He II liquid/vapour phase separator for large dynamic range operation

The world’s first 1000MW double reheat ultrasupercritical unit has been in operation since September 25th, 2015 in Taizhou, China. The thermal efficiency at turbine heat-rate acceptance (THA) condition is around 51%, which is the highest among all condensing units in coal-fired power plants around the world. However, the resultant superheat degree of the extraction steam is relatively high, leading to a large temperature difference in heat transfer process in the regenerative system, thereby a great exergy loss. In order to utilize the superheat of turbine bleeds more effectively, we present a scheme by employing an outer steam cooler (OSC) after the last high pressure heater in series to use the superheat to heat the feed water. Based on the newly installed unit in Taizhou, we examine the energy saving effect of the superheat utilization of different bleeds and their possible combinations respectively. The influencing factors of the mass flow rate, superheat, and effective superheat of the extraction steam are studied. Thermodynamic analyses revealed that the second extraction steam has not only high effective superheat, but also large mass flow rate, so in the overall efficiency improvement it ranks first and the third extraction steam ranks second. Although the fourth extraction steam has the largest superheat, it ranks third as the result of relatively lower mass flow rate. It was found that at nominal load, by adopting OSC’s to utilize the superheat of the second to sixth extraction steam, temperature of the feed water can increase by 8.1 °C, 3.5 °C, 2.6 °C, 1.1 °C, and 1 °C respectively, and the net coal consumption reduces by 0.73g/kWh, 0.47g/kWh, 0.40g/kWh, 0.21g/kWh and 0.22g/kWh accordingly. Consequently, three possible schemes are recommended for future design: one is to adopt one OSC to utilize the superheat of the second extraction steam, in return of 8.1°C increment in feed water temperature and 0.73g/kWh reduction of the net coal consumption; the second is to adopt two OSC’s to utilize the superheat of the second and third extraction steam at the same time, in return of 11.4 °C increment in feed water temperature and 1.21g/kWh reduction of the net coal consumption; and the last is to apply three OSC’s to utilize the superheat of the second to the fourth extraction steam simultaneously, to achieve 13.9°C increment in feed water temperature, and 1.62g/kWh reduction of the net coal consumption.

The elliptical hole has the potential to promote jet breakup and improve atomization and mixing quality. The flow behavior in the nozzle and the outflow characteristics have an important influence on the atomization and mixing process of the downstream fuel. Therefore, the inner cavitation flow processes and outflow characteristics for elliptical holes for scramjet engine at injection pressures of 3 MPa and 5 MPa were investigated using the Volume of Fluid (VOF) method and Large Eddy Simulation (LES). The numerical findings demonstrated that the cavitation region and vortex structures of elliptical holes mainly appeared on the long axial plane. The cavitation intensity of elliptical holes was weaker than that of the circular hole. Compared with the circular hole, the gas phase volume fraction of elliptical hole with the aspect ratio of 4 was reduced by about 22.1 and 21.1% respectively under 3MPa and 5MPa injection pressure. The application of elliptical holes inhibited the generation of cavitation and had large mass flow rate and flow coefficient. The mass flow rate of the elliptical hole with the aspect ratio of 4 was the largest, and the mass flow rate increased by about 5.1% compared with that of the circular hole. Elliptical holes showed a smaller velocity coefficient and a larger area coefficient when cavitation existed. The elliptical hole with the aspect ratio of 4 held the smallest velocity coefficient and largest area coefficient, with the difference of 12.1% and 18.9% respectively from the circular hole. Another important point was that elliptical holes enhanced the turbulent disturbance at the hole outlet, which facilitated the breakup of downstream jet.

