Internal
flow analysis of a porous
burner via CFD
Ali H. Abdulkarim
Department of Mechanical Engineering, University of Kirkuk, Kirkuk, Iraq, and
Ali Ates
,
Kemal Altinisik
and
Eyüb Canli
Department of Mechanical Engineering, Selcuk Universitesi, Konya, Turkey
Abstract
Purpose – This study aims to introduce a metal porous burner design. Literature is surveyed in a comprehensive manner to relate the current design with ongoing research. A demonstrative computational fluid dynamics (CFD) analysis is presented with projected flow conditions by means of a common commercial CFD code and turbulence model to show theflow-related features of the proposed burner. The porous metal burner has a novel design, and it is not commercially available.
Design/methodology/approach – Based on the field experience about porous burners, a metal, cylindrical, two-staged, homogenous porous burner was designed. Literature was surveyed to lay out research aspects for the porous burners and porous media. Three dimensional solid computer model of the burner was created. Theflow domain was extracted from the solid model to use in CFD analysis. A commercial computationalfluid dynamics code was utilized to analyze the flow domain. Projected flow conditions for the burner were applied to the CFD code. Results were evaluated in terms of homogenousflow distribution at the outer surface andflow mixing. Quantitative results are gathered and are presented in the present report by means of contour maps.
Findings– There aren’t any flow sourced anomalies in the flow domain which would cause an inefficient combustion for the application. An accumulation of gas is evident around the top flange of the burner leading to higher static pressure. Generally, very low pressure drop throughout the proposed burner geometry is found which is regarded as an advantage for burners. About 0.63 Pa static pressure increase is realized on theflange surface due to the accumulation of the gas. The passage between inner and outer volumes has a high impact on the total pressure and leads to about 0.5 Pa pressure drop. About 0.03 J/kg turbulent kinetic energy can be viewed as the highest amount. Together with the increase in total enthalpy, total amount of energy drawn from theflow is 0.05 J/kg. More than half of it spent through turbulence and remaining is dissipated as heat. Outflow from burner surface can be regarded homogenous though the top part has slightly higher outflow. This can be changed by gradually increasing pore sizes toward inlet direction.
Research limitations/implications– Combustion via a porous medium is a complex phenomenon since it involves multiple phases, combustion chemistry, complex pore geometries and fast transient responses. Therefore, experimentation is used mostly. To do a precise computational analysis, strong computational power, parallelizing, elaborate solid modeling, veryfine meshes and small time steps and multiple models are required.
Practical implications–Findings in the present work imply that a homogenous gas outflow can be attained through the burner surfaces while very small pressure drop occurs leading to less pumping power requirement which is regarded as an advantage. Flow mixing is realizable since turbulent kinetic energy is distinguished at the interface surface between inner and outer volumes. The porous metal matrix burner offers fluid mixing and therefore better combustion efficiency. The proposed dimensions are found appropriate for real-world application.
A brief presentation of this content was done in ICCE-NANO 24 (www.icce-nano.org/) [3440] and not published elsewhere. The content is prepared from a study of a PhD dissertation (Abdulkarim, 2016).
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Received 9 August 2018 Revised 26 November 2018 Accepted 6 December 2018International Journal of Numerical Methods for Heat & Fluid Flow Vol. 29 No. 8, 2019
pp. 2666-2683
© Emerald Publishing Limited 0961-5539
DOI10.1108/HFF-07-2018-0413
The current issue and full text archive of this journal is available on Emerald Insight at: www.emeraldinsight.com/0961-5539.htm
Originality/value–Conducted analysis is for a novel burner design. There are opportunities both for scientific and commercial fields.
