Subcooled Flow Boiling Heat Transfer Research Articles

Experimental Investigation of Subcooled Flow Boiling in a Minichannel

The present work presents experimental results of subcooled flow boiling heat transfer phenomena in minichannels. The experiments were conducted with high surface tension liquid (water), which clearly indicates that inertial effects are important for both momentum and thermal transport in minichannels. The test section is a horizontal AISI 316 stainless-steel minitube with internal diameter of 1.13 mm with uniform heated length of 250 mm and the tube is heated by AC electric current. The variation in wall temperature and pressure drop gives an indication of the different boiling regions along the axis of the horizontal tube. Based on sudden oscillations of the pressure drop and wall temperature, onset of nucleate boiling (ONB) is identified in the tube. During ONB, pressure drop and the fluctuations of fluid outlet temperature are observed near the exit of the test section. Predictive models for ONB were evaluated and Kandlikar's ONB model was found to agree well with the present data. Comparisons of experimental heat transfer coefficient in the subcooled boiling regime with the existing correlations are also presented.

An experimental study of bubble departure diameter in subcooled flow boiling including the effects of orientation angle, subcooling, mass flux, heat flux, and pressure

The effects of orientation angle, subcooling, heat flux, mass flux, and pressure on bubble departure diameter in the isolated bubble regime of subcooled flow boiling were studied by high-speed video in a two-phase flow loop that can accommodate a wide range of flow conditions. Specifically, the following ranges were explored: orientation angles of 0° (downward-facing horizontal), 30°, 45°, 60°, 90° (vertical), and 180° (upward-facing horizontal); mass flux values of 250, 300, 350, and 400kg/m2s, corresponding to Froude numbers between 0.42 and 1.06; pressures of 101 (atmospheric), 202, and 505kPa; two values of the subcooling degrees (10 and 20°C); and two heat fluxes (0.05 and 0.10MW/m2). The combination of the test section design, high-speed video camera and LED lighting results in high accuracy (order of 20μm) in the determination of the bubble departure diameter. The data indicate that the bubble departure diameter increases with increasing heat flux, decreasing mass flux, decreasing subcooling, and decreasing pressure. Also, the bubble departure diameter increases with decreasing orientation angle, i.e. the largest bubbles are found to detach from a downward-facing horizontal surface. The mechanistic bubble departure diameter model of Klausner et al. and its recent modification by Yun et al. were found to correctly predict all the observed parametric trends, but with large average errors and standard deviation: 65.5±75.8% for Klausner's and 37.9±51.2% for Yun's. Since the cube of the bubble departure diameter is used in subcooled flow boiling heat transfer models, such large errors are clearly unacceptable, and underscore the need for more accurate bubble departure diameter models.

Flow boiling heat transfer in volumetrically heated packed bed

The volumetrically heated packed bed has been widely utilized in modern industry. However, due to the variability and randomness of packed bed channels, flow boiling heat transfer characteristics becomes complex, and there are no published research regarding this topic. To study flow boiling heat transfer characteristics of volumetrically heated packed beds, electromagnetic induction heating method is used to heat oxidized carbon steel balls adopted to stack the packed bed, with water as coolant in the experiment. The experimental results indicate that heat flux at onset of nucleate boiling (ONB) increases as mass flux and inlet subcooling are increased. A new correlation is developed to predict the ONB heat flux qONB in volumetrically heated packed bed, the predictions by which agree well with the experimental data, and the deviation remains less than 15%. Subcooled flow boiling heat transfer coefficient (hsub) increases with increasing mass flux, and equilibrium quality is slightly affected by heat flux. The saturated flow boiling heat transfer coefficient (hsat) increases with mass flux and equilibrium quality when equilibrium quality is lower than about 0.05, while the nucleate boiling is suppressed when the equilibrium quality exceeds a certain value.

Subcooled flow boiling of ethylene glycol/water mixtures in a bottom-heated tube

Coolant subcooled boiling in the cylinder head regions of heavy-duty vehicle engines is unavoidable at high thermal loads due to high metal temperatures. However, theoretical and experimental studies of coolant boiling under these specific application conditions are generally lacking. In the present study, subcooled flow boiling heat transfer experiments were performed with water and ethylene glycol/water mixtures at volume ratios of 40/60 and 50/50 in turbulent flow in a specifically designed and fabricated test facility with its experimental test section simulating the heating conditions of the coolant channels in the cylinder head regions of heavy-duty vehicle engines. Boiling curves and subcooled flow boiling heat transfer coefficients for the tested fluids were determined based on the experimental results. Comparisons between the experimental data and the predicted values from existing correlation equations in the engineering literature are presented.