The thermal transpiration of molecular gas is investigated based on the model of Wu et al. [“A kinetic model of the Boltzmann equation for non-vibrating polyatomic gases,” J. Fluid Mech. 763, 24–50 (2015)], which is solved by a synthetic iterative scheme efficiently and accurately. A detailed investigation of the thermal slip coefficient, Knudsen layer function, and mass flow rate for molecular gas interacting with the inverse power-law potential is performed. It is found that (i) the thermal slip coefficient and Knudsen layer function increase with the viscosity index determined by the intermolecular potential. Therefore, at small Knudsen number, gas with a larger viscosity index has a larger mass flow rate; however, at late transition and free molecular flow regimes, this is reversed. (ii) The thermal slip coefficient is a linear function of the accommodation coefficient in Maxwell’s diffuse–specular boundary condition, while its variation with the tangential momentum accommodation coefficient is complicated in Cercignani–Lampis’s boundary condition. (iii) The ratio of the thermal slip coefficients between monatomic and molecular gases is roughly the ratio of their translational Eucken factors, and thus, molecular gas always has a lower normalized mass flow rate than monatomic gas. (iv) In the transition flow regime, the translational Eucken factor continues to affect the mass flow rate of thermal transpiration, but in the free molecular flow regime, the mass flow rate converges to that of monatomic gas. Based on these results, accommodation coefficients were extracted from thermal transpiration experiments of air and carbon dioxide, which are found to be 0.9 and 0.85, respectively, rather than unity used in the literature. The methodology and data presented in this paper are useful, e.g., in the pressure correction of capacitance diaphragm gauge when measuring low gas pressures.

Mass flow rate through a horizontal opening at small pressure differences

This work concerns the modeling of buoyancy-dominated turbulent diffusion flames. In these flames, ambient air is entrained just above the burning surface into the reactive zone at large mass flow rates. To describe the buoyancy-induced flow, both large-eddy simulation (LES) and the k–ϵ turbulence model are used. The two models, including direct effects of buoyancy, have been validated using experimental data from three (pool-like, vertical, and interaction between pool-like and vertical wall fires) turbulent diffusion flames. It is found that LES successfully predicted the variation of both the mean and fluctuating velocity/temperature. Important features of highly oscillating buoyancy-induced flows (recirculation zone, narrowing and broadening of the flame) have been correctly reproduced by LES, whereas the standard Smagorinsky model should be improved to model turbulent mixing near the vertical burning wall. It is found that interaction between pool-like and vertical wall fires induces an entraining ambient air at larger mass flow rates compared to a pool-like fire or a single vertical burning wall.

A supercritical carbon dioxide (S-CO2) gas turbine can generate power at a high cycle thermal efficiency, even at a modest temperature level of 500–550°C. Its high thermal efficiency is attributed to markedly reduced compressor work at the vicinity of the critical point. Furthermore, the reaction between Na and CO2 is milder than that between H2O and Na. Consequently, a more reliable and economically advantageous power generation system is achieved by coupling with a Na cooled fast reactor. In a typical design, the reactor thermal power, a turbine inlet pressure and an inlet temperature are, respectively, 600 MW, 20 MPa and 527°C. In the S-CO2 gas turbine system, a partial cooling cycle is used to compensate a difference in heat capacity for the high-temperature – low-pressure side and the low-temperature – high-pressure side of the recuperators to achieve high cycle thermal efficiency. The flow is divided into two streams before the precooler. One stream goes to recuperator 2 via a main compressor (MC); the other goes to recuperator 1 via a bypass compressor (BC). The performance and integrity of these two compressors are crucial. As described herein, an aerodynamic design of BC is given. The inlet temperature, inlet pressure, exit pressure and mass flow rate are, respectively, 77°C, 8 MPa, 20 MPa and 1392 kg/s. The salient features of this compressor are its compact size and a large bending stress caused by the large mass flow rate. The number of stages is numerous associated with the large enthalpy rise compared with MC. To achieve as high efficiency as possible, not a centrifugal type but an axial type is examined first. The aerodynamic design was conducted using one-dimensional design method, where the loss model of Cohen et al. is used. Its aerodynamic design enables the use of several stages and provides total adiabatic efficiency of 21 and 87%, respectively. Then, CFD analysis was conducted using “FLUENT”. Blade shapes were prepared based on flow angles and chord length obtained in the aerodynamic design. The CO2 properties in a fluid computer dataset “PROPATH” were used. The features of gas velocity distribution and pressure distribution were confirmed to the fundamental knowledge. The value of the calculated flow rate coincided very well with that of the design.

