Periodic Inlet Research Articles

Transient prediction model of finned tube energy storage system based on thermal network

This paper proposed a two-dimensional thermal network model to predict the output of the finned tube energy storage system during the melting stage to solve the fast calculation of latent heat storage cooling equipment for high-energy airborne weapons. The thermal network model introduces effective thermal conductivity to consider the effect of natural convection on the melting of PCM. Numerical simulation and experiment verify the accuracy of a single tube under various working conditions and actual size heat exchangers, respectively. Compared with single-tube numerical simulation, the maximum average outlet temperature error is 0.634 °C, saving 99% of the simulation time. Compared with the experiment, the maximum temperature error of outlet temperature is 4.91%, and the average temperature error is 1.18%. Results are found to be in good agreement with numerical simulation and experiment. The performance of heat exchangers is evaluated by effectiveness under different conditions, including fin geometry, material, and periodic inlet. The results show that the structure with smaller fin thickness and interval has better effectiveness and latent heat storage, more benefits from increasing the fin thermal conductivity, and better stabilizes the outlet under the periodic inlet. It provides a reference for designing and optimizing energy storage systems.

충전층 축열시스템의 성능 특성

An analytical model is presented, for packed-bed thermal storage systems. The model is based on a mathematical solution for packed-bed storage systems, subject to arbitrary periodic inlet temperature conditions and heat loss to ambient. The model is validated via comparison with well-known theoretical and experimental data in the literature. It is also demonstrated that the model can be used to analyze the experimental results, and to predict the performance of packed beds in general. The model is simple and flexible, so that it can facilitate design and analysis of the packed-bed systems, subjected to various boundary conditions in the field.

Numerical Study of the Fluid-Structure Interaction During CNT-Water Nanofluid Mixed Convection in a Micro-Channel Equipped with Elastic Fins Under Periodic Inlet Velocity Conditions

Numerical Study of the Fluid-Structure Interaction During CNT-Water Nanofluid Mixed Convection in a Micro-Channel Equipped with Elastic Fins Under Periodic Inlet Velocity Conditions

주기적으로 작동하는 평행류 열교환기의 해석해와 그 응용

The objective of this study is to present an analytical model for a parallel-flow rotary wheel heat exchanger. The model is based on mathematical solution for an ideal parallel-flow heat exchanger with a periodic inlet temperature condition. Air and wheel matrix temperatures are given in an infinite series of the product of trigonometric and exponential functions. An analytical expression is also presented for the effectiveness of the heat exchanger, which has been validated using literature and numerical data. The model is confirmed to give an exact result when the NTU ratio between hot and cold region is unity. It tends to give a higher error for smaller heat capacity rate ratio as it deviates from the unity. Ranges of parameters are presented with distribution of the maximum absolute error. This study shows that the model is applicable to cross-flow heat exchangers when the split ratio approaches zero.

CFD study on crossflow and heat transfer characteristics of single phase flow in rod bundle with spacer grid under rolling conditions

In order to study the thermal and hydraulic characteristics of nuclear reactors under ocean conditions and meet the urgent needs of the design of floating nuclear power plants, the crossflow and heat transfer characteristics of rod bundle with the spacer grid under rolling conditions are studied by using computational fluid dynamics (CFD) code. Firstly, revise and verify CFD codes suitable for rolling conditions. According to CFD results, the influence of heat flux on heat transfer coefficient is greater than that of pressure drops under this condition. And with the spacer grid, the total pressure drop between inlet and outlet is higher. Furthermore, the heat transfer coefficient is also higher due to the mixing effect of spacer grid. Under periodic inlet velocity, the fluctuation amplitude of total pressure, the temperature difference between inlet and outlet and heat transfer coefficient is increased obviously with the spacer grid. Under the condition of nonuniform heat flux, the variation law of crossflow with rolling motion will become more obvious and the spacer grid can enhance the crossflow mixing and improve the heat transfer capacity of the fluid under rolling conditions.

Modeling VOCs emission/sorption with variable operating parameters and general boundary conditions.

An integrated model of VOCs emission/sorption from/on dry building materials with a general boundary condition, variable air exchange rate and inlet concentration is developed. An analytical solution is obtained by using the generalized integral transform technique. Good agreements are obtained between the present model and the experimental data. The effects of environmental conditions on the emission are investigated. The emission from two surfaces can increase the concentration of hexanal in the air and decrease the initial emission rate at x=δ with the increase in mass transfer coefficient at x=0. Periodical inlet concentration can lead to the periodic variation of materials between a source and a sink. Ventilation can keep the concentration in the air at a low level and help to decrease the concentration of hexanal in materials. The present model is capable of simulating indoor air quality due to the VOCs emission and sorption.

