Research on the Working Performance of Carbonized Micro-wood Fiber DPF

Along with the increasingly stringent emissions regulations, the Diesel Particulate Filter (DPF) is one of the leading technologies for reducing particulate matter in diesel exhaust. In this study, it is presented that the Carbonized Micro-Wood Fiber (CMWF) is used as filter material of DPF. Firstly, the permeability and the inertia coefficient are obtained by numerical simulation method, then the geometric relationship between the Δp/H (the ratio of a pressure difference of the front and back of a filter body and the thickness) and the v1 (exhaust flow velocity in front of the filter body), lastly, the test is performed utilizing the self-designed detachable DPF and the flow characteristics test bed. Theory and test proves that the numerical simulation method can be consistent with the test very well, which shows the DPF with bigger pore diameter and longer axial length should take priority over the others.


INTRODUCTION
Along with the more and more strictly emission regulations, Diesel Particulate Filter (DPF) technology is one of the most effective and simple methods to control the diesel particulate emission ultimately (Juan et al., 2009).The current DPF filter materials mainly include: cordierite honeycomb ceramics, ceramic foam, woven ceramic fiber, metal mesh, metal fiber, silicon carbide, silicon nitride, activated carbon fiber and so on, in which the most commonly used filter materials are cordierite and silicon carbide (Adler, 2005).However, in view of the specific national conditions of our country, the DPF filter materials always exist some kinds of shortcomings, so the vast majority of diesels will face the problem that they cannot run on the roads in their service lives if the Euro V standard is executed (Fang et al., 2009;Li et al., 2009;Wang et al., 2010).In order to make our country's exhaust emission standards conforms with international ones as soon as possible, the researchers are spare no effort on new materials and new technology for diesel vehicle energy saving and emission reduction.Along with the development of the diesel engine machine cleaning technology, the particulate materials emitted by diesel engines become more and more small, a large number of studies show that these small particles are very harmful for human health (He et al., 2007).Therefore, more and more countries pay attention to the small particulate emission and studies show that Carbonized Micron Wood Fiber (CMWF) DPF is expected to become one of the effective methods to solve the problem of small particle emission in future.
The theory and experiment prove that, The DPF made by CMWF has high filtration efficiency, less exhaust back pressure and long service life, which open a new direction for the purification of diesel engine exhaust gas (Ma et al., 2009).
The permeability and inertial of porous media are two important parameters related to the characteristics of fluid flow in the porous medium (ISO4022, 1977;Kaviany, 1991).Therefore, it is very important for the research on porous materials to determine the permeability and inertial coefficient of porous mediaexactly.In this study, the numerical simulation and experimental study of exhaust flow characteristics in CMWF DPF was investigated and the relationship of the permeability, inertial coefficient and ∆p/H (the ratio of the pressure difference to the material length) with v 1 (the gas flow velocity before filter material) was obtained, which could provide an effective basis for the structure design and the theory or experimental research.

CARBONIZED MICRON WOOD FIBER FILTER MATERIAL
Processing the high strength wood fiber to microns thickness can form micron wood fiber, then applying such micron wood fiber materials and other materials in appropriate proportion could form DPF filter by a specific carbonization process, micron wood fiber and CMWF filter as shown in Fig. 1.In the research on lumber nano-micron processing technology, the restructure of fiber structure would change the arrangement shape of wood structure when the wood fiber is processed into the micron thickness, so the ultrathin processing method could apply to adjust make the wood fiber structure to increase the strength and toughness and the super finishing could apply to eliminate natural defect of wood itself (Ma, 2001;Ma et al., 2003).Through depth carbonation by use some special method, micron wood fiber produced by this technology processing has not only high porosity, large specific surface area, faster adsorption desorption speed and larger adsorption capacity, but also different active functional groups on carbide surface, which have catalytic conversion function to pollutants and could also remove nitrogen oxides and carbon and smoke particles in a certain temperature.

