矩形带肋通道中蒸汽与空气换热特性比较试验研究

模拟实际燃气轮机叶片内冷通道几何及传热结构,研究了蒸汽和空气在两面带有肋片的矩形通道中当雷诺数为10000⁸0000时的换热和摩擦特性。试验通道宽高比(肋片在宽面上)为0.5,肋间距p/e为10,通道阻塞比为0.047,试验通道长度L为1000mm。试验结果显示,蒸汽和空气在带肋通道中的平均换热系数,平均摩擦系数和换热性能随雷诺数的变化趋势几乎相同;在相同试验条件下,蒸汽在带肋通道中的平均换热系数比空气高30.2%,平均摩擦系数比空气高18.4%;蒸汽在带肋通道带肋面和光滑面上的换热性能比空气分别高8.4%和7.3%。     

Contrastive experimental study on heat transfer and friction characteristics in steam cooled and air cooled rectangular channels with rib turbulators

The present experiment compares the heat transfer and friction characteristics in steam cooled and air cooled rectangular channels (simulating a gas turbine blade cooling passage) with two opposite rib-roughened walls. The Reynolds number (Re) whose length scale is the hydraulic diameter of the passage is set within the range of 10000–60000. The channel length is 1000 mm. The pitch-to-rib height ratio, the channel aspect ratio and the channel blockage ratio is 10, 0.5 and 0.047, respectively. It is found that the average Nu, the average friction coefficient, and the heat transfer performance of both steam and air in the ribbed channels show almost the same change trend with the increase of Re. Under the same test conditions, the average Nu of steam is 30.2% higher than that of air, the average friction coefficient is 18.4% higher, and the heat transfer performances of steam on the ribbed and the smooth walls are 8.4% and 7.3% higher than those of air, respectively. In addition, semi-empirical correlations for the two test channels are developed, which can predict the Nu under the given test condition. The correlations can be used in the design of the internal cooling passage of new generation steamcooled gas turbine blade/vane.     

An investigation on friction factors and heat transfer coefficients in a rectangular duct with surface roughness

An investigation on the fully developed heat transfer and friction factor characteristics has been made in rectangular ducts with one-side roughened by five different shapes. The effects of rib shape geometries as well as Reynolds numbers are examined. The rib height-to-duct hydraulic diameter, pitch-to-height ratio, and aspect ratio of channel width to height are fixed ate/De=0.0476,P/e=8, andW/H=2.33, respectively. To understand the characteristics of heat transfer enhancements, the friction factors are also measured. The data indicates that the triangular type rib has a substantially higher heat transfer performance than any other ones.     

Effect of hole shape on the heat transfer in a rectangular duct with perforated blockage walls

Heat transfer coefficients were measured by the improved hue detection based liquid crystal technique in a turbine blade internal cooling passage model with blockage walls. In the experiments, blockages with 9 holes of circular, wide, narrow shapes were used and for the circular shape, the number of hole of 7, 9, and 11 were tested. For all cases, the perforated area was kept same. Results showed that the staggered impingement jets increased heat transfer coefficient, however, pressure drop also increased greatly. Generally, Nusselt number ratio and the thermal performance factor decreased as Reynolds number increased. For all Reynolds numbers tested, the blockage wall with wide holes gave more uniform heat transfer coefficient and higher thermal performance factor. As the number of hole increased from 7 to 11, the distribution of heat transfer coefficient became uniform and the thermal performance factor increased. This paper was presented at the 9^th Asian International Conference on Fluid Machinery (AICFM9), Jeju, Korea, October 16–19, 2007.     

Heat transfer and friction characteristics in steam cooled rectangular channels with rib turbulators

We studied the heat transfer and friction characteristics in steam-cooled rectangular channels with rib turbulators on W side or H side walls in the Reynolds number (Re) range of 10000-80000. Each of the test channels was welded by four stainless steel plates to simulate the actual geometry and heat transfer structure of blade/vane internal cooling passage. The length of the channel L was 1000 mm, the cross section of the channel was 40 mm × 80 mm, and the pitch-to-rib height ratio p/e was kept at 10. The channel blockage ratio (W/H) was 0.047. Results showed that the Nusselt number (Nu) distributions displayed different trends at the entrance region with the increase of Re for the rib turbulators on the W side walls. The heat transfer performance of the rib turbulators on the H side walls was about 24-27% higher than that on the W side walls at the same pumping power. In addition, semi-empirical correlations for the two cases, rib turbulators on W side walls and rib turbulators on H side walls, were developed based on the heat transfer results, which could be used in the design of the internal cooling passage of new generation steam-cooled gas turbine blade/vane.     

