1,720,987 research outputs found
Identification of nucleation site interactions
No full textSimple models of nucleate boiling consider nucleation sites in
isolation. In practice, they interact in ways that depend on the
distance between them. In this paper; statistical evidence of
interactions is obtained for a large number of non-uniformly distributed
sites with different activation superheats during pool boiling of water
on a thin plate at a heat flux of 51 kW/m(2). By analysis of
spatio-temporal data for wall temperature obtained by liquid crystal
thermography over a period of 30 s, the timing, position, activation
superheat and radius of cooled region are obtained for every nucleation
event, without need for direct observations of the bubbles. For each
event, the number of subsequent events at all other sites during
different delayed time intervals is obtained as a function of distance
from the original site. The number is compared with a null hypothesis
obtained by assigning random times to all events: a higher number
indicates promotion, a lower number inhibition. It is found that there
is promotion during very short time delays of the order of the bubble
growth time at distances less than the bubble radius and inhibition for
slightly longer delays and shorter distances. There is no statistically
significant effect at distances greater than the bubble radius. This
finding may be influenced by the low bubble frequencies characteristic
of these particular experiments. (C) 2003 Elsevier Inc. All rights
reserved
Flow boiling of R134a in vertical mini-diameter tubes
No full textThis thesis was submitted for the degree of Doctor of Philosophy and awarded by Brunel University, 21/03/2011.The current study is a part of a long term experimental project devoted to investigate flow boiling heat transfer, pressure drop and flow visualization of R134a in small to mini/micro-diameter tubes. The experimental facility was first designed and constructed by X. Huo (2005) with the contribution of L. Chen (2006). In the present study, the experimental facility was upgraded by changing the heating system from AC to DC heating and also upgrading the logging system through using a faster data logger and developing a new Labview program. The objectives of the current study include (i) contribute in identifying the reasons behind the wide scatter in the published flow boiling heat transfer results, (ii) contribute in understanding the fundamentals of flow boiling heat transfer in mini/micro-diameter tubes and (iii) evaluation of the existing heat transfer and pressure drop prediction methods. Two sizes of stainless steel tubes were investigated in the current study; 0.52 mm and 1.1 mm diameter. In the current study, the 0.52 mm tube was roughly called a “micro-tube” whilst the 1.1 mm tubes were called “mini-tubes”.
The present study proposes two possible reasons for the scatter in the published heat transfer results. The first reason is the variations in the heated length from one study to another–there is no criterion for choosing the heated length. The second reason is the variations in the inner surface characteristics of the channels from one study to another. These two important parameters were not taken into consideration by researchers in the past studies. Accordingly, the effect of the heated length was investigated in the current study using a seamless cold drawn tube with diameter of 1.1 mm and heated length ranging from 150 to 450 mm. The effect of the tube inner surface was also tested here by conducting the test in two stainless steel tubes with diameter of 1.1 mm and manufactured by two different processes. The first tube was manufactured by welding technique whilst the second tube was a seamless cold drawn tube. Both tubes were identical in design and dimensions. The inner surface of each tube was examined first using SEM analysis and demonstrated that, the surface morphology is completely different. The local heat transfer coefficient was determined through measuring the local wall temperature using 14 K-type thermocouples attached to the wall using thermally conducting but electrically insulating epoxy supplied by Omega. Pressure drop was measured directly across the heated section and a high speed camera was used for the flow visualization at 1000 frames/s. All measurements were recorded after the system attained steady state. The experimental conditions include mass flux range of 100 – 500 kg/m2 s, system pressure range of 6 – 10 bar, inlet sub-cooling of about 5K and exit quality up to about 0.9.
The most frequently observed flow regimes in the 0.52 mm tube were found to be slug (elongated bubble), transition to annular and annular flow regimes. In the 1.1 mm tube, the observed regimes were found to be slug, churn and annular. The transition from slug flow to annular flow in the 0.52 mm tube occurred smoothly with little disturbances at the liquid vapour interface compared to the 1.1 mm tube. Additionally, increasing the heated length of the 1.1 mm tube was found to shift the transition to annular flow to occur at lower vapour quality.
