EFFECT OF GAS-COOLER GEOMETRY ON HEAT TRANSFER AND PRESSURE DROP CHARACTERISTICS

摘要

To understand the effects of tube geometry on the heat transfer and pressure drop characteristics of the CO_2 and water sides in the gas coolers of CO_2 water heaters, two tubes of different geometry were tested in this study. The tested tubes consisted of a pair of twisted smooth tubes or a pair of twisted dimple tubes and an outer tube. CO_2 flows in the pair of twisted inner tubes, while water flows in the annulus between the inner tubes and the outer tubes with a counter flow configuration. The CO_2 and the water side heat transfer coefficients and the pressure drops were measured at CO_2 pressures ranging from 8 to 10 MPa, mass flow rates ranging from 19.2 to 34.6 kg·h~(-1) and temperatures ranging from 20 to 70 ℃. The heat transfer rate and volumetric flow rate of the water were set to 300 W and 1.50 L min~(-1). The thermal resistance was broken down into CO_2 and water side components to quantify the role of each thermal resistance in the total resistance. The experimental results indicate that the CO_2 side heat transfer coefficient strongly depends on the CO_2 temperature and is maximized at the pseudo-critical temperature. However, the heat transfer enhancement achieved by the dimples is very slight for the CO_2 side. On the other hand, due to the dimples, the water side heat transfer coefficient is increased by a factor of approximately two over all of the test conditions. Thus, the total thermal resistance is minimized near the CO_2 pseudo-critical temperature and reduced by approximately 0.5