Keywords CFD, Burner, Porous metal matrix, Pressure drop Paper type Research paper
Nomenclature
a = Thermal diffusivity; c = Specific heat (J/kg·K); C = Constant;
r = Del operator;
e = Turb. kinetic energy dissipation (J/kg); f = Force (N);
U = Generation (J/kg);
G = Turbulence generation (J/kg);
k = Turbulent kinetic energy (J/kg) or heat conduction coefficient (W/m·K);
m = Dynamic viscosity (kg/m·s); p = Pressure (Pa);
r = Density (kg/m3);
s = Turbulence model coefficient; T = Temperature (K);
t = Time (s); u = x velocity (m/s); x = x coordinate (m);
V = Velocity in vector convention; v = y velocity (m/s); = Kinematic viscosity (m2 /s); w = z velocity (m/s); y = y coordinate (m); and z = z coordinate (m). Subscript i = number indice; p = constant pressure; T = Turbulence; and
k = for turbulence kinetic energy.
1. Introduction
Interactions between solid boundaries andfluids lead to some phenomena that can be utilized in
engineering designs. Engineers mostly focus on geometrical and material parameters considering these numerous opportunities. Foam-like structures and porous media are part of
this effort. For instance, metal foams are used for applications in the field of petroleum
reservoirs, cryogenics, catalytic beds, burners, heat exchangers, condensers, heat sinks, strain
isolation, cladding on buildings, buffer between a stiff structure and afluctuating temperature
field and geothermal applications. Although metal foams are well known presently, research on them in respect of heat and mass transfer, or in other words, engineering designs is still new
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analysis
and has a huge research potential, considering its complex structure and various parameters. Their superior offerings such as light weight, electrical and acoustic properties, mechanical and
thermal behaviors make them an excellent alternative for conventional means such as plate-fin
heat exchangers, conventional burners etc. Porous solids can be thought where performance improvement becomes a hard task with conventional means (Sertkaya et al., 2015).
It can be seen that metal foams can also be named as solid sponges in the literature. They are mostly visible in thermal applications. For instance, porous burners and volumetric solar receivers are used as parts of thermal systems. It is a well-known fact that heat transfer depends on mostly to the surface area of the solid boundaries. Accordingly, a solid sponge or metal foam can offer much more surface area with much less weight. The increase in surface area can be as high as 75-95 per cent comparing to conventional means compactness up to
2,500 m2/m3. The only drawback can be increased pressure drop. While there are two
components of pressure drop, namely because of friction drag and form drag, the structures constituting the porous media have small length scales and mostly avoids form drag. Pressure loss is mostly due to friction. Therefore the tradeoff between heat transfer improvement and pressure drop should be evaluated carefully for each application. There have been various experimental investigations on single-phase hydrodynamics and heat transfer showing the
solid sponges’ potential to be applied as novel heat exchanger types (Meinicke et al., 2017).
Some recent examples of porous media and heat transfer can be viewed fromMiansari
et al. (2015). A unique application is reported inAskari et al. (2017)as a thermal conduction
analysis for pore scale evaluation.
As mentioned above, burners can be made of porous media to obtain high mixing rates and improved heat transfer. Porous medium as an alternative to conventional burners
attracts industrial and scientific communities. Their compact and complex inner structure
leads to efficient burning of the fuels. Its general geometry can be adapted literally to any
volume. Exhaust emissions are formed in lower temperatures and hence, their compositions are less harmful to environment. The reason of lower combustion temperature is the more homogenous mixture of air and fuel. Also generated heat is distributed to gas and solid phases, leading to lower temperature values comparing to conventional burners. In general,
combustion efficiency can be attained better (Omar et al., 2015). Porous media are especially
important for combustion where the combustion volume can be considered constant orfixed
(it is different in internal combustion engines for instance). Stability of the combustion is another advantage of the material. For the metal materials, heat transfer mostly occurs in radiation to the environment because temperature of the metal increases above the intensive radiation level due to the absorbed heat by the porous medium (Omar et al., 2015).
Combustion can take place with lean air fuel mixtures, still NOx, CO and HC emissions are
lower. Flow inside porous medium burners is highly three dimensional.
Metal foams can also be used for cooling purposes since heat transfer mechanisms act in both direction effectively (Altinisik et al., 2007). In the cooling applications, they have rather long operational life because of absence of exhaust emissions. The major effect of exhaust emissions is not the hazardous effect but the fouling that blocks the pores. Therefore, it can be said that fouling is a factor to be considered for porous media. Another interesting work that involves porous
media in refrigeration includes several circular porousfins (Hatami and Ganji, 2014). Authors
compare differentfin geometries containing porosity by using computational fluid dynamics
(CFD). They utilize a new wetfin parameter that enables them to calculate the wet fin parameter
without knowing thefin tip condition. They present results for several dimensionless numbers.
