Experimental and Numerical Simulation Study of the Microscale

Article pubs.acs.org/IECR

Experimental and Numerical Simulation Study of the Microscale Laminar Flow Diffusion Combustion of Liquid Ethanol Tao Xu,†,⊥ Xue-nong Gao,*,† Jing Yang,‡ Yun-hua Gan,§ Ze-liang Yang,§ and Zheng-guo Zhang† †

Key Laboratory of Enhanced Heat Transfer and Energy Conservation, Ministry of Education, China, South China University of Technology, Guangzhou 510641, China ‡ Management Engineering Department, Guangdong Light Industry Technical College, Guangzhou 510300, China § School of Electric Power, South China University of Technology, Guangzhou 510641, China ABSTRACT: Laminar flow diffusion combustion appears when liquid ethanol passes through the ceramic tubes with inner diameter of 1.0 mm, 0.6 mm and 0.4 mm and outer diameter of 2.0 mm. Due to the limitation of measurement tools, it is very difficult to measure the microflame characteristic parameters and the numerical simulation method for microflame is always employed. By comparing the simulation results and measured values, liquid droplet radiation heat transfer and boundary slip have significant effect on numerical simulation and the numerical simulation values agree well with the experimental data, which proves that the numerical simulation method is reliable. The numerical simulation results show that the flame height and width increase almost linearly with the volume flow while the maximal temperature of flame increases first and then decreases. Because the microscale effect can improve the combustion efficiency, A flame with smaller height and width and lower maximal temperature is observed at a lager inner tube diameter.

1. INTRODUCTION Miniaturization of energy systems has been accelerated dramatically with the development of nano and microfabrication technologies. The power requirements of these energy systems extend what traditional batteries can provide over an extended operation. A conventional hydrocarbon fuel has energy density of 40 kJ/g, which is about 100 times higher than that of the most advanced batteries. Thus, a hydrocarbon fuel-based microcombustor can replace the batteries to deliver much higher power for the development of micro-electromechanical devices for biomedical applications, chemical sensing, telecommunication, and micropropulsion.1,2 In the last 10 years, various microthrusters,3−5 microengines,6,7 and microreactors8 have been developed. However, sustaining combustion in a microscale combustor will be largely affected by the increased heat losses due to the high surface to volume ratio that tends to suppress ignition and quench the reaction.9 In order to test the feasibility of combustion in microdevices and determine the relevant factors affecting microcombustion, numerical and experimental work should be performed. Experimental research and numerical simulation on microscale diffusion flames have greatly promoted the development of microscale combustion systems. There are various fundamental studies to observe the characteristics of diffusion flames. The effect of buoyancy on microdiffusion flames has been studied by examining the flame shape, and a phenomenological model has been developed (based on experimentally determined flame shapes) to compare diffusion and convection transport effects.10 The prediction of laminar jet diffusion flame sizes was analyzed by theoretical model and experimental verification. The diffusion flame sizes can be predicated for two different burner geometries (circular and slotted ports) by taking the diffusion coefficient of oxygen at a characteristic flame temperature of 1500 K.11,12 The soot© XXXX American Chemical Society

luminosity boundaries (near the laminar smoke-point condition) of steady nonbuoyant round hydrocarbon/air laminar jet diffusion flames at microgravity were found from color video images obtained by Aalburg et al.13 The contradiction between the experimental data and theoretical concepts on gasification of disperse carbon in the diffusion flame of hydrocarbon fuels is examined. As a result of the heterogeneous reaction of carbon with molecules of carbon dioxide and water inside a laminar diffusion flame, complete gasification of particles within the time of their residence in the flame is impossible.14 The behavior of laminar jet diffusion flames in the presence of nonuniform magnetic fields has been investigated, and the results are presented by Baker et al.15,16 Chen et al.17 investigated the stabilization mechanisms of the lifted laminar propane flames in an axisymmetric jet flow configuration. A numerical study on fluid dynamics and the thermal and chemical structure of the laminar methane−air microflames established under quiescent atmospheric conditions is conducted by Nakamura et al.18 Effects of oxygen enhancement, microgravity, and inverse burning on flame appearance and sooting behavior have been emphatically discussed for ethanolfueled laminar gas jet diffusion flames.19 Flame heights of coflowing cylindrical ethylene−air and methane−air laminar inverse diffusion flames are measured.20 However, those studies mainly focus on diffusion flames of gaseous fuels, and very few studies have been conducted to understand microdiffusion flames with liquid fuels for liquid fuel combustion, including liquid fuel gasification, which is more complex than gaseous fuel combustion, However, many higher energy density liquid fuels Received: November 19, 2012 Revised: March 20, 2013 Accepted: April 22, 2013