LES of Turbulent Co-current Taylor Bubble Flow

Turbulent co-current Taylor bubble flow was modeled with LES alongside the VOF interface tracking solver. The method is first validated in the context of single phase turbulent pipe flow and three distinct laminar Taylor bubble flow cases. The turbulent co-current Taylor bubble flow is simulated using a moving frame of reference which is attached to the bubble, allowing for a straight-forward time averaging and Dirichlet-type flow recycling inlet boundary condition. In contrast to periodic inlet and outlet boundary conditions, the Dirichlet one assures that small bubbles which are lost by the Taylor bubble are removed from the computation once they leave the domain, therewith not interfering with the nose of the Taylor bubble. The transient loss of small bubbles which is observed in our simulations implies that the problem is inherently unsteady. We therefore present an averaging strategy which consists of combined time, space and ensemble averaging. This strategy allows systematic comparison of results across different computational meshes. The results show that the approach captures distinct features of turbulent co-current Taylor bubble flow well, in the sense that good agreement is observed with experimental data from literature in terms of the toroidal vortex flow field in the wake of the Taylor bubble. Agreement with measured velocity fluctuation profiles in the wake of the Taylor bubble is less pronounced, and is discussed in the paper. Nevertheless, the simulation approach of co-current Taylor bubble flow proposed in this paper may serve as a tool that will provide deeper insights in two-phase turbulent flow for future research.

Integral transform method solution for heat transfer in polymer melt flow in a parallel plate single screw channel with periodic inlet temperature

This work investigates the capacity of a particular class of flows, namely drag and pressure driven flows, to propagate and distorts a transient sinusoidal temperature disturbance occurring at the inlet of the metering zone of a polymer processing screw channel. The integral transform method is applied to the forced convection heat transfer problem in a parallel plate channel with a general drag and pressure driven flow velocity profile for a Newtonian fluid, with the pressure gradient as an adjustable flow parameter. We follow closely the procedure outlined by Cotta and Özişik in their seminal paper [5]. We found that the knowledge of the lowest eigenvalue in the solution is sufficient to characterise the dampening efficiency of an inlet temperature fluctuation by the flow and that a higher level of positive pressure difference is indeed improving the thermal homogeneity in the flow. Moreover, we found that for any given flow, the higher frequency disturbances are dispersed more efficiently than the lower frequency disturbances.

Film Control to Study Contributions of Waves to Droplet Impact Dynamics on Thin Flowing Liquid Films.

Droplet impact is a very common phenomenon in nature and attracts attention due to its aesthetic fascination and wide-ranging applications. Previous studies on flowing liquid films have neglected the contributions of spatial structures of waves to the impact outcome, while this has recently been shown to have a significant influence on the drop impact dynamics. In this report, we outline a step-by-step procedure to investigate the effect of periodic inlet forcing of a flowing liquid film leading to the production of spatiotemporally regular wave structures on drop impact dynamics. A function generator in connection with a solenoid valve is used to excite these spatiotemporally regular wave structures on the film surface while the impact dynamics of uniform-sized droplets are captured using a high-speed camera. Three distinct regions are then studied; viz. the capillary wave region preceding the large wave peak, the flat film region, and the wave hump region. The effects of important dimensionless quantities such as film Reynolds, drop Weber and Ohnesorge numbers parameterized by the film flow rate, drop speed, and drop size are also examined. Our results show interesting, hitherto undiscovered dynamics brought about by this application of film inlet forcing of the flowing film for both low and high inertia drops.

Film Control to Study Contributions of Waves to Droplet Impact Dynamics on Thin Flowing Liquid Films

Droplet impact is a very common phenomenon in nature and attracts attention due to its aesthetic fascination and wide-ranging applications. Previous studies on flowing liquid films have neglected the contributions of spatial structures of waves to the impact outcome, while this has recently been shown to have a significant influence on the drop impact dynamics. In this report, we outline a step-by-step procedure to investigate the effect of periodic inlet forcing of a flowing liquid film leading to the production of spatiotemporally regular wave structures on drop impact dynamics. A function generator in connection with a solenoid valve is used to excite these spatiotemporally regular wave structures on the film surface while the impact dynamics of uniform-sized droplets are captured using a high-speed camera. Three distinct regions are then studied; viz. the capillary wave region preceding the large wave peak, the flat film region, and the wave hump region. The effects of important dimensionless quantities such as film Reynolds, drop Weber and Ohnesorge numbers parameterized by the film flow rate, drop speed, and drop size are also examined. Our results show interesting, hitherto undiscovered dynamics brought about by this application of film inlet forcing of the flowing film for both low and high inertia drops.