Combustion in porous burners does not occur through freeflames, but takes place in 3 D
form, withoutflame, and inside the openings of porous body (Dehaj et al., 2017). The absence
of freeflames is a part of avoiding or reducing hazardous exhaust emissions.
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There are remarkable studies in the literature about modeling porous combustion media.
These studies focus on geometrical modeling of the medium, proper models for theflow and
combustion and comparison of the obtained results with literature and experimental results.
A new technique of combustion in porous medium was developed byBakry et al. (2010).
They correlateflow rate and flame rate after an experimental work. They obtain a stabilized
flame by means of this technique at high pressure rates (1-9 bar) and temperatures about
400°C. Exhaust emissions are improved in terms of CO and NOx.
Ceramic materials can also be used as combustion foams or sponges.Yu et al. (2013)
compare three different porous burners which are metalfiber, ceramic and stainless steel.
The lowest CO emission occurs with metalfiber burner while stainless steel burner emits the
highest CO. In other words, ceramic porous burner yields a moderate performance among others.
Metal porous burners can also be used in conventional stoves. It is a different application
because when they are used in stoves, they act rather aflow regulator of free flames.Wu
et al. (2014) experimentally investigate such a burner for a stove and find significant
performance improvement.
Keramiotis and Founti (2013)report an experimental work in which a two layered porous
burner from different materials is analyzed. Lower layer consists of Al2O3and upper layer
consists of SiSiC foam. The fuel mixture contains 60 per cent methane and 40 per cent carbon dioxide. Their results show stable and complete combustion.
A semi spherical, premixed, porous metal matrix burner was developed byOmar et al.
(2015)to be used in combi-boilers, boilers and other combustion systems. A stainless steel
material was used alongside a novel design. Flow rate is changed as a parameter (0.048 kg/s, 0.075 kg/s, 0.105 kg/s and 0.125 kg/s). Tests show the temperature on the burner surface reaches up to 1174 K. Value of CO emission drops under the limit values, and the value of
99.2 per cent is obtained for the efficiency of combustion. As a result, the emission values
decrease because of excellent combustion, and the energy saving is reported (Omar et al., 2015). The content of this paper is part of the dissertation of Omar (Omar, 2014). Author not only conducted experimental studies but also used computational means. Some other general results from that work are the per cent 43 energy saved by recycling of exhaust gas by a compact condenser unit and the CO emission decreases below the per cent 77.5 limit value.
Kim et al. (2000) measured the heat transfer coefficient for forced convection of air
through aluminum foams. The authors tested six foams: three of those presented are 10, 20
and 40 Pores Per Inch (PPI) with a constant porosity of 0.92.Bai and Chung (2011)carried
out a study on analytical and numerical prediction of heat transfer and pressure drop in open-cell metal foams.
Radiation heat transfer is very important for open-celled porous metal burners since a big portion of the heat is transferred to the surroundings via this mechanism. A model of
radiation heat transfer from a porous burner is reported byZhao et al. (2008).
An empirical model was developed by Writz to model heat transfer mechanisms in a thin porous wall. Porous medium is found to have 1.5 times improved heat transfer comparing to
an offset stripfin heat exchanger (Writz, 1997). Singh and Kasana study computational
aspects of thermal conductivity of highly porous metal foams. They note that experimental data for the effective thermal conductivity of metal foams with well characterized
micro-structures having wide range of porosity are still insufficient (Singh and Kasana, 2004).
Aluminum foams are placed in a vertical channel in the report ofKamath et al. (2011).
They conducted experiments to assess the porosity changing between 0.9 and 0.95. The
main focus of their study is to analyze theflow assisted mixed convection.
