Reduction In Mass Flow Rate Research Articles

Improved Scaling Analysis for Heat Transfer in a Circular Tube With Various Supercritical Fluids Using Computational Fluid Dynamics Simulations

ABSTRACTThe operating conditions of supercritical water cooler reactor (SCWR) are well above the critical point of water, so it is not possible to investigate its heat transfer aspects through laboratory experiments without industry-scale support. The most feasible alternative can be to scale-down the operating parameters by fluid-to-fluid scaling with a suitably chosen scaling fluid. However, it is impossible to incorporate all phenomenological factors of an intricate system like the SCWR through simple analytical scaling. This study demonstrates the limitation of fluid-to-fluid scaling in such situations and suggests the incorporation of computational fluid dynamics simulation as a subsequent step for better scaling. A scaling methodology from the published literature is adopted. Carbon dioxide and R134a have been considered as scaling fluids to identify the parameter ranges suitable for lab-scale simulation of the SCWR. A circular tube of 8 mm diameter and 1500 mm length is taken for simulation. A grid dependency test is done and the standard κ − ϵ turbulence model is selected. The developed computational model showed amicable agreement with existing experimental data. Analytically scaled-down parameters failed to simulate the axial and radial temperature profiles of the prototype. Increase in wall heat flux and reduction in mass flow rate are suggested as two possible options for achieving better profile matching. The modified values of scaled parameters with respect to a particular prototypical condition are reported. Profiles with CO2 as model fluid show better agreement with water as compared to R134a and hence this is recommended for use in lab experiments.

CFD analysis of localized crud effects on the flow of coolant in nuclear rod bundles

It has been suggested that crud deposits on a number of adjacent fuel rods might reduce coolant flow rates in associated sub-channels. Such reduced flow rates could then worsen thermal-hydraulic conditions, such as margin to saturated boiling, fuel surface temperature, and the DNB ratio. We report the results of a detailed computational fluid dynamics study of the flow pattern in a partially crudded rod bundle. Values obviously depend on, for example, the thickness of crud assumed, but sub-channel flow rate reductions of ∼10% were predicted by this analysis. However, this mass flow rate reduction was found to be more than offset by improved heat transfer induced by the relatively rough surface of the crud. Cladding temperatures were predicted to be essentially unchanged, and the DNBR was similarly little altered. We conclude that such flow reduction and diversion is not likely to be of concern.

Effect of guide wall on the potential of a solar chimney power plant

A solar chimney power plant (SCPP) converts solar thermal energy into electricity by generating a buoyant flow in a chimney. To assist the air flow in shifting its direction from horizontal to vertical, a guide wall (GW) is usually set in the collector-to-chimney transition region. The primary objective of this study is to examine the impact of the GW geometry on the power output of a SCPP. A reduction in mass flow rate after adding a GW in the system was observed in a small-scale experimental prototype. Numerical simulations on a large-scale SCPP further found that the mass flow rate was linearly and inversely proportional to the increase of GW height. The driving force, however, nonlinearly increased with increasing the GW height. Subsequently, the potential maximum power output, which was mainly governed by the driving force, increased with increasing the GW height. Furthermore, a divergent-chimney system which can improve the performance of SCPPs had different reactions with the geometry of GWs compared with a cylindrical-chimney system. Under the optimal GW configuration, the power output of the SCPP increased by ∼40% in a cylindrical-chimney system and by ∼9.0% in a divergent-chimney system with respect to the system without a guide wall.

Flow dynamical behavior and performance of a micro viscous pump with unequal inlet and outlet areas

ABSTRACTThe micro viscous pump is an important type of fluidic device. Optimizing the working performance of the pump is crucial for its wider application. A micro viscous pump design with unequal inlet and outlet areas is proposed in this paper. The flow field of the viscous pump is investigated using 2D laminar simulations. The mass flow rate and driving power are studied with different opening angles. The effects of the Reynolds number and the pressure load on the working performance are discussed in detail. Flow structures and vortex evolution are analyzed. With larger inlet and outlet areas, a higher mass flow rate is obtained and less driving power is achieved. A high pressure load results in a reduction in mass flow rate and an increase in driving power. Pumps with large opening angles are more susceptive to the Reynolds number and the pressure load. The adverse impact of the pressure load can be reduced by increasing the rotor speed. The vortex structure is affected by the geometric and operating parameters in the flow field. The flow dynamical behavior of the viscous pump exerts significant influence on its pumping ability. The present work gives rise to performance improvements for the micro viscous pump.

