Nucleate Flow Boiling Heat Transfer Research Articles

Experimental and theoretical investigation on the effect of surface porosity on the nucleate flow boiling heat transfer

The surface porosity of accident tolerant fuel cladding materials is different from the fuel material inservice at nuclear power plant, which leads to differences in the heat transfer characteristics. An experiment was conducted to investigate the effect of surface porosity on the nucleate flow boiling heat transfer. The results showed that bubbles on the SiC surface appear to be more uniform and remain isolated longer after bubble generation, and the average diameter of bubbles on the SiC surface tends to be smaller than that on the FeCrAl alloy surface. The nucleation site density is power-law dependent on porosity and superheat, and negative power-law dependent on critical cavity diameter and coolant subcooling. For the same degree of temperature increase on both surfaces, the number of nucleation sites on the surface with larger porosity increases much more than that on the surface with lower porosity, while the coolant subcooling degree inhibits the bubble nucleation and shrinks the bubble detachment diameter accordingly. A new correlation was proposed based on dimensional analysis, and the predictions were compared with the present experimental data and the other research data, with an 86% confidence interval of ± 50% for the consolidated datasets. Furthermore, the side-by-side comparison of the performance on these correlations showed that the present correlation, which includes surface porosity and roughness factors, significantly reduces the prediction error compared to the previous correlations.

Data-driven approach to predict the flow boiling heat transfer coefficient of liquid hydrogen aviation fuel

The airline industry targets net-zero carbon emissions by 2050. Hydrogen, a fuel with high energy density and clean combustion products, is poised to substitute kerosene as a new kind of future aviation fuel. Nonetheless, hydrogen aviation fuel is extremely sensitive to heat leakage during the transportation from the fuel tank to the engine on account of the liquid hydrogen flow boiling. Up to now, a reliable tool to predicate the hydrogen flow boiling heat transfer coefficient with high accuracy remains inaccessible. Herein, we propose a data-driven machine-learning model to predict this coefficient via a non-linear regression approach. To this end, we first collected over 864 data points with a hydraulic diameter ranging from 4 mm to 6 mm and with a flow velocity ranging from 1.33 m/s to 11.56 m/s. Then, eight machine learning regression models are developed and compared with empirical correlations. Among them, the Extra Tree model exhibited the best prediction performance with a MSE <0.01% and a R2 = 0.9933, significantly outperforming previously reported generalized prediction correlations. Finally, we found that the Ja number, indispensable to the input parameters, served as a fundamental descriptor in the accurate prediction of the hydrogen flow boiling heat transfer coefficient. The machine learning-based technique provides a potent tool to predict the hydrogen nucleate flow boiling heat transfer coefficients with great precision.

Saturated hydrogen nucleate flow boiling heat transfer coefficients study based on artificial neural network

Flow boiling heat transfer of liquid hydrogen is very important in many applications such as hydrogen storage and transportation, cryogenic cooling. However, effective tool for accurate prediction of hydrogen flow boiling heat transfer coefficient is still absent due to the large property disparity between hydrogen and room temperature fluids. In this study, a hydrogen flow boiling heat transfer database consisting of 366 data points is amassed from different sources. An Artificial Neural Network (ANN) is employed to identify the key parameters influencing the boiling heat transfer. Based on the identified key parameters, a new concise correlation is proposed for predicting hydrogen flow boiling Nusselt number. The new correlation is capable in predicting the Nusselt number with an overall Mean Absolute Error (MAE) of 12.2%. Around 93.2% and 99.5% of the data fall within the ±30% and ±50% error bands, respectively. The application range of the new correlation is Reynolds number 64873~660000, mass flux 76~1136 kg/(m2s), saturation temperature 22~29 K, Boiling number 1.39 × 10−5~2.20 × 10−3. It is found that the Boiling number is a dominant factor in determining the hydrogen nucleate flow boiling heat transfer coefficient. Compared to the flow rate of liquid hydrogen, the flow boiling heat transfer coefficient is more influenced by the saturated pressure.

