Subcooled Pool Boiling Heat Transfer Research Articles

Subcooled pool boiling heat transfer enhancement on hierarchically biomimetic laser-structured surfaces with gradient pores

Subcooled pool boiling heat transfer enhancement on hierarchically biomimetic laser-structured surfaces with gradient pores

Experimental study on subcooled pool boiling heat transfer under hypergravity

The pool boiling heat transfer experiment was conducted with pure water under subcooled and saturation conditions, both in hypergravity (1–3 g) and normal gravity (1 g) at atmospheric pressure. A turntable was employed to simulate the hypergravity environment. Subcooling was varied at four different levels: 2 °C, 5 °C, 8 °C, and 10 °C, corresponding to gravity levels ranging from 1 g to 3 g. Heat transfer enhancement was observed in the fully developed region under saturation conditions, whereas it was less pronounced in the natural convection region. Specifically, increasing the subcooling degrees leads to an expansion of the natural convection region, where the effects of hypergravity become noticeable at higher heat flux levels. The effect of hypergravity diminishes as the bubble detachment diameter in the fully developed region decreases with increasing subcooling degree, implying that the main effect of hypergravity on heat transfer is due to its influence on the bubble behavior. Additionally, a detailed analysis is presented to further explain the effect of hypergravity on heat transfer, focusing on the bubble dynamical behavior and the impact of buoyancy under hypergravity conditions.

Saturated and subcooled pool boiling heat transfer in mixtures of water and glycerin

ABSTRACT Heat transfer coefficient (HTC) was experimentally measured for saturated and subcooled pool boiling of binary mixtures of water and glycerin. Saturated boiling was studied for mixtures with water mass fractions from to on horizontal flat nickel-plated surfaces at heat fluxes from 50 to at atmospheric pressure. Subcooled boiling was investigated in the range of subcooling from 0 to at heat fluxes of approximately 250, 450 and . It was found that mixture effects have a significant impact on saturated boiling HTC even for mixtures with very low content of glycerin as significant drops of HTC were observed for subtle changes in composition for mixtures of high . Measured HTC was successfully correlated with the combination of Yagov (1999) and Inoue and Monde (2009) correlations with a mean relative error of . A simple empirical HTC correlation is also proposed. For subcooled boiling, developed subcooled boiling regime was reached for all investigated heat fluxes. For this regime, correlations, which were able to predict HTC for saturated boiling, were employed to predict subcooled boiling HTCs for all investigated concentrations, heat fluxes and subcoolings. Effect of subcooling and effect of liquid composition on total HTC were of the same importance for mixtures with higher water content. With the increase in concentration of glycerin in the mixture, decrease in total HTC with increasing subcooling became more significant.

Wettability effect on pool boiling heat transfer using a multiscale copper foam surface

To improve thermal performance, the wettability effect on pool boiling heat transfer using copper foam is experimentally studied. A surface oxidation and chemical modification method is employed to modify copper foam surface’s wettability. After wettability treatment, the copper foam surface is covered with nanosheet. The average contact angle on 50 PPI super-hydrophobic and super-hydrophilic copper foam surface is 148.7° and nearly 0°, respectively. An experimental platform regarding the thermal performance of subcooled pool boiling heat transfer for deionized water on copper foam with a modified wettability surface is conducted. Results showed that the super-hydrophilic copper foam’s surface achieves better boiling heat transfer performance in a medium- or high-heat flux region (q ≥ 20 W⋅cm−2), while super hydrophobic copper foam surface shows a better performance when q < 20 W⋅cm−2. The pool boiling enhancement of wettability-modified surface has been verified compared with flat surface from public work. The maximum heat transfer coefficient in this experiment reaches 19,238 W⋅m−2⋅K−1 for super-hydrophilic surface (50 PPI, 2 mm thickness) at a heat flux of 96 W⋅cm−2. The bubble dynamic behavior during pool boiling is observed through a high-speed camera. The visualization showed that the shape, size and number of bubbles on super-hydrophilic copper foam surface during pool boiling are different from that on super-hydrophobic copper foam surface due to different adhesion force and surface tension. As copper foam thickness increases, more bubbles are trapped in the copper foam. By analyzing the heat transfer mechanism in copper foam, it is deduced that the specific copper foam structure conveys a larger heat transfer area, and meanwhile, the micro-nanostructure provides an abundance of nucleation sites. Moreover, a super-hydrophilic surface could promote liquid replenishment for the copper foam. Therefore, wettability-modified copper foam shows great promise in thermal management.

