Change In Mass Flow Rate Research Articles

Response of a proton exchange membrane fuel cell to step changes in mass flow rates

AbstractTransient regime effects are particularly important in fuel cells designed for vehicles. Three‐dimensional modeling of a proton exchange membrane fuel cell with a serpentine channel is presented, and the response of the fuel cell to a step‐change in the mass flow rates is analyzed by using the computational fluid dynamics techniques. After a validation study of the mathematical and numerical model, step increases of 20% in mass flow rates are applied to the inlet boundary conditions, and time dependent power and current density responses of the fuel cell are analyzed. Polarization curves are generated for the assessment of the fuel cell performance, and their variations in time are presented. The results show that current and power densities increase with time at low cell voltage values due to concentration losses; however, increases in power and current are negligible at high voltages.

Modelling Bubble Lifetime of Thin Film Surfactants Solution on Fuel Spillage

Aims / Objectives: To find the lifetime of the bubble by plotting the rate of mass flow rate change against time. Place and Duration of Study: Department of Mathematics and Applied Science, Catholic University of Eastern Africa, Nairobi, Kenya, between February 2020 and March 2021.
 Methodology: The maximum lifetime of the bubble is assumed to match the time when the mass flow rate change is zero. The study also assumes the velocity of flow rate and other fluid properties at the interface of fuel-surfactant constant other than Re. Re is varied from 0.01 to 100.
 Results: The graphical plots show that for Re ! 1, and Re " 1, the stability depends on diffusive viscosity and linearized convection, respectively. The simulation suggested that the bubble formed at the fuel-surfactant interface may have Re “ 1 and its lifetime is tb » 0.28.
 Conclusion: The lifetime of surfactant depends on Re while assuming other interface properties constant.
 Recommendation: Future studies in the area need to consider the effect of variation in temperature, velocity, and Reynolds number in determining the lifetime of a bubble in the thin foam of the surfactant-fuel interface.

Transient numerical modelling of a thermoelectric generator system used for automotive exhaust waste heat recovery

The automotive thermoelectric generator system is a promising technology of exhaust waste heat recovery, but reasonable theoretical models to predict its dynamic performance are lacking. In this work, a transient fluid-thermal-electric multiphysics coupling field numerical model is proposed for the first time, and the model is used to evaluate the dynamic performance of a simplified automotive thermoelectric generator system under vehicle driving cycles. The transient numerical model, which takes into account the dynamic characteristics, fluid-thermal-electric multiphysics field coupling effects, and material temperature dependence, is thus far the most complete model ever. Numerical results reveal that there is a delay in output response with the change of exhaust temperature, and the change of output voltage and output power is often accompanied by the change of exhaust mass flow rate. The small and short-term fluctuation of exhaust gases has a slight influence on output performance. With the transient variation of exhaust characteristics, the output voltage and output power show more stable changes and slower responses, but the situation is the opposite for conversion efficiency. The output power predicted by steady-state numerical simulation is 12.6% higher than that of transient numerical simulation. Moreover, the proposed transient numerical model is recommended to investigate the dynamic performance of automotive thermoelectric generator systems.

Parametric investigation of a nanofluid-NEPCM based spectral splitting photovoltaic/thermal system

