NUMERICAL ANALYSIS OF TWO-PHASE FLOW IN SPACE OF HORIZONTAL TUBE BUNDLE DURING NUCLEATE BOILING

摘要

Shell-side boiling heat transfer in horizontal tube bundles is popular in kettle reboiler and flooded evaporator used for air conditioning, refrigeration, water chilling, and chemical industry. Heat transfer characteristics of each tube arrayed in such a bundle are quite different from boiling heat transfer of a single isolated tube. For a moderate level of heat flux, as usual in reboiler, nucleation sites are sparse and boiling is not so intensive that each tube in a bundle will be directly exposed to two-phase bubbly flow rising up through narrow bundle space. As a result, heat transfer is expected to differ from that for a single isolated tube. In general, two-phase bubbly flow is expected to have two opposite influence on heat transfer; an enhancement of heat transfer due to enforced circulation in bundle space and a deterioration in heat transfer due to stagnation of bubbles within narrow bundle space. Their relative magnitude seems to change not only with various system parameters such as the tube location in a bundle, tube arrangement, pitch-to-tube diameter ratio, number of heated tubes located below a tube of interest, but also with operation parameters such as heat flux, boiling fluid, and pressure. Hence heat transfer characteristics are also a function of these many parameters. There have been already a number of experimental studies on heat transfer in bundles. However, it is quite difficult to empirically investigate probable influence of all parameters one by one. For improvement, optimization or minimization of equipment such as reboilers, two phase flow and heat transfer in narrow bundle space is needed to be properly described. For this purpose, various two-phase flow models, i.e., homogeneous, two-fluid, or drift-flux models, are often used to perform numerical analysis. A conventional drift flux model treats two phases as a homogeneous mixture while making allowance for the relative movement of vapor and liquid. The vapor phase translates relatively to an average volumetric mixture as a result of the acceleration force of gravity. Hence the drift-flux model is of limited use to one-dimensional flow in the direction of gravity force. Two-phase flow in tube bundles is a typical multi-dimensional system because flow passage is curved around tubes. Taking account of the drift velocity in the horizontal direction, Zhang et al. (1995) applied a drift flux model to multi-dimensional system. Based on their model but making some revise, we performed a two-dimensional analysis of two-phase flow in tube bundles arrayed in a vertical channel. The tube bundles are composed of three vertical columns and four horizontal rows in the equilateral inline and staggered arrangements. A pitch-to-tube diameter ratio is 1.20, 1.40, and 1.60. Boiling fluid is refrigerant R-113 saturated at atmospheric pressure and fluid approach velocity to the bundles is 0.20 m/s. Heat flux is 170 and 119 kW/m{sup}2. The predicted void fraction was compared with the measured one both for a single tube and twin tubes, confirming qualitative agreement. Velocity field and void fraction in bundle space were numerically analyzed for the inline and staggered arrangements. The drift flux model is expected as a promising tool for predicting two-phase flow field in bundle space.