Modeling and numerical investigation of hydrogen nucleate flow boiling heat transfer

Liquid hydrogen flow boiling heat transfer in tubes is of great importance in the hydrogen applications such as superconductor cooling, hydrogen fueling. In the present study, a numerical model for hydrogen nucleate flow boiling based on the wall partition heat flux model is established. The key parameters in the model such as active nucleation site density, bubble departure diameter and frequency are carefully discussed and determined to facilitate the modeling and simulation of hydrogen flow boiling. Simulation results of the numerical model show reasonably well agreement with experimental data from different research groups in a wide operation condition range with the means absolute error (MAE) of 10.6% for saturated and 5.3% for subcooled flow boiling. Based on the model, wall heat flux components and void fraction distribution of hydrogen flow boiling are studied. Effects of mass flow rate and wall heat flux on the flow boiling heat transfer performance are investigated. It is found that in the hydrogen nucleate flow boiling, the predominated factor is the Boiling number, rather than the vapor quality. A new simple correlation is proposed for predicting hydrogen saturated nucleate flow boiling Nusselt number. The MAE between the correlation predicted and experimentally measured Nusselt number is 13.6% for circular tubes and 12.5% for rectangular tubes. The new correlation is applicable in the range of channel diameter 4–6.35 mm, Reynolds number 64000–660,000, saturation temperature 22–29 K, Boiling number 8.37 × 10−5–2.33 × 10−3.

Wall heat flux partitioning during subcooled flow boiling of NaCl solutions and pure water

A careful evaluation on the wall heat flux partitioning is of great importance for the deep understanding the nucleate flow boiling heat transfer to saline solution. In this work, the portion of each heat transfer contribution was analyzed through wall heat flux partitioning model after model validation. The key parameters used in the model were discussed and the reasonable values were determined. The experimental data of bubble departure diameter, departure frequency, active nucleation site density, bubble waiting time and growth time, were combined into the model to eliminate the uncertainties of empirical closure sub-models. The comparison of heat flux partitioning of subcooled flow boiling between NaCl solution and pure water was presented under different operating conditions. The results showed that for the operating conditions of Qw > 180 kW/m2 or G < 400 kg/(m2·s), the contribution of single-phase liquid convection in the heat transfer of saline solution is greater than that in pure water, due to the bigger bubble diameter and more intensive bubble-induced turbulence. Quenching heat flux for NaCl solution is lower than that in pure water, owing to the smaller bubble departure frequency. For the lower heat flux or higher mass flux conditions, however, the contrary trend was observed. The dominant heat transfer mechanisms for solution and pure water during subcooled nucleate flow boiling were revealed. The interactions between bubble dynamic and heat transfer characteristics were carefully investigated.

Comparison of the Evaporation and Condensation Heat Transfer Coefficients on the External Surface of Tubes in the Annulus of a Tube-in-Tube Heat Exchanger

An experimental study was carried out to explore the heat transfer characteristics on the outside of smooth and enhanced tubes, during evaporation and condensation of R134A in the annulus of a tube-in-tube heat exchanger. The three-dimensional enhanced surface tube consisted of primary enhancement patterns and secondary patterns; results were compared to the performance of an equivalent smooth tube. The equivalent external diameter of the inside horizontal copper tubes used in this study was 19.05 mm, while the outer tube varied in size, allowing a comparison of heat transfer for different annulus dimensions. Tests were conducted with a fixed inlet/outlet vapor quality and a constant saturation temperature for varied mass velocities in the range of 30 to 100 kg/(m2∙s). For condensation, the ratio of heat transfer coefficient enhancement (enhanced tube/ smooth tube) was up to 1.78; this can be attributed to the turbulence increase, as well as liquid film re-distribution, produced from the dimples. Furthermore, the condensation heat transfer coefficient increased rapidly with increasing mass flux. For flow boiling in the annulus between the 1EHT tube and outer tube, the heat transfer coefficient during boiling was 11–36% higher when compared to the smooth tube at xave = 0.35, while the performance of the 1EHT tube was not as good as the smooth tube at xave = 0.5. The heat transfer deterioration can be explained by decreased effective nucleate flow boiling heat transfer area and the flow pattern transition between a slug/wavy-stratified flow to wavy-stratified flow.

Flow boiling in microchannel with synthetic jet in cross-flow

Flow boiling heat transfer in a microchannel with a micro synthetic jet powered by liquid/vapor phase change was studied. The synthetic jet had no moving parts and used the motion of the interfacial layer between vapor and liquid to propel liquid. Bubbles were generated on a micro heater in a chamber, 3.5 mm in radius and 220 µm tall, which was connected to the microchannel through a 300 µm nozzle opening. Periodic growth and collapse of bubbles in the chamber powered the synthetic jet. Using high-speed photography and a microscope, the sequence of bubble growth and collapse in the chamber, bubble nucleation in the microchannel, and their interaction with the synthetic jet were captured and analyzed. The jet velocity was estimated using image processing and its momentum coefficient was used to characterize the strength of the synthetic jet. The results showed that the synthetic jet enhanced nucleate flow boiling heat transfer in the microchannel by up to 20%. dry-out spots formed over the heated surface for high heat fluxes were mitigated by the synthetic jet as it assisted rewetting the surface and maintaining the integrity of the thin liquid film adjacent to the heated surface.

