Film Boiling Heat Transfer Coefficients Research Articles

Influence of water temperature on spray cooling at high surface temperatures

Spray cooling is a common cooling method used in many high-temperature metallurgical processes. Heat transfer in the spray cooling of hot surfaces depends on many parameters, and one of them is cooling water temperature. Especially in the steel industry, the cooling water temperature is influenced by the season, the location in the world and also by the intensity of the manufacturing process. Using an experimental approach, the influence of water temperature on the Leidenfrost temperature and film boiling heat transfer coefficient in spray cooling is investigated. Extensive measurements, taken for two types of nozzles, different water flow rates and different water temperatures (12 °C to 78 °C), are presented. It is shown that the film boiling heat transfer coefficient and the Leidenfrost temperature decrease linearly as the water temperature increases. The obtained results are generalised into correlations and transformation functions, allowing extension of existing correlations, which are mostly valid only for water temperatures around 20 °C, to arbitrary water temperatures. All existing papers, which deal with the influence of the water temperature on spray cooling, only present the measured data and do not provide a generalisation of the results. This is a research gap, and this paper tries to fill it.

Universal Correlations for Post-CHF Saturated and Superheated Flow Film Boiling Heat Transfer Coefficient, Minimum Heat Flux and Rewet Temperature for Cryogenic Fluids in Uniformly Heated Tubes

Despite the many experiments conducted throughout the globe during the past sixty years for a variety of cryogens to determine film boiling heat transfer coefficient (HTC) in a uniformly heated round tube, experimental data are either rarely published or published only for a few cryogens, with majority of the data remaining in archives of original authors, or in obscure technical reports of an organization or other inaccessible sources. In the present study, a very comprehensive data mining effort is undertaken to develop a consolidated database for Post Critical Heat Flux (CHF) flow boiling HTC for cryogens from world literature dating back to 1959. With 1730 local Dispersed Flow Film Boiling (DFFB) and 1310 local Inverted Annular Film boiling (IAFB) HTC data points for LHe, LH2, LN2, and LCH4, it represents the largest cryogen post-CHF HTC database assembled to date. Using this database, new universal cryogen correlations for DFFB and IAFB HTCs are constructed and verified in terms of both predictive accuracy and trend. Similar efforts are carried out to collect a relatively small Minimum Heat Flux (MHF) and local Re-wet Temperature (RW) database, and construct correlations for both.

Buoyancy effects on film boiling heat transfer from a sphere at low velocities

A theoretical model is developed for the forced convection film boiling phenomenon over a heated sphere moving vertically downwards in the water. Unprecedented compared with the previous analytical studies, this model accounts for the buoyancy effects while solving the momentum and energy equations in the vapour phase to obtain the velocity and the temperature distribution in terms of the vapour boundary layer thickness. To calculate the vapour boundary layer thickness, an energy balance is applied at the vapour–liquid interface. The flow of liquid around the sphere is considered to be governed by potential theory, and the energy equation in liquid is then solved for the known velocity distribution. We find that the vapour boundary layer thickness increases with an increase in the sphere temperature, the bulk water temperature and a decrease in the free stream velocity. This further results in a decrease in the film boiling heat transfer coefficient. The present study concludes that at low free stream velocities ( ${<}0.5\, {\rm m}\,{\rm s}^{-1}$ ) buoyancy becomes significant in delaying the separation, and when the velocity is further reduced the separation angle approaches $180^{\circ}$ .

