Heat Transfer Characteristics Of Pool Research Articles

Experimental study on bubble dynamics and heat transfer of pool boiling at sub-atmospheric pressures

Pool boiling, as an efficient phase-change heat transfer method, has attracted extensive attention in recent years. However, the bubble dynamics behavior (BDB) and heat transfer characteristics of pool boiling at sub-atmospheric pressure (sub-ATM) are significantly different from those at or above standard atmospheric pressure (std-ATM). This paper presents experimental results of BDB and heat transfer characteristics of pool boiling at sub-ATM in the range of 30–120 kPa. High-speed backlight imaging technique was adopted. The results show that with the decrease of boiling pressure, the diameter and near circularity coefficient of bubble departure increase, and the frequency and aspect ratio decrease. With the decline in pressure, the critical heat flux (CHF) of pool boiling on the surface of the electric heater decreases. Specifically, the CHF at 120 kPa is 23.2% higher than that at 40 kPa. At sub-ATM, the boiling heat transfer coefficient (HTC) grows with the increase of liquid height. However, the HTC hardly changes with liquid height at std-ATM and higher.

Experimental study on subcooled pool boiling of FC-72 on a flat plate in normal and microgravity

A series of experimental studies on the heat transfer characteristics of pool boiling were conducted aboard the Chinese satellite SJ-10 in April 2016. Ground comparative experiments under the same conditions were carried out using the same apparatus. FC-72 was used as the working fluid containing non-condensable gas. The total quasi-stable-state acceleration aboard satellite SJ-10 during its in-orbit flight is approximately 2 × 10−6 g0. The integrated microheater fabricated by the microelectromechanical system (MEMS) technique was used to investigate the boiling characteristics. The boiling surface of the integrated microheater is on top of a flat plate quartz glass wafer substrate 2 mm thick, heated by a serpentine Pt thin film (main heater) below the substrate. The effective area of the boiling surface in direct contact with FC-72 is 5.5 mm in diameter. Ten temperature sensors are located around the boiling surface center and uniformly distributed in the circumferential direction. The experimental results show that single-phase convection could still be maintained under lower Rayleigh number conditions, showing consistency with the predictions based on the available correlations. The generated bubble volume determines the boiling state. At high liquid subcooling, the bubbles formed after the liquid vaporizes are relatively small, and steam condensation at the bubble cap promotes the phase change efficiency, which maintains nucleate boiling in microgravity. When subcooling is low, bubbles are larger and cover most of the heating surface, nucleate boiling is difficult to maintain, and the boiling becomes transition boiling. High liquid subcooling improves the heat transfer efficiency, thereby contributing to maintaining the progress of nucleate boiling in microgravity. However, boiling is less affected by liquid subcooling on the ground than in microgravity. A comparison of pool boiling curves between microgravity and normal gravity experiments indicates that nucleate boiling curves under the same subcooling partially overlap, while the heat flux corresponding to the initiation of the nucleate state is lower than that on the ground. In microgravity, lower liquid subcooling and higher heating power will result in larger bubble volumes and easier complete coverage of the heating surface, resulting in an easier transition from nucleate boiling to film boiling.

Heat Transfer Characteristics of Pool Boiling with Scalable Plasma-Sprayed Aluminum Coatings