The heat transfer results demonstrated that the behaviour of the local heat transfer coefficient in the 0.52 mm diameter tube is different compared to that in the 1.1 mm tubes. Also, the tube inner surface characteristics and the heated length were found to strongly influence the local behaviour of the heat transfer coefficient. Flow boiling hysteresis was investigated and the results indicated that hysteresis exists only at very low heat fluxes near the boiling incipience.
Existing heat transfer and pressure drop correlations were examined using the results of the 0.52 and 1.1 mm seamless cold drawn tubes. The pressure drop data were predicted very well using the Muller-Stienhagen and Heck (1986) correlation, the homogeneous flow model and the correlation of Mishima and Hibiki (1996). On the contrary, all macro and microscale heat correlations failed to predict the current experimental data. The mechanistic models failed to predict the data of all tubes with the same accuracy. Accordingly, two heat transfer correlations were proposed in the current study. The first correlation is based on dimensionless groups whilst the second is based on the superposition model of Chen (1966). Both correlations predicted the current experimental data and the data of Huo (2005) and Shiferaw (2008) very well.Egyptian Ministry of Higher Educatio
Spatio-temporal analysis of nucleate pool boiling: identification of nucleation sites using non-orthogonal empirical functions
No full textThe analysis of spatio-temporal data and the physical understanding of
the systems generating them are often limited by the available
techniques. These limitations are especially evident in nucleate
boiling. This paper investigates the analysis of a sequence of
temperature fields obtained from a pool nucleate boiling experiment.
Spatio-temporal data for the wall temperature in pool nucleate boiling
of water on a thin, horizontal, stainless steel plate were obtained by
liquid crystal thermography and high speed video recording. A previous
analysis provided examples of the thermal conditions for activation of
individual nucleation sites, for the heat transfer mechanisms during
bubble growth and for the consequent interactions between adjacent
sites. Principal component analysis (PCA) is shown to provide a
reconstruction of the temperature fields that is accurate in the root
mean square sense but which obscures information about the underlying
physics, such as positions of the nucleation sites. In contrast, a new
approach using non-orthogonal empirical functions (NEFs) encodes the
relevant physical constraints (e.g., each NEF has a radially symmetrical
form as suggested by the pattern of cooling during bubble growth). NEFs
provide an efficient identification of the positions of active sites in
successive frames; they are better suited to the analysis of
non-stationary dynamics than PCA and allow for information compression.
(C) 2001 Elsevier Science Ltd. All rights reserved
Two-phase flow boiling in small to micro-diameter tubes
No full textThis thesis was submitted for the degree of Doctor of Philosophy and awarded by Brunel University.This thesis is dedicated to the experimental and theoretical study of flow boiling in small to micro diameter tubes using R 134a. Flow pattern, heat transfer and pressure drop studies were conducted in stainless steel cold drawn tubes with internal diameter
2.88,1.1, and 0.52 mm using an existing facility that was designed with a long term research objective of improving the fundamental understanding of flow boiling in small metallic tubes. The facility was moved to the present location from London South Bank University and re-commissioned before carrying out the experiments. The test sections were heated by a direct passage of alternating current and wall temperatures were measured at 15 axial locations by miniature thermocouples that were directly spotwelded at the tube outer wall. A digital high-speed camera was used to simultaneously observe the flow patterns (during the heat transfer tests) directly at a borosilicate glass tube located immediately downstream of the heat transfer test section. The purpose of the flow visualization study was to support understanding of the heat transfer characteristics and development of flow regime-specific models. The heat transfer and pressure drop data of X. Huo (2005) in the 4.26 and 2.01 mm tubes and the