A

dx.doi.org/10.1021/ie303171v | Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Industrial & Engineering Chemistry Research

Article

Figure 1. Test facility of microscale liquid ethanol laminar flow diffusion combustion. (1) Syringe pump. (2) Syringe. (3) Plastic tube. (4) Pedestal. (5) Wood sleeve. (6) Ceramic tube. (7) Microflame. (8) Stereo microscope. (9) Digital camera. (10) Computer. (11) Platinum rhodium probe. (12) Thermocouple.

are more easily transported and kept in storage than gaseous fuels. Our research group has already carried out the experimental study of the microscale laminar flow diffusion combustion of liquid ethanol in free space and restricted space. Some experimental results have already been published,21−23 but the numerical simulation of liquid ethanol has not been carried out yet. For the microscale burner, it is very difficult to measure the internal parameters of the combustion flame. As the numerical models and software become more reliable, numerical simulation has been an important tool to study fluid flow, heat transfe,r and combustion. This paper will present an experimental and numerical simulation on the combustion characteristic of microscale liquid fuel laminar flow diffusion combustion and discuss the relationship between the ceramic tube inner diameter, fuel flow rate, characteristic size, and temperature of the microflame. The mechanism of the microscale liquid fuel laminar diffusion combustion has been revealed, which will lay the foundation for the development of efficient liquid fuel microscale burners.

Table 1. Parameters of Ceramic Tubes type

tube 1

tube 2

tube 3

inner diameter outer diameter

1.0 mm 2 mm

0.6 mm 2 mm

0.4 mm 2 mm

the wooden insulation coat to reduce heat loss. The microfuel supply system consists of a microsyringe pump (TS2−60, Baoding Longer Precision Pump Co., Ltd., China), syringe with a 10 mm inner diameter, and plastic tube. The syringe pump controls the fuel flow rates from 1 mL/h to 300 mL/h with a deviation of less than 1%. The microflame observation system includes a stereo microscope (Zoom460T, Nanjing Jiangnan Xinxing Optical instrument Co., Ltd., China), digital camera (ProgRes C12plus, Eyelike Instruments, Germany), and computer. The microflame can be captured by the stereo microscope, which is connected to the digital camera (2580 pixels × 1944 pixels), and the signal is transferred to the computer. An S type thermocouple with a platinum−rhodium probe, with a temperature range of −50°C∼1300°C and a measurement error of ±1°C, is selected to measure the temperature of the microflame.

2. EXPERIMENT METHOD The test facility of microscale liquid ethanol laminar flow diffusion combustion shown in Figure 1 is made up of a microcombustion system, microfuel supply system, microflame observation system, and microtemperature measurement system. Three different size capillary tubes are used for the test section. In the experiments, the capillary tubes are installed vertically and used as the combustion nozzles. The liquid fuel combustion in the microdiffusion flame occurs at the tube exit. Three tubes are ceramic tubes with inner diameters ranging from 0.4 to 1.0 mm, and their sizes and materials are listed in Table 1. The liquid fuel used in the experiments is ethanol at atmospheric temperature. In all experiments, the liquid ethanol is burned in a quiet room temperature and air environment. The microcombustion system mainly includes a pedestal, wooden insulation coat, and microceramic tube. The pedestal is made by the square insulation board with four adjustable bolts. The microceramic tube is embedded in the inner round hole of

