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1.
Sang Houn Han Min-Kyun Na Si-Doek Oh Ho-Young Kwak 《Journal of Mechanical Science and Technology》1999,13(4):376-385
The effect of D. C electric field on nucleate boiling heat transfer for refrigerants of R-11, HCFC-123 and FC-72 was investigated
experimentally by using a single lo-fin tube shell-and-tube heat exchanger. The lo-fin tube which brought two times increase
in the heat transfer area provided about 150% of boiling heat transfer enhancement compared to that of smooth surface. This
experimental study has revealed that the electrical charge relaxation time was an important parameter for the boiling heat
transfer enhancement under electric field. Boiling heat transfer enhancement was obtained up to 40% for R-11 which had moderate
relaxation time of 1.3s. However remarkable boiling heat transfer enhancement has been obtained up to three fold increase(300%)
for HCFC-123 which has the electrical charge relaxation time of 0.89 x 10-3s. For FC-72 having longer relaxation time than the bubble detachment one, no appreciable effect on the nucleate boiling heat
transfer was observed. 相似文献
2.
This study investigates the heat transfer characteristics and entropy generation rate of a condensate film formed on a horizontal
plate with suction at the wall. Applying the minimum mechanical energy principle, the dimensionless liquid film thickness
along the plate is found to vary as a function of the Rayleigh number, the Jakob number, the Prandtl number and the suction
parameter. The governing differential equation of the condensate thickness is solved numerically by using a finite-difference
shooting method. Closed-form analytical expressions are derived for the Nusselt number and the dimensionless overall entropy
generation number. When there is no suction at the wall, the results obtained from the analytical expression for the Nusselt
number are found to be in good agreement with those presented in the literature.
This paper was recommended for publication in revised form by Associate Editor Dae Hee Lee
Tong-Bou Chang received the Ph.D. degree in Mechanical En-gineering from National Cheng Kung University, Tainan, Taiwan, in 1997. From 1997
to 2001, he was a researcher at Yuloon-Motor Group (Taiwan), whose job function includes design and characterization of the
thermal and fluid flow systems for vehicle. Since 2002, he has been as a Professor at the Department of Mechanical Engineering,
Southern Taiwan University. His current research interests include heat transfer with phase change, energy-system optimization,
heat and mass transfer in porous medium, enhancement heat transfer and high performance heat exchangers. 相似文献
3.
The evaporation heat transfer coefficienthr and frictional pressure drop δpf of refrigerant R-134a flowing in the oblong shell and plate heat exchanger were investigated experimentally in this study.
Four vertical counterflow channels were formed in the oblong shell and plate heat exchanger by four plates of geometry with
a corrugated sinusoid shape of a 45° chevron angle. Upflow of refrigerant R-134a boils in two channels receiving heat from
downflow of hot water in other channels. The effects of the refrigerant mass flux, average heat flux, refrigerant saturation
temperature and vapor quality of R- 134a were explored in detail. Similar to the case of a plate heat exchanger, even at a
very low Reynolds number, the flow in the oblong shell and plate heat exchanger remains turbulent. The results indicate that
the evaporation heat transfer coefficienthr and pressure drop Δpf increase with the vapor quality. A rise in the refrigerant mass flux causes an increase in theh
r
and Δpf. But the effect of the average heat flux does not show significant effect on the hr and Δpf. Finally, at a higher saturation temperature, both theh
r
and Δpf are found to be lower. The empirical correlations are also provided for the measured heat transfer coefficient and pressure
drop in terms of the Nusselt number and friction factor. 相似文献
4.