Photovoltaic cells can convert incoming sunlight within their responsive wavelength range into electricity. Unfortunately, the majority of the incident radiation is reflected, absorbed directly as heat, or indirectly converted to heat due to internal losses (i.e., ≥ 52.9%, even world-record efficiency cells). In concentrated photovoltaic systems, this added heat can further degrade the electrical conversion efficiency of the cells if it is not removed, resulting from an elevated operating temperature. Spectral splitters and absorptive filters can remove the unused portion of the solar spectrum before it is converted to heat in the photovoltaic cells and harvest this energy for active use in thermal applications. In this paper, an innovative concentrated photovoltaic/thermal system that utilizes a layered design containing an optical fluid and a phase change material was investigated as a new alternative absorptive filter. Five possible configurations of this filter were compared from energy, exergy, and energy storage perspectives. These layers consist of a phase change material (dotriacontane, a paraffin) layer and fluid channel layers (a glycerol fluid) with suspended Ag and/or Au nanoparticles. The optical properties of the nanofluid and the nano-enhanced phase change material (NEPCM) were modeled using Mie and Rayleigh scattering theories. The comparison showed that the configuration containing the nanofluid channel at the top of the nano-enhanced phase change material performs well in all aspects of energy, exergy, and energy storage. A parametric study was conducted to determine the effects of nanoparticles volume fraction, concentration ratio, mass flow rate, nanofluid thickness and phase change material thickness on PV/T system performance with this filter configuration. It was found that volume fractions lower than 0.3 × 10−5 and 0.2 × 10−5 for Ag and Au, respectively, provided the best system performance for the assumed geometric and operational conditions. At an optimal mass flow of 0.005 kg s−1 for the nanofluid, this design achieved a maximum exergetic efficiency of 24% for a concentration ratio of 30. The results also revealed that the nanofluid-NEPCM filter is not suitable for concentration ratios of less than 10. Overall, this study finds that nanofluid-NEPCM-based spectral splitter, in addition to successfully filtering radiation, can store energy efficiently.

Effect of pre-swirl nozzle closure modes on unsteady flow and heat transfer characteristics in a pre-swirl system of aero-engine

The pre-swirl system is of great importance for temperature drop and cooling air supply. This study aims to investigate the influencing mechanism of heat transfer, nonuniform thermodynamic characteristics, and cooling air supply sensitivity in a pre-swirl system by the application of the flow control method of the pre-swirl nozzle. A novel test rig was proposed to actively control the supplied cooling air mass flow rate by three adjustable pre-swirl nozzles. Then, the transient problem of the pre-swirl system was numerically conducted by comparison with 60°, 120°, and 180° rotating disk cavity cases, which were verified with the experiment results. Results show that the partial nozzle closure will aggravate the fluctuation of air supply mass flow rate and temperature. When three parts of nozzles are closed evenly at 120° in the circumferential direction, the maximum value of the nonuniformity coefficient of air supply mass flow rate changes to 3.1% and that of temperature changes to 0.25%. When six parts of nozzles are closed evenly at 60° in the circumferential direction, the maximum nonuniformity coefficient of air supply mass flow rate changes to 1.4% and that of temperature changes to 0.20%. However, different partial nozzle closure modes have little effect on the average air supply parameters. Closing 14.3% of the nozzle area will reduce the air supply mass flow rate by 9.9% and the average air supply temperature by about 1 K.

헬륨을 사용하는 코일형 열교환기의 형상 및 유량 변화에 따른 열전달 특성

헬륨을 사용하는 코일형 열교환기의 형상 및 유량 변화에 따른 열전달 특성

Analysis and optimization of triple tube phase change material based energy storage system

We present an analysis and optimization of triple tube phase change material (PCM) based energy storage system. The study considers a triple tube energy storage system with PCM stored in the inner cylinder and outer annular region while heated water is passed through the inner annular region. Simulations are performed to analyze the effect of inner cylinder diameter on the total time required for complete melting of PCM while keeping the volume of PCM constant. Results show that the triple cylinder configuration leads to considerably faster melting as compared to the double cylinder system for the same mass flow rate of water and same quantity of PCM. It is found that the inner diameter, inlet temperature and mass flow rate of heat transfer fluid have considerable effect on the melting time while the effect of cylinder length is negligible. The main outcome of the study is that it shows that there exists an optimum diameter of the inner cylinder for which the melting rate is highest and considerably faster than that for the maximum and minimum diameter. Subsequent studies for different mass flow rates of water show that the optimum diameter does not vary significantly with change in mass flow rate.