THE NUMERICAL SIMULATION METHOD OF EXHAUST FLOW IN CMWF FILTER
Porous medium, pure fluid coupling flow area, could be formed when the diesel exhaust flows through the DPF filter.For these kind problems of coupling area flow, the traditional numerical simulation mainly has two methods: the first one is the numerical simulation method which uses Darcy or Forchheimer equation and slip boundary condition (Beavers and Joseph, 1967); the second one is the numerical simulation method which uses Brinkman equation or Brinkman-Forchheimer equation (Beavers and Joseph, 1967).Due to the diesel engine exhaust has high emission rate and great pressure, the steady non Darcy flow by using the compressible gas through the porous media was used to take a flow analysis, the flow diagram as shown in Fig. 2. In the figure, p 1 , ρ 1 and v 1 are pressure, density and speed of exhaust before the DPF filtration device whose x axis length is H, p 0 , ρ 0 and v 0 are ones of exhaust after the DPF.Because of the exhaust would be released into the atmosphere from the DPF directly, the In order to establish the mathematical model of exhaust flow in CMWF DPF, the study make the following assumptions about the flow analysis: Flow steady non-Darcy flow which meet the Darcy-Forchheimer law; the fluid temperature T is constant, when Klinkenberg effect is ignored; the porous medium is rigid porous medium.
Considering the motion equation (Darcy-Forchheimer equation) corrected by Forchheimer is: where, p = The gas pressure expressed in Pa v = The flow rate expressed in m/s ρ = The gas density expressed in kg/m 3 µ = The viscosity coefficient β = The inertial coefficient Because in the particular container, the high speed flow of the exhaust meets mass conservation equation, the relationship between mass average flow and crosssectional area of the filter meets the following equation (Zuo, 2007): where, S = The cross-sectional area expressed in m 2 Q m = The mass average flowexpressed in kg/s The state equation is: where, p = The absolute gas pressure expressed in N/m 2 R = The gas constant, the value of dryair is 287.1 N.m/ (kg.K) T = The air thermodynamic temperature expressed K With reference to Eq. ( 2) and ( 3): Substituted in Eq. ( 1): Because the exhaust flows in CMWF DPF steady flow given by straight pipe inlet parameters, Q m is a constant.For rigid porous media, k and β are all constants.For the gas isothermal flow, T and µ are all constants: The equation above is flow pressure equation, which could be integrated: x = 0 and p = p 1 , the boundary condition, was substituted to determine the integration constant: ଶ , which substituted in Eq. ( 8): The equation above is the flow pressure distribution.Formula (4) shows that: So the velocity distribution of the gas in the porous medium can be obtained as follow: According to formula ( 9) and ( 6), when x = H, p = p 0 , the follows could be known as: Finishing could be obtained: For the condition of that the gas pressure is p 2 , velocity is v 2 before the porous medium: Simultaneous ( 14) and ( 15): In the research on flow characteristics of exhaust in the CMWF DPF, the relationship of ∆p/H (the ratio of pressure difference to the length of DPF filter) with the exhaust flow rate v 1 is very important.According to it the relationship of pressure difference with exhaust flow could be obtained easily.By using above formulas, the analytical expression of the relationship of ∆p/H and v 1 which suitable for the compressible gas can be obtained.According to the formula ( 9) and ( 6): Formula ( 18) is quadratic equation with variable p 1 , solve the equation: According to formula (13): can be adopted as basis of contrast enhancement methods.So, ଶுఉ௩ భ మ ோ் < 1, the reasonable solution of formula (18): Formula ( 21) shows the relationship of the pressure before CMWF DPF with flow velocity.Then the relationship of ∆p/H and v 1 could be obtained: The simulation experiment of the flow characteristics of CMWF DPF: The method to study the flow characteristics of CMWF filter material by using diesel engine bench test has high fuel consumption, long test cycle, the different effect factors for exhaust flow characteristics caused by the model and running condition of engine and particle deposition.So the results of bench test have some certain limitations.In order to measure the exhaust flow characteristic in CMWF DPF accurately and economically, the measurement device for exhaust flow characteristic in CMWF DPF is designed.
The test instrument mainly includes: LGU22A fixed twin-screw air compressor with 3.8 m 3 /min maximum displacement and 0.7 M maximum working pressure; the air reservoir with the heating system; TF turbine flow sensor, with 0.7 M maximum pressure and ±3% FS measurement accuracy; HJ69-JPG3 pressure sensor with 0~350 kPa~70 MPa measuring range; PT100 temperature sensor; self-development detachable DPF, whose photograph is shown in Fig. 3.The length of intermediate pipe selectable mainly includes 50, 100, 150 and 200 mm, respectively.
In the test, three kinds of CMWF filter with different apertures include crude holes, holes and pores (respectively marked as filter (I, II, III) were studied.These filters are all, in clean state, whose crosssectional area and length are all 1.539×10 -2 m 2 and 100 mm respectively.The mainly parameters of CMWF filter are shown in Table 1.
In order to obtain the average temperature of the airflow in CMWF DPF, the American Raiterrayteck non-cantact infrared thermometer was apply to measure the actual exhaust temperatures of 205 diesel vehicles driven into Harbin at Harbin highway toll stations, which measuring position is DPF installation position, the tail pipe.By averaging the test results, we could obtain that the average temperature at tail pipe T is 367.39K, corresponding gas dynamic viscosity coefficient µ = 2.43×10 -5 kg/m.s.By using the exhaust flow characteristic measure device for CMWF DPF, we could control the temperature of the airflow, change the pressure and flow of the airflow in front of the filter, simulate the flow characteristics of diesel engine exhaust and also measure a series of pressure and velocity in front of the filter.The data about filter I are shown in Table 2; the ones about II and III are omitted here.

