Liquid Saturation Temperature Research Articles

Power law exponents for single bubbles growth in nucleate pool boiling at zero gravity

Nucleate pool boiling is an important boiling mode used in various fields of application. Bubble detachment and new bubble generation at the nucleation site is an important aspect of the process under terrestrial conditions. This sequence prevents prolonged observation of bubble growth phenomena. The problem can be circumvented by performing single bubble growth experiments under zero gravity conditions. Many experiments of this type are performed in a specially designed cell on board the International Space Station. Several combinations of boiling experimental parameters (pressure, heat flux, waiting time, degrees of subcooling) are tested. The bubble size evolution curve was found to represent the influences off the individual parameters best. Fitting attempts show that the experimental bubble growth curves can be nicely described by power law correlations. An analytical solution is given for the evolution of the liquid temperature field within the experimental test cell, in the absence of boiling incidents. It is shown that the time evolving difference between the solid/liquid interfacial temperature and the liquid saturation temperature is the basic factor that determines the bubble growth exponent. As an outcome, the whole experimental information on the bubble growth curves of 160 different boiling conditions is compressed in the form of tables that include the corresponding power law factors. These tables not only allow the reconstruction of the examined boiling curves but can also generate boiling curves in a dense grid of boiling conditions through interpolation of existing boiling data. The latter offers coherence and continuity in boiling computer codes over a broad range of conditions.

Boiling regimes of a single droplet impinging on a superheated surface: Effect of the surrounding medium

Impinging droplets on a hot surface is a significant phenomenon in many industrial applications, such as spray cooling. Previous studies reported various morphological behaviours affecting the droplet's wetting, heat transfer, and evaporation characteristics. However, there is an ambiguity in understanding the influence of the surrounding medium on the resulting boiling regimes. The present work aims to study this aspect through an experimental investigation using high-speed photography and infrared thermography. During the experiments, the degassed FC-72 liquid droplets have impinged onto a heated chromium-plated calcium fluoride (CaF2) surface with a constant impact velocity. Three ambient conditions, such as saturated vapour (heated vapour), air at liquid saturation temperature (heated air), and air at room temperature (ambient air), are used. The surface temperature is varied in each ambience until the complete droplet dewetting, i.e., the Leidenfrost phenomenon has appeared. In this study, the stages of boiling are perceived as film evaporation, excessive and nominal bubbly boiling, transition, and Leidenfrost. With different surrounding media, the onset of a regime and its boiling morphology are distinct. During the film evaporation regime, the droplet heat transfer is high for the impact in ambient air, and the contact line evaporation is dominant in the vapour medium. In contrast, heat transfer is improved in a vapour medium during the bubbly boiling process, reaching up to 30% enhancement compared to the droplets in air surroundings. It is attributed to increased nucleation sites and excessive bubbling compared to air media. Correlations from the literature are also revisited to understand the onset of bubbly boiling and the droplet breakup at the Leidenfrost point. An overall comparison of droplet heat transfer during the boiling regimes in surrounding media is discussed in this paper.

Numerical study of drop spread and rebound on heated surfaces with consideration of high pressure

The impact of a drop on a solid surface has been studied for many years. However, most of the previous numerical simulations were focused on the drop impact on a surface at room temperature and standard atmospheric pressure. This paper presents a numerical study of n-heptane and n-decane drops impacting solid surfaces with the consideration of high temperature and high pressure using smoothed particle hydrodynamics (SPH). The SPH method is validated against experiments from our work and literature. This work is focused on two typical drop-impact regimes, namely, spread and rebound. Different drop impact sequences were simulated at the wall temperature in the range of 27–400 °C and the ambient pressure between 1–20 bars. The difference between the inception of film boiling and liquid saturation temperature was found to decrease with elevating ambient pressure. The spread factor and apex height are investigated for the regime of spread. The results indicate that the lower viscosity fluid has a smaller spread factor as compared to the fluid with higher viscosity. The variation of Leidenfrost temperature with ambient pressure for both n-heptane and n-decane droplets is established numerically and compared with the trend observed in the experiment. The simulation outcomes of drop rebound for high boiling point liquid (n-decane) in the film boiling regime at atmospheric pressure show that with the increasing wall temperature, the drop rebound height and vapor layer height increase. Finally, the effect of ambient pressure on drop rebound height and velocity is investigated. The numerical results indicate that the increase in ambient pressure reduces the droplet rebound velocity and rebound height.