flow visualization results of Chen (2006) for the tubes of diameter 4.26,2.88,2.01, and 1.1 mm were included with the new data in an extensive analysis of flow boiling heat transfer and pressure drop in five vertical tubes with internal diameters 4.26, 2.88,2.01, 1.1 and 0.52 mm. The wide range of tube diameter was chosen to investigate the influence of tube size and possibly identify the threshold where the effect of small or micro diameter effects become significant. In the experiments, parameters were varied in the ranges: mass flux 100 to 700 kg/m2s; heat flux 1.6 to 150 kW/m; pressure 6 to 14 bar; quality up to 0.9 and the inlet temperature was controlled at a subcooling of 1-5K. There was no clear significant difference between the characteristics and magnitude of the heat transfer coefficients in the 4.26 mm and 2.88 mm tubes but the coefficients in the 2.01 and 1.1 mm tube were higher. The heat transfer results suggested that a tube size of about 2 mm might be considered as a critical diameter to distinguish small and conventional tubes. Further differences have now been observed in the 0.52 mm tube. These differences, both in flow patterns and heat transfer, indicate a possible second change from small to micro behaviour at diameters less than 1 mm for R 134a. Also, the results showed axial variations in heat transfer characteristics marking the importance of surface conditions on heat transfer. This calls for a further detail investigation to understand the underlying physics in the initiation of boiling, effect of surface condition on nucleation, and structure of newly emerging flow patterns, particularly in very small tubes. Existing correlations were examined using the results of the five tubes and indicated that these correlations do not predict the present small diameter data to a satisfactory degree. Therefore, two new correlations that take into account both magnitude and characteristic effect of tube diameter have been proposed covering the 4.26 mm-1.1 mm and the smallest 0.52 mm tube, respectively. A detailed comparison was also made with the state-of-the-art flow regime-specific model of Thome et al. (2004) and verified that the mechanistic modelling approach has a promising capability of predicting two phase heat transfer in small diameter tubes, although it still requires further development. Some improvements have been proposed and tested against the current data. Using a similar approach, a new two phase pressure drop model has been proposed and compared with the current data with encouraging results.Funding was obtained from the School of Engineering and Design, Brunel Universit
Numerical investigation of saturated flow boiling on thin walls
No full textThis thesis was submitted for the degree of Doctor of Philosophy and awarded by Brunel University.Boiling heat transfer provides a means of removing high heat fluxes at low temperature differences in many applications in the power and process industries. A strong interest has been also developed for the cooling of silicon-based devices, such as
electronic chips. However, a complete model to describe the processes involved has not
been developed as yet.
This PhD project focused on the study of nucleate pool boiling via numerical simulations for a solid plate horizontally immersed in a saturated liquid with a large
number of potential nucleation sites. The simulations were developed by a FORTRAN
code based on a hybrid approach, combining the 3-dimensional time-dependent solution
for the temperature field on the substrate with semi-empirical models for phenomena
occurring on the liquid side. The starting point of the project was the modification of a previous version of the code in order to reduce the computational time (in collaboration with Dr. Nelson at Los Alamos National Laboratory) and improve the modelling of the physics of the processes.
One of the key features of the code is the flexibility in adapting to different
conditions. In fact the code was used to study bubble growth, site activation frequency and superheat variations, as well as the interactions between nucleation sites. The differences in behaviour between very thin metal foils immersed in water and thicker silicon substrates in FC-72 were studied. The results were compared with experimental results produced at the University of Edinburgh and the University of Ljubljana, both partners of this project.