Internal
flow
analysis
One dimensional heat transfer of open cell aluminum foams with 10, 20 and 30 PPI pore
densities was investigated both numerically and experimentally bySertkaya et al. (2015). An
in house computer code was developed by the team to solve the one dimensional heat transfer equation set numerically. Numerical solution was done in the light of Gauss-Seidel iteration scheme. The distribution of the temperature values are compared for each burner
porosity value. They also conducted an experimental work to see the effect offlow rate and
velocity values. Theyfind that temperature drop is inversely proportional to the surface area
per unit volume according to their experimental and theoretical results. Again theirflow rate
parameter shows that temperature drop is proportional withflow rate and pore density.
Altinisik et al. propose a theoretical scheme to analyze the transient heat transfer for
compact heat exchangers and calculate the latent heat of water condensation influe gases.
They look for overall thermal permeability by changing inlet temperature, relative humidity
andflow rate (Altinisik et al., 2012). They also note that the temperature of theflue gases
should be below about 55°C.
Altinisik et al. (2007) also present details of a discretization scheme for heat transfer in aluminum foams. It is three dimensional and two heat transfer mechanisms are considered, namely, forced convection and conduction, in other words conjugate heat transfer. Since it is a hard task to model a porous structure, they used information provided by the manufacturer such as porosity, and areal density which is the ratio of the surface to the
volume. The proposed model does not need solution of theflow.
Although experiments can assist researchers with spatial and global data about engineering designs, local and time resolved physical phenomena can be evaluated by
means of computational approaches if the boundary conditions can be defined.
Computationalfluid dynamics is a comprehensive tool for heat transfer and pressure drop
examinations. There are vast varieties of examples in the related literature. A semi-spherical
ceramic foam burner for a small capacity boiler was developed byAltinisik et al. (2006). The
solid model of the semi-spherical foam burner was obtained and the numerical solution was done using commercial CFD code. The software of PrePDF, GAMBIT and FLUENT 6.0 are used. The turbulence ratio is reported to be 95 per cent, and the inlet velocity is 10 m/s for the uniform temperature on the surface of the semispherical ceramic foam burner.
The work ofFursenko et al. (2016)is considered to be directly related to the present work.
They investigate a cylindrical porous burner and premixed gas combustion is considered. The main heat transfer is radiation and thickness of the porous layers is taken as a
parameter of investigation. They define two different combustion regimes, namely, internal
and external combustions. For methane air mixture, they establish one of these combustion
regimes while porous layer temperatures, radiation power and efficiency values are
examined with respect to their effects on the firing rate. It is found that the internal
combustion mode is characterized by higher temperatures of the outer burner surface, higher
thermal radiation power and radiation efficiency, as well as more uniform glow of the porous
medium compared to the external combustion mode. Theflame has a stabilized manner
about the proximity of the outer surface for external combustion. They propose internal combustion as a preferable mode comparing to the external combustion. The maximum
radiation efficiency is found to be 62 per cent for the firing rate 150 kW/m2for internal
combustion. The differences between 3-9 mm in layer thicknesses show no significant
differences (Fursenko et al., 2016).
Pore-level simulations can be done if regularity can be assumed for a solid sponge.
Parthasarathy et al. (2016)report such a simulation to provide some new values to the Ergun
equation. No simplifications are done in Navier-Stokes equations. Three dimensional
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tomography scans are used for the mesh. Finally they propose a modified and improved new Ergun equation.
Meinicke et al. (2017) present a combined experimental and numerical approach to
analyze single-phase hydrodynamics inside porous SiO2glass sponges. Their particle image
velocimetry (PIV) results are in good agreement with the simulation results.
Boggavarapu et al. (2014)report a CFD analysis and experiments of a domestic burner to
investigate the thermal efficiency for LPG and natural gas. They offer various modification
based on theirfindings and at different levels considering the fuel.
Another detailed simulation of Meinicke et al. shows that their simplification approach in
CFD for an irregular and highly porous structure yields results that are in agreement with the results in the literature (Meinicke et al., 2017). Their results are in terms of heat transfer
and pressure drop. For the simplification, they divide the whole structure into small areas
(elementary volumes) and connect computational domains with a workflow.