A high-fidelity approach to correlate the nucleate pool boiling data of roughened surfaces

The existing nucleate pool boiling correlations have theoretical footings and their usefulness is restricted by the failure to effectively account for the surface effect. To tackle this problem, a high-fidelity approach based on deep learning has been developed to predict the nucleate boiling surfaces subject to various surface roughness. The proposed model accounts for the effect of surface roughness, roughness fabrication method, surface material, surface inclination, saturation temperature, and pressure on the pool boiling performance of dielectric liquids, water, and refrigerants. The proposed method can accurately predict the boiling heat transfer performance of roughened surfaces by including the most influential surface characteristics, testing conditions, and liquid thermophysical properties into the architecture of the developed model. Correlation matrix identifies that heat flux, surface inclination, surface roughness, thermal conductivity of surface material, liquid saturation temperature, and pressure are the prime factors to affect the nucleate boiling heat transfer coefficient. Different neural networks (DNNs) are built and tested in order to find an optimal model based on an experimental dataset of 13000 data points. The final selected model can estimate the investigated parameter with a coefficient of determination (R2) = 0.994 and mean absolute error (MAE) = 0.65. The suggested method can be utilized to predict the boiling heat transfer performance of a variety of roughened surfaces subject to different working fluids and testing conditions.

Heat transfer mechanisms of single droplet impacting onto flowing film

The experimental study of heat transfer and film dynamics of droplet impacting onto flowing film using Novec-7100 as the cooling liquid was presented in this article. The droplet velocity and the liquid film velocity could be adjusted in this experimental study. The Reynolds number of the flowing liquid film varied from 1257 to 6290. In this study, the substrate of heater was Zinc Selenide (ZnSe), and the heating element was a thin layer of Indium Tin Oxide (ITO). The overflow plate was used to make the liquid film thickness fixed at 1 mm. The heat transfer mechanisms of flowing liquid film (flowing film only), droplet impinging on the dry surface (droplet-dry surface) and droplet impinging on the flowing liquid film (droplet-flowing film) were discussed in this paper respectively. The results showed that when the averaged temperature of the heated surface does not reach the liquid saturation temperature, bubbles appear in the liquid film, and the bubbles are more dense downstream. When the averaged heat flux of the heated surface ranges from 6.2 × 103 W/m2 to 3.4 × 104 W/m2, the averaged heat transfer coefficient of the droplet-flowing film can be calculated by the superposition of the averaged heat transfer coefficient of the flowing film only and the averaged heat transfer coefficient of the droplet-dry surface, the maximum error is 14%. When the averaged heat flux of the heated surface ranges from 3.4 × 104 W/m2 to 6.8 × 104 W/m2, the Reynolds number of the flowing liquid film and the effect of the coupling between droplet impingement and flow boiling should be considered in the calculation of the averaged heat transfer coefficient of the droplet-flowing film. In addition, boiling heat transfer is the main heat transfer mechanism of droplet impinging on thin liquid film when the averaged heat flux of the heated surface ranges from 2 × 104 W/m2 to 6.8 × 104 W/m2.

Dynamics of propagation of the self-sustaining evaporation front in a mixture of Freons under the natural convection conditions

The results of experimental studies on dynamics of propagation of the self-sustaining evaporation front at nonstationary heat release are presented in the paper. The experiments were carried out in a large volume at liquid saturation temperature and subcooling on a horizontal cylindrical heat-releasing surface with the diameter of 3 mm under nonstationary heat release. Freons R21, R114 and their mixtures were used as the working liquid. The experiments carried out in the range of concentrations of high-volatile component 0 C C = 0.62). At a fixed value of the temperature head in the range of high values of Δ T , liquid subcooling relative to saturation line leads to a decrease in the velocity of evaporation front propagation.

Computational study of a 2D capillary pump evaporator

Capillary pump loop (CPL) is a two-phase heat transfer device considered as a useful solution for thermal control applications in spacecrafts, satellites and electronic components. The purpose of this paper is to study various aspects of the working state of the CPL evaporator. A two-dimensional computational model was developed in order to analyze the flow and the phase transformation inside a cylindrical evaporator. At the present analysis, different cases were studied by changing liquid saturation temperature, inclination angle of the evaporator and the length of the porous heated wick. Water, ammonia, acetone and freon-134 were used as working liquids for numerical experiments. Results present the evaporator capability to vaporize each working liquid and find out its possibility of dry-out causing failure. This information is useful in choosing the best working liquid for an evaporator, enabling bigger amounts of heat to be transferred.

Influence of the Wettability on the Boiling Onset

Experimental investigation of pool boiling is conducted in stationary conditions over very smooth bronze surfaces covered by a very thin layer of gold presenting various surface treatments to isolate the role of wettability. We show that even with surfaces presenting mean roughness amplitudes below 10 nm the role of surface topography is of importance. The study shows also that wettability alone can trigger the boiling and that the boiling position on the surface can be controlled by chemical grafting using for instance alkanethiol. Moreover, boiling curves, that is, heat flux versus the surface superheat (which is the difference between the solid surface temperature and the liquid saturation temperature), are recorded and enabled to quantify, for this case, the significant reduction of the superheat at the onset of incipient boiling due to wettability.

