Two-phase Flow Heat Transfer Research Articles

Analysis of flow boiling incipience models in computational fluid dynamics

Recent advances in the computational fluid dynamics and heat transfer (CFD/HT) modeling of flow boiling have enabled a more detailed analysis and understanding of thermal management in relatively complex microsystems. Most advanced modeling approaches allow the transient analysis of two-phase flow and phase-change processes, providing the capability to post-process in detail the temperature, phase and pressure distributions in microscale applications with the use of moderate computational resources. Although these models have significantly evolved in recent years, there are still several idealizations and assumptions that have yet to be improved or better justified, the boiling incipience treatment being a commonly questioned topic. In the present investigation, a boiling incipience model is incorporated into an existing CFD/HT model for the analysis of flow boiling mechanisms, where the superheat temperature is derived from fundamental thermodynamic relations and compared to the original CFD/HT model that does not account for such effects. Results are compared using a non-trivial case of a silicon micro-cooling layer with variable density of pin fins that has been demonstrated to be an efficient thermal control device in high-power applications. The dielectric fluid HFE-7200 is used as the coolant in flow boiling conditions. The two-phase flow regimes and heat transfer results for both models are compared.

A Novel Geothermal Wellbore Model Based on the Drift-Flux Approach

Geothermal energy, being a clean energy source, has immense potential, and accurate wellbore modeling is crucial for optimizing the drilling process and ensuring safety. This paper presents a novel geothermal wellbore model based on the drift-flux approach, tested under three different temperature and pressure well conditions. The proposed model integrates the conservation equations of mass, momentum, and energy, incorporating the gas–liquid two-phase flow drift-flux model and heat transfer model. The key features include handling the heat transfer between the formation and the wellbore, addressing the slip relationship between the gas and liquid phases, and accounting for wellbore friction. The nonlinear equations are discretized using the finite difference method, and the highly nonlinear system is solved using the Newton–Raphson method. The numerical simulation, validation, and comparison with existing models demonstrate the enhanced accuracy of this model. In our tests, the model achieved a high accuracy in calculating the bottom-hole pressure and temperature, with mean relative errors (MREs) significantly lower than those of other models. For example, the MREs for the bottom-hole pressure and temperature of the Rongxi area well in Xiongan, calculated by this model, are 1.491% and 1.323%, respectively. These results offer valuable insights for optimizing drilling parameters and ensuring drilling safety. Comparisons indicate that this approach significantly outperforms others in capturing the complex dynamics of geothermal wellbores, making it a superior tool for geothermal energy development.

Research on heat transfer property of air-air micro channel evaporator

Heat exchanger with air-air micro channel is widely used in the refrigeration and air-conditioning industries due to its excellent energy saving and low cost. Recent researches on the air-air micro channel has achieved some available correlations to solve heat transfer coefficient in the channel. However, the available correlations cannot correspond our heat exchanger because the heat transfer for two-phase flow in the micro channel is complex. In this paper, we manufactured an evaporator with the aluminum micro channel heat exchanger and tested its heat transfer efficiency. The heat exchanger, which was filled with R32 as the working fluid, was characterized with the liquid filling ratio [Formula: see text] ranging from 0.21 to 0.61. We also investigated the thermal performance of the heat exchanger under various temperature differences of the cold and hot air inlets, which ranges from 3°C to 21°C. The experimental results show that the internal boiling heat transfer coefficient increases first and then decreases, with the increasing of liquid filling ratio and the increasing of temperature. On basis of the experimental data, we obtained a new correlation associating with the liquid filling ratio and the temperature difference, which can be used to predict the heat transfer coefficient in the channel. Comparing the predicted value from our correlation with the experimental data, around 88% experimental data falls in the ±15% error bands and the mean absolute error is 7.91%.