Nozari et al. (2017) focus on premixed ammonia-hydrogen-air flames under standard
temperature and pressure conditions using an inert silicon-carbide (SiC) porous block as a
practical and effective medium forflame stabilization. Noticeable power output values are
achieved.
Porous burners and heat exchangers are also part of novel and different applications. For
instance,Wang et al. (2016)use a two-layer porous media burner on a fuel call. The stable
operation limits of the burner are obtained by adjusting the inlet gas velocity and the
equivalence ratio. Methane fuel-rich flames are stabilized inside the burner from the
equivalence ratio of 1.4-1.8.
Dehaj et al. (2017) evaluate a novel porous medium burner for household heating systems. It is an experimental work and changes in temperature for the porous burner
are recorded. Theyfind that the maximum efficiency can be attained by ½ excess air
ratios. Another importantfinding is the reduction of NOxwith increasing excess air and
temperature.
Literature also comprises very complex cases including porous media. It is beneficial to
view those studies to evaluate and asses research potential for advanced engineering
applications. One of the cases is dealing magneticfield acting on the flow through porous
media. Two important papers for dealing the case with approximate solutions are reported
by Ziabakhsh and Domairry (2009) and Shekholeslami et al. (2012). Those papers give
“homotopy asymptotic method (HAM)” and an improvement on it as “Optimal HAM
(OHAM)” as approximate solution methods while comparing them to numerical means.
Further advancement can be done by adding nano particles tofluids to enhance the heat
transfer. When nano particles are involved, together with the porosity and magneticfield, a
very complex parametric problem occurs. A series of reports in time highlight these
phenomena (Sheikholeslami and Ganji, 2014; Ghadikolaei et al., 2017; Ghadikolaei et al.,
2018a;Ghadikolaei et al., 2018b;Ghadikolaei et al., 2018c). Authors deal with laminar viscous
flow, various nano particles that are frequently encountered in recent studies, Heat Permeable Stretching Surface, porous medium, Joule heating, inclination and radiation by using fourth order Runge-Kutta integration scheme and some enhanced models. The
research parameters are nanoparticle volume fraction, kind of nanofluid, nanofluid hybrids,
magnetic parameter, Reynolds number and some additional parameters in respect of heat
transfer andflow dynamics.
A metal porous burner with two stages was designed based on the past experience. It is introduced after literature is surveyed in respect of the recent research aspects in this paper.
A common CFD code is used to demonstrate effect of its structure on theflow. Both the code
and the turbulence model are selected as highly credited and validated in the literature.
Internal
flow
analysis
Therefore, focus is on the effect of the geometry to theflow structure rather than the CFD method. Boundary conditions are selected according to the projected real world application
values of theflow. By this way, static pressure distribution, velocity profiles, density of the
fluid, enthalpy, total pressure, turbulent kinetic energy and dynamic viscosity changes are presented versus radius of the examined body. Star CCM was employed for the modeling and calculation software (Abdulkarim, 2016). This work contributes to the literature by the proposed burner design. Future work can be planned to test the design and further improvement of it.
2. The methodology
Before details of the simulation are laid out, the burner design is desired to be introduced. An engineering design was carried out to develop a good performance porous metal matrix burner for combi-boilers and boilers. The design has a cylindrical shape and has an internal structure that enables mixing the fuel prior to the combustion. The two porous layers (inner and outer volumes) is projected to lead to further mixing of air and fuel and regulate the combustion. The porosity is homogenous and regularly distributed. The burner has unique dimensions and design parameters.
Figure 1shows the parts of the burner.
A solid model of the burner can be seen inFigure 2, while burner in operation is given in
Figure 3.
The burner is designed to be placed in a boiler. The boiler is produced in the workshop where the study was conducted; therefore, boiler also has a novel design. It is presented in Figure 4.