3. NUMERICAL SIMULATION PROCESS FLUENT software is an effective analytic tool to simulate flow, heat transfer, and combustion of fluid. Operating FLUENT is made up of two steps: first, to establish a physical model and mesh generation by the preprocessing GAMBIT software of FLUENT, and second, to import generated gridding into FLUENT. The numerical simulation is based on the boundary condition and material property. 3.1. Physical Model and Mesh Generation. Because the microscale combustion has the characteristics of short residence time, high surface−volume ratio, significant viscous effect, large quantity of heat loss, and so on, it is different from the traditional combustion. At present, microscale combustion numerical simulation researches are mostly concentrated on 2D models. However, 2D models cannot comprehensively simulate microscale combustion characteristics, so a 3D model is selected in this paper. Combining the characteristics of laminar flow diffusion combustion, a 3D cylinder combustion model B



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Table 2. Parameters of Mesh Generation fluid zone in ceramic tube

solid zone of ceramic tube

combustion zone

gridding size

gridding number

gridding size

gridding number

gridding size

gridding number

0.2 0.2 0.2

18,471 58,004 37,063

0.1 0.08 0.06

1615 33,505 75,485

0.5 0.8 0.8

39,864 109,142 95,803

tube 1 tube 2 tube 3

with height of 25 mm and diameter of 10 mm is created. On the basis of the experiment, the height of tube 1, tube 2, and tube 3 are set as 8 mm. The physical model is divided into a solid zone, fluid flow zone, and combustion zone, and they are meshed by different gridding sizes. Tet/Hybrid gridding and the TGrid method are chosen to mesh the volumes. The parameters of mesh generation are shown in Table 2. 3.2. Material Properties and Boundary Conditions. The liquid ethanol laminar flow diffusion combustion in turn includes four processes: atomization, evaporation, mixing of the liquid fuel and air, and combustion with mutual influence. In order to simplify the problem, the liquid ethanol laminar flow diffusion combustion is regarded as a stable process, and composition chemical properties are unchanged. The initial temperature of liquid ethanol, ceramic tube, and air is set as 300 K. The physical properties for liquid ethanol, microscale ceramic tube, and air are defined in Table 3. In addition, liquid ethanol will turn into gaseous ethanol and generate heat of phase change, so the latent heat of liquid ethanol needs be set to 85,5237 J/kg.

flows. The source Sm is the mass added to the continuous phase from the dispersed second phase and any user-defined sources. The momentum conservation equation is described by

Table 3. Physical Properties for Liquid Ethanol, Microscale Ceramic Tube, and Air

There is radiation between the ceramic tube and combustion gas. The radiation heat transfer equation can be solved as

material ethanol ceramic tube air

density kg/m3

specific heat J/kg K

coefficient of thermal conductivity W/m K

790 2872

2470 910

0.182 1.75

1225

1006

0.0242

∂ (ρu ⃗) + ∇ × (ρuu⃗ ⃗) = −∇p + ∇ × (τ ) + ρg ⃗ + F ⃗ ∂t (2)

The energy equation can be solved as ∂ (ρE) + ∇ × (u ⃗(ρE + p)) ∂t = ∇ × (keff ∇T − ∑ hjJj ⃗ + (τeff × u ⃗)) + S h

In eq 3, the first three terms at the right side are energy transfer due to heat conduction, component diffusion, and viscous dissipation. Sh includes the heat of chemical reaction and any other volumetric heat sources to be defined. E=h−

p u2 + ρ 2

(4)

−∇qr = aG − 4aσT 4

(5)

The component equation should be followed when ethanol is burning. It can be expressed as ∂ (ρYi ) + ∇ × (ρuY ⃗ i ) = −∇ × Ji ⃗ + R i + Si ∂t

(6)

Si is the rate of creation by addition from dispersed phase plus any user-defined sources. Because the heat transfer of a microscale ceramic tube can affect the liquid ethanol combustion directly, diffusion of multicomponents and heat diffusion must be considered. Heat transfer to the liquid ethanol particles must follow the following equation