Intrinsic effects of Cr-layered accident-tolerant fuel cladding surface on reflood heat transfer

A transient flow quench experiment was conducted to investigate the reflood heat transfer of a Cr-layered vertical tube, which is considered to be the most promising candidate for an accident-tolerant fuel cladding application. Cr was physically deposited on a stainless-steel tube by using DC magnetron sputtering. Owing to the increased nanoscale features of the Cr coating, the surface showed an enhanced capillary wicking potential with superhydrophilic characteristics. In addition, an oxidized Cr-coated specimen was prepared to examine the effects of oxidation during normal operations and blowdown phases in a loss-of-coolant accident. After being heated to 740 °C, the specimens were subjected to a continuous flow of deionized water and heat generation with varying coolant subcooling. The temperature history during quenching was recorded, and the quench behavior was visualized using a high-speed camera. The quench performance, including the quench temperature, critical heat flux, film boiling heat transfer coefficient, and quench front velocity, was analyzed for each specimen by performing an inverse heat conduction analysis. The effects of the Cr coating and oxidation were found to have different impacts on the quench performance, depending on the coolant subcooling and boiling regimes.

An Experimental Study on Cryogenic Spray Quenching of a Circular Metal Disk, Part I: Effects of Mass Flux, Spray Fluid Inlet Subcooling, and Gravity

Abstract This is the first part of a two-paper series that reports the design, experimentation, and results of a spray quenching experiment of a circular metal disk in terrestrial gravity condition. The objective of this experiment is to provide experimental data and corresponding analysis on the heat transfer characteristics and chilldown performance of the cryogenic spray quenching process. In this paper, the presented continuous-flow spray quenching results include the spray-cone angle visualization, spray cooling heat transfer characteristics represented by chilldown curves and boiling curves, gravity effects, and Leidenfrost rewet point temperatures. Additionally, detailed discussion is given on the film boiling heat transfer and rewet temperature in terms of various contributing factors such as gravitational acceleration, spray mass flux, and radial position on the plate. Based on the experimental data, empirical correlations for film boiling heat transfer coefficient, and rewet temperatures are provided. We expect that the current terrestrial study would offer invaluable information for the design of a robust in-space cryogenic propellant storage tank spray chilldown system.

Experimental Investigation of the Transient Pool Boiling Heat Transfer on the Quenching of Vertical Rodlet in Water

Abstract Quenching is an important phenomenon when the emergency core cooling system is put into operation during loss of coolant accident (LOCA) in a nuclear reactor. In this study, an experimental apparatus is designed and constructed with the purpose of conducting transient pool boiling experiments with quenching method for zirconium (Zr-4) cylindrical test samples. Three thermocouples are inserted in the test sample to investigate the effect of axial distance on the minimum film boiling temperature. The Zr-4 rodlet is heated up to a temperature well above the minimum film boiling temperature (up to 600 °C), and then plunged vertically in a quiescent pool of subcooled water. A data acquisition system is used to record the temperature history of the embedded thermocouples. Data reduction is performed by an inverse heat conduction code to calculate the surface temperature and corresponding surface heat flux. A visualization study is conducted to record the quench behavior of the test sample by using a high-speed camera. It is found that the minimum film boiling temperature decreases with the axial distance, while temperature at critical heat flux (CHF) is relatively insensitive to the axial distance. The film boiling heat transfer coefficient decreases with surface temperature, and seems to be independent of axial distance. The quench front is observed to originate from the bottom and move upward. It is found that the quench front velocity remains nearly constant in the lower region of the test sample, and significantly increases in the upper region.

Sensitivity analysis of in-pile CHF experiments in the TREAT facility: Characterization of impacts of fuel system thermal properties