Entropy parameters for falling film absorber optimization

A local entropy generation analysis, for water vapor absorption in LiBr-H2O solution, is performed referring to velocity, temperature and concentration fields obtained from the numerical solution of mass and energy transport equations. The hydrodynamic description is based on Nusselt boundary layer assumption and the actual amount of irreversibility introduced is determined for an absorptive falling film over a cooled horizontal tube inside the absorber. Results are explored in different operative conditions in order to examine the impact of the various irreversibility sources in a wide operative range. A least irreversible solution mass flow-rate can always be identified. Furthermore, a simple and general thermodynamic analysis, carried out regarding a refrigerating and a heat boosting applications, makes evidence of a dimensionless group “Q/σT” that separates the weight of the irreversibilities and gives the way to an optimization criterion applied to the absorber in order to improve the whole system efficiency. Both thermodynamic equilibrium and sub-cooling conditions of the solution at the inlet are considered for typical temperature and concentration of refrigerators' absorbers and heat transformers' absorbers. Results suggest the importance to work at reduced mass flow-rates with a thin uniform film. In practice, tension-active additives are required to realize this condition. Also, it is highlighted that the two parameters defined with reference to the dimensionless group “Q/σT” can be maximized by specific values of the tube radius, operative Reynolds number, solution sub-cooling and temperature difference between the wall and the inlet solution.

Experimental Measurements of Pressure Losses in the Inter-Turbine Duct of a Gas Turbine

The paper presents the total pressure experimental measurements carried out at the Romanian Research and Development Institute for Gas Turbines COMOTI in order to determine the total pressure losses in the Inter - Turbine Duct of a two spools gas turbine, as a function of the gas turbine operating regime (mass flow rate) and rotational speed. The Inter - Turbine Duct experimental assembly has been designed, manufactured and tested at COMOTI. The total pressures were measured as a function of the pre-swirling angle, which simulates the influence of the high pressure turbine rotational speed located upstream of the Inter turbine duct in the real gas turbine, as well as for three operational regimes, without the pre-swirlers modules. The results indicate that the total pressure loss along the Inter - Turbine Duct is of maximum 0.9 %. The lowest overall total pressure loss occurs at 0o pre-swirling angle, around 0.8%, while along the ITD struts, the lowest pressure loss is obtained for a 15o pre-swirling, below 0.1%. The influence of the operating regime on the total pressure loss was found to be linearly, the pressure loss increasing with the reduced mass flow rate, between 1% and 1.9% overall, and between about 0.1% and 0.4 % along the struts.

Irreversibility analysis of falling film absorption over a cooled horizontal tube

Based on a numerical study of the water vapour absorption process in LiBr–H2O solution, for a laminar, gravity driven, viscous, incompressible liquid film, flowing over a horizontal cooled tube, irreversibilities related to fluid friction, heat transfer, mass transfer and their coupling effects have been locally and globally examined. The hydrodynamic description is based on Nusselt boundary layer assumptions. The tangential and normal velocity components, respectively obtained from momentum and continuity equations, have been used for the numerical solution of mass and energy transport equations in the two-dimensional domain defined by the film thickness and the position along the tube surface. Local entropy generation calculation can be performed referring to the calculated velocity, temperature and concentration fields. Results have been explored in different operative conditions, in order to examine comprehensively the impact of the various irreversibility sources and to identify the least irreversible solution mass flow-rate for the absorber. As a parallel, a refined understanding of the absorption process can be obtained. Considering absorption at the film interface and cooling effect at the tube wall, the analysis thermodynamically characterises the absorption process which occurs inside actual falling film heat exchangers and establishes a criterion for their thermodynamic optimisation. Results suggest the importance to operate at reduced mass flow rates with a thin uniform film. Meanwhile, tension-active additives are required to realise this condition.

