Secondary Flow Rate Research Articles

Measurement of Specific Enthalpy Under Very Low Pressure Plasma Spray Condition

VLPPS has attracted much attention in recent years, especially concerning its potential applications but not so much concerning the properties of the plasma jet. Operating parameters such as the chamber pressure and the secondary gas flow rate have a strong influence on specific enthalpy and temperature of the plasma jet, and then on the efficient use of the system. In this work, thermodynamic properties measurements under very low pressure (i.e., around 1 mbar) were performed using a modified enthalpy probe (increase of the internal diameter and thermal insulation of the head). Several parameters such as arc current intensity, hydrogen gas flow rate and working distance were considered in order to check their effect on the characteristics of the plasma jet and to determine the variation of the specific enthalpy, temperature, and heat flux versus these parameters.

Numerical Simulation of a Hot Gas Injection Thrust Vector Control System Performance

The complex three-dimensional flow field produced by secondary injection of hot gases in nozzle of a rocket motor for the thrust vector control is analyzed by solving three-dimensional governing equations, using Fluent code, with appropriate boundary conditions. Various system performance parameters like secondary jet amplification factor, axial thrust augmentation are computed as part of the flow field solution and compared with the previous experimental and computational studies. The agreement is good within the practical range of secondary injectant flow rates for thrust vector control applications.

Spraying of Metallic Powders by Hybrid Gas/Water Torch and the Effects of Inert Gas Shrouding

A hybrid DC arc plasma torch, combining water and gas stabilization, offers a high flexibility in plasma characteristics. These can be controlled in a wide range by the torch operational parameters, such as arc current and secondary gas flow rate. In this study, their influence on plasma spraying of tungsten and copper was investigated. To suppress the in-flight oxidation of the metals, inert gas shrouding was applied. In-flight particle diagnostics and analysis of free-flight particles and coatings was performed for spraying experiments in the open atmosphere and with argon shrouding. Both in-flight particle behavior and coating properties were found to be sensitive to the torch parameters. The application of shrouding was found to affect particle in-flight parameters, reduce the oxide content in the coatings and generally improve their properties, such as thermal conductivity. However, a different degree of these effects was observed for copper and tungsten.

용량 가변 및 유량변화에 따른 지열원 물대물 열펌프 유닛의 충전량 변화에 따른 성능 특성

The objective of this study is to investigate the effects of the refrigerant charge amount on the performance of a water-to-water ground source heat pump with a variation of compressor speed and the secondary fluid flow rate. The water-to-water ground source heat pump was tested by varying refrigerant charge amount from -40% to 20% of full charge. Compressor speed was changed from 30 Hz to 75 Hz and the secondary fluid flow rate was adjusted from 6 LPM to 14 LPM. For all test conditions, EWTs of an indoor heat exchanger and an outdoor heat exchanger were maintained at standard conditions of ISO 13256-2. The slope of the COP with the variation of charge amount is much steeper at undercharged conditions than that at overcharged conditions. For all compressor speed, the variation of the system performance according to charge amounts showed the similar trends. However, the optimum charge amount of the system increased a little with an increment of compressor speed. When the secondary fluid flow rate decreased, the system was optimized at higher refrigerant charge amount conditions.

Variable-speed heat pump model for a wide range of cooling conditions and loads

A modular variable-speed heat pump model is developed from first principles. The system model consists of steady-state evaporator, compressor, and condenser component sub-models. The compressor model currently implemented accounts for re-expansion, valve pressure drop, and back leakage using empirical coefficients obtained from tests covering a wide range of pressure ratio and shaft speed. Variations in heat transfer coefficients with refrigerant and secondary fluid flow rates are modeled over a wide range of capacity. Pressure drops in piping and heat exchangers are also modeled. The resulting heat pump model is flexible and fast enough for use in finding optimal compressor, fan, and pump speeds and optimal subcooling for any specified capacity fraction and operating condition. To confirm the model's accuracy, simulation results are compared to experimental data with condenser inlet air temperatures ranging from 15°C to 45°C, evaporator inlet air (dry) from 14°C to 34°C, and cooling capacity from 1.1 kW to 3.9 kW. The refrigerant charge balance has not been modeled; instead, it is assumed that a liquid receiver maintains the necessary charge balance. Over this wide range of conditions, the coefficient of performance prediction errors are found to be ±10%. An example of configuring the HPM with a different evaporator demonstrates the benefits of a modular approach.

