scholarly journals An accurate lumped convective plus radiative heat exchanger model for performance-based modelling

2021 ◽  
Vol 347 ◽  
pp. 00007
Author(s):  
Wim Fuls
Keyword(s):  
Heat Exchanger ◽  
Gas Flow ◽  
Scaling Laws ◽  
Scaling Law ◽  
Process Conditions ◽  
Particle Radiation ◽  
Gas Radiation ◽  
Base Process ◽  
Fluid Process ◽  
Improved Model

There are several thermo-fluid process modelling tools available on the market which can be used to analyze the off-design performance of thermal plants. These tools all offer the user with a simple convective heat exchanger component that requires the design-base process conditions as inputs. The tools would then calculate an effective overall heat transfer factor (UA) and make use of gas flow mass ratios to scale the UA value for off-design conditions. The models employed in these tools assume that the contribution of gas radiation is insignificant, hence only applies convection scaling laws. This paper presents an improved model which considers the contribution of the gas and particle radiation, as is often encountered in the first few heaters in coal fired boilers and heat recovery steam generators. A more fundamental scaling law is applied for the convection scaling and incorporates a cleanliness factor which allows for the consideration of fouling of the heater surfaces. The model’s performance was validated against a discretized tube-level heater model that solves the fundamental convection and radiation terms. The model is accurate within 1% for the cases considered, as compared to more than 20% error if radiation contribution is not considered.

Hydrodynamic Performance of a Novel Design of Pressurized Fluidized Bed Combustor

2003 ◽  
Author(s):  
Alan L. T. Wang ◽  
John F. Stubington
Keyword(s):  
Fluidized Bed ◽  
Gas Flow ◽  
Scaling Laws ◽  
Scaling Law ◽  
Gas Injection ◽  
Hydrodynamic Performance ◽  
Silica Sand ◽  
Particle Movement ◽  
Cold Model ◽  
Fluidized Bed Combustor

A bench-scale fluidized bed combustor with a novel fluidizing gas injection manifold was successfully built for characterization of Australian black coals under pressurized fluidized bed combustion (PFBC) conditions. The bed of silica sand (mean size 1.3 mm and density 2700 kg/m3) was 40 mm ID with a static height of 75 mm. This facility was designed to operate at 1.6 MPa, 850°C and a fluidizing velocity of 0.9 m/s, identical to those used industrially, in order to match as closely as possible the local hydrodynamic environment around each coal particle in an industrial PFBC. To verify satisfactory hydrodynamic performance with the novel gas injection manifold, the fluidization was directly investigated by measuring differential pressure fluctuations under both ambient and PFBC conditions. In addition, a Perspex cold model was built to simulate at ambient conditions the hydrodynamics of the hot bed in this PFBC facility. The cold model was constructed to a geometric scale of 1.431:1, determined by Glicksman’s scaling law. Under PFBC conditions of 1.6 MPa, 850°C and 0.9 m/s, the bed in UNSW’s PFBC facility operated in a stable bubbling regime and the solids were very well mixed. The bubbles in this PFBC were effectively cloudless and no gas backmixing or slugging occurred; so the gas flow in this bed could be modeled by assuming two phases (bubble and particulate) with plug flow through each phase. The results from the cold model showed that the ratio of Umf for the simulated bed to Umf for the hot PFBC bed matched the conditions proposed by Glicksman’s scaling laws. The bubbles rose along the bed with axial and lateral movements (moving both towards and from the wall), and erupted from the bed surface evenly and randomly at different locations. Two patterns of particle movement were observed in the cold model bed: a circular pattern near the top section, and a rising and falling pattern dominating the particle movement in the lower section created by the rising bubbles.

