Critical Heat Flux Temperature Research Articles

Capillary limit of a sodium screen-wick heat pipe

High-temperature alkali-metal heat pipes are passive heat transfer devices, where the capillary pressure created by the menisci in the wick pumps the condensed fluid back to the evaporator. Thus, the heat pipe heat transfer capacity is limited by the maximum capillary pressure at high heat fluxes. This work presents a two-dimensional heat and mass transfer model for alkali metal heat pipes. In addition to the traditional wall heat conduction model, the model also incorporates gas–liquid flow heat transfer and capillary structure models, enabling the model to simulate both steady-state operation and transient capillary limit processes. The predictions of the model agreed well with experimental data. The predicted critical heat flux temperature has an error of less than 20℃ and a relative error of 6%. The predicted transient sequences are consistent with the measurements. The model shows that capillary limit is affected by inclination angle, heat flux, and wick structure. For inclinations, the critical heat flux at the capillary limit is linearly related to the inclination angle with the predicted temperature rising 2–10℃/s during the early stage of the capillary limit critical heat flux condition. The evaporator temperature oscillates before the capillary limit with positive inclinations. For heat fluxes, increasing the heat flux by 17% increases the temperature increase rate 10 fold with a longer dryout length. The evaporator exhibits hysteresis after the capillary limit due to partial dryout of evaporator. For wick structures, reducing the mesh screen pitch from 80 μm to 15 μm reduces the critical heat flux to 10% of the CHF at 80 μm. This model can accurately predict the heat pipe temperature variations at the capillary limit and can explain the physical mechanisms for the dryout oscillations and recovery hysteresis at the capillary limit. Thus, the measurements and model provide analytical guidelines for designing heat pipe cooling systems.

Numerical Investigation of the Heat Transfer Characteristics and Wall Film Formation of Spray Impingement in SCR Systems

This work established a numerical model to investigate the heat transfer characteristics and wall film formation of spray impinging on the wall in the selective catalytic reduction (SCR) system. The model is developed by the Eulerian–Lagrangian approach, where the Lagrangian approach is used to represent the spray generated by a commercial non-air-assisted pressure-driven injector and the Eulerian approach is adopted to represent exhaust gas. The Stochastic Kuhnke Model is applied to spray/wall interaction. The model considers relevant processes, which include mass transfer, momentum transfer, heat transfer, droplet phase change, spray/wall interaction, and wall film formation. The numerical results compared with that of the experiment indicate that the model can accurately estimate the heat transfer characteristics of the wall surface during the spray impingement. Based on the numerical results, the causes of the spray local cooling effect and the rapid cooling effect are analyzed. The correlation between the critical transition temperature and the critical heat flux temperature for wall film formation is derived from the trends of wall temperature and heat flux. In this work, the Stochastic Kuhnke Model is applied and compared with the Kuhnke Model, which proves that it can improve the disadvantage of sudden change during the wall film formation. When the wall temperature is below the critical transition temperature, the wall film mass is sensitive to the wall temperature and increases as the wall temperature decreases.

The role mechanism of vapor-liquid behavior on boiling crisis triggering

Revealing the physical mechanism of critical heat flux (CHF) triggering is an essential basis for building accurate prediction models. Photographic observations assist in comprehending the physical mechanism of CHF during subcooled flow boiling. The vapor-liquid behavior obtained from experimental observations shows that, when heat fluxes extremely approaching CHF, a large amount of vapor condenses into a wavy vapor layer; simultaneously, a wavy liquid film layer forms beneath the vapor layer that propagates along the heating wall. The wavy vapor layer directly impedes liquid supplement from the liquid core to the liquid film adjacent to the heating wall. As the heat flux increases, the bubble departure diameter increases and the residual vapor phase in the detachment process directly leads to the formation of local dry spot, which prevents the rewetting of the adjacent liquid due to the vapor recoil effects. At approaching CHF conditions, dry patches are formed after the remaining wetted areas rapidly evaporate. Then, boiling crisis is postulated to be triggered when the dry spots initially form, grow and finally merge into dry patches. This observation sheds light on the triggering mechanism of the flow boiling crisis. The role mechanism for the CHF triggering is proposed through studying the boiling behavior of subcooled flow boiling near a heating wall, including just before CHF (CHF-), during CHF onset (CHF transient), and after CHF appearance (CHF+). Besides, a new correlation is presented for predicting critical heat flux temperature of water under a wide range of operating conditions with a mean absolute error of 3.81%. These results will provide guidance for developing mechanistic theoretical model and enhancing CHF.