Analysing the transport phenomena of novel dew-point evaporative coolers with different flow configurations considering condensation

This paper entails a study on the transport phenomena of a novel dew-point evaporative cooler with counter-flow closed-loop configuration, a potential alternative to the conventional mechanical vapor compression system. In contrast to the conventional indirect evaporative cooler and the regenerative indirect evaporative cooler, the proposed novel dew-point evaporative cooler is capable of achieving the simultaneous goals of pre-cooling, energy recovery and dehumidification. The novel dew-point evaporative cooler is classified as either type 1 or type 2 based on the relative flow direction between the water and the supply air. A two-dimensional computational fluid dynamics model is judiciously developed and employed to conduct a comparative study which evaluates the performance of the conventional indirect evaporative cooler vis-a-vis both novel type 1 and novel type 2 configurations. Experimental data is then employed to validate the predictive accuracy of the mathematical model. The distributions of both temperature and humidity ratio are plotted for a typical case of the novel cooler incorporating both flow configurations and considering the effects of moisture condensation. Additionally, the impacts made by key parameters on the cooling performance under condensation state are analyzed by using three evaluation indexes, namely, latent efficiency, wet-bulb efficiency, and enlargement coefficient. By regulating several operating parameters, the variations of the three evaluation indexes are plotted via the proposed numerical model. Key results from this study revealed that the novel type 1 flow configuration is capable of achieving the highest latent and wet-bulb efficiencies, and enlargement coefficient. It is also observed that the relative flow direction between the water and the supply air exerts a stronger influence on the cooler performance compared to the changes of the water mass flow rate.

Two-phase flow distribution in a refrigerant distributor having four indoor-unit connections of a variable refrigerant flow system

• Multiple indoor units are connected to a refrigerant distributor of a VRF system. • Distribution of two-phase flow was experimentally and numerically investigated. • Maldistribution of two-phase flow is significantly influenced by void fraction. • CFD simulation was performed to estimate distribution of R410A. • Maldistribution of R410A appeared to be less than maldistribution of air-water. In a variable refrigerant flow (VRF) system, the refrigerant is supplied from one outdoor unit to multiple indoor units through a header or refrigerant distributor. When the two-phase refrigerant flows into the distributor, uneven distribution occurs and deteriorate system performance. An experimental apparatus simulating the refrigerant distributor of a VRF system was constructed and air-water two-phase flow distribution characteristics were investigated. The air-water two-phase flow was more mal-distributed as the void fraction increased. A change in void fraction affected the distribution more than a change in mass flow rate in the experimental range. The flow distribution in the header was little affected by variation of the downstream flow resistance. Previous empirical correlations were investigated to predict the air-water two-phase flow distribution measured in this experiment, but performance was poor. To estimate the refrigerant two-phase flow distribution, computational fluid dynamics (CFD) simulations were performed using R410A as a working fluid. The mal-distribution of R410A appeared to be mitigated compared with air-water flow due to the large vapor phase density of R410A.

Photovoltaic thermal building skin: effect of condensing and evaporating temperature on flow rate and heat transfer

PurposeThis paper aims to present a numerical study on the natural convection, which operates either as an evaporator or condenser unit of the heat pump system to pre-cool and pre-heat the ambient fresh air.Design/methodology/approachThis study focuses on natural air cooling or heating within the air channel considering the double skin configuration. Particular focus is given to the analysis of airflow and the heat transfer processes in an air channel to cool or heat the ambient fresh air. In this study, the physical model consists of one wall, either heated uniformly or cooled uniformly, whereas the other wall is adiabatic.FindingsThe results show that the variation of both velocity and temperature is observed as the flow transition occurs at the evaporator or condenser wall. In either case, the temperature rises in the range of 6.3–8.4°C with an increase in mass flow rate from 0.07–0.08 kg/s in the photovoltaic thermal condenser part, while in the photovoltaic thermal evaporator part, the change in mass flow rate from 0.048–0.061 kg/s causes a decrease in temperature from 7.1–4.5°C.Practical implicationsThe solar-assisted photovoltaic thermal heat pump system, in building façade having an air layer application, is feasible for pre-heating and pre-cooling the ambient fresh air and also reduces the energy needed to treat the fresh air.Originality/valueThe influence of condensing and evaporating temperature under natural convection mode in double skin conformation is considered for pre-heating and pre-cooling of ambient fresh air.