Optimization and Standardization of Flanged and Flued Expansion Joint Design

2019 ◽  
Vol 141 (3) ◽  
Author(s):  
Kamlesh M. Chikhaliya ◽  
Bhaveshkumar P. Patel
Keyword(s):  
Heat Exchanger ◽  
Optimum Design ◽  
Design Methodology ◽  
Thick Wall ◽  
Process Conditions ◽  
Expansion Joint ◽  
Standard Requirement ◽  
Spring Rate ◽  
Tube Heat Exchanger ◽  
Heat Exchanger Design

Flanged and flued type expansion joint (thick wall expansion bellow) used as an integral part of many shell and tube heat exchanger where process conditions produce differential expansion between shell and tubes. It provides flexibility for thermal expansion and also functions as a pressure retaining part. Design of expansion joints is usually based on trial and error method in which initial geometry must be assumed, and accordingly maximum stresses and spring rate are be calculated. Inadequate selection of geometry leads to higher tubesheet and bellow thickness, which increases cost of equipment. This paper presents standardization and optimum design approach of flange and flued expansion bellow fulfilling ASME VIII-1 and TEMA standard requirement. Methodology to define expansion bellow geometry is developed, and geometry dimensions are tabulated for expansion bellow diameter from 300 to 2000 mm and thickness from 6 to 30 mm. Each defined geometry is analyzed using finite element method, and maximum von Mises stresses are calculated for bellow axial displacement from 0.5 to 1.5 mm and internal pressure from 0.1 to 6.5 MPa. Spring rate is also calculated for each defined geometry for consideration in tubesheet calculation. Accordingly, optimum design methodology is developed, tested, and compared with existing design. Results depicted that proposed standardization approach and design methodology will optimize expansion bellow and tubesheet thickness and will also save considerable time in finalization of heat exchanger design.

scholarly journals Asymmetric Unimodal Maps with Non-universal Period-Doubling Scaling Laws

2020 ◽  
Vol 379 (1) ◽  
pp. 103-143
Author(s):  
Oleg Kozlovski ◽  
Sebastian van Strien
Keyword(s):  
Scaling Laws ◽  
Scaling Law ◽  
Period Doubling ◽  
Cantor Sets ◽  
Unimodal Maps ◽  
Renormalization Theory

Abstract We consider a family of strongly-asymmetric unimodal maps $$\{f_t\}_{t\in [0,1]}$$ { f t } t ∈ [ 0 , 1 ] of the form $$f_t=t\cdot f$$ f t = t · f where $$f:[0,1]\rightarrow [0,1]$$ f : [ 0 , 1 ] → [ 0 , 1 ] is unimodal, $$f(0)=f(1)=0$$ f ( 0 ) = f ( 1 ) = 0 , $$f(c)=1$$ f ( c ) = 1 is of the form and $$\begin{aligned} f(x)=\left\{ \begin{array}{ll} 1-K_-|x-c|+o(|x-c|)&{} \text{ for } x<c, \\ 1-K_+|x-c|^\beta + o(|x-c|^\beta ) &{} \text{ for } x>c, \end{array}\right. \end{aligned}$$ f ( x ) = 1 - K - | x - c | + o ( | x - c | ) for x < c , 1 - K + | x - c | β + o ( | x - c | β ) for x > c , where we assume that $$\beta >1$$ β > 1 . We show that such a family contains a Feigenbaum–Coullet–Tresser $$2^\infty $$ 2 ∞ map, and develop a renormalization theory for these maps. The scalings of the renormalization intervals of the $$2^\infty $$ 2 ∞ map turn out to be super-exponential and non-universal (i.e. to depend on the map) and the scaling-law is different for odd and even steps of the renormalization. The conjugacy between the attracting Cantor sets of two such maps is smooth if and only if some invariant is satisfied. We also show that the Feigenbaum–Coullet–Tresser map does not have wandering intervals, but surprisingly we were only able to prove this using our rather detailed scaling results.

scholarly journals Scaling and Similarity of the Anisotropic Coherent Eddies in Near-Surface Atmospheric Turbulence