Natural convection heat transfer estimation from a longitudinally finned vertical pipe using CFD 总被引:1,自引:0,他引:1
Hyo Min Jeong Yong Hun Lee Myoung Kuk Ji Kang Youl Bae Han Shik Chung 《Journal of Mechanical Science and Technology》2009,23(6):1517-1527
In this study, CFD analysis of air-heating vaporizers was conducted. A longitudinally finned vertical pipe was used to represent
the air-heating vaporizer in the CFD model. Nitrogen gas was used as the working fluid inside the vertical pipe, and it was
made to flow upward. Ambient air, which was the heat source, was assumed to contain no water vapor. To validate the CFD results,
the convective heat transfer coefficients inside the pipe, hi-c, derived from the CFD results were first compared with the heat transfer coefficients inside the pipe, hi-p, which were derived from the Perkins correlation. Second, the convection heat transfer coefficients outside the pipe, ho-c, derived from the CFD results were compared with the convection heat transfer coefficients, ho-a, which were derived from an analytical solution of the energy equation. Third, the CFD results of both the ambient-air flow
pattern and temperature were observed to determine whether they were their reasonability. It was found that all validations
showed good results. Subsequently, the heat transfer coefficients for natural convection outside the pipe, ho-c, were used to determine the Nusselt number outside the pipe, Nuo.. This was then correlated with the Rayleigh number, Ra. The results show that Ra and Nuo have a proportional relationship in the range of 2.7414×1012 ≤ Ra ≤ 2.8263×1013. Based on this result, a relation for the Nusselt number outside the pipe, Nuo, was proposed.
This paper was recommended for publication in revised form by Associate Editor Man Yeong Ha
Hyomin Jeong is currently a professor of Mechanical and Precision Engineering at Gyeongsang Nation University. He received his ph.D. in
mechanical engineering from the University of Tokyo in 1992 and he joined Arizona State University as a visiting professor
from 2008 to 2009. His research interests are in fluid engineering, CFD, cryogenic system, cascade refrigeration system and
ejector system, mechanical vapor compression
Hanshik Chung is a professor of Mechanical and Precision Engineering at Gyeongsang National University. He obtianed his Ph.D. in Mechanical
Engineering from Donga University. He joined Changwon Master’s College and Tongyeong Fisher National College as an assistant
Professor in 1988 and 1993, respectively. His research fields extend into the thermal engineering, heat transfer, solar heating
& cooling system, LNG vaporizer optimum, solar cell, hydrogen compressor for fuel cell and making fresh water system from
sea water 相似文献
5.
Influence of a ring flat zone in the point contact surface on thermal elastohydrodynamic lubrication
This paper describes a geometrical profile, an elastohydrodynamically lubricated point contact surface with a ring flat zone, aimed at building up local line contact elastohydrodynamic lubrication (EHL) in point contact conjunctions to reduce the influence of side-leakage on the central film thickness. Effects of the ring flat zone on the thermal EHL characteristics are studied. A dimensionless coefficient, rW, is defined to represent the relative half width of the ring flat zone in a point contact EHL surface. Thermal EHL numerical simulations have been performed to investigate the influence of rW on the film thickness as well as pressure, temperature and friction coefficients under different operating conditions. In the range of 0≤rW≤1.0 results show that the minimum film thickness decreases with increasing rW and the central film thickness increases with increasing rW, and the influence of rW on the film thickness is more pronounced than those on the maximum pressure, the maximum temperature and the friction coefficients. It is revealed that the proposed ring flat zone with appropriate width is beneficial to the thermal lubrication. 相似文献
6.
The characteristics of the two-phase flow heat transfer of R-134a in microtubes with inner diameters of 430 μm and 792 μm
were experimentally investigated. The effect of the heat flux on the heat transfer coefficient for microtubes was significant
before the transition quality. The boiling number expressed the interrelation between the heat flux and the mass about the
heat transfer coefficients. The smaller microtube had greater heat transfer coefficients; the average heat transfer coefficient
for the tube A (D
i
= 430 μm) was 47.0% greater than that for the tube B (D
i
= 792 μm) at G = 370 kg/m2·s and q″ = 20 kW·m2. A new correlation for the evaporative heat transfer coefficients in microtubes was developed by considering the following
factors: the laminar flow heat transfer coefficient of liquid-phase flow, the enhancement factor of the convective heat transfer,
and the nucleate boiling correction factor. The correlation developed in present study predicted the experimental heat transfer
coefficients within an absolute average deviation of 8.4%. 相似文献
7.
A sinusoidal surface roughness model is adopted for the analysis of the effects of roughness amplitude and wavelength on pressure profile, film shape, minimum film thickness and coefficient of friction in a steady state EHL line contact. The influence coefficients used for the evaluation of surface displacements are calculated by utilizing a numerical method based on Fast Fourier Transform. Significant reduction is observed in the minimum film thickness due to surface roughness. Such reduction is quantified by roughness correction factor, CR, and a relationship between CR and non-dimensional surface roughness amplitude A is derived as: CR=1−0.7823A0.8213. This equation may prove to be of interest from designer's viewpoint. The friction coefficient is found to increase appreciably with increasing amplitude and decreasing wavelength of surface roughness. 相似文献
8.
