Enhancement of Critical Heat Flux in Tight Rod Bundle with Wire Spacer

Transcription

1 Journal of Energy an Power Engineering 10 (2016) oi: / / D DAVID PUBLISHING Enhancement of Critical Heat Flux in Tight Ro Bunle with Wire Spacer Dan Tri Le 1 an Minoru Takahashi 2 1. Department of Nuclear Engineering, Tokyo Institute of Technology, Tokyo , Japan 2. Laboratory for Avance Nuclear Energy, Tokyo Institute of Technology, Tokyo , Japan Receive: December 04, 2015 / Accepte: December 22, 2015 / Publishe: February 29, Abstract: The experiment of CHF (critical heat flux) was conucte for water boiling two-phase flow in three-pin tight ro bunle. The effects of with an without wire spacers an the pitch to iameter ratio p/ on CHF were investigate uner the conitions of mass flux range kg/(m 2 s), inlet temperature from 70 C to 92 C an the pressure of 0.1 MPa. The CHF was enhance by wire spacers in comparison with the results of CHF without wire spacers. The CHF was enhance by reucing the p/ from 1.18 to 1.10 uner the same flow rate conition, although it i not change appreciably with the change of p/ uner the same mass flux conition. Key wors: Critical heat flux, tight lattice core, boiling two-phase flow, wire spacer. Nomenclature Outer iameter of heater pin (m) D he Heate equivalent iameter (m) D hy Hyraulic iameter (m) wire Wire iameter (m) G Mass flux (kg/m 2 s) H Axial pitch of wire spacer (m) L Heate length (m) P Pressure (MPa) p Pitch (m) q CHF Critical heat flux (kw/m 2 ) W Mass flow rate (kg/s) X Thermal equilibrium steam quality Distance from the upstream en of the heater Z pin to CHF position (m) Abbreviations CHF Critical heat flux LWR Light water reactor RMWR Reuce-moeration water reactor 1. Introuction Conversion ratio of plutonium is generally lower than unity in thermal neutron reactors such as conventional LWR (light water reactors). However, if Corresponing author: Dan Tri Le, octoral stuent, research fiel: thermal hyraulic in nuclear reactor. the neutron spectrum is mae harer by reucing moerator-to-fuel ratio, that is, a tight lattice core, the conversion ratio can be mae nearly unity even in LWRs [1]. Such LWRs can be not only fissile breeer but also actinie transmutator. It has been reporte that, the conversion ratio can be higher than unity in tight lattice cores with a pitch-to-iameter ratio, p/, of 1.09 [2] an 1.06 [3]. Coolability is one of the most important issues for the tight lattice core, since flow areas are very narrow. The thermal-hyraulic performance in tight ro bunle with gri spacer has been investigate experimentally for the stuy of RMWR (reuce-moeration water reactor) [4]. The experimental results showe that, the coolability of the tight lattice core was goo uner the reasonable flow conition. CHF (critical heat flux) is the most important feature for the jugment of coolability. CHF behavior in tight ro bunle with gri spacer was investigate experimentally for tight 37-ros bunle using Freon-12 [5] an water [6] as a coolant. The results with Freon-12 showe that, the effects of pressure, mass flux an steam quality on CHF were similar to