This study aims to develop a reliable numerical model for predicting the supercritical heat transfer of aviation kerosene RP-3 in a tube under heating conditions, thereby providing a reference for revealing the mechanism behind the experimental phenomena. Based on validation studies between seven turbulence models and experiments, a numerical method using the Yang-Shih turbulence model is proposed. A detailed prediction of the turbulent flow process is obtained, and the heat transfer characteristics of RP-3 are analyzed. The evolution of parameters and properties in axial and radial directions is demonstrated, followed by investigations of the effects of system pressure, fuel inlet temperature, and mass flow rate. The drastic change in the specific heat of the fuel when its temperature is close to the pseudocritical value and the temperature difference between the area near the wall and the center of the tube are the main causes of the enhancement and deterioration of the heat exchange. A higher inlet temperature increases the heat transfer coefficient, but due to its different effects on decreasing the density and the viscosity, it increases the pressure drop. In addition, larger mass flow rates can promote turbulence intensity and heat transfer, but cause a higher pressure drop across the tube.

Piston gauges are well-known standard instruments in the field of pressure metrology. But it is difficult to use them for small differential pressure measurements, because the pressure must be balanced against at least the weight of the piston, which generally corresponds to a pressure of several kilopascals. It is also difficult to reduce the pressure fluctuations caused by the rotation of the piston or the cylinder to lower than 0,1 Pa. A novel type of piston gauge has been developed in order to solve these problems and to measure small differential gas pressures in the range 1 Pa to 10 kPa with a sensitivity of about 5 mPa. This piston gauge uses two special features, of which one compensates for the weight of the piston and the other centralizes the piston in the cylinder without rotation. These mechanisms enable an electronic balance to measure the force produced on the piston by a small differential pressure.

This paper researches the parametric optimization of a two-stage transonic compressor having a large air bypass at partial rotating speed according to flow analysis for a turbine-based combined cycle engine (TBCC). To obtain adequate thrust, the inlet transonic compressor of the turbofan part of the TBCC is required to have a wider frequently used corrected rotating speed range and a larger mass-flow rate at low rotating speed, which is different from a typical transonic compressor. The one-dimensional blade design parameters and flow path of the baseline two-stage transonic compressor are introduced. With the widely used CFD software Numeca, the three-dimensional flow fields of the baseline transonic compressor and effects of the flow path between Stage 1 and Stage 2 on the inlet mass flow rate are analyzed for indicating the further improvement direction. For design speed (NC = 1.0), to improve the efficiency at the design point, parametric research is carried out on Rotor 2 to optimize the shock structure and strength, resulting in enhanced efficiency at the design point due to reduced shock loss of Rotor 2. For partial speed (NC = 0.8 and 0.7), since the flow field analysis indicates that the flow blockage in S1 limits the entire mass flow rate, the parametric redesign of stator S1 aims at obtaining an increased blade throat width to enhance the flow capacity of S1. Simulation confirms the increase in the mass-flow rate and efficiency at partial speed due to the reduction in flow blockage and related viscous losses. Aerodynamic analysis at representative operation points indicates that the modifications of R2 and S1 lead to obvious aerodynamic improvement at all rotating speeds (NC = 1.0 to 0.7), while maintaining sufficient stall margin.

Experiments of density wave instability in a sodium boiling two-phase flow in an annulus were carried out with the parameters of heat flux from 80 to 976kW/m2, inlet subcooling from 25.6 to 226.8°C, mass flow rate from 7.92 to 68.9 kg/h, and system pressure from 2,600 Pa to 0.06 MPa. It was found that the density wave instability occurred in the case of low exit quality, and the oscillation of flow rate was so large that the flow would be reversal. The lower inlet temperature, the higher system pressure and the larger mass flow rate could result in a more stable boiling two-phase flow. The oscillation period of the instability increased with the system pressure and the inlet subcooling, but it decreased with the mass flow rate. A correlation for the onset condition of the density wave instability was obtained from the experimental data.

