Two-phase flow in hydrodynamic torque converter.

Manea, Adriana Sida ; Dobanda, Eugen ; Barglazan, Mircea 等

1. INTRODUCTION

The hydrodynamic torque converter is a transmission which consists from a combination between a hydrodynamic pump and a turbine in the same carcass (envelope). Hydrodynamic transmission found a large area of applications in automotive industry by cars, busses and heavy civil engineering machines, in mining, for ships and railway engines (locomotives) etc. In this article was investigated a special torque converter, Lysholm-Smith type, composed from a pump impeller, three stages of turbine runners and between them two stages of stators. The main advantages of this constructive solution are a greater speed reduction and an extended range of high efficiencies.

To control the torque converter operation were developed different methods and means. Between them, two-phase flow, namely air mixed with oil, was studied here.

The maximum temperature rise of the hydraulic transmision is one limiting parameter for the safe operation of the machine (Barglazan, 2001). To avoid this situation usually the torque converters are provided with cooling circuit.

It is important to establish what happens when a blocking (closing) of the cooling circuit occurs.

In the next chapters are investigated different aspects related to this situation.

2. THEORETIC CHARACTERISTICS

Theoretical model alloweded us to analyse the behavior of a hydrodynamic torque converter in complex regimes: in normal working regime--obtaining the characteristic curves, or in transient regimes--generated during starting up or stopping process (Milos, 2000). Main parameters of the model are: rotational speed of the pump shaft, rotational speed of the turbine shaft and the physic parameters of the fluid.

H p = 1/g x ([u.sub.2] x [v.sub.u2] - [u.sub.1] x [v.sub.u1]) =

= [r.sub.2] x [omega]/g x ([r.sub.2] x [omega] - Q/[[rho].sub.2] x [s.sub.2] x tg ([[beta].sub.2]) x Q/[[rho].sub.1] x [s.sub.1] x tg ([[alpha].sub.1])) (1)

The main step in modeling process are: calculation of kinematic elements of machines cascades, i.e. speed triangle elements. In this purpose, will consider the elements geometriey of the hydrodynamic torque converter as a constant and as variable the rotational speed at the primary shaft and the load to the secondary shaft. The specific energy transfered to the fluid by the pump is given by (1):

The specific energy transfered in the turbines, are given by (2):

[H.sub.Tj] = 1/g x ([u.sub.1j] x [v.sub.u1j] - [u.sub.2j] - [v.sub.u2j]) =

= [r.sub.1j] x [omega]/g x ([r.sub.1j] x [omega] - Q/[[rho].sub.1j] x [s.sub.1j] x tg ([[beta].sub.1j]) - [r.sub.2j]/[r.sub.1j] x Q/[[rho].sub.2j] x [s.sub.2j] x tg ([[alpha].sub.2j]) (2)

with "j" the number of the turbine stages (3):

[H.sub.T] = [summation over (j)]H [T.sub.j] (3)

calculation of hydraulic losses: hydraulic losses through shock at the entrance in blade cascades, losses dues to sudden modification of cross sections, losses dues to interblades channels curvature, friction losses between working fluid and solid walls, friction losses between adjacent fluid layers. The hydraulic losses are estimated using the Carnot--Borda relation (4):

[h.sub.p] = [zeta] x [v.sup.2.sub.0]/2 x g (4)

with the characteristic speed, [v.sub.0], given with the volumic flow thought the machine, and the loses coefficient, [zeta], calculated with the Reynolds number in wich the modification of the fluid with temperature and the degree of filling is taking into account;

--calculation of mechanical losses, dues to solid--to--solid friction.

Modeling the normal working regime, at standard asinchronous rotational speed of 975 rev/min shows a variation of torques at primary and secondary shaft machines, in function of speed ratio i = [n.sub.T]/[n.sub.P] as is presented in fig. 1.

[FIGURE 1 OMITTED]

3. WORKING FLUID PROPRIETIES

An important problem which appears in operating regimes of hydrodynamic torque converters is the modification of working fluid temperature. The fluid proprieties of two--phase flow oil--air--becomes variables in time (Stepanoff, 1965). As a consequence of temperature rising, the fluid proprieties would be affected. In particular way, the density and viscosity will be affected, and, also, the machine energetic parameters. Working fluid, being considered as a two--phase fluid, composed by mineral oil and air, impose to determine his proprieties and more, to analyze the modification of those proprieties as a function of temperature, how it was shown in previous observations. In fig. 2 is plotted the variation of kinematic constitutive viscosity coefficient of the mixed fluid as function of temperature and the degree of filling and fig. 3 represents the variation of specific mass (density) of the mixed fluids as function of temperature and the degree of filling (Soo, 1974).

[FIGURE 2 OMITTED]

[FIGURE 3 OMITTED]

Using the presented analytical model, was simulated several working regimes for a torque converter of a special type Lysholm--Smith, which has a primary machine (pomp), three secondary machines (turbines), and two reactors (composed of fix blade cascade) (Brennen et all, 1978). Fig. 4 shows some of the results obtained, regarding the behavior of this complex machine in special working regimes, with partial degree of filling and considering, also, the rise of the temperature.

[FIGURE 4 OMITTED]

4. TEMPERATURE MEASUREMENTS

For temperature determination in the two-phase fluid of the hydrodynamic transmission was used Chromel--Alumel type K probe and digital sensor type DS 1820. The Chromel--Alumel probe has 1.5 mm diameter. The advantages of measurements with thermocouples are: ready for connection to the measuring instrument, not sensible at shock and vibration, resistant to extreme pressures and also corrosion resistant.

The thermocouple output is an electrical signal obtained in mV, measured by a voltmeter, and the temperature value is obtained from the calibration curve. For DS 1820 sensor, the temperature is read from an electronic screen.

The comparison between the two methods of temperature measurement is presented in fig. 5., for 97,5 % degree of filling.

[FIGURE 5 OMITTED]

5. CONCLUSION

On the base of the theoretical results obtained, it can be told that the mathematical model allowed us to analyze the behavior of a complex machine as a torque converter in dynamic regimes generated in exploitation. The experimental results, that were obtained and were selectively presented, permit the wording of some outstanding conclusions concerning the two-phase mediums, the thermo-hydrodynamic properties of these. The knowledge of the unic degree of filling for different entrance speeds of rotation [n.sub.1] = const permits the establishment of stationary working regimes of the hydrodynamic transmission, from thermodynamic standpoint.

6. FUTURE WORK

In our next work we will try to obtain the transfer functions for the same torque converter, using the methods from the system identification studies.

7. AKNOWLEDGEMENT

This paper was possible trough the CNCSIS Grant IDEI cod 929/679/2008 director dr. ing. Adriana Sida MANEA and CNCSIS Grant IDEI 35/68/2007 director dr. ing. Victor BALASOIU.

8. REFERENCES

Barglazan, M. (2001). Hydraulic turbine and hydrodynamic torque converters, Politehnica, ISBN 973-9389-39-2, Timisoara

Brennen, C.; Cooper, P. & Runstadler P.W. (1978). Polyphase Flow in Turbomachinery, ASME, San Francisco

Milos, TC. (2000). Energetic Characteristics of an Improved Hydrofoil Cascade Inducer. Scientific bulletin of the Politehnica University of Timisoara, Vol.2, No.45(59), (October 2000) page numbers (309-316), ISSN-1224-6077

Soo, S. (1974). Fluid dynamics of multiphase systems, Mir, Moskva

Stepanoff, A.J. (1965). Pumps and blowers. Two phase flow, John Wiley&Sons, New York