Performance modeling of industrial gas turbines with inlet air filtration system

The effect of inlet air filtration on the performance of two industrial gas turbines (GT) is presented. Two GTs were modeled similar to GE LM2500+ and Alstom GT13 E2-2012, using TURBOMATCH and chosen to operate at environmental conditions of Usan offshore oilfield and Maiduguri dessert in Nigeria. The inlet pressure recovered (Precov) from the selected filters used in Usan offshore, and Maiduguri ranged between 98.36≤Precov≤99.51% and 98.67≤Precov≤99.56% respectively. At reduced inlet Precov by 98.36% (1.66kPa) and, at a temperature above 15°C (ISA), a reduction of 16.9%, and 7.3% of power output and efficiency was obtained using GT13 E2-2012, while a decrease of 14.8% and 4.7% exist for power output and efficiency with GE LM2500+. In addition, a reduction in mass flow rate of air and fuel under the same condition was between 4.3≤mair≤10.6% and 10.4≤mfuel≤11.5% for GT13 E2-2012 and GE LM2500+, correspondingly. However, the GE LM2500+ was more predisposed to intake pressure drops since it functioned at a higher overall pressure ratio. The results obtained were found worthwhile and could be the basis for filter selection and efficient compressor housing design in the locations concerned.

Nanofluid application in post SB-LOCA transient in VVER-1000 NPP

This paper is the third in a series of four papers that the application of nanofluid as a coolant to improve heat transfer in a VVER-1000 nuclear reactor is investigated. In the first and second papers, neutronics and thermo-hydraulic behavior of nano-particles in normal operation mode of the reactor were reported. In this study, the effects of nanofluids in Small Break Loss of Coolant Accident (SB-LOCA) to complement the existing safety systems is investigated. During SB-LOCA transient, due to reduced mass flow rate and Reynolds number, flow boiling along with vapor formation around the fuel rods occur. The fuel assembly coolant channel of a VVER-1000 core is modeled using a CFD code and heat transfer coefficients, pressure drop and volume fraction distribution of phases are computed for water/Al2O3 nanofluid. We observe that with the escalation of heat transfer enhancement, due to reduction in void fraction, pressure drop along the channel is reduced. This new phenomenon of lower pressure drop along with heat transfer enhancement, would be a significant factor for the use of nanofluids during reactor SB-LOCA transients.

Predictions of Flow Separation at the Valve Seat for Steady-State Port-Flow Simulation

Steady-state port-flow simulations with static valve lift are often utilized to optimize the performance of intake system of an internal combustion engine. Generally, increase in valve lift results in higher mass flow rate through the valve. But in certain cases, mass flow rate can actually decrease with increased valve lift, caused by separation of turbulent flow at the valve seat. Prediction of this phenomenon using computational fluid dynamics (CFD) models is not trivial. It is found that the computational mesh significantly influences the simulation results. A series of steady-state port-flow simulations are carried out using a commercial CFD code. Several mesh topologies are applied for the simulations. The predicted results are compared with available experimental data from flow bench measurements. It is found that the flow separation and reduction in mass flow rate with increased valve lift can be predicted when high mesh density is used in the proximity of the valve seat and the walls of the intake port. Higher mesh density also gives better predictions of mass flow rate compared to the experiments, but only for high valve lifts. For low valve lifts, the error in predicted flow rate is close to 13%.