In this study, the physical characteristics of ethanol vapor behind the incident and reflected shock waves in a vertical diaphragmless shock tube are measured. To verify the excellent properties of the vertical diaphragmless shock tube, the experimental data are compared with the theoretical Rankine-Hugoniot curves. Combined with the shock wave visualization imaging, it is verified that the designed shock tube can be effectively used for investigating the phase-change heat transfer behind the shock waves. According to measurements based on a He–Ne laser beam optical interference system, the growth rate of the liquid film behind the reflected shock wave (Vfr) is smaller than that behind the incident shock wave (Vfi). The experimental value of condensation parameter is approximately 0.025, which is in good agreement with the theoretical value determined using the molecular dynamics approach. When the Mach number of the incident shock wave (Mi) is employed as a factor influencing liquid film growth, it is observed that when all the other parameters remain constant, the larger the Mi, the faster the growth of condensed liquid film. The vertical diaphragmless shock tube facilitates strong reproducibility of results under the same conditions, which can increase the accuracy of experiments such as phase-change heat transfer. 相似文献
9.
Tong-Bou Chang Wen-Yu Yeh Go-Long Tsai 《Journal of Mechanical Science and Technology》2009,23(12):3399-3406
This study performs a theoretical investigation into the problem of two-dimensional steady filmwise condensation flow on a
horizontal tube with suction effects at the tube surface. An effective suction function is introduced to model the effect
of the wall suction on the thickness of the liquid condensate film. The local condensate film thickness and the local Nusselt
number are then derived using a simple series numerical method. The results show that the Nusselt number varies as a function
of the Jakob number Ja, the Rayleigh number Ra, and the suction parameter Sw. It is found that the wall suction effect has a significant influence on the heat transfer performance. An analytical solution
is derived for the mean Nusselt number for the case in which the wall suction effect is ignored. Finally, a closed-form correlation
is presented for the mean Nusselt number subject to a wall suction effect. 相似文献
10.
H. Christensen 《摩擦学汇刊》2013,56(3):219-226
Systematic measurements with a disk machine of the thickness of the lubricating film separating the surfaces of heavily loaded rollers have been made, and the variation of film thickness with applied load and rolling speed has been studied. It was found that the results can be expressed by: H = hp + 0.2 = 0.7 W?0.2 (ηU)0.83 where H is the minimum film thickness in the contact in microns, W is the applied load per unit width in dynes per centimeter, and ηU is expressed in dynes per centimeter. 相似文献
11.
This study is focused on the comparison of heat transfer performance of two thermosyphons having 60 straight and helical internal grooves. Distilled water has been used as working fluid. Liquid fill charge ratio defined by the ratio of working fluid volume to total internal volume of thermosyphon, the inclination angle and operating temperature were used as experimental parameters. The heat flux and heat transfer coefficient are estimated from experimental results. The conclusions of this study may be summarized as follows; Liquid fill charge ratio, inclination angle and geometric shape of grooves were very important factors for the operation of thermosyphon. The optimum liquid fill charge ratio for the best heat flux were 30%. The heat transfer performance of helically grooved tube was higher than that of straight grooved tube in low inclination angle (less than 30°), but the results were opposite in high inclination angle (more than 30°). As far as optimum inclination angle concerns, range of 25°-30° for a helically grooved tube and about 40° for a straight grooved tube are suggested angles for the best results. 相似文献
12.