Design and performance study of the ground-coupled heat pump system with an operating parameter

In the present study, the performance of a heat pump unit is measured by varying the secondary fluid flow rate and compressor speed. Test results are applied to design the GCHP system, and then the effects of the flow rate and the thermal conductivity on the performance of the system and the size of GLHX are analyzed. For all compressor speed and operating modes, the COP of the heat pump unit with an increment of flow rate is enhanced, while the increasing rate of that decrease. For low flow rate conditions, the unit COP and the system COP according to the flow rate increase due to a rise of heating and cooling capacity and a drop of power consumption. When the flow rate increases under high flow rate conditions, heat pump unit COP is slightly enhanced in the heating and cooling modes. However, the variation of the system COP in the cooling mode is negligible and that in the heating mode decreases even though the flow rate increases. The GLHX length increases with the increase of the flow rate, while it decreases with an increment of the thermal conductivity. As the thermal conductivity decreases, the increasing rate of the GLHX length according to the flow rate rises. A higher secondary fluid flow rate can increase the performance of the heat pump unit, while it makes the length of the GLHX longer. As a result, it is suggested that the flow rate of the heat pump unit have to be checked to optimize the system, and consequently, the system COP and the flow rate should be considered as the important design parameters in order to reduce the installation cost and save energy in the GCHP system.

지열 히트펌프에서 운전변수가 냉난방 성능에 미치는 영향에 대한 실험연구

In this research, an experimental study is performed to investigate the effects of system operating variables on the cooling and heating characteristics of heat pump system using geothermal heat source and carbon dioxide as a refrigerant. System variables analyzed include compressor frequency, electronic expansion valve opening, refrigerant charge, secondary fluid temperature and flow rate. Results show that optimum refrigerant charge and electronic expansion valve opening position exist at the maximum point of COP curve, and both cooling and heating capacity increase but COPs decrease with the increase of compressor frequency. The change of a secondary fluid temperature leads to variation of overheat area and enthalpy difference in the evaporator and gas cooler. which again results in considerable variations of cooling and heating capacity and COP. In the case of effects of secondary water fluid flow rate, both cooling capacity and COP increase with the increase of secondary flow in evaporator or gas cooler, whereas heating capacity and COP decrease with the increase of flow rate in gas cooler.

Steady hydromagnetic Couette flow in a rotating system with nonconducting walls

Steady hydromagnetic Couette flow of class-II of a viscous incompressible electrically conducting fluid in a rotating system with non-conducting walls is studied. Exact solution of the governing equations is obtained in closed form. Expressions for the shear stress at the lower and upper plates due to primary and secondary flows and mass flow rates in primary and secondary flow directions are also derived. Asymptotic behavior of the solution for fluid velocity and induced magnetic field is analyzed for small and large values of rotation parameter K2 and magnetic parameter M2 to gain further insight into the flow pattern. Heat transfer characteristics of the flow are considered taking viscous and Joule dissipations into account. The numerical solution of energy equation and numerical values of rate of heat transfer at both plates are obtained with the help of MATLAB software. The numerical values of fluid velocity, induced magnetic field and fluid temperature are depicted graphically versus channel width variable c for various values of M2 and K2 while numerical values of primary and secondary shear stress at the lower and upper plates, mass flow rates in primary and secondary flow directions and rate of heat transfer at the lower and upper plates are presented in tabular form for various values of K2 and M2.

