Subcooled flow boiling of R-134a in vertical channels of small diameter

Subcooled flow boiling heat transfer for refrigerant R-134a in vertical cylindrical tubes with 0.83, 1.22 and 1.70 mm internal diameter was experimentally investigated. The effects of the heat flux, q″ = 1–26 kW/m², mass flux, G = 300–700 kg/m² s, inlet subcooling, ΔT_sub,i = 5–15 °C, system pressure, P = 7.70–10.17 bar, and channel diameter, D, on the subcooled boiling heat transfer were explored in detail. The results are presented in the form of boiling curves and heat transfer coefficients. The boiling curves evidenced the existence of hysteresis when increasing the heat flux until the onset of nucleate boiling, ONB. The wall superheat at ONB was found to be essentially higher than that predicted with correlations for larger tubes. An increase of the mass flux leads, for early subcooled boiling, to an increase in the heat transfer coefficient. However, for fully developed subcooled boiling, increases of the mass flux only result in a slight improvement of the heat transfer. Higher inlet subcooling, higher system pressure and smaller channel diameter lead to better boiling heat transfer. Experimental heat transfer coefficients are compared to predictions from classical correlations available in the literature. None of them predicts the experimental data for all tested conditions.     

Flow boiling heat transfer of R134a,R236fa and R245fa in a horizontal 1.030 mm circular channel

This research focuses on acquiring accurate flow boiling heat transfer data and flow pattern visualization for three refrigerants, R134a, R236fa and R245fa in a 1.030 mm channel. We investigate trends in the data, and their possible mechanisms, for mass fluxes from 200 to 1600 kg/m²s, heat fluxes from 2.3 kW/m² to 250 kW/m² at T_sat = 31 °C and ΔT_sub from 2 to 9 K. The local saturated flow boiling heat transfer coefficients display a heat flux and a mass flux dependency but no residual subcooling influence. The changes in heat transfer trends correspond well with flow regime transitions. These were segregated into the isolated bubble (IB) regime, the coalescing bubble (CB) regime, and the annular (A) regime for the three fluids. The importance of nucleate boiling and forced convection in these small channels is still relatively unclear and requires further research.     

Natural convection boiling of water and surfactant solutions having negligible environmental impact in vertical confined space

Natural convection boiling of water and surfactant solutions in a confined space between two vertical plates was studied experimentally in the range of heat flux 19–170 kW/m². The surfactant used was Alkyl (8–16) Glucoside having negligible environmental impact. The gap size was changed in the range of s = 1–80 mm, the concentration of surfactant solutions was changed in the range of C = 200–600 ppm. Generally, an addition of surfactant leads to an enhancement of heat transfer compared to water boiling at the same gap size. Enhancement of the heat transfer depending on the solution concentration at fixed gap size yields maximum value at the solution concentration close to the critical micelle concentration. The effect of confined space on a bubble dynamic was studied. Temperature field on the heater was determined using the infrared thermography technique. Quasi periodic wall temperature fluctuations were observed in the regime of high heat flux. An increase in the Bond number leads to an increase in the dimensionless frequency of the heated wall temperature fluctuations. The correlation between the dimensionless parameter of heat transfer and the Bond number under condition of quasi periodic boiling was derived and discussed.     

Effect of surfactant concentration on saturated flow boiling in vertical narrow annular channels

Saturated flow boiling of environmentally acceptable nonionic surfactant solutions of Alkyl (8–16) was compared to that of pure water. The concentration of surfactant solutions was in the range of 100–1000 ppm. The liquid flowed in an annular gap of 2.5 and 4.4 mm between two vertical tubes. The heat was transferred from the inner heated tube to two-phase flow in the range of mass flux from 5 to 18 kg/m² s and heat flux from 40 to 200 kW/m². Boiling curves of water were found to be heat flux and channel gap size dependent but essentially mass flux independent. An addition of surfactant to the water produced a large number of bubbles of small diameter, which, at high heat fluxes, tend to cover the entire heater surface with a vapor blanket. It was found that the heat transfer increased at low values of relative surfactant concentration C/C₀, reaches a maximum close to the value of C/C₀ = 1 (where C₀ = 300 ppm is the critical micelle concentration) and decreased with further increase in the amount of additive. The dependence of the maximal values of the relative heat transfer enhancement, obtained at the value of relative concentration of C/C₀ = 1, on the boiling number Bo may be presented as single curve for both gap sizes and the whole range of considered concentrations.     

