Nucleate boiling of FC-87/FC-72 zeotropic mixtures on a horizontal copper disc

This paper presents nucleate boiling experimental results, at atmospheric pressure, for heat fluxes q ≤ 40 kW/m², for FC-87/FC-72 binary mixtures in molar fractions of 0/100, 25/75, 50/50, 75/25, 85/15 and 100/0, at saturation temperatures for pure fluids and bubble points for mixtures. The test section was an upward facing copper disc of 12 mm diameter and 1 mm thickness. The experimental heat transfer coefficient was compared with the correlations of Rohsenow (1952), as reported by Rohsenow et al. (Handbook of heat transfer, McGraw-Hill, New York, 1998), Stephan and Abdelsalam (Int J Heat Mass Transfer 23;73–78, 1978) and Cooper (Int Chem Eng Symp Ser 86:785–792, 1984) for pure fluids and the semi-empirical models of Stephan and Körner (Chem Ing Tech Jahrg 7:409–484, 1969), Thome (J Heat Transfer 104:474–478, 1982), Fujita et al. (1996), as reported by Rohsenow et al. (Handbook of heat transfer, McGraw-Hill, New York, 1998), Fujita and Tsutsui (Int J Heat Mass Transfer 37(1):291–302, 1994) and Calus and Leonidopoulos (Int J Heat Mass Transfer 17:249–256, 1973) for mixtures.     

Oscillating heat pipe simulation considering dryout phenomena

In heat transport devices such as oscillating heat pipe (OHP), dryout phenomena is very important and avoided in order to give the optimum performance. However, from the previous studies (including our studies), the dryout phenomena in OHP and its mechanism are still unclear. In our studies of OHP (Senjaya and Inoue in Appl Thermal Eng 60:251–255, 2013; Int J Heat Mass Transfer 60:816–824, 2013; Int J Heat Mass Transfer 60:825–835, 2013), we introduced the importance and roles of liquid film in the operating principle of OHP. In our previous simulation, the thickness of liquid film was assumed to be uniform along a vapor plug. Then, dryout never occurred because there was the liquid transfer from the liquid film in the cooling section to that in the heating section. In this research, the liquid film is not treated uniformly but it is meshed similarly with the vapor plugs and liquid slugs. All governing equations are also solved in each control volume of liquid film. The simulation results show that dryout occurs in the simulation without bubble generation and growth. Dryout is started in the middle of vapor plug, because the liquid supply from the left and right liquid slugs cannot reach until the liquid film in the middle of vapor plug, and propagates to the left and right sides of a vapor plug. By inserting the bubble generation and growth phenomena, dryout does not occur because the wall of heating section is always wetted during the bubble growth and the thickness of liquid film is almost constant. The effects of meshing size of liquid film and wall temperature of heating section are also investigated. The results show that the smaller meshing size, the smaller liquid transfer rate and the faster of dryout propagation. In the OHP with higher wall temperature of heating section, dryout and its propagation also occur faster.     

Two-phase flow of refrigerants in 85 μm-wide multi-microchannels: Part II – Heat transfer with 35 local heaters

This article is the second part of a study on flow boiling of R236fa and R245fa. This part presents the heat transfer coefficients obtained in a 12.7 mm silicon evaporator composed of 135 microchannels with 85 μm wide and 560 μm high channels separated by 46 μm wide fins. There were 35 local heaters and temperature measurements arranged in a 5 × 7 array. The heat transfer results were uniform in the lateral direction to the flow (attributable to the inlet restriction) and a function of the heat flux, vapor quality and mass flux. The steady-state standard deviation of the local base temperature was less than 0.2 °C, inferring that the boiling process was very stable. For wall heat fluxes over 45 kW/m², the heat transfer coefficient curves were V-shaped, decreasing for intermittent flow regimes and increasing for annular flow. The three-zone model of Thome et al. (2004) was the best heat transfer prediction method when setting the dryout thickness equal to the channel roughness.     