A thermal and non-Newtonian fluid model under elastohydrodynamic lubrication conditions is proposed, integrating some particularities, such as the separation between hydrodynamic and dissipative phenomena inside the contact. The concept of apparent viscosity is used to introduce the non-Newtonian behaviour of the lubricant and the thermal behaviour of the contact into the Reynolds equation, acting as a link element between the hydrodynamic and dissipative components of the EHD film, independently of the rheological and thermal models considered. The apparent viscosity enables the application of the rheological model better adapted to each lubricant, without appealing to special formulations of the EHD problem.The Newton–Raphson technique is used to obtain the lubricant film geometry and the pressure distribution inside the EHD contact. The shear stresses developed in the fluid film are evaluated assuming the non-linear Maxwell rheological model. The surfaces and lubricant temperature distributions are determined using the simplified Houpert's method, applied to the inlet contact zone, and the thermal method proposed by Tevaarwerk is applied in the high pressure contact zone.The non-Newtonian thermal EHD model is applied to the analysis of a contact lubricated with MIL-L-23699 oil. Significant results are obtained for the centre and minimum film thickness, for the inlet shear heating and film thickness reduction factor (φT), for the temperature rise of the lubricant and of the surfaces and for the friction coefficient inside the contact, considering wide ranges of the operating conditions (maximum Hertzian pressure, inlet oil temperature, rolling speed and slide-to-roll ratio).Finally, the numerical traction curves determined are compared with the corresponding experimental results, showing very good correlation. 相似文献
13.
In this work, we consider several types of lubricants—including non-Newtonian fluids—that were studied in EHD pure rolling point contacts under various operating conditions, leading us to explore a wide range of dimensionless parameters. The experimental results are compared with predictions given by the usual analytical EHL relationships and by more recently developed models. This broad comparison conducted with particular emphasis on minimum film thickness (hm) showed a fair agreement between experimental data and a few predictions including some obtained from extended models. Commonly used elastohydrodynamic lubrication (EHL) models did not systematically give accurate hm estimation, whereas minimum film thickness not only is a yield value but also serves as a key parameter in estimating lubrication regimes. 相似文献
14.
Thomas Touche Tomasz Woloszynski Pawel Podsiadlo Gwidon W. Stachowiak Juliette Cayer-Barrioz Denis Mazuyer 《Tribology Letters》2017,65(3):113
The effects of depth and top width of transverse rectangular grooves on film thickness and friction in elastohydrodynamic lubrication (EHL) regime were investigated through numerical simulations. Results were obtained in the form of pressure profiles and Stribeck curves for central and minimum film thickness and for friction coefficient. The results indicate that grooves with narrow top widths reduce the minimum film thickness and that this reduction is greater for deeper grooves. Lubricant shearing inside these grooves was further identified as a dominant factor contributing to friction. Near the groove edges, however, no evidence of micro-EHL effect was observed. Based on the results, a groove volume parameter was proposed to characterise the groove lubrication efficiency. We found that the parameter was linearly related to the average central film thickness and by increasing the groove wavelength the film could be made thicker than that of a smooth contact. 相似文献
15.
An experimental and numerical study of cold rolling lubricated by O/W emulsion has been carried out. The strip rolling experiment
was carried out on a Hille experimental rolling mill with a view to study the performance of emulsion lubrication in terms
of practical rolling parameters. Accordingly, rolling parameters such as rolling force and torque were measured. The experimental
measurements compare favourably with the computed results from a numerical scheme developed by the authors. The scheme, based
on a two-phase lubricant model, is capable of calculating the oil concentration at any point within the inlet zone and work
zone, rolling pressure, film thickness, and fractional contact area ratio associated with strip rolling under mixed film lubrication
at different rolling speeds. Using this scheme, the intertwined effects of an emulsion’s parameters such as: oil concentration,
mean oil droplet size, and rolling speed on strip rolling were investigated. The numerical study encompassed the mixed film
regime for speed, S ranges from 10−4 to 10−2, supply oil concentration level λds from 1 to 10%, and oil droplet size D
S from 5 to 10. Experimentally, the differences between water, oil and emulsion-lubricated rolling are not discernible except
for film thickness. At a low speed of 10 RPM, force and torque of water-lubricated rolling are marginally higher than oil-
or emulsion-lubricated ones. However, the difference between emulsion and neat oil is not apparent. The numerical results
show the occurrence of a moderate oil concentration increase in the inlet zone followed by a sharp one at the beginning of
the work zone. The effect of the concentration process is predominantly seen in the film thickness and the lubricant pressure
whilst its effect on the total pressure is less pronounced. The analysis of the results suggests that it is possible to lower
the emulsion oil concentration without any adverse effect on the rolling process. This principle can be used to control the
outlet lubricant film thickness and hence the surface quality of the rolled strip. 相似文献
16.