Experimental Research on Flow and Heat Transfer in Microchannel with Refrigerant HFO1234yf

An experiment was carried out to study the effect of refrigerant mass flow rate, heat flux density, and saturation temperature on boiling heat transfer coefficient and frictional pressure drop in microchannel. Flow patterns are captured by a high-speed camera. HFO1234yf was used in rectangular microchannels with a hydraulic diameter of 0.5 mm. The results showed that there were eight flow patterns in the experiment, and the change of flow pattern is closely related to the change of heat flux and mass flow rate. The higher the heat flux and mass flow rate, the more the flow patterns appear. Different flow patterns have different heat transfer mechanisms. The heat transfer mechanism is mostly nuclear boiling at the region of bubble flow, slug flow, elongated slug flow, and agitated flow; the heat transfer mechanism is film convection boiling at the region of annular flow, and it is vapor convection at the region of dryout flow. When the dryness is about 0.4, the heat transfer mechanism changes from nuclear boiling to film boiling. The friction resistance decreases with the increase of dryness and saturation temperature.

Controlling the Thermal Power of a Wall Heating Panel with Heat Pipes by Changing the Mass Flowrate and Temperature of Supplying Water—Experimental Investigations

Renewable energy sources for the purpose of heating buildings cooperate perfectly with so-called low-temperature heating systems. Water loop surface heating systems had been thoroughly tested. In contrast, thermal performance of wall panels with heat pipes have not been fully recognized, yet. The determination of the thermal power as well as the control of panels thermal performance cannot be performed with the methods developed for water loop systems. In this paper, the novel heating panels with heat pipes were tested to analyze the possibility of controlling their performance by changing the mass flowrate of heating water and its temperature. Specific heating power of the investigated panels varies from 16.9 W/m2 to 93.8 W/m2 when supplying a water temperature ranging from 35 °C to 65 °C and mass flowrate from 10 g/s to 47.5 g/s. Investigations revealed that the thermal performance of the panels is more sensitive to the changes of temperature than to the changes of mass flowrate of supplying water, and thus, should be controlled by changing the supply water temperature at low mass flowrates to obtain a low energy usage of pumps (diminished pressure losses) and good quality of controlling.

Shear Rate Coat-Hanger Die Using Casson Viscosity Model

Coat-hanger die design aims for optimization of the die geometry of the body and the flow distribution manifold, such that through the exit at the die lip homogeneous distribution of the polymer melt is achieved. This paper proposes a novel methodology for deriving the design equations of the coat-hanger die geometry for some specific extrusion materials and provides fluid–solid interaction simulations for validations. The basis for the calculations is the Casson rheological model, analytic velocity profiles for the pseudoplastic flow through circular pipe and slit, and the constant shear rate coat-hanger die design methodology developed by Winter and Fritz. The geometry obtained was then evaluated using the fluid-structure interaction numerical simulation approach. The sensitivity of the outlet velocity uniformity and die body deformation due to the material and mass flow rate change were investigated using the finite element software, Ansys. It was found that the homogeneity of the outlet velocity is very sensitive to the extrusion materials. The structural analysis of the solid die body also resulted in higher deformations when using some other extrusion materials different from the initial design. Mass flow rate increase only resulted in large zones of stagnation, which occurred around the melt as it passes from the manifold to the slit region. Therefore, it is recommended to define the required range of mass flow rate to prevent the formation of stagnation zones.

THERMO-HYDRAULIC PERFORMANCE ANALYSIS OF PARABOLIC CONCENTRATING SOLAR WATER HEATER

This paper concerns the thermo-hydraulic performance analysis of a PCSWH (parabolic concentrating solar water heater) with placing the PTT (perforated twisted tape) in the absorber tube. The experiments are performed using water as testing fluid. In this analysis, the mass flow rate changes from 0.0326 kg/s to 0.0667 kg/s and Reynolds number ranges from 2100 to 4250 respectively. The analysis is based on the effect of porosity of the PTT and mass flow rate over the pressure drop and heat transfer enhancement during the flow into the absorber tube of the PCSWH. At last to guess the increment in pressure drop and enhancement of heat transfer the empirical correlations for friction factor and the Nusselt number have been formulated considering the turbulent effect caused by PTT inside the absorber tube. The major observations of this analysis are that PTT enhances the pressure drop and heat transfer and also both enhances with increasing porosity of the PTT.