Simulation of pressure oscillations in a combustion chamber under periodic inlet disturbances

The working process of a solid rocket motor (SRM) is a transient problem. The burning rate of the propellant is related to several factors, which results in variations in the mass injection rate. To date, the simulation of the flow field in an SRM generally comprises the use of constant inlet boundary conditions, a simulation that does not consider inlet disturbance conditions. This paper is mainly concerned with the influence of periodic disturbances on the pressure oscillation. In this study, a numerical simulation was performed with a periodic disturbance inlet based on the typical Von Karman Institute for Fluid Dynamics subscale motor without an obstacle. It was found that the inlet conditions of the periodic disturbance greatly affect the amplitude and frequency of the pressure oscillation in the combustion chamber. When the mass flow inlet disturbance frequency is close in value to the acoustic modal frequency of the combustion chamber, the amplitude of the pressure oscillations increases. Similarly, when the disturbance frequency is far in value from the acoustic modal frequency, the amplitude of the pressure oscillation decreases. When the inlet disturbance changes from 650 Hz to 400 Hz, the amplitude of the pressure oscillation increases by 451.5%. In addition, at certain frequencies, the inlet disturbance can excite pressure oscillations at acoustic modes near the disturbance frequency in the combustion chamber.

Analysis of inter-channel sweeping flow in wire wrapped 19-rod bundle

Helical wire spacers are widely used in sodium cooled fast reactors. One of the main effects of wire spacers is enhancing the inter-channel cross flow in triangular arrayed rod bundles. To study the characteristic of sweeping flow rate across the gaps between the sub-channels, the computational fluid dynamics (CFD) method is used in the present study to analyze the flow field in a 19-rod bundle with wire wraps. This CFD work only considers the fluid zone of one section of the rod bundle. The height of the CFD model with periodic inlet and outlet boundary conditions is one wire pitch. The k-ω SST model is used as the turbulent model. The wire effect on the velocity distribution of the flow across the interface between two sub-channels is investigated in the CFD study. Based on the force balance equation between the wire wake force and the surface friction force of rods in the circumferential direction, a theoretical model for estimating the average swirling flow rate driven by one single wire is proposed. The simulation shows that the ratio of rod pitch (P) to rod diameter (D) has a strong effect on the velocity of the sweeping flow. The flow rate increases with the P/D ratio. The simulation agrees well with the proposed model. In addition, the effect of the ratio of wire pitch (H) to rod diameter has also an influence on the cross flow rate: A small H/D ratio leads to higher cross flow rate. A correlation as a function of both P/D ratio and H/D ratio is proposed to predict the sweeping flow velocity by the present study.

Resonance response in the catalytic combustion of methane and propane binary mixture in reverse-flow reactor

A numerical study was performed of the resonance behavior of a reverse-flow reactor that conducts methane and propane binary-mixture catalytic oxidation. A one-dimensional heterogeneous model was used. A resonance response occurs when the periodic inlet concentration variation cycle is the odd part of the flow-reversal cycle, which results in temperature excursion and a sharp increasing in maximum temperature within the reactor. Decreasing in average temperature implies thermal stability of the reactor is impaired. Temperature variation in the reactor is induced by both inlet concentration variation and flow-reversal operation, which may contain low frequency variation. Such resonance behavior is attributed to the breakdown of spatial and temporal symmetry of the reactor. Thus, ways to compensate such asymmetry can be used to maintain stable reactor operation, including adjusting the initial phase difference between the flow-reversal cycle and the inlet concentration variation cycle or the addition of supplemental fuel.

Combined numerical and experimental investigations of heat transfer of a highly loaded low-pressure turbine blade under periodic inlet flow condition

