TRANSIENT OPERATION OF SENSIBLE FIXED-BED REGENERATORS

2022 ◽  
pp. 1-16
Author(s):  
Hadi Ramin ◽  
Easwaran N Krishnan ◽  
Gurubalan Annadurai ◽  
Carey J. Simonson
Keyword(s):  
Steady State ◽  
Fixed Bed ◽  
Operating Conditions ◽  
Design Parameters ◽  
Transient Temperature ◽  
Quasi Steady State ◽  
Transient Operation ◽  
Steady State Condition ◽  
Test Standards ◽  
Wide Range

Abstract Fixed-bed regenerator is a type of air-to-air energy exchanger and recently introduced for energy recovery application in HVAC systems because of their high heat transfer effectiveness. Testing of FBRs is essential for performance evaluation and product development. ASHRAE and CSA recently included guidelines for testing of FBRs in their respective test standards. The experiments on FBRs are challenging as they never attain a steady state condition, rather undergoes a quasi-steady state operation. Before reaching the quasi-steady state, FBRs undergo several transient cycles. Hence, the test standards recommend getting measurements after one hour of operation, assuming FBR attains the quasi-steady state regardless of test conditions. However, the exact duration of the initial transient cycles is unknown and not yet studied so far. Hence, in this paper, the duration of FBR's transient operation is investigated for a wide range of design and operating conditions. The test standards' recommendation for the transient duration is also verified. The major contributions of this paper are (i) quantifying the effect of design parameters (NTUo and Cr*) on the duration of transient operation and (ii) investigation of the effect of sensor time constant on the transient temperature measurements. The results will be useful to predict and understand the transient behavior of FBRs accurately.

Transient Performance of Fixed-Bed Regenerators for Energy Recovery in Building Applications

2020 ◽  
Author(s):  
Hadi Ramin ◽  
Easwaran N. Krishnan ◽  
Gurubalan Annadurai ◽  
Carey J. Simonson
Keyword(s):  
Steady State ◽  
Fixed Bed ◽  
Performance Testing ◽  
State Condition ◽  
Test Facility ◽  
Small Scale ◽  
Outlet Temperature ◽  
Quasi Steady State ◽  
Steady State Condition ◽  
Steady State Operation

Abstract A small-scale test facility is developed to determine the sensible effectiveness of a Fixed-Bed Regenerator (FBR) and the results are used to validate a numerical model. The numerical and experimental results for quasi-steady-state conditions are in a good agreement within the experimental uncertainty bounds. At quasi-steady-state condition, the outlet temperature of FBR varies with time but cyclically repeats itself; this is an important difference between FBR (regenerator) and recuperator heat exchangers. The outlet temperature of recuperator heat exchangers reaches a constant value during the steady-state operation. The quasi-steady-state temperature profile is used to determine the sensible effectiveness of FBRs. However, FBRs undergo several cycles to reach the quasi-steady-state condition. The prediction of the duration of the transient duration of FBR is important for performance testing that could save money and time. CSA (Canadian Standards Association) recommends operating FBR for at least one hour to achieve a quasi-steady-state condition. This paper addresses the heat transfer behavior of FBRs during their transient operation. The initial transient cycles depend on the cycle period of FBR, air flow rate and the thermal condition of the exchanger at the beginning of the test. The small-scale FBR test facility is used to study the transient behavior of FBRs and this is the main focus of this paper. The temperature profile during the transient condition of FBR is obtained and the results are compared with the numerical model. The effects of the mass flow rate of air and the cycle duration on the transient period of FBR are studied. The results show that FBR reaches a quasi-steady state operation in less than 30 minutes. The results will be useful for understanding the time required for performance testing, which will reduce the cost and time of each test.

scholarly journals A criterion based on computational singular perturbation for the construction of reduced mechanism for dimethyl ether oxidation

2013 ◽  
Vol 78 (8) ◽  
pp. 1177-1188
Author(s):  
Zuozhu Wu ◽  
Xinqi Qiao ◽  
Zhen Huang
Keyword(s):  
Steady State ◽  
Singular Perturbation ◽  
Dimethyl Ether ◽  
Reduction Process ◽  
Operating Conditions ◽  
Quasi Steady State ◽  
Reduced Mechanism ◽  
Wide Range ◽  
Computational Singular Perturbation ◽  
Premixed Laminar