The characteristics of boiling heat transfer and pressure drop of R-600a in a circular tube with porous inserts

This investigation was undertaken in order to better understand the characteristics of sub-cooled flow boiling heat transfer and pressure drop of a hydrocarbon refrigerant R600a (isobutane) flowing in a circular pipe within dispersed-copper porous inserts. The test was conducted at a saturation temperature of 10 °C, vapor qualities were from 0.076 to 0.87, and there was shown to be a mass flux of 120–1100 kg/m2 s and a heat flux of 12–65 kW/m2. The effects of Reynolds number and the sizes of inserts (the porosity, mean pore diameters and permeability) on the heat transfer and pressure drop of R-600a are presented. In addition, empirical correlations for estimating the boiling heat transfer coefficients and pressure drop for pure R-600a under these conditions have been proposed.

Experimental studies on the effect of water contaminants in convective boiling heat transfer

Experimental investigations on the influences of different contaminants to deionized water have been conducted under the sub-cooled flow boiling heat transfer inside the vertical annulus. Many experiments have been performed to investigate the influence of different operating parameters on the flow boiling heat transfer coefficient in the upward flow of contaminated water under the atmospheric pressure. The experimental apparatus provides the particular conditions to investigate the influence of heat flux (up to 132kW/m2), flow rate (1.5–3.5 l/min), sub-cooling level (Max. 30°C), and concentration of contaminants (1–5% by volume). According to the results, with increasing the heat flux and flow rate, the flow boiling heat transfer coefficient and rate of bubble formation significantly increase. Results also demonstrated that adding contaminants to the deionized water causes the flow boiling heat transfer coefficient to be deteriorated. Likewise, sub-cooling level may only influence on the onset of nucleate boiling and heat flux corresponding to beginning of nucleate boiling phenomenon which is called inception heat flux.

Scale formation and subcooled flow boiling heat transfer of CuO–water nanofluid inside the vertical annulus

The forced convective and subcooled flow-boiling heat transfer of CuO/water nanofluid as well as fouling rate of nanofluid are experimentally quantified for different dilute concentrations of CuO nanoparticles in water over a range of mass fluxes (353kg/m2s<G<1059kg/m2s). Stabilization of nanofluid was also carried out using pH control method and examined using time-spend experiment. In the best case of stabilization (about 216h), experimental results demonstrate that two discrete regions of heat transfer are seen namely force convective and nucleate boiling. Results also show that when concentration of nanofluid increases, heat transfer coefficient in both regions significantly decreases in comparison to pure water. Beside, results reveal that increase of heat and mass flux significantly increases the heat transfer coefficient in both heat transfer mechanisms. In the range of these experiments; nanoparticles have an insignificant effect on the flow pressure drop with the CuO/water nanofluid. Beside, Influence of some operating conditions on flow boiling heat transfer coefficient and fouling rate is discussed.

Experimental study on subcooled flow boiling heat transfer to water–diethylene glycol mixtures as a coolant inside a vertical annulus

Experimental studies on subcooled nucleate flow boiling heat transfer of dilute water–diethylene glycol (DEG) mixtures inside a vertical annulus have been conducted. The experimental apparatus provides the particular conditions to investigate the influence of heat flux, flow velocity, degree of subcooling, and concentration of mixture on heat transfer coefficient in both the convection and nucleate boiling regimes. Results reveal that increasing heat flux causes the single phase heat transfer coefficient increases slightly until the incipience of bubble formation. Then, the flow boiling heat transfer coefficient dramatically increases when heat flux increases. Increasing the flow velocity increases the flow boiling heat transfer coefficient. Contrary to the expectations; little increase in the heat transfer coefficient is clearly observed in both of the convective and the nucleate boiling regimes when small amount of DEG was added to the mixture. In addition, liquid inlet temperature to the annulus significantly influences the inception heat flux such that increasing the inlet temperature decreases the inception heat flux.

