Thermal Analysis on Dynamic Braking Resistor Unit

Transcription

1 Thermal Analysis on Dynamic Braking Resistor Unit B.Praveen Kumar Swami Vivekananda Institute of Technology, Hyderabad. B.Venkatesh Vardhaman College of Engineering, Hyderabad ABSTRACT Dynamic brakes are used for braking the locomotive when the locomotive is at higher speeds. As the braking torque required at higher speeds are large, air brakes are not effective and wear and tear is quite large resulting in frequent maintenance. In dynamic brakes, tractive motor is activated as generator and the output is fed to a set of resistors which in turn is cooled by axial blower drawing air from under the frame, blowing through the resistors. The hot air is delivered from the roof top into the ambient. The braking power is of the order of 2.5 MW. The exit temperature of the air is of the order of 300 C. The entire heat load is dissipated in every compact size hence the surface loading is very high. Added to that the elements undergo a severe thermal loading in a short span of time thereby it results in heavy thermal stresses. The elements at an approximate temperature of 800 C vibrate due to locomotive movement. The expansion of the resistors as the temperature of the elements rises 40 C to 800 C also pose a severe problem in the design. In this work calculations are carried out considering the combined effect of forced convection and radiation for different air flow rates and for different surface loading for uniform velocities. The effects of constant flow, variable load are also studied. From the comparison made between the calculated values and the rest results available with BHEL the variation taking into account the limitations of the measurements is for the order of +- 2 to 3% hence the calculations are considered to be quite accurate. From the results of the detailed thermal calculations carried out for D.C blower and A.C blower motor operations, it is felt for such a condition as that of a locomotive that it is ideal to go for A.C. Blower motor operation as the temperature rise in the element comes down by about 18 0 C. Hence the life of the DBR can be enhanced. As radiation is also taken into account in addition to forced convection for the forced flow operation. The objective of studies is extremely useful for BHEL in the developments of DBR unit being supplied to Indian Railways. Keywords Dynamic brake, DBR, Brake Resistor, Constant Flow, Variable Heat load. INTRODUCTION Cast-iron brake blocks, rubbing against the wheel treads have been the most common form of braking on railway vehicles for nearly 150 years. In recent years disc brakes have become more widely fitted, but cast iron blocks are still the most common form of railway braking even today. Today, there is a large range of non-asbestos composition brake blocks available from a number of manufactures worldwide, catering for a whole range of friction levels, however, despite the number of materials available, there still are not any direct replacements for cast iron brake blocks which do not have some disadvantages, particularly with regard to adhesion, track circuitry and to lesser extent thermal damage to the wheel. These factors led to the development of a wheel-mounted disc brake for a high speed train. The disc consists of two rings bolted on to the wheel hub, one on either side of the wheel. Modeling of bulk thermal effects was found to be most effective for shape optimization and prediction of global disc behavior, with more sophisticated analysis required to determine tests and vehicle trails gave good correlation with theoretical results and proved suitability of the design for the required duty. The design is capable of high performance braking of power cars and other vehicles where space around the axle is restricted. The major reasons for the introduction of the wheel mounted discs on trains were duty increase and noise reduction and noise reduction at high speeds (generated by wheel conditioned by the tread brake). The basic requirement is for the disc to be bolted to the wheel hub, which substantially simplifies wheel design and minimizes the wheel loading from the brake. Although some of the existing wheel (Hub) mounted disc brakes 68 B.Praveen Kumar, B.Venkatesh

