Instability of subcooled boiling film on a vertical wall

Consider a vertical plate with a leading edge. The temperature of the plate is above the spontaneous nucleation temperature, so that vapor completely covers the plate. Two-dimensional quasi-parallel theory is used to examine the stability of the two-phase system. Numerical calculations show that the main parameter determining the thickness of the vapor film and its stability is the difference between the temperature of the heated vertical plate and the saturation temperature of the liquid. As the thickness of the vapor film is made smaller, the nose of the neutral curve approaches that of the corresponding one-phase liquid. The overall temperature difference between the plate and liquid bulk does not strongly influence stability properties.     

Wall heat flux partitioning during subcooled forced flow film boiling of water on a vertical surface

Subcooled flow film boiling experiments were conducted on a vertical flat plate, 30.5 cm in height, and 3.175 cm wide with forced convective upflow of subcooled water at atmospheric pressure. Data have been obtained for mass fluxes ranging from 0 to 700 kg/m²s, inlet subcoolings ranging from 0 to 25 °C and wall superheats ranging from 200 to 400 °C. Correlations for wall heat transfer coefficient and wall heat flux partitioning were developed as part of this work. These correlations derive their support from simultaneous measurements of the wall heat flux, fluid temperature profiles, liquid side heat flux and interfacial wave behavior during steady state flow film boiling. A new correlation for the film collapse temperature was also deduced by considering the limiting case of heat flux to the subcooled liquid being equal to the wall heat flux. The premise of this deduction is that film collapse under subcooled conditions occurs when there is no net vapor generation. These correlations have also been compared with the data and correlations available in the literature.     

CFD simulation of upward subcooled boiling flow of refrigerant-113 using the two-fluid model

In this paper, a modified two-fluid model considering temperature dependent properties and saturation temperature variation was adapted to accurately simulate the process of subcooled boiling flow. The calculations were performed by a CFD code CFX-10 with an extended user defined FORTRAN. The refrigerant-113 boiling flow experiments in a vertical concentric annulus from reference were used to validate the modified two-fluid model of subcooled boiling flow. Compared with the previous published predicted results, the results in the present model show better and reasonable approximation with the experimental data, including the local distribution of void fraction, liquid temperature, axial liquid and vapor velocity. Results show that with increasing the wall heat flux, the bubble boundary layer will become thicker, the liquid temperature gradient at the near-wall region will be smoother and the profiles of axial liquid velocity will gradually depart from those of single-phase flow. Decreasing the inlet liquid subcooling or the mass flux will obtain the same results.     

Bubble dynamics at boiling incipience in subcooled upward flow boiling

Bubble dynamics in water subcooled flow boiling was investigated through visualization using a high-speed camera. The test section was a vertical rectangular channel, and a copper surface of low contact angle was used as a heated surface. Main experimental parameters were the pressure, mass flux and liquid subcooling. Although all the experiments were conducted under low void fraction conditions close to the onset of nucleate boiling, no bubbles stayed at the nucleation sites at which they were formed. Depending on the experimental conditions, the following two types of bubble behavior were observed after nucleation: (1) lift-off from the heated surface followed by collapsing rapidly in subcooled bulk liquid due to condensation, and (2) sliding along the vertical heated surface for a long distance. Since the bubble lift-off was observed only when the wall superheat was high, the boundary between the lift-off and the sliding could be determined in terms of the Jakob number. Based on the present experimental results, discussion was made for the possible mechanisms governing the bubble dynamics.     

An experimental study of subcooled film boiling of refrigerants in vertical up-flow

Subcooled film boiling has been investigated experimentally for vertical up-flow in a directly heated tube using the refrigerants R-12, R-22 and R-134a as test fluids. The data cover a mass flux range of 530–3000 kgm⁻² s⁻¹, an inlet subcooling range of 8–28°C and a pressure range of 0.83-1.6 MPa (corresponding to an approximate water pressure range of 5–7 MPa, based on an equal liquid-to-vapour density ratio). To the authors' knowledge, these are the first flow film boiling data obtained for R-134a and R-22. The results show strong effects of mass flux, inlet subcooling and pressure on the heat transfer coefficient. Also, the data exhibit complex trends of the heat transfer coefficient as a function of thermodynamic equilibrium quality. Because of the wide range of conditions covered in this study, a systematic examination of the effect of flow parameters and fluid properties on the heat transfer coefficient was performed, and this has provided a unique insight into the heat transfer mechanisms.     

CFD modeling for nucleate pool boiling of nanofluids

Boiling is one of the most important processes in almost every industrial heat exchanger arrangement. The present study examines the role played by nanofluids in increasing the heat transfer rate which could improve process efficiency as well as operational cost. The setup consists of a stainless steel vertical cylinder pressure vessel having a horizontal heating rod made of stainless steel submerged in a pool of working fluid (distilled water, alumina/water nanofluid of variable concentration). Simulations were carried out using a 2D geometrical domain in order to calculate values of heat transfer coefficient for different constant heat flux applied on the heater at atmospheric as well as sub atmospheric pressures. For the simulations, a transient Eulerian multiphase involving boiling model was used along with various sub-models involving drag, lift, heat and mass transfer models. The simulated results for the value of heat transfer coefficient were compared and validated from the experimental results.     

