Moderate Heat Fluxes Research Articles

Jet flows from bubbles during subcooled pool boiling on micro wires

An experimental investigation was conducted on subcooled nucleate boiling on ultra-small wires having diameters of 25―100 μm. High-speed photography and laser PIV (Particle Image Velocimetry) technology were used to visually observe the bubble dynamics. For highly subcooled boiling at moderate heat fluxes, the bubbles generally remained attached to the micro heating wires and bubble-top jet flows were clearly observed. Smaller bubbles usually had stronger bubble-top jet flows, while larger bubbles seemed to produce multi-jet flows. The structures of the bubble-top jet flows, as well as multi-jet flows, were proposed from the experimental observation. A model was developed to describe jet flow phenomena from bubbles on micro wires. Numerical simulations for bubbles having diameter of 0.03 and 0.06 mm showed that both the bubble-top and multi-jet flows were induced by a strong Marangoni effect due to high temperature gradients near the wire. The predicted velocity magnitudes and flow structures agreed very well with experimental measurements. The bubble size relative to the wire is an important factor affecting the jet flow structure. For a 0.03 mm bubble on a 0.1 mm wire, only a bubble-top jet flow forms, while a complex multi-jet flow pattern forms around the bubble with a weak bubble-top jet and two side jet flows for a 0.06 mm bubble.

Structure of Moist Layer and Sources of Water over the Southern Region Far from the Meiyu/Baiu Front

We investigate the structure of a moist layer and sources of water in the southern region far from the Meiyu/Baiu front using GAME (GEWEX Asian Monsoon Experiment, GEWEX: Global Energy and Water Cycle Experiment) reanalysis data. In the period from July 4 to 15, 1998, the Meiyu/Baiu front was located over northern China (the Yellow River Valley) and the Korean Peninsula. Both regions over the plain area of mainland China (the eastern region of 112°E line) and the East China Sea have very moist air masses in this period; the total precipitable water over the regions is about 45 kgm−2. However, the structure of these moist layers is quite different. The moist layer over mainland China is deep, reaching 600 hPa. On the other hand, the moist layer over the East China Sea is shallow, with a thickness of up to 800 hPa. This difference in the structure of the moist layers can be attributed to the development of a moist convective mixing layer and the generation of shallow convective clouds in the daytime over mainland China, as a result of the supply of abundant latent and moderate sensible heat fluxes from the land surface.We also examine the sources of water supplied into the Meiyu front using Colored Moisture Analysis (CMA). Though water vapor to generate the deep moist layer over mainland China is mainly transported from the upstream regions of the Asian summer monsoon, which include the Indian Ocean, the Indochina Peninsula, and the South China Sea, the rate of the contribution of water from the southeastern region of mainland China is over 15% of the total precipitable water, when the Meiyu front is located over northern China. This shows that the land surface over southeastern China is regarded as a major source of water to the Meiyu front.

Heat Transfer to Water-Oxygen Mixtures at Supercritical Pressure

The constant pressure heat capacity and forced convection heat transfer coefficient of water/oxygen mixtures were measured in a horizontal, smooth, electrically-heated tube. For the supercritical pressure (25 MPa) considered, flow rates (0.76–2.04 kg/min), heat fluxes (21–290 kW/m2) and temperatures (330–430°C), the flow in the 6.2 mm ID tube was fully turbulent. The fluid was distilled water and up to 9 wt % oxygen. The water/oxygen mixture and the above experimental conditions are relevant to supercritical water oxidation systems (SCWO). At subcritical temperatures the oxygen/water mixture is almost immiscible and the flow is two-phase. Just below the critical temperature, the fluid becomes single-phase. By measuring bulk and surface temperatures, for a given flow rate, heat flux and oxygen content, both the heat capacity and heat transfer coefficient for the mixture were measured. The water-oxygen system is a highly non-ideal mixture, and small amounts of oxygen significantly reduce the temperature at which maximum heat transfer occurs. Despite the multi-phase nature of the flow at temperatures well below the critical temperature (i.e., <360°C), the presence of small quantity of oxygen has little effect on the heat transfer. At supercritical temperatures where the flow is single-phase and gas-like, the presence of oxygen has little effect on the heat transfer coefficient. However, at near-critical temperatures, the addition of small amounts of oxygen results in a dramatic change in the heat transfer. Firstly, the magnitude and temperature for the peak heat transfer decrease, consistent with changes in heat capacity. Secondly, heat transfer is deteriorated at moderate heat flux, mostly but not exclusively on the top surface of the tube.