A criterion based on the computational singular perturbation (CSP) method is proposed in order to determine the number of quasi-steady state (QSS) species. This criterion is employed for the reduction of a detailed chemical kinetics mechanism for the oxidation of dimethyl ether (DME), involving 55 species and 290 reactions, leading to a 20 steps reduced mechanism which involves 26 species. A software package, named I-CSP, was developed to make the reduction process algorithmic. Input to the I-CSP includes (i) the detailed mechanism, (ii) the numerical solution of the problem for a specific set of operating conditions, (iii) the number of quasi steady state (QSS) species. The resulting reduced mechanism was validated both in homogenous reactor, including auto-ignition and PSR, over a wide range of pressures and equivalence ratios, and in a one-dimensional, unstretched, premixed, laminar steady DME/Air flame. Comparison of the results calculated with the detailed and the reduced mechanisms shows excellent agreement in the case of homogenous reactor, but discrepancies can be observed in the case of the premixed laminar flame.

Analysis of Transient Temperatures in Grinding

1995 ◽  
Vol 117 (4) ◽  
pp. 571-577 ◽  
Author(s):  
C. Guo ◽  
S. Malkin
Keyword(s):  
Steady State ◽  
Transient Behavior ◽  
Grinding Wheel ◽  
Transient Temperature ◽  
Quasi Steady State ◽  
Workpiece Material ◽  
Steady State Condition ◽  
Transient Temperature Distribution ◽  
Creep Feed ◽  
Grinding Conditions

Temperatures generated in the workpiece during straight surface plunge grinding follow a transient behavior as the grinding wheel engages with and disengages from the workpiece, and throughout the entire grinding pass for workpieces which are shorter than needed to reach a quasi-steady state condition. In the present paper, a thermal model is developed for the transient temperature distribution under regular and creep-feed grinding conditions. Numerical results obtained using a finite difference method indicate that the workpiece temperature rises rapidly during initial wheel-workpiece engagement (cut in), subsequently reaches a quasi-steady state value if the workpiece is sufficiently long, and increases still further during final wheel-workpiece disengagement (cut out) as workpiece material is suddenly unavailable to dissipate heat. Cooling by a nozzle directed at the end face of the workpiece should significantly reduce the temperature rise during cut out.

Thermo-structural analysis of a honeycomb-type volumetric absorber for concentrated solar power applications

2021 ◽  
Vol ahead-of-print (ahead-of-print) ◽  
Author(s):  
Masoud Behzad ◽  
Benjamin Herrmann ◽  
Williams R. Calderón-Muñoz ◽  
José M. Cardemil ◽  
Rodrigo Barraza
Keyword(s):  
Heat Transfer ◽  
Thermal Stress ◽  
Steady State ◽  
Discrete Model ◽  
Continuum Model ◽  
Transient State ◽  
Operating Conditions ◽  
Steady State Condition ◽  
Content Type ◽  
The Continuum

Purpose Volumetric air receivers experience high thermal stress as a consequence of the intense radiation flux they are exposed to when used for heat and/or power generation. This study aims to propose a proper design that is required for the absorber and its holder to ensure efficient heat transfer between the fluid and solid phases and to avoid system failure due to thermal stress. Design/methodology/approach The design and modeling processes are applied to both the absorber and its holder. A multi-channel explicit geometry design and a discrete model is applied to the absorber to investigate the conjugate heat transfer and thermo-mechanical stress levels present in the steady-state condition. The discrete model is used to calibrate the initial state of the continuum model that is then used to investigate the transient operating states representing cloud-passing events. Findings The steady-state results constitute promising findings for operating the system at the desired airflow temperature of 700°C. In addition, we identified regions with high temperatures and high-stress values. Furthermore, the transient state model is capable of capturing the heat transfer and fluid dynamics phenomena, allowing the boundaries to be checked under normal operating conditions. Originality/value Thermal stress analysis of the absorber and the steady/transient-state thermal analysis of the absorber/holder were conducted. Steady-state heat transfer in the explicit model was used to calibrate the initial steady-state of the continuum model.

scholarly journals A Computationally Efficient Multidiode Model for Optimizing the Front Grid of Multijunction Solar Cells under Concentration

2020 ◽  
Vol 2020 ◽  
pp. 1-10
Author(s):  
Chris H. van de Stadt ◽  
Pilar Espinet Gonzalez ◽  
Harry A. Atwater ◽  
Rebecca Saive
Keyword(s):  
Solar Cells ◽  
Operating Conditions ◽  
Design Parameters ◽  
Computationally Efficient ◽  
Fast Determination ◽  
Wide Range ◽  
Multijunction Solar Cells ◽  
Efficiency Increase ◽  
Light Concentration