Understanding fuel-to-coolant heat transfer mechanisms under transient heating irradiation conditions is an important aspect of pressurized-water reactor (PWR) safety. The work here aims to elucidate the potential impacts of critical heat flux (CHF) events on nuclear fuel systems, utilizing the heat transfer time constant (HTTC) as the fundamental basis. A set of thermophysical material and heat transfer coolant properties was established to represent the HTTCs of two Transient Reactor Test (TREAT) facility experimental fuel/cladding designs. Pool and flow boiling conditions were used to model these fuel systems within TREAT experimental capabilities, and a power pulse was used to represent a transient insertion of energy. Using Sobol sensitivity analysis and the Reactor Excursion and Leak Analysis Program (RELAP5-3D) code, an investigation was conducted to identify the most important HTTC parameters pertaining to the output of the peak outer cladding temperature (POCT), maximum fuel centerline temperature, and the time of CHF.The maximum fuel centerline temperature was shown to be highly dependent on the UO2 volumetric heat capacity. Regarding the POCTs, DNB was determined to have a dominant effect on the heat transfer mechanisms of these fuel systems. Further, the HTTC significantly impacted POCT predictions with the UO2 volumetric heat capacity, the film boiling heat transfer coefficient, and the CHF multiplier being most influential regardless of coolant flow conditions. Gap variations were shown to influence the POCT in cases that experienced the DNB event. In non-DNB cases, the HTTC had no significant impact on POCT. For the time of CHF, variations in gap thickness, and the CHF multiplier were overwhelmingly dominant.

Study of the film boiling heat transfer and two-phase flow interface behavior using image processing

In the current study, based on a small-scale quench test facility, the two-phase flow interface behavior during quench transients is visualized and analyzed utilizing an image processing framework. The high-fidelity experimental results obtained for two-phase flow in the current framework can support various studies both in the time domain and in the frequency domain. In particular, visualization of the data obtained from different heating surfaces under different test conditions are used to perform a full-scale transient 2-D vapor film reconstruction. The liquid-vapor interface variations in various heat transfer regimes as well as at the initial film breakup point can be directly obtained through the processed data. Moreover, the temporal variation of the interfacial wave frequency approaching quench is investigated in detail. Based on the high-resolution data obtained for the liquid-vapor interface, the detailed phase velocity and temperature profiles are obtained through theoretic analysis, based on which the film boiling heat transfer coefficient (HTC) can be determined. In addition, an improved film boiling HTC model is developed considering the effects of wall superheat, liquid subcooling temperature, vapor film thickness as well as fluid properties. The model is found to predict film boiling HTC well within 15% error.

Experimental investigation on the transient film boiling heat transfer during quenching of FeCrAl

As one of the promising accident tolerant fuel (ATF) cladding materials, FeCrAl can efficiently reduce the hydrogen generation and heat release under accident conditions due to its excellent oxidation resistance. In the case of postulated accidents, such as the loss of coolant accident (LOCA), coolant is immediately ejected to achieve rapid cooling of reactor core. In order to investigate the liquid-vapour interface behavior and film boiling heat transfer of FeCrAl, experiments are carried out with FeCrAl. The rodlets are first heated up to 600 °C before plunging into the liquid pool. The quench behavior is recorded by a high-speed camera, and the temperature variation of the test section is measured by thermocouples. A stable vapor layer is formed preventing the direct contact between the heated surface and liquid, and K-H instability wave is recognized due to the velocity difference between the liquid and vapor. It is found that as the liquid subcooling degree increases, the quenching time is decreased, and minimum film boiling temperature is increased. The quenching time and minimum film boiling temperature is also influenced by surface roughness and surface wettability. Based on the image processing method, it is found that the vapor layer thickness decreases with the increase of liquid subcooling degree and surface roughness. Based on the one-dimensional inverse heat conduction code, the surface heat flux and temperature is calculated. The film boiling heat transfer coefficient is found to increase with liquid subcooling, and it is relatively insensitive with surface roughness. Taking into account of the effect of liquid subcooling, a generalized correlation for film boiling heat transfer is developed.

Mechanistic method to predicting minimum heat flux point wall temperature in saturated pool boiling