This paper experimentally and numerically investigates heat transfer characteristics of a low-pressure turbine blade under steady/unsteady flow conditions. Generally, the low-pressure turbine blades are not exposed to excessive temperatures that require detailed heat transfer predictions. In aircraft engines, they operate at low Re-numbers causing the inception of large separation bubbles on their suction surface. As documented in previous papers, the results of detailed aerodynamic simulations have shown significant discrepancies with experiments. It was the objective of the current investigation to determine the discrepancies between the experimental and numerical heat transfer results. It is shown that small errors in aero-calculation results in large deviations of heat transfer results. The characteristics of the blades mentioned above, make low-pressure turbine blades suitable candidates for evaluating the predictive capability of any numerical method. Documenting the scope of these discrepancies defines the framework of the current paper. The periodic flow inside the gas turbine engine was simulated using the cascade facility at the Turbomachinery Performance and Flow Research Laboratory (TPFL) of Texas A&M University. In this study, the wakes that originate from stator blades were simulated by moving rods. The instrumented blade was covered with a liquid crystal sheet and it was used to measure heat transfer coefficient. Reynolds-averaged Navier–Stokes equations were used for numerical investigation purposes. Measurements and simulations were conducted at three different Reynolds numbers (110,000, 150,000, and 250,000). Furthermore, for unsteady flow condition, reduced frequencies of the incoming wakes were varied. The current paper includes a comprehensive heat transfer assessment of the predictive capability of Reynolds-averaged Navier–Stokes based tools. The effect of the separation bubbles on heat transfer is thoroughly discussed in this paper. Comparisons of the experimental and numerical results detail the differences and identify the sources of error that leads to in accurate calculations in terms of predicting heat transfer calculation results.

Instability modes on a solid-body-rotation flow in a finite-length pipe

Numerical solutions of the incompressible Navier-Stokes equations are obtained to study the time evolution of both axisymmetric and three-dimensional perturbations to a base solid-body-rotation flow in a finite-length pipe with non-periodic boundary conditions imposed at the pipe inlet and outlet. It is found that for a given Reynolds number there exists a critical swirl number beyond which the initial perturbations grow, in contrast to the solid-body rotation flow in an infinitely-long pipe or a finite-length pipe with periodic inlet and exit boundary conditions for which the classical Kelvin analysis and Rayleigh stability criterion affirm neutrally stable for all levels of swirl. This paper uncovers for the first time the detailed evolution of the perturbations in both the axisymmetric and three-dimensional situations. The computations reveal a linear growth stage of the perturbations with a constant growth rate after a brief initial period of decay of the imposed initial perturbations. The fastest growing axisymmetric and three-dimensional instability modes and the associated growth rates are identified numerically for the first time. The computations show that the critical swirl number increases and the growth rate of instability decreases at the same swirl number with decreasing Reynolds number. The growth rate of the axisymmetric mode at high Reynolds number agrees well with previous stability theory for inviscid flow. More importantly, three-dimensional simulations uncover that the most unstable mode is the spiral type m = 1 mode, which appears at a lower critical swirl number than that for the onset of the axisymmetric mode. This spiral mode grows faster than the unstable axisymmetric mode at the same swirl. Moreover, the computations reveal that after the linear growing stage of the perturbation the flow continues to evolve nonlinearly to a saturated axisymmetric vortex breakdown state.

Human heart preservation analyses using convective cooling

– Currently, human hearts destined for transplantation can be used for 4.5 hours which is often insufficient to test the heart, the purpose of this paper is to find a compatible recipient and transport the heart to larger distances. Cooling systems with simultaneous internal and external liquid cooling were numerically simulated as a method to extend the usable life of human hearts. , – Coolant was pumped inside major veins and through the cardiac chambers and also between the heart and cooling container walls. In Case 1, two inlets and two outlets on the container walls steadily circulated the coolant. In the Case 2, an additional inlet was specified on the container wall thus creating a steady jet impinging one of the thickest parts of the heart. Laminar internal flow and turbulent external flow were used in both cases. Unsteady periodic inlet velocities at two frequencies were applied in Case 3 and Case 4 that had four inlets and four outlets on walls with turbulent flows used for internal and external circulations. , – Computational results show that the proposed cooling systems are able to reduce the heart temperature from +37°C to almost uniform +5°C within 25 min of cooling, thus reducing its metabolic rate of decay by 95 percent. Calculated combined thermal and hydrodynamic stresses were below the allowable threshold. Unsteady flows did not make any noticeable difference in the speed of cooling and uniformity of temperature field. , – This is the pioneering numerical study of conjugate convective cooling schemes capable of cooling organs much faster and more uniformly than currently practiced.