Bubble Dynamic Parameters and Pool Boiling Heat Transfer on Plasma Coated Tubes in Saturated R-134a and R-600a

An optical method for measuring the bubble dynamic data subject to an isolated bubble model is presented at low heat flux q⩽1kW/m2; while the operating heat fluxes are up to 30 kW/m2. By simultaneous measurements of departure diameters, velocities, frequencies and nucleation site densities, the heat transfer contribution of an individual active site is evaluated. A single phase heat transfer correlation was used to model the present heat transfer data. The test specimens consisted of tubes with porous copper (Cu) and molybdenum (Mo) plasma coated surfaces. The porosity (ε), the thickness of the porous layer (δ), and the mean pore diameter (η) of the tested tubes are the following: 0.055⩽ε⩽0.057,100⩽δ⩽300μm, and 3⩽η⩽4μm. The tests were carried out using R-134a and R-600a as working fluid at a saturation temperature of 18°C and with low and moderate heat fluxes (⩽1 kW/m2) for boiling visualization and related measurements (⩽30 kW/m2).

Pool boiling heat transfer in vertical annular crevices

Effects of vertical annuli on nucleate pool boiling heat transfer of water at atmospheric pressure have been obtained experimentally. Experiments were performed for annuli with a height of 570 mm and gap sizes of 3.9 and 15 mm. Through the tests, tube bottom confinement (open or closed) has been investigated, too, and the whole results are compared with a single unconfined tube. According to the results, the annular condition gives much increase in heat transfer coefficients at moderate heat fluxes. Its effect is observed much greater for the bottom-closed tube condition.

Effects of annular crevices on pool boiling heat transfer

Effects of gap sizes (3.9–44.3 mm) of vertical annuli on nucleate pool boiling heat transfer of water at atmospheric pressure have been obtained experimentally. Through the tests, the conditions of tube bottom confinement (open or closed) have also been investigated and the whole results are compared with those of a single unconfined tube. According to the results, the annular condition gives much increase in heat transfer at moderate heat fluxes. The increase is much enhanced as the gap size decreases. At the same tube wall superheat (about 3.2 K) the heat flux of the least gap size is more than four times greater than that of the unconfined tube. Its effect is observed much greater for the bottom-closed tube condition. However, a deterioration of heat transfer occurs at high heat flux for confined boiling.

High speed flow visualization of pool boiling from structured tubular heat transfer surfaces

Abstract Experimental investigations of pool boiling from novel tubular heat transfer surfaces (structured tubes with re-entrant cavities) are carried out with the hydrocarbon propane as working fluid operating under moderate heat fluxes ( 100 W / m 2 ) and saturation conditions. The heat transfer coefficients are presented. The evaporation phenomena are visualized by a high-speed video system and by means of digital image processing techniques (e.g., Fourier-analyses, correlation-techniques). The bubble departure diameter at the surface, the bubble generation frequency at corresponding nucleation sites and the bubble upward flow velocity are quantitatively determined.