TRANSIENT ANALYSIS OF LIQUID-HYDROGEN TRANSFER IN CRYOGENIC STORAGE TANKS

Efficient transfer of liquid hydrogen is critical for a myriad of processes in terrestrial and extraterrestrial applications. The objective of this study is to model and quantify the amount of liquid parahydrogen vaporized during a discharging/charging process in a cryogenic storage system. The effects that are included include geometrical effects (storage tank volume), two-phase compressible flow, choking/unchoking, and wall-fluid heat transfer. A brief description of other physical boil-off mechanisms is also presented. This study provides a foundation for applicationspecific optimization. The storage system studied comprises two interconnected tanks whose transfer line has a variation in the flow area. This simulates current liquid-hydrogen storage and transportation systems. The model tracks the temperature- and pressure-time histories of liquid hydrogen as it flows between storage tanks, in both the choked and unchoked flow regimes. The transfer of liquid is induced solely by the pressure differential that exists between the two storage tanks. An iterative technique helps account for choking, which is likely to exist at the throat in two-phase flow. Cases of zero and infinite tank-wall thermal mass are also discussed. By analyzing the behavior of fundamental variables during the transient transfer of liquid hydrogen, boil-off losses may be minimized if the variables with the greatest effect on boil-off losses are controlled.

Optimizing Two-Phase Flow Heat Transfer: DCS Hybrid Modeling and Automation in Coal-Fired Power Plant Boilers

Optimizing Two-Phase Flow Heat Transfer: DCS Hybrid Modeling and Automation in Coal-Fired Power Plant Boilers

Numerical prediction of the two-phase flow and radiation effects on the thermal environment and ablation of solid rocket nozzle

The effects of two-phase flow and radiative heat transfer have a significant impact on the internal thermal environment and thermochemical ablation process in aluminized solid rocket motor (SRM). However, these two effects were not revealed systematically in previous studies. In this paper, a multiscale methodology combined radiative properties of molten alumina particles at microscale calculated by Mie theory and the two-phase flow and radiative heat transfer at macroscale calculated by Eulerian-Lagrangian method coupling with discrete ordinate method is constructed within aluminized SRM. The convection and radiation are coupled with the chemical kinetics to predict the thermochemical ablation and thermal environment. The influence of two-phase fluid flow and thermal radiation on the internal thermal environment and thermochemical ablation of a SRM nozzle is analyzed quantitatively. Comparisons are made among the methodologies that models the alumina particles as gas species, considers the effect of two-phase flow and considers the effects of two-phase flow and radiative heat transfer. The results show that the erosion rate and total heat flux at the inner wall in the convergent section increase noticeably when considering the effect of two-phase flow. Meanwhile, the erosion rate in the divergent section also increases and the thermal environment is dependent on the particle size and aluminum content. Radiative heat transfer considerably enhances the total heat flux, especially in the convergent section. On the contrary, its impact on the erosion rate is very small except for the inner wall with very low temperature.