2018 ◽  
Vol 75 (3) ◽  
pp. 943-964 ◽  
Author(s):  
Khaled Ghannam ◽  
Gabriel G. Katul ◽  
Elie Bou-Zeid ◽  
Tobias Gerken ◽  
Marcelo Chamecki
Keyword(s):  
Scaling Laws ◽  
Velocity Structure ◽  
Scaling Law ◽  
Longitudinal Velocity ◽  
Structure Functions ◽  
Length Scale ◽  
Near Surface ◽  
Vegetation Canopies ◽  
Velocity Spectra ◽  
Attached Eddies

Abstract The low-wavenumber regime of the spectrum of turbulence commensurate with Townsend’s “attached” eddies is investigated here for the near-neutral atmospheric surface layer (ASL) and the roughness sublayer (RSL) above vegetation canopies. The central thesis corroborates the significance of the imbalance between local production and dissipation of turbulence kinetic energy (TKE) and canopy shear in challenging the classical distance-from-the-wall scaling of canonical turbulent boundary layers. Using five experimental datasets (two vegetation canopy RSL flows, two ASL flows, and one open-channel experiment), this paper explores (i) the existence of a low-wavenumber k−1 scaling law in the (wind) velocity spectra or, equivalently, a logarithmic scaling ln(r) in the velocity structure functions; (ii) phenomenological aspects of these anisotropic scales as a departure from homogeneous and isotropic scales; and (iii) the collapse of experimental data when plotted with different similarity coordinates. The results show that the extent of the k−1 and/or ln(r) scaling for the longitudinal velocity is shorter in the RSL above canopies than in the ASL because of smaller scale separation in the former. Conversely, these scaling laws are absent in the vertical velocity spectra except at large distances from the wall. The analysis reveals that the statistics of the velocity differences Δu and Δw approach a Gaussian-like behavior at large scales and that these eddies are responsible for momentum/energy production corroborated by large positive (negative) excursions in Δu accompanied by negative (positive) ones in Δw. A length scale based on TKE dissipation collapses the velocity structure functions at different heights better than the inertial length scale.

scholarly journals Improving the performance of the air-cooling unit in the cooling system of a radar

2021 ◽  
Vol 2057 (1) ◽  
pp. 012004
Author(s):  
Yu A Borisov ◽  
V V Volkov-Muzilev ◽  
D A Kalashnikov ◽  
H S Khalife
Keyword(s):  
Heat Exchanger ◽  
Gas Flow ◽  
Cooling System ◽  
Experimental Studies ◽  
Air Cooling ◽  
Axial Fan ◽  
Gas Dynamic ◽  
Cooling Unit ◽  
Calculation Results ◽  
Axial Fans

Abstract The article discusses the issues of reducing the size of the cooling unit of the antenna of a radar station by improving the gas-dynamic processes occurring in the air-cooling unit. The results of the experimental studies of the gas flow in a plate-fin heat exchanger, being blown by one axial fan are presented. The feasibility of changing the number of axial fans for organizing a more uniform flow around the heat-exchange surfaces has been determined by calculation and theoretical methods. The calculation results are confirmed by experimental studies of the air flow in the segment of the heat exchanger, which is provided by a smaller fan.

Thermal analysis of a three-layered radiant porous heat exchanger including fluid flow simulation

2013 ◽  
Vol 228 (8) ◽  
pp. 1375-1390 ◽  
Author(s):  
M Sajedi ◽  
SA Gandjalikhan Nassab ◽  
E Jahanshahi Javaran
Keyword(s):  
Thermal Analysis ◽  
Porous Medium ◽  
Heat Exchanger ◽  
Thermal Radiation ◽  
Optical Thickness ◽  
Gas Flow ◽  
Flow Simulation ◽  
Local Thermal Equilibrium ◽  
Outlet Section ◽  
Energy Equations