We have developed a computationally efficient simulation model for the optimization of redirecting electrical front contacts for multijunction solar cells under concentration, and we present its validation by comparison with experimental literature results. The model allows for fast determination of the maximum achievable efficiency under a wide range of operating conditions and design parameters such as the contact finger redirecting capability, period and width of the fingers, the light concentration, and the metal and emitter sheet resistivity. At the example of a state-of-the-art four-junction concentrator solar cell, we apply our model to determine ideal operating conditions for front contacts with different light redirection capabilities. We find a 7% relative efficiency increase when enhancing the redirecting capabilities from 0% to 100%.

Analysis of Contact Mechanics for Rotor-Bristle Interference of Brush Seal

2003 ◽  
Vol 125 (2) ◽  
pp. 414-421 ◽  
Author(s):  
R. J. Stango ◽  
H. Zhao ◽  
C. Y. Shia
Keyword(s):  
Contact Force ◽  
Flexural Rigidity ◽  
Operating Conditions ◽  
Contact Forces ◽  
Design Parameters ◽  
Attractive Alternative ◽  
Seal Design ◽  
Brush Seal ◽  
Wide Range ◽  
Deformation Mechanics

Brush seals have proven to be an attractive alternative to labyrinth seals for turbomachinery applications. This innovation in seal technology utilizes both the high temperature capability of special-alloy wire and the flexural adaptability of fibers to accommodate a wide range of operating conditions that are encountered during service. The effectiveness of the seal is principally derived from the bristles ability to endure forces imparted by both the fluid and shaft, and yet maintain contact between the filament tips and the surface of the rotor. Consequently, contact forces generated along the interface of the fiber tip and rotor are an important consideration for both the design and performance of the rotor-seal assembly. This paper focuses on evaluating brush seal forces that arise along the surface of the rotor due to the dimensional disparity or interference between the rotor-fiber. Filament tip contact forces are computed on the basis of an in-plane, large deformation mechanics analysis of a cantilever beam, and validation of the model is assessed by using an electronic balance for measuring the shear and normal force exerted by a bristle tip onto a flat, hardened surface. Formulation of the mechanics problem is briefly reviewed, and includes the effect of Coulombic friction at the interface of the fiber tip and rotor. Filament contact force is used as a basis for computing bearing stress along the fiber-rotor interface. Results are reported for a range of brush seal design parameters in order to provide a better understanding of the role that seal geometry, friction, and bristle flexural rigidity play in generating rotor contact force.

Simulation of an Industrial Linear Low Density Polyethylene Plant

2011 ◽  
Vol 6 (1) ◽  
Author(s):  
Ali Farhangiyan Kashani ◽  
Hossein Abedini ◽  
Mohammad Reza Kalaee
Keyword(s):  
Steady State ◽  
Average Molecular Weight ◽  
Molar Fraction ◽  
Operating Conditions ◽  
Low Density Polyethylene ◽  
Process Equipment ◽  
Flow Index ◽  
Low Density ◽  
Linear Low Density Polyethylene ◽  
Wide Range

In this paper, an industrial linear low density polyethylene (LLDPE) production process including two serried fluidized bed reactors (FBR) and other process equipment was completely simulated in steady state mode. Both of FBRs were considered like two serried continuous stirred tank reactors (CSTR). In this simulation, a kinetic model that is based on a multiple active site heterogeneous Ziegler-Natta catalyst was used for simulation of reactions in two FBRs. Simulator by using this model is able to predict the important attributes of LLDPE like melt flow index (MFI), density (ρ), polydispersity (PDI), numerical and weight average molecular weight (Mn, Mw) and co-polymer molar fraction (SFRAC). On the other hand, this simulator can be applied in wide range of changing in inlet operating conditions. The results of the simulation are compared with industrial data of LLDPE plant. A good agreement is observed between the simulator predictions and actual plant data. Finally, by using of the simulator, the steady state operating conditions for producing different grades of polyethylene are obtained.