Choked Flow Behavior of CO2 Refrigerant Flowing Through the Spiral Capillary Tube

The flow characteristics of CO2 refrigerant are numerically studied for an adiabatic spirally coiled capillary tube employing choked flow conditions. The mass, momentum and energy conservation equations are used to develop a numerical model. The existing model is verified with the published results. The choked flow behavior at various geometric parameters viz. tube diameter and spiral pitch is studied. Similarly, the influence of these parameters on the mass flow rate through the tube is observed. A significant change in mass flow rate is due to a change in tube diameter, whereas a minimal variation is observed with the change in surface roughness and spiral pitch. Moreover, it is observed that the coiling effect has a significant influence on the flow behavior of the spiral capillary tube. As the pressure decreases, from unchoked to the choked pressure in the evaporator by 63.46%, the mass flow rate increases by 9.46% only. The capillary tube choking is circumvented by increasing spiral pitch, tube diameter and decreasing the length of the tube. A unique nomogram is developed that gives the best understanding of choked and unchoked flow conditions, that graphical representation is useful to design the spirally coiled capillary tube. By using that, the choked length is identified for the known mass flow rate, even more, the choked mass flow rate is known for a given tube length. Moreover, for the given tube length and evaporator temperature, a nomogram is useful to the known choked values of mass flow rate and exit values of the evaporator pressure and quality of refrigerant.

Three-dimensional numerical simulation of external fluid flow and heat transfer of a heat exchanger in a wind tunnel using porous media model

Notwithstanding the widespread use of wind tunnel to investigate the performance of radiators and heat exchangers, has never been considered a numerical analysis of radiator inside the wind tunnel with complete three-dimensional geometry of tunnel. In the present study, three-dimensional numerical simulation of thermal performance of a car radiator inside a wind tunnel is performed to predict and improve the air flow behavior in a wind tunnel. The effect of the Reynolds number of air and the mass flow rate of hot water supply into the heat exchanger to preheat the air is investigated on heat transfer and fluid flow. Radiator, heat exchanger, honeycomb and flow straighteners in the diffusers are defined as porous media due to their complex geometry. To solve this problem, the $$k - \varepsilon$$ turbulence model with a high-resolution discretization was used along with the (RMS) convergence criterion. Moreover, the results of the present numerical analysis are compared with the performance results of a car radiator in a thermal wind tunnel and demonstrated that the results are in good agreement. It can be seen in the results that in formation of vortex and reverse flows, there are the test section and the corner of diffusers. But the maximum variety of pressure, velocity and temperature is observed in the test section. The agitation is enhanced by increasing the Reynolds number and the difference between the size of radiator and test chamber. That can lead to undesired errors during the measurement process in experimental tests. In addition, the air non-dimensional temperature decreases by 30.1% and the air total pressure drop increases by 230 times by increasing the Reynolds number of air from 1e5 to 1.5e6 at a constant water mass flow rate of 8 kg s−1. Also, at Reynolds number of 1.5e6, air non-dimensional temperature decreases by 54.03% as the water mass flow rate changes from 18 to 4 kg s−1.

Mathematical modeling of transition processes due to a change in gas consumption at the ends of the inclined section of the gas pipeline

The paper presents a solution to the problem of the gas-dynamic state of an elementary section of a gas pipeline with variable input and output mass gas outflows, which is obtained by the method of separation of variables. The quasi-one-dimensional mathematical model of pipeline transport of real gas takes into account factors such as friction, gravity and the local component of the inertia of the gas. By introducing the mass flow rate and averaging the velocity in the quadratic law of resistance, the autonomous equations of the telegraph equation with respect to pressure and mass flow are compiled from a system of equations. Taking into account possible abrupt changes of the unknowns in time and distance, the solution is sought in the form of functional series. The demonstrativeness of this method lies in the fact that the perturbation frequencies relevant to the parabolic and hyperbolic types of equations and the intermediate variant are distinguished. A description is given of a general method for solving the problem when temporary changes in the gas mass flow rate are set at the ends of the section. The exact solutions of mass flow rate and hydrostatic pressure are obtained for the case of a spasmodic change in mass flow rate at the boundaries of the site with a constant slope from the horizon. Numerical results related to the case of instantaneous closure of the output section of a linear section are presented. The results of the work are useful for assessing the energy intensity and reliability of gas pipelines in conditions of large diameters, high working pressures, and also when the gas pipeline is laid along a relief line.

