共查询到20条相似文献,搜索用时 31 毫秒
1.
Ki-Jung Park Dong-Gyu Kang Dongsoo Jung Sang Eun Shim 《Journal of Mechanical Science and Technology》2011,25(10):2647-2655
In this study, pool boiling heat transfer coefficients (HTCs) and critical heat fluxes (CHF) are measured on a smooth square
flat copper heater in a pool of pure water with and without carbon nanotubes (CNTs) dispersed at 60°C. Tested aqueous nanofluids
are prepared using multi-walled CNTs whose volume concentrations are 0.0001, 0.001, 0.01, and 0.05%. For the dispersion of
CNTs, DISPERBYK 184 is used in distilled water. Pool boiling HTCs are taken from 10 kW/m2 to critical heat flux for all tested fluids. Test results show that the pool boiling HTCs of the nanofluids are lower than
those of pure water in entire nucleate boiling regime. On the other hand, critical heat flux is enhanced greatly showing up
to 150% increase at the CNT concentration of 0.001% as compared to that of pure water. This is related to the change in surface
characteristics by the deposition of CNTs. This deposition makes a thin CNT layer on the surface and the active nucleation
sites of the surface are decreased due to this layer. The thin CNT layer acts as the thermal resistance and also decreases
the bubble generation rate resulting in a decrease in pool boiling HTCs. The same layer, however, decreases the contact angle
on the test surface and extends the nucleate boiling regime to very high heat flux range and reduces the formation of large
vapor canopy at near CHF. Thus, a significant increase in CHF results. 相似文献
2.
This paper reports an experimental study on flow boiling of pure refrigerants R134a and R123 and their mixtures in a uniformly
heated horizontal tube. The flow pattern was observed through tubular sight glasses with an internal diameter of 10 mm located
at the inlet and outlet of the test section. Tests were run at a pressure of 0.6 MPa in the heat flux ranges of 5–50 kW/m2, vapor quality 0–100 percent and mass velocity of 150–600 kg/m2s. Both in the nucleate boiling-dominant region at low quality and in the two-phase convective evaporation region at higher
quality where nucleation is supposed to be fully suppressed, the heat transfer coefficient for the mixture was lower than
that for an equivalent pure component with the same physical properties as the mixture. The reduction of the heat transfer
coefficient in mixture is explained by such mechanisms as mass transfer resistance and non-linear variation in physical properties
etc. In this study, the contribution of convective evaporation, which is obtained for pure refrigerants under the suppression
of nucleate boiling, is multiplied by the composition factor by Singal et al. (1984). On the basis of Chen’s superposition
model, a new correlation is presented for heat transfer coefficients of mixture. 相似文献
3.
Agus Sunjarianto Pamitran Kwang-Il Choi Jong-Taek Oh Ki-Won Park 《Journal of Mechanical Science and Technology》2009,23(3):599-606
Experiments were performed on the convective boiling heat transfer in horizontal minichannels using propane. The test section
was made of stainless steel tubes with inner diameters of 1.5 mm and 3.0 mm and lengths of 1000 mm and 2000 mm, respectively,
and it was uniformly heated by applying an electric current directly to the tubes. Local heat transfer coefficients were obtained
for a heat flux range of 5–20 kW m−2, a mass flux range of 50–400 kg m−2 s−1, saturation temperatures of 10, 5, and 0°C and quality ranges of up to 1.0. The nucleate boiling heat transfer contribution
was predominant, particularly at the low quality region. Decreases in the heat transfer coefficient occurred at a lower vapor
quality with a rise of heat flux and mass flux, and with a lower saturation temperature and inner tube diameter. Laminar flow
appeared in the minichannel flows. A new boiling heat transfer coefficient correlation that is based on the superposition
model for propane was developed with 8.27% mean deviation.