Natural circulation oscillation in a seawater loop during startup

Aiming to improve nuclear safety by using seawater as an alternative emergency coolant during an accident, the knowledge of two-phase flow instabilities and heat transfer performances of seawater is needed. Startup experiments from the quiescent state at room temperature of 20–23 °C are carried out by raising heating power with 3.5 wt% artificial seawater in a natural circulation loop. De-ionized water is adopted as well for the comparative study. In this paper, a type of natural circulation oscillation at the modified heat fluxes of 383.70–530.62 kW/m2 and 404.05–594.97 kW/m2, respectively, for seawater and de-ionized water, with intermittent inlet mass flow rates is exclusively investigated, as a part of the experimental studies of two-phase flow instabilities during the startup of a natural circulation loop. Visualizations of two-phase flow behaviors at the power of 2206 W in seawater reveal the unique bubble foam regimes at the outlet of the heated section, tiny bubbles at the inlet of the riser, and the possible elongated slug bubbles at the central region of the riser.The natural circulation oscillations for two fluids are characterized by four distinct stages in sequence: calm state (stage 1), accumulations of foam/slug bubbles in seawater/de-ionized water (stage 2), propagations of foam/slug bubbles in seawater/de-ionized water (stage 3), and restorations (stage 4). The same period for stages 1, 3, and 4 for both fluids are 25 s, 4 s, and 5 s, respectively. For stage 2, the period measured as 12.5 s in seawater is significantly shorter than that recorded as 45 s in de-ionized water, leading to earlier systematic natural circulation rates. It is possibly caused by the much stronger accumulation effect of tiny bubbles in the riser, resulting in vast buoyant forces by draining more liquid out of the riser. The intermittent peak inlet mass flow rates of 0.017 kg/s and 0.023 kg/s are recorded, respectively, for seawater and de-ionized water. It would not induce a pressure drop increase over the riser while cause a very high pressure drop oscillation of 4.67 kPa in de-ionized water. Slower circulation rates induce smaller wall temperature drops of 3.6 °C in seawater than that of 5.2 °C in de-ionized water. Under zero flow rate conditions, the wall temperatures are mainly kept at 117–119 °C, accompanied by bubble foam without ruptures in seawater, while that are about 115 °C with slug bubbles due to bubble coalescences in de-ionized water. It demonstrates that the immobile foam may deteriorate the heat transfer under pool-like conditions.

A new paradigm for the role of disturbance waves on film dryout and wall heat transfer in annular two-phase flow

The entire liquid-film dryout process in vertical annular two-phase boiling flow is characterized experimentally, from inception to completion, in a newly defined flow regime between annular two-phase flow and mist flow, called ‘intermittent dryout’. Experiments are conducted using saturated R245fa flowing vertically through a heated transparent channel. Liquid-film thickness measurements are taken during dryout events. The state (wet or dry) of the heated surface is determined using a laser reflectance measurement, and the signal is used to calculate the time-averaged dry fraction, fdry. The local heat transfer coefficient (HTC) is characterized as a function of fdry. For all the investigated mass fluxes, optimum boiling conditions (OBC), corresponding to the heat flux that results in maximum HTC, consistently occur when the time-averaged dry fraction is fdry≈0.05. Subsequently, the critical boiling transition (CBT), corresponding to the heat flux that results in a significantly lower HTC, can occur for dry fractions of fdry>0.1. Further insight into the liquid film behavior within the intermittent dryout regime is obtained by combining analyses of high-speed videos, time-resolved liquid-film thickness signals, and statistics about the duration of and time between dryout events. In general, the dryout mechanism is dominated by disturbance waves.Mechanistic model frameworks are presented for the prediction of two-phase HTC and heat flux associated with dryout. The model inputs are local flow quantities (pressure, mass flux, vapor quality, and liquid-film thickness) and output a prediction for local heat transfer coefficient. The model framework accurately predicts the local two-phase HTC for a range of mass flux and vapor quality in the annular flow regime. The model framework captures the observed initial increase in the local HTC and subsequent decrease that is associated with liquid-film dryout. The mechanistic approach provides predictions for time-averaged HTC and OBC heat flux that both agree well with experimentally measured values.

Effect of surface engineering methods on refrigerant evaporating flow characteristics in annular tubes: Experimental approach