Determination of air-to-air heat wheel sensible effectiveness using temperature step change data

The determination of the effectiveness of large heat wheels using standard measured data and test conditions can be very expensive and time consuming. The main contribution of this paper is that it tests heat wheel components rather than the wheel itself. In addition, this paper deals directly with the question of what accuracy can be achieved for the determination of sensible energy effectiveness by using a transient step change method for the exchanger matrix materials in a test cell. In this study, the sensible effectiveness of heat wheels is predicted by performing a number of cyclic and single step change transient experiments on a parallel-plate heat exchanger. A new experimental facility is developed to cause a step change for the inlet air temperature of the exchanger. In the cyclic tests, the heat exchanger is exposed to a periodic inlet temperature steps; afterward, the sensible effectiveness of parallel-flow and counter-flow heat wheel, comprised of the same material as parallel-plate exchanger, is determined using the obtained temperature profiles. It is find that this method can be used to determine the sensible effectiveness of parallel-flow heat wheel. However, the high uncertainty found in the sensible effectiveness of counter-flow heat wheel, ±32%, makes the results unreliable. In the single step-change test, a time constant is assigned to the exchanger response when it is subjected to a step change in inlet temperature. The time constant is obtained by fitting the experimental data to a first order exponential time response curve. An analytical solution posits that the effectiveness of the heat wheel depends only on the product of the time constant and the wheel angular speed, or angle ratio. Comparing values of the sensible effectiveness calculated through available empirical correlations and the ones obtained by single step change experiment showed less than 3% difference in results when the heat capacity rate ratio is greater than 5. It is concluded that due to simplicity, accuracy, and low cost of the single step change experiments, it is the preferred method to determine the effectiveness of heat wheels operating at a specified rotary speeds, provided the flow channel geometries and Reynolds numbers for both the wheel and the small-scale test cell.

Numerical simulation and mathematical analysis of flow-wall interaction in the large deformation application to the dynamics of the aneurysms

A numerical method is derived to take account of full flowwall interaction in la large deformation domain. To this end, a simplified Lagrangian and nonlinear model is derived to describe the wall motion. the flow is described by two dimensional Naiver stokes equation. The projection method is used to solve for the flow and fourth Rung-Kutta method is used to solve wall equation. The formulation of the problem allows full flow and wall interaction via the boundary conditions at the interface flow-wall. Some numerical simulation will be presented with periodic inlet flow. The method is applied to study the dynamics of aneurysms in arteries and veins. The flow inside the aneurysm is examined under the effects of a steady inlet flow as well as a pulsatile inlet flow for different aneurysm sizes. The wall model is analyzed when the wall is subjected to a constant transmural pressure and a quasi uniform inviscid flow. For a steady constant transmural pressure, a formal solution of the non linear integral-partial differential equation governing the wall motion is derived. For a steady and a quasi uniform inviscid flow, a first integral of the wall equation is obtained, then the solution is found to satisfy an integral non linear equation which is solved by numerical iteration

Numerical Study of Self-Induced Rotary Sloshing Caused by an Upward Periodic Jet in a Cylindrical Container

Jet-induced rotary sloshing caused by an upward periodic jet in a partially filled cylindrical container is computationally simulated for investigating a stable sloshing condition and its basic characteristics under various values of AQ, Tjet and QL0 in a given function of the inlet jet: QL(t) = AQ sin[(2π /Tjet)t] + QL0. Then, the stable rotary sloshing is found not to be observed when either the minimum or maximum flow rate of QL(t) reaches a threshold value of the sloshing-occurrence boundary. The free surface motion like a beat phenomenon caused by interference between the sloshing wave and the periodicity of the inlet jet is also observed in the signal of the free surface elevation. Furthermore, the period of the envelope seems to match with Tjet, and this phenomenon is successfully formulated by the superposition between the sloshing wave and the free surface wave due to the periodic inlet jet.

An iterative method for the computation of the response of linearised Navier–Stokes equations to harmonic forcing and application to forced cylinder wakes

SUMMARYThe central aim of this paper is the development and application of an efficient, iterative methodology for the computation of the perturbation fields induced by harmonic forcing of the linearised Navier–Stokes equations. The problem is formulated directly in the frequency domain, and the resulting system of equations is solved iteratively until convergence. The method is easily implemented to any implicit code that can solve iteratively the steady‐state Navier–Stokes equations. In this paper, it is applied to investigate the flow around a static cylinder with pulsating approaching flow and a cylinder undergoing forced stream‐wise oscillations. All terms of the perturbation kinetic energy equation are computed, and it is shown that perturbations grow by extracting energy from two sources: the underlying base flow field and the externally provided energy that maintains the imposed oscillation. The periodic drag force acting on the cylinder is also computed, and it is demonstrated that Morrison's equation is a simple model that can estimate with good accuracy the amplitude and phase of this force with respect to the approaching flow. The perturbation fields induced by periodic inlet flow (static cylinder) and forced stream‐wise cylinder oscillation are closely related: the velocity fields are identical in the appropriate reference frames, and a simple expression is derived, which links the pressures in the two flow cases. Copyright © 2014 John Wiley & Sons, Ltd.