共查询到20条相似文献,搜索用时 31 毫秒
1.
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
2.
In this study, the effect of carbon nanotubes (CNTs) on nucleate boiling heat transfer is investigated. Three refrigerants
of R22, R123, R134a, and water were used as working fluids and 1.0 vol.% of CNTs was added to the working fluids to examine
the effect of CNTs. Experimental apparatus was composed of a stainless steel vessel and a plain horizontal tube heated by
a cartridge heater. All data were obtained at the pool temperature of 7°C for all refrigerants and 100°C for water in the
heat flux range of 10–80 kW/m 2. Test results showed that CNTs increase nucleate boiling heat transfer coefficients for all fluids. Especially, large enhancement
was observed at low heat fluxes of less than 30 kW/m 2. With increasing heat flux, however, the enhancement was suppressed due to vigorous bubble generation. Fouling on the heat
transfer surface was not observed during the course of this study. Optimum quantity and type of CNTs and their dispersion
should be examined for their commercial application to enhance nucleate boiling heat transfer in many applications. 相似文献
3.
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/m 2 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
4.
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
5.
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/m 2·s and q″ = 20 kW·m 2. 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
6.
Continuing efforts to achieve increased circuit performance in electronic package have resulted in higher power density at chip and module level. As a result, the thermal management of electronic package has been important in maintaining or improving the reliability of the component. An experimental investigation of thermosyphonic boiling in vertical tube and channel made by two parallel rectangular plates was carried out in this study for possible application of the direct immersion cooling. Fluorinert FC-72 as a working fluid was used in this experiment. Asymmetric heated channel of open periphery with gap size of 1, 2, 4 and 26 mm and uniformly heated vertical tubes with diameter of 9, 15 and 20 mm were boiled at saturated condition. The boiling curves from tested surfaces exhibited the boiling hysteresis. It was also found that the gap size is not a significant parameter for the thermosyphonic boiling heat transfer with this Fluorinert. Rather pool boiling characteristics appeared for larger gap size and tube diameter. The heat transfer coefficients measured were also compared with the calculation results by Chen’s correlation. 相似文献
7.
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
8.
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
9.
This study concerns the performance of the heat transfer of the thermosyphons having 60, 70, 80, 90 axial internal low-fins
in which boiling and condensation occurr. Water, HCFC-141b and CFC-11 have been used as the working fluids. The operating
temperature, the liquid charge ratio and the inclination angle of thermosyphons have been used as the experimental parameters.
The heat flux and heat transfer coefficient at the condenser are estimated from experimental results. The experimental results
have been assessed and compared with existing theories. As a result of the experimental investigation, it was found that the
maximum heat flow rate in the thermosyphons is dependent upon the liquid charge ratio and inclination angle. A relatively
high rate of heat transfer has been achieved by the thermosyphon with axial internal low-fins. The inclination of a thermosyphon
has a notable influence on the condensation. In addition, the overall heat transfer coefficients and the characteristics at
the operating temperature are obtained for the practical applications. 相似文献
10.
以乙醇为工作液体,对声空化作用下直径为20 mm的水平铜管的沸腾传热进行试验研究.试验中对空化强度及超声换能棒和试件间的距离进行测定.试验研究发现,声空化对过冷沸腾有显著强化作用;在核态沸腾区,声空化对沸腾起始区域的强化率最大,但是随热流密度的增加,强化率逐渐减小;当热流密度相等时,强化率随空化强度的增加而增大.声空化引起的液体的宏观湍动及由于声冲流的存在导致的边界层厚度的减薄,直接或间接地影响加热表面上气泡胚胎的生成、长大和脱离. 相似文献
11.
Journal of Mechanical Science and Technology - An experimental study was performed to measure FC-72(C6F14) flow boiling heat transfer and pressure drop in heat sinks for electronics cooling. The... 相似文献
12.
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/m 2, vapor quality 0–100 percent and mass velocity of 150–600 kg/m 2s. 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
13.
In a flooded refrigerant evaporator, where enhanced tubes are frequently used, lubrication oil inevitably circulates with the refrigerant. However, the literature shows that systematic studies on this subject are lacking. In this study, the effects of oil on the pool boiling of pored surfaces having a range of pore diameter (0.1 to 0.3 mm) and pitch (0.75 to 3.0 mm) were investigated using R-134a/polyester oil (POE) and R-123/mineral oil (MO) mixtures. The saturation temperature was 26.7 °C, and the oil concentration was varied up to 10 %. The results showed that the oil reduced the heat transfer coefficient. This was true for all the pored surface as well as the smooth surface. Overall, the samples having a ‘small open area’ yielded a significant degradation at a low heat flux, whereas the samples having a ‘large open area’ yielded a noticeable degradation at a high heat flux. Meanwhile, the heat transfer degradation was larger for R-134a/POE than R-123/MO, and the reason was attributed to the oil effect on the surface tension, which was stronger for R-134a/POE. The flow visualization results showed that, with the increase of oil concentration, the bubble departure diameter decreased. Similarly, the bubble generation frequency and nucleation site density decreased. These changes of the bubble dynamic parameters certainly were responsible for the heat transfer degradation. A model was developed extending that of Pastuszko et al. [27] to predict the heat transfer coefficients as well as bubble dynamic parameters. The model predicted 92 % of the heat transfer coefficients within ± 40 %. 相似文献
14.