High Resolution Heat Transfer Measurements on the Stator Endwall of an Axial Turbine

In order to continue increasing the efficiency of gas turbines, an important effort is made on the thermal management of the turbine stage. In particular, understanding and accurately estimating the thermal loads in a vane passage is of primary interest to engine designers looking to optimize the cooling requirements and ensure the integrity of the components. This paper focuses on the measurement of endwall heat transfer in a vane passage with a three-dimensional (3D) airfoil shape and cylindrical endwalls. It also presents a comparison with predictions performed using an in-house developed Reynolds-Averaged Navier–Stokes (RANS) solver featuring a specific treatment of the numerical smoothing using a flow adaptive scheme. The measurements have been performed in a steady state axial turbine facility on a novel platform developed for heat transfer measurements and integrated to the nozzle guide vane (NGV) row of the turbine. A quasi-isothermal boundary condition is used to obtain both the heat transfer coefficient and the adiabatic wall temperature within a single measurement day. The surface temperature is measured using infrared thermography through small view ports. The infrared camera is mounted on a robot arm with six degrees of freedom to provide high resolution surface temperature and a full coverage of the vane passage. The paper presents results from experiments with two different flow conditions obtained by varying the mass flow through the turbine: measurements at the design point (ReCax=7.2×105) and at a reduced mass flow rate (ReCax=5.2×105). The heat transfer quantities, namely the heat transfer coefficient and the adiabatic wall temperature, are derived from measurements at 14 different isothermal temperatures. The experimental data are supplemented with numerical predictions that are deduced from a set of adiabatic and diabatic simulations. In addition, the predicted flow field in the passage is used to highlight the link between the heat transfer patterns measured and the vortical structures present in the passage.

Experimental Investigation of the Flow Field and the Heat Transfer on a Scaled Cooled Combustor Liner With Realistic Swirling Flow Generated by a Lean-Burn Injection System

Lean-burn swirl stabilized combustors represent the key technology to reduce NOx emissions in modern aircraft engines. The high amount of air admitted through a lean-burn injection system is characterized by very complex flow structures, such as recirculations, vortex breakdown, and processing vortex core, which may deeply interact in the near wall region of the combustor liner. This interaction and its effects on the local cooling performance make the design of the cooling systems very challenging, accounting for the design and commission of new test rigs for detailed analysis. The main purpose of the present work is the characterization of the flow field and the wall heat transfer due to the interaction of a swirling flow coming out from real geometry injectors and a slot cooling system which generates film cooling in the first part of the combustor liner. The experimental setup consists of a nonreactive three sector planar rig in an open loop wind tunnel; the rig, developed within the EU project Low Emissions Core-Engine Technologies (LEMCOTEC), includes three swirlers, whose scaled geometry reproduces the real geometry of an Avio Aero partially evaporated and rapid mixing (PERM) injector technology, and a simple cooling scheme made up of a slot injection, reproducing the exhaust dome cooling mass flow. Test were carried out imposing realistic combustor operating conditions, especially in terms of reduced mass flow rate and pressure drop across the swirlers. The flow field is investigated by means of particle image velocimetry (PIV), while the measurement of the heat transfer coefficient is performed through thermochromic liquid crystals (TLCs) steady state technique. PIV results show the behavior of flow field generated by the injectors, their mutual interaction, and the impact of the swirled main flow on the stability of the slot film cooling. TLC measurements, reported in terms of detailed 2D heat transfer coefficient maps, highlight the impact of the swirled flow and slot film cooling on wall heat transfer.

Mechanism and influencing factors analysis of flowing instability of supercritical endothermic hydrocarbon fuel within a small-scale channel

Regenerative cooling technology is widely used in advanced engines which use fuel as coolant. The occurrence of fuel flow instability during cooling process should be forbidden because it brings harm to the safe operation of the engine. The instability of supercritical endothermic hydrocarbon fuel flow during cooling is experimentally studied. Mechanism of instability is interpreted through the simulation results of a zero-dimension homogeneous model. Stability criterion is established and influencing factors are analyzed using the small deviation linearization theory and stability analysis method in the automatic control field. The experimental results indicate the instability of supercritical hydrocarbon fuel flow occurs in critical temperature region and cracking temperature region. The acute decrease in fuel density near the critical temperature region and cracking temperature region is the primary cause for instability of fuel flow. And the instability can be weakened by increasing tube heat capacity and compressible volume stiffness coefficient, reducing mass flow rate and using a fuel with density changing smoothly with temperature. The investigation on the instability mechanism and influencing factors offer guidance on inhibition methods, which can secure the stable fuel flow of cooling system.