This paper was recommended for publication in revised form by Associate Editor Jae Young Lee
Jong-Taek Oh received his B.S., M.S. and Ph.D. degrees in Refrigeration Engineering from Pukyong National University, Korea. Dr. Oh is
currently a Professor at Department of Refrigeration and Air Conditioning Engineering, Chonnam National University at Yeosu,
Korea. Dr. Oh’s research interests are in the area of boiling and condensation heat transfer and pressure drop of refrigerants
with small tubes, heat pump and transportation refrigeration. 相似文献
4.
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
5.
Kyoo Woan Kim Nguyen Ba Chien Kwang-Il Choi Jong-Taek Oh 《Journal of Mechanical Science and Technology》2014,28(10):4301-4308
We report experimental data of boiling heat transfer of R-1234yf in horizontal small tubes. The experimental data obtained in the horizontal circular small tubes of 1.5 and 3.0 mm inner diameter, the lengths of 1000 and 2000 mm, the mass flux range from 200–650 kg/m2s, the heat flux range from 5–40 kW/m2 and saturation temperature of 10 and 15°C, was used to develop a modified correlation for the heat transfer coefficient. The flow pattern of the experimental data was mapped and analyzed with existing flow pattern maps. The heat transfer coefficient was also compared with some well-known correlations. 相似文献
6.
Direct immersion cooling has been considered as one of the promising methods to cool high power density chips. A fluorocarbon
liquid such as FC-72, which is chemically and electrically compatible with microelectronic components, is known to be a proper
coolant for direct immersion cooling. However, boiling in this dielectric fluid is characterized by its small value of the
critical heat flux. In this experimental study, we tried to enhance the critical heat flux by increasing the nucleate boiling
area in the heat spreader (Conductive Immersion Cooling Module). Heat flux of 2 MW/m2 was successfully removed at the heat source temperature below 78°C in FC-72. Some modified boiling curves at high heat flux
were obtained from these modules. Also, the concept of conduction path length is very important in enhancing the critical
heat flux by increasing the heat spreader surface area where nucleate boiling occurs. 相似文献
7.
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
8.
In this study, external condensation heat transfer coefficients (HTCs) of six flammable refrigerants of propylene (R1270),
propane (R290), isobutane (R600a), butane (R600), dimethylether (RE170), and HFC32 were measured at the vapor temperature
of 39°C on a 1023 fpm low fin and Turbo-C tubes. All data were taken under the heat flux of 32- 116 and 42-142 kW/m2 for the Iow fin and Turbo-C tubes respectively. Flammable refrigerants’ data obtained on enhanced tubes showed a typical
trend that external condensation HTCs decrease with increasing wall subcooling. HFC32 and DME showed up to 30% higher HTCs
than those of HCFC22 due to their excellent thermophysical properties. Propylene, propane, isobutane, and butane showed similar
or lower HTCs than those of HCFC22. Beatty and Katz’ correlation predicted the HTCs of the flammable refrigerants obtained
on a low fin tube within a mean deviation of 7.3%. Turbo-C tube showed the best performance due to its 3 dimensional surface
geometry for fast removal of condensate. 相似文献
9.
Nucleate boiling heat transfer for refrigerants, R113, and R113+ wt4% ethanol mixture, an azeotropic mixture under electric
field was investigated experimentally in a single-tube shell/ tube heat exchanger. A special electrode configuration which
provides a more uniform electric field that produces more higher voltage limit against the dielectric breakdown was used in
this study. Experimental study has revealed that the electrical charge relaxation time is an important parameter for the boiling
heat transfer enhancement under electric field. Up to 1210% enhancement of boiling heat transfer was obtained for R113+wt4%
ethanol mixture which has the electrical charge relaxation time of 0.0053 sec whereas only 280% enhancement obtained for R113
which has relaxation time of 0.97 sec. With artificially machined boiling surface, more enhancement in the heat transfer coefficient
in the azeotropic mixture was obtained. 相似文献
10.
