atw 2017-06

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atw Vol. 62 (2017) | Issue 6 ı June RESEARCH AND INNOVATION 414 | | Fig. 1. Schematic of the Passive Containment Cooling System using the Multi-Pod Heat Pipe. 2 Experiment procedure 2.1 Experimental apparatus design As illustrated in Figure 2, an experimental facility was designed and installed to acquire various types of information related to the heat transfer capacity of MPHP. The facility consists of three major parts: a pressure vessel, a coolant tank, and an experimental TPCT assembly. An experimental TPCT assembly is a key part of the experimental apparatus used in this study. It conducts a heat transfer from the heater in the pressure tank to the coolant in the coolant tank. This assembly is made up of seven TPCTs, which are a 1-m long boiling region and condensation region, respectively, and has a hexagonal array. 2.1.1 Design of TPCT assembly The operation of TPCT is based on the force of gravity and the temperature differences between its parts; one side is heated while the other side is cooled. Heat transfer occurs in TPCT due to these temperature differences. The thermal resistance (or the heat transfer coefficient) is calculated for each region. These are combined in a thermal resistance circuit, as shown in Figure 3, to calculate the total thermal resistance between the pressure tank inside and the coolant water inside the coolant tank (the heat transfer coefficient) for one TPCT. If R tot and ∆T are the total thermal resistance and the temperature difference between the pressure tank inside, which heats the boiling region, and the water cooling the condensation region, respectively, then it holds that: (1) where, ˙Q is the heat removal rate for a TPCT. To obtain an explicit expression for R tot , the heat transfer coefficient of each region was calculated first, and then the heat removal of one TPCT was calculated from this. The total heat transfer coefficient (resistance) for a given value of T h , T c and ∆T bc was calculated by summing up the aforementioned thermal resistances in each region. We first assumed that the temperature distribution was uniform, that is, there was no temperature difference between the air in the assembly center and the air inside the containment, as mentioned above. In fact, a considerable temperature drop is expected, and it is difficult to predict the specific value. This needs to be researched through additional experiments or a review of the literature. For convenience, we denote the overall number of pipes in the boiling and condensation regions as N b and N c , respectively, and the heat removal rate per TPCT as ˙Q i . The following equation holds for the temperature drop at the inner boundary of the boiling region (where the resultant thermal resistance R5 can also be determined): (2) | | Fig. 2. Experimental apparatus for heat transfer performance test of MPHP. | | Fig. 3. The thermal resistance circuit in a TPCT. Research and Innovation Experimental Investigation of a Two-Phase Closed Thermosyphon Assembly for Passive Containment Cooling System ı Kyung Ho Nam and Sang Nyung Kim
atw Vol. 62 (2017) | Issue 6 ı June Similarly, R 3 can be determined by dividing the thermal resistance of a single pipe by the number of pipes in the condensation region. (3) The resulting thermal resistance of the pipe walls can be determined by dividing the value for one TPCT by the number of TPCTs. At the same time, the heat transfer increases by the same factor. (4) (5) (6) where, ˙Q i is the heat transfer rate per pipe. If we assume that there is no heat transfer or temperature drop in the adiabatic region, then R 4 = 0 and R 8 = 0. The total thermal power extracted from the containment by one assembly can be calculated by multiplying the reciprocal of the sum of the thermal resistance values found above by the temperature difference between the containment atmosphere and the cooling water on the top of the containment dome shell and taking into account the difference ∆T bc as well. Here R MPHP is the total thermal resistance of the MPHP. (7) (8) where, R i is the thermal resistance component of a single TPCT. Therefore, the total heat transfer coefficient of the MPHP assembly is: (9) Section Material Height, m Diameter, m P/D ratio Thickness, m FR Pipe Stainlesssteel 1 0.03 2 0.0005 Adiabatic 304 0.3 0.2 − 0.013 | | Tab. 1. Specifications of experimental thermosyphon assembly. This equation will be applied to compare the theoretical and experimental results in Chapter 3 of this paper. The heat transfer mechanisms in the boiling region occur in various patterns, which are the natural convection, evaporation, nucleate boiling, and a combination of each to the fill charge ratio (FR), heat flux, etc. The fill charge ratio indicates the percentage of the boiling region volume that is filled by the working fluids. In this study, it is determined that a fill charge ratio is 30 % of the boiling region based on a previous study performed by Imura [2, 3, 4]. Thus, an experimental TPCT assembly is partially filled with distilled water and then sealed. The specifications of the experimental TPCT assembly are presented in Table 1. 2.1.2 Pressure tank and Coolant tank A pressure tank simulates the inner containment building, and steam is generated at the bottom of the vessel by two horizontally mounted immersion electric heaters with a total capacity of 30 kW. Air and makeup water can be injected into the vessel, and a drain line is located at the bottom of the vessel. The maximum rated operating pressure for the tank is 1 MPa, which is insured by a safety relief valve. The coolant tank is an open type, and the height is sufficiently high so that the condensation region of the pipes can be submerged in the coolant. The pressure tank is fully insulated with fiberglass to reduce heat loss so that sufficient power can be delivered to the TPCT assembly. Measurements have shown that the vessel heat loss is less than 1 kW, which can be easily compensated by the heaters, which have a capacity of 30 kW. The capacity of a heater is determined by considering the predicted performance limitation of the TPCT assembly. The maximum heat transfer rate owing to entrainment limitations can be calculated through flooding correlations that are expressed in terms of the Kutateladze dimensionless groups [5]. (10) where, (11) (12) The maximum heat transfer rate is predicted to be about 4 kW per pipe. Therefore, an experimental TPCT assembly that has seven pipes is considered, in which the maximum heat transfer rate is about 28 kW. However, this limitation is a conservative value, and thus this value is only considered to determine the capacity of an electrical heater. The coolant tank is an open type and the height is sufficiently high so that the condensation region of pipes can be submerged in coolant as shown in figure 2. As shown in figure 3, the heat generated in pressure tank transfers and this coolant in coolant tank will be external heat sink during the operation. 2.3 Experiment procedure The experiment was conducted in the following manners. Two types of instrumentation devices were used in the experiment: thermocouples for temperature measurement, and pressure gauges/transducers for pressure measurement. K-type thermocouples were placed at each point to provide temperature readings. Temperature data were continuously recorded during operation with the thermocouples. The temperatures at the pressure tank, pipe wall surface, inner pipe, and coolant tank were recorded. One pressure transducer was installed to measure the overall pressure of the vessel. A communication-based Data Acquisition System (DAS) was set up for this experiment. All thermocouple leads and pressure transducer output cables were wired into the measurement channels. Additionally, a Silicon Controlled Rectifier (SCR) was installed to control the electrical power of the heater so that the electrical power was consistently fixed to the heater. 0.3 RESEARCH AND INNOVATION 415 Research and Innovation Experimental Investigation of a Two-Phase Closed Thermosyphon Assembly for Passive Containment Cooling System ı Kyung Ho Nam and Sang Nyung Kim
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