors are errors in measurement that lead to measured values being inconsistent when repeated.The word random indicates that they are inherently unpredictable, and have null expected value, namely, they are scattered about the true value, and tend to have null arithmetic mean when a measurement is repeated several times with the same instrument.Random errors are also can be defined as an error that results from unpredictable variations from one or more influenced quantities.Random error is mainly caused by unpredictable fluctuations in the data of a measurement apparatus, or in the human interpretation of the instrumental reading; these fluctuations may be in part due to interference of the environment with the measurement process.The concept of random error is closely related to the concept of precision.There are no way of perfect experiment to be done without any random error, but the higher the precision of a measurement instrument in an experiment, the smaller the variability (standard deviation) of the fluctuations in the readings.

Systematic errors
Systematic errors are also known as bias error in measurement which leads to the situation where the mean of many separate measurements differs significantly from the actual value of the measured attribute.Often of several different types of measurement are prone to systematic errors.Sources of systematic error may be imperfect calibration of measurement instruments and sometimes imperfect methods of observation can be either zero error or percentage error.
Systematic errors may also be present in the result of an estimate based on a mathematical model or physical law.For instance, the estimated oscillation frequency of a pendulum will be systematically in error if slight movement of the support is not accounted for.Systematic errors can be either constant, or be related (e.g.proportional or a percentage) to the actual value of the measured quantity, or even to the value of a different quantity.

Comparison of random errors and systematic errors
Many are still confused between random and systematic errors.Random error will always appear in a measurement and being caused by unpredictable fluctuations in the readings of a measurement apparatus or with experiment conductor's interpretation of the instrumental reading.
On the other side, systematic error is quite predictable, and always either constant or proportional to the true value.It can be eliminated if the cause of the systematic error can be identified.Systematic errors are caused by imperfect calibration of measurement instruments or imperfect methods of observation, and always affect the results of an experiment in a predictable direction.Inversely for random errors, its unpredictable criteria makes the error hardly identified, yet eliminate it.Example, distance measured by radar will be systematically overestimated if the slight slowing down of the waves in air is not accounted for.Incorrect zeroing of an instrument leading to a zero error is an example of systematic error in instrumentation.

Measurement devices error analysis
To gather all information regarding systematic errors, user manuals for all equipment were researched.Below is the table describing the channels representing hardware, basic description about the function of the hardware and systematic errors.When calculating for combined errors for certain formulas or equations, the errors for standard operation is chosen.The errors also selected when the variables are in 100% condition, example for flow meter which having two condition of error selection.The first condition of ±1.5% of measured value is chosen because flow rate is 100%, which indicating no disturbance in the flow.For pressure transducer, 0.1% error is chosen in calculation of error analysis, because the Experimental Rig is assumed working under standard operation.Other errors are taken directly, which had given in the respective user manuals that suitable for all kind of condition.

Pitot-static measurement
The flow measurement is first measured using pitot-static tube.The schematic of the experimental rig illustrated in Fig. .As shown, to constraint the flow through the disk specimen, a custom-made brake disk casing is fabricated.In order to measure the air inflow velocity, an circular duct was built.Additional tapping is also made on the brake disk casing to tap the inlet and outlet pressure between the disk blades in radial direction.This experimental rig can be tested using various brake disk specimen with constant outer diameter.Another constraint to this test rig is the maximum revolution shall go up to 2600 rpm which is equivalent to 320km/h road speed.
The equation 1 is for laminar flow and equation 2 is for turbulent flow.The highest Reynolds Number is turbulent.Thus, equation 2 is used to calculate the entrance length.The entrance length e l of a 2.5m ducting is constructed for the pitot -static tube velocity measurement.Fig shows the pitot -static tube measuring position.The velocity in traverse direction (see Fig is measured using pitot tube with reference to the BS1042.Then, the flow rate is calculated using multiple-application trapezoidal rule.An incline liquid manometer is used to measure the small static pressure difference.

Oscillation flow meter (model: DOG)
This device works without any moving parts.An orifice in the lower section generates a flow resistance through which part of the flow is fed into the measuring head.There is then a spontaneous oscillation of the gas in the measuring head.The oscillation frequency is proportional to the flow velocity and consequently to the volume flow.Since the relationship between the flow through the measuring head and the flow through the measuring casing is constant the oscillation frequency also remains directly proportional to the overall volume flow through the device.A hot wire sensor determines the oscillations in the measuring head.A signal transducer feeds the sensor with power supply and amplifies and scales the oscillations.This flow meter manufactured by Kobold.Flow meter's technical information listed as below:-

Calculation of convective heat transfer coefficient
The cooling results from test rig were used to determine the convective heat transfer coefficient.The radiation heat transfer was deducted from the disc total heat transfer coefficient and heat conduction to shaft is neglected as the disk brake housing was insulated.At steady state the disk temperature is recorded T d .The disk temperature will vary in accordance to the convective heat transfer performance of the ventilated blades.The heater is set to be on constant heat flux mode.Thus the heat transfer coefficient is calculated as below: (3) The equation 3 indicates the heat transfer coefficient, h c calculated at steady state heating of the brake disk.

