Experimental Study And CFD Modeling On The Process Of Heat Transfer And Cracking Reactions From Hydrocarbon Fuel In Microchannel

As a key technology to solve the problem of thermal protection from scramjet,the design of regenerative cooling channel is of great importance. For this, acomprehensive understanding to the characteristics of heat transfer from endothermichydrocarbon fuel in regenerative cooling channel is needed. Actually, the process ofheat transfer from endothermic hydrocarbon fuel in regenerative cooling channel is astrongly couping process of multiple phenomenon containing flow, heat transfercracking, coking and etc. At present, not enough information is available on theresearch of this process. In this paper, electrically heated tube is used to simulate thecooling channel, while jet fuel model compounds–decane is used as the researchobject. Effect on the couping process from different conditions is multi-dimensionallyinvestigated through both the experiment al and calculational methodology.The CFD model used to simulate the flow and heat transfer process of decane inthe electrically heated tube was established. During this section, the effect on theabove process from heating electric current, operating pressure, rate of flow, pipediameter was investigated. It is found that, increasing heating electric current canincrease both wall temperature and bulk temperature, which not only have axialdifference, but also have radial difference, as well as flow and heat transferperformance. Decreasing pressure can increase heat transfer performance, and reducewall temperature and bulk temperature near the critical pressure area(above thecritical pressure). Further more, the effect becomes more obvious when the pressure isclose to the critical pressure area. At the same time, the flow performance is weakenby decreasing pressure. Increasing mass flow rate can reduce wall temperature andbulk temperature, however, heat transfer performance is enhanced with increasingmass flow rate. In the low temperature area, increasing flow rate can increase flowperformance while in the high temperature area, increasing flow rate will decrease theflow performance, and in the middle temperature area, flow rate has no influence onthe flow performance. Increasing tube diameter reduces both wall temperature andbulk temperature, as well as flow and heat transfer performance.By the introduction of secondary reaction from C5-C91-alkenes, modified PPDmodel which can predict the thermal cracking of decane at higher conversion wasdeveloped. the CFD model used to simulate the coupling process among flow, heat transfer and cracking reaction of n-decane in the electrically heated tube wasestablished. Using the modified PPD kinetics model, the effect on thermophysicalproperties, flow and heat transfer from cracking reactions was evaluated. It is foundthat, cracking reactions have an significant impact on the thermophysical properties,including decreasing density, declining specific heat, reducing heat conductivity, andincreasing viscosity. Cracking reactions have little influence on the flow performanceof fluid, but significantly improve the heat transfer performance between wall andfluid, in turn, decrease the wall and bulk temperature. At the outlet of tube, Nuincreases by26.7%, and wall temperature drops by about139.5℃, which contributes12.2%of the total wall temperature rise. Secondary cracking reaction has little effecton the wall temperature, bulk temperature and cracking conversion, however,obviously increases the yield of small molecule cracking products. At the outlet oftube, the yield of propylene (coking precursor) increases by about7.5%. It is alsofound that, the distribution of bulk temperature and composition in the electricallyheated tube not only has axial difference, but also has radial difference. At the outletof tube, compared with the value from axis, the bulk temperature and mass fraction ofpropylene from inwall increase by about50.9℃and3.9%respectively.