Adiabatic and conjugate cooling effectiveness analysis of a new hybrid scheme

The adiabatic film cooling effectiveness and flow characteristics for a new hybrid scheme and a circular hole were investigated numerically. The new film cooling scheme is proposed for high temperature gas turbine engines in aerospace and electric power generation applications. The hybrid scheme included two consecutive film hole configurations with interior bending to direct the secondary flow in stream-wise direction. The film cooling performance with flow-field analysis for the hybrid scheme and the circular hole were investigated at blowing ratio of 0.5, 1.0 and 2.0 for 0.95 density ratios. The results showed that the hybrid scheme provided a superior local and average film cooling effectiveness performance that was enhanced for further increase in blowing ratio. Moreover, the new scheme enhanced the downstream overall area-average film cooling effectiveness compared to the circular film hole. Subsequently, film cooling and conjugate heat transfer were combined to investigate the cooling effectiveness of the hybrid scheme at Br = 0.5 and 1.0 for different flow arrangements, specifically: parallel flow and jet impingement with two different gap heights (0.8 d and 1.2 d). The hybrid scheme presented a high cooling effectiveness by combining film cooling and conjugate heat transfer. The jet impingement configuration enhanced the upstream flow circulation which has a significant effect on convective heat transfer. Furthermore, a large gap height with jet impingement enhanced the downstream cooling effectiveness compared to other conjugate and adiabatic cases studied. The conjugate configuration with large gap height provided the highest overall laterally averaged cooling effectiveness compared to other conjugate and adiabatic cases studied. As a result, the hybrid scheme is able to minimize the cooling flow rate significantly since it provided superior cooling effectiveness at a lesser secondary flow rate, thus increasing overall engine efficiency.

Experimental and numerical analysis of a variable area ratio steam ejector

In the present paper, experimental and CFD results for a 5 kW capacity steam ejector with variable primary nozzle geometry are presented and compared. The variable geometry was achieved by applying a movable spindle at the primary nozzle inlet. Operating conditions were considered in a range that would be suitable for an air-conditioning application, with thermal energy supplied by vacuum tube solar collectors. The CFD model was based on the axi-symmetric representation of the experimental ejector, using water as working fluid. The experimental entrainment ratio varied in the range of 0.1–0.5 depending on operating conditions and spindle tip position. It was found that the primary flow rate can be successfully adjusted by the spindle. CFD and experimental primary flow rates agreed well, with an average relative error of 7.7%. CFD predicted the secondary flow rate and entrainment ratio with good accuracy only in 70% of the cases.

Comparison of CFD and experimental performance results of a variable area ratio steam ejector

The advantages of numerical modelling compared with experimental studies (e.g. reduced cost, easy control of the variables, high yield etc.) are well known. Theoretical studies where experimental validation is also presented provide an important added value to numerical investigations. In the present paper, experimental and computational fluid dynamics (CFD) results for a 5-kW-rated capacity steam ejector, with a variable primary nozzle geometry, are presented and compared. The variable geometry was achieved by applying a movable spindle at the primary nozzle inlet. Relatively low operating temperatures and pressures were considered, so that the cooling system could be operated with thermal energy supplied by solar collectors (solar air-conditioning). The CFD model was based on the axi-symmetric representation of the experimental ejector, using water as a working fluid. The experimental entrainment ratio varied in the range of 0.1–0.5, depending on operating conditions and spindle tip position. It was found that the primary flow rate can be successfully adjusted by the spindle. CFD and experimental primary flow rates agreed well, with an average relative error of 8%. CFD predicted the secondary flow rate and entrainment ratio with good accuracy only in 70% of the cases.

Variation of splat shape with processing conditions in plasma sprayed alumina coatings

This paper is on various splat shapes obtained using three alumina based powders sprayed on various substrates. The parameters considered were substrate preheating temperature, nozzle diameter, and secondary and primary gas flow rates. The splat shape was found to be strongly dependent on spraying conditions. The substrate preheating temperature determined the degree of substrate wetting by the splat. A change in either nozzle diameter or primary gas flow rate brought about a change in the particle momentum and subsequently, a change in splat shape. The splat shape differed widely on an as – sprayed bond coat as compared to a polished one, owing to splat confinement by surface asperities. Sub-microscale surface roughness of polished substrate surfaces showed an increase with the preheating temperature and this in turn, resulted in better substrate wetting by the splats.