The Effect of Droplets Evaporation on Turbulence Modification and Heat Transfer Enhancement in a Two-Phase Mist Flow Downstream of a Pipe Sudden Expansion

Turbulent droplet-laden flow downstream of a sudden pipe expansion is numerically studied using an Eulerian two-fluid model. The model is used to investigate the effect of droplet evaporation on the particle dispersion and on the gas phase turbulence modification. Turbulence suppression in the case of evaporating droplets is hardly observed near the wall, and the level of turbulence tends to the corresponding value for the single-phase flow regime. In the flow core, where evaporation is insignificant, a decrease in the level of gas turbulence (to 20 % as compared to a single-phase flow) can be observed. The maximal effect of droplet evaporation is obtained in the wall region of the tube. A considerable increase in the maximal value of heat exchange on adding the evaporating droplets to the separated flow is shown (more than 1.5-fold as compared to the single-phase flow at a small value of droplet mass concentration of M _L1≤ 0.05). The addition of the solid non-evaporating particles causes a slight increase in the maximum value of heat transfer in the case of small particles and a decrease in heat transfer in the case of large particles.     

Experimental research on transient critical heat flux in vertical tube under oscillatory flow condition

The transient critical heat flux (CHF) experiments with forced sinusoidal inlet flow oscillation (oscillation period in 1–11 s, normalized amplitude of inlet flow oscillation in 0–3.0) were conducted in a vertical tube under low pressure condition. To analyze the triggering mechanism and aftermath of periodic dryout, the wall temperature fluctuation characteristics at the onset of periodic dryout and during post-periodic dryout were investigated. Under inlet flow oscillation condition, periodic dryout would be triggered at the wave trough of liquid film oscillation as wall heat flux far below the stable-flow CHF. The transient periodic dryout would give rise to temperature fluctuations on the tube wall, the amplitude of which increased with oscillation period and heat flux. The large wall temperature fluctuation during long-playing periodic dryout could significantly pre-trigger continuous dryout. The changing trends of the periodic dryout heat flux show a reasonable agreement with Okawa’s theoretical model, in which the liquid film oscillation was supposed be weakened by the axial mixing of liquid film. Moreover, the droplet entrainment at the oscillatory interface also has noticeable influence on the oscillation characteristics of liquid film. Based on the analysis of parameter effects on periodic dryout, a semi-empirical correlation was proposed to predict the periodic dryout heat flux under inlet flow oscillation condition.     

An experimental study of the evaporation characteristics of emulsified liquid droplets

This paper presents the results of an experimental investigation, into the effect of water in diesel and kerosene emulsions, on the evaporation time of a single droplet, on hot surfaces (stainless-steel and aluminum). Experiments are performed at atmospheric pressure, and initial water volume concentrations of 10, 20, 30, and 40%. The wall temperatures ranging from 100–460 °C, to cover the entire spectrum of heat transfer characteristics from evaporation to film boiling. Results show that, qualitatively, the shapes of emulsion evaporation curves are very similar to that of pure liquids. Quantitavely, there are significant differences. The total evaporation time, for the emulsion droplets is lower than that for diesel and kerosene fuels, and decreased as water initial concentration increases, up to surface temperatures less than the critical temperature. The value of the critical surface temperature (maximum heat transfer rate), decreases as initial concentration of water increases. In the film-boiling region, the evaporation time for the emulsion droplets is higher than for diesel and kerosene droplets, at identical conditions.List of Symbols h_fg latent heat of vaporization, KJ/kg - m mass of the droplet, gm - T_b boiling temperature, °C - T_c critical temperature, °C - T_L Leidenfrost temperature, °C - T_s initial surface temperature of the hot surface, °C     

Numerical simulation of the deformation and breakup of a two-core compound droplet in an axisymmetric T-junction channel