This simulation is done via Star CCM commercial CFD code. Afine mesh setting was
applied to the cylindrical porous medium model geometry. Mesh was composed of 35,000
Figure 1. Exploded burner assembly
Figure 2. Solid model of the premixed metal matrix porous burner
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structured elements. The general view of the mesh structure is provided inFigure 5. k-e
turbulence model was utilized as a general and justified turbulence model for the examined
geometry. Since internal boundaries are avoided for the present preliminary investigation,
the utilization of the k-e turbulence model is appropriate. Cartesian coordinates were used
whereas the mesh was adapted for the cylindrical geometry. This doesn’t lead to any
complexities in the major fraction of the computational domain and the CFD code can handle
this situation. Air was selected as thefluid and thermo physical properties were selected
accordingly.
The governing equations of k-e turbulence model and the energy equation are provided
inequations (1)-(9)(Anderson et al., 1984;Versteeg and Malalasekera, 2007). Some of these
equations are given in vector form and can be applied to any coordinate system. This denotation is widely used in the literature:
@r @t þ r ðrVÞ ¼ 0 or Dr Dtþrr V ¼ 0 (1) rDVDt ¼ f rp þmr2V (2) Figure 3. Combustion on the surface of the burner
Figure 4. The boiler designed for the test of the burner
Internal
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analysis
rcp @T @t þ v @T @y þ w @T @z þ u @T @x ¼ k @2T @y2þ @2T @z2 þ @2T @x2 " # þ U (3) @k @tþ ui@k @xi ¼ @ @xi T sk @k @xi þ G e (4) @e @t þ ui@e @xi¼ @ @xi T se @e @xi þ C1e ekG C2e e 2 k (5) G¼ T @ui @xjþ @uj @xi ! @ui @xj (6) T¼ Cm k2 e (7) Cm¼ 0:09; C1e ¼ 1:44; C2e ¼ 1:92; sk¼ 1:0; se ¼ 1:3 (8) aT¼ vT sT (9) Figure 5. General view of the structured grid
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3. Results
Quantitative results are presented below for static pressure inFigure 6, density distribution
in Figure 7, total enthalpy distribution in Figure 8, velocity magnitude distribution in
Figure 9, total pressure distribution inFigure 10, turbulent kinetic energy distribution in
Figure 11and dynamic viscosity distribution inFigure 12, respectively.
Allfigures contain a contour map on the cross-sectional surface when cylinder axis and
inlet circle radius are used for the cross-section and a x-y line plot containing several point data on a line drawn at the bottom part of the geometry from outer edges of the circle. A
curve is alsofitted for the point samples in all x-y line plots.
Normally, in a pipeflow, static pressure gets its highest values at the solid boundaries
and static pressure profile is an inverse of the developed velocity profile. However, in this
case, outer surface of the cylinder except the topflange is porous and the lowest value of the
Figure 6. Static pressure: contour map and line plot between outer edges at the inlet
Figure 7. Density: contour map and line plot between outer edges at the inlet
Internal
flow
analysis
Figure 8. Total enthalpy: contour map and line plot between outer edges at the inlet
Figure 9. Velocity magnitude: contour map and line plot between outer edges at the inlet
Figure 10. Total pressure: contour map and line plot between outer edges at the inlet
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static pressure is on the outer surface. Sinceflow accumulates on the flange, as the inlet flow
direction is perpendicular to theflange surface, static pressure reaches to its maximum at the
center on theflange. Pressure decreases as a gradient toward inlet as if a shockwave. The
outer volume surrounding the inner volume has a similar trend in respect of the static
pressure. It is understood that the high pressure core next to theflange surface builds up to
0.29 Pa and then 0.22 Pa pressure value spreads toward inlet direction and outer volume.
Thisfigure implies a very low pressure drop due to the porous medium. Still, the flow rate
seems low since theflow only passes to outer volume significantly after a static pressure
level asflow accumulates on the top flange. Higher flow rates would lead to higher static
pressure values and hence early transition to the outer volume. Another reason of theflow
accumulation is the dimensions of the homogenous porosity on the interface surface between inner and outer volumes. Smaller size porosity at the top region and bigger size porosity close to the inlet region may lead to a better distribution of the gas in the outer volume. In its
Figure 11. Turbulent kinetic energy: contour map and line plot between outer edges at the inlet
Figure 12. Dynamic viscosity: contour map and line plot between outer edges at the inlet
Internal
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analysis
present form, a big portion of the burner has a uniform outflow while the top part next to the flange shows higher outflow due to the higher static pressures. However, the difference
should be small, otherwise,flow accumulation would not be seen by the static pressure map.