This study details the development of a new mechanistic approach to predicting heating wall temperature corresponding to the minimum heat flux (MHF) point for saturated pool boiling from a horizontal flat surface. The model is constructed by performing a force balance on a unit cell of the wavy liquid-vapor interface, incorporating the effects of pressure difference across the interface, surface tension, gravity and stagnation pressure. It is shown how, in the film boiling regime, where a continuous vapor film insulates the heating surface, a decrease in wall heat flux towards the MHF point causes the liquid to approach the surface but without yielding any contact. A further decrease in heat flux triggers the MHF condition once the downward forces pushing the interface towards the wall overcome the upward forces lifting the interface away from the wall. This causes the wavy liquid-vapor interface to contact and wet the heating surface; vigorous bubble nucleation and vapor production ensue, signaling onset of the transition boiling regime. To achieve closure, the model adopts a previous relation for film boiling heat transfer coefficient, which is used to determine the wall temperature . The new model is shown to provide good agreement with MHF-point wall temperature data for many liquids and a broad range of pressures, evidenced by a mean absolute error of 9.35%. This study also includes an assessment of previous thermodynamic and hydrodynamic MHF models.

Falling film boiling and pool boiling on plain circular tubes: Influence of surface roughness, surface material and saturation temperature on heat transfer and dryout

Falling film evaporators in the refrigeration industry offer a number of advantages over their flooded counterparts such as improved heat transfer and lower refrigerant charge. Existing literature has not characterised the influence of surface roughness and material on the falling film boiling process. The purpose of this study was therefore to experimentally measure the influence of surface material and roughness on the heat transfer of falling film boiling and pool boiling on the outside of horizontal plain tubes. The falling film enhancement factor and total dryout threshold were also measured in the study. The study was conducted on an experimental set-up at saturation temperatures of 5 °C and 25 °C using refrigerant R-134a at heat fluxes from 20 to 90 kW/m2 and film Reynolds numbers from 2000 to 0. The outside of plain copper, stainless steel and mild steel tubes were roughened with various grades of sandpaper to achieve roughnesses between 0.1 and 1.9 μm. The tubes were heated by water and as such Wilson plots were conducted to characterize the internal heat transfer. Increases in surface roughness were found to increase both pool boiling and falling film boiling heat transfer coefficients. Changes in surface material decreased the heat transfer coefficients in line with the decrease of the material’s thermal effusivity. The falling film enhancement factor was found to increases as surface roughness was increased, but changes in material had no discernible effect. Decreases in saturation temperature decreased pool boiling and falling film boiling heat transfer coefficients and weakly increased the falling film enhancement factor. Changes in surface roughness and material had no discernible influence on total dryout threshold.

Film boiling heat transfer properties of liquid hydrogen flowing inside of heated pipe

The knowledge of heat transfer properties of liquid hydrogen is important for designing and developing superconducting devices. In this study, film boiling heat transfers of liquid hydrogen flowing inside of heated pipe were measured under saturated conditions at the absolute pressures of 700 kPa for various mass flow rates. Test pipe heater made of SS310S with a diameter of 8 mm and a length of 200 mm was used. The test heater was once heated up to the film boiling regime with exponential heat generation rate. And then, while the heat generation rate was decreased exponentially down to the minimum heat flux, the film boiling heat transfer coefficient, mass flow rate per unit area and the degree of superheat of pipe length direction were measured. It was observed that though the mass flow rate decreased according to increase of the heat generation rate, the heat transfer coefficient increased. Discussions on the experimental results of various conditions were carried out to clarify the phenomenon of film boiling of liquid hydrogen flowing inside of heated pipe. It was considered that since the void fraction in the flow path was high, the effect of improving the heat transfer rate was also observed in the film boiling region due to the acceleration of the liquid phase.

Film Boiling Heat Transfer from a Wire to Upward Flow of Liquid Hydrogen: Effect of Wire Diameter

Film boiling heat transfer coefficients were measured for the test wires of 0.5 and 0.7 mm in diameters and 200mm in length located at the centre of 8 mm diameter conduit with upward flow of liquid hydrogen. Pressures are ranged from 0.4 to 1.1 MPa, liquid subcoolings from 0 to 11 K and flow velocities up to 5 m/s. The experimental data were compared with the authors’ correlation for forced flow film boiling already presented based on the data for 1.2 mm diameter test wire. The experimental data are higher than the predicted values by the correlation. Discussion is made on the effect of wire diameter observed and a new correlation was presented.

