Istraživanje termičkih i hidrauličkih uslova na grejnoj površini pri ključanju i krizi razmene toplote

Abstract

Prediction of thermal-hydraulic conditions on a heated surface of heat exchangers during boiling and boiling crisis when the surface’s dry out can occur, because it is no more in contact with liquid phase, strongly depends on micro-conditions in nucleation site, liquid superheat, mass of evaporation per unit volume and time as well as void fraction and a two-phase mixture swell level. Due to inability of experimental research to include all of the above mentioned heat and mass transfer aspects on the interface and to realize the consequences of accidental conditions that can occur during heat exchanger's tube overheat, it is very important their numerical investigation.Prediction of two-phase thermal-hydraulic conditions on heat exchanger’s heated surface during boiling and boiling crisis is based on the two-fluid model and it consists of mass, momentum and energy fluid flow conservation equations for both liquid and vapor phase, while interface transfer processes are modeled by “closure laws”. Governing equations are solved by the “SIMPLE – Semi-Implicit-Method for Pressure-Linked Equations” type pressure-correction method that is derived for the multiphase flow conditions. Calculated are pressure field, velocities of both phases, enthalpy of liquid phase, void fraction as well as temperature field into and onto the heated wall. Developed numerical method represents significant contribution to the development of nucleate boiling researchviimethods, regarding previous investigation methods were largely experimental, while developed empirical methods could be reliably applied only within narrow range of flow and thermal parameters of importance for process. This model provides information on steam generation and enables direct numerical simulation of boiling process, as well as quantification of the impact of certain parameters of two-phase flow and heating wall conditions on the creation and running of nucleate boiling.