Power loss reduction of the magneto hydro dynamic naval truster.

Moroianu, Corneliu ; Grozeanu, Silvestru ; Ciobanu, Camelia 等

1. INTRODUCTION

An M.H.D. naval thruster is a homo polar electrical engine using a pipe full with sea water as a moving drive. The pipe is parallel with ship longitudinal axis. The bow has an inlet nozzle and the stern an outlet nozzle. Perpendicular on pipe axis two electrodes produces an electric current through water. A magnetic field is also applied. The result is a Laplace force. It moves the sea water through this pipe--fig. 1.

[FIGURE 1 OMITTED]

The length of the electrodes and the magnetic field application area being shorter than the pipe, a certain part of the current will be dispersed in the the exterior of this area producing a boundary effect.

It consists in a current circulation outside of electrodes. This dispersed current escapes from magnetic field influence. In this way it increases the lost power. To limit this effect, some restrictive blades from insulating materials are placed parallel with electrodes. This specific zone is extra electrode space (Shatrov, 2006).

These blades reduce dispersed currents, increase system electrical resistance and decrease power loses with several percents--Fig.2.

The [R.sub.i] is the resistence between electrodes when the dispersed current is blocked.This space is the intra electrode gap. The gap between electrodes is 2a, the gap length is 2b, [eta]=a/b, the power developed in the intra electrode gap is [P.sub.i] and restrictive blade length is l. Solving the Laplace equation results that the intra electrodes gap potential is (Gavrila, 1999):

[FIGURE 2 OMITTED]

V{x, y) = [[summation].sup.[infinity].sub.n=1] [A.sub.n] e - n[pi]x/a sin n[pi]x/a - U/2a (1)

and extra electrode gap potential is:

[V'.sup.(x,y)] = [[summation].sup.[infinity].sub.m=1,3,5..] [B.sub.m] e -m[pi]y/2a sin m[pi]x/2a (2)

Boundary conditions determine [A.sub.n] and [B.sub.m] coefficients. Total electrodes--seawater system resistance with restrictive blades is:

R = U/I = - U/2[sigma]h ([[integral].sup.b.sub.0] [partial derivative]V(x,y)/[partial derivative]x dx + [[integral].sup.[infinity].sub.l] [partial derivative]V'(x,y)/[partial derivative]x dx) (3)

where [sigma] is the electrical conductivity of seawater and h is the electrode height.

Intra electrode gap resistance with circulation of outside current limited by blades is:

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (4)

Power loses through dispersed currents using limitation blades

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (5)

Previous relation reveals without doubts that resistance increases and the power loss decreases when limitation blades length is increased. Increasing limitation blades length also increases frictions and hydro dynamic loses. The optimum limitation blades length should be find and used (Lin, 1991).

This study presents several experiments in order to confirm the mathematical model of increasing resistance of electrodes--sea water system with restrictive blades increase. The aim is blades optimal length.

2. EXPERIMENTAL DEVICES

We performed several experiments which relate resistance and blades length. Blades were fixed in vertical position, parallel with pipe axis outside intra electrode gap. In Fig.3.a and 3.b, there are an upper view and a frontal section.

[FIGURE 3 OMITTED]

Replacing separation blades "l" length was realized using longer ones. Resistance measurements were performed in A.C. with a 500Hz frequency so that the results not are being perturbed by electrochemical processes from DC. In order not to affect sea water conductivity by temperature gradient during experiments, the experimantal system was built having a thermostat. The resistance between the pair of electrodes placed in seawater is compared with a reference resistance.

A system for small current variation measurement (using a compensate miliammeter with the aid of D.C source) was used in order to measure the finest variation resistance produced by parameter modifications. (Antoniu, 1998).

A diode was introduced into miliammeter and this compensation being in DC, in such way that in this circuit branch (were the miliammeter is fitted) only continuous current will exist--Fig.4.

3. EXPERIMENTAL RESULTS

The distance 2a between electrodes was a constant 80 mm value, the depth 25 mm, the salinity 25 g/l and the electrodes length between 0.5-8 cm. Experimental resistence values for different blades and electrodes lengths are presented in Fig.5.a. Powers loses are presented in Fig 5.b.

Diagrams reveal that increasing limitation blades length reduces the power loss and also increases sea water-electrodes system resistance.

[FIGURE 4 OMITTED]

[FIGURE 5 OMITTED]

4. CONCLUSIONS

A bigger [eta] = a/b value produces a significant resistance increase. For small lengths of limitation blades (aproximative equal with electrodes length 2b) the increasing of the resistence is small. An signifiant resistence increase was obtained making limitation blade length double to electrode length. If electrodes length is bigger than gap between them, the resistance is bigger and lost power is smaller.

Limitation blade length must be no more than twice electrodes length. Overcomming this value increase the hydro dynamic loses (Mocanu, 1981). The utilization of restrictive blades increase the system efficiency. Future researches must establish the optimal length of restrictive blades so that the electrical power loss to be minimal without increasing too much the hydro dynamic loss.

5. REFERENCES

Antoniu M. (1998). Masurari Electronice (Electronical Measurements) Vol.I, Editura "Gh.Asachi", p.256-259, ISBN 973-9178-22-7, Iasi

Gavrila, H., Centea, O. (1999). Teoria moderna a campului electromagnetic si aplicatii, (Modern theory of electromagnetic field and applications) Editura All, p.221-224, ISBN, 973-571-257-1, Bucuresti

Lin T.F.; Gilbert J.B.; Roy G.D.(1991). Analyses of magnetohydrodynamic propulsion with seawater for underwater vehicles. Journal of Propulsion and Power, Vol.7 Nov.-Dec., p. 1081-1083, ISSN 0748-4698,

Mocanu C.I., (1981). Teoria cimpului electromagnetic,(Theory of electromagnetic field) Editura Didactica si Pedagogica, p. 821-840, Bucuresti

Shatrov V.; Gerbert G. (2006). On magnetohydrodynamic drag reduction and its efficiency, Magnetohydrodynamics, Vol. 42, No. 2/3, p. 181-186, ISSN 0024-998X