SIMULATION AND EXPERIMENTAL RESULT ANALYSIS
Combined with known conditions, one of the total 16 sets of data was banded together with any other one of all and the penetration and inertia coefficient could be calculated by using formula ( 15) and ( 16), then by omitting unreasonable results and averaging the remaining ones, the penetration and inertia coefficient of filter I could be obtained: k = 3.000×10 -8 m 2 ; β = 701.1 m -1 .The ones of filter II and III could be obtained by using the above method.Substituting the permeabilities and inertia coefficients of filter I, II and III into formula, respectively, three parabolas could be obtained.By drawing the coordinate points of ∆p/H (∆p is the pressure difference of the values detected by two pressure sensors) and v 1 and the three parabolas into the same coordinate system, the relationship of ∆p/H and v 1 of CMWF DPF could be obtained, which was shown in Fig. 4.
Figure 4 shows that the experimental results agree with theoretical ones excellently, so the numerical simulation method is feasible.When the porosities of the filters are similar, the smaller the average pore size is, the longer the focal of the parabola, which means the faster the increasing speed of filter resistance for exhaust flow with the increase of exhaust flow velocity.According to the references (Ma et al., 2009;Neale and Nader, 1974;Hua and Yang, 1985), the total remove efficiency of DPF for soot is inversely proportional to Fig. 4: Relation between ∆p/H and s v 1 of CWMF DPF pore diameter; the pressure in filter and filtration efficiency are all proportional to the length of filter.So the determine of pore diameter is a very important factor which could keep the balance of filtration efficiency and exhaust resistance when we design a CMWF DPF.In the permission of vehicle chassis structure, the filter media with longer length and bigger pore diameter is sure to win out over ones with shorter length and smaller pore diameter, which could keep smaller resistance in filter under the same filtration efficiency.

CONCLUSION
• In this study, CMWF was used in the purification of the diesel exhaust.According to the characteristic of higher pressure and velocity when exhaust flow through the filter, the steady non Darcy flow by using the compressible gas through the porous media was used to take an flow analysis for CMWF DPF and the permeability, inertia coefficient and the geometric relationship of ∆p/H (the ratio of pressure difference to the length of DPF filter) with v 1 (the exhaust flow rate in front of filter).The test shows that the results detected by the self-developed CMWF flow characteristic simulation test-bed are agree with theoretical results greatly, which shows the numerical simulation method is feasible.• The determination of the pore diameter is a very important factor.Theory and practice show that, in the permission of vehicle chassis structure, the filter media with longer length and bigger pore diameter is sure to win out over ones with shorter length and smaller pore diameter, which could keep smaller resistance in filter under the same filtration efficiency.
Fig. 1: Micron wood fiber and CMWF DPF filter

Fig. 2 :
Fig. 2: Flow diagram of diesel engine exhaust gas flowing through DPF value of p 0 equals approximate standard atmospheric pressure, p 0 = 1.013×10 5 Pa.In order to establish the mathematical model of exhaust flow in CMWF DPF, the study make the following assumptions about the flow analysis: Flow steady non-Darcy flow which meet the Darcy-Forchheimer law; the fluid temperature T is constant, when Klinkenberg effect is ignored; the porous medium is rigid porous medium.Considering the motion equation (Darcy-Forchheimer equation) corrected by Forchheimer is:

Table 1 :
Major parameters of CMWF filters