Unsteady Simulations of a Low NOx LDI Combustor for Environmentally Responsible Aviation Engines

2015 ◽  
Author(s):  
Scott A. Drennan ◽  
Gaurav Kumar ◽  
Erlendur Steinthorsson ◽  
Adel Mansour
Keyword(s):  
Experimental Data ◽  
Complex Geometry ◽  
Mesh Refinement ◽  
Operating Conditions ◽  
Design Parameters ◽  
Nox Emissions ◽  
Fuel Injector ◽  
Cfd Simulations ◽  
Wide Range ◽  
Environmentally Responsible

A key objective of NASA’s Environmentally Responsible Aviation (ERA) research program is to develop advanced technologies that enable 75% reduction of LTO NOx emissions of N+2 aviation gas turbine engines relative to the CAEP 6 standard. To meet this objective, a new advanced multi-point fuel injector was proposed and tested under the NASA ERA program. The new injector, called the three-zone injector, or 3ZI, uses fifteen spray cups arranged in three zones. Swirling air flows into each cup and fuel is introduced via pressure swirl atomizers within the cup. Multiple design parameters impact the performance of the injector, such as the location of the atomizer within the spray cup, the spray angle and cup-to-cup spacing. To fully understand the benefits and trade-offs of various injector design parameters and to optimize the performance of the injector, detailed CFD simulations are an essential tool. Furthermore, the CFD methodology must allow easy changes in design parameters and guarantee consistent and comparable accuracy from one design iteration to the next. This paper investigates the use of LES in reacting and non-reacting flows and compares against the NOx experimental data for the multi-point atomization strategy of the injector. The CFD simulations employ an automatically generated Cartesian cut-cell meshing approach with mesh refinement applied near complex geometry and spray regions. Adaptive Mesh Refinement (AMR) is used to refine mesh in regions of high gradients in velocity and temperature. The CFD simulations use boundary and operating conditions based on experimental data for air flow and spray atomization obtained from LDV and PDPA characterizations of the spray respectively. The results are extended to reacting flow using a detailed reaction mechanism and predictions of NOx emissions are compared to experimental data. Overall NOx predictions were consistently less than experimental values. However, the NOx prediction trends showed excellent agreement with experimental data across the wide range of equivalence ratios investigated.

scholarly journals On the application of storage coefficient determination by quasi-steady-state flow

2005 ◽  
Vol 36 (1) ◽  
pp. 13-20 ◽  
Author(s):  
Mehmet E. Birpinar ◽  
Ayhan Gazioglu
Keyword(s):  
Steady State ◽  
Quasi Steady State ◽  
Confined Aquifer ◽  
Steady State Flow ◽  
Steady State Condition ◽  
Storage Coefficient ◽  
Unconfined Aquifers ◽  
Aquifer Test ◽  
Order Of Magnitude ◽  
Observation Wells

A simple analytical method has been used for estimating the storage coefficient provided that transmissivity of the aquifer is known at the quasi-steady-state condition in confined or unconfined aquifers. The application of the method has been performed for unconfined and confined aquifer test data in Chaj Doab, Pakistan with observation wells and compared with conventional methods in the groundwater flow literature dealing with pumping tests. The results from the methodology presented in this paper conform well in practice with the results obtained from the traditional methods on the basis of order of magnitude.

An Investigation of Heat Generation Characteristics of Brush Seals

2007 ◽  
Author(s):  
Mehmet Demiroglu ◽  
John A. Tichy
Keyword(s):  
Contact Force ◽  
Heat Generation ◽  
Operating Conditions ◽  
Frictional Heat ◽  
Design Parameters ◽  
Frictional Heat Generation ◽  
Operation Conditions ◽  
Seal Design ◽  
Wide Range ◽  
Brush Seals

Brush seals are considered as a category of compliant seals, which tolerate a great high level of interference between the seal and the rotor or shaft. Their superior leakage characteristics have opened many application fields in the turbo-machinery world, ranging from industrial steam turbines to jet engines. However, brush seal designers have to find a trade-off between the lower parasitic leakage but higher heat generation properties of brush seals for given operation conditions. As brush seals can maintain contact with the rotor for a wide range of operating conditions, the contact force/pressure generated at the seal-rotor interface becomes an important design parameter for sustained seal performance and longevity of its service life. Furthermore, due to this contact force at the interface, frictional heat generation is inevitable and must be evaluated for various design and operating conditions. In this paper, frictional heat generation at the sealrotor interface is studied. To capture temperature rise at the interface, a thermal image of the seal and rotor is taken with an infrared camera under various operating conditions. The temperature map of the rotor is compared to results from thermal finite element analysis of the rotor to back calculate the heat flux to the rotor. A closed form equation for frictional heat generation is suggested as a function of seal design parameters, material properties, friction coefficient and empirical factors from testing.

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