The laminar flow diffusion combustion of liquid ethanol is a complicated process with droplet evaporation, mixing, and combustion, so only the dispersed phase model can be selected to calculate droplets combustion. Inject type is set for plainorifice-atomizer. Evaporating species are C2H5OH, CO2, H2O, and O2. Ambient atmosphere and pressure outlet are selected as the outlet boundary condition with 0 Pa gauge pressure, 300 K initial temperature, and 22% mass fraction of oxygen. The operation pressure is 101,325 Pa, and the gravity acceleration in each directions is X = 0 m/s2, Y = 0 m/s2, Z = −9.81 m/s2. Coupled wall condition is chosen because of the heat transfer between solid and fluid. 3.3. Numerical Simulation Method. Because the laminar flow diffusion combustion of liquid ethanol includes fluid flow, heat transfer, mass transfer, chemical reactions, etc., a mathematical model should follow the mass conservation equation, momentum conservation equation, energy conservation equation, and component equation. The mass conservation equation can be written as follows ∂ρ + ∇ × (ρu ⃗) = Sm ∂t

(3)

j

m pc p

dTp dt

= kA p(T∞ − Tp) +

dm p dt

hfg + εpA pσ(θr4 − Tp4) (7)

The pressure-based approach is developed for low-speed incompressible flows, while the density-based approach is mainly used for high-speed compressible flows. In this paper, the pressure-based solver is selected to simulate the laminar flow diffusion combustion. The pressure is tightly coupled with velocity, and the SIMPLEC algorithm is chosen to accelerate convergence. Also, under-relaxation factors need be modified as in Table 4. Table 4. Modifications of Under-Relaxation Factors

(1)

Equation 1 is the general form of the mass conservation equation and is valid for incompressible as well as compressible C

pressure

body forces

momentum

energy

discrete phase sources

0.25

0.9

0.6

0.9

0.4



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4. DATA PROCESSING Flame experimental photos are gained by the microflame observation system. An image distinguished and magnified by a stereo microscope can be captured by the CCD of a digital camera. It is converted by A/D and stored point-by-point of pixel that contains information on the color values red, green, and blue and the brightness values Re, Ge, and Be, which are proportional to the radiant energy. The brightness of the flame in the area of higher temperature will be higher. Thus, the flame temperature contour chart of numerical simulation can be considered as flame structure similarly. Characteristic size of the experimental flame structure is shown in Figure 2, which can be

equal absolute speed, namely, the traditional no-slip boundary hypothesis. With the growth of micronanometer testing technology, it was recognized that microscale fluid clearance flow was different from macroscopic fluid flow constitutionally, and scale effect was obvious, so that the most representative of a class of problems was the boundary slip. Boundary slip implies that the relative movement between the fluid adjacent to the solid surface and surface of the solid occurs. The existing research results show that boundary slip has an effect on interstitial fluid flow under the condition of microscale. Thus, the boundary slip effect on the combustion process is being focused in researching. Further more, liquid ethanol droplets are pushed into the combustion zone, so droplet radiation heat also needs to be incorporated into the liquid ethanol microscale laminar diffusion combustion process. Four numerical simulation models with and without considering the boundary slip and droplet radiation heat transfer are shown in Table 5. Table 5. Type of Boundary Slip and Particle Diffusion Model boundary slip droplet radiation heat transfer

model 1

model 2

model 3

model 4

√ ×

× √

× ×

√ √

The flame structure for tube 3 at flow rate of 1.8 mL/h is shown in Figure 4. Simulation values with four numerical simulation models are compared with experimental results, according to the deviation of flame characteristics, to determine the influence degree. The comparison results are listed in Table 6. For models 2 and 4, the simulation values of the maximal temperature only are positive deviations, and close to the experimental values, it is implied that the droplet radiation heat transfer has a significant effect on microscale combustion. During the burning process, the liquid ethanol needs to be heated for vaporization, though this is conducted by droplet radiation heat transfer. For models 1 and 3, the deviations with model 1 are less than those with model 2. This suggests that boundary slip also has an effect on the microscale combustion. According to the hypothesis of classical fluid mechanics and lubrication mechanics, boundary slip cannot appear in the solid−liquid interface while liquid ethanol flows into the ceramic tube or the deviations will be larger. However, the flow mechanism of a microscale ceramic tube is very different from that of conventional size. The deviations for model 4 are the lowest because both boundary slip and droplet radiation heat transfer are considered. In conclusion, boundary slip and droplet radiation heat transfer cannot be neglected in the process of microscale combustion, and the flame characteristics of the numerical simulation are analyzed here: (1) Flame characteristic values of the numerical simulation are larger than those of the experiment, which include flame height, width, and highest temperature. The first reason is that the heat loss of the wooden insulation coat and ceramic tube is unavoidable in the actual experiment but ignored in the numerical simulation process. The second reason is that the effect of wind speed and direction in the experimental environment is not taken into account in the numerical simulation. (2) Maximum deviations of the height and width of the flames are 6.2% and 5.7%, respectively, and for the maximal temperature is 1.8%. It is indicated that the numerical simulation values agree quite well with the