Modelling of a Single Passage Air PV/T Solar Collector: Experimental and Simulation Design

The hybrid photovoltaic/thermal solar collector has attracted research attention for more than five decades. Its capability to produce thermal energy simultaneously with electrical energy is considered attractive since it provides higher total efficiency than stand-alone photovoltaic or thermal systems separately. This paper describes theoretical and experimental studies of a finned single pass air-type photovoltaic/thermal (PV/T) solar collector. The performance of the system is calculated based on one dimensional (1D) steady-state analysis using one dimensional energy balance equations, where simulation was carried out using MATLAB. Experiments were carried out to observe the performance of the solar collector under changes in air mass flow rate. Experimental values on photovoltaic panel temperature and air temperature on both air inlet and outlet, together with the ambient temperature and solar radiation were measured. The simulation results were validated against the results obtained from experiments using the error analysis method, Root Mean Square Error. At a solar irradiance level of 800 to 900 W/m2, the thermal efficiency increases to 20.32% while the electrical efficiency increases to 12.01% when the air mass flow rate increases from 0.00015 kg/s to 0.01 kg/s. The error analysis shows that both experimental and simulation results are in good agreement.

Identification of the Wear Margin of a Pipeline–Machine Subsystem

The quality of company asset management is significantly dependent on the quality of a system for asset wear margin identification. A pipeline–machine subsystem may be an essential part of assets in many production companies. It is necessary to build models of pipeline–machine subsystems and models of a system for the identification of subsystem wear margin. The method used consists of a decomposition of desired characteristics of an enterprise into desired characteristics of a pipeline–machine subsystem. Methods for the identification of real characteristics of a subsystem depend on the character of subsystem operation. In this study, hydrodynamic and thermodynamic models of the subsystem are built. Tests are conducted on industrial and laboratory objects. The boundaries of the subsystem are defined and changes in pressure, temperature and mass flow rate in the pipeline are presented. Causes of changes in the mentioned quantities are described. Desired characteristics of the subsystem resulting from decomposition are described. The presented methods of determining efficiency for steady working conditions and open flow use hydrodynamic and thermodynamic models. Energy efficiency of the subsystem is decomposed into efficiencies of main elements of the subsystem. A method is proposed for determining the subsystem’s energy efficiency in the case of the flow into a closed vessel. It is possible to determine the hydraulic efficiency of the subsystem components: the suction pipe, the discharge pipe and the machine. The efficiency of the machine determined by the hydrodynamic method is complementary to the efficiency obtained by the thermodynamic method. The machine set efficiency is composed of hydraulic efficiency of the machine; mechanical efficiency of the machine, the gearbox and motor; and electric efficiency of the motor. Hydraulic efficiency of the pipeline is related to substitute measure of wear margin—the coefficient of resistance. Pressure drop is a diagnostic symptom. Thermal efficiency of the heat exchanger is related to a substitute measure of wear margin—the coefficient of heat penetration. Temperatures and mass flow rate in the heat exchanger are diagnostic symptoms. It is possible to determine the capacity and efficiency of subsystems with one side closed—such as those filling hydrophore and gas vessels.

Performance Parameter Analysis of a Hall Effect Thruster With Modified Bohm Parameter Model

The Hall effect thruster (HET) is an electric propulsion system in which electrons confined in a certain region collide with neutral particles due to the Hall effect. The neutral particles are ionized, and the ions are accelerated by the electric field to produce thrust. Thus, a Hall thruster has a simpler structure than an ion thruster. To improve the accuracy of the HET performance analysis, the energy change of electrons is predicted by considering thermal conduction. The number of ions produced by ionization and the change in electron energy via multivalent ionization are investigated. To minimize the independent variables, the boundary condition is modified after considering the physical correlation of each variable, and a semi-experimental model equation for the Bohm parameter is proposed. The performance results have greater similarity to the experimental results than those of the previous analytical model. Based on this, the performance changes according to the thruster operating conditions are predicted and compared. It is confirmed that the thrust increases by about 10–20% according to the change of propellant mass flow rate due to the increase of ion exit velocity.