Interfacial heat transfer during subcooled flow boiling

In order to develop a mechanistic model for the subcooled flow boiling process, the key issues which must be addressed are wall heat flux partitioning and interfacial (condensation) heat transfer. The sink term in the two-fluid models for void fraction prediction is provided by the condensation rate at the vapor-liquid interface. Low pressure subcooled flow boiling experiments, using water, were performed using a vertical flat plate heater to investigate the bubble collapse process. A high-speed CCD camera was used to record the bubble collapse in the bulk subcooled liquid. Based on the analyses of these digitized images, bubble collapse rates and the associated heat transfer rate were determined. The experimental data were in turn used to correlate the bubble collapse rate and the interfacial heat transfer rate. These correlations are functions of bubble Reynolds number, liquid Prandtl number, Jacob number, and Fourier number. The correlations account for both the effect of forced convection heat transfer and thickening of the thermal boundary layer as the vapor bubble condenses which in turn makes the condensation heat transfer time dependent. Comparison of the measured experimental data with those predicted from the correlations show that predictions are well within ±25% of the experimentally measured values. These correlations have also been compared with those available in the literature.     

Two-fluid modeling for low-pressure subcooled flow boiling

In the advanced electronic packaging, low-pressure subcooled flow boiling has been applied in design of compact heat exchangers for the effective electronic cooling. Through literature survey it is noted that little studies were carried out on the low-pressure and low-flow velocity subcooled flow boiling. In this paper a one-dimensional, non-equilibrium two-fluid model is proposed. The model has been validated with existing data in literature for both vertical up-flow and down-flow configurations. The simulated results show that under low-flow velocity the single phase heat transfer fraction is insignificant in vapor generation rate. The predicted results indicate that buoyancy force plays an important role on the void fraction evolvement, especially under low-flow velocity in vertical down-flow configuration.     

High Heat Flux Burnout in Subcooled Flow Boiling

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Photographic study of high-flux subcooled flow boiling and critical heat flux

This study examines both high-flux flow boiling and critical heat flux (CHF) under highly subcooled conditions using FC-72 as working fluid. Experiments were performed in a horizontal flow channel that was heated along its bottom wall. High-speed video imaging and photomicrographic techniques were used to capture interfacial features and reveal the sequence of events leading to CHF. At about 80% of CHF, bubbles coalesced into oblong vapor patches while sliding along the heated wall. These patches grew in size with increasing heat flux, eventually evolving into a fairly continuous vapor layer that permitted liquid contact with the wall only in the wave troughs between vapor patches. CHF was triggered when this liquid contact was finally halted. These findings prove that the CHF mechanism for subcooled flow boiling is consistent with the interfacial lift-off mechanism proposed previously for saturated flow boiling.     

Study on flow characteristics and pressure distribution along a heated channel in subcooled flow boiling

In the present work, an experimental investigation is presented which is focused on the pressure drop along the heated as well as the unheated zone of a flow channel. The experiments were performed by using a 16 mm stainless steel ID tube with a 350 mm heated length followed by a 500 mm unheated extension downstream. Pressure drops and pressure distribution were measured along the flow channel, with water, at mass flow velocities of 6342–9513 kg/m² s and at heat fluxes of 3.37–4.1 MW/m². The whole experiments were conducted near atmospheric pressure by keeping the outlet of the flow channel open to the atmospheric pressure. The experimental results show a reduction of the pressure at the outlet of the heated zone at the onset of significant void (OSV) conditions, caused by the increasing of the average velocity at that zone. That reduction of the pressure, at the outlet of the heated zone, decreases as the inlet (and also the outlet) temperature increases, due to the increasing of the length of the two phase flow at the unheated zone. That phenomenon is backed by photographing the flow in the unheated zone.     

Bubble lift-off size in forced convective subcooled boiling flow

Forced convective subcooled boiling flow experiments were conducted in a BWR-scaled vertical upward annular channel. Water was used as the testing fluid, and the tests were performed at atmospheric pressure. A high-speed digital video camera was applied to capture the dynamics of the bubble nucleation process. Bubble lift-off diameters were obtained from the images for a total of 91 test conditions. A force balance analysis of a growing bubble was performed to predict the bubble lift-off size. The dimensionless form of the bubble lift-off diameter was formulated to be a function of Jacob number and Prandtl number. The proposed model agreed well with the experimental data within the averaged relative deviation of ±35.2%.     

Experimental investigation of bubble behaviors in subcooled flow boiling

The experimental investigation on vapor bubble growth is performed for analyzing subcooled boiling in a vertical annular channel with inner heating surface and upward water flow under atmospheric pressure. Bulk liquid mass flux ranges from 79 kg/m2s to 316 kg/m2s, and subcooling is from 40 K to 60 K. The bubble behaviors from inception to collapse are captured by High-speed photography. The performance of bubble growth recorded by the high-speed photography is given in this paper. The bubble behaviors, effect of the bubble slippage on the heat transfer, and various forces acting on the bubble are discussed.     