Natural convection boiling of water and surfactants in narrow horizontal annular channels

Natural convection boiling of water and surfactants at atmospheric pressure in narrow horizontal annular channels was studied experimentally in the range of Bond numbers Bo = 0.185–1.52. The flow pattern was visualized by high-speed video recording to identify the different regimes of boiling of water and surfactants. The channel length was 24 mm and 36 mm, the gap size was 0.45, 1.2, 2.2, and 3.7 mm. The heat flux was in the range of 20–500 kW/m², the concentration of surfactant solutions was varied from 10 to 600 ppm. For water boiling at Bond numbers Bo < 1 the CHF in restricted space is lower than that in unconfined space. This effect increases with increasing the channel length. For water at Bond number Bo = 1.52, boiling can almost be considered as unconfined. Additive of surfactant led to enhancement of heat transfer compared to water boiling in the same gap size, however, this effect decreased with decreasing gap size. For the same gap size, CHF in surfactant solutions was significantly lower than that in water. Hysteresis was observed for boiling in degraded surfactant solutions.     

Heat transfer in nucleate pool boiling of aqueous SDS and triton X-100 solutions

Variation in degree of surface wettability is presented through the application of Cooper’s correlative approach (h ∝ M ^−0.5 q _w ^″0.67) for computing enhancement (ϕ) in nucleate pool boiling of aqueous solutions of SDS and Triton X-100 and its presentation with Marangoni parameter (χ) that represents the dynamic convection effects due to surface tension gradients. Dynamic spreading coefficient defined as σ _dyn N _a , which relates spreading and wetting characteristics with the active nucleation site density on the heated surface and bubble evolution process, represents cavity filling and activation process and eliminates the concentration dependence of nucleate pool boiling heat transfer in boiling of aqueous surfactant solutions. Using the dynamic spreading coefficient (σ _dyn N _a  = 0.09q _w ^″0.71), correlation predictions within ±15% for both SDS and Triton X-100 solutions for low heat flux boiling condition (q _w^″ ≤ 100 kW/m²) characterised primarily by isolated bubble regime are presented.     

Periodic boiling in parallel micro-channels at low vapor quality

Based on experimental investigations the present study evaluates instability and heat transfer phenomenon under condition of periodic flow boiling of water and ethanol in parallel triangular micro-channels. Tests were performed in the range of hydraulic diameter 100–220 μm, mass flux 32–200 kg/m² s, heat flux 120–270 kW/m², vapor quality x = 0.01–0.08. The period between successive events depends on the boiling number and decreases with an increase in the boiling number. The initial film thickness decreases with increasing heat flux. When the liquid film reached the minimum initial film thickness CHF regime occurred. Temporal variations of pressure drop, fluid and heater temperatures were periodic. Oscillation frequency is the same for the pressure drop, for the fluid temperature at the outlet manifold, and for the mean and maximum heater temperature fluctuations. All these fluctuations are in phase. The CHF phenomenon is different from that observed in a single channel of conventional size. A key difference between micro-channel heat sink and single conventional channel is amplification of parallel-channel instability prior to CHF. The dimensionless experimental values of the heat transfer coefficient are presented as the Nusselt number dependence on the Eotvos number and the boiling number.     

Influence of the aspect ratio on boiling flows in rectangular mini-channels

This article presents experiments conducted with two single rectangular mini-channels of same hydraulic diameter (1.4 mm) and different aspect ratios for conditions of horizontal boiling flow. The Forane® 365 HX used was subcooled (ΔT_sub = 15 °C) for all the boiling curves presented in the paper. Local heat transfer coefficients were measured for heat flux ranging from 25 to 62 kW m⁻² and mass flux from 200 kg m⁻² s⁻¹ to 400 kg m⁻² s⁻¹. The boiling flows were observed with two different cameras (depending on the flow velocity) through a visualization window. The flow patterns in the two channels were compared for similar conditions. The results show that the boiling heat transfer coefficient and the pressure drop values are different for the two single mini-channels. For low heat flux condition, the channel with lowest aspect ratio (H/W = 0.143) has a higher heat transfer coefficient. On the other hand, for high heat flux condition, the opposite situation occurs, namely the heat transfer coefficient becomes higher for the channel with highest aspect ratio (H/W = 0.43). This is probably due to the earlier onset of dryout in the channel with lowest aspect ratio. For the two cases of heating, the pressure drop for the two-phase flow remains lower for the channel with lowest aspect ratio. These results show that the aspect ratio plays a substantial role for boiling flows in rectangular channels. As for single-phase flows, the heat transfer characteristics are significantly influenced (even though the hydraulic diameter remains the same) by this parameter.     