Falling film evaporation heat transfer of water/salt mixtures from roll-worked enhanced tubes and tube bundle

An experimental study is carried out for enhancement of falling film evaporation heat transfer of pure water and water/salt mixtures on horizontal smooth tube and two kinds of structured tube bundles under atmospheric pressure. The experimental results show that the low-cost roll-worked tube can greatly enhance the evaporation heat transfer performance of the falling film, and make it comparable to that of expensive commercial enhanced tubes such as GEWA-T tubes, TE tubes and HF tubes, even at low and moderate heat flux levels. The average evaporation heat transfer coefficients for the roll-worked tube bundle are basically independent from the parameters tested such as flow and heating conditions, salt-concentrations, as well as geometries of the tube bundles. The present experimental data result in a constant heat transfer coefficient; α≈20 kW/m 2 K, in the convective heat transfer range of the heat fluxes <10 5 W/m 2.

Enhanced evaporation heat transfer of water and R-11 falling film with the roll-worked enhanced tube bundle

An experimental study was carried out for the evaporation heat transfer of water and R-11 falling films across horizontal smooth tube and two kinds of structured tube bundles under atmospheric pressure. The experimental results reveal that the low-cost roll-worked tube can greatly enhance the falling-film evaporation heat transfer either for water or for R-11 at low and moderate heat fluxes. The average evaporation heat transfer coefficients for the roll-worked tube bundle are basically independent of the falling film mass flow rate, the heat flux and the geometry parameters of the tube bundle, but only rely on the kind of liquids in the convective evaporation regime.

Two-phase flow structure near a heated vertical wall during nucleate pool boiling

An experimental study has been carried out in order to investigate the two-phase flow structure in the vicinity of a vertical heated wall (120 mm high) on which boiling of pentane takes place. Measurements of the frequency and time of bubble passages and also of void fraction have been performed by means of hot-wire anemometry (HWA). It has been shown that, at high heat flux, bubbles mainly remain close to the wall and form large coalesced bubbles. On the contrary, at low or moderate heat flux, they are propelled into the fluid core prior to rising in a bubble column located about 1.5–2 mm away from the heated wall and sometimes they coalesce during their rising motion in the upper part of the column. Using a classification of bubbles based on their passage time, the contribution from short, average and long bubbles to the void fraction was also discussed. This classification allows to determine the transition between the boiling two-phase flow patterns defined previously from objective criteria: the regime of isolated bubbles or partial nucleate boiling occurs for q ′/ q ′ crit<0.2, the fully developed nucleate boiling for 0.2< q ′/ q ′ crit<0.7 and the transition to critical heat flux (CHF) for q ′/ q ′ crit>0.7.

Experimental Verification of Two New Theories Predicting Temperature-Dependent Viscosity Effects on the Forced Convection in a Porous Channel

Experimental Verification of Two New Theories Predicting Temperature-Dependent Viscosity Effects on the Forced Convection in a Porous Channel

Nucleate Pool Boiling From Coated and Spirally Wrapped Tubes in Saturated R-134a and R-600a at Low and Moderate Heat Flux

Pool nucleate boiling heat transfer experiments from coated surfaces with porous copper (Cu) and molybdenum (Mo) and spirally wrapped with helical wire on copper surfaces with micro-roughness immersed in saturated R-134a and R-600a were conducted. The influence of coating thickness, porosity, wrapped helical angle, and wire pitch on heat transfer and boiling characteristics including bubble parameters were studied. The enhanced surface heat transfer coefficients with R-600a as refrigerant found are 2.4 times higher than those of the smooth surfaces. Photographs indicate that the average number of bubbles and bubble departure diameters has been found to increase linearly with heat flux, while the bubble diameters exhibit opposite trend in both refrigerants. Furthermore, the heat transfer of the boiling process for the present enhanced geometry (coated and wrapped) was modeled and analyzed. The experimental data for plasma coating and spirally wrapped surfaces were correlated in terms of relevant parameters, respectively to provide a thermal design basis for engineering applications.

