Experimental study of the flow between triangular ribs in a square channel

An experimental study of the flow features of a channel with triangular ribs was carried out using particle image velocimetry technique. The experiments were performed for three Reynolds numbers, including 2900, 8400 and 15000, and were based on the hydraulic diameter of the channel and the mean velocity. The mean and instantaneous velocity fields and turbulence statistics were also depicted in detail.

     

Direct numerical simulation of flow characteristics and heat transfer enhancement in a rib-dimpled cooling channel

The present study reports the numerical investigation on the flow characteristics and heat transfer enhancement of the rib-dimpled channels. Two geometric variables were considered: the rib angle, θ, and the length between the rib center and the dimple rim, l. Nine cases were investigated by combining three different rib angles with three different lengths. Direct numerical simulations were conducted with a Reynolds number of 2800. As θ and l changed, the flow characteristics of the rib-dimpled channel were altered, which lead to different characteristics in the flow mixing and heat transfer rate. The span-wise rotating flow and the up-wash counter rotating vortices played an important role in the augmentation of heat transfer rate. The rib-dimpled channel with l = 0.15 and θ = 70° showed the maximum increase of 32 % in the volume goodness factor, in comparison with the general dimpled channel.

     

Forced convection heat transfer of steam in a square ribbed channel

An experimental study of heat transfer characteristics of steam in a square channel (simulating a gas turbine blade cooling passage) with two opposite surfaces roughened by 60 deg parallel ribs was performed. The ranges of key governing parameters were: Reynolds numbers (Re) based on the channel hydraulic diameter (30000–140000), entry gauge pressure (0.2Mpa–0.5Mpa), heat flux of heat transfer surface area (5kWm⁻²–20kWm⁻²), and steam superheat (13°C–51°C). The test channel length was 1000mm, while the rib spacing (p/e) was 10, and the ratio of rib height (e) to hydraulic diameter (D) was 0.048. The test channel was heated by passing current through stainless steel walls instrumented with thermocouples. The local heat transfer coefficients on the ribbed wall from the channel entrance to the fully developed regions were measured. The semi-empirical correlation was fitted out by using the average Nusselt numbers in the fully developed region to cover the range of Reynolds number. The correlation can be used in the design of new generation of gas turbine blade cooled by steam.     

Experimental and numerical crashworthiness investigation of combined circular and square sections

Quasi-static experimental and nonlinear finite element analyses are performed to compare the energy absorption and initial peak load of combined circular and square sections with those of regular circular and square sections. The combined circular and square sections have higher energy absorption and lower initial peak load. These tubes can be widely used to ensure passenger safety in automotive and aerospace landing structures. The predicted numerical crushing load and fold pattern are in good agreement with experimental results. The specific energy absorption capability of the combined tube is significantly higher than that of the square tube and is close to that of the circular tube. The initial peak load of combined tube is significantly lower than that of the circular tube and somewhat lower than that of the square tube. Changing the section dimensions and their length results in higher energy absorption of the combined section than that of circular and square sections. Moreover, the initial peak load in the combined section is lower than that of the circular and square sections in all cases.     

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 square heat sink having staggered and inline pin fin arrays placed at different orientation

Journal of Mechanical Science and Technology - The present problem is devoted to establish the effect of orientation on thermohydraulic performance of a horizontal heat sink with vertical...     

Experimental investigation of heat and mass transfer in absorber with enhanced tubes

In this paper, an experimental study of the absorption process of water vapor into lithium bromide solution is reported. For the purpose of developing high performance absorption chiller/heater utilizing lithium bromide solutions as working fluid, it brings the largest contribution to improve the performance of the absorber which normally requires the largest surface area among the four heat exchangers of the system. The performance of four types of absorber tubes; bare tube, bumping bare tube, floral tube and twisted floral tube, have been experimentally evaluated. The results show that the floral tube and the twisted floral tube show about 40% higher heat and mass transfer performance than the bare tube which is conventionally used in absorbers.     

A numerical study on heat transfer characteristics in a spray column direct contact heat exchanger

A reliable computational heat transfer model has been investigated to define the heat transfer characteristics of a spray column direct contact heat exchanger, which is often utilized in the process involving counterflows for heat and mass transfer operations. Most of the previous studies investigated are one-dimensional unsteady solutions based on rather fragmentary experimental data. Development of a multidimensional numerical model and a computational algorithm are essential to analyze the inherent multidimensional characteristics of a direct contact heat exchanger. The present study has been carried out numerically and establishes a solid simulation algorithm for the operation of a direct contact heat exchanger. Operational and system parameters such as the speed and direction of working fluid droplets at the injection point, and the effects of aspect ratio and void fraction of continuous fluid are examined thoroughly as well to assess their influence on the performance of a spray column.     