Pool boiling heat transfer of FC-72 on pin-fin silicon surfaces with nanoparticle deposition

In the present study, two types of micro-pin–fin configurations were fabricated on silicon surfaces by a dry etching method, i.e., staggered pin fins (#1) and aligned pin fins with empty areas (#2). The micro-pin–fin surfaces were then further modified by depositing FeMn oxide nanoparticles (∼35 nm) electrostatically for 8 h and 16 h, respectively, namely #1-8h, #1-16h, #2-8h and #2-16h. Subcooled pool boiling heat transfer was experimentally studied on these surfaces at atmospheric pressure, using FC-72 as the working fluid. The results showed that in comparison to the smooth surface, pool boiling heat transfer was significantly enhanced by the micro-pin-fin surfaces and the maximum superheat was considerably decreased. Additionally, critical heat fluxes were also greatly improved, e.g., the critical heat flux on #1 was almost twice of that on the smooth surface. Generally, the nanoparticle deposition could further enhance pool boiling heat transfer, including the heat transfer coefficient and critical heat flux (CHF). High speed visualizations were taken to explore the mechanisms behind the heat transfer performance. The bubble behavior on the micro-pin–fin surfaces with and without nanoparticles was compared at low, moderate and high heat fluxes, respectively. The wickability of FC-72 on the test surfaces was measured, based on which, a modified CHF model was proposed to predict the experimental CHFs. Accordingly, a possible mechanism of CHF enhancement was described.

Experimental study of the heater size effect on subcooled pool boiling heat transfer of FC-72 in microgravity

Experiments of highly subcooled nucleate pool boiling of FC-72 with dissolved air were conducted on a large scale silicon chip (2×2×0.05cm3, denoted as chip S 2×2) in short-term microgravity condition and normal gravity condition by utilizing the drop tower in Beijing respectively. The results were compared with published results of a smaller scale chip (1×1×0.05cm3, denoted as chip S 1×1) both in normal gravity condition and microgravity condition, to study the heater size effect on boiling heat transfer. It is indicated that in microgravity, the input heat flux range in which bubble departure took place for chip S 2×2 is wider than that for chip S 1×1, and an interesting phenomenon observed is that coalesced bubbles departed at a continuously smaller radii at a given heat flux. Initially, the average bubble departure radii for chip S 2×2 increased linearly with heat flux while later remained constant. At a same heat flux, chip S 2×2 showed a bubble departure radius larger than that of chip S 1×1. By comparison, it is found that nucleate boiling performance deteriorates with increase in heater size in both earth gravity condition and microgravity condition. However, in microgravity the qCHF of chip S 2×2 is 20% greater than that of chip S 1×1, contrary to the CHF characteristic in earth ground condition. Moreover, boiling patterns in microgravity are different in high heat flux region: a smooth hemispherical bubble was generated on chip S 1×1, while on chip S 2×2 an oblate vapor blanket was formed, which indicates that different dominated boiling heat transfer mechanisms exist in both cases. It is found that in microgravity, the boiling was dominated by buoyancy for chip S 2×2, but it was in surface tension dominated boiling regime for chip S 1×1, which proved the above speculation. Moreover, the range of transition heater size criteria is 1.45<LhLc<2.89 in the present work. By using the updated model developed by Raj and Kim, it is discovered that predicted values are generally lower than experimental values in both experiments. Moreover, differences were more evident for chip S 2×2.

Subcooled pool boiling heat transfer in fractal nanofluids: A novel analytical model

A novel analytical model to determine the heat flux of subcooled pool boiling in fractal nanofluids is developed. The model considers the fractal character of nanofluids in terms of the fractal dimension of nanoparticles and the fractal dimension of active cavities on the heated surfaces; it also takes into account the effect of the Brownian motion of nanoparticles, which has no empirical constant but has parameters with physical meanings. The proposed model is expressed as a function of the subcooling of fluids and the wall superheat. The fractal analytical model is verified by a reasonable agreement with the experimental data and the results obtained from existing models.

