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1.
The jet boiling heat transfer of a bar water–CuO particle suspensions (nanofluids) jet impingement on a large flat surface
was experimentally investigated. The experimental results were compared with those from water. The quantificational effects
of the nanoparticles concentration and the flow conditions on the nucleate boiling heat transfer and the critical heat flux
(CHF) were investigated. The experimental data showed that the jet boiling heat transfer for the water–CuO nanofluid is significantly
different from those for water. The nanofluids have poor nucleate boiling heat transfer compared with the base fluid due to
that a very thin nanoparticle sorption layer was formed on the heated surface. The CHF for the nanofluid increased compared
with that of water. The reasons were that the solid–liquid contact angle decreased due to a very thin sorption layer on the
heated surface and the jet and agitating effect of the nanoparticles on the subfilm layer enhance supply of liquid to the
surface. 相似文献
2.
《International Journal of Multiphase Flow》2005,31(5):618-642
An experimental study was performed to investigate the pool boiling critical heat flux (CHF) in one-dimensional inclined rectangular channels by changing the orientation of a copper test heater assembly. In a pool of saturated water under the atmospheric pressure, the test parameters included the gap sizes of 1, 2, 5, and 10 mm, and the surface orientation angles from the downward-facing position (180°) to the vertical position (90°). Tests were conducted on the basis of the visualization of boiling phenomena in the narrowly confined channel and open periphery utilizing a high-speed digital camera. To prevent the heat loss from the water pool and copper test heater, a state-of-the-art vacuum pumping technique was introduced. It was observed that the CHF generally decreased as the surface inclination angle increased and as the gap size decreased. In the downward-facing position (180°), however, the vapor movement was enhanced by the gap structure, which produced the opposing result; that is, the CHF increased as the gap size decreased. Phenomenological characteristics regarding the interfacial instability of vapor layer were addressed in terms of visualization approaching the CHF. It was found that there exists a transition angle, around which the CHF changes with a rapid slope. 相似文献
3.
Pool boiling heat transfer characteristics on treated surfaces were investigated experimentally. Surface treatments were performed
with sandpapers, building micro structures by etching, and micro-porous coating. Copper blocks of 20 mm × 20 mm were used
as test sections and PF5060 was used as a working fluid. The effect of wall superheat (0–35 K), surface orientation (0°, 45°,
90°), and subcooling (0, 5, 10 K) on treated surfaces were also investigated. Heat transfer performance on inclined surface
was better than that of horizontal surface in all test sections. Because bubble generation was suppressed by subcooling, higher
wall superheat was needed to initiate boiling in all surfaces. Micro-porous coated surface showed the highest heat transfer
enhancement among tested surfaces. 相似文献
4.
The process of contact melting of the solid phase change material (PCM) around a hot sphere, which is driven by the temperature
difference between the PCM and the sphere, is analyzed in this paper. Considering the difference of the normal angle between
the sphere surface and the solid–liquid interface of the melting PCM, the fundamental equations of the melting process are
derived with the film theory. The new film thickness and pressure distribution inside the liquid film and the variation law
of the normal angle of the solid–liquid interface and the melting velocity of the sphere are also obtained. It is found that
(1) while normal angle at sphere surface φ is within a certain value φ0, which is related to Ste number and the outside force F, it has no obvious effect on the pressure distribution inside the liquid film and the numerical results by the present model
are in accordance with the analytical results in the published literature, (2) the film thickness at φ = ±90° is constringent
to a certain value and not the infinity, (3) the analytical results can be employed approximately to analyze the contact melting
process except for the film thickness at φ = ±90°. 相似文献
5.
Ho Seon Ahn Hyungdae Kim HangJin Jo SoonHo Kang WonPyo Chang Moo Hwan Kim 《International Journal of Multiphase Flow》2010
Enhancements of nucleate boiling critical heat flux (CHF) using nanofluids in a pool boiling are well-known. Considering importance of flow boiling heat transfer in various practical applications, an experimental study on CHF enhancements of nanofluids under convective flow conditions was performed. A rectangular flow channel with 10-mm width and 5-mm height was used. A 10 mm-diameter disk-type copper surface, heated by conduction heat transfer, was placed at the bottom surface of the flow channel as a test heater. Aqueous nanofluids with alumina nanoparticles at the concentration of 0.01% by volume were investigated. The experimental results showed that the nanofluid flow boiling CHF was distinctly enhanced under the forced convective flow conditions compared to that in pure water. Subsequent to the boiling experiments, the heater surfaces were examined with scanning electron microscope and by measuring contact angle. The surface characterization results suggested that the flow boiling CHF enhancement in nanofluids is mostly caused by the nanoparticles deposition of the heater surface during vigorous boiling of nanofluids and the subsequent wettability enhancements. 相似文献
6.
