Modeling a direct contact heat exchanger for a supercritical water loop.
详细信息
文摘
In the last thirty years,Direct Contact Heat Exchangers DCHX) have found success in different power engineering applications. In fact,due to their configuration,which allows the direct contact between the hot and cold fluids,it is possible to reach very high mass and energy transfer efficiencies. Despite their high performance,it remains,to this day,difficult to correctly predict the thermal power as a function of plant operation conditions. In fact,this problematic constitutes a fundamental parameter to correctly operate heat exchangers. In order to study super-critical water choked flow in a super-critical water loop,a heat exchanger of this type has been recently installed in the "Altan Tapucu" Thermo-hydraulic Laboratory. It consists of a fluid mixer called "quenching chamber",i.e. a vessel where super- heated steam coming from a test section where choked flow conditions occur) mixes with sub-cooled water. This component can safely work in a wide range of pressures 5 bar ＜ p ＜ 40 bar). However,on the top of the vessel,a nozzle is set so that the cooling water is sprayed into the chamber under the form of tiny droplets i.e.,about 200 mum in diameter). Within the frame work of this Masters thesis,we developed a thermodynamic model capable of describing the thermal power in the aforementioned DCHX for different working conditions. The main idea is to apply an energy balance to every single droplet in order to evaluate the total heat transfer. In order to do that,we focused our attention on two problems: Droplet size: to perform any energy balance,it is necessary to know the droplet size,however,the quenching chamber working conditions affect this parameter. That is,the droplet dimensions vary depending on steam pressure,liquid flow rate and temperature. Moreover,for a given condition,droplets are expected to have non-uniform dimensions. This means that firstly,a statistical distribution describing the droplet size is to be found,and secondly,the working conditions have to be considered when evaluating this statistical law. Heat transfer: Since there is a mutual interaction between the sub-cooled liquid dis- perse phase) and the super-heated steam continuous phase),we analyzed two heat transfer modes: convection and evaporation. However,this study cannot be performed without a preliminary evaluation of the droplet velocity. That is,the velocity field needs to be known since it affects the amount of energy released. . In this work,the experimental data collected at Ecole Polytechnique de Montreal l are compared with the predictions of our model. We found a very good agreement for steam pressures of 1.6 and 2.1 MPa however,at higher pressures,it over estimates the experimental trends. Hence,we performed an analysis in order to explain the model behavior. Thus,we have justified the observed over predictions at high pressure due to physical variables which are not taken into account in the model such as droplet collision and break-up). Despite the fact that our modeling approach may be questionable on several points,it gives us the possibility to analyze the quenching chamber behavior by linking the dynamics of liquid droplets to the total thermal power. This way,we are able to predict some working conditions that may optimize the thermal power in our DCHX. However,this aspect has not been proven yet and should be the research subject of a future work.