Preliminary thermal hydraulic analysis of various accident tolerant fuels and claddings for control rod ejection accidents in LWRs

The present study assesses the effect of various accident tolerant fuel (ATF) materials on thermal hydraulic behaviors of Light Water Reactors (LWRs) in a Reactivity-Initiated-Accident (RIA). The study focuses on the impact of ATF thermal physical properties and cladding boiling heat transfer enhancement. The results show that without the heat transfer enhancement, peak cladding temperature (PCT) can be reduced by the utilization of FeCrAl, SA3-PyC150-A and HNLS/ML-A claddings but peak fuel centerline temperature (PFCT) rises at the same time. However, the reverse situation happens to the utilization of UO2 + BeO and UO2 + SiC composite fuels. In addition, to identify the potential impact of nucleate and film boiling heat transfer coefficient and critical heat flux (CHF), a sensitivity analysis to these parameters is conducted. The results show PCT is particularly affected by CHF. PFCT can be reduced and heat transfer deterioration is eliminated by the application of 90UO2 + 10SiC assembled with all the three ATF claddings as well as 90UO2 + 10BeO assembled with HNLS/ML-A cladding when 10% boiling heat transfer enhancement is added.

Potential impact of accident tolerant fuel cladding critical heat flux characteristics on the high temperature phase of reactivity initiated accidents

The present paper details scoping RELAP5-3D simulations to assess the potential impact of boiling heat transfer coefficients and critical heat flux (CHF) on peak cladding temperature (PCT) of key accident tolerant fuel (ATF) cladding types in a design basis accident (DBA). The accident event studied is a Hot Zero Power (HZP) Reactivity-Initiated Accident (RIA) in a Pressurized Water Reactor (PWR), so the present study is focused on bubble crowding CHF. Results are presented for the sensitivity of PCT and the duration time of film boiling to the key parameters in the boiling curve: nucleate, transition, and film boiling heat transfer coefficient, and CHF. This included a generic study of perturbations in boiling heat transfer characteristics for Zircaloy-4 cladding as well as a study which focused on the key candidate ATF cladding types. For Zircaloy-4 cladding, the simulation results show that both PCT and duration of film boiling are particularly sensitive to the value of CHF. We also compared the performance of FeCrAl claddings and SiC/SiC claddings with Zircaloy-4 cladding under the same RIA scenario. A 10% CHF enhancement for FeCrAl cladding can significantly reduce the PCT of FeCrAl claddings. Lower irradiated thermal conductivity of SiC/SiC claddings does cause a high temperature gradient in the radial direction.

Film boiling heat transfer from a wire to upward flow of liquid hydrogen and liquid nitrogen

Film boiling heat transfer coefficients in liquid hydrogen were measured for the heater surface superheats to 300 K under pressures from 0.4 to 1.1 MPa, liquid subcoolings to 11 K and flow velocities to 8 m/s. Two test wires were both 1.2 mm in diameter, 120 mm and 200 mm in lengths and were made of PtCo alloy. The test wires were located on the center of 8 mm and 5 mm diameter conduits of FRP (Fiber Reinforced Plastics). Furthermore film boiling heat transfer coefficients in liquid nitrogen were measured only for the 200 mm long wire. The film boiling heat transfer coefficients are higher for higher pressure, higher subcooling, and higher flow velocity. The experimental data were compared with a conventional equation for forced flow film boiling in a wide channel. The data for the 8 mm diameter conduit were about 1.7 times and those for the 5 mm conduit were about 1.9 times higher than the predicted values by the equation. A new equation was presented modifying the conventional equation based on the liquid hydrogen and liquid nitrogen data. The experimental data were expressed well by the equation.