The rheological behaviors of a compound droplet in a confined geometry are of importance in many industrial and natural processes. However, a detailed numerical simulation of the finite deformation and its transition to the breakup of the multi-core compound droplet in an axisymmetric T-junction channel is still lacking. The present study is to fill this gap through the numerical simulations of a two-core compound droplet that deforms and breaks up in this channel configuration. The numerical results are obtained by the axisymmetric front-tracking method. Our new finding is that the compound droplet in the channel can experience the finite deformation or the breakup depending on the flow condition or the configuration of the channel. In the finite deformation mode (i.e. non-breakup mode), the droplet rapidly reaches the maximum deformation before approaching the perpendicular rigid wall. The most deformation occurs with the outer droplet, and the inner droplet closer to the wall is less deformed than the other inner core droplet. In the breakup mode, three breakup patterns are recognized: (i) breakup type I occurring when breaking up only the outer droplet; (ii) breakup type 2 occurring when breaking up only the inner droplets; (iii) breakup type 3 occurring when breaking up both inner and outer droplets. The transition from the non-breakup mode to the breakup mode is available when increasing the Reynolds number Re (from 0.16 to 40.0), the capillary number Ca (from 0.04 to 4.0), the size R_o of the outer droplet and the middle-to-outer fluid viscosity ratio μ₂₁, or decreasing the size R_i of the inner droplets, the radial size C₂ of the channel (normalized by the channel axial size C₁) and the interfacial tension ratio of the inner to the outer droplets. The transition diagrams based on some of these parameters are also proposed to provide a more complete picture of the two-core compound droplet behaving in the axisymmetric T-junction channel..     

Combined convection and radiation heat transfer to thermally developing laminar droplet flow in concentric annuli

This paper presents numerical results for combined convection and radiation heat transfer to a laminar mist flow in the thermal entrance region of a concentric annulus with a heated core at constant wall temperature and an insulated outer wall. The saturated droplets in the mist flow are considered as equivalent heat sinks distributed in the superheated vapor stream. Numerical calculations are performed for the variations of droplet size, mean vapor velocity, and the local Nusselt number in the streamwise direction until the single-phase fully-developed condition is reached. The important role of the saturated droplets on combined convection and radiation heat transfer to mist flow is clearly demonstrated.

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Kombinierte Wärmeübertragung durch Konvektion und Strahlung im thermischen Einlauf einer laminaren Tröpfchenströmung in einem konzentrischen Ringspalt

Zusammenfassung Dieser Artikel stellt numerische Ergebnisse für kombinierte Wärmeübertragung durch Konvektion und Strahlung im thermischen Einlauf einer laminaren Tröpfchenströmung in einem konzentrischen Ringraum mit beheiztem Kern bei konstanter Wandtemperatur und isolierter Außenwand dar. Die gesättigten Tröpfchen wirken als verteilte Wärmesenken im überhitzten Dampfstrom. Numerische Berechnungen werden unter Variation des Tröpfchendurchmessers, der durchschnittlichen Dampfgeschwindigkeit und der Nusselt-Zahl durchgeführt, bis eine einphasige vollausgebildete Strömung erreicht ist. Der wichtige Einfluß der gesättigten Tröpfchen auf die kombinierte Wärmeübertragung durch Konvektion und Strahlung wird klar gezeigt.