FC72 and FC87 nucleate boiling inside a narrow horizontal space

This paper presents new experimental results for saturated nucleate boiling of FC72 and FC87 on a horizontal copper disc, at atmospheric pressure, for different degrees of confinement, s, in the range of 0.1-13 mm, and with two kinds of confining element, for low and moderated heat fluxes (?40 kW/m²), on both a downward and an upward facing heating surface. For low heat flux a decrease of the confinement gap causes an enhancement of the boiling and a decrease in the dryout heat flux. A visualization of the boiling phenomenon shows the effect of confinement and heat flux on the liquid-vapor configuration.     

A self-consistent,physics-based boiling heat transfer modeling framework for use in computational fluid dynamics

Computational Fluid Dynamics (CFD) offers the opportunity to investigate physically and geometrically complex systems with high fidelity. Its applicability to multiphase flow, and particularly boiling heat transfer, is currently limited by the lack of appropriate closure models to describe all relevant phenomena. In this paper, we present an original subcooled flow boiling modeling framework for CFD, which aims at consistently and accurately characterizing the key physics that affect heat transfer at the boiling surface. The new framework introduces a fully mechanistic representation of heterogeneous boiling that improves numerical robustness and reduces sensitivity to closure coefficients. The proposed formulation is inspired by new experimental insight, and significantly extends the existing boiling models by capturing the effects of (i) the microlayer on surface evaporation, (ii) the boiling surface, and (iii) bubbles sliding along the boiling surface. A new statistical treatment of the location and mutual interactions of bubbles on the surface allows for mechanistic prediction of the dry surface area, an important quantity that affects the boiling heat transfer coefficient. This approach lends itself naturally to extension to very high heat fluxes, potentially up to the critical heat flux. An assessment and sensitivity study of the model is presented for a range of mass fluxes (500–1250 kg/m²/s), heat fluxes (100–1600 kW/m²), inlet subcoolings (5, 10, 15 K), and pressures (1, 1.5, 2 bars), demonstrating improved robustness and predictive accuracy at all tested conditions in comparison to traditional heat partitioning approaches, including high heat fluxes, where classic models often fail to converge. Lastly, the framework proposed here should not be viewed as another heat partitioning model, but rather as a general platform that allows incorporation of advanced models for each physical phenomenon considered, leveraging the growing insight generated by modern experimental diagnostics for boiling heat transfer.     

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.     

PTV experiments of subcooled boiling flow through a vertical rectangular channel

Time resolved Particle Tracking Velocimetry (PTV) experiments were carried out to investigate turbulent, subcooled boiling flow of refrigerant HFE-301 through a vertical rectangular channel with one heated wall. Measurements were performed with liquid Reynolds numbers (based on the hydraulic diameter) of Re = 3309, 9929 and 16,549 over a wall heat flux range of 0.0–64.0 kW/m². Turbulence statistics are inferred from PTV full-field velocity measurements. Quantities such as: instantaneous 2D velocity fields, time-averaged axial and normal velocities, axial and normal turbulence intensities, and Reynolds stresses are obtained. The present results agree well with previous studies and provides new information due to the full-field nature of the technique. This work is an attempt to provide turbulent subcooled boiling flow data for validation and improvement of two-phase flow computational models.     

Freon R141b flow boiling in silicon microchannel heat sinks: experimental investigation

This paper presents experimental investigations on Freon R141b flow boiling in rectangular microchannel heat sinks. The main aim is to provide an appropriate working fluid for microchannel flow boiling to meet the cooling demand of high power electronic devices. The microchannel heat sink used in this work contains 50 parallel channels, with a 60 × 200 (W × H) μm cross-section. The flow boiling heat transfer experiments are performed with R141b over mass velocities ranging from 400 to 980 kg/(m² s) and heat flux from 40 to 700 kW/m², and the outlet pressure satisfying the atmospheric condition. The fluid flow-rate, fluid inlet/outlet temperature, wall temperature, and pressure drop are measured. The results indicate that the mean heat transfer coefficient of R141b flow boiling in present microchannel heat sinks depends heavily on mass velocity and heat flux and can be predicted by Kandlikar’s correlation (Heat Transf Eng 25(3):86–93, 2004). The two-phase pressure drop keeps increasing as mass velocity and exit vapor quality rise.     