Techniques such as mechanical, electrochemical, and chemical etching are recognized as functional strategies for modifying surface roughness, which in turn manipulates surface energy. This research aims to delve into the influence of these factors on the heat transfer and pressure drop characteristics of two-phase R134a refrigerant flow during boiling inside horizontal annular concentric tubes with internal heat flux. The study scrutinizes the heat transfer coefficient and frictional pressure drop for R134a evaporating flow in an annular tube featuring an inner diameter of 28.6 mm and an outer diameter of 42 mm. The exploration spans a broad range of parameters, which include vapor quality from 0.15 to 0.8, mass flux from 6.72 to 20.16 kg/m2s, internal heat flux from 0.608 to 2.432 kW/m2, and varying surface characteristics. The results show that escalating surface roughness via mechanical techniques introduces greater resistance to flow shear stress in comparison to other methods. Moreover, an increase in surface roughness contributes to a reduction in contact angle and an expansion of the heating area, thereby enhancing both heat transfer coefficient and pressure drop. However, the implementation of low-energy coating holds an insignificant influence on R134a heat transfer characteristics, potentially due to its diminutive surface tension. This results in minor variations in the refrigerant contact angle in relation to the coatings. Performance evaluation criteria have been employed to assess the optimal thermal performance of the surface engineering method, with peak performance achieved at a mass flux of 20.16 Kg/m2s and an internal heat flux of 2.432 kW/m2. Ultimately, the study corroborates the experimental procedure by comparing the observed outcomes with established empirical correlations for R134a flow boiling.

Numerical Investigation Into the Gas–Liquid Two-Phase Flow Regime and Heat Transfer Characteristics in a Gravity Heat Pipe

Abstract In order to reveal the gas–liquid distribution, heat and mass transfer characteristics inside a gravity heat pipe, the bubble behavior and the flow regime transition during the phase-change process were examined by employing a copper-water heat pipe, with the length of 500 mm and Φ22 × 1.5 mm. The results indicate that in the process of phase change, the typical flow regimes of bubble flow, slug flow, and churn flow can be observed in the evaporator, and the presence of bubbles has an obvious disturbance on the flow field. In addition, the discontinuous liquid film plays an important role in the heat transfer mechanism in the condenser, which allows the vapor to contact the wall directly and reduces the heat transfer resistance. The temperature difference between the evaporator and the condenser can be reduced by adjusting the saturation temperature, so as to effectively improve the heat transfer performance of the heat pipe and contribute to the practical engineering design.

Correction: Experimental and numerical study of forced convection heat transfer in a upward two-phase flow of air–water/SiO2 nanofluid with slug flow regime

Correction: Experimental and numerical study of forced convection heat transfer in a upward two-phase flow of air–water/SiO2 nanofluid with slug flow regime

Two-Phase Flow Visualization and Heat Transfer Characteristics Analysis in Ultra-Long Gravity Heat Pipe

The ultra-long gravity heat pipe has a long heat transfer distance and narrow working fluid flow channel within its tube. Due to these unique design features, the vapor–liquid counter-flow and heat transfer characteristics of these heat pipes are more complex than those found in conventional-size heat pipes. This paper innovatively proposes the design of a segmented visualization window structure of an ultra-long gravity heat pipe, which successfully overcomes the challenge of visualizing the internal flow during operations. A visualization experimental platform, measuring 40 m in height with an inner diameter of 7 mm and the aspect ratio up to 5714, was built to investigate the evolving characteristics of two-phase flows with an increasing heat input and the impact of the phase change flow characteristics on the thermal performance of ultra-long gravity heat pipes. The results obtained can provide guidance for the development of the internal structure of ultra-long gravity heat pipes that are being applied in exploiting geothermal energy. The results show that, at low heat input (200 W, 250 W), there are separate flow paths between the condensate return and the steam, but the high hydrostatic pressure due to the height of the liquid injection results in the presence of an unsaturated working fluid with a higher temperature in the liquid pool area, which has a lower evaporation rate, limiting the heat transfer through the heat pipe. It is found that if increasing the heat input up to 300 W, the evaporative phase change in the heating section becomes intense and stable. At the same time, despite the intermittent formation of liquid columns in the adiabatic section due to the vapor–liquid rolls, which increases the resistance to the vapor–liquid counter-flow, the liquid columns are blocked for a short period of time, and the path of steam rises and the condensate return is smooth, which does not seriously affect the steam condensation and liquid return evaporation. At this point, the overall temperature of the heat pipe is evenly distributed along the tube and the heat transfer performance is optimal. When the heat input further increases (350 W, 400 W), a large amount of condensate is trapped in the upper part of the adiabatic section and the condensing section by long liquid plugs for a long time. At this point, the condensate flow back to the heating section is significantly reduced, and the steam is seriously prevented from entering the condensation section, resulting in a significant increase in the temperature gradient between the lower part of the evaporating section and the upper part of the adiabatic section and deterioration of the heat transfer performance.