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Nomenclature A liquid loading parameter, defined in Eq. (3) - A _d heat transfer area of droplets per unit volume of vapor - A _w heat transfer area of heated wall per unit volume of vapor - C wall superheat parameter, defined in Eq. (5) - C _p specific heat of vapor - D dimensionless droplet diameter,d/d ₀ - D _h hydraulic diameter, 2(r ₀–r _i) - d droplet diameter - d ₀ droplet diameter at thermal entrance (x=0) - E dimensionless parameter, defined in Eq. (6) - H dimensionless parameter, defined in Eq. (7) - F _w–d geometric view factor - h _d heat transfer coefficient for evaporating droplets - h _p0 heat transfer coefficient of non-evaporating droplet or solid sphere with diameter ofd ₀ - k thermal conductivity of vapor - n droplet number density (number of droplets per unit volume of vapor) - n ₀ droplet number density at thermal entrance (x=0) - Nu _x local Nusselt number, defined by Eq. (17) - Pr Prandtl number of vapor,C _p/k - Q _r radiative heat transfer to droplets (per unit volume of vapor) - q _w heat flux at the inner wall - R dimensionless radial position,r/r _i - Re Reynolds number of vapor, 2 _v V₀ r _i/ - r radial position - r _i radius of inner tube - r _o radius of outer tube - S heat sink parameter, defined in Eq. (4) - T temperature of vapor - T _m bulk mean temperature of vapor - T _s saturated temperature - T _w inner wall temperature - V mean vapor velocity - V fully-developed vapor velocity, given in Eq. (12) - V ₀ mean vapor velocity atx=0 - x axial position in thermal entrance region - X dimensionless axial position, (x/r _i)/(Re·Pr) - z ₀ flow quality atx=0 Greek symbols ₀ vapor void fraction atx=0 - ratio of radius,r _i/r₀ - _d emissivity of droplets - _w emissivity of inner heated wall - dimensionless vapor temperature, defined in Eq. (9) - _m dimensionless vapor mean temperature, given by Eq. (14) - _wi dimensionless inner wall temperature - _wo dimensionless outer wall temperature - dynamic viscosity of vapor - _l liquid density - _v vapor density - Stefan-Boltzmann constant     

Quadrature method of moments for modeling multi-component spray vaporization

This article puts forward the quadrature method of moments (QMoM) for modeling droplet composition during the spray vaporization process. This method is implemented for solving the Continuous Thermodynamic Model (CTM) of multi-component droplet vaporization, an advantageous alternative to the classical Discrete Component Model (DCM) when the droplet is formed of a great number of components. The CTM approach consists in modeling the droplet’s composition using a probability density function (PDF). This method was first tried out for vaporizing droplets by Hallett, who assumed a Gamma-function for the PDF. However, Harstadt et al. underlined some problems in the case of vapor condensation on the droplet surface, since the Gamma-PDF model presumes the PDF’s mathematical form. The QMoM which does not require this hypothesis is studied in this article, according to Lage’s research dealing with QMoM application to phase equilibria. The numerical features of QMoM are investigated in detail, and then the method is implemented for the difficult test case of vapor condensation. The results are analyzed to illustrate the application of QMoM to multi-component droplet vaporization modeling and to provide a better understanding of the QMoM main advantages and limitations.     

Carbon dioxide flow boiling in a single microchannel - Part II: Heat transfer

Flow boiling heat transfer coefficients of CO₂ have been measured in a single microchannel. Experiments were carried out in a horizontal stainless steel tube of 0.529 mm inner diameter, for three temperatures (−10, −5 and 0 °C), with the mass flux ranging from 200 to 1200 kg/m² s and the heat flux varying from 10 to 30 kW/m². The investigation covered qualities from zero to the dryout inception, i.e. pre-dryout conditions. Compared to larger microchannels and positive temperatures, a higher contribution of convective boiling was found, with a larger heat transfer coefficient than for pure nucleate boiling. Mainly two heat transfer regimes were found, depending on the boiling number (Bo). For Bo > 1.1 × 10⁻⁴, the heat transfer coefficient was highly dependent on the heat flux and moderately influenced by the quality and the mass flux. For Bo < 1.1 × 10⁻⁴, the heat transfer coefficient was hardly affected by the heat flux but strongly influenced by the quality and the mass flux. In addition, dryout results were reported. The effect of the mass flux on the dryout inception quality was found to be highly dependent on the heat flux and the saturation temperature.     