Impeller inlet geometry effect on performance improvement for centrifugal pumps

This research treats the effect of impeller inlet geometry on performance improvement for a boiler feed pump, who is a centrifugal pump having specific speed of 183 m·m³min⁻¹·min⁻¹ and close type impeller with exit diameter of 450 mm. The hydraulic performance and cavitation performance of the pump have been tested experimentally. In order to improve the pump, five impellers have been considered by extending the blade leading edge or applying much larger blade angle at impeller inlet compared with the original impeller. The 3-D turbulent flow inside those pumps has been analyzed basing on RNG k-ɛ turbulence model and VOF cavitation model. It is noted that the numerical results are fairly good compared with the experiments. Based on the experimental test and numerical simulation, the following conclusions can be drawn: (1) Impeller inlet geometry has important influence on performance improvement in the case of centrifugal pump. Favorite effects on performance improvement have been achieved by both extending the blade leading edge and applying much larger blade angle at impeller inlet; (2) It is suspected that the extended leading edge have favorite effect for improving hydraulic performance, and the much larger blade angle at impeller inlet have favorite effect for improving cavitation performance for the test pump; (3) Uniform flow upstream of impeller inlet is helpful for improving cavitation performance of the pump. This paper was presented at the 9^th Asian International Conference on Fluid Machinery (AICFM9), Jeju, Korea, October 16–19, 2007.     

Measurement of the heat transfer coefficient in the dimpled channel: effects of dimple arrangement and channel height

Heat transfer coefficients were measured in a channel with one side dimpled surface. The sphere type dimples were fabricated, and the diameter (D) and the depth of dimple was 16 mm and 4 mm, respectively. Two channel heights of about 0.6D and 1.2D, two dimple configurations were tested. The Reynolds number based on the channel hydraulic diameter was varied from 30000 to 50000. The improved hue detection based transient liquid crystal technique was used in the heat transfer measurement. Heat transfer measurement results showed that high heat transfer was induced downstream of the dimples due to flow reattachment. Due to the flow recirculation on the upstream side in the dimple, the heat transfer coefficient was very low. As the Reynolds increased, the overall heat transfer coefficients also increased. With the same dimple arrangement, the heat transfer coefficients and the thermal performance factors were higher for the lower channel height. As the distance between the dimples became smaller, the overall heat transfer coefficient and the thermal performance factors increased. This paper was recommended for publication in revised form by Associate Editor Yong Tae Kang Jae Su Kwak received his B.S. and M.S. degrees in Mechanical Engineering from Korea University in 1996 and 1998, respectively. He then received his Ph.D. from Texas A&M University in 2002. Dr. Kwak is currently an Assistant Professor at the School of Aerospace and Mechanical Engineering at Korea Aerospace University in Goyang-City, Korea. His main research interests include gas turbine heat transfer, compact heat exchanger, and enhancement of heat transfer.     

Heat transfer and friction factor enhancement in a square channel having integral inclined discrete ribs on two opposite walls

The influence of a gap provided in integral inclined ribs on heat transfer and friction factor enhancement is investigated. Experiments are conducted to obtain heat transfer and friction factor characteristics in a square channel with two opposite in-line ribbed walls for Reynolds numbers from 5000 to 40000. The test section of square channel composed of integral inclined ribs with a gap and has a length-tohydraulic diameter ratio (L/D _h ) of 20. The rib pitch-to-height ratio (p/e) is 10, the rib height-to-hydraulic diameter ratio (e/D _h ) is 0.060 and rib attack angle (α) varies in the range of 30⁰ to 90⁰ (4 steps). The relative gap position (d/W) and relative gap width (g/e) is varied in the range of 1/5–2/3 (5 steps) and 0.5–2.0 (4 steps), respectively. The enhancement in heat transfer and friction factor of this roughened duct was compared with smooth duct and duct roughened with continuous inclined ribs (with no gap) under similar flow condition. Presence of inclined ribs with a gap yields about 4-fold enhancements in Nusselt number and about 8-fold increase in the friction factor compared with smooth duct and about 1.3 times and 1.4 times higher than the case of continuous ribs (without gaps) for the entire range of parameters investigated. Ribs with relative gap width of 1.0 at relative gap position of 1/3 and attack angle of 60° provides maximum heat transfer and friction factor enhancement.     