Numerical analysis of the performance of a thermal ejector in a steam evaporator

Ejectors have been widely used in many applications such as water desalination, steam turbine, refrigeration systems, and chemical plants. The advantage of an ejector system lies in its extremely reliable operation due to the complete absence of moving parts. However, the performance depends on a number of factors, among which the flow channel configuration/arrangement is critical. To improve the performance of an existing thermal compressor in a steam evaporator, a comprehensive study was conducted in this paper with a main focus on the sensitivity of performance to the geometric arrangement. Numerical simulation was employed to investigate the thermal-flow behavior. The performance is measured by the entrainment ratio, i.e., the secondary (suction) flow rate from a vapor plenum over the primary steam jet flow. It is observed that any downstream resistance will seriously impede the suction flow rate. In addition, the entrainment ratio is sensitive to the location of the jet exit, and there is an optimum location where the primary flow should be issued. A well-contoured diffuser can increase the entrainment ratio significantly. However, the size of suction opening to the plenum is less important, and a contoured annular passage to guide the entrained flow shows little effect on the overall performance. Based on the numerical results the steam entrainment rate of the best case in the confinement of the current study is approximately 430% of the jet flow rate, while some cases with mediocre design can only produce an entrainment of 24% of the primary jet flow.

Analysis of phase transformation in plasma sprayed alumina coatings using Rietveld refinement

During plasma spraying of alumina with the stable α phase in the starting powder, metastable phases tend to form in the final coating. This is attributed to the rapid quenching associated with the process. In this paper the weight fraction of metastable phase formed, i.e., stable phase retained, and has been estimated using Rietveld refinement of X-ray diffraction data. This weight fraction depends on the process parameters like standoff distance, primary and secondary gas flow rate, nozzle size, etc., which in turn control particle melting. Under favourable melting conditions the weight fraction of the metastable phases approaches 1.

Numerical simulation of single and multiple gas jets in bubbling fluidized beds

The behavior of single and multiple horizontal gas jet injections into a rectangular bubbling fluidized bed are studied with three-dimensional numerical simulations. The jet penetrations as well as the interactions between the jet and the surrounding gas, solids, bubbles, and other jets are investigated. As far as the average jet penetration length is concerned, limited influence of each jet on the others is observed in multiple jet injections until the jets start to overlap. The effect of the secondary gas injection on the flow hydrodynamics in the bed is examined for multiple jet injections with different jetting velocities and arrangements. It is found that the secondary gas injection mainly affects the hydrodynamics of the upper section above the injection level, while its effect is nearly negligible below the injection. Above the injection level, more particles are brought upward and the circulation of solids is promoted. Furthermore, the tendency of slugging inside the bed is observed at high secondary injection flow rates.

Mixing of secondary gas injection in a bubbling fluidized bed

In this work, three-dimensional numerical simulations with the aim of investigating the mixing of secondary gas in a bubbling fluidized bed are performed. Single and multiple horizontal gas jet injections into a small scale rectangular bubbling fluidized bed are studied. A tracer gas is introduced through the jet orifice to study the gas mixing in the system. Both transient and time-averaged results are analyzed. The effect of gas injection velocity and jet arrangement on the gas–solid contact is evaluated. It is found that the tracer distribution is non-uniform at the injection level. Within a short distance above the injection, the tracer becomes uniformly distributed. Gas back-mixing is observed in all simulations, which is prominent near the wall due to the downward flow of solids. For the cases studied, the gas back-mixing tends to decrease as the secondary gas flow rate increases. For the same secondary gas flow rate, it has been demonstrated that a better mixing between the tracer gas and solid particles is achieved when the secondary gas is injected through distributed jet arrangement.

Structural evolution of mechanically alloyed Al–12Si/TiB 2/h-BN composite powder coating by atmospheric plasma spraying

In the present study, mechanically alloyed Al–12Si/TiB 2/h-BN composite powder was deposited onto an aluminum substrate by atmospheric plasma spraying. The effect of mechanical alloying (MA) and plasma spray parameters on composite powder and coating structure were investigated. It has been observed that the MA process has a significant effect on the composite powder morphology and reactivity between the selective powders. Results also demonstrate that, at relatively high milling time h-BN decomposes into B and N and forms a solid solution. Also, it has been found that, the relative amount of the in-situ formed AlN through the reaction between h-BN and Al and/or the decomposition of Al–B–N solid solution is independent from the plasma parameters (arc current and secondary gas flow rate). However, spray parameters remarkably affects the coating hardness due to coarsening of Si during the solidification of the coating.