Flow and heat transfer characteristics of water in eccentric micro annuli

The secondary air system plays important roles in gas turbines, such as cooling hot-end components, sealing the rim, and balancing axial forces. In this paper, the flow structure and the heat transfer characteristics of the rotating disk cavity with two inlets and single outlet is studied by CFD (Computational Fluid Dynamics) approach. The effect and mechanism under higher rotational speed and larger mass flow rate are also discussed. The results show that a large-scale vortex is induced by the central inlet jet in the low-radius region of the cavity, while the flow structure in the high-radius region is significantly influenced by rotational speed and flow rate. Increasing the rotational speed generally enhances heat transfer because it amplifies the differential rotational linear velocity between the disk surface and nearby wall flow, consequently thinning the boundary layer. Increasing the mass flow rate enhances heat transfer through two primary mechanisms: firstly, it elevates the turbulence intensity of the near-wall fluid; secondly, the higher radial velocity results in a thinner boundary layer.

The present paper investigates the effects of a fan-shaped hole endwall cooling geometry on the aero-thermal performance of a nozzle vane cascade. Two endwall cooling geometries with four rows of holes were tested, for different mass flow rate ratios: the first configuration is made of cylindrical holes, whereas the second one features conical expanded exits and a reduced number of holes. The experimental analysis is mainly focused on the variations of secondary flow phenomena related to different injection rates, as they have a strong relationship with the film cooling effectiveness. Secondary flow assessment was performed through downstream 3D aerodynamic measurements, by means of a miniaturized 5-hole probe. The results show that at high injection rates, the passage vortex and the 3D effects tend to become weaker, leading to a strong reduction of the endwall cross flow and to a more uniform flow in spanwise direction. This is of course obtained at the expense of a significant increase of losses. The thermal behavior was then investigated through the analysis of adiabatic effectiveness distributions on the two endwall configurations. The wide-banded thermochromic liquid crystals (TLC) technique was used to determine the adiabatic wall temperature. Using the measured distributions of film-cooling adiabatic effectiveness, the interaction between the secondary flow vortices and the cooling jets can be followed in good detail all over the endwall surface. Fan-shaped holes have been shown to perform better than cylindrical ones: at low injection rates, the cooling performance is increased only in the front part of the vane passage. A larger improvement of cooling coverage all over the endwall is attained with a larger mass flow rate, about 1.5% of core flow, without a substantial increase of the aerodynamic losses.

Numerical investigations for a solid oxide electrolyte cell stack

In porous resistances, Darcy’s law provides a good approximation of mass flow rate when the differences between upstream and downstream pressures are sufficiently small. In this range, the mass flow rates are proportional to the porous resistance’s permeability. For gas bearings, the pressure difference is normally higher, and it is known experimentally that the mass flow rates are lower than would result from Darcy’s law. Forchheimer’s law adds an inertial term to Darcy’s law and, when an appropriate coefficient is selected for this term, provides a good approximation of flow rates for the same applications even with the highest pressure differences. This paper presents an experimental and theoretical investigation of porous resistances used in gas bearing supply systems. Cylindrical sintered bronze inserts featuring lengths, diameters and particle sizes commonly used in gas bearings and thrust pads were examined. The paper describes the test set-up and experimental results obtained for: a) Mass flow rate through single porous resistances at different upstream and downstream pressures; and b) Mass flow rate and pressure distribution on a pneumatic pad featuring the same porous resistances. The theoretical permeability of the chosen porous resistances was calculated, and the results from set-up a) were then used to obtain experimental permeability and to determine the inertial coefficients. The results, which are expressed as a function of the Reynolds number, confirmed the validity of using Forchheimer’s law. The mass flow rates from set-up b) were compared with those from set-up a) at the same pressure differentials across the insert.