Experimental study of heat transfer and bubble behaviors of NaCl solutions during nucleate flow boiling

Concentrated salt solutions are used as working fluids in heat exchangers for heavy oil drilling to meet the requirements of environmental protection. Deep understanding the nucleate flow boiling heat transfer mechanism of salt solutions is of great importance for the design and safe operation of modern steam power equipment. In this work, the nucleate flow boiling for NaCl solution and pure water was experimentally studied within the concentration (C) range of 0%–6% in a vertical heated tube. To simultaneously obtain the heat transfer coefficient (HTC) and bubble parameters, the test section was built to be transparent and the energy for boiling was provided by a transparent ITO heater. The bubble parameters, including bubble departure diameter (Dw), bubble departure frequency (f), bubble growth time (tg) and waiting time (tw), were measured in the experiments. A wide range of operating conditions is considered in this study, covering the inlet fluid temperature (Tinlet) between 337 and 365 K, the mass flux (G) between 200 and 600 kg/m2s and the heat flux between 10 and 180 kW/m2. Based on the experimental results, the difference of nucleate flow boiling heat transfer performance between pure water and solutions was compared. It was found that the HTC of pure water was higher than that of salt solutions under the operating conditions of Tinlet < 337 K or Tinlet > 351 K and q > 180 kW/m2, in some cases, it was lower than that of solutions. Dw in the solution is bigger than that in pure water while f in the solution is lower. The corresponding reasons for those heat transfer differences between pure water and solutions were revealed by considering the differences in bubble behaviors, physical properties and mass transfer rate. In addition, the influences of operating parameters on the heat transfer characteristics and bubble behaviors of salt solution during nucleate flow boiling were demonstrated.

Forced Convective and Subcooled Flow Boiling Heat Transfer to Water–Al2O3 Microfluid in an Annular Heat Exchanger

An experimental study has been conducted on subcooled nucleate flow boiling heat transfer of water–Al2O3 particulate liquid inside a vertical annulus. The experimental apparatus provides the particular conditions to investigate the influence of heat flux, flow rate, inlet temperature, and particle concentration on the heat transfer coefficient in both single-phase convection and two-phase nucleate boiling regimes. Operating conditions are subcooling temperature (30–50 °C), volume flow rate (2–4 l/min), heat flux (8–110 kW/m2), and particle concentration (0.01–0.03 vol%). The experimental data is in good agreement with well-known Chen model with the accuracy of 10.84%.

Saturated flow boiling heat transfer investigation for nanofluid in minichannel

In the present paper, a theoretical saturated flow boiling heat transfer coefficient correlation for nanofluid in minichannel has been proposed based on the contributions of convective boiling and nucleate boiling. For convective heat transfer, the absorbed heat amount of nanoparticles from fluid has been deduced by the fractal distribution theory. For nucleate boiling, the probability density equation of bubbles random growth is obtained on the basis of a Markov process. As a result, the nucleate flow boiling heat transfer coefficient formula is derived. In addition, three mass fractions for 0.2%, 0.5% and 1.0% Al2O3/H2O nanofluid are prepared by ultrasonic vibration and saturated flow boiling heat transfer coefficients in different rectangular minichannels with cross section 0.3×2mm, 0.6×2mm, 2×2mm are experimentally investigated. The results show that the heat transfer performance is enhanced after adding nanoparticles in base fluid and the average saturated flow boiling heat transfer coefficients of 0.2wt.%, 0.5wt.% and 1.0wt.% Al2O3/H2O nanofluid respectively increase by 11.2%, 15.4% and 18.7% compared with that of deionized water. Meanwhile, the average saturated flow boiling heat transfer coefficients for 1.0wt.% Al3O2/H2O nanofluid are increased by 21.1%, 26.2% in channels with cross section 0.3mm×2mm, 0.6mm×2mm compared with the average heat transfer coefficients in channels with cross section 2mm×2mm, which illustrates that the capillary effects are strengthened with the hydraulic diameter decreasing. Furthermore, a comparison is made between the experimental and theoretical data. The results show that the theoretical heat transfer model has a good universal prediction for nanofluid saturated flow boiling heat transfer in minichannel.

