VGB POWERTECH 11 (2019)

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Sub-cooled boiling of natural circulation in narrow rectangular channels VGB PowerTech 11 l 2019 1 experiment channel – 2 thermal insulation material – 3 high speed camera – 4thermocouples – 5 photography lamp Fig. 3. Measure device of experiment channel. 1 Tank of deionized water – 2 Nitrogen gas bottle – 3 Pressure regulator – 4 Cooling tank – 5 Preheater – 6 Rectangular heater with visible window – 7 Thermocouples – 8 Condenser – 9 Pressure sensors – Fig. 1. Experiment facility of natural circulation system. 1 cover plate – 2 2-5mm mica – 3 gasket – 4 glass – 5 thin gasket – 6 heating panel – Fig. 2a. Cross-section of experiment channel. 6 5 1 2 4 3 and a descending channel. The deionized water is used as fluid medium, which is driven through the preheater channel, rectangular heater channel and condenser channel by density difference. For this natural circulation system, the flow rate is measured by a turbine flowmeter with a tolerance of ±0.001 l/min and the pressure is balanced by a pressure regulator. Visual experiment channel with a narrow rectangular slit F i g u r e 2 ( a ) shows the cross-sectional view of the visual experiment channel. F i g u r e 2 ( b ) shows its three-dimensional view. It has a size of 2 to 5 mm × 40 mm with a length of 1,000 mm. One side consists of a heating surface made of stainless steel, and the other side is the visual window made of quartz glass. The flow pattern is well-observable through the quartz glass. The effective heating power can be adjusted continuously, in the range of 0 to 30 kW. Measurement device F i g u r e 3 shows the measurement device, used alongside the experiment channel. The temperature is measured by sheathed thermocouples, and the measurement accuracy is ±0.25 %. From the inlet to the outlet of the experiment channel, 20 temperature thermocouples which are used to transfer the collected temperature signals to the data acquisition unit, are installed on the side of the metal heat surface. There are 2 additional temperature test points installed at the inlet and outlet of the experiment channel, in order to measure the fluid temperature. The flow pattern in experiment channel is recorded by using high speed camera (1,000 fps). It allows the behavior of bubbles to be captured easily within the experiment channel. All instruments have been shown in Ta - b l e 1 , alongside their maximum errors. Experiment parameters and procedure Experiment parameters The experiment parameters of the natural circulation system are shown in Ta b l e 2 . Experiment procedure The following procedure has been followed in this experiment. Tab. 1. Instruments and errors. Parameter Name Model Range Errors Pressure Pressure transmitter HSLT-P 0 to 6.0 MPa 0.25 % Temperature Thermocouple WRNK101 0 to 600 o C 0.25 % Volume flow Turbine flowmeter LW-10 0 to 600 l/H 0.2 % Voltage Voltmeter HC-300/C 0 ∼ 380 V 0.2 % Current Ammeter T23-A 0 ∼ 5 A 0.2 % Data Data acquisition instrument KPCI-1813 0.1 % Tab. 2. Experiment parameters. Fig. 2b. Three-dimension view of the experiment channel. Inlet subcooling Preheater power Heater power Gap size Device height Device width 60 to 15 o C 0 to 30 kW 0 to 10 kW 2 mm to 5 mm 3.3 m 2 m 64
VGB PowerTech 11 l 2019 Sub-cooled boiling of natural circulation in narrow rectangular channels –– At the beginning of the experiments, deionized water is added into the whole loop, and the pressure is regulated using a pressure stabilizer. –– A preheater and a rectangular heater are used to heat the fluid medium. The power of preheater is maintained so that the fluid enters the rectangular heater at a certain temperature. –– Once natural circulation begins, the power of rectangular heater is increased by a certain amount of time step. In fact, the power should be added gradually, in order to allow the system to balance itself. –– All requisite operational parameters are recorded. The above procedure is repeated by changing the fluid temperature at the inlet of the rectangular heater, the heat flux and size of experiment channel. In practice, the heating power is lost due to the glass wall of the experiment channel and a heat transfer efficiency of 0.75 is used. This is calculated using a heat balance experiment. Heat transfer coefficient kW/(m 2 K) 4.0 3.5 3.0 2.5 2.0 1.5 80 90 100 110 120 130 140 Heat flux kW/m 2 Experiment calculations Wall temperature calculation The effective heating power of the experiment channel is defined as equation (1). (1) In this equation, q is the effective heating power of experiment channel measured in kW/m 2 . U is the voltage of experiment channel measured in V. I is the electric current of experiment channel measured in A. eff is the heat transfer efficiency. b is the width of rectangular heating surface measured in m. L is the length of rectangular heating surface measured in m. In addition, there is an offset between the position of thermocouples and the inner wall surface of the experiment channel. Because the offset is small, it can be assumed that the temperature varies linearly along the thickness of inner wall. This temperature can be calculated by Fourier heat conduction law, which is showed as equation (2). (2) Fig. 4. The influence of heat flux on heat transfer coefficient. In the above equation, T wi is the inner wall temperature of test position i measured in K. T i is the thermocouple measuring temperature of i test position measured in K. λ w is thermal conductivity of heating plate measured in kW/(m K). δ is the gap w between the thermocouple and inner wall of experimental channel measured in m. Heat transfer coefficient of sub-cooled boiling calculation Through observation and experiment data analysis, it is found that the ONB occurs at the lower part in the mid-section of experiment channel. Hence, the average heat transfer coefficient of 9-12 test positions which are in the mid-section of experiment channel is regard as the standard heat transfer coefficient of sub-cooled boiling shown as equation (3). (3) Heat transfer coefficient kW/(m 2 K) 2.6 2.5 2.4 2.3 2.2 2.1 2.0 1.9 1.8 In the above equation, h is the heat transfer coefficient of sub-cooled boiling measured in kW/(m 2 K). T fi is the temperature of fluid at i th test position measured in K. It has been assumed [22] that the fluid’s temperature varies linearly along the axial direction from inlet to the outlet of the experiment channel. Experiment results and analysis The influence of heat flux on heat transfer coefficient F i g u r e 4 shows the influence of heat flux on heat transfer coefficient in a 3 mm experiment channel. The inlet sub-cooling is 50 o C. As it is depicted in F i g u r e 4 , the heat transfer coefficient of sub-cooled boiling in natural circulation is 1.9 kW/(m 2 K) when the heat flux is at 80 kW/m 2 , and which in- 2.0 2.5 3.0 3.5 4.0 4.5 5.0 Experiment channel size mm Fig. 5. The influence of experiment channel on heat transfer coefficient. 65
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