Effect of EHD on heat transfer enhancement during two-phase condensation of R-134a at high mass flux in a horizontal smooth tube

In this study, effect of electrohydrodynamic (EHD) on the condensation heat transfer enhancement and pressure drop of pure R-134a are experimentally investigated. The test section is a 2.5 m long counterflow double tube heat exchanger with refrigerant flowing in the inner tube and cooling water flowing in the annulus. The inner tube is made from smooth horizontal copper tubing of 9.52 mm outer diameter. The electrode is made from stainless steel wire of 1.47 mm diameter. The test runs are performed at average saturated temperatures ranging between 40 and 60°C, mass flux ranging between 200 and 600 kg/m² s, heat flux ranging between 10 and 20 kW/m² and applied voltage at 2.5 kV. For the presence of the electrode, the experimental results indicate that the maximum heat transfer enhancement ratio is around 30% while the maximum increase in pressure drop is about 25%.     

The influence of oil on nucleate pool boiling heat transfer

The influence of various oil contents in R134a is investigated for nucleate pool boiling on copper tubes either sandblasted or with enhanced heating surfaces (GEWA-B tube). Polyolester oils (POE) (Reniso Triton) with medium viscosity 55 cSt (SE55) and high viscosity 170 cSt (SE170) were used. Heat transfer coefficients were obtained for boiling temperatures between −28.6 and +20.1°C. The oil content varied from 0 to 5% mass fraction. For the sandblasted tube and the SE55 oil the heat transfer coefficients for the refrigerant/oil-mixture can be higher or lower than those for the pure refrigerant, depending on oil mass fraction, boiling temperature and heat flux. In some cases the highest heat transfer coefficients were obtained at a mass fraction of 3%. For the 170 cSt oil there is a clear decrease in heat transfer for all variations except for a heat flux 4,000 W/m² and −10.1°C at 0.5% oil content. The heat transfer coefficients are compared to those in the literature for a smooth stainless steel tube and a platinum wire. For the enhanced tube and 55 cSt oil the heat transfer coefficients are clearly below those for pure refrigerant in all cases. The experimental results for the sandblasted tube are compared with the correlation by Jensen and Jackman. The calculated values are within +20 and −40% for the medium viscosity oil and between +50% and −40% for the high viscosity oil. A correlation for predicting oil-degradation effects on enhanced surfaces does not exist.     

Effect of force fields on pool boiling flow patterns in normal and reduced gravity

This paper reports the observations of boiling flow patterns in FC-72, performed during a microgravity experiment, recently flown aboard of Foton-M2 satellite, in some instances with the additional aid of an electrostatic field to replace the buoyancy force. The heater consisted of a flat plate, 20 × 20 mm², directly heated by direct current. Several levels of liquid subcooling (from 20 to 6 K) and heat fluxes up to 200 kW/m² were tested. A complete counterpart test, carried out on ground before the mission, allowed direct comparison with terrestrial data. The void fraction in microgravity revealed much larger than in normal gravity condition: this may be attributed to increased bubble coalescence that hinders vapor condensation in the bulk of the subcooled fluid. In several cases, an oscillatory boiling behavior was detected, leading to periodical variation of average wall overheating of some degrees. The electric field confirmed to be very effective, even at low values of applied voltage, in reducing bubble size, thus improving their condensation rate in the bulk fluid, and in enhancing heat transfer performance, suppressing the boiling oscillations and preventing surface dryout.     

Saturated nucleate boiling to binary and ternary mixtures on horizontal cylinder

In this investigation, a large number of experiments have been performed to determine saturated nucleate pool boiling heat transfer coefficients of MEA/water and DEA/water binary mixtures and that of water/MEA/DEA ternary mixtures. These heat transfer coefficients have been measured at atmospheric pressure and over a wide range of heat fluxes and solution concentrations. The heat flux has been varied in 14 different levels from 7 to about 230 kW/m² and amines concentration has been changed in 10 different levels from zero to 84 wt%. Results show that strong reduction of heat transfer coefficient occurs as a result of mass transfer interference in this phenomenon. Furthermore, in this study, all the correlations proposed during the last years for the prediction of nucleate boiling heat transfer coefficient of mixtures have been categorized in three groups. Some experimental results have been compared with the most accurate representatives of these three groups and the corresponding RMS errors have been calculated. Also, impacts of important existing parameters in these correlations like ideal heat transfer coefficient (h_id.) on the prediction have been discussed.     