Hysteretic heat transfer study of liquid–liquid two-phase flow in a T-junction microchannel

Liquid–liquid two-phase flow in microchannels is capable of boosting the heat removal rate in cooling processes. Formation of different two-phase flow patterns which affect the heat transfer rate is numerically investigated here in a T-junction containing water-oil flow. For this purpose, the finite element method (FEM) is applied to solve the unsteady two-phase Navier–Stokes equations along with the level set (LS) equation in order to capture the interface between phases. It is shown that the two-phase flow pattern in microchannels depends on the flow initial condition which causes hysteresis effect in two-phase flow. In this study, the hysteresis is observed in flow pattern and consequently in the heat transfer rate. The effect of wall contact angle on the hydrodynamics and heat transfer in the microchannel is investigated to gain useful insight into the hysteresis phenomenon. It is observed that the hysteresis is significant in super-hydrophilic microchannels, while it disappears at the contact angle of 75^°. The effect of water to oil flow rate ratio (Q_wat/Q_oil) on the heat transfer is also studied. The flow rate ratio has a negligible effect on the Nusselt number (Nu) in the dripping regime, while the Nu decreases with an increase of Q_wat/Q_oil in the co-flow regime. The thickness of the oil film, velocity, and temperature distribution are studied in the co-flow regime. It is revealed that the normalized slip velocity reduces at higher values of Q_wat/Q_oil, which causes a reduction in the averaged Nu. In dripping regimes, higher flow rate ratios lead to a more frequent generation of droplet/slugs at a smaller size. The passage of the slugs or droplets increases the local Nu. Larger droplets generated at lower flow rate ratios cause a larger increase in the local Nu than smaller droplets. The temperature and velocity field around the droplets are also illustrated to investigate the heat transfer improvement. The generated vortex at the tip of the oil jet causes an increase in the velocity and Nu on the water side.     

Flow pattern and boiling heat transfer of CO2 in horizontal small-bore tubes

Increasing attention has been focused on carbon dioxide (CO₂) heat pump system where the temperature level is rather low, while the operating pressure is rather high. In this system, the density difference between vapor and liquid becomes rather small, which significantly affects flow patterns. Low surface tension and latent heat also have significant influence on two-phase flow patterns and heat transfer. This paper describes experimental and numerical investigation on flow patterns and heat transfer characteristics of boiling flow CO₂ at high pressure in horizontal small-bore tubes ranging from 1.0 mm to 3.0 mm I.D. Even though the density difference is rather small at high pressure, phase stratification takes place, which leads to the intermittent dryout at the upper wall. So far developed discrete bubble model by the authors for vertical flows is modified so as to include horizontal flow mechanisms. The predicted flow patterns with this new model agree on the whole with the experimental observation.     

Numerical simulation of annular two-phase flow considering the four involved mass transfers

In the present study, a numerical model is developed for simulation of annular two-phase flow considering bubbly flow regime in the liquid film along with the four involved mechanisms of mass transfer those are evaporation, entrainment, deposition and condensation. In the numerical approach, liquid film accompanied by fine nucleated bubbles are simulated with innovative model named suction model, the whole domain containing liquid film and the vapor core is simulated by volume of fluid model. While the vapor and the entrained droplets are treated as homogeneous flow. The interface between the liquid and the vapor is traced by level set formulation. The model is then validated by experimental models of Lee & Lee and Stevanovic et al. and shows a good precision such that it predicts the experimental results of Stevanivic et al. Better than their own numerical model. This issue is due to the least possible simplifying assumptions along with considering the effect of boiling in liquid film and all mechanisms of mass transfer in the fluid flow.     

Comparison of the Eulerian and Lagrangian approaches in studying the flow pattern and heat transfer in a separated axisymmetric turbulent gas-droplet flow

The Eulerian and Lagrangian approaches are used to perform a numerical study of the disperse phase dynamics, turbulence, and heat transfer in a turbulent gas-droplet flow in a tube with sudden expansion with the following ranges of two-phase flow parameters: initial droplet size d ₁ = 0–200 µm and mass fraction of droplets M _L1 = 0–0.1. The main difference between the Eulerian and Lagrangian approaches is the difference in the predictions of the droplet mass fraction: the Eulerian approach predicts a smaller value of M _L both in the recirculation region and in the flow core (the difference reaches 15–20%). It is demonstrated that the disperse phase mass fraction calculated by the Lagrangian approach agrees better with measured data than the corresponding value predicted by the Eulerian approach.     