Sensitivity study of the influence of the water boiling parameters on aluminum semi-continuous DC casting

The thermal aspect is dominant in semi-continuous direct chill (DC) casting. In steady state, 80% of the total energy contained inside the ingot is evacuated by secondary cooling. For this reason, water is often suspected as a potential cause for the issues related to ingot quality. In the secondary cooling zone, the ingot is cooled by jet impingement boiling. Opinions in the literature about the influence of water quality on the boiling curve are conflicting. This can be associated mainly to the difficulties that experimenters meet during the measurement of the boiling curve repeatedly. Furthermore, published works present the entire boiling curve rather than identifying what are the significant parameters in it. This understanding is nonetheless essential in order to improve the measurement repeatability by focusing on the most significant boiling parameters as well as to ensure that the measured variations of the boiling curve are those that have an impact on the process. Considering this lack of knowledge, a sensitivity study has been completed to compare the relative influence of four parameters that characterize the boiling curve (Leidenfrost temperature, film boiling heat flux, critical heat flux and the temperature that belongs to the critical heat flux) on three issues responsible for the diminution of the recovery rate (hot tearing, bleed-out, butt curl). The study has shown that except the critical heat flux temperature, every tested boiling parameter plays a significant role on the three recovery rate obstacles. Moreover, for every case, a mutual interaction exists between the three significant boiling parameters. The latter demonstrates that the variation of each of the three most important boiling parameters is relevant and needs to be measured following a modification of the water quality. This study is the first step in the framework of a project aimed at the conception of an appropriate device for measuring the variation of the cooling capacity of water and its role on ingot quality.

Investigation on heat transfer in line chill-down process with various cryogenic fluids

This paper focuses on the transient process of the pipe cooling by the internal flow of cryogenic fluid, which is a so-called cryogenic line chill-down process. Due to the increasing use of cryogenic fluid in various industries, the heat transfer characteristics during the line chill-down process are followed with great interest. One of the biggest concern is that there are no universal heat transfer correlations for simulating the chill-down process. This research aims to extend the quenching database with liquid oxygen and predict the cryogenic line chill-down process with a one-dimensional homogeneous model. The line chill-down experiments are performed by liquid oxygen with the mass flux range from 15.9 kg/m2-s to 75.9 kg/m2-s. A 7 m long stainless steel 316 horizontal tube is used as the test section. The empirical correlations for heat transfer coefficient of single-phase vapor convection and film boiling are suggested based on the chill-down experimental data of liquid oxygen, liquid argon, and liquid nitrogen. The line chill-down process is simulated by applying the empirical correlations in a one-dimensional homogeneous model. The feasibility of the dynamic simulation model is verified in the mass flux up to 868 kg/m2-s by simulating the chill-down experimental data in the literature. Moreover, the characteristics of the critical heat flux, critical heat flux temperature, and minimum heat flux temperature are also investigated by comparing various experimental data and the empirical correlations. This study provides not only the open experimental data with liquid oxygen, which is rarely found in the literature but also the numerical simulation method, which predicts the line chill-down time within ± 10% error.