Based on an effective energy conversion method between flowing gas enthalpy and thermal radiation, a three-layered type of porous heat exchanger (PHE) has been proposed. The PHE has one high temperature (HT) and two heat recovery (HR1 and HR2) sections. In HT section, the enthalpy of gas flow converts to thermal radiation and the opposite process happens in HR1 and HR2. In each section, a 2-D rectangular porous medium which is assumed to be absorbing, emitting and scattering is presented. For theoretical analysis of the PHE, the gas and solid phases are considered in non-local thermal equilibrium and separate energy equations are used for these two phases. Besides, in the gas flow simulation, the Fluent code is used to obtain the velocity distribution in the PHE from inlet to outlet section. For thermal analysis of the PHE, the coupled energy equations for gas and porous layer at each section are numerically solved using the finite difference method. In the computation of radiative heat flux distribution, the radiative transfer equation (RTE) is solved by the discrete ordinates method (DOM). The effects of scattering albedo, optical thickness, particle size of porous medium and inlet gas temperature on the efficiency of PHE are explored. Numerical results show that this type of PHE has high efficiency especially when the porous layers have high optical thickness. The present results are compared with those reported theoretically by other investigators and reasonable agreement is found.

A Study on the Effect of Process Parameters on Surface Topography of Al Thin Films on Various Substrates Using AFM

2011 ◽  
Vol 383-390 ◽  
pp. 903-908
Author(s):  
S. Shanmugan ◽  
D. Mutharasu ◽  
Z. Hassan ◽  
H. Abu. Hassan
Keyword(s):  
Thin Films ◽  
Surface Topography ◽  
Flow Rate ◽  
Gas Flow ◽  
Process Conditions ◽  
Gas Flow Rate ◽  
Glass Substrates ◽  
Sputtering Power ◽  
Large Particles ◽  
Ar Gas

Al thin films were prepared over different substrates at various process conditions using DC sputtering. The surface topography of all prepared films was examined using AFM technique. Very smooth, uniform and dense surface were observed for Al films coated over Glass substrates. The observed particle size was nano scale (20 -70 nm) for Glass substrates. Sputtering power showed immense effect on surface roughness with respective to Ar gas flow rate. Noticeable change on surface with large particles was observed in Copper substrates at various sputtering power and gas flow rate.

Ultrasonic Nondestructive Testing for In-Line Monitoring of Wire-Arc Additive Manufacturing (WAAM)

2020 ◽  
Author(s):  
Md Shahjahan Hossain ◽  
Hossein Taheri ◽  
Niraj Pudasaini ◽  
Alexander Reichenbach ◽  
Bishal Silwal
Keyword(s):  
Quality Assurance ◽  
Additive Manufacturing ◽  
Nondestructive Testing ◽  
Large Scale ◽  
Gas Flow ◽  
Complex Structure ◽  
Process Condition ◽  
Process Conditions ◽  
Ultrasonic Signals ◽  
Wire Arc Additive Manufacturing

Abstract The applications for metal additive manufacturing (AM) are expanding. Powder-bed, powder-fed, and wire-fed AM are the different kinds of AM technologies based on the feeding material. Wire-Arc AM (WAAM) is a wire-fed technique that has the potential to fabricate large-scale three-dimensional objects. In WAAM, a metallic wire is continuously fed to the deposition location and is melted by an arc-welding power source. As the applications for WAAM expands, the quality assurance of the parts becomes a major concern. Nondestructive testing (NDT) of AM parts is necessary for quality assurance and inspection of these materials. The conventional method of inspection is to perform testing on the finished parts. There are several limitations encountered when using conventional methods of NDT for as-built AM parts due to surface conditions and complex structure. In-situ process monitoring based on the ultrasound technology is proposed for WAAM material inspection during the manufacturing process. Ultrasonic inline monitoring techniques have the advantages of providing valuable information about the process and parts quality. Ultrasonic technique was used to detect the process condition deviations from the normal. A fixture developed by the authors holds an ultrasonic sensor under the build platform and aligned with the center of the base plate. Ultrasonic signals were measured for different process conditions by varying the current and gas flow rate. Features (indicators) from the radio frequency (RF) signal were used to evaluate the difference in signal clusters to identify and classify different build conditions. Results show that the indicator values of the ultrasonic signals in the region of interest (ROI) changes with different process conditions and can be used to classify them.