2 110 Enhancement of Critical Heat Flux in Tight Ro Bunle with Wire Spacer those of ata in roun tube channel [7]. On the other han, the results with water coole type showe that, the effects of flow parameters on CHF were the same with those in 7-ro bunle [8]. The subchannel analysis was performe for the experimental result [9]. The effect of gap with on CHF in tight ro bunle with gri spacer was investigate experimentally for 37-ros bunle [10]. It was foun that the CHF values were lower in the channel with a gap with of 1.0 mm than in the channel with a gap with of 1.3 mm at the same mass flux. It can be seen that CHF in tight ro bunle with gri spacer were well investigate, an the effects of mass flux, pressure, steam quality an gri spacer on CHF were mae clear experimentally an analytically. Wire spacers are recommene rather than gri spacers for fuel assemblies in the tight lattice core, in particular, with smaller values of p/ for the breeer type of LWRs. The increase of CHF by wire spacer was foun in an experiment for a single fuel pin with an without wire spacer [11]. The CHF values increase up to 25% by wire spacer compare with that without wire spacer. The effect of gap size on CHF was also investigate in the stuy. The CHF coul reach the values aroun 100% higher when the gap size was reuce from 2.0 mm to 1.1 mm. The CHF experiment using Freon-12 was one for a hexagonal tight bunle with wire an gri spacers an with p/ of 1.14 [12]. It was reporte that the CHF in tight ro bunle with wire spacer was higher than that with gri spacer. The flui-to-flui moel [13] was use to create the CHF in water conition base on the experimental ata in Freon-12 conition. As the stuy of thermal-hyraulic performance in tight ro bunle for PWR (pressurize water reactor), the effect of wire spacer on the improvement of CHF was investigate analytically [14]. However, the effect of wire spacer on CHF in tight ro bunle was not investigate sufficiently. Particularly as one of the most important factors that influence the CHF, the effect of a pitch to iameter ratio on CHF was not investigate. The purpose of the present stuy is to clarify the CHF behavior in boiling two-phase flow in three-pins tight ro bunle with wire spacer. In particular, the effects of wire spacer an the pitch to iameter ratio on CHF were investigate experimentally. 2. Experimental Apparatus an Proceure 2.1 Experimental Apparatus The experimental apparatus of a water circulation loop has been escribe in the previous paper [11]. The schematic rawings of the test section are shown in Fig. 1. The flow channel ha the triangular shape which was mae from three Pyrex glass plate. The triangular flow channel was supporte by two polycarbonate plate at both ens. Three pin-tight ro bunles were mae by assembling three heater pins mae of stainless steel tubes in the triangular arrangement. The heater pins were 4.57 mm in outer iameter an 400 mm in length. The both ens of the heater pin was connecte to the copper electroes by silver solering. The heater pins were irectly heate by Joule-heating an it ha an uniform heat flux on the heater pin surface. The experiment can reach the maximum power of 12 kw an current of 630 A. The wire spacer was electrically insulate from the heater pin by a Teflon tube or Holmal coating so that no heat was generate in the wire. The pin pitch, p, was 5 mm an 5.4 mm, which correspone to the pitch to iameter ratio, p/, of 1.10 an 1.18, respectively. The wire spacer for p/ = 1.18 was a Teflon tube in outer iameter w = 0.76 mm in which a stainless steel wire was inserte, an that for p/ = 1.10 was a Holmal coate wire in outer iameter w = 0.4 mm. The reason for the use of Holmal coate wire is to satisfy the gap size of 0.5 mm in the case of p/ of A wire spacer was wrappe aroun each of the heater pin with the axial wining pitch H = 200 mm.

3 Enhancement of Critical Heat Flux in Tight Ro Bunle with Wire Spacer 111 Copper electroe Teflon tube Thermocouples Wire wrap Outlet p Wire Wire spacer 10 mm Heater pin Triangular glass flow channel A A H Heater pin Glass plate L= 400 mm Gravity irection = 5 mm Glass a Thermocouples A A (b) Cross section Stainless steel wire Inlet Teflon tube Copper electroe (a) Teflon tube (c) Wire structure Fig. 1 Test section: (a) test section; (b) cross section of the flow channel with wire an without wire spacer; (c) wire structure. The value of axial pitch of wire met the requirement of H/ less than 50 which was the upper limit of wire correlation for both CHF an pressure rop [14]. Thermocouple elements were spot-wele on the surface of each heater pin. The surface temperature of the heater pin rapily rose at the CHF conition. To etermine the CHF, the temperature rise at the CHF conition was etecte by thermocouples at the positions 10 mm upstream from the ownstream en of heate length: T1, T2 an T3. Thin type K thermocouple elements with the iameter of 100 µm were use, so that the thermocouples i not isturb the flow to change the CHF behavior. 2.2 Experimental Conitions The experimental parameters an conitions are given in Table 1. From the thermal-hyraulic point of view, the effect of a wire spacer on CHF in tight ro bunle at constant