Development of an Approach to Measure the Inner Wall Temperature of a Tubular Window for Experiments of Two-Phase Flow Heat Transfer on the China Space Station

Development of an Approach to Measure the Inner Wall Temperature of a Tubular Window for Experiments of Two-Phase Flow Heat Transfer on the China Space Station

Experimental and numerical study of forced convection heat transfer in a upward two-phase flow of air–water/SiO2 nanofluid with slug flow regime

Experimental and numerical study of forced convection heat transfer in a upward two-phase flow of air–water/SiO2 nanofluid with slug flow regime

Numerical simulation of heat transfer of two-phase flow in mini-channel heat sink and investigation the effect of pin-fin shape on flow maldistribution

Nowadays, given the rising power of processors and the increase in operating temperature of these objects, the cooling and heat loss process is necessary to prevent the breakdown of these devices. Considering the ability to dissipate high heat flux in mini-channels and low pressure drop, in this study, the performance of mini-channels with different inlet headers was studied. Accordingly, 3 pin fins with elliptical, circular and V-shaped cross-sections in the inlet header were examined in terms of velocity distribution uniformity and heat sink surface temperature reduction. The lowest maldistribution and surface temperature were obtained for the header with elliptical pin fins having a diameter ratio of a/b = 2. Also, the effect of employing the nanofluid in mini-channel on pressure drop and heat transfer was investigated by utilizing the two-phase mixture model. The results showed that with the rise of volume fraction (∅) and Reynolds number (Re), thermal resistance was reduced significantly. Also, the highest Nusselt number (Nu) with the Re of 3000 and volume fraction of 0.5% was obtained to be equal to 77.3.

A point particle source model for conjugate heat and mass transfer in dispersed two-phase flows by BEM based methods

The paper reports on development of a Boundary Element Method (BEM) based two phase dispersed flow model, that allows an accurate and efficient heat and mass transfer coupling by using a moving point source model to account for particle–fluid interaction. The two-way coupling model highlights the efficient use of the elliptic fundamental solution and the Dirac delta distribution properties to accurately evaluate the heat and mass point particle source impact on the continuous phase, solved by the Boundary Domain Integral Method (BDIM). In addition to the BDIM model of the particle–fluid heat and mass transfer interaction, the two-phase flow case under consideration is extended to the case of porous spherical particle drying with internal moving drying front, which is solved by the Boundary Element Method. As the two-phase flow is considered to be dilute, the particle–fluid momentum exchange is covered by a one-way coupling algorithm, with Lagrangian particle tracking used for determination of particle positions and velocities in the flow. Two computational cases are presented, where 1000 and 10000 particles are dried in a stream of hot air. Comparison between the obtained drying times for the cases of the one-way and the two-way heat and mass transfer coupling results shows, that the developed two-way coupling model accurately captures the effect of moisture accumulation and temperature decrease in the fluid phase, leading to realistic computations of drying rates of particles in the flow.

Numerical method for gas-liquid two-phase flow with phase change heat transfer considering compressibility using OpenFOAM