Hypersonic Viscous Drag Reduction via Multiporthole Injector Arrays

A numerical study has been performed to investigate the film-cooling drag reduction performance of a small-scale multiport injector array, in addition to its potential for improved boundary-layer combustion-induced drag reduction. Hydrogen fuel is injected on a flat plate through an array consisting of four streamwise aligned flush circular portholes into a Mach 4.5 crossflow. Parametric studies were conducted on injectant mass flow rate and streamwise jet-to-jet spacing using the Reynolds-averaged Navier–Stokes equations with Menter’s Shear Stress Transport turbulence model. Complex jet interactions were found in the injection region with a variety of flow features dependent upon the specific configuration. These flow features were found to have subtle effects on the overall system performance. Total viscous drag reductions of up to 60% over a plate length of 0.5 m were achieved, with local drag reductions of over 90% in the near field. Significant wall heat transfer reductions were also found in all cases. Drag reduction and wall heat transfer rates were strongly influenced by injectant mass flow rate, and only moderately effected by streamwise spacing. The maximum drag reduction performance was found for the highest injectant mass flow rate and closest streamwise jet-to-jet spacing. In contrast, mixing performance generally improved with reduced mass flow rate and increased streamwise spacing.

Evaluation of simultaneous effects of inlet stagnation pressure and heat transfer on condensing water-vapor flow in a supersonic Laval nozzle

In supersonic two-phase flows of steam, under the influence of rapid expansion, the vapor becomes supersaturated. Following this condition, nucleation happens during the vapor phase; formed tiny droplets grow along the passage and, therefore, the condensation phenomenon occurs. The effects of the condensation phenomenon in power steam turbines include efficiency drop and mechanical damage. In the previous work of the authors, volumetric heating was introduced as an approach towards reducing the mentioned damage and loss. However, further investigations revealed that heating decreases the mass flow rate, which can be increased by adjusting the inlet stagnation pressure. In this paper, using a semi-analytical and a one-dimensional modeling approach, the simultaneous effects of volumetric heat transfer and inlet stagnation pressure variation are investigated in order to remedy the mass flow rate reduction. The results show that increasing the inlet stagnation pressure up to 5% can fix the mass flow rate of the non-adiabatic flow, compared to the adiabatic flow under the same conditions.

Experimental investigation of symmetric and asymmetric heating of pressure tube under accident conditions for Indian PHWR

In pressurized heavy water reactor (PHWR), under postulated scenario of small break Loss of Coolant Accident (LOCA) coincident with the failure of Emergency Core Cooling System (ECCS), a situation may arise under which reduction in mass flow rate of coolant through individual reactor channel can lead to stratified flow. Such stratified flow condition creates partial uncover of fuel bundle, which creates a circumferential temperature gradient over PT. The present investigation has been carried out to study thermo-mechanical behaviour of PT under asymmetric heating conditions for a 220MWe PHWR. A 19-pin fuel simulator has been developed in which preferential heating of elements could be done by supplying power to the selected pins. The asymmetric heating of PT has been carried out at pressure 2MPa and 1MPa, respectively, by supplying power to upper region heating elements thus creating an half filled stratified flow conditions. The temperature difference up to 425°C has been observed along top to bottom periphery of PT. A comparison is made between thermo-mechanical behaviour of PT under asymmetrical and symmetrical heat-up, expected from a large break LOCA condition. The radial expansion rate during symmetrical heating is found to be much faster as compared to that for asymmetric ballooning of PT at the same internal pressure. Integrity of PT is found to be maintained under both loading conditions. Heat sink around of test section, simulating moderator is found to be helpful in arresting the rise in temperature for both fuel pins and PT, thus establishing moderator as an effective heat sink under accident conditions.

