The CFD Simulation And Optimization Of Flow/Heat Transfer Process In "Waste Heat Recovery-Methanol Cracking" Bottom Circulation

Currently, the world is facing enormous challenge in energy crisis and environmental pollution. As a major equipment of energy utilization and conversion, internal combustion engine has become the main target of energy saving and emission reduction. Application of alternative fuels, improvement of in-cylinder combustion process, recovery of waste heat are the most promising energy saving technology of internal combustion engine. According to Chinese basic national conditions of "lack of oil, less gas and rich in coal", while the industrial application of coal-to-methanol is very mature, our project team have proposed a new joint thermodynamic cycle which combined engine waste heat recovery with methanol fuel improvement. This new joint thermodynamic cycle is expected to solve the problem of the direct application of methanol on internal combustion engine and realize the goals of the improvement of total energy efficiency and the reduction of pollution emission. In this paper, we are working on some core issues of "waste heat recovery- methanol cracking". These crucial scientific issues are design and optimization of methanol cracking heat exchanger and numerical simulation of the flow/heat transfer process in bottom circulation..This paper proposed the design method of methanol cracking heat exchanger of coupling one-dimensional thermodynamic performance calculation with three-dimensional CFD numerical simulation. Based on a new type model of methanol cracking heat exchanger which proposed by our project team, we completed the one-dimensional and three-dimensional design of methanol cracking heat exchanger and produced the sample of methanol cracking heat excha nger. Then, we built a "waste heat recovery- methanol cracking" bottoming cycle test bench. Recalibration simulation model based on the measured data, the flow/ heat transfer process of methanol in the methanol cracking heat exchangers have been carried out several times of simulation optimization, and methanol flow and heat transfer process variation with parameters of engine exhaust was obtained.The test results of "waste heat recovery- methanol cracking" bottom cycle show that the recycling rates of bottoming cycle system are as high as 35%-45% considered in the state of the methanol cracking heat exchanger outlet, and the recycling rates of bottoming cycle system up to 17.76% considered in the inlet state of internal combustion engine. Methanol cracking rate has no effect on the recycling rates considered in the state of the methanol cracking heat exchanger outlet. But it has direct influence on the recycling rates considered in the inlet state of internal combustion engine.Improving the flow and heat transfer process of methanol cracking heat exchanger is bound to improve the utilization of waste heat recovery of bottoming cycle. Thus, based on the respective boundary conditions provided by the test, using the CFD simulation method, this article anal yzed the flow and heat transfer process of exhaust and methanol vapor flow field, and designed an optimization model. The calculated results show that the pressure loss of exhaust gas flow field and flow field of methanol decreased by 439 Pa and 3945 Pa, while the average surface heat transfer coefficient increased by 4W/(m~2·K) and 7.6W/(m~2·K). The results of the analysis provide a feasible way for the design of high efficient methanol cracking heat exchanger in the future.Through this study, we have solved some problems of flow/heat transfer process in "waste heat recovery- methanol cracking" bottom circulation, completed the design and optimization of methanol cracking heat transfer, achieved synergic optimization of methanol flow and heat transfer process, reduced the heat transfer and flow loss in the methanol cracking heat exchanger and provided a new way of designing high-efficiency and low-resistance of heat transfer equipment.