The numerical simulation of phase change heat transfer is important for predicting the gas-liquid two-phase flow and heat transfer characteristics and optimizing the phase change heat dissipation system. For the isovolumetric phase change heat transfer process, the compressibility of the gas phase has a significant impact on the simulation accuracy. A numerical simulation method of flow and phase change heat transfer considering gas-liquid two-phase compressibility is established based on VOF method using OpenFOAM in this paper. This method introduces a generalized phase change source term and a compressible source term to improve the isoAdvector geometric reconstruction algorithm and establishes a method for capturing the phase interface of a compressible two-phase flow. The phase change flux based on the phase interface is converted into a phase change source term based on the unit volume by calculating the phase interface density. The compressible term of the governing equation is constructed and its implicit discrete method is proposed. The coupling between velocity and pressure is realized based on the PISO algorithm. A numerical calculation method for phase change heat transfer of compressible two-phase flow is established. The effectiveness and accuracy of the proposed numerical method for predicting the heat transfer characteristics of phase change flows are verified by the simulation of vertical flat film condensation and film boiling. The numerical simulation results are in good agreement with the experiment. Compared with the other numerical simulation methods, the proposed method considering the influence of the compressibility of the gas phase on the flow and heat transfer characteristics in the heat pipe has high accuracy in predicting the gas-liquid two-phase flow, capturing the evolution characteristics of the phase interface and predicting the phase change heat transfer. The numerical simulation method can provide algorithm support for the numerical simulation of compressible two-phase flow phase change heat transfer and the design of a phase change heat dissipation system.

Flow Boiling in Flexible Polymer Microgaps for Embedded Cooling in High-Power Applications

Abstract Structural flexibility has become a common feature in emerging microsystems with increasing heat fluxes. The thermal control of such applications is a significant challenge because of both structural and volumetric requirements, where standard cooling solutions are not applicable. Flexible polymer microlayers are a promising solution for the embedded cooling of such microsystems. In the present investigation, a flexible polydimethylsiloxane (PDMS) microgap is proposed and assessed in an effort to prove its viability for thermal management in the aforementioned applications. The analyzed polymer microgap features a dedicated vapor pathway design which is proven to assist in the efficient removal of vapor from the microsystem. The dielectric refrigerant HFE-7100 is used as the working fluid under flow boiling conditions, reporting on the two-phase flow regime, heat transfer, and pressure drop. In addition to experimental results, the numerical modeling of the relevant features of flow boiling is explored with the use of a mechanistic phase-change model that is proven to accurately predict the flow variables and constitutes a valuable tool in the analysis and design of such microsystems. The results from this study demonstrate that this approach is feasible for the removal of relatively high heat fluxes which are comparable to metallic-based or silicon microchannels, with the added advantage of structural flexibility while also providing a stable two-phase cooling mechanism.

Flow and heat transfer model for turbulent-laminar/turbulent gas-liquid annular flows

• Physical process of heat transfer for annular flow was mathematically formulated. • Effect of flow orientations and parameters on heat transfer process was analyzed. • Heat transfer was significantly enhanced by gas in flow regime of annular flow. • Dependence of heat transfer multiplier on void fraction and pressure drop multiplier was identified. • A heat transfer correlation was developed based on Chilton & Colburn analogy. Two-phase gas-liquid annular flows are characterized by the liquid flowing in the annular-shaped channels as liquid film and gas flowing in the center as gas core. Adequate understanding of the flow and heat transfer mechanism is important. This paper aims at developing a model for flow and heat transfer in two-phase annular flows. First, the physical process of flow and heat transfer for two-phase annular flow is mathematically formulated and a new heat transfer model is developed based on two-fluid concept. Then, the new model is well validated using the experimental void fraction, liquid film thickness, and heat transfer coefficient collected from various sources. Third, the effect of the orientations and flow parameters on the heat transfer of two-phase annular flows are comprehensively investigated. Finally, the dependence of the two-phase heat transfer multipliers on the void fractions and pressure multipliers is identified quantitatively and a simple heat transfer correlation is developed based on Chilton & Colburn analogy.

Analytical Solution for Heat and Mass Transfer of two-phase Nanofluid Flow with Magnetic Field in a rotating System using Adomian Decomposition Method

Analytical Solution for Heat and Mass Transfer of two-phase Nanofluid Flow with Magnetic Field in a rotating System using Adomian Decomposition Method