CHF enhancement through rewetting and heating time change via controlling heater configuration, surface characteristics and discretized heating elements

Abstract

The boiling phenomena can induce much higher cooling performances than other cooling methods based on the characteristics of using the latent heat of the coolant. Therefore, the boiling phenomenon is being utilized in various applications that emit high energy density and large amounts of energy, such as cooling integrated circuit (IC) chips and nuclear power plants. In the systems that use the boiling phenomenon, the critical heat flux (CHF) is one of the major interests. If the heat flux exceeds CHF, a rapid temperature rise within a short period occurs which eventually induces system failure (deformation or rupture). This is caused by the regime transition from the nucleate boiling regime, which has a much higher heat transfer coefficient, to the film boiling regime, which has a very low heat transfer coefficient due to the vapor film all over the heating surface. As heat flux is directly related to the system performances, it is very important to accurately predict and enhance CHF from the perspective of system safety and efficiency. Various models have been proposed to accurately predict CHF. In literatures, CHF enhancement were widely reported and explained based on the previous models. Based on the recent literature, it was confirmed that CHF is significantly related to the critical temperature that prevent surface rewetting and heating time scale to reach the critical temperature. Based on this, I set the purpose of this study as to identify the CHF enhancement mechanism by controlling various factors and to present a generally valid CHF model based on the mechanism. In this study, heater configuration, surface characteristics and heating method were set as factors that mainly affect CHF, and CHF enhancement was experimentally confirmed by changing them. Among all experiments with fixed working fluid (water), the lowest CHF was confirmed to be 696 kW/m2 whereas the highest CHF was confirmed to be 3043 kW/m2. By changing the heater configuration and surface characteristics, significant CHF changes were confirmed between the corresponding values, and dry-spot rewetting model was proposed to explain the CHF. In the model, we compared the rewetting time scale and heating time scale of the dry spot created at the center of the heater and assumes that the moment when heating time scale is shorter than the rewetting time scale is the CHF triggering point. The predicted values by the model relatively matches well with the experimental data with MAPE of 11.8 % and data from the literatures with MAPE of 26.3 %. Based on the modeling, we confirmed that CHF can be much enhanced if the heating scale becomes longer, which indicates that it takes longer time to reach the critical temperature. To validate it, I developed the discretized heating elements (DHE), which can control heat flux locally based on the measured temperature. Using DHE, we confirmed 18 % enhancement of CHF, which matches well with the modified equation for DHE based on the dry spot rewetting model. Further CHF enhancement can be obtained where magnitude differs depending on surface characteristics and heater configuration and temporal, spatial resolution of DHE.