Effect of inclination angle on the pool boiling heat transfer of ultra-light copper foams

Effect of inclination angles on the pool boiling heat transfer on ultra-light copper foam covers was studied using acetone as the working fluid. The inclination angle was from 0° to 90°. It is found that copper foam covers decrease the surface superheat at the onset of nucleate boiling and extend the operation ranges of surface superheats and heat fluxes, significantly. Boiling curves are crossed between low and high inclination angles. Heat transfer coefficients are increased, attain maximum values, and then are decreased with continuous increases in heat fluxes. The thermal performance is very insensitive to inclination angles at low pool liquid temperatures. The thermal performance is better for the saturation pool boiling heat transfer at small surface superheats, but it is better for the subcooled pool boiling heat transfer at high surface superheats. The Nusselt number is well correlated using the 812 data points, with the maximum error of 20%.

Heater size and heater aspect ratio effects on subcooled pool boiling heat transfer in low-g

Pool boiling heat transfer measurements using heaters of varying aspect ratio were obtained in low-g (0.01 g ± 0.025 g) aboard the KC-135 aircraft. The heater aspect ratio was varied by selectively powering 2 × 2, 2 × 4, 2 × 6, 2 × 8, and 2 × 10 arrays of heaters in a 10 × 10 heater array containing individual heaters 0.7 × 0.7 mm 2 in size. Electronic control circuitry was used to maintain an isothermal boundary condition on the heater surface while the power dissipated by the heater was measured. The working fluid was FC-72 at 101 kPa and three different bulk subcoolings. Low-g boiling behavior was governed by the dynamics of the primary bubble. For both square and rectangular heaters, CHF appeared to be a result of the competition between increasing heat transfer associated with the satellite bubbles and the decrease in heat transfer due to growth of the dry area under the primary bubble as the wall superheat increases. At low subcooling on rectangular heaters, surface tension acted to pull the bubble into a spherical shape, allowing liquid to rewet the surface. At high subcooling and high superheat, thermocapillary convection caused the large bubbles that formed on the surface to shrink by increasing the condensation on the bubble cap, resulting in more wetted area. The presence of thermocapillary convection at higher subcooolings may be due to FC-72 being a mixture of various components.

高濡れ性液体の沸騰伝熱現象に関する実験的研究

Thermal energy conversion systems for ships on the environment in ocean are commonly limited by the space or equipment. It is very important to make more effective use of the energy systems such as heat exchangers. For the purpose of the improvement of performance for these energy systems, the utilization of boiling heat transfer is the most effective method to solve these kinds of problems. In this research, steady-state and transient subcooled pool boiling heat transfer was carried out using exponentially increasing heat inputs to a horizontal cylinder in a wetting liquid of ethanol. The photographic observations on the vapor behavior at the boiling heat transfer processes were performed using a high-speed video camera.

Highly subcooled pool boiling heat transfer at various gravity levels

A microscale heater array 2.7 mm×2.7 mm in size along with a video camera was used to provide subcooled pool boiling heat transfer measurements at gravity levels ranging from 1.8 g to 10 −6 g. The fluid was FC-72 at 1 atm and subcooled by nominally 36 °C for all cases. A Terrier–Orion sounding rocket and the KC-135 aircraft were used to provide the microgravity and low- g environments, respectively. The data indicate that there is little effect of gravity on boiling heat transfer at wall superheats below 25 °C, even though there are large differences in bubble behavior between gravity levels. At higher superheats, the low- g data fell significantly below the 1 g and 1.8 g data due to a large part of the heater surface drying out. A large primary bubble was observed to form and move over the surface in low- g, occasionally causing nucleation to occur. This primary bubble was surrounded by smaller bubbles which eventually merged with it. The primary bubble initially formed by the coalescence of smaller bubbles generated on the surface, but then remained constant in size for a given superheat, indicating a balance between evaporation at the bubble base and condensation on the bubble cap. The size of the primary bubble increased with increasing wall superheat. Most of the heaters under the primary bubble indicated low heat transfer, consistent with dryout on the heater surfaces. Strong Marangoni convection around the bubble was observed to develop in low- g, forming a “jet” of heated fluid into the bulk fluid.

223 水平円柱のサブクールプール沸騰熱伝達における蒸気泡の可視化

Subcooled Pool boiling heat transfer from a horizontal wire to distilled water is experimentally studied under an atmospheric pressure condition. The horizontal cylinder is made of platinum. Diameter an length of the wire are 0.8 mm and 59.7 mm respectively. The horizontal wire is heated by a D.C. electric power source. Vaper bubbles on horizontal cylinder is photographed by a frash (25μ s) of storoboscope.