Turbulent forced convective heat transfer in two-phase evaporating droplet flow through a vertical pipe

A physical model was developed to study heat transfer in turbulent dispersed flow at very high vapor quality in a vertical pipe by numerically solving the coupling governing differential equations for both phases. Major heat transfer mechanisms included in the model were the thermal nonequilibrium effects, droplet vaporization, droplet deposition on the duct wall and thermal radiative transfer. The predicted results indicated that vapor superheating is dominant for the cases with high wall superheat, otherwise droplet vaporization dominates the energy transport processes. Heat transfer during the droplet-wall interaction only exists at low wall superheat but in small amounts.     

Investigation of droplet collisions for solutions with different solids content

The collision behaviour of droplets and the collision outcome are investigated for high viscous polymer solutions. For that purpose, two droplet chains produced by piezoelectric droplet generators are directed towards each other at a certain angle so that individual droplet pairs collide. For recording the collision event, one double-image and one high-speed CCD camera were used. One camera is positioned perpendicular to the collision plane recording the outcome of the collision, and the second camera is aligned parallel to the collision plane to assure that the droplet chains are exactly in one plane. A new approach for tracking droplets in combination with an extended particle tracking velocimetry algorithm has been developed. Time-resolved series of pictures were used to analyse the dynamics of droplet collisions. The three different water soluble substances were saccharose and 1-Ethenyl-2-pyrrolidone (PVP) with different molecular weights (K17, K30). The solvent was demineralised water. The solids contents ranged from 20 to 60 %, 5 to 25 % and 5 to 35 %, yielding dynamic viscosities in the range of 2–60 mPa s. Results were collected for different pairs of impact angles and Weber numbers in order to establish common collision maps for characterising the outcomes. Here, relative velocities between 0.5 and 4 m/s and impact parameters in the interval from 0 to 1 for equal-sized droplets (Δ = 1) have been investigated. Additionally, satellite formation will be discussed exemplarily for K30. A comparison with common models of different authors (Ashgriz and Poo in J Fluid Mech 221:183–204, 1990; Estrade et al. in Int J Heat Fluid Flow 20:486–491, 1999) mainly derived for low viscous droplets revealed that the upper limit of their validity is given by an Ohnesorge number of Oh = 0.115 and a capillary number of Ca = 0.577. For higher values of these non-dimensional parameters and hence higher dynamic viscosities, these models are unable to predict correctly the boundaries between collision scenarios. The model proposed by Jiang et al. (J Fluid Mech 234:171–190, 1992), which includes viscous dissipation, is able to predict the boundary between coalescence and stretching separation for higher viscosities (i.e. Oh > 0.115 and Ca > 0.577). However, the model constants are not identical for different solution properties. As a conclusion, an alteration of the collision appearance takes place because of the relative importance between surface tension and viscosity.     

Boundary layer and heat transfer in a two-phase gas-evaporating droplet medium

An asymptotic model of the flow in the laminar boundary layer of a gas-evaporating droplet mixture is constructed within the framework of the two-continuum approximation. The case of evaporation of the droplets into an atmosphere of their own vapor is examined in detail with reference to the example of longitudinal flow over a hot flat plate. Numerical and asymptotic solutions of the boundary layer equations constructed are found for a number of limiting situations (low droplet concentration, no droplet deposition, significant droplet deposition). The development of the flow with respect to the longitudinal coordinate is studied and it is shown that in the absence of droplet deposition a region of pure vapor may be formed near the surface. Similarity criteria are established and the mechanism of surface heat transfer enhancement is studied for a low evaporating droplet concentration in the boundary layer. In the inertial deposition regime the results of calculating the integral heat transfer coefficient are found to correspond with the experimental data [1].Translated from Izvestiya Rossiiskoi Akademii Nauk, Mekhanika Zhidkosti i Gaza, No.3, pp. 42–50, May–June, 1992.     