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
15.
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
16.
建立单面加热垂直矩形窄通道流动沸腾换热试验装置,针对截面250mm×3.5mm的窄缝通道,对水流动沸腾换热特性进行试验研究。通过试验分析可知:(1)随着干度的增加,局部换热系数先增加后减小,有一个最大值,此时处于饱和核沸腾区域,其蒸汽干度也接近于0,同时也接近于沸腾起始点。相应地流体从单相流-泡状-块状流-搅拌-环状流转变。(2)在流动沸腾换热中,热流密度对核态沸腾换热有明显影响,而对流动沸腾液膜蒸发的影响甚小,所以可以认为由热流密度的变化而引起的换热变化,主要表现在核态沸腾。(3)入口温度的变化对单相流动的换热系数有影响,而沸腾换热系数与流型及汽泡的产生及扰动有极大关系,入口温度对流动沸腾局部换热系数基本没有影响。 相似文献
17.
An analytic approach has been employed to study condensate film thickness distribution inside cave-shaped cavity of a flat
plate heat pipe. The results indicate that the condensate film thickness largely depends on mass flow rate and local velocity
of condensate. The increasing rate of condensate film for circular region reveals about 50% higher value than that of vertical
region. The physical properties of working fluid affect significantly the condensate film thickness, such as the condensate
film thickness for the case of FC-40 are 5 times larger than that of water. In comparison with condensation on a vertical
wall, the average heat transfer coefficient in the cave-shaped cavity presented 10-15% lower values due to the fact that the
average film thickness formed inside the cave-shaped cavity was larger than that of the vertical wall with an equivalent flow
length. A correlation formula which is based on the condensate film analysis for the cave-shaped cavity to predict average
heat transfer coefficient is presented. Also, the critical minimum fill charge ratio of working fluid based on condensate
film analysis has been predicted, and the minimum fill charge ratios for FC-40 and water are about Ψ crit= 3-7%, Ψ crit=0.5-1.3%, respectively, in the range of heat flux q” = 5-90kW/ 2 相似文献
18.
对微通道中液氮流动沸腾换热特性进行试验研究和分析。给出典型的沸腾曲线,分析壁温、干度和换热系数沿微通道管程的变化规律,考察热流密度、质量流量和压力对流动沸腾换热的影响。将126个试验数据点与四个换热关联式比较,并对微通道中流动沸腾换热机理进行分析。结果表明,在多数情况下干度和热流密度对沸腾换热系数的影响较小,换热系数主要决定于质量流量和压力,随两者增加而增加,换热以对流蒸发为主导机理。KLIMENKO关联式预测效果最好,TRAN微通道关联式次之,对常规管道得到广泛使用的CHEN关联式和SHAH关联式都远远高估了试验值。基于两相流压降和换热特性分析,推知微通道中的两相流流型不同于常规管道:在低干度情况下,流型以弥散泡状流为主;而在高干度情况下,流型以由雾状汽芯和不规则液膜组成的环状流为主。 相似文献
19.
磨削接触区材料去除厚度是不一致的,同时,在微量润滑过程中,雾滴之间的运动特征存在差异且易受其他因素的影响,致使整个接触区的磨削温度分布呈现出非线性,换热机理也异常复杂。从雾化机理出发,对影响换热效果的两个关键因素--雾滴直径和雾滴速度进行了分析。依据雾滴在不同壁温处表现出的不同换热特性,将磨削区划分为无沸腾换热、核态沸腾换热、过渡沸腾换热和稳定膜态沸腾换热四个不同的换热区域,建立了微量润滑磨削区的换热系数数学模型。在此基础上,运用有限元技术对微量润滑磨削表面的温度场进行了仿真分析,采用单级热电偶技术测量了磨削温度,发现磨削区仿真温度值与实验测量值吻合较好,表明通过该理论获得的微量润滑磨削表面换热系数是可信的。 相似文献
20.
强化传热技术自20世纪70年代以来获得了迅速发展和广泛应用。换热管作为强化传热的关键传热元件亦获得了迅速的发展。本文综述了换热管表面强化传热结构制造方法及存在的不足,在此基础上,分析了今后的发展趋势。 相似文献
|