| Summary: | 碩士 === 國立臺灣海洋大學 === 機械與機電工程學系 === 101 === Abstract
The objective of this study is to investigate the thermal performance of double-pipe heat exchangers powered by supercritical water. An experimental facility built by TPRI was used to conduct the tests. To analyze the data, a computer program has been developed to calculate the heat transfer coefficients in both steady and transient states.
Three groups of experimental data were taken under the conditions of Case1: 180kg/hr, 20MPa; Case2: 180kg/hr, 30MPa and Case3: 300kg/hr, 30MPa. During the heating process, the heat exchanger HE3 closest to the resistance heater was firstly heated, and then extended to HE2 and HE1. Eventually, the whole system reached a steady state and the history of temperatures at the inlet and exit at each heat exchanger were recorded. The maximum temperature in the resistance heater is also noted, which provides the safety consideration in a practical running.
The energy balance in each heat exchanger is made to assure the consistence with the first law in thermodynamics. Due to the same structure in HE1 and HE2, they can be taken as a combined exchanger during the analysis. Five kinds of heat transfer correlations based on the experimental observations at ordinary pressures were used to predict the overall heat transfer coefficient and with which compared with the experimental data. The error was found to be around 80, 56 and 35% in each case. This deviation is suspected to the low values of pressure and flow rate. At the same pressure and working fluid as the present study, the Chinese scholars Yang whose results in heat transfer coefficient are higher than the present data by 12% and 32%. However the tube used in that paper is a inner threaded pipe and three times larger in diameter, comparing with the present study with bare tube, the results are comparable.
The program was coded in Visual-Basic combined with Microsoft Excel, program owns query hot physical properties of features. Once the measured temperatures, pressure, flowrate and temperature change rate have been determined, the energy balance is validate first. Then the overall heat transfer coefficient can be found by LMTD method. Finally, the results are made to compared with those theoretically calculated values in both of tube side and shell-side heat transfer coefficient and the overall heat transfer coefficients. Through the computer program work, one can improve the computational efficiency , reduce errors, and save computing time to achieve the best results.
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