An experimental study was carried out to understand the nucleate boiling characteristics and the critical heat flux (CHF)
of water, the water based nanofluids and the water based nanoparticle-suspensions in vertical small heated tubes with a closed
bottom. Here, the nanofluids consisted of the base liquid, the CuO nanoparticles and the surfactant. The nanoparticle-suspensions
consisted of the base liquid and CuO nanoparticles. The surfactant was sodium dodecyl benzene sulfate. The study focused on
the influence of the nanoparticles and surfactant on the nucleate boiling characteristics and the CHF. The experimental results
indicated that the nanoparticle concentrations of the nanofluids and nanoparticle-suspensions in the tubes do not change during
the boiling processes; the nanoparticles in the evaporated liquid are totally carried away by the steam. The boiling heat
transfer rates of nanofluids are poorer than that of the base liquid. However, the boiling heat transfer rates of nanoparticle-suspensions
are better than that of the base liquid. Comparing with the base liquid, the CHF of the nanofluids and the nanoparticle-suspensions
is higher. The CHF is only related to nanoparticle mass concentration when the tube length and the tube diameter are fixed.
The experiment confirm that there is a thin nanoparticle coating layer on the heated surface after the nanofluids boiling
test but there is no coating layer on the heated surface after the nanoparticle-suspensions boiling test. This coating layer
is the main reason that deteriorates the boiling heat transfer rates of nanofluids. An empirical correlation was proposed
for predicting the CHF of nanofluids boiling in the vertical tubes with closed bottom. 相似文献
7.
This paper presents an experimental study dealing with the basic nucleate boiling concerning two finned surfaces placed in
a narrow channel. The influence of both the channel width and the orientation of the base surface (horizontal or vertical)
are discussed. The experiments were performed in a saturated pool of FC-72 while the channel widths investigated were 2.0 mm
and 0.5 mm. The experimental data are compared with those obtained in the case of the unconfined situation of the extended
surfaces. Channel width reduction does not affect the heat transferred to the liquid in the case of vertical orientation of
the base surface, while it causes a drastic reduction in the heat transfer behavior in the case of a horizontal base surface.
For the latter situation, vapor stagnation in the gap was observed after the maximum heat flux had been reached.
Received on 13 August 1998 相似文献
8.
9.
Andrea Luke 《Heat and Mass Transfer》2009,45(7):909-917
The processes of the phase change in boiling occur at the solid–liquid interface by heat transfer from a solid heating surface
to the boiling liquid. The characteristic features of the heating surfaces are therefore of great interest to optimize the
design of evaporators. The microstructure with all its peaks and cavities influences directly the wetting and rewetting conditions
of the heated surface by the boiling liquid and hence bubble formation and heat transfer. The roughness structures of different
evaporator copper tubes with 8 or 25 mm diameter are characterized quantitatively with regard to the cavities offered to nucleation.
The surfaces of the heating elements are sandblasted by different means resulting in a stochastic microstructure. The surfaces
are investigated by a three-dimensional contactless roughness measurement technique combining the stylus technique with the
near field acoustic microscopy. The method opens the possibility to obtain results according to standard for practical applications
and additionally delivers detailed information about the three-dimensional shape of each cavity within the surface investigated.
The analysis of the microstructure implies the total number of cavities, their local and size distribution calculated by the
method of the envelope area. The results of the surface analysis are linked to those of heat transfer and bubble formation
discussed in a contribution by Kotthoff and Gorenflo. 相似文献
10.
In this work a simplified calculation method taking into account the effect of mass transport on the heat transfer coefficient
(HTC) during boiling of multicomponent mixture has been elaborated. The calculation results were compared with own experimental
data for ternary system methanol–isopropanol–water and Grigoriev data [1] (acetone–methanol–water). The experiments were performed in different hydrodynamic conditions such as: pool boiling and
liquid evaporation at the free surface of the falling film. The experimental data covered wide range of heat fluxes from 6
to 30 kW/m2 in the case of liquid evaporation from the falling film and from 30 to 240 kW/m2 for pool boiling. The analysis of the results indicates that the mass transfer resistance in the liquid phase caused a significant
reduction of experimental value HTC in comparison to so-called ideal HTC. 相似文献
11.
