Thermal-hydraulic Assessment of Seven Wire-wrapped rod Bundle

: Lead bismuth eutectic (LBE) is one of the most potential materials for coolant and spallation target for Accelerator Driven Systems (ADS). Thermal-hydraulic behavior of LBE in fuel assembly is a key issue for development of the systems. To get a deeper understanding on the complex thermal-hydraulic features of wire-wrapped rod bundle cooled by upward LBE, an electrically bundle with 7 rods wrapped with helical wire was developed in KYLIN-II thermal-hydraulic forced circulation loop. The flow resistance, thermal entrance characteristic and heat transfer coefficient were investigated. As for the entrance characteristics, during the full heating length (exceeding 140 times the hydraulic diameter), the thermal field did not reach a fully developed and stable condition which is contrary to the ducted flows. The experimental heat transfer coefficient showed that the hexagonal shell has a great influence on the heat transfer coefficient in rod bundle geometry. For this reason the application of empirical correlation should be kept cautious in rod bundle analysis.

1996; W. Gudowski, 2001). With respect to low melting temperature, high boiling temperature, outstanding heat transfer performance, chemical stability, and good neutron economy, Lead-Bismuth Eutectic (LBE) are currently attracting more and more attention word wildly to be both the coolant for sub-critical reactor core and spallation target in transmutation systems (T. Takizuka, 2002;N. Li, 2008). In 2011, the ADS project was launched in China by Chinese Academy of Sciences (CAS), and a 10MWth experimental reactor cooled by LBE named CLEAR-I (Y. Wu and Y. Bai) was designed by FDS team which has been strongly involved on advanced reactor concepts, advanced materials and nuclear code development. In the frame of this project, to support the design and construction of CLEAR-I, KYLIN-II multi-functional LBE facility, including material, thermal-hydraulic and safety loops has been established based on the former experience on lead-lithium experimental technology.
One of the most interesting topics in thermal-hydraulic fields is the turbulent heat transfer, especially for complex geometries, e.g. fuel assemblies and high power spallation target. Owing to the lower prandtl number, the heat transfer to heavy liquid metal is very different from the heat transfer to water or gas (X. Cheng, 2006). That is, compared to the contribution from convection, the contribution from thermal conductivity in low prandtl number fluid like lead-bismuth eutectic is much higher than water even through at the conditions with high Reynolds number. Thus existing heat transfer numerical models and experimental experience are not applicable to systems cooled with liquid metal directly.
Moreover, due to the compact arrangement of rod bundle and the difficulties of heavy liquid metal measurement, the experimental data on the rod bundle cooled by heavy liquid metal is scare. Several experimental studies were conducted in the sixties using mercury (Hg) and sodium-potassium alloy (NaK), which has the similar scale prandtl number with lead or lead bismuth. Unfortunately, as mentioned by W. Pfrang (2007), in most publications about these former experiments, no detailed description was given, For the time being, it is well agreed that there still exists big deficiency in understanding and describing special thermal-hydraulic behavior in tight rod bundles, especially related to heat transfer behavior.
The 7-heated rod bundle test section was installed in the vertical direction with upward flow at position 4 in Fig.1. The coolant LBE enters the rod bundle at the lower end and flows upward to move the thermal generated by heated pins. A general view of this test section is depicted in Fig.2. In the flow direction, upstream of test section, the flow developing length with 1.4 meters and a flow restrictor were placed to obtain a uniform inlet flow rate around each sub-channel.

Fig.1. General view of the 7-rod bundle test section
The rod bundle were manufactured referring to the preliminary structure design of fuel assembly of CLEAR-I. Table 1 shows the main parameters of the experimental bundle. Each rod has a heated length of 800mm, and an unheated length of 1100mm.
The nominal input voltage is 220V while the power of each pin is 5000W. To obtain the accurate power of experimental bundle, a direct-current power supply system consisting of a thristor rectifier and a power distribution system was employed.
To evaluate the thermal-hydraulic performance of the bundle, the wall temperature of heated rod and pressure drop of wire-wrapped bundle were measured in the test section.
The bulk temperature in the bundle section was not measured. This parameter would be calculated from an energy balance throughout the test section (from inlet until axil position z). The wall thermocouples with 0.5mm diameter were embedded in the heated wall to measure the heated wall temperature which depicted in Fig.3. The number of the wall thermocouples is 16, and the detailed position is also depicted in Fig.2