Study of Injection Angle and Carrier Gas Flow Rate Effects on Particles In-Flight Characteristics in Plasma Spray Process: Modeling and Experiments

This paper investigates the influence of particle injection angle on particle in-flight behaviors and characteristics at different primary and carrier gas flow rates through an integrated modeling and experimental approach. Particle in-flight status such as temperature, velocity, size and their distribution are analyzed to examine particle’s melting status before impact. Results from the experiments and numerical simulations both show that, when carrier gas flow rate is fixed, a small injection angle favors the particle melting and flattening. This behavior is independent of primary and secondary gas flow rates, spray distance and carrier gas flow rate. When both carrier gas flow and injection angle vary, a high carrier gas flow rate and a small injection angle are recommended for high particle temperature and velocity.

Low conductivity plasma sprayed thermal barrier coating using hollow psz spheres: Correlation between thermophysical properties and microstructure

Life and thermal properties of plasma sprayed TBCs – widely used in gas turbine engines – are closely related to the microstructure of the ceramic top coating. Especially, the thermal behaviour of this coating is induced by the void shapes and networks which are in turn determined by both the spraying conditions and the feedstock material. A specific hollow yttria partially stabilised zirconia powder was produced in a one-step process by spray drying and an experimental statistical design study was conducted to investigate the influence of spraying variables (primary and secondary gas flow rates, arc current, spraying distance, spraying angle and traverse speed) on structure and properties of resulting plasma sprayed coatings. The coatings were characterized with respect to deposition efficiency, roughness, porosity and thermal conductivity. A reduction of 25% of the thermal conductivity was achieved by improving the spray and powder parameters. A quantitative characterization of the porous structure using image analysis of polished cross-sections was implemented. The parameters that have relevant influence on the coating porous structure were identified, and their relative importance was determined. An attempt was made to identify morphological criteria of the porous network (coarse/fine porosity ratio, cracks total length, cracks orientation) correlating with the thermal conductivity values.

Determining jet mixing characteristics using electrical resistance tomography

Coaxial jet and side entry mixers are used in a wide range of industries for a variety of processes including precipitation, polymerization and neutralization duties [Wei H, Garside J. Application of CFD modelling to precipitation systems. Chem Eng Res Des 1995; 75 (2): 219–27; Kolhapure NH, Tilton JN, Pereira CJ. Integration of CFD and condensation polymerization chemistry for a commercial multijet tubular reactor. Chem Eng Sci 2004; 59: 5177–84; Pipino M, Fox RO. Reactive mixing in a tubular jet reactor: A comparison of PDF simulations with experimental data. Chem Eng Sci 1994; 49 (24B): 5229–41]. Jet mixers are characterized by short contact times between the fluids and can be operated in continuous or semi-batch modes. Coaxial and side entry jets can be designed in order to deliver rapid turbulent mixing using short sections of pipeline. As the energy required for mixing is provided by the addition stream, the process-side pressure drop required for homogeneity is very low. A key design parameter for jet mixers is the mixing length, i.e. the length of pipe downstream of the injection point required to achieve a given degree of homogeneity. The mixing length can be affected by the addition geometry (for example, coaxial or side entry), orifice size and shape, operating conditions and material properties. This paper presents the use of electrical resistance tomography (ERT) to monitor jet mixing via the addition of a conductivity tracer through coaxial and side entry jets. Multiple ERT sensors are fitted along the pipe downstream of the jet addition point. The ERT sensors enable real time, noninvasive measurement of conductivity within the pipe, utilizing a 3D finite element mesh consisting of ∼2500 tetrahedral elements. The effect of secondary (main pipe) flow rate and jet configuration on the nature of the tracer plume evolution and axial mixing is determined using this technique.