Measurements of heat transfer coefficients at low contact pressures for actively cooled bolted armour concepts in Tore Supra

For the future upgrade of inner vessel components (CIEL project) a guard limiter for plasma ramp-up and disruption protection will be installed on the high field side of the vacuum vessel. Among transient heat loads, this structure has to sustain a moderate heat flux in the range of ≤0.5 MW/m 2 during quasi steady state operation (1000 s). A bolted carboncarbon (CC) tile is preferred compared with a brazed tile solution due to the expected moderate heat fluxes, costs and the possibility of rapid replacement of individual tiles. Large flat tile assemblies require a sufficient soft and conductive compliant layer enclosed between tile and heat sink in order to avoid thermal contact loss of the assembly during heat loads and therefore minimising the tile surface temperature. The global heat transfer coefficient ( H gl) under vacuum at low contact pressures (0.5–1.5 MPa) between CC and CuCrZr heat sink substrata has been measured in the experimental device, installation of contact heat transfer measurements (ITTAC), using different compliant materials. It appears that the best compliant layer is a graphite sheet (PAPYEX), compared with copper-felt/foam material. As an example, a H gl number of ∼10 4 W/m 2 K at an average contact pressure of 0.5 MPa has been measured near room temperature between CC (SEP N11) and CuCrZr substrata using a 0.5-mm thick PAPYEX layer. Thermohydraulic calculations (2D) of the guard limiter design show an expected tile surface temperature of about 550°C in steady state regime for an incident heat flux of 0.5 MW/m 2.

Analysis of pool boiling within smooth and grooved tubes

This work presents experimental data for R-113 pool boiling, at atmospheric pressure and moderate heat flux, inside vertical and horizontal aluminum open-ended tubes, whose internal surfaces are either smooth or grooved. The tube length is 40 mm and the internal diameters of the smooth and the grooved tubes are 16.64 and 15.88 mm, respectively. The results are presented and discussed in relation to the heat transfer coefficients and compared with the Cooper and the Forster and Zuber empirical correlations. For all the tests, the heat transfer coefficient of the grooved tube is higher than that of the smooth tube. For horizontal tubes, an elongated bubble at the top region promotes a better local heat transfer coefficient as is indicated by the lower temperature than that of the bottom region, where the bubbles are isolated.

Influence of heat conduction in the wall on nucleate boiling heat transfer

On the basis of a single bubble model on heat transfer in a wedge shaped micro region the influence of heat conduction in the wall on heat transfer in nucleate boiling is studied. The wall thermal conductivity λ W is varied, whereas all other parameters, particularly the bubble site density, are kept constant. Wall conductivity is set to the values of copper, stainless steel and a ceramic material. The results of the parameter study show that even large variations of λ W have only moderate influence on heat transfer to a growing vapour bubble. Simulation results for various boiling liquids are then compared with experimental data of others to test the model and examine the influence of liquid properties upon boiling heat transfer. Although the model is based on some simplifying assumptions and thus not all heat transfer mechanisms can be described in detail, the boiling curve can be predicted in the nucleate boiling regime with good accuracy for low to moderate heat fluxes.

A visual study of phase-change heat transfer in a two-dimensional porous structure with a partial heating boundary

A visual study on the phase-change behaviors in a vertical two-dimensional porous structure made of staggered miniature silver–copper circular cylinders has been carried out. Subcooled water was pumped into the porous structure from its bottom due to the capillary action developed in the vicinity of a grooved heating block placed on the top of the porous structure. Using a high-speed video imaging system, both pore-scale bubble-growth behaviors and continuum-scale distributions of two-phase zone in the porous structure were observed. Photographic results reveal that for a small or moderate heat flux, isolated bubbles formed, grew, and collapsed in the pores in a cyclic manner with a nearly constant frequency. In the macroscopic view, it is found that the periodic downflows of dispersed bubbles and upflows of the liquid phase in the porous structure led to a quasi-steady liquid–vapor two-phase zone. As the imposed heat flux was increased, both the frequency of the bubble growth–collapse cycle and the number of isolated bubbles increased while in the macroscopic view, the two-phase zone expanded laterally but shrank vertically. When the imposed heat flux was sufficiently high, a vapor film was observed beneath the heated fin. These visual observations explain heat transfer measurements: with an increase of the imposed heat flux, the heat transfer coefficient increases to a maximum value and then rapidly decreases afterwards.