The flow and heat transfer in an inclined and horizontal rectangular duct with a heated plate longitudinally mounted in the
middle of cross section was experimentally investigated. The heated plate and rectangular duct were both made of highly conductive
materials, and the heated plate was subjected to a uniform heat flux. The heat transfer processes through the test section
were under various operating conditions: Pr ≈ 0.7, inclination angle ϕ = −60° to +60°, Reynolds number Re = 334–1,911, Grashof number Gr = 5.26 × 102–5.78 × 106. The experimental results showed that the average Nusselt number in the entrance region was 1.6–2 times as large as that
in the fully developed region. The average Nusselt numbers and pressure drops increased with the Reynolds number. The average
Nusselt numbers and pressure drops decreased with an increase in the inclination angle from −60° to +60° when the Reynolds
number was less than 1,500. But when the Reynolds number increased to over about 1,800, the heat transfer coefficients and
pressure drops were independent of inclination angles. 相似文献
12.
An experimental investigation was carried out to study the enhancement of the heat transfer from a heated flat plate fitted
with rectangular blocks of 1 × 2 × 2 cm3 dimensions in a channel flow as a function of Reynolds number (Reh), spacing (S
y
) of blocks in the flow direction, and the block orientation angle (α) with respect to the main flow direction. The experiments were performed in a channel of 18 cm width and 10 cm height, with
air as the working fluid. For fixed S
x
=3.81 cm, which is the space between the blocks in transverse to the flow direction, the experimental ranges of the parameters
were S
y
=3.33–4.33 cm, α=0–45°, Reh=7625–31550 based on the hydraulic diameter and the average velocity at the beginning of the test section in the channel.
Correlations for Nusselt number were developed, and the ratios of heat transfer with blocks to those with no blocks were given.
The results indicated that the heat transfer could be enhanced or reduced depending on the spacing between blocks, and the
block orientation angle. The maximum heat transfer rate was obtained at the orientation angle of 45°.
Received on 13 December 2000 / Published online: 29 November 2001 相似文献
13.
Film boiling of binary liquid mixtures may be significantly different from that of single-component liquids due to the mass diffusion effect. A theoretical analysis is performed to outline the effects of mass diffusion phenomena on film boiling heat transfer process from a horizontal cylinder heating surface to the binary liquid mixtures of ethylene oxide/water and ethanol/benzene over whole range of compositions. These two binary systems are chosen for illustrating the strong and weak mass diffusion effects, respectively, on film boiling. Furthermore, a simple correlation for predicting heat transfer coefficient is proposed to demonstrate the idea that the dimensionless F factor can satisfactorily account for the mass diffusion effect on film boiling heat transfer of binary mixtures. 相似文献
14.
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%. 相似文献
15.
An experimental study of transient boiling heat transfer during a cooling of a hot cylindrical block with an impinging water
jet has been made at atmospheric pressure. The experimental data were taken for the following conditions: a degree of subcooling
of ΔT
sub = 20–80 K, a jet velocity of u
j
= 5–15 m/s, a nozzle diameter of d
j
= 2 mm and three materials of copper, brass and carbon steel. The block was initially and uniformly heated to about 250 °C
and the transient temperatures in the block were measured at eight locations in r-direction at two different depths from the surface during the cooling of hot block. The surface heat flux distribution with
time was evaluated using a numerical analysis of 2-D heat conduction. Behavior of the wetting front, which is extending the
nucleate boiling region outward, is observed with a high-speed video camera. A position of wetting region is measured and
it is correlated well with a power function of time. The changes in estimated heat flux and temperature were compared with
the position of wetting region to clarify the effects of subcooling, jet velocity and thermal properties of block on the transient
cooling.
Received on 17 March 2000 相似文献
16.
In pool boiling, the electrically heated tube releases the energy non-uniformly to the liquid, due to different surface roughness
and flowing liquid. The heat transfer coefficient therefore varies with axial and azimuthal position on the tube. Hence a
finite element analysis has been carried out on a horizontal 1in. copper tube for evaporation in pool boiling for three-dimensional
conduction heat transfer. A test tube has been made with different surface structures, tested and analysed for heat conduction
effects. It has been observed that significant amount of heat flows in azimuthal and axial directions in addition to the heat
flow in radial direction. 相似文献
17.