To ensure adequate reliability in two-phase cooling systems involving boiling, it is essential to enhance the heat transfer coefficient and maximize the critical heat flux (CHF) limit. A key technique to avoid surface burnout and increase the CHF limit in pool boiling is the frequent coolant supply to the probable dry-out locations. In the present work, we have explored the plasma-spray coating as a surface modification technique for enhancing heat transfer coefficient and CHF value in pool boiling applications. Three plasma-coated aluminum surfaces (C-15, C-20, and C-25) are fabricated on a copper substrate at three different plasma powers of 15, 20, and 25 kW, respectively. Detailed surface morphologies of the plasma-sprayed coatings are presented, and their roles in pool boiling heat transfer mechanisms are analyzed. Plasma-coated surfaces exhibit wickability characteristics and enhanced wettability compared to the plain copper surface. Saturated pool boiling experiments are performed with DI (deionized) water at atmospheric pressure. Plasma spray-coated surfaces show favorable boiling incipience with less wall superheat and more active nucleation sites than the plain copper surface. Compared to the plain copper surface, enhancement values of nearly 68, 60.7, and 55.5% in the heat transfer coefficient are observed for C-15, C-20, and C-25 plasma-coated surfaces, respectively. Experiments could not be performed beyond the heat flux of 197 W/cm2 due to repeated failure of the cartridge heaters. Based on the experimental measurement of wickabilities, the CHF values of plasma-coated surfaces have been theoretically calculated. Compared to the plain copper surface, a maximum 2.39 times higher CHF value is observed for C-15 plasma-coated surface. Improved wettability and wickability are responsible for CHF enhancement in the case of plasma-coated surfaces. At higher heat flux, capillary wicking and frequent rewetting of the dryout locations delay the burnout phenomenon, enhancing CHF in plasma-coated surfaces. The plasma-spray coating is a robust and scalable process, which can be a potential candidate for high heat flux dissipation in various industrial applications.

Effects of geometric arrangement on pool boiling heat exchange in the tubular bundle

Examination of bubble nucleation and detachment mechanisms in tubular pool boiling is important for designing passive heat exchangers in nuclear power plants. This study investigated the heat transfer characteristics of pool boiling in deionized water using an experimental method combined with a simulation approach. Based on the change in the experimental conditions, a prediction model of the heat transfer coefficient was constructed, and the model was verified using experimental data with a maximum error of 13%. The effects of different inclination angles and lower tubes (two rods: the upper and lower tubes) on the boiling heat transfer, wall temperature uniformity, and bubble departure diameter of the tubular bundle pool were studied. The results showed that an increase in the heat flux of the lower tube and a decrease in the inclination angle increased the heat transfer coefficient of the upper tube. The main reason for the enhanced heat transfer in the upper tube was the convective flow and liquid stirring caused by the rising bubbles in the lower tube. The boiling process changed from natural convection to forced convection, which strengthened the heat transfer in the upper tube. Finally, the simulation was verified using the volume of fluid (VOF) and Lee phase change model, and the results agreed with the experimental results (within 10% of the maximum error).

Enhanced pool boiling of propane on horizontal U-shaped tubes in a large-scale confined space

• The experimental data of propane pool boiling on a fin-enhanced tube was obtained. • The heat transfer coefficients demonstrated increasing with heat flux and saturation temperature. • The enhancement factors were found in the range of 1.25∼2.28. • Optimum correlations were validated to predict the experimental data within 11.48%. This paper aims to experimentally investigate the heat transfer characteristics of pool boiling on horizontal U-shaped tubes in a large-scale confined space. The saturation temperature ranges from 20 to 40°C and the heat flux varies from 2.5 to 10.5 kWm −2 . Two types of tubes with an outside diameter of 19.05 mm are tested: a smooth tube and a fin-enhanced tube with 0.4 mm fin height, 1220 fpm (fins per meter) and a 0.12 mm fin gap. The tubes are made of titanium with a length of 1165 mm. In case of pool boiling, the heat transfer coefficient for the tested tubes increases with increasing saturation temperature and heat flux. The fin-enhanced tube facilitates the bubble nucleation with the heat transfer coefficient enhancement factor varying in the range of 1.25∼2.28. Furthermore, the experimental heat transfer coefficients are compared with four correlations obtained in the open literature. R-J correlation and Copper correlation are found to be the optimum correlation for the smooth tube and the enhanced tube with the mean relative error of 10.93% and 11.48%, respectively. Findings from this study can supply a fundamental basis for validation and improvement propane heat transfer performances for the intermediate fluid vaporizer (IFV) engineering design.