Ramakrishna N. Hegde Srikanth S. Rao R. P. Reddy 《Journal of Mechanical Science and Technology》2011,25(11):2927-2934
The pool boiling characteristics of dilute dispersions of CuO nanoparticles in water were studied at atmospheric pressure
on a vertical heating surface. Experimental investigation of different weight concentrations of nanoparticles revealed significant
enhancement in heat flux and deterioration in pool boiling. Out of many reasons, nanoparticles coating the heater surface
was believed to be the reason behind this. Subsequent inspection of the heater surface showed nanoparticles coating the surface,
forming a porous layer. To substantiate the nanoparticle deposition and its effect on heat flux, an investigation was performed
by measuring the surface roughness of the heater surface before and after the experiment. While SEM images of the heater surface
revealed nanoparticle deposition, measurement of surface roughness of the heater surface confirmed it. Formation of the porous
layer on the heater surface as revealed by SEM images provided an excellent location for nucleation sites enhancing heat transfer.
However, deterioration in nucleate boiling at different weight concentrations indicated some phenomenon is working behind
this. 相似文献
11.
非圆截面小通道内R113的流动沸腾换热特性 总被引:1,自引:0,他引:1
针对非圆截面小通道流动沸腾换热研究报道较少的现状,以R113为工质,对4种不同水力直径的正方形、三角形截面小通道内的流动沸腾换热特性进行试验研究,试验参数范围:入口干度,过冷~1.0;质量流速400~ 3 300 kg/(m2?s);热流密度20~150 kW/m2,并将试验结果与相近水力直径的圆通道内流动沸腾试验结果进行了对比分析。试验结果表明:非圆小通道内饱和流动沸腾局部壁面温度与质量流速密切相关,并受热负荷与流动沸腾换热状况的影响;质量流速和壁面热负荷是非圆小通道内流动沸腾换热特性的主要影响因素;与相近水力直径的圆通道内流动沸腾试验数据对比显示,非圆截面小通道具有明显的强化传热作用。 相似文献
12.
针对传统的满液型蒸发换热器,将蒸发器中的水平加热管束按叉排方式紧凑排列形成窄缝空间,在大气压和低压运行条件下,利用窄缝空间沸腾强化换热机理,可以将在低壁温/低热负荷条件下的自然对流换热转化为核态沸腾换热,能有效提高满液式蒸发器的换热性能。和传统的满液型蒸发换热器相比,这种紧凑式蒸发器平均换热系数能提高一倍以上。紧凑蒸发器的管距、管位置,工作压力都对蒸发器的换热性能有显著影响,管距的影响是最大的。不同的压力条件下存在一个对应的最佳管距。在此管距下,蒸发换热器的强化换热性能达到最大。最佳管距对应的管束水力当量直径近似等于池内沸腾时的气泡脱离直径。随着压力减小,最佳管距逐渐增大。同时,紧凑式管束布置引起的窄缝空间内沸腾强化换热强化效果也逐步降低。 相似文献
13.
Nucleate pool boiling heat transfer coefficient (HTCs) were measured with one nonazeotropic mixture of propane/isobutane and two azeotropic mixtures of HFC134a/isobutane and propane/HFC 134a. All data were taken at the liquid pool temperature of 7°C on a horizontal plain tube of 19.0 mm outside diameter with heat fluxes of 10kW/m2 to 80 kW/m2 with an interval of 10 kW/m2 in the decreasing order of heat flux. The measurements were made through electrical heating by a cartridge heater. The nonazeotropic mixture of propane/isobutane showed a reduction of HTCs as much as 41% from the ideal values. The azeotropic mixtures of HFC134a/isobutane and propane/HFC 134a showed a reduction of HTCs as much as 44% from the ideal values at compositions other than azeotropic compositions. At azeotropic compositions, however, the HTCs were even higher than the ideal values due to the increase in the vapor pressure. For all mixtures, the reduction in heat transfer was greater with larger gliding temperature difference. Stephan and Körner’s and Jung et al’s correlations predicted the HTCs of mixtures with a mean deviation of 11 %. The largest mean deviation occurred at the azeotropic compositions of HFC 134a/isobutane and propane/HFC 134a. 相似文献
14.