Density changes very little since the changes in gauge pressure values are very small and there is no combustion in the volume. Density distribution is in line with the static pressure distribution as expected. Only energy dissipation due to friction changes density at the close proximity of the interface between inner and outer volumes. Density increases at the upper part of the cylindrical body due to the compression at the closed upper part. Therefore, the main mechanism determining the density distribution is the
compression of theflow. The friction and viscous dissipation during the mass transfer
from inner volume to outer volume heats the gas and therefore, density gets its lowest values. To validate and justify the temperature increase around the boundaries between inner and outer volumes, local enthalpy values can be viewed. Still, the differences in
the values of density are very slow and increase in the volumetricflow rate should
compensate the reduction in the values of the density for mass flow rate. Energy
dissipation increases the enthalpy, thus enthalpy values are higher at the close proximity of the wall. Also maximum enthalpy occurs near the outlet of the cylindrical
body. One can view the position of the walls by thefigure. Also maximum enthalpy
occurs near the outlet of the cylindrical body. The highest enthalpy values and the lowest density values coincide at the same locations. The increase in total enthalpy is 0.02 J/kg. This change is acceptable in terms of safe operation, avoiding self ignition. It is also regarded as an indicator of gas mixing.
If static pressure is deducted from the total pressure, one can acquire the dynamic pressure. In this respect, velocity distribution can be assessed and evaluated
considering the static and total pressure distributionfigures. Velocity magnitude is
relatively small where static pressure gets higher values since the kinetic energy is converted into potential energy as static pressure and compresses the local domain in a
denser gas region. Also, it is seen that the part of the inner volume close to theflange
and the passage between inner and outer volumes pose a resistant region toflow. This
flow resistance reduces the speed and increases the static pressure. This can be
justified by the density distribution graphic. It is seen that most of the flow energy is
conserved due to the absence of the solid boundaries in the burner core. Then flow
accumulates at the top flange and approaches to stagnation. The porous interface
surface acts as a medium that makes the flow laminar. Flow develops in the outer
volume toward inlet direction and is developed in the middle part of the outer volume.
Velocity magnitude map also justifies the comment for static pressure map as the top
part slightly exhibit moreflow rate toward outer surface. It is also seen that much of
theflow energy is spent on the interface surface between inner and outer volumes.
Accordingly, total pressure is decreasing asflow continues further due to the friction
and viscous dissipation. The passage between inner and outer volumes has a great impact on the total pressure. Also, the effect of the friction can be viewed in the total pressure map. Actually it is an expected phenomenon since the porosity acts as a
resistance to theflow. It also regulates flow velocity for lower amounts and aids a
more homogenous distribution and mixing. Again, changing porosity dimensions from homogenous to gradually increasing toward inlet direction seems logical for more homogenous gas distribution.
As expected, turbulence occurs at the close proximity of the interface walls due to boundary effects and the mixing in the near proximity. About 0.03 J/kg turbulent kinetic energy can be viewed as the highest amount. Together with the increase in total enthalpy,
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total amount of energy drawn from theflow is 0.05 J/kg. More than half of it spent through
turbulence and remaining is dissipated as heat. Decrease of theflow energy can also be
justified by the total pressure map. Nevertheless, turbulence is anticipated here for gas
mixing. The level of the turbulence can be correlated with the combustion efficiency
considering the gas mixing in the future. By this way, CFD can be utilized for obtaining
desired turbulence level prior to experimenting. The curvefit in the line plot of the figure
may mislead as it reaches to a maximum in one side and then reduces to zero on the other
side. One should only consider the point data here. Curvefitting tool tried to catch zero
values outside of the calculation domain and misprinted the curve. The two points for the turbulence kinetic energy have very close values to each other since they denote the turbulence values near the interface surface.