Experimental and numerical investigation of heat transfer characteristics in a square channel with various truncated ribs

Oblique ribs are widely applied to the internal cooling of turbine blades to promote the heat transfer between blade wall and coolant. In this study, the effect of several new types of truncated ribs on the heat transfer characteristics in 45° oblique rib channels is investigated experimentally and numerically. The numerical results obtained by the SST k-ω turbulence model agree well with the experimental data for the Reynolds number ranging from 10000 to 60000. The results indicate a significant entrance effect on the heat transfer in truncated rib channels. The numerical results show that ribs continuously truncated at 3.8 mm gives the best heat transfer performance among the newly truncated ribs. Compared with the original structure, the Nusselt number and heat transfer enhancement factor of newly truncated ribs increased by 24.6 % and 17.8 %, respectively. Concurrently, the friction factor is reduced by 5.1 %.

     

Two-phase flow heat transfer of propane vaporization in horizontal minichannels

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.     

涡轮动叶表面气膜冷却换热实验研究

这里在大尺寸低速叶栅传热风洞中，对某型涡轮动叶表面有无气膜冷却的换热情况进行了详细的实验研究，实验结果表明，不同孔位出流的换热由于孔排下游表面来流速度及叶片表面曲率的不同而有着不同的规律，即主流雷诺数对叶片表面特别是压力面和前缘区域的换热系数比的影响较小，吹风比对换热系数影响较大，并且随气膜孔位置和来流雷诺数的变化而情况复杂。     

Mobile platform for hydraulic turbine blade repair robot

The wall-climbing mobile platform (MP) of a robot for repairing a hydraulic turbine blade onsite is developed. The MP is equipped with ferromagnetic adhesive devices and can work on a spatial curved surface. The contradiction between mobility and load-bearing ability is analyzed, and the problem of self-adaptation to the curved face is solved using differential-driven wheeled locomotion with ferromagnetic adhesive devices. The platform adheres to the blade surface through the force provided by the ferromagnetic devices, and a certain gap exists between the magnetic devices and the blade’s surface. A mechanism of three revolution degrees of freedom, which connects the magnetic devices with the platform’s chassis, is developed to make the platform self-adapt to the complex curved surface of the turbine blade. A proof-of-principle prototype has been manufactured, and experiments prove the success of the MP. The payload of the zero-turn-radius MP with excellent maneuverability exceeds 80 kg. The platform can automatically adapt to complex spatial surfaces, which satisfy the requirements of a hydraulic turbine blade in-situ repair robot. __________ Translated from Chinese Journal of Mechanical Engineering, 2006, 42(11): 156–161 [译自:机械工程学报]     

An experimental investigation of heat transfer in forced convective boiling of R134a,R123 and R134a/R123 in a horizontal tube

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.     

Experimental measurement and numerical computation of the air side convective heat transfer coefficients in a plate fin-tube heat exchanger

The air-side forced convective heat transfer of a plate fin-tube heat exchanger is investigated by experimental measurement and numerical computation. The heat exchanger consists of a staggered arrangement of refrigerant pipes with a diameter of 10.2 mm and a fin pitch of 3.5 mm. In the experimental study, the forced convective heat transfer was measured at Reynolds numbers of 1082, 1397, 1486, 1591 and 1649 based on the diameter of the refrigerant piping and on the maximum velocity. The average Nusselt number for the convective heat transfer coefficient was also computed for the same Reynolds number by using the commercial software STAR-CD with the standard k - ɛ turbulent model. It was found that the relative errors of the average Nusselt numbers between the experimental and numerical data were less than 6 percent in a Reynolds number range of 1082∼1649. The errors between the experiment and other correlations from literature ranged from 7% to 32.4%. However, the literature correlation of Kim et al. is closest to the experimental data within a relative error of 7%. This paper was recommended for publication in revised form by Associate Editor Man-Yeong Ha Jin-Gi Paeng recieved a bachelor’s degree in Aero Mechanical Engineering from Gyeong-sang National University in 2000. He then went on to recieve his M.S. degrees from Changwon National University in 2004. Currently, he completed the doc-tor’s course and a doctoral dissertation in 2007 and 2008, respectively. He will take a doctorate in 2008.     