2 used a simpler fixing method, the duty and certain design requirements make this disc design quite new. The design is considered to be suitable for wider range of vehicles, form power cars and locomotives, to freight vehicles with space restriction around axles. In the modern age, travel time/transport time is becoming increasingly important. There is a need to increase the speed of the locomotive and haulage of larger weights. This calls for increased speed control and consequently higher braking torque. This leaves large amounts of energy to be dissipated quickly and safely during braking either by friction or by the use of the traction motors as known as electric or dynamic braking. In general, friction brakes are fitted on all mass transit rolling stock as their high reliability makes them suitable for use, usually exclusively, in emergencies. The degree of usage in service braking, however, tends to be minimized due to the ongoing costs associated with wearing friction surfaces. In order to minimize friction braking, two forms of electric braking are used, either regenerative or rheostatic. Regenerative braking enables a braking train to return energy to supply rails to be utilized by accelerating trains elsewhere on the system. It relies, however, on those trains being close enough for the energy to be transmitted to them and is dependent upon a large number of designs and operational variables of the rolling stools, power and signaling systems. The ability of the supply to absorb a trains braking energy, known as its receptivity, is a time and position dependant variable which can range between 0 and 100%. Should the supply not be fully receptive the excess electrical energy is usually dissipated in train carried resistors, rheostat braking. Thus, dynamic brakes are used for braking the locomotive when the locomotive is at higher speeds. In dynamic brakes, tractive motor is activated into generator and the output is fed to a set of resistor girds. These resistor grids are in turn cooled by vertical axial blower, which is placed below the resistors and mounted on the locomotive flat. BASIC THEORY The thermal calculations of DBR Unit are performed considering two modes of heat transfer. (1) Forced Convection and (2) Radiation At high velocities, forced convection predominates free convection. Therefore the effect of free convection is not considered for these calculations and only the effect of forced convection is taken into account for one tray layout. There is a rise o temperature of air from 40 O C (at the inlet of the 1 st tray) to about 240 O C (at the out of the 6 th tray) in the DBR unit (given in fig.1) for a rated heat load 2.43MW. Due to this there is a large variation in the physical properties of air such as density, thermal conductivity, specific heat, kinematics viscosity etc. Hence it is required to calculate the combined effect of forced convection and radiation heat transfer for each tray in the DBR unit. There are totally 6 trays in one design of the DBR unit. DESIGN VARIANTS AND PRACTICAL UTILITIES Thermal studies on DBR in this case are calculated for different variations in parameters such as constant flow, variable flow and losses etc. The present work is carried out on constant flow and variable heat load. CONSTANT FLOW, VARIABLE HEAT LOAD: The blower motor which is used to blow air through the resistors in this case is an A.C. blower motor. The supply to these blowers is arranged through static converter / rotary converter and therefore the air discharge is practically constant at all levels of braking current. The thermal calculations are therefore performed for different heat loads, keeping the low through the DBR unit constant. The calculations are carried out for two conditions; Static condition and dynamic condition. 69 B.Praveen Kumar, B.Venkatesh

3 (m 3 /s) (m 3 /s) Fig.1 RESULTS AND DISCUSSIONS The results of the calculations carried out are analyzed as follows: CONSTANT FLOW, VARIABLE HEAT LOAD: Static Condition: the element temperatures rise for each tray, in C for different losses are shown in tables no 1,2 and 5. It can be seen that the element temperatures of tray 5 greater than the elements temperature of all the other trays. Therefore that elements temperature is considered as the hot spot temperature. TABLE.1 Constant Flow, Variable Heat Load (Static Condition) From the results in the table, the hot spot temperature for this condition is around 603 C for rated loss of KW. it is minimum for a loss of being around 433 C. The exit temperature of air each tray has also been 70 B.Praveen Kumar, B.Venkatesh

4 S.No S.No calculated. The final exit air temperature (from tray 6), is around 217 C for a rated loss KW. It is again minimum for a loss of KW being around 147 C. TABLE.2 Constant Flow, Variable Heat Load (Static Condition) Dynamic Condition: The element temperatures for each tray in dynamic condition are given in tables no 3, 4 and 6. It can be seen that the hot spot temperature (temperature of tray 5) in this condition is around 690 C which is greater than in static condition. It is minimum for a loss of KW being around 558 C which is still greater than the static condition. Different graphs to study the variation of elements and other and the other governing parameters in both static and dynamic conditions are drawn. TABLE.3 Constant Flow, Variable Heat Load (Dynamic Condition) TABLE 4 Constant Flow, Variable Heat Load (Dynamic Condition) B.Praveen Kumar, B.Venkatesh