Application of the Spray Quenching to T6 Heat Treatment of Thick A6061 Hollow Cylinders

Aluminum allay A6061 is commonly applied T6 heat treatment and ultimate tensile strength above 300 MPa is obtained by solid solution strengthening. The T6 treatment consists of solution treatment, water quenching and artificial aging. Lower cooling rate during quenching is very important because impurity phase deposition in solution phasedue to longer quenching delay results in growth of grains in the aging process and less strengthening. The quenching delay time strongly depends on boiling transition from film boiling to nucleate boiling as well as heat capacity of the solid. Stable wetted situation hardly occurs in the film boiling region and film boiling heat transfer coefficient is one or two digit lower than that of the nucleate boilingheat transfer. In this research, dip (bath) and spray quenches were applied to thick A6061 hollow cylinders to get comparable spray cooling rate with the conventional dip quenching. Temperature histories in the cylinder were measured during quenching and inside temperature distribution was estimated with inverse heat conduction analysis. After the T6 treatment, tensile test pieces were cut away from the cylinder and effectiveness of the spray quench was accessed. The spray quenchingtests were done for water temperature of 10-22 oC and the maximum total spray flow rate of 20 L/min with four or eight spray nozzles around the cylinder. For the dip quenching, test piece was soaked into well stirred water pool at a fixed temperature of 18 oC.From the spray quenchtests, the quenching delay time was difficult to keep within the allowable maximum delay time requested from existing metallurgical studies. It was found that higher water flow rate or larger number of nozzle arrangement will be required to obtain comparable cooling rate with the dip quenching result. However, each tensile test showed that the strength of spray quenched materials is almost equivalent to that of dip quenching and satisfied the minimum tensile strength of A6061-T6 regulated by JIS (Japanese Industrial Standards) B 8270.

Film Boiling Heat Transfer from a Round Wire to Liquid Hydrogen Flowing Upward in a Concentric Annulus

Hydrogen film boiling heat transfer coefficients were measured for the heater surface superheats up to 400K under pressures from 400 to 1100kPa, liquid subcoolings from 0 to 11K and flow velocities up to 7 m/s. The test wire used was 1.2mm in diameter and 120mm in length made of PtCo (0.5wt.%) alloy, which was located at the center of 8mm diameter conduit made of FRP (Fiber Reinforced Plastics). The heat transfer coefficients were higher for higher pressure, higher subcooling and higher flow velocity. The heat transfer coefficients were about 1.6 times higher than those predicted by Shiotsu-Hama equation for forced flow film boiling in a wide channel. Discussions were made on the mechanism of difference between them.

Film Boiling Heat Transfer Properties of Liquid Hydrogen in Natural Convection

Film boiling heat transfer properties of LH2 for various pressures and subcooling conditions were measured by applying electric current to give an exponential heat input to a PtCo wire with a diameter of 1.2mm submerged in LH2. The heated wire was set to be horizontal to the ground. The heat transfer coefficient in the film boiling region was higher for higher pressure and higher subcooling. The experimental results are compared with the equation of pool film boiling heat transfer. It is confirmed that the pool film boiling heat transfer coefficients in LH2 can be expressed by this equation.

Subcooled pool film boiling heat transfer from small horizontal cylinders at near-critical pressures

Experimental results for subcooled pool film boiling on small horizontal cylinders at near-critical pressures are presented. Experiments were performed with CO2 was the test liquid and platinum wires (25.4, 76.2 and 100μm diameter, D) as the test heaters. The pressure (P), liquid subcooling (ΔTsub) and bulk liquid temperature (Tb) were varied parametrically. For the range of parameters investigated, the heat transfer coefficient (h) was found to be proportional to D−0.5. Because the dependence of h on D varies, a criterion has been developed to predict a priori what this dependence would be any given situation. Subcooled pool film boiling heat transfer coefficient data have been correlated. The correlation developed predicts almost all the experimental data from the current study to within ±10%. The strength of the new correlation is that it is applicable to a much wider range of experimental conditions (including level of gravity) and fluids as demonstrated by the fact that it predicts all of the data reported in the literature to within ±20%. Visual observations during experiments showed that the transitions in film boiling patterns are a function of the dimensionless cylinder diameter.