Numerical investigation of fluid flow and heat transfer characteristics in a helically-finned tube

In order to investigate the characteristics of flow and heat transfer rate in a Helically-finned tub (HFT), we used continuity, momentum and energy equations under a steady, three-dimensional and incompressible fluid flow assumptions. For the performance metrics, we considered the Darcy friction factor, Colburn j-factor, volume goodness factor and area goodness factor of the HFT. We could also evaluate the effect of geometry parameters on the results of local pressure coefficient, fluid vorticity and Nusselt number of the HFT. We carried out the CFD calculation for a range of laminar flow (Re = 100) and turbulent flow (Re = 2000 and 10000). In a laminar and turbulent flow regime, the friction factor increases with increasing the each geometric parameter. While the Colburn j-factor decreases as increasing these geometric parameters. Consequently, the thermal performance of HFT is poorer than that of single straight circular tube type because of having a small volume and area goodness factor as increasing the Reynolds numbers.     

Experimental and numerical investigation of self-priming siphon side weir on a straight open channel

Siphons are basic and powerful hydraulic instruments which can be used as dam spillway or weir. In a siphon, atmospheric pressure pushes the water into the region of vacuum at the crest of the siphon, and then water falls towards the outlet of siphon. In this study, the siphon used as a side weir was investigated to determine hydrodynamic characteristics experimentally, theoretically and numerically. First, the flow properties of main channel were examined for subcritical flow condition. Then, the velocity and pressures distributions inside the siphon; finally the discharge performance of siphon side weir was determined comparatively, and the results were discussed.     

Experimental investigation of heat transfer enhancement in a circular duct with circumferential fins and circular disks

The characteristics of heat transfer and pressure drop for fully developed turbulent flow in a tube with circumferential fins and circular disks were experimentally studied. The various spacing and sizes of circumferential fins and circular disks were selected as design parameters, while the effects of these parameters on heat transfer enhancement and pressure drop were investigated. In order to quantify the effect of heat transfer enhancement and the increase of pressure drop due to the fins and disks in a tube, the Nusselt numbers and the friction factors for various configurations and operating conditions were compared to those for a corresponding smooth tube. The results showed that the heat transfer rate was significantly enhanced by increasing the height of circumferential fins and decreasing the pitch of circumferential fins. On the other hand, the influence of the disk size and the fin-disk spacing were not significant. Based on the experimental results, a correlation for estimating the Nusselt number was suggested.     

Experimental study on heat transfer characteristics of water-spray-bed heat exchanger

An experimental study was conducted on a water-spray-bed heat exchanger to investigate the heat transfer characteristics. A laboratory-scale test rig was built and its heat transfer characteristics were investigated with respect to various design and operation parameters such as the water spray flow rate, exhaust gas flow rate and number of tube rows. It was found that the implementation of the water spray increased the heat transfer rate to about 1.3 - 2.2 times that of a heat exchanger without water spray, although with a slight increase in the pressure loss. It was thus confirmed that the water spray was effective for enhancing condensing heat recovery from an exhaust gas.

     

A numerical study on flow and heat transfer characteristics of film cooling with a compound angle hole

A numerical simulation has been performed for the investigation of flow and heat transfer characteristics of a film cooling system injected through a hole with compound angle orientation. The finite volume method is employed to discretize the governing equations based on the non-orthogonal coordinate with non-staggered variable arrangement. In order to analyze flow and heat transfer characteristics, velocity, temperature, aerodynamic loss coefficient, skin friction and vorticity are calculated with the variation of the skew angle. The maximum longitudinal vorticity and aerodynamics loss depend strongly on the skew angle. For the symmetric case of β=0 deg, a pair of counter-rotating vortices are formed and the maximum value of the film cooling effectiveness has appeared in the center plane where the skin friction is the minimum. For the skew angle of β=30 deg and above, only one strong counter-clockwise vortex remains in the downstream region and the maximum value of the film cooling effectiveness are obtained on the right side of the vortex. The predicted results for the film cooling effectiveness show good agreements with previous experimental data except the near-hole region.     