Figure 2. Characteristic size of experimental flame structure.

calibrated by Origin software with an allowable error of ±0.01 mm. Figure 3 is the processing method of the temperature field

Figure 3. Processing method of temperature field of numerical simulation.

of numerical simulation. Tecplot software can estimate the flame structure size and temperature of numerical simulation within an error of ±0.01 mm for flame structure sizes and ±1 K for temperatures. H and W are defined as the actual height and width of the flame, respectively.

5. NUMERICAL SIMULATION OF IMPACT FACTOR ANALYSIS Classical fluid mechanics and lubrication mechanics assumed that the molecules adjacent to the solid and fluid interface have D



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Figure 4. Flame characteristics for tube 1 at flow rate of 1.8 mL/h.

Table 6. Comparisons of Flame Characteristics for Tube 1 at Flow Rate of 1.8 mL/h

experimental value model 1 model 2 model 3 model 4

simulation deviation simulation deviation simulation deviation simulation deviation

H

W

highest temperature

3.72 mm

2.65 mm

1445 K

2.25 mm −39.5% 3.30 mm −11.3% 1.50 mm −59.7% 3.95 mm +6.2%

2.40 mm −9.4% 2.45 mm −7.5% 2.30 mm −13.2% 2.80 mm +5.7%

1270 K −12.1% 1480 K +2.4% 1280 K −11.4% 1471 K +1.8%

experimental values, and the employed numerical simulation method is reliable. Figure 5. Variation of normalized flame heights.

6. NUMERICAL SIMULATION RESULTS 6.1. Flame Height and Width. The flame heights and widths are measured directly from the temperature field contour by Tecplot software. Typical results are presented in Figures 5 and 6, where Q is fuel flow rate. The flame height and width increase almost linearly with the fuel flow rate, which are shown in Table 7. For flame height, traditional gas jet diffusion flame theory indicates that the flame height is directly proportional to flow rate and independent of the tube diameter. However, in this simulation, the flame height increases as the tube inner diameter decreases at a given flow rate. It is also found that the increases with flow rate are faster for a smaller tube, and the

slopes of the lines in Figure 5 are 1.98, 4.87, and 5.55 for tube 1, tube 2, and tube 3 respectively. When the ceramic tube inner diameter is smaller, the microscale effect is stronger and combustion efficiency is improved. For flame width, increases in the fuel flow rate lead to linear increases in the flame widths. With the growth of flow rate, more ethanol can spread toward the radial direction of tube nozzle and make the flame wide. Also, the flame width increases as the tube inner diameter decreases for a given flow rate. The linear slopes in Figure 6 are 0.76, 1.12, and 1.21 for tube 1, tube 2, and tube 3, respectively. Further more, smaller inner tube diameters increase the exit velocity of the ethanol vapor and E



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more quantity of heat, which makes the temperature of the ceramic tube higher to overcome the capillary force of the microscale tube and improve the evaporation rate of ethanol to burn more sufficiently. It is undoubtedly the maximal temperature of the flame becoming higher. As the flow rate arrives at a certain value, the maximal temperature of the flame decreases with increasing flow rate. The main reason is that the evaporation particles are too many to be burnt completely, and the particles not participating in the combustion absorb some heat from the reaction zone. Additionally, the maximal temperature of flame also increases with decreasing the ceramic tube inner diameter. The maximum values of flame temperature and its corresponding flow rate are presented in Table 8. As the Table 8. Maximum Value of Flame Temperature and Its Corresponding Flow Rate Figure 6. Variation of normalized flame widths.

