Pool Boiling Critical Heat Flux in Dielectric Liquids and Nanofluids

Many theories predicting the critical (or burnout) heat flux in saturation pool boiling have been proposed during the past decade. Although most are able to correlate existing experimental data, it is evident that the theoretical models upon which they are based are widely different in concept. Moreover, owing to lack of experimental evidence it is not clear which, if any, are based upon the more correct models. Certain of the theories implicitly contain the acceleration due to gravity raised to a different power; by performing a relatively simple experiment it may be possible to accept or reject certain of the theories. An experiment of this type is described in the present paper. On the basis of the results of this experiment it is possible to reject several of the above theories. The extent to which the acceleration of the boiling system is important as a parameter in determining the critical heat flux was ascertained experimentally. A simple centrifugal apparatus is described which was used for determining the critical heat flux in pool boiling to water at atmospheric pressure and accelerations in the range 1 < ( a/g) < 160. A two-variable non-linear correlation involving a logarithmic transformation was made upon the experimental results after allowing for the possible effect of pressure and potential sub-cooling by extrapolation to zero subcooling. The resulting equation is of the form:

Micro/nanostructuring of metal additively manufactured aluminum alloy for enhanced pool boiling of dielectric fluids

Layer-by-layer carbon nanotube coatings for enhanced pool boiling heat transfer on metal surfaces

Time effect on wetting transition of smart surface and prediction of the wetting transition for critical heat flux in pool boiling

The synergistic effect of micro/nanostructure length scale and fluid thermophysical properties on pool boiling heat transfer

An experimental study was carried out to investigate the effects of heat transfer surface orientation and the solid–liquid contact angle on the boiling heat transfer and critical heat flux (CHF) in water pool boiling using a smooth heat-transfer surface under atmospheric pressure. The orientation angle was ranged from 0° (up-facing horizontal position) to 180° (down-facing horizontal position) with a pace of 45°. The three kinds of heat transfer surfaces having different solid–liquid contact angles were the normal surface with a contact angle of 55°, the hydrophilic surface with a contact angle of 30° and the superhydrophilic surface with a contact angle of 0°. The experimental results indicate that orientation and contact angle have complex, coupling effects on heat transfer and CHF. A predicting correlation for the CHF which takes the effects of both orientation and contact angle into account is established. The predicting correlation agrees reasonably well with the experimental data.

Enhanced critical heat flux in pool boiling applying the method of different-mode-interacting boiling for water

Nanofluids are suspensions of nanoparticles with small concentration spread in base fluids such as water, oil and ethylene glycol. Nanofluid boiling is an important research area which provides many chances to explore new frontiers but also poses great challenges. Over the last decade, various studies have been carried out on pool boiling of nanofluids for the enhancement of critical heat flux which is otherwise limited by the use of base fluids. Several efforts have been made in the literature on nanofluid boiling, however, data on the boiling heat transfer coefficient and the critical heat flux have been unpredictable. Current study is a review of the status of research work on effects of nanofluids on heat transfer coefficient and critical heat flux. An emphasis is put in a review form on the recent progresses in nanofluid heat transfer coefficient and critical heat flux of pool boiling. This study also focuses on advancements in nanofluids, their properties and various parameters affecting boiling critical heat flux and heat transfer coefficient. At the end correlations used by different researchers to find out the critical heat flux and heat transfer coefficient are listed.

In order to study the mechanism of critical heat flux of pool boiling, measurement of heat flux fluctuation and high speed photographic observation were carried out simultaneously on a constant temperature surface heated by electrically controlled Joule heating, in the vicinity of a critical heat flux point. They revealed the following : (1) when the surface is covered by a coalesced bubble, heat flux begins to increase because evaporation is enhanced ; (2) heat flux begins to decrease either while the surface is covered by a coalesced bubble due to dryout of the liquid on the surface, or just after nucleation begins under the rising bubble due to a relatively low heat transfer rate of nucleation ; and (3) heat applied from the time the surface is covered by a bubble until the time heat flux begins to ducrease does not vary widely and corresponds to latent heat of liquid about 11 -μm thick.