C.L. OngJ.R. Thome 《Experimental Thermal and Fluid Science》2011,35(6):873-886
This part of the paper presents the current experimental flow boiling heat transfer and CHF data acquired for R134a, R236fa and R245fa in single, horizontal channels of 1.03, 2.20 and 3.04 mm diameters over a range of experimental conditions. The aim of this study is to investigate the effects of channel confinement, heat flux, flow pattern, saturation temperature, subcooling and working fluid properties on the two-phase heat transfer and CHF. Experimentally, it was observed that the flow boiling heat transfer coefficients are a significant function of the type of two-phase flow pattern. Furthermore, the monotonically increasing heat transfer coefficients at higher vapor qualities, corresponding to annular flow, signifies convective boiling as the dominant heat transfer mechanism in these small scale channels. The decreasing heat transfer trend at low vapor qualities in the slug flow (coalescing bubble dominated regime) was indicative of thin film evaporation with intermittent dry patch formation and rewetting at these conditions. The coalescing bubble flow heat transfer data were well predicted by the three-zone model when setting the dryout thickness to the measured surface roughness, indicating for the first time a roughness effect on the flow boiling heat transfer coefficient in this regime. The CHF data acquired during the experimental campaign indicated the influence of saturation temperature, mass velocity, channel confinement and fluid properties on CHF but no influence of inlet subcooling for the conditions tested. When globally comparing the CHF values for R134a in the 0.51-3.04 mm diameter channels, a peak in CHF peak was observed lying in between the 0.79 (Co ≈ 0.99) and 1.03 (Co ≈ 0.78) mm channels. A new CHF correlation has been proposed involving the confinement number, Co that is able to predict CHF for R134a, R236fa and R245fa in single-circular channels, rectangular multichannels and split flow rectangular multichannels. In summary, the present flow boiling and CHF trends point to a macro-to-microscale transition as indicated by the results presented in Ong and Thome (2011) [1]. 相似文献
18.
Jet impingement boiling is very efficient in cooling of hot surfaces as a part of the impinging liquid evaporates. Several
studies have been carried out to measure and correlate the heat transfer to impinging jets as a function of global parameters
such as jet subcooling, jet velocity, nozzle size and distance to the surface, etc. If physically based mechanistic models
are to be developed, studies on the fundamentals of two-phase dynamics near the hot surface are required. In the present study
the vapor–liquid structures underneath a subcooled (20 K) planar (1 mm × 9 mm) water jet, impinging the heated plate vertically
with a velocity of 0.4 m/s, were analyzed by means of a miniaturized optical probe. It has a tip diameter of app. 1.5 μm and
is moved toward the plate by a micrometer device. The temperature controlled experimental technique enabled steady-state experiments
in all boiling regimes. The optical probe data provides information about the void fraction, the contact frequencies and the
distribution of the vapor and liquid contact times as a function of the distance to the surface. The measured contact frequencies
range from 40 Hz at the onset of nucleate boiling to nearly 20,000 Hz at the end of the transition boiling regime. Due to
condensation in the subcooled jet vapor disappears at a distance to the surface of app. 1.2 mm in nucleate boiling. This vapor
layer becomes smaller with increasing wall superheat. In film boiling a vapor film thickness of 8 ± 2 μm was found. 相似文献
19.
Experiments of flow boiling heat transfer and two-phase flow frictional pressure drop in a spirally internally ribbed tube
(φ22×5.5 mm) and a smooth tube (φ19×2 mm) were conducted, respectively, under the condition of 6×105 Pa (absolute atmosphere pressure). The available heated length of the test sections was 2500 mm. The mass fluxes were selected,
respectively, at 410, 610 and 810 kg/m2 s. The maximum heat flux was controlled according to exit quality, which was no more than 0.3 in each test run. The experimental
results in the spirally internally ribbed tube were compared with that in the smooth tube. It shows that flow boiling heat
transfer coefficients in the spirally internally ribbed tube are 1.4–2 times that in the smooth tube, and the flow boiling
heat transfer under the condition of smaller temperature differences can be achieved in the spirally internally ribbed tube.
Also, the two-phase flow frictional pressure drop in the spirally internally ribbed tube increases a factor of 1.6–2 as compared
with that in the smooth tube. The effects of mass flux and pressure on the flow boiling heat transfer were presented. The
effect of diameters on flow boiling heat transfer in smooth tubes was analyzed. Based on the fits of the experimental data,
correlations of flow boiling heat transfer coefficient and two-phase flow frictional factor were proposed, respectively. The
mechanisms of enhanced flow boiling heat transfer in the spirally internally ribbed tube were analyzed.
Received on 1 December 1999 相似文献
20.
The dynamic behavior of a water droplet impinging upon a heated surface was shown to be significantly different, depending
on the normal momentum of the impinging droplet before impact. This experimental study focused mainly on the effects of the
impinging angle of the droplet on impact dynamics and its dependence on surface temperature. At the surface temperature of
the nucleate boiling regime, disintegration of the droplet did not occur, whereas the deforming droplet adhered to the surface.
The liquid film was spread and contracted several times on the horizontal surface, but the expanded droplet merely slipped
without noticeable contraction on the inclined surfaces. In the film boiling regime, the impinging droplet spread over the
surface as a liquid film separated from the surface by the vapor produced. Depending on the magnitude of the normal momentum
of droplet, disintegration into several irregular shapes of liquid elements occurred in the case of the horizontal and 30°-inclined
surfaces. The impinging droplet in the case of the 60°-inclined surface did not break up and tended to recover its original
spherical shape.
Received: 16 February 1999/Accepted: 9 November 1999 相似文献