The study focuses on the heat transfer performance of two-phase closed thermosyphons with plain copper tube and tubes having
50, 60, 70, 80, 90 internal grooves. Three different working fluids (distilled water, methanol, ethanol) are used with various
volumetric liquid fill charge ratio from 10 to 40%. Additional experimental parameters such as operating temperature and inclination
angle of zero to 90 degrees are used for the comparison of heat transfer performance of the thermosyphon. Condensation and
boiling heat transfer coefficients, heat flux are obtained using experimental data for each case of specific parameter. The
experimental results are assessed and compared with existing correlations. The results show that working fluids, liquid fill
charge ratio, number of grooves and inclination angle are very important factors for the operation of thermosyphons. The relatively
high rate of heat transfer is achieved when the thermosyphon with internal grooves is used compared to that with plain tube.
The optimum liquid fill charge ratio for the best heat transfer performance lies between 25% and 30%. The range of the optimum
inclination angle for this study is 20°-30° from the horizontal position. 相似文献
15.
Kookjeong Seo Yongchan Kim Kyu-Jung Lee Youn cheol Park 《Journal of Mechanical Science and Technology》2001,15(8):1156-1164
Evaporation heat transfer coefficients and pressure drops were measured for smooth and micro-fin tubes with R-22 and R-410A. Heat transfer measurements were performed for 3.0 m long horizontal tubes with nominal outside diameters of 9.52 and 7.0 mm over an evaporating temperature range of ?15 to 5°C, a mass flux range of 68 to 211 kg/m2s, and a heat flux range of 5 to 15 kW/m2. It was observed that the heat transfer coefficient increased with mass flux. Evaporation heat transfer coefficients of R-22 and R-410A increased as the evaporating temperature dropped at a lower heat flux. Generally, R-410A showed the higher heat transfer coefficients than R-22 in the range of low mass flux, high heat flux and high evaporating temperature. Pressure drop increased with a decrease of evaporating temperature and a rise of mass flux. Pressure drop of R-22 was higher than that of R-410A at the same mass flux. 相似文献
16.
Jae-Ho Hong Cheon-Ho Park Ho-Young Kwak 《Journal of Mechanical Science and Technology》1998,12(3):493-503
An experimental study was carried out on convective boiling heat transfer for mixtures of R11 and R113 flowing in a uniformly heated vertical tube by measuring the wall and bulk temperatures, and the results were compared with an existing correlation. A reduction of the average heat transfer coefficient for mixtures was verified for flow boiling. It was observed that two kinds of boiling behavior existed depending on mass flux. It was also found that the Chen's correlation was particularly successful for the case of high mass rate flow in which convective boiling prevailed. However in the case of low mass rate flow where nucleate boiling was dominant, the Chen's correlation was found to be inappropriate. Mass transfer resistance in the liquid film played a vital role for determining the heat transfer coefficient of refrigerant mixtures. It has been also found that the equilibrium assumption was hardly applicable to the convective boiling phenomena. 相似文献
17.
Suriyan Laohalertdecha Somchai Wongwises 《Journal of Mechanical Science and Technology》2007,21(12):2168-2177
The results of the condensation heat transfer enhancement and pressure drop of HFC-134a by using the electrohydrodynamic (EHD)
technique are presented. The test section is a horizontal tube-in-tube heat exchanger where the refrigerant flows in the inner
tube and water flows in the annulus. The outer tube is a smooth copper tube havign outer diameter of 21.2 mm. Two types of
inner tubes, smooth and micro-fin copper tubes, are tested. The outer diameter and length of both inner tubes is 9.52 mm and
2.5 m, respectively. A stainless steel cylindrical electrode of 1.47 mm in diameter is placed in the center of the tube. Experiments
are conducted under conditions providing mass flux of 400 kg/m2s, saturated temperature of 40°C, heat flux of 20 kW/m2 and applied voltage of 2.5 kV. The experimental results indicate that the EHD enhancements of the smooth tube are higher
than those of the micro-fin tube over the range of average quality. The maximum heat transfer enhancements for smooth and
micro-fin tubes are 1.1. times and 1.08 times, respectively. For a smooth tube, the pressure drop induced by EHD is considerably
small. However, the application of EHD in a micro-fin tube can lead to 10% increase in the pressure drop. 相似文献
18.
