(368i) Experimental and Numerical Analysis of the Isothermal Two-Phase Flow over a Tube Bundle

Heat exchangers are present in various sectors of industry. However, once its design and operation are satisfactory for single-phase flows, cases with two-phase flow have higher complexity associated to the several possible flow patterns this systems can reach. Several flow pattern maps were suggested over the last years, but always regarding only horizontal or only vertical flow across the tube bundle, often without the presence of baffles. Therefore, in this work a numerical and physical study of the two-phase flow over a tube bundle of a shell and tube heat exchanger closely designed as a industrial two-phase heat exchanger is carried out. The presence of the baffles is taken into account, as well as the direction of the flow. Also, the device has two shell passes, what makes the synergy between horizontal and vertical regions important in the flow, and the tubes are arranged in the parallel direction to the flow. Four experimental conditions were tested, all of them falling in the intermittent flow, according to vertical flow patterns maps, and in the stratified flow, according to horizontal flow pattern maps. Water and air at ambient conditions for temperature and pressure were used to simulate the liquid and vapor phases, without considering heat transfer. All the experiments showed two clearly different behaviors: an oscillatory behavior of the inlet and outlet pressure, but with small amplitude, which was present throughout the experiment; and high pressure peaks, caused by big waves of liquid similar to which are seem in pipe slug flows, occurring only once in a while. A wavelet analysis showed that the pressure oscillations frequency in the first behavior lies bellow 15.6 Hz, with dominance in the range of 0.488 to 0.244 Hz; but for the second behavior, the dominance lies bellow 0.244 Hz. CFD numerical simulations with VOF approach (which is capable of tracking the fluids interface) were also performed in parallel to physical experiments. In the VOF approach, the volume fraction conservation equation is solved for each mesh cell, at the end of each time-step. In this equation, the density and viscosity are a linear relationship between these properties according to the phases volume fraction. For the interpolation of the volume fraction across the face cells and reconstruction of the interface, the geometric reconstruction method was chosen. Two simulations were performed with different boundary conditions at the inlet for comparison with one physical experiment: the first BC used the mean liquid and gas flow rates as inlet for the phases; the second BC used the instantaneous flow rates for each phase. After comparing the inlet and outlet pressures, the numerical model used in this work has proved itself a suitable tool for predicting this flow, because it is capable of predicting the oscillatory pressure signals of high frequency in the equipment, as its main fluid dynamics characteristics.