Thermocapillary migration of liquid droplets in a temperature gradient in a density matched system

An experimental investigation of thermocapillary flow in droplets of a vegetable oil (partially hydrogenated soybean oil) immersed in silicone oil was conducted in a test cell with a heated top wall and a cooled bottom wall. The liquids are nearly immiscible and have equal densities at a temperature below the room temperature, thus providing a simulation of low-gravity conditions by reducing the buoyancy forces. The interfacial tension between the two oils was measured in the temperature range 20–50°C using a capillary tube and d/dT was determined to be negative. Droplets ranging in sizes from 3 mm to 1 cm diameter were injected into the silicone oil. The vertical temperature profile in the bulk liquid (silicone oil) produces temperature variations along the interface which induce variations in the interfacial tension. The flow inside the droplet driven by the resulting interfacial shear stresses was observed using a laser light-sheet flow visualization technique. The flow direction is consistent with the sign of d/dT. The observed maximum surface velocities are compared to the theoretical predictions of Young et al. (1959).For short times after injection, the droplets were driven by this flow towards the hot wall above the matched-density temperature until the droplets reached a point where the forces due to the flow and buoyancy were equal. After longer times, the droplets moved to the cold side due to suspected density changes caused by mass transfer from the droplets to the silicone oil. This was confirmed by tests under isothermal conditions, where it was observed that droplets of all sizes fell to the cold bottom eventually.Thus, even though the thermocapillary flow inside the droplets persisted for long times in spite of the mass transfer, the migration of droplets towards the hot side was eventually affected by uncontrolled buoyancy forces resulting from density changes due to mass transfer. While additional liquids are being tried, it is suggested from the present experience that reduced gravity experiments will probably be necessary to provide unambiguous data for the migration of droplets.     

Macro-to-microchannel transition in two-phase flow: Part 2 - Flow boiling heat transfer and critical heat flux

This part of the paper presents the current experimental flow boiling heat transfer and CHF data acquired for R134a, R236fa and R245fa in single, horizontal channels of 1.03, 2.20 and 3.04 mm diameters over a range of experimental conditions. The aim of this study is to investigate the effects of channel confinement, heat flux, flow pattern, saturation temperature, subcooling and working fluid properties on the two-phase heat transfer and CHF. Experimentally, it was observed that the flow boiling heat transfer coefficients are a significant function of the type of two-phase flow pattern. Furthermore, the monotonically increasing heat transfer coefficients at higher vapor qualities, corresponding to annular flow, signifies convective boiling as the dominant heat transfer mechanism in these small scale channels. The decreasing heat transfer trend at low vapor qualities in the slug flow (coalescing bubble dominated regime) was indicative of thin film evaporation with intermittent dry patch formation and rewetting at these conditions. The coalescing bubble flow heat transfer data were well predicted by the three-zone model when setting the dryout thickness to the measured surface roughness, indicating for the first time a roughness effect on the flow boiling heat transfer coefficient in this regime. The CHF data acquired during the experimental campaign indicated the influence of saturation temperature, mass velocity, channel confinement and fluid properties on CHF but no influence of inlet subcooling for the conditions tested. When globally comparing the CHF values for R134a in the 0.51-3.04 mm diameter channels, a peak in CHF peak was observed lying in between the 0.79 (Co ≈ 0.99) and 1.03 (Co ≈ 0.78) mm channels. A new CHF correlation has been proposed involving the confinement number, Co that is able to predict CHF for R134a, R236fa and R245fa in single-circular channels, rectangular multichannels and split flow rectangular multichannels. In summary, the present flow boiling and CHF trends point to a macro-to-microscale transition as indicated by the results presented in Ong and Thome (2011) [1].     

Towards the problem of droplet injection in a vapor with the aim of reducing the pressure

As applied to the analysis of sprinkler systems which inject droplets into a vapor in the case of emergency pressure increases, the process of vapor condensation on a single droplet is considered. For the specification of the intensity of interphase heat and mass transfer, the solution of an unsteady heat conduction problem is used. Approximate formulas describing the laws of the pressure drop in a vapor-droplet system due to the condensation of the vapor phase are obtained.