狭缝中流动沸腾传热过冷沸腾起始点的实验研究

以间隙为1．0mm和1．5mm的环形狭缝通道中流动沸腾传热的实验数据为基础，分析了影响过冷沸腾起始点热负荷的主要因素，给出了计算环形狭缝通道中流动沸腾传热过冷沸腾起始点的经验关联式，并将计算结果与实验值进行了比较。该关联式可以用来预测实验范围内的过冷沸腾起始点的热负荷。     

Micro‐bubble emission boiling from horizontal and vertical surfaces to subcooled parallel flow water

Heat removal of more than 10 MW/m² in heat flux has been required in high‐heat‐generation equipment in nuclear fusion reactors. In some conditions of water subcooling and velocity, there appears an extraordinary high heat flux boiling in the transition boiling region. This boiling regime is called micro‐bubble emission boiling (MEB) because many micro‐bubbles are spouted from the heat transfer surface accompanying a huge sound. The study intent is to obtain heat transfer performance of MEB in horizontal and vertical heated surfaces to parallel flow of subcooled water, comparing with CHF of this system. Three types of MEB with different heat transfer performance and bubble behavior are observed according to the flow velocity and liquid subcooling. © 2003 Wiley Periodicals, Inc. Heat Trans Asian Res, 32(2): 130–140, 2003; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.10077     

Numerical investigation of subcooled flow boiling in segmented finned microchannels

Bubble growth behavior and heat transfer characteristics during subcooled flow boiling in segmented finned microchannels have been numerically investigated. Simulations have been performed for a single row of segmented finned microchannel and predicted results are compared with experimental investigations. Onset of nucleation, formation of bubbles, their growth and movements have been investigated for different values of applied heat flux. Mechanism of bubble expansion without clogging resulting in enhanced heat transfer in segmented finned microchannels has been explained. Temperature and pressure fluctuations during subcooled flow boiling condition have been investigated. It is observed that at high heat flux, thin liquid film trapped between the bubble and channel wall is evaporated leading to localized heating effect. Predicted flow patterns are similar to experimental results. However, simulations over predict the bubble growth rate and heat transfer coefficient.     

A wall heat transfer model for subcooled boiling flow

High compactness, low weight and little space requirement are gaining attention as prominent design criteria in the development of modern cooling systems in many applications. The resulting demand for highest possible heat transfer rates has lead to the very promising concept of providing for a controlled transition from pure single-phase convection to subcooled boiling flow in thermally highly loaded regions. For its application in modern engineering design this approach requires a realistic modeling of the complex phenomena associated with the two-phase flow heat transfer. The present work proposes for the computation of the specific wall heat transfer rate a modified superposition model, where the total heat flux is assumed to be additively composed of a forced convective and a nucleate boiling component. Since the present model requires only local input quantities, it is well suited to CFD of geometrically very complex coolant flows, where the definition of global length or velocity scales would be impractical. The wall heat fluxes predicted by the present model were compared against experimental data which were obtained by in-house measurements with water being the working fluid. The overall agreement is very good, particularly, in the partially nucleate boiling regime, where the effect of the bulk flow rate on the heat transfer is significant. Deviations are primarily observed at higher wall superheats, where a strong two-way coupling between the motion of the liquid and the motion of the bubbles as well as considerable bubble–bubble interactions typically occur.     

Fundamental consideration of wall heat partition of vertical subcooled boiling flows

Improved wall heat flux partitioning accounting sliding bubbles and a mechanistic model that incorporates the fundamental consideration of bubble frequency during low-pressure subcooled flow boiling is presented. A model considering the forces acting on departing bubbles at the heated surface is employed. Coupled with a three-dimensional two-fluid and population balance equations based on the modified MUSIG (MUltiple-SIze-Group) model, the behavior of an upward forced convective subcooled boiling flows in a vertical annular channel is simulated. Comparison of model predictions against local and axial measurements (heat fluxes ranged from 152.9 to 705.0 kW/m²) is made for the void fraction, Sauter mean bubble diameter and interfacial area concentration covering a range of different mass and heat fluxes and inlet subcoolings. Good agreement is achieved between the predicted and measured profiles. Reasonable agreement with recent experimental measurements is also attained for the predicted growth and waiting times of bubble frequency at particular local wall superheat and subcooling temperatures.     

An investigation of dryout/rewetting in subcooled two-phase flow boiling

An investigation of a flow reversal that was observed to occur during narrow channel flow boiling is reported in this paper. The investigation was undertaken to determine the conditions under which the flow reversal occurred and to determine its cause. High-speed photography was used to study the sequence of events that occurred during each cycle of the flow reversal.Two-phase flow instability models involving boiling crisis were examined and correlations for the occurrence of boiling crisis based on the flow conditions and the test section geometry were tested with the experimental data. Strong agreement was found between the onset of the flow reversal and the prediction of CHF under subcooled flow boiling conditions as well as between the predicted rewetting times and the period of the flow reversal. As a result, the instability was deemed to be caused by the onset of CHF and to be the result of dryout and rewetting of the heated surface.