Heat Transfer Characteristics of Pool Boiling under Low Pressure Condition

Heat Transfer Characteristics of Pool Boiling under Low Pressure Condition

Molecular dynamics simulation on the effect of nanoparticles on the heat transfer characteristics of pool boiling

ABSTRACTMolecular dynamics simulation was performed to investigate pool boiling of nanofluids on the metal wall. Nanoparticles were placed near the wall. Results showed that with the addition of nanoparticles the fluid temperature, net evaporation number and heat flux were increased, indicating that the boiling heat transfer was enhanced. In addition, the nanoparticles were able to move around the wall disorderly but did not move with the fluid. The effects of heated temperature and nanoparticle size on the boiling heat transfer were also investigated. By increasing heated temperature and nanoparticle size, the boiling heat transfer enhancement increased.

Heat transfer characteristics of pool boiling under low pressure condition

Heat transfer characteristics of pool boiling under low pressure condition

Bubble growth, departure and rising of ethane at nucleate pool boiling condition

Boiling heat transfer exists in many industrial heat exchangers such as the evaporator of the refrigerator and the air conditioner, the reboiler of the chemical equipment and steam generator of the nuclear reactor. In the process of boiling, bubble movement can speed up the heat transfer between liquid and solid, leading to the improvement in heat transfer efficiency. Traditional refrigerants used in the industrial facilities have serious environmental problems, therefore, new substitutes are needed. Hydrocarbons are good alternatives. Ethane was suggested as one component of a mixed- refrigerant for its zero ozone depletion potential, low global warming potential and high thermodynamic properties. Therefore, it is important to investigate the bubble behavior of ethane in saturated nucleate pool boiling to design and optimize the industrial equipment like evaporator, improve natural gas liquefaction technology, and research characteristics of heat transfer and multiphase flow as well. Although there are a lot of researches on the heat transfer characteristics of pool boiling, the research on the bubble behavior in ethane is still lacking. In this paper, visualization experiments on the pool boiling heat transfer characteristics of ethane were carried out at 0.2 MPa. The heat fluxes varied from 14.65 kW m−2 to 80.79 kW m−2. Boiling occurred on the upward facing side of a smooth vertical copper cylinder with 20 mm diameter, connected to a DC power supply to control the heat flux on the surface. The measured roughness of the surface was 68.1 nm (root mean square average of the height deviations) and 50.7 nm (the arithmetic average of the absolute values of the surface height deviations). Acquisition of the temperature and pressure was accomplished by using a Keithley 2700 and six thermocouples. The experimental apparatus has a steel ruler inside the pool as the reference length to measure the bubble sizes and the maximum uncertainty for length measurement is 0.060 mm. Some conclusions can be drawn. (1) According to the change of diameter with time, the growth process was divided into three stages. The bubble growth process has obvious segmentation characteristics, and piecewise prediction models are recommended. (2) The bubbles in boiling pool exhibit the characteristics of axial symmetry and uneven distribution; the shape of separation of ethane bubbles has ellipsoidal and irregular shapes relative to normal temperature liquids. (3) There were trail changes and additional movements of large bubbles. Compared to normal temperature liquids, the trail changes were relatively simple; and the bubbles are bubbly and slug flow of two flow regimes. (4) System pressure, heating wall properties like surface roughness, liquid properties like contact angle and surface tension have an important influence on bubble behavior. This paper suggests that the heat transfer prediction equation should consider them.

Numerical study of pool boiling heat transfer in a large-scale confined space

The article aims to numerically investigate the heat transfer characteristics of pool boiling on large-scale tube bundles, according to the VOF method. We build a numerical model of single tube in a rectangle confined space to simulate the tube bundles based on the Passive Residual Heat Removal Heat Exchanger, which is applied in nuclear power. The numerical model presents the solid substrate coupled heat transfer between the convection in tube and boiling outside tube. We analyze the dynamic and local characteristic of boiling heat transfer along the tube and the effect of the tube pitch on the heat transfer. Heat transfer characteristics vary along the tube vertical direction. The time-average heat fluxes outside surface increase along the tube. The numerical results are compared with the classical correlation and reliable to predict the pool boiling heat transfer for tubes in the intermediate region of bundle.