Oscillatory subcooled flow boiling heat transfer of R-134a and associated bubble characteristics in a narrow annular duct due to flow rate oscillation

An experiment is conducted here to investigate how an imposed time periodic flow rate oscillation in the form of a triangular wave affects the long time subcooled flow boiling heat transfer and associated bubble characteristics of refrigerant R-134a in a horizontal narrow annular duct. In the experiment the mean R-134a mass flux G¯ varies from 200 to 500kg/m2s, imposed heat flux ranges from 0 to 45kW/m2, and the amplitude of the mass flux oscillation changes from 0 to 30% of G¯ with the period of the mass flux oscillation varied from 20 to 120s for the inlet liquid subcooling ranging from 0 to 6°C. The duct gap is fixed at 2.0mm. The results indicate that the inlet liquid subcooling significantly affects the oscillatory flow boiling heat transfer characteristics. Besides, when the imposed heat flux is close to that for the onset of stable flow boiling, intermittent flow boiling appears. The intermittent boiling prevails in a very different range of the Boiling number for a change in the inlet subcooling. Moreover, in the subcooled boiling the heated wall temperature, bubble departure diameter and frequency, and active nucleation site density also oscillate periodically in time. Furthermore, in the persistent boiling at high imposed heat flux the resulting Tw oscillation is stronger for a higher inlet liquid subcooling and for a longer period and a larger amplitude of the mass flux oscillation. And for a larger amplitude of the mass flux oscillation, stronger temporal oscillations in dp, f and nac are noted. Finally, a flow regime map is provided to delineate the boundaries separating different boiling regimes for the oscillatory R-134a subcooled flow boiling in the annular duct.

Heating surface material’s effect on subcooled flow boiling heat transfer of R134a

In this study, subcooled flow boiling of R134a on copper (Cu) and stainless steel (SS) heating surfaces was experimentally investigated from both macroscopic and microscopic points of view. By utilizing a high-speed digital camera, bubble growth rate, bubble departure size, and nucleation site density, were able to be observed and analyzed from the microscopic point of view. Macroscopic characteristics of the subcooled flow boiling, such as heat transfer coefficient, were able to be measured as well. Experimental results showed that there are no obvious difference between the copper and the stainless surface with respect to bubble dynamics, such as contact angle, growth rate and departure size. On the contrary, the results clearly showed a trend that the copper surface had a better performance than the stainless steel surface in terms of heat transfer coefficient. It was also observed that wall heat fluxes on both surfaces were found highly correlated with nucleation site density, as bubble hydrodynamics are similar on these two surfaces. The difference between these two surfaces was concluded as results of different surface thermal conductivities.

Forced convective and subcooled flow boiling heat transfer to pure water and n-heptane in an annular heat exchanger

In this research, subcooled flow boiling heat transfer coefficients of pure n-heptane and distilled water at different operating conditions have been experimentally measured and compared. The heat exchanger consisted of vertical annulus which is heated from the inner cylindrical heater with variable heat flux (less than 140kW/m2). Heat flux is varied so that two different flow regimes from single phase forced convection to nucleate boiling condition are created. Meanwhile, liquid flow rate is changed in the range of 2.5×10−5–5.8×10−5m3/s to create laminar up to transition flow regimes. Three subcooling levels including 10, 20 and 30°C are also considered. Experimental results demonstrated that subcooled flow boiling heat transfer coefficient increases when higher heat flux, higher liquid flow rate and greater subcooling level are applied. Furthermore, influence of the operating conditions on the bubbles generation on the heat transfer surface is also discussed. It is also shown that water is better cooling fluid in comparison with n-heptane.

Influences of bubble formation on different types of heat exchanger fouling

In this paper, a systematic comparison is performed to investigate fouling of suspended particles under forced convective and subcooled flow boiling heat transfer. For this purpose, two different types of fouling are separately considered: crystallization fouling of dissolved CaSO4 particles in water and particulate fouling of suspended Al2O3 particles in n–heptane. The effect of hydraulic parameters such as fluid velocity and also bubble generation under subcooled flow boiling are studied. Results of the experiments demonstrate that creation of boiling condition in the heat exchanger has opposite influence in these two types of fouling. It means that bubble generation on the heat transfer surface promotes scale formation under crystallization fouling. This is due to the fact that increased bubble generation creates higher supersaturation beneath the vapor bubble, therefore, increasing the crystal concentration in the boundary layer. On the other hand, boiling condition inhibits the scale formation under particulate fouling because the suspended particles are repelled from the boundary layer by the strong turbulences created by the swarm of bubbles.

