A Comparative Analysis of Evaporative Heat Transfer Effect in Nucleate Pool Boiling Process on Copper Substrate (original) (raw)

Experimental Study for Investigating the Mechanism of Heat Transfer Near the Critical Heat Flux in Nucleate Pool Boiling

International Journal of Engineering

The regime of nucleate pool boiling near critical heat flux (60-98% of CHF) is known as vapor mushroom regime. Understanding the mechanism of heat transfer in regime of vapor mushroom of nucleate pool boiling is not only helpful to explain high rate heat transfer, but also useful to explain boiling crisis phenomenon. In this paper, an experimental setup is designed and fabricated to study the mechanism of heat transfer from boiling surface to bulk liquid near critical heat flux (vapor mushroom regime). In addition, vapor mass frequency and thickness of macro-layer are measured at various heat fluxes in this regime of boiling. The experimental study reveals that individual bubbles coalesce due to very high bubble site density and form vapor mass entrapping a relatively thicker film of liquid known as 'macro-layer' between the growing vapor mass and the heating surface of pool boiling near the high heat flux region. The rate of evaporation of macro-layer and transient conduction through macro-layer is the prime parameter to transfer heat from the heated surface. The thickness of the macro-layer is found to be lying in the range of 153-88 µm, respectively, for range of 60-98% critical heat flux. As the heat flux increases, the thickness of macro-layer decreases. The vapor mass frequency varies from 4 to 8.80 Hz for water in the range of 60-98 % of critical heat flux. The vapor mass frequency increases as heat flux increases due to higher evaporation rate associated with higher heat flux. The data reported in this manuscript are more consistent as comparing to data available in the literature and these data are useful in modeling heat transfer in nucleate pool boiling near critical heat flux.

Numerical investigation of nucleate pool boiling heat transfer

Thermal Science, 2016

Multi-dimensional numerical simulation of the atmospheric saturated pool boiling is performed. The applied modelling and numerical methods enable a full representation of the liquid and vapour two-phase mixture behaviour on the heated surface, with included prediction of the swell level and heated wall temperature field. In this way the integral behaviour of nucleate pool boiling is simulated. The micro conditions of bubble generation at the heated wall surface are modelled by the bubble nucleation site density, the liquid wetting contact angle and the bubble grow time. The bubble nucleation sites are randomly located within zones of equal size, where the number of zones equals the nucleation site density. The conjugate heat transfer from the heated wall to the liquid is taken into account in wetted heated wall areas around bubble nucleation sites. The boiling curve relation between the heat flux and the heated wall surface temperature in excess of the saturation temperature is predicted for the pool boiling conditions reported in the literature and a good agreement is achieved with experimentally measured data. The influence of the nucleation site density on the boiling curve characteristic is confirmed. In addition, the influence of the heat flux intensity on the spatial effects of vapour generation and two-phase flow are shown, such as the increase of the swell level position and the reduced wetting of the heated wall surface by the heat flux increase.

Numerical investigation of nucleate boiling heat transfer on thin substrates

International Journal of Heat and Mass Transfer, 2014

The objective of this paper is to define the guidelines for the design of new boiling test sections with a large number of artificial nucleation sites during nucleate boiling for thin substrates horizontally immersed in a saturated liquid with artificial cavities located on the upper surface. The findings of numerical simulations of pool boiling heat transfer for a single bubble and for a large number of nucleation sites based on the analysis of experimental cases were analysed. Dedicated test sections were used in experiments for the study of boiling mechanisms and interactions between active sites so that the numerical models representing the physics of the problem could be improved. The hybrid nature of the code used in this study, combining the complete solution of the three-dimensional time-dependent energy equation in the solid substrate with semi-empirical models representing the physical phenomena occurring in the liquid side, in a simplified way, allows a large number of simulations in a reasonable computational time. The present paper focuses in the first part on the capability of the model to reproduce the experimental results for various conditions, while in the second part, the results for a large number of nucleation sites are analysed. Regarding the single bubble growth, two series of simulations will be presented in this paper: the first one analyses the mechanisms of nucleate boiling on a silicon substrate immersed in the dielectric fluid FC-72. The second series studies the behaviour of bubbles on metallic substrates, platinum and titanium, in saturated water. In the last section, the effect of the position of a site during simulations of a large population of sites (of the order of 100) on the waiting time, growth time, type and occurrence of coalescence and the thermal characteristics is presented.

Improvement in Heat Transfer Characteristic of Nucleate-Pool Boiling of Water

Now-a-days, to save energy and to save environment are the great issues of the world.The enhancementof boiling heat transfer is one of the most important andadvanced research fields to meet up the partial crisis of energy. But the way of research should beenvironmentally friendly. Various inferences have been drawn based on the existing parameters like operating pressure, temperature, and types ofworking fluids are Nano fluids by different researchers for enhancement of heat transferrate. Here is an experimental study which is conducted to investigate the pool boilingheat transfer of environmentally friendly water as base fluid and nichrome wire as aheating surfaces. The goal of the Experiment was to understand the characteristics anddesign of a pool boiling heat transfer, as well as evaluate the effect of addition of wateras a base fluid, in nucleate pool boiling heat transfer coefficient. The experiments werecarried out a nichrome wire as a heating surface.The results showed that, the heat transfer coefficient increases with increasing thickness of the nichrome wire.