Error analysis for non-dimensional groups
Error analysis is important to be done for the equations because few variables are involved, which carrying their own errors.So calculation is necessary in order to determine new error   Once temperature reached steady state, stop the process and save the data.
To convert raw data to a readable Excel file, 'Convert' button clicked and the file is saved.

Conclusion
The experimental rig is a control environment to focus on the heat transfer enhancement of the disk brake alone.The parameters like mass flow rate and Nusselt number is used for classification of the trend of heat transfer characteristics.
For instance for the above Figure 14 nusselt number plot, an expression can be derived with with curve fitting R 2 value less than 1.
For the low speed region where Reynolds number < 2.4 X 10 5 the Nusselt is correlated with Reynolds number with the relationship is given by: 0. .
The Nusselt number against Rotational Reynolds number is given for references.
For the laminar flow condition, the Kreith et.al. ( 7) correlation is given in equation 10: The best fit is given by power of 0.5 for laminar flow condition which agrees well with Kreith (Kreith, Taylor, and Chong, 1959) as in the equation 10.
The experimental procedure described in this chapter can be cost effective to design an improved brake disk for industrial application on vehicles.The rig also demonstrates confident results to be used as significant contribution in disk brake heat transfer area.

Fig. 1 .
Fig. 1.Schematic of Experimental Rig and the Main Components Test rig shown above used to predict the flow range using pitot tube which is labeled as no.8.The upgraded version of the experimental rig has ability to measure sophisticated measurements, such as total flow rate, surface temperature and pressure which collected by DAQ.The enhanced version of test rig circuit drawing shown in Fig to observe the position of the measuring device which placed at the test specimen.Moreover the brief explanation of the thermocouple location also has been discussed in Table.There are Pt-type, K-type and infrared type thermocouples involved in the measurements of temperature.

Fig. 2 .
Fig. 2. Ventilated brake disk experimental rig Fig. illustrates the photographic view of the upgraded experimental rig used.The Fig. show the overall view of the test rig with long intake duct to ensure developed flow at the flow meter.The Figure (a) shows the long air flow ducting to ensure a fully developed flow passes through the flow meter.Figure 3(b) shows the disk brake stationary casing and the manual display of the parameter measurements.The design shows the custom-made Aluminum material casing that guides the outflow radially out and axially directed in through the center of the brake disk with almost zero leakage.The ventilated brake disk of 255mm diameter is fitted with only 1mm clearance between rotating disk and the casing.The tube around the casing is to measure the outlet static pressure.

Fig. 4 .
Fig. 4. Traversed Pitot tube positioning at 8 radial points in pipe cross-sectionThe experimental rig is mainly developed to measure the amount of air metered into the ventilated brake disk and the pressure between the rotating blades.The inlet velocity is measured in the by taking the velocity integral in the inlet ducting.The screen which keeps the flow attached in a ducting is not needed because there is no sudden change in the diameter of the ducting(Mehta, (34)).Then, the entrance length e l was calculated with reference toMunson et.al. (35)

Fig. 9 .
Fig. 9. Raw results with analog signal upon capturing started for all channel

Fig. 12 .
Fig. 12. Mass flow rate measured using flow meter and pitot-static manometer for straight blade brake Disk

Table 2 .
Table of representative channel, description and systematic error for hardware in Disc Brake Experimental Rig

Table channel 3
, 4 and 8 are not connected to any hardware, and might be used in future if more measurement is needed.Similar brand and model is used for each type of sensors, so similar errors are obtained.
This model is a low cost, super compact infrared transmitter.It measures temperature via non contact, and provides an analog output proportional to the measured temperature.This unit has a fixed Emissivity of 0.95 which makes it easy to measure temperature.The range is larger than the optical field of view of the transmitter.The design, 19mm OD x 89mm length is ideal to measure temperature in confined, and hard to reach places (shown in Fig).This transmitter can operate in an ambient temperature of 0 °C to 70 °C.There is a warm up period of 2 minute after power up.This flow meter manufactured by Omega Engineering.