A comprehensive study on the effect of cavitation on injection velocity in diesel nozzles

Results when testing cavitating injection nozzles show a strong reduction in mass flow rate when cavitation appears (the flow is choked), while the momentum flux is reduced to a lesser extent, resulting in an increase in effective injection velocity. So as to better understand the origin of this increase in effective injection velocity, the basic equations for mass and momentum conservation were applied to an injection nozzle in simplified conditions.The study demonstrated that the increase in injection velocity provoked by cavitation is not a direct effect of the latter, but an indirect effect. In fact, the vapor appearance inside the injection hole produces a decrease in the viscosity of the fluid near the wall. This leads to lower momentum flux losses and to a change in the velocity profile, transforming it into a more “top hat” profile type. This change in the profile shape allows explaining why the momentum flux reduction is not so important compared to that of the mass flow rate, thus explaining why the effective injection velocity increases.

An Investigation on Turbocharger Turbine Performance Parameters Under Inlet Pulsating Flow

Three-dimensional steady and unsteady (pulsating) compressible flows in a vane-less turbocharger turbine of a 1.7 liter SI engine are simulated numerically, and the results are validated experimentally using a turbocharged on-engine test cell. Simulations are carried out for a 720° engine cycle at three engine speeds, and the complete forms of volute and rotor vanes are modeled. Two ways for modeling the rotating wheel, multiple reference frames (MRF), and sliding mesh (SM) techniques are also examined. Finally, the effects of pulsating flow on the turbocharger turbine performance parameters (TTPP) such as the inlet static pressure, reduced mass flow rate, and efficiency are obtained and compared with their values under steady flow. The results show that the accuracy of steady characteristic map to estimate the TTPPs has some source of ambiguity, which should be considered for detailed analysis. TTPP values under steady flow conditions are found to be significantly deviated from the unsteady results. These deviations are decreased as the engine speed increases.

Dynamic performance assessment of MOX and metallic fuel core options for a Gen-IV LFR demonstrator

A preliminary study concerning the responses of a Generation IV (GEN-IV) Lead Fast Reactor (LFR) demonstrator (DEMO) core to externally-induced reactivity perturbations has been carried out with the aim at assessing and comparing the dynamic performances of MOX and metallic fuel alternative options. Reactivity coefficients and kinetics parameters have been calculated for both Beginning of Cycle (BoC) and End of Cycle (EoC) configurations by means of ERANOS deterministic code ver. 2.1 associated with the JEFF-3.1 data library. A simplified lumped-parameter model has been developed to treat both neutronics (point-kinetics approximation) and thermal-hydraulics (average temperature heat-exchange model); the latter have been then coupled to analyze MOX and metallic fuel behaviors following operational transient initiators, such as control rod partial extraction (reactivity insertion), coolant inlet temperature perturbation (simulating a loss of heat sink), and mass flow rate reduction (loss of flow), using the MATLAB/SIMULINK ® tool. The analysis of DEMO core sub-system open-loop stability has ultimately been performed. Results have shown that the model is stable and evidences a satisfactory capability of predicting the response to the reactivity perturbations considered.

Cold gas dual-bell tests in high-altitude simulation chamber

An experimental investigation has been carried out to study dual-bell transition behavior in different set ups inside a high-altitude test facility. Cold gas tests were carried out under two different operating conditions namely (i) test nozzle operating in self-evacuation mode and, (ii) test nozzle operating with an additional ejector nozzle (pre-evacuated condition). Although forward transition nozzle pressure ratio does not show any change in its value irrespective of the type of test facility and test set up, the re-transition nozzle pressure ratio shows a significant increase (7–8%) in its value when tested in the high-altitude facility. The latter is caused due to plume blowback effect which dominates during shut down transients in such facilities. Driven by the high atmospheric pressure, the jet exhaust is pushed backwards into the altitude chamber causing the re-transition to occur earlier than that observed in sea-level tests. Further the reduced mass flow rates for nozzle operation in different test set ups in a high-altitude test facility also reduces the magnitude of side-load peaks during the dual-bell transitions.