tube 1 tube 2 tube 3

Table 7. Relationship between Flame Characteristic Size and Q tube 1 tube 2 tube 3

H

W

H = 0.02 + 1.98Q H = −0.27 + 4.87Q H = −0.12 + 5.55Q

W = 1.43 + 0.76Q W = 1.75 + 1.12Q W = 2.44 + 1.21Q

flow rate

maximum value of flame temperature

3.9 mL/h 1.9mL/h 1.1 mL/h

1546 K 1572 K 1635 K

inner diameter of the ceramic tube is smaller, the maximum value of the flame temperature increases, and its corresponding flow rate is also smaller. It implies that a microscale effect can improve fuel combustion efficiency. Possibly the smaller the inner diameter of ceramic tube is, the greater the microscale effect and the stronger the capillary forces are, and there is a longer residence time for liquid ethanol to be evaporated in the ceramic tube to take part in combustion.

move the widest flame away from the flame base. It is also implied that the microscale effect is more significant when the inner diameter of tube is smaller. 6.2. Flame Temperature. Heat generated from burning fuel at constant pressure is consumed in two parts: one part to the surrounding environment and another part to the increase temperature of the combustion products. If there is no heat loss from radiation, heat conduction, or heat diffusion, the flame will obtain the maximal temperature, which is also called adiabatic burning temperature. For diffusion flames, especially laminar flow diffusion flames, the maximal temperature is associated with the flow rate and the inner diameter of ceramic tube. The maximal temperature of the flame is very difficult to measure with experimental tools, but it can be shown in the flame temperature contour chart of numerical simulation clearly. The relationship between the maximal temperature and liquid ethanol flow rate is presented in Figure 7. With the increase in liquid ethanol flow rate, fuel combustion can release

7. CONCLUSIONS Through experimental study and numerical simulation, the following conclusions can be drawn: (1) Numerical simulation can solve the problem in the measurement of microscale flame height, width, and highest temperature. Numerical simulation values of the microscale flame height, width, and the maximal temperature agree well with the experimental results, which proves that the numerical simulation is reliable. (2) Liquid droplet radiation transfer heat and boundary slip have a significant effect on the characteristics of microscale combustion in the process of numerical simulation of the microscale combustion so that traditional gas jet diffusion flame theory is unavailable for the microscale flame. (3) The flame height and width increase almost linearly with the volume flow of liquid ethanol, but the linear slope increases as the tube inner diameter decreases. For the maximal temperature, it increases first and then decreases while arriving at a certain volume flow. It is implied that the microscale effect can improve the combustion efficiency of liquid ethanol.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +86-20-87113870. Fax: +86-20-87113870. Present Address ⊥

Marine Engineering Department, Guangzhou Maritime Institute, Guangzhou 510725, China.

Figure 7. Variation of highest temperature of flame. F



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Notes

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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research is supported by the National Natural Science Foundation of China (50806022). This material is available free of charge via the Internet at http://pubs.acs.org.

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NOMENCLATURE u = velocity (m/s) ρ = density (kg/m3) μ = coefficient of kinetic viscosity (Pa s) p = pressure (Pa) F = external body forces (N) g = gravitational acceleration (m/s2) t = time (s) ∇ = vector differential operator τ = stress tensor (N/m2) k = heat transfer coefficient (W/m2 K) h = specific enthalpy (kJ/kg) J = diffusion flux T = temperature (K) q = quantity of heat (J) a = absorption coefficient G = incident radiation (W) σ = dissipation coefficient R = net reaction rate of production by chemical reaction (mol/m3·s) Y = mass fraction M = molecular weight (g/mol) m = mass (kg) c = specific heat capacity (kg K) ε = particle emissivity θ = radiation temperature H = flame height (mm) W = flame width (mm) d = diameter of ceramic tube (mm) Re = Reynolds number Q = volume flux (mL/h)

Subscripts

r = radiation heat transfer eff = effective X = coordinate X Y = coordinate Y Z = coordinate Z i = material component j = material component p = constant pressure ∞ = continuous phase fg = liquid phase to gas phase

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REFERENCES

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Experimental and Numerical Simulation Study of the Microscale

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