Numerical simulation of mixing characteristics of CNG-air mixer for dual-fuel vehicle
A compressed natural gas (CNG)-air mixer is a device like a carburettor positioned at the air intake manifold of the engine to mix CNG with incoming air at proper amounts of CNG and air prior to entering the combustion chamber. According to literature, the best design of CNG-air mixer is one that...
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Format: | Thesis |
Language: | English |
Published: |
2016
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Online Access: | http://psasir.upm.edu.my/id/eprint/67083/1/FK%202016%20146%20IR.pdf http://psasir.upm.edu.my/id/eprint/67083/ |
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Summary: | A compressed natural gas (CNG)-air mixer is a device like a carburettor positioned at
the air intake manifold of the engine to mix CNG with incoming air at proper
amounts of CNG and air prior to entering the combustion chamber. According to
literature, the best design of CNG-air mixer is one that is able to meet the conditions
of 1) supply the engine with a homogeneous mixture of CNG and air, 2) with air and
CNG with the required air fuel ratio (AFR), and 3) without reduction of the air intake
manifold size. The homogeneous mixture occurs when the uniformity index (UI) of
methane mass fraction (MCh4) =1.0. From previous studies, there is no design of
CNG-air mixer which could satisfy all the above three conditions at the same time.
This research carried out a computational fluid dynamics (CFD) study to design a
CNG-air mixer for CNG-diesel dual fuel (DDF) engine. The objectives of this study
were to examine the performance of existing Secondary Fuel Premixing Controller
(SFPMC) commercial mixer and modify it in terms of air fuel ratio (AFR) and CNGair
mixture homogeneity (CAMH).
The validity and reliability procedures of the simulation results were carried out
using the grid independent test, and verification by comparing the results with the
literature.
Results from simulation indicated that the original mixer (model 1) was unable to
control AFR due to the shaft design of the control valve. Furthermore, this mixer
could not provide a homogeneous mixture of CNG and air due to the mixer’s internal design, position and directions of the CNG holes. Therefore, the mixer design was
modified in terms of AFR and CNG-air mixture homogeneity. Design modification
of the control valve shaft was the first method to control AFR, while the second
modification involved removing the control valve and also assuming that the gas
flow rate is controlled by the electronic flow controller and electronic control unit
(ECU). For the mixture homogeneity, there were 10 alternative designs that were
tested (models 2-11) to achieve the desired design. The uniformity index (UI) of
methane mass fraction (MCh4) was the approach used to quantify CNG spread on the
mixer outlet. The UI of MCh4 represents how MCh4 varies over a plane surface, where
a value of 0 and 1 indicate the lowest and highest uniformity of gas spread,
respectively. Based on the results obtained from mixer models 1-11, it can be
concluded that models 8 and 11 showed a superior performance in terms of UI. The
minimum UI of MCh4 at the outlet of these two mixer models was not less than 0.96
at various engine speeds of 1000, 1500, 2000, 3000, and 3600 rpm, and 0.87 at
various AFRs of 10, 17.2, 20, 30, and 40. On the other hand, at the outlet of original
mixer, the maximum UI of MCh4 was not higher than 0.57 at various engine speeds,
and 0.58 at various AFRs. In terms of AFR, the optimized mixer with a new control
valve shaft showed better controlling of AFR at the various engine speeds compared
with the original mixer. |
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