4 112 Enhancement of Critical Heat Flux in Tight Ro Bunle with Wire Spacer Table 1 Experimental conitions. Parameter Run. 1 Run. 2 Run. 3 Run. 4 Wire No wire Wire Wire Pitch to iameter ratio, p/ Wire iameter, w (mm) Wire axial pitch, H (mm) Mass flux, G (kg/(m 2 s)) Mass flow rate, W (kg/s) Hyraulic iameter of flow channel, D hy (m) Outer iameter of heater pin, (mm) 4.57 Pressure, P (MPa) 0.1 Inlet temperature (K) mass flux is interesting. Thus, the CHF experiments for both cases of heater pin with an without a wire spacer were conucte uner the same mass flux conition. On the other han, from the viewpoint of the esign of the practical reactor, it is important to know how the coolability changes with the change of core tightness uner constant mass flow rate. Thus, the experiments were conucte for two ifferent values of p/ at the same flow rate conition. 3. Results 3.1 Effect of Wire Spacer Fig. 2 shows the result of CHF with an without wire spacer. The CHF values with wire spacer were higher than that without wire spacer. The ifference in CHF value was larger at the quality region from x local = to 0 than at the quality region from x local = 0 to , Without wire 200 G = 435 kg/(m 2. s) p/ = Local quality, x local Fig. 2 Critical heat flux in bunle with an without wire spacer. The CHF was enhance by up to 50% with wire spacer compare with it without wire spacer uner the constant mass flux conition. The enhancement was more significant at the low quality region. Therefore, the coolability in tight lattice core coul be optimize by using wire spacer. 3.2 Effect of Pitch to Diameter Ratio Fig. 3 shows the results of CHF values for three-pin bunle experiment with p/ = 1.10 an 1.18 uner the same mass flux conition. It is foun that, the CHF values have a similar tenency even with ifferent p/. Aitionally, the CHF ata with p/ = 1.10 was slightly lower than that in the case of p/ = 1.18 uner the same mass flux conition. The comparison of CHF between two ifferent values of p/ uner constant mass flow rate is shown in Fig. 4. 1, G = 435 kg/(m 2. s) 200 p/ = 1.10 p/ = Local quality, x local Fig. 3 CHF of ifferent values of p/ uner the constant mass flux conition W = kg/s 200 p/ = 1.10 p/ = Local quality, x local Fig. 4 CHF of ifference values of p/ with the same flow rate conition.

5 Enhancement of Critical Heat Flux in Tight Ro Bunle with Wire Spacer 113 It can be seen that, the CHF values for p/ = 1.10 were much higher than that for p/ = 1.18 uner the same mass flow rate conition particularly in lower range of x local. 3.3 Axial CHF Position Figs. 5 an 6 show the position where CHF occur re in all the experiment. Fig. 5 shows that the positions of most of CHF at p/ = 1.18 an G = 280 kg/(m 2 s) were locate in the ownstream region of the flow channel. However, when the mass flux was increase from 280 kg/(m 2 s) to 435 kg/(m 2 s), the CHF occurre in the mile of the heater pin. Fig. 6 shows that, uner constant mass flux conition of W = 435 kg/(m 2 s), CHF occurre at nearly the same position at p/ = 1.10 an 1.8. Axial CHF location, z (m) p/ = 1.18 G = 280 kg/(m 2. s) G = 435 kg/(m 2. s) ,000 Fig. 5 CHF position along the heate length with the effect of mass flux. Axial CHF location, z (m) G = 435 kg/(m 2. s) p/ =1.18 p/ = ,000 Fig. 6 CHF position along the heate length with the effect of p/. 4. Discussion It was reveale that the existence of wire spacer enhance the CHF from the result in Fig. 2. The reason for the enhancement can be explaine by two parts base on the local vapor quality region. In the range of the quality from to 0, the enhancement of CHF was very clear, which was possibly cause by the increase of isturbance by wire spacer. In the low quality region, the turbulent mixing cause by the isturbance became higher, which enhance the bubble removal from the heate surfaces. As a result, CHF increase. The existence of the wire spacer inuce a spiral flow, which increase coolant velocity. Therefore, the CHF increase in comparison with the case of straight flow. Besies, in the range of positive local vapor quality, the CHF was nearly the same in the cases of bunle pins with wire an without wire spacer. It has been known that, the liqui film flow becomes thinner compare with it in the low quality region in the high quality region. In the conition of the thin liqui film flow, the wire spacer may break the liqui film flow, an consequently the ry-out more easily occurs. Since the boiling heat transfer rate becomes higher in the high quality region than in low quality region, the isturbances of the bubbles become higher an ten to break the liqui film, which leas to the ecrease of CHF values. The effect of p/ on CHF value was investigate both uner constant mass flux conitions an at constant mass flow rate conitions. In the case of constant mass flux conitions, it was foun in Fig. 3 that, the CHF values for p/ = 1.10 were nearly the same as those for p/ = However, the ifference became larger in the region of higher quality x > 0.01, that is, the CHF values for p/ = 1.18 were higher than those for p/ = The reason for this result is relate to the geometric factorial. For p/ = 1.10, the flow area is smaller than that for p/ = Hence, in the flow channel with p/ = 1.10, the bubbles coalesce with each other into larger bubbles more