The experiments were performed by using PF-5060 and water to investigate the thermal characteristics from an in-line 6 x 1
array of discrete heat sources for simulating the multichip module which were flush mounted on the top wall of a horizontal,
rectangular channel of aspect ratio 0.2. The inlet temperature was 15°C for all experiments, and the parameters were the heat
flux of simulated VLSI chips with 10, 20, 30, and 40W/cm2 and the Reynolds numbers ranging from 3,000 to 20,000. The measured friction factors for PF-5060 and water gave a good agreement
with the values predicted by the modified Blasius equation within ±6%. The chip surface temperatures for water were lower
by 14.4-21.5°C than those for PF-5060 at the heat flux of 30W/cm2. From the boiling curve of PF-5060, the temperature overshoot at the first heater was 3.5°C and was 2.6°C at the sixth heater.
The local heat transfer coefficients for water were larger by 5.5-11.2% than those for PF-5060 at the heat flux of 30W/cm2, and the local heat transfer coefficients for PF-5060 and water reached a uniform value after the fourth row. This meant
that the thermally fully developed condition was reached after the fourth row. The local Nusselt number data gave the best
agreement with the values predicted by the Malina and Sparrow’s correlation and the empirical correlations for Nusselt number
were provided at the first, fourth and sixth rows for a channel Reynolds number over 3,000. 相似文献
19.
Kwang-II Choi A. S. Pamitran Jong-Taek Oh Hoo-Kyu Oh 《Journal of Mechanical Science and Technology》2005,19(1):156-163
An experimental study of boiling heat transfer with refrigerants R-410A and R-407C is presented. The present paper is focused
on pressure drop and boiling heat transfer coefficient of the refrigerants inside a horizontal smooth minichannel. To evaluate
the diameter size effect on pressure and heat transfer characteristics, minichannels with inner diameters of 1.5 mm and 3.0
mm and with lengths of 1500 mm and 3000 mm respectively are used. The pressure drop increases with mass flux and heat flux
for both inner tube diameters and for both the refrigerants. The pressure drop of R-407C is higher than that of R-410A, but
the heat transfer coefficient of R-410A is higher than of R-407C at the low quality region. The heat transfer coefficient
in the tube with an inner diameter of 1.5 mm is higher than that of 3.0 mm diameter tube at the low quality region. The comparison
of present heat transfer coefficient with the predictions of some previous correlations shows a large deviation. Therefore,
there is a necessity to develop a new correlation. 相似文献
20.
Boiling heat transfer characteristics of a two-phase closed thermosyphons with various helical grooves are studied experimentally
and a mathematical correlation is developed to predict the performance of such thermosyphons. The study focuses on the boiling
heat transfer characteristics of two-phase closed thermosyphons with copper tubes having 50, 60, 70, 80, 90 internal helical
grooves. A two-phase closed thermosyphon with plain copper tube having the same inner and outer diameter as those of grooved
tubes is also tested for comparison. Water, methanol and ethanol are used as working fluid. The effects of the number of grooves,
various working fluids, operating temperature and heat flux are investigated experimentally. From these experimental results,
a mathematical model is developed. In the present model, boiling of liquid pool in the evaporator is considered for the heat
transfer mechanism of the thermosyphons. And also the effects of the number of grooves, the various working fluids, the operating
temperature and the heat flux are brought into consideration. A good agreement between the boiling heat transfer coefficient
of the thermosyphon estimated from experimental results and the predictions from the present mathematical correlation is obtained.
The experimental results show that the number of grooves, the amount of the working fluid and the various working fluids are
very important factors for the operation of thermosyphons. Also, the thermosyphons with internal helical grooves can be used
to achieve some inexpensive and compact heat exchangers in low temperature. 相似文献