The analysis in this Paper is concerned with the piezoviscous-rigid regime of lubrication for the general case of elliptical contacts. In this regime, several, formulas of the lubricant film thickness have been proposed. However, either the load parameter W is not included, which has a strong effect on film thickness, or the film thickness is overestimated by using the Barus formula for pressure viscosity characteristics. In the current study, the Roelands formula has been used for the pressure-viscosity relationship. The effects of the dimensionless load, speed, and materials parameters, the radius ratio, and the lubricant entrainment direction have been investigated. Forty-one cases were used in obtaining the minimum film thickness formula: H0 = 178G0.386U1.266W?0.880 (1 ? e?0.0387α Contour Plots indicate in detail the pressure developed between the contacting solids. 相似文献
17.
18.
Woo-Seog Song Seungbae Lee Dong-Shin Shin Yang Na 《Journal of Mechanical Science and Technology》2006,20(11):1993-2001
The piezoelectric bimorph film, which, as an actuator, can generate more effective displacement than the usual PVDF film,
is used to control the turbulent boundary-layer flow. The change of wall pressures inside the turbulent boundary layer is
observed by using the multi-channel microphone array flush-mounted on the surface when actuation at the non-dimensional frequency
fb+=0.008 and 0.028 is applied to the turbulent boundary layer. The wall pressure characteristics by the actuation to produce
local displacement are more dominantly influenced by the size of the actuator module than the actuation frequency. The movement
of large-scale turbulent structures to the upper layer is found to be the main mechanism of the reduction in the wallpressure
energy spectrum when the 700v/uτ-long bimorph film is periodically actuated at the non-dimensional frequency fb+ =0.008 and 0.028. The biomorph actuator is triggered with the time delay for the active forcing at a single frequency when
a 1/8″ pressuretype, pin-holed microphone sensor detects the large-amplitude pressure event by the turbulent spot. The wall-pressure
energy in the late-transitional boundary layer is partially reduced near the convection wavenumber by the open-loop control
based on the large amplitude event. 相似文献
19.
Hyo-Min Jeong Han-Shik Chung Sang-Chul Lee Tae-Woo Kong Chung-Seub Yi 《Journal of Mechanical Science and Technology》2006,20(4):545-553
Generally, the temperature drop under 0°C on vaporizer surface creates frozen dews. This problem seems to increase as the
time progress and humidity rises. In addition, the frozen dews create frost deposition. Consequently, heat transfer on vaporizer
decreases because frost deposition causes adiabatic condition. Therefore, it is very important to solve this problem. This
paper aims to study of the optimum design of used vaporizer at local LNG station. In this paper, experimental results were
compared with numerical results. Geometries of numerical and experimental vaporizers were identical. Studied parameters of
vaporizer are angle between two fins (Φ) and fin thickness (THF). Numerical analysis results were presented through the correlations between the ice layer thickness (THICE) on the vaporizer surface to the temperature distribution of inside vaporizer (TIN), fin thickness (THF), and angle between two fins (Φ). Numerical result shows good agreement with experimental outcome. Finally, the correlations
for optimum design of vaporizer are proposed on this paper. 相似文献
20.
In this paper we report molecular dynamics simulation results of lubricant films only a few nanometres thick confined between atomically smooth gold or model mica surfaces. We have studied dodecane (C12H26) of various film thicknesses. We show below a critical film thickness structural transitions take place with the formation of crystal bridges. We demonstrate this critical film thickness is larger with Au(1 0 0) surface. Below this critical film thickness we observe a large enhancement of apparent shear viscosity. This enhancement, however, is much lower for films confined by gold. We find the extrapolated zero shear viscosity of a ∼2.4-nm-thick film confined by gold is almost two orders of magnitude lower than that of a film, of the same thickness, confined by model mica. We find the source of this difference is the weaker pinning of the layers next to the surface of the gold. This leads to stronger slip, at the lubricant-gold interface, which persists at solid-like and liquid-like states. For mica a larger part of shearing takes place inside the film. The solid-like structure of the films confined by gold shows stronger resilience to shear and melt at considerably larger shear rates than those confined by model mica surfaces. These differences could be important in developing low friction devices at microscale. 相似文献