Results and discussion
The friction factor of wire-wrapped rod bundle was calculated according to the following formula: Where ΔP is the pressure drop of flow resistance, dh is the hydraulic diameter of rod assembly, LB is the length of wire-wrapped bundle, and v is the average velocity. The experimental friction factor profiles as function of Reynolds number for the bundle are displayed in Fig.4. The unheated test was performed at ~260 ℃ with adiabatic running, while the heated test was conducted at ~260℃ of average inlet and outlet temperature with maximum temperature difference 40℃. From the experimental results depicted in Fig.2, there is no significant difference between the unheated and heated test due to their similar reference temperature. Furthermore, the experimental friction factor compared with empirical correlations (E.H. Novendstern, 1972;K. Rehme, 1972) which derived from the air or water experiments is also depicted in Fig.4. All the experimental values were located between the Novendstern and Rehme correlations.
That could be seen that LBE has similar flow resistance characteristics of traditional fluid like water or air. In detailed the Rehme correlation presented a better agreement with the experimental results than Novendstern correlation. The probable reason is that the effect of the rod number is concluded in the Rehme correlation.

Fig.3. Thermal entrance characteristic of experimental bundle
The thermal entrance characteristic of experimental bundle is depicted in Fig.3.
The blue squares are the wall temperature of PIN4 along the heating length, and the black line is the average bulk temperature derived from energy balance. It could be seen that along the flow direction the discrepancy between the wall temperature and bulk temperature became larger. This phenomenon could be concluded that the thermal field did not reach a fully developed and stable condition during the full heated bundle section, although the overall heating length reached to 140 times the hydraulic diameter. The conclusion was different with the ducted flows. This phenomenon was also predicted through CFD calculations by other publications (N. Govindha Rasu, 2013). They mentioned that the reason was the weak communication among the sub-channels in tightly packed bundle. In addition, the wall temperature at axial position z=525mm was abnormal lower. The reason was that the thermocouple probably didn't embed well in the heated wall in this experiment. Fig.6 shows the temperature deviations in the cross section of the bundle, respectively for two different Peclet numbers. To characterize the temperature deviations, a non-dimensional parameter Θ (M. Berthoux, 2010) is defined as following: Where Tw max-Tw min is the measured maximum wall temperature deviation, Tzf-Tin is the temperature difference between the local bulk temperature (calculated on heat balance) and inlet bulk temperature.
The wall temperature deviations between center rod and periphery rod could be detected obviously in Fig.6. The temperature deviations were caused by the adiabatic hexagonal shell and the different local hydraulic diameters between the inner and periphery sub-channels. Furthermore, the comparison between the Fig.6 (a) and (b) revealed that the temperature non-uniformity was more significant for lower Peclet number. That is because in the higher Peclet regimes, the coolant mixing is stronger, which can induce the temperature distribution more uniform. The wall temperature of the center rod was used to evaluate the heat transfer coefficeient. The local Nussult number (Nu) is defined as in Eq (3), where ds is the hydraulic diameter of sub-channel, Tb is the average bulk temperture driven by energy balance, Tw is the measured wall temperature of center rod. The Nu profiles as function of Peclet number (Pe) are depicted in Fig.7. At the given Pe, the Nu at axial position z=400mm (z/ds≈71.25) was remarkably larger than the value at axial position z=775mm (z/ds≈138). The phenomenon could be attributed to thermal developing condition which has discussed in Fig.3.
Furthermore, the deviation became larger and larger along the flowing direction.
The empirical correlation K. Mikityuk (2009) which is valid for full developed condition of infinite rod bundle is also depicted in Fig.7. It could be seen that the Nu at section z=775mm had a similar rising slope of K. Mikityuk correlation, but the Nu at section z=400mm had a relatively steep slope. Moreover, the magnitude of experimental Nu at section z=775mm is smaller than K. Mikityuk correlation. That is because of the non-uniform distribution of wall temperature which discussed in Fig.6. Thus the local Nu which based on center rod wall temperature and average bulk temperature would be lower than the average Nu like K. Mikityuk. This indicates that the hexagonal shell has a great influence on the heat transfer coefficient of rod bundle geometry. For this reason the application of empirical correlation should be kept cautious.