Both the numerical and physical modifications introduced made it possible to have simulations for a large number of sites, of the order of 100, in reasonable times, of the order of days, so that the code can be now used as a tool for the design of new test sections.Engineering and Physical Sciences Research Council (EPSRC) grant EP/C532805/
Comparison of a mechanistic model for nucleate boiling with experimental spatio-temporal data
No full textMechanistic numerical simulations have been developed for pool nucleate
boiling involving large groups of nucleation sites that are
non-uniformly distributed spatially and have different activation
superheats. The simulations model the temperature field in the heated
wall accurately and use approximations for events in the liquid-vapour
space. This paper describes the first attempt to compare the numerical
simulations with spatio-temporal experimental data at a similar level of
detail. The experimental data were obtained during pool boiling of water
at atmospheric pressure on a horizontal, electrically heated stainless
steel plate 0.13 mm thick. They consist of wall temperature fields
measured on the back of the plate by liquid crystal thermography at a
sampling rate of 200 Hz over a period of 30 s. Methods of image analysis
have been developed to deduce the time, position, nucleation superheat
and size of the cooled area for every bubble nucleation event during
this period. The paper discusses the methodology of using some of the
experimental data as input for the simulations and the remainder for
validation. Because of the high-dimensional dynamics and possibly
chaotic nature of nucleate boiling, the validation must be based on
statistical properties over a large area and a long period. This
preliminary study is restricted to a single heat flux
Combined effect of electric field and surface modification on pool boiling of R-123
No full textThis thesis was submitted for the degree of Doctor of Philosophy and awarded by Brunel University.The effect of surface modification and high intensity electric field (uniform and non – uniform) acting separately or in combination on pool boiling of R-123 is presented in this thesis. The effect of surface modification was investigated on saturated pool boiling of R-123 for five horizontal copper surfaces modified by different treatments, namely: an emery polished surface, a fine sandblasted surface, a rough sandblasted surface, an electron beam (EB) enhanced surface and a sintered surface. Each 40 mm diameter heating surface formed the upper face of an oxygen-free copper block, electrically heated by embedded cartridge heaters. The experiments were performed from the convective heat transfer regime to the critical heat flux, with both increasing and decreasing heat flux, at 1.01 bar, and additionally at 2 bar and 4 bar for the emery polished surface. Significant enhancement of heat transfer with increasing surface modification was demonstrated, particularly for the EB enhanced and sintered surfaces. The emery polished and sandblasted surface results are compared with nucleate boiling correlations and other published data. The effect of uniform and non-uniform electric fields on saturated pool boiling of R-123 at 1.01 bar pressure was also examined. This method of heat transfer enhancement is known as electrohydrodynamic abbreviated as EHD-enhancement. A high voltage potential was applied at the electrode located above the heating surface, which was earthed. The voltage was varied from 0 to 30 kV. The uniform electric field was provided through a 40 mm diameter circular electrode of stainless steel 304 wire mesh having an aperture of 5.1 mm, while the non-uniform electric field was obtained by using a 40 mm diameter circular rod electrode with rods 5 and 8 mm apart. The effect of uniform electric field was investigated using all five modified surfaces, i.e. emery polished, fine sandblasted, rough sandblasted, EB enhanced and sintered surfaces, while non – uniform electric field was tested using the emery polished, fine sandblasted, EB enhanced and sintered surfaces. The effect of pressure on EHD enhancement was also examined using emery polished surface at saturation pressure of 2 and 4 bars while the electric field was fix at 20 kV corresponding to 2 MV/m. Further, the bubble dynamics is presented for the emery polished surface obtained using a high-speed high – resolution camera.The Government of Pakista
Single-phase flow and flow boiling of water in horizontal rectangular microchannels
Full text linkThis thesis was submitted for the degree of Doctor of Philosophy and awarded by Brunel UniversityThe current study is part of a long term experimental project devoted to investigating
single-phase flow pressure drop and heat transfer, flow boiling pressure drop and heat
transfer, flow boiling instability and flow visualization of de-ionized water flow in
microchannels. The experimental facility was first designed and constructed by S.
Gedupudi (2009) and in the present study; the experimental facility was upgraded by
changing the piping and pre-heaters so as to accommodate the objectives of the research.
These objectives include (i) modifying the test rig, to be used for conducting
experiments in microchannels in single and two-phase flow boiling heat transfer,
pressure drop and visualization, (ii) redesign metallic single microchannels using copper as the material. The purpose of the redesign is to provide microchannels with strong heaters, high insulation performance and with test sections easy to dismantle and
reassemble, (iii) obtaining the effect of hydraulic diameter on single-phase flow, flow
pattern, heat transfer and pressure drop, (iv) studying the effects of heat flux, mass flux,and vapour quality on flow pattern, flow boiling heat transfer and pressure drop, (v)comparing experimental results with existing correlations. However, the main focus in this present study is to investigate the effects of hydraulic diameter, heat flux, mass flux and vapour quality on flow pattern, flow boiling heat transfer coefficient and pressure drop.
