Analysis of Two-Phase Flow in a Marine Waste Heat Recovery Boiler: Pressure Loss and Heat Transfer Relation

Skou, Michael ; Galindo Asuar, Beltrán

4. term

Energy Engineering, Master

2014

2014-06-03

92 pages

Boilers are an essential component in the marine WHR field and their performance accounts for a great part of the WHR system efficiency. Although two-phase flow effects are considered in their design it is usually done with very simplistic approximations that come from rules of thumb developed in the industry. This thesis deals with the ellaboration of computer models for the rigurous analysis of the effect two-phase flow has in the essential variables of the boiler operation. Based on a real boiler case both analytical and numerical models are built for the two main two-phase flow study approaches: homogeneous and separated flow. The models are tested for the known working conditions of the boiler on board the ship in order to determine the pressure, quality, temperature and heat flow profiles along the boiler and compare the results with the total pressure loss and heat transfer obtained with the usual design rules of thumb. It is concluded that for a given boiler the analytical homogeneous flow model provides results within a 2\% difference of those obtained with the numerical separated flow model for the total heat transfer, but the total pressure loss results differ up to a 60\%. The obtained pressure profiles are used to explain the development of the frictional, gravitational and accelerational components of the pressure gradient as evaporation occurs. The most precise of the computer models is also tested under different steam drum pressures, relative positions of the boiler and the drum and mass flows through the system. The characteristic curve of the system is determined and it is concluded that the system is stable and far from reaching the critical heat flux. At the end of the paper some design recommendations to maximize the heat transfer are given: to use lower drum pressures, as a 10\% decrease in pressure accounts for 8\% more power transfered, and to minimize the distance (pressure loss) between the boiler and the drum, as in the working region an increase of 1\,bar in this pressure loss results in 4\% heat transfered.

Waste Heat Recovery ; Two-Phase Flow ; Boiler Design

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