Heat transfer characteristics during subcooled flow boiling of a kerosene kind hydrocarbon fuel in a 1 mm diameter channel

The subcooled flow boiling heat transfer characteristics of a kerosene kind hydrocarbon fuel were investigated in an electrically heated horizontal tube with an inner diameter of 1.0mm, in the range of heat flux: 20–1500kW/m2, fluid temperature: 25–400°C, mass flux: 1260–2160kg/m2s, and pressure: 0.25–2.5MPa. It was proposed that nucleate boiling heat transfer mechanism is dominant, as the heat transfer performance is dependent on heat flux imposed on the channel, rather than the fuel flow rate. It was found that the wall temperatures along the test section kept constant during the fully developed subcooled boiling (FDSB) of the non-azeotropic hydrocarbon fuel. After the onset of nucleate boiling, the temperature differences between inner wall and bulk fluid begin to decrease with the increase of heat flux. Experimental results show that the complicated boiling heat transfer behavior of hydrocarbon fuel is profoundly affected by the pressure and heat flux, especially by fuel subcooling. A correlation of heat transfer coefficients varying with heat fluxes and fuel subcooling was curve fitted. Excellent agreement is obtained between the predicted values and the experimental data.

Subcooled flow boiling heat transfer of ethanol aqueous solutions in vertical annulus space

The subcooled flow boiling heat-transfer characteristics of water and ethanol solutions in a vertical annulus have been investigated up to heat flux 132kW/m2. The variations in the effects of heat flux and fluid velocity, and concentration of ethanol on the observed heat-transfer coefficients over a range of ethanol concentrations implied an enhanced contribution of nucleate boiling heat transfer in flow boiling, where both forced convection and nucleate boiling heat transfer occurred. Increasing the ethanol concentration led to a significant deterioration in the observed heat-transfer coefficient because of a mixture effect, that resulted in a local rise in the saturation temperature of ethanol/water solution at the vapor-liquid interface. The reduction in the heat-transfer coefficient with increasing ethanol concentration is also attributed to changes in the fluid properties (for example, viscosity and heat capacity) of tested solutions with different ethanol content. The experimental data were compared with some well-established existing correlations. Results of comparisons indicate existing correlations are unable to obtain the acceptable values. Therefore a modified correlation based on Gnielinski correlation has been proposed that predicts the heat transfer coefficient for ethanol/water solution with uncertainty about 8% that is the least in comparison to other well-known existing correlations.

Enhanced flow boiling heat transfer with jet impingement on micro-pin-finned surfaces

Experiments were conducted to study the performance of subcooled flow boiling heat transfer with jet impingement of FC-72 over silicon chips (10 × 10 × 0.5 mm3). Four kinds of micro-pin-fins with dimensions of 30 × 60, 30 × 120, 50 × 60, 50 × 120 μm2 (thickness t× height h) were fabricated on the chip surfaces by using the dry etching technique. The experiments were made at two different liquid subcooling (25 °C and 35 °C), three different crossflow velocities Vc (0.5, 1, 1.5 m/s) and three different jet velocities Vj (0, 1, 2 m/s). A smooth surface was also tested for comparison. The results show that both the microstructure and impingement give a large enhancement on heat transfer. The maximum allowable heat flux increases with the velocity and liquid subcooling. For a fixed Vc, the enhancement degree increases with Vj especially for Vc = 0.5 m/s, Vj = 2 m/s. As Vc increases, the heat transfer enhancement of jet impingement weakens and the increase rate of CHF (the critical heat flux) also decreases. The largest value of qmax (the maximum allowable heat flux) can reach 167 W/cm2 for chip with the fin dimension of 50 × 120 μm2 at the condition of Vc = 1.5 m/s, Vj = 2 m/s and liquid subcooling of 35 °C.