In addressing (iii) many possible reasons exist for the discrepancies between published
results and conventional theory and for the scatter of data in published flow boiling heat
transfer results:
1. Accuracy in measuring the dimensions of the test section, namely the width, depth and length and in the tested variables of temperature, pressure, heat flux
and mass flux.
2. Variations in hydraulic diameter and geometry between different studies.
3. Differences in working fluids.
4. Effects of hydrodynamic and thermal flow development
5. Inner surface characteristics of the channels. Three different hydraulic diameters of copper microchannels were investigated: 0.438mm, 0.561 mm and 0.635 mm. For single-phase flow the experimental conditions
included mass fluxes ranging from 278 – 5163 kg/m2 s, heat fluxes from 0 - 537 kW/m²,
and inlet temperatures of 30, 60 and 90°C. In the flow boiling experiments the
conditions comprised of an inlet pressure of 125 kPa (abs), inlet temperature of 98°C
(inlet sub-cooling of 7 K), mass fluxes ranging from 200 to 1100 kg/m²s, heat fluxes
ranging from 0 to 793 kW/m² and qualities up to 0.41. All measurements were recorded
after the system attained steady states.
The single-phase fluid flow results showed that no deviation of friction factors was
found from the three different hydraulic diameters. The effect of fluid temperature on
friction factor was insignificant and the friction factors themselves were in reasonable agreement with developing flow theory. The typical flow patterns observed in all three test sections were bubbly, slug/confined churn and annular, however, based on the observation performed near the outlet, the bubbly flow was not detected. The effects of mass flow and hydraulic diameter on flow pattern for the three test sections investigated in the range of experimental conditions were not clear.
The single-phase heat transfer results demonstrated that smaller test sections result in higher heat transfer coefficients. However, for heat transfer trends presented in the form of Nusselt number versus Reynolds number, the effect of hydraulic diameter was insignificant.The flow boiling experiments gave similar heat transfer results; they exhibited that the smaller hydraulic diameter channels resulted in higher heat transfer coefficients. The
nucleate boiling mechanism was found for all three test sections, evidenced by the
significant effect of heat flux on the local heat transfer coefficient. Moreover, the heat
flux had a clear effect on average heat transfer coefficient for the 0.561 mm and 0.635mm test sections, whilst for the 0.438 mm test section, there was no discernible effect. At the same heat flux, increases in mass flux caused heat transfer coefficients to decrease. This could be due to the decrease of pressure inside the test section. When a higher mass flux was tested, the inlet pressure increased, and in reducing the inlet pressure to the original value, a decrease in system pressure resulted. Consequently, the outlet pressure and local pressure became lower. Existing flow pattern maps, flow boiling heat transfer and pressure drop correlations
were compared with the experimental results obtained for all three test sections. The
comparison showed that the flow pattern map proposed by Sobierska et al. (2006) was
the most successful in predicting the experimental data. The local heat transfer
coefficient data were compared with existing published correlations. The correlations of
Yu et al. (2002), Qu and Mudawar (2003) and Li and Wu (2010) are found to predict the current local heat transfer coefficient better than other correlations tested.
Pressure drop results showed that as the heat flux and mass flux were increased, the
two-phase pressure drop increased too. These were due to the increase in bubble
generations and the inertia momentum effect. As the channel was reduced, the twophase
pressure drop increased because the pressure drop related inversely with the channel hydraulic diameter. The pressure and pressure drop fluctuations were indentified in this project, however, the maximum pressure fluctuation was found in the 0.438 mm channel whilst the minimum fluctuation was attained in the 0.561 mm channel. This indicated that the effect of decreasing in hydraulic diameter on pressure and pressure drop fluctuations is not clear and needs to be investigated further. The two-phase pressure drop data were compared with selected correlations. The Mishima and Hibiki (1996)’s correlation was found to predict the current two-phase pressure drop better than the other correlations examined in this study
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
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