Using molecular dynamics simulation to investigate the number of wall layers and pyramidal surface roughness on atomic behavior and boiling characteristics of water/Fe nanofluid flow

The electrohydrodynamically enhanced heat transfer in pool boiling in the nucleate regime was studied using R-123 as the working fluid. An experimental apparatus was designed and built which allowed accurate measurements. The evaporator consisted of an electrically heated single horizontal smooth tube. Several different electrode designs were investigated. This study included higher heat fluxes than most of those previously reported in the literature. A summary of the previous work is provided. The results indicated that the heat transfer coefficient at a heat flux of 1.6 kW/m{sup 2} and a voltage of 10 kV was 4.6 times higher than the heat transfer coefficient without the electric field presence. However, the heat transfer coefficient at 52 kW/m{sup 2} and 10 kV was improved only by 38%. These enhancements are significant even at large heat flux levels. The power consumption for establishing the electric fields was on the order of 0.1% of the heat transfer power in the evaporator. When R-123 fluid was contaminated with a few percent ethanol, the boiling heat transfer at 3.3 kW/m{sup 2} was increased by a factor of 12.6 at 15 kV compared to zero kV. Finally, the presence of the electric fields nearly eliminated the hysteresis effect.

Enhancement of the critical heat flux in saturated pool boiling using honeycomb porous media

The objective of this research work is to experimentally determine and analyze the effect of geometry and orientation of strips (heating element) on critical heat flux (CHF) in pool boiling under atmospheric pressure. Studies concerning the effect of geometry, orientation, and thermophysical properties of the heating element on CHF are limited. In the present work, Stainless Steel 304 is used as strip material. The experiments are conducted with strips of length 50, 75, 100, and 150 mm each having width 5, 10, and 15 mm. The experiments are performed with strips of two thicknesses 0.1 and 0.2 mm. For each of the aforementioned combination of dimensions, the experiment is conducted by orienting it in horizontally, vertically over width and vertically over length, respectively. DC power supply is used to supply controlled amount of heat accurately. K-type thermocouples are used to measure the temperature of fluid. Datalogger is used to record the current, voltage, and temperature values. DSLR video camera is used to record the boiling mechanism as well as the location of critical heat flux on strips. The experimental setup was tested for repeatability, and it was observed that under same experimental conditions, CHF value obtained was within a relative deviation of ±6%. It is observed that thickness of strip has negligible effect on CHF for given dimensions and orientation. However, for a fixed length of strip, the magnitude of CHF decreases with increase in its width. It is observed that CHF decreases with increase in the length of strip for a fixed width irrespective of orientation. Out of the three orientations of strip, vertical over width has maximum CHF followed by horizontal and vertical over length, respectively.

While the role of the liquid properties, surface morphology, and operating conditions on critical heat flux (CHF) in pool boiling is well investigated, the effect of the properties of the heater material is not well understood. Previous studies indicate that the heater thickness plays an important role on the CHF phenomenon. However, beyond a certain thickness, called the asymptotic thickness, the local temperature fluctuations on the heater surface caused by the periodic bubble ebullition cycle are evened out, and the CHF is not influenced by further increasing the thickness. In the present work, data from literature and pool boiling experiments conducted in this study with seven substrates—aluminum, brass, copper, carbon steel, Monel 400, silver, and silicon—are used to determine the effect of the thermophysical property of the material on CHF for thick heaters that are used in industrial pool boiling applications. The results indicate that the product of density (ρ) and specific heat (cp) represents an important substrate property group that affects the CHF, and that the thermal conductivity is not an important parameter. A well-established force-balance-based CHF model (Kandlikar model) is modified to account for the thermal properties of the substrate. The predicted CHF values are within 15% of the experimental results.

The concept of critical heat flux (CHF) is discussed based on the mechanism that the CHF is caused by the dryout of a liquid layer formed on a heating surface. It is suggested that a liquid macrolayer is formed due to the coalescence of bubbles for most boiling systems, and that the dryout of the macrolayer is controlled by the hydrodynamic behavior of coalesced bubbles on the macrolayer. Based on these considerations, a new CHF model is proposed for saturated pool boiling at higher pressures. The idea of this model comes from a close examination of the measured diameters of various bubbles and the photographic records obtained by Semeria (1963) for water boiling under higher pressures. In the model, a liquid macrolayer is formed due to coalescence of the secondary bubbles formed from the primary bubbles. The detachment of the tertiary bubbles formed from the secondary bubbles determines the frequency of the liquid macrolayer formation. The CHF occurs when the macrolayer is dried out before the departure of the tertiary bubbles from the heating surface. One of the formulations of the model gives the well-known Kutateladze or Zuber correlation for CHF in saturated pool boiling.