Subcooled flow boiling heat transfer from microporous surfaces in a small channel

The continuously increasing requirement for high heat transfer rate in a compact space can be met by combining the small channel/microchannel and heat transfer enhancement methods during fluid subcooled flow boiling. In this paper, the sintered microporous coating, as an efficient means of enhancing nucleate boiling, was applied to a horizontal, rectangular small channel. Water flow boiling heat transfer characteristics from the small channel with/without the microporous coating were experimentally investigated. The small channel, even without the coating, presented flow boiling heat transfer enhancement at low vapor quality due to size effects of the channel. This enhancement was also verified by underpredictions from macroscale correlations. In addition to the enhancement from the channel size, all six microporous coatings with various structural parameters were found to further enhance nucleate boiling significantly. Effects of the coating structural parameters, fluid mass flux and inlet subcooling were also investigated to identify the optimum condition for heat transfer enhancement. Under the optimum condition, the microporous coating could produce the heat transfer coefficients 2.7 times the smooth surface value in subcooled flow boiling and 3 times in saturated flow boiling. The combination of the microporous coating and small channel led to excellent heat transfer performance, and therefore was deemed to have promising application prospects in many areas such as air conditioning, chip cooling, refrigeration systems, and many others involving compact heat exchangers.

Effects of water contamination on sub-cooled flow boiling heat transfer

An experimental investigation has been carried out to study the effect of adding four different contaminants to distilled water on heat transfer under sub-cooled flow boiling conditions. Flow boiling experimental test rig has been designed and constructed to study the effect of changing the contaminant concentration and flow velocity. Lube oil, Nile river water, tap water and sea water were added at different concentrations to distilled water under sub-cooled flow boiling testing at constant bulk temperature. The effect of flow velocity was also studied for three different concentrations of 1%, 3% and 5% as compared to pure distilled water case. The heat flux applied was in the range of 100–400 kW/m 2. Flow velocities were changed from 1, 2 to 2.5 m/s at constant bulk temperature of 70 °C. It was found that adding any of the contaminants at all considered concentrations to distilled water impairs the heat transfer process substantially.

E207 地震振動が冷却限界および沸騰気泡挙動に及ぼす影響に関する研究 : 加熱面振動下のCHF発生機構の考察(OS9 熱・流動(試験))

E207 地震振動が冷却限界および沸騰気泡挙動に及ぼす影響に関する研究 : 加熱面振動下のCHF発生機構の考察(OS9 熱・流動(試験))

Subcooled Boiling Heat Transfer of CO2 in A Horizontal Tube at Low Temperatures

This article presents an experimental investigation on the subcooled flow boiling heat transfer characteristics of CO2 in a horizontal tube with inner diameter of 6.16 mm below −30°C. The effects of mass and heat fluxes and saturation temperature on the heat transfer coefficient are discussed. Large deviations are noted between the predictions from previous empirical correlations and the current CO2 experimental data. Hence a new empirical correlation is developed, which agrees within ±30% with the current CO2 experimental data. It is expected that the data presented in this study would be beneficial to industry and designers of compact heat exchangers for CO2 at low temperatures.

Subcooled flow boiling heat transfer and associated bubble characteristics of FC-72 on a heated micro-pin-finned silicon chip

Experiments are conducted here to investigate subcooled flow boiling heat transfer and associated bubble characteristics of FC-72 on a heated micro-pin-finned silicon chip flush-mounted on the bottom of a horizontal rectangular channel. In the experiments the mass flux is varied from 287 to 431 kg/m 2 s, coolant inlet subcooling from 2.3 to 4.3 °C, and imposed heat flux from 1 to 10 W/cm 2. Besides, the silicon chips contain three different geometries of micro-structures, namely, the smooth, pin-finned 200 and pin-finned 100 surfaces. The pin-finned 200 and 100 surfaces, respectively, contain micro-pin-fins of size 200 μm × 200 μm × 70 μm (width × length × height) and 100 μm × 100 μm × 70 μm. The measured data show that the subcooled flow boiling heat transfer coefficient is reduced at increasing inlet liquid subcooling but is little affected by the coolant mass flux. Besides, adding the micro-pin-fin structures to the chip surface can effectively raise the single-phase convection and flow boiling heat transfer coefficients. Moreover, the mean bubble departure diameter and active nucleation site density are reduced for rises in the FC-72 mass flux and inlet liquid subcooling. Increasing coolant mass flux or reducing inlet liquid subcooling results in a higher mean bubble departure frequency. Furthermore, larger bubble departure diameter, higher bubble departure frequency, and higher active nucleation site density are observed as the imposed heat flux is increased. Finally, empirical correlations for the present data for the heat transfer and bubble characteristics in the FC-72 subcooled flow boiling are proposed.