It is known that increasing temperature and pressure increase viscosity of gases. Since air was used to simulate gas mixture, increasing density due to the increasing static pressure and increasing temperature affect viscosity in a directly proportional way. At the upper part, higher values of viscosity can be observed due to relatively higher gas density and static
pressure. After all, viscosity and static pressure maps are very similar. The curvefit line
indicates the only read value of the dynamic viscosity in the inlet region but since the distribution is almost equal everywhere in this region, it shows the quantitative
result. The increase in dynamic viscosity also contributes to the decrease of theflow energy
during the mixing procedure. But this is indispensable for gases and the change has an ignorable level.
4. Conclusion
This work is an introduction of a newly designed burner. Related literature is surveyed comprehensively. Flow through the burner and mixing procedure is demonstrated by means of a common commercial CFD code. The main aim is to introduce the burner and provide some aspects of potential studies. The geometry for the CFD analysis contains details of the homogenous porous medium which is designated for usage as a burner. Boundary conditions were selected according to the projected application operational parameters of the so called porous metal matrix
burner. By this way, static pressure distribution, velocity profiles, density of the fluid,
enthalpy, total pressure, turbulent kinetic energy and dynamic viscosity changes were presented versus radius of the examined body. Star CCM was employed for the modeling and calculation software.
Some points drawn from the evaluation are provided below:
Flow accumulates on theflange; as the inlet flow direction is perpendicular to the flange surface, static pressure reaches to its maximum at the center on the flange. Pressure decreases as a gradient toward inlet as if a shockwave.
The porous medium causes very low pressure drop which means low pumping power requirement which is regarded as an advantage. Usually, the main drawback of porous media is the pressure drop.
In its present form, the top region close to the topflange exhibits higher outflow through the porous outer surface. It is inferred from static pressure and velocity magnitude maps.
Although the burner has homogenous porosity in the axial direction, a gradient increase in the porosity size toward inlet direction can be studied in future works. It is inferred from static pressure, total pressure and velocity magnitude maps.
Internal
flow
analysis
The main mechanism determining the density distribution is the compression of the flow. The friction and viscous dissipation during the mass transfer from inner volume to outer volume heats the gas and therefore, density gets its lowest values. The differences in the values of density are very slow and increase in the volumetric flow rate should compensate the reduction in the values of the density for mass flow rate.
The increase in total enthalpy is 0.02 J/kg. This change is acceptable in terms of safe operation, avoiding self ignition. It is also regarded as an indicator of gas mixing.
Most of theflow energy is conserved due to the absence of the solid boundaries in the burner core.
The porous interface surface acts as a medium that makes theflow laminar. Flow develops in the outer volume toward inlet direction and is developed in the middle part of the outer volume.
Much of theflow energy is spent on the interface surface between inner and outer volumes.
As expected, turbulence occurs at the close proximity of the interface walls due to boundary effects and the mixing in the near proximity. About 0.03 J/kg turbulent kinetic energy can be viewed as the highest amount. Together with the increase in total enthalpy, total amount of energy drawn from theflow is 0.05 J/ kg. More than half of it spent through turbulence and remaining is dissipated as heat.
Turbulence is anticipated here for gas mixing. The level of the turbulence can be correlated with the combustion efficiency considering the gas mixing in the future. By this way, CFD can be utilized for obtaining desired turbulence level prior to experimenting.
The change in dynamic viscosity has an ignorable level.
General geometry of the porous medium model is found to be favorable in respect of its
operationalflow conditions.
In the future, it is desired to test the burner experimentally and to use the results for evaluating and assessing CFD models. Combustion is also desired to be modeled and
calculated. Also transient analyses will be beneficial in a comprehensive manner to have an
idea of system startup.
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Further reading
Abdulkarim, A.H., Ates, A., Altinisik, K., Canli, E. and Arslan, O.M. (2016),“Computational modeling of flow through circular volume representing a porous medium”, 24th Annual International Conference on Composites or Nano Engineering, ICCE-24 Haikou.
Corresponding author
Ali H. Abdulkarim can be contacted at:alihuseyin81@yahoo.comanddr.ali@uokirkuk.edu.iq
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