超臨界二氧化碳在板式熱交換器中的熱流場之數值分析

Abstract

摘要 本研究開發以二氧化碳為工作流體的板式熱交換器模擬程式，模擬內容包括二氧化碳在板式熱交換器中的流速分佈、壓降及熱傳等。模擬條件包括冷媒及水側的質量流率、入口壓力、板片數量以及不同的板片等。模擬結果顯示在水對二氧化碳板式熱交換器中，由於水的物性隨溫度變化並不明顯，因此影響其流場分佈結果主要為溫度變化所導致。而在二氧化碳流場中，由於超臨界二氧化碳的物性變化相當劇烈，因此密度及黏滯係數所造成的影響將不可忽略。除此之外，二氧化碳在靠近臨界溫度時，其熱傳效果將有顯著的提升，未來在設計熱交換器時，必須將此一特性考慮進去。透過此一程式，能夠對二氧化碳應用於熱交換器上能夠有更進一步的瞭解，為熱交換的設計及改進性能有相當大的幫助。

Abstract

In the present study, a numerical simulation program is developed to calculate the pressure drop, velocity distribution and temperature of both water and supercritical carbon dioxide in a plate heat exchanger. The critical state of carbon dioxide is at 7.8 MPa and 330 K, respectively. Therefore the heat exchanger is operated above critical point as a gas cooler. Since the physical properties of carbon dioxide vary drastically near the critical point, therefore the conventional numerical simulation methods for plate heat exchanger are not available. The objective of this study is to develop a carbon dioxide based program which includes the pressure drop, velocity distribution, temperature and physical properties such as density, viscosity and heat capacity. By using appropriate correlations of the Nusselt number and the friction coefficient, the flow and the temperature distribution can be correctly calculated. The effect of the pressure drop, velocity distribution and temperature was calculated with different mass flow rate, inlet pressure, inlet velocity, inlet temperature, number of channels and different plates. In a water to carbon-dioxide plate heat changer, the simulation results show that the flow distribution of water did not change significantly with temperature. Due to the physical properties of carbon dioxide vary drastically at supercritical state, the density and viscosity of the impact cannot be ignored. For an example, when temperature variation between the plates change rapidly, it will cause considerable flow distribution effect ,and may change the flow velocity distribution pattern change from decreasing type into increasing type. In addition, the heat transfer effect will be significantly improved when the carbon dioxide near the critical temperature, when designing of heat transfer, it could be taken into account of the characteristics.