Addressing Fouling by Qualifying Chemical Additives Using Novel Technologies

2021 ◽  
Author(s):  
Andrew Robert Farrell ◽  
Dario Marcello Frigo ◽  
Gordon Michael Graham ◽  
Robert Stalker ◽  
Ernesto Ivan Diestre Redondo ◽  
...  
Keyword(s):  
Heat Exchanger ◽  
Heat Exchangers ◽  
Emulsion Stability ◽  
Ambient Pressure ◽  
Screening Tests ◽  
Process Conditions ◽  
Screening Methods ◽  
Chemical Additives ◽  
Laboratory Techniques ◽  
Prediction And Prevention

Abstract Fouling of heat exchangers and production of stable emulsions in desalting units can present significant challenges in refinery operations. Often these difficulties occur due to the concurrent processing of two or more crude oils that are incompatible under process conditions. This paper describes a significant development in laboratory techniques for studying these issues and evaluating mitigation strategies. Asphaltenes compatibility was evaluated for oil mixtures that may be co-processed in the refinery using a deposition flow rig, and the results were compared with those obtained with more conventional tests: blending stability analysis by light scattering and various screening methods. The flow rig mimics the process conditions (elevated pressure, high temperature, flow-induced shear) and identifies whether deposition or precipitation will occur. The former can cause fouling of heat exchangers whereas the latter produces solids that can stabilize emulsions in the desalter. By varying the proportions of oils that were co-injected into the deposition flow rig, the range within which mixtures were unstable was found. By flowing through a capillary (to mimic a heat exchanger) and in-line filter, it was possible to identify whether precipitation of suspended flocs or fouling of the heat exchanger itself was the likely issue for each mixture. Emulsion-stability tests were conducted using a pressurized rig with an ersatz separator to mimic the desalting unit; results were compared with those obtained in conventional, ambient-pressure bottle tests. Oil(s) and refinery wash water were injected, mixed under representative shear, and allowed to separate within the typical residence time of the desalter. Chemical additives were tested to identify those that were effective at controlling any observed problems. Results obtained in either flow rig (using representative pressure, temperature, and shear) did not always match those obtained using conventional methods. Asphaltenes fouling occurred under conditions where it was not predicted by screening tests that were conducted at conditions not representative of the process and did not occur under conditions where it was predicted. Differences were also observed between the emulsion stability observed in bottle versus rig tests, though these should be viewed as complementary techniques. This paper presents new laboratory techniques for the prediction and prevention of refinery fouling and emulsion stability. They mimic conditions in the facilities much better than those typically used to date.

scholarly journals Numerical Simulations of Molten Breakup Behaviors of a de Laval-Type Nozzle, and the Effects of Atomization Parameters on Particle Size Distribution

Processes ◽  
2020 ◽  
Vol 8 (9) ◽  
pp. 1027
Author(s):  
Lianghui Xu ◽  
Xianglin Zhou ◽  
Jinghao Li ◽  
Yunfei Hu ◽  
Hang Qi ◽  
...  
Keyword(s):  
Particle Size ◽  
Particle Size Distribution ◽  
Size Distribution ◽  
Gas Flow ◽  
Nozzle Geometry ◽  
Particle Model ◽  
Process Conditions ◽  
Operating Pressure ◽  
Gas Temperature ◽  
Tracking Model

In this work, an atomizer with a de Laval-type nozzle is designed and studied by commercial computational fluid dynamics (CFD) software, and the secondary breakup process during atomization is simulated by two-way coupling and the discrete particle model (DPM) using the Euler-Lagrange method. The simulation result demonstrates that the gas flow patterns greatly change with the introduction of liquid droplets, which clearly indicates that the mass loading effect is quite significant as a result of the gas-droplet interactions. An hourglass shape of the cloud of disintegrating molten metal particles is observed by using a stochastic tracking model. Finally, this simulation approach is used for the quantitative evaluation of the effects of altering the atomizing process conditions (gas-to-melt ratio, operating pressure P, and operating gas temperature T) and nozzle geometry (protrusion length h, half-taper angle α, and gas slit nozzle diameter D) on the particle size distribution of the powders produced.

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