6 114 Enhancement of Critical Heat Flux in Tight Ro Bunle with Wire Spacer easily, an covere the heate surface, an as a result hiner the cooling of heater pin by the liqui film, which lea to lower CHF values for p/ = 1.10 compare with those for p/ = In contrast, it was shown uner constant mass flow rate conitions in Fig. 4 that, the CHF values for p/ = 1.10 were higher than those for p/ = It can be explaine by the increase of mass flux. With the same flow rate conition, the flow channel which has a smaller value of p/ woul have a smaller flow area, which leas to a higher mass flux. Therefore, the coolant velocity in case of p/ = 1.10 was higher than that in the case of p/ = 1.18 at the same flow rate conition. Therefore, the coolability was enhance by tighter core lattice uner the constant mass flow rate conition, which turns to a higher CHF values. The result of the measure position at which CHF occurre along the heate length suggests that CHF mechanism change with changing the value of p/ an mass flux. In the case of the lowest mass flux of 280 kg/(m 2 s) in the flow channel with p/ = 1.18, the CHF positions were mostly in the ownstream region of the flow channel, which suggeste the occurrence of liqui film ryout along the heater pin. On the other han, at higher mass flux, 435 kg/(m 2 s), CHF positions shifte to the mile of the flow channel, which suggeste the DNB (eparture from nucleate boiling) phenomenon. In short wors, the CHF mechanism change from the liqui film ryout phenomenon to DNB phenomenon with an increase of mass flux. It is shown in Fig. 6 that, uner the same mass flux conition, the CHF positions were nearly the same even with two ifferent values of p/. It can be conclue that, the CHF position, or CHF mechanism, was just epenent on mass flux. 5. Conclusions Critical heat flux in boiling water flow in a tight ro bunle was investigate experimentally for a three-pin bunle with an without wire spacers. Two ifferent values of pitch to iameter ratio were chosen: 1.10 an The conclusions are as follows: (1) The CHF was enhance by up to 50% with wire spacer compare with it without wire spacer uner constant mass flux conition. The enhancement was more significant at the low quality region. Thus, the coolability in tight lattice core coul be optimize by using wire spacer. (2) Uner constant flow rate conitions, the CHF values increase by up to 150% by ecreasing the pitch to iameter ration from 1.18 to (3) Uner constant mass flux conitions, the tenency of CHF values i not change even with ifferent values of p/. The CHF ecrease by aroun 5% by changing the p/ from 1.18 to (4) The positions of CHF at p/ = 1.18 an low mass flux were locate in the ownstream region of the flow channel. However, when the mass flux was increase, the CHF occurre in the mile of the heater pin. Uner constant mass flux conition, CHF occurre at nearly the same position for both of p/ = 1.10 an 1.8. Acknowlegments The stuy was supporte by Hitachi Nuclear Scholarship project. The authors woul like to give the thanks to Dr. Masatoshi Kono for his iscussion an to Mr. Naoyuki Tsuruoka an Mr. Shoji Matsui for their assistance in the technical work. References [1] Siik, P., Takaki, N., an Sekimoto, H Impact of Different Moerator Ratios with Light an Heavy Water Coole Reactor in Equilibrium States. Annals of Nuclear Energy 33 (7): [2] Uchikawa, S., Okubo, T., Kugo, T., Akie, H., Takea, R., Nakano, Y., Ohnuki, A., an Iwamura, T Conceptual Design of Innovative Water Reactor for Flexible Fuel Cycle (FLWR) an Its Recycle Characteristics. Journal of Nuclear Science an Technology 44 (3): [3] Siik, P., Takaki, N., an Sekimoto, H Preliminary Stuy on Feasibility of Large an Small Water Coole Thorium Breeer Reactor in Equilibrium Sates. Progress in Nuclear Energy 50 (2-6): [4] Ohnuki, A., Takase, K., Kureta, M., Yoshia, H., Tamai,

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