Simulation of nanochannel membrane formation

Monte Carlo simulations of atomic processes on the surface of silicon nanochannel membranes during molecular-beam epitaxy and subsequent thermal oxidation are performed. It is demonstrated that silicon deposition on Si(001) wafers with 1–100 nm cylindrical pores results in constriction of channel inlets. The rates of reduction of the nanochannel diameter are estimated as functions of the wafer temperature, silicon deposition rate, and initial nanochannel diameter. Optimal conditions of silicon deposition on nanochannel membranes are determined: the wafer temperature of 250–450°C and silicon flux intensity of 10⁻² to 10 monolayers (ML) per second. Under these conditions, the rate of reduction of the nanochannel inlet diameter is 0.13–0.15 nm/ML, which allows membrane channel modifications over a wide range down to several nanometers. Simulations of nanochannel membrane oxidation in an oxygen flux shows that precise reduction of nanochannel inlet diameters down to complete sealing of the channel due to oxide growth is only possible for small diameters of the initial pores. For channels with large lateral sizes, the effect of reduction of the channel inlet diameter due to oxidation is insignificant. Oxidation of pores enhances their stability to subsequent high-temperature treatment.     

A numerical study of impingement heat transfer in a confined circular jet

Heat transfer characteristics of a submerged circular jet impingement with a confined plate was studied numerically. The continuity, momentum and energy equations were solved simultaneously. FIDAP, a finite element code, was used to formulate and solve the matrix equations for fluid elements. The effects of channel height and Reynolds number on the local Nusselt number were considered in the range of H=0.5–1.5 and Re=100–900, respectively. It was found that the channel height influenced strongly on the surface temperature, shear stress and pressure drop. The peak temperature was observed and gradually moved outward to the rim of the heated circular plate with increasing the Reynolds number, which may be related to flow recirculation region in the channel. It is also noted that the pressure drop increased more than the average heat transfer coefficient as the Reynolds number increased. For Pr=7, the Nusselt number was much more dependent on the Reynolds number than the channel height, and the magnitude of the second peak in the Nusselt number distribution increased as the Reynolds number increased. The local Nusselt number calculated based on a mixing-cup temperature was considerably different from that using the inlet nozzle temperature for H=0.5 and Re=100. The present study showed that the local Nusselt number of a confined submerged jet was significantly larger than that of the unconfined free jet which was available in the literature.     

Thermal characteristics of a multichip module using PF-5060 and water

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.     

Test investigation on hydraulic losses in the discharge passage of an axial-flow pump

In a discharge passage with a guide blade discharge circulation and secondary flow because of bend pipe, the flow in a 1-channel discharge passage of an axial flow pump is a complicated spiral flow. For a 2-channel passage, the discharge in the left channel is bigger than that in the right, and the passage hydraulic losses are abnormal. In this study, the section current energy of the passage is accurately measured and determined with a 5-hole probe. The hydraulic loss characteristics are determined and analyzed. The methods deducing the hydraulic losses are investigated. The results indicate that the passage hydraulic losses are not proportional to the flow discharge. Compared with a circular pipe, the hydraulic losses of a divergent discharge passage are smaller and the pump assembly efficiency is 10%–30% higher. As for the 1-channel passage, the axial-flow pump outlet circulation is usually too big; the passage hydraulic losses are also big, but a small circulation can slightly reduce hydraulic losses. As for the 2-channel passage, discharges in the two channels are not equal and the hydraulic losses increase. The outlet guide blade with a small discharge circulation or without circulation could reduce discharge passage hydraulic losses and increase pump assembly efficiency by 6%–11%. __________ Translated from Chinese Journal of Mechanical Engineering, 2006, 42(5): 39–44 [译自: 机械工程学报]     

Influences of mach number and flow incidence on aerodynamic losses of steam turbine blade

An experiment was conducted to investigate the aerodynamic losses of high pressure steam turbine nozzle (526A) subjected to a large range of incident angles (−34° to 26°) and exit Mach numbers (0.6 and 1.15). Measurements included downstream Pitot probe traverses, upstream total pressure, and endwall static pressures. Flow visualization techniques such as shadowgraph and color oil flow visualization were performed to complement the measured data. When the exit Mach number for nozzles increased from 0.9 to 1.1 the total pressure loss coefficient increased by a factor of 7 as compared to the total pressure losses measured at subsonic conditions (M₂<0.9). For the range of incidence tested, the effect of flow incidence on the total pressure losses is less pronounced. Based on the shadowgraphs taken during the experiment, it’s believed that the large increase in losses at transonic conditions is due to strong shock/ boundary layer interaction that may lead to flow separation on the blade suction surface.     

基于Pro/E软件平台的混流式水轮机转轮叶片形状拟合

根据混流式水轮机叶片设计方程，基于Pro／ENGEINEER软件设计平台，对混流式水轮机叶片进行了形状拟合，得到了叶片拟合形状，完全符合叶片加工的工艺要求，也为研制水轮机修复专用机器人提供了研制条件。