An investigation of local heat transfer characteristics in a ventilated disc brake with helically fluted surfaces

Local Nusselt numbers in the cooling flow passage of the automobile disc brake with helically fluted surfaces are presented. The flat surface in the flow passage is modified to the helically fluted surface for the purpose of enhancing the heat transfer rate, thereby reducing the thermal stress and deformation in the disc brake. Thermochromic liquid crystals and shroud-transient technique are used to measure spatially-resolved surface temperature distributions, which are used to deduce local Nusselt numbers. The Reynolds number Re ranges from 30,000 to 70,000, the helix angle θ is fixed at 45° and the dimensionless streamwise distance z/d ranges from 1.5 to 4.5. The results show that in general, local Nusselt numbers monotonically decrease with a distance away from both windward and leeward crests of the helical flute and reach a minimum value near its valley for all Re’s and z/d’s tested. The local Nusselt numbers on the helically fluted grooves are maximum 51.6 to 93.7% higher than values measured on the flat surface. The heat transfer enhancement magnitudes become more pronounced with smaller Re and z/d. The largest enhancement occurs at the windward side of the helical flute at z/d=1.5 and Re=30,000. It is also found that at Re=30,000 the average Nusselt numbers on the helically fluted surface are maximum 37% higher than those on the flat surface. The numerical results show that with 10 braking cycles, the temperatures with helically fluted surface are maximum 44.3%, 36.8%, and 36.6% lower than those with the flat surface in the inlet, the center, and the outlet, respectively.     

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.     

涡轮工作叶片内冷通道换热特性数值模拟

采用非结构化网格和SST紊流模型,求解三维N-S方程,对带90°肋和气膜孔的矩形通道在入口雷诺数60000,罗斯贝数0.11,气膜孔总出流比为0.22时的三维流场进行了数值模拟。分析了通道旋转和静止时各个面的换热变化规律。结果表明,通道静止时,不但布置了粗糙肋的上、下壁面换热得到了增强,光滑的侧壁换热同样获得了增强;通道旋转时流场更加复杂,旋转所产生的二次流动使各个壁面的换热进一步增强。     

A numerical study on the flow and heat transfer characteristics in a noncontact glass transportation unit

Vertical sputtering systems are key equipment in the manufacture of liquid crystal display (LCD) panels. During the sputtering process for LCD panels, a glass plate is transported between chambers for various processes, such as deposition of chemicals on the surface. The minimization of surface scratches and damage to the glass, the rate of consumption of gas, and the stability of the floating glass-plate are key considerations in the design of a gas pad. To develop new, non-contact systems of transportation for large, thin glass plates, various shapes of the nozzle of a gas pad unit were considered in this study. In the proposed nozzle design, negative pressure was used to suppress undesirable fluctuations of the glass plate. After the nozzle’s shape was varied through numerical simulations in two dimensions, we determined the optimal shape, after which three-dimensional analyses were carried out to verify the results from the two-dimensional analyses. The rate of heat transfer from the glass plate, as a result of the gas jet, was also investigated. The average Nusselt number at the glass surface varied from 22.7 to 26.6 depending on the turbulence model, while the value from the correlation for the jet array was 23.5. It was found that the well-established correlation equation of the Nusselt number for the circular jet array can be applied to the cooling of the glass plates.     

Investigation of boiling heat transfer characteristics of two-phase closed thermosyphons with various internal grooves

The boiling heat transfer characteristics of two-phase closed thermosyphons with internal grooves are studied experimentally and a simple mathematical model is developed to predict the performance of such thermosyphons. The study focuses on the boiling heat transfer characteristics of a two-phase closed thermosyphons with copper tubes having 50, 60, 70, 80, 90 internal grooves. A two-phase closed thermosyphon with plain copper tube having the same inner and outer diameter as those of grooved tube is also tested for comparison. Methanol is used as working fluid. The effects of the number of grooves, the operating temperature, the heat flux are investigated experimentally. From these experimental results, a simple mathematical model is developed. In the present model, boiling of liquid pool in the evaporator is considered for the heat transfer mechanism of the thermosyphon. And also the effects of the number of grooves, the operating temperature, 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 model is obtained. The experimental results show that the number of grooves and the amount of the working fluid are very important factors for the operation of thermosyphons. The two-phase closed thermosyphon with copper tubes having 60 internal grooves shows the best boiling heat transfer performance.     

Numerical investigation on frictional pressure loss in a perfect square micro channel with roughness and particles

A numerical study is performed to investigate the effect of inner surface roughness and microparticles on adiabatic single phase frictional pressure drop in a perfect square micro channel. With the variation of particles sizes (0.1 to 1 μm) and occupied volume ratio (0.01 to 10%) by particles, the Eulerian multi-phase model is applied to a 100 μm hydraulic diameter perfect square micro channel in laminar flow region. Frictional pressure loss is affected significantly by particle size than occupied volume ratio by particles. The particle properties like density and coefficient of restitution are investigated with various particle materials and the density of particle is found as an influential factor. Roughness effect on pressure drop in the micro channel is investigated with the consideration of roughness height, pitch, and distribution. Additionally, the combination effect by particles and surface roughness are simulated. The pressure loss in microchannel with 2.5% relative roughness surface can be increased more than 20% by the addition of 0.5 μm diameter particles.