Bubble dynamics and nucleate pool boiling heat transfer on microporous copper surfaces

International Journal of Heat and Mass Transfer, 2015

Nucleate pool boiling experiments were performed on microporous copper surfaces and plain surfaces using saturated HFE-7100 as the working fluid. Quantitative measurements of the bubble dynamics, such as the nucleation site density, bubble diameter at departure, and bubble departure frequency, were obtained using high-speed visualization. The microporous surfaces, with coating thicknesses in the range of 100 µm to 700 µm, porosity of 55% to 60%, and cavity sizes in the range of 0.5 µm to 5 µm, showed a significantly lower boiling incipience temperature, which enhanced the heat transfer coefficient by 50% to 270% and enhanced the critical heat fluxes by 33% to 60% when compared to the plain surface. At low heat flux levels, the surface with a thicker microporous coating showed better performance than the thinner one. However, the thinner microporous coating resulted in higher critical heat flux than the thicker surface. The site density, departure diameter, and departure frequency were compared against the

Experimental studies on nucleate pool boiling heat transfer enhancement for composite nano-structure coated copper heating surface

Journal of physics, 2019

The present paper is based on experimental studies on nucleate pool boiling heat transfer enhancement of different surfaces using water as a base fluid at atmospheric pressure. The test surfaces for the experiments include untreated, treated, and treated with Aluminumsilver oxide composite thin film surfaces having nano-layer thickness of 180 nm and 260 nm. The thin films are prepared on copper substrate by electron beam evaporation technique. The characterization of the heated surfaces is done by using optical surface profilometer for surface roughness and sessile drop method for contact angle measurement. The experiment is conducted in a closed boiling chamber and the heat flux is varied from 141.524-1244.101 kW/m 2 in time steps. The enhancement of heat transfer coefficient is found as 22.8%, 17.27% and 11.81% from the 260 nm, 180 nm composite nanostructured coated and treated surfaces respectively compared to plain surface. Enhancement in nanostructured coated surfaces is found higher due to the capillary effect, increased wettability and high active nucleate site density and the increased rate of bubble frequency.

Numerical study of heat transfer during nucleate pool boiling

Advanced technologies, 2016

In this paper three-dimensional numerical simulation of the atmospheric saturated pool boiling was performed. The applied modelling and numerical methods enable full representation of the two-phase mixture behaviour on the heating surface with the inclusion of the swell level prediction. The three-dimensional investigation presented here was performed in order to take into account a convective heat transfer on the heated surface, as well as spatial effects of the vapour generation and a twophase flow such as phase dispersion within the two-phase mixture. The results are presented for a short period of time after the initiation of the heat supply and vapor generation on the heating surface. The replenishment of the heating surface with water and partial surface wetting for lower heat fluxes is shown. The influence of the density of nucleation sites and the bubble residence time on the wall on the pool boiling dynamics is discussed. Also, the influence of the heat flux intensity on the pool boiling dynamics is investigated. The applied numerical and modelling method showed robustness by allowing stable calculations for wide ranges of applied modelling boiling parameters.

Experimental investigation on nucleate pool boiling heat transfer enhancement for nano-structured copper oxide coated heating surface

Journal of Physics: Conference Series, 2019

The present paper is based on experimental studies of augmentation of pool boiling heat transfer characteristics of unlike surfaces using water as a working fluid at atmospheric pressure. The test surfaces for the experiments include untreated, treated, copper oxide (CuO) thin film coated copper heating surfaces having coating thickness of 200 nm and 400 nm. The thin film coating is fabricated by sol-gel spin coating technique. The characterization of surfaces is done by considering wettability, surface roughness and topography study by the sessile droplet method, optical surface profile meter and X-ray diffraction [XRD]. The experiment is conducted in a closed boiling chamber and heat flux varied from 526.3 kW/m2 to 2546.689 kW/m2. The augmentation of heat transfer coefficients is found more than 40.60% of the higher thickness of copper oxide thin film coated copper heating surfaces. This is happened due to enhanced wettability, roughness and increase in active nucleation site dens...

Experimental study of pool temperature effects on nucleate pool boiling

International Journal of Multiphase Flow, 2006

Nucleate pool boiling experiments with constant wall temperature were performed using pure R113 for subcooled, saturated, and superheated pool conditions. A microscale heater array and Wheatstone bridge circuits were used to maintain the constant wall temperature and to measure the instantaneous heat flow rate accurately with high temporal and spatial resolutions. Images of bubble growth were taken at 5000 frames per second using a high-speed CCD camera synchronized with the heat flow rate measurements. The bubble geometry was obtained from the captured bubble images. The effect of the pool conditions on the bubble growth behavior was analyzed using dimensionless parameters for the initial and thermal growth regions. The effect of the pool conditions on the heat flow rate behavior was also examined. The bubble growth behaviors during subcooled, saturated, and superheated pool boiling were analyzed using a modified Jakob number that we newly defined. Dimensionless time and bubble radius parameters with the modified Jakob number characterized the bubble growth behavior well. These phenomena require further analysis for various pool temperature conditions, and this study will provide good experimental data with precise constant wall temperature boundary condition for such works.