Forced Convective and Nucleate Flow Boiling Heat Transfer to Alumnia Nanofluids

A large number of experiments have been performed to quantify the forced convective and nucleate flow boiling heat transfer coefficient of Al 2 O 3 water based nanofluid. The employed test loop provides conditions to investigate the influence of operating parameters such as heat flux, flow mass flux and volumetric concentration of nanofluids (0.5, 1 and 1.5%). Results demonstrate that two heat transfer regions are observed namely forced convective and nucleate boiling region. Investigating on the operating parameters illustrated that with increasing the heat flux and flow rate of nanofluid, heat transfer coefficient of nanofluid dramatically increases. In contrast, with increasing the volumetric concentration of nanofluid, controversial condition is observed such that increases the heat transfer coefficient in forced convective region is reported while reduction of heat transfer coefficient is seen for nucleate boiling zone. Obtained results were then compared to Chen and also Gungor-Winterton well-known correlations. Results of this comparison show that experimental data are in a good agreement with those of obtained by correlations.

Application of thermodynamic models to estimating the convective flow boiling heat transfer coefficient of mixtures

A large number of experiments has been performed to measure the forced convective and nucleate flow boiling heat transfer coefficient of three different none-volatile mixtures at different heat fluxes (up to 175kWm−2) and five different volumetric concentrations (10–50% of heavier component). The test mixtures include water/glycerol, water/monoethylene glycol (MEG), and water/diethylene glycol (DEG). The experimental apparatus provides conditions to investigate the influence of the main operating parameters such as: heat flux, concentration, and flow rate of fluid on the forced convective and flow boiling heat transfer coefficient. It is shown that physical properties of the mixtures have a considerable effect on the prediction of flow boiling heat transfer coefficients by the predictive correlations. In almost all of the predictive correlations, physical properties are strongly involved which can be estimated by different thermodynamic models. This work demonstrates that thermodynamic models for the calculation of specific heat, liquid density and heat of vaporization do not obtain identical results and consequently, the heat transfer coefficient obtained from a specified predictive correlation (Chen type model) can be tolerated according to the used thermodynamic model for the calculation of the physical properties. This point has been ignored by the investigators and they compare their experimental data with the correlations without specifying that, which one of the thermodynamic models has to be used for the obtaining of the thermo-physical properties. After reading the present study, a new vision can be opened to the readers interested in prediction of the flow boiling heat transfer coefficient and may help the researchers to reliably predict the thermo-physical properties of fluids particularly for forced convective and boiling phenomena.

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.

Horizontal Nucleate Flow Boiling Heat Transfer Coefficient Measurements and Visual Observations for R12, R134a and R134a/Ester Lubricant Mixtures

Horizontal Nucleate Flow Boiling Heat Transfer Coefficient Measurements and Visual Observations for R12, R134a and R134a/Ester Lubricant Mixtures

Nucleate boiling of non-Newtonian fluids in a stirred tank reactor

The nucleate boiling of non-Newtonian fluids has been investigated in a mechanically stirred tank. The wall heated reactor is stirred with an anchor. The case of high consistency fluids has been more specifically studied. It was shown that when the fluid consistency increases, the predominant mechanisms of boiling heat transfer change, resulting in the fact that the correlations which can be used as a design estimate for low viscosity fluids cannot be used any more. A method was proposed for these fluids to estimate nucleate flow boiling heat transfer from the forced convection predictions.

Flow boiling heat transfer to saturated pure components and non-azeotropic mixtures in a horizontal tube Wärmeübergang beim Strömungssieden von gesättigten reinen Komponenten und binären nicht azeotropen Gemischen im horizontalen Rohr

In horizontal flow boiling conditions the heat transfer is affected by the saturation pressure, the composition of the mixture, the vapour fraction, the heat flux and the mass velocity. Furthermore, the heat transfer coefficients are different at different axial and circumferential positions on the heat transfer surface. In this work the calculation of averaged heat transfer coefficients is based on local (point) values. For the prediction of local heat transfer coefficients of pure substances a new correlation is proposed. Local heat transfer coefficients for two pure substances (SF 6 and CCl 2F 2) and mixtures were measured for the condition of uniform wall temperature over the whole composition range and various values of the affecting parameters. In the nucleate flow boiling region a pronounced reduction of the local heat transfer coefficients occurs, which is dependent on the liquid composition, the heat flux and the saturation pressure. This reduction of nucleate flow boiling heat transfer coefficients of mixtures is predicted by the model of Schlünder. However, from the experimental results it can be concluded, that the reduction in vaporization heat transfer is smaller for high mass velocities. With a simple model for the evaluation of the wetting boundary circumferential averaged heat transfer coefficients can be computed.