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

Maritime varmegenvindingssystemer (WHR) udnytter ellers spildt varme til at producere damp; kedler er afgørende for systemernes effektivitet. Selvom tofaset strømning (en blanding af vand og damp) indgår i designet, bygger det ofte på simple tommelfingerregler. Dette speciale udvikler computermodeller til en mere grundig analyse af, hvordan tofaset strømning påvirker kedlens vigtigste driftsparametre. Med en virkelig skibskedel som case er der opbygget både analytiske og numeriske modeller efter to udbredte tilgange: homogen strømning (behandler væske og damp som én blandet væske) og separeret strømning (følger væske og damp hver for sig). Modellerne blev kørt under kendte driftsforhold ombord for at forudsige, hvordan tryk, dampkvalitet/tørhedsgrad (andelen af damp i blandingen), temperatur og varmeflow udvikler sig langs kedlen, og resultaterne blev sammenlignet med skøn fra almindelige designregler. For denne kedel giver den analytiske, homogene model total varmeoverførsel inden for 2% af den numeriske, separerede model, mens det totale tryktab kan afvige med op til 60%. De opnåede trykprofiler bruges til at forklare, hvordan friktion, tyngdekraft og acceleration bidrager til trykgradienten, efterhånden som fordampningen skrider frem. Den mest præcise model blev også brugt til at undersøge forskellige damptromletryk, de relative positioner mellem kedel og tromle, og masseflow. Heraf blev systemets karakteristiske kurve bestemt, hvilket viser, at systemet er stabilt og langt fra den kritiske varmeflux (en grænse, hvor kogning bliver ustabil). På den baggrund anbefales det at anvende lavere tromletryk (et trykfald på 10% svarer til ca. 8% mere overført effekt) og at reducere afstanden—og dermed tryktabet—mellem kedel og tromle; i det undersøgte arbejdsområde ændrer en stigning på 1 bar i dette tab varmeoverførslen med omkring 4%.

Marine waste heat recovery (WHR) systems reuse otherwise lost heat to produce steam, and boilers are central to their efficiency. Although two-phase flow (a mixture of liquid water and steam) is considered in design, it is often handled with simple rules of thumb. This thesis develops computer models for a more rigorous analysis of how two-phase flow affects key boiler operating variables. Using a real shipboard boiler as a case study, both analytical and numerical models were built following two common approaches: homogeneous flow (treating liquid and vapor as a single mixed fluid) and separated flow (tracking liquid and vapor separately). The models were run under the vessel’s known operating conditions to predict how pressure, steam quality (vapor fraction), temperature, and heat flow evolve along the boiler, and the results were compared with estimates from the usual design rules. For this boiler, the analytical homogeneous-flow model predicts total heat transfer within 2% of the numerical separated-flow model, while total pressure loss can differ by up to 60%. The pressure profiles help explain the roles of friction, gravity, and acceleration in the pressure gradient as evaporation takes place. The most precise model was also used to examine different steam drum pressures, the relative positions of the boiler and the drum, and mass flow rates. From this, the system’s characteristic curve was obtained, indicating that the system is stable and far from the critical heat flux (a limit where boiling becomes unstable). Based on these results, the thesis recommends using lower drum pressures (a 10% reduction corresponds to about 8% more power transferred) and reducing the distance—and thus pressure loss—between the boiler and the drum; in the operating range studied, a 1 bar increase in this loss changes heat transfer by about 4%.

[This abstract was generated with the help of AI]

Waste Heat Recovery ; Two-Phase Flow ; Boiler Design

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