5 The temperature rise of elements in C Vs. Tray number in airflow direction gives a curve for both static and dynamic conditions shown in fig.1 and fig.2. The graph shows that the temperature rise of the elements is greater in dynamic condition than in static condition for different losses. Fig.1 It is the exit temperature of air in C Vs. tray number in air flow direction, for different losses in both static and dynamic conditions as given in fig.3 and fig.4. TABLE.5 Constant Flow, Variable Heat Load (Static Condition) Fig.2 Governing Parameters Avg.Temp rise of 6 trays 0 C Avg.temp rise of 4,5&6 trays 0 C Hot Spot Temp Rise (tray5) 0 C Final Exit Air Temp. 0 C B.Praveen Kumar, B.Venkatesh

6 Gross Flow TABLE.6 Constant Flow, Variable Heat Load (Dynamic Condition) Governing Parameters Avg.Temp Rise of 6 trays 0 C Avg.Temp Rise of 4,5&6 trays 0 C Hot Spot Temp Rise (tray5) 0 C Final Exit Air Temp. 0 C There is almost a linear variation of exit temperature of air, for each tray with respect to tray number in the direction of airflow. The graph shows that the exit temperatures of air are greater in dynamic condition that in static condition for different losses. It is due to the reduction of flow by about 10% in the dynamic condition. Fig.3 Fig.4 CONCLUSIONS 1. Detailed thermal calculations were carried out for D.C blower and A.C blower motor operations. The D.C blower motor operation is for variable air flow and variable heat load and the A.C. blower motor operation is for constant air flow and variable heat load. It is felt from the results for such a condition as that of a locomotive that is ideal to go for A.C. blower by about 18 C. Hence the life of the DBR can be enhanced. 2. It is always from the calculations, the hot spot occurs in the 5 th tray. Hence it may be advisable to connect a D.C blower motor in parallel with the 5 th tray. 3. A comparison has been made between the calculated values and the test results available with BHEL and the variations taking into account the limitations of the measurements is of the order of +/- 2 to 3%. Hence the calculations are considered to be accurate. 73 B.Praveen Kumar, B.Venkatesh

7 4. In forced flow operation it is essential that both forced convection and radiation are to be taken into account as the radiation plays a role to the extent of 25 to 30% of the total heat transfer. 5. Calculations have been carried out for free convection also. In the presence of strong forced convection the part played by free convection is negligible. Hence for further calculations the effect of free convection is neglected. It is concluded from the calculations that a minimum air flow required for a temperature rise of 600 C of the element should not be less than 9.5 m 3 /s. REFERENCES [1] X. M. Huang and G. B.Ning, 2013, Study on Mechanical Braking System Development Method and Validation for Electric Vehicles, Proceedings of the 3rd International Conference on Advanced design and manufacturing engineering, Anshan, China. [2] G. Y. Zhang and Q. Z. Li, 2013, Applied Mechanics and Materials, vol. 380/384. [3] P.L.Rongmei,Shimi S.L, S. Chatterji and Vinod K. Sharma, 2012, A Novel Fast Braking System For Induction Motor, International Journal Of Engineering and Innovative Technology, Volume 1, Issue 6. [4] M. J. Chung and Y. J. Kim, 2014,Design of Roof Type Dynamic Braking Resistor for Railway Carriage by using Thermal Analysis, Proceedings of the 2014 International Workshops, Jeju, Korea. [5] W. Freitas, A. Morelato and W. Xu, 2004, Improvement of induction generator stability using braking resistors, IEEE Trans. Power Syst., vol. 19, no. 2. [6] D. F. Peelo, D. W. Hein and F. Peretti, 1994, Application of a 138 kv 200 MW braking resistor, Power Eng. J. vol. 8. [7] Yunusa.Cengel, Afshinj.Ghajars, 2013, Heat and Mass Transfer, Tata McGraw-Hill Education Pvt. Ltd. [8] Kothandaraman, C.P and Subramanian, S, 2014, Heat and Mass Transfer Data Book, New age International,Edition 8. [9] Myung-Jin Chung, 2014,Design Method Using Thermal Analysis in the Development of Electric Braking Resistor for Railway Carriage, International Journal of Control and Automation Vol.7, No.8. [10] Frank P. Incropera, David P. DeWitt, Theodore L. Bergman, Adrienne S. Lavine, 2012,Principles of Heat and Mass Transfer, 7th Edition International Student Version. 74 B.Praveen Kumar, B.Venkatesh