Heat and mass transfer of single droplet/wet particle drying

A theoretical model, which considers the fully unsteady character of both heat and mass transfer during the drying of single droplet/wet particle, is presented. The model enables prediction of pressure and fraction distributions of air–vapour mixture within the capillary pores of the wet particle crust. The simulations of the drying process of a single silica droplet under different conditions show a permanent rising of pressure within the capillary pores, but the corresponding vapour fraction remains less than unity. The comparison between the drying histories of the silica droplet, predicted by the present model with the data, calculated by the model which assumes a quasi-steady-state mass transfer and linear pressure profile within the capillary pores, shows inconsiderable differences between the droplet/wet particle temperature and mass time-changes. At the same time, the present model predicts pressure build-up and temperature rising within the particle wet core. However, in the studied cases the temperature of the wet core temperature does not exceed the liquid saturation temperature and therefore no boiling of liquid within the particle wet core is observed.

Combined effects of sub-cooling and operating pressure on the performance of a two-chamber thermosyphon

The heat dissipation rates at the chip level are projected to reach the 50-100 W/cm/sup 2/ mark for some future high performance electronic systems. Liquid cooling with phase change has been demonstrated to be a very efficient technique for thermal management of such high heat dissipation rates. Past work on liquid immersion cooling using fluorocarbons has shown the advantage of using enhanced structures to reduce boiling incipience excursion and raise the critical heat flux (CHF). Thermosyphons, employing these enhanced structures are an alternative to liquid immersion and are suitable for point cooling applications, where very compact evaporators are needed. This study investigates the combined effect of sub-cooling and pressure on the performance of an enhanced microstructure based thermosyphon, which has shown very high heat transfer rates (up to 100 W/cm/sup 2/ with a wall superheat of 27.8/spl deg/C). The pressure levels tested were partial vacuum (40-101.3 kPa), atmospheric pressure (101.3 kPa) and high pressure (101.3-370 kPa). The experiments were initiated at room temperature, and hence the sub-cooling corresponded to the difference in the liquid saturation temperature at the starting system pressure and room temperature. The results show a reduction in wall superheat values at higher pressures, at a given heat flux. The performance of the system was evaluated by defining a surface-to-ambient resistance. Results show that a partial vacuum at all heat fluxes results in better performance compared to higher pressures. The combined effect of pressure and sub-cooling was also tested for a compact evaporator and the results obtained were similar to the baseline case (larger evaporator).

Heat Transfer Characteristics in Partial Boiling, Fully Developed Boiling, and Significant Void Flow Regions of Subcooled Flow Boiling

Subcooled flow boiling covers the region beginning from the location where the wall temperature exceeds the local liquid saturation temperature to the location where the thermodynamic quality reaches zero, corresponding to the saturated liquid state. Three locations in the subcooled flow have been identified by earlier investigators as the onset of nucleate boiling, the point of net vapor generation, and the location where x = 0 is attained from enthalpy balance equations. The heat transfer regions are identified as the single-phase heat transfer prior to ONB, partial boiling (PB), and fully developed boiling (FDB). A new region is identified here as the significant void flow (SVF) region. Available models for predicting the heat transfer coefficient in different regions are evaluated and new models are developed based on our current understanding. The results are compared with some of the experimental data available in the literature.

Contact angle temperature dependence for water droplets on practical aluminum surfaces

Abstract This paper presents an experimental investigation of the temperature dependence of the quasistatic advancing contact angle of water on an aluminum surface polished in accordance with surface preparation techniques commonly employed in boiling heat transfer studies. The surface, speculated to contain aluminum oxide and organic residue left behind from the polishing process, was characterized with scanning electron microscopy, surface contact profilometry, and ellipsometry. By utilizing a pressure vessel to raise the liquid saturation temperature, contact angles were measured with the sessile drop technique for surface temperatures ranging from 25 to 170°C and pressures from 101.3 to 827.4 kPa. Two distinct temperature-dependent regimes were observed. In the lower temperature regime, below 120°C, a relatively constant contact angle of 90° was observed. In the high temperature regime, above 120°C, the contact angle decreased in a fairly linear manner. Empirical correlations were developed to describe this behavior which emulated previous experimental data for nonmetallic surfaces as well as theoretical trends.

Oscillation of a liquid-vapor interface during flow film boiling

For both pool and flow film boiling, the liquid-vapor interface is rarely stationary, stable film boiling exhibits a continuously wavy interface with an amplitude of oscillation dependent on parameters such as liquid saturation temperature, amount of superheat, amount of subcooling, liquid velocity, and heater geometry. The present analysis treats from a theoretical point of view stagnation point flow film boiling on a sphere and on a cylinder immersed in a subcooled liquid. The effects of system parameters on the dynamic behavior as well as the response of a liquid-vapor interface to changes in the temperature and velocity fields are investigated.