On capillary-driven flow and phase-change heat transfer in a porous structure heated by a finned surface: measurements and modeling

Characteristics of capillary-driven flow and phase-change heat transfer in a porous structure heated with a permeable heating source at the top were studied experimentally and theoretically in this paper. The experiments show that for small and moderate heat fluxes, the whole porous structure was fully saturated with liquid except adjacent to the horizontal heated surface where evaporation took place uniformly. For higher heat fluxes, a two-phase zone developed in the upper portion of the porous structure while the lower portion of the porous structure was saturated with subcooled liquid. When the imposed heat flux was further increased, a vapor blanket formed below the heated surface and the corresponding critical heat flux was reached. The heat transfer coefficient was modeled by simultaneously solving the problem of evaporating capillary meniscus in the pore level and the problem of fluid flow through a porous medium. The model is in good agreement with the experimental data, predicting the variations of the heat transfer coefficient with the increasing heating load.

Transient radiative initiation of the heterogeneous flame in self-propagating high-temperature materials synthesis: Theory and experimental comparisons

Radiative initiation of a combustion wave in the self-propagating high-temperature materials synthesis (SHS) process was analyzed for a mixture compact by use of a heterogeneous theory which accounts for the premixed mode of bulk flame propagation as well as the nonpremixed mode of consumption of the non-metal (or higher-melting-point metal) particles. In the realistic limit of diffusion-controlled combustion, which nevertheless is still highly temperature sensitive because of the Arrhenius nature of mass diffusion, the analysis successfully identified the functional dependence of the various system parameters related to flame initiation on the particle size, a feature of which was not captured in previous theoretical works based on homogeneous flame propagation. Specifically, calculated results showed that the ignition delay increases with decreasing mixture ratio, increasing dilution, decreasing intensity and beam diameter of the heat flux, and increasing particle size. Furthermore, in the presence of heat loss which is inevitable, there also exist absolute ignition limits beyond which the mixture compact is not ignitable. Such nonignition events are manifested by the failure to either establish a combustion wave, for large particles or moderate heat fluxes, or sustain the propagation of an established wave even though the particle size is small and the heat flux is strong. Comparison with experimental data showed satisfactory agreement, both qualitatively and quantitatively, thereby substantiating the viability of the present heterogeneous theory of flame initiation.

Nucleate Pool Boiling on Ribbed Surfaces With Micro-Roughness at Low and Moderate Heat Flux

Nucleate pool boiling correlation was developed for five different rib-type roughened tube geometries (including plain tube) with different rib angles of 30 deg, 45 deg, 60 deg, and 90 deg for both distilled water and R-134a as the working media. A scanning electron micrograph (SEM) examination was made for these horizontal roughened tubes. Bubble departure diameter, frequency of bubble emission, and the active nucleation site density with the influence of the rib angle for this type of roughened surface were obtained. Boiling heat flux incorporating natural convection, nucleate boiling, and microlayer evaporation mechanisms following Benjamin and Balakrishnan (1996) was predicted. Heat transfer correlation was also developed in terms of the degree superheat and active nucleation site density. The dependence for these two parameters was found in favorable agreement with that of previous study for smooth surfaces.

Nucleate Boiling Heat Transfer of Binary Mixtures at Low to Moderate Heat Fluxes

A model for nucleate pool boiling heat transfer of binary mixtures has been proposed based on an additive mechanism. The contributing modes of heat transfer are (i) the heat transferred by microlayer evaporation, (ii) the heat transferred by transient conduction during the reformation of the thermal boundary layer, and (iii) the heat transferred by turbulent natural convection. The model takes into account the microroughness of the heating surface which has been defined quantitatively. The model compares satisfactorily with data obtained in the present study and in the literature. These data were obtained on a variety of heating surfaces such as a vertical platinum wire, a horizontal stainless steel tube and flat horizontal aluminium, and stainless steel surfaces (with various surface finishes) thereby demonstrating the validity of the model.