Experimental And Numerical Investigations Of Combustion Process In Micro Catalyst Combustor

There has been major advancement in micro processing technology in recent years leading to the development of micro-electromechanical systems(MEMS).The MEMS technology has caught the attention of many researchers as its application is of national interest and has broader prospects.The world is now tilting to the use of power devices with high energy densities,longer service life and small scaled to replace the traditional lithium-ion batteries.Micro-power systems based on combustion meets these requirements.As the major component of micro-power units,the performance of micro-combustor is critical to the integral performance of the system.However,there are major problems associated with the reduction of the micro combustors.For example,the combustion space is very small leading to a significant reduction in the residence time of fuel and oxidant in the micro-combustor which makes it difficult to achieve a higher energy conversion rate.In addition,reduction in the size of the micro combustor leads to high surface-to-volume ratio and increase in heat loss.This ultimately causes flame instability and low combustion efficiency.Catalyst imbedded on the micro combustor has proven to be an effective way of solving the problems enumerated above.However,there have been very few researches in that regards.Therefore,it is necessary to carry out research on the combustion characteristics of micro-scale catalytic combustion and to reveal the interaction mechanism between the catalytic and gas-phase reactions.In this work,experimental studies and numerical simulation of hydrogen/oxygen premixed combustion in a micro catalytic/non-catalytic combustors were carried out.The combustion characteristics under different parameters were analyzed.The interaction between the catalytic and gas-phase reactions on combustion was studied.The influence between combustor structure and catalytic combustion characteristics was proposed.The following summarizes the major conclusion derived from the thesis:(1)On the basis of detailed literature search,a parallel plate micro combustor and catalytic platinum segments were designed and fabricated.A cross-section and sudden expansion combustors were also assembled with different combinations of platinum and steel segments.The test bench was set up;the models of the three micro combustors;parallel plate,sudden expansion and variable cross-section micro combustor were established,and the grid-independent inspection and experimental verification were completed.(2)The characteristics of the catalytic and gas-phase combustion were analyzed and the relevant laws were obtained by changing the inlet velocity,equivalent ratio,the height of the inlet channel,catalytic position and dimensions.The results show that the temperature of the centerline of the outer wall of the combustor increased first and then decreased under different working conditions.With the increase of inlet velocity,the outer wall temperature increased.With the increase of the equivalent ratio,the outer wall temperature first increased to the maximum value at the equivalent ratio of 1.0 and then decreased.Under the same inlet flow rates,the wall temperature was lower at a smaller inlet height.A greater effect was obtained on the micro-scale combustion when the catalytic platinum segment was close to the combustor's inlet,but the influence of catalytic area was not obvious.(3)Based on the experimental verification,the simulation model about micro catalyst/non-catalyst combustion was established and the combustion characteristics were achieved corresponding to microscale gas-phase/couple/catalytic combustion under different inlet parameters.The reaction relationship between the catalytic and gas-phase reaction in couple combustion were analyzed.The result showed that: As the velocity increased,maximum fuel conversion efficiency was achieved in the couple combustion.The effect of catalyst from platinum segment decreased the combustion chamber's average temperature and the outlet average temperature.With the increase of the equivalent ratio and at fuel-lean condition,the temperature of the combustor first increased to the maximum value when the equivalent ratio was 1.0,and then the temperature decreased.The maximum fuel conversion efficiency was achieved in the couple combustion,but it was adverse at fuel-rich condition.Because of the competition of reactant and intermediate products and the absorption/desorption of gas-phase components between the catalytic and gas-phase reactions,the catalytic reaction has obvious competitive advantages on and around the catalytic surface.These were the reasons which led to the differences between couple combustion and the others.(4)The combustion characteristics in the catalytic combustor with different structures were discussed,and the influencing mechanisms of the structure and combustion characteristics were achieved.The result showed that the recirculation zone in the sudden-expansion and variable-section micro combustor was beneficial for the mixture and transport of reactant and intermediate products,and could promote combustion and enhance fuel conversion efficiency.It could achieve higher conversion efficiency if the catalytic segment was placed near the combustor's inlet.The sudden-expansion surface in the combustor transfers the heat back to the upstream of the inlet.This heat is used to pre-heat the cold gas emanating from the inlet of the combustor and thus enhances the catalytic surface temperature and then improves the heat flux and catalytic reaction velocity.The maximum heat value in the sudden-expansion combustor has the highest conversion efficiency.The thesis provided an experimental and numerical basis for micro-scale combustion theory,and provided theoretical supports for the improvement of combustion performance.The research methods and conclusions proposed in this thesis also have certain reference values for the design and optimization of micro combustors.