Study of the circulation theory of the cooling system in vertical evaporative cooling generator

Transcription

1 358 Science in China: Series E Technological Sciences 006 Vol.49 No DOI: /s Study of the circulation theory of the cooling system in vertical evaorative cooling generator YU Shunzhou 1, CAI Jing, GUO Chaohong 1 & GU Guobiao 1 1. Institute of Electrical Engineering, Chinese Academy of Sciences, Beijing , China;. Beijing Changcheng Institute of Metrology and Measurement, Beijing , China Corresondence should be addressed to Yu Shunzhou ( ezhi@mail.iee.ac.cn) Received October 13, 004; acceted December 9, 005 Abstract The article briefly states the current develoment of evaorative cooling generator and its advantages comaring with generators of traditional cooling. Vertical evaorative cooling generator, which adots Close-Loo-Self-Cycle with no-um and free convection boil in the hollow stator bar, is one of the great develoments in generator design. This article emhasizes the imortance of cooling system in generator; exatiates the circulation theory in two asects, energy and flow; and analyzes the essential reason, motivity and stability of Close-Loo-Self-Cycle. The article oints out that the motivity of the circulation is the heat absorbed by coolant. After absorbing heat the coolant will have the ability of doing wor because of the hase change. In another words, it is the buoyancy causing by density difference leads to the Close-Loo-Self-Cycle. This conclusion is validated by exerimental data. Keywords: vertical evaorative cooling generator, cooling system, circulation theory. The evaorative cooling system is the most imortant art differing from other traditional cooling generator, and its design directly determines the evaorative generator s erformance [1]. The circle method is Close-Loo-Self-Cycle with no um by maing full use of its vertical structure, and stator bars are cooled by coolant s otential heat. Evaorative cooling system does not need additional machines. The aaratus is simler and wor noise is reduced much. The wor reliability is imroved, and the insulation wor life is rolonged, too. Comaring with air-cooling generator, the temerature of stator bars in evaorative cooling generator decreases K at rated load, and the roblems such as insulation exuviations and heat distortion caused by large temerature difference are solved []. The evaorative cooling hydro-generator at Lijiaxia Power Station has wored suc-

2 Study of the circulation theory of the cooling system in vertical evaorative cooling generator 359 cessfully for four years, which roves that the evaorative cooling technology is very favorable [3]. The evaorative cooling generator was araised as one of the most advanced develoments of rotating generator on the conference of CIGRE in 000. In recent years constructing velocity of large ower stations becomes quicer, the technology of evaorative cooling generator develos quicer, too. It is necessary to go on a dee research on the cooling system [4]. This article analyses the circulation theory of the cooling system from two asects, energy and flow. 1 Theoretical analysis The setch ma of self-circle system in hollow stator bar of the evaorative cooling hydro-generator is shown in Fig. 1. Fig. 1. Setch ma of the natural self circulation theory in evaorative hydro-generator stator bar. 1.1 Energy equation Using a certain coolant as study object, with no regard to the status of startu and shutdown, the flow is steady. According to the first thermodynamic law, we obtain 1 q u = v v 1 1+ ( c c1 ) + g( z z1) + w i, where q is heat load, u is inner energy, c is velocity, z is height, is ressure, v is secific volume, wi is inner wor, v v 1 1 is imellent wor, 1 ( ) 1 ( 1) c c + g z z + w i is technique wor. w = 0. The equation above can be sim- As the coolant does not wor, the inner wor lified as i

3 360 Science in China: Series E Technological Sciences 1 q u = v v 1 1+ g( z z1) + ( c c1). (1) The circle system is comosed with three segments: ascending ie, condenser and descending ie. Ascending ie is from evaorative oint 1 to the inlet of condenser ; descending ie is from outlet of condenser 3 to liquid surface 4; from liquid surface 4 to inlet of ascending ie 1 is unused coolant. The three segments are analyzed with eq. (1) resectively. 1. Ascending segment 1 q u = v v 1 1+ g( z z1) + ( c c1). () During flow, the coolant absorbs heat and boils away. Its density becomes smaller and secific volume becomes larger. According to the continuous equation ρ ca = ρ c A, TP with a certain flow area, the ressure dro increases with flow velocity increasing, so the change trend of ressure dro and secific volume is contrary, which is, v > v. The value of imellent wor is determined by the multilication of and v. The exeriment and calculation result shows that v v; > z, which means the otential energy increases; and c z > < 1 1 c, which means inetic energy increases. > 1 From eq. (), we can see that the heat absorbed by coolant has two uses: some of the heat is used to increase inner energy u (temerature increase before evaoration and then boiling away); the other is used to do wor, including imellent wor and technique wor. The value of technique wor is equivalent to mechanical energy, which includes otential energy and inetic energy. According to the thermodynamic first law q = u+ w, where w is exansion wor. The wor comes from the heat absorbed by coolant. By changing the coolant s thermodynamic state, the coolant exands and transforms heat energy to mechanical energy. Namely, heat is the essential flow motivity. Similarly, using eq. (1) to condenser, 1 q u = 3v3 v+ g( z3 z) + ( c3 c). In the condenser, assuming the wor ressure is constant, namely is equal to 3 ; the coolant is condensed, so q is minus; otential energy increment is zero; and the density increases. According to continuous equation, the flow velocity decreases to c 3. The equation above can be simlified as follows: 1 q = ( v3 v) u ( c c3 ). (3) From eq. (3), we can see that the heat absorbed by coolant in ascending ie is taen