Experimental investigation on line chill-down process by liquid argon

The cryogenic line chill-down process is an essential part of the cryogenic system application. As the heat transfer characteristics during the transient cooling down are highly affected by the fluid properties, universal heat transfer correlations, which are valid for the chill-down process with various kinds of cryogenic fluid, are required in many industries. This paper investigates cryogenic line chill-down process by using a 7 m long stainless steel horizontal pipe. Liquid argon is selected as the working fluid to examine the fluid property effects. The histories of the wall temperature, pressure, and mass flow rate are measured during the chill-down process. The heat transfer characteristics of liquid argon are analyzed and compared with those of chill-down process with liquid nitrogen. At the same time, the empirical correlations for the critical heat flux, critical heat flux temperature, and minimum heat flux temperature, which can well estimate the parameters for both of liquid nitrogen and liquid argon cases, are suggested. As a result, the critical heat flux is estimated with the mean absolute error (MAE) of 14.8%. The critical heat flux temperature and minimum heat flux temperature are predicted with the MAE of 3.4% and 1.9%, respectively. The empirical correlations are essential to construct a numerical model for simulating the cryogenic line chill-down process.

Improvement and validation of the wall heat transfer package of RELAP5/MOD3.3

The process of energy transfer from heat structure to control volume is determined by the wall-to-fluid heat transfer package, which is crucial for nuclear reactor safety analysis codes. The current logic for selection of heat transfer modes of RELAP5/MOD3.3 code is too complex and may result in incorrect heat transfer mode judgment. Also, the narrow application scope of film boiling heat transfer correlations may result in large errors in film boiling region which is of paramount importance for the predicted peak clad temperatures during hypothetical LB-LOCAs in PWRs. In this study, a new heat transfer package has been developed and incorporated into the RELAP5/MOD3.3 code. Differing from the original package, the modified one consists of twelve heat transfer modes and proposes a new logic for selection of heat transfer modes. For each mode, the models in the existing safety analysis codes and the leading models in literature have been reviewed in order to determine the best model which can easily be applicable to the RELAP5/MOD3.3 code. Particularly (1) a new package of heat transfer correlations are produced; (2) a new logic for selection of film boiling and transition boiling heat transfer modes is proposed which use minimum film boiling temperature and critical heat flux temperature as distinguished points. The modified code has been validated by comparing the analysis results with available experimental data from tube post dryout experiments and loss-of-fluid test (LOFT) facility. The calculation results showed that the improved package could better predict the experimental phenomena with higher prediction accuracy.

Comment from Europe. Completing a silly fuel cycle

The rewetting or quench temperature is the temperature of a hot solid surface at which a liquid can reestablish contact with the dry surface. An estimation of this temperature is essential in predicting the rate at which the coolant quenches the core of a light-water reactor (LWR) after a loss-of-coolant accident. The present study reviews and evaluates previous work in this area and presents a model relating experiments to theory for the different possible types of reflood in LWRs. It is postulated that, with the exception of those cases of top reflood by a film in a single-rod geometry and bottom reflood with a very low mass flow rate, the quench temperature corresponds to either the minimum film boiling temperature or the Leidenfrost temperature. In cases where there are such exceptions, the quench temperature corresponds to the critical heat flux temperature. New correlations for the rewetting or quench temperature are presented.

A study on the rewetting temperature

The rewetting or quench temperature is the temperature of a hot solid surface at which a liquid can reestablish contact with the dry surface. An estimation of this temperature is essential in predicting the rate at which the coolant quenches the core of a light-water reactor (LWR) after a loss-of-coolant accident. The present study reviews and evaluates previous work in this area and presents a model relating experiments to theory for the different possible types of reflood in LWRs. It is postulated that, with the exception of those cases of top reflood by a film in a single-rod geometry and bottom reflood with a very low mass flow rate, the quench temperature corresponds to either the minimum film boiling temperature or the Leidenfrost temperature. In cases where there are such exceptions, the quench temperature corresponds to the critical heat flux temperature. New correlations for the rewetting or quench temperature are presented.