Effect of surfactant additives on nucleate pool boiling heat transfer of refrigerant-based nanofluid

Effect of surfactant additives on nucleate pool boiling heat transfer of refrigerant-based nanofluid was investigated experimentally. Three types of surfactants including Sodium Dodecyl Sulfate (SDS), Cetyltrimethyl Ammonium Bromide (CTAB) and Sorbitan Monooleate (Span-80) were used in the experiments. The refrigerant-based nanofluid was formed from Cu nanoparticles and refrigerant R113. The test surface is horizontal with the average roughness of 1.6 μm. Test conditions include a saturation pressure of 101.3 kPa, heat fluxes from 10 to 80 kW m⁻², surfactant concentrations from 0 to 5000 ppm (parts per million by weight), and nanoparticle concentrations from 0 to 1.0 wt.%. The experimental results indicate that the presence of surfactant enhances the nucleate pool boiling heat transfer of refrigerant-based nanofluid on most conditions, but deteriorates the nucleate pool boiling heat transfer at high surfactant concentrations. The ratio of nucleate pool boiling heat transfer coefficient of refrigerant-based nanofluid with surfactant to that without surfactant (defined as surfactant enhancement ratio, SER) are in the ranges of 1.12-1.67, 0.94-1.39, and 0.85-1.29 for SDS, CTAB and Span-80, respectively, and the values of SER are in the order of SDS > CTAB > Span-80, which is opposite to the order of surfactant density values. The SER increases with the increase of surfactant concentration and then decreases, presenting the maximum values at 2000, 500 and 1000 ppm for SDS, CTAB and Span-80, respectively. At a fixed surfactant concentration, the SER increases with the decrease of nanoparticle concentration. A nucleate pool boiling heat transfer correlation for refrigerant-based nanofluid with surfactant is proposed, and it agrees with 92% of the experimental data within a deviation of ±25%.     

Flow boiling heat transfer of R134a and R404A in a microfin tube at low mass fluxes and low heat fluxes

An experimental investigation of flow boiling heat transfer in a commercially available microfin tube with 9.52 mm outer diameter has been carried out. The microfin tube is made of copper with a total fin number of 55 and a helix angle of 15°. The fin height is 0.24 mm and the inner tube diameter at fin root is 8.95 mm. The test tube is 1 m long and is electrically heated. The experiments have been performed at saturation temperatures between 0 and −20°C. The mass flux was varied between 25 and 150 kg/m²s, the heat flux from 15,000 W/m² down to 1,000 W/m². All measurements have been performed at constant inlet vapour quality ranging from 0.1 to 0.7. The measured heat transfer coefficients range from 1,300 to 15,700 W/m²K for R134a and from 912 to 11,451 W/m²K for R404A. The mean heat transfer coefficient of R134a is in average 1.5 times higher than for R404A. The mean heat transfer coefficient has been compared with the correlations by Koyama et al. and by Kandlikar. The deviations are within ±30% and ±15%, respectively. The influence of the mass flux on the heat transfer is most significant between 25 and 62.5 kg/m²s, where the flow pattern changes from stratified wavy flow to almost annular flow. This flow pattern transition is shifted to lower mass fluxes for the microfin tube compared to the smooth tube.     

Study of transient heat transfer and synchronized flow visualizations during sub-cooled flow boiling in a small aspect ratio microchannel

The challenges that microchannel flow boiling technology faces are the lack of understanding of underlying mechanisms of heat transfer during various flow boiling regimes and a dearth of analytical models that can predict heat transfer. This paper aims to understand flow boiling heat transfer mechanisms by analyzing results obtained by synchronously captured high-speed flow visualizations with local, transient temperature data. Using Inverse Heat Conduction Problem (IHCP) solution methodology, the transient wetted surface heat flux and temperature as well as heat transfer coefficient are calculated. These are then correlated with the visual data. Experiments are performed on a single microchannel embedded with fast response temperature sensors located (630 µm) below the wetted surface. The height, width and length of the microchannel are 0.42 mm, 2.54 mm and 25.4 mm respectively. De-ionized, de-gassed water is used as the working fluid. Two heat fluxes are tested at each of the mass fluxes of 182 kg/(m²s) and 380 kg/(m²s). Because of vapor confinement, slug flow is observed for the tested conditions. The present study provides detailed insights into the effect of various events such as passage of vapor slug, 3-phase contact line, partial-dry-out and liquid slug on transient heat transfer coefficient. Transient heat transfer coefficient peaks when thin film evaporation mechanism is prevalent. The peak value is influenced by the distance of bubble incipience as well as downstream events obstructing the flow. Heat transfer coefficient during the passage of liquid slug and 3-phase contact line were relatively lower for the tested experimental conditions.