4 Study of the circulation theory of the cooling system in vertical evaorative cooling generator 361 away by cooling water, and the coolant s inner energy, secific volume and inetic energy decrease. 1.3 Descending ie q u = 4v4 3v3+ g( z4 z3) + 1 ( c ) 4 c3. There is no heat exchange during flow, so its temerature and hase does not change, u = 0, v1 = v 3. Some of otential energy of the coolant transforms to inetic energy, c4 > c 3, others transforms to imellent wor. Simlifying the above equation, 1 0 = ( 4 3) v3 + g( z4 z3) + ( c4 c3 ). (4) Using Bernoulli equation we can obtain the same result. For ideal incomressible liquid flow with a certain length considering gravity, Bernoulli equation is where 1 is static ressure. ( c4 c3 ) g( z4 z3) + =, (4 ) ρ 4 3 Eqs. (4) and (4 ) are the same which show that the coolant s otential energy decreases, inetic energy increases and static ressure also increases. The unused coolant between 4 and 1 can imrove flow stability caused by c c. 1 4 The more unused coolant is, the more stable the cooling system is. The thermodynamic first law is about energy equilibrium. According to the analysis above, we can see that from the asect of energy, the coolant absorbs heat in ascending ie to change its state, and then it has the caability of doing wor. The result of doing wor is inetic and otential energy increase. The three segments, ascending ie, condenser and descending segment, comose the cycle system. 1.4 Flow theory From the asect of flow, the motivity is comosed with 3 arts. The one is buoyancy in ascending ie, which is the most imortant. In ascending ie, the coolant absorbs heat and boils away, so the density is less than that in descending ie. Because of the density difference, buoyancy comes into being and the coolant in the ascending ie flows u into condenser, after cooled the coolant flows down into descending ie by gravity and comlements to ascending ie. The one is flowing coolant above the unused coolant in descending ie. During flow the otential energy artially transforms to inetic energy, the other transforms to static ressure acting on the coolant surface. In the condenser, because the temerature of coolant is higher than the cooling water and the ressure of gas is higher than the gas around the cooling ie, with the difference of temerature and ressure, gas is cooled into liquid, and flow into descending ie. This is the third art of the flow motivity.

5 36 Science in China: Series E Technological Sciences Exeriment analysis In the evaorative cooling system of generator, the flow character in ascending ie is the most comlicated, and it is wanted to now urgently by designers, the flow energy equation in ascending ie is roved by exeriment data. Exeriment model is shown in Fig.. Fig.. Setch ma of the exeriment system. 1, Single hase segment;, two-hase segment. As the boiling oint in self circle is difficult to confirm, the horizontal comel circle system is adoted. The exeriment ie is Φ6 1, which is the same as the equivalent diameter of hollow stator bar in Lijiaxia Power Station. By adjusting the current in single hase segment, the coolant becomes saturated in the exit of single hase segment. The ressure difference of two-hase segment is the total of acceleration and friction ressure dro. The gravity ressure dro can be calculated by density variety. The total of the three arts is the ressure dro in ascending ie in self circle system. Basal arameter: Measure arameter: heat load (current), volume flux, the total of friction and acceleration ressure dro, inlet ressure Calculated arameter: resistant, exit vaor quantity, gravity ressure dro, outlet ressure Form arameter: inlet saturated temerature, inlet saturated liquid enthaly, outlet saturated temerature, outlet saturated gas enthaly, and outlet saturated liquid enthaly In Fig. 3, and E are both magnified 100 times for the convenience of E drawing figure. Because the heat load and flux are different, h, q with different flux are incomarable. The total of h and q are equivalent in theoretic, but unavoidable some quantity of heat may be dissiate to environment, so q is always

6 Study of the circulation theory of the cooling system in vertical evaorative cooling generator 363 larger than the total of h. The error 10% as shown in Fig. 3(b). h+ E + E q q is less than Fig. 3. (a) Energy in unit mass fluent with different flux; (b) error curve. The results show that: (1) The datum measured in this article is exact, and the thermodynamic analysis of ascending ie is correct. () During flow, inetic energy variety is little. According to mass conservation, the velocity variety is little, so the acceleration ressure dro caused by velocity variety is little, which can be neglected in cursory calculation or considered with friction ressure dro, but in exact calculation, the outlet thermal arameter can be influenced directly by flow resistant, so acceleration ressure dro must be considered. (3) The calculation result shows that 88.9% of enthaly is used to increase the internal energy of coolant, which is the foundation of exansion wor. From this oint it is the enthaly which causes the flow of coolant. Because it is a flowing system, enthaly is at than internal energy.

7 364 Science in China: Series E Technological Sciences 3 Conclusion The article analysis the circle theory of vertical evaorative cooling generator from two asect energy and flow, and oints out: (1) The heat absorbed by coolant is the resource of flow motivity, and the enthaly is the essential reason. () From the asect of flow, the coolant absorbs heat and boils away, so the coolant density decreases. The buoyancy caused by density difference is the essential motivation. The unused coolant in descending ie can imrove the circle stability. The circular coolant above the unused coolant can also rovide a certain static ressure. References 1 Ding S N. Heat and Cool of the Large Generator (in Chinese). Beijing: Science Press, 199 Gu G B, Tian X D. The develoment of inner cooling technique in hydro-generator. Electric Machines and Control (in Chinese), 1997, 1(1): Gu G B. Develoment and alication of evaoration-cooling technology. China Electrical Equiment Industry (in Chinese), 003, 1: 5 4 Xiong N. Main tass for research and ractice of evaoration-cooling technology. Advanced Technology of Electrical Engineering and Energy (in Chinese), 000, 19(3): 36 40