Tieyu Gao

An Experimental Investigation of Heat Transfer Characteristics in a Steam-Cooled Square Channel With Rib Turbulators

In recent years, steam is used in vane internal cooling ducts as a new coolant to replace compressed air. Much research has been carried out into the closed circuit steam cooling for vanes. But most of the existing literatures only provide information on the overall cooling effect of vanes. The effects of steam parameters on the cooling effectiveness of the vane internal channel are still not very clear. A steam heat transfer enhancement test platform of turbine blade (vane) internal channel at Equipment Cooling Technology Laboratory, Xi’an Jiaotong University was built to study the steam cooling characteristics in blade (vane) internal cooling channel. In this paper, heat transfer data of a steam-cooled duct have been experimentally obtained under different operating conditions. The range of key governing parameters is presented as follows: Reynolds numbers based on the channel hydraulic diameter (10000–60000), entry absolute pressures (0.3Mpa–0.6Mpa), heat flux of heat transfer surface area (5.5kWm−2 −22kWm−2 ), steam superheat (0K–40K). The results show that the average Nusselt numbers along the centerline of the ribbed wall of the steam-cooled square duct are about 15 percent higher than that of air, but the overall heat transfer characteristics of steam in the square duct are similar to that of air under the same conditions.Copyright

Performance Analysis of 15 kW Closed Cycle Ocean Thermal Energy Conversion System With Different Working Fluids

Closed cycle ocean thermal energy conversion (CC-OTEC) is a way to generate electricity by the sea water temperature difference from the upper surface to the different depth. This paper presents the performance of a 15 kW micropower CC-OTEC system under different working fluids. The results show that both butane and isobutane are not proper working fluids for the CC-OTEC system because the inlet stable operating turbine pressure is in a very narrow range. R125, R143a, and R32, especially R125, are suggested to be the transitional working fluids for CC-OTEC system for their better comprehensive system performance. Moreover, it is recommended that propane should be a candidate for the working fluid because of its excellent comprehensive properties and environmental friendliness. However, propane has inflammable and explosive characteristics. As for the natural working fluid ammonia, almost all performance properties are not satisfactory except the higher net output per unit sea water mass flow rate. But ammonia has relative broader range of the stable operating turbine inlet pressure, which has benefits for the practical plant operation.

The Effect of Vortex Core Distribution on Heat Transfer in Steam Cooling of Gas Turbine Blade Internal Ribbed Channels

Steam has already been used as coolant of gas turbine blade internal cooling. A lot of investigations have been carried out to research the heat transfer performance of steam in ribbed rectangular channels. However, the micro-structure of the flow field especially the vortex distribution is not very clear. As the vortex caused by ribs is one of the main factors that enhance heat and mass transfer, it is very necessary to investigate the distribution of vortex in steam-cooled ribbed channels. The numerical simulation of steam flow field in 45° rib channels were carried out by using ANSYS CFX commercial program. The inlet Reynolds numbers are 30000 and 60000. The wall heat flux and inlet static pressure is 10kW/m2 and 0.3MPa, respectively. In order to find out the distribution and shape of all the main vortices, the technology of vortex core is applied, which is based on the critical point theory and Eigen-values of velocity gradient tensor. The distribution and shape of vortex core clearly indicates that the heat transfer strength of vortices positions is relatively higher than other places. There are four high strength vortices at the near-wall region between every two neighbored ribs. At the side wall which is located at the front side of ribs, high strength vortex exists and washes over the most part of the side wall. In the main flow region, the secondary flow caused by angled ribs can also be seen. The heat and mass transfer performance of steam in angled rib channels can be illustrated by the location and shape of vortex core.Copyright

Numerical and Experimental Study on a New Separation Device for Gas-Solid Two-Phase Flows

A novel separation device is designed to separate solid particles from the high-temperature, high-pressure dusty gas based on oblique shock wave theory. Two-dimensional numerical simulations and a series of experiments of the separation of the supersonic gas-solid two-phase flow are carried out. The aim is to validate the conditions for realizing the gas dedusting and to study the performance of the separation device. It is found that the gas dedusting can be realized only when the physical interface which basically overlaps the geometrical interface are stable and the design flow velocities at the dust-carrying and dust-absorbing nozzles are close to each other. Besides, the experimental results show that 57.83%∼67.28% particles with a sauter average diameter of 9.84 µm can be separated by the experimental device. 75.73%∼80.15% particles with diameters above 8.31 µm and 96%∼100% particles with diameters above 15 µm can also be separated. Larger particles correspond to higher separation efficiency. The same conclusions are also drawn from the numerical simulation results. The study indicates that the present method for the separation of gas-solid two-phase flow is feasible and highly efficient.

Numerical study of the influence of rib orientation on heat transfer enhancement in two-pass ribbed rectangular channel

ABSTRACT Strong secondary flow generated by ribbed channel and U-shaped bend is the key for forced convection performance and energy dissipation in U-shaped cooling passage. This investigation studies the coupling of nine different rib orientation and the 180-degree bend on overall friction loss and forced convection in the U-shaped passage by ANSYS CFX commercial CFD package when Re = 30000. The comprehensive evolution of secondary flow is visualized by vortex core method and colored by turbulence kinetic energy. The qualitative results show that the Nu ratio and overall pressure loss in the downstream passage (Passage 2) is highly affected by the upstream geometry. The N-type rib orientation in Passage 1 delivers more disturbance energy into Passage 2 where P-type rib orientation can reduce the momentum loss of the upstream secondary flows and pressure loss. Based on the understanding of interaction of secondary flow near the bend, modified bend geometry is proposed with 9% thermal performance gain over the existent optimized rib orientation. This investigation suggests vortex core method is a promising visualization tool for the flow control and optimization in U-shaped cooling channel with angled ribs.

Effect of aspect ratio on the flow and heat transfer characteristics of mist/steam in rectangular ribbed channels

ABSTRACT The effects of Reynolds number from 10,000 to 80,000, mist mass ratios from 1 to 6%, and droplet sizes from 5 to 20 µm on flow and heat transfer behaviors of mist/steam in rectangular channels with various aspect ratios of 1/4, 1/2, 1/1, 2/1, and the rib angle of 60° are numerically studied in this paper. Additionally, secondary flow distribution in the four ribbed channels and its effect on heat transfer are analyzed in detail. The 3D steady Reynolds-averaged Navier–Stokes equations with a SST k-ω turbulent model are solved by using ANSYS CFX. The CFD model has been verified by the experimental data for steam-only case with a good agreement. The results indicate that similar secondary flow pattern can be observed in the four ribbed channel except for the size of main secondary flow; the heat transfer augmentation of mist/steam raises as Reynolds number and mist mass ratio increase; a peak value of average Nu is obtained in the case of 15 µm mist among all the sizes of droplets. The friction coefficient decays with increase of Reynolds number and mist mass ratio but is insensitive to droplet sizes. The case of AR = 1/2 obtains the best thermal performance in mist/steam cooling channels.

Numerical Investigation of Secondary Flow Vortex Core Structure in the Two-Pass Rectangular Channel With 45° Ribs

The secondary flow which is generated by the angled rib is one of the key factors of heat transfer enhancement in gas turbine blade cooling channels. However, the current studies are all based on the velocity vector and streamline, which limit the research on the detailed micro-structure of secondary flow. In order to make further targeted optimization on the flow and heat transfer in the cooling channels of gas turbine blade, it is necessary to firstly investigate the generation, interaction, dissipation and the influence on heat transfer of secondary flow with the help of new topological method. This paper reports the numerical study of the secondary flow and the effect of secondary flow on heat transfer enhancement in rectangular two-pass channel with 45° ribs. Based on the vortex core technology, the structure of secondary flow can be clearly shown and studied. The results showed that the main flow secondary flow is thrown to the outer side wall after the corner due to the centrifugal force. Then it is weakened in the second pass and a new main flow secondary flow is generated at the same time near the opposite side wall in the second pass. The Nusselt number distribution has also been compared with the secondary flow vortex core distribution. The results shows that the heat transfer strength is weakened in the second pass due to the interaction between the old main flow secondary flow and the new one. These two secondary flows are in opposite rotation direction, which reduces the disturbance and mass transfer strength in the channel.Copyright

Lattice Boltzmann Simulation of Frost Formation Process

Compared with the conventional mathematical and physical models, the lattice Boltzmann (LB) method is an effective method to simulate the heat and mass transfer in porous media. Frost crystallization aggregation is a very complex process involving inconsistency of frost structures, crystal size distributions, the complex transient shapes, and other numerous influential factors. Assuming the frost is a special porous medium consists of ice crystals and humid air, a mesoscopic model is established to predict the behavior of frost formation based on the lattice Boltzmann equation. The moving boundary condition is adopted in the two-dimensional nine-speed (D2Q9) lattices. The influences of the cold flat surfaces temperature on frost formation process are investigated. The variation laws of frost density and frost layer height are obtained and discussed. Simulation results by the LB model are in agreement with the experiment data from the references.Copyright

Investigation on the Performance of the 15kW OTEC System

In the paper, the performance of two cycle systems, Rankine and Kalina-11 (KSC-11) are compared. The conclusion of this comparison yields the Kalina-11 cycle superior. In the KSC-11 cycle, it is found that the optimal ammonia fraction exists in the system under a fixed turbine inlet pressure. And the corresponding optimal turbine exists in the system under a constant ammonia fraction. When the ocean temperature difference is 19°C, the ammonia fraction has little effect on the 15kW OTEC KSC-11 system maximal net power with the corresponding optimal turbine inlet pressure of 0.6∼0.75MPa. When the ammonia fraction is 0.95, the system has the best combined performance. And the corresponding optimal turbine inlet pressure is revealed to be 0.75MPa